PATENT DOCUMENT

Publication Number: US-12148332-B1
Application Number: US-202318478796-A
Country: US
Kind Code: B1

Title: Head mountable display

Abstract:
A head mountable device includes a front cover defining an external forward facing surface and a nasal bridge, a first rearward facing display, a second rearward facing display, a first forward facing camera, a second forward facing camera, and a controller electrically coupled to the first forward facing camera, the second forward facing camera, the first rearward facing display, and the second rearward facing display. The nasal bridge is disposed between the first forward facing camera and the second forward facing camera and the controller is configured to cause mixed reality video pass-through from the first forward facing camera to the first rearward facing display including a first image and from the second forward facing camera to the second rearward facing display including a second image.

Claims:
What is claimed is: 
     
       1. A head mountable device, comprising:
 a front cover defining:
 an external forward facing surface; and 
 a nasal bridge configured to accommodate a nose when the head mountable device is donned; 
 
 a first rearward facing display configured to project light in a rearward direction opposite the forward facing surface; 
 a second rearward facing display configured to project light in the rearward direction; 
 a first forward facing camera configured to capture a first image through the front cover; 
 a second forward facing camera configured to capture a second image through the front cover; and 
 a controller coupled to memory including instructions, the controller electrically coupled to the first forward facing camera, the second forward facing camera, the first rearward facing display, and the second rearward facing display; wherein: 
 the nasal bridge is disposed between the first forward facing camera and the second forward facing camera; and 
 the controller is configured to, when executing the instructions, cause mixed reality video pass-through from the first forward facing camera to the first rearward facing display including the first image and from the second forward facing camera to the second rearward facing display including the second image. 
 
     
     
       2. The head mountable device of  claim 1 , further comprising a shroud disposed between the first forward facing camera and the front cover. 
     
     
       3. The head mountable device of  claim 2 , wherein the shroud defines a first transparent portion and a second transparent portion. 
     
     
       4. The head mountable device of  claim 3 , wherein:
 the first forward facing camera is configured to capture a first image through the first transparent portion; and 
 the second forward facing camera is configured to capture a second image through the second transparent portion. 
 
     
     
       5. The head mountable device of  claim 3 , further comprising a front facing display configured to project light through the first transparent portion and the front cover. 
     
     
       6. The head mountable device of  claim 5 , wherein the first forward facing camera is configured to capture an image through the second transparent portion. 
     
     
       7. The head mountable device of  claim 2 , wherein:
 the head mountable device further comprises a sensor bracket; 
 the first forward facing camera and the second forward facing camera are coupled to the sensor bracket; and 
 the shroud is configured to hide a view of the sensor bracket through the front cover. 
 
     
     
       8. The head mountable device of  claim 7 , further comprising:
 a depth projector coupled to the sensor bracket; and 
 a depth sensor coupled to the bracket. 
 
     
     
       9. A wearable electronic device, including:
 a structural frame; 
 a display coupled to the structural frame for projecting light in a first direction; 
 a sensor mounting bracket coupled to the structural frame, the bracket including:
 a middle portion coupled to the structural frame; 
 a first arm cantilevered from the middle portion; and 
 a second arm cantilevered from the middle portion; 
 
 a first camera mounted to the first arm and configured to capture images in a second direction opposite the first direction; 
 a second camera mounted to the second arm and configured to capture images in the second direction; 
 a third camera mounted to the structural frame and configured to capture images in the second direction; and 
 a fourth camera mounted to the structural frame and configured to capture images in the second direction; 
 wherein the sensor mounting bracket is disposed between the third camera and the fourth camera. 
 
     
     
       10. The wearable electronic device of  claim 9 , further comprising a depth sensor coupled to the sensor mounting bracket. 
     
     
       11. The wearable electronic device of  claim 10 , further comprising a depth projector coupled to the sensor mounting bracket. 
     
     
       12. The wearable electronic device of  claim 9 , further comprising a first infrared illuminator mounted to the structural frame. 
     
     
       13. The wearable electronic device of  claim 12 , further comprising a second infrared illuminator mounted to the structural frame. 
     
     
       14. The wearable electronic device of  claim 13 , wherein the sensor mounting bracket is disposed between the first infrared illuminator and the second infrared illuminator. 
     
     
       15. The wearable electronic device of  claim 9 , further comprising a depth sensor mounted at the central middle portion of the sensor mounting bracket. 
     
     
       16. A head mountable device, comprising:
 a structural frame; 
 a front cover coupled to the structural frame, the front cover defining an external forward facing surface; 
 a forward facing display configured to project light in a forward direction through the front cover; 
 a rearward facing display configured to project light in a rearward direction opposite the forward direction; 
 an outward facing sensor assembly configured to capture images from an external environment in front of the head mountable device, the outward facing sensor assembly comprising:
 a first camera pointed in a first direction; and 
 a second camera pointed in a second direction; 
 
 an inward facing sensor assembly configured to capture images of the user when the user dons the head mountable device, the inward facing sensor assembly comprising an eye tracking camera pointed in the rearward direction; and 
 a controller communicatively coupled to a memory device including instructions, the controller electrically coupled to the outward facing sensor assembly and the inward facing sensor assembly, the controller, when executing the instructions, configured to:
 cause the rearward facing display to project first images captured by the outward facing sensor assembly; and 
 cause the forward facing display to project second images captured by the eye tracking camera. 
 
 
     
     
       17. The head mountable device of  claim 16 , wherein the inward facing sensor assembly includes a jaw camera pointed downward and configured to capture the jaw of the user. 
     
     
       18. The head mountable device of  claim 17 , wherein:
 the eye tracking camera is a first eye tracking camera; and 
 the inward facing sensor assembly further comprises a second eye tracking camera. 
 
     
     
       19. The head mountable device of  claim 18 , wherein the first eye tracking camera and the second eye tracking camera are configured to capture images of a single eye of the user. 
     
     
       20. The head mountable device of  claim 18 , wherein:
 the first eye tracking camera is configured to capture first images of a first user eye; and 
 
       the second eye tracking camera is configured to capture second images of a second user eye.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This claims priority to U.S. Provisional Patent Application No. 63/586,403, filed 28 Sep. 2023, and entitled “HEAD MOUNTABLE DISPLAY,” to U.S. Provisional Patent Application No. 63/506,020, filed 2 Jun. 2023, and entitled “HEAD MOUNTABLE DISPLAY,” and to U.S. Provisional Patent Application No. 63/502,408, filed 15 May 2023, and entitled “HEAD MOUNTABLE DISPLAY,” the entire disclosures of which are hereby incorporated by reference in their entirety. 
    
    
     FIELD 
     The present disclosure relates generally to head mountable computer systems that provide computer-generated experiences, including, but not limited to, electronic devices that provide virtual reality and mixed reality experiences via a display. 
     BACKGROUND 
     The development of computer systems for augmented reality, including head mountable computer systems, has increased significantly in recent years. Example augmented reality environments include at least some virtual elements that replace or augment the physical world. Input devices, such as cameras, controllers, joysticks, touch-sensitive surfaces, and touch-screen displays for computer systems and other electronic computing devices are used to interact with virtual/augmented reality environments. Example virtual elements include virtual objects, such as digital images, video, text, icons, and control elements such as buttons and other graphics. 
     SUMMARY 
     In at least one example of the present disclosure, a head mountable device includes a front cover defining an external forward facing surface and a nasal bridge configured to accommodate a nose when the head mountable device is donned, a first rearward facing display configured to project light in a rearward direction opposite the forward facing surface, a second rearward facing display configured to project light in the rearward direction, a first forward facing camera configured to capture a first image through the front cover, a second forward facing camera configured to capture a second image through the front cover, and a controller coupled to memory including instructions, the controller electrically coupled to the first forward facing camera, the second forward facing camera, the first rearward facing display, and the second rearward facing display. The nasal bridge is disposed between the first forward facing camera and the second forward facing camera and the controller is configured to, when executing the instructions, cause mixed reality video pass-through from the first forward facing camera to the first rearward facing display including the first image and from the second forward facing camera to the second rearward facing display including the second image. 
     In one example, the device further includes a shroud disposed between the first forward facing camera and the front cover. In one example, the shroud defines a first transparent portion and a second transparent portion. In one example, the first forward facing camera is configured to capture a first image through the first transparent portion and the second forward facing camera is configured to capture a second image through the second transparent portion. In one example, the device further includes a front facing display configured to project light through the first transparent portion and the front cover. In one example, the first forward facing camera is configured to capture an image through the second transparent portion. In the head mountable device further includes a sensor bracket, the first forward facing camera and the second forward facing camera are coupled to the sensor bracket, and the shroud is configured to hide a view of the sensor bracket through the front cover. In one example, the device further includes a depth projector coupled to the sensor bracket and a depth sensor coupled to the bracket. 
     In at least one example of the present disclosure, a wearable electronic device includes a structural frame, a sensor bracket coupled to the structural frame and including a central portion coupled to the structural frame, a first arm cantilevered from the central portion, and a second arm cantilevered from the central portion, a first camera mounted to the first arm, a second camera mounted to the second arm, a third camera mounted to the structural frame, and a fourth camera mounted to the structural frame. In such an example, the sensor bracket is disposed between the third camera and the fourth camera. 
     In one example, the device further includes a depth sensor coupled to the sensor bracket. In one example, the device further includes a depth projector coupled to the sensor bracket. In one example, the device further includes a first infrared illuminator mounted to the structural frame. In one example, the device further includes a second infrared illuminator mounted to the structural frame. In one example, the sensor bracket is disposed between the first infrared illuminator and the second infrared illuminator. In one example, the device further includes a depth sensor mounted at the central portion of the sensor bracket. 
     In at least one example of the present disclosure, a head mountable device includes a structural frame, a front cover coupled to the structural frame, the front cover defining an external forward facing surface, a forward facing display configured to project light in a forward direction through the front cover, a rearward facing display configured to project light in a rearward direction opposite the forward direction, an outward facing sensor assembly configured to capture images from an external environment in front of the head mountable device, the outward facing sensor assembly including, a first camera pointed in a first direction and a second camera pointed in a second direction, an inward facing sensor assembly configured to capture images of the user when the user dons the head mountable device, the inward facing sensor assembly including an eye tracking camera pointed in the rearward direction, and a controller communicatively coupled to a memory device including instructions, the controller electrically coupled to the outward facing sensor assembly and the inward facing sensor assembly, the controller, when executing the instructions, configured to cause the rearward facing display to project first images captured by the outward facing sensor assembly and cause the forward facing display to project second images captured by the eye tracking camera. 
     In one example, the inward facing sensor assembly includes a jaw camera pointed downward and configured to capture the jaw of the user. In one example, the eye tracking camera is a first eye tracking camera and the inward facing sensor assembly further includes a second eye tracking camera. In one example, the first eye tracking camera and the second eye tracking camera are configured to capture images of a single eye of the user. In one example, the first eye tracking camera is configured to capture first images of a first user eye and the second eye tracking camera is configured to capture second images of a second user eye. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which: 
       I: Overall System 
         FIG.  1 - 1 A  illustrates a front, perspective view of an example of a head-mountable device (HMD). 
         FIG.  1 - 1 B  illustrates rear, perspective view of an example of an HMD. 
         FIG.  1 - 2    illustrates an example of an HMD. 
         FIG.  1 - 3    illustrates a display module of an HMD. 
         FIG.  1 - 4    illustrates a display module of an HMD. 
       II: Cover Glass 
         FIGS.  2 . 0 - 1    illustrates a view of an example of an HMD. 
       2.1: Systems with Transparent Layers 
         FIGS.  2 . 1 - 1    is a perspective view of an illustrative system with a transparent layer in accordance with an embodiment. 
         FIGS.  2 . 1 - 2    is a cross-sectional side view of an illustrative transparent layer overlapping optical components that operate through the transparent layer. 
         FIGS.  2 . 1 - 3    is a cross-sectional side view of an illustrative transparent layer in accordance with an embodiment. 
       2.2: Systems with Displays and Sensors 
         FIGS.  2 . 2 - 1    is a side view of an illustrative electronic device such as a head-mounted device in accordance with an embodiment. 
         FIGS.  2 . 2 - 2    is schematic diagram of an illustrative system with an electronic device in accordance with an embodiment. 
         FIGS.  2 . 2 - 3    is a front view of an illustrative head-mounted device in accordance with an embodiment. 
         FIGS.  2 . 2 - 4    is a cross-sectional top view of an illustrative head-mounted device in accordance with an embodiment. 
         FIGS.  2 . 2 - 5 A  is a cross-sectional side view of an illustrative head-mounted device in accordance with an embodiment. 
         FIGS.  2 . 2 - 5 B  is a cross-sectional side view of another illustrative head-mounted device in accordance with an embodiment. 
         FIGS.  2 . 2 - 6    is a front view of an upper left portion of an illustrative head-mounted device with a publicly viewable display in accordance with an embodiment. 
         FIGS.  2 . 2 - 7 ,  2 . 2 - 8 ,  2 . 2 - 9 ,  2 . 2 - 10 ,  2 . 2 - 11 , and  2 . 2 - 12    are front views of portions of an illustrative head-mounted device in accordance with embodiments. 
         FIGS.  2 . 2 - 13    is a cross-sectional top view of a portion of an illustrative head-mounted device in accordance with an embodiment. 
         FIGS.  2 . 2 - 14    is a cross-sectional side view of a portion of an illustrative head-mounted device with a display in accordance with an embodiment. 
         FIGS.  2 . 2 - 15 ,  2 . 2 - 16 , and  2 . 2 - 17    are cross-sectional side views of illustrative display cover layers overlapping illustrative optical components in accordance with embodiments. 
       2.3: Systems with Supplemental Illumination 
         FIGS.  2 . 3 - 1    is a cross-sectional side view of a portion of an illustrative electronic device with an environmental illumination system in accordance with an embodiment. 
         FIGS.  2 . 3 - 2    is a top view of an illustrative electronic device with an environmental illumination system in accordance with an embodiment. 
         FIGS.  2 . 3 - 3 ,  2 . 3 - 4 ,  2 . 3 - 5 , and  2 . 3 - 6    are cross-sectional side view of illustrative light sources for a supplemental illumination system in accordance with an embodiment. 
         FIGS.  2 . 3 - 7 ,  2 . 3 - 8 , and  2 . 3 - 9    are graphs showing illustrative illumination patterns that may be produced by a supplemental illumination system in accordance with an embodiment. 
         FIGS.  2 . 3 - 10    is a flow chart of illustrative operations involved in using an electronic device such as a head-mounted device with a supplemental illumination system in accordance with an embodiment. 
       2.4: Systems with Displays and Sensor-Hiding Structures 
         FIGS.  2 . 4 - 1    is a front view of an illustrative head-mounted device in accordance with an embodiment. 
       2.5: Systems with Cover Layer Sealing Structures 
         FIGS.  2 . 5 - 1    is a side view of an illustrative electronic device such as a head-mounted device in accordance with an embodiment. 
         FIGS.  2 . 5 - 2    is schematic diagram of an illustrative system with an electronic device in accordance with an embodiment. 
         FIGS.  2 . 5 - 3    is a front view of an illustrative head-mounted device in accordance with an embodiment. 
         FIGS.  2 . 5 - 4    is a front view of an illustrative shroud in accordance with an embodiment. 
         FIGS.  2 . 5 - 5    is a top view of a portion of an illustrative head-mounted device with a display, cover layer, and shroud in accordance with an embodiment. 
         FIGS.  2 . 5 - 6    is a side view of an illustrative cover layer with encapsulation material that seals an edge surface of the cover layer and overlaps a laminate on the cover layer in accordance with an embodiment. 
         FIGS.  2 . 5 - 7    is a side view of an illustrative cover layer with encapsulation material that seals an edge surface of the cover layer in accordance with an embodiment. 
         FIGS.  2 . 5 - 8    is a side view of an illustrative cover layer having an edge surface that is spaced apart from a head-mounted device housing in accordance with an embodiment. 
         FIGS.  2 . 5 - 9    is a side view of an illustrative cover layer with a bumper ring or overmold structure that seals an edge surface of the cover layer in accordance with an embodiment. 
         FIGS.  2 . 5 - 10    is a side view of an illustrative cover layer with an upper laminate that wraps an edge surface of the cover layer in accordance with an embodiment. 
         FIGS.  2 . 5 - 11    is a side view of an illustrative cover layer with a lower laminate that wraps an edge surface of the cover layer in accordance with an embodiment. 
         FIGS.  2 . 5 - 12    is a side view of an illustrative cover layer and glue that fills a gap between an edge surface of the cover layer and a housing structure in accordance with an embodiment. 
         FIGS.  2 . 5 - 13    is a side view of an illustrative cover layer with an upper laminate that extends over the cover layer to a housing structure to separate an edge surface of the cover layer from an exterior of the device in accordance with an embodiment. 
         FIGS.  2 . 5 - 14    is a side view of an illustrative cover layer and a lip formed from a shroud or housing member that overlaps an edge portion of the cover layer in accordance with an embodiment. 
         FIGS.  2 . 5 - 15    is a side view of an illustrative cover layer and a lip formed from a shroud or housing member that overlaps an edge portion of the cover layer, along with an upper laminate that wraps around the edge portion in accordance with an embodiment. 
       2.6: Electronic Devices with Antennas and Optical Components 
         FIGS.  2 . 6 - 1    is a top view of a head-mounted device. 
         FIGS.  2 . 6 - 2    is a rear view of a head-mounted device. 
         FIGS.  2 . 6 - 3    is a schematic diagram of a head-mounted device. 
         FIGS.  2 . 6 - 4    is a view of a portion of a head-mounted device with a head-mounted housing frame and a camera support member. 
         FIGS.  2 . 6 - 5    is a front view of a portion of a head-mounted device with a camera support structure. 
         FIGS.  2 . 6 - 6    is a cross-sectional side view of a portion of a head-mounted device with a camera support structure. 
         FIGS.  2 . 6 - 7    is a schematic diagram of wireless communications circuitry. 
         FIGS.  2 . 6 - 8    is a diagram of an antenna. 
         FIGS.  2 . 6 - 9 ,  2 . 6 - 10 ,  2 . 6 - 11 , and  2 . 6 - 12    are cross-sectional side views of portions of a support structure, such as a camera support structure with antennas. 
         FIGS.  2 . 6 - 13    is a top view of a camera support structure. 
         FIGS.  2 . 6 - 14    is a cross-sectional side view of a camera support structure. 
         FIGS.  2 . 6 - 15    is a cross-sectional side view of a portion of a camera support structure with a bend sensor to detect camera misalignment. 
         FIGS.  2 . 6 - 16    is a cross-sectional side view of a portion of a camera support structure with an adjustable-orientation camera. 
       III: Display Integration Assembly 
         FIG.  3 - 1    is a view of a display and front-cover assembly of an HMD. 
         FIG.  3 - 2    is a cross-sectional view of a portion of a display assembly of an example HMD. 
         FIG.  3 - 3    is a side view of an example of a display assembly of an HMD. 
         FIG.  3 - 4    is a side cross-sectional view of a portion of a display assembly of an example HMD. 
         FIG.  3 - 4 A  is a side cross-sectional view of a portion of a display assembly of an example HMD. 
         FIG.  3 - 4 B  is a perspective cross-sectional view of a portion of a display assembly of an example HMD. 
         FIG.  3 - 4 C  is a perspective cross-sectional view of a portion of a display assembly of an example HMD. 
         FIG.  3 - 5    is a perspective cross-sectional view of a portion of a display assembly of an example HMD. 
         FIG.  3 - 6    is a perspective cross-sectional view of a portion of a display assembly of an example HMD. 
       IV: Shroud 
       4.0: Systems with Displays and Sensor-Hiding Structures 
         FIG.  4 - 1    is a front view of an illustrative shroud in accordance with an embodiment. 
         FIG.  4 - 2    is a front view of a portion of an illustrative shroud with a curved periphery in accordance with an embodiment. 
         FIG.  4 - 3    is a front view of a portion of an illustrative forward-facing display in accordance with an embodiment. 
         FIG.  4 - 4    is a cross-sectional top view of a portion of an illustrative display in accordance with an embodiment. 
         FIG.  4 - 5    is a cross-sectional top view of a portion of an illustrative head-mounted device with a display and shroud in accordance with an embodiment. 
         FIG.  4 - 6    is a cross-sectional side view of a portion of an illustrative shroud with a through-hole opening to accommodate an optical component in accordance with an embodiment. 
         FIG.  4 - 7    is a cross-sectional side view of a portion of an illustrative shroud with a window member in a through-hole opening in accordance with an embodiment. 
         FIG.  4 - 8    is a cross-sectional side view of a portion of a head-mounted device with a shroud covering a display in accordance with an embodiment. 
         FIG.  4 - 9    is a cross-sectional side view of an illustrative head-mounted device optical component mounting arrangement with an optical component window coating in accordance with an embodiment. 
         FIG.  4 - 10    is a cross-sectional side view of an illustrative head-mounted device optical component mounting arrangement using shroud through-hole openings in accordance with an embodiment. 
         FIG.  4 - 11    is a cross-sectional side view of an illustrative head-mounted device optical component mounting arrangement with a window formed from a transparent window member such as a layer of glass or clear polymer with a coating in accordance with an embodiment. 
       4.1: System with Cover Layer Sealing Structures 
         FIGS.  4 . 1 - 1    is a side view of an illustrative electronic device such as a head-mounted device in accordance with an embodiment. 
         FIGS.  4 . 1 - 2    is schematic diagram of an illustrative system with an electronic device in accordance with an embodiment. 
         FIGS.  4 . 1 - 3    is a front view of an illustrative head-mounted device in accordance with an embodiment. 
         FIGS.  4 . 1 - 4    is a front view of an illustrative shroud in accordance with an embodiment. 
         FIGS.  4 . 1 - 5    is a top view of a portion of an illustrative head-mounted device with a display, cover layer, and shroud in accordance with an embodiment. 
         FIGS.  4 . 1 - 6    is a side view of an illustrative cover layer with encapsulation material that seals an edge surface of the cover layer and overlaps a laminate on the cover layer in accordance with an embodiment. 
         FIGS.  4 . 1 - 7    is a side view of an illustrative cover layer with encapsulation material that seals an edge surface of the cover layer in accordance with an embodiment. 
         FIGS.  4 . 1 - 8    is a side view of an illustrative cover layer having an edge surface that is spaced apart from a head-mounted device housing in accordance with an embodiment. 
         FIGS.  4 . 1 - 9    is a side view of an illustrative cover layer with a bumper ring or overmold structure that seals an edge surface of the cover layer in accordance with an embodiment. 
         FIGS.  4 . 1 - 10    is a side view of an illustrative cover layer with an upper laminate that wraps an edge surface of the cover layer in accordance with an embodiment. 
         FIGS.  4 . 1 - 11    is a side view of an illustrative cover layer with a lower laminate that wraps an edge surface of the cover layer in accordance with an embodiment. 
         FIGS.  4 . 1 - 12    is a side view of an illustrative cover layer and glue that fills a gap between an edge surface of the cover layer and a housing structure in accordance with an embodiment. 
         FIGS.  4 . 1 - 13    is a side view of an illustrative cover layer with an upper laminate that extends over the cover layer to a housing structure to separate an edge surface of the cover layer from an exterior of the device in accordance with an embodiment. 
         FIGS.  4 . 1 - 14    is a side view of an illustrative cover layer and a lip formed from a shroud or housing member that overlaps an edge portion of the cover layer in accordance with an embodiment. 
         FIGS.  4 . 1 - 15    is a side view of an illustrative cover layer and a lip formed from a shroud or housing member that overlaps an edge portion of the cover layer, along with an upper laminate that wraps around the edge portion in accordance with an embodiment. 
         FIGS.  4 . 1 - 16  and  4 . 1 - 17    are side views of illustrative cover layers with upper and lower laminates in accordance with some embodiments. 
         FIGS.  4 . 1 - 18    is a side view of an illustrative cover layer with a laminate and a seal that covers an edge of the laminate in accordance with some embodiments. 
       V: Dust Seal 
       5.1: Seal for an Electronic Device 
         FIG.  5 - 1    shows a cross-sectional view of a portion of an electronic device, according to an example; 
         FIG.  5 - 2    shows a cross-sectional view of a seal, according to an example; 
         FIG.  5 - 3    shows a cross-sectional view of an electronic device, according to an example; 
         FIG.  5 - 4 A  shows a top perspective view of an electronic component and a seal, according to an example; 
         FIG.  5 - 4 B  shows a cross-sectional view of a portion of an electronic device, according to an example; and 
         FIG.  5 - 4 C  shows a cross-sectional view of a portion of an electronic device, according to an example. 
       VI: Sensor System 
         FIG.  6 - 0    illustrates a view of an example of an HMD. 
         FIG.  6 - 1    illustrates a front perspective view of an example of a sensor system for an HMD. 
         FIG.  6 - 2    illustrates a lower perspective view of an example of a sensor system for and HMD. 
         FIG.  6 - 3    illustrates a lower perspective view of an example of a sensor system for and HMD without a front cover assembly. 
         FIG.  6 - 4    illustrates a lower perspective view of an example of a sensor system of and HMD. 
       VII: Antennas 
         FIGS.  7 . 0 - 1    illustrates a view of an example of a display unit of and HMD. 
       7.1: Electronic Devices with Antenna Mounting Structures 
         FIGS.  7 . 1 - 1    is a top view of an illustrative electronic device such as a head-mounted device in accordance with an embodiment. 
         FIGS.  7 . 1 - 2    is a diagram of an illustrative antenna for an electronic device in accordance with an embodiment. 
         FIGS.  7 . 1 - 3    is a perspective view of an illustrative antenna on an illustrative unidirectional structured foam antenna biasing structure in accordance with an embodiment. 
         FIGS.  7 . 1 - 4    is a top view of an illustrative structured foam member in accordance with an embodiment. 
         FIGS.  7 . 1 - 5    is a diagram illustrating how a structured foam member may exhibit preferential unidirectional compression and expansion characteristics in accordance with an embodiment. 
         FIGS.  7 . 1 - 6    is a cross-sectional top view of a right-hand edge portion of an illustrative head-mounted device in which a unidirectional structured foam antenna biasing member (antenna biasing structure) is being used to mount an antenna against a surface of an overlapping layer such as a display cover layer in accordance with an embodiment. 
       7.2: Electronic Devices with Millimeter Wave Antennas 
         FIGS.  7 . 2 - 1    is a top view of an illustrative electronic device with an antenna in accordance with an embodiment. 
         FIGS.  7 . 2 - 2    is a front view of an illustrative antenna for an electronic device in accordance with an embodiment. 
         FIGS.  7 . 2 - 3    is a side view of an illustrative millimeter wave antenna having an array of patch antenna elements in accordance with an embodiment. 
         FIGS.  7 . 2 - 4    is a cross-sectional side view of a corner portion of an illustrative head-mounted device with an antenna in accordance with an embodiment. 
         FIGS.  7 . 2 - 5    is a cross-sectional view of a front portion of an illustrative head-mounted device with an antenna in accordance with an embodiment. 
       7.3: Electronic Devices with Antennas Having Compound Curvature 
         FIGS.  7 . 3 - 1    is a top view of an illustrative electronic device with an antenna in accordance with an embodiment. 
         FIGS.  7 . 3 - 2    is a diagram of an illustrative antenna for an electronic device in accordance with an embodiment. 
         FIGS.  7 . 3 - 3    is a perspective view of an illustrative flexible printed circuit antenna with compound curvature in accordance with an embodiment. 
         FIGS.  7 . 3 - 4    is a side view of illustrative equipment for laminating a flexible printed circuit antenna to a dielectric member such as a polymer layer in accordance with an embodiment. 
         FIGS.  7 . 3 - 5    is a side view of an illustrative printed circuit antenna with compound curvature attached to a compound curvature surface of dielectric member with compound curvature in accordance with an embodiment. 
         FIGS.  7 . 3 - 6    is a perspective view of an illustrative printed circuit antenna with compound curvature laminated to the inner surface of a dielectric member with compound curvature in accordance with an embodiment. 
       VIII: Bent MLB 
         FIG.  8 - 0    illustrates a view of an HMD including a logic board. 
         FIG.  8 - 1    illustrates a plan view of an example of a logic board. 
         FIG.  8 - 2    illustrates a top view of an example of a logic board. 
         FIG.  8 - 3    illustrates a close up view of the logic board shown in  FIG.  8 - 2   . 
         FIG.  8 - 4    illustrates an example of a logic board. 
         FIG.  8 - 5    illustrates a perspective view of a logic board coupled with a fan assembly of an HMD. 
       IX: Thermals 
         FIGS.  9 . 0 - 1    illustrates a view of an HMD. 
       9.1: Air Deflector for a Cooling System in a Head-Mounted Device 
         FIGS.  9 . 1 - 1    illustrates a schematic diagram of an example of a head-mounted device. 
         FIGS.  9 . 1 - 2    illustrates a front view of an example of a head-mounted device. 
         FIGS.  9 . 1 - 3    illustrates a side view of an example of a cooling system. 
         FIGS.  9 . 1 - 4    illustrates a side view of an example of a cooling system having an air deflector. 
         FIGS.  9 . 1 - 5    illustrates a side view of an example of a cooling system having an air deflector. 
         FIGS.  9 . 1 - 6    illustrates a side view of an example of a cooling system having an air deflector. 
         FIGS.  9 . 1 - 7    illustrates a side view of an example of air flow in a cooling system. 
         FIGS.  9 . 1 - 8    illustrates a side view of an example of air flow in a cooling system. 
         FIGS.  9 . 1 - 9    illustrates a block diagram of an example of a head-mounted device. 
       9.2: Fan with Debris Mitigation 
         FIGS.  9 . 2 - 1    illustrates a side view of a head-mountable device, according to some embodiments of the present disclosure. 
         FIGS.  9 . 2 - 2    illustrates a perspective view of a fan for a head-mountable device, according to some embodiments of the present disclosure. 
         FIGS.  9 . 2 - 3    illustrates a sectional view of an assembly of the head-mountable device of  FIGS.  9 . 2 - 1    including the fan of  FIGS.  9 . 2 - 2    in operation to generate a flow, according to some embodiments of the present disclosure. 
         FIGS.  9 . 2 - 4    illustrates another sectional view of the assembly of  FIGS.  9 . 2 - 3    with the fan of  FIGS.  9 . 2 - 2    in stasis and particles entering through the outlet, according to some embodiments of the present disclosure. 
         FIGS.  9 . 2 - 5    illustrates a perspective and sectional view of a fan having an annular ring to direct incoming particles, according to some embodiments of the present disclosure. 
         FIGS.  9 . 2 - 6    illustrates a perspective and sectional view of a fan having an annular ring to direct incoming particles, according to some embodiments of the present disclosure. 
         FIGS.  9 . 2 - 7    illustrates a sectional view of a fan having a base plate with variable thickness, according to some embodiments of the present disclosure. 
         FIGS.  9 . 2 - 8    illustrates a view of a fan having a base plate forming openings, according to some embodiments of the present disclosure. 
         FIGS.  9 . 2 - 9    illustrates a bottom view of a fan having a base plate forming openings and an adhesive pad, according to some embodiments of the present disclosure. 
         FIGS.  9 . 2 - 10    illustrates a sectional view of the fan of  FIGS.  9 . 2 - 9   , according to some embodiments of the present disclosure. 
         FIGS.  9 . 2 - 11    illustrates a view of a fan having a base plate forming openings, according to some embodiments of the present disclosure. 
         FIGS.  9 . 2 - 12    illustrates a perspective and sectional view of the fan of  FIGS.  9 . 2 - 11   , according to some embodiments of the present disclosure. 
         FIGS.  9 . 2 - 13    illustrates a block diagram of a head-mountable device, in accordance with some embodiments of the present disclosure. 
       9.3: Ventilation 
         FIGS.  9 . 3 - 1    illustrates a view of an example of an HMD. 
         FIGS.  9 . 3 - 2    illustrates a rear perspective view of an example of a ventilation assembly of and HMD. 
         FIGS.  9 . 3 - 3    illustrates a perspective cross-sectional view of an example of a fan assembly of an HMD. 
         FIGS.  9 . 3 - 4    illustrates a cross-sectional view of an example of a fan assembly of an HMD. 
         FIGS.  9 . 3 - 5    illustrates a top plan view of an example of a fan for an HMD. 
         FIGS.  9 . 3 - 6    illustrates a bottom plan view of an example of a fan for an HMD. 
         FIGS.  9 . 3 - 7    illustrates an exploded view of an example of a fan for and HMD. 
         FIGS.  9 . 3 - 8    illustrates a rear perspective view of an example of a fan and circuit board assembly of an HMD. 
         FIGS.  9 . 3 - 9    illustrates a perspective view of an example of a fan and circuit board assembly of an HMD. 
         FIGS.  9 . 3 - 10    illustrates a close up perspective view of an example of a fan and circuit board assembly of an HMD. 
         FIGS.  9 . 3 - 11    illustrates a side cross-sectional view of an example of a fan and circuit board assembly of an HMD. 
       X: Chassis 
         FIG.  10 - 0    illustrates a view of an example of an HMD. 
         FIG.  10 - 1    illustrates a view of an example of an HMD. 
         FIG.  10 - 2    illustrates a rear perspective view of an example of an HMD. 
         FIG.  10 - 3    illustrates a front perspective view of an example of a frame assembly of an HMD. 
         FIG.  10 - 4    illustrates a front plan view of an example of a frame assembly of an HMD. 
         FIG.  10 - 5    illustrates a front plan view of an example of a frame assembly of an HMD. 
         FIG.  10 - 6    illustrates a close-up cross-sectional view of a portion of an example of an HMD. 
       XI: Optical Module 
         FIG.  11 - 1    illustrates a view of an example of an HMD. 
       11.1: IPD Adjust 
         FIGS.  11 . 1 - 1    illustrates a partial perspective view of an example of an HMD including an optical module adjustment system. 
       11.1.1: Crown 
         FIG.  11 . 1   . 1 - 1  illustrates a partial perspective view of an example of an HMD including an optical module adjustment system. 
       11.1.1.1: Adjustment Mechanism for Head-Mounted Display 
         FIG.  11 . 1   . 1 . 1 - 1  is a top view of a head-mounted display. 
         FIG.  11 . 1   . 1 . 1 - 2 A is a detail view of an actuator disposed within a head-mounted display similar to the head-mounted display of  FIG.  11 . 1   . 1 . 1 - 1 . 
         FIG.  11 . 1   . 1 . 1 - 2 B is a partially-exploded sectional view of the actuator of  FIG.  11 . 1   . 1 . 1 - 2 A. 
         FIG.  11 . 1   . 1 . 1 - 3 A is a detail view of another actuator disposed within a head-mounted display similar to the head-mounted display of  FIG.  11 . 1   . 1 . 1 - 1 . 
         FIG.  11 . 1   . 1 . 1 - 3 B is a partially-exploded sectional view of the actuator of  FIG.  11 . 1   . 1 . 1 - 3 A. 
         FIG.  11 . 1   . 1 . 1 - 4 A is a detail view of another actuator disposed within a head-mounted display similar to the head-mounted display of  FIG.  11 . 1   . 1 . 1 - 1 . 
         FIG.  11 . 1   . 1 . 1 - 4 B is a partially-exploded sectional view of the actuator of  FIG.  11 . 1   . 1 . 1 - 4 A. 
         FIG.  11 . 1   . 1 . 1 - 5 A is a detail view of an electromagnetic dampening mechanism for an actuator similar to the actuators of  FIGS.  11 . 1   . 1 . 1 - 2 A,  11 . 1 . 1 . 1 - 2 B,  11 . 1 . 1 . 1 - 3 A,  11 . 1 . 1 . 1 - 3 B,  11 . 1 . 1 . 1 - 4 A, and  11 . 1 . 1 . 1 - 4 B. 
         FIG.  11 . 1   . 1 . 1 - 5 B is a detail view of another electromagnetic dampening mechanism for an actuator similar to the actuator of  FIGS.  11 . 1   . 1 . 1 - 2 A,  11 . 1 . 1 . 1 - 2 B,  11 . 1 . 1 . 1 - 3 A,  11 . 1 . 1 . 1 - 3 B,  11 . 1 . 1 . 1 - 4 A, and  11 . 1 . 1 . 1 - 4 B. 
         FIG.  11 . 1   . 1 . 1 - 6 A is a detail view of a mechanical dampening mechanism for an actuator similar to the actuator of  FIGS.  11 . 1   . 1 . 1 - 2 A,  11 . 1 . 1 . 1 - 2 B,  11 . 1 . 1 . 1 - 3 A,  11 . 1 . 1 . 1 - 3 B,  11 . 1 . 1 . 1 - 4 A, and  11 . 1 . 1 . 1 - 4 B. 
         FIG.  11 . 1   . 1 . 1 - 6 B is a detail view of another mechanical dampening mechanism for an actuator similar to the actuator of  FIGS.  11 . 1   . 1 . 1 - 2 A,  11 . 1 . 1 . 1 - 2 B,  11 . 1 . 1 . 1 - 3 A,  11 . 1 . 1 . 1 - 3 B,  11 . 1 . 1 . 1 - 4 A, and  11 . 1 . 1 . 1 - 4 B. 
         FIG.  11 . 1   . 1 . 1 - 7  is a flowchart depicting a process of operation for an actuator disposed within a head-mounted display similar to the head-mounted display of  FIG.  11 . 1   . 1 . 1 - 1 . 
         FIG.  11 . 1   . 1 . 1 - 8  is a schematic hardware configuration for a controller in the head-mounted display of  FIG.  11 . 1   . 1 . 1 - 1 . 
       11.1.1.2: Crown Input and Feedback for Head-Mountable Devices 
         FIG.  11 . 1   . 1 . 2 - 1  illustrates a top view of a head-mountable device, according to some embodiments of the present disclosure. 
         FIG.  11 . 1   . 1 . 2 - 2  illustrates a top exploded view of a head-mountable device, according to some embodiments of the present disclosure. 
         FIG.  11 . 1   . 1 . 2 - 3  illustrates a sectional view of a crown module of the head-mountable device of  FIG.  11 . 1   . 1 . 2 - 2 , according to some embodiments of the present disclosure. 
         FIG.  11 . 1   . 1 . 2 - 4  illustrates a partial sectional view of a crown module of the head-mountable device of  FIG.  11 . 1   . 1 . 2 - 2 , according to some embodiments of the present disclosure. 
         FIG.  11 . 1   . 1 . 2 - 5  illustrates a sectional view of the crown module of  FIG.  11 . 1   . 1 . 2 - 4  taken along line A-A, according to some embodiments of the present disclosure. 
         FIG.  11 . 1   . 1 . 2 - 6  illustrates a side view of the crown module of  FIG.  11 . 1   . 1 . 2 - 4 , according to some embodiments of the present disclosure. 
         FIG.  11 . 1   . 1 . 2 - 7  illustrates a circuit diagram of a sensor of the crown module of  FIG.  11 . 1   . 1 . 2 - 4 , according to some embodiments of the present disclosure. 
         FIG.  11 . 1   . 1 . 2 - 8  illustrates a block diagram of a head-mountable device, in accordance with some embodiments of the present disclosure. 
       11.1.2: Wishbone and Mustache 
         FIG.  11 . 1   . 2 - 1  illustrates a front perspective view of an HMD with the front cover and display assembly omitted to show an example of a sensor system. 
         FIG.  11 . 1   . 2 - 2  illustrates a perspective view of a portion of a sensor system including sensors coupled to a bracket. 
         FIG.  11 . 1   . 2 - 3  illustrates a rear perspective view of a portion of an example HMD including a display module bracket. 
         FIG.  11 . 1   . 2 - 4  illustrates a top view of a portion of a display assembly of an example HMD. 
         FIG.  11 . 1   . 2 - 5  illustrates a side cross-sectional view of an example HMD. 
       11.1.3: Upper Guide Rod System 
         FIG.  11 . 1   . 3 - 1  illustrates a rear perspective view of an example HMD including a display adjustment system. 
         FIG.  11 . 1   . 3 - 2  illustrates a close-up view thereof with the display module omitted. 
         FIG.  11 . 1   . 3 - 3  illustrates a close-up view of the system shown in  FIG.  11 . 1   . 3 - 1  with the display module omitted. 
       11.1.3.1: Motors 
         FIG.  11 . 1   . 3 . 1 - 1  illustrates a rear perspective view of an example HMD including a display adjustment system. 
         FIG.  11 . 1   . 3 . 1 - 2  illustrates a perspective view of an example of a motor of a display adjustment system of an example HMD. 
         FIG.  11 . 1   . 3 . 1 - 3  illustrates a cross-sectional view of an example of a motor of a display adjustment system of an example HMD. 
       11.1.3.1.1: Electronic Devices with Optical Module Positioning Systems 
         FIG.  11 . 1   . 3 . 1 . 1 - 1  is a top view of an illustrative head-mounted device in accordance with an embodiment. 
         FIG.  11 . 1   . 3 . 1 . 1 - 2  is a rear view of an illustrative head-mounted device in accordance with an embodiment. 
         FIG.  11 . 1   . 3 . 1 . 1 - 3  is a schematic diagram of an illustrative head-mounted device in accordance with an embodiment. 
         FIG.  11 . 1   . 3 . 1 . 1 - 4  is a rear view of an interior portion of an illustrative head-mounted device in accordance with an embodiment. 
         FIG.  11 . 1   . 3 . 1 . 1 - 5  is a side view of an illustrative portion of an optical module that is configured to receive a guide rail and a threaded actuator rod in accordance with an embodiment. 
         FIG.  11 . 1   . 3 . 1 . 1 - 6  is an exploded cross-sectional view of an illustrative guide rod and end cap in accordance with an embodiment. 
         FIG.  11 . 1   . 3 . 1 . 1 - 7  is a side view of the illustrative guide rod of  FIG.  11 . 1   . 3 . 1 . 1 - 6  following attachment of the end cap in accordance with an embodiment. 
         FIG.  11 . 1   . 3 . 1 . 1 - 8  is a cross-sectional top view of the illustrative guide rod of  FIGS.  11 . 1   . 3 . 1 . 1 - 6  and  11 . 1 . 3 . 1 . 1 - 7  showing how the guide rod may be mounted to a housing structure such as a frame in head-mounted support structures in accordance with an embodiment. 
         FIGS.  11 . 1   . 3 . 1 . 1 - 9 ,  11 . 1 . 3 . 1 . 1 - 10 ,  11 . 1 . 3 . 1 . 1 - 11 , and  11 . 1 . 3 . 1 . 1 - 12  are views of illustrative guide rods in accordance with embodiments. 
         FIG.  11 . 1   . 3 . 1 . 1 - 13  is a cross-sectional side view of an illustrative guide rod tube that has been partially filled with a core in accordance with an embodiment. 
         FIG.  11 . 1   . 3 . 1 . 1 - 14  is a top view of a portion of an illustrative guide rod formed from fiber-composite material in accordance with an embodiment. 
         FIG.  11 . 1   . 3 . 1 . 1 - 15  is a cross-sectional end view of an illustrative portion of a guide rod formed from fiber-composite material in accordance with an embodiment. 
         FIG.  11 . 1   . 3 . 1 . 1 - 16  is a cross-sectional side view of an illustrative end portion of a guide rod in accordance with an embodiment. 
         FIG.  11 . 1   . 3 . 1 . 1 - 17  is a cross-sectional side view of an illustrative tapered end portion of a guide rod in accordance with an embodiment. 
       11.1.3.1.2: Electronic Device with Lens Positioning Sensing 
         FIG.  11 . 1   . 3 . 1 . 2 - 1  is a schematic diagram of an illustrative electronic device such as a head-mounted display device in accordance with an embodiment. 
         FIG.  11 . 1   . 3 . 1 . 2 - 2  is a top view of an illustrative head-mounted device in accordance with an embodiment. 
         FIG.  11 . 1   . 3 . 1 . 2 - 3  is a front view of an illustrative lens assembly having a force or position sensor in accordance with an embodiment. 
         FIG.  11 . 1   . 3 . 1 . 2 - 4 A is a front view of an illustrative direct force sensor in accordance with an embodiment. 
         FIG.  11 . 1   . 3 . 1 . 2 - 4 B is a top view of an illustrative sensor woven into a fabric in accordance with an embodiment. 
         FIG.  11 . 1   . 3 . 1 . 2 - 4 C is a cross-sectional side view of an illustrative nasal flap with an air bladder sensor in accordance with an embodiment. 
         FIG.  11 . 1   . 3 . 1 . 2 - 5  is a front view of an illustrative lens assembly having a proximity sensor in accordance with an embodiment. 
         FIG.  11 . 1   . 3 . 1 . 2 - 6  is a front view of an illustrative lens assembly having movable components that block a light-emitting component to indicate a position of the lens assembly in accordance with an embodiment. 
         FIG.  11 . 1   . 3 . 1 . 2 - 7  is a circuit diagram of an illustrative control circuit for controlling a positioner motor while monitoring for feedback from the motor in accordance with an embodiment. 
         FIG.  11 . 1   . 3 . 1 . 2 - 8  is a flow chart of illustrative steps involved operating a head-mounted device in accordance with an embodiment. 
       11.1.3.2: Sensors/Encoders 
         FIG.  11 . 1   . 3 . 2 - 1  illustrates a perspective view of an example encoder for an HMD display adjustment system. 
         FIG.  11 . 1   . 3 . 2 - 2  illustrates a top perspective view of an example display adjustment system for an HMD. 
         FIG.  11 . 1   . 3 . 2 - 3  illustrates a top view of an example encoder assembly for an HMD display adjustment system. 
       11.1.3.2.1: Sensor Assembly 
         FIG.  11 . 1   . 3 . 2 . 1 - 1  illustrates a side view of a head-mountable device, according to some embodiments of the present disclosure. 
         FIG.  11 . 1   . 3 . 2 . 1 - 2  illustrates an exploded perspective view of a sensor assembly of the head-mountable device of  FIG.  11 . 1   . 3 . 2 . 1 - 1 , according to some embodiments of the present disclosure. 
         FIG.  11 . 1   . 3 . 2 . 1 - 3  illustrates a side sectional view of a sensor assembly, according to some embodiments of the present disclosure. 
         FIG.  11 . 1   . 3 . 2 . 1 - 4  illustrates a side sectional view of a sensor assembly, according to some embodiments of the present disclosure. 
         FIG.  11 . 1   . 3 . 2 . 1 - 5  illustrates a block diagram of a head-mountable device, in accordance with some embodiments of the present disclosure. 
       11.1.3.2.2: Electronic Devices with Movable Optical Assemblies 
         FIG.  11 . 1   . 3 . 2 . 2 - 1  is a diagram of an illustrative head-mounted device in accordance with an embodiment. 
         FIGS.  11 . 1   . 3 . 2 . 2 - 2  and  11 . 1 . 3 . 2 . 2 - 3  are rear views of portions of illustrative head-mounted devices in accordance with embodiments. 
         FIG.  11 . 1   . 3 . 2 . 2 - 4  is a graph in which illustrative optical assembly adjustment values are plotted as a function of measured eye relief for multiple different illustrative measured interpupillary distances in accordance with an embodiment. 
         FIG.  11 . 1   . 3 . 2 . 2 - 5  is a flow chart of illustrative operations involved in using a head-mounted device in accordance with an embodiment. 
       11.1.3.2.3: Electronic Devices with Movable Optical Assemblies 
         FIG.  11 . 1   . 3 . 2 . 3 - 1  is a diagram of an illustrative head-mounted device in accordance with an embodiment. 
         FIGS.  11 . 1   . 3 . 2 . 3 - 2  and  11 . 1 . 3 . 2 . 3 - 3  are flow charts of illustrative operations involved in using a head-mounted device with movable optical assemblies in accordance with embodiments. 
         FIG.  11 . 1   . 3 . 2 . 3 - 4  is a cross-sectional end view of an illustrative clutch based on a split nut may be used in limiting how much force is applied to an optical assembly in accordance with an embodiment. 
         FIGS.  11 . 1   . 3 . 2 . 3 - 5  and  11 . 1 . 3 . 2 . 3 - 6  are diagrams showing how magnetic clutches may be used in limiting the force applied to optical assemblies in accordance with embodiments. 
         FIGS.  11 . 1   . 3 . 2 . 3 - 7 ,  11 . 1 . 3 . 2 . 3 - 8 ,  11 . 1 . 3 . 2 . 3 - 9 , and  11 . 1 . 3 . 2 . 3 - 10  are diagrams of illustrative mechanical clutch mechanisms that may be used in moving optical assemblies in accordance with embodiments. 
         FIG.  11 . 1   . 3 . 2 . 3 - 11  is a diagram showing how force-sensitive switches may be used in coupling a nut to an optical assembly in accordance with an embodiment. 
         FIG.  11 . 1   . 3 . 2 . 3 - 12  is a diagram showing how torque-sensitive switches may be coupled between a rotating motor and a portion of a rotating shaft in accordance with an embodiment. 
         FIG.  11 . 1   . 3 . 2 . 3 - 13  is a circuit diagram showing how motor load may be measured electrically while moving optical assemblies in accordance with an embodiment. 
         FIG.  11 . 1   . 3 . 2 . 3 - 14  is a diagram of an illustrative motor with a rotary encoder in accordance with an embodiment 
         FIG.  11 . 1   . 3 . 2 . 3 - 15  is a diagram of an illustrative motor, movable optical assembly, and associated linear magnetic encoder in accordance with an embodiment. 
         FIG.  11 . 1   . 3 . 2 . 3 - 16  is a graph showing how motor stalling may be detected while controlling a motor to move an optical assembly in accordance with an embodiment. 
         FIG.  11 . 1   . 3 . 2 . 3 - 17  is a flow chart of illustrative operations involved in using a head-mounted device with motors to move optical assemblies in accordance with an embodiment. 
       11.1.3.3: Hard Stops 
         FIG.  11 . 1   . 3 . 3 - 1  illustrates a perspective view of a portion of an example HMD including a hard stop. 
         FIG.  11 . 1   . 3 . 3 - 2  illustrates a perspective view of a portion of an example HMD including a hard stop. 
       11.1.3.4: Upper Biasing Members 
         FIG.  11 . 1   . 3 . 4 - 1  illustrates a perspective view of a portion of a display adjustment system of an example HMD. 
         FIG.  11 . 1   . 3 . 4 - 2  illustrates a perspective view of a portion of a display adjustment system of an example HMD. 
       11.1.4: Lower Guide Rod System 
       11.1.4.1: Electronic Devices with Biased Guide Rails 
         FIG.  11 . 1   . 4 . 1 - 1  is a top view of an illustrative electronic device in accordance with an embodiment. 
         FIG.  11 . 1   . 4 . 1 - 2  is a schematic diagram of an illustrative electronic device in accordance with an embodiment. 
         FIG.  11 . 1   . 4 . 1 - 3  is top view of an illustrative electronic device with optical module guide rails in accordance with an embodiment. 
         FIG.  11 . 1   . 4 . 1 - 4  is a rear view of illustrative electronic device with optical module guide rails in accordance with an embodiment. 
         FIG.  11 . 1   . 4 . 1 - 5  is a side view of an illustrative optical module with guide rails in accordance with an embodiment. 
         FIGS.  11 . 1   . 4 . 1 - 6 A,  11 . 1 . 4 . 1 - 6 B, and  11 . 1 . 4 . 1 - 7  are cross-sectional side views of illustrative guide rail biasing mechanisms in accordance with embodiments. 
         FIG.  11 . 1   . 4 . 1 - 8  is a cross-sectional side view of a portion of a kinematic guide rail mounting system in accordance with an embodiment. 
         FIG.  11 . 1   . 4 . 1 - 9  is a side view of a kinematic optical module guide rail mounting system in accordance with an embodiment. 
         FIG.  11 . 1   . 4 . 1 - 10  is a perspective view of an illustrative guide rail sensor based on a strain gauge in accordance with an embodiment. 
         FIG.  11 . 1   . 4 . 1 - 11  is a cross-sectional side view of an illustrative optical module with a guide rail sensor in accordance with an embodiment. 
       11.1.4.2: Lower Guide Rods 
         FIG.  11 . 1   . 4 . 2 - 1  illustrates a plan view of a portion of an example HMD including a guide system for an adjustable display. 
       11.1.4.2.1: Electrical Contacts 
         FIG.  11 . 1   . 4 . 2 . 1 - 1  illustrates a perspective view of a portion of an example HMD. 
       11.1.4.2.2: Biasing Members 
         FIG.  11 . 1   . 4 . 2 . 2 - 1  illustrates a perspective view of a portion of an example HMD. 
       11.2: Barrels and Baskets 
       11.2.1: Lens Mounting Systems 
         FIG.  11 . 2   . 1 - 1  is a diagram of an illustrative head-mounted device in accordance with an embodiment. 
         FIG.  11 . 2   . 1 - 2  is a front view of an illustrative lens in accordance with embodiments. 
         FIGS.  11 . 2   . 1 - 3  and  11 . 2 . 1 - 4  are cross-sectional side view of peripheral portions of illustrative lenses and associated mounting structures in accordance with embodiments. 
         FIGS.  11 . 2   . 1 - 5  and  11 . 2 . 1 - 6  are top views of illustrative flexures for mounting a lens in accordance with embodiments. 
         FIGS.  11 . 2   . 1 - 7 ,  11 . 2 . 1 - 8 ,  11 . 2 . 1 - 9 , and  11 . 2 . 1 - 10  are cross-sectional side views of additional illustrative flexure arrangements for mounting a lens in accordance with embodiments. 
         FIG.  11 . 2   . 1 - 11  is a diagram showing how adhesive may be introduced into a gap between an illustrative flexure and a lens in accordance with an embodiment. 
       11.3: Rear-Facing Cameras 
       11.3.1: Optical Module for Head-Mounted Device 
         FIG.  11 . 3   . 1 - 1  is a block diagram that shows an example of a hardware configuration for a head-mounted device. 
         FIG.  11 . 3   . 1 - 2  is a top view illustration that shows the head-mounted device, including a device housing and a support structure. 
         FIG.  11 . 3   . 1 - 3  is a rear view illustration taken along line A-A of  FIG.  11 . 3   . 1 - 2  that shows the device housing. 
         FIG.  11 . 3   . 1 - 4  is a perspective view illustration that shows an optical module of the head-mounted device. 
         FIG.  11 . 3   . 1 - 5  is an exploded side view diagram showing components of an optical module according to an example. 
         FIG.  11 . 3   . 1 - 6  is a front view that shows the lens according to an example. 
         FIG.  11 . 3   . 1 - 7  is a cross-section view taken along line B-B of  FIG.  11 . 3   . 1 - 6  showing the lens. 
         FIG.  11 . 3   . 1 - 8  is a front view illustration that shows a housing body of an optical module housing assembly 
         FIG.  11 . 3   . 1 - 9  is a cross-section view illustration taken along line C-C of  FIG.  11 . 3   . 1 - 8  showing the housing body. 
         FIG.  11 . 3   . 1 - 10  is a front view illustration that shows a retainer of the optical module housing assembly. 
         FIG.  11 . 3   . 1 - 11  is a cross-section view illustration taken along line D-D of  FIG.  11 . 3   . 1 - 10  showing the retainer. 
         FIG.  11 . 3   . 1 - 12  is a front view illustration that shows an infrared emitter. 
         FIG.  11 . 3   . 1 - 13  is a cross-section view illustration showing a portion of the infrared emitter and a peripheral wall of the housing body. 
         FIG.  11 . 3   . 1 - 14  is a cross-section view illustration that shows the optical module. 
         FIG.  11 . 3   . 1 - 15  is a cross-section view illustration that shows the optical module according to an alternative implementation in which an optical axis of the eye camera is angled toward an optical axis of the optical module. 
         FIG.  11 . 3   . 1 - 16  is a cross-section view illustration that shows the optical module according to an alternative implementation in which the infrared emitter is located outside of the housing body of the optical module housing assembly. 
         FIG.  11 . 3   . 1 - 17  is a side-view illustration that shows a display module according to an implementation. 
         FIG.  11 . 3   . 1 - 18  is a top-view illustration that shows interpupillary adjustment mechanisms that each support one of the optical modules. 
         FIG.  11 . 3   . 1 - 19  is a side view illustration that shows one of the interpupillary adjustment mechanisms. 
         FIG.  11 . 3   . 1 - 20  is a top-view cross-section illustration that shows front-facing cameras that are supported by each of the optical modules. 
         FIG.  11 . 3   . 1 - 21  is an illustration that shows connection of the eye camera and the infrared emitter to a computing device by an optical module jumper board. 
       11.3.2: Cameras and Leds 
         FIG.  11 . 3   . 2 - 1  illustrates a perspective view of a portion of an example of an optical module of an HMD. 
         FIG.  11 . 3   . 2 - 2  illustrates a top view of a portion of an example of an optical module of an HMD. 
         FIG.  11 . 3   . 2 - 3  illustrates a perspective cutaway view of a portion of an example of an optical module of an HMD. 
         FIG.  11 . 3   . 2 - 4  illustrates a plan view of a portion of an example of an optical module of and HMD. 
         FIG.  11 . 3   . 2 - 5  illustrates a cutaway view of a portion of an example of an optical module of an HMD. 
       11.4: Display 
       11.4.1: Display System with Interchangeable Lens 
         FIG.  11 . 4   . 1 - 0  illustrates a view of an HMD. 
         FIG.  11 . 4   . 1 - 1  is a side view of a display system with hidden components illustrated in dashed lines. 
         FIG.  11 . 4   . 1 - 2  is a cross-sectional view of the display system of  FIG.  11 . 4   . 1 - 1  taken along line  2 - 2  in  FIG.  11 . 4   . 1 - 1 . 
         FIG.  11 . 4   . 1 - 3 A is a cross-sectional view of a display unit and interchangeable lens assembly of the display system of  FIG.  11 . 4   . 1 - 1  taken along line  3 - 3  in  FIG.  11 . 4   . 1 - 2  and shown in an assembled state. 
         FIG.  11 . 4   . 1 - 3 B is a cross-sectional view of the display unit and the interchangeable lens assembly of  FIG.  11 . 4   . 1 - 3 A shown in a disassembled state. 
         FIG.  11 . 4   . 1 - 4  is a rear view of a removable lens of the display system of  FIG.  11 . 4   . 1 - 1  with light emission, entry, and exit points illustrated by dashed lines (i.e., dash-dot lines). 
         FIG.  11 . 4   . 1 - 5  is a rear view of another embodiment of a removable lens. 
         FIG.  11 . 4   . 1 - 6  is a rear view of another embodiment of a removable lens. 
         FIG.  11 . 4   . 1 - 7  is a cross-sectional view another embodiment of a removable lens. 
         FIG.  11 . 4   . 1 - 8  is a cross-sectional view another embodiment of a removable lens. 
         FIG.  11 . 4   . 1 - 9  is a cross-sectional view another embodiment of a removable lens. 
         FIG.  11 . 4   . 1 - 10 A is a cross-sectional view of another display unit and another interchangeable lens assembly for the display system of  FIG.  11 . 4   . 1 - 1  shown in a disassembled state. 
         FIG.  11 . 4   . 1 - 10 B is a cross-sectional view of the display unit and the interchangeable lens assembly of  FIG.  11 . 4   . 1 - 10 A shown in an assembled state. 
         FIG.  11 . 4   . 1 - 11 A is a cross-sectional view of another display unit and another interchangeable lens assembly for the display system of  FIG.  11 . 4   . 1 - 1  shown in a disassembled state. 
         FIG.  11 . 4   . 1 - 11 B is a cross-sectional view of the display unit and the interchangeable lens assembly of  FIG.  11 . 4   . 1 - 10 A shown in an assembled state. 
         FIG.  11 . 4   . 1 - 12 A is a side view of a display module for use in the display system. 
         FIG.  11 . 4   . 1 - 12 B is a front view of a display module for use in the display system. 
         FIG.  11 . 4   . 1 - 12 C is a front view of a display module for use in the display system. 
         FIG.  11 . 4   . 1 - 12 D is a front view of a display module for use in the display system. 
         FIG.  11 . 4   . 1 - 13 A is a front view of a front view of a display module for use in the display system. 
         FIG.  11 . 4   . 1 - 13 B is a front view of a removable lens assembly for use with the display module of  FIG.  11 . 4   . 1 - 13 A. 
         FIG.  11 . 4   . 1 - 13 C is a cross-sectional view of the display module of  FIG.  11 . 4   . 1 - 13 A taken along line  11 . 4 . 1 - 13 A- 11 . 4 . 1 - 13 A. 
         FIG.  11 . 4   . 1 - 13 D is a cross-sectional view of the removable lens assembly of  FIG.  11 . 4   . 1 - 13 B taken along line  11 . 4 . 1 - 13 B- 11 . 4 . 1 - 13 B. 
         FIG.  11 . 4   . 1 - 13 E is a cross-sectional view of the display module of  FIG.  11 . 4   . 1 - 13 A and the removable lens assembly of  FIG.  11 . 4   . 1 - 13 B in a partially coupled state. 
         FIG.  11 . 4   . 1 - 13 F is a cross-sectional view of the display module of  FIG.  11 . 4   . 1 - 13 A and the removable lens assembly of  FIG.  11 . 4   . 1 - 13 B in a coupled state. 
         FIG.  11 . 4   . 1 - 14 A is a schematic view of a display system. 
         FIG.  11 . 4   . 1 - 14 B is a flow chart of a method for operating the display system. 
         FIG.  11 . 4   . 1 - 15  is a flow chart of a process for determining compatibility of a removable lens and a user. 
         FIG.  11 . 4   . 1 - 16  is a flow chart of a method for determining compatibility of the removable lens and the user. 
         FIG.  11 . 4   . 1 - 17  is a schematic of an example hardware configuration of a controller of the display system. 
       11.4.2: Electronic Device System with Supplemental Lenses 
         FIG.  11 . 4   . 2 - 1  is a schematic diagram of an illustrative electronic device such as a head-mounted display device in accordance with an embodiment. 
         FIG.  11 . 4   . 2 - 2  is a top view of an illustrative head-mounted device in accordance with an embodiment. 
         FIG.  11 . 4   . 2 - 3  is a diagram of an illustrative removable supplemental lens in accordance with an embodiment. 
         FIG.  11 . 4   . 2 - 4  is a flow chart of illustrative operations associated with using a head-mounted device in accordance with an embodiment. 
       11.4.3: Rx Lenses 
         FIG.  11 . 4   . 3 - 1  illustrates a perspective view of a portion of an optical assembly of an example HMD. 
         FIG.  11 . 4   . 3 - 2  illustrates a perspective view of a portion of an optical assembly of an example HMD. 
         FIG.  11 . 4   . 3 - 3  illustrates a perspective view of a portion of an optical assembly of an example HMD. 
         FIG.  11 . 4   . 3 - 4  illustrates a plan and exploded view of a portion of an optical assembly of an example HMD. 
         FIG.  11 . 4   . 3 - 5  illustrates a magnet array for an example display module of an HMD. 
         FIG.  11 . 4   . 3 - 6  illustrates a perspective view of an example lens of an HMD. 
         FIG.  11 . 4   . 3 - 7  illustrates a side view of an example lens of an HMD. 
         FIG.  11 . 4   . 3 - 8  illustrates a side view of an example lens of an HMD. 
         FIG.  11 . 4   . 3 - 9  illustrates a side view of an example lens of an HMD. 
         FIG.  11 . 4   . 3 - 10  illustrates a side view of an example lens of an HMD. 
         FIG.  11 . 4   . 3 - 11  illustrates a side view of an example lens of an HMD. 
       XII: Curtain 
         FIGS.  12 . 0 - 1    illustrates a view of an HMD. 
       12.1: Electronic Devices with Stretchable Fabric Covers 
         FIGS.  12 . 1 - 1    is a top view of an illustrative head-mounted device in accordance with an embodiment. 
         FIGS.  12 . 1 - 2    is a rear view of an illustrative head-mounted device in accordance with an embodiment. 
         FIGS.  12 . 1 - 3    is a schematic diagram of an illustrative head-mounted device in accordance with an embodiment. 
         FIGS.  12 . 1 - 4    is a top view of an illustrative head-mounted device in which left-eye and right-eye optical modules have been placed close to each other to accommodate a user with a small interpupillary distance in accordance with an embodiment. 
         FIGS.  12 . 1 - 5    is a top view of the illustrative head-mounted device of  FIGS.  12 . 1 - 4    in which the optical modules have been moved away from each other to accommodate a user with a large interpupillary distance in accordance with an embodiment. 
         FIGS.  12 . 1 - 6    is a front view of an illustrative cover layer with stretchable fabric in an unstretched state in accordance with an embodiment. 
         FIGS.  12 . 1 - 7    is a front view of the illustrative cover layer of  FIGS.  12 . 1 - 6    with stretchable fabric in a stretched state in accordance with an embodiment. 
         FIGS.  12 . 1 - 8    is a side view of an illustrative first strand that may be used in a cover layer of the type shown in  FIGS.  12 . 1 - 6  and  12 . 1 - 7    in accordance with an embodiment. 
         FIGS.  12 . 1 - 9    is a side view of an illustrative second strand that may be used in a cover layer of the type shown in  FIGS.  12 . 1 - 6  and  12 . 1 - 7    in accordance with an embodiment. 
         FIGS.  12 . 1 - 10    is a front view of an illustrative cover layer having regions with different levels of stretch and opacity in accordance with an embodiment. 
         FIGS.  12 . 1 - 11    is a perspective view of an illustrative cover layer formed from a three-dimensional fabric in accordance with an embodiment. 
       12.2: Curtain Assembly 
         FIGS.  12 . 2 - 1    illustrates a view of an example of an HMD. 
         FIGS.  12 . 2 - 2    illustrates a rear perspective view of an example HMD including a curtain assembly. 
         FIGS.  12 . 2 - 3    illustrates a rear view of an example HMD including a curtain assembly. 
         FIGS.  12 . 2 - 4    illustrates a side cross-sectional view of an example HMD including a curtain assembly. 
         FIGS.  12 . 2 - 5    illustrates a perspective view of an example of a curtain assembly of an HMD. 
         FIGS.  12 . 2 - 6    illustrates an exploded view of an example of a curtain assembly of an HMD. 
         FIGS.  12 . 2 - 7    illustrates a rear view of an example of a curtain assembly of an HMD. 
         FIGS.  12 . 2 - 8    illustrates a partial view of an example curtain assembly. 
         FIGS.  12 . 2 - 9    illustrates a partial view of an example curtain assembly. 
         FIGS.  12 . 2 - 10    illustrates a partial view of an example curtain assembly. 
         FIGS.  12 . 2 - 11    illustrates a partial view of an example curtain assembly. 
         FIGS.  12 . 2 - 12    illustrates a partial view of an example curtain assembly. 
         FIGS.  12 . 2 - 13    illustrates a partial view of an example curtain assembly. 
       XIII: Light Seal 
         FIGS.  13 . 0 - 1    illustrates a view of an HMD. 
         FIGS.  13 . 0 - 2 A  illustrates a front perspective view of a device seal according to one embodiment. 
         FIGS.  13 . 0 - 2 B  illustrates a bottom-rear perspective view of the device seal of  FIG.  13 . 0 - 2 A . 
         FIGS.  13 . 0 - 2 C  illustrates a rear view of the device seal of  FIGS.  13 . 0 - 2 A . 
       13.1: Electronic Devices with Covering Structures 
         FIGS.  13 . 1 - 1    is a top view of a head-mounted device. 
         FIGS.  13 . 1 - 2    is a rear view of a head-mounted device. 
         FIGS.  13 . 1 - 3    is a schematic diagram of a head-mounted device. 
         FIGS.  13 . 1 - 4    is a top view of a head-mounted device with left-eye and right-eye optical modules. 
         FIGS.  13 . 1 - 5    is a top view of the head-mounted device of  FIGS.  13 . 1 - 4    with the optical modules spaced further apart. 
         FIGS.  13 . 1 - 6    is a cross-sectional side view of a head-mounted device with a fan. 
         FIGS.  13 . 1 - 7    is an exploded perspective view of a curtain having a frame and a cover layer supported on the frame. 
         FIGS.  13 . 1 - 8    is a top view of an optical module and a cover layer. 
         FIGS.  13 . 1 - 9    is a view of a cover layer with a peripheral elastic band. 
         FIGS.  13 . 1 - 10    is a view of a cover layer with woven elastic strands forming a peripheral elastic band. 
         FIGS.  13 . 1 - 11    is a diagram of a cover layer formed from a material that stretches. 
         FIGS.  13 . 1 - 12    is a view of a frame for a curtain. 
         FIGS.  13 . 1 - 13    is a cross-sectional side view of a cover layer with a peripheral elastic band that moves relative to a rigid frame. 
         FIGS.  13 . 1 - 14    is a cross-sectional top view of a head-mounted device with a floating curtain. 
         FIGS.  13 . 1 - 15    is a rear view of a curtain with locations for attaching the curtain to a head-mounted device housing member. 
         FIGS.  13 . 1 - 16    is a cross-sectional side view of a portion of a head-mounted device showing with a curtain attached to a head-mounted device housing member. 
         FIGS.  13 . 1 - 17    is a top view of an apparatus with a movable member surrounded by a curtain. 
       13.2: Device with a Removable Cushion 
         FIGS.  13 . 2 - 1    is a top view of an electronic device such as a head-mounted device. 
         FIGS.  13 . 2 - 2    is top view of an optical module for an electronic device. 
         FIGS.  13 . 2 - 3 A  is a cross-sectional top view of a head-mounted device with a removable cushion in an unattached state. 
         FIGS.  13 . 2 - 3 B  is a cross-sectional top view of a head-mounted device with a removable cushion in an attached state. 
         FIGS.  13 . 2 - 4    is a perspective view of a head-mounted support structure. 
         FIGS.  13 . 2 - 5 A  is a rear view of a flexible structure of a head-mounted support structure attached to support posts. 
         FIGS.  13 . 2 - 5 B  is a rear view of a removable cushion with high-rigidity portions configured to overlap support posts in a corresponding head-mounted support structure. 
         FIGS.  13 . 2 - 6 A  is a rear view of a flexible structure with primary attachment structures and auxiliary attachment structures. 
         FIGS.  13 . 2 - 6 B  is a rear view of a removable cushion with primary attachment structures and auxiliary attachment structures. 
         FIGS.  13 . 2 - 7    is a cross-sectional top view of a head-mounted device with a removable cushion with magnets and recesses. 
         FIGS.  13 . 2 - 8    is a rear view of a removable cushion with hinge structures. 
         FIGS.  13 . 2 - 9    is a schematic diagram of a system that includes a head-mounted support structure and multiple removable cushions. 
       13.3: Electronic Devices with Light-Blocking Fabrics 
         FIGS.  13 . 3 - 1    is a top view of a head-mounted device. 
         FIGS.  13 . 3 - 2    is a rear view of a head-mounted device. 
         FIGS.  13 . 3 - 3    is a schematic diagram of a head-mounted device. 
         FIGS.  13 . 3 - 4    is a perspective view of a head-mounted device with a fabric-covered face frame. 
         FIGS.  13 . 3 - 5 A  is a schematic diagram of a knitting system. 
         FIGS.  13 . 3 - 5 B  is a schematic diagram of a knitting system. 
         FIGS.  13 . 3 - 6    is a diagram of a portion of a weft knit fabric layer. 
         FIGS.  13 . 3 - 7    is a cross-sectional side view of a light seal. 
         FIGS.  13 . 3 - 8    is a perspective view of an inner fabric layer for a light seal. 
         FIGS.  13 . 3 - 9    is a cross-sectional side view of a light seal. 
       13.4: Electronic Devices with Stretchable Fabrics 
         FIGS.  13 . 4 - 6    is a diagram of a portion of a fabric layer with knit stitches. 
         FIGS.  13 . 4 - 7    is a diagram of a portion of a fabric layer with knit stitches and miss stitches. 
         FIGS.  13 . 4 - 8    is a diagram of a portion of a fabric layer with knit stitches and tuck stitches. 
         FIGS.  13 . 4 - 9    is a knitting chart of a fabric layer may have a four-row-repeat-pattern with knit stitches and tuck stitches. 
       13.5: Contactless Sensors for a Head-Mountable Device 
         FIGS.  13 . 5 - 1    shows a top view profile of a head-mountable device including a facial interface. 
         FIGS.  13 . 5 - 2 A  shows a side view of a head-mountable device including a facial interface. 
         FIGS.  13 . 5 - 2 B  shows a front view of a head-mountable device including a facial interface. 
         FIGS.  13 . 5 - 3    shows a top view of a facial interface with a sensor. 
         FIGS.  13 . 5 - 4    shows a top view of a facial interface with multiple sensors at various positions. 
         FIGS.  13 . 5 - 5    shows yet another top view of a facial interface with multiple sensors at various positions. 
         FIGS.  13 . 5 - 6 A  shows a top view of a facial interface with various components, including a sensor. 
         FIGS.  13 . 5 - 6 B  shows a top view of a facial interface with various components, including a sensor. 
         FIGS.  13 . 5 - 7 A- 13 . 5 - 7 B  shows non-exploded and exploded perspective views of a facial interface with sensors. 
       13.6: Integrated Health Sensors 
         FIGS.  13 . 6 - 1    shows a block diagram of a head-mountable device. 
         FIGS.  13 . 6 - 2    shows a top view of an example head-mountable device. 
         FIGS.  13 . 6 - 3    shows a rear perspective view of an example head-mountable device including a facial interface incorporated with sensors. 
         FIGS.  13 . 6 - 4    shows a cross-sectional view of a facial interface with sensors disposed at various locations. 
         FIGS.  13 . 6 - 5    shows a perspective view of a head-mountable device including sensors. 
         FIGS.  13 . 6 - 6    shows a perspective view of a head-mountable device including a facial interface, a frame, and a plurality of electronic components. 
       13.7: Health Sensing Retention Band 
         FIGS.  13 . 7 - 1    shows a schematic block diagram of a head-mountable device. 
         FIGS.  13 . 7 - 2    shows a top view of a head-mountable device. 
         FIGS.  13 . 7 - 3    shows a cross-sectional side view of a head-mountable device. 
         FIGS.  13 . 7 - 4 A  shows a rear perspective view of a retention band. 
         FIGS.  13 . 7 - 4 B  shows a side view of the retention band of  FIGS.  13 . 7 - 4 A  in an articulated position. 
         FIGS.  13 . 7 - 4 C  shows a side view of the retention band of  FIGS.  13 . 7 - 4 A  in an articulated position. 
         FIGS.  13 . 7 - 5    shows a perspective exploded view of a head-mountable device. 
         FIGS.  13 . 7 - 6    shows a side view of a retention band having sensors. 
       13.8: Conductive Fabric Architecture 
         FIGS.  13 . 8 - 1 A  shows a schematic block diagram of a head-mountable device. 
         FIGS.  13 . 8 - 1 B  shows a top view of a head-mountable device. 
         FIGS.  13 . 8 - 2    shows a bottom perspective view of a light seal. 
         FIGS.  13 . 8 - 3    shows a top view of a head-mountable device. 
         FIGS.  13 . 8 - 4 A  shows a conductive fabric in a neutral state. 
         FIGS.  13 . 8 - 4 B  shows the conductive fabric of  FIGS.  13 . 8 - 4 A  in a compressed state. 
         FIGS.  13 . 8 - 4 C  shows the conductive fabric of  FIGS.  13 . 8 - 4 A  in a stretched state. 
         FIGS.  13 . 8 - 5 A  shows a conductive component on an exterior of a cover. 
         FIGS.  13 . 8 - 5 B  shows a conductive component interwoven into a cover. 
         FIGS.  13 . 8 - 5 C  shows a conductive component on an interior of a cover. 
         FIGS.  13 . 8 - 5 D  shows free-floating conductive components. 
         FIGS.  13 . 8 - 6    shows a side perspective view of a light seal. 
         FIGS.  13 . 8 - 7    shows a bottom perspective view of a light seal. 
       13.9: Facial Interface Having Integrated Health Sensors 
         FIGS.  13 . 9 - 1    shows a block diagram of a head-mountable device. 
         FIGS.  13 . 9 - 2 A  shows a top view of a head-mountable device. 
         FIGS.  13 . 9 - 2 B  shows a rear view of a facial interface for a head-mountable device. 
         FIGS.  13 . 9 - 3    shows a rear perspective view of a facial interface with sensors disposed near a nasal region of a head-mountable device. 
         FIGS.  13 . 9 - 4 A  shows an exploded perspective view of pressure sensor assembly of a head-mountable device. 
         FIGS.  13 . 9 - 4 B  shows an assembled perspective view of a pressure sensor assembly of a head-mountable device. 
         FIGS.  13 . 9 - 5 A  shows sensors disposed on a forehead region of a facial interface of a head-mountable device. 
         FIGS.  13 . 9 - 5 B  shows sensors disposed on a forehead region of a facial interface of a head-mountable device. 
         FIGS.  13 . 9 - 6    shows a cross-sectional view of a pressure sensors assembly of a head-mountable device. 
       13.10: Touch Sensitive Input Surface 
         FIGS.  13 . 10 - 1 A  shows a schematic block diagram of a head-mountable device. 
         FIGS.  13 . 10 - 1 B  shows a top view of a head-mountable device. 
         FIGS.  13 . 10 - 2    shows a bottom perspective view of a light seal. 
         FIGS.  13 . 10 - 3 A  shows a top view of a head-mountable device with conducive fabric in a light seal of the head-mountable device. 
         FIGS.  13 . 10 - 3 B  shows a top view of a head-mountable device with a user engaging a touch sensitive surface of a light seal of the head-mountable device. 
         FIGS.  13 . 10 - 4    shows a touch sensitive surface of a light seal of a head-mountable device. 
         FIGS.  13 . 10 - 5    shows a touch sensitive surface of a light seal of a head-mountable device. 
         FIGS.  13 . 10 - 6    shows a touch sensitive surface of a light seal of a head-mountable device. 
         FIGS.  13 . 10 - 7    shows a head-mountable device with a sensor incorporated with a frame of the head-mountable device. 
         FIGS.  13 . 10 - 8 A  shows a head-mountable device with a sensor incorporated with a frame of the head-mountable device. 
         FIGS.  13 . 10 - 8 B  shows the head-mountable device of  FIGS.  13 . 10 - 8 A  with a user mechanically deflecting the frame of the head-mountable device. 
       13.11: Face Engaging Structures 
         FIGS.  13 . 11 - 1    shows a top view of an example head-mountable device. 
         FIGS.  13 . 11 - 2 A  shows a side view of an example head-mountable device. 
         FIGS.  13 . 11 - 2 B  shows a front view of an example head-mountable device. 
         FIGS.  13 . 11 - 3 A  shows a perspective view of a head-mountable device including a connector positioned at a forehead location. 
         FIGS.  13 . 11 - 3 B- 3 E  show various connector types. 
         FIGS.  13 . 11 - 4 A  shows a perspective view of a head-mountable device including a connector positioned at the zygoma location. 
         FIGS.  13 . 11 - 4 A- 4 H  show various connector types. 
         FIGS.  13 . 11 - 5 A  shows a perspective view of a head-mountable device including a facial interface. 
         FIGS.  13 . 11 - 5 B- 5 G  show various facial interfaces. 
         FIGS.  13 . 11 - 6 A- 6 B  show another variation of a facial interface. 
         FIGS.  13 . 11 - 7 A  shows a perspective view of a display including a display frame. 
         FIGS.  13 . 11 - 7 B  shows an exploded perspective view of a display including display frame. 
         FIGS.  13 . 11 - 8 A- 8 B  show a display frame with a relief cutout. 
         FIGS.  13 . 11 - 9 A  shows a head-mountable device with no relief cutout. 
         FIGS.  13 . 11 - 9 B  shows a head-mountable device with a relief cutout. 
         FIGS.  13 . 11 - 10 A- 10 B  show a head-mountable device with a relief cutout at various locations. 
         FIGS.  13 . 11 - 11 A- 11 C  show a display frame with a relief cutout. 
         FIGS.  13 . 11 - 12    shows a display frame with thru-holes. 
         FIGS.  13 . 11 - 13    shows a display frame with stiffeners. 
         FIGS.  13 . 11 - 14 A  is a top view of a frame for a device seal including stiffeners. 
         FIGS.  13 . 11 - 14 B  is a cross-sectional view of the frame of  FIGS.  13 . 11 - 14 A . 
         FIGS.  13 . 11 - 14 C  is a bottom view of the frame of  FIGS.  13 . 11 - 14 A . 
         FIGS.  13 . 11 - 14 D  is a top view of the frame of  FIGS.  13 . 11 - 14 A . 
         FIGS.  13 . 11 - 14    shows a perspective view of an example connector. 
         FIGS.  13 . 11 - 15 A  shows a side view of an example connector positioned between a display frame and a facial interface. 
         FIGS.  13 . 11 - 15 B  shows an example facial interface. 
         FIGS.  13 . 11 - 15 C- 13 . 11 - 15 D  show example cross-sections of the facial interface shown in  FIGS.  13 . 11 - 15 B . 
         FIGS.  13 . 11 - 16    shows a cross-sectional view of an example connector with a connector frame and post. 
         FIGS.  13 . 11 - 17    shows a top view of an example connector. 
         FIGS.  13 . 11 - 18    shows a side perspective view of a base of an example connector where 
       attached to an example display frame. 
         FIGS.  13 . 11 - 19    shows another cross-sectional view of an example connector. 
         FIGS.  13 . 11 - 20 - 13 . 11 - 21    respectively show perspective and top views of example adhesives in an example head-mountable device. 
       13.12: Face Engaging Structure 
         FIGS.  13 . 12 - 1    shows a top view of a head-mountable device including a facial interface. 
         FIGS.  13 . 12 - 2 A  shows a side view of a head-mountable device including a facial interface connected to a display. 
         FIGS.  13 . 12 - 2 B  shows a top view of a head-mountable device including a facial interface connected to a display. 
         FIGS.  13 . 12 - 3    shows a perspective view of a head-mountable device including a facial interface and an example connector. 
         FIGS.  13 . 12 - 4 A  shows a perspective view of a head-mountable device with an example connector between a display and a facial interface. 
         FIGS.  13 . 12 - 4 B  shows a front view of an example connector. 
         FIGS.  13 . 12 - 4 C  shows a side view of an example connector portion. 
         FIGS.  13 . 12 - 5 A- 13 . 12 - 5 B  show views of a connector in example positional states. 
         FIGS.  13 . 12 - 6 A  shows a perspective view of a head-mountable device including a facial interface and another example connector. 
         FIGS.  13 . 12 - 6 B  shows a top view of an example connector. 
         FIGS.  13 . 12 - 7 A- 13 . 12 - 7 B  show side views of another connector in example positional states. 
         FIGS.  13 . 12 - 8 A- 13 . 12 - 8 B  show schematic views of an example sliding connector. 
         FIGS.  13 . 12 - 9 A  shows a bottom view of another example head-mountable device. 
         FIGS.  13 . 12 - 9 B- 13 . 12 - 9 F  show various positions of a connector of a head-mountable device. 
         FIGS.  13 . 12 - 10    shows a cutaway view of an example connector. 
         FIGS.  13 . 12 - 11    shows a perspective view of another example connector. 
         FIGS.  13 . 12 - 12    shows a side view of yet another example connector. 
       13.13: Adjustment Mechanism 
         FIGS.  13 . 13 - 1    shows a top view profile of a head-mountable device including a facial interface. 
         FIGS.  13 . 13 - 2 A  shows a side view profile of a head-mountable device including a facial interface. 
         FIGS.  13 . 13 - 2 B  shows a top view profile of a head-mountable device including a facial interface. 
         FIGS.  13 . 13 - 3 A- 13 . 13 - 3 D  show exemplary locations of adjustment mechanism of a head-mountable device. 
         FIGS.  13 . 13 - 4 A- 13 . 13 - 4 C  show exemplary translatable positions of an adjustment mechanism. 
         FIGS.  13 . 13 - 5 A- 13 . 13 - 5 C  show exemplary rotatable positions of an adjustment mechanism of a head-mountable device. 
         FIGS.  13 . 13 - 6 A- 13 . 13 - 6 B  show an exemplary adjustment mechanism. 
         FIGS.  13 . 13 - 7 A- 13 . 13 - 7 B  show an exemplary rotatable adjustment mechanism. 
         FIGS.  13 . 13 - 8    shows another exemplary adjustment mechanism. 
         FIGS.  13 . 13 - 9 A- 13 . 13 - 24 B  respectively show example head-mountable devices with an actuator control. 
         FIGS.  13 . 13 - 25 A- 13 . 13 - 25 D  show an example head-mountable device with an example connection and corresponding actuator control. 
         FIGS.  13 . 13 - 26    shows an example connection of a head-mountable device. 
         FIGS.  13 . 13 - 27    shows another example connection of a head-mountable device. 
         FIGS.  13 . 13 - 28 - 13 . 13 - 30    respectively show top, front, and side views of another example head-mountable device. 
         FIGS.  13 . 13 - 31 - 13 . 13 - 33    respectively show a perspective view of lock-slider disengagement, a front view of lock-slider disengagement, and a front view of lock-slider engagement of a portion of a linear adjustment connection. 
         FIGS.  13 . 13 - 34    illustrates a perspective view of a portion of a head-mountable device having multiple linear adjustment connections, according to one exemplary embodiment. 
       13.14: Nosepiece 
         FIGS.  13 . 14 - 1    is a diagram of an illustrative electronic device in accordance with an embodiment. 
         FIGS.  13 . 14 - 2    is a front view of an illustrative electronic device with a light-shielding structure in accordance with an embodiment. 
         FIGS.  13 . 14 - 3    is a diagram of an illustrative light-shielding structure having a fabric cover in accordance with an embodiment. 
         FIGS.  13 . 14 - 4    is a front view of an illustrative light-shielding structure having a structural frame in accordance with an embodiment. 
         FIGS.  13 . 14 - 5    is a side view of an illustrative light-shielding structure having a fabric and elastomer layers in accordance with an embodiment. 
         FIGS.  13 . 14 - 6    is a front view of an illustrative light-shielding structure having an extension in accordance with an embodiment. 
         FIGS.  13 . 14 - 7    is a side view of an illustrative light-shielding structure having an embedded service loop in accordance with an embodiment. 
         FIGS.  13 . 14 - 8    is a side view of an illustrative light-shielding structure having an embedded deformable stiffener in accordance with an embodiment. 
         FIGS.  13 . 14 - 9 A  is a side view of an illustrative light-shielding structure having a rolled edge in accordance with an embodiment. 
         FIGS.  13 . 14 - 9 B  is a side view of an illustrative light-shielding structure having embedded foam in accordance with an embodiment. 
         FIGS.  13 . 14 - 9 C  is a top view of an illustrative light-shielding structure having a crumple zone in accordance with an embodiment. 
         FIGS.  13 . 14 - 9 D  is a side view of an illustrative light-shielding structure having a hemmed edge in accordance with an embodiment. 
         FIGS.  13 . 14 - 9 E  is a top view of an illustrative light-shielding structure having a foam in corner regions in accordance with an embodiment. 
         FIGS.  13 . 14 - 9 F  is a side view of an illustrative light-shielding structure having segmented foam or elastomeric regions in accordance with an embodiment 
         FIGS.  13 . 14 - 9 G  is a side view of an illustrative light-shielding structure having a stiffener and a foam layer in accordance with an embodiment. 
         FIGS.  13 . 14 - 10    is a front view of an illustrative light-shielding structure having a semi-rigid stiffener in accordance with an embodiment. 
       13.15: Removable Facial Interface 
         FIGS.  13 . 15 - 1 A  is a schematic block diagram of an example of a head-mountable device. 
         FIGS.  13 . 15 - 1 B  is a top view of an example of a head-mountable device. 
         FIGS.  13 . 15 - 2 A  is a perspective view of an example of a device seal. 
         FIGS.  13 . 15 - 2 B  is a perspective view of an example of a facial interface frame. 
         FIGS.  13 . 15 - 2 C  is a perspective view of an example of a facial interface frame and a removable facial interface. 
         FIGS.  13 . 15 - 2 D  is a cross-sectional view of an example of a facial interface shim. 
         FIGS.  13 . 15 - 3 A  is a perspective view of an example of a device seal. 
         FIGS.  13 . 15 - 3 B  is a plan view of an example of a removable facial interface. 
         FIGS.  13 . 15 - 4    is a cross-sectional view of an example of a magnetic attachment mechanism. 
         FIGS.  13 . 15 - 5 A  is a cross-sectional view of an example of an interlocking attachment mechanism. 
         FIGS.  13 . 15 - 5 B  is a cross-sectional view of an example of an interlocking attachment mechanism. 
         FIGS.  13 . 15 - 6    is a cross-sectional view of an example of a magnetic slide attachment mechanism. 
         FIGS.  13 . 15 - 7    is a cross-sectional view of an example of a hook-and-loop attachment mechanism. 
         FIGS.  13 . 15 - 8    is a cross-sectional view of an example of a magnetic attachment mechanism. 
         FIGS.  13 . 15 - 9    is a cross-sectional view of an example of a spring snap attachment mechanism. 
         FIGS.  13 . 15 - 10    is a cross-sectional view of an example of an interlocking attachment mechanism. 
         FIGS.  13 . 15 - 11    is a cross-sectional view of an example of a suction attachment mechanism. 
         FIGS.  13 . 15 - 12    is a cross-sectional view of an example of a bi-stable attachment mechanism. 
         FIGS.  13 . 15 - 13 A  is a plan view of an example of a removable facial interface. 
         FIGS.  13 . 15 - 13 B  is a plan view of an example of a removable facial interface. 
         FIGS.  13 . 15 - 14    is a cross-sectional view of an example facial interface. 
         FIGS.  13 . 15 - 15 A  is a cross-sectional view of a compressible portion. 
         FIGS.  13 . 15 - 15 B  is a cross-sectional view of a compressible portion. 
         FIGS.  13 . 15 - 15 C  is a cross-sectional view of a compressible portion. 
       13.16: Electronic Devices with Light Blocking Structures 
         FIGS.  13 . 16 - 1    is a diagram of an illustrative electronic device in accordance with an embodiment. 
         FIGS.  13 . 16 - 2    is a front view of an illustrative electronic device with a light-shielding structure in accordance with an embodiment. 
         FIGS.  13 . 16 - 3    is a diagram of an illustrative light-shielding structure having a fabric cover in accordance with an embodiment. 
         FIGS.  13 . 16 - 4 A and  13 . 16 - 4 B  are front views of illustrative elastomeric layers that may be used in a nosepiece in accordance with some embodiments. 
         FIGS.  13 . 16 - 5    is a front view of an illustrative light-shielding structure having a structural frame in accordance with an embodiment. 
         FIGS.  13 . 16 - 6    is a side view of an illustrative light-shielding structure having a fabric and elastomer layers in accordance with an embodiment. 
         FIGS.  13 . 16 - 7    is a front view of an illustrative light-shielding structure having an extension in accordance with an embodiment. 
         FIGS.  13 . 16 - 8    is a side view of an illustrative light-shielding structure having an embedded service loop in accordance with an embodiment. 
         FIGS.  13 . 16 - 9    is a side view of an illustrative light-shielding structure having an embedded deformable stiffener in accordance with an embodiment. 
         FIGS.  13 . 16 - 10 A  is a side view of an illustrative light-shielding structure having a rolled edge in accordance with an embodiment. 
         FIGS.  13 . 16 - 10 B  is a side view of an illustrative light-shielding structure having embedded foam in accordance with an embodiment. 
         FIGS.  13 . 16 - 10 C  is a top view of an illustrative light-shielding structure having a crumple zone in accordance with an embodiment. 
         FIGS.  13 . 16 - 10 D  is a side view of an illustrative light-shielding structure having a hemmed edge in accordance with an embodiment. 
         FIGS.  13 . 16 - 10 E  is a top view of an illustrative light-shielding structure having a foam in corner regions in accordance with an embodiment. 
         FIGS.  13 . 16 - 10 F  is a side view of an illustrative light-shielding structure having segmented foam or elastomeric regions in accordance with an embodiment. 
         FIGS.  13 . 16 - 10 G  is a side view of an illustrative light-shielding structure having a stiffener and a foam layer in accordance with an embodiment. 
         FIGS.  13 . 16 - 11    is a front view of an illustrative light-shielding structure having a semi-rigid stiffener in accordance with an embodiment. 
         FIGS.  13 . 16 - 12    is a perspective view of an illustrative light-shield structure formed from multiple fabric layers in accordance with an embodiment. 
       XIV: Powerstraps and Securement Band 
         FIGS.  14 . 0 - 1    illustrates a view of an HMD. 
       14.1: Electrical Connectors 
         FIGS.  14 . 1 - 1 A  shows a perspective side view of an electronic device. 
         FIGS.  14 . 1 - 1 B  shows a perspective side view of the electronic device of  FIGS.  14 . 1 - 1 A . 
         FIGS.  14 . 1 - 2    shows a perspective view of a display, a support, and a plug connector. 
         FIGS.  14 . 1 - 3 A  shows a perspective view of a receptacle connector. 
         FIGS.  14 . 1 - 3 B  shows a perspective view of a plug connector. 
         FIGS.  14 . 1 - 4    shows an exploded view of a receptacle connector. 
         FIGS.  14 . 1 - 5 A  shows a front view of a receptacle connector. 
         FIGS.  14 . 1 - 5 B  shows a partial cut-away front view of the receptacle connector of  FIGS.  14 . 1 - 5 A . 
         FIGS.  14 . 1 - 6 A  shows a side sectional view of a receptacle connector. 
         FIGS.  14 . 1 - 6 B  shows a side sectional view of a receptacle connector. 
         FIGS.  14 . 1 - 7 A  shows a detail perspective view of a receptacle connector. 
         FIGS.  14 . 1 - 7 B  shows a detail perspective view of a plug connector. 
         FIGS.  14 . 1 - 8 A  shows a sectional view of a plug connector inserted into a receptacle connector. 
         FIGS.  14 . 1 - 8 B  shows a detail sectional view of the plug connector inserted into the receptacle connector of  FIGS.  14 . 1 - 8 A . 
         FIGS.  14 . 1 - 9 A  shows a detail sectional view of a plug connector inserted into a receptacle connector. 
         FIGS.  14 . 1 - 9 B  shows a detail sectional view of the plug connector inserted into the receptacle connector of  FIGS.  14 . 1 - 9 A . 
         FIGS.  14 . 1 - 9 C  shows a detail sectional view of a plug connector inserted into a receptacle connector. 
         FIGS.  14 . 1 - 9 D  shows a detail sectional view of the plug connector inserted into the receptacle connector of  FIGS.  14 . 1 - 9 C . 
         FIGS.  14 . 1 - 9 E  shows a detail sectional view of a plug connector inserted into a receptacle connector. 
         FIGS.  14 . 1 - 9 F  shows a detail sectional view of the plug connector inserted into the receptacle connector of  FIGS.  14 . 1 - 9 E . 
         FIGS.  14 . 1 - 10    shows a sectional view of a tool ejecting a plug connector from a receptacle connector. 
         FIGS.  14 . 1 - 11 A through  14 . 1 - 11 D  show perspective views of tools for ejecting plug connectors from receptacle connectors. 
         FIGS.  14 . 1 - 12 A  shows a front view of a receptacle connector. 
         FIGS.  14 . 1 - 12 B  shows a front view of a plug connector. 
         FIGS.  14 . 1 - 13 A  shows a sectional view of a receptacle connector. 
         FIGS.  14 . 1 - 13 B through  14 . 1 - 13 D  show detail sectional views of seals of the receptacle connector of  FIGS.  14 . 1 - 13 A . 
         FIGS.  14 . 1 - 14 A  shows a perspective view of a receptacle connector. 
         FIGS.  14 . 1 - 14 B  shows a detail sectional view of a fastener of the receptacle connector of  FIGS.  14 . 1 - 14 A . 
         FIGS.  14 . 1 - 15    shows a perspective view of a receptacle connector, a plug connector, and a housing. 
         FIGS.  14 . 1 - 16 A  shows a side sectional view of a receptacle connector and a plug connector. 
         FIGS.  14 . 1 - 16 B  shows a side view of the plug connector of  FIGS.  14 . 1 - 16 A . 
         FIGS.  14 . 1 - 17 A  shows a side view of a plug connector. 
         FIGS.  14 . 1 - 17 B  shows a bottom-up view of the plug connector of  FIGS.  14 . 1 - 17 A . 
         FIGS.  14 . 1 - 18 A  shows a perspective view of a receptacle connector. 
         FIGS.  14 . 1 - 18 B  shows a top-down view of the receptacle connector of  FIGS.  14 . 1 - 18 A . 
         FIGS.  14 . 1 - 18 C  shows a side sectional view of the receptacle connector of  FIG.  14 . 1 - 18 A . 
         FIGS.  14 . 1 - 18 D  shows a side sectional view of the receptacle connector of  FIG.  14 . 1 - 18 A  and a plug connector. 
         FIGS.  14 . 1 - 18 E  shows a side sectional view of the receptacle connector of  FIG.  14 . 1 - 18 A  and the plug connector of  FIGS.  14 . 1 - 18 D . 
         FIGS.  14 . 1 - 19 A  shows a detail top-down view of a receptacle connector. 
         FIGS.  14 . 1 - 19 B  shows a perspective view of a detent of the receptacle connector of  FIGS.  14 . 1 - 19 A . 
         FIGS.  14 . 1 - 19 C  shows a sectional view of the detent of  FIGS.  14 . 1 - 19 B  and a plug connector. 
         FIGS.  14 . 1 - 19 D  shows a detail top-down view of a receptacle connector. 
         FIGS.  14 . 1 - 19 E  shows a perspective view of a detent of the receptacle connector of  FIGS.  14 . 1 - 19 D . 
         FIGS.  14 . 1 - 19 F  shows a sectional view of the detent of  FIGS.  14 . 1 - 19 E  and a plug connector. 
         FIGS.  14 . 1 - 19 G  shows a detail top-down view of a receptacle connector. 
         FIGS.  14 . 1 - 19 H  shows a perspective view of a detent of the receptacle connector of  FIGS.  14 . 1 - 19 G . 
         FIGS.  14 . 1 - 19 I  shows a detail top-down view of a receptacle connector. 
         FIGS.  14 . 1 - 19 J  shows a perspective view of a detent of the receptacle connector of  FIGS.  14 . 1 - 19 I . 
         FIGS.  14 . 1 - 19 K  shows a sectional view of the detent of  FIGS.  14 . 1 - 19 J  and a plug connector. 
         FIGS.  14 . 1 - 19 L  shows a detail top-down view of a receptacle connector. 
         FIGS.  14 . 1 - 19 M  shows a perspective view of a detent of the receptacle connector of  FIGS.  14 . 1 - 19 L . 
         FIGS.  14 . 1 - 20 A  shows a bottom view of a receptacle connector and a plug connector. 
         FIGS.  14 . 1 - 20 B  shows a bottom view of a receptacle connector and a plug connector. 
         FIGS.  14 . 1 - 21 A  shows a bottom view of a receptacle connector and a plug connector. 
         FIGS.  14 . 1 - 21 B  shows a bottom view of a receptacle connector and a plug connector. 
         FIGS.  14 . 1 - 22 A through  14 . 1 - 22 E  show sectional side views of a receptacle connector, a plug connector, and seals between the receptacle connector and the plug connector. 
         FIGS.  14 . 1 - 23 A through  14 . 1 - 23 G  show sectional side views of a receptacle connector, a plug connector, and seals between the receptacle connector and the plug connector. 
         FIGS.  14 . 1 - 24 A and  14 . 1 - 24 B  show exploded views of receptacle connectors. 
         FIGS.  14 . 1 - 25 A and  14 . 1 - 25 B  show exploded views of receptacle connectors. 
         FIGS.  14 . 1 - 26 A  shows a perspective view of an electronic device. 
         FIGS.  14 . 1 - 26 B  shows a perspective view of the electronic device of  FIGS.  14 . 1 - 26 A  and a plug connector. 
         FIGS.  14 . 1 - 27 A  shows a perspective view of a plug connector inserted into an electronic device. 
         FIGS.  14 . 1 - 27 B  shows a partially exploded view of the plug connector of  FIG.  14 . 1 - 27 A , a trim ring, and a receptacle connector. 
         FIGS.  14 . 1 - 28 A  shows a sectional view of a plug connector being inserted into a trim ring and a receptacle connector. 
         FIGS.  14 . 1 - 28 B  shows a detail sectional view of the plug connector and a latch of the trim ring of  FIGS.  14 . 1 - 28 A . 
         FIGS.  14 . 1 - 28 C  shows a sectional view of the plug connector being inserted into the trim ring and the receptacle connector of  FIGS.  14 . 1 - 28 A . 
         FIGS.  14 . 1 - 28 D  shows a sectional view of the plug connector inserted into the trim ring and the receptacle connector of  FIGS.  14 . 1 - 28 A . 
         FIGS.  14 . 1 - 28 E  shows a sectional view of the plug connector being un-latched from the trim ring and the receptacle connector of  FIGS.  14 . 1 - 28 A . 
         FIGS.  14 . 1 - 28 F  shows a perspective view of a lever arm of the trim ring of  FIG.  14 . 1 - 28 A . 
         FIGS.  14 . 1 - 29 A  shows a sectional view of a plug connector in a trim ring and a receptacle connector. 
         FIGS.  14 . 1 - 29 B  shows a sectional view of a tool used to un-latch the plug connector from the trim ring and the receptacle connector of  FIGS.  14 . 1 - 29 A . 
         FIGS.  14 . 1 - 29 C  shows a perspective view of the plug connector and the trim ring of  FIGS.  14 . 1 - 29 A . 
         FIGS.  14 . 1 - 29 D  shows a sectional view of the trim ring of  FIGS.  14 . 1 - 29 A . 
         FIGS.  14 . 1 - 29 E  shows a perspective view of a lever arm of the trim ring of  FIG.  14 . 1 - 29 A . 
         FIGS.  14 . 1 - 30 A  shows a perspective view of a plug connector and a trim ring. 
         FIGS.  14 . 1 - 30 B  shows a sectional view of the plug connector inserted in the trim ring of  FIGS.  14 . 1 - 30 A  and a receptacle connector. 
         FIGS.  14 . 1 - 30 C  shows a sectional view of the plug connector inserted in the trim ring and the receptacle connector of  FIGS.  14 . 1 - 30 B . 
         FIGS.  14 . 1 - 31 A  shows a sectional view of a plug connector being inserted into a trim ring and a receptacle connector. 
         FIGS.  14 . 1 - 31 B  shows a sectional view of the plug connector being un-latched from into the trim ring and the receptacle connector of  FIGS.  14 . 1 - 31 A . 
         FIGS.  14 . 1 - 32 A through  14 . 1 - 32 C  show perspective views of a trim ring and a receptacle connector being assembled in a housing. 
         FIGS.  14 . 1 - 33 A  shows a top-down view of a plug connector being inserted into a trim ring and a receptacle connector. 
         FIGS.  14 . 1 - 33 B  shows a detail view of the trim ring and the plug connector of  FIGS.  14 . 1 - 33 A  prior to the trim ring latching with the receptacle connector. 
         FIGS.  14 . 1 - 33 C  shows a top-down view of the plug connector latched in the trim ring and the receptacle connector of  FIGS.  14 . 1 - 33 A . 
         FIGS.  14 . 1 - 33 D  shows a detail view of the trim ring and the plug connector of  FIGS.  14 . 1 - 33 C  with the plug connector latched in the trim ring. 
         FIGS.  14 . 1 - 33 E  shows a top-down view of the plug connector being un-latched from the trim ring and the receptacle connector of  FIGS.  14 . 1 - 33 A . 
         FIGS.  14 . 1 - 33 F  shows a detail view of the trim ring and the plug connector of  FIGS.  14 . 1 - 33 E  with the plug connector un-latched from the trim ring. 
         FIGS.  14 . 1 - 34 A and  14 . 1 - 34 B  show perspective views of a plug connector inserted into a trim ring and a receptacle connector. 
         FIGS.  14 . 1 - 35 A  shows an exploded view of a receptacle connector. 
         FIGS.  14 . 1 - 35 B  shows a side sectional view of the receptacle connector of  FIG.  14 . 1 - 35 A . 
         FIGS.  14 . 1 - 36 A and  14 . 1 - 36 B  respectively illustrate a semi-transparent view and a solid view of an electrical connector portion. 
         FIGS.  14 . 1 - 37  through  14 . 1 - 38    illustrate respective top and bottom views of an electrical connector portion. 
         FIGS.  14 . 1 - 39    illustrates example method steps for manufacturing an electrical connector portion. 
         FIGS.  14 . 1 - 40    depicts example method steps of providing an interface connector to an electrical connector portion. 
         FIGS.  14 . 1 - 41 A and  14 . 1 - 41 B  show side schematic views of the assembling an interface connector to an electrical connector portion. 
       14.2: Modular Components for Wearable Electronic Devices 
         FIGS.  14 . 2 - 1 A  shows a wearable electronic device being worn by a user. 
         FIGS.  14 . 2 - 1 B  shows a top view of the wearable electronic device of  FIGS.  14 . 2 - 1 A . 
         FIGS.  14 . 2 - 1 C  shows an exploded view of the wearable electronic device of  FIG.  14 . 2 - 1 A . 
         FIGS.  14 . 2 - 2 A  shows an exploded view of a wearable electronic device. 
         FIGS.  14 . 2 - 2 B  shows a side view of a component of the wearable electronic device of  FIGS.  14 . 2 - 2 A . 
         FIGS.  14 . 2 - 2 C  shows a side view of a component of the wearable electronic device of  FIGS.  14 . 2 - 2 A . 
         FIGS.  14 . 2 - 2 D  shows a cross-sectional view of the component of  FIGS.  14 . 2 - 2 C . 
         FIGS.  14 . 2 - 3    shows a side view of a component of a wearable electronic device. 
         FIGS.  14 . 2 - 4    shows a side view of a component of a wearable electronic device. 
         FIGS.  14 . 2 - 5 A  shows a top view of a component of a wearable electronic device. 
         FIGS.  14 . 2 - 5 B  shows a side view of the component of  FIGS.  14 . 2 - 5 A . 
         FIGS.  14 . 2 - 5 C  shows a cross-sectional view of the component of  FIGS.  14 . 2 - 5 A . 
         FIGS.  14 . 2 - 6 A  shows a top view of a component of a wearable electronic device. 
         FIGS.  14 . 2 - 6 B  shows a side view of the component of  FIGS.  14 . 2 - 6 A . 
         FIGS.  14 . 2 - 6 C  shows a cross-sectional view of the component of  FIGS.  14 . 2 - 6 A . 
         FIGS.  14 . 2 - 7 A  shows a top view of a component of a wearable electronic device. 
         FIGS.  14 . 2 - 7 B  shows a side view of the component of  FIGS.  14 . 2 - 7 A . 
         FIGS.  14 . 2 - 7 C  shows a cross-sectional view of the component of  FIGS.  14 . 2 - 7 A . 
         FIGS.  14 . 2 - 8 A  shows a top view of a component of a wearable electronic device. 
         FIGS.  14 . 2 - 8 B  shows a side view of the component of  14 . 2 - FIG.  8 A . 
         FIGS.  14 . 2 - 8 C  shows a cross-sectional view of the component of  FIGS.  14 . 2 - 8 A . 
         FIGS.  14 . 2 - 9 A  shows a top view of a component of a wearable electronic device. 
         FIGS.  14 . 2 - 9 B  shows a side view of the component of  FIGS.  14 . 2 - 9 A . 
         FIGS.  14 . 2 - 9 C  shows a cross-sectional view of the component of  FIGS.  14 . 2 - 9 A . 
         FIGS.  14 . 2 - 10 A  shows a top view of a component of a wearable electronic device. 
         FIGS.  14 . 2 - 10 B  shows a side view of the component of  FIGS.  14 . 2 - 10 A . 
         FIGS.  14 . 2 - 10 C  shows a cross-sectional view of the component of  FIGS.  14 . 2 - 10 A . 
         FIGS.  14 . 2 - 11    shows an exploded view of a wearable electronic device. 
         FIGS.  14 . 2 - 12    shows an exploded view of a wearable electronic device. 
         FIGS.  14 . 2 - 13    exploded view of a wearable electronic device. 
       14.3: Modular Strap for an Electronic Device 
         FIGS.  14 . 3 - 1    shows a top view of an example of an electronic device donned by a user. 
         FIGS.  14 . 3 - 2    shows a perspective view of an example of an electronic device. 
         FIGS.  14 . 3 - 3    shows an exploded perspective view of an example of an electronic device. 
         FIGS.  14 . 3 - 4    shows a side profile view of an example removable strap of an HMD system. 
         FIGS.  14 . 3 - 5    shows a top cross-sectional profile view of an example electronics pod. 
         FIGS.  14 . 3 - 6    shows a top view of another example of an electronic device donned by a user. 
         FIGS.  14 . 3 - 7  and  14 . 3 - 8    show example cable management mechanisms of an example HMD system. 
       14.4: Devices with Detachable Headbands 
         FIGS.  14 . 4 - 1    is a side view of an electronic device with a detachable headband. 
         FIGS.  14 . 4 - 2    is a view of a detachable headband. 
         FIGS.  14 . 4 - 3    is a cross-sectional side view of a portion of a detachable headband. 
         FIGS.  14 . 4 - 4    is a top view of a spring. 
         FIGS.  14 . 4 - 5    is a diagram of a detachable headband having a latch with a release tab. 
         FIGS.  14 . 4 - 6    is a cross-sectional side view of a detachable headband with a release tab. 
         FIGS.  14 . 4 - 7    is a top view of a magnet arrangement. 
         FIGS.  14 . 4 - 8 ,  14 . 4 - 9 , and  14 . 4 - 10    are diagrams showing latch biasing mechanisms. 
         FIGS.  14 . 4 - 11    is a cross-sectional side view of a latch biasing mechanism. 
         FIGS.  14 . 4 - 12  and  14 . 4 - 13    are cross-sectional side views of detachable headbands. 
         FIGS.  14 . 4 - 14    is a perspective view of a detachable headband with recesses. 
         FIGS.  14 . 4 - 15    is a top view of a headband attachment post. 
         FIGS.  14 . 4 - 16    is a cross-sectional side view of a headband attachment post. 
         FIGS.  14 . 4 - 17    is a cross-sectional side view of a headband attachment post with a recess and a corresponding detachable headband. 
       14.5: Cable Tensioning System and Dial 
         FIGS.  14 . 5 - 1    is a side view of an example of a head-mountable display device with an adjustable headband; 
         FIGS.  14 . 5 - 2    is a plan view of an example of an adjustable headband; 
         FIGS.  14 . 5 - 3    is a perspective view of an example of a tensioning system for an adjustable headband; 
         FIGS.  14 . 5 - 4    is a partially exploded view of an example of a tensioning system for an adjustable headband; 
         FIGS.  14 . 5 - 5    is a partial cross-sectional view of an example of a tensioning system for an adjustable headband; 
         FIGS.  14 . 5 - 6    is a partially exploded view of an example of a dial cap for a tensioning system; 
         FIGS.  14 . 5 - 7 A and  14 . 5 - 7 B  are partial cross-sectional views of an example of a disc-type angular restraint system; 
         FIGS.  14 . 5 - 8    is partial cross-sectional view of an example of a dial cap including a spring detent mechanism; and 
         FIGS.  14 . 5 - 9 A through  14 . 5 - 9 C  are perspective views of an example of angular restraint systems. 
       14.6: Two-Part Speaker System 
         FIGS.  14 . 6 - 1 A  shows a side view of an electronic device. 
         FIGS.  14 . 6 - 1 B  shows a perspective view of an electronic device. 
         FIGS.  14 . 6 - 1 C  shows a perspective view of an electronic device. 
         FIGS.  14 . 6 - 1 D  shows a perspective view of an electronic device. 
         FIGS.  14 . 6 - 2    shows a cross-sectional side view of a speaker assembly. 
         FIGS.  14 . 6 - 3 A  shows a perspective view of a speaker assembly. 
         FIGS.  14 . 6 - 3 B  shows a cross-sectional side view of a speaker assembly. 
         FIGS.  14 . 6 - 3 C  shows a cross-sectional perspective view of a speaker assembly. 
         FIGS.  14 . 6 - 3 D  shows a top perspective view of a speaker assembly. 
         FIGS.  14 . 6 - 3 E  shows a bottom perspective view of a speaker assembly. 
         FIGS.  14 . 6 - 4    shows a perspective exploded view of a port barrier. 
       14.7: Bifurcated Band 
         FIGS.  14 . 7 - 1    is a side view of an illustrative electronic device such as a head-mounted display device with an adjustable headband in accordance with some embodiments. 
         FIGS.  14 . 7 - 2 A and  14 . 7 - 2 B  are side views of opposing sides of an illustrative headband in accordance with some embodiments. 
         FIGS.  14 . 7 - 3    is an illustrative front view of an edge of a headband in accordance with some embodiments. 
         FIGS.  14 . 7 - 4    is a side view of an illustrative headband with a seam that is invisible to a naked eye in accordance with some embodiments. 
         FIGS.  14 . 7 - 5    is a side view of an illustrative headband having stiffeners on a surface of the headband in accordance with some embodiments. 
         FIGS.  14 . 7 - 6 A- 14 . 7 - 6 C  are side views of illustrative stiffeners that may be incorporated onto a surface of a headband in accordance with some embodiments. 
         FIGS.  14 . 7 - 7    is a side view of an illustrative headband having embedded stiffeners in accordance with some embodiments. 
         FIGS.  14 . 7 - 8    is a perspective view of an illustrative stiffener in a channel of a headband in accordance with some embodiments. 
         FIGS.  14 . 7 - 9 A and  14 . 7 - 9 B  are side views of illustrative headbands with local stiffeners that change the curvature of the headband when under tension in accordance with some embodiments. 
       14.8: Over the Head Strap 
         FIGS.  14 . 8 - 1    is a side view of an illustrative electronic device such as a head-mounted display device with a detachable headband in accordance with some embodiments. 
         FIGS.  14 . 8 - 2    is a perspective view of an illustrative headband with a post that couples to a post on a head-mounted structure in accordance with some embodiments. 
         FIGS.  14 . 8 - 3    is a cross-sectional side view of an illustrative headband with a post that couples to a post on a head-mounted structure in accordance with some embodiments. 
         FIGS.  14 . 8 - 4    is a cross-sectional side view of an illustrative detachable headband with a release tab in accordance with some embodiments. 
         FIGS.  14 . 8 - 5    is a perspective view of an illustrative headband with a magnet that couples to a post on a head-mounted structure in accordance with some embodiments. 
         FIGS.  14 . 8 - 6    is a cross-sectional side view of an illustrative headband with a magnet that couples to a post on a head-mounted structure in accordance with some embodiments. 
         FIGS.  14 . 8 - 7    is a cross-sectional side view of an illustrative headband with a magnet and a protrusion that couples to a post with a recess on a head-mounted structure in accordance with some embodiments. 
         FIGS.  14 . 8 - 8    is a perspective view of an illustrative headband that has portions that wrap around a head-mounted support structure to attach to the support structure in accordance with some embodiments. 
         FIGS.  14 . 8 - 9    is a perspective view of an illustrative headband that attaches to a head-mounted support structure with a lug and socket system in accordance with some embodiments. 
         FIGS.  14 . 8 - 10    is a cross-sectional side view of two illustrative headbands that attach to a head-mounted support structure with latches in accordance with some embodiments 
         FIGS.  14 . 8 - 11    is a cross-sectional side view of two illustrative headbands, one of which attaches to a head-mounted support structure with a latch, and one of which attaches to the head-mounted support structure with a protrusion, in accordance with some embodiments 
         FIGS.  14 . 8 - 12    is a diagram of an illustrative headband that attaches to a head-mounted support structure with a twist-to-lock system in accordance with some embodiments. 
         FIGS.  14 . 8 - 13    is a perspective view of an illustrative headband that has an opening to surround a post of a head-mounted support structure in accordance with some embodiments. 
         FIGS.  14 . 8 - 14    is a cross-sectional side view of an illustrative headband that has an opening to surround a post of a head-mounted support structure in accordance with some embodiments. 
         FIGS.  14 . 8 - 15 A and  14 . 8 - 15 B  are perspective views of an illustrative post with an extendable magnet in accordance with some embodiments. 
         FIGS.  14 . 8 - 16    is a perspective view of an illustrative headband with an opening to receive a magnet and couple to a head-mounted support structure in accordance with some embodiments. 
         FIGS.  14 . 8 - 17 A and  14 . 8 - 17 B  are cross-sectional side views of an illustrative headband engaging with an extendable magnet of a post in accordance with some embodiments. 
       XV: User Interface 
         FIG.  15 - 1    shows an example user interface displayed by a display module of an HMD. 
         FIG.  15 - 2 A  shows an example user interface displayed by a display module of an HMD. 
         FIG.  15 - 2 B  shows an example user interface displayed by a display module of an HMD. 
         FIG.  15 - 3 A  shows an example of a user interface of a display module of an electronic device. 
         FIG.  15 - 3 B  shows an example of a user interface of a display module of an electronic device. 
         FIG.  15 - 4 A  shows an example of users interacting with a user interface of two display modules of an electronic device. 
         FIG.  15 - 4 B  shows an example of users interacting with a user interface of two display modules of an electronic device. 
         FIG.  15 - 5 A  shows an example user interface displayed by a display module of an HMD. 
         FIG.  15 - 5 B  shows an example user interface displayed by a display module of an HMD. 
         FIG.  15 - 5 C  shows an example user interface displayed by a display module of an HMD. 
         FIG.  15 - 6 A  shows an example user interface displayed by a display module of an HMD. 
         FIG.  15 - 6 B  shows an example user interface displayed by a display module of an HMD. 
     
    
    
     DETAILED DESCRIPTION 
     I: Overall System 
       FIG.  1 - 1 A  illustrates a front, top, perspective view of an example of a head-mountable display (HMD) device  1 - 100  configured to be donned by a user and provide virtual and altered/mixed reality (VR/AR) experiences. The HMD  1 - 100  can include a display unit  1 - 102  or assembly, an electronic strap assembly  1 - 104  connected to and extending from the display unit  1 - 102 , and a band assembly  1 - 106  secured at either end to the electronic strap assembly  1 - 104 . The electronic strap assembly  1 - 104  and the band  1 - 106  can be part of a retention assembly configured to wrap around a user&#39;s head to hold the display unit  1 - 102  against the face of the user. 
     In at least one example, the band assembly  1 - 106  can include a first band  1 - 116  configured to wrap around the rear side of a user&#39;s head and a second band  1 - 117  configured to extend over the top of a user&#39;s head. The second strap can extend between first and second electronic straps  1 - 105   a ,  1 - 105   b  of the electronic strap assembly  1 - 104  as shown. The strap assembly  1 - 104  and the band assembly  1 - 106  can be part of a securement mechanism extending rearward from the display unit  1 - 102  and configured to hold the display unit  1 - 102  against a face of a user. 
     In at least one example, the securement mechanism includes a first electronic strap  1 - 105   a  including a first proximal end  1 - 134  coupled to the display unit  1 - 102 , for example a housing  1 - 150  of the display unit  1 - 102 , and a first distal end  1 - 136  opposite the first proximal end  1 - 134 . The securement mechanism can also include a second electronic strap  1 - 105   b  including a second proximal end  1 - 138  coupled to the housing  1 - 150  of the display unit  1 - 102  and a second distal end  1 - 140  opposite the second proximal end  1 - 138 . The securement mechanism can also include the first band  1 - 116  including a first end  1 - 142  coupled to the first distal end  1 - 136  and a second end  1 - 144  coupled to the second distal end  1 - 140  and the second band  1 - 117  extending between the first electronic strap  1 - 105   a  and the second electronic strap  1 - 105   b . The straps  1 - 105   a - b  and band  1 - 116  can be coupled via connection mechanisms or assemblies  1 - 114 . In at least one example, the second band  1 - 117  includes a first end  1 - 146  coupled to the first electronic strap  1 - 105   a  between the first proximal end  1 - 134  and the first distal end  1 - 136  and a second end  1 - 148  coupled to the second electronic strap  1 - 105   b  between the second proximal end  1 - 138  and the second distal end  1 - 140 . 
     In at least one example, the first and second electronic straps  1 - 105   a - b  include plastic, metal, or other structural materials forming the shape the substantially rigid straps  1 - 105   a - b . In at least one example, the first and second bands  1 - 116 ,  1 - 117  are formed of elastic, flexible materials including woven textiles, rubbers, and the like. The first and second bands  1 - 116 ,  1 - 117  can be flexible to conform to the shape of the user′ head when donning the HMD  1 - 100 . 
     In at least one example, one or more of the first and second electronic straps  1 - 105   a - b  can define internal strap volumes and include one or more electronic components disposed in the internal strap volumes. In one example, as shown in  FIG.  1 - 1 A , the first electronic strap  1 - 105   a  can include an electronic component  1 - 112 . In one example, the electronic component  1 - 112  can include a speaker. In one example, the electronic component  1 - 112  can include a computing component such as a processor. 
     In at least one example, the housing  1 - 150  defines a first, front-facing opening  1 - 152 . The front-facing opening is labeled in dotted lines at  1 - 152  in  FIG.  1 - 1 A  because the front display assembly  1 - 108  is disposed to occlude the first opening  1 - 152  from view when the HMD  1 - 100  is assembled. The housing  1 - 150  can also define a rear-facing second opening  1 - 154 . The housing  1 - 150  also defines an internal volume between the first and second openings  1 - 152 ,  1 - 154 . In at least one example, the HMD  1 - 100  includes the display assembly  1 - 108 , which can include a front cover and display screen (shown in other figures) disposed in or across the front opening  1 - 152  to occlude the front opening  1 - 152 . In at least one example, the display screen of the display assembly  1 - 108 , as well as the display assembly  1 - 108  in general, has a curvature configured to follow the curvature of a user&#39;s face. The display screen of the display assembly  1 - 108  can be curved as shown to compliment the user&#39;s facial features and general curvature from one side of the face to the other, for example from left to right and/or from top to bottom where the display unit  1 - 102  is pressed. 
     In at least one example, the housing  1 - 150  can define a first aperture  1 - 126  between the first and second openings  1 - 152 ,  1 - 154  and a second aperture  1 - 130  between the first and second openings  1 - 152 ,  1 - 154 . The HMD  1 - 100  can also include a first button  1 - 128  disposed in the first aperture  1 - 126  and a second button  1 - 132  disposed in the second aperture  1 - 130 . The first and second buttons  1 - 128 ,  1 - 132  can be depressible through the respective apertures  1 - 126 ,  1 - 132 . In at least one example, the first button  1 - 126  and/or second button  1 - 130  can be twistable dials as well as depressible buttons. In at least one example, the first button  1 - 126  is a depressible and twistable dial button and the second button  1 - 132  is a depressible button. 
       FIG.  1 - 1 B  illustrates a rear, perspective view of the HMD  1 - 100 . The HMD  1 - 100  can include a light seal  1 - 110  extending rearward from the housing  1 - 150  of the display assembly  1 - 108  around a perimeter of the housing  1 - 150  as shown. The light seal  1 - 110  can be configured to extend from the housing  1 - 150  to the user&#39;s face around the user&#39;s eyes to block external light from being visible. In one example, the HMD  1 - 100  can include first and second display assemblies  1 - 120   a ,  1 - 120   b  disposed at or in the rearward facing second opening  1 - 154  defined by the housing  1 - 150  and/or disposed in the internal volume of the housing  1 - 150  and configured to project light through the second opening  1 - 154 . In at least one example, each display assembly  1 - 120   a - b  can include respective display screens  1 - 122   a ,  1 - 122   b  configured to project light in a rearward direction through the second opening  1 - 154  toward the user&#39;s eyes. 
     In at least one example, referring to both  FIGS.  1 - 1 A and  1 - 1 B , the display assembly  1 - 108  can be a front-facing, forward display assembly including a display screen configured to project light in a first, forward direction and the rear facing display screens  1 - 122   a - b  can be configured to project light in a second, rearward direction opposite the first direction. As noted above, the light seal  1 - 110  can be configured to block light external to the HMD  1 - 100  from reaching the user&#39;s eyes, including light projected by the forward facing display screen of the display assembly  1 - 108  shown in the front perspective view of  FIG.  1 - 1 A . In at least one example, the HMD  1 - 100  can also include a curtain  1 - 124  occluding the second opening  1 - 154  between the housing  1 - 150  and the rear-facing display assemblies  1 - 120   a - b . In at least one example, the curtain  1 - 124  can be elastic or at least partially elastic. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  1 - 1 A and  1 - 1 B  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  1 - 2 - 1 - 4    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  2 - 4    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  1 - 1 A and  1 - 1 B . 
       FIG.  1 - 2    illustrates a view of an example of an HMD  1 - 200  including various portions or parts thereof separated according to the modularity and selective coupling of those parts. For example, the HMD  1 - 200  can include a band  1 - 216  which can be selectively coupled to first and second electronic straps  1 - 205   a ,  1 - 205   b . The first securement strap  1 - 205   a  can include a first electronic component  1 - 212   a  and the second securement strap  1 - 205   b  can include a second electronic component  1 - 212   b . In at least one example, the first and second straps  1 - 205   a - b  can be removably coupled to the display unit  1 - 202 . 
     In addition, the HMD  1 - 200  can include a light seal  1 - 210  configured to be removably coupled to the display unit  1 - 202 . The HMD  1 - 200  can also include lenses  1 - 218  which can be removably coupled to the display unit  1 - 202 , for example over first and second display assemblies including display screens. The lenses  1 - 218  can include customized prescription lenses configured for corrective vision. As noted, each part shown in the view of  FIG.  1 - 2    and described above can be removably coupled, attached, re-attached, and changed out to update parts or swap out parts for different users. For example, bands such as the band  1 - 216 , light seals such as the light seal  1 - 210 , lenses such as the lenses  1 - 218 , and electronic straps such as the straps  1 - 205   a - b  can be swapped out depending on the user such that these parts are customized to fit and correspond to the individual user of the HMD  1 - 200 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  1 - 2    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  1 - 1 A,  1 - 1 B , and  1 - 3 - 1 - 4  and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  1 - 1 A,  1 - 1 B, and  1 - 3 - 1 - 4    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  1 - 2   . 
       FIG.  1 - 3    illustrates a view of an example of a display unit  1 - 306  of an HMD. The display unit  1 - 306  can include a front display assembly  1 - 308 , a frame/housing assembly  1 - 350 , and a curtain assembly  1 - 324 . The display unit  1 - 306  can also include a sensor assembly  1 - 356 , logic board assembly  1 - 358 , and cooling assembly  1 - 360  disposed between the frame assembly  1 - 350  and the front display assembly  1 - 308 . In at least one example, the display unit  1 - 306  can also include a rear-facing display assembly  1 - 320  including first and second rear-facing display screens  1 - 322   a ,  1 - 322   b  disposed between the frame  1 - 350  and the curtain assembly  1 - 324 . 
     In at least one example, the display unit  1 - 306  can also include a motor assembly  1 - 362  configured as an adjustment mechanism for adjusting the positions of the display screens  1 - 322   a - b  of the display assembly  1 - 320  relative to the frame  1 - 350 . In at least one example, the display assembly  1 - 320  is mechanically coupled to the motor assembly  1 - 362 , with at least one motor for each display screen  1 - 322   a - b , such that the motors can translate the display screens  1 - 322   a - b  to match an interpupillary distance of the user&#39;s eyes. 
     In at least one example, the display unit  1 - 306  can include a dial or button  1 - 328  depressible relative to the frame  1 - 350  and accessible to the user outside the frame  1 - 350 . The button  1 - 328  can be electronically connected to the motor assembly  1 - 362  via a controller such that the button  1 - 328  can be manipulated by the user to cause the motors of the motor assembly  1 - 362  to adjust the positions of the display screens  1 - 322   a - b.    
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  1 - 3    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  1 - 1 A- 1 - 2  and  1 - 4    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  1 - 1 A- 1 - 2  and  1 - 4    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  1 - 3   . 
       FIG.  1 - 4    illustrates a view of another example of a display unit  1 - 406  of an HMD device similar to other HMD devices described herein. The display unit  1 - 406  can include a front display assembly  1 - 402 , a sensor assembly  1 - 456 , a logic board assembly  1 - 458 , a cooling assembly  1 - 460 , a frame assembly  1 - 450 , a rear-facing display assembly  1 - 421 , and a curtain assembly  1 - 424 . The display unit  1 - 406  can also include a motor assembly  1 - 462  for adjusting the positions of first and second display sub-assemblies  1 - 420   a ,  1 - 420   b  of the rear-facing display assembly  1 - 421 , including first and second respective display screens for interpupillary adjustments, as described above. 
     The various parts, systems, and assemblies shown in the view of  FIG.  1 - 4    are described in greater detail herein with reference to  FIGS.  1 - 1 A- 1 - 3    as well as subsequent figures referenced in the present disclosure. The display unit  1 - 406  shown in  FIG.  1 - 4    can be assembled and integrated with the securement mechanisms shown in  FIGS.  1 - 1 A- 1 - 3   , including the electronic straps, bands, and other components including light seals, connection assemblies, and so forth. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  1 - 4    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  1 - 1 A- 1 - 3    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  1 - 1 A- 1 - 3    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  1 - 4   . 
     II: Cover Glass 
       FIGS.  2 . 0 - 1    illustrates a view of an HMD  2 . 0 - 100  including a front cover and display assembly  2 . 0 - 102 , including one or more transparent layers, display integration assemblies, shroud, and dust seal. The transparent layers, display assemblies, shroud, and dust seal are described below in sections II, III, IV, and V. 
     2.1: Systems with Transparent Layers 
     Transparent layers may be used to form windows in buildings, vehicles, and/or other systems. Transparent layers may also be used in forming protective cover layers such as cover layers for optical components. 
       FIGS.  2 . 1 - 1    is a cross-sectional side view of an illustrative system containing a transparent layer. System  2 . 1 - 10  of  FIGS.  2 . 1 - 1    has a support such as support  2 . 1 - 12  in which one or more transparent layers such as transparent layer  2 . 1 - 14  may be mounted. System  2 . 1 - 10  may be a building (e.g., support  2 . 1 - 12  may include building walls), may be a vehicle (e.g., support  2 . 1 - 12  may be vehicle body), may be an electronic device (e.g., support  2 . 1 - 12  may be an electronic device housing such as a head-mounted housing for a head-mounted device), and/or may be any other suitable system. In arrangement in which system  2 . 1 - 10  is a building or vehicle, layer  2 . 1 - 14  may serve as a window. In arrangements in which system  2 . 1 - 10  is an electronic device, layer  2 . 1 - 14  may overlap and protect components in the device. For example, layer  2 . 1 - 14  may serve as a protective cover layer that overlaps optical components. In an illustrative configuration, system  2 . 1 - 10  is a portable electronic device (e.g., a cellular telephone, head-mounted device, tablet computer, laptop computer, wristwatch, etc.). 
     Transparent layer  2 . 1 - 14  and support  2 . 1 - 12  may separate interior region  2 . 1 - 16  of system  2 . 1 - 10  from exterior region  2 . 1 - 18 . System components may be mounted in interior region  2 . 1 - 16 . Layer  2 . 1 - 14  may have opposing inner and outer surfaces. The outer surface of layer  2 . 1 - 14  may face exterior region  2 . 1 - 18  and the inner surface of layer  2 . 1 - 14  may face interior region  2 . 1 - 16 . The surfaces of layer  2 . 1 - 14  may include planar portions and/or portions that are curved. For example, layer  2 . 1 - 14  may have a shape with a curved cross-sectional profile such as shape  2 . 1 - 20 . In arrangements in which layer  2 . 1 - 14  is curved, the inner and outer surfaces may be parallel to each other (e.g., the thickness of layer  2 . 1 - 14  may be constant across layer  2 . 1 - 14 ). If desired, some or all of the surfaces of layer  2 . 1 - 14  may have compound curvature (surfaces that can only be flattened into a plane with distortion). Surface areas of compound curvature may be bent about both the X and Y axes of  FIGS.  2 . 1 - 1   . 
       FIGS.  2 . 1 - 2    shows how layer  2 . 1 - 14  may overlap components in interior region  2 . 1 - 16  such as illustrative components  2 . 1 - 20  and  2 . 1 - 22 . Components  2 . 1 - 20  and  2 . 1 - 22  may include optical components that emit and/or detect light. As an example, component  2 . 1 - 22  may be a display that emits visible light that passes though layer  2 . 1 - 14 . This allows a viewer in exterior region  2 . 1 - 18  to view an image on the display through layer  2 . 1 - 14  (e.g., layer  2 . 1 - 14  may serve as a display cover layer). Components such as component  2 . 1 - 20  may include, for example, visible and/or infrared cameras and/or other optical sensors that receive light through layer  2 . 1 - 14 . By overlapping components  2 . 1 - 20  and  2 . 1 - 22  as shown in  FIGS.  2 . 1 - 2   , layer  2 . 1 - 14  may serve as a protective cover layer for components  2 . 1 - 20  and  2 . 1 - 22 . 
     During events such as drop events in which system  2 . 1 - 10  abruptly contacts the ground or other hard surface, layer  2 . 1 - 14  may be subjected to undesirably large amounts of stress. To help enhance durability, layer  2 . 1 - 14  may be provided with one or more layers of polymer. As an example, a polymer layer may be used to laminate multiple layers of transparent material together and/or polymer layers may be formed on exposed inner and/or outer surfaces of layer  2 . 1 - 14 . 
       FIGS.  2 . 1 - 3    is a cross-sectional side view of layer  2 . 1 - 14 . As shown in  FIGS.  2 . 1 - 3   , layer  2 . 1 - 14  may include multiple layers of transparent material such as layers  2 . 1 - 40 ,  2 . 1 - 34 ,  2 . 1 - 32 , and  2 . 1 - 30 . In an illustrative configuration, layer  2 . 1 - 14  includes two layers of hard transparent material and one or more softer layers that are attached to the harder layers. The softer layers may be, for example, polymer layers that help enhance durability. 
     In the example of  FIGS.  2 . 1 - 3   , layer  2 . 1 - 34  may be a hard layer such as a layer of glass (including glass ceramic) or sapphire or other crystalline material. Illustrative configurations in which layer  2 . 1 - 34  is a layer of glass may sometimes be described herein as an example. Layer  2 . 1 - 34  may be formed from alumina silicate glass or other glass materials and may optionally be chemically strengthened using an ion-exchange chemical strengthening process that places the surfaces of layer  2 . 1 - 34  in compression relative to the core of layer  2 . 1 - 34 . Layer  2 . 1 - 34  may have a thickness that is sufficient to provide layer  2 . 1 - 14  with some or all of its structural strength, so layer  2 . 1 - 34  may sometimes be referred to as a structural layer, structural transparent layer, or structural glass layer. Layer  2 . 1 - 34  may, as an example, have a thickness of 700 microns, at least 400 microns, at least 500 microns, at least 600 microns, less than 1200 microns, less than 1000 microns, less than 900 microns, less than 800 microns, 400-1200 microns, 400-1100 microns, 400-1000 microns, 400-800 microns, and/or other suitable thickness. 
     One or more polymer layers may be attached to layer  2 . 1 - 34 . In an illustrative configuration, polymer layer  2 . 1 - 40  is attached to inner surface  2 . 1 - 42  of layer  2 . 1 - 34 . Layer  2 . 1 - 40  may include a first layer such as layer  2 . 1 - 38  and a second layer such as layer  2 . 1 - 36 . Layer  2 . 1 - 38  may be a polymer film (e.g., a film of polycarbonate, polyethylene terephthalate, or other polymer film) and may have a thickness of 50 microns, 10-250 microns, 25-100 microns, at least 20 microns, less than 200 microns, less than 150 microns, or other suitable thickness. Layer  2 . 1 - 36  may be a polymer layer such as a layer of polymer adhesive (e.g., epoxy, acrylic adhesive, cured liquid adhesive, pressure sensitive adhesive, and/or other adhesive) that attaches layer  2 . 1 - 38  to layer  2 . 1 - 34  and may have a thickness of 100 microns, 20-500 microns, at least 30 microns, less than 250 microns, less than 300 microns, or other suitable thickness. 
     If desired, an additional polymer layer such as polymer layer  2 . 1 - 32  may be attached to upper surface outer surface  1 . 3 - 44  of layer  2 . 1 - 34 . Layer  2 . 1 - 32  may be formed from an elastomeric polymer or other soft polymer material. Examples of materials that may be used in forming polymer layer  2 . 1 - 32  include polyvinyl butyral and ethylene vinyl acetate. Other polymers may be used in forming layer  2 . 1 - 32 , if desired. Layer  2 . 1 - 32  may be the outermost layer of material of layer  2 . 1 - 14  (e.g., the outer surface of layer  2 . 1 - 32  may be exposed to region  2 . 1 - 18 ) or layer  2 . 1 - 32  may be covered with a harder outer layer. 
     As shown in  FIGS.  2 . 1 - 3   , for example, layer  2 . 1 - 32 , which may sometimes be referred to as an elastomeric polymer layer or polymer interlayer, may be used to attach a thin hard layer such as outer layer  2 . 1 - 30  to layer  2 . 1 - 34 . The thickness of layer  2 . 1 - 32  may be 50 microns, 25-100 microns, at least 20 microns, at least 40 microns, at least 50 microns, less than 400 microns, 25-400 microns, less than 300 microns, less than 200 microns, 20-200 microns, 50-400 microns, or other suitable thickness. Layer  2 . 1 - 30  be formed from glass (including glass ceramic), a crystalline material such as sapphire, or hard polymer (e.g., hardened acrylic). The thickness of layer  2 . 1 - 30  is preferably less than the thickness of layer  2 . 1 - 34  to help minimize the weight of layer  2 . 1 - 14 . 
     In an illustrative arrangement, layer  2 . 1 - 30  is formed as a separate layer (e.g., a separate glass layer from layer  2 . 1 - 34 ) that is attached to layer  2 . 1 - 34  by laminating layers  2 . 1 - 30  and  2 . 1 - 34  together using polymer layer  2 . 1 - 32 . The thickness of layer  2 . 1 - 30  in this type of arrangement may be at least 50 microns, at least 75 microns, at least 100 microns, less than 300 microns, less than 250 microns, less than 200 microns, less than 150 microns, less than 100 microns, 50-200 microns, 25-300 microns, 50-150 microns, or other suitable thickness (e.g., a thickness that provides the outermost surface of layer  2 . 1 - 14  with sufficient hardness to resist scratches). In addition to resisting scratches, the inclusion of a hard outer layer such as layer  2 . 1 - 30  to layer  2 . 1 - 14  may help enhance the strength of layer  2 . 1 - 14  and thereby allow the thickness of layer  34  to be reduced. To help match the curvature of layers  2 . 1 - 30  and  2 . 1 - 34  in this type of arrangement, layers  2 . 1 - 30  and  2 . 1 - 34  may be formed into desired shapes using molding operations (e.g., glass molding), machining and/or polishing operations, etching (wet and/or dry chemical etching), and/or other suitable shaping operations. 
     In some embodiments, layer  2 . 1 - 30  may be deposited as a coating on layer  2 . 1 - 32 . As an example, deposition techniques such as physical vapor deposition and sol-gel deposition may be used to deposit an inorganic dielectric layer of a hard material (e.g., a glass coating formed of silicon nitride, silicon oxynitride, zirconia, alumina, and/or other hard dielectric coating deposited by physical vapor deposition or a glass coating formed of an inorganic dielectric based on silicon oxide deposited by sol-gel deposition techniques). The thickness of this coating may be sufficient to allow the coating to enhance durability (e.g., to help prevent scratches in layer  2 . 1 - 32 ). As an example, layer  2 . 1 - 30  may have a thickness of at least 20 microns, at least 25 microns, at least 35 microns, and/or other suitable thickness). If desired, a liquid polymer (e.g., liquid acrylic) may be deposited and cured to form an acrylic-based hard coat (e.g., layer  2 . 1 - 30  may be a polymer hard coat that is harder than layer  2 . 1 - 32  and that therefore helps resist scratching). 
     2.2: Systems with Displays and Sensors 
       FIGS.  2 . 2 - 1    is a side view of an illustrative head-mounted electronic device. As shown in  FIGS.  2 . 2 - 1   , head-mounted device  2 . 2 - 10  may include head-mounted support structure  2 . 2 - 26 . Support structure  2 . 2 - 26  may have walls or other structures that separate an interior region of device  2 . 2 - 10  such as interior region  2 . 2 - 42  from an exterior region surrounding device  2 . 2 - 10  such as exterior region  2 . 2 - 44 . Electrical components  2 . 2 - 40  (e.g., integrated circuits, sensors, control circuitry, light-emitting diodes, lasers, and other light-emitting devices, other control circuits and input-output devices, etc.) may be mounted on printed circuits and/or other structures within device  2 . 2 - 10  (e.g., in interior region  2 . 2 - 42 ). 
     To present a user with images for viewing from eye boxes such as eye box  2 . 2 - 34 , device  2 . 2 - 10  may include rear-facing displays such as display  2 . 2 - 14 R and lenses such as lens  2 . 2 - 38 . These components may be mounted in optical modules such as optical module  2 . 2 - 36  (e.g., a lens barrel) to form respective left and right optical systems. There may be, for example, a left rear-facing display for presenting an image through a left lens to a user&#39;s left eye in a left eye box and a right rear-facing display for presenting an image to a user&#39;s right eye in a right eye box. The user&#39;s eyes are located in eye boxes  34  at rear side R of device  2 . 2 - 10  when structure  2 . 2 - 26  rests against the outer surface (face surface  2 . 2 - 30 ) of the user&#39;s face. 
     Support structure  2 . 2 - 26  may include a main support structure such as main housing portion  2 . 2 - 26 M (sometimes referred to as a main portion). Main housing portion  2 . 2 - 26 M may extend from front side F of device  2 . 2 - 10  to opposing rear side R of device  2 . 2 - 10 . On rear side R, main housing portion  2 . 2 - 26 M may have cushioned structures to enhance user comfort as portion  2 . 2 - 26 M rests against face surface  2 . 2 - 30 . If desired, support structure  2 . 2 - 26  may include optional head straps such as strap  2 . 2 - 26 B and/or other structures that allow device  2 . 2 - 10  to be worn on a head of a user. 
     Device  2 . 2 - 10  may have a publicly viewable front-facing display such as display  2 . 2 - 14 F that is mounted on front side F of main housing portion  2 . 2 - 26 M. Display  2 . 2 - 14 F may be viewable to the user when the user is not wearing device  2 . 2 - 10  and/or may be viewable by others in the vicinity of device  2 . 2 - 10 . Display  2 . 2 - 14 F may, as an example, be visible on front side F of device  2 . 2 - 10  by an external viewer such as viewer  2 . 2 - 50  who is viewing device  2 . 2 - 10  in direction  2 . 2 - 52 . 
     A schematic diagram of an illustrative system that may include a head-mounted device is shown in  FIGS.  2 . 2 - 2   . As shown in  FIGS.  2 . 2 - 2   , system  2 . 2 - 8  may have one or more electronic devices  2 . 2 - 10 . Devices  2 . 2 - 10  may include a head-mounted device (e.g., device  2 . 2 - 10  of  FIGS.  2 . 2 - 1   ), accessories such as controllers and headphones, computing equipment (e.g., a cellular telephone, tablet computer, laptop computer, desktop computer, and/or remote computing equipment that supplies content to a head-mounted device), and/or other devices that communicate with each other. 
     Each electronic device  2 . 2 - 10  may have control circuitry  2 . 2 - 12 . Control circuitry  2 . 2 - 12  may include storage and processing circuitry for controlling the operation of device  2 . 2 - 10 . Circuitry  2 . 2 - 12  may include storage such as hard disk drive storage, nonvolatile memory (e.g., electrically-programmable-read-only memory configured to form a solid-state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Processing circuitry in control circuitry  2 . 2 - 12  may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors, power management units, audio chips, graphics processing units, application specific integrated circuits, and other integrated circuits. Software code may be stored on storage in circuitry  2 . 2 - 12  and run on processing circuitry in circuitry  2 . 2 - 12  to implement control operations for device  2 . 2 - 10  (e.g., data gathering operations, operations involving the adjustment of the components of device  2 . 2 - 10  using control signals, etc.). Control circuitry  2 . 2 - 12  may include wired and wireless communications circuitry. For example, control circuitry  2 . 2 - 12  may include radio-frequency transceiver circuitry such as cellular telephone transceiver circuitry, wireless local area network transceiver circuitry (e.g., WiFi® circuitry), millimeter wave transceiver circuitry, and/or other wireless communications circuitry. 
     During operation, the communications circuitry of the devices in system  2 . 2 - 8  (e.g., the communications circuitry of control circuitry  2 . 2 - 12  of device  2 . 2 - 10 ) may be used to support communication between the electronic devices. For example, one electronic device may transmit video data, audio data, control signals, and/or other data to another electronic device in system  2 . 2 - 8 . Electronic devices in system  2 . 2 - 8  may use wired and/or wireless communications circuitry to communicate through one or more communications networks (e.g., the internet, local area networks, etc.). The communications circuitry may be used to allow data to be received by device  2 . 2 - 10  from external equipment (e.g., a tethered computer, a portable device such as a handheld device or laptop computer, online computing equipment such as a remote server or other remote computing equipment, or other electrical equipment) and/or to provide data to external equipment. 
     Each device  2 . 2 - 10  in system  2 . 2 - 8  may include input-output devices  2 . 2 - 22 . Input-output devices  2 . 2 - 22  may be used to allow a user to provide device  2 . 2 - 10  with user input. Input-output devices  2 . 2 - 22  may also be used to gather information on the environment in which device  2 . 2 - 10  is operating. Output components in devices  2 . 2 - 22  may allow device  2 . 2 - 10  to provide a user with output and may be used to communicate with external electrical equipment. 
     As shown in  FIGS.  2 . 2 - 2   , input-output devices  2 . 2 - 22  may include one or more displays such as displays  2 . 2 - 14 . Displays  2 . 2 - 14  may include rear facing displays such as display  2 . 2 - 14 R of  FIGS.  2 . 2 - 1   . Device  2 . 2 - 10  may, for example, include left and right components such as left and right scanning mirror display devices or other image projectors, liquid-crystal-on-silicon display devices, digital mirror devices, or other reflective display devices, left and right display panels based on light-emitting diode pixel arrays (e.g., organic light-emitting displays with polymer or semiconductor substrates or display devices based on pixel arrays formed from crystalline semiconductor light-emitting diode dies), liquid crystal display panels, and/or or other left and right display devices that provide images to left and right eye boxes for viewing by the user&#39;s left and right eyes, respectively. Display components such as these (e.g., an organic light-emitting display with a flexible polymer substrate or a display based on a pixel array formed from crystalline semiconductor light-emitting diode dies on a flexible substrate) may also be used in forming a forward-facing display for device  2 . 2 - 10  such as forward-facing display  2 . 2 - 14 F of  FIGS.  2 . 2 - 1    (sometimes referred to as a front-facing display, front display, or publicly viewable display). 
     During operation, displays  2 . 2 - 14  (e.g., displays  2 . 2 - 14 R and/or  2 . 2 - 14 F) may be used to display visual content for a user of device  2 . 2 - 10  (e.g., still and/or moving images including pictures and pass-through video from camera sensors, text, graphics, movies, games, and/or other visual content). The content that is presented on displays  2 . 2 - 14  may, for example, include virtual objects and other content that is provided to displays  2 . 2 - 14  by control circuitry  2 . 2 - 12 . This virtual content may sometimes be referred to as computer-generated content. Computer-generated content may be displayed in the absence of real-world content or may be combined with real-world content. In some configurations, a real-world image may be captured by a camera (e.g., a forward-facing camera, sometimes referred to as a front-facing camera) and computer-generated content may be electronically overlaid on portions of the real-world image (e.g., when device  2 . 2 - 10  is a pair of virtual reality goggles). 
     Input-output circuitry  2 . 2 - 22  may include sensors  2 . 2 - 16 . Sensors  2 . 2 - 16  may include, for example, three-dimensional sensors (e.g., three-dimensional image sensors such as structured light sensors that emit beams of light and that use two-dimensional digital image sensors to gather image data for three-dimensional images from dots or other light spots that are produced when a target is illuminated by the beams of light, binocular three-dimensional image sensors that gather three-dimensional images using two or more cameras in a binocular imaging arrangement, three-dimensional LIDAR (light detection and ranging) sensors, sometimes referred to as time-of-flight cameras or three-dimensional time-of-flight cameras, three-dimensional radio-frequency sensors, or other sensors that gather three-dimensional image data), cameras (e.g., two-dimensional infrared and/or visible digital image sensors), gaze tracking sensors (e.g., a gaze tracking system based on an image sensor and, if desired, a light source that emits one or more beams of light that are tracked using the image sensor after reflecting from a user&#39;s eyes), touch sensors, capacitive proximity sensors, light-based (optical) proximity sensors, other proximity sensors, force sensors (e.g., strain gauges, capacitive force sensors, resistive force sensors, etc.), sensors such as contact sensors based on switches, gas sensors, pressure sensors, moisture sensors, magnetic sensors, audio sensors (microphones), ambient light sensors, flicker sensors that gather temporal information on ambient lighting conditions such as the presence of a time-varying ambient light intensity associated with artificial lighting, microphones for gathering voice commands and other audio input, sensors that are configured to gather information on motion, position, and/or orientation (e.g., accelerometers, gyroscopes, compasses, and/or inertial measurement units that include all of these sensors or a subset of one or two of these sensors), and/or other sensors. 
     User input and other information may be gathered using sensors and other input devices in input-output devices  2 . 2 - 22 . If desired, input-output devices  2 . 2 - 22  may include other devices  2 . 2 - 24  such as haptic output devices (e.g., vibrating components), light-emitting diodes, lasers, and other light sources (e.g., light-emitting devices that emit light that illuminates the environment surrounding device  2 . 2 - 10  when ambient light levels are low), speakers such as ear speakers for producing audio output, circuits for receiving wireless power, circuits for transmitting power wirelessly to other devices, batteries and other energy storage devices (e.g., capacitors), joysticks, buttons, and/or other components. 
     As described in connection with  FIGS.  2 . 2 - 1   , electronic device  2 . 2 - 10  may have head-mounted support structures such as head-mounted support structure  2 . 2 - 26  (e.g., head-mounted housing structures such as housing walls, straps, etc.). The head-mounted support structure may be configured to be worn on a head of a user (e.g., against the user&#39;s face covering the user&#39;s eyes) during operation of device  2 . 2 - 10  and may support displays  2 . 2 - 14 , sensors  2 . 2 - 16 , other components  2 . 2 - 24 , other input-output devices  2 . 2 - 22 , and control circuitry  2 . 2 - 12  (see, e.g., components  2 . 2 - 40  and optical module  2 . 2 - 36  of  FIGS.  2 . 2 - 1   ). 
       FIGS.  2 . 2 - 3    is a front view of device  2 . 2 - 10  in an illustrative configuration in which device  2 . 2 - 10  has a publicly viewable display such as forward-facing display  2 . 2 - 14 F. As shown in  FIGS.  2 . 2 - 3   , support structure  2 . 2 - 26 M of device  2 . 2 - 10  may have right and left portions such as portions  2 . 2 - 26 R and  2 . 2 - 26 L that are coupled by an interposed nose bridge portion such as portion  2 . 2 - 26 NB. Portion  2 . 2 - 26 NB may have a curved exterior surface such as nose bridge surface  2 . 2 - 90  that is configured to receive and rest upon a user&#39;s nose to help support main housing portion  2 . 2 - 26 M on the head of the user. 
     Display  2 . 2 - 14 F may have an active area such as active area AA that is configured to display images and an inactive area IA that does not display images. The outline of active area AA may be rectangular, rectangular with rounded corners, may have teardrop shaped portions on the left and right sides of device  2 . 2 - 10 , may have a shape with straight edges, a shape with curved edges, a shape with a peripheral edge that has both straight and curved portions, and/or other suitable outlines. As shown in  FIGS.  2 . 2 - 3   , active area AA may have a curved recessed portion at nose bridge portion  2 . 2 - 26 NB of main housing portion  2 . 2 - 26 . The presence of the nose-shaped recess in active area AA may help fit active area AA within the available space of housing portion  2 . 2 - 26 M without overly limiting the size of active area AA. 
     Active area AA contains an array of pixels. The pixels may be, for example, light-emitting diode pixels formed from thin-film organic light-emitting diodes or crystalline semiconductor light-emitting diode dies (sometimes referred to as micro-light-emitting diodes) on a flexible display panel substrate. Configurations in which display  2 . 2 - 14 F uses other display technologies may also be used, if desired. Illustrative arrangements in which display  2 . 2 - 14  is formed from a light-emitting diode display such as an organic light-emitting diode display that is formed on a flexible substrate (e.g., a substrate formed from a bendable layer of polyimide or a sheet of other flexible polymer) may sometimes be described herein as an example. The pixels of active area AA may be formed on a display device such as display panel  2 . 2 - 14 P of  FIGS.  2 . 2 - 3    (e.g., a flexible organic light-emitting diode display panel). In some configurations, the outline of panel  2 . 2 - 14 P may have a peripheral edge that contains straight segments or a combination of straight and curved segments. Configurations in which the entire outline of panel  2 . 2 - 14 P is characterized by a curved peripheral edge may also be used. 
     Display  2 . 2 - 14 F may have an inactive area such as inactive area IA that is free of pixels and that does not display images. Inactive area IA may form an inactive border region that runs along one more portions of the peripheral edge of active area AA. In the illustrative configuration of  FIGS.  2 . 2 - 3   , inactive area IA has a ring shape that surrounds active area AA. In this type of arrangement, the width of inactive area IA may be relatively constant and the inner and outer edges of area IA may be characterized by straight and/or curved segments or may be curved along their entire lengths. For example, the outer edge of area IA (e.g., the periphery of display  2 . 2 - 14 F) may have a curved outline that runs parallel to the curved edge of active area AA. 
     In some configurations, device  2 . 2 - 10  may operate with other devices in system  2 . 2 - 8  (e.g., wireless controllers and other accessories). These accessories may have magnetic sensors that sense the direction and intensity of magnetic fields. Device  2 . 2 - 10  may have one or more electromagnets configured to emit a magnetic field. The magnetic field can be measured by the wireless accessories near device  2 . 2 - 10 , so that the accessories can determine their orientation and position relative to device  2 . 2 - 10 . This allows the accessories to wirelessly provide device  2 . 2 - 10  with real-time information on their current position, orientation, and movement so that the accessories can serve as wireless controllers. The accessories may include wearable devices, handled devices, and other input devices. 
     In an illustrative configuration, device  2 . 2 - 10  may have a coil such as illustrative coil  2 . 2 - 54  that runs around the perimeter of display  2 . 2 - 14 F (e.g., under inactive area IA or other portion of display  2 . 2 - 14 F). Coil  2 . 2 - 54  may have any suitable number of turns (e.g., 1-10, at least 2, at least 5, at least 10, 10-50, fewer than 100, fewer than 25, fewer than 6, etc.). These turns may be formed from metal traces on a substrate, may be formed from wire, and/or may be formed from other conductive lines. During operation, control circuitry  2 . 2 - 12  may supply coil  2 . 2 - 54  with an alternating-current (AC) drive signal. The drive signal may have a frequency of at least 1 kHz, at least 10 kHz, at least 100 kHz, at least 1 MHz, less than 10 MHz, less than 3 MHz, less than 300 kHz, or less than 30 kHz (as examples). As AC current flows through coil  2 . 2 - 54 , a corresponding magnetic field is produced in the vicinity of device  2 . 2 - 10 . Electronic devices such as wireless controllers with magnetic sensors that are in the vicinity of device  2 . 2 - 10  may use the magnetic field as a reference so that the wireless controllers can determine their orientation, position, and/or movement while being moved relative to device  2 . 2 - 10  to provide device  2 . 2 - 10  with input. 
     Consider, as an example, a handheld wireless controller that is used in controlling the operation of device  2 . 2 - 10 . During operation, device  2 . 2 - 10  uses coil  2 . 2 - 54  to emit a magnetic field. As the handheld wireless controller is moved, the magnetic sensors of the controller can monitor the location of the controller and the movement of the controller relative to device  2 . 2 - 10  by monitoring the strength, orientation, and change to the strength and/or orientation of the magnetic field emitted by coil  2 . 2 - 54  as the controller is moved through the air by the user. The electronic device can then wirelessly transmit information on the location and orientation of the controller to device  2 . 2 - 10 . In this way, a handheld controller, wearable controller, or other external accessory can be manipulated by a user to provide device  2 . 2 - 10  with air gestures, pointing input, steering input, and/or other user input. 
     Device  2 . 2 - 10  may have components such as optical components (e.g., optical sensors among sensors  2 . 2 - 16  of  FIGS.  2 . 2 - 2   ). These components may be mounted in any suitable location on head-mounted support structure  2 . 2 - 26  (e.g. on head strap  2 . 2 - 26 B, on main housing portion  2 . 2 - 26 M, etc.). Optical components and other components may face rearwardly (e.g., when mounted on the rear face of device  2 . 2 - 10 ), may face to the side (e.g. to the left or right), may face downwardly or upwardly, may face to the front of device  2 . 2 - 10  (e.g., when mounted on the front face of device  2 . 2 - 10 ), may be mounted so as to point in any combination of these directions (e.g., to the front, to the right, and downward) and/or may be mounted in other suitable orientations. In an illustrative configuration, at least some of the components of device  2 . 2 - 10  are mounted so as to face outwardly to the front (and optionally to the sides and/or up and down). For example, forward-facing cameras for pass-through video may be mounted on the left and right sides of the front of device  2 . 2 - 10  in a configuration in which the cameras diverge slightly along the horizontal dimension so that the fields of view of these cameras overlap somewhat while capturing a wide-angle image of the environment in front of device  2 . 2 - 10 . The captured image may, if desired, include portions of the user&#39;s surroundings that are below, above, and to the sides of the area directly in front of device  2 . 2 - 10 . 
     To help hide components such as optical components from view from the exterior of device  2 . 2 - 10 , it may be desirable to cover some or all of the components with cosmetic covering structures. The covering structures may include transparent portions (e.g., optical component windows) that are characterized by sufficient optical transparency to allow overlapped optical components to operate satisfactorily. For example, an ambient light sensor may be covered with a layer that appears opaque to an external viewer to help hide the ambient light sensor from view, but that allows sufficient ambient light to pass to the ambient light sensor for the ambient light sensor to make a satisfactory ambient light measurement. As another example, an optical component that emits infrared light may be overlapped with a visibly opaque material that is transparent to infrared light. 
     In an illustrative configuration, optical components for device  2 . 2 - 10  may be mounted in inactive area IA of  FIGS.  2 . 2 - 3    and cosmetic covering structures may be formed in a ring shape overlapping the optical components in inactive area IA. Cosmetic covering structures may be formed from ink, polymer structures, structures that include metal, other materials, and/or combinations of these materials. In an illustrative configuration, a cosmetic covering structure may be formed from a ring-shaped member having a footprint that matches the footprint of inactive area IA. If, for example, active area AA has left and right portions with teardrop shapes, the ring-shaped member may have curved edges that follow the curved periphery of the teardrop-shaped portions of active area AA. The ring-shaped member may be formed from one or more polymer structures (e.g., the ring-shaped member may be formed from a polymer ring). Because the ring-shaped member can help hide overlapped components from view, the ring-shaped member may sometimes be referred to as a shroud or ring-shaped shroud member. The outward appearance of the shroud or other cosmetic covering structures may be characterized by a neutral color (white, black, or gray) or a non-neutral color (e.g., blue, red, green, gold, rose gold, etc.). 
     Display  2 . 2 - 14 F may, if desired, have a protective display cover layer. The cover layer may overlap active area AA and inactive area IA (e.g., the entire front surface of device  2 . 2 - 10  as viewed from direction  2 . 2 - 52  of  FIGS.  2 . 2 - 1    may be covered by the cover layer). The cover layer, which may sometimes be referred to as a housing wall or transparent housing wall, may have a rectangular outline, an outline with teardrop portions, an oval outline, or other shape with curved and/or straight edges. 
     The cover layer may be formed from a transparent material such as glass, polymer, transparent crystalline material such as sapphire, clear ceramic, other transparent materials, and/or combinations of these materials. As an example, a protective display cover layer for display  2 . 2 - 14 F may be formed from safety glass (e.g., laminated glass that includes a clear glass layer with a laminated polymer film). Optional coating layers may be applied to the surfaces of the display cover layer. If desired, the display cover layer may be chemically strengthened (e.g., using an ion-exchange process to create an outer layer of material under compressive stress that resists scratching). In some configurations, the display cover layer may be formed from a stack of two or more layers of material (e.g., first and second structural glass layers, a rigid polymer layer coupled to a glass layer or another rigid polymer layer, etc.) to enhance the performance of the cover layer. 
     In active area AA, the display cover layer may overlap the pixels of display panel  2 . 2 - 14 P. The display cover layer in active area AA is preferably transparent to allow viewing of images presented on display panel  2 . 2 - 14 P. In inactive area IA, the display cover layer may overlap the ring-shaped shroud or other cosmetic covering structure. The shroud and/or other covering structures (e.g., opaque ink coatings on the inner surface of the display cover layer and/or structures) may be sufficiently opaque to help hide some or all of the optical components in inactive area IA from view. Windows may be provided in the shroud or other cosmetic covering structures to help ensure that the optical components that are overlapped by these structures operate satisfactorily. Windows may be formed from holes, may be formed from areas of the shroud or other cosmetic covering structures that have been locally thinned to enhance light transmission, may be formed from window members with desired light transmission properties that have been inserted into mating openings in the shroud, and/or may be formed from other shroud window structures. 
     In the example of  FIGS.  2 . 2 - 3   , device  2 . 2 - 10  includes optical components such as optical components  2 . 2 - 60 ,  2 . 2 - 62 ,  2 . 2 - 64 ,  2 . 2 - 66 ,  2 . 2 - 68 ,  2 . 2 - 70 ,  2 . 2 - 72 ,  2 . 2 - 74 ,  2 . 2 - 76 ,  2 . 2 - 78 , and  2 . 2 - 80  (as an example). Each of these optical components (e.g., optical sensors selected from among sensors  2 . 2 - 16  of  FIGS.  2 . 2 - 2   , light-emitting devices, etc.) may be configured to detect light and, if desired to emit light (e.g., ultraviolet light, visible light, and/or infrared light). 
     In an illustrative configuration, optical component  2 . 2 - 60  may sense ambient light (e.g., visible ambient light). In particular, optical component  2 . 2 - 60  may have a photodetector that senses variations in ambient light intensity as a function of time. If, as an example, a user is operating in an environment with an artificial light source, the light source may emit light at a frequency associated with its source of wall power (e.g., alternating-current mains power at 60 Hz). The photodetector of component  2 . 2 - 60  may sense that the artificial light from the artificial light source is characterized by 60 Hz fluctuations in intensity. Control circuitry  2 . 2 - 12  can use this information to adjust a clock or other timing signal associated with the operation of image sensors in device  2 . 2 - 10  to help avoid undesired interference between the light source frequency and the frame rate or other frequency associated with image capture operations. Control circuitry  2 . 2 - 12  can also use measurements from component  2 . 2 - 60  to help identify the presence of artificial lighting and the type of artificial lighting that is present. In this way, control circuitry  2 . 2 - 12  can detect the presence of lights such as fluorescent lights or other lights with known non-ideal color characteristics and can make compensating color cast adjustments (e.g., white point adjustments) to color-sensitive components such as cameras and displays. Because optical component  2 . 2 - 60  may measure fluctuations in light intensity, component  2 . 2 - 60  may sometimes be referred to as a flicker sensor or ambient light frequency sensor. 
     Optical component  2 . 2 - 62  may be an ambient light sensor. The ambient light sensor may include one or more photodetectors. In a single-photodetector configuration, the ambient light sensor may be a monochrome sensor that measures ambient light intensity. In a multi-photodetector configuration, each photodetector may be overlapped by an optical filter that passes a different band of wavelengths (e.g. different visible and/or infrared passbands). The optical filter passbands may overlap at their edges. This allows component  2 . 2 - 62  to serve as a color ambient light sensor that measures both ambient light intensity and ambient light color (e.g., by measuring color coordinates for the ambient light). During operation of device  2 . 2 - 10 , control circuitry  2 . 2 - 12  can take action based on measured ambient light intensity and color. As an example, the white point of a display or image sensor may be adjusted or other display or image sensor color adjustments may be made based on measured ambient light color. The intensity of a display may be adjusted based on light intensity. For example, the brightness of display  2 . 2 - 14 F may be increased in bright ambient lighting conditions to enhance the visibility of the image on the display and the brightness of display  2 . 2 - 14 F may be decreased in dim lighting conditions to conserve power. Image sensor operations and/or light source operations may also be adjusted based on ambient light readings. 
     The optical components in active area IA may also include components along the sides of device  2 . 2 - 10  such as components  2 . 2 - 80  and  2 . 2 - 64 . Optical components  2 . 2 - 80  and  2 . 2 - 64  may be pose-tracking cameras that are used to help monitor the orientation and movement of device  2 . 2 - 10 . Components  2 . 2 - 80  and  2 . 2 - 64  may be visible light cameras (and/or cameras that are sensitive at visible and infrared wavelengths) and may, in conjunction with an inertial measurement unit, form a visual inertial odometry (VIO) system. 
     Optical components  2 . 2 - 78  and  2 . 2 - 66  may be visible-light cameras that capture real-time images of the environment surrounding device  2 . 2 - 10 . These cameras, which may sometimes be referred to as scene cameras or pass-through-video cameras, may capture moving images that are displayed in real time to displays  2 . 2 - 14 R for viewing by the user when the user&#39;s eyes are located in eye boxes  2 . 2 - 34  at the rear of device  2 . 2 - 10 . By displaying pass-through images (pass-through video) to the user in this way, the user may be provided with real-time information on the user&#39;s surroundings. If desired, virtual content (e.g. computer-generated images) may be overlaid over some of the pass-through video. Device  2 . 2 - 10  may also operate in a non-pass-through-video mode in which components  2 . 2 - 78  and  2 . 2 - 66  are turned off and the user is provided only with movie content, game content, and/or other virtual content that does not contain real-time real-world images. 
     Input-output devices  2 . 2 - 22  of device  2 . 2 - 10  may gather user input that is used in controlling the operation of device  2 . 2 - 10 . As an example, a microphone in device  2 . 2 - 10  may gather voice commands. Buttons, touch sensors, force sensors, and other input devices may gather user input from a user&#39;s finger or other external object that is contacting device  2 . 2 - 10 . In some configurations, it may be desirable to monitor a user&#39;s hand gestures or the motion of other user body parts. This allows the user&#39;s hand locations or other body part locations to be replicated in a game or other virtual environment and allows the user&#39;s hand motions to serve as hand gestures (air gestures) that control the operation of device  2 . 2 - 10 . User input such as hand gesture input can be captured using cameras that operate at visible and infrared wavelengths such as tracking cameras (e.g., optical components  2 . 2 - 76  and  2 . 2 - 68 ). Tracking cameras such as these may also track fiducials and other recognizable features on controllers and other external accessories (additional devices  2 . 2 - 10  of system  2 . 2 - 8 ) during use of these controllers in controlling the operation of device  2 . 2 - 10 . If desired, tracking cameras can help determine the position and orientation of a handheld controller or wearable controller that senses its location and orientation by measuring the magnetic field produced by coil  2 . 2 - 54 . The use of tracking cameras may therefore help track hand motions and controller motions that are used in moving pointers and other virtual objects being displayed for a user and can otherwise assist in controlling the operation of device  2 . 2 - 10 . 
     Tracking cameras may operate satisfactorily in the presence of sufficient ambient light (e.g., bright visible ambient lighting conditions). In dim environments, supplemental illumination may be provided by supplemental light sources such as supplemental infrared light sources (e.g., optical components  2 . 2 - 82  and  2 . 2 - 84 ). The infrared light sources may each include one or more light-emitting devices (light-emitting diodes or lasers) and may each be configured to provide fixed and/or steerable beams of infrared light that serve as supplemental illumination for the tracking cameras. If desired, the infrared light sources may be turned off in bright ambient lighting conditions and may be turned on in response to detection of dim ambient lighting (e.g., using the ambient light sensing capabilities of optical component  2 . 2 - 62 ). 
     Three-dimensional sensors in device  2 . 2 - 10  may be used to perform biometric identification operations (e.g., facial identification for authentication), may be used to determine the three-dimensional shapes of objects in the user&#39;s environment (e.g., to map the user&#39;s environment so that a matching virtual environment can be created for the user), and/or to otherwise gather three-dimensional content during operation of device  2 . 2 - 10 . As an example, optical components  2 . 2 - 74  and  2 . 2 - 70  may be three-dimensional structured light image sensors. Each three-dimensional structured light image sensor may have one or more light sources that provide structured light (e.g., a dot projector that projects an array of infrared dots onto the environment, a structured light source that produces a grid of lines, or other structured light component that emits structured light). Each of the three-dimensional structured light image sensors may also include a flood illuminator (e.g., a light-emitting diode or laser that emits a wide beam of infrared light). Using flood illumination and structured light illumination, optical components  2 . 2 - 74  and  2 . 2 - 70  may capture facial images, images of objects in the environment surrounding device  2 . 2 - 10 , etc. 
     Optical component  2 . 2 - 72  may be an infrared three-dimensional time-of-flight camera that uses time-of-flight measurements on emitted light to gather three-dimensional images of objects in the environment surrounding device  2 . 2 - 10 . Component  2 . 2 - 72  may have a longer range and a narrower field of view than the three-dimensional structured light cameras of optical components  2 . 2 - 74  and  2 . 2 - 70 . The operating range of component  2 . 2 - 72  may be 30 cm to 7 m, 60 cm to 6 m, 70 cm to 5 m, or other suitable operating range (as examples). 
       FIGS.  2 . 2 - 4    is a top view of device  2 . 2 - 10  in an illustrative arrangement in which display  2 . 2 - 14 F and main housing portion  2 . 2 - 26 M have been configured to curve about the curved surface of a user&#39;s face (curved face surface  2 . 2 - 30 ). In particular, rear surface  2 . 2 - 96  of housing portion  2 . 2 - 26 M on rear side R of device  2 . 2 - 10  may have a curved shape that is bent about axis  2 . 2 - 98  (e.g., an axis parallel to the vertical Z axis in the example of  FIGS.  2 . 2 - 4   ). By wrapping housing portion  2 . 2 - 26 M smoothly about the curved surface of the user&#39;s head, comfort may be enhanced when wearing device  2 . 2 - 10 . 
     As shown in  FIGS.  2 . 2 - 4   , display  2 . 2 - 14 F and other structures on the front of device  2 . 2 - 10  may have a protective cover layer such as display cover layer  2 . 2 - 92  (e.g., a front portion of housing portion  2 . 2 - 26 M, which may sometimes be referred to as a front housing wall, transparent dielectric housing wall, or dielectric housing member). In some embodiments, display cover layer  2 . 2 - 92  may include areas that are characterized by curved surfaces that can be flattened into a plane without distortion (sometimes referred to as developable surfaces or curved surfaces without compound curvature). Display cover layer  2 . 2 - 92  may also include areas that are characterized by compound curvature (e.g., surfaces that can only be flattened into a plane with distortion, sometimes referred to as non-developable surfaces). 
     In active area AA of display  2 . 2 - 14 F, cover layer  2 . 2 - 92  overlaps an array of pixels P in display panel  2 . 2 - 14 P. In inactive area IA, cover layer  2 . 2 - 92  does not overlap any pixels, but may overlap optical components such as the optical components shown in  FIGS.  2 . 2 - 3   . To help reduce the size and weight of device  2 . 2 - 10 , display  2 . 2 - 14 F may have a curved shape that wraps around the front of the user&#39;s head parallel to face surface  2 . 2 - 30  and parallel to curved rear surface  2 . 2 - 96  of housing portion  2 . 2 - 26 M. For example, display panel  2 . 2 - 14 P may have a flexible substrate that allows panel  2 . 2 - 14 P to bend about bend axis  2 . 2 - 94  (e.g., a bend axis that is parallel to the Z axis in the example of  FIGS.  2 . 2 - 4   ). In active area AA of display  2 . 2 - 14 F, display cover layer  2 . 2 - 92  may have an inner surface with a curved cross-sectional profile that conforms to bent display panel  2 . 2 - 14 P and a corresponding curved outer surface. In inactive area IA, display cover layer  2 . 2 - 92  may also be curved (e.g., with a tighter bend radius and more curvature than in active area AA). If desired, a polymer layer (sometimes referred to as a shroud canopy or polymer member) may be interposed between display cover layer  2 . 2 - 92  and display panel  2 . 2 - 14 P. The polymer layer may be separated from the pixels of panel  2 . 2 - 14 P by an air gap and may be separated from the inner surface of display cover layer  2 . 2 - 92  by an air gap (as an example). 
       FIGS.  2 . 2 - 5 A  is a cross-sectional side view of display  2 . 2 - 14 F viewed in the X direction. As shown in  FIGS.  2 . 2 - 5 A , the cross-sectional profile of display panel  2 . 2 - 14 P (in planes parallel to the YZ plane) may, in an illustrative configuration, be straight rather than curved. This may help prevent wrinkling or other distortion to the flexible substrate material of display panel  2 . 2 - 14 P as display panel  2 . 2 - 14 P is bent about bend axis  2 . 2 - 94  to wrap around the curved surface of the user&#39;s face. Display panel  2 . 2 - 14 P may, in this example, have a developable surface (e.g., a surface that has a curved cross-sectional profile but that does not have any compound curvature). Panel  2 . 2 - 14 P of  FIGS.  2 . 2 - 5 A  may be attached to the inner surface of layer  2 . 2 - 92  (e.g., with adhesive). In this scenario, the inner surface of layer  2 . 2 - 92  may be a developable surface that mates with the outwardly facing developable surface of panel  2 . 2 - 14 P. The corresponding outer surface of layer  2 . 2 - 92  in active area AA may be a developable surface or may be a surface of compound curvature. In inactive area IA, layer  2 . 2 - 92  may have inner and/or outer surfaces of compound curvature and/or the inner and/or outer surfaces may be developable surfaces. If desired, the entire outer surface of layer  2 . 2 - 92  may have compound curvature (both in active area AA and in inactive area IA), the inner surface of layer  2 . 2 - 92  in active area AA may be a developable surface to which panel  2 . 2 - 14 P is laminated with adhesive, and the inner surface of layer  2 . 2 - 92  in inactive area IA may have compound curvature and/or may be a developable surface. 
     Another illustrative configuration for display  2 . 2 - 14 F is shown in  FIGS.  2 . 2 - 5 B . As shown in the cross-sectional side view of  FIGS.  2 . 2 - 5 B , display cover layer  2 . 2 - 92  may, if desired, have a cross-sectional profile that is curved across all of layer  2 . 2 - 92 . With this type of arrangement, the surface of inactive area IA of display cover layer  2 . 2 - 92  may have compound curvature and active area AA of display cover layer  2 . 2 - 92  may have compound curvature (e.g., layer  2 . 2 - 92  may be free of any areas with developable surfaces). A polymer layer such as polymer layer  2 . 2 - 130 , which may sometimes be referred to as a shroud or shroud canopy, may be interposed between the inner surface of display cover layer  2 . 2 - 92  and the opposing outer surface of display panel  2 . 2 - 14 P. The outer surface of display panel  2 . 2 - 14 P may be a developable surface (e.g., display panel  2 . 2 - 14 P may be bent about axis  2 . 2 - 94 ). In active area AA, where polymer layer  2 . 2 - 130  overlaps the pixels of panel  2 . 2 - 14 P, polymer layer  2 . 2 - 130  may also be bent about axis  2 . 2 - 94  (e.g., the inner and outer surfaces of polymer layer  2 . 2 - 130  in active area AA may be developable surfaces). In inactive area IA, the inner and outer surfaces of polymer layer  2 . 2 - 130  may have compound curvature. Air gaps may separate panel  2 . 2 - 14 P from the inner surface of layer  2 . 2 - 130  and may separate the outer surface of layer  2 . 2 - 130  from the inner surface of layer  2 . 2 - 92 . 
     If desired, other arrangements for layer  2 . 2 - 130  may be used. For example, the side of layer  2 . 2 - 130  facing display panel  2 . 2 - 14 P may have a developable surface in active area AA, whereas the side of layer  2 . 2 - 130  facing layer  2 . 2 - 92  may have compound curvature in active area AA (e.g., layer  2 . 2 - 130  may have a non-uniform thickness). Layer  2 . 2 - 92  may also have different configurations. For example, the outer surface of layer  2 . 2 - 92  may have compound curvature, whereas the inner surface of layer  2 . 2 - 92  in active area AA and/or in area IA may be a developable surface. Other arrangements in which layer  2 . 2 - 92  and/or layer  2 . 2 - 130  have variable thicknesses may also be used. In inactive area IA, multiple polymer structures may be joined. For example, in area IA, a ring-shaped polymer member, sometimes referred to as a shroud trim, may be joined to layer  2 . 2 - 130 , which may form a shroud canopy member that extends across the entire front face of device  2 . 2 - 10 . The shroud trim and shroud canopy may, if desired, sometimes be referred to individually or collectively as forming a shroud, shroud member(s), etc. Tinting (e.g., dye, pigment, and/or other colorant) may be included in layer  2 . 2 - 130 . For example, layer  2 . 2 - 130  may be tinted to exhibit a visible light transmission of 30-80% to help obscure internal structures in device  2 . 2 - 10  such as display panel  2 . 2 - 14 P from view when not in use. 
       FIGS.  2 . 2 - 6    is a front view of a portion of display  2 . 2 - 14 F and display cover layer  2 . 2 - 92 . The inner and outer surfaces of display cover layer  2 . 2 - 92  that directly overlap active area AA and display panel  2 . 2 - 14 P may be developable surfaces and/or may include areas of compound curvature. In an illustrative configuration, the inner surface of cover layer  2 . 2 - 92  in area AA may, as described in connection with  FIGS.  4  and  5 A , bend about bend axis  2 . 2 - 94  without exhibiting curvature about any axis orthogonal to axis  2 . 2 - 94 . The outer surface of layer  2 . 2 - 92  in area AA may be a developable surface or a surface of compound curvature. The use of a developable surface for the inwardly facing side of display cover layer  2 . 2 - 92  (and, if desired, the use of a developable surface for the inwardly facing side of optional layer  2 . 2 - 130  of  FIGS.  2 . 2 - 5 B ) may help ensure that display panel  2 . 2 - 14 P is not wrinkled or otherwise damaged during the bending of panel  2 . 2 - 14 P to form a curved display shape that conforms to the shape of the user&#39;s head. 
     Display panel  2 . 2 - 14 P may have an outwardly facing surface in active area AA that is a developable surface. This display panel surface may be adhered to the corresponding inner developable surface of layer  2 . 2 - 130  or a corresponding inner developable surface of layer  2 . 2 - 92  or may be spaced apart from the layer  2 . 2 - 130  and/or the inner surface of layer  2 . 2 - 92  by an air gap (as examples). 
     Some or all portions of the inner and outer surfaces of display cover layer  2 . 2 - 92  in inactive area IA may, if desired, be characterized by compound curvature. This allows the periphery of display  2 . 2 - 14 F to smoothly transition away from the active area and provides an attractive appearance and compact shape for device  2 . 2 - 10 . The compound curvature of display cover layer  2 . 2 - 92  in inactive area IA may also facilitate placement of the optical components under inactive area IA in desired orientations. If desired, all areas of layer  2 . 2 - 92  may have compound curvature (e.g., the inner and outer surfaces of layer  2 . 2 - 92  may have compound curvature in both area IA and area AA). 
     In the illustrative configuration of  FIGS.  2 . 2 - 6   , in which display cover layer  2 . 2 - 92  has a curved peripheral edge and in which the inwardly facing and outwardly facing surfaces of display cover layer  2 . 2 - 92  have compound curvature in inactive area IA, the cross-sectional profiles of display cover layer  2 . 2 - 92  taken along each of illustrative lines  2 . 2 - 100  of  FIGS.  2 . 2 - 6    are curved (e.g., the entire peripheral ring-shaped inactive area of display  2 . 2 - 14 F in the  FIGS.  2 . 2 - 6    example is covered by a portion of display cover layer  2 . 2 - 92  with inner and outer surfaces of compound curvature). This type of shape for display cover layer  2 . 2 - 92  may be produced by glass forming, polymer molding, machining, and/or other display cover layer fabrication techniques. Other arrangements (e.g., configurations in which display cover layer  2 . 2 - 92  has at least some developable surfaces (inner and/or outer surfaces) in inactive area IA) may also be used. The arrangement of  FIGS.  2 . 2 - 6    is illustrative. 
       FIGS.  2 . 2 - 7 ,  2 . 2 - 8 , and  2 . 2 - 9    are front views of illustrative upper left portions of display cover layer  2 . 2 - 92 . Device  2 . 2 - 10  may have symmetrical right-hand cover layer portions. The example of  FIGS.  2 . 2 - 7    shows how the peripheral edge of display cover layer  2 . 2 - 92  may have straight edges (e.g., a generally rectangular shape with straight edges) and rounded corners. In the example of  FIGS.  2 . 2 - 8   , display cover layer  2 . 2 - 92  has teardrop shapes on the upper left and right sides.  FIGS.  2 . 2 - 9    shows how the upper corners of display cover layer  2 . 2 - 92  may have sweeping curves (e.g., to help soften the visual appearance of device  2 . 2 - 10  when viewed from the front). 
       FIGS.  10 ,  11 , and  12    are front views of illustrative lower left portions of display cover layer  2 . 2 - 92 . As shown in  FIGS.  2 . 2 - 10   , the lower half of cover layer  2 . 2 - 92  may be characterized by a rectangular shape with rounded corners. Cover layer  2 . 2 - 92  of  FIGS.  2 . 2 - 10    may have an upper portion with a shape of the type shown in  FIGS.  2 . 2 - 7    (as an example). In the nose bridge portion of device  2 . 2 - 10 , cover layer  2 . 2 - 92  may have a recessed curved nose-bridge edge shape (see, e.g., curved edge surface  2 . 2 - 90 ). In the illustrative arrangement of  FIGS.  2 . 2 - 11   , display cover layer  2 . 2 - 92  has lower left and right sides with teardrop shapes (e.g., shapes that may be used with a display cover layer having upper left and right teardrop shapes of the type shown in  FIGS.  2 . 2 - 8   ).  FIGS.  2 . 2 - 12    shows how the lower portion of display cover layer  2 . 2 - 92  may have a more gradually curved outline. 
     In general, the upper and lower portions of cover layer  2 . 2 - 92  may have any suitable outlines when viewed from the front of device  2 . 2 - 10 . The shape used for cover layer  2 . 2 - 92  may be determined by factors such as aesthetics, size, the ability to facilitate suitable placement for optical components in inactive area IA, the ability to provide desired active area coverage (overlap over active area AA), etc. Any of the illustrative shapes for the upper portion of device  2 . 2 - 10  shown in  FIGS.  7 ,  8   , and/or  9  may be used in combination with any of the illustrative shapes for the lower portion of device  2 . 2 - 10  shown in  FIGS.  10 ,  11 , and  12   . The overall shape for cover layer  2 . 2 - 92  may be symmetric about the nose bridge (e.g., left and right halves of layer  2 . 2 - 92  may exhibit mirror symmetry). The shapes of  FIGS.  7 ,  8 ,  9 ,  10 ,  11 , and  12    are illustrative. Other shapes may be used, if desired. 
       FIGS.  2 . 2 - 13    is an exploded cross-sectional top view of a portion of device  2 . 2 - 10  showing how display cover layer  2 . 2 - 92  may have a portion overlapping display panel  2 . 2 - 14 P and a portion overlapping a cosmetic covering structure such as shroud  2 . 2 - 130  (e.g., a ring-shaped shroud portion sometimes referred to as a shroud trim or shroud trim member, which may optionally be attached in area IA to a shroud canopy that covers display  2 . 2 - 14 F such as optional polymer layer  2 . 2 - 130 ). Cosmetic covering structures in inactive area IA may be formed from opaque masking layers (e.g., black ink layers) and/or other coatings on the inner surface of display cover layer  2 . 2 - 92  and/or on the shroud, from separate structures formed from metal, polymer, glass, or other materials, and/or other structures that can help hide overlapped components  2 . 2 - 104 . Components  2 . 2 - 104  may include sensors  16  and other input-output devices  2 . 2 - 22  of  FIGS.  2 . 2 - 2   . For example, components  2 . 2 - 104  may be optical components such as components  2 . 2 - 60 ,  2 . 2 - 62 ,  2 . 2 - 64 ,  2 . 2 - 84 ,  2 . 2 - 66 ,  2 . 2 - 68 ,  2 . 2 - 70 ,  2 . 2 - 72 ,  2 . 2 - 74 ,  2 . 2 - 76 ,  2 . 2 - 78 ,  2 . 2 - 82 , and  2 . 2 - 80  of  FIGS.  2 . 2 - 3   . In inactive area IA, cover layer  2 . 2 - 92  may have curved inner and outer surfaces (e.g., surfaces with compound curvature). Shroud  2 . 2 - 102  (and, if desired, layer  2 . 2 - 130  in area IA) may optionally have corresponding inner and outer surfaces (e.g., surfaces with compound curvature). Components  2 . 2 - 104  may operate through optical component windows in shroud  2 . 2 - 102  (and optionally in layer  2 . 2 - 130  in area IA) and corresponding areas in layer  2 . 2 - 92 . These windows may be formed by recesses and/or through-hole openings in shroud  2 . 2 - 102  (and optionally in layer  2 . 2 - 130 ) and/or layer  2 . 2 - 92 , by window members that are installed within openings in shroud  2 . 2 - 102  (and optionally in layer  2 . 2 - 130 ) and/or layer  2 . 2 - 92 , by portions of shroud  2 . 2 - 102  (and optionally portions of layer  2 . 2 - 130 ) and/or layer  2 . 2 - 92  that exhibit optical transparency sufficient for satisfactory operation of overlapped components, and/or by other structures in shroud  2 . 2 - 102  (and optionally in layer  2 . 2 - 130 ) and/or window  2 . 2 - 92 . 
     If desired, components  2 . 2 - 104  may include components such as cameras (e.g., visible and/or infrared image sensors, time-of-flight sensors, structured light three-dimensional sensors, etc.) that are sensitive to optical distortion imposed by the curved shapes of the curved inner and/or outer surface of cover layer  2 . 2 - 92 . For example, a camera or other optical component  104  may operate through a portion of cover layer  2 . 2 - 92  in inactive area IA that is characterized by an outer surface that has compound curvature and an inner surface with compound curvature or a developable inner surface. In this type of situation, the control circuitry of device  2 . 2 - 10  may be configured to digitally compensate for the optical distortion introduced as light (e.g., real-world image light) passes through layer  2 . 2 - 92  to the camera or other optical sensor. As an example, the amount of image distortion imposed by layer  2 . 2 - 92  (e.g., stretching, shifting, keystoning, barrel distortion, pincushion distortion, and/or other optical distortion) may be measured and characterized for each optical component that operates through layer  2 . 2 - 92  (e.g., through a portion of layer  2 . 2 - 92  in inactive area IA that has inner and/or outer surfaces of compound curvature). During operation of device  2 . 2 - 10 , the image data captured by a camera and/or other sensor data that is gathered by an optical component overlapped by layer  2 . 2 - 92  may be compensated accordingly (e.g., an equal and opposite amount of digital image warping may be applied to the captured image data, thereby removing the known distortion effects of layer  2 . 2 - 92 ). In this way, high quality (undistorted) images and/or other sensor data may be gathered by cameras and/or other optical components that operate through curved portions of layer  2 . 2 - 92 . This allows layer  2 . 2 - 92  to be provided with an attractive shape (e.g., a shape with one or more surfaces characterized by compound curvature). 
     When assembled into device  2 . 2 - 10 , display cover layer  2 . 2 - 92  and shroud  2 . 2 - 102  (and optionally layer  2 . 2 - 130 ) may be mounted to an exposed edge portion of a polymer housing structure, a metal housing wall, or other housing structure in main housing portion  2 . 2 - 26 M. As an example, main housing portion  2 . 2 - 26 M may have a polymer sidewall member that runs around the periphery of display cover layer  2 . 2 - 92  and that supports the peripheral edge of display cover layer  2 . 2 - 92 . Shroud  2 . 2 - 102  may have a ring shape that runs along the edge of display cover layer  2 . 2 - 92  in inactive area IA. In an illustrative configuration, adhesive is used to attach display cover layer  2 . 2 - 92  to shroud  2 . 2 - 102  (and/or layer  2 . 2 - 130 ) and adhesive is used to attach shroud  2 . 2 - 102  (and/or layer  2 . 2 - 130 ) to the exposed front edge of the sidewall in main housing portion  2 . 2 - 26 M. Components  2 . 2 - 104  may be attached to shroud  2 . 2 - 102  (and/or layer  2 . 2 - 130 ) and/or may be supported on internal housing structures (e.g., brackets, frame members, etc.) in alignment with optical windows in shroud  2 . 2 - 102  (and/or layer  2 . 2 - 130 ) and corresponding portions of layer  2 . 2 - 92 . 
       FIGS.  2 . 2 - 14    is a cross-sectional side view of a portion of display  2 . 2 - 14 F. In the example of  FIGS.  2 . 2 - 14   , display panel  2 . 2 - 14 P is a three-dimensional display panel having an array of pixels P overlapped by lenticular lenses  2 . 2 - 106  (e.g., display panel  2 . 2 - 14 P is an autostereoscopic display that produces glasses-free three-dimensional images for viewers such as viewer  2 . 2 - 50  of  FIGS.  2 . 2 - 1   ). Lenses  2 . 2 - 106  may, as an example, be formed from semicylindrical lens elements that are elongated along columns of pixels (e.g., lens elements that extend parallel to the Z dimension in the example of  FIGS.  2 . 2 - 14   ). If desired, lenses  2 . 2 - 106  may be omitted (e.g., display panel  2 . 2 - 14 P may have an array of pixels P that are not overlapped by lenses  2 . 2 - 106  to form a two-dimensional display). 
     An air gap such as gap  2 . 2 - 114  may separate display panel  2 . 2 - 14 P of display  2 . 2 - 14 F from display cover layer  2 . 2 - 92 . Optional layer  2 . 2 - 130  may be formed within gap  2 . 2 - 114  of  FIG.  2 . 2 - 14   , so that layer  2 . 2 - 130  has an outer surface that is separated from layer  2 . 2 - 92  by a first air gap and an opposing inner surface that is separated from lenses  2 . 2 - 106  and pixels P of display panel  2 . 2 - 14 P by a second air gap. In arrangements in which lenses  2 . 2 - 106  are present, air gap  2 . 2 - 114  (and the resulting absence of direct contact between the inner surface of layer  2 . 2 - 130  and lenses  2 . 2 - 106 ) may allow lenses  2 . 2 - 106  to operate satisfactorily. Display cover layer  2 . 2 - 92  and optional layer  2 . 2 - 130  may be formed from transparent material such as glass, polymer, clear ceramic, crystalline material such as sapphire, one or more sublayers of these materials and/or other materials that have been laminated together (e.g., using adhesive, etc.), etc. Configurations in which layer  2 . 2 - 92  is a glass layer and layer  2 . 2 - 130  is a polymer layer may sometimes be described herein as an example. 
     Coatings may be provided on one or more of the layers in display cover layer  2 . 2 - 92 . As shown in the illustrative configuration of  FIGS.  2 . 2 - 14   , display cover layer  2 . 2 - 92  may include, for example, a layer such as layer  2 . 2 - 108  that is formed from one or more sublayers (e.g., layer(s) of glass and/or polymer), a polymer layer that helps provide layer  2 . 2 - 92  with safety glass functionality (see, e.g., illustrative polymer film  112 , which has been attached to the inner surface of glass layer  2 . 2 - 108  to form a layer of laminated glass), and coating  2 . 2 - 110  on the front (outwardly facing) surface of layer  2 . 2 - 92  (e.g., the outer surface of glass layer  2 . 2 - 108 ). Coating  2 . 2 - 110  may be, for example, an antireflection coating formed from one or more inorganic dielectric layers and/or other layers with thicknesses and refractive index values selected to minimize visible light reflections from the outermost surface of layer  2 . 2 - 92  and help maintain a desired appearance (e.g., a neutral tint) for layer  2 . 2 - 92 . If desired, display panel  2 . 2 - 14 P may be a touch sensitive display (e.g., a display that is overlapped by or incorporates capacitive touch sensor circuitry). In configurations in which display  2 . 2 - 14 F is touch sensitive, the outermost surface of layer  2 . 2 - 92  may be coated with an oleophobic coating layer (e.g., a fluoropolymer layer). 
     To help strengthen layer  2 . 2 - 92 , layer  2 . 2 - 108  may be formed from chemically strengthened glass (e.g., a glass layer that has been treated in an ion-exchange bath to place the exterior surfaces of the glass layer under compression relative to the interior of the glass layer). This may help layer  2 . 2 - 108  resist scratching and cracks. Layer  2 . 2 - 108  may be formed from a single glass layer, a single polymer layer, a stack of two laminated glass layers (e.g., first and second glass layers laminated together with a layer of polymer), a stack of two polymer layers, three or more polymer and/or glass layers, etc. If desired, layer  2 . 2 - 108  may be formed from a hybrid stack of layers that includes one or more glass layers attached to one or more polymer layers. As an example, layer  2 . 2 - 92  may include a rigid structural polymer layer that is covered with a thin glass layer (e.g., a glass layer attached to the structural polymer layer using heat and/or pressure or a glass layer attached to the structural polymer layer using a layer of polymer adhesive). The thin glass layer in this type of arrangement may help protect the structural polymer layer from scratches. 
     One or more of the structures in layer  2 . 2 - 92  (e.g., coating  2 . 2 - 110 , the layer(s) forming layer  2 . 2 - 108 , layer  2 . 2 - 112 , optional layer  2 . 2 - 130 , etc.) may, if desired, be provided with a dye, pigment, or other colorant that creates a desired neutral tint (e.g., gray or black) or non-neutral tint (e.g., red). Thin metal coatings, polarizers, and/or other structures may also be incorporated into layer  2 . 2 - 92  to help provide layer  2 . 2 - 92  with desired optical properties and/or to provide layer  2 . 2 - 92  with a desired external appearance. 
     If desired, the portion of layer  2 . 2 - 92  that overlaps optical components  2 . 2 - 104  and/or other portions of layer  2 . 2 - 92  may be provided with a coating that helps prevent scratches that could adversely affect optical quality for components  2 . 2 - 104 . As shown in  FIGS.  2 . 2 - 15   , for example, display cover layer  2 . 2 - 92  may have a transparent layer such as transparent layer  2 . 2 - 116  (e.g., one or more layers of polymer, glass, and/or other transparent layers such as layer  2 . 2 - 108  of  FIGS.  2 . 2 - 14   ). Transparent layer  2 . 2 - 116  may be covered with one or more coating layers such as coating layer  2 . 2 - 118 . Layer  2 . 2 - 118  may be a thin-film layer formed from an inorganic material (e.g., an oxide, nitride, diamond-like carbon etc.) that helps resist scratches. This type of approach may be used, for example, to ensure that the portion of display cover layer  2 . 2 - 92  that overlaps optical component  2 . 2 - 104  does not become hazy from scratches when layer  2 . 2 - 116  is formed from a material such as polymer that may be prone to scratching when exposed to excessive rubbing from sharp external objects. Layer  2 . 2 - 118  may sometimes be referred to as a hard coat and may have a higher hardness (e.g., a higher Mohs hardness) than layer  2 . 2 - 116 . Layer  2 . 2 - 118  may be a thin-film coating with a thickness of less than 3 microns, less than 2 microns, less than 1 micron, less than 0.5 microns, or other suitable thickness. 
     Another way in which to help prevent undesired scratches on the surface of display cover layer  2 . 2 - 92  where layer  2 . 2 - 92  overlaps optical components  2 . 2 - 104  is illustrated in the cross-sectional side view of display cover layer  2 . 2 - 92  of  FIGS.  2 . 2 - 16   . As this example demonstrates, the outer surface of display cover layer  2 . 2 - 92  may be provided with a recess such as recess  2 . 2 - 120  (e.g., a shallow circular depression or a depression with a rectangular shape or other footprint). This places recessed display cover layer surface  2 . 2 - 124  of recess  2 . 2 - 120  below surrounding external surfaces  2 . 2 - 122  of layer  2 . 2 - 92 . When device  2 . 2 - 10  is laid on a tabletop or other surface, the unrecessed portion of the surface of layer  2 . 2 - 92  (external surface  122 ) will contact the tabletop surface and will thereby help prevent the tabletop surface from contacting the recessed portion of the surface of layer  2 . 2 - 92  (surface  2 . 2 - 124 ). As a result, recessed surface  2 . 2 - 124 , which overlaps component  104 , will remain free of scratches. Haze will therefore not generally develop in the area of layer  2 . 2 - 92  that overlaps component  104 , even when layer  2 . 2 - 92  is exposed to excessive wear. 
     Layer  2 . 2 - 92  may be formed from materials having optical properties that are compatible with overlapped optical components  2 . 2 - 104 . For example, if an optical component that is overlapped by a portion of layer  2 . 2 - 92  in inactive area IA is configured to operate at visible and infrared wavelengths, that portion of layer  2 . 2 - 92  may be provided with sufficient visible light and infrared light transparency to allow the overlapped component to operate satisfactorily at visible and infrared wavelengths. In arrangements in which the material from the bulk of layer  2 . 2 - 92  does not have desired optical properties for an optical component, an optical component window member (e.g., a disk of window material such as a disk of infrared-transparent and, if desired, visible-transparent glass or other inserted window member) may be mounted within an opening in layer  2 . 2 - 92  overlapping the optical component. 
     Consider, as an example, an arrangement in which layer  2 . 2 - 92  is transparent to visible light but has low transmission at infrared wavelengths. An optical component in this type of arrangement may operate at infrared wavelengths. To ensure that the optical component can transmit and/or receive infrared light through layer  2 . 2 - 92 , layer  2 . 2 - 92  may be provided with a through-hole opening and an infrared-transparent optical component window member such as an infrared-transparent disk. The infrared-transparent window member may be formed from a different material than the material forming layer  2 . 2 - 92  and may be mounted within the through-hole opening in layer  2 . 2 - 92 . This type of arrangement is shown in the cross-sectional side view of  FIGS.  2 . 2 - 17    in which display cover layer  2 . 2 - 92  has been provided with optical component window member  2 . 2 - 92 W in a through-hole opening in layer  2 . 2 - 92 . Member  2 . 2 - 92 W may be a glass optical component window member that is transparent to infrared light (and optionally transparent to visible light), whereas surrounding portions of layer  2 . 2 - 92  may be formed from different material (e.g., polymer, different glass material, etc.). By providing an infrared-transparent window in layer  2 . 2 - 92 , the infrared optical component (e.g., optical component  2 . 2 - 102  of  FIGS.  2 . 2 - 17   ) can transmit and/or received infrared light through display cover layer  2 . 2 - 92  (e.g., through the window in the display cover layer), even when layer  2 . 2 - 92  has been formed from materials that are not infrared-transparent. This approach may be used to provide an optical component window with any suitable optical properties that are different than those of the rest of layer  2 . 2 - 92  (e.g., desired amounts of opacity, light transmission, reflection, absorption, and/or haze level, desired polarization properties, etc.). 
     2.3: Systems with Supplemental Illumination 
       FIGS.  2 . 3 - 1    is a cross-sectional side view of a head-mounted device in an illustrative configuration in which the device includes an illumination system for providing environmental illumination. Head-mounted device  2 . 3 - 10  of  FIGS.  2 . 3 - 1    may have optical sensors. These sensors may include cameras. The cameras of device  2 . 3 - 10  may have lenses and image sensors that are configured to capture images at ultraviolet light wavelengths, visible light wavelengths, and/or infrared wavelengths. 
     Some cameras (e.g., cameras of the type that may sometimes be referred to as scene cameras) may be used for capturing images of a user&#39;s environment that are displayed on displays  2 . 3 - 14  in real time (e.g., real-time pass-through video). Cameras in device  2 . 3 - 10  may also be used in tracking the positions and movements of external objects. As an example, tracking cameras may track a user&#39;s hand (see, e.g., hand  2 . 3 - 30 H) or the user&#39;s torso or other body part (see, e.g., user body part  2 . 3 - 30 B). Hand gesture input may, as an example, be used in controlling operation of device  2 . 3 - 10 . Body part monitoring may be used to allow a user&#39;s body motions to be replicated by content displayed in a virtual environment. If desired, cameras may also be used in tracking the position of external accessories (e.g., the position and movement of controllers that are moved by a user to control device  2 . 3 - 10 ). In some scenario, visual inertial odometry (VIO) systems or other systems that determine the position, movement, and/or orientation of device  2 . 3 - 10  relative to the environment surrounded by device  2 . 3 - 10  may be formed by combining data from one or more cameras in device  2 . 3 - 10  with additional sensor data (e.g., data from an inertial measurement unit). Cameras may perform dedicated functions (tracking, visual inertial odometry functions, scene capture, ranging, three-dimensional image capture for facial recognition and environment mapping, etc.) or two or more of these operations may be performed by a shared camera. 
     It may be desirable to allow a user of device  2 . 3 - 10  to operate device  2 . 3 - 10  in low lighting conditions. As an example, a user may be viewing content on displays  14  while in a dark room or dark vehicle interior. To ensure that camera tracking functions such as hand tracking, body tracking, accessory tracking, and optionally other camera-based functions (e.g., visual inertial odometry, etc.) can be performed satisfactorily, device  2 . 3 - 10  may provide supplemental illumination. The supplemental illumination may be provided by light sources that produce supplemental ultraviolet light, supplemental visible light, and/or supplemental infrared light to augment any ambient light that is available. In an illustrative configuration, supplemental illumination is provided at infrared wavelengths, as this light is detectable by tracking cameras or other cameras with infrared sensing capabilities and is invisible to the human eye. Because supplemental infrared illumination is invisible, people in the vicinity of the user of device  2 . 3 - 10  (e.g., people in the same room or vehicle as the user) will not be disturbed by the presence of the supplemental illumination. 
     Any suitable light sources may be used in forming the supplemental illumination system for device  2 . 3 - 10  (e.g., light-emitting didoes, lasers, etc.). In an illustrative configuration, these light-emitting devices are laser diodes or light-emitting diodes that emit infrared light at a wavelength of 940 nm or other infrared wavelength (e.g., one or more wavelengths such as 740-1500 nm, at least 800 nm, 940 nm, at least 900 nm, 800-1200 nm, 900-1000 nm, 750-1100 nm, 800-1100 nm, less than 1500 nm, etc.). There may be N cameras that use supplemental illumination in device  2 . 3 - 10  and M supplemental light sources. The values of N and M may be 1-10, at least 2, at least 3, at least 4, at least 6, at least 8, 2-10, 4-6, 2-4, less than 10, less than 5, less than 4, or other suitable values. The value of N may be larger than the value of M, the value of N may be equal to the value of M, or the value of N may be less than the value of M. As one example, there may be four cameras that use supplemental infrared illumination and there may be two light sources that emit supplemental illumination. 
     The cameras that use the supplemental infrared illumination may be configured to be sensitive at the wavelengths illuminated by the supplemental illumination system (e.g., the infrared light wavelengths associated with the M supplemental light sources). The cameras may also be sensitive at visible light wavelengths so that when ample visible ambient light illumination is present, the cameras can operate without any supplemental illumination. To help avoid infrared interference during normal ambient lighting conditions, the supplemental illumination system may, as an example, be configured to emit light in a narrow infrared band (e.g., 940 nm) and the cameras may be provided with filters that pass visible light while blocking all infrared light except light in the narrow infrared band. In another illustrative configuration, the cameras are sensitive across the visible spectrum (e.g., 380 to 740 nm) and into the infrared spectrum (e.g., 740-1000 nm, or other suitable broader infrared wavelength band in which the infrared supplemental illumination is produced). If desired, switchable filters may be sued to block infrared light from the cameras when supplemental infrared illumination is not being used and that pass infrared light when supplemental infrared illumination is being used. 
     As shown in  FIGS.  2 . 3 - 1   , the right-hand side of device  2 . 3 - 10  may contain a first camera such as camera  2 . 3 - 50  that faces in a direction such as direction  2 . 3 - 54  (e.g., in the −Z direction and slightly in the +Y direction as an example) and may contain a second camera such as camera  2 . 3 - 52  (sometimes referred to as a forward-facing camera) that faces in a forward direction such as direction  2 . 3 - 56  (e.g., in the +Y direction and slightly in the −Z direction as an example). The left-hand side of device  2 . 3 - 10  may have a corresponding pair of cameras that are oriented in the same way. The angles of view of the cameras on the left and right sides may be configured to overlap in front of device  2 . 3 - 10 , so that there are no gaps in coverage in front of the user. If desired, cameras  2 . 3 - 50  and  2 . 3 - 52  may be replaced by a single camera (e.g., a camera in the position of camera  2 . 3 - 52 , a camera in the position of camera  2 . 3 - 50 , or a camera in another suitable forward-facing and/or downward-facing orientation that captures images while viewing outwardly from a location on front side F of device  2 . 3 - 10 ). There may be, for example, a single tracking camera (e.g., camera  2 . 3 - 52 ) on the right side of device  2 . 3 - 10  and a corresponding single tracking camera on the left side of device  2 . 3 - 10 . 
     Regardless of the number of tracking cameras provided on each side of device  2 . 3 - 10 , there may be a right-hand infrared light source such as light source  2 . 3 - 58  that provides supplemental illumination (infrared light) in direction  2 . 3 - 60  to illuminate objects such as hand  2 . 3 - 30 H, body  2 . 3 - 30 B, and other external objects for the tracking camera(s) on the right-hand side of device  2 . 3 - 10  and there may be a corresponding left-hand infrared light source that provides supplemental infrared light for the tracking camera(s) on the left side of device  2 . 3 - 10 . The use of a single supplemental infrared light source on each side of device  2 . 3 - 10  to provide supplemental illumination for the tracking camera(s) on that side of device  2 . 3 - 10  may help to conserve space within the tight confines of housing  2 . 3 - 26 . 
     The supplemental illumination system of device  2 . 3 - 10  may provide infrared illumination in an area (range of angles) that is larger than the area (range of angles) covered by the tracking camera(s) of device  2 . 3 - 10 , that is equal in area to the area covered by the camera(s), or that is smaller than the area covered by the camera(s). 
     Consider, as an example the coverage of the supplemental illumination system of device  2 . 3 - 10  of  FIGS.  2 . 3 - 1    within the YZ plane. As shown in the side view of  FIGS.  2 . 3 - 1   , downward-facing camera  2 . 3 - 50  may be characterized by an angle of view A 1  in the YZ plane and forward-facing camera  2 . 3 - 52  may be characterized by an angle of view A 3  in the YZ plane. These angles of view may overlap to provide continuous tracking coverage in the YZ plane. If desired, the same amount of coverage in the YZ plane or another suitable amount of coverage may be provided using a single tracking camera. The example of  FIGS.  2 . 3 - 1    is illustrative. 
     Supplemental illumination from light source  2 . 3 - 58  may be characterized by an illumination angle A 2  in the YZ plane. The value of A 2  may be larger than, equal to, or smaller than the combined angle-of-view of cameras  2 . 3 - 50  and  2 . 3 - 52  or may be larger than, equal to, or smaller than the angle-of-view of a single tracking camera being used in place of cameras  2 . 3 - 50  and  2 . 3 - 52 . In an illustrative configuration, A 2  is smaller than the overall angle of view of the tracking camera(s) and is directed outwardly in a forward and downward direction in front of device  2 . 3 - 10  (where hand and body tracking is most likely to take place). The use of a somewhat reduced illumination area for the supplemental illumination system (e.g., an area of illumination that is less than the area covered by the tracking camera system) may help to conserve power when operating for extended periods of time in dark operating environments while preserving the ability to track objects in all but peripheral areas. 
       FIGS.  2 . 3 - 2    is a top view of device  2 . 3 - 10  showing how device  2 . 3 - 10  may contain cameras on both the left and right sides of support structure  2 . 3 - 26 . The center of housing portion  2 . 3 - 26 M may contain nose bridge portion  2 . 3 - 26 NB. Nose bridge portion  2 . 3 - 26 NM may have a lower edge with a curved shape configured to rest on a user&#39;s nose while device  2 . 3 - 10  is worn on a user&#39;s face. Nose bridge portion  2 . 3 - 26 NB may couple right housing portion  2 . 3 - 26 R to left housing portion  2 . 3 - 26 L. Optical components  2 . 3 - 62  may include side-facing visible light cameras, forward-facing visible light cameras, a time-of-flight camera (e.g., a time-of-flight sensor in nose bridge portion  2 . 3 - 26 NM that faces forward), three-dimensional structured light cameras (e.g., left and right structured light cameras adjacent to nose bridge portion  2 . 3 - 26 NB), a flicker sensor for detecting ambient light fluctuations (e.g., 60 Hz fluctuations associated with indoor artificial lighting), an ambient light sensor, etc. 
     Right camera  2 . 3 - 52  may be supported in right housing portion  2 . 3 - 26 R and corresponding left camera  2 . 3 - 52 ′ may be supported in left housing portion  2 . 3 - 26 L. Similarly, an optional additional right camera such as camera  2 . 3 - 50  of  FIGS.  2 . 3 - 1    may be supported in right housing portion  2 . 3 - 26 R and a corresponding optional additional left camera may be supported in left housing portion  2 . 3 - 26 L. In this type of configuration, supplemental illumination for the single right-side tracking camera or the pair of right side tracking cameras may be provided by right supplemental light source  2 . 3 - 58  and supplemental illumination for the left side camera(s) may be provided by left supplemental light source  2 . 3 - 58 ′. 
     During supplemental illumination operations, light sources  2 . 3 - 58  and  2 . 3 - 58 ′ produce supplemental illumination in directions  2 . 3 - 60  and  2 . 3 - 60 ′, respectively. As described in connection with the relative coverage areas of the cameras and light source of  FIGS.  2 . 3 - 1   , it is not necessary for the illumination coverage area of the supplemental illumination system to exactly match the coverage area of the cameras. For example, the tracking cameras on each side of device  2 . 3 - 10  may be characterized by an angle of view that is larger in the XY plane than the angle of coverage of the associated light source. Arrangements in which the illumination from the supplemental light source on each side of device  2 . 3 - 10  is provided over the same range of angles as the angle of view of the cameras or in which the illumination is provided over a wider range of angles than the cameras&#39; angle-of-view may also be used. 
     Supplemental illumination may be provided over a relatively large fixed area in a global fashion or a desired area may be covered by activating or moving a narrower beam of illumination towards or across the desired area. If desired, a dynamic illumination system with a steered or addressable beam of supplemental illumination may steer or activate the illumination beam so that the beam follows a user&#39;s hand or other object of interest. In this way, power is not needlessly expended illuminating areas that do not contain objects to track. 
       FIGS.  2 . 3 - 5  and  2 . 3 - 6    are side views of illustrative fixed-area supplemental illumination light sources. Illustrative light source  2 . 3 - 58  of  FIGS.  2 . 3 - 3    has a semiconductor light-emitting device  2 . 3 - 70 . Device  2 . 3 - 70  may be a solid state light-emitting device such as a light-emitting diode, a superluminous light-emitting diode, a resonant cavity light-emitting diode, an edge-emitting light-emitting diode, or a vertical-cavity-surface-emitting diode, may be a diode-pumped laser such as a diode-pumped fiber laser or other diode-pumped laser, etc. As shown in  FIG.  2 . 3 - 3   , device  2 . 3 - 70  may be mounted on optional interposer interposer  2 . 3 - 72  (e.g., using solder). Interposer interposer  2 . 3 - 72  may be mounted to package substrate  2 . 3 - 74  (e.g., a printed circuit). During operation, device  2 . 3 - 70  may emit infrared light that is spread over a desired illumination area by one or more optical structures that overlap device  2 . 3 - 70 . In the example of  FIGS.  2 . 3 - 3   , these optical structures include optional overmolded polymer lens  3 . 3 - 76  and optional secondary optical structures such as peanut lens  3 . 3 - 78 . It is also possible to form curved reflective optical structures on interposer  2 . 3 - 76  or substrate  2 . 3 - 74  to enhance side and/or back light recollection. The optical structures that overlap device  2 . 3 - 70  may be used to shape the light intensity to produce a desirable far-field distribution different than the native light source intensity distribution (e.g., a light-emitting diode with a Lambertian intensity distribution). If desired, safety enhancement structures such as resistive safety traces or capacitive traces may be embedded in optics or may overlap optics or a photodetector may be used to form a closed loop with a safety interlock on light-source drivers (e.g., in connection with module architectures of the types shown in connection with  FIGS.  2 . 3 - 5  through  2 . 3 - 8   ). 
     In the illustrative configuration of  FIGS.  2 . 3 - 4   , light-emitting device  2 . 3 - 70  (e.g., a laser) has been mounted under a light spreading structure such as beam shaping layer  2 . 3 - 82 . Layer  2 . 3 - 82  may be supported in light source package  2 . 3 - 80 . Device  2 . 3 - 70  may be mounted in package  2 . 3 - 80  on optional interposer interposer  2 . 3 - 72  on a printed circuit or other substrate. During operation, device  2 . 3 - 70  of  FIGS.  2 . 3 - 4    may emit infrared light in an upward direction that is spread out laterally by beam shaping layer  2 . 3 - 82  to cover a desired illumination area (e.g., +/−60° or other suitable range of angles). 
     In general, any suitable optical components that serve as light spreading structures may overlap device  2 . 3 - 70  of  FIGS.  2 . 3 - 5  and  2 . 3 - 6   . These optical components may include optical components such as refractive beam shaping optical components, diffractive optics, diffusive optics, optical nanostructures (e.g., thin two-dimensional metamaterial layers such as patterned structures of clear dielectric with subwavelength dimensions that form metasurfaces that are configured to spread the emitted beam), curved reflectors, etc. Multiple devices  2 . 3 - 70  may be mounted in a common package and/or multiple packaged devices  2 . 3 - 70  may be mounted on a printed circuit adjacent to each other when forming light source  2 . 3 - 58 . The use of a single light-emitting device  2 . 3 - 70  in forming light source  58  in the examples of  FIGS.  2 . 3 - 5  and  2 . 3 - 6    is illustrative. 
       FIGS.  2 . 3 - 7  and  2 . 3 - 8    are side views of illustrative dynamic pattern illuminators that may be used in an illumination system for device  2 . 3 - 10 . Using light source of the types shown in  FIGS.  2 . 3 - 7  and  2 . 3 - 8   , control circuitry  2 . 3 - 12  can selectively activate or steer an emitted beam of infrared light so that one or more objects of interest can be provided with targeted supplemental illumination. 
     In the example of  FIGS.  2 . 3 - 5   , light source  2 . 3 - 58  has an array of light-emitting devices  2 . 3 - 70 . Devices  2 . 3 - 70  may include multiple semiconductor dies mounted on a substrate such as printed circuit  2 . 3 - 84  in package  2 . 3 - 86 , may include multiple individually addressable emitters, or may include multiple individually addressable segments of emitters mounted on a substrate such as silicon, ceramic, printed circuit board  2 . 3 - 84 , or other substrate in package  2 . 3 - 86 . A zoned beam shaper layer or other optical component such as layer  2 . 3 - 88  may overlap devices  2 . 3 - 70 . Layer  2 . 3 - 88  may have multiple zones each with a respective beam steering and beam shaping optical structure. These structures may be refractive structures, diffractive structures, nanostructures, etc. Structures on both surfaces of layer  2 . 3 - 88  and/or multiple layers of layer  2 . 3 - 88  with vertically aligned or misaligned zones may be employed. Each zone may be used to steer and shape a beam of light emitted from a respective light-emitting device in a different respective direction. For example, a first zone may direct a beam of light that has been emitted vertically from a first device  2 . 3 - 70  to the left, whereas a second zone may direct a beam of light that has been emitted vertically from a second device  2 . 3 - 70  to the right. By overlapping an array of individually controlled devices  70  with a corresponding array of individualized beam steering structures, each device  2 . 3 - 70  can be configured to emit a beam of light in a different respective direction (see, e.g., illustrative beams  2 . 3 - 90 ), providing light source  2 . 3 - 58  of  FIGS.  2 . 3 - 5    with the ability to emit a steered beam of light. The emission area of each beam may overlap with adjacent beams to avoid potential gaps in coverage. Beams  2 . 3 - 90  may all be emitted simultaneously or one or more selected beams  2 . 3 - 90  may be emitted at a time. If desired, beams  2 . 3 - 90  may be emitted in sequence (e.g., to scan the emitted beam from light source  58  across an area of interest). 
     Another illustrative light source that may be used in forming a dynamic pattern illuminator for the supplemental illumination system of device  2 . 3 - 10  is shown in  FIGS.  2 . 3 - 6   . Light source  2 . 3 - 58  of  FIGS.  2 . 3 - 6    may have one or more light-emitting devices such as device  2 . 3 - 70  that emit one or more beams of light such as light beam  2 . 3 - 92  (e.g., an infrared light beam). Device  2 . 3 - 70  may be mounted on a printed circuit or other substrate  2 . 3 - 94  in package  2 . 3 - 96 . Electrically controlled beam steerer  2 . 3 - 98  may have one or more beam steerers such as steerable microelectromechanical systems mirror  2 . 3 - 100  or other electrically adjustable beam steering element(s) controlled by control signals from control circuitry  2 . 3 - 12 . When it is desired to emit light in a first direction, mirror  2 . 3 - 100  may be placed in a first orientation that reflects beam  2 . 3 - 92  to create first emitted beam  2 . 3 - 102 . When it is desired to emit light in a second direction, mirror  2 . 3 - 100  may be placed in a second orientation that is different than the first orientation, thereby reflecting beam  2 . 3 - 92  to create second emitted beam  2 . 3 - 104 . Mirror  2 . 3 - 100  may be placed in any suitable number of different orientations (e.g., at least 2, at least 10, at least 25, at least 100, less than 5000, less than 1000, less than 500, or other suitable number). Mirror  2 . 3 - 100  may be rotated about a single axis (to change the angle of emitted light beams along a single dimension) or may be rotated about two axes (e.g., to change the angle of emitted light beams arbitrarily in two dimensions). If desired, beam shaping optics (e.g., beam collimating lenses, etc.) may be incorporated into beam steerer  2 . 3 - 98  to help ensure that the steered beam has a desired intensity profile. 
     If desired, a hybrid illuminator architecture may be employed, such that multiple channels of device  2 . 3 - 70  or multiple devise  2 . 3 - 70  as described in connection with  FIGS.  2 . 3 - 5    can be selectively activated to provide one or more additional dimensions of dynamic illumination to beam steering optics such as mirror  2 . 3 - 100  of  FIGS.  2 . 3 - 6   . 
     Light sources that emit static wide-area beams (see, e.g., illustrative light sources  2 . 3 - 58  of  FIGS.  2 . 3 - 5  and  2 . 3 - 6   ) may be configured to emit light beams of any suitable shape to help provide supplemental illumination for the tracking cameras of device  2 . 3 - 10 .  FIGS.  2 . 3 - 7    is a graph showing how light source  2 . 3 - 58  may be configured to emit a circular beam field of regards (FoG) such as circular beam  2 . 3 - 110  (e.g., a beam of infrared light with full width half maximum (FWHM) intensity characterized by an angular spread of +/−60° or other suitable coverage area) or may be configured to emit a rectangular beam FoG such as rectangular beam  2 . 3 - 112  with a similar angular spread vertically and a smaller angular spread horizontally. Two rectangular beams such as beam  2 . 3 - 112  may be produced side by side to provide sufficient horizontal illumination coverage for both the left and right cameras in device  2 . 3 - 10  (as an example). 
     In most general use cases, a goal of the illumination system is to provide a uniform signal-to-noise ratio for the illuminated scene captured by one or more cameras. Within the desired FWHM 2-D FOG, a uniform far-field intensity at each instantaneous FOG (iFoG) can be achieved to provide uniform illumination and working range for the cameras. However, there are cases when non-uniform far-field intensity distributions may be desired. For example, when a target of the illumination is flat or when camera vignetting is significant, a symmetric “bat-wing” intensity distribution may be used to compensate for the relative intensity fall-off of the camera image sensor. Further examples include asymmetric intensity distribution for cameras that are aligned with a non-co-axial orientation relative to the illumination system, for targets such as hands that have asymmetric occurrence/residence across FoGs, for multiple illuminators with overlapping FoGs, for multiple non-co-axial cameras, for irregular occlusions at certain FOG regions, etc. 
     The graphs of  FIGS.  2 . 3 - 10  and  2 . 3 - 11    show illustrative beam outputs (angular beam distributions) associated with a dynamically adjustable illumination system. In the example of  FIGS.  2 . 3 - 8   , a light source such as light source  58  of  FIG.  2 . 3 - 5    or light source  58  of  FIGS.  2 . 3 - 6    has been configured to produce a beam with an elongated rectangular shape (e.g., a rectangle having a larger horizontal spread than vertical spread). Using beam steering, light source  2 . 3 - 58  may emit this elongated rectangular beam in one or more vertical locations, such as illustrative location  2 . 3 - 114  of  FIGS.  2 . 3 - 8   . In an arrangement of the type shown in  FIGS.  2 . 3 - 5   , each light-emitting device  2 . 3 - 70  may produce a different respective elongated rectangular beam each of which is associated with a different vertical position in the output of light source  2 . 3 - 58 . One or more of these beams may be emitted at the same time by turning on one or more respective light-emitting devices  2 . 3 - 70 . In an arrangement of the type shown in  FIGS.  2 . 3 - 6   , light-emitting device  2 . 3 - 70  may produce a beam such as beam  2 . 3 - 92  of  FIGS.  2 . 3 - 6    that is steered to a desired location (e.g., illustrative location  2 . 3 - 114  of  FIGS.  2 . 3 - 8   ) and/or to other locations by beam steerer  2 . 3 - 98 , thereby providing a desired coverage for light source  2 . 3 - 58 . 
     In the illustrative example of  FIGS.  2 . 3 - 8   , light is output over a larger vertical angular range than horizontal range. Additional horizontal coverage may be supplied using an additional light source (e.g., a light source on an opposing side of device  2 . 3 - 10 ). In this way, a desired angular output range (e.g., +/−60° in both horizontal and vertical dimensions or other suitable angular output range) may be covered. 
     In the illustrative configuration of  FIGS.  2 . 3 - 9   , light source  2 . 3 - 58  (e.g., a dynamically configured light source such as light source  58  of  FIGS.  2 . 3 - 5    or  FIGS.  2 . 3 - 6   ) is configured to supply a relatively small circular or square output beam that can be steered in both horizontal and vertical dimensions so that a desired overall amount of coverage is produced. 
     In both those light sources that are static and do not have steerable beams and in those light sources with dynamically patterned output, beam power can be controlled in a binary fashion (on/off) or in an analog fashion (e.g., by adjusting output power continuously or in a stepwise fashion between more than two different output levels). As shown in  FIGS.  2 . 3 - 9   , for example, no light may be output in certain portions of a coverage area such as areas  2 . 3 - 116  (e.g., beam power may be zero for these areas), full power light may be output in areas such as areas  2 . 3 - 118  (e.g., beam power may be maximized for these areas), and an intermediate power level may be used when supplying output light to other areas such as areas  2 . 3 - 120  that are immediately adjacent to areas  2 . 3 - 118 . 
     Arrangements in which full-power light is only output in a subset of the total coverage area for light source  58  may help device  2 . 3 - 10  use power efficiently. As shown in the diagram of  FIGS.  2 . 3 - 9   , for example, there may be one or more external objects of interest such as objects  2 . 3 - 122  within the coverage area of a given light source. Device  2 . 3 - 10  may, as an example, be tracking a user&#39;s hands or other external objects. When these objects are relatively small compared to the overall angle-of-view of the cameras in device  2 . 3 - 10 , power can be conserved by restricting the output of supplemental illumination (or at least restricting the output of full-power supplemental illumination) to only those regions that overlap the tracked external objects. 
     In the  FIGS.  2 . 3 - 9    example, objects  2 . 3 - 122  (e.g., the user&#39;s hands or other body part or other objects in the user&#39;s environment) are being actively tracked by device  2 . 3 - 10 . As a result, the supplemental illumination system of device  2 . 3 - 10  is being used to provide full-power illumination to areas  2 . 3 - 118  that overlap objects  2 . 3 - 122 . Elsewhere in the coverage area of light-emitting device  58 , beam power is reduced (see, e.g., intermediate power areas  2 . 3 - 120 ) or shut off entirely (see, e.g., unilluminated areas  2 . 3 - 116 ). This type of approach may be used for either scanned beam arrangements (e.g., using a scanning mirror device or other beam steerer as described in connection with  FIG.  2 . 3 - 6   ) or using light sources with addressable arrays of devices  70  each of which can provide output in different directions (e.g., light source  58  of  FIGS.  2 . 3 - 5   ). 
     In areas such as areas  2 . 3 - 116  of  FIGS.  2 . 3 - 9   , no supplemental illumination is present, so items in those areas will not receive supplemental illumination. Nevertheless, once objects such as objects  2 . 3 - 122  are being tracked, device  2 . 3 - 10  can monitor the position and direction of movement of objects  2 . 3 - 122  in real time. This allows device  2 . 3 - 10  to provide supplemental illumination of full power to the areas overlapping objects  2 . 3 - 122  and intermediate power (or, if desired, full power), to portions of the output area of light source  58  that are immediately adjacent to objects  2 . 3 - 122  (e.g., areas where objects  2 . 3 - 122  may possibly move and/or are predicted to occupy in the near future based on tracked movements). In the event that the positions of objects  2 . 3 - 122  move into one of those adjacent areas, device  2 . 3 - 10  can increase the supplemental illumination on those areas to full power and can update the beam powers so that adjacent areas again have intermediate power level coverage. 
     Although the multi-power-level beam scheme of  FIGS.  2 . 3 - 9    has been described in connection with a two-dimensional scanning light beam from light sources  2 . 3 - 58  of  FIGS.  7  and  8   , such adjustable power output schemes may also be used with light sources  2 . 3 - 58  that provide one-dimensional adjustable direction light sources (e.g., light sources that produce slices of supplemental illumination of the type shown in  FIGS.  2 . 3 - 8   ) and/or may be used with fixed-area light sources. In a fixed-area light source scheme, for example, a right-hand light source  58  of the type shown in  FIGS.  2 . 3 - 3    or  FIGS.  2 . 3 - 4    may be used to supply supplemental illumination for tracking objects  2 . 3 - 122  that are in front of the right-hand camera(s) of device  2 . 3 - 10 , whereas a left-hand light source  58  of the type shown in  FIGS.  2 . 3 - 3    or  FIGS.  2 . 3 - 4    may be used to supply supplemental illumination for tracking objects  2 . 3 - 122  that are in front of the left-hand camera(s) of device  2 . 3 - 10 . Device  2 . 3 - 10  can activate either the right-hand light-source or the left-hand light source or both depending on the current and expected locations of objects  2 . 3 - 122 . 
     Another way in which to help use power efficiently for the supplemental illumination system involves using light sources  2 . 3 - 58  to produce supplemental illumination only when the cameras for which the supplemental illumination is being provided will benefit from the supplemental illumination. For example, in bright lighting conditions, ambient visible light will provide sufficient illumination, so supplemental infrared light beams can be turned off (or at least reduced in power to a lower level than otherwise used) to help conserve power. The activation of supplemental lighting may take place when dim ambient lighting conditions are detected or when other suitable conditions are detected to trigger the production of supplemental lighting. 
       FIGS.  2 . 3 - 10    is a flow chart of illustrative operations involved in using electronic device  2 . 3 - 10 . During the operations of block  2 . 3 - 150 , device  2 . 3 - 10  may be used to provide a user with content such as visual content, audio content, and other output. Device  2 . 3 - 10  may, as an example be worn on a user&#39;s head while images are presented for viewing. The operations of block  2 . 3 - 150  may be performed while device  2 . 3 - 10  is in a normal operating environment with satisfactory visible ambient light levels. 
     Visual content may be presented for the user on displays  2 . 3 - 14 . This visual content may include camera images from cameras in device  2 . 3 - 10  (e.g., pass-through video) and/or other content. In some scenarios, computer-generated content (sometimes referred to as virtual content) may be overlaid on top of real-world content from cameras in device  2 . 3 - 10 . In this type of mixed reality environment, camera data may be used to help track the locations of the user&#39;s hands and other real-world objects and thereby help register the overlaying of virtual content on real-world images. For example, by tracking the location of a user&#39;s hand, a computer-generated image of a glove may be accurately overlaid on top of a real-world image of the user&#39;s hand. By tracking the location of a table surface, a computer-generated image may be placed on top of the table surface. Camera data can be used to track the motion of a user&#39;s hands, fingers, and/or other body parts in real time. In this way, hand gestures, finger gestures, and/or other body part motions that serve as user input (sometimes referred to as air gestures) can be used in controlling the operation of device  2 . 3 - 10  (e.g., in a mixed-reality or completely virtual environment). 
     Device  2 . 3 - 10  may have any suitable number of cameras including three-dimensional cameras (e.g., structured light cameras, time-of flight cameras, etc.), cameras for capturing real-world visible-light images (e.g., for video passthrough), and/or cameras that perform tracking operations, that serve as parts of visual inertial odometry systems, and/or that otherwise support the operation of device  2 . 3 - 10 . The cameras of device  2 . 3 - 10  may face forward, down, to the side, up, to the rear, and/or in multiple directions. Some cameras may operate only at visible wavelengths. Other cameras may operate at visible and infrared wavelengths. 
     As described in connection with  FIGS.  3  and  4   , device  2 . 3 - 10  may, as an example, have a one or more tracking cameras on each side of device  2 . 3 - 10 . These cameras may be sensitive at visible and infrared wavelengths and may be used for tracking operations (e.g., hand and body tracking, air gesture input tracking, accessory tracking) and optionally additional functions such as imaging structures in the user&#39;s environment for a visual inertial odometry system). Tracking cameras may be sensitive at visible and infrared wavelengths such as wavelengths from 400-1000 nm, 400-740 nm and 940 nm, or other suitable visible and infrared wavelengths. The infrared sensitivity of the tracking cameras preferably coincides with the wavelength or wavelengths emitted by light sources  2 . 3 - 58  in the supplemental illumination system, allowing these cameras to operate when most or all available illumination is being provided by light sources  2 . 3 - 58  rather than ambient light sources. 
     Supplemental illumination may, if desired, be provided continuously. Arrangements in which power is conserved by at least occasionally depowering the supplemental illumination system are described herein as an example. In configurations for device  2 . 3 - 10  in which supplemental illumination is turned on and off, device  2 . 3 - 10  may, during the operations of block  2 . 3 - 150 , monitor for the occurrence of conditions indicating that supplemental illumination should be switched on for satisfactory operation of the cameras (e.g., the tracking cameras). These monitoring activities may take place while the cameras of device  2 . 3 - 10  (e.g., the tracking cameras) are operating normally in the absence of supplemental illumination from the supplemental illumination system. 
     Any suitable trigger criteria may be used to determine when to activate the supplemental illumination system by turning on light sources  2 . 3 - 58 . As an example, device  2 . 3 - 10  may contain an ambient light sensor. The ambient light sensor may measure the amount of visible ambient light that is present in the environment surrounding device  2 . 3 - 10 . A threshold or other criteria may be applied to ambient light readings from the ambient light sensor. In response to determining that ambient light levels are below a predetermined ambient light threshold or are otherwise too dim for satisfactory operation of the tracking cameras, control circuitry  12  can turn on light sources  2 . 3 - 58  to provide supplemental illumination (e.g., infrared light). 
     Another illustrative criteria that may be used in determining when to activate supplemental illumination involves evaluating an image processing algorithm quality metric. During the operations of bock  2 . 3 - 150 , captured images may be proceed by one or more image processing algorithms. These algorithms may include, as an example, a hand tracking algorithm. The hand tracking algorithm may produce a quality factor or other metric that is indicative of the ability of the hand tracking algorithm to satisfactorily track the user&#39;s hands. In response to detecting that the tracking algorithm quality metric is below a desired threshold value, control circuitry  12  can turn on light sources  2 . 3 - 58  to provide supplemental illumination for the cameras. 
     If desired, the tracking cameras or other image sensor hardware may supply information indicating that performance is being adversely affected by low ambient lighting levels. As an example, frames of image data may be evaluated to determine whether lighting levels are low. The output of the tracking camera hardware of device  2 . 3 - 10  may also indicate whether signal-to-noise levels are satisfactory. If the tracking cameras are producing only dark and/or noisy image data, control circuitry  12  can determine that light sources  2 . 3 - 58  should be turned on. 
     In some arrangements, device  2 . 3 - 10  may be configured to determine the location of a user relative to walls and other obstructions in the user&#39;s environment. As an example, device  2 . 3 - 10  may contain a map of known wall locations (e.g., a map obtained from an external source or a map based on previous map-building operations performed by device  2 . 3 - 10  when a user wore device  2 . 3 - 10  while walking throughout a building or other environment). Satellite navigation system circuitry (e.g., Global Positioning System circuitry) may use satellite signals to determine the location of device  2 . 3 - 10  (e.g. the location of device  2 . 3 - 10  relative to building walls and other obstructions). From the user&#39;s known location and movement and using information on the locations of known obstructions such as walls, device  2 . 3 - 10  can predict when a user is likely to approach a wall or other obstruction. Sensors  16  in device  2 . 3 - 10  (such as proximity sensors, time of flight sensors, radar, LIDAR, etc.) may also be used in monitoring the user&#39;s movements relative to walls and other obstructions. By using some or all of this information in combination with additional information on the operating environment for device  2 . 3 - 10  (e.g., ambient light readings indicating that ambient lighting is dim), device  2 . 3 - 10  can determine when light sources  2 . 3 - 58  should be turned on to provide supplemental illumination to help ensure that the tracking cameras of device  2 . 3 - 10  will operate satisfactorily. This may help ensure that the cameras of device  2 . 3 - 10  can track the locations of obstructions in the user&#39;s environment using the infrared illumination of light sources  2 . 3 - 58 . By tracking the locations of obstructions accurately, these obstructions or alerts regarding the presence of the obstructions can be displayed on displays  2 . 3 - 14  to help the user avoid undesired collisions with the obstructions. 
     If desired, multiple electronic devices  2 . 3 - 10  in system  2 . 3 - 8  may monitor for conditions indicating that supplemental illumination is needed. For example, multiple users may be wearing head-mounted devices and one device may detect low levels of ambient lighting before another. In this type of system, any of the devices that detect a low level of ambient lighting can signal the other devices in the system to request that supplemental illumination be provided. In response, one or more of the other devices may provide supplemental illumination to assist the cameras of the requesting device in gathering images. The supplemental illumination systems of different devices may therefore assist each other by contributing shared supplemental illumination. This may allow a wall-powered device to help provide supplemental illumination for a battery powered device or may allow an electronic device that is close to a tracked object to provide supplemental illumination to that object more efficiently than an electronic device that is farther from the tracked object (as examples). 
     So long as conditions for triggering supplemental illumination are not detected, device  2 . 3 - 10  (e.g., control circuitry  12 ) may continue to monitor for conditions that satisfy supplemental illumination trigger criteria (e.g., dim ambient lighting, reduction of tracking camera image processing quality, reduction of camera hardware performance, criteria based on obstruction proximity, requests from other devices, etc.) during the operations of block  2 . 3 - 150 . 
     In the event that the trigger criteria are satisfied, processing may proceed to block  2 . 3 - 152 . During the operations of block  2 . 3 - 152 , control circuitry  2 . 3 - 14  can use the supplemental illumination system to provide supplemental illumination for the cameras (e.g., infrared light emitted by light sources  2 . 3 - 58  that illuminates exterior objects in the field of view of the tracking cameras). In providing the supplemental illumination, the power of the infrared light emitted by each light source  2 . 3 - 58  and/or the direction of the light beam(s) emitted by each light source  2 . 3 - 58  may be adjusted. For example, some devices  2 . 3 - 70  may be turned on while other devices  2 . 3 - 70  remain off, beams of emitted light may be directed to areas containing tracked objects (e.g., the known locations of the user&#39;s hands or other external objects of interest being tracked by the tracking cameras) and/or adjacent areas, emitted power levels may be adjusted in a stepwise fashion or continuously (e.g., so that sufficient supplemental illumination is provided to ensure satisfactory tracking camera operation without providing excess illumination), etc. 
     Light sources such as light sources  2 . 3 - 58  of  FIGS.  2 . 3 - 5  and  2 . 3 - 6    that are configured to provide illumination over a fixed area may be turned on to ensure that objects in those fixed areas are illuminated. Light sources that emit steerable beams such as light sources  2 . 3 - 58  of  FIGS.  2 . 3 - 5  and  2 . 3 - 8    may be used to emit supplemental illumination over a relatively large area (e.g. by scanning a beam across the large area or by simultaneously using multiple smaller beams to illuminate different respective parts of the larger area) or may be used to emit supplemental illumination to particular locations such as the location(s) containing the user&#39;s hands or other objects being tracked. 
     Supplemental illumination may be provided for cameras that track user body parts, cameras that track the locations of accessories, cameras that capture pass-through video, cameras that form part of a visual inertial odometry system, and/or other optical components that gather light from objects in the vicinity of device  2 . 3 - 10 . If desired, light sources  2 . 3 - 58  may be configured to emit structured light (e.g., lines, dots, features distributed in pseudorandom patterns, etc.). Structured light may be used, for example, in scenarios in which the tracking cameras gather three-dimensional images. 
     During the operations of block  2 . 3 - 152 , device  2 . 3 - 10  may monitor for conditions that indicate that supplemental illumination is no longer needed. Control circuitry  2 . 3 - 12  may, for example, monitor to determine whether supplemental illumination trigger conditions cease to be satisfied. So long as dim ambient lighting conditions or other conditions indicating that supplemental illumination should be provided continue to be present, device  2 . 3 - 10  can continue to use light sources  2 . 3 - 58  to provide supplemental illumination. In the event that dim lighting conditions cease or that other conditions in which supplemental illumination is desired are determined to no longer be present, device  2 . 3 - 10  can turn off the supplemental illumination system. In particular, control circuitry  2 . 3 - 12  can turn off light sources  2 . 3 - 58  during the operations of block  156 . As indicated by line  2 . 3 - 152 , operations may then return to block  2 . 3 - 150 . 
     2.4: Systems with Displays and Sensor-Hiding Structures 
       FIGS.  2 . 4 - 1    is a front view of device  2 . 4 - 10  in an illustrative configuration in which device  2 . 4 - 10  has a publicly viewable display such as forward-facing display  2 . 4 - 14 F. As shown in  FIGS.  2 . 4 - 1   , support structure  2 . 4 - 16 M of device  2 . 4 - 10  may have right and left portions such as portions  2 . 4 - 16 R and  2 . 4 - 16 L that are coupled by an interposed nose bridge portion such as portion  2 . 4 - 16 NB. Portion  2 . 4 - 16 NB may have a curved exterior surface such as nose bridge surface  2 . 4 - 90  that is configured to receive and rest upon a user&#39;s nose to help support main housing portion  2 . 4 - 16 M on the head of the user. 
     Display  2 . 4 - 14 F may have an active area such as active area AA that is configured to display images and an inactive area IA that does not display images. The outline of active area AA may be rectangular, rectangular with rounded corners, may have teardrop shaped portions on the left and right sides of device  2 . 4 - 10 , may have a shape with straight edges, a shape with curved edges, a shape with a peripheral edge that has both straight and curved portions, and/or other suitable outlines. As shown in  FIGS.  2 . 4 - 1   , active area AA may have a curved recessed portion at nose bridge portion  2 . 4 - 16 NB of main housing portion  2 . 4 - 16 . The presence of the nose-shaped recess in active area AA may help fit active area AA within the available space of housing portion  2 . 4 - 16 M without overly limiting the size of active area AA. 
     Active area AA contains an array of pixels. The pixels may be, for example, light-emitting diode pixels formed from thin-film organic light-emitting diodes or crystalline semiconductor light-emitting diode dies (sometimes referred to as micro-light-emitting diodes) on a flexible display panel substrate. Configurations in which display  2 . 4 - 14 F uses other display technologies may also be used, if desired. Illustrative arrangements in which display  14  is formed from a light-emitting diode display such as an organic light-emitting diode display that is formed on a flexible substrate (e.g., a substrate formed from a bendable layer of polyimide or a sheet of other flexible polymer) may sometimes be described herein as an example. The pixels of active area AA may be formed on a display device such as display panel  2 . 4 - 14 P of  FIGS.  2 . 4 - 1    (e.g., a flexible organic light-emitting diode display panel). In some configurations, the outline of active area AA (and, if desired, panel  2 . 4 - 14 P) may have a peripheral edge that contains straight segments or a combination of straight and curved segments. Configurations in which the entire outline of active area AA (and optionally panel  2 . 4 - 14 P) is characterized by a curved peripheral edge may also be used. 
     Display  2 . 4 - 14 F may have an inactive area such as inactive area IA that is free of pixels and that does not display images. Inactive area IA may form an inactive border region that runs along one more portions of the peripheral edge of active area AA. In the illustrative configuration of  FIGS.  2 . 4 - 1   , inactive area IA has a ring shape that surrounds active area AA and forms an inactive border. In this type of arrangement, the width of inactive area IA may be relatively constant and the inner and outer edges of area IA may be characterized by straight and/or curved segments or may be curved along their entire lengths. For example, the outer edge of area IA (e.g., the periphery of display  2 . 4 - 14 F) may have a curved outline that runs parallel to the curved edge of active area AA. 
     In some configurations, device  2 . 4 - 10  may operate with other devices in system  2 . 4 - 8  (e.g., wireless controllers and other accessories). These accessories may have magnetic sensors that sense the direction and intensity of magnetic fields. Device  2 . 4 - 10  may have one or more electromagnets configured to emit a magnetic field. The magnetic field can be measured by the wireless accessories near device  2 . 4 - 10 , so that the accessories can determine their orientation and position relative to device  2 . 4 - 10 . This allows the accessories to wirelessly provide device  2 . 4 - 10  with real-time information on their current position, orientation, and movement so that the accessories can serve as wireless controllers. The accessories may include wearable devices, handled devices, and other input devices. 
     In an illustrative configuration, device  2 . 4 - 10  may have a coil such as illustrative coil  2 . 4 - 54  that runs around the perimeter of display  2 . 4 - 14 F (e.g., under inactive area IA or other portion of display  2 . 4 - 14 F). Coil  2 . 4 - 54  may have any suitable number of turns (e.g., 1-10, at least 2, at least 5, at least 10, 10-50, fewer than 100, fewer than 25, fewer than 6, etc.). These turns may be formed from metal traces on a substrate, may be formed from wire, and/or may be formed from other conductive lines. During operation, control circuitry  2 . 4 - 12  may supply coil  2 . 4 - 54  with an alternating-current (AC) drive signal. The drive signal may have a frequency of at least 1 kHz, at least 10 kHz, at least 100 kHz, at least 1 MHz, less than 10 MHz, less than 3 MHz, less than 300 kHz, or less than 30 kHz (as examples). As AC current flows through coil  2 . 4 - 54 , a corresponding magnetic field is produced in the vicinity of device  2 . 4 - 10 . Electronic devices such as wireless controllers with magnetic sensors that are in the vicinity of device  2 . 4 - 10  may use the magnetic field as a reference so that the wireless controllers can determine their orientation, position, and/or movement while being moved relative to device  2 . 4 - 10  to provide device  2 . 4 - 10  with input. 
     Consider, as an example, a handheld wireless controller that is used in controlling the operation of device  2 . 4 - 10 . During operation, device  2 . 4 - 10  uses coil  2 . 4 - 54  to emit a magnetic field. As the handheld wireless controller is moved, the magnetic sensors of the controller can monitor the location of the controller and the movement of the controller relative to device  2 . 4 - 10  by monitoring the strength, orientation, and change to the strength and/or orientation of the magnetic field emitted by coil  2 . 4 - 54  as the controller is moved through the air by the user. The electronic device can then wirelessly transmit information on the location and orientation of the controller to device  2 . 4 - 10 . In this way, a handheld controller, wearable controller, or other external accessory can be manipulated by a user to provide device  2 . 4 - 10  with air gestures, pointing input, steering input, and/or other user input. 
     Device  2 . 4 - 10  may have components such as optical components (e.g., optical sensors among sensors  2 . 4 - 16  of  FIGS.  2 . 4 - 1   ). These components may be mounted in any suitable location on head-mounted support structure  2 . 4 - 16  (e.g., on head strap  2 . 4 - 16 B, on main housing portion  2 . 4 - 16 M, etc.). Optical components and other components may face rearwardly (e.g., when mounted on the rear face of device  2 . 4 - 10 ), may face to the side (e.g., to the left or right), may face downwardly or upwardly, may face to the front of device  2 . 4 - 10  (e.g., when mounted on the front face of device  2 . 4 - 10 ), may be mounted so as to point in any combination of these directions (e.g., to the front, to the right, and downward) and/or may be mounted in other suitable orientations. In an illustrative configuration, at least some of the components of device  2 . 4 - 10  are mounted so as to face outwardly to the front (and optionally to the sides and/or up and down). For example, forward-facing cameras for pass-through video may be mounted on the left and right sides of the front of device  2 . 4 - 10  in a configuration in which the cameras diverge slightly along the horizontal dimension so that the fields of view of these cameras overlap somewhat while capturing a wide-angle image of the environment in front of device  2 . 4 - 10 . The captured image may, if desired, include portions of the user&#39;s surroundings that are below, above, and to the sides of the area directly in front of device  2 . 4 - 10 . 
     To help hide components such as optical components from view from the exterior of device  2 . 4 - 10 , it may be desirable to cover some or all of the components with cosmetic covering structures. The covering structures may include transparent portions (e.g., optical component windows) that are characterized by sufficient optical transparency to allow overlapped optical components to operate satisfactorily. For example, an ambient light sensor may be covered with a layer that appears opaque to an external viewer to help hide the ambient light sensor from view, but that allows sufficient ambient light to pass to the ambient light sensor for the ambient light sensor to make a satisfactory ambient light measurement. As another example, an optical component that emits infrared light may be overlapped with a visibly opaque material that is transparent to infrared light. 
     In an illustrative configuration, optical components for device  2 . 4 - 10  may be mounted in inactive area IA of  FIGS.  2 . 4 - 1    and cosmetic covering structures may be formed in a ring shape overlapping the optical components in inactive area IA. Cosmetic covering structures may be formed from ink, polymer structures, structures that include metal, glass, other materials, and/or combinations of these materials. In an illustrative configuration, a cosmetic covering structure may be formed from a ring-shaped member having a footprint that matches the footprint of inactive area IA. If, for example, active area AA has left and right portions with teardrop shapes, the ring-shaped member may have curved edges that follow the curved periphery of the teardrop-shaped portions of active area AA. The ring-shaped member may be formed from one or more polymer structures (e.g., the ring-shaped member may be formed from a polymer ring). Because the ring-shaped member can help hide overlapped components from view, the ring-shaped member may sometimes be referred to as a shroud or ring-shaped shroud member. The outward appearance of the shroud or other cosmetic covering structures may be characterized by a neutral color (white, black, or gray) or a non-neutral color (e.g., blue, red, green, gold, rose gold, etc.). 
     Display  2 . 4 - 14 F may, if desired, have a protective display cover layer. The cover layer may overlap active area AA and inactive area IA (e.g., the entire front surface of device  2 . 4 - 10  as viewed from direction  2 . 4 - 52  of  FIGS.  2 . 4 - 1    may be covered by the cover layer). The cover layer, which may sometimes be referred to as a housing wall or transparent housing wall, may have a rectangular outline, an outline with teardrop portions, an oval outline, or other shape with curved and/or straight edges. 
     The cover layer may be formed from a transparent material such as glass, polymer, transparent crystalline material such as sapphire, clear ceramic, other transparent materials, and/or combinations of these materials. As an example, a protective display cover layer for display  2 . 4 - 14 F may be formed from safety glass (e.g., laminated glass that includes a clear glass layer with a laminated polymer film). Optional coating layers may be applied to the surfaces of the display cover layer. If desired, the display cover layer may be chemically strengthened (e.g., using an ion-exchange process to create an outer layer of material under compressive stress that resists scratching). In some configurations, the display cover layer may be formed from a stack of two or more layers of material (e.g., first and second structural glass layers, a rigid polymer layer coupled to a glass layer or another rigid polymer layer, etc.) to enhance the performance of the cover layer. 
     In active area AA, the display cover layer may overlap the pixels of display panel  2 . 4 - 14 P. The display cover layer in active area AA is preferably transparent to allow viewing of images presented on display panel  2 . 4 - 14 P. In inactive area IA, the display cover layer may overlap the ring-shaped shroud or other cosmetic covering structure. The shroud and/or other covering structures (e.g., opaque ink coatings on the inner surface of the display cover layer and/or structures) may be sufficiently opaque to help hide some or all of the optical components in inactive area IA from view. Windows may be provided in the shroud or other cosmetic covering structures to help ensure that the optical components that are overlapped by these structures operate satisfactorily. Windows may be formed from holes, may be formed from areas of the shroud or other cosmetic covering structures that have been locally thinned to enhance light transmission, may be formed from window members with desired light transmission properties that have been inserted into mating openings in the shroud, and/or may be formed from other shroud window structures. 
     In the example of  FIGS.  2 . 4 - 1   , device  2 . 4 - 10  includes optical components such as optical components  2 . 4 - 60 ,  2 . 4 - 62 ,  2 . 4 - 64 ,  2 . 4 - 66 ,  2 . 4 - 68 ,  2 . 4 - 70 ,  2 . 4 - 72 ,  2 . 4 - 74 ,  2 . 4 - 76 ,  2 . 4 - 78 , and  2 . 4 - 80  (as an example). Each of these optical components (e.g., optical sensors selected from among sensors  2 . 4 - 16  of  FIGS.  2 . 4 - 1   , light-emitting devices, etc.) may be configured to detect light and, if desired to emit light (e.g., ultraviolet light, visible light, and/or infrared light). 
     In an illustrative configuration, optical component  2 . 4 - 60  may sense ambient light (e.g., visible ambient light). In particular, optical component  2 . 4 - 60  may have a photodetector that senses variations in ambient light intensity as a function of time. If, as an example, a user is operating in an environment with an artificial light source, the light source may emit light at a frequency associated with its source of wall power (e.g., alternating-current mains power at 60 Hz). The photodetector of component  2 . 4 - 60  may sense that the artificial light from the artificial light source is characterized by 60 Hz fluctuations in intensity. Control circuitry  2 . 4 - 12  can use this information to adjust a clock or other timing signal associated with the operation of image sensors in device  2 . 4 - 10  to help avoid undesired interference between the light source frequency and the frame rate or other frequency associated with image capture operations. Control circuitry  2 . 4 - 12  can also use measurements from component  2 . 4 - 60  to help identify the presence of artificial lighting and the type of artificial lighting that is present. In this way, control circuitry  2 . 4 - 12  can detect the presence of lights such as fluorescent lights or other lights with known non-ideal color characteristics and can make compensating color cast adjustments (e.g., white point adjustments) to color-sensitive components such as cameras and displays. Because optical component  2 . 4 - 60  may measure fluctuations in light intensity, component  2 . 4 - 60  may sometimes be referred to as a flicker sensor or ambient light frequency sensor. 
     Optical component  2 . 4 - 62  may be an ambient light sensor. The ambient light sensor may include one or more photodetectors. In a single-photodetector configuration, the ambient light sensor may be a monochrome sensor that measures ambient light intensity. In a multi-photodetector configuration, each photodetector may be overlapped by an optical filter that passes a different band of wavelengths (e.g., different visible and/or infrared passbands). The optical filter passbands may overlap at their edges. This allows component  2 . 4 - 62  to serve as a color ambient light sensor that measures both ambient light intensity and ambient light color (e.g., by measuring color coordinates for the ambient light). During operation of device  2 . 4 - 10 , control circuitry  2 . 4 - 12  can take action based on measured ambient light intensity and color. As an example, the white point of a display or image sensor may be adjusted or other display or image sensor color adjustments may be made based on measured ambient light color. The intensity of a display may be adjusted based on light intensity. For example, the brightness of display  2 . 4 - 14 F may be increased in bright ambient lighting conditions to enhance the visibility of the image on the display and the brightness of display  2 . 4 - 14 F may be decreased in dim lighting conditions to conserve power. Image sensor operations and/or light source operations may also be adjusted based on ambient light readings. 
     The optical components in active area IA may also include components along the sides of device  2 . 4 - 10  such as components  2 . 4 - 80  and  2 . 4 - 64 . Optical components  2 . 4 - 80  and  2 . 4 - 64  may be pose-tracking cameras that are used to help monitor the orientation and movement of device  2 . 4 - 10 . Components  2 . 4 - 80  and  2 . 4 - 64  may be visible light cameras (and/or cameras that are sensitive at visible and infrared wavelengths) and may, in conjunction with an inertial measurement unit, form a visual inertial odometry (VIO) system. 
     Optical components  2 . 4 - 78  and  2 . 4 - 66  may be visible-light cameras that capture real-time images of the environment surrounding device  2 . 4 - 10 . These cameras, which may sometimes be referred to as scene cameras or pass-through-video cameras, may capture moving images that are displayed in real time to displays  2 . 4 - 14 R for viewing by the user when the user&#39;s eyes are located in eye boxes  2 . 4 - 24  at the rear of device  2 . 4 - 10 . By displaying pass-through images (pass-through video) to the user in this way, the user may be provided with real-time information on the user&#39;s surroundings. If desired, virtual content (e.g., computer-generated images) may be overlaid over some of the pass-through video. Device  2 . 4 - 10  may also operate in a non-pass-through-video mode in which components  2 . 4 - 78  and  2 . 4 - 66  are turned off and the user is provided only with movie content, game content, and/or other virtual content that does not contain real-time real-world images. 
     Input-output devices  2 . 4 - 12  of device  2 . 4 - 10  may gather user input that is used in controlling the operation of device  2 . 4 - 10 . As an example, a microphone in device  2 . 4 - 10  may gather voice commands. Buttons, touch sensors, force sensors, and other input devices may gather user input from a user&#39;s finger or other external object that is contacting device  2 . 4 - 10 . In some configurations, it may be desirable to monitor a user&#39;s hand gestures or the motion of other user body parts. This allows the user&#39;s hand locations or other body part locations to be replicated in a game or other virtual environment and allows the user&#39;s hand motions to serve as hand gestures (air gestures) that control the operation of device  2 . 4 - 10 . User input such as hand gesture input can be captured using cameras that operate at visible and infrared wavelengths such as tracking cameras (e.g., optical components  2 . 4 - 76  and  2 . 4 - 68 ). Tracking cameras such as these may also track fiducials and other recognizable features on controllers and other external accessories (additional devices  2 . 4 - 10  of system  2 . 4 - 8 ) during use of these controllers in controlling the operation of device  2 . 4 - 10 . If desired, tracking cameras can help determine the position and orientation of a handheld controller or wearable controller that senses its location and orientation by measuring the magnetic field produced by coil  2 . 4 - 54 . The use of tracking cameras may therefore help track hand motions and controller motions that are used in moving pointers and other virtual objects being displayed for a user and can otherwise assist in controlling the operation of device  2 . 4 - 10 . 
     Tracking cameras may operate satisfactorily in the presence of sufficient ambient light (e.g., bright visible ambient lighting conditions). In dim environments, supplemental illumination may be provided by supplemental light sources such as supplemental infrared light sources (e.g., optical components  2 . 4 - 82  and  2 . 4 - 84 ). The infrared light sources may each include one or more light-emitting devices (light-emitting diodes or lasers) and may each be configured to provide fixed and/or steerable beams of infrared light that serve as supplemental illumination for the tracking cameras. If desired, the infrared light sources may be turned off in bright ambient lighting conditions and may be turned on in response to detection of dim ambient lighting (e.g., using the ambient light sensing capabilities of optical component  2 . 4 - 62 ). 
     Three-dimensional sensors in device  2 . 4 - 10  may be used to perform biometric identification operations (e.g., facial identification for authentication), may be used to determine the three-dimensional shapes of objects in the user&#39;s environment (e.g., to map the user&#39;s environment so that a matching virtual environment can be created for the user), and/or to otherwise gather three-dimensional content during operation of device  2 . 4 - 10 . As an example, optical components  2 . 4 - 74  and  2 . 4 - 70  may be three-dimensional structured light image sensors. Each three-dimensional structured light image sensor may have one or more light sources that provide structured light (e.g., a dot projector that projects an array of infrared dots onto the environment, a structured light source that produces a grid of lines, or other structured light component that emits structured light). Each of the three-dimensional structured light image sensors may also include a flood illuminator (e.g., a light-emitting diode or laser that emits a wide beam of infrared light). Using flood illumination and structured light illumination, optical components  2 . 4 - 74  and  2 . 4 - 70  may capture facial images, images of objects in the environment surrounding device  2 . 4 - 10 , etc. 
     Optical component  2 . 4 - 72  may be an infrared three-dimensional time-of-flight camera that uses time-of-flight measurements on emitted light to gather three-dimensional images of objects in the environment surrounding device  2 . 4 - 10 . Component  2 . 4 - 72  may have a longer range and a narrower field of view than the three-dimensional structured light cameras of optical components  2 . 4 - 74  and  2 . 4 - 70 . The operating range of component  2 . 4 - 72  may be 30 cm to 7 m, 2.4-60 cm to 6 m, 70 cm to 5 m, or other suitable operating range (as examples). 
     2.5: Systems with Cover Layer Sealing Structures 
     A head-mounted device may include a head-mounted support structure that allows the device to be worn on the head of a user. The head-mounted device may have displays that are supported by the head-mounted support structure for presenting a user with visual content. The displays may include rear-facing displays that present images to eye boxes at the rear of the head-mounted support structure. The displays may also include a forward-facing display. The forward-facing display may be mounted to the front of the head-mounted support structure and may be viewed by the user when the head-mounted device is not being worn on the user&#39;s head. The forward-facing display, which may sometimes be referred to as a publicly viewable display, may also be viewable by other people in the vicinity of the head-mounted device. 
     Optical components such as image sensors and other light sensors may be provided in the head-mounted device. In an illustrative configuration, optical components are mounted under peripheral portions of a display cover layer that protects the forward-facing display. The display cover layer, or other layers within the head-mounted device, may be formed from materials, such as glass, that are prone to shattering. Because the head-mounted device is near a user&#39;s eyes during operation, it may be desirable to reduce the likelihood that these layers will shatter into the user&#39;s eyes. Therefore, laminates, such as plastic laminates, may be formed on top and bottom surfaces of the cover layer. To protect the edges of the cover layer, encapsulation material may be coupled to the edge surface, or the head-mounted device housing structures may be modified to reduce the likelihood that glass from the cover layer exits the device. 
       FIGS.  2 . 5 - 1    is a side view of an illustrative head-mounted electronic device. As shown in  FIGS.  2 . 5 - 1   , head-mounted device  2 . 5 - 10  may include head-mounted support structure  2 . 5 - 26 . Support structure  2 . 5 - 26  may have walls or other structures that separate an interior region of device  2 . 5 - 10  such as interior region  2 . 5 - 42  from an exterior region surrounding device  2 . 5 - 10  such as exterior region  2 . 5 - 44 . Electrical components  2 . 5 - 40  (e.g., integrated circuits, sensors, control circuitry, light-emitting diodes, lasers, and other light-emitting devices, other control circuits and input-output devices, etc.) may be mounted on printed circuits and/or other structures within device  2 . 5 - 10  (e.g., in interior region  2 . 5 - 42 ). 
     To present a user with images for viewing from eye boxes such as eye boxes  2 . 5 - 34 , device  2 . 5 - 10  may include rear-facing displays such as displays  2 . 5 - 14 R, which may have associated lenses that focus images for viewing in the eye boxes. These components may be mounted in optical modules (e.g., a lens barrel) to form respective left and right optical systems. There may be, for example, a left rear-facing display for presenting an image through a left lens to a user&#39;s left eye in a left eye box and a right rear-facing display for presenting an image to a user&#39;s right eye in a right eye box. The user&#39;s eyes are located in eye boxes  2 . 5 - 34  at rear side R of device  2 . 5 - 10  when structure  2 . 5 - 26  rests against the outer surface of the user&#39;s face. 
     Support structure  2 . 5 - 26  may include a main support structure (sometimes referred to as a main portion or housing). The main housing support structure may extend from front side F of device  2 . 5 - 10  to opposing rear side R of device  2 . 5 - 10 . On rear side R, support structure  2 . 5 - 26  may have cushioned structures to enhance user comfort as support structure  2 . 5 - 26  rests against the user&#39;s face. If desired, support structure  2 . 5 - 26  may include optional head straps and/or other structures that allow device  2 . 5 - 10  to be worn on a head of a user. 
     Device  2 . 5 - 10  may have a publicly viewable front-facing display such as display  2 . 5 - 14 F that is mounted on front side F of support structure  2 . 5 - 26 . Display  2 . 5 - 14 F may be viewable to the user when the user is not wearing device  2 . 5 - 10  and/or may be viewable by others in the vicinity of device  2 . 5 - 10 . Display  2 . 5 - 14 F may, as an example, be visible on front side F of device  2 . 5 - 10  by an external viewer who is viewing device  2 . 5 - 10  from front side F. 
     A schematic diagram of an illustrative system that may include a head-mounted device is shown in  FIGS.  2 . 5 - 2   . As shown in  FIGS.  2 . 5 - 2   , system  2 . 5 - 8  may have one or more electronic devices  2 . 5 - 10 . Devices  2 . 5 - 10  may include a head-mounted device (e.g., device  2 . 5 - 10  of  FIGS.  2 . 5 - 1   ), accessories such as controllers and headphones, computing equipment (e.g., a cellular telephone, tablet computer, laptop computer, desktop computer, and/or remote computing equipment that supplies content to a head-mounted device), and/or other devices that communicate with each other. 
     Each electronic device  2 . 5 - 10  may have control circuitry  2 . 5 - 12 . Control circuitry  2 . 5 - 12  may include storage and processing circuitry for controlling the operation of device  2 . 5 - 10 . Circuitry  2 . 5 - 12  may include storage such as hard disk drive storage, nonvolatile memory (e.g., electrically-programmable-read-only memory configured to form a solid-state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Processing circuitry in control circuitry  2 . 5 - 12  may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors, power management units, audio chips, graphics processing units, application specific integrated circuits, and other integrated circuits. Software code may be stored on storage in circuitry  2 . 5 - 12  and run on processing circuitry in circuitry  2 . 5 - 12  to implement control operations for device  2 . 5 - 10  (e.g., data gathering operations, operations involving the adjustment of the components of device  2 . 5 - 10  using control signals, etc.). Control circuitry  2 . 5 - 12  may include wired and wireless communications circuitry. For example, control circuitry  2 . 5 - 12  may include radio-frequency transceiver circuitry such as cellular telephone transceiver circuitry, wireless local area network transceiver circuitry (e.g., WiFi® circuitry), millimeter wave transceiver circuitry, and/or other wireless communications circuitry. 
     During operation, the communications circuitry of the devices in system  2 . 5 - 8  (e.g., the communications circuitry of control circuitry  2 . 5 - 12  of device  2 . 5 - 10 ) may be used to support communication between the electronic devices. For example, one electronic device may transmit video data, audio data, control signals, and/or other data to another electronic device in system  2 . 5 - 8 . Electronic devices in system  2 . 5 - 8  may use wired and/or wireless communications circuitry to communicate through one or more communications networks (e.g., the internet, local area networks, etc.). The communications circuitry may be used to allow data to be received by device  2 . 5 - 10  from external equipment (e.g., a tethered computer, a portable device such as a handheld device or laptop computer, online computing equipment such as a remote server or other remote computing equipment, or other electrical equipment) and/or to provide data to external equipment. 
     Each device  2 . 5 - 10  in system  2 . 5 - 8  may include input-output devices  2 . 5 - 22 . Input-output devices  2 . 5 - 22  may be used to allow a user to provide device  2 . 5 - 10  with user input. Input-output devices  2 . 5 - 22  may also be used to gather information on the environment in which device  2 . 5 - 10  is operating. Output components in devices  2 . 5 - 22  may allow device  2 . 5 - 10  to provide a user with output and may be used to communicate with external electrical equipment. 
     As shown in  FIGS.  2 . 5 - 2   , input-output devices  2 . 5 - 22  may include one or more displays such as displays  2 . 5 - 14 . Displays  2 . 5 - 14  may include rear facing displays such as display  2 . 5 - 14 R of  FIGS.  2 . 5 - 1   . Device  2 . 5 - 10  may, for example, include left and right components such as left and right scanning mirror display devices or other image projectors, liquid-crystal-on-silicon display devices, digital mirror devices, or other reflective display devices, left and right display panels based on light-emitting diode pixel arrays (e.g., thin-film organic light-emitting displays with polymer or semiconductor substrates such as silicon substrates or display devices based on pixel arrays formed from crystalline semiconductor light-emitting diode dies), liquid crystal display panels, and/or or other left and right display devices that provide images to left and right eye boxes for viewing by the user&#39;s left and right eyes, respectively. Display components such as these (e.g., a thin-film organic light-emitting display with a flexible polymer substrate or a display based on a pixel array formed from crystalline semiconductor light-emitting diode dies on a flexible substrate) may also be used in forming a forward-facing display for device  2 . 5 - 10  such as forward-facing display  2 . 5 - 14 F of  FIGS.  2 . 5 - 1    (sometimes referred to as a front-facing display, front display, or publicly viewable display). 
     During operation, displays  2 . 5 - 14  (e.g., displays  2 . 5 - 14 R and/or  2 . 5 - 14 F) may be used to display visual content for a user of device  2 . 5 - 10  (e.g., still and/or moving images including pictures and pass-through video from camera sensors, text, graphics, movies, games, and/or other visual content). The content that is presented on displays  2 . 5 - 14  may, for example, include virtual objects and other content that is provided to displays  2 . 5 - 14  by control circuitry  2 . 5 - 12 . This virtual content may sometimes be referred to as computer-generated content. Computer-generated content may be displayed in the absence of real-world content or may be combined with real-world content. In some configurations, a real-world image may be captured by a camera (e.g., a forward-facing camera, sometimes referred to as a front-facing camera) and computer-generated content may be electronically overlaid on portions of the real-world image (e.g., when device  2 . 5 - 10  is a pair of virtual reality goggles). 
     Input-output circuitry  2 . 5 - 22  may include sensors  2 . 5 - 16 . Sensors  2 . 5 - 16  may include, for example, three-dimensional sensors (e.g., three-dimensional image sensors such as structured light sensors that emit beams of light and that use two-dimensional digital image sensors to gather image data for three-dimensional images from dots or other light spots that are produced when a target is illuminated by the beams of light, binocular three-dimensional image sensors that gather three-dimensional images using two or more cameras in a binocular imaging arrangement, three-dimensional LIDAR (light detection and ranging) sensors, sometimes referred to as time-of-flight cameras or three-dimensional time-of-flight cameras, three-dimensional radio-frequency sensors, or other sensors that gather three-dimensional image data), cameras (e.g., two-dimensional infrared and/or visible digital image sensors), gaze tracking sensors (e.g., a gaze tracking system based on an image sensor and, if desired, a light source that emits one or more beams of light that are tracked using the image sensor after reflecting from a user&#39;s eyes), touch sensors, capacitive proximity sensors, light-based (optical) proximity sensors, other proximity sensors, force sensors (e.g., strain gauges, capacitive force sensors, resistive force sensors, etc.), sensors such as contact sensors based on switches, gas sensors, pressure sensors, moisture sensors, magnetic sensors, audio sensors (microphones), ambient light sensors, flicker sensors that gather temporal information on ambient lighting conditions such as the presence of a time-varying ambient light intensity associated with artificial lighting, microphones for gathering voice commands and other audio input, sensors that are configured to gather information on motion, position, and/or orientation (e.g., accelerometers, gyroscopes, compasses, and/or inertial measurement units that include all of these sensors or a subset of one or two of these sensors), and/or other sensors. 
     User input and other information may be gathered using sensors and other input devices in input-output devices  2 . 5 - 22 . If desired, input-output devices  2 . 5 - 22  may include other devices  2 . 5 - 24  such as haptic output devices (e.g., vibrating components), light-emitting diodes, lasers, and other light sources (e.g., light-emitting devices that emit light that illuminates the environment surrounding device  2 . 5 - 10  when ambient light levels are low), speakers such as ear speakers for producing audio output, circuits for receiving wireless power, circuits for transmitting power wirelessly to other devices, batteries and other energy storage devices (e.g., capacitors), joysticks, buttons, and/or other components. 
     As described in connection with  FIGS.  2 . 5 - 1   , electronic device  2 . 5 - 10  may have head-mounted support structures such as head-mounted support structure  2 . 5 - 26  (e.g., head-mounted housing structures such as housing walls, straps, etc.). The head-mounted support structure may be configured to be worn on a head of a user (e.g., against the user&#39;s face covering the user&#39;s eyes) during operation of device  2 . 5 - 10  and may support displays  2 . 5 - 14 , sensors  2 . 5 - 16 , other components  2 . 5 - 24 , other input-output devices  2 . 5 - 22 , and control circuitry  2 . 5 - 12  (see, e.g., components  2 . 5 - 40  and displays  2 . 5 - 14 R and  2 . 5 - 14 F of  FIGS.  2 . 5 - 1   , which may include associated optical modules). 
       FIGS.  2 . 5 - 3    is a front view of device  2 . 5 - 10  in an illustrative configuration in which device  2 . 5 - 10  has a publicly viewable display such as forward-facing display  2 . 5 - 14 F. As shown in  FIGS.  2 . 5 - 3   , support structure  2 . 5 - 26  of device  2 . 5 - 10  may have right and left portions on either side of nose bridge  2 . 5 - 90 . Nose bridge  2 . 5 - 90  may be a curved exterior surface that is configured to receive and rest upon a user&#39;s nose to help support housing  2 . 5 - 26  on the head of the user. 
     Display  2 . 5 - 14 F may have an active area such as active area AA that is configured to display images and an inactive area IA that does not display images. The outline of active area AA may be rectangular, rectangular with rounded corners, may have teardrop shaped portions on the left and right sides of device  2 . 5 - 10 , may have a shape with straight edges, a shape with curved edges, a shape with a peripheral edge that has both straight and curved portions, and/or other suitable outlines. As shown in  FIGS.  2 . 5 - 3   , active area AA may have a curved recessed portion at nose bridge  2 . 5 - 90 . The presence of the nose-shaped recess in active area AA may help fit active area AA within the available space of housing  2 . 5 - 26  without overly limiting the size of active area AA. 
     Active area AA contains an array of pixels. The pixels may be, for example, light-emitting diode pixels formed from thin-film organic light-emitting diodes or crystalline semiconductor light-emitting diode dies (sometimes referred to as micro-light-emitting diodes) on a flexible display panel substrate. Configurations in which display  2 . 5 - 14 F uses other display technologies may also be used, if desired. Illustrative arrangements in which display  2 . 5 - 14  is formed from a light-emitting diode display such as an organic light-emitting diode display that is formed on a flexible substrate (e.g., a substrate formed from a bendable layer of polyimide or a sheet of other flexible polymer) may sometimes be described herein as an example. The pixels of active area AA may be formed on a display device such as a display panel (e.g., a flexible organic light-emitting diode display panel). In some configurations, the outline of active area AA may have a peripheral edge that contains straight segments or a combination of straight and curved segments. Configurations in which the entire outline of active area AA is characterized by a curved peripheral edge may also be used. 
     Display  2 . 5 - 14 F may have an inactive area such as inactive area IA that is free of pixels and that does not display images. Inactive area IA may form an inactive border region that runs along one more portions of the peripheral edge of active area AA. In the illustrative configuration of  FIGS.  2 . 5 - 3   , inactive area IA has a ring shape that surrounds active area AA and forms an inactive border. In this type of arrangement, the width of inactive area IA may be relatively constant and the inner and outer edges of area IA may be characterized by straight and/or curved segments or may be curved along their entire lengths. For example, the outer edge of area IA (e.g., the periphery of display  2 . 5 - 14 F) may have a curved outline that runs parallel to the curved edge of active area AA. 
     In some configurations, device  2 . 5 - 10  may operate with other devices in system  2 . 5 - 8  (e.g., wireless controllers and other accessories). These accessories may have magnetic sensors that sense the direction and intensity of magnetic fields. Device  2 . 5 - 10  may have one or more electromagnets configured to emit a magnetic field. The magnetic field can be measured by the wireless accessories near device  2 . 5 - 10 , so that the accessories can determine their orientation and position relative to device  2 . 5 - 10 . This allows the accessories to wirelessly provide device  2 . 5 - 10  with real-time information on their current position, orientation, and movement so that the accessories can serve as wireless controllers. The accessories may include wearable devices, handled devices, and other input devices. 
     In an illustrative configuration, device  2 . 5 - 10  may have a coil that runs around the perimeter of display  2 . 5 - 14 F (e.g., under inactive area IA along the periphery of active area AA). The coil may have any suitable number of turns (e.g., 1-10, at least 2, at least 5, at least 10, 10-50, fewer than 100, fewer than 25, fewer than 6, etc.). These turns may be formed from metal traces on a substrate, may be formed from wire, and/or may be formed from other conductive lines. During operation, control circuitry  2 . 5 - 12  may supply the coil with an alternating-current (AC) drive signal. The drive signal may have a frequency of at least 1 kHz, at least 10 kHz, at least 100 kHz, at least 1 MHz, less than 10 MHz, less than 3 MHz, less than 300 kHz, or less than 30 kHz (as examples). As AC current flows through the coil a corresponding magnetic field is produced in the vicinity of device  2 . 5 - 10 . Electronic devices such as wireless controllers with magnetic sensors that are in the vicinity of device  2 . 5 - 10  may use the magnetic field as a reference so that the wireless controllers can determine their orientation, position, and/or movement while being moved relative to device  2 . 5 - 10  to provide device  2 . 5 - 10  with input. 
     Consider, as an example, a handheld wireless controller that is used in controlling the operation of device  2 . 5 - 10 . During operation, device  2 . 5 - 10  uses the coil to emit a magnetic field. As the handheld wireless controller is moved, the magnetic sensors of the controller can monitor the location of the controller and the movement of the controller relative to device  2 . 5 - 10  by monitoring the strength, orientation, and change to the strength and/or orientation of the magnetic field emitted by the coil as the controller is moved through the air by the user. The electronic device can then wirelessly transmit information on the location and orientation of the controller to device  2 . 5 - 10 . In this way, a handheld controller, wearable controller, or other external accessory can be manipulated by a user to provide device  2 . 5 - 10  with air gestures, pointing input, steering input, and/or other user input. 
     Device  2 . 5 - 10  may have components such as optical components (e.g., optical sensors among sensors  2 . 5 - 16  of  FIGS.  2 . 5 - 2   ). These components may be mounted in any suitable location on head-mounted support structure  2 . 5 - 26  (e.g. on a head strap, on housing  2 . 5 - 26 , etc.). Optical components and other components may face rearwardly (e.g., when mounted on the rear face of device  2 . 5 - 10 ), may face to the side (e.g. to the left or right), may face downwardly or upwardly, may face to the front of device  2 . 5 - 10  (e.g., when mounted on the front face of device  2 . 5 - 10 ), may be mounted so as to point in any combination of these directions (e.g., to the front, to the right, and downward) and/or may be mounted in other suitable orientations. In an illustrative configuration, at least some of the components of device  2 . 5 - 10  are mounted so as to face outwardly to the front (and optionally to the sides and/or up and down). For example, forward-facing cameras for pass-through video may be mounted on the left and right sides of the front of device  2 . 5 - 10  in a configuration in which the cameras diverge slightly along the horizontal dimension so that the fields of view of these cameras overlap somewhat while capturing a wide-angle image of the environment in front of device  2 . 5 - 10 . The captured image may, if desired, include portions of the user&#39;s surroundings that are below, above, and to the sides of the area directly in front of device  2 . 5 - 10 . 
     To help hide components such as optical components from view from the exterior of device  2 . 5 - 10 , it may be desirable to cover some or all of the components with cosmetic covering structures. The covering structures may include transparent portions (e.g., optical component windows) that are characterized by sufficient optical transparency to allow overlapped optical components to operate satisfactorily. For example, an ambient light sensor may be covered with a layer that appears opaque to an external viewer to help hide the ambient light sensor from view, but that allows sufficient ambient light to pass to the ambient light sensor for the ambient light sensor to make a satisfactory ambient light measurement. As another example, an optical component that emits infrared light may be overlapped with a visibly opaque material that is transparent to infrared light. 
     In an illustrative configuration, optical components for device  2 . 5 - 10  may be mounted in inactive area IA of  FIGS.  2 . 5 - 3    and cosmetic covering structures may be formed in a ring shape overlapping the optical components in inactive area IA. Cosmetic covering structures may be formed from ink, polymer structures, structures that include metal, glass, other materials, and/or combinations of these materials. In an illustrative configuration, a cosmetic covering structure may be formed from a ring-shaped member having a footprint that matches the footprint of inactive area IA. If, for example, active area AA has left and right portions with teardrop shapes, the ring-shaped member may have curved edges that follow the curved periphery of the teardrop-shaped portions of active area AA. The ring-shaped member may be formed from one or more polymer structures (e.g., the ring-shaped member may be formed from a polymer ring). Because the ring-shaped member can help hide overlapped components from view, the ring-shaped member may sometimes be referred to as a shroud or ring-shaped shroud member. The outward appearance of the shroud or other cosmetic covering structures may be characterized by a neutral color (white, black, or gray) or a non-neutral color (e.g., blue, red, green, gold, rose gold, etc.). 
     Display  2 . 5 - 14 F may, if desired, have a protective display cover layer. The cover layer may overlap active area AA and inactive area IA (e.g., the entire front surface of device  2 . 5 - 10  as viewed from front F of  FIGS.  2 . 5 - 1    may be covered by the cover layer). The cover layer, which may sometimes be referred to as a housing wall or transparent housing wall, may have a rectangular outline, an outline with teardrop portions, an oval outline, or other shape with curved and/or straight edges. 
     The cover layer may be formed from a transparent material such as glass, polymer, transparent crystalline material such as sapphire, clear ceramic, other transparent materials, and/or combinations of these materials. As an example, a protective display cover layer for display  2 . 5 - 14 F may be formed from safety glass (e.g., laminated glass that includes a clear glass layer with a laminated polymer film). Optional coating layers may be applied to the surfaces of the display cover layer. If desired, the display cover layer may be chemically strengthened (e.g., using an ion-exchange process to create an outer layer of material under compressive stress that resists scratching). In some configurations, the display cover layer may be formed from a stack of two or more layers of material (e.g., first and second structural glass layers, a rigid polymer layer coupled to a glass layer or another rigid polymer layer, etc.) to enhance the performance of the cover layer. 
     In active area AA, the display cover layer may overlap the pixels of display panel  2 . 5 - 14 P. The display cover layer in active area AA is preferably transparent to allow viewing of images presented on display panel  2 . 5 - 14 P. In inactive area IA, the display cover layer may overlap the ring-shaped shroud or other cosmetic covering structure. The shroud and/or other covering structures (e.g., opaque ink coatings on the inner surface of the display cover layer and/or structures) may be sufficiently opaque to help hide some or all of the optical components in inactive area IA from view. Windows may be provided in the shroud or other cosmetic covering structures to help ensure that the optical components that are overlapped by these structures operate satisfactorily. Windows may be formed from holes, may be formed from areas of the shroud or other cosmetic covering structures that have been locally thinned to enhance light transmission, may be formed from window members with desired light transmission properties that have been inserted into mating openings in the shroud, and/or may be formed from other shroud window structures. 
     In the example of  FIGS.  2 . 5 - 3   , device  2 . 5 - 10  includes optical components such as optical components  2 . 5 - 60 ,  2 . 5 - 62 ,  2 . 5 - 64 ,  2 . 5 - 66 ,  2 . 5 - 68 ,  2 . 5 - 70 ,  2 . 5 - 72 ,  2 . 5 - 74 ,  2 . 5 - 76 ,  2 . 5 - 78 , and  2 . 5 - 80  (as an example). Each of these optical components (e.g., optical sensors selected from among sensors  2 . 5 - 16  of  FIGS.  2 . 5 - 2   , light-emitting devices, etc.) may be configured to detect light and, if desired to emit light (e.g., ultraviolet light, visible light, and/or infrared light). 
     In an illustrative configuration, optical component  2 . 5 - 60  may sense ambient light (e.g., visible ambient light). In particular, optical component  2 . 5 - 60  may have a photodetector that senses variations in ambient light intensity as a function of time. If, as an example, a user is operating in an environment with an artificial light source, the light source may emit light at a frequency associated with its source of wall power (e.g., alternating-current mains power at 60 Hz). The photodetector of component  2 . 5 - 60  may sense that the artificial light from the artificial light source is characterized by 60 Hz fluctuations in intensity. Control circuitry  2 . 5 - 12  can use this information to adjust a clock or other timing signal associated with the operation of image sensors in device  2 . 5 - 10  to help avoid undesired interference between the light source frequency and the frame rate or other frequency associated with image capture operations. Control circuitry  2 . 5 - 12  can also use measurements from component  2 . 5 - 60  to help identify the presence of artificial lighting and the type of artificial lighting that is present. In this way, control circuitry  2 . 5 - 12  can detect the presence of lights such as fluorescent lights or other lights with known non-ideal color characteristics and can make compensating color cast adjustments (e.g., white point adjustments) to color-sensitive components such as cameras and displays. Because optical component  2 . 5 - 60  may measure fluctuations in light intensity, component  2 . 5 - 60  may sometimes be referred to as a flicker sensor or ambient light frequency sensor. 
     Optical component  2 . 5 - 62  may be an ambient light sensor. The ambient light sensor may include one or more photodetectors. In a single-photodetector configuration, the ambient light sensor may be a monochrome sensor that measures ambient light intensity. In a multi-photodetector configuration, each photodetector may be overlapped by an optical filter that passes a different band of wavelengths (e.g. different visible and/or infrared passbands). The optical filter passbands may overlap at their edges. This allows component  2 . 5 - 62  to serve as a color ambient light sensor that measures both ambient light intensity and ambient light color (e.g., by measuring color coordinates for the ambient light). During operation of device  2 . 5 - 10 , control circuitry  2 . 5 - 12  can take action based on measured ambient light intensity and color. As an example, the white point of a display or image sensor may be adjusted or other display or image sensor color adjustments may be made based on measured ambient light color. The intensity of a display may be adjusted based on light intensity. For example, the brightness of display  2 . 5 - 14 F may be increased in bright ambient lighting conditions to enhance the visibility of the image on the display and the brightness of display  2 . 5 - 14 F may be decreased in dim lighting conditions to conserve power. Image sensor operations and/or light source operations may also be adjusted based on ambient light readings. 
     The optical components in active area IA may also include components along the sides of device  2 . 5 - 10  such as components  2 . 5 - 80  and  2 . 5 - 64 . Optical components  2 . 5 - 80  and  2 . 5 - 64  may be pose-tracking cameras that are used to help monitor the orientation and movement of device  2 . 5 - 10 . Components  2 . 5 - 80  and  2 . 5 - 64  may be visible light cameras (and/or cameras that are sensitive at visible and infrared wavelengths) and may, in conjunction with an inertial measurement unit, form a visual inertial odometry (VIO) system. 
     Optical components  2 . 5 - 78  and  2 . 5 - 66  may be visible-light cameras that capture real-time images of the environment surrounding device  2 . 5 - 10 . These cameras, which may sometimes be referred to as scene cameras or pass-through-video cameras, may capture moving images that are displayed in real time to displays  2 . 5 - 14 R for viewing by the user when the user&#39;s eyes are located in eye boxes  2 . 5 - 34  at the rear of device  2 . 5 - 10 . By displaying pass-through images (pass-through video) to the user in this way, the user may be provided with real-time information on the user&#39;s surroundings. If desired, virtual content (e.g. computer-generated images) may be overlaid over some of the pass-through video. Device  2 . 5 - 10  may also operate in a non-pass-through-video mode in which components  2 . 5 - 78  and  2 . 5 - 66  are turned off and the user is provided only with movie content, game content, and/or other virtual content that does not contain real-time real-world images. 
     Input-output devices  2 . 5 - 22  of device  2 . 5 - 10  may gather user input that is used in controlling the operation of device  2 . 5 - 10 . As an example, a microphone in device  2 . 5 - 10  may gather voice commands. Buttons, touch sensors, force sensors, and other input devices may gather user input from a user&#39;s finger or other external object that is contacting device  2 . 5 - 10 . In some configurations, it may be desirable to monitor a user&#39;s hand gestures or the motion of other user body parts. This allows the user&#39;s hand locations or other body part locations to be replicated in a game or other virtual environment and allows the user&#39;s hand motions to serve as hand gestures (air gestures) that control the operation of device  2 . 5 - 10 . User input such as hand gesture input can be captured using cameras that operate at visible and infrared wavelengths such as tracking cameras (e.g., optical components  2 . 5 - 76  and  2 . 5 - 68 ). Tracking cameras such as these may also track fiducials and other recognizable features on controllers and other external accessories (additional devices  2 . 5 - 10  of system  2 . 5 - 8 ) during use of these controllers in controlling the operation of device  2 . 5 - 10 . If desired, tracking cameras can help determine the position and orientation of a handheld controller or wearable controller that senses its location and orientation by measuring the magnetic field produced by coil  2 . 5 - 54 . The use of tracking cameras may therefore help track hand motions and controller motions that are used in moving pointers and other virtual objects being displayed for a user and can otherwise assist in controlling the operation of device  2 . 5 - 10 . 
     Tracking cameras may operate satisfactorily in the presence of sufficient ambient light (e.g., bright visible ambient lighting conditions). In dim environments, supplemental illumination may be provided by supplemental light sources such as supplemental infrared light sources (e.g., optical components  2 . 5 - 82  and  2 . 5 - 84 ). The infrared light sources may each include one or more light-emitting devices (light-emitting diodes or lasers) and may each be configured to provide fixed and/or steerable beams of infrared light that serve as supplemental illumination for the tracking cameras. If desired, the infrared light sources may be turned off in bright ambient lighting conditions and may be turned on in response to detection of dim ambient lighting (e.g., using the ambient light sensing capabilities of optical component  2 . 5 - 62 ). 
     Three-dimensional sensors in device  2 . 5 - 10  may be used to perform biometric identification operations (e.g., facial identification for authentication), may be used to determine the three-dimensional shapes of objects in the user&#39;s environment (e.g., to map the user&#39;s environment so that a matching virtual environment can be created for the user), and/or to otherwise gather three-dimensional content during operation of device  2 . 5 - 10 . As an example, optical components  2 . 5 - 74  and  2 . 5 - 70  may be three-dimensional structured light image sensors. Each three-dimensional structured light image sensor may have one or more light sources that provide structured light (e.g., a dot projector that projects an array of infrared dots onto the environment, a structured light source that produces a grid of lines, or other structured light component that emits structured light). Each of the three-dimensional structured light image sensors may also include a flood illuminator (e.g., a light-emitting diode or laser that emits a wide beam of infrared light). Using flood illumination and structured light illumination, optical components  2 . 5 - 74  and  2 . 5 - 70  may capture facial images, images of objects in the environment surrounding device  2 . 5 - 10 , etc. 
     Optical component  2 . 5 - 72  may be an infrared three-dimensional time-of-flight camera that uses time-of-flight measurements on emitted light to gather three-dimensional images of objects in the environment surrounding device  2 . 5 - 10 . Component  2 . 5 - 72  may have a longer range and a narrower field of view than the three-dimensional structured light cameras of optical components  2 . 5 - 74  and  2 . 5 - 70 . The operating range of component  2 . 5 - 72  may be 30 cm to 7 m, 60 cm to 6 m, 70 cm to 5 m, or other suitable operating range (as examples). 
       FIGS.  2 . 5 - 4    is a front view of an illustrative ring-shaped cosmetic covering structure for device  2 . 5 - 10 . Illustrative ring-shaped shroud  2 . 5 - 100  of  FIGS.  2 . 5 - 4    may be mounted under the inner surface of the display cover layer for display  2 . 5 - 14 F in inactive area IA. This may help hide the optical components and other internal portions of device  2 . 5 - 10  from view from the exterior of device  2 . 5 - 10 . Shroud  2 . 5 - 100  may be formed from one or more unbroken ring-shaped members and/or may be formed from multiple shroud segments that are attached using adhesive, fasteners, or other attachment structures. If desired, shroud  2 . 5 - 100  may be formed from multiple members that are sandwiched together along some or all of their lengths. In an illustrative configuration, which may sometimes be described herein as an example, shroud  2 . 5 - 100  may be formed from an inner piece (e.g., an inner full or partial ring), which may sometimes be referred to as an inner shroud member, shroud trim, or shroud trim member and may be formed from an outer piece or pieces (e.g., one or more strips of material or covering members, an full ring, one or more partial rings, etc.), which may sometimes be referred to as a shroud cover, canopy, or shroud canopy. 
     As shown in  FIGS.  2 . 5 - 4   , shroud  2 . 5 - 100  may have optical component windows to accommodate components  2 . 5 - 60 ,  2 . 5 - 62 ,  2 . 5 - 64 ,  2 . 5 - 84 ,  2 . 5 - 66 ,  2 . 5 - 68 ,  2 . 5 - 70 ,  2 . 5 - 72 ,  2 . 5 - 74 ,  2 . 5 - 76 ,  2 . 5 - 78 ,  2 . 5 - 82 , and  2 . 5 - 80 . The optical component windows may be formed from through-hole openings in shroud  2 . 5 - 100 , from recesses or other partial openings that do not pass entirely through shroud  2 . 5 - 100 , from inserted optical window members in shroud through-hole openings, and/or from other shroud optical component window structures. Display  2 . 5 - 14 F may have a display cover layer that has corresponding optical component windows (through-hole openings, recessed areas, inserted window members in through-hole openings, etc.) and/or that is formed from bulk material that has desired optical properties (e.g., a display cover layer formed from one or more layers of material such as glass and/or polymer with sufficient transparency at the operating wavelength range of the overlapped optical component to allow the optical component to operate satisfactorily through the cover layer without forming openings or other window structures in the cover layer). 
     Shroud  2 . 5 - 100  may have any suitable shape. For example, the outline of shroud  2 . 5 - 100  may be rectangular with rounded corners as shown in  FIGS.  2 . 5 - 4   , may have teardrop shapes on the left and right sides of device  2 . 5 - 10 , may have an oval outline, and/or may have other outlines with curved and/or straight edge segments. For example, the inner and outer edges of shroud  2 . 5 - 100  may be curved (e.g., to follow a teardrop shape). Shroud  2 . 5 - 100  may, if desired, have a peripheral edge that is curved along most or all of its length. 
     The width of shroud  2 . 5 - 100  may be constant along its length or shroud  2 . 5 - 100  may have portions that are wider than others. The thickness of shroud  2 . 5 - 100  (e.g., the dimension of shroud  2 . 5 - 100  into the page in the orientation of  FIGS.  2 . 5 - 4   ) may be smaller than the width of shroud  2 . 5 - 100  (the lateral dimension of shroud  2 . 5 - 100  within the page in the orientation of  FIGS.  2 . 5 - 4   ) or the thickness of the shroud may be equal to or greater than the width of the shroud. The shroud may have a two-dimensional shape (e.g., shroud  2 . 5 - 100  may have a planar shape) or may have a three-dimensional shape (e.g., a shape with a curved cross-sectional profile and/or a shape characterized by inner and/or outer surfaces of compound curvature). In an illustrative configuration, most or all of the inner and outer surfaces of shroud have a compound-curvature surface. 
     The optical components under inactive area IA may include components on the left and right sides of device  2 . 5 - 10  that operate in conjunction with each other. For example, scene cameras, tracking cameras, and/or structured light cameras in device  2 . 5 - 10  may be formed in pairs, each of which includes a left camera and a corresponding right camera. A left scene camera and a right scene camera may, as an example, operate together to capture overlapping images that provide device  2 . 5 - 10  with a wide field of view for gathering pass-through video. Left and right tracking cameras may operate together to track a user&#39;s hands or other external objects. Left and right structured light cameras or other three-dimensional cameras may be used together to capture three-dimensional images of the user&#39;s environment. To enhance performance of the left and right optical components in these types of paired component arrangements, it may be desirable to maintain accurate alignment between the left and right optical components. To help maintain left and right optical components on the respective left and right sides of device  2 . 5 - 10  in alignment with each other, device  2 . 5 - 10  may be provided with one or more housing structures that help support the optical components. An illustrative example of device  2 . 5 - 10  having housing structures that support the optical components and a cover layer that overlaps the optical components is shown in  FIGS.  2 . 5 - 5   . 
     As shown in  FIGS.  2 . 5 - 5   , shroud  2 . 5 - 100  and display cover layer  2 . 5 - 92  may be attached to housing  2 . 5 - 26  using adhesive, screws and other fasteners, press-fit connections, and/or other attachment mechanisms. An illustrative configuration in which shroud  2 . 5 - 100  and cover layer  2 . 5 - 92  are attached to forward-facing edge of a housing wall in the main housing portion of structure  2 . 5 - 26  using adhesive is shown in  FIGS.  2 . 5 - 5   . In the example of  FIGS.  2 . 5 - 5   , shroud  2 . 5 - 100  has an inner shroud member such as shroud trim  2 . 5 - 100 A and has a corresponding outer shroud member such as shroud canopy  2 . 5 - 100 B. Shroud trim  2 . 5 - 100 A and shroud canopy  2 . 5 - 100 B may be formed from metal, polymer, ceramic, glass, other materials, and/or combinations of these materials. In an illustrative example, shroud trim  2 . 5 - 100 A is formed from black polymer or other dark material and shroud canopy  2 . 5 - 100 B is formed from clear polymer. The outer surface of shroud canopy  2 . 5 - 100 B may be smooth to provide shroud  2 . 5 - 100  with a cosmetically attractive appearance. 
     A layer of pressure sensitive adhesive may be used in attaching canopy  2 . 5 - 100 B to trim  2 . 5 - 100 A, or canopy  2 . 5 - 100 B may be formed integrally with trim  2 . 5 - 100 A. Adhesive may also be used in attaching cover layer  2 . 5 - 92  and shroud  2 . 5 - 100  to housing portion  2 . 5 - 26 . As shown in  FIGS.  2 . 5 - 5   , for example, a first adhesive such as adhesive  2 . 5 - 122  may be used to attach display cover layer  2 . 5 - 92  to shroud  2 . 5 - 100  (e.g., to a ledge in shroud trim  2 . 5 - 100 A). A second adhesive such as adhesive  2 . 5 - 124  may, in turn, be used to attach shroud  2 . 5 - 100  (e.g., shroud trim  2 . 5 - 100 A) to an adjacent lip of a wall in housing  2 . 5 - 26 . 
     In some configurations, adhesives  2 . 5 - 122  and  2 . 5 - 124  may be formed from the same type of material. In an illustrative configuration, adhesives  2 . 5 - 122  and  2 . 5 - 124  are different. Housing portion  2 . 5 - 26  may have a wall with a lip shape that creates a shearing force on adhesive  2 . 5 - 124  as display  2 . 5 - 14 F is attached to housing  2 . 5 - 26  by pressing display  2 . 5 - 14 F against housing  2 . 5 - 26 . In this type of scenario, it may be desirable to form adhesive  2 . 5 - 124  from an adhesive that can bond satisfactorily in the presence of shear forces such as a molten hot melt glue (thermoplastic adhesive) or other liquid adhesive rather than pressure sensitive adhesive. Adhesive  2 . 5 - 124  may, if desired, be exposed to a curing agent (ultraviolet light, moisture, etc.) before display  2 . 5 - 14 F is assembled into housing  2 . 5 - 26 . 
     It may be desirable to repair device  2 . 5 - 10 . For example, if a user exposes display  2 . 5 - 14 F to excessive force during a drop event, it may be desirable to replace display  2 . 5 - 14 F with a new display. This can be accomplished by heating adhesive  2 . 5 - 124  to loosen the adhesive bond formed by adhesive  2 . 5 - 124 . To help prevent display cover layer  2 . 5 - 92  from detaching from shroud  2 . 5 - 100  while softening adhesive  2 . 5 - 124  with heat, adhesive  2 . 5 - 122  may be provided with a higher-temperature softening point than adhesive  2 . 5 - 124  (e.g., adhesive  2 . 5 - 122  may be a two-part hot melt glue with a higher melting point than adhesive  2 . 5 - 124 ). 
     Optical components that are overlapped by display cover layer  2 . 5 - 92  and shroud  2 . 5 - 100  in inactive area IA may transmit and/or receive light through shroud  2 . 5 - 100  and display cover layer  2 . 5 - 92 . Layer  2 . 5 - 92  may be formed from a single layer of glass, laminated glass, or other clear material that allows light for each overlapped optical component  2 . 5 - 104  to pass through layer  2 . 5 - 92 . If desired, a partial recess or a through-hole opening may be formed in the portion of layer  2 . 5 - 92 . An optional optical component window member may then be inserted within layer  2 . 5 - 92  (e.g., a window that overlaps component  2 . 5 - 104 ). As an example, layer  2 . 5 - 92  may be formed from one or more layers of glass and/or polymer and may be characterized by a first level of light transmission at operating wavelength(s) for component  2 . 5 - 104 . A window member in layer  2 . 5 - 92  may be formed from polymer, glass, and/or other materials that are characterized by a second level of light transmission at the operating wavelength(s) that is greater than the first level of light transmission. In other illustrative arrangements, no window member is inserted in layer  2 . 5 - 92  (e.g., when layer  2 . 5 - 92  alone is sufficiently transparent to pass light for component  2 . 5 - 104 ). 
     Shroud  2 . 5 - 100  may be provided with an optical component window that overlaps optical component to help accommodate overlapped optical component  2 . 5 - 104 . Component  2 . 5 - 104  may operate at ultraviolet light wavelengths, visible light wavelengths, and/or infrared light wavelengths. To accommodate component  2 . 5 - 104 , shroud trim  2 . 5 - 100 A has been provided with a through-hole opening, whereas shroud canopy  2 . 5 - 100 B has no openings overlapping component  2 . 5 - 104 . This effectively forms a window recess in shroud  2 . 5 - 100  in alignment with components  2 . 5 - 104 . Trim  2 . 5 - 100 A may be formed from black polymer or other light-absorbing material, so the formation of opening  120  in trim  2 . 5 - 100 A may help ensure that sufficiently light may pass through to allow component  2 . 5 - 104  to operate satisfactorily. The portion of canopy  2 . 5 - 100 B that overlaps component  2 . 5 - 104  may be transparent (e.g., clear polymer). Alternatively, canopy  2 . 5 - 100 B may be formed from light-absorbing material, and a portion of canopy  2 . 5 - 100 B overlapping component  2 . 5 - 104  may be removed. 
     To help hide component  2 . 5 - 104  from view, the inner surface of shroud canopy  2 . 5 - 100 B may be covered with one or more coatings, which may be used to provide region the region overlapping component  2 . 5 - 104  with a desired outward appearance and optical properties that ensure that component  2 . 5 - 104  can operate satisfactorily. The coatings may include a thin-film-interference filter formed from a stack of thin-film dielectric layers of alternating refractive index values (with indices and thicknesses selected to create a desired transmission spectrum and a desired reflection spectrum for the filter), may include a layer of ink (e.g., a polymer layer including dye, pigment, and/or other colorant), and/or may include any other suitable coating with desired optical properties. 
     Consider, as an example, a scenario in which component  2 . 5 - 104  transmits and/or receives infrared light. In this type of arrangement, canopy  2 . 5 - 100 B may be coated with a coating that is opaque at visible wavelengths and transparent at infrared wavelengths. This helps to hide component  2 . 5 - 104  from view from the exterior of device  2 . 5 - 10  while allowing infrared light associated with the operation of component  2 . 5 - 104  to pass through shroud  2 . 5 - 100  and layer  2 . 5 - 92 . 
     As another example, consider a scenario in which component  2 . 5 - 104  is an ambient light sensor. In this configuration, canopy  2 . 5 - 100 B may be coated with a coating that exhibits a visible light transmission of 1-8% (as an example). This may allow sufficient visible ambient light to reach the ambient light sensor for the ambient light sensor to make an ambient light reading. At the same time, the transmission of the coating may be sufficiently low to reduce the visibility of component  2 . 5 - 104  from the exterior of device  2 . 5 - 10 . 
     As these examples demonstrate, regions of display  2 . 5 - 14 F that overlap optical components such as component  2 . 5 - 104  of  FIGS.  2 . 5 - 5    may be provided with optical component window structures in layer  2 . 5 - 92  and/or shroud  2 . 5 - 100  that help accommodate the optical component. 
     As described in connection with  FIGS.  2 . 5 - 3  and  2 . 5 - 4   , there may be numerous optical components such as component  2 . 5 - 104  in inactive area IA. Each optical component may potentially have a different type of optical component window structure in shroud  2 . 5 - 100  and/or layer  2 . 5 - 92  to accommodate that component. For example, some areas of shroud  2 . 5 - 100  may have openings that receive components, other areas of shroud  2 . 5 - 100  may have inserted optical window member, and/or other areas of shroud  2 . 5 - 100  may have partial shroud openings (e.g., non-through-hole recesses) such as the opening of  FIGS.  2 . 5 - 8    (which may optionally be coated to modify the optical properties of shroud  2 . 5 - 100 ). 
     Because of the proximity of cover layer  2 . 5 - 92  to a user&#39;s eyes, it may be desirable to reduce the likelihood of the cover layer material (e.g., glass) from shattering and injuring the user&#39;s eyes. An illustrative example of cover layer  2 . 5 - 92  with encapsulation to reduce the likelihood of such an event is shown in  FIGS.  2 . 5 - 6   . 
     As shown in  FIGS.  2 . 5 - 6   , cover layer  2 . 5 - 92  may be coupled to shroud  2 . 5 - 100 . Cover layer  2 . 5 - 92  may include glass layer  2 . 5 - 126 , front laminate  2 . 5 - 128  and rear laminate  2 . 5 - 130 . Front laminate  2 . 5 - 128  and rear laminate  2 . 5 - 128  may be for example, layers of plastic that are laminated to cover layer  2 . 5 - 92 , layers of plastic that are adhesively attached to cover layer  2 . 5 - 92 , or other shatter-resistant material that is attached to the front and rear surfaces of glass layer  2 . 5 - 126 . Although not shown in  FIGS.  2 . 5 - 6   , multiple layers, such as antireflection coatings, antismudge coatings, acrylic layers, or other desired layers, may be included as part of cover layer  2 . 5 - 92 . 
     Although front laminate  2 . 5 - 128  and rear laminate  2 . 5 - 130  may reduce the chances of glass layer  2 . 5 - 126  shattering toward the front or rear of device  2 . 5 - 10 , an edge surface of glass layer  2 . 5 - 126  may still be exposed. In the case of a shatter event, such as if device  2 . 5 - 10  were dropped, glass layer  2 . 5 - 126  could shatter and glass could exit device  2 . 5 - 10  from the edge surface of glass layer  2 . 5 - 126 . To mitigate this risk, encapsulation material  2 . 5 - 132  may be attached to the edge surface of glass layer  2 . 5 - 126 . Encapsulation material  2 . 5 - 132  may be an epoxy material, such as a ductile epoxy, that seals the edge surface of glass layer  2 . 5 - 126  and prevents glass layer  2 . 5 - 126  from shattering at the edge surface. Alternatively, acrylate, polyvinyl butyral (PVB), polyurethane, or moisture cure materials may be used for encapsulation material  2 . 5 - 132 . In general, encapsulation material  2 . 5 - 132  may be formed from material that adheres to glass layer  2 . 5 - 126 , while preventing glass layer  2 . 5 - 126  from shattering. 
     Encapsulation material  2 . 5 - 132  may substantially fill the opening between the edge surface of glass layer  2 . 5 - 126  and shroud  2 . 5 - 100 . For example, encapsulation material  2 . 5 - 132  may extend approximately 150 microns from the edge surface. In general, however, any amount of encapsulation material  2 . 5 - 132  may be applied to the edge surface. 
     As shown in  FIGS.  2 . 5 - 6   , encapsulation material  2 . 5 - 132  may cover the edge surface and may also cover an edge portion of laminate  2 . 5 - 128 . However, this is merely illustrative. If desired, encapsulation material  2 . 5 - 132  may cover the edge surface of glass layer  2 . 5 - 126  without covering an edge portion of laminate  2 . 5 - 128 . For example, as shown in  FIGS.  2 . 5 - 7   , encapsulation material may cover only the edge surface of glass layer  2 . 5 - 126 . In the example of  FIG.  7   , laminate  2 . 5 - 128  may extend over encapsulation material  2 . 5 - 132 . However, this is merely illustrative. Laminate  2 . 5 - 128  may be flush with the edge surface of glass layer  2 . 5 - 126 , if desired. 
     In some embodiments, it may be determined that the shattering risk of the edge surface of glass layer  2 . 5 - 126  can be mitigated by modifying the position of glass layer  2 . 5 - 126  relative to shroud  2 . 5 - 100  (or support structure  2 . 5 - 126 ). For example, as shown in the illustrative embodiment of  FIGS.  2 . 5 - 8   , the edge surface of glass layer  2 . 5 - 126  may be left unencapsulated, but the size of opening  2 . 5 - 134  between the edge surface and shroud  2 . 5 - 100  may be adjusted to reduce the risk of glass escaping through opening  2 . 5 - 134  in the case of a shatter event. For example, if glass layer  2 . 5 - 126  is sufficiently close to shroud  2 . 5 - 100  (e.g., if opening  2 . 5 - 134  is sufficiently small), glass from layer  2 . 5 - 126  may not escape through opening  2 . 5 - 134  if layer  2 . 5 - 126  shatters. 
     Instead of, or in addition to, adding material to the edge surface of layer  2 . 5 - 126 , it may be desirable to add material in the gap between layer  2 . 5 - 126  and the shroud/support structure. An illustrative example of adding material in this gap is shown in  FIGS.  2 . 5 - 9   . 
     As shown in  FIGS.  2 . 5 - 9   , material  2 . 5 - 136  may be included between the edge surface of glass layer  2 . 5 - 126  and shroud  2 . 5 - 100 . Material  2 . 5 - 136  may be, for example, a bumper ring. The bumper ring may be formed from elastomer, rigid plastic, or another material that helps protect the edge surface of layer  2 . 5 - 126 . 
     As an alternative to material  2 . 5 - 136  being a bumper ring between layer  2 . 5 - 126  and shroud  2 . 5 - 100 , material  2 . 5 - 136  may be an overmolded structure on layer  2 . 5 - 126 , on shroud  2 . 5 - 100 , or on a chassis that is coupled to support structure  26 . In general, the overmolded structure may fill the gap between layer  2 . 5 - 126  and the support structure, shroud, and/or chassis and help to prevent the edge surface of layer  2 . 5 - 126  from shattering out of device  2 . 5 - 10 . 
     Although not shown in  FIGS.  2 . 5 - 9   , a portion of material  2 . 5 - 136  may extend underneath layer  2 . 5 - 126  if desired. In particular, there may be a portion of material  2 . 5 - 136  between the bottom surface of layer  2 . 5 - 126  and shroud  2 . 5 - 100 . 
     Instead of, or in addition to, adding material between layer  2 . 5 - 126  and shroud  2 . 5 - 100 , upper laminate  2 . 5 - 128  and/or lower laminate  2 . 5 - 130  may wrap around the edge surface of layer  2 . 5 - 126 . Illustrative examples of the laminates wrapping the edge surface are shown in  FIGS.  2 . 5 - 10  and  2 . 5 - 11   . 
     As shown in  FIGS.  2 . 5 - 10   , upper laminate  2 . 5 - 128  may wrap around the edge surface of layer  2 . 5 - 126 . In particular, upper laminate  2 . 5 - 128  may have portion  2 . 5 - 128 A that extends around and covers the edge surface of layer  2 . 5 - 126 . Layer  2 . 5 - 126  may have a rounded edge surface to allow upper laminate  2 . 5 - 128  to wrap the edge surface and sufficiently adhere to the surface, as shown in  FIGS.  2 . 5 - 10   . By forming layer  2 . 5 - 126  with a rounded edge, the curve of laminate  2 . 5 - 128  around the edge may be reduced, thereby reducing stress on laminate  2 . 5 - 128 . However, layer  2 . 5 - 126  may have a planar edge surface, or a surface with any other desired profile, around which upper laminate  2 . 5 - 128  wraps, if desired. By wrapping upper laminate around the edge surface of layer  2 . 5 - 126 , glass may be prevented from shattering out of the edge surface. 
     As shown in  FIGS.  2 . 5 - 11   , lower laminate  2 . 5 - 130  may wrap around the edge surface of layer  2 . 5 - 126 . In particular, lower laminate may have portion  2 . 5 - 130 A that extends around and covers the edge surface of layer  2 . 5 - 126 . Layer  2 . 5 - 126  may have a rounded edge surface to allow lower laminate  2 . 5 - 130  to wrap the edge surface and sufficiently adhere to the surface, may have a planar edge surface, or may have a surface with any other desired profile, around which lower laminate  2 . 5 - 130  wraps. By wrapping lower laminate  2 . 5 - 130  around the edge surface of layer  2 . 5 - 126 , glass may be prevented from shattering out of the edge surface. 
     In the example of  FIGS.  2 . 5 - 11   , lower laminate portion  2 . 5 - 130 A wraps entirely around the edge surface of layer  2 . 5 - 126  and partially overlaps upper laminate  2 . 5 - 128 . This arrangement may ensure that no glass can escape if layer  2 . 5 - 126  shatters. However, the arrangement is merely illustrative. If desired, lower laminate portion  2 . 5 - 130 A may wrap around only the edge surface of layer  2 . 5 - 126  without overlapping or extending over upper laminate  2 . 5 - 128 . 
     Another example of material that may be used to prevent layer  2 . 5 - 126  from shattering and glass from exiting through the edge surface is shown in  FIGS.  2 . 5 - 12   . In the example of  FIGS.  2 . 5 - 12   , glue (or another similar material)  2 . 5 - 138  may be used to completely fill the gap between layer  2 . 5 - 126  and shroud  2 . 5 - 100 . For example, the glue may be inserted into the gap after cover layer  2 . 5 - 92  has been assembled into the head-mounted device. Glue  2 . 5 - 138  may help prevent glass from exiting through the edge surface of layer  2 . 5 - 126  if layer  2 . 5 - 126  shatters. 
     Rather than wrapping the upper or lower laminate around the edge surface of layer  2 . 5 - 126 , upper laminate  2 . 5 - 128  may extend to shroud  2 . 5 - 100  to cover the gap between the edge surface and shroud  2 . 5 - 100 . For example, as shown in  FIGS.  2 . 5 - 13   , upper laminate  2 . 5 - 128  may have portion  2 . 5 - 128 B that extends to shroud  2 . 5 - 100  (or support structure  2 . 5 - 26  or another portion of device  2 . 5 - 10 ). By covering the gap between the edge surface of layer  2 . 5 - 126  and shroud  2 . 5 - 100 , glass may prevented from exiting device  2 . 5 - 10  during a shatter event. 
     Instead of, or in addition to, adding material or extending the laminates to prevent glass from shattering out of device  2 . 5 - 10 , shroud  2 . 5 - 100  or a chassis attached to support structure  2 . 5 - 26  may be modified to reduce the risk of layer  2 . 5 - 126  shattering. Illustrative examples of modifying these components to reduce the risk of layer  2 . 5 - 126  shattering are shown in  FIGS.  2 . 5 - 14  and  2 . 5 - 15   . 
     As shown in  FIGS.  2 . 5 - 14   , structure  2 . 5 - 140  may have a lip that covers the gap between layer  2 . 5 - 126  and shroud  2 . 5 - 100 /support structure  2 . 5 - 126 . Structure  2 . 5 - 140  may be formed from a portion of shroud  2 . 5 - 100  or from a portion of support structure  2 . 5 - 126  (e.g., a chassis of support structure  2 . 5 - 126 ). The lip of structure  2 . 5 - 140  may help prevent glass that would otherwise shatter from layer  2 . 5 - 126  and exit device  2 . 5 - 10  from exiting device  2 . 5 - 10 , thereby protecting users of device  2 . 5 - 10 . 
     If desired, the lip of structure  2 . 5 - 140  may be combined with the extension of laminate around the edge surface of layer  2 . 5 - 126 . For example, as shown in  FIGS.  2 . 5 - 15   , upper laminate  2 . 5 - 128 A may wrap the edge surface of layer  2 . 5 - 126  to prevent glass from exiting through the edge surface, and the lip of structure  2 . 5 - 140  may provide additional protection should some glass get through the laminate. 
     Although cover layer  2 . 5 - 92  has been described as being coupled to shroud  2 . 5 - 100 , this is merely illustrative. In some embodiments, cover layer  2 . 5 - 92  may be coupled directly to support structure  2 . 5 - 26 . In other embodiments, device  2 . 5 - 10  may include a chassis attached to support structure  2 . 5 - 26  (e.g., a chassis to support various components in device  2 . 5 - 10 ), and cover layer  2 . 5 - 92  may be coupled to the chassis. 
     Moreover, although cover layer  2 . 5 - 92  has been described as including a glass layer that may shatter, this material is merely illustrative. Layer  2 . 5 - 126  may be formed from ceramic, sapphire, or any other desired material. 
     2.6: Electronic Devices with Antennas and Optical Components 
     An electronic device such as a head-mounted device may have a front face that faces away from a user&#39;s head and may have an opposing rear face that faces the user&#39;s head. Optical modules may be used to provide images to a user&#39;s eyes. The positions of the optical modules may be adjusted to accommodate different user interpupillary distances. The head-mounted device may have actuators and optical module guide structures to allow the optical module positions to be adjusted. 
     The head-mounted device may have wireless communications circuitry to communicate with external equipment such as a computer, cellular telephone, or other computing device. This allows the external equipment to provide the head-mounted device with content for viewing on the head-mounted device and/or allows the head-mounted device to otherwise interact with the remote equipment. The wireless communications circuitry may include multiple antennas. 
     The head-mounted device may have one or more cameras. For example, forward-facing (front-facing) cameras may allow the head-mounted device to monitor movement of the head-mounted device relative to the environment surrounding the head-mounted device (e.g., the cameras may be used in forming a visual odometry system or part of a visual inertial odometry system). Forward-facing cameras may also be used to capture images of the environment that are displayed to a user of the head-mounted device. If desired, images from multiple forward-facing cameras may be merged with each other and/or forward-facing camera content can be merged with computer-generated content for a user. 
     A top view of an illustrative head-mounted device is shown in  FIGS.  2 . 6 - 1   . As shown in  FIGS.  2 . 6 - 1   , head-mounted devices such as electronic device  2 . 6 - 10  may have head-mounted support structures such as housing  2 . 6 - 12 . Housing  2 . 6 - 12  may include portions (e.g., head-mounted support structures  2 . 6 - 12 T) to allow device  2 . 6 - 10  to be worn on a user&#39;s head. Support structures  2 . 6 - 12 T may be formed from fabric, polymer, metal, and/or other material. Support structures  2 . 6 - 12 T may form a strap or other head-mounted support structures to help support device  2 . 6 - 10  on a user&#39;s head. A main support structure (e.g., a head-mounted housing such as main housing portion  2 . 6 - 12 M) of housing  2 . 6 - 12  may support electronic components such as displays  2 . 6 - 14 . 
     Main housing portion  2 . 6 - 12 M may include housing structures formed from metal, polymer, glass, ceramic, and/or other material. For example, housing portion  2 . 6 - 12 M may have housing walls on front face F and housing walls on adjacent top, bottom, left, and right side faces that are formed from rigid polymer or other rigid support structures and these rigid walls may optionally be covered with electrical components, fabric, leather, or other soft materials, etc. Housing portion  2 . 6 - 12 M may also have internal support structures such as a frame and/or structures that perform multiple functions such as controlling airflow while providing structural support. The walls of housing portion  2 . 6 - 12 M may enclose internal components  2 . 6 - 38  in interior region  2 . 6 - 34  of device  2 . 6 - 10  and may separate interior region  2 . 6 - 34  from the environment surrounding device  2 . 6 - 10  (exterior region  2 . 6 - 36 ). Internal components  2 . 6 - 38  may include integrated circuits, actuators, batteries, sensors, and/or other circuits and structures for device  2 . 6 - 10 . Housing  2 . 6 - 12  may be configured to be worn on a head of a user and may form glasses, a hat, a helmet, goggles, and/or other head-mounted device. Configurations in which housing  2 . 6 - 12  forms goggles may sometimes be described herein as an example. 
     Front face F of housing  2 . 6 - 12  may face outwardly away from a user&#39;s head and face. Opposing rear face R of housing  2 . 6 - 12  may face the user. Portions of housing  2 . 6 - 12  (e.g., portions of main housing  2 . 6 - 12 M) on rear face R may form a cover such as cover  2 . 6 - 12 C (sometimes referred to as a curtain). The presence of cover  2 . 6 - 12 C on rear face R may help hide internal housing structures, internal components  2 . 6 - 38 , and other structures in interior region  2 . 6 - 34  from view by a user. 
     Device  2 . 6 - 10  may have one more cameras such as cameras  2 . 6 - 46 . For example, device  2 . 6 - 10  may have K cameras, where the value of K is at least one, at least two, at least four, at least six, at least eight, at least ten, at least 12, less than 20, less than 14, less than 12, less than 10, from 4 to 10, or other suitable value. Cameras  2 . 6 - 46  may be sensitive at infrared wavelengths (e.g., cameras  2 . 6 - 46  may be infrared cameras), may be sensitive at visible wavelengths (e.g., cameras  2 . 6 - 46  may be visible cameras), and/or cameras  2 . 6 - 46  may be sensitive at other wavelengths. If desired, cameras  2 . 6 - 46  may be sensitive at both visible and infrared wavelengths. 
     Cameras  2 . 6 - 46  that are mounted on front face F and that face outwardly (towards the front of device  2 . 6 - 10  and away from the user) may sometimes be referred to herein as forward-facing or front-facing cameras. Forward-facing cameras (e.g., cameras  2 . 6 - 46  of  FIGS.  2 . 6 - 1   ) may include a first set of two or more front-facing cameras on the left side of front face F of device  2 . 6 - 10  and/or may include a second set of two or more front-facing cameras on the right side of front face F of device  2 . 6 - 10 . Cameras  2 . 6 - 46  may also be provided elsewhere in housing portion  2 . 6 - 12 M. Cameras  2 . 6 - 46  may, if desired, include cameras that are oriented at a slight angle relative to the −Z axis of  FIGS.  2 . 6 - 1   . For example, some of cameras  2 . 6 - 46  may be oriented directly ahead, whereas some cameras  2 . 6 - 46  along the left and right edges of front face F may be respectively angled slightly to the left and right of the −Z axis to capture peripheral images on the left and right. Cameras  2 . 6 - 46  may capture visual odometry information, image information that is processed to locate objects in the user&#39;s field of view (e.g., so that virtual content can be registered appropriately relative to real-world objects), image content that is displayed in real time for a user of device  2 . 6 - 10 , and/or other suitable image data. 
     Device  2 . 6 - 10  may have left and right optical modules  2 . 6 - 40 . Optical modules  2 . 6 - 40  support electrical and optical components such as light-emitting components and lenses and may therefore sometimes be referred to as optical assemblies, optical systems, optical component support structures, lens and display support structures, electrical component support structures, or housing structures. Each optical module may include a respective display  2 . 6 - 14 , lens  2 . 6 - 30 , and support structure such as support structure  2 . 6 - 32 . Support structure  2 . 6 - 32 , which may sometimes be referred to as a lens support structure, optical component support structure, optical module support structure, or optical module portion, or lens barrel, may include hollow cylindrical structures with open ends or other supporting structures to house displays  2 . 6 - 14  and lenses  2 . 6 - 30 . Support structures  2 . 6 - 32  may, for example, include a left lens barrel that supports a left display  2 . 6 - 14  and left lens  2 . 6 - 30  and a right lens barrel that supports a right display  2 . 6 - 14  and right lens  2 . 6 - 30 . 
     Displays  2 . 6 - 14  may include arrays of pixels or other display devices to produce images. Displays  2 . 6 - 14  may, for example, include organic light-emitting diode pixels formed on substrates with thin-film circuitry and/or formed on semiconductor substrates, pixels formed from crystalline semiconductor dies, liquid crystal display pixels, scanning display devices, and/or other display devices for producing images. 
     Lenses  2 . 6 - 30  may include one or more lens elements for providing image light from displays  2 . 6 - 14  to respective eyes boxes  2 . 6 - 13 . Lenses may be implemented using refractive glass lens elements, using mirror lens structures (catadioptric lenses), using Fresnel lenses, using holographic lenses, and/or other lens systems. 
     When a user&#39;s eyes are located in eye boxes  2 . 6 - 13 , displays (display panels)  2 . 6 - 14  operate together to form a display for device  2 . 6 - 10  (e.g., the images provided by respective left and right optical modules  2 . 6 - 40  may be viewed by the user&#39;s eyes in eye boxes  2 . 6 - 13  so that a stereoscopic image is created for the user). The left image from the left optical module fuses with the right image from a right optical module while the display is viewed by the user. 
     It may be desirable to monitor the user&#39;s eyes while the user&#39;s eyes are located in eye boxes  2 . 6 - 13 . For example, it may be desirable to use a camera to capture images of the user&#39;s irises (or other portions of the user&#39;s eyes) for user authentication. It may also be desirable to monitor the direction of the user&#39;s gaze. Gaze tracking information may be used as a form of user input and/or may be used to determine where, within an image, image content resolution should be locally enhanced in a foveated imaging system. To ensure that device  2 . 6 - 10  can capture satisfactory eye images while a user&#39;s eyes are located in eye boxes  2 . 6 - 13 , each optical module  2 . 6 - 40  may be provided with a camera such as camera  2 . 6 - 42  and one or more light sources such as light-emitting diodes  2 . 6 - 44  or other light-emitting devices such as lasers, lamps, etc. Cameras  2 . 6 - 42  and light-emitting diodes  2 . 6 - 44  may operate at any suitable wavelengths (visible, infrared, and/or ultraviolet). As an example, diodes  2 . 6 - 44  may emit infrared light that is invisible (or nearly invisible) to the user. This allows eye monitoring operations to be performed continuously without interfering with the user&#39;s ability to view images on displays  2 . 6 - 14 . 
     Not all users have the same interpupillary distance IPD. To provide device  2 . 6 - 10  with the ability to adjust the interpupillary spacing between modules  2 . 6 - 40  along lateral dimension X and thereby adjust the spacing IPD between eye boxes  2 . 6 - 13  to accommodate different user interpupillary distances, device  2 . 6 - 10  may be provided with optical module positioning systems in housing  2 . 6 - 12 . The positioning systems may have guide members and actuators  2 . 6 - 43  that are used to position optical modules  2 . 6 - 40  with respect to each other. 
     Actuators  2 . 6 - 43  can be manually controlled and/or computer-controlled actuators (e.g., computer-controlled motors) for moving support structures (lens barrels)  2 . 6 - 32  relative to each other. Information on the locations of the user&#39;s eyes may be gathered using, for example, cameras  2 . 6 - 42 . The locations of eye boxes  2 . 6 - 13  can then be adjusted accordingly. 
     As shown in the rear view of device  2 . 6 - 10  of  FIGS.  2 . 6 - 2   , cover  2 . 6 - 12 C may cover rear face R while leaving lenses  2 . 6 - 30  of optical modules  2 . 6 - 40  uncovered (e.g., cover  2 . 6 - 12 C may have openings that are aligned with and receive modules  2 . 6 - 40 ). As modules  2 . 6 - 40  are moved relative to each other along dimension X to accommodate different interpupillary distances for different users, modules  2 . 6 - 40  move relative to fixed housing structures such as the walls of main portion  2 . 6 - 12 M and move relative to each other. 
     A schematic diagram of an illustrative electronic device such as a head-mounted device or other wearable device is shown in  FIGS.  2 . 6 - 3   . Device  2 . 6 - 10  of  FIGS.  2 . 6 - 3    may be operated as a stand-alone device and/or the resources of device  2 . 6 - 10  may be used to communicate with external electronic equipment. As an example, communications circuitry in device  2 . 6 - 10  may be used to transmit user input information, sensor information, and/or other information to external electronic devices (e.g., wirelessly or via wired connections). Each of these external devices may include components of the type shown by device  2 . 6 - 10  of  FIGS.  2 . 6 - 3   . 
     As shown in  FIGS.  2 . 6 - 3   , a head-mounted device such as device  2 . 6 - 10  may include control circuitry  2 . 6 - 20 . Control circuitry  2 . 6 - 20  may include storage and processing circuitry for supporting the operation of device  2 . 6 - 10 . The storage and processing circuitry may include storage such as nonvolatile memory (e.g., flash memory or other electrically-programmable-read-only memory configured to form a solid state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Processing circuitry in control circuitry  2 . 6 - 20  may be used to gather input from sensors and other input devices and may be used to control output devices. The processing circuitry may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors and other wireless communications circuits, power management units, audio chips, application specific integrated circuits, etc. During operation, control circuitry  2 . 6 - 20  may use display(s)  2 . 6 - 14  and other output devices in providing a user with visual output and other output. 
     To support communications between device  2 . 6 - 10  and external equipment, control circuitry  2 . 6 - 20  may communicate using communications circuitry  2 . 6 - 22 . Circuitry  2 . 6 - 22  may include antennas, radio-frequency transceiver circuitry, and other wireless communications circuitry and/or wired communications circuitry. Circuitry  2 . 6 - 22 , which may sometimes be referred to as control circuitry and/or control and communications circuitry, may support bidirectional wireless communications between device  2 . 6 - 10  and external equipment (e.g., a companion device such as a computer, cellular telephone, or other electronic device, an accessory such as a point device, computer stylus, or other input device, speakers or other output devices, etc.) over a wireless link. For example, circuitry  2 . 6 - 22  may include radio-frequency transceiver circuitry such as wireless local area network transceiver circuitry configured to support communications over a wireless local area network link, near-field communications transceiver circuitry configured to support communications over a near-field communications link, cellular telephone transceiver circuitry configured to support communications over a cellular telephone link, or transceiver circuitry configured to support communications over any other suitable wired or wireless communications link. Wireless communications may, for example, be supported over a Bluetooth® link, a WiFi® link, a wireless link operating at a frequency between 10 GHz and 400 GHz, a 60 GHz link, or other millimeter wave link, a cellular telephone link, or other wireless communications link. Device  2 . 6 - 10  may, if desired, include power circuits for transmitting and/or receiving wired and/or wireless power and may include batteries or other energy storage devices. For example, device  2 . 6 - 10  may include a coil and rectifier to receive wireless power that is provided to circuitry in device  2 . 6 - 10 . 
     Device  2 . 6 - 10  may include input-output devices such as devices  2 . 6 - 24 . Input-output devices  2 . 6 - 24  may be used in gathering user input, in gathering information on the environment surrounding the user, and/or in providing a user with output. Devices  2 . 6 - 24  may include one or more displays such as display(s)  2 . 6 - 14 . Display(s)  2 . 6 - 14  may include one or more display devices such as organic light-emitting diode display panels (panels with organic light-emitting diode pixels formed on polymer substrates or silicon substrates that contain pixel control circuitry), liquid crystal display panels, microelectromechanical systems displays (e.g., two-dimensional mirror arrays or scanning mirror display devices), display panels having pixel arrays formed from crystalline semiconductor light-emitting diode dies (sometimes referred to as microLEDs), and/or other display devices. 
     Sensors  2 . 6 - 16  in input-output devices  2 . 6 - 24  may include force sensors (e.g., strain gauges, capacitive force sensors, resistive force sensors, etc.), audio sensors such as microphones, touch and/or proximity sensors such as capacitive sensors such as a touch sensor that forms a button, trackpad, or other input device), and other sensors. If desired, sensors  2 . 6 - 16  may include optical sensors such as optical sensors that emit and detect light, ultrasonic sensors, optical touch sensors, optical proximity sensors, and/or other touch sensors and/or proximity sensors, monochromatic and color ambient light sensors, image sensors (e.g., cameras), fingerprint sensors, iris scanning sensors, retinal scanning sensors, and other biometric sensors, temperature sensors, sensors for measuring three-dimensional non-contact gestures (“air gestures”), pressure sensors, sensors for detecting position, orientation, and/or motion (e.g., accelerometers, magnetic sensors such as compass sensors, gyroscopes, and/or inertial measurement units that contain some or all of these sensors), health sensors such as blood oxygen sensors, heart rate sensors, blood flow sensors, and/or other health sensors, radio-frequency sensors, three-dimensional camera systems such as depth sensors (e.g., structured light sensors and/or depth sensors based on stereo imaging devices that capture three-dimensional images) and/or optical sensors such as self-mixing sensors and light detection and ranging (LIDAR) sensors that gather time-of-flight measurements (e.g., time-of-flight cameras), humidity sensors, moisture sensors, gaze tracking sensors, electromyography sensors to sense muscle activation, facial sensors, and/or other sensors. In some arrangements, device  2 . 6 - 10  may use sensors  2 . 6 - 16  and/or other input-output devices to gather user input. For example, buttons may be used to gather button press input, touch sensors overlapping displays can be used for gathering user touch screen input, touch pads may be used in gathering touch input, microphones may be used for gathering audio input (e.g., voice commands), accelerometers may be used in monitoring when a finger contacts an input surface and may therefore be used to gather finger press input, etc. 
     If desired, electronic device  2 . 6 - 10  may include additional components (see, e.g., other devices  2 . 6 - 18  in input-output devices  2 . 6 - 24 ). The additional components may include haptic output devices, actuators for moving movable housing structures, audio output devices such as speakers, light-emitting diodes for status indicators, light sources such as light-emitting diodes that illuminate portions of a housing and/or display structure, other optical output devices, and/or other circuitry for gathering input and/or providing output. Device  2 . 6 - 10  may also include a battery or other energy storage device, connector ports for supporting wired communication with ancillary equipment and for receiving wired power, and other circuitry. 
     Housing  2 . 6 - 12  may include support structures for optical modules  2 . 6 - 40  and other components of device  2 . 6 - 10 . In an illustrative configuration, housing  2 . 6 - 12  may include a head-mounted support structure such as frame  2 . 6 - 12 I of  FIGS.  2 . 6 - 4   . Frame  2 . 6 - 12 I may have support structures that run vertically (e.g., frame portion  2 . 6 - 12 I-M in the middle of device  2 . 6 - 10  that are aligned with the user&#39;s nose bridge) and may have support structures that run horizontally across the top edge of housing  2 . 6 - 12 , along the bottom of edge of housing  2 . 6 - 12 , and along the left and right edges of housing  2 . 6 - 12  (see, e.g., peripheral edge portion  2 . 6 - 12 I-E). This forms left and right openings in frame  2 . 6 - 12 I that receive, respectively, left and right optical modules  2 . 6 - 40 . There may, in general, be one or more supporting members in device housing  2 . 6 - 12  that help create housing portion  2 . 6 - 12 M and that support the components in housing portion  2 . 6 - 12 M. The frame  2 . 6 - 12 I of  FIGS.  2 . 6 - 4    is illustrative. 
     As shown in  FIGS.  2 . 6 - 4   , one or more component support structures such as camera support structure  2 . 6 - 50  may be coupled to frame  2 . 6 - 12 I (e.g., left and right camera support structures  2 . 6 - 50  may be attached to respective left and right peripheral edges such as edge portions  2 . 6 - 12 I-E of frame  2 . 6 - 12 I). Support structures for device  2 . 6 - 10  such as frame  2 . 6 - 12 I and camera support structure  2 . 6 - 50  may be formed from polymer, glass, ceramic, metal, carbon-fiber composite material or other fiber-composite material, other materials, and/or combinations of these materials (e.g., sheets of rigid polymer or other material, and/or other structural members). 
     There may be multiple component support structures coupled to frame  2 . 6 - 12 I. For example, there may be a right-hand camera support structure  2 . 6 - 50  coupled to a right side of frame  2 . 6 - 12 I and a left-hand camera support structure  2 . 6 - 50  coupled to a left side of frame  2 . 6 - 12 I. A single side of frame  2 . 6 - 12 I and corresponding camera support structure  2 . 6 - 50  is shown in the example of  FIGS.  2 . 6 - 4   . 
     Camera support structure  2 . 6 - 50  may be coupled to frame  2 . 6 - 12 I using adhesive, welds, screws or other fasteners, mating engagement structures (e.g., recesses and protrusions for forming a snap fit), press-fit connections, and/or other coupling arrangements. In the example of  FIGS.  2 . 6 - 4   , fasteners  2 . 6 - 56  (e.g., threaded fasteners such as screws) pass through through-hole openings  2 . 6 - 54  of camera support structure  2 . 6 - 50  and are received in corresponding openings  2 . 6 - 52  of frame  2 . 6 - 12 I. Openings  2 . 6 - 52  may be threaded openings or may be unthreaded through-hole openings in configurations in which fasteners  2 . 6 - 56  are supplied with corresponding threaded nuts (as examples). 
     Camera support structure  2 . 6 - 50  may be configured to receive cameras  2 . 6 - 46  (e.g., structure may have recesses, openings, and/or other structures configured to receive front-facing cameras). As an example, camera support structures  2 . 6 - 50  may have at least two openings  2 . 6 - 58  (e.g., through-hole openings), each of which is configured to receive an associated camera. Each camera  2 . 6 - 46 , which may sometimes be referred to as a camera module, may have a camera module housing and may have a lens and image sensor coupled to the camera module housing. Cameras  2 . 6 - 46  may be sensitive to any suitable wavelengths of light (e.g., infrared, visible, both infrared and visible, and/or other wavelengths), may be stereoscopic (three-dimensional) cameras or two-dimensional cameras, may be time-of-flight cameras, may be structured light three-dimensional cameras may be cameras that gather information for use in placing virtual objects in a scene containing real-world and virtual content, may be cameras that are used as part of a visual odometry system, and/or may be other imaging systems. If desired, other optical components may be mounted to camera mounting structure  2 . 6 - 50 . For example, ambient light sensors, proximity sensors, and/or other components that emit and/or detect light may be mounted to structure  2 . 6 - 50 . Configurations in which two or more cameras  2 . 6 - 46  are attached to each camera mounting structure  2 . 6 - 50  may sometimes be described herein as an example. 
     When cameras  2 . 6 - 46  are received within respective openings  2 . 6 - 58  of a rigid unitary camera support structure  2 . 6 - 50  and/or are otherwise mounted to camera support structure  2 . 6 - 50 , the relative position of these cameras becomes fixed. This ensures that the direction in which each camera is pointing (e.g., the orientation of the camera&#39;s field of view) is fixed relative to the other, thereby helping to avoid misalignment issues arising from cameras orientations that vary during use of device  2 . 6 - 10 . By attaching camera support structure  2 . 6 - 50  to frame  2 . 6 - 12 I, the rigidity and strength of frame  2 . 6 - 12 I may be enhanced. This helps ensure that housing portion  2 . 6 - 12 M is sturdy and able to maintain sensitive components such as optical modules  2 . 6 - 40  in alignment with each other in the event that device  2 . 6 - 10  is subjected to an undesired drop event. 
     Camera support structure  2 . 6 - 50  may be formed from a layer of polymer or other material with optional ribs and/or other features to help strengthen structure  2 . 6 - 50  without adding excessive weight. To help maintain the rigidity and strength of camera support structure  2 . 6 - 50 , support structure  2 . 6 - 50  may be partly or completely free of large notches along the periphery of structure  2 . 6 - 50 . This may help ensure that there are no portions with locally narrowed widths along the length of structure  2 . 6 - 50  that could compromise the rigidity of structure  2 . 6 - 50 . The width of support structure may be relatively large near the middle of structure  2 . 6 - 50 . For example, support structure  2 . 6 - 50  may have a maximum width across its shorter lateral dimension that is at least 2 mm, at least 4 mm, at least 8 mm, at least 16 mm, at least 32 mm, less than 40 mm, less than 25 mm, less than 18 mm, less than 15 mm, less than 10 mm, less than 7 mm, or other suitable value. The longitudinal dimension (length) of support structure  2 . 6 - 50  may be at least 2 cm, at least 4 cm at least 8 cm, at least 16 cm, less than 20 cm, less than 14 cm, less than 10 cm, less than 6 cm, less than 4 cm, or other suitable value. The minimum thickness of support  2 . 6 - 50  may be at least 0.3 mm, at least 0.6 mm, at least 1.2 mm, at least 2.4 mm, less than 5 mm, less than 2.5 mm, less than 1.3 mm, less than 0.8 mm, less than 0.5 mm, or other suitable value. 
     In addition to supporting cameras  2 . 6 - 54  and/or other optical components, camera support structure  2 . 6 - 50  may serve as a support for wireless communications components such as antennas  2 . 6 - 60 . In the example of  FIGS.  2 . 6 - 4   , camera support structure  2 . 6 - 50  serves as a support member for a pair of antennas  2 . 6 - 60 . In general, camera support structure  2 . 6 - 50  may support at least one antenna, at least two antennas, at least three antennas, fewer than ten antennas, from 2 to 5 antennas, or other suitable number of antennas. Antennas  2 . 6 - 60  may be formed using any suitable antenna types. For example, antennas  2 . 6 - 60  may include antennas with resonating elements that are formed from loop antenna structures, patch antenna structures, inverted-F antenna structures, slot antenna structures, planar inverted-F antenna structures, monopoles, dipoles, helical antenna structures, Yagi (Yagi-Uda) antenna structures, hybrids of these designs, etc. If desired, one or more of antennas  2 . 6 - 60  may be cavity-backed antennas. Different types of antennas may be used for different bands and combinations of bands. For example, one type of antenna may be used in forming a local wireless link antenna and another type of antenna may be used in forming a remote wireless link antenna. Dedicated antennas may be used for receiving satellite navigation system signals or, if desired, antennas  2 . 6 - 60  can be configured to receive both satellite navigation system signals and signals for other communications bands (e.g., wireless local area network signals and/or cellular telephone. Antennas  2 . 6 - 60  may be formed from metal members, patterned thin-film metal layers, and/or other conductive structures. 
     The front face of device  2 . 6 - 10  may be covered with an inactive housing wall (e.g., a polymer layer). In the example of  FIGS.  2 . 6 - 5   , front face F of device  2 . 6 - 10  is covered by display  2 . 6 - 14 F (e.g., an organic light-emitting diode display, a microLED display, an electrophoretic display, liquid crystal display, etc.). The pixels of display  2 . 6 - 14 F may be covered with an outer protective display cover layer (e.g., a layer of glass, a layer of clear polymer, etc.). 
     Optical windows such as camera windows  2 . 6 - 62  may be provided in the display cover layer. Camera windows  2 . 6 - 62  may be formed from portions of the display cover layer or from clear window structures that are mounted in openings in the display cover layer. Each optical window may overlap a corresponding optical component and may allow light from the component to be emitted through the optical window and/or may allow ambient light from the environment to pass to the optical component. Camera windows  2 . 6 - 62  (e.g., camera windows in the display cover layer for display  2 . 6 - 14 F and/or optical windows formed in other portions of housing  2 . 6 - 12 ) may have optical characteristics that allow an associated optical component to operate satisfactorily. Consider, as an example, a camera window  2 . 6 - 62  that overlaps one of forward-facing cameras  2 . 6 - 46 . As shown in  FIGS.  2 . 6 - 5   , camera support structure  2 . 6 - 50  may be mounted in the interior of device  2 . 6 - 10  so that cameras  2 . 6 - 46  are aligned with camera windows  2 . 6 - 62  and so that antennas  2 . 6 - 60  are overlapped by the display cover layer for display  2 . 6 - 14 F. Each camera window  2 . 6 - 62  may have a visible-light and/or infrared-light transparency level sufficient to allow the forward-facing camera  2 . 6 - 46  that is overlapped by that window to capture images of real-world objects in the user&#39;s environment and/or to gather other image data. The transmission of camera window  2 . 6 - 62  may be, as an example, at least 50%, at least 90%, at least 95%, or other suitable value (at visible and/or infrared wavelengths). Non-camera components (e.g., an ambient light sensor, an optical proximity sensor, etc.) may have optical windows with other transmission values. 
       FIGS.  2 . 6 - 6    is a cross-sectional view of a portion of device  2 . 6 - 10  in an illustrative configuration in which display  2 . 6 - 14 F is formed on front face F of device  2 . 6 - 10 . As shown in  FIGS.  2 . 6 - 6   , display  2 . 6 - 14 F includes pixel array  2 . 6 - 14 M (e.g., a display layer such as an organic light-emitting diode display layer, an array of crystalline light-emitting diodes, an electrophoretic display layer, a liquid crystal display layer, etc.). Display cover layer CG of display  2 . 6 - 14 F may cover and protect pixel array  2 . 6 - 14 M. During operation, display  2 . 6 - 14 F may present images to a user (while device  2 . 6 - 10  is or is not being worn on a user&#39;s head). If desired, display  2 . 6 - 14 F may have touch screen functionality, so that a user may supply touch input to front face F of device  2 . 6 - 10 . 
     Camera  2 . 6 - 46  may be located at the edge of display  2 . 6 - 14 F (e.g., outside of the active area of the display), camera  2 . 6 - 46  may operate through an opening in pixel array  2 . 6 - 14 M, and/or camera  2 . 6 - 46  may sense light that passes through gaps in the opaque structures of pixel array  2 . 6 - 14 M. In the illustrative configuration of  FIGS.  2 . 6 - 6   , camera  2 . 6 - 46  is located in an inactive display border region that is free of pixels. As shown in  FIGS.  2 . 6 - 6   , camera window  2 . 6 - 62  may be formed from an opening in opaque masking layer  2 . 6 - 64  that allows light to pass through display cover layer CG. Opaque masking layer  2 . 6 - 64  may be, as an example, a layer of black ink that is formed on the inner surface of display cover layer CG. 
     Camera  2 . 6 - 46  may be mounted to an opening in camera support structure  2 . 6 - 50  using bonds  2 . 6 - 66  (e.g., adhesive bonds, welds, etc.), using screws or other fasteners such as illustrative fastener  2 . 6 - 68 , or using other attachment mechanisms (press-fit connections, mating engagement structures, etc.). In turn, camera support structure  2 . 6 - 50  may be attached to frame  2 . 6 - 12 I by heat stakes (e.g., heat staked protrusions extending from camera support structure  2 . 6 - 50  into mating openings in frame  2 . 6 - 12 I and/or heat staked protrusions extending from frame  2 . 6 - 12 I into openings), adhesive, welds (e.g., laser welds joining a metal camera support structure to a metal frame, laser welds joining polymer camera support structure to a polymer frame, and/or other welds), press-fit connections, mating engagement structures (e.g., snaps), or other attachment structures  2 . 6 - 70  and/or screws or other fasteners  2 . 6 - 56  (e.g., screws that are received within threaded openings in camera support structure  2 . 6 - 50  and/or frame  2 . 6 - 12 I, screws that are received within insert nuts, etc.). 
     As shown in  FIGS.  2 . 6 - 6   , antenna  2 . 6 - 60  may be formed from conductive antenna structures (e.g., metal traces, stamped metal foil, etc.) supported by camera support structure  2 . 6 - 50 . During operation, antenna  2 . 6 - 60  may transmit and/or receive wireless signals that pass through display cover layer CG and other portions of housing  2 . 6 - 12 M. 
     A schematic diagram of an illustrative antenna (antenna  2 . 6 - 60 ) coupled to illustrative radio-frequency transceiver circuitry  2 . 6 - 90  is shown in  FIGS.  2 . 6 - 7   . Communications circuitry  2 . 6 - 22  of  FIG.  3    may include transceiver circuitry  2 . 6 - 90  ( FIGS.  2 . 6 - 7   ) and/or other wireless circuitry formed from one or more integrated circuits, power amplifier circuitry, low-noise input amplifiers, passive radio-frequency (RF) components, one or more antennas  2 . 6 - 60 , transmission lines, and other circuitry for handling RF wireless signals. 
     Radio-frequency transceiver circuitry  2 . 6 - 90  of  FIGS.  2 . 6 - 7    may use antenna  2 . 6 - 60  for handling various radio-frequency communications bands. For example, circuitry  2 . 6 - 90  may include wireless local area network transceiver circuitry (e.g., circuitry  2 . 6 - 90  may handle 2.4 GHz and 5 GHz bands for WiFi® (IEEE 802.11) communications) and may handle the 2.4 GHz Bluetooth® communications band. If desired, circuitry  2 . 6 - 90  may use cellular telephone transceiver circuitry or other circuitry for handling cellular telephone wireless communications and/or other wireless communications in frequency ranges such as a communications band from 700 to 2700 MHz, from 3.4 to 3.6 GHz, from 450 MHz to 6 GHz, from 24 to 53 GHz, from 5 to 8 GHz, from 60 to 90 GHZ, and/or other communications bands. Circuitry  2 . 6 - 90  may handle voice data and non-voice data. 
     Transceiver circuitry  2 . 6 - 90  may include satellite navigation system circuitry such as Global Positioning System (GPS) receiver circuitry for receiving GPS signals at 1575 MHz or for handling other satellite positioning data (e.g., GLONASS signals at 1609 MHz). Satellite navigation system signals are received from a constellation of satellites orbiting the earth. 
     In satellite navigation system links, cellular telephone links, and other long-range links, wireless signals are typically used to convey data over thousands of feet or miles. In WiFi® and Bluetooth® links at 2.4 and 5 GHz and other short-range wireless links, wireless signals are typically used to convey data over tens or hundreds of feet. If desired, device  2 . 6 - 10  may include millimeter wave wireless transceiver circuitry. To enhance signal reception for millimeter wave communications, phased antenna arrays and beam steering techniques may be used (e.g., schemes in which antenna signal phase and/or magnitude for each antenna in an array is adjusted to perform beam steering). Antenna diversity schemes may also be used to ensure that the antennas that have become blocked or that are otherwise degraded due to the operating environment of device  2 . 6 - 10  can be switched out of use and higher-performing antennas used in their place. Circuitry  2 . 6 - 90  can include circuitry for other short-range and long-range wireless links if desired. For example, circuitry  2 . 6 - 90  may include circuitry for receiving television and radio signals, paging system transceivers, near field communications (NFC) circuitry, etc. If desired, circuitry  2 . 6 - 90  and/or other wireless circuitry may use antennas such as antenna  2 . 6 - 60  for radio-frequency sensing (e.g., to determine the orientation and/or distance between device  2 . 6 - 10  and other wireless equipment, to form radar-based sensors, etc.). 
     As shown in  FIGS.  2 . 6 - 7   , radio-frequency transceiver circuitry  2 . 6 - 90  may be coupled to antenna feed  2 . 6 - 102  of antenna  2 . 6 - 60  using transmission line  2 . 6 - 92 . Antenna feed  2 . 6 - 102  may include a positive antenna feed terminal such as positive antenna feed terminal  2 . 6 - 98  and may have a ground antenna feed terminal such as ground antenna feed terminal  2 . 6 - 100 . Transmission line  2 . 6 - 92  may be formed from metal traces on a printed circuit or other conductive structures and may have a positive transmission line signal path such as path  2 . 6 - 94  that is coupled to terminal  2 . 6 - 98  and a ground transmission line signal path such as path  2 . 6 - 96  that is coupled to terminal  2 . 6 - 100 . Transmission line paths such as path  2 . 6 - 92  may be used to route antenna signals within device  2 . 6 - 10 . For example, transmission line paths may be used to couple antenna structures such as one or more antennas in an array of antennas to transceiver circuitry  2 . 6 - 90 . Transmission lines in device  2 . 6 - 10  may include coaxial cable paths, microstrip transmission lines, stripline transmission lines, edge-coupled microstrip transmission lines, edge-coupled stripline transmission lines, transmission lines formed from combinations of transmission lines of these types, etc. Filter circuitry, switching circuitry, impedance matching circuitry, and other circuitry may be interposed within transmission line  2 . 6 - 92  and/or circuits such as these may be incorporated into antenna  2 . 6 - 60  (e.g., to support antenna tuning, to support operation in desired frequency bands, etc.). 
     Device  2 . 6 - 10  may contain multiple antennas  2 . 6 - 60 . The antennas may be used together or one of the antennas may be switched into use while other antenna(s) are switched out of use. If desired, control circuitry  2 . 6 - 20  may be used to select an optimum antenna to use in device  2 . 6 - 10  in real time and/or to select an optimum setting for adjustable wireless circuitry associated with one or more of antennas  2 . 6 - 60 . Antenna adjustments may be made to tune antennas to perform in desired frequency ranges, to perform beam steering with a phased antenna array, and to otherwise optimize antenna performance. Sensors may be incorporated into antennas  2 . 6 - 60  to gather sensor data in real time that is used in adjusting antennas  2 . 6 - 60 . 
       FIGS.  2 . 6 - 8    is a diagram of an illustrative antenna that may be used in device  2 . 6 - 10 . In the example of  FIGS.  2 . 6 - 8   , antenna  2 . 6 - 60  is an inverted-F antenna. As shown in  FIGS.  2 . 6 - 8   , antenna  2 . 6 - 60  may include an antenna resonating element such as antenna resonating element  2 . 6 - 110  and an antenna ground such as antenna ground  2 . 6 - 112 . Antenna resonating element  2 . 6 - 110  may have one or more branches such as antenna resonating element arm  2 . 6 - 116  and optional antenna resonating element arm  2 . 6 - 116 ′. Return path  2 . 6 - 118  (sometimes referred to as a short circuit path) may be coupled between resonating element arm  2 . 6 - 116  and ground  2 . 6 - 112 . Antenna feed  2 . 6 - 102  may include positive antenna feed terminal  2 . 6 - 98  and ground antenna feed terminal  2 . 6 - 100  and may be coupled between element  2 . 6 - 110  (e.g., arm  2 . 6 - 116 ) and ground  2 . 6 - 112  in parallel with return path  2 . 6 - 118 . One or more optional components (switches, tunable circuits such as tunable capacitors, tunable inductors, etc.) may be coupled between antenna ground  2 . 6 - 112  and resonating element arm  2 . 6 - 116  and may be adjusted to tune antenna  2 . 6 - 60 . The configuration of  FIGS.  2 . 6 - 8    in which no tunable components are coupled between arm  2 . 6 - 116  and ground  2 . 6 - 112  is merely illustrative. 
     Antenna resonating element arm  2 . 6 - 116  may be separated from ground  2 . 6 - 112  by dielectric opening  2 . 6 - 122 . If desired, opening  2 . 6 - 122  may form a slot antenna element that contributes to the antenna response of antenna  2 . 6 - 60 . In the example of  FIGS.  2 . 6 - 8   , antenna  2 . 6 - 40  is an inverted-F antenna that does not include a slot antenna element. 
     Optional parasitic antenna elements such as optional parasitic element  2 . 6 - 124  may be included in antenna  2 . 6 - 60  to adjust the frequency response of antenna  2 . 6 - 60 . 
     Antennas such as antenna  2 . 6 - 60  of  FIGS.  2 . 6 - 8    (e.g., inverted-F antennas, slot antennas, hybrid inverted-F slot antennas, etc.) and/or other types of antenna  2 . 6 - 60  (e.g., patch antennas, loop antennas, etc.) may be used in supporting any suitable operations involving transmission and/or reception of wireless signals. 
     Antennas (e.g., antenna resonating elements, parasitic elements, antenna ground structures, feed structures, and/or other structures for each antenna  2 . 6 - 60 ) may be formed from conductive structures such as metal members (e.g., metal structures formed from wireless, machined metal parts, stamped sheet metal, etc.), metal traces (e.g., patterned metal deposited by physical vapor deposition or laser-assisted deposition techniques), other conductive materials (e.g., carbon nanowires, etc.), and/or other conductive antenna structures. These conductive structures may be supported by substrates such as rigid and/or flexible printed circuit substrates, by polymer housing structures (e.g., by portions of camera support structure  2 . 6 - 50 ), dielectric members formed from glass, ceramic, and/or other dielectric, and/or other antenna support structures. 
       FIGS.  2 . 6 - 9 ,  2 . 6 - 10 ,  2 . 6 - 11 , and  2 . 6 - 12    are cross-sectional side views of illustrative conductive antenna structures  2 . 6 - 126  for use in forming antennas  2 . 6 - 60 . 
     In the illustrative configuration of  FIGS.  2 . 6 - 9   , laser direct structuring (LDS) techniques are being used to form antenna structures  2 . 6 - 126 . Laser beam  2 . 6 - 128  is used to selectively illuminate area  2 . 6 - 130  on the surface of a dielectric antenna support structure such as structure  2 . 6 - 50 . Structure  2 . 6 - 50  in the example of  FIGS.  2 . 6 - 9    may be formed from polymer with additives to help sensitize structure  2 . 6 - 50  to laser light exposure. After laser light exposure with beam  2 . 6 - 128 , electroplating operations are used to selectively electrodeposit conductive structures  2 . 6 - 126  on area  2 . 6 - 130  without depositing the conductive structures elsewhere on the exposed surface of structure  2 . 6 - 50 , thereby forming structures  2 . 6 - 126  with a desired antenna shape (e.g., to form an antenna resonating element, parasitic element, ground, and/or other patterned antenna structures as shown in  FIGS.  2 . 6 - 8   ). 
     In the example of  FIGS.  2 . 6 - 10   , conductive antenna structures  2 . 6 - 126  are metal traces deposited on printed circuit  2 . 6 - 132 . These metal traces may be deposited by physical vapor deposition and patterned using photolithography, and/or may be formed using other deposition and patterning techniques. Metal traces  2 . 6 - 134  of printed circuit  2 . 6 - 132  may help convey radio-frequency signals to and/or from antenna structures  2 . 6 - 126 . Adhesive  2 . 6 - 136  may be used to attach printed circuit  2 . 6 - 132  to a surface of support structure  2 . 6 - 50 . 
     If desired, conductive antenna structures  2 . 6 - 126  can be formed from metal structures embedded in support structure  2 . 6 - 50 . For example, metal antenna structures (wire, metal foil, structural metal members, sheet metal parts, and/or other conductive antenna structures forming antenna structures  2 . 6 - 126 ) can be embedded in polymer that forms support structure  2 . 6 - 50 , as shown in  FIGS.  2 . 6 - 11    (e.g., one or more shots of polymer for support structure  2 . 6 - 50  may be molded over conductive antenna structures  2 . 6 - 126 ). 
     In the illustrative example of  FIGS.  2 . 6 - 12   , printed circuit  2 . 6 - 132  has metal traces forming conductive antenna structures  2 . 6 - 126  and metal traces  2 . 6 - 134  forming signal paths such as transmission lines. As shown in  FIGS.  2 . 6 - 12   , printed circuit  2 . 6 - 132  may be embedded within support structure  2 . 6 - 50  (e.g., polymer forming support structure  2 . 6 - 50  may be molded over printed circuit  2 . 6 - 132 ). 
     The arrangements of  FIGS.  2 . 6 - 9 ,  2 . 6 - 10 ,  2 . 6 - 11   , and/or  2 . 6 - 12  and/or other arrangements may be used in forming antennas  2 . 6 - 60  on camera support structure  2 . 6 - 50 , while camera support structure  2 . 6 - 50  simultaneously serves as a support and alignment member for cameras  2 . 6 - 46 . 
       FIGS.  2 . 6 - 13    is a top view of an illustrative camera support structure formed using multiple shots of polymer. One shot of polymer forms portion  2 . 6 - 50 - 1  of camera support structure  2 . 6 - 50  and another shot of polymer forms portion  2 . 6 - 50 - 2  of camera support structure  2 . 6 - 50 . Portion  2 . 6 - 50 - 1  may, as an example, include fibers or other filler embedded in the shot of polymer forming portion  2 . 6 - 50 - 1  or portion  2 . 6 - 50 - 1  may have an embedded fiber-composite member (e.g., a stiffening member formed from a rod, strip, or other elongated member of carbon-fiber material or other stiffening member). This may help to locally stiffen and strengthen portion  2 . 6 - 50 - 1  (e.g., to enhance the stiffness of portion  2 . 6 - 50 - 1  relative to portion  2 . 6 - 50 - 2 ). As shown in  FIGS.  2 . 6 - 13   , stiffening member  2 . 6 - 50 M may extend between openings  2 . 6 - 58  (and therefore cameras  2 . 6 - 46 ) to prevent bending of the intervening portion of structure  2 . 6 - 50  (e.g., to prevent bending of structure  2 . 6 - 50  out of the X-Y plane of  FIGS.  2 . 6 - 13   ) and thereby prevent undesired bending-induced camera misalignment. Portion  2 . 6 - 50 - 1  may, if desired, be free of conductive material such as conductive carbon fibers (e.g., to reduce the presence of conductive material that could interfere with the operation of overlapping antennas). 
       FIGS.  2 . 6 - 14    is a cross-sectional side view of camera support structure  2 . 6 - 50  taken along line  2 . 6 - 140  of  FIGS.  2 . 6 - 13    and viewed in direction  2 . 6 - 142  of  FIGS.  2 . 6 - 13   . As shown in  FIGS.  2 . 6 - 14   , camera support structure  2 . 6 - 50  may include an embedded stiffening structure such as fiber-composite stiffening member  2 . 6 - 50 M (e.g., an elongated strip-shaped carbon-fiber stiffening member). Member  2 . 6 - 50 M may be embedded within portion  2 . 6 - 50 - 2 . Portions  2 . 6 - 50 - 1  and  2 . 6 - 50 - 2  may be formed from first and second shots of molded polymer material or may be formed using other techniques. 
     It may be desirable to detect misalignment of cameras  2 . 6 - 46  due to deformation of camera support structure  2 . 6 - 50 . As shown in the cross-sectional side view of structure  2 . 6 - 50  of  FIGS.  2 . 6 - 15   , a bend sensor such as sensor  2 . 6 - 16 B may be mounted to camera support structure  2 . 6 - 50  between cameras  2 . 6 - 46 . Sensor  2 . 6 - 16 B may be a strain gauge or other sensor that is configured to detect bending of structure  2 . 6 - 50  (e.g., bending about bend axis  2 . 6 - 150 ). Flexible printed circuit  2 . 6 - 152  may have signal lines that carry bending measurements to control circuitry  2 . 6 - 20  ( FIG.  3   ). In response to measuring bending in structure  2 . 6 - 50 , control circuitry  2 . 6 - 20  can take corrective action to compensate for any predicted misalignment between cameras  2 . 6 - 46 . For example, if cameras  2 . 6 - 46  are detected as being misaligned by lo from data gathered by sensor  2 . 6 - 16 B, control circuitry  2 . 6 - 20  can digitally compensate for the measured misalignment (e.g., by shifting and/or warping the camera image data gathered by cameras  2 . 6 - 46  to ensure that the images from cameras  2 . 6 - 46  can be stitched together as desired or otherwise used as desired in operating device  2 . 6 - 10 ). 
     If desired, device  2 . 6 - 10  may have one or more camera positioning devices such as actuator  2 . 6 - 160  of  FIGS.  2 . 6 - 16   . Actuator  2 . 6 - 160  can change the angular orientation of camera  2 . 6 - 46  relative to structure  2 . 6 - 50 . In response to detecting with sensor  2 . 6 - 16 B that structure  2 . 6 - 50  has bent about axis  150  of  FIGS.  2 . 6 - 15    by  20 , for example, control circuitry  2 . 6 - 20  may direct actuator  2 . 6 - 160  to move camera  2 . 6 - 46  to compensate. For example, camera  2 . 6 - 46  may be tilted in an opposing direction by a compensating amount (e.g., −20), thereby ensuring that cameras  2 . 6 - 46  remain aligned even if structure  2 . 6 - 50  experiences deformation during operation of device  2 . 6 - 10 . 
     The use of a strain gauge to detect bending is illustrative. Any suitable sensor  2 . 6 - 16  may be used to detect camera misalignment due to deformation of support structure  2 . 6 - 50 . The effects of camera misalignment may be compensated by physically steering optical components such as cameras  2 . 6 - 46  (as described in connection with  FIGS.  2 . 6 - 16   ), by processing the image data from cameras  2 . 6 - 46  (e.g., image warping, etc.), and/or by otherwise compensating for detected misalignment. The examples of  FIGS.  2 . 6 - 13 ,  2 . 6 - 14 ,  2 . 6 - 15 , and  2 . 6 - 16    are illustrative. 
     III: Display Integration Assembly 
       FIG.  3 - 1    illustrates a perspective view of a front cover assembly  3 - 100  of an HMD device described herein, for example the front cover assembly  3 - 1  of the HMD  3 - 100  shown in  FIG.  3 - 1    or any other HMD device shown and described herein. The front cover assembly  3 - 100  shown in  FIG.  1    can include a transparent or semi-transparent cover  3 - 102 , shroud  3 - 104  (or “canopy”), adhesive layers  3 - 106 , display assembly  3 - 108  including a lenticular lens panel or array  3 - 110 , and a structural trim  3 - 112 . The adhesive layer  3 - 106  can secure the shroud  3 - 104  and/or transparent cover  3 - 102  to the display assembly  3 - 108  and/or the trim  3 - 112 . The trim  3 - 112  can secure the various components of the front cover assembly  3 - 100  to a frame or chassis of the HMD device. 
     In at least one example, as shown in  FIG.  3 - 1   , the transparent cover  3 - 102 , shroud  3 - 104 , and display assembly  3 - 108 , including the lenticular lens array  3 - 110 , can be curved to accommodate the curvature of a user&#39;s face. The transparent cover  3 - 102  and the shroud  3 - 104  can be curved in two or three dimensions, e.g. vertically curved in the Z-direction in and out of the Z-X plane and horizontally curved in the X-direction in and out of the Z-X plane. In at least one example, the display assembly  3 - 108  can include the lenticular lens array  3 - 110  as well as a display panel having pixels configured to project light through the shroud  3 - 104  and the transparent cover  3 - 102 . The display assembly  3 - 108  can be curved in at least one direction, for example the horizontal direction, to accommodate the curvature of a user&#39;s face from one side (e.g., left side) of the face to the other (e.g., right side). In at least one example, each layer or component of the display assembly  3 - 108 , which will be shown in subsequent figures and described in more detail, but which can include the lenticular lens array  3 - 110  and a display layer, can be similarly or concentrically curved in the horizontal direction to accommodate the curvature of the user&#39;s face. 
     In at least one example, the shroud  3 - 104  can include a transparent or semi-transparent material through which the display assembly  3 - 108  projects light. In one example, the shroud  3 - 104  can include one or more opaque portions, for example opaque ink-printed portions or other opaque film portions on the rear surface of the shroud  3 - 104 . The rear surface can be the surface of the shroud  3 - 104  facing the user&#39;s eyes when the HMD device is donned. In at least one example, opaque portions can be on the front surface of the shroud  3 - 104  opposite the rear surface. In at least one example, the opaque portion or portions of the shroud  3 - 104  can include perimeter portions visually hiding any components around an outside perimeter of the display screen of the display assembly  3 - 108 . In this way, the opaque portions of the shroud hide any other components, including electronic components, structural components, and so forth, of the HMD device that would otherwise be visible through the transparent or semi-transparent cover  3 - 102  and/or shroud  3 - 104 . 
     In at least one example, the shroud  3 - 104  can define one or more apertures transparent portions  3 - 120  through which sensors can send and receive signals. In one example, the portions  3 - 120  are apertures through which the sensors can extend or send and receive signals. In one example, the portions  3 - 120  are transparent portions, or portions more transparent than surrounding semi-transparent or opaque portions of the shroud, through which sensors can send and receive signals through the shroud and through the transparent cover  3 - 102 . In one example, the sensors can include cameras, IR sensors, LUX sensors, or any other visual or non-visual environmental sensors of the HMD device. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  3 - 1    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  3 - 2 - 3 - 6    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  3 - 2 - 3 - 6    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  3 - 1   . 
       FIG.  3 - 2    illustrates a partial, cross-sectional view of an example of a front cover assembly  3 - 200 , including a shroud  3 - 204  and a display assembly  3 - 208  coupled to the shroud  3 - 204  via a display bracket  3 - 214 . The shroud  3 - 204  can be coupled to a shroud bracket  3 - 216 , which can couple the front cover assembly  3 - 200  to one or more structural frame members  3 - 218  of an HMD device. The display bracket  3 - 214  can extend behind the display assembly  3 - 208  and fixe the display assembly  3 - 208  adjacent to the shroud  3 - 204  such that pixels of the display screen of the display assembly  3 - 208  project light outward through the shroud  3 - 204 . 
     In at least one example, the display assembly  3 - 208  is curved in one direction, e.g., the horizontal direction, but not the vertical direction, e.g. up and down in the orientation shown in  FIG.  3 - 2   . In such an example, the shroud  3 - 204  can be curved in both vertical and horizontal directions, as shown in  FIG.  1    and described above. 
       FIG.  3 - 3    shows a side view of another front cover assembly  3 - 300  similar to the front cover assembly  3 - 200  shown in  FIG.  3 - 2   . In  FIG.  3 - 3   , the front cover assembly  3 - 300  is shown without an exterior transparent cover but includes a shroud  3 - 304  defining mechanical or visual apertures  3 - 320  through which sensors can send and receive signals as well as a display assembly  3 - 308  secured to a back or rear side of the shroud  3 - 304 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  3 - 2  and  3 - 3    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  3 - 1  and  3 - 4  through  3 - 6    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  3 - 1  and  3 - 4  through  3 - 6    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  3 - 2  and  3 - 3   . 
       FIG.  3 - 4    illustrates a partial, cross-sectional view of an example of a front cover assembly  3 - 400 , including an outer transparent or semi-transparent cover  3 - 402 , a shroud  3 - 404 , a blinker film  3 - 422  secured to a back/rear side of the shroud  3 - 404 , a display assembly  3 - 408  including a lenticular lens array  3 - 410 , a display bracket  3 - 414  securing the display assembly relative to the shroud  3 - 404  such that an air gap  3 - 424  is defined between the lenticular lens array  3 - 410  and the blinker film  3 - 422 , and a graphite layer  3 - 426  disposed on a back/rear side of the display bracket  3 - 414 . The air gap  3 - 424  can define an open space or volume between the lenticular lens array  3 - 410  and the blinker film  3 - 422  such that no other components are disposed within the gap  3 - 424  between the lenticular lens array  3 - 410  and the blinker film  3 - 422 . In at least one example, the blinker film  3 - 422  can be configured to hide the display from view when the display of the display assembly  3 - 408  is not on or projecting light and/or when someone views the front display assembly from certain predetermined angles (e.g., wide angles instead of narrow, more “straight on” viewing angles in front of the display  3 - 408 ). In at least one example, the lenticular lens array  3 - 410  can be used to generate a three-dimensional effect of those not donning the HMD and viewing the outward facing display  3 - 408  through the cover  3 - 402  and shroud  3 - 404 . The gap  3 - 424  can be present to allow light transmitted through the cover  3 - 402 , shroud  3 - 404 , and blinker film  3 - 422  to pass appropriately through the lenses of the lenticular lens array  3 - 410  to create the desired three-dimensional effect. 
     In at least one example, the blinker film  3 - 422  can be adhered to the shroud  3 - 404  via an optically clear adhesive  3 - 423  shown in  FIG.  3 - 4 A . In one example, the lenticular lens array  3 - 410  can be adhered to the display assembly  3 - 408  via an optically clear adhesive  3 - 411  shown in  FIG.  3 - 4 A . In at least one example, the blinker film  3 - 422  can diffuse and darken light, including specular highlights from the lenticular lens array  3 - 410 , passing through from the display assembly  3 - 408 . In at least one example, diffusion particles can be formed in the optically clear adhesive layer  3 - 423 , which can be referred to as a haze film  3 - 432 , between the blinker film  3 - 422  and the shroud  3 - 404 . In at least one example, the diffusion particles can be formed within the blinker film  3 - 422  itself. In at least one example, the diffusion particles can be formed within the shroud  3 - 404 . In at least one example, the diffusion particles can include titanium dioxide. In at least one example, the display assembly  3 - 408  can include an LCD and/or OLED display layer rather than the lenticular lens array  3 - 410 , e.g., with a flat display assembly (as opposed to the curved display assembly  3 - 408  shown) to provide 3-d visual effects. The front cover assembly of the HMD can also include a dust seal  3 - 409  disposed between the shroud  3 - 404  and the lenticular lens array  3 - 422  and/or other components of the assembly, including the display assembly  3 - 408 , to prevent dust and other particles/pollutants from entering the air gap  3 - 424  between the lenticular lens array  3 - 410  and the blinker film  3 - 422 . 
     In at least one example, the graphite layer  3 - 426  can be one of multiple graphite layers, for example two layers, three layers, four layers, or more than four layers. The graphite layers  3 - 426  can be configured to spread heat from the display assembly  3 - 408  coupled to the display bracket  3 - 414 . In at least one example, the display bracket  3 - 414  is a metal material that acts as a heat sink to dissipate heat from the display assembly  3 - 408 . In one example, the display bracket  3 - 414  includes a thermally conductive material. In one example, the display bracket  3 - 414  includes magnesium. 
     In at least one example, the cover  3 - 402  includes a clear core material  3 - 427 . In one example, the core material  3 - 427  can include polycarbonate. In one example, the core material includes glass or other ceramic materials. In at least one example, optical clear adhesive (OCA) layers  3 - 428   a ,  3 - 428   b  can be disposed on either side, respectively, of the core material  3 - 427 . In at least one example, first and second protective coating layers/films  3 - 430   a ,  3 - 430   b  can be disposed on either side, respectively, of the core material  3 - 427 , with the OCA layers  3 - 428   a ,  3 - 428   b  disposed between respective protective coating films  3 - 430   a ,  3 - 430   b  and the core material  3 - 4271 . In at least one example, an additional hard-coat layer and or an additional anti-reflective coating, which can be part of a hard-coat layer, can be disposed on top of the first polycarbonate film  3 - 430   a  to define an external surface of the transparent cover  3 - 402 . 
     In at least one example, the cover assembly  3 - 400  can include a haze film  3 - 432  disposed on the back/rear side of the shroud  3 - 404 . In one example, an optical bonding material, such as an optically clear bonding material, can be disposed between the blinker film  3 - 422  and the shroud  3 - 404 . In one example, the bonding material can include or form the haze film  3 - 432  from particles in the bonding material. In at least one example, the front cover assembly  3 - 400  can also include a bracket  3 - 434  disposed against a rear/back side of the shroud  3 - 404  outside a perimeter of the display assembly  3 - 408 . The display bracket  3 - 414  can be mounted to the shroud  3 - 404  via the bracket  3 - 434  as shown in  FIG.  3 - 4   . In at least one example, one or more opaque painted portions  3 - 436  can be applied or formed on the rear/back side of the shroud  3 - 404 , for example, between the bracket  3 - 434  and the shroud  3 - 404  to visually hide the bracket  3 - 434  from being seen from outside or in front of the front cover assembly  3 - 400 . The shroud can also include an opaque portion or multiple portions disposed generally around an outer perimeter area of the shroud to hide components behind the perimeter portion, for example cameras, sensors, brackets, circuitry, frame and structural components of the HMD, and so forth. The opaque portions can be applied to the shroud  3 - 404 . 
     In at least one example, the bracket  3 - 434  can be referred to as a shroud canopy. The shroud  3 - 404  can include a layer of ink, paint, film, or other opaque layer  3 - 435  disposed between the bracket  3 - 434  and the shroud  3 - 404  to visually hide any glue or other connection mechanisms securing the bracket  3 - 434  to the shroud  3 - 404  from someone looking in through the front of the device. The bracket  3 - 434  can also be opaque and provide a visual barrier to components behind/within the HMD device. The film or paint layer  3 - 435  can be disposed on the shroud  3 - 404  around a peripheral edge or edge area surrounding the display assembly  3 - 408  or around a more opaque area of the shroud  3 - 404  through which the display assembly  3 - 408  is configured to project light. 
       FIG.  3 - 4 B  illustrates an exploded view of a front cover assembly  3 - 400  of an HMD device, including a transparent cover  3 - 402 , shroud  3 - 404 , and a frame  3 - 448  to which the cover glass  3 - 402  and shroud  3 - 404  can be secured. The shroud  3 - 404  can include or define an aperture  3 - 440  through which one or more cameras or other sensors of the HMD can send and receive signals. The shroud  3 - 404  can also include a visually opaque and infrared-transparent window  3 - 438  through which an infrared sensor and/or emitter can send and receive infrared light. The shroud  3 - 404  can also include another window or opening  3 - 442  through which another sensor of the HMD can send and receive signals. 
     In at least one example, the cover  3 - 402  can include back-painted, opaque trim layer  3 - 444  disposed on a back side of the cover  3 - 402  between the cover  3 - 402  and the shroud  3 - 404 . The trim layer  3 - 444  can include opaque ink, paint, film, or other opaque layers to reduce stray signals, for example light signals, from the various emitters of the HMD from undesirably bouncing between the cover  3 - 402  and the shroud  3 - 404 , thus reducing cross-talk between sensors and emitters. The trim layer  3 - 444  can include a sensor peripheral trim portion  3 - 402  on a back side of the cover  3 - 402  as well to limit stray light directly around a sensor disposed behind the window  3 - 438 . The trim portion  3 - 402  can include the same or different layers and materials as that of the trim layer  3 - 444  but be aligned with the window  3 - 438 , and thus disposed directly around a periphery of the emitter or sensor of the HMD sending and/or receiving signals through the window  3 - 438 , to prevent stray signals, including stray light signals, from cross-talking with other emitters/sensors. In this way, stray cross-talk between the cover  3 - 402  and the shroud  3 - 404  can be reduced. 
       FIG.  3 - 4 C  illustrates another exploded, perspective view of a front cover assembly  3 - 400 , including the shroud  3 - 404 , cover  3 - 402 , trim layer  3 - 444 , trim portion  3 - 446  defining a sensor window  3 - 445 , windows  3 - 440 , and sensor/emitter window  3 - 438 . In at least one example, the assembly  3 - 400  can also include a light seal  3 - 450  disposed around the window  3 - 438  on a front side of the shroud  3 - 404  between the shroud  3 - 404  and the cover  3 - 402 . The light seal  3 - 450  can be a physical barrier extending between the shroud  3 - 404  and the cover  3 - 402 , for example physically contacting both the shroud  3 - 404  and the cover  3 - 402 , and around the window  3 - 438 , such that light or other signals sent by an emitter or received by a sensor of the HMD through the window  3 - 438  is directed through the window  3 - 445  of the cover  3 - 402  to reduce stray signals and cross-talk. The seal  3 - 450  can be aligned with the trim portion  3 - 446  of the cover  3 - 402 . 
     In at least one example, the shroud  3 - 404  and cover  3 - 402  can include radio-frequency transparent materials and the HMD can include antennas and emitters configured to pass radio-frequency signals through shroud  3 - 404  and cover  3 - 402 . 
     In at least one example, the shroud  3 - 404  can include a notch or cutout recessed into or disposed on a back side of the shroud  3 - 404  to accommodate one or more antennas. The cutouts can be shaped and positioned to accommodate the placement of antennas adjacent to or against the shroud to allow more distance between the antennas and one or more grounding planes relative to the antennas. The increased resonating distance of the antennas provided by the shroud cutouts can improve the performance of the antennas. In one example, an increase of about 0.8 mm of distance between an antenna disposed in the cutout and the grounding plane can improve the antenna performance up to about 3 dB. 
     In at least one example, the backside of shroud  3 - 404  (e.g., the side of the shroud  3 - 404  facing inward toward an internal volume of the HMD device) can include posts, brackets, or other datum features to align cameras within the HMD device when assembled. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  3 - 4 - 3 - 4 C  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  3 - 1 - 3 - 3  and  3 - 5 - 3 - 6    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  3 - 1 - 3 - 3  and  3 - 5 - 3 - 6    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  3 - 4 - 3 - 4 C . 
       FIG.  3 - 5    shows another cutaway view of an example of a front cover display  3 - 500  without the shroud or external transparent cover shown. In the illustrated example of  FIG.  3 - 5   , the assembly  3 - 500  includes a blinker film  3 - 522  separated from the lenticular lens array  3 - 510  by the gap  3 - 524 , the lenticular lens array  3 - 510  being part of or disposed on/against the display assembly  3 - 508 .  FIG.  3 - 5    also shows the display bracket  3 - 514  coupling the display assembly  3 - 508  to the bracket  3 - 534 .  FIG.  3 - 5    illustrates the curvature of the display assembly  3 - 508 , including the lenticular lens  3 - 510 , and the curvature of the blinker film  3 - 522 . 
       FIG.  3 - 6    illustrates a close-up view of a lenticular lens array  3 - 610  similar to the array  3 - 510  shown in  FIG.  3 - 5   , with individual lens or array portions  3 - 611  shown. In at least one example, because the lens array  3 - 610  can be curved, as well as the display screen  3 - 609  of the display assembly  3 - 608 , as noted above in other examples and as shown in  FIG.  3 - 6   , the angle of the array portions  3 - 611  can vary across a horizontal width or length of the lenticular lens array  3 - 610 . The variation of the angles of the array portions  3 - 611  can be such that they do not block light from the display screen as the display screen  3 - 609  curves from the point of view of someone external to and in front of the front cover assembly  3 - 600 , to which the light from the display screen  3 - 609  is projected. In one example, the display screen  3 - 609  can include a panel of pixels. In one example, the display screen  3 - 609  includes and OLED panel. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  3 - 5  and  3 - 6    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  3 - 1 - 3 - 4    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  3 - 1 - 3 - 4    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  3 - 5  and  3 - 6   . 
     IV: Shroud 
     4.0: Systems with Displays and Sensor-Hiding Structures 
       FIG.  4 - 1    is a front view of an illustrative ring-shaped cosmetic covering structure for device  4 - 10 . Illustrative ring-shaped shroud  4 - 100  of  FIG.  4 - 1    may be mounted under the inner surface of the display cover layer for display  4 - 14 F in inactive area IA. This may help hide the optical components and other internal portions of device  4 - 10  from view from the exterior of device  4 - 10 . Shroud  4 - 100  may be formed from one or more unbroken ring-shaped members and/or may be formed from multiple shroud segments that are attached using adhesive, fasteners, or other attachment structures. If desired, shroud  4 - 100  may be formed from multiple members that are sandwiched together along some or all of their lengths. In an illustrative configuration, which may sometimes be described herein as an example, shroud  4 - 100  may be formed from an inner piece (e.g., an inner full or partial ring), which may sometimes be referred to as an inner shroud member, shroud trim, or shroud trim member and may be formed from an outer piece or pieces (e.g., one or more strips of material or covering members, an full ring, one or more partial rings, etc.), which may sometimes be referred to as a shroud cover, canopy, or shroud canopy. 
     As shown in  FIG.  4 - 1   , shroud  4 - 100  may have optical component windows to accommodate components  4 - 60 ,  4 - 62 ,  4 - 64 ,  4 - 84 ,  4 - 66 ,  4 - 68 ,  4 - 70 ,  4 - 72 ,  4 - 74 ,  4 - 76 ,  4 - 78 ,  4 - 82 , and  4 - 80 . The optical component windows may be formed from through-hole openings in shroud  4 - 100 , from recesses or other partial openings that do not pass entirely through shroud  4 - 100 , from inserted optical window members in shroud through-hole openings, and/or from other shroud optical component window structures. Display  4 - 14 F may have a display cover layer that has corresponding optical component windows (through-hole openings, recessed areas, inserted window members in through-hole openings, etc.) and/or that is formed from bulk material that has desired optical properties (e.g., a display cover layer formed from one or more layers of material such as glass and/or polymer with sufficient transparency at the operating wavelength range of the overlapped optical component to allow the optical component to operate satisfactorily through the cover layer without forming openings or other window structures in the cover layer). 
     Shroud  4 - 100  may have any suitable shape. For example, the outline of shroud  4 - 100  may be rectangular with rounded corners as shown in  FIG.  4 - 1   , may have teardrop shapes on the left and right sides of device  4 - 10 , may have an oval outline, and/or may have other outlines with curved and/or straight edge segments.  FIG.  4 - 2    is a front view of a portion of shroud  4 - 100  showing how the inner and outer edges of shroud  4 - 100  may be curved (e.g., to follow a teardrop shape). Shroud  4 - 100  may, if desired, have a peripheral edge that is curved along most or all of its length. 
     The width of shroud  4 - 100  may be constant along its length or shroud  4 - 100  may have portions that are wider than others. The thickness of shroud  4 - 100  (e.g., the dimension of shroud  4 - 100  into the page in the orientation of  FIG.  4 - 1   ) may be smaller than the width of shroud  4 - 100  (the lateral dimension of shroud  4 - 100  within the page in the orientation of  FIG.  4 - 1   ) or the thickness of the shroud may be equal to or greater than the width of the shroud. The shroud may have a two-dimensional shape (e.g., shroud  4 - 100  may have a planar shape that lies in the XZ plane in the example of  FIG.  4 - 1   ) or may have a three-dimensional shape (e.g., a shape with a curved cross-sectional profile and/or a shape characterized by inner and/or outer surfaces of compound curvature). In an illustrative configuration, most or all of the inner and outer surfaces of shroud have a compound-curvature surface. 
     The optical components under inactive area IA may include components on the left and right sides of device  4 - 10  that operate in conjunction with each other. For example, scene cameras, tracking cameras, and/or structured light cameras in device  4 - 10  may be formed in pairs, each of which includes a left camera and a corresponding right camera. A left scene camera and a right scene camera may, as an example, operate together to capture overlapping images that provide device  4 - 10  with a wide field of view for gathering pass-through video. Left and right tracking cameras may operate together to track a user&#39;s hands or other external objects. Left and right structured light cameras or other three-dimensional cameras may be used together to capture three-dimensional images of the user&#39;s environment. To enhance performance of the left and right optical components in these types of paired component arrangements, it may be desirable to maintain accurate alignment between the left and right optical components. To help maintain left and right optical components on the respective left and right sides of device  4 - 10  in alignment with each other, device  4 - 10  may be provided with one or more housing structures that help support the optical components. 
     As shown in  FIG.  4 - 3   , for example, device  4 - 10  may be provided with an internal support structure such as bracket  4 - 102  that helps support optical components  4 - 104  on the left and right sides of device  4 - 10 . Components  4 - 104  may be, for example, optical components of the type shown under inactive area IA of  FIG.  4 - 1   . Bracket  4 - 102  may be formed from stiff metal and/or other rigid materials (e.g., rigid polymer, carbon fiber composite material or other fiber-composite material, etc.). A nose-bridge recess in bracket  4 - 102  (e.g., in the portion of bracket  4 - 102  near nose-bridge portion  4 - 16 NB) may help bracket  4 - 102  conform to the shape of the user&#39;s face. Bracket  4 - 102  may have an elongated strip shape that runs along a portion of the length of inactive area IA (e.g., on the lower edge of device  4 - 10 ). 
     Bracket  4 - 102  may be coupled to device  4 - 10  with attachment structures (adhesive, fasteners, press-fit connections, and/or other attachment mechanism) that allow bracket  4 - 102  to float with respect to the rest of housing portion  4 - 16 M during a drop event. The stiffness of bracket  4 - 102  and the ability of bracket  4 - 102  to shift in position somewhat relative to other housing structures without deforming the shape of bracket  4 - 102  significantly may help hold components on the left and right sides of device  4 - 10  in alignment with each other during periods of excessive stress such as when device  4 - 10  experiences high stress during an unexpected drop event. 
     In the example of  FIG.  4 - 3   , bracket  4 - 102  is mounted under inactive area IA and has a nose bridge recess with a curved edge that is configured to accommodate a user&#39;s nose when device  4 - 10  is worn on a user&#39;s head. Bracket  4 - 102  may have other shapes, if desired. Components  4 - 104  may be attached to respective left and right sides of bracket  4 - 102  and/or other supporting structures in device  4 - 10  (e.g., shroud  4 - 100 ) using adhesive, fasteners, press fit connections, and/or other attachment structures. 
       FIG.  4 - 4    is a cross-sectional top view of a portion of device  4 - 10 . As shown in  FIG.  4 - 4   , shroud  4 - 100  may overlap one or more optical components  4 - 104  in inactive area IA. Inactive area IA may form a ring-shaped border that surrounds active area AA. Display  4 - 14 F may have a display cover layer such as display cover layer  4 - 92 . Layer  4 - 92  may be formed from glass, polymer, ceramic, crystalline material such as sapphire, other materials, and/or combinations of these materials. Layer  4 - 92  may include a single layer of material or multiple stacked layers of material. In active area AA, pixels P in display panel  4 - 14 P display images that are viewable through display cover layer  92 . Shroud  4 - 100  may be absent from active area AA (e.g., shroud may have a ring shape that surrounds an opening over panel  4 - 14 P as shown in  FIG.  4 - 4   ) or shroud  4 - 100  may optionally have a portion (sometimes referred to as a canopy or shroud structure) that overlaps display panel  4 - 14 P. The canopy may be fully or partly transparent. In inactive area IA, shroud  4 - 100  overlaps components  4 - 104 . Components  4 - 104  may be optical components that emit and/or detect light that passes through transparent portions of layer  92  and shroud  4 - 100  and/or through optical component windows formed from recesses, through-hole openings, window members, and/or other window structures in layer  4 - 92  and shroud  4 - 100 . 
     Display cover layer  4 - 92  may include planar surfaces and/or curved surfaces. In an illustrative configuration, most or all of the inner and outer surfaces of display cover layer  4 - 92  have curvature. 
     The curved surfaces of display cover layer  4 - 92  may include curved surfaces that can be flattened into a plane without distortion (sometimes referred to as developable surfaces or curved surfaces without compound curvature). Surfaces such as these may, as an example overlap active area AA. The curved surfaces of display cover layer  4 - 92  may also include curved surfaces that are characterized by compound curvature (e.g., surfaces that can only be flattened into a plane with distortion, sometimes referred to as non-developable surfaces). Some or all portions of the inner and outer surfaces of display cover layer  4 - 92  in inactive area IA may, as an example, be characterized by compound curvature. This allows the periphery of display  4 - 14 F to smoothly transition away from the active area and provides an attractive appearance and compact shape for device  4 - 10 . The compound curvature of display cover layer  4 - 92  in inactive area IA may also facilitate placement of the optical components under inactive area IA in desired orientations. The inner and outer surfaces of display cover layer  4 - 92  in active area AA may have compound curvature, may be developable surfaces, or may include both developable surface areas and compound curvature areas. 
     Image data and other data gathered by optical components can be warped digitally to compensate for optical distortion associated with display cover layer  4 - 92 . To help minimize optical distortion, one or more of the optical components may optionally be oriented in a direction that is parallel or close to parallel to the surface normal of the portion of the display cover layer surface that is overlapping the optical component. 
     Consider, as an example, optical components  4 - 104  of  FIG.  4 - 4   . As shown in  FIG.  4 - 4   , some optical components such as illustrative optical component  4 - 104 B, which operates in direction  4 - 112 , may face forward (e.g., direction  4 - 112  may be parallel to or nearly parallel to the Y axis of  FIG.  4 - 4   ) in portions of display cover layer  4 - 92  where the surface normal of layer  4 - 92  is oriented parallel to the Y axis or close to parallel to the Y axis. Other optical components such as illustrative optical component  4 - 104 A, which operates in direction  4 - 110 , may be angled away from the forward direction by a non-zero angle (e.g., by an angle of at least 10°, at least 20°, less than 90°, less than 50°, or other suitable amount). Direction  4 - 110  may be parallel or closely parallel (e.g., aligned within 30°, within 20°, within 10° or other suitable amount) to the surface normal of the overlapping surface of display cover layer  92  and may lie in the XY plane of  FIG.  4 - 4    or be angled out of the XY plane (e.g., by orienting component  4 - 104 A so that direction  4 - 110  is angled upwards in the +Z direction or downwards in the −Z direction in addition to angling direction  4 - 110  away from the +Y direction as shown in  FIG.  4 - 4   ). 
     In this type of arrangement, display cover layer  4 - 92  may have compound curvature in inactive area IA and shroud  4 - 100  may have a shape with a cross-sectional profile that mirrors that of display cover layer  92  in inactive area IA (e.g., the outer and/or inner surfaces of shroud  4 - 100  in inactive area IA may be compound-curvature surfaces). When components such as components  4 - 104 A and  4 - 104 B are mounted to shroud  4 - 100  and/or are otherwise supported by the support structures of device  4 - 10  to operate through shroud  4 - 100  and display cover layer  4 - 92 , the curved shape of display cover layer  4 - 92  and shroud  4 - 100  may help allow these components to face in desired orientations (e.g., in a forward direction for components such as component  4 - 104 B or angled away from the forward direction for components such as component  4 - 104 A). 
     As an example, optical components that are mounted to the left and right sides of nose bridge portion  4 - 16 NB may be oriented respectively somewhat to the left and somewhat to the right of the +Y forward direction (e.g., to ensure an adequate angle-of-view for a pair of cameras). As another example, the curved shape of display cover layer  4 - 92  and shroud  4 - 100  along the lower edge of device  4 - 10  may allow the components in this portion to point somewhat downward out of the XY plane, which may help orient cameras such as tracking cameras towards the user&#39;s hands. 
     Display panel  4 - 14 P may be a flexible display such as a flexible organic light-emitting diode display with a flexible substrate or a light-emitting diode display formed from crystalline semiconductor light-emitting diode dies mounted on a flexible substrate. This allows display panel  4 - 14 P and the pixels of panel  4 - 14 P that form active area AA to be bent about a bend axis that runs parallel to vertical axis Z, thereby helping to wrap display  4 - 14 F and housing portion  4 - 16 M about the curved surface of the user&#39;s face. If desired, display panel  4 - 14 P may be a lenticular display configured to display three-dimensional images (e.g., an autostereoscopic display having a series of parallel lenticular lenses, each of which overlaps a respective group of multiple columns of pixels). 
     The outer and inner surfaces of display cover layer  4 - 92  may have the same shape (e.g., these surfaces may be parallel to each other) or the outer surface and inner surfaces may have different shapes. In arrangements in which display panel  4 - 14 P of display  4 - 14 F is flexible, it may be desirable to configure the inner surface of display cover layer  4 - 92  in active area AA to exhibit a bent surface shape that matches the bent outwardly-facing surface of display panel  4 - 14 P (e.g., the inner and, if desired, the outer surface of display cover layer  4 - 92  in active area AA may be developable surfaces without compound curvature to match the developable outward-facing surface of display panel  4 - 14 P). 
     Shroud  4 - 100  and display cover layer  4 - 92  may be attached to main housing portion  4 - 16 M using adhesive, screws and other fasteners, press-fit connections, and/or other attachment mechanisms. An illustrative configuration in which shroud  4 - 100  and cover layer  4 - 92  are attached to forward-facing edge of a housing wall in main housing portion  4 - 16 M using adhesive is shown in  FIG.  4 - 4  through  4 - 6   . In the example of  FIG.  4 - 5   , shroud  4 - 100  has an inner shroud member such as shroud trim  4 - 100 A and has a corresponding outer shroud member such as shroud canopy  4 - 100 B. Shroud trim  4 - 100 A and shroud canopy  4 - 100 B may be formed from metal, polymer, ceramic, glass, other materials, and/or combinations of these materials. In an illustrative example, shroud trim  4 - 100 A is formed from black polymer or other dark material and shroud canopy  4 - 100 B is formed from clear polymer. The outer surface of shroud canopy  4 - 100 B may be smooth to provide shroud  4 - 100  with a cosmetically attractive appearance. 
     A layer of pressure sensitive adhesive (see, e.g., adhesive  114 ) may be used in attaching canopy  4 - 100 B to trim  4 - 100 A. Adhesive may also be used in attaching cover layer  92  and shroud  4 - 100  to housing portion  4 - 16 M. As shown in  FIG.  4 - 5   , for example, a first adhesive such as adhesive  4 - 122  may be used to attach display cover layer  92  to shroud  4 - 100  (e.g., to a ledge in shroud trim  4 - 100 A). A second adhesive such as adhesive  4 - 124  may, in turn, be used to attach shroud  4 - 100  (e.g., shroud trim  4 - 100 A) to an adjacent lip of a wall in main housing portion  4 - 16 M. 
     In some configurations, adhesives  4 - 122  and  4 - 124  may be formed from the same type of material. In an illustrative configuration, adhesives  4 - 122  and  4 - 124  are different. Housing portion  4 - 16 M may have a wall with a lip shape that creates a shearing force on adhesive  4 - 124  as display  4 - 14 F is attached to housing portion  4 - 16 M by pressing display  4 - 14 F against housing portion  4 - 16 M in the −Y direction. In this type of scenario, it may be desirable to form adhesive  4 - 124  from an adhesive that can bond satisfactorily in the presence of shear forces such as a molten hot melt glue (thermoplastic adhesive) or other liquid adhesive rather than pressure sensitive adhesive. Adhesive  4 - 124  may, if desired, be exposed to a curing agent (ultraviolet light, moisture, etc.) before display  4 - 14 F is assembled into housing  4 - 16 M. 
     It may be desirable to repair device  4 - 10 . For example, if a user exposes display  4 - 14 F to excessive force during a drop event, it may be desirable to replace display  4 - 14 F with a new display. This can be accomplished by heating adhesive  4 - 124  to loosen the adhesive bond formed by adhesive  4 - 124 . To help prevent display cover layer  4 - 92  from detaching from shroud  4 - 100  while softening adhesive  4 - 124  with heat, adhesive  4 - 122  may be provided with a higher-temperature softening point than adhesive  4 - 124  (e.g., adhesive  4 - 122  may be a two-part hot melt glue with a higher melting point than adhesive  4 - 124 ). 
     Optical components that are overlapped by display cover layer  4 - 92  and shroud  4 - 100  in inactive area IA may transmit and/or receive light through shroud  4 - 100  and display cover layer  4 - 92 . Layer  4 - 92  may be formed from laminated glass or other clear material that allows light for each overlapped optical component  4 - 104  to pass through layer  4 - 92 . If desired, a partial recess or a through-hole opening may be formed in the portion of layer  4 - 92 . An optional optical component window member  4 - 116  may then be inserted within layer  4 - 92  (e.g., in window region  4 - 118 ). As an example, layer  4 - 92  may be formed from one or more layers of glass and/or polymer and may be characterized by a first level of light transmission at operating wavelength(s) for component  4 - 104 , whereas window member  4 - 116  may be formed from polymer, glass, and/or other materials that are characterized by a second level of light transmission at the operating wavelength(s) that is greater than the first level of light transmission. In other illustrative arrangements, no window member is inserted in layer  4 - 92  (e.g., optional window member  4 - 116  of  FIG.  4 - 5    can be omitted when layer  4 - 92  alone is sufficiently transparent to pass light for component  4 - 104 ). 
     Shroud  4 - 100  may be provided with an optical component window in region  4 - 118  to accommodate overlapped optical component  4 - 104 . Component  4 - 104  may operate at ultraviolet light wavelengths, visible light wavelengths, and/or infrared light wavelengths. To accommodate component  4 - 104  in the example of  FIG.  4 - 5   , shroud trim  4 - 100 A has been provided with a through-hole opening such as opening  120 , whereas shroud canopy  4 - 100 B has no openings in region  4 - 118 . This effectively forms a window recess in shroud  4 - 100  in alignment with components  4 - 104 . Trim  4 - 100 A may be formed from black polymer or other light-absorbing material, so the formation of opening  120  in trim  4 - 100 A may help ensure that sufficiently light may pass through region  4 - 118  to allow component  4 - 104  to operate satisfactorily. The portion of canopy  4 - 100 B that overlaps opening  4 - 120  may be transparent (e.g., clear polymer). 
     To help hide component  4 - 104  from view, the inner surface of shroud canopy  4 - 100 B of  FIG.  4 - 5    has been covered with coating  4 - 126 . Coating  4 - 126  may be used to provide region  4 - 118  with a desired outward appearance and optical properties that ensure that component  104  can operate satisfactorily. Coating  4 - 126  may be a thin-film-interference filter formed from a stack of thin-film dielectric layers of alternating refractive index values (with indices and thicknesses selected to create a desired transmission spectrum and a desired reflection spectrum for the filter), may be a layer of ink (e.g., a polymer layer including dye, pigment, and/or other colorant), and/or may be any other suitable coating with desired optical properties. 
     Consider, as an example, a scenario in which component  4 - 104  transmits and/or receives infrared light. In this type of arrangement, coating  4 - 126  may be opaque at visible wavelengths and transparent at infrared wavelengths. This helps to hide component  4 - 104  from view from the exterior of device  4 - 10  while allowing infrared light associated with the operation of component  4 - 104  to pass through shroud  4 - 100  and layer  4 - 92 . 
     As another example, consider a scenario in which component  4 - 104  is an ambient light sensor. In this configuration, coating  4 - 126  may exhibit a visible light transmission of 1-8% (as an example). This may allow sufficient visible ambient light to reach the ambient light sensor for the ambient light sensor to make an ambient light reading. At the same time, the transmission of coating  4 - 126  may be sufficiently low that coating  4 - 126  helps reduce the visibility of component  4 - 104  from the exterior of device  4 - 10 . 
     As these examples demonstrate, regions of display  4 - 14 F that overlap optical components such as component  4 - 104  of  FIG.  4 - 5    may be provided with optical component window structures in layer  4 - 92  and/or shroud  4 - 100  that help accommodate the optical component. 
     If desired, shroud  4 - 100  may be provided with a through-hole opening to accommodate an overlapped optical component. As shown in  FIG.  4 - 6   , for example, shroud  4 - 100  may contain one or more sublayers (e.g., a trim, a canopy, and/or other layers). Through-hole opening  4 - 130  may pass from the inner surface of shroud  4 - 100  to the outer surface of shroud  4 - 100 . Opening  4 - 130  may be aligned with optical component  104 . Component  104  may be mounted behind opening  4 - 130  and/or may be partly or fully receive within opening  4 - 130  as shown in  FIG.  4 - 6   . This allows light to be emitted and/or received by component  104  without being blocked by shroud  4 - 100 . 
     In the illustrative configuration of  FIG.  4 - 7   , shroud  4 - 100  also contains one more sublayers (e.g., a trim, a canopy, and/or other layers). As shown in  FIG.  4 - 7   , a through-hole opening may formed in shroud  4 - 100  in alignment with optical component  104  and may be filled with optical component window member  4 - 132  (e.g., a glass or polymer member or a window structure formed from other material and/or combinations of these materials). Optical component window member  4 - 132  has optical characteristics (e.g., light transmission, reflection, absorption, haze, etc.) that allow component  4 - 104  to transmit and/or receive light satisfactorily through region  4 - 118 . As an example, member  4 - 130  may be formed from glass that is transparent to infrared light and that is opaque or transparent to visible light. 
     As described in connection with  FIGS.  4 - 2  and  4 - 3   , there may be numerous optical components such as component  4 - 104  in inactive area IA. Each optical component may potentially have a different type of optical component window structure in shroud  4 - 100  and/or layer  4 - 92  to accommodate that component. For example, some areas of shroud  4 - 100  may have openings that receive components as described in connection with  FIG.  4 - 6   , other areas of shroud  4 - 100  may have inserted optical window member such as member  4 - 132  of  FIG.  4 - 7   , and/or other areas of shroud  4 - 100  may have partial shroud openings (e.g., non-through-hole recesses) such as opening  4 - 120  of  FIG.  4 - 5    (which may optionally be covered with a layer such as coating  4 - 126  to modify the optical properties of shroud  4 - 100 ). 
       FIG.  4 - 8    is a cross-sectional side view of a portion of a head-mounted device with a fully or partly transparent shroud covering the front face of the device. As shown in  FIG.  4 - 8   , head-mounted device  4 - 10  may include display panel  4 - 14 P for forward-facing display  4 - 14 . Panel  4 - 14 P may be a lenticular display (e.g., an autostereoscopic display with lenticular lenses  4 - 14 P′ configured to display three-dimensional images for a user). 
     In the arrangement of  FIG.  4 - 8   , display cover layer  92  has inner and outer surfaces with compound curvature in inactive area IA (e.g., a ring-shaped area running along the periphery of layer  4 - 92 ). The inner and outer surfaces of display cover layer  4 - 92  in active area AA may also have compound curvature or one or both of these surfaces may be developable surfaces. In the example of  FIG.  4 - 8   , the inner and outer surfaces of layer  4 - 92  have compound curvature in both inactive area IA and active area AA (e.g., these surfaces may be free of any developable surfaces), which may help provide device  4 - 10  with an attractive appearance. 
     The shroud of device  4 - 10  of  FIG.  4 - 8    includes a shroud trim  4 - 100 A and shroud canopy  4 - 100 B. Trim  4 - 100 A may have a ring shape and may extend around the periphery of display  14 . Canopy  4 - 100 B, which may be formed from a material such as polymer, may have an outline equal to or nearly equal to that of display cover layer  4 - 92  and may cover substantially the entire front face of device  4 - 10 . With this type of arrangement, shroud canopy  4 - 100 B overlaps all of display panel  4 - 14 P. The polymer that makes up canopy  4 - 100 B may have a bulk tint (e.g., a colorant such as dye and/or pigment that provides canopy  4 - 100 B with a desired optical transmission characteristic). For example, canopy  4 - 100 B may be tinted so that canopy  4 - 100 B exhibits a visible light transmission of 30-80%, at least 20%, at least 40%, less than 95%, less than 90%, less than 85%, less than 75%, 4-60%, or other suitable amount. By configuring canopy  10 B to exhibit partial light transmission (e.g., 30-80% or other suitable value), canopy  4 - 100 B may help visually hide internal components such as lenses  4 - 14 P′ and other structures of display  4 - 14 P from view (e.g., when display  4 - 14 P is not in use). 
     The inner surface of canopy  4 - 100 B may also be provided with an optical layer such as optical layer (optical film)  4 - 146 . Layer  4 - 146  may have texture and/or light-scattering particles that create haze. The haze may help hide the structures of display panel  4 - 14 P from view from the exterior of device  4 - 10 . Layer  4 - 146  may also have microlouvers or other features that help suppress off-axis light transmission (e.g., layer  4 - 146  may have privacy structures that reduce light transmission for light rays that are not parallel to the Y axis). Because layer  4 - 146  may contain haze and/or privacy structures, layer  4 - 146  may sometimes be referred to as a privacy layer, a haze layer, and/or a privacy and haze layer. 
     In an illustrative configuration, layer  4 - 146  may have a flexible substrate layer covered with a hazy coating. The hazy coating may be a pad-printed polymer coating that contains embedded light-scattering particles (e.g., inorganic light-scattering particles such as titanium oxide particles, etc.). The flexible substrate layer may be a privacy film such as a microlouver film or other privacy layer that prevents off-axis (away from the Y axis) viewing of display panel  4 - 14 P). 
     Haze for layer  4 - 146  may be provided using any suitable haze structures (e.g., a coating of hazy polymer having a thickness of 3-10 microns on a flexible privacy film or other substrate, a laminated hazy film, or other layer that exhibits 3%-40% haze or other suitable value, sometimes referred to as a haze coating). Haze may be provided by embedded light-scattering particles and/or surface texture (e.g., texture in layer  4 - 146  or optionally texture on the surface of canopy  4 - 100 B). The haze provided by the hazy coating of layer  4 - 146  and/or other haze structures is preferably provided sufficiently close to display  4 - 14 P that the resolution of display  4 - 14 P is not significantly affected. At the same time, the presence of the haze (e.g., the hazy coating of layer  4 - 146 ) may help hide lenses and other structures in layer  4 - 14 P from view when not in use. 
     Device  4 - 10  may have an air gap between display panel  4 - 14 P and canopy  4 - 100 B (e.g., an air gap such as air gap  4 - 144  may be present between the inwardly facing side of canopy  4 - 100 B and any coatings and/or films on this side of canopy  4 - 100 B such as haze layer  4 - 146  and the opposing upper surface of display panel  4 - 14 P (and lenses  4 - 14 P′ and the pixels on panel  4 - 14 P). The presence of air gap  4 - 144  may help ensure that lenses  4 - 14 P′ operate satisfactorily. Bracket  4 - 156  may help support display panel  4 - 14 P. 
     To help hide internal components from view, an opaque masking layer such as layer BM-1 may be formed on the inner surface of display cover layer  4 - 92  in inactive area IA. Adhesive  4 - 122  may attach layer  4 - 92  to the edge of canopy  4 - 100 B. Additional opaque masking material (see, e.g., canopy opaque masking layer BM-2) may be formed on the inner surface of canopy  4 - 100 B in inactive area IA. Adhesive  4 - 114  may be used to attach shroud trim  4 - 100 A to shroud canopy  4 - 100 B. Adhesive  4 - 124  may be used to attach shroud trim  4 - 100 A to housing portion  4 - 16 M. Adhesive  14 - 60  may be used to attach bracket  4 - 156  (which is attached with adhesive to the rear of panel  4 - 14 P) to canopy  4 - 100 B. 
     In the example of  FIG.  4 - 8   , outer surface  4 - 148  and inner surface  4 - 150  of display cover layer  4 - 92  have compound curvature in inactive area IA and in active area AA. Outer surface  4 - 152  and opposing inner surface coil  4 - 54  of shroud canopy  4 - 100 B may have matching compound curvature in inactive area IA. In active area AA, outer surface  4 - 152  and inner surface 1 coil  4 - 54  of shroud canopy  4 - 100 B may be developable surfaces (e.g., surfaces without compound curvature that exhibit a curved cross-sectional profile that bends about a single bend axis such as axis  4 - 142 ). Axis  4 - 142  is an axis that runs parallel to the Z axis in this example. Display panel  4 - 14 P may exhibit the same amount of bending about axis  4 - 142  and may also be characterized by a developable surface (e.g., the pixel array on the outer surface of panel  4 - 14 P may have a developable surface). 
     The amount of bending of canopy  4 - 100 B and the corresponding amount of bending of display panel  4 - 14 P about axis  4 - 142  may be selected to help device  4 - 10  conform to the curved shape of a user&#39;s face. 
     In the illustrative configuration of  FIG.  4 - 8   , canopy  4 - 100 B does not have any areas of compound curvature that overlap display panel  4 - 14 P. Rather, the portion of canopy  4 - 100 B that overlaps panel  4 - 14 P has inner and outer developable surfaces. If desired, one or both of surfaces  4 - 152  and  1  coil  4 - 54  may have compound curvature. For example, outer surface  4 - 152  may have compound curvature and may be configured to establish a uniform thickness for air gap  4 - 140  under some or all of inner surface  4 - 150  of layer  4 - 92 . In the example of  FIG.  4 - 9   , there is an air gap  4 - 140  of uneven thickness between layer  92  and canopy  4 - 100 B. 
     Bracket  4 - 156  may be formed from a metal sheet or other support structure and may be characterized by inner and outer surfaces that are developable surfaces (e.g., surfaces that bend about axis  4 - 142  and that do not contain areas of compound curvature). By avoiding compound curvature in the structures that support and immediately overlap display panel  4 - 14 P, display panel  4 - 14 P may be formed from a bent flexible substrate such as a polyimide substrate that bends about axis  4 - 142  without risk of creating wrinkles or other artifacts of the type that might be introduced if panel  4 - 14 P had areas of compound curvature. 
     The shroud and other structures of device  4 - 10  of  FIG.  4 - 8    (e.g., the opaque masking layer coatings such as layers BM-1 and BM-2 which may be, for example, black ink layers) may be configured to form optical windows for optical components  4 - 104 . 
       FIG.  4 - 9    shows how opaque masking layer BM-2 on canopy  4 - 100 B may have a window opening that is filled with a coating layer such as coating  4 - 170 . Optical component  4 - 104  (e.g., a flicker sensor, an ambient light sensor, and/or other photodetector) may be aligned with the window opening. A transparent canopy portion may overlap this window opening or a canopy opening may overlap this window opening. Layer BM-2 may be opaque, which helps prevent internal components in device  4 - 10  from being viewed from the exterior of device  4 - 10 . The presence of the opening in layer BM-2 allows optical component  4 - 104  to operate satisfactorily (e.g., to receive and measure ambient light). Coating  4 - 170  may be configured to allow component  104  to operate, while helping to visually hide component  4 - 104 . As an example, coating  4 - 170  may be formed from a layer of ink with a visible light transmission of 2-25%, at least 1%, at least 2%, at least 4%, less than 80%, less than 30%, or other suitable amount, whereas layer BM-2 may have a visible transmission of less than 2%, less than 1%, or less than 0.5% (as examples). 
       FIG.  4 - 10    is a cross-sectional side view of another illustrative head-mounted device optical component mounting arrangement. The arrangement of  FIG.  4 - 10    uses shroud through-hole openings in trim  4 - 100 A and canopy  4 - 100 B. These through-hole openings are aligned with an opening in display opaque masking layer BM-1 (and are optionally aligned with a corresponding opening in canopy opaque masking layer BM-2). An optional coating layer such as layer  4 - 164  may cover the optical window formed from these openings. Layer  4 - 164  and the other openings of  FIG.  4 - 11    may be aligned with optical component  4 - 104 , which may be mounted behind the shroud and/or which may have portions protruding into the through-hole openings of the shroud. In a first illustrative configuration, component  4 - 104  of  FIG.  4 - 10    is an infrared illuminator (e.g., an infrared light-emitting diode). In this type of arrangement, coating layer  4 - 164  may be formed from a layer of ink, a thin-film interference filter, or other filter layer that blocks visible light and that is transparent to infrared light (e.g., a visible-light-blocking-and-infrared-light-transmitting filter layer). In a second illustrative configuration, component  4 - 104  of  FIG.  4 - 10    is a camera (e.g., a visible pass-through camera, an infrared camera, and/or other camera operating at visible and/or infrared wavelengths). In this arrangement, coating  4 - 164  may be omitted (to pass visible and/or infrared light), may be configured to form an antireflection coating, and/or may otherwise be configured to operate with the camera. 
       FIG.  4 - 11    is a cross-sectional side view of an illustrative head-mounted device optical component mounting arrangement with an optical component window formed from a transparent window member. Transparent window member  4 - 166  (e.g., a layer of glass or polymer) may be mounted in through-hole openings in trim  4 - 100 A and canopy  4 - 100 B and may be aligned with optical component  4 - 104  and an opening in opaque masking layer BM-1 on layer  4 - 92  (and, if desired, may be aligned with an opening in opaque masking layer BM-2 on canopy  4 - 100 B). Filter coating  4 - 168  may be provided on window member  4 - 166 . In an illustrative configuration, component  4 - 104  of  FIG.  4 - 11    is a three-dimensional camera such as a time-of-flight camera or a structured light camera and may operate at infrared wavelengths. Filter  4 - 168  in this type of arrangement may be transparent to infrared light and may be transparent to visible light or may be opaque to visible light (e.g., filter  4 - 168  may be an infrared-light-transparent-and-visible-light-blocking filter). Filter coating  4 - 168  may be formed from ink, from a thin-film interference filter, or other filter structures. 
     The presence of window member  4 - 166 , which may be configured to exhibit relatively small amounts of optical distortion, may help enhance the optical performance of component  4 - 104 . If desired, optical-component-compatible surface areas for an optical component window for component  4 - 104  may be formed directly in canopy  4 - 100 B (e.g., so that canopy  4 - 100 B may overlap component  4 - 104  without forming a through-hole opening in canopy  4 - 100 B). 
     4.1: System with Cover Layer Sealing Structures 
     A head-mounted device may include a head-mounted support structure that allows the device to be worn on the head of a user. The head-mounted device may have displays that are supported by the head-mounted support structure for presenting a user with visual content. The displays may include rear-facing displays that present images to eye boxes at the rear of the head-mounted support structure. The displays may also include a forward-facing display. The forward-facing display may be mounted to the front of the head-mounted support structure and may be viewed by the user when the head-mounted device is not being worn on the user&#39;s head. The forward-facing display, which may sometimes be referred to as a publicly viewable display, may also be viewable by other people in the vicinity of the head-mounted device. 
     Optical components such as image sensors and other light sensors may be provided in the head-mounted device. In an illustrative configuration, optical components are mounted under peripheral portions of a display cover layer that protects the forward-facing display. The display cover layer, or other layers within the head-mounted device, may be formed from materials, such as glass, and, laminates, such as plastic laminates, may be formed on top and bottom surfaces of the cover layer. To protect the edges of the cover layer, encapsulation material may be coupled to the edge surface, or the head-mounted device housing structures may be modified. 
       FIGS.  4 . 1 - 1    is a side view of an illustrative head-mounted electronic device. As shown in  FIGS.  4 . 1 - 1   , head-mounted device  4 . 1 - 10  may include head-mounted support structure  4 . 1 - 26 . Support structure  4 . 1 - 26  may have walls or other structures that separate an interior region of device  4 . 1 - 10  such as interior region  4 . 1 - 42  from an exterior region surrounding device  4 . 1 - 10  such as exterior region  4 . 1 - 44 . Electrical components  4 . 1 - 40  (e.g., integrated circuits, sensors, control circuitry, light-emitting diodes, lasers, and other light-emitting devices, other control circuits and input-output devices, etc.) may be mounted on printed circuits and/or other structures within device  4 . 1 - 10  (e.g., in interior region  4 . 1 - 42 ). 
     To present a user with images for viewing from eye boxes such as eye boxes  4 . 1 - 34 , device  4 . 1 - 10  may include rear-facing displays such as displays  4 . 1 - 14 R, which may have associated lenses that focus images for viewing in the eye boxes. These components may be mounted in optical modules (e.g., a lens barrel) to form respective left and right optical systems. There may be, for example, a left rear-facing display for presenting an image through a left lens to a user&#39;s left eye in a left eye box and a right rear-facing display for presenting an image to a user&#39;s right eye in a right eye box. The user&#39;s eyes are located in eye boxes  4 . 1 - 34  at rear side R of device  4 . 1 - 10  when structure  4 . 1 - 26  rests against the outer surface of the user&#39;s face. 
     Support structure  4 . 1 - 26  may include a main support structure (sometimes referred to as a main portion or housing). The main housing support structure may extend from front side F of device  4 . 1 - 10  to opposing rear side R of device  4 . 1 - 10 . On rear side R, support structure  4 . 1 - 26  may have cushioned structures to enhance user comfort as support structure  4 . 1 - 26  rests against the user&#39;s face. If desired, support structure  4 . 1 - 26  may include optional head straps and/or other structures that allow device  4 . 1 - 10  to be worn on a head of a user. 
     Device  4 . 1 - 10  may have a publicly viewable front-facing display such as display  4 . 1 - 14 F that is mounted on front side F of support structure  4 . 1 - 26 . Display  4 . 1 - 14 F may be viewable to the user when the user is not wearing device  4 . 1 - 10  and/or may be viewable by others in the vicinity of device  4 . 1 - 10 . Display  4 . 1 - 14 F may, as an example, be visible on front side F of device  4 . 1 - 10  by an external viewer who is viewing device  4 . 1 - 10  from front side F. 
     A schematic diagram of an illustrative system that may include a head-mounted device is shown in  FIGS.  4 . 1 - 2   . As shown in  FIGS.  4 . 1 - 2   , system  4 . 1 - 8  may have one or more electronic devices  4 . 1 - 10 . Devices  4 . 1 - 10  may include a head-mounted device (e.g., device  4 . 1 - 10  of  FIGS.  4 . 1 - 1   ), accessories such as controllers and headphones, computing equipment (e.g., a cellular telephone, tablet computer, laptop computer, desktop computer, and/or remote computing equipment that supplies content to a head-mounted device), and/or other devices that communicate with each other. 
     Each electronic device  4 . 1 - 10  may have control circuitry  4 . 1 - 12 . Control circuitry  4 . 1 - 12  may include storage and processing circuitry for controlling the operation of device  4 . 1 - 10 . Circuitry  4 . 1 - 12  may include storage such as hard disk drive storage, nonvolatile memory (e.g., electrically-programmable-read-only memory configured to form a solid-state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Processing circuitry in control circuitry  4 . 1 - 12  may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors, power management units, audio chips, graphics processing units, application specific integrated circuits, and other integrated circuits. Software code may be stored on storage in circuitry  4 . 1 - 12  and run on processing circuitry in circuitry  4 . 1 - 12  to implement control operations for device  4 . 1 - 10  (e.g., data gathering operations, operations involving the adjustment of the components of device  4 . 1 - 10  using control signals, etc.). Control circuitry  4 . 1 - 12  may include wired and wireless communications circuitry. For example, control circuitry  4 . 1 - 12  may include radio-frequency transceiver circuitry such as cellular telephone transceiver circuitry, wireless local area network transceiver circuitry (e.g., WiFi® circuitry), millimeter wave transceiver circuitry, and/or other wireless communications circuitry. 
     During operation, the communications circuitry of the devices in system  4 . 1 - 8  (e.g., the communications circuitry of control circuitry  4 . 1 - 12  of device  4 . 1 - 10 ) may be used to support communication between the electronic devices. For example, one electronic device may transmit video data, audio data, control signals, and/or other data to another electronic device in system  4 . 1 - 8 . Electronic devices in system  4 . 1 - 8  may use wired and/or wireless communications circuitry to communicate through one or more communications networks (e.g., the internet, local area networks, etc.). The communications circuitry may be used to allow data to be received by device  4 . 1 - 10  from external equipment (e.g., a tethered computer, a portable device such as a handheld device or laptop computer, online computing equipment such as a remote server or other remote computing equipment, or other electrical equipment) and/or to provide data to external equipment. 
     Each device  4 . 1 - 10  in system  4 . 1 - 8  may include input-output devices  4 . 1 - 22 . Input-output devices  4 . 1 - 22  may be used to allow a user to provide device  4 . 1 - 10  with user input. Input-output devices  4 . 1 - 22  may also be used to gather information on the environment in which device  4 . 1 - 10  is operating. Output components in devices  4 . 1 - 22  may allow device  4 . 1 - 10  to provide a user with output and may be used to communicate with external electrical equipment. 
     As shown in  FIGS.  4 . 1 - 2   , input-output devices  4 . 1 - 22  may include one or more displays such as displays  4 . 1 - 14 . Displays  4 . 1 - 14  may include rear facing displays such as display  4 . 1 - 14 R of  FIGS.  4 . 1 - 1   . Device  4 . 1 - 10  may, for example, include left and right components such as left and right scanning mirror display devices or other image projectors, liquid-crystal-on-silicon display devices, digital mirror devices, or other reflective display devices, left and right display panels based on light-emitting diode pixel arrays (e.g., thin-film organic light-emitting displays with polymer or semiconductor substrates such as silicon substrates or display devices based on pixel arrays formed from crystalline semiconductor light-emitting diode dies), liquid crystal display panels, and/or or other left and right display devices that provide images to left and right eye boxes for viewing by the user&#39;s left and right eyes, respectively. Display components such as these (e.g., a thin-film organic light-emitting display with a flexible polymer substrate or a display based on a pixel array formed from crystalline semiconductor light-emitting diode dies on a flexible substrate) may also be used in forming a forward-facing display for device  4 . 1 - 10  such as forward-facing display  4 . 1 - 14 F of  FIGS.  4 . 1 - 1    (sometimes referred to as a front-facing display, front display, or publicly viewable display). 
     During operation, displays  4 . 1 - 14  (e.g., displays  4 . 1 - 14 R and/or  4 . 1 - 14 F) may be used to display visual content for a user of device  4 . 1 - 10  (e.g., still and/or moving images including pictures and pass-through video from camera sensors, text, graphics, movies, games, and/or other visual content). The content that is presented on displays  4 . 1 - 14  may, for example, include virtual objects and other content that is provided to displays  4 . 1 - 14  by control circuitry  4 . 1 - 12 . This virtual content may sometimes be referred to as computer-generated content. Computer-generated content may be displayed in the absence of real-world content or may be combined with real-world content. In some configurations, a real-world image may be captured by a camera (e.g., a forward-facing camera, sometimes referred to as a front-facing camera) and computer-generated content may be electronically overlaid on portions of the real-world image (e.g., when device  4 . 1 - 10  is a pair of virtual reality goggles). 
     Input-output circuitry  4 . 1 - 22  may include sensors  4 . 1 - 16 . Sensors  4 . 1 - 16  may include, for example, three-dimensional sensors (e.g., three-dimensional image sensors such as structured light sensors that emit beams of light and that use two-dimensional digital image sensors to gather image data for three-dimensional images from dots or other light spots that are produced when a target is illuminated by the beams of light, binocular three-dimensional image sensors that gather three-dimensional images using two or more cameras in a binocular imaging arrangement, three-dimensional lidar (light detection and ranging) sensors, sometimes referred to as time-of-flight cameras or three-dimensional time-of-flight cameras, three-dimensional radio-frequency sensors, or other sensors that gather three-dimensional image data), cameras (e.g., two-dimensional infrared and/or visible digital image sensors), gaze tracking sensors (e.g., a gaze tracking system based on an image sensor and, if desired, a light source that emits one or more beams of light that are tracked using the image sensor after reflecting from a user&#39;s eyes), touch sensors, capacitive proximity sensors, light-based (optical) proximity sensors, other proximity sensors, force sensors (e.g., strain gauges, capacitive force sensors, resistive force sensors, etc.), sensors such as contact sensors based on switches, gas sensors, pressure sensors, moisture sensors, magnetic sensors, audio sensors (microphones), ambient light sensors, flicker sensors that gather temporal information on ambient lighting conditions such as the presence of a time-varying ambient light intensity associated with artificial lighting, microphones for gathering voice commands and other audio input, sensors that are configured to gather information on motion, position, and/or orientation (e.g., accelerometers, gyroscopes, compasses, and/or inertial measurement units that include all of these sensors or a subset of one or two of these sensors), and/or other sensors. 
     User input and other information may be gathered using sensors and other input devices in input-output devices  4 . 1 - 22 . If desired, input-output devices  4 . 1 - 22  may include other devices  4 . 1 - 24  such as haptic output devices (e.g., vibrating components), light-emitting diodes, lasers, and other light sources (e.g., light-emitting devices that emit light that illuminates the environment surrounding device  4 . 1 - 10  when ambient light levels are low), speakers such as ear speakers for producing audio output, circuits for receiving wireless power, circuits for transmitting power wirelessly to other devices, batteries and other energy storage devices (e.g., capacitors), joysticks, buttons, and/or other components. 
     As described in connection with  FIGS.  4 . 1 - 1   , electronic device  4 . 1 - 10  may have head-mounted support structures such as head-mounted support structure  4 . 1 - 26  (e.g., head-mounted housing structures such as housing walls, straps, etc.). The head-mounted support structure may be configured to be worn on a head of a user (e.g., against the user&#39;s face covering the user&#39;s eyes) during operation of device  4 . 1 - 10  and may support displays  4 . 1 - 14 , sensors  4 . 1 - 16 , other components  4 . 1 - 24 , other input-output devices  4 . 1 - 22 , and control circuitry  4 . 1 - 12  (see, e.g., components  4 . 1 - 40  and displays  4 . 1 - 14 R and  4 . 1 - 14 F of  FIGS.  4 . 1 - 1   , which may include associated optical modules). 
       FIGS.  4 . 1 - 3    is a front view of device  4 . 1 - 10  in an illustrative configuration in which device  4 . 1 - 10  has a publicly viewable display such as forward-facing display  4 . 1 - 14 F. As shown in  FIGS.  4 . 1 - 3   , support structure  4 . 1 - 26  of device  4 . 1 - 10  may have right and left portions on either side of nose bridge  4 . 1 - 90 . Nose bridge  4 . 1 - 90  may be a curved exterior surface that is configured to receive and rest upon a user&#39;s nose to help support housing  4 . 1 - 26  on the head of the user. 
     Display  4 . 1 - 14 F may have an active area such as active area AA that is configured to display images and an inactive area IA that does not display images. The outline of active area AA may be rectangular, rectangular with rounded corners, may have teardrop shaped portions on the left and right sides of device  4 . 1 - 10 , may have a shape with straight edges, a shape with curved edges, a shape with a peripheral edge that has both straight and curved portions, and/or other suitable outlines. As shown in  FIGS.  4 . 1 - 3   , active area AA may have a curved recessed portion at nose bridge  4 . 1 - 90 . The presence of the nose-shaped recess in active area AA may help fit active area AA within the available space of housing  4 . 1 - 26  without overly limiting the size of active area AA. 
     Active area AA contains an array of pixels. The pixels may be, for example, light-emitting diode pixels formed from thin-film organic light-emitting diodes or crystalline semiconductor light-emitting diode dies (sometimes referred to as micro-light-emitting diodes) on a flexible display panel substrate. Configurations in which display  4 . 1 - 14 F uses other display technologies may also be used, if desired. Illustrative arrangements in which display  4 . 1 - 14  is formed from a light-emitting diode display such as an organic light-emitting diode display that is formed on a flexible substrate (e.g., a substrate formed from a bendable layer of polyimide or a sheet of other flexible polymer) may sometimes be described herein as an example. The pixels of active area AA may be formed on a display device such as a display panel (e.g., a flexible organic light-emitting diode display panel). In some configurations, the outline of active area AA may have a peripheral edge that contains straight segments or a combination of straight and curved segments. Configurations in which the entire outline of active area AA is characterized by a curved peripheral edge may also be used. 
     Display  4 . 1 - 14 F may have an inactive area such as inactive area IA that is free of pixels and that does not display images. Inactive area IA may form an inactive border region that runs along one more portions of the peripheral edge of active area AA. In the illustrative configuration of  FIGS.  4 . 1 - 3   , inactive area IA has a ring shape that surrounds active area AA and forms an inactive border. In this type of arrangement, the width of inactive area IA may be relatively constant and the inner and outer edges of area IA may be characterized by straight and/or curved segments or may be curved along their entire lengths. For example, the outer edge of area IA (e.g., the periphery of display  4 . 1 - 14 F) may have a curved outline that runs parallel to the curved edge of active area AA. 
     In some configurations, device  4 . 1 - 10  may operate with other devices in system  4 . 1 - 8  (e.g., wireless controllers and other accessories). These accessories may have magnetic sensors that sense the direction and intensity of magnetic fields. Device  4 . 1 - 10  may have one or more electromagnets configured to emit a magnetic field. The magnetic field can be measured by the wireless accessories near device  4 . 1 - 10 , so that the accessories can determine their orientation and position relative to device  4 . 1 - 10 . This allows the accessories to wirelessly provide device  4 . 1 - 10  with real-time information on their current position, orientation, and movement so that the accessories can serve as wireless controllers. The accessories may include wearable devices, handled devices, and other input devices. 
     In an illustrative configuration, device  4 . 1 - 10  may have a coil that runs around the perimeter of display  4 . 1 - 14 F (e.g., under inactive area IA along the periphery of active area AA). The coil may have any suitable number of turns (e.g., 1-10, at least 2, at least 5, at least 10, 10-50, fewer than 100, fewer than 25, fewer than 6, etc.). These turns may be formed from metal traces on a substrate, may be formed from wire, and/or may be formed from other conductive lines. During operation, control circuitry  4 . 1 - 12  may supply the coil with an alternating-current (AC) drive signal. The drive signal may have a frequency of at least 1 kHz, at least 10 kHz, at least 100 kHz, at least 1 MHz, less than 10 MHz, less than 3 MHz, less than 300 kHz, or less than 30 kHz (as examples). As AC current flows through the coil a corresponding magnetic field is produced in the vicinity of device  4 . 1 - 10 . Electronic devices such as wireless controllers with magnetic sensors that are in the vicinity of device  4 . 1 - 10  may use the magnetic field as a reference so that the wireless controllers can determine their orientation, position, and/or movement while being moved relative to device  4 . 1 - 10  to provide device  4 . 1 - 10  with input. 
     Consider, as an example, a handheld wireless controller that is used in controlling the operation of device  4 . 1 - 10 . During operation, device  4 . 1 - 10  uses the coil to emit a magnetic field. As the handheld wireless controller is moved, the magnetic sensors of the controller can monitor the location of the controller and the movement of the controller relative to device  4 . 1 - 10  by monitoring the strength, orientation, and change to the strength and/or orientation of the magnetic field emitted by the coil as the controller is moved through the air by the user. The electronic device can then wirelessly transmit information on the location and orientation of the controller to device  4 . 1 - 10 . In this way, a handheld controller, wearable controller, or other external accessory can be manipulated by a user to provide device  4 . 1 - 10  with air gestures, pointing input, steering input, and/or other user input. 
     Device  4 . 1 - 10  may have components such as optical components (e.g., optical sensors among sensors  4 . 1 - 16  of  FIGS.  4 . 1 - 2   ). These components may be mounted in any suitable location on head-mounted support structure  4 . 1 - 26  (e.g. on a head strap, on housing  4 . 1 - 26 , etc.). Optical components and other components may face rearwardly (e.g., when mounted on the rear face of device  4 . 1 - 10 ), may face to the side (e.g. to the left or right), may face downwardly or upwardly, may face to the front of device  4 . 1 - 10  (e.g., when mounted on the front face of device  4 . 1 - 10 ), may be mounted so as to point in any combination of these directions (e.g., to the front, to the right, and downward) and/or may be mounted in other suitable orientations. In an illustrative configuration, at least some of the components of device  4 . 1 - 10  are mounted so as to face outwardly to the front (and optionally to the sides and/or up and down). For example, forward-facing cameras for pass-through video may be mounted on the left and right sides of the front of device  4 . 1 - 10  in a configuration in which the cameras diverge slightly along the horizontal dimension so that the fields of view of these cameras overlap somewhat while capturing a wide-angle image of the environment in front of device  4 . 1 - 10 . The captured image may, if desired, include portions of the user&#39;s surroundings that are below, above, and to the sides of the area directly in front of device  4 . 1 - 10 . 
     To help hide components such as optical components from view from the exterior of device  4 . 1 - 10 , it may be desirable to cover some or all of the components with cosmetic covering structures. The covering structures may include transparent portions (e.g., optical component windows) that are characterized by sufficient optical transparency to allow overlapped optical components to operate satisfactorily. For example, an ambient light sensor may be covered with a layer that appears opaque to an external viewer to help hide the ambient light sensor from view, but that allows sufficient ambient light to pass to the ambient light sensor for the ambient light sensor to make a satisfactory ambient light measurement. As another example, an optical component that emits infrared light may be overlapped with a visibly opaque material that is transparent to infrared light. 
     In an illustrative configuration, optical components for device  4 . 1 - 10  may be mounted in inactive area IA of  FIGS.  4 . 1 - 3    and cosmetic covering structures may be formed in a ring shape overlapping the optical components in inactive area IA. Cosmetic covering structures may be formed from ink, polymer structures, structures that include metal, glass, other materials, and/or combinations of these materials. In an illustrative configuration, a cosmetic covering structure may be formed from a ring-shaped member having a footprint that matches the footprint of inactive area IA. If, for example, active area AA has left and right portions with teardrop shapes, the ring-shaped member may have curved edges that follow the curved periphery of the teardrop-shaped portions of active area AA. The ring-shaped member may be formed from one or more polymer structures (e.g., the ring-shaped member may be formed from a polymer ring). Because the ring-shaped member can help hide overlapped components from view, the ring-shaped member may sometimes be referred to as a shroud or ring-shaped shroud member. The outward appearance of the shroud or other cosmetic covering structures may be characterized by a neutral color (white, black, or gray) or a non-neutral color (e.g., blue, red, green, gold, rose gold, etc.). 
     Display  4 . 1 - 14 F may, if desired, have a protective display cover layer. The cover layer may overlap active area AA and inactive area IA (e.g., the entire front surface of device  4 . 1 - 10  as viewed from front F of  FIGS.  4 . 1 - 1    may be covered by the cover layer). The cover layer, which may sometimes be referred to as a housing wall or transparent housing wall, may have a rectangular outline, an outline with teardrop portions, an oval outline, or other shape with curved and/or straight edges. 
     The cover layer may be formed from a transparent material such as glass, polymer, transparent crystalline material such as sapphire, clear ceramic, other transparent materials, and/or combinations of these materials. As an example, a protective display cover layer for display  4 . 1 - 14 F may be formed from safety glass (e.g., laminated glass that includes a clear glass layer with a laminated polymer film). Optional coating layers may be applied to the surfaces of the display cover layer. If desired, the display cover layer may be chemically strengthened (e.g., using an ion-exchange process to create an outer layer of material under compressive stress that resists scratching). In some configurations, the display cover layer may be formed from a stack of two or more layers of material (e.g., first and second structural glass layers, a rigid polymer layer coupled to a glass layer or another rigid polymer layer, etc.) to enhance the performance of the cover layer. 
     In active area AA, the display cover layer may overlap the pixels of display panel  4 . 1 - 14 P. The display cover layer in active area AA is preferably transparent to allow viewing of images presented on display panel  4 . 1 - 14 P. In inactive area IA, the display cover layer may overlap the ring-shaped shroud or other cosmetic covering structure. The shroud and/or other covering structures (e.g., opaque ink coatings on the inner surface of the display cover layer and/or structures) may be sufficiently opaque to help hide some or all of the optical components in inactive area IA from view. Windows may be provided in the shroud or other cosmetic covering structures to help ensure that the optical components that are overlapped by these structures operate satisfactorily. Windows may be formed from holes, may be formed from areas of the shroud or other cosmetic covering structures that have been locally thinned to enhance light transmission, may be formed from window members with desired light transmission properties that have been inserted into mating openings in the shroud, and/or may be formed from other shroud window structures. 
     In the example of  FIGS.  4 . 1 - 3   , device  4 . 1 - 10  includes optical components such as optical components  4 . 1 - 60 ,  4 . 1 - 62 ,  4 . 1 - 64 ,  4 . 1 - 66 ,  4 . 1 - 68 ,  4 . 1 - 70 ,  4 . 1 - 72 ,  4 . 1 - 74 ,  4 . 1 - 76 ,  4 . 1 - 78 , and  4 . 1 - 80  (as an example). Each of these optical components (e.g., optical sensors selected from among sensors  4 . 1 - 16  of  FIGS.  4 . 1 - 2   , light-emitting devices, etc.) may be configured to detect light and, if desired to emit light (e.g., ultraviolet light, visible light, and/or infrared light). 
     In an illustrative configuration, optical component  4 . 1 - 60  may sense ambient light (e.g., visible ambient light). In particular, optical component  4 . 1 - 60  may have a photodetector that senses variations in ambient light intensity as a function of time. If, as an example, a user is operating in an environment with an artificial light source, the light source may emit light at a frequency associated with its source of wall power (e.g., alternating-current mains power at 4.1-60 Hz). The photodetector of component  4 . 1 - 60  may sense that the artificial light from the artificial light source is characterized by 60 Hz fluctuations in intensity. Control circuitry  4 . 1 - 12  can use this information to adjust a clock or other timing signal associated with the operation of image sensors in device  4 . 1 - 10  to help avoid undesired interference between the light source frequency and the frame rate or other frequency associated with image capture operations. Control circuitry  4 . 1 - 12  can also use measurements from component  4 . 1 - 60  to help identify the presence of artificial lighting and the type of artificial lighting that is present. In this way, control circuitry  4 . 1 - 12  can detect the presence of lights such as fluorescent lights or other lights with known non-ideal color characteristics and can make compensating color cast adjustments (e.g., white point adjustments) to color-sensitive components such as cameras and displays. Because optical component  4 . 1 - 60  may measure fluctuations in light intensity, component  4 . 1 - 60  may sometimes be referred to as a flicker sensor or ambient light frequency sensor. 
     Optical component  4 . 1 - 62  may be an ambient light sensor. The ambient light sensor may include one or more photodetectors. In a single-photodetector configuration, the ambient light sensor may be a monochrome sensor that measures ambient light intensity. In a multi-photodetector configuration, each photodetector may be overlapped by an optical filter that passes a different band of wavelengths (e.g. different visible and/or infrared passbands). The optical filter passbands may overlap at their edges. This allows component  4 . 1 - 62  to serve as a color ambient light sensor that measures both ambient light intensity and ambient light color (e.g., by measuring color coordinates for the ambient light). During operation of device  4 . 1 - 10 , control circuitry  4 . 1 - 12  can take action based on measured ambient light intensity and color. As an example, the white point of a display or image sensor may be adjusted or other display or image sensor color adjustments may be made based on measured ambient light color. The intensity of a display may be adjusted based on light intensity. For example, the brightness of display  4 . 1 - 14 F may be increased in bright ambient lighting conditions to enhance the visibility of the image on the display and the brightness of display  4 . 1 - 14 F may be decreased in dim lighting conditions to conserve power. Image sensor operations and/or light source operations may also be adjusted based on ambient light readings. 
     The optical components in active area IA may also include components along the sides of device  4 . 1 - 10  such as components  4 . 1 - 80  and  4 . 1 - 64 . Optical components  4 . 1 - 80  and  4 . 1 - 64  may be pose-tracking cameras that are used to help monitor the orientation and movement of device  4 . 1 - 10 . Components  4 . 1 - 80  and  4 . 1 - 64  may be visible light cameras (and/or cameras that are sensitive at visible and infrared wavelengths) and may, in conjunction with an inertial measurement unit, form a visual inertial odometry (VIO) system. 
     Optical components  4 . 1 - 78  and  4 . 1 - 66  may be visible-light cameras that capture real-time images of the environment surrounding device  4 . 1 - 10 . These cameras, which may sometimes be referred to as scene cameras or pass-through-video cameras, may capture moving images that are displayed in real time to displays  4 . 1 - 14 R for viewing by the user when the user&#39;s eyes are located in eye boxes  4 . 1 - 34  at the rear of device  4 . 1 - 10 . By displaying pass-through images (pass-through video) to the user in this way, the user may be provided with real-time information on the user&#39;s surroundings. If desired, virtual content (e.g. computer-generated images) may be overlaid over some of the pass-through video. Device  4 . 1 - 10  may also operate in a non-pass-through-video mode in which components  4 . 1 - 78  and  4 . 1 - 66  are turned off and the user is provided only with movie content, game content, and/or other virtual content that does not contain real-time real-world images. 
     Input-output devices  4 . 1 - 22  of device  4 . 1 - 10  may gather user input that is used in controlling the operation of device  4 . 1 - 10 . As an example, a microphone in device  4 . 1 - 10  may gather voice commands. Buttons, touch sensors, force sensors, and other input devices may gather user input from a user&#39;s finger or other external object that is contacting device  4 . 1 - 10 . In some configurations, it may be desirable to monitor a user&#39;s hand gestures or the motion of other user body parts. This allows the user&#39;s hand locations or other body part locations to be replicated in a game or other virtual environment and allows the user&#39;s hand motions to serve as hand gestures (air gestures) that control the operation of device  4 . 1 - 10 . User input such as hand gesture input can be captured using cameras that operate at visible and infrared wavelengths such as tracking cameras (e.g., optical components  4 . 1 - 76  and  4 . 1 - 68 ). Tracking cameras such as these may also track fiducials and other recognizable features on controllers and other external accessories (additional devices  4 . 1 - 10  of system  4 . 1 - 8 ) during use of these controllers in controlling the operation of device  4 . 1 - 10 . If desired, tracking cameras can help determine the position and orientation of a handheld controller or wearable controller that senses its location and orientation by measuring the magnetic field produced by coil  4 . 1 - 54 . The use of tracking cameras may therefore help track hand motions and controller motions that are used in moving pointers and other virtual objects being displayed for a user and can otherwise assist in controlling the operation of device  4 . 1 - 10 . 
     Tracking cameras may operate satisfactorily in the presence of sufficient ambient light (e.g., bright visible ambient lighting conditions). In dim environments, supplemental illumination may be provided by supplemental light sources such as supplemental infrared light sources (e.g., optical components  4 . 1 - 82  and  4 . 1 - 84 ). The infrared light sources may each include one or more light-emitting devices (light-emitting diodes or lasers) and may each be configured to provide fixed and/or steerable beams of infrared light that serve as supplemental illumination for the tracking cameras. If desired, the infrared light sources may be turned off in bright ambient lighting conditions and may be turned on in response to detection of dim ambient lighting (e.g., using the ambient light sensing capabilities of optical component  4 . 1 - 62 ). 
     Three-dimensional sensors in device  4 . 1 - 10  may be used to perform biometric identification operations (e.g., facial identification for authentication), may be used to determine the three-dimensional shapes of objects in the user&#39;s environment (e.g., to map the user&#39;s environment so that a matching virtual environment can be created for the user), and/or to otherwise gather three-dimensional content during operation of device  4 . 1 - 10 . As an example, optical components  4 . 1 - 74  and  4 . 1 - 70  may be three-dimensional structured light image sensors. Each three-dimensional structured light image sensor may have one or more light sources that provide structured light (e.g., a dot projector that projects an array of infrared dots onto the environment, a structured light source that produces a grid of lines, or other structured light component that emits structured light). Each of the three-dimensional structured light image sensors may also include a flood illuminator (e.g., a light-emitting diode or laser that emits a wide beam of infrared light). Using flood illumination and structured light illumination, optical components  4 . 1 - 74  and  4 . 1 - 70  may capture facial images, images of objects in the environment surrounding device  4 . 1 - 10 , etc. 
     Optical component  4 . 1 - 72  may be an infrared three-dimensional time-of-flight camera that uses time-of-flight measurements on emitted light to gather three-dimensional images of objects in the environment surrounding device  4 . 1 - 10 . Component  4 . 1 - 72  may have a longer range and a narrower field of view than the three-dimensional structured light cameras of optical components  4 . 1 - 74  and  4 . 1 - 70 . The operating range of component  4 . 1 - 72  may be 30 cm to 7 m, 60 cm to 6 m, 70 cm to 5 m, or other suitable operating range (as examples). 
       FIGS.  4 . 1 - 4    is a front view of an illustrative ring-shaped cosmetic covering structure for device  4 . 1 - 10 . Illustrative ring-shaped shroud  4 . 1 - 100  of  FIGS.  4 . 1 - 4    may be mounted under the inner surface of the display cover layer for display  4 . 1 - 14 F in inactive area IA. This may help hide the optical components and other internal portions of device  4 . 1 - 10  from view from the exterior of device  4 . 1 - 10 . Shroud  4 . 1 - 100  may be formed from one or more unbroken ring-shaped members and/or may be formed from multiple shroud segments that are attached using adhesive, fasteners, or other attachment structures. If desired, shroud  4 . 1 - 100  may be formed from multiple members that are sandwiched together along some or all of their lengths. In an illustrative configuration, which may sometimes be described herein as an example, shroud  4 . 1 - 100  may be formed from an inner piece (e.g., an inner full or partial ring), which may sometimes be referred to as an inner shroud member, shroud trim, or shroud trim member and may be formed from an outer piece or pieces (e.g., one or more strips of material or covering members, an full ring, one or more partial rings, etc.), which may sometimes be referred to as a shroud cover, canopy, or shroud canopy. 
     As shown in  FIGS.  4 . 1 - 4   , shroud  4 . 1 - 100  may have optical component windows to accommodate components  4 . 1 - 60 ,  4 . 1 - 62 ,  4 . 1 - 64 ,  4 . 1 - 84 ,  4 . 1 - 66 ,  4 . 1 - 68 ,  4 . 1 - 70 ,  4 . 1 - 72 ,  4 . 1 - 74 ,  4 . 1 - 76 ,  4 . 1 - 78 ,  4 . 1 - 82 , and  4 . 1 - 80 . The optical component windows may be formed from through-hole openings in shroud  4 . 1 - 100 , from recesses or other partial openings that do not pass entirely through shroud  4 . 1 - 100 , from inserted optical window members in shroud through-hole openings, and/or from other shroud optical component window structures. Display  4 . 1 - 14 F may have a display cover layer that has corresponding optical component windows (through-hole openings, recessed areas, inserted window members in through-hole openings, etc.) and/or that is formed from bulk material that has desired optical properties (e.g., a display cover layer formed from one or more layers of material such as glass and/or polymer with sufficient transparency at the operating wavelength range of the overlapped optical component to allow the optical component to operate satisfactorily through the cover layer without forming openings or other window structures in the cover layer). 
     Shroud  4 . 1 - 100  may have any suitable shape. For example, the outline of shroud  4 . 1 - 100  may be rectangular with rounded corners as shown in  FIGS.  4 . 1 - 4   , may have teardrop shapes on the left and right sides of device  4 . 1 - 10 , may have an oval outline, and/or may have other outlines with curved and/or straight edge segments. For example, the inner and outer edges of shroud  4 . 1 - 100  may be curved (e.g., to follow a teardrop shape). Shroud  4 . 1 - 100  may, if desired, have a peripheral edge that is curved along most or all of its length. 
     The width of shroud  4 . 1 - 100  may be constant along its length or shroud  4 . 1 - 100  may have portions that are wider than others. The thickness of shroud  4 . 1 - 100  (e.g., the dimension of shroud  4 . 1 - 100  into the page in the orientation of  FIGS.  4 . 1 - 4   ) may be smaller than the width of shroud  4 . 1 - 100  (the lateral dimension of shroud  4 . 1 - 100  within the page in the orientation of  FIGS.  4 . 1 - 4   ) or the thickness of the shroud may be equal to or greater than the width of the shroud. The shroud may have a two-dimensional shape (e.g., shroud  4 . 1 - 100  may have a planar shape) or may have a three-dimensional shape (e.g., a shape with a curved cross-sectional profile and/or a shape characterized by inner and/or outer surfaces of compound curvature). In an illustrative configuration, most or all of the inner and outer surfaces of shroud have a compound-curvature surface. 
     The optical components under inactive area IA may include components on the left and right sides of device  4 . 1 - 10  that operate in conjunction with each other, for example, scene cameras, tracking cameras, and/or structured light cameras in device  4 . 1 - 10  may be formed in pairs, each of which includes a left camera and a corresponding right camera. A left scene camera and a right scene camera may, as an example, operate together to capture overlapping images that provide device  4 . 1 - 10  with a wide field of view for gathering pass-through video. Left and right tracking cameras may operate together to track a user&#39;s hands or other external objects. Left and right structured light cameras or other three-dimensional cameras may be used together to capture three-dimensional images of the user&#39;s environment. To enhance performance of the left and right optical components in these types of paired component arrangements, it may be desirable to maintain accurate alignment between the left and right optical components. To help maintain left and right optical components on the respective left and right sides of device  4 . 1 - 10  in alignment with each other, device  4 . 1 - 10  may be provided with one or more housing structures that help support the optical components. An illustrative example of device  4 . 1 - 10  having housing structures that support the optical components and a cover layer that overlaps the optical components is shown in  FIGS.  4 . 1 - 5   . 
     As shown in  FIGS.  4 . 1 - 5   , shroud  4 . 1 - 100  and display cover layer  4 . 1 - 92  may be attached to housing  4 . 1 - 26  using adhesive, screws and other fasteners, press-fit connections, and/or other attachment mechanisms. An illustrative configuration in which shroud  4 . 1 - 100  and cover layer  4 . 1 - 92  are attached to forward-facing edge of a housing wall in the main housing portion of structure  4 . 1 - 26  using adhesive is shown in  FIGS.  4 . 1 - 5   . In the example of  FIGS.  4 . 1 - 5   , shroud  4 . 1 - 100  has an inner shroud member such as shroud trim  4 . 1 - 100 A and has a corresponding outer shroud member such as shroud canopy  4 . 1 - 100 B. Shroud trim  4 . 1 - 100 A and shroud canopy  4 . 1 - 100 B may be formed from metal, polymer, ceramic, glass, other materials, and/or combinations of these materials. In an illustrative example, shroud trim  4 . 1 - 100 A is formed from black polymer or other dark material and shroud canopy  4 . 1 - 100 B is formed from clear polymer. The outer surface of shroud canopy  4 . 1 - 100 B may be smooth to provide shroud  4 . 1 - 100  with a cosmetically attractive appearance. 
     A layer of pressure sensitive adhesive may be used in attaching canopy  4 . 1 - 100 B to trim  4 . 1 - 100 A, or canopy  4 . 1 - 100 B may be formed integrally with trim  4 . 1 - 100 A. Adhesive may also be used in attaching cover layer  4 . 1 - 92  and shroud  4 . 1 - 100  to housing portion  4 . 1 - 26 . As shown in  FIGS.  4 . 1 - 5   , for example, a first adhesive such as adhesive  4 . 1 - 122  may be used to attach display cover layer  4 . 1 - 92  to shroud  4 . 1 - 100  (e.g., to a ledge in shroud trim  4 . 1 - 100 A). A second adhesive such as adhesive  4 . 1 - 124  may, in turn, be used to attach shroud  4 . 1 - 100  (e.g., shroud trim  4 . 1 - 100 A) to an adjacent lip of a wall in housing  4 . 1 - 26 . 
     In some configurations, adhesives  4 . 1 - 122  and  4 . 1 - 124  may be formed from the same type of material. In an illustrative configuration, adhesives  4 . 1 - 122  and  4 . 1 - 124  are different. Housing portion  4 . 1 - 26  may have a wall with a lip shape that creates a shearing force on adhesive  4 . 1 - 124  as display  4 . 1 - 14 F is attached to housing  4 . 1 - 26  by pressing display  4 . 1 - 14 F against housing  4 . 1 - 26 . In this type of scenario, it may be desirable to form adhesive  4 . 1 - 124  from an adhesive that can bond satisfactorily in the presence of shear forces such as a molten hot melt glue (thermoplastic adhesive) or other liquid adhesive rather than pressure sensitive adhesive. Adhesive  4 . 1 - 124  may, if desired, be exposed to a curing agent (ultraviolet light, moisture, etc.) before display  4 . 1 - 14 F is assembled into housing  4 . 1 - 26 . 
     Adhesive  4 . 1 - 124  may be heated to loosen the adhesive bond formed by adhesive  4 . 1 - 124 , if desired. To help prevent display cover layer  4 . 1 - 92  from detaching from shroud  4 . 1 - 100  while softening adhesive  4 . 1 - 124  with heat, adhesive  4 . 1 - 122  may be provided with a higher-temperature softening point than adhesive  4 . 1 - 124  (e.g., adhesive  4 . 1 - 122  may be a two-part hot melt glue with a higher melting point than adhesive  4 . 1 - 124 ). 
     Optical components that are overlapped by display cover layer  4 . 1 - 92  and shroud  4 . 1 - 100  in inactive area IA may transmit and/or receive light through shroud  4 . 1 - 100  and display cover layer  4 . 1 - 92 . Layer  4 . 1 - 92  may be formed from a single layer of glass, laminated glass, or other clear material that allows light for each overlapped optical component  4 . 1 - 104  to pass through layer  4 . 1 - 92 . If desired, a partial recess or a through-hole opening may be formed in the portion of layer  4 . 1 - 92 . An optional optical component window member may then be inserted within layer  4 . 1 - 92  (e.g., a window that overlaps component  4 . 1 - 104 ). As an example, layer  4 . 1 - 92  may be formed from one or more layers of glass and/or polymer and may be characterized by a first level of light transmission at operating wavelength(s) for component  4 . 1 - 104 . A window member in layer  4 . 1 - 92  may be formed from polymer, glass, and/or other materials that are characterized by a second level of light transmission at the operating wavelength(s) that is greater than the first level of light transmission. In other illustrative arrangements, no window member is inserted in layer  4 . 1 - 92  (e.g., when layer  4 . 1 - 92  alone is sufficiently transparent to pass light for component  4 . 1 - 104 ). 
     Shroud  4 . 1 - 100  may be provided with an optical component window that overlaps optical component to help accommodate overlapped optical component  4 . 1 - 104 . Component  4 . 1 - 104  may operate at ultraviolet light wavelengths, visible light wavelengths, and/or infrared light wavelengths. To accommodate component  4 . 1 - 104 , shroud trim  4 . 1 - 100 A has been provided with a through-hole opening, whereas shroud canopy  4 . 1 - 100 B has no openings overlapping component  4 . 1 - 104 . This effectively forms a window recess in shroud  4 . 1 - 100  in alignment with components  4 . 1 - 104 . Trim  4 . 1 - 100 A may be formed from black polymer or other light-absorbing material, so the formation of opening  4 . 1 - 120  in trim  4 . 1 - 100 A may help ensure that sufficiently light may pass through to allow component  4 . 1 - 104  to operate satisfactorily. The portion of canopy  4 . 1 - 100 B that overlaps component  4 . 1 - 104  may be transparent (e.g., clear polymer). Alternatively, canopy  4 . 1 - 100 B may be formed from light-absorbing material, and a portion of canopy  4 . 1 - 100 B overlapping component  4 . 1 - 104  may be removed. 
     To help hide component  4 . 1 - 104  from view, the inner surface of shroud canopy  4 . 1 - 100 B may be covered with one or more coatings, which may be used to provide region the region overlapping component  4 . 1 - 104  with a desired outward appearance and optical properties that ensure that component  4 . 1 - 104  can operate satisfactorily. The coatings may include a thin-film-interference filter formed from a stack of thin-film dielectric layers of alternating refractive index values (with indices and thicknesses selected to create a desired transmission spectrum and a desired reflection spectrum for the filter), may include a layer of ink (e.g., a polymer layer including dye, pigment, and/or other colorant), and/or may include any other suitable coating with desired optical properties. 
     Consider, as an example, a scenario in which component  4 . 1 - 104  transmits and/or receives infrared light. In this type of arrangement, canopy  4 . 1 - 100 B may be coated with a coating that is opaque at visible wavelengths and transparent at infrared wavelengths. This helps to hide component  4 . 1 - 104  from view from the exterior of device  4 . 1 - 10  while allowing infrared light associated with the operation of component  4 . 1 - 104  to pass through shroud  4 . 1 - 100  and layer  4 . 1 - 92 . 
     As another example, consider a scenario in which component  4 . 1 - 104  is an ambient light sensor. In this configuration, canopy  4 . 1 - 100 B may be coated with a coating that exhibits a visible light transmission of 1-8% (as an example). This may allow sufficient visible ambient light to reach the ambient light sensor for the ambient light sensor to make an ambient light reading. At the same time, the transmission of the coating may be sufficiently low to reduce the visibility of component  4 . 1 - 104  from the exterior of device  4 . 1 - 10 . 
     As these examples demonstrate, regions of display  4 . 1 - 14 F that overlap optical components such as component  4 . 1 - 104  of  FIGS.  4 . 1 - 5    may be provided with optical component window structures in layer  4 . 1 - 92  and/or shroud  4 . 1 - 100  that help accommodate the optical component. 
     As described in connection with  FIGS.  4 . 1 - 3  and  4 . 1 - 4   , there may be numerous optical components such as component  4 . 1 - 104  in inactive area IA. Each optical component may potentially have a different type of optical component window structure in shroud  4 . 1 - 100  and/or layer  4 . 1 - 92  to accommodate that component. For example, some areas of shroud  4 . 1 - 100  may have openings that receive components, other areas of shroud  4 . 1 - 100  may have inserted optical window member, and/or other areas of shroud  4 . 1 - 100  may have partial shroud openings (e.g., non-through-hole recesses) such as the opening of  FIGS.  4 . 1 - 8    (which may optionally be coated to modify the optical properties of shroud  4 . 1 - 100 ). 
     In some embodiments, it may be desirable to provide encapsulation material over cover layer  4 . 1 - 92 . An illustrative example of cover layer  4 . 1 - 92  with encapsulation is shown in  FIGS.  4 . 1 - 6   . 
     As shown in  FIGS.  4 . 1 - 6   , cover layer  4 . 1 - 92  may be coupled to shroud  4 . 1 - 100 . Cover layer  4 . 1 - 92  may include glass layer  4 . 1 - 126 , front laminate  4 . 1 - 128  and rear laminate  4 . 1 - 130 . Front laminate  4 . 1 - 128  and rear laminate  4 . 1 - 128  may be for example, layers of plastic that are laminated to cover layer  4 . 1 - 92 , layers of plastic that are adhesively attached to cover layer  4 . 1 - 92 , or other protective material that is attached to the front and rear surfaces of glass layer  4 . 1 - 126 . Although not shown in  FIGS.  4 . 1 - 6   , multiple layers, such as antireflection coatings, antismudge coatings, acrylic layers, or other desired layers, may be included as part of cover layer  4 . 1 - 92 . 
     Although front laminate  4 . 1 - 128  and rear laminate  4 . 1 - 130  may protect the front and rear of glass layer  4 . 1 - 126 , an edge surface of glass layer  4 . 1 - 126  may still be exposed. To further protect the edge surface of glass layer  4 . 1 - 126 , encapsulation material  4 . 1 - 132  may be attached to the edge surface of glass layer  4 . 1 - 126 . Encapsulation material  4 . 1 - 132  may be an epoxy material, such as a ductile epoxy, that seals the edge surface of glass layer  4 . 1 - 126  and protects glass layer  4 . 1 - 126  at the edge surface. Alternatively, acrylate, polyvinyl butyral (PVB), polyurethane, or moisture cure materials may be used for encapsulation material  4 . 1 - 132 . 
     In some embodiments, encapsulation material  4 . 1 - 132  may be an epoxy that adheres to glass layer  4 . 1 - 126  without a primer (e.g., encapsulation material  4 . 1 - 132  may be a primer-less adhesive). Moreover, encapsulation material  4 . 1 - 132  may have adequate ductility to elongate and not fracture, such as a Young&#39;s modulus of less than 3 GPa, less than 4 GPa, less than 2.5 GPa, or other suitable modulus. Additionally, encapsulation material  4 . 1 - 132  may be chemically resistant to chemicals and resistant to degradation due to solar exposure. It may also be desirable for encapsulation material  4 . 1 - 132  to match an appearance of the shroud. For example, encapsulation material  4 . 1 - 132  may have a black appearance, a white appearance, a gray appearance, a shiny appearance, and/or a matte appearance. In general, encapsulation material  4 . 1 - 132  may be formed from material that adheres to glass layer  4 . 1 - 126 , while protecting the edge surface of glass layer  4 . 1 - 126 . 
     Encapsulation material  4 . 1 - 132  may substantially fill the opening between the edge surface of glass layer  4 . 1 - 126  and shroud  4 . 1 - 100 . For example, encapsulation material  4 . 1 - 132  may extend approximately 150 microns from the edge surface. In general, however, any amount of encapsulation material  4 . 1 - 132  may be applied to the edge surface. 
     As shown in  FIGS.  4 . 1 - 6   , encapsulation material  4 . 1 - 132  may cover the edge surface and may also cover an edge portion of laminate  4 . 1 - 128 . However, this is merely illustrative. If desired, encapsulation material  4 . 1 - 132  may cover the edge surface of glass layer  4 . 1 - 126  without covering an edge portion of laminate  4 . 1 - 128 . For example, as shown in  FIGS.  4 . 1 - 7   , encapsulation material may cover only the edge surface of glass layer  4 . 1 - 126 . In the example of  FIGS.  4 . 1 - 7   , laminate  4 . 1 - 128  may extend over encapsulation material  4 . 1 - 132 . However, this is merely illustrative. Laminate  4 . 1 - 128  may be flush with the edge surface of glass layer  4 . 1 - 126 , if desired. 
     In some embodiments, it may be determined that glass layer  4 . 1 - 126  can be protected by modifying the position of glass layer  4 . 1 - 126  relative to shroud  4 . 1 - 100  (or support structure  4 . 1 - 126 ). For example, as shown in the illustrative embodiment of  FIGS.  4 . 1 - 8   , the edge surface of glass layer  4 . 1 - 126  may be left unencapsulated, but the size of opening  4 . 1 - 134  between the edge surface and shroud  4 . 1 - 100  may be adjusted. By increasing or decreasing the size of opening  4 . 1 - 134 , glass layer  4 . 1 - 126  may be protected. 
     Instead of, or in addition to, adding material to the edge surface of layer  4 . 1 - 126 , it may be desirable to add material in the gap between layer  4 . 1 - 126  and the shroud/support structure. An illustrative example of adding material in this gap is shown in  FIGS.  4 . 1 - 9   . 
     As shown in  FIGS.  4 . 1 - 9   , material  4 . 1 - 136  may be included between the edge surface of glass layer  4 . 1 - 126  and shroud  4 . 1 - 100 . Material  4 . 1 - 136  may be, for example, a bumper ring. The bumper ring may be formed from elastomer, rigid plastic, or another material that helps protect the edge surface of layer  4 . 1 - 126 . 
     As an alternative to material  4 . 1 - 136  being a bumper ring between layer  4 . 1 - 126  and shroud  4 . 1 - 100 , material  4 . 1 - 136  may be an overmolded structure on layer  4 . 1 - 126 , on shroud  4 . 1 - 100 , or on a chassis that is coupled to support structure  4 . 1 - 26 . In general, the overmolded structure may fill the gap between layer  4 . 1 - 126  and the support structure, shroud, and/or chassis and help protect the edge surface of layer  4 . 1 - 126 . 
     Although not shown in  FIGS.  4 . 1 - 9   , a portion of material  4 . 1 - 136  may extend underneath layer  4 . 1 - 126  if desired. In particular, there may be a portion of material  4 . 1 - 136  between the bottom surface of layer  4 . 1 - 126  and shroud  4 . 1 - 100 . 
     Instead of, or in addition to, adding material between layer  4 . 1 - 126  and shroud  4 . 1 - 100 , upper laminate  4 . 1 - 128  and/or lower laminate  4 . 1 - 130  may wrap around the edge surface of layer  4 . 1 - 126 . Illustrative examples of the laminates wrapping the edge surface are shown in  FIGS.  4 . 1 - 10  and  4 . 1 - 11   . 
     As shown in  FIGS.  4 . 1 - 10   , upper laminate  4 . 1 - 128  may wrap around the edge surface of layer  4 . 1 - 126 . In particular, upper laminate  4 . 1 - 128  may have portion  4 . 1 - 128 A that extends around and covers the edge surface of layer  4 . 1 - 126 . Layer  4 . 1 - 126  may have a rounded edge surface to allow upper laminate  4 . 1 - 128  to wrap the edge surface and sufficiently adhere to the surface, as shown in  FIGS.  4 . 1 - 10   . By forming layer  4 . 1 - 126  with a rounded edge, the curve of laminate  4 . 1 - 128  around the edge may be reduced, thereby reducing stress on laminate  4 . 1 - 128 . However, layer  4 . 1 - 126  may have a planar edge surface, or a surface with any other desired profile, around which upper laminate  4 . 1 - 128  wraps, if desired. By wrapping upper laminate around the edge surface of layer  4 . 1 - 126 , the edge surface may be protected 
     As shown in  FIGS.  4 . 1 - 11   , lower laminate  4 . 1 - 130  may wrap around the edge surface of layer  4 . 1 - 126 . In particular, lower laminate may have portion  4 . 1 - 130 A that extends around and covers the edge surface of layer  4 . 1 - 126 . Layer  4 . 1 - 126  may have a rounded edge surface to allow lower laminate  4 . 1 - 130  to wrap the edge surface and sufficiently adhere to the surface, may have a planar edge surface, or may have a surface with any other desired profile, around which lower laminate  4 . 1 - 130  wraps. By wrapping lower laminate  4 . 1 - 130  around the edge surface of layer  4 . 1 - 126 , the edge surface may be protected. 
     In the example of  FIGS.  4 . 1 - 11   , lower laminate portion  4 . 1 - 130 A wraps entirely around the edge surface of layer  4 . 1 - 126  and partially overlaps upper laminate  4 . 1 - 128 . However, the arrangement is merely illustrative. If desired, lower laminate portion  4 . 1 - 130 A may wrap around only the edge surface of layer  4 . 1 - 126  without overlapping or extending over upper laminate  4 . 1 - 128 . 
     Another example of material that may be used to protect layer  4 . 1 - 126  is shown in  FIGS.  4 . 1 - 12   . In the example of  FIGS.  4 . 1 - 12   , glue (or another similar material)  4 . 1 - 138  may be used to completely fill the gap between layer  4 . 1 - 126  and shroud  4 . 1 - 100 . For example, the glue may be inserted into the gap after cover layer  4 . 1 - 92  has been assembled into the head-mounted device. Glue  4 . 1 - 138  may help protect the edge surface of layer  4 . 1 - 126 . 
     Rather than wrapping the upper or lower laminate around the edge surface of layer  4 . 1 - 126 , upper laminate  4 . 1 - 128  may extend to shroud  4 . 1 - 100  to cover the gap between the edge surface and shroud  4 . 1 - 100 . For example, as shown in  FIGS.  4 . 1 - 13   , upper laminate  4 . 1 - 128  may have portion  4 . 1 - 128 B that extends to shroud  4 . 1 - 100  (or support structure  4 . 1 - 26  or another portion of device  4 . 1 - 10 ). By covering the gap between the edge surface of layer  4 . 1 - 126  and shroud  4 . 1 - 100 , the edge surface may be protected. 
     Instead of, or in addition to, adding material or extending the laminates to protect the edge surface of layer  4 . 1 - 126 , shroud  4 . 1 - 100  or a chassis attached to support structure  4 . 1 - 26  may be modified to protect the edge surface. Illustrative examples of modifying these components to protect layer  4 . 1 - 126  are shown in  FIGS.  4 . 1 - 14  and  4 . 1 - 15   . 
     As shown in  FIGS.  4 . 1 - 14   , structure  4 . 1 - 140  may have a lip that covers the gap between layer  4 . 1 - 126  and shroud  4 . 1 - 100 /support structure  4 . 1 - 26 . Structure  4 . 1 - 140  may be formed from a portion of shroud  4 . 1 - 100  or from a portion of support structure  4 . 1 - 26  (e.g., a chassis of support structure  4 . 1 - 26 ). The lip of structure  4 . 1 - 140  may help protect the edge surface of layer  4 . 1 - 126 . 
     If desired, the lip of structure  4 . 1 - 140  may be combined with the extension of laminate around the edge surface of layer  4 . 1 - 126 . For example, as shown in  FIGS.  4 . 1 - 15   , upper laminate  4 . 1 - 128 A may wrap the edge surface of layer  4 . 1 - 126  to protect the edge surface, and the lip of structure  4 . 1 - 140  may provide additional protection. 
     Although cover layer  4 . 1 - 92  has been described as being coupled to shroud  4 . 1 - 100 , this is merely illustrative. In some embodiments, cover layer  4 . 1 - 92  may be coupled directly to support structure  4 . 1 - 26 . In other embodiments, device  4 . 1 - 10  may include a chassis attached to support structure  4 . 1 - 26  (e.g., a chassis to support various components in device  4 . 1 - 10 ), and cover layer  4 . 1 - 92  may be coupled to the chassis. 
     Moreover, although cover layer  4 . 1 - 92  has been described as including a glass layer, this material is merely illustrative. Layer  4 . 1 - 126  may be formed from ceramic, sapphire, or any other desired material. 
     In general, laminates  4 . 1 - 128  and  4 . 1 - 130  may protect layer  4 . 1 - 126 . An illustrative stackup of cover layer  4 . 1 - 92  including detailed laminates  4 . 1 - 128  and  4 . 1 - 130  is shown in  FIGS.  4 . 1 - 16   . 
     As shown in  FIGS.  4 . 1 - 16   , cover layer  4 . 1 - 92  may include layer  4 . 1 - 126 , which may be glass, sapphire, or other material, and laminates  4 . 1 - 128  and  4 . 1 - 130 . Although laminates  4 . 1 - 128  and  4 . 1 - 130  are shown as planar in  FIGS.  4 . 1 - 16    for illustrative purposes, laminate  4 . 1 - 128  may be a convex laminate, and laminate  4 . 1 - 130  may be a concave laminate (as shown in  FIGS.  4 . 1 - 6 - 4 . 1 - 15   ), if desired. 
     Laminate  4 . 1 - 128  may include polymer layer  4 . 1 - 134  coupled to layer  4 . 1 - 126  with adhesive  4 . 1 - 132 . Polymer layer  4 . 1 - 134  may be a polycarbonate (PC) layer, a polymethyl methacrylate (PMMA) layer, or other suitable polymer layer. Adhesive  4 . 1 - 132  may be a pressure-sensitive adhesive (PSA), an optically clear adhesive (OCA), or other suitable adhesive. In some illustrative embodiments, adhesive  4 . 1 - 132  may be an ultraviolet-curable OCA. Adhesive  4 . 1 - 132  may have a thickness of at least 100 microns, at least 200 microns, 150-250 microns, or other suitable thickness. 
     Hard coat  4 . 1 - 136  may be formed on polymer layer  4 . 1 - 134 . Hard coat  4 . 1 - 136  may be an acrylic layer, a thin glass layer, a sapphire layer, or other material. In some embodiments, if polymer layer  4 . 1 - 134  is a PMMA layer, hard coat  4 . 1 - 136  may be a polycarbonate layer or a blend of polycarbonate and acrylic material. For example, hard coat  4 . 1 - 136  may be formed from an ultraviolet-curable film. Hard coat  4 . 1 - 136  may be at least 2 microns thick, three microns thick, 3-5 microns thick, or other suitable thickness. 
     Coating layers  4 . 1 - 138  may be formed on hard coat  4 . 1 - 136 . Coating layers  4 . 1 - 138  may include an antireflection coating (e.g., a layer that matches an index of refraction of glass  4 . 1 - 126  to the air outside of glass  4 . 1 - 126 ) and an antismudge coating (e.g., a fluoropolymer or other oleophobic material), as examples. 
     By including adhesive  4 . 1 - 132  between glass  4 . 1 - 126  and polymer layer  4 . 1 - 134 , polymer  4 . 1 - 134  and glass  4 . 1 - 126  may be decoupled. As a result, less stress may be applied to glass  4 . 1 - 126  from polymer  4 . 1 - 134 . To reduce the amount of stress further, a UV-curable OCA may be used, as such an OCA may be soft when applied. However, this is merely illustrative. Any suitable adhesive may be used to decouple polymer  4 . 1 - 134  from glass  4 . 1 - 126  and reduce the stress applied to glass  4 . 1 - 126 . 
     In some illustrative embodiments, polymer layer  4 . 1 - 134  may be formed from PMMA, which may match the optical properties of glass  4 . 1 - 126 , particularly when cover layer  4 . 1 - 92  is curved. However, any suitable material may be used to form polymer layer  4 . 1 - 134  and match the appearance of polymer layer  4 . 1 - 134  to glass  4 . 1 - 126 . 
     Although hard coat  4 . 1 - 136  has been described as being overlapped by coating layers  4 . 1 - 138 , this is merely illustrative. In some embodiments, hard coat  4 . 1 - 136  may be formed as the outermost layer of cover layer  4 . 1 - 92 . For example, as shown in  FIGS.  4 . 1 - 17   , hard coat  4 . 1 - 136  may be formed as the outermost layer. 
     If desired, hard coat  4 . 1 - 136  may include antismudge properties. For example, hard coat  4 . 1 - 136  may have fluoropolymer or other oleophobic material incorporated into the hard coat material. Additionally, if an antireflection coating is desired, antireflection coating  4 . 1 - 137  may be formed as the lowermost layer of laminate  4 . 1 - 128 , directly on glass  4 . 1 - 126 . 
     Returning to  FIGS.  4 . 1 - 16   , laminate  4 . 1 - 130  may include adhesive  4 . 1 - 140  that couples polymer  4 . 1 - 142  to glass  4 . 1 - 126 . Adhesive  4 . 1 - 140  and polymer  4 . 1 - 142  may correspond with adhesive  4 . 1 - 132  and polymer layer  4 . 1 - 132 . For example, adhesive  4 . 1 - 140  may be a PSA or OCA (e.g., a UV-curable OCA), and polymer layer  4 . 1 - 132  may be polycarbonate or PMMA. Polymer layer  4 . 1 - 142  may be acrylic, sapphire, glass, UV-curable material, or other suitable material. 
     Antireflection coating  4 . 1 - 146  may be formed on polymer layer  4 . 1 - 142 . Although not shown, a UV stabilizer material, such as alumina, may be formed between polymer layer  4 . 1 - 142  and antireflection coating  4 . 1 - 146 . The UV stabilizer material may have a thickness of at least 10 nm, at least 20 nm, or other suitable thickness, and may stabilize the surrounding layers when adhesives  4 . 1 - 140  and  4 . 1 - 132 , and/or polymer layers  4 . 1 - 134  and  4 . 1 - 142  are cured. 
     Laminate  4 . 1 - 130  may also include ink  4 . 1 - 148 . Ink  4 . 1 - 148  may be formed over an optical component in a head-mounted device, such as one of components  4 . 1 - 60 ,  4 . 1 - 62 ,  4 . 1 - 64 ,  4 . 1 - 66 ,  4 . 1 - 68 ,  4 . 1 - 70 ,  4 . 1 - 72 ,  4 . 1 - 74 ,  4 . 1 - 76 ,  4 . 1 - 78 , and  4 . 1 - 80  of  FIGS.  4 . 1 - 3   . In an illustrative embodiment, ink  4 . 1 - 148  may overlap a flood illuminator, and ink  4 . 1 - 148  may be an infrared-transparent-visible-light-blocking ink, such as an ink with 10% visible light transmission or less. However, this is merely illustrative. In general, ink  4 . 1 - 148  may overlap any component in a head-mounted device and may have a corresponding transmission spectrum. 
     Although  FIGS.  4 . 1 - 16  and  4 . 1 - 17    show laminate  4 . 1 - 130  without a hard coat layer, this is merely illustrative. If desired, laminate  4 . 1 - 130  may have a hard coat layer, such as hard coat layer  4 . 1 - 136 , either within laminate  4 . 1 - 130  or as an outermost layer of laminate  4 . 1 - 130 . 
     An edge seal, such as the seals of  FIGS.  4 . 1 - 6 - 4 . 1 - 9   , may protect one or more layers of laminate  4 . 1 - 128  and/or laminate  4 . 1 - 130 . An illustrative example of a seal overlapping the edges of layers in laminate  4 . 1 - 128  is shown in  FIGS.  4 . 1 - 18   . 
     As shown in  FIGS.  4 . 1 - 18   , seal  4 . 1 - 150  may overlap the edges of laminate  4 . 1 - 128 . In particular, seal  4 . 1 - 150  may overlap and bond to a PMMA layer, a polycarbonate layer, and/or a hard coat layer in laminate  4 . 1 - 128 . In some embodiments, seal  4 . 1 - 150  may form chemical bonds with an upper surface of laminate  4 . 1 - 128 . Moreover, in the illustrative example of  FIGS.  4 . 1 - 18   , seal  4 . 1 - 150  may extend to glass layer  4 . 1 - 126 . Seal  4 . 1 - 150  may be formed from polyurethane, epoxy, acrylate, PVB, or other suitable material to protect the edges of laminate  4 . 1 - 128 . 
     The foregoing is merely illustrative and various modifications can be made to the described embodiments. The foregoing embodiments may be implemented individually or in any combination. 
     V: Dust Seal 
     5.1: Seal for an Electronic Device 
     The electronic devices described in the present disclosure include components and arrangements configured to protect the LEDs, antennas, sensors, and/or other components communicating with the external environment noted above, from light and debris that may otherwise harm or negatively affect the components. More specifically, the present disclosure describes seals and electronic components that ensure internal components can sufficiently function without interference from an environment external to the electronic device, as needed, without the risk of damage to the internal component. Such interferences can include light, debris, or moisture. Such internal components can include, but are not limited to, cameras, LEDs, ambient light sensors, flicker sensors, and other sensors, and the like. These and other internal components can be disposed adjacent to, or aligned below, one or more apertures/ports of the electronic device, with a seal disposed between the aperture/vent and the internal electronic component. In this way, the seals described herein can act as a barrier between the internal component and the external environment or the internal component and other internal components while also enabling the proper functioning of the internal component. 
     In a particular example, an electronic device includes a housing that defines an internal volume and an aperture defined by the housing. An electronic component can be disposed within the internal volume and a seal can be disposed between the aperture and the electronic component, the seal being configured to protect the electronic component within the housing. 
     In a particular example, an electronic component can include a light emitting component coupled to a base support. The electronic device can include other electronic components that are light sensitive. The seal can be disposed and configured between the light emitting component and light sensitive component to prevent and/or minimize light interference. 
     The seals described herein obstruct light having wavelengths in the visible range, as well as those having wavelengths extending into to ultraviolet and infrared ranges. The seals also inhibit dust and/or debris from traversing into the housing from apertures and/or any vents disposed on the housing of the electronic device. The seals described herein are also compressible to aid in assembly of the electronic device. In addition, the seals and assemblies described herein are durable and resistant to damage. The seal can be die cut and the components of the electronic device can be made of materials intended to minimize the weight of the components and/or the electronic device. 
     For example, a seal described herein can include an open or closed cell foam. The seal can engage a bracket or base support of the electronic device in an interference fit. In some examples, the seal can include a seal configured to surround the light emitting component. Because the light emitting component is mounted to an end of the base support in some examples, the seal can be die cut to have a precise interference fit with the base support to separate the light emitting component from other electronic components disposed in the electronic device and to facilitate a top-down assembly orientation. 
       FIG.  5 - 1    illustrates a cross-section of an electronic device  5 - 100 . The electronic device can include any of the electronic devices described above. The electronic device can include a housing  5 - 105 . The housing can include a first wall  5 - 110 . In some examples, the first wall  5 - 110  can include an exterior wall of the electronic device  5 - 100  or can include a wall that separate chambers or compartments within electronic device  5 - 100 . The housing  5 - 105  can include a base support  5 - 115  coupled to the first wall  5 - 110 . The base support  5 - 115  can include a base bracket or support piece for the electronic device  5 - 100  and/or electronic components disposed within the housing  5 - 105 . In some examples, the base support  5 - 115  can include one or more stiff metal materials, including steel, stainless steel, magnesium, and/or titanium. In some examples, the base support  5 - 115  can include plastic and/or plastic components to reduce weight of the electronic device  5 - 100 . The base support  5 - 115  can be cylindrical shaped or can include a cubic shape and may be a solid material or hollow to reduce weight of the electronic device  5 - 100 . The base support  5 - 115  can include a first surface  5 - 120  coupled to the first wall  5 - 110  by adhesive or any non-limiting fastening method and/or device. An electronic component can be coupled to a second surface  5 - 125  of the base support  5 - 115 . The second surface  5 - 125  of the base support  5 - 115  can be configured opposite the first surface  5 - 120 . The electronic component can include a light emitting component  5 - 130  (e.g., a light emitting diode or LED). 
     The electronic device  5 - 100  can further include a second wall  5 - 135  disposed over the base support  5 - 115 . The first wall  5 - 110  and the second wall  5 - 135  can form an enclosure  5 - 140  within the housing  5 - 105  and the base support  5 - 115  can be disposed within the enclosure  5 - 140 . In some examples, an electronic component  5 - 145  can also be disposed within the enclosure  5 - 140 . The electronic component  5 - 145  can include a light sensitive component, in some examples, subject to light interference from the light emitting component  5 - 130 . The electronic component  5 - 145  can be mounted to a second base support or can be coupled to either the first wall  5 - 110  and/or the second wall  5 - 135  directly. 
     The electronic device  5 - 100  can further include a seal  5 - 150  surrounding the light emitting component  5 - 130 . The seal  5 - 150  can be disposed between the base support  5 - 115  and the second wall  5 - 135 . In some examples, the seal  5 - 150  separates the electronic component  5 - 145  from the light emitting component  5 - 130 . The seal  5 - 150  can be configured to obstruct light having wavelengths between about 10-8 m and about 10-3 m. In other words, the seal  5 - 150  can obstruct light in the visible region, the ultraviolet region, and the infrared region. The seal  5 - 150  can also inhibit dust and/or debris from traversing between the base support  5 - 115  and the second wall  5 - 135 . In some examples, the seal  5 - 150  surrounds the light emitting component  5 - 130  on all sides. As such, the seal  5 - 150  engages the base support  5 - 115  either around a perimeter of an upper portion of the base support  5 - 115  or the second surface  5 - 125  of the base support  5 - 115 . In some examples, the second surface  5 - 125  includes the same surface having the light emitting component  5 - 130  disposed thereon. 
     In some examples, the base support  5 - 115  defines a ledge  5 - 155 . The seal  5 - 150  can surround an upper portion of the base support  5 - 115  and contact or engage an upper surface of the ledge  5 - 155 . As such, the seal  5 - 150  and the base support  5 - 115  can be engaged in an interference fit. For example, the radial seal  5 - 150  can be disposed in a press fit or friction fit between the base support  5 - 115  and the second wall  5 - 135 . The seal  5 - 150  and the base support  5 - 115  can be held together by friction after the housing  5 - 105  and other components are assembled. For example, the seal  5 - 150  can be compressed between the second wall  5 - 135  and the second surface  5 - 125  or the ledge  5 - 155  of the base support  5 - 115  to separate or seal the light emitting component  5 - 130  from the electronic component  5 - 145 . In some examples, the seal  5 - 150  can separate and/or seal off the enclosure  5 - 140  from an area or environment outside the housing  5 - 105  or external to the electronic device  5 - 100 . 
     In some examples, the seal  5 - 150  can be die cut. The seal  5 - 150  can be cut into a precise shape by a metal “die” during production. Because of the precise shape (e.g., radial) and dimensions of the seal  5 - 150  to properly seal against light, dust, and other debris and to fit properly against the second wall  5 - 135  and the base support  5 - 115 , including the ledge  5 - 155  defined thereby, the die cutting process can provide the exact shape and properties of the seal  5 - 150  for proper assembly of the electronic device  5 - 100 . 
     In some examples, the seal  5 - 150  can include a silicone material. The silicone seal  5 - 150  can be manufactured inexpensively and when shaped appropriately, provide the required compressibility and interference fit to the base support  5 - 115 . In some examples, the silicone seal  5 - 150  can include a foaming agent and/or be dyed to obstruct light from passing through from either the external environment and/or the light emitting component  5 - 130  to the electronic component  5 - 145  within the enclosure  5 - 140 . The seal  5 - 150  can include portions formed of different materials. In some examples, the seal  5 - 150  can include a plastic shim  5 - 160 . The plastic shim  5 - 160  can be included as a component of the seal  5 - 150  to reduce weight of the seal  5 - 150 , and thus, can reduce the weight of the electronic device  5 - 100 . In some examples, the plastic shim  5 - 160  can contact the base support  5 - 115 . In some examples, the plastic shim  5 - 160  can contact the ledge  5 - 155  and/or the upper portion of the base support  5 - 115 . The plastic shim  5 - 160  can also be impenetrable or resistant to light, dust, and/or debris. The seal can further include a foam material  5 - 165  coupled to the plastic shim  5 - 160 . 
     Referring now to  FIG.  5 - 2   , a cross-section of the seal  5 - 150  illustrated in  FIG.  5 - 1    is shown. In some examples, the plastic shim  5 - 160  can be coupled to the foam material  5 - 165  with a pressure sensitive adhesive  5 - 170 , or any suitable adhesive. The plastic shim  5 - 160  can form a precision seal with the base support  5 - 115  and can be less expensive to manufacture than the silicone and/or a seal including only foam material. The foam material  5 - 165  can be compressed between the plastic shim  5 - 160  and the second wall  5 - 135 , as shown in  FIG.  5 - 1   . The plastic shim  5 - 160  can include a bottom surface  5 - 162  coupled to the base support  5 - 115 . The plastic shim  5 - 160  can be joined to the base support  5 - 115  with the pressure sensitive adhesive  5 - 170 . A top surface of the plastic shim  5 - 160  can also include a pressure sensitive adhesive  5 - 170  to couple the plastic shim  5 - 160  to the foam material  5 - 165 . In some examples, the foam material  5 - 165  can include a compressible open or closed cell foam to inhibit light, dust, and/or debris. 
       FIG.  5 - 3    illustrates an electronic device  5 - 200  having a housing  5 - 205  and electronic components therein. The electronic device  5 - 200  can include a head mountable device (e.g., device  30 ). In some examples, the electronic device  5 - 200  can include a first enclosure  5 - 210  having a light emitting component  5 - 222  disposed therein. The first enclosure can be defined, at least in part, by the housing  5 - 205 , an internal wall  5 - 255 , and an external wall  5 - 240  defining an external surface  5 - 242 . The housing  5 - 205  can include a first wall  5 - 215 . In some examples, the first wall  5 - 215  can include an exterior wall of the electronic device  5 - 200  or can include a wall that separate chambers or compartments within the electronic device  5 - 200 . The housing  5 - 205  can include a base support  5 - 220  coupled to the first wall  5 - 215 . The base support  5 - 220  can include a first surface  5 - 225  coupled to the first wall  5 - 215  by an adhesive or any non-limiting fastening method and/or device. An electronic component can be coupled to a second surface  5 - 230  of the base support  5 - 220 . The second surface  5 - 230  of the base support  5 - 220  can be opposite the first surface  5 - 225 . The light emitting component  5 - 222  can include a light emitting diode (LED). 
     The housing  5 - 205  can further include a second enclosure  5 - 230 . The second enclosure  5 - 230  can include at least one light sensitive component  5 - 235  disposed therein. The second enclosure  5 - 230  can be defined by the housing  5 - 205 , including various internal and external walls. In some examples, the light sensitive component  5 - 235  can include an optically sensitive device. In some examples, the light sensitive component  5 - 235  can include at least one of a camera, an ambient light sensor, or a flicker sensor. In some examples, the second enclosure  5 - 230  can include an exterior surface  5 - 240  of the housing  5 - 205 . In other words, one of the walls of the second enclosure  5 - 230  includes the exterior surface  5 - 240  of the housing  5 - 205 . 
     In some examples, the housing  5 - 205  can further define a third enclosure  5 - 245 . The base support  5 - 220  can be disposed within the third enclosure  5 - 245 . In some examples, the base support  5 - 220  can include a metal or plastic material. In some examples, the third enclosure  5 - 245  can include an interior surface  5 - 250  of the housing  5 - 205 . In other words, one of the walls of the third enclosure  5 - 245  includes the interior surface  5 - 250  of the housing  5 - 205 . In some examples, the housing  5 - 205  of the electronic device  5 - 200  includes an internal wall  5 - 255  separating at least a portion of the second enclosure  5 - 230  and the third enclosure  5 - 245 . In some examples, a seal  5 - 260  is included. The seal  5 - 260  can be coupled to a top portion or the second surface  5 - 230  of the base support  5 - 220 . The seal  5 - 260  can separate the first enclosure  5 - 210 , the second enclosure  5 - 230 , and the third enclosure  5 - 245 . In other words, the seal  5 - 260  includes at least a portion of a wall or divider that separates the first enclosure  5 - 210 , the second enclosure  5 - 230 , and the third enclosure  5 - 245  from each other. In some examples, the seal  5 - 260  is impenetrable to dust, debris, and/or light. The seal can extend from the second surface  5 - 230  of the base support  5 - 220  past the internal wall  5 - 255  and press fit to the exterior surface  5 - 240  of the housing  5 - 205 . In such a configuration as shown in  FIG.  6   , the seal  5 - 260  functions to both protect the light sensitive component  5 - 235  from the light emitting component  5 - 222  and protects both the light sensitive component  5 - 235  and the light emitting component  5 - 222  from debris and/or dust  5 - 265  present either outside the housing  5 - 205  or within the third enclosure  5 - 245 . 
     In some examples, the dust  5 - 265  can ingress into the third enclosure  5 - 245  through an air vent  5 - 270  or other aperture. The seal  5 - 260  prevents or substantially prevents the dust  5 - 265  or other debris, such as, but in no way limited to, water, dirt, and smoke, from passing through to the first enclosure  5 - 210  or second enclosure  5 - 230 , which can include sensitive electronic components. The air vent  5 - 270  can be included on the electronic device  5 - 200  for temperature control and/or heat dissipation. In some examples, the air vent  5 - 270  can be included for comfort and/or safety for the user of the electronic device  5 - 200 . According to one example, the air vent  5 - 270  can include a perforated material to cover the air vent  5 - 270  or other apertures in the electronic device. The size, locations, and number of perforations extending through such a material can vary from one example to another. Such perforations can include machined, laser cut, or otherwise manufactured openings defined by and extending through the material. Such openings can be sized and arranged to prevent a certain size particle from the external environment from passing through the air vent  5 - 270 . Such openings can also be sized and include other types of filters to prevent moisture or other harmful debris from passing through the air vent  5 - 270 . 
     In some examples, the seal  5 - 260  can be cut and/or shaped to seal around the internal wall  5 - 255  and between the second surface  5 - 230  of the base support  5 - 220  and the exterior surface  5 - 240  of the housing  5 - 205 . The seal  5 - 260  can include a compressible foam and can compress in an interference or press fit between the exterior surface  5 - 240  of the housing  5 - 205  and a top portion or the second surface  5 - 230  of the base support  5 - 220 . The seal  5 - 260  can include an open cell polyurethane foam, which is lightweight and more compressible compared to a closed cell foam. In other examples, the seal  5 - 260  can include a silicone material. The seal  5 - 260  can include a reticulated Polyurethane, PVC/Nitrile, Ethylene Propylene Diene Monomer (EPDM) rubber, or other suitable material. The materials which can be used to produce the closed cell foam for the seal  5 - 260  can vary greatly from ethylene-vinyl acetate (EVA), polyethylene, polystyrene, rubber to polypropylene etc. The closed cell foam can include trapped gas bubbles which are formed during the expansion and cure of the foam. The bubbles are permanently locked to a place, as the trapped gas is very efficient in increasing the insulation capability of the foam. The foam which is formed is strong and is usually of a greater density than open cell foam, which enables the gas bubbles to lock into place. The nature of the foam enables it to be vapor retardant and resist liquid water. In some examples, the materials for the seal can be open celled, but can include a sufficiently tortuous path so as to restrict or eliminate the passage of contaminants between enclosures. 
     In some examples, the top portion of the base support  5 - 220  can include a ledge (e.g., ledge  5 - 155  shown in  FIG.  5 - 1   ). The seal  5 - 260  can surround an upper portion of the base support  5 - 220  and contact the ledge. In some examples, the seal  5 - 260  can further include a plastic shim (e.g., plastic shim  5 - 160 ) that contacts the ledge. The seal  5 - 260  can fit tightly to the base support  5 - 220 . The tight fit can function not only to seal the enclosures within the housing, but also to control and/or retain electronic components in place, which can improve the design of the electronic device  5 - 200 , minimize noise and/or vibrations, and reduce manufacturing costs. 
       FIGS.  5 - 4 A- 5 - 4 C  illustrate an electronic device  5 - 300 .  FIG.  5 - 4 A  shows a top perspective view of an electronic component  5 - 305  and a dust seal  5 - 310  surrounding the electronic component  5 - 305  within the electronic device  5 - 300 , according to one example. In some examples, the dust seal  5 - 310  can be coupled to a base support  5 - 315 . The dust seal  5 - 310  can couple to the base support  5 - 315  via an interference fit. In some examples, the dust seal  5 - 310  can be adhered to the base support  5 - 315  with an adhesive (e.g., pressure sensitive adhesive  5 - 170  shown in  FIG.  5 - 2   ). In some examples, the electronic component  5 - 305  can include a light emitting component (e.g., light emitting component  5 - 222  shown in  FIG.  5 - 3   ). The electronic device  5 - 300  can further include a flexible printed circuit board or flexible printed circuit (flex PCB)  5 - 320 . At least a portion of the flex PCB  5 - 320  can be disposed between the dust seal  5 - 310  and the base support  5 - 315 . In some examples, a flex PCB can include an arrangement of printed circuitry and/or components that utilize flexible materials with a flexible overlay. A flexible printed circuit board can include a metallic layer of traces, usually copper, bonded to a dielectric layer. Thickness of the metallic layer can be very thin (&lt;0.0001″) to very thick (&gt;0.010″) and the dielectric thickness can also vary. The flex PCB can be included in the electronic device  5 - 300  to reduce the assembly process and improve reliability. The flex PCB can be used as a connector, power supply, and also as full circuits assembled with components such as electronic component  5 - 305 . A benefit of including the flex PCB  5 - 320  in the electronic device  5 - 300  is connecting the electronic component  5 - 305  to other components and/or a power source (not shown) and having the flex PCB  5 - 320  disposed between the dust seal  5 - 310  and the base support  5 - 315 . In some examples, the dust seal  5 - 310  retains the flex PCB adjacent to the base support  5 - 315 . 
       FIG.  5 - 4 B  shows a cross-sectional view of a portion of electronic device  5 - 300 . The flex PCB  5 - 320  can be coupled to the electronic component  5 - 305 . In some examples, electronic component  5 - 305  can include a light emitting device mounted to the base support  5 - 315 . In some examples, the light emitting device can be coupled to a top surface  5 - 325  of the flex PCB  5 - 320  and a bottom surface  5 - 330  of the flex PCB  5 - 320  can be coupled to the base support  5 - 315 . The dust seal  5 - 310  can encircle the light emitting device. In some examples, the dust seal  5 - 310  is impenetrable or resistant to light, dust, and/or debris.  FIG.  5 - 4 B  shows the base support  5 - 315  having a ledge  5 - 335 . The dust seal  5 - 310  can extend from the upper portion of the base support  5 - 315  adjacent to the base support  5 - 315  and past the dust seal  5 - 310 . In some examples, the ledge  5 - 335  can include a cut-out portion. The cut-out portion can have the width of the flex PCB to minimize folding of the PCB and retain the flex PCB  5 - 320  adjacent to the base support  5 - 315 . Referring to  FIG.  5 - 4 C , the flex PCB  5 - 320  can further include an adhesive  5 - 340  on the bottom surface  5 - 330  to retain the flex PCB  5 - 320  adjacent to the base support  5 - 315 .  FIG.  7 C  shows components of the dust seal, each component configured to retain the flex PCB  5 - 320  and/or prevent dust and/or debris ingress into the electronic device. 
     In some examples, the dust seal  5 - 310  can act as a radial seal to apply a radial force  5 - 345  to the flex PCB  5 - 320  to retain the flex PCB  5 - 320  adjacent the base support  5 - 315 . In other words, the dust seal  5 - 310  prevents ingress path and aids adhesive  5 - 340  in flex management by applying compressive normal force that keeps the flex PCB  5 - 320  from delaminating from the base support  5 - 315 . The dust seal  5 - 310  can also act as a compressive seal to apply a compressive force  5 - 350  to compress the dust seal  5 - 310  between a housing wall  5 - 355  and the base support  5 - 315 . As such, the dust seal  5 - 310  can include a compressible foam. The dust seal  5 - 310  can be coupled to the base support  5 - 315  and engaged to the base support and the housing wall  5 - 355  in an interference fit. As described in other examples above, the housing wall  5 - 355  of the electronic device  5 - 300  can include a vent  5 - 360  and/or other apertures.  FIG.  7 C  includes the dust ingress route  5 - 365  that the dust seal  5 - 310  is configured to prevent. Because the dust seal  5 - 310  includes a compressible foam and is engaged in a compressive seal and a radial seal, the dust ingress route  5 - 365  is closed off, the dust seal  5 - 310  being impenetrable to dust and/or debris. In some examples, the dust seal  5 - 310  is also impenetrable to light. 
     In some examples, the components described above for the electronic device examples (e.g., electronic device  5 - 100 ,  5 - 200 , and/or  5 - 300 ) can aid in the assembly of the electronic device housing (e.g. housing  5 - 305 ). The assembly process can, in some examples, demand the use of a minimal amount of metal material for the base support  5 - 315  to maximize the available distance that can be used between the components. The assembly methods can also demand that the ledge  5 - 335  be minimized so that the base support  5 - 315  does not interfere with the inclusion/installation of other electronic components (e.g., electronic component  5 - 145 ) during assembly. In other words, due to the materials used for the base support  5 - 315  and/or the shape of the base support  5 - 315 , the base support  5 - 315  can be installed first, then the other electronic components (e.g., electronic component  5 - 145 ) can be installed after the base support  5 - 315 , followed by the dust seal  5 - 310  being placed on or around the base support, and finally the housing wall (e.g., housing wall  5 - 355 ) can be installed to compress the dust seal  5 - 310  in an interference fit between the housing wall  5 - 355  and the base support  5 - 315 . In other words, the present exemplary sealing systems and methods facilitate a top or front assembly of the device, while ensuring a secure light and/or dust seal between sections. 
     VI: Sensor System 
       FIG.  6 - 0    illustrates a view of an example of an HMD device  6 - 100 . The HMD device  6 - 100  can include a sensor array or system  6 - 102  including one or more sensors, cameras, projectors, and so forth mounted to one or more components of the HMD  6 - 100 . In at least one example, the sensor system  6 - 102  can include a bracket  1 - 338  on which one or more sensors of the sensor system  6 - 102  can be fixed/secured. 
       FIG.  6 - 1    illustrates a portion of an HMD device  6 - 100  including a front transparent cover  6 - 104  and a sensor system  6 - 102 . The sensor system  6 - 102  can include a number of different sensors, emitters, receivers, including cameras, IR sensors, projectors, and so forth. The transparent cover  6 - 104  is illustrated in front of the sensor system  6 - 102  to illustrate relative positions of the various sensors and emitters as well as the orientation of each sensor/emitter of the system  6 - 102 . As referenced herein, “sideways,” “side,” “lateral,” “horizontal,” and other similar terms refer to orientations or directions as indicated by the X-axis shown in  FIG.  6 - 2   . Terms such as “vertical,” “up,” “down,” and similar terms refer to orientations or directions as indicated by the Z-axis shown in  FIG.  6 - 2   . Terms such as “frontward,” “rearward,” “forward,” backward,” and similar terms refer to orientations or directions as indicated by the Y-axis shown in  FIG.  6 - 2   . 
     In at least one example, the transparent cover  6 - 104  can define a front, external surface of the HMD device  6 - 100  and the sensor system  6 - 102 , including the various sensors and components thereof, can be disposed behind the cover  6 - 104  in the Y-axis/direction. The cover  6 - 104  can be transparent or semi-transparent to allow light to pass through the cover  6 - 104 , both light detected by the sensor system  6 - 102  and light emitted thereby. 
     As noted elsewhere herein, the HMD device  6 - 100  can include one or more controllers including processors for electrically coupling the various sensors and emitters of the sensor system  6 - 102  with one or more mother boards, processing units, and other electronic devices such as display screens and the like. In addition, as will be shown in more detail below with reference to other figures, the various sensors, emitters, and other components of the sensor system  6 - 102  can be coupled to various structural frame members, brackets, and so forth of the HMD device  6 - 100  not shown in  FIG.  6 - 1   .  FIG.  6 - 1    shows the components of the sensor system  6 - 102  unattached and un-coupled electrically from other components for illustrative clarity sake. 
     In at least one example, the device can include one or more controllers having processors configured to execute instructions stored on memory components electrically coupled to the processors. The instructions can include, or cause the processor to execute, one or more algorithms for self-correcting angles and positions of the various cameras described herein overtime with use as the initial positions, angles, or orientations of the cameras get bumped or deformed due to unintended drop events or other events. 
     In at least one example, the sensor system  6 - 102  can include one or more scene cameras  6 - 106 . The system  6 - 102  can include two scene cameras  6 - 102  disposed on either side of the nasal bridge or arch of the HMD device  6 - 100  such that each of the two cameras  6 - 106  correspond generally in position with left and right eyes of the user behind the cover  6 - 103 . In at least one example, the scene cameras  6 - 106  are oriented generally forward in the Y-direction to capture images in front of the user during use of the HMD  6 - 100 . In at least one example, the scene cameras are color cameras and provide images and content for MR video pass through to the display screens facing the user&#39;s eyes when using the HMD device  6 - 100 . The scene cameras  6 - 106  can also be used for environment and object reconstruction. 
     In at least one example, the HMD  6 - 100  can include a controller electrically coupled to the various sensors and displays of the HMD  6 - 100 . In one example, the controller is configured to cause mixed-reality video passthrough from the first and second scene cameras  6 - 106  to the rearward facing displays, respectively, including one or more images captured by the first and second scene cameras  6 - 106 . 
     In at least one example, the sensor system  6 - 102  can include a first depth sensor  6 - 108  pointed generally forward in the Y-direction. In at least one example, the first depth sensor  6 - 108  can be used for environment and object reconstruction as well as user hand and body tracking. In at least one example, the sensor system  6 - 102  can include a second depth sensor  6 - 110  disposed centrally along the width (i.e., along the X-axis) of the HMD device  6 - 100 . For example, the second depth sensor  6 - 110  can be disposed above the central nasal bridge or accommodating features over the nose of the user when donning the HMD  6 - 100 . In at least one example, the second depth sensor  6 - 110  can be used for environment and object reconstruction as well as hand and body tracking. In at least one example, the second depth sensor can include a LIDAR sensor. 
     In at least one example, the sensor system  6 - 102  can include a depth projector  6 - 112  facing generally forward to project electromagnetic waves, for example in the form of a predetermined pattern of light dots, out into and within a field of view of the user and/or the scene cameras  6 - 106  or a field of view including and beyond the field of view of the user and/or scene cameras  6 - 106 . In at least one example, the depth projector can project electromagnetic waves of light in the form of a dotted light pattern to be reflected off objects and back into the depth sensors noted above, including the depth sensors  6 - 108 ,  6 - 110 . In at least one example, the depth projector  6 - 112  can be used for environment and object reconstruction as well as hand and body tracking. 
     In at least one example, the sensor system  6 - 102  can include downward facing cameras  6 - 114  with a field of view pointed generally downward relative to the HDM device  6 - 100  in the Z-axis. In at least one example, the downward cameras  6 - 114  can be disposed on left and right sides of the HMD device  6 - 100  as shown and used for hand and body tracking, headset tracking, and facial avatar detection and creation for display a user avatar on the forward facing display screen of the HMD device  6 - 100  described elsewhere herein. The downward cameras  6 - 114 , for example, can be used to capture facial expressions and movements for the face of the user below the HMD device  6 - 100 , including the cheeks, mouth, and chin. 
     In at least one example, the sensor system  6 - 102  can include jaw cameras  6 - 116 . In at least one example, the jaw cameras  6 - 116  can be disposed on left and right sides of the HMD device  6 - 100  as shown and used for hand and body tracking, headset tracking, and facial avatar detection and creation for display a user avatar on the forward facing display screen of the HMD device  6 - 100  described elsewhere herein. The jaw cameras  6 - 116 , for example, can be used to capture facial expressions and movements for the face of the user below the HMD device  6 - 100 , including the user&#39;s jaw, cheeks, mouth, and chin. for hand and body tracking, headset tracking, and facial avatar 
     In at least one example, the sensor system  6 - 102  can include side cameras  6 - 118 . The side cameras  6 - 118  can be oriented to capture side views left and right in the X-axis or direction relative to the HMD device  6 - 100 . In at least one example, the side cameras  6 - 118  can be used for hand and body tracking, headset tracking, and facial avatar detection and re-creation. 
     In at least one example, the sensor system  6 - 102  can include a plurality of eye tracking and gaze tracking sensors for determining an identity, status, and gaze direction of a user&#39;s eyes during and/or before use. In at least one example, the eye/gaze tracking sensors can include nasal eye cameras  6 - 120  disposed on either side of the user&#39;s nose and adjacent the user&#39;s nose when donning the HMD device  6 - 100 . The eye/gaze sensors can also include bottom eye cameras  6 - 122  disposed below respective user eyes for capturing images of the eyes for facial avatar detection and creation, gaze tracking, and iris identification functions. 
     In at least one example, the sensor system  6 - 102  can include infrared illuminators  6 - 124  pointed outward from the HMD device  6 - 100  to illuminate the external environment and any object therein with IR light for IR detection with one or more IR sensors of the sensor system  6 - 102 . In at least one example, the sensor system  6 - 102  can include a flicker sensor  6 - 126  and an ambient light sensor  6 - 128 . In at least one example, the flicker sensor  6 - 126  can detect overhead light refresh rates to avoid display flicker. In one example, the infrared illuminators  6 - 124  can include light emitting diodes and can be used especially for low light environments for illuminating user hands and other objects in low light for detection by infrared sensors of the sensor system  6 - 102 . 
     In at least one example, multiple sensors, including the scene cameras  6 - 106 , the downward cameras  6 - 114 , the jaw cameras  6 - 116 , the side cameras  6 - 118 , the depth projector  6 - 112 , and the depth sensors  6 - 108 ,  6 - 110  can be used in combination with an electrically coupled controller to combine depth data with camera data for hand tracking and for size determination for better hand tracking and object recognition and tracking functions of the HMD device  6 - 100 . In at least one example, the downward cameras  6 - 114 , jaw cameras  6 - 116 , and side cameras  6 - 118  described above and shown in  FIG.  6 - 1    can be wide angle cameras operable in the visible and infrared spectrums. In at least one example, these cameras  6 - 114 ,  6 - 116 ,  6 - 118  can operate only in black and white light detection to simplify image processing and gain sensitivity. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  6 - 1    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  6 - 2 - 6 - 4    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  6 - 2 - 6 - 4    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  6 - 1   . 
       FIG.  6 - 2    illustrates a lower perspective view of an example of an HMD  6 - 200  including a cover or shroud  6 - 204  secured to a frame  6 - 230 . In at least one example, the sensors  6 - 203  of the sensor system  6 - 202  can be disposed around a perimeter of the HDM  6 - 200  such that the sensors  6 - 203  are outwardly disposed around a perimeter of a display region or area  6 - 232  so as not to obstruct a view of the displayed light. In at least one example, the sensors can be disposed behind the shroud  6 - 204  and aligned with transparent portions of the shroud allowing sensors and projectors to allow light back and forth through the shroud  6 - 204 . In at least one example, opaque ink or other opaque material or films/layers can be disposed on the shroud  6 - 204  around the display area  6 - 232  to hide components of the HMD  6 - 200  outside the display area  6 - 232  other than the transparent portions defined by the opaque portions, through which the sensors and projectors send and receive light and electromagnetic signals during operation. In at least one example, the shroud  6 - 204  allows light to pass therethrough from the display (e.g., within the display region  6 - 232 ) but not radially outward from the display region around the perimeter of the display and shroud  6 - 204 . 
     In some examples, the shroud  6 - 204  includes a transparent portion  6 - 205  and an opaque portion  6 - 207 , as described above and elsewhere herein. In at least one example, the opaque portion  6 - 207  of the shroud  6 - 204  can define one or more transparent regions  6 - 209  through which the sensors  6 - 203  of the sensor system  6 - 202  can send and receive signals. In the illustrated example, the sensors  6 - 203  of the sensor system  6 - 202  sending and receiving signals through the shroud  6 - 204 , or more specifically through the transparent regions  6 - 209  of the (or defined by) the opaque portion  6 - 207  of the shroud  6 - 204  can include the same or similar sensors as those shown in the example of  FIG.  6 - 1   , for example depth sensors  6 - 108  and  6 - 110 , depth projector  6 - 112 , first and second scene cameras  6 - 106 , first and second downward cameras  6 - 114 , first and second side cameras  6 - 118 , and first and second infrared illuminators  6 - 124 . These sensors are also shown in the examples of  FIGS.  6 - 3  and  6 - 4   . Other sensors, sensor types, number of sensors, and relative positions thereof can be included in one or more other examples of HMDs. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  6 - 2    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  6 - 1  and  6 - 3 - 6 - 4    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  6 - 1  and  6 - 3 - 6 - 4    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  6 - 2   . 
       FIG.  6 - 3    illustrates a front view of a portion of an example of an HMD device  6 - 300  including a display  6 - 334 , brackets  6 - 336 ,  6 - 338 , and frame or housing  6 - 330 . The example shown in  FIG.  6 - 3    does not include a front cover or shroud in order to illustrate the brackets  6 - 336 ,  6 - 338 . For example, the shroud  6 - 204  shown in  FIG.  6 - 2    includes the opaque portion  6 - 207  that would visually cover/block a view of anything outside (e.g., radially/peripherally outside) the display/display region  6 - 334 , including the sensors  6 - 303  and bracket  6 - 338 . 
     In at least one example, the various sensors of the sensor system  6 - 302  are coupled to the brackets  6 - 336 ,  6 - 338 . In at least one example, the scene cameras  6 - 306  include tight tolerances of angles relative to one another. For example, the tolerance of mounting angles between the two scene cameras  6 - 306  can be 0.5 degrees or less, for example 0.3 degrees or less. In order to achieve and maintain such a tight tolerance, in one example, the scene cameras  6 - 306  can be mounted to the bracket  6 - 338  and not the shroud. The bracket can include cantilevered arms on which the scene cameras  6 - 306  and other sensors of the sensor system  6 - 302  can be mounted to remain un-deformed in position and orientation in the case of a drop event by a user resulting in any deformation of the other bracket  6 - 226 , housing  6 - 330 , and/or shroud. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  6 - 3    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  6 - 1 - 6 - 2  and  6 - 4    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  6 - 1 - 6 - 2  and  6 - 4    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  6 - 3   . 
       FIG.  6 - 4    illustrates a bottom view of an example of an HMD  6 - 400  including a front display/cover assembly  6 - 404  and a sensor system  6 - 402 . The sensor system  6 - 402  can be similar to other sensor systems described above and elsewhere herein, including in reference to  FIGS.  6 - 1 - 6 - 3   . In at least one example, the jaw cameras  6 - 416  can be facing downward to capture images of the user&#39;s lower facial features. In one example, the jaw cameras  6 - 416  can be coupled directly to the frame or housing  6 - 430  or one or more internal brackets directly coupled to the frame or housing  6 - 430  shown. The frame or housing  6 - 430  can include one or more apertures/openings  6 - 415  through which the jaw cameras  6 - 416  can send and receive signals. 
     In at least one example, an outward facing sensor assembly configured to capture images from an external environment in front of the head mountable device can include the first scene camera  6 - 106  pointed in a first direction and the second scene camera  6 - 106  pointed in a second direction, and an inward facing sensor assembly configured to capture images of the user when the user dons the head mountable device  6 - 100 . The inward facing sensor assembly can include the various eye tracking camera  6 - 120  and  6 - 122  pointed in the rearward direction. In at least one example, the device  6 - 100  can include a controller electrically coupled to the outward facing sensor assembly and the inward facing sensor assembly. The controller can be configured to cause the rearward facing display to project first images captured by the outward facing sensor assembly and cause the forward facing display to project second images captured by the various eye tracking cameras  6 - 120 ,  6 - 122 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  6 - 4    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  6 - 1 - 6 - 3    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  6 - 1 - 6 - 3    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  6 - 4   . 
     VII: Antennas 
       FIGS.  7 . 0 - 1    illustrates a view of an example of an HMD display unit  100  including an antenna assembly  7 . 0 - 102 . Section VII of the present application describes the antennas and various components, assemblies, and systems associated with the antenna system  7 . 0 - 102  of the HMD display unit  100 . 
     7.1: Electronic Devices with Antenna Mounting Structures 
     Electronic devices may be provided with component mounting structures. The electronic devices may include portable electronic devices, wearable devices, desktop devices, embedded systems, and other electronic equipment. Illustrative configurations in which the electronic devices include a head-mounted device with a component mounting system may sometimes be described herein as an example. 
     Component mounting systems may be used to help mount electrical components in a device. As an example, a biasing member (sometimes referred to as a basing structure) may be used to help mount an antenna in the housing of a device. The biasing member, the antenna, and additional structures such as an antenna support member (sometimes referred to as an antenna support structure or antenna support) may form an antenna assembly that helps mount the antenna in a location in the device in which antenna signals are not blocked by conductive housing structures. If desired, biasing members may be used in mounting components other than antennas in a device. The use of biasing members to help mount antennas in head-mounted devices is illustrative. 
     A biasing member for an antenna assembly may be formed from a material such as structural foam or other compressible material that exhibits unidirectional compression and expansion characteristics. With this type of arrangement, the foam preferentially compresses and expands along a particular direction and exhibits little or no expansion and compression along orthogonal directions. The direction of preferential compression and expansion, which may sometimes be referred to as a preferential compression direction, unidirectional compression direction, preferential compression/expansion axis, axis of preferential compression, etc., need not be parallel with the surface normals of the biasing member. For example, a biasing member may be formed from a layer of foam with first and second opposing surfaces that are characterized by surface normals and the preferential compression direction may be oriented at a non-zero angle (e.g., an angle of at least 10°, at least 20°, less than 80°, less than 70°, or other suitable angle) with respect to the surface normals. 
       FIGS.  7 . 1 - 1    is a top view of an illustrative electronic device that may include a component biasing member. In the example of  FIGS.  7 . 1 - 1   , device  7 . 1 - 10  is a head-mounted device. As shown in  FIGS.  7 . 1 - 1   , head-mounted device  7 . 1 - 10  may include housing  7 . 1 - 12 . Housing  7 . 1 - 12  is configured to be worn on a user&#39;s head and may sometimes be referred to as a head-mounted housing or head-mounted support structure. Housing  7 . 1 - 12  may have curved head-shaped surfaces, a nose-bridge portion such as portion NB that is configured to rest on a user&#39;s nose when device  7 . 1 - 10  is on a user&#39;s head, may have a headband such as strap  7 . 1 - 12 T for supporting device  7 . 1 - 10  on the user&#39;s head, and/or may have other features that allow device  7 . 1 - 10  to be worn by a user. Housing  7 . 1 - 12  may have walls or other structures that separate an interior region of device  7 . 1 - 10  such as interior region  7 . 1 - 42  from an exterior region surrounding device  7 . 1 - 10  such as exterior region  7 . 1 - 44 . As an example, housing  7 . 1 - 12  may include a transparent layer that forms a housing wall on front F of device  7 . 1 - 10  such as display cover layer  7 . 1 - 12 CG. Display cover layer  7 . 1 - 12 CG may overlap a forward-facing display such as display  7 . 1 - 20  (e.g., a pixel array based on organic light-emitting diodes or other display panel). Electrical components  7 . 1 - 36  (e.g., integrated circuits, sensors, control circuitry, light-emitting diodes, lasers, and other light-emitting devices, other control circuits and input-output devices, etc.) may be mounted on printed circuits and/or other structures within device  7 . 1 - 10  (e.g., in interior region  7 . 1 - 42 ). 
     To present a user with images for viewing from eye boxes such as eye box  7 . 1 - 34 , device  7 . 1 - 10  may include rear-facing displays in optical modules  7 . 1 - 16 . There may be, for example, a left rear-facing display in a left optical module  7 . 1 - 16  for presenting an image through a left lens to a user&#39;s left eye in a left eye box  7 . 1 - 34  and a right rear-facing display in right optical module  7 . 1 - 16  for presenting an image through a right lens to a user&#39;s right eye in a right eye box  7 . 1 - 34 . 
     The user&#39;s eyes are located in eye boxes  7 . 1 - 34  at rear R of device  7 . 1 - 10  when inwardly facing surface  7 . 1 - 18  of housing  7 . 1 - 12  rests against the outer surface of the user&#39;s face. On rear R, housing  7 . 1 - 12  may have cushioned structures (sometimes referred to as light seal structures) to enhance user comfort as surface  7 . 1 - 18  rests against the user&#39;s face. Device  7 . 1 - 10  may have forward-facing components such has forward-facing cameras and other sensors on front F that face outwardly away from the user. These components may generally be oriented in the +Y (forward) direction of  FIGS.  7 . 1 - 1   . 
     During operation, device  7 . 1 - 10  may receive image data (e.g., image data for video, still images, etc.) and may present this information on the displays of optical modules  7 . 1 - 16 . Device  7 . 1 - 10  may also receive other data, control commands, user input, etc. Device  7 . 1 - 10  may transmit data to accessories and other electronic equipment. For example, image data from a forward-facing camera may be provided to an associated device, audio output may be provided to a device with speakers such as a headphone device, user input and sensor readings may be transmitted to remote equipment, etc. 
     Communications such as these may be supported using wired and/or wireless communications. In an illustrative configuration, components  7 . 1 - 36  may include wireless communications circuitry for supporting wireless communications between device  7 . 1 - 10  and remote wireless equipment (e.g., a cellular telephone, a wireless base station, a computer, headphones or other accessories, a remote control, peer devices, internet servers, and/or other equipment). Wireless communications may be supported using one or more antennas operating at one or more wireless communications frequencies. In an illustrative configuration, one or more antennas may be coupled to wireless transceiver circuitry. The wireless transceiver circuitry may include transmitter circuitry configured to transmit wireless communications signals using the antenna(s) and receiver circuitry configured to receive wireless communications signals using the antenna(s). 
     The wireless circuitry of device  7 . 1 - 10  may be formed from one or more integrated circuits, power amplifier circuitry, low-noise input amplifiers, passive RF components, one or more antennas, transmission lines, and other circuitry for handling RF wireless signals. The wireless circuitry may include radio-frequency transceiver circuitry for handling various radio-frequency communications bands. For example, the wireless circuitry of device  7 . 1 - 10  may include wireless local area network (WLAN) and wireless personal area network (WPAN) transceiver circuitry. This transceiver circuitry may handle 2.4 GHz and 5 GHz bands for WiFi® (IEEE 802.11) communications and other WLAN communications and the 2.4 GHz Bluetooth® communications band or other WPAN bands and may sometimes be referred to herein as WLAN/WPAN transceiver circuitry or local transceiver circuitry. 
     The wireless circuitry of device  7 . 1 - 10  may use remote wireless circuitry such as cellular telephone transceiver circuitry for handling wireless communications in frequency ranges (communications bands) such as a cellular low band (LB) from 600 to 960 MHz, a cellular low-midband (LMB) from 1410 to 1510 MHz, a cellular midband (MB) from 1710 to 2170 MHz, a cellular high band (HB) from 2300 to 2700 MHZ, a cellular ultra-high band (UHB) from 3300 to 5000 MHz, or other communications bands between 600 MHz and 5000 MHz. If desired, the cellular telephone transceiver circuitry may support 5G communications using a low band at 600-850 MHz, a mid-band at 2.5-3.7 GHZ, and a high band at 25-39 GHz. Wireless communications may also be provided using other frequency ranges (e.g., frequencies above 100 MHz, above 1 GHz, 1-30 GHz, 100 Mhz-300 GHz, 24 GHz, less than 300 GHz, less than 100 GHz, 10-300 GHz or other mm-wave frequencies, and/or other suitable frequencies). WLAN/WPAN transceiver circuitry and/or cellular transceiver circuitry may handle voice data and non-voice data. 
     If desired, the antennas and other wireless circuitry of device  7 . 1 - 10  may include satellite navigation system circuitry such as Global Positioning System (GPS) receiver circuitry for receiving GPS signals at 1575 MHz or for handling other satellite positioning data (e.g., GLONASS signals at 1609 MHz). Satellite navigation system signals are received from a constellation of satellites orbiting the earth. Wireless circuitry in device  7 . 1 - 10  can include circuitry for other short-range (local) and long-range (remote) wireless links if desired. For example, wireless circuitry in device  7 . 1 - 10  may be provided to receive television and radio signals, paging signals, near field communications (NFC) signals at 13.56 MHz or other suitable NFC frequencies, ultrawideband (UWB) signals (e.g., UWB signals from 6-8.5 GHZ, UWB signals from 3.5-9 GHZ, etc.). Wireless circuitry in device  7 . 1 - 10  may also include antennas and transceiver for handling sensing applications (e.g., radar). If desired, antennas may be provided in arrays (e.g., phased antenna arrays) that support beam steering. These arrangements and other arrangements may be used in supporting wireless communications, wireless sensing, wireless location services, wireless power, and other wireless operations. 
     The wireless circuitry of device  7 . 1 - 10  may include antennas that are formed using any suitable antenna types. For example, the antennas of device  7 . 1 - 10  may include antennas with resonating elements that are formed from slot antenna structures, loop antenna structures, patch antenna structures, stacked patch antenna structures, antenna structures having parasitic elements, inverted-F antenna structures, planar inverted-F antenna structures, helical antenna structures, monopole antennas, dipole antenna structures, Yagi (Yagi-Uda) antenna structures, surface integrated waveguide structures, coils, hybrids of these designs, etc. If desired, one or more of the antennas may be cavity-backed antennas. 
     Different types of antennas may be used for different bands and combinations of bands. For example, one type of antenna may be used in forming a local wireless link antenna whereas another type of antenna is used in forming a remote wireless link antenna. If desired, space may be conserved within device  7 . 1 - 10  by using a single antenna to handle two or more different communications bands. For example, a single antenna in device  7 . 1 - 10  may be used to handle communications in a WiFi® or Bluetooth® communication band while also handling communications at one or more cellular telephone frequencies. In some configurations, some cellular telephone communications (e.g., low-band and mid-band communications) may be handled using a first antenna (e.g., an inverted-F antenna), whereas other communications (e.g., high-band communications) may be handled using one or more phased antenna arrays (e.g., multiple linear patch antenna arrays each of which is mounted in a different orientation and each of which has a different angle of view so that a desired amount of angular coverage is achieved). 
     To provide antenna structures in device  7 . 1 - 10  with the ability to cover different frequencies of interest, one or more of the antennas of device  7 . 1 - 10  may be provided with circuitry such as filter circuitry (e.g., one or more passive filters and/or one or more tunable filter circuits). Discrete components such as capacitors, inductors, and resistors may be incorporated into the filter circuitry. Capacitive structures, inductive structures, and resistive structures may also be formed from patterned metal structures (e.g., part of an antenna). If desired, antenna(s) in device  7 . 1 - 10  may be provided with adjustable circuits such as tunable components that tune the antenna over communications (frequency) bands of interest. The tunable components may be part of a tunable filter or tunable impedance matching network, may be part of an antenna resonating element, may span a gap between an antenna resonating element and antenna ground, etc. 
     Radio-frequency transmission line paths may be used to convey antenna signals between the radio-frequency transceiver circuitry of device  7 . 1 - 10  and the antenna(s) of device  7 . 1 - 10 . These paths may include one or more radio-frequency transmission lines (sometimes referred to herein simply as transmission lines). Radio-frequency transmission line paths may each include a positive signal conductor and a ground signal conductor. Transmission lines in device  7 . 1 - 10  may include coaxial cable transmission lines, stripline transmission lines, microstrip transmission lines, edge-coupled microstrip transmission lines, edge-coupled stripline transmission lines, transmission lines formed from waveguide structures (e.g., coplanar waveguides or grounded coplanar waveguides), and combinations of these types of transmission lines and/or other transmission line structures. 
     If desired, matching networks may be used to help match impedances in the wireless circuitry of device  7 . 1 - 10 . A matching network may, for example, include components such as inductors, resistors, and capacitors configured to match the impedance of an antenna to the impedance of an associated radio-frequency transmission line path that is used in coupling the antenna to a transceiver. Matching network components may be provided as discrete components (e.g., surface mount technology components) or may be formed from housing structures, printed circuit board structures, traces on plastic supports, etc. Components such as these may also be used in forming antenna filter circuitry and may be tunable and/or fixed components. 
     Radio-frequency transmission line paths may be coupled to antenna feed structures associated with antennas in device  7 . 1 - 10 . As an example, an antenna in device  7 . 1 - 10  such as an inverted-F antenna, a planar inverted-F antenna, a patch antenna, a loop antenna, or other antenna may have an antenna feed with a positive antenna feed terminal and a ground antenna feed terminal. The positive antenna feed terminal may be coupled to an antenna resonating (radiating) element within the antenna. The ground antenna feed terminal may be coupled to an antenna ground in the antenna. The positive feed terminal may be coupled to a positive signal line in a transmission line and the ground feed terminal may be coupled to a ground signal line in the transmission line. 
     Other types of antenna feed arrangements may be used if desired. For example, an antenna may be fed using multiple feeds each coupled to a respective port of a transceiver over a corresponding transmission line. If desired, a given transmission line signal conductor may be coupled to multiple locations on an antenna and/or switches may be interposed within the paths between a transceiver and the feed terminals of an antenna. 
       FIGS.  7 . 1 - 2    is a diagram of illustrative wireless communications circuitry for device  7 . 1 - 10 . As shown in  FIGS.  7 . 1 - 2   , the wireless circuitry includes radio-frequency transceiver  7 . 1 - 60 , which is coupled to antenna  7 . 1 - 40  by transmission line  7 . 1 - 62 . Antenna  7 . 1 - 40  may have an antenna resonating element  7 . 1 - 52  and antenna ground  7 . 1 - 50 . Antenna resonating element  7 . 1 - 52  may be formed from any suitable antenna resonating element structures. In the example of  FIG.  2   , antenna resonating element  7 . 1 - 52  is an inverted-F antenna resonating element having resonating element arm  7 . 1 - 56 , which is coupled to ground  7 . 1 - 50  by return path  7 . 1 - 54  and which is fed using antenna feed  7 . 1 - 58 . Feed  7 . 1 - 58  has positive and ground feed terminals coupled respectively to positive and ground signal lines in transmission line  7 . 1 - 62 . Conductive structures making up antenna  7 . 1 - 40  may be formed from thin-film metal traces on printed circuits (e.g., rigid printed circuit boards formed from fiberglass-filled epoxy and other rigid printed circuit board substrate material and/or flexible printed circuits formed from sheets of polyimide or other flexible polymer substrates), metal traces on molded polymer antenna substrates, metal traces on other dielectric substrates, metal foil, conductive structural members such as portions of a housing for device  7 . 1 - 10  (e.g., a metal chassis and/or other internal and/or external frame structures, metal housing walls, metal component support brackets, and/or other conductive housing structures), and/or other structures in device  7 . 1 - 10  that are formed from metal and/or other conductive material. 
     Antennas may be mounted within device  7 . 1 - 10  using mounting brackets, using biasing structures that press antenna components against housing structures, using adhesive, using screws and other fasteners, using press-fit connections, using solder, welds, conductive adhesive, and/or other conductive attachment mechanisms, and/or other mounting arrangements. Due to the order of assembly of device components (e.g., due to the desire to assemble some components such as a display cover layer and/or other housing structures after other structures such as the rear of housing  7 . 1 - 12  and optical modules  7 . 1 - 16  have been assembled), it may be desirable to mount an antenna using a compressible structure such as a layer of foam or other biasing member that helps bias the antenna towards a known (reference) location in the device when the display cover layer or other housing structure is attached to other device structures. 
     Consider, as an example, the illustrative antenna mounting structures of  FIGS.  7 . 1 - 3   . As shown in  FIGS.  7 . 1 - 3   , antenna  7 . 1 - 40  may be formed from antenna substrate  7 . 1 - 64  (e.g., a flexible printed circuit or other substrate, which may be planar, as shown in  FIGS.  7 . 1 - 3   , or which may have a curved developable surface and/or a surface of compound curvature). Antenna substrate  7 . 1 - 64  contains metal traces  7 . 1 - 66  that have been patterned to form antenna resonating element  7 . 1 - 52  and/or other antenna structures. 
     Antenna  7 . 1 - 40  may be mounted in interior  7 . 1 - 42  using an antenna support member such as support member  7 . 1 - 68  (sometimes referred to as a support structure or support). Support member  7 . 1 - 68  may be formed from polymer, glass, ceramic, or other dielectric and/or other materials (e.g., metal, etc.) and/or combinations of these materials. A biasing member such as member  7 . 1 - 70  (sometimes referred to as a biasing structure) may be located between antenna  7 . 1 - 40  and support member  7 . 1 - 68 . Member  7 . 1 - 70  may be formed from a compressible structure (e.g., one or more springs, foam layers, elastomeric polymer layers, and/or other structures that can exhibit a restoring force when compressed). This allows member  7 . 1 - 70  to provide a biasing force that helps hold antenna  7 . 1 - 40  (e.g., substrate  7 . 1 - 64 ) against the inner surface of cover layer  7 . 1 - 12 CG and/or other portions of device  7 . 1 - 10  such as other housing structures. As an example, member  7 . 1 - 68  may be mounted in device  7 . 1 - 10  so that antenna  7 . 1 - 40  faces outwardly towards front F (e.g., in the +Y direction away from eye boxes  7 . 1 - 34 ). After mounting member  7 . 1 - 68  in this way, one or more additional layers of material may be installed in device  7 . 1 - 10  (e.g., by attaching such layer(s) to housing sidewalls and/or other housing structures). The layers of material that are installed in this way (which may sometimes be referred to as housing layers) may include cosmetic covering layers (e.g., a ring-shaped cover, sometimes referred to as a shroud or shroud trim, that runs around the border of display  7 . 1 - 20 , a tinted polymer layer that covers display  7 . 1 - 20  on the front of device  7 . 1 - 10  (sometimes referred to as a shroud canopy), display cover layer  7 . 1 - 12 CG, and/or other layer(s) of material. During installation of one or more of these housing layers (e.g., layer  7 . 1 - 12 CG, shroud layer(s), etc.), a curved inner surface or other inner surface of layer  7 . 1 - 12 CG and/or other housing layer(s) may contact antenna  7 . 1 - 40  and may press antenna  7 . 1 - 40  towards member  7 . 1 - 68 . This compresses biasing member  7 . 1 - 70 , which generates a restoring force (biasing force) that helps hold antenna  7 . 1 - 40  in place against the inner surface of layer  7 . 1 - 12 CG and/or other overlapping housing layers. In this way, the location of antenna  7 . 1 - 40  relative to layer  7 . 1 - 12 CG and/or other housing layers may be reliably established. 
     Substrate  7 . 1 - 64  may be formed from a layer with opposing first and second sides characterized by respective surface normals (see, e.g., surface normal n). In configurations in which antenna  7 . 1 - 40  (e.g., substrate  7 . 1 - 64 ) and member  7 . 1 - 70  are compressed in a direction parallel that is parallel to the surface normal n of antenna  7 . 1 - 40  and substrate  7 . 1 - 64 , member  7 . 1 - 70  will compress in the absence of sheering forces (off-axis forces with respect to surface normal n). In some configurations, however, the inner surface of layer  7 . 1 - 12 CG (or other structure against which antenna  7 . 1 - 40  is being placed) may exert force on antenna  7 . 1 - 40  and member  7 . 1 - 70  in a direction that is orientated at a non-zero angle (see, e.g., angle A 2  of  FIGS.  7 . 1 - 4   , which may be between 10° and 80°, between 20° and 70°, etc.) with respect to surface normal n. For example, the inner surface of layer  7 . 1 - 12 CG may be angled with respect to the Y axis. As a result, sheering forces may be generated that give rise to uncertainty in the lateral placement (location along the X axis) of antenna  7 . 1 - 40  after an overlapping layer such as layer  7 . 1 - 12 CG and/or other overlapping housing layer(s) has been assembled into device  7 . 1 - 10 . 
     To overcome undesired lateral movement of antenna  7 . 1 - 40  in response to installation of layer  7 . 1 - 12 CG or other overlapping layer(s) and associated application of force from layer  7 . 1 - 12 CG or other overlapping layer(s) onto antenna  7 . 1 - 40  and member  7 . 1 - 70 , member  7 . 1 - 70  may be formed from a compressible structure that exhibits preferential compression along an axis (sometimes referred to as a compression axis, unidirectional compression axis, axis of compression, axis of preferential compression, preferential axis of compression, or unidirectional axis of compression) that is not parallel to surface normals such as surface normal n that are associated with the inner surface of the display cover layer, the surfaces of the antenna substrate, and the adjacent surface of member  7 . 1 - 70 . 
     The use of a biasing member with a preferential axis of compression that is not parallel to surface normal n shown in  FIGS.  7 . 1 - 4   . In the example of  FIGS.  7 . 1 - 4   , member  7 . 1 - 70  has been formed from a polymer such as polymer foam (e.g., elastomeric open-cell and/or closed-cell foam). Fibers  7 . 1 - 72  (e.g., strands of polymer, glass, carbon, or other materials) have been embedded into member  7 . 1 - 70 . Most or all of fibers  7 . 1 - 72  extend parallel to the X axis of  FIGS.  7 . 1 - 4   , which is perpendicular to the Y axis of  FIGS.  7 . 1 - 4   , and/or the lengths of fibers  7 . 1 - 72  that run parallel to the X axis are longer than those of fibers  7 . 1 - 72  that run parallel to the Y axis. The presence of fibers  7 . 1 - 72  helps preferentially stiffen and reduce compressibility in member  7 . 1 - 70  along the direction in which fibers  7 . 1 - 72  are aligned). As a result, the foam material of member  7 . 1 - 70  of  FIGS.  7 . 1 - 4    exhibits elevated resistance to compression along the X axis (oriented at a non-zero angle A 1  with respect to surface normal n in the example of  FIG.  4   ) and exhibits lowered (e.g., less) resistance to compression along the Y axis (oriented at a non-zero angle A 2  with respect to surface normal n in the example of  FIGS.  7 . 1 - 4   ). The Y axis in this example serves as the preferential axis of compression for member  7 . 1 - 70 . Because member  7 . 1 - 70  compresses and stretches along the Y axis but does not significantly compress or stretch along the X axis, which is orthogonal to the Y axis, member  7 . 1 - 70  may sometimes be referred to as unidirectional structural foam antenna biasing member (unidirectional structural foam antenna biasing structure) or unidirectional antenna biasing member (unidirectional antenna biasing structure). 
       FIGS.  7 . 1 - 5    is a diagram illustrating the behavior of member  7 . 1 - 70  of  FIGS.  7 . 1 - 4    when exposed to applied force along the Y axis (e.g., a direction that is at a non-zero angle A 2  with respect to surface normal n). Initially, member  7 . 1 - 70  is uncompressed and has thickness T. After compression, member  7 . 1 - 70  has reduced thickness TR. Due to the presence of fibers  7 . 1 - 72  ( FIGS.  7 . 1 - 4   ) and/or other structures that promote uniaxial expansion and contraction, when force is applied to the surfaces of member  7 . 1 - 70  along the Y axis, member  7 . 1 - 70  compresses in direction  7 . 1 - 74  along the Y axis from its initially uncompressed state to a compressed state (see, e.g., compressed member shape  7 . 1 - 70 ′ of  FIGS.  7 . 1 - 5   ). During these compression activities, compression occurs along the Y axis and not the orthogonal X axis. As a result, the opposing outwardly facing and inwardly facing surfaces of member  7 . 1 - 70  do not shift laterally (e.g., there is no movement of these surfaces relative to each other along the X axis). This helps ensure that antenna  7 . 1 - 40  is mounted in a desired location in device  7 . 1 - 10  and does not experience undesirable lateral movement during assembly. 
       FIGS.  7 . 1 - 6    shows how the uniaxial compression properties of member  7 . 1 - 70  may be used to help ensure satisfactory placement of antenna  7 . 1 - 40  within device  7 . 1 - 10 . As shown in  FIGS.  7 . 1 - 6   , forward-facing display  7 . 1 - 20  may be mounted under display cover layer  7 . 1 - 12 CG, so that images on display  7 . 1 - 20  may be viewed on front F of device  7 . 1 - 10 . If desired, an air gap may separate display  7 . 1 - 20  from display cover layer  7 . 1 - 12 CG. One or more additional structures (e.g., a shroud having a ring-shaped trim portion that surrounds the pixels of display  7 . 1 - 20  and having a canopy portion that covers the pixels of display  7 . 1 - 20 ) may optionally be located between display cover layer  7 . 1 - 12 CG and antenna  7 . 1 - 40 , as shown by illustrative dielectric layer(s)  7 . 1 - 85 . As an example, device  7 . 1 - 10  may include an internal housing structure such as a polymer structure forming a shroud (e.g., layer  7 . 1 - 85  may be a polymer should layer) and this layer may be interposed between layer  7 . 1 - 12 CG and antenna  7 . 1 - 40 . The shroud in this type of arrangement may have a ring shape that extends around the periphery of display  7 . 1 - 20  and/or may have portions that overlap display  7 . 1 - 20 . In general, antenna  7 . 1 - 40  may be overlapped by any structure having a surface (e.g., an inner surface) against which antenna  7 . 1 - 40  is mounted. The overlapping structure, which may be a housing structure such as display cover layer  7 . 1 - 12 CG, a shroud trim member, a shroud canopy, or other polymer layer, a dielectric housing wall, and/or any other dielectric member (sometimes referred to as a housing structure or housing layer), may have an inwardly facing surface against which antenna  7 . 1 - 40  is mounted. In the illustrative configuration of  FIGS.  7 . 1 - 6   , the housing layer that overlaps antenna  7 . 1 - 40  is display cover layer  7 . 1 - 12 CG (or, in the situation where optional interposed layer  7 . 1 - 85  is present, the layer that overlaps antenna  7 . 1 - 40  is a polymer layer that is between layer  7 . 1 - 12 CG and antenna  7 . 1 - 40 ). These are illustrative examples. In general, any suitable polymer layer or other dielectric structure may overlap antenna  7 . 1 - 40  and may have a surface against which antenna  7 . 1 - 40  may be mounted during assembly of device  7 . 1 - 10 . 
     In the illustrative configuration of  FIGS.  7 . 1 - 6   , antenna  7 . 1 - 40  is mounted in device  7 . 1 - 10  in peripheral (edge) region  7 . 1 - 80  (on the right side of device  7 . 1 - 10  in the example of  FIG.  7 . 1 - 6   ) under layer  7 . 1 - 12 CG. Layer  7 . 1 - 12 CG (and/or other housing structures overlapping antenna  7 . 1 - 40 ) may be planar or may be curved and may be tilted to the right sufficiently to create a region of surface  7 . 1 - 84  with a surface normal that is angled with respect to the X and Y axes of  FIGS.  7 . 1 - 6    (see, e.g., surface normal n of antenna  7 . 1 - 40 , which is parallel to the surface normal of surface  7 . 1 - 84  in region  7 . 1 - 80 ). Because region  7 . 1 - 80  is angled away from the central portion of layer  7 . 1 - 12 CG (or other overlapping dielectric layer) and may have a curved cross-sectional profile, region  7 . 1 - 80  may sometimes be referred to as forming a curved edge portion of layer  7 . 1 - 12 CG or a curved edge portion of other overlapping housing layer. 
     Antenna  7 . 1 - 40  is attached (e.g., with adhesive) to biasing member  7 . 1 - 70  (e.g., a layer of unidirectional structural foam of the type descried in connection with  FIGS.  7 . 1 - 4  and  7 . 1 - 5    that is configured to serve as an antenna biasing member). Member  7 . 1 - 70  has a first side that faces antenna  7 . 1 - 40  and an opposing second side that faces support member  7 . 1 - 68 . Member  7 . 1 - 70  is attached (e.g., with adhesive) to support member  7 . 1 - 68 . Support member  7 . 1 - 68 , in turn, is mounted to housing  7 . 1 - 12  (e.g., fastener  7 . 1 - 82  may be used to attach member  7 . 1 - 68  to portion  7 . 1 - 12 P of housing  7 . 1 - 12  and/or other attachment mechanisms may be used to secure member  7 . 1 - 68  relative to housing  7 . 1 - 12  on rear R and/or elsewhere in device  7 . 1 - 10 ). When device  7 . 1 - 10  is assembled, there is generally slight compression of member  7 . 1 - 70  (e.g., along the Y axis), which creates a restoring force outward against inner surface  7 . 1 - 84 . The arrangement of  FIGS.  7 . 1 - 6    therefore places antenna  7 . 1 - 40  in a known spatial relationship with overlapping structures such display cover layer  7 . 1 - 12 CG (e.g., in direct contact with surface  7 . 1 - 84 ), thereby eliminating uncertainty in the distance between antenna  7 . 1 - 40  and layer  7 . 1 - 12 CG. This may help avoid the possibility of forming variable-size air gaps between antenna  7 . 1 - 40  and surface  7 . 1 - 84 , which could have varying impacts on antenna performance. Layer  7 . 1 - 12 CG and/or other overlapping housing structures (e.g., a polymer shroud layer or other polymer layer) is preferably formed from a dielectric such as glass or polymer, so radio-frequency antenna signals for antenna  7 . 1 - 40  may pass through portion  7 . 1 - 80  of layer  7 . 1 - 12 CG or other overlapping layer. 
     Antenna performance, which is affected by the distance between antenna  7 . 1 - 40  and the structures of device  7 . 1 - 10 , and the reliability of the mounting arrangement shown in  FIGS.  7 . 1 - 6    could potentially be adversely affected by undesired lateral movement of antenna  7 . 1 - 40  relative to its nominal position under portion  7 . 1 - 80 . This lateral movement is prevented by using uniaxial foam in forming member  7 . 1 - 70 . During assembly of device  7 . 1 - 10 , antenna  7 . 1 - 40 , member  7 . 1 - 70 , and member  7 . 1 - 68  are initially mounted to housing  7 . 1 - 12 . In this initial state, the front wall of housing  7 . 1 - 12  (e.g., cover layer  7 . 1 - 12 CG in the current example) may not be present. After components  7 . 1 - 36  have been installed within housing  7 . 1 - 12 , cover layer  7 . 1 - 12 CG and/or other overlapping housing layer(s) may be moved in the −Y direction and mounted to housing  7 . 1 - 12  (e.g., using adhesive layer  7 . 1 - 86 , using fasteners, and/or using other attachment structures). Because surface normal n of antenna  7 . 1 - 40  and the opposing surface normal of surface  7 . 1 - 84  in edge region  7 . 1 - 80  are at a non-zero angle with respect to the Y axis (see, e.g., angle A 2  of  FIGS.  7 . 1 - 4   ), movement of layer  7 . 1 - 12 CG in the −Y direction creates a lateral force on antenna  7 . 1 - 40  and member  7 . 1 - 70  along the X axis that has the potential to compress member  7 . 1 - 70  laterally. Nevertheless, because member  7 . 1 - 70  is formed from unidirectional structural foam that preferentially compresses along the Y axis, this lateral movement (movement parallel to the X axis) is prevented. Rather, as layer  7 . 1 - 12 CG (or other overlapping dielectric housing structure) is mounted to housing  7 . 1 - 12  and presses inwardly on antenna  7 . 1 - 40 , member  7 . 1 - 70  compresses only along the Y axis. As a result, antenna  7 . 1 - 40  moves slightly in the −Y direction when pressed by surface  7 . 1 - 84 , but does not shift position relative to the X axis. This ensures that antenna  7 . 1 - 40  is located satisfactorily in device  7 . 1 - 10  relative to housing  7 . 1 - 12  (which may include, for example, metal structures such as a metal chassis that forms some or all of antenna ground  7 . 1 - 50  of  FIGS.  7 . 1 - 2   ). Satisfactory performance of antenna  7 . 1 - 40  may therefore be achieved. 
     Although sometimes described herein in the context of an antenna biasing member in a head-mounted device, unidirectional structural foam or other compressible structures with a unidirectional axis of compression may be used in other contexts and/or devices. As an example, unidirectional structural foam may be used in device  7 . 1 - 10  for sound management (e.g., to exhibit preferential sound channeling along a particular direction), may be used for directional vibration absorption (perpendicular to the axis of fibers  7 . 1 - 72 ), may be used to exhibit directional electrical conductivity (e.g., for sensing and signal routing applications), and/or may otherwise be used in electronic devices that can benefit from elevated control of antenna placement and/or elevated control of the placement of other device components. 
     7.2: Systems with Transparent Layers 
     Electronic devices may be provided with components such as antennas. The electronic devices may include portable electronic devices, wearable devices, desktop devices, embedded systems, and other electronic equipment. Illustrative configurations in which the electronic devices include a head-mounted device may sometimes be described herein as an example. 
     The antennas in an electronic device may be configured to cover communications bands of interest (e.g., local area network bands, cellular telephone bands, etc.). To handle some communications such as 5G cellular communications, the antennas may include millimeter wave antennas (e.g., antennas operating at one or more frequencies between 20 GHz and 300 GHz, as an example). A millimeter wave antenna may use a phased-antenna array architecture in which multiple antenna elements such as patch antenna elements are arranged in an array (e.g., multiple patches in a row). During operation, the relative phases of each of the elements may be adjusted (e.g., so that the phased-antenna array performs beam steering). 
     Electronic device housing structures and other parts of an electronic device may include areas that are characterized by curved surfaces that can be flattened into a plane without distortion (sometimes referred to as developable surfaces or curved surfaces without compound curvature). Electronic device housing structures and other parts of an electronic device may also include areas that are characterized by compound curvature (surfaces that can only be flattened into a plane with distortion, sometimes referred to as non-developable surfaces). Mounting millimeter wave antennas and/or other antennas in an electronic device with curved surfaces can be challenging, because the presence of a curved surface adjacent to an antenna may give rise to different amounts of loading for different antenna elements in the antenna. 
     To help ensure satisfactory antenna operation when integrating a millimeter wave antenna into an electronic device with a curved structure, a dielectric member (sometimes referred to as a dielectric structure or dielectric layer) may be provided between the curved structure and the antenna. The dielectric member may have a planar surface facing the antenna. As an example, in a head-mounted device with a curved display cover layer, a polymer layer with a planar surface facing an antenna may be placed between the antenna and the curved display cover layer. In this way, the dielectric structure may help even out the amount of loading experienced by each antenna element in the antenna. The dielectric structure may also assist in impedance matching. 
       FIGS.  7 . 2 - 1    is a top view of an illustrative electronic device having antennas such as one or more millimeter wave antennas. In the example of  FIGS.  7 . 2 - 1   , device  7 . 2 - 10  is a head-mounted device. In general, device  7 . 2 - 10  may be any suitable electronic equipment. 
     As shown in  FIGS.  7 . 2 - 1   , head-mounted device  7 . 2 - 10  may include housing  7 . 2 - 12 . Housing  7 . 2 - 12  is configured to be worn on a user&#39;s head and may sometimes be referred to as a head-mounted housing or head-mounted support structure. Housing  7 . 2 - 12  may have curved head-shaped surfaces, a nose-bridge portion such as portion NB that is configured to rest on a user&#39;s nose when device  7 . 2 - 10  is on a user&#39;s head, may have a headband such as strap  7 . 2 - 12 T for supporting device  7 . 2 - 10  on the user&#39;s head, and/or may have other features that allow device  7 . 2 - 10  to be worn by a user. 
     Housing  7 . 2 - 12  may have walls or other structures that separate an interior region of device  7 . 2 - 10  such as interior region  7 . 2 - 42  from an exterior region surrounding device  7 . 2 - 10  such as exterior region  7 . 2 - 44 . As an example, housing  7 . 2 - 12  may include a transparent layer that forms a housing wall on front F of device  7 . 2 - 10  such as display cover layer  7 . 2 - 12 CG. Housing  7 . 2 - 12  may also include internal frame structures (e.g., a metal chassis), cosmetic covering members, polymer layers (e.g., fully or partly transparent polymer layers), housing walls formed from polymer and/or other materials, and/or other housing structures. In an illustrative configuration, housing  7 . 2 - 12  includes a dielectric structure such as dielectric member  7 . 2 - 13  that is overlapped by display cover layer  7 . 2 - 12 CG. Dielectric member  7 . 2 - 13 , which may sometimes be referred to as a polymer layer, shroud, dielectric layer, or dielectric structure, may be formed from one or more individual dielectric structures (e.g., structures formed from polymer, glass, ceramic, and/or other dielectric). Member  7 . 2 - 13  may be formed in a ring shape that runs along the periphery of cover layer  7 . 2 - 12 CG (e.g., under peripheral edge portion E of cover layer  7 . 2 - 12 CG) or may, as shown in  FIGS.  7 . 2 - 1   , overlap substantially all of display cover layer  7 . 2 - 12 CG. 
     Display cover layer  7 . 2 - 12 CG and member  7 . 2 - 13  may overlap a forward-facing display such as display  7 . 2 - 20  (e.g., a flexible display panel formed from a pixel array based on organic light-emitting diodes or other display panel). The portion of member  7 . 2 - 13  that overlaps display  7 . 2 - 20  may be formed from fully transparent polymer or partly transparent polymer that helps hide display  7 . 2 - 20  from view. The portion of member  7 . 2 - 13  in edge portion E may be opaque or transparent. Display cover layer  7 . 2 - 12 CG may be formed from transparent polymer or glass (as examples). 
     Portions of display cover layer  7 . 2 - 12 CG and member  7 . 2 - 13  such as edge portions of display cover layer  7 . 2 - 12 CG and member  7 . 2 - 13  that surround display  7 . 2 - 20  may have curved cross-sectional profiles. As an example, edge portion E of cover layer  7 . 2 - 12 CG and the underlying edge portion of member  7 . 2 - 13  may have one or more surfaces characterized by compound curvature (e.g., non-developable surfaces). The central portions of display cover layer  7 . 2 - 12 CG and member  7 . 2 - 13  that overlap pixels of display  7 . 2 - 20  may have compound curvature and/or may have developable surfaces. In an illustrative arrangement, cover layer  7 . 2 - 12 CG has inner and outer surfaces with compound curvature and member  7 . 2 - 13  has an outer surface of compound curvature around the edges of device  7 . 2 - 10  (e.g., the portion of member  7 . 2 - 13  surrounding display  7 . 2 - 20 ) and has developable inner and outer surfaces overlapping display  7 . 2 - 20 . In the areas of compound curvature, at least some portions of the curved surface of layer  7 . 2 - 12 CG and/or member  7 . 2 - 13  may be characterized by a radius of curvature R of 4 mm to 250 mm, 8 mm to 200 mm, 10 mm to 150 mm, at least 5 mm, at least 12 mm, at least 16 mm, at least 20 mm, at least 30 mm, less than 200 mm, less than 100 mm, less than 75 mm, less than 55 mm, less than 35 mm, and/or other suitable amount of curvature. In this illustrative configuration, display  7 . 2 - 20  may be a flexible display panel that is bent into a curved shape (e.g., a curved shape following the curved face of a user) and that is characterized by inner and outer developable surfaces. The portion of member  7 . 2 - 13  overlapping display  7 . 2 - 20  may have corresponding inner and outer developable surfaces. The innermost surface of member  7 . 2 - 13  in edge portion E may be planar to accommodate millimeter wave antennas. Other arrangements for the shapes of display cover layer  7 . 2 - 12 CG and member  7 . 2 - 13  may be used in device  7 . 2 - 10 , if desired. 
     Device  7 . 2 - 10  may include millimeter wave antennas and other antennas. Millimeter wave antennas may use phased-antenna arrays to implement beam steering. Each millimeter wave antenna may have an associated angle of view. To help provide satisfactory antenna coverage for device  7 . 2 - 10  at millimeter wave frequencies, it may be desired to provide device  7 . 2 - 10  with multiple millimeter wave antennas and to orient each of these antennas in a different direction so that the angular coverage of each of the antennas overlaps. 
     Consider, as an example, illustrative device  7 . 2 - 10  of  FIGS.  7 . 2 - 1   , which has three millimeter wave antennas, each of which is pointed in a different direction. A first of the three antennas (millimeter wave antenna  7 . 2 - 40 - 1 ) is located under edge portion E of member  7 . 2 - 13  and cover layer  7 . 2 - 12 CG on the left side of device  7 . 2 - 10 . Antenna  7 . 2 - 40 - 1  is oriented in direction  7 . 2 - 72 , which is rotated counter clockwise by angle AX relative to the Y axis (where the Y axis is oriented in the forward direction facing outwardly in front of device  7 . 2 - 10 ). A second of the three antennas (millimeter wave antenna  7 . 2 - 40 - 2 ) is located in the center of device  7 . 2 - 10 , and, in this illustrative example, points straight ahead (in direction  7 . 2 - 74 , along the Y axis). A third of the three antennas (millimeter wave antenna  7 . 2 - 40 - 3 ) is oriented in direction  7 . 2 - 76 , which is rotated clockwise by an angle AX relative to the Y axis. Directions  7 . 2 - 72 ,  7 . 2 - 74 , and  7 . 2 - 76  may each lie in the XY plane of  FIG.  1    or may be angled above or below the XY plane. With this type of arrangement, each antenna has a respective angle of view VA (e.g., a value within the range of 15°-90°, as an example). By overlapping the angle-of-view coverage of each antenna (e.g., by pointing antenna  7 . 2 - 40 - 1  slightly to the left of center, by pointing antenna  7 . 2 - 40 - 2  straight ahead, and by pointing antenna  7 . 2 - 40 - 3  slightly to the right of center), device  7 . 2 - 10  can be provided with a larger angular coverage at millimeter wave frequencies than if only one of these antennas were to be used. 
     During operation, device  7 . 2 - 10  may receive image data (e.g., image data for video, still images, etc.) and may present this information on the displays of optical modules  7 . 2 - 16 . Device  7 . 2 - 10  may also receive other data, control commands, user input, etc. Device  7 . 2 - 10  may transmit data to accessories and other electronic equipment. For example, image data from a forward-facing camera may be provided to an associated device, audio output may be provided to a device with speakers such as a headphone device, user input and sensor readings may be transmitted to remote equipment, etc. 
     Communications such as these may be supported using wired and/or wireless communications. In an illustrative configuration, components  7 . 2 - 36  may include wireless communications circuitry for supporting wireless communications between device  7 . 2 - 10  and remote wireless equipment (e.g., a cellular telephone, a wireless base station, a computer, headphones or other accessories, a remote control, peer devices, internet servers, and/or other equipment). Wireless communications may be supported using one or more antennas operating at one or more wireless communications frequencies (see, e.g., antennas  7 . 2 - 40 - 1 ,  7 . 2 - 40 - 2 , and  7 . 2 - 40 - 3  of  FIGS.  7 . 2 - 1   ). In an illustrative configuration, one or more antennas may be coupled to wireless transceiver circuitry. The wireless transceiver circuitry may include transmitter circuitry configured to transmit wireless communications signals using the antenna(s) and receiver circuitry configured to receive wireless communications signals using the antenna(s). 
     The wireless circuitry of device  7 . 2 - 10  may be formed from one or more integrated circuits, power amplifier circuitry, low-noise input amplifiers, passive RF components, one or more antennas, transmission lines, and other circuitry for handling RF wireless signals. The wireless circuitry may include radio-frequency transceiver circuitry for handling various radio-frequency communications bands. For example, the wireless circuitry of device  7 . 2 - 10  may include wireless local area network (WLAN) and wireless personal area network (WPAN) transceiver circuitry. This transceiver circuitry may handle 2.4 GHz and 5 GHz bands for WiFi® (IEEE 802.11) communications and other WLAN communications and the 2.4 GHz Bluetooth® communications band or other WPAN bands and may sometimes be referred to herein as WLAN/WPAN transceiver circuitry or local transceiver circuitry. 
     The wireless circuitry of device  7 . 2 - 10  may use remote wireless circuitry such as cellular telephone transceiver circuitry for handling wireless communications in frequency ranges (communications bands) such as a cellular low band (LB) from 600 to 960 MHz, a cellular low-midband (LMB) from 1410 to 1510 MHz, a cellular midband (MB) from 1710 to 2170 MHz, a cellular high band (HB) from 2300 to 2700 MHZ, a cellular ultra-high band (UHB) from 3300 to 5000 MHz, or other communications bands between 600 MHz and 5000 MHz. If desired, the cellular telephone transceiver circuitry may support 5G communications using a low band at 600-850 MHz, a mid-band at 2.5-3.7 GHZ, and a high band at 25-39 GHz. Wireless communications may also be provided using other frequency ranges (e.g., frequencies above 100 MHz, above 1 GHz, 1-30 GHz, 100 Mhz-300 GHz, 24 GHz, less than 300 GHz, less than 100 GHz, 10-300 GHz or other millimeter wave frequencies, and/or other suitable frequencies). WLAN/WPAN transceiver circuitry and/or cellular transceiver circuitry may handle voice data and non-voice data. 
     If desired, the antennas and other wireless circuitry of device  7 . 2 - 10  may include satellite navigation system circuitry such as Global Positioning System (GPS) receiver circuitry for receiving GPS signals at 1575 MHz or for handling other satellite positioning data (e.g., GLONASS signals at 1609 MHz). Satellite navigation system signals are received from a constellation of satellites orbiting the earth. Wireless circuitry in device  7 . 2 - 10  can include circuitry for other short-range (local) and long-range (remote) wireless links if desired. For example, wireless circuitry in device  7 . 2 - 10  may be provided to receive television and radio signals, paging signals, near field communications (NFC) signals at 13.56 MHz or other suitable NFC frequencies, ultrawideband (UWB) signals (e.g., UWB signals from 6-8.5 GHZ, UWB signals from 3.5-9 GHZ, etc.). Wireless circuitry in device  7 . 2 - 10  may also include antennas and transceiver for handling sensing applications (e.g., radar). If desired, antennas may be provided in arrays (e.g., phased antenna arrays) that support beam steering. These arrangements and other arrangements may be used in supporting wireless communications, wireless sensing, wireless location services, wireless power, and other wireless operations. 
     The wireless circuitry of device  7 . 2 - 10  may include antennas that are formed using any suitable antenna types. For example, the antennas of device  7 . 2 - 10  may include antennas with resonating elements that are formed from slot antenna structures, loop antenna structures, patch antenna structures, stacked patch antenna structures, antenna structures having parasitic elements, inverted-F antenna structures, planar inverted-F antenna structures, helical antenna structures, monopole antennas, dipole antenna structures, Yagi (Yagi-Uda) antenna structures, surface integrated waveguide structures, coils, hybrids of these designs, etc. If desired, one or more of the antennas may be cavity-backed antennas. 
     Different types of antennas may be used for different bands and combinations of bands. For example, one type of antenna may be used in forming a local wireless link antenna whereas another type of antenna is used in forming a remote wireless link antenna. If desired, space may be conserved within device  7 . 2 - 10  by using a single antenna to handle two or more different communications bands. For example, a single antenna in device  7 . 2 - 10  may be used to handle communications in a WiFi® or Bluetooth® communication band while also handling communications at one or more cellular telephone frequencies. In some configurations, some cellular telephone communications (e.g., low-band and mid-band communications) may be handled using a first antenna (e.g., an inverted-F antenna), whereas other communications (e.g., high-band cellular communications) may be handled using one or more phased antenna arrays (e.g., multiple linear patch antenna arrays each of which is mounted in a different orientation and each of which has a different angle of view so that a desired amount of angular coverage is achieved). 
     To provide antenna structures in device  7 . 2 - 10  with the ability to cover different frequencies of interest, one or more of the antennas of device  7 . 2 - 10  may be provided with circuitry such as filter circuitry (e.g., one or more passive filters and/or one or more tunable filter circuits). Discrete components such as capacitors, inductors, and resistors may be incorporated into the filter circuitry. Capacitive structures, inductive structures, and resistive structures may also be formed from patterned metal structures (e.g., part of an antenna). If desired, antenna(s) in device  7 . 2 - 10  may be provided with adjustable circuits such as tunable components that tune the antenna over communications (frequency) bands of interest. The tunable components may be part of a tunable filter or tunable impedance matching network, may be part of an antenna resonating element, may span a gap between an antenna resonating element and antenna ground, etc. 
     Radio-frequency transmission line paths may be used to convey antenna signals between the radio-frequency transceiver circuitry of device  7 . 2 - 10  and the antenna(s) of device  7 . 2 - 10 . These paths may include one or more radio-frequency transmission lines (sometimes referred to herein as transmission lines). Radio-frequency transmission line paths may each include a positive signal conductor and a ground signal conductor. Transmission lines in device  7 . 2 - 10  may include coaxial cable transmission lines, stripline transmission lines, microstrip transmission lines, edge-coupled microstrip transmission lines, edge-coupled stripline transmission lines, transmission lines formed from waveguide structures (e.g., coplanar waveguides or grounded coplanar waveguides), and combinations of these types of transmission lines and/or other transmission line structures. 
     If desired, matching networks may be used to help match impedances in the wireless circuitry of device  7 . 2 - 10 . A matching network may, for example, include components such as inductors, resistors, and capacitors configured to match the impedance of an antenna to the impedance of an associated radio-frequency transmission line path that is used in coupling the antenna to a transceiver. Matching network components may be provided as discrete components (e.g., surface mount technology components) or may be formed from housing structures, printed circuit board structures, traces on plastic supports, etc. Components such as these may also be used in forming antenna filter circuitry and may be tunable and/or fixed components. 
     Radio-frequency transmission line paths may be coupled to antenna feed structures associated with antennas in device  7 . 2 - 10 . As an example, an antenna in device  7 . 2 - 10  such as an inverted-F antenna, a planar inverted-F antenna, a patch antenna, a loop antenna, or other antenna may have an antenna feed with a positive antenna feed terminal and a ground antenna feed terminal. The positive antenna feed terminal may be coupled to an antenna resonating (radiating) element within the antenna. The ground antenna feed terminal may be coupled to an antenna ground in the antenna. The positive feed terminal may be coupled to a positive signal line in a transmission line and the ground feed terminal may be coupled to a ground signal line in the transmission line. 
     Other types of antenna feed arrangements may be used if desired. For example, an antenna may be fed using multiple feeds each coupled to a respective port of a transceiver over a corresponding transmission line. If desired, a given transmission line signal conductor may be coupled to multiple locations on an antenna and/or switches may be interposed within the paths between a transceiver and the feed terminals of an antenna. 
       FIGS.  7 . 2 - 2    is a front view of device  7 . 2 - 10  showing illustrative locations for millimeter wave antennas  7 . 2 - 40 - 1 ,  7 . 2 - 40 - 2 , and  7 . 2 - 40 - 3 . As shown in  FIGS.  7 . 2 - 2   , front-facing display  7 . 2 - 20  may be surrounded by peripheral edge portions E of display cover layer  7 . 2 - 12 CG (which may be formed from dielectric materials such as glass and/or polymer) and dielectric member  7 . 2 - 13 , and these dielectric structures may overlap antennas  7 . 2 - 40 - 1 ,  7 . 2 - 40 - 2 , and  7 . 2 - 40 - 3 . During operation, transmitted antenna signals from the millimeter wave antennas and received antenna signals for the millimeter wave antennas may pass through display cover layer  7 . 2 - 12 CG and member  7 . 2 - 13 . To enhance antenna efficiency, conductive structures such as conductive pixel structures and other conductive structures associated with display  7 . 2 - 20  may be present only in the center of device  7 . 2 - 10  (e.g., edge portions E may be free of any conductive display structures overlapping the millimeter wave antennas). 
       FIGS.  7 . 2 - 3    is a cross-sectional side view of an illustrative millimeter wave antenna. As shown in  FIGS.  7 . 2 - 3   , the wireless circuitry of device  7 . 2 - 10  includes radio-frequency transceiver  7 . 2 - 60 . Transceiver  7 . 2 - 60  may be coupled to antenna  7 . 2 - 40  by signal path  7 . 2 - 62  (e.g., one or more transmission lines). The configuration of millimeter wave antenna  7 . 2 - 40  of  FIGS.  7 . 2 - 3    may be used for antenna  7 . 2 - 40 - 1 , may be used for antenna  7 . 2 - 40 - 2 , and may be used for antenna  7 . 2 - 40 - 3 . As shown in  FIGS.  7 . 2 - 3   , millimeter wave antenna  7 . 2 - 40  may have multiple antenna elements  7 . 2 - 40 E. Elements  7 . 2 - 40 E may be formed from millimeter wave antenna resonating elements such as patch antenna elements (e.g., patch antennas formed from thin-film metal structures). The patch antennas may be arranged in a linear array (e.g., a line) on antenna substrate  7 . 2 - 40 B (e.g., a printed circuit substrate, a ceramic or glass layer, or other dielectric substrate). A ground for antenna  7 . 2 - 40  may be formed from a ground antenna trace in substrate  7 . 2 - 40 B and/or other conductive structures in device  7 . 2 - 10  (e.g., a metal chassis in device  7 . 2 - 10 , a heat sink in device  7 . 2 - 10 , a support bracket in device  7 . 2 - 10 , etc.). Antenna  7 . 2 - 40  (e.g., elements  7 . 2 - 40 E and substrate  7 . 2 - 40 B) may have a planar surface that is characterized by a surface normal (see, e.g., surface normal nb of  FIGS.  7 . 2 - 3   ). During operation, the control circuitry of device  7 . 2 - 10  may perform beam steering operations by adjusting the relative phases of the signals for each respective element  7 . 2 - 40 E. In this way, angle BA of antenna beam direction  7 . 2 - 92  relative to antenna surface normal nb (e.g., the direction in which antenna  7 . 2 - 40  is nominally pointed) may be adjusted (e.g., to ensure that antenna signals are transmitted and received along a direction that provides satisfactory antenna performance). 
     Antennas may be mounted within device  7 . 2 - 10  using mounting brackets, using biasing structures that press antenna components against housing structures, using adhesive, using screws and other fasteners, using press-fit connections, using solder, welds, conductive adhesive, and/or other conductive attachment mechanisms, using one or more frames, carriers, and/or other internal support structures, and/or other mounting arrangements. 
     To ensure even loading for each of elements  7 . 2 - 40 E, antenna  7 . 2 - 40  may be mounted adjacent to a planar dielectric structure that is evenly spaced from each of elements  7 . 2 - 40 E. In arrangements such as the arrangement of  FIGS.  7 . 2 - 1  and  7 . 2 - 2   , in which edge portion E of display cover layer  7 . 2 - 12 CG has an inner surface with a curved cross-sectional profile (e.g., an inwardly facing concave surface of compound curvature), a dielectric structure such as dielectric member  7 . 2 - 13  that is located between display cover layer  7 . 2 - 12 CG and antenna  7 . 2 - 40  may be used to form the planar dielectric structure. (Arrangements in which the planar inner surface is formed directly on the inner side of layer  7 . 2 - 12 CG and in which member  7 . 2 - 13  is omitted may also be used, if desired). 
     Consider, as an example, the top cross-sectional view of the left front corner of device  7 . 2 - 10  that is shown in  FIGS.  7 . 2 - 4   . As shown in  FIGS.  7 . 2 - 4   , the inner and outer surfaces of display cover layer  7 . 2 - 12 CG (e.g., the surfaces of edge portion E of layer  7 . 2 - 12 CG) may have curved cross-sectional profiles. These surfaces may have compound curvature. For example, inner surface  7 . 2 - 83  of display cover layer  7 . 2 - 12 CG may be a concave surface of compound curvature. Antenna elements  7 . 2 - 40 E of antenna  7 . 2 - 40  are supported on planar outer surface  7 . 2 - 82  of substrate  7 . 2 - 40 B. To equalize antenna loading for each of the elements  7 . 2 - 40 E in antenna  7 . 2 - 40  and thereby facilitate beam forming by antenna  7 . 2 - 40 , dielectric member  7 . 2 - 13  may be provided with planar inner surface  7 . 2 - 70  that is parallel to planar surface  7 . 2 - 82  (e.g., surface normal na of surface  7 . 2 - 70  may be parallel to surface normal nb of surface  7 . 2 - 82 ). This ensures that each antenna element  7 . 2 - 40 E will be separated from surface  7 . 2 - 70  by an air gap of equal size, thereby ensuring equal loading on each element  7 . 2 - 40 E. 
     The outer surface of member  7 . 2 - 13  may be curved (e.g., the outer surface of member  7 . 2 - 13  that is attached to or adjacent to inner surface  7 . 2 - 83  of layer  7 . 2 - 12 CG may have a convex shape such as a convex shape with compound curvature that matches the concave shape of surface  7 . 2 - 83 ). The permittivity of member  7 . 2 - 13 , the thickness of member  7 . 2 - 13 , and the size of the air gap between antenna  7 . 2 - 40  and member  7 . 2 - 13  may be selected to help match the impedance of antenna  7 . 2 - 40  to the impedance of layer  7 . 2 - 12 CG, thereby reducing antenna signal reflections. In an illustrative configuration, the permittivity of layer  7 . 2 - 12 CG has a first permittivity value, the permittivity of air has a second permittivity value that is lower than the first permittivity value, and the permittivity of the polymer that makes up member  7 . 2 - 13  has a third permittivity value that is between the first and second values. This configuration may help match the impedance of antenna  7 . 2 - 40  to that of layer  7 . 2 - 12 CG. The presence of the air gap between antenna  7 . 2 - 40  and member  7 . 2 - 13  may help reduce surface waves and may facilitate assembly of device  7 . 2 - 10  (e.g., by physically decoupling antenna  7 . 2 - 40  from overlapping structures such as member  7 . 2 - 13 ). 
     Antenna  7 . 2 - 40  may be supported on internal housing structures and/or other support structures (see, e.g., illustrative support structures  7 . 2 - 86  of  FIGS.  7 . 2 - 4   ). The support structures under antenna  7 . 2 - 40  may include a metal plate and/or other heatsink structure (see, e.g., heat sink  7 . 2 - 84 ). In the illustrative configuration of  FIGS.  7 . 2 - 4   , antenna  7 . 2 - 40  is located on the left front corner of device  7 . 2 - 10  (e.g., antenna  7 . 2 - 40  of  FIGS.  7 . 2 - 4    may serve as antenna  7 . 2 - 40 - 1  of  FIGS.  7 . 2 - 1   ). If desired, the approach of  FIGS.  7 . 2 - 4    may be used to form antenna  7 . 2 - 40 - 3  on the right front corner of device. 
     In the center of device  7 . 2 - 10 , antenna  7 . 2 - 40 - 2  may be installed using an arrangement of the type shown in the top view of  FIGS.  7 . 2 - 5   . A shown in  FIGS.  7 . 2 - 5   , antenna  7 . 2 - 40  (e.g., antenna  7 . 2 - 40 - 2  of  FIGS.  7 . 2 - 1   ) may be mounted on heatsink  7 . 2 - 84  and support structure  7 . 2 - 86  so that the planar outer surface of antenna  7 . 2 - 40  faces an opposing inwardly facing planar surface of member  7 . 2 - 13 . The adjacent surfaces of member  7 . 2 - 13  and antenna  7 . 2 - 40  may be parallel to each other (e.g., surface normal na may be parallel to surface normal nb). 
     7.3: Systems with Transparent Layers 
     Electronic devices may be provided with components such as antennas. The electronic devices may include portable electronic devices, wearable devices, desktop devices, embedded systems, and other electronic equipment. Illustrative configurations in which the electronic devices include a head-mounted device may sometimes be described herein as an example. 
     Antennas may be formed from thin flexible substrates such as flexible printed circuits. A flexible printed circuit antenna may have metal traces that are patterned to form an antenna resonating element (sometimes referred to as an antenna resonating structure or antenna resonator). The metal traces may be supported by a flexible printed circuit substrate layer. The flexible printed circuit substrate layer may be formed from one or more sheets of polyimide or layers of other polymer. 
     Electronic device housing structures and other parts of an electronic device may include areas that are characterized by curved surfaces that can be flattened into a plane without distortion (sometimes referred to as developable surfaces or curved surfaces without compound curvature). Electronic device housing structures and other parts of an electronic device may also include areas that are characterized by compound curvature (surfaces that can only be flattened into a plane with distortion, sometimes referred to as non-developable surfaces). 
     To help conform a flexible printed circuit antenna to a surface of an electronic device housing structure or other dielectric member in an electronic device and/or to otherwise provide the antenna with a shape that facilitates installation and use of the antenna in a device with potentially complex shapes such as surfaces with compound curvature, a flexible printed circuit antenna may be formed into a three-dimensional shape (e.g., an unwrinkled shape characterized by surfaces of compound curvature). A flexible printed circuit antenna that has been provided with compound curvature surfaces in this way may then be attached to a supporting housing structure with compound curvature. For example, a flexible printed circuit antenna with compound curvature may be laminated to a dielectric member having matching compound curvature using a layer of adhesive. 
       FIGS.  7 . 3 - 1    is a top view of an illustrative electronic device that may include a flexible printed circuit antenna with compound curvature. In the example of  FIGS.  7 . 3 - 1   , device  7 . 3 - 10  is a head-mounted device. In general, device  7 . 3 - 10  may be any suitable electronic equipment. 
     As shown in  FIGS.  7 . 3 - 1   , head-mounted device  7 . 3 - 10  may include housing  7 . 3 - 12 . Housing  7 . 3 - 12  is configured to be worn on a user&#39;s head and may sometimes be referred to as a head-mounted housing or head-mounted support structure. Housing  7 . 3 - 12  may have curved head-shaped surfaces, a nose-bridge portion such as portion NB that is configured to rest on a user&#39;s nose when device  7 . 3 - 10  is on a user&#39;s head, may have a headband such as strap  7 . 3 - 12 T for supporting device  7 . 3 - 10  on the user&#39;s head, and/or may have other features that allow device  7 . 3 - 10  to be worn by a user. 
     Housing  7 . 3 - 12  may have walls or other structures that separate an interior region of device  7 . 3 - 10  such as interior region  7 . 3 - 42  from an exterior region surrounding device  7 . 3 - 10  such as exterior region  7 . 3 - 44 . As an example, housing  7 . 3 - 12  may include a transparent layer that forms a housing wall on front F of device  7 . 3 - 10  such as display cover layer  7 . 3 - 12 CG. Housing  7 . 3 - 12  may also include internal frame structures (e.g., a metal chassis), cosmetic covering members, polymer layers (e.g., fully or partly transparent polymer layers), housing walls formed from polymer and/or other materials, and/or other housing structures. In an illustrative configuration, housing  7 . 3 - 12  includes a dielectric structure such as dielectric member  7 . 3 - 13  that is overlapped by display cover layer  7 . 3 - 12 CG. Dielectric member  7 . 3 - 13 , which may sometimes be referred to as a polymer layer, shroud, dielectric layer, or dielectric structure, may be formed from one or more individual dielectric structures (e.g., structures formed from polymer, glass, ceramic, and/or other dielectric). In the example of  FIGS.  7 . 3 - 1   , dielectric member  7 . 3 - 13 , includes a first dielectric layer such as polymer layer  7 . 3 - 13 - 1  that extends across substantially all of front F of device  7 . 3 - 10  (e.g., layer  7 . 3 - 13 - 1  of  FIGS.  7 . 3 - 1    has a footprint similar to or the same as that of layer  7 . 3 - 12 CG). With this arrangement, layer  7 . 3 - 13 - 1 , which may sometimes be referred to as a shroud canopy or shroud, has a central portion that overlaps display  7 . 3 - 20  and has a peripheral portion (e.g., the portion under edge portion E of display cover layer  7 . 3 - 12 CG) with a ring shaped footprint that surrounds display  7 . 3 - 20 . Dielectric member  7 . 3 - 13  of  FIGS.  7 . 3 - 1    also has a second polymer layer such as layer  7 . 3 - 13 - 2 . Layer  7 . 3 - 13 - 2 , which may sometimes be referred to as a shroud trim or shroud, may have a ring shape that surrounds display  7 . 3 - 20 . In the peripheral portion of member  7 . 3 - 13 , layers  7 . 3 - 13 - 1  and  7 . 3 - 13 - 2  may be attached to each other using adhesive, press-fit connections, screws or other fasteners, and/or other attachment mechanisms. 
     Display cover layer  7 . 3 - 12 CG and member  7 . 3 - 13  (e.g., layer  7 . 3 - 13 - 1 ) may overlap a forward-facing display such as display  7 . 3 - 20  (e.g., a flexible display panel formed from a pixel array based on organic light-emitting diodes or other display panel). Layer  7 . 3 - 13 - 1  may be formed from fully transparent polymer or partly transparent polymer that helps hide display  7 . 3 - 20  from view. Display cover layer  7 . 3 - 12 CG may be formed from transparent polymer or glass (as examples). 
     Portions of display cover layer  7 . 3 - 12 CG and member  7 . 3 - 13  such as edge portions of display cover layer  7 . 3 - 12 CG and member  7 . 3 - 13  that surround display  7 . 3 - 20  may have curved cross-sectional profiles. As an example, edge portion E of cover layer  7 . 3 - 12 CG and the underlying edge portion of member  7 . 3 - 13  may have inner and/or outer surfaces characterized by compound curvature (e.g., non-developable surfaces). The central portions of display cover layer  7 . 3 - 12 CG and member  7 . 3 - 13  may have compound curvature and/or may have developable surfaces. In an illustrative arrangement, cover layer  7 . 3 - 12 CG has inner and outer surfaces with compound curvature and member  7 . 3 - 13  has surfaces of compound curvature around the edges of device  7 . 3 - 10  (e.g., the portion of member  7 . 3 - 13  surrounding display  7 . 3 - 20 ) and has developable surfaces overlapping display  7 . 3 - 20 . In this illustrative configuration, display  7 . 3 - 20  may be a flexible display panel that is bent into a curved shape (e.g., a curved shape following the curved face of a user) and that is characterized by inner and outer developable surfaces. The portion of member  7 . 3 - 13  overlapping display  7 . 3 - 20  may have corresponding inner and outer developable surfaces. Other arrangements for the shapes of display cover layer  7 . 3 - 12 CG and member  7 . 3 - 13  may be used in device  7 . 3 - 10 , if desired. 
     Device  7 . 3 - 10  may have one or more antennas. As an example, antenna  7 . 3 - 40  may be mounted in device  7 . 3 - 10  along the edge of display  7 . 3 - 20 . As shown in  FIGS.  7 . 3 - 1   , antenna  7 . 3 - 40  may, as an example, be mounted to the inner surface of dielectric member  7 . 3 - 13  under edge portion E of display cover layer  7 . 3 - 12 CG. During operation, antenna signals may pass through these overlapping dielectric structures. 
     Antenna  7 . 3 - 40  may be attached to the surface of member  7 . 3 - 13  (e.g., the inner surface of layer  7 . 3 - 13 - 2  in the example of  FIGS.  7 . 3 - 1   ) using adhesive  7 . 3 - 15 . The portion of the inner surface of member  7 . 3 - 13  to which antenna  7 . 3 - 40  is mounted in this way may have compound curvature. Antenna  7 . 3 - 40  may be formed from a flexible printed circuit with matching compound curvature. 
     Device  7 . 3 - 10  may include electrical components  7 . 3 - 36  (e.g., integrated circuits, sensors, control circuitry, light-emitting diodes, lasers, and other light-emitting devices, other control circuits and input-output devices, etc.). Components  7 . 3 - 36  may be mounted on printed circuits and/or other structures within device  7 . 3 - 10  (e.g., in interior region  7 . 3 - 42 ). 
     To present a user with images for viewing from eye boxes such as eye box  7 . 3 - 34 , device  7 . 3 - 10  may include rear-facing displays in optical modules  7 . 3 - 16 . There may be, for example, a left rear-facing display in a left optical module  7 . 3 - 16  for presenting an image through a left lens to a user&#39;s left eye in a left eye box  7 . 3 - 34  and a right rear-facing display in right optical module  7 . 3 - 16  for presenting an image through a right lens to a user&#39;s right eye in a right eye box  7 . 3 - 34 . 
     The user&#39;s eyes are located in eye boxes  7 . 3 - 34  at rear R of device  7 . 3 - 10  when inwardly facing surface  7 . 3 - 18  of housing  7 . 3 - 12  rests against the outer surface of the user&#39;s face. On rear R, housing  7 . 3 - 12  may have cushioned structures (sometimes referred to as light seal structures) to enhance user comfort as surface  7 . 3 - 18  rests against the user&#39;s face. Device  7 . 3 - 10  may have forward-facing components such has forward-facing cameras and other sensors on front F that face outwardly away from the user. These components may generally be oriented in the +Y (forward) direction of  FIGS.  7 . 3 - 1   . 
     During operation, device  7 . 3 - 10  may receive image data (e.g., image data for video, still images, etc.) and may present this information on the displays of optical modules  7 . 3 - 16 . Device  7 . 3 - 10  may also receive other data, control commands, user input, etc. Device  7 . 3 - 10  may transmit data to accessories and other electronic equipment. For example, image data from a forward-facing camera may be provided to an associated device, audio output may be provided to a device with speakers such as a headphone device, user input and sensor readings may be transmitted to remote equipment, etc. 
     Communications such as these may be supported using wired and/or wireless communications. In an illustrative configuration, components  7 . 3 - 36  may include wireless communications circuitry for supporting wireless communications between device  7 . 3 - 10  and remote wireless equipment (e.g., a cellular telephone, a wireless base station, a computer, headphones or other accessories, a remote control, peer devices, internet servers, and/or other equipment). Wireless communications may be supported using one or more antennas operating at one or more wireless communications frequencies (see, e.g., antenna  7 . 3 - 40  of  FIGS.  7 . 3 - 1   ). In an illustrative configuration, one or more antennas may be coupled to wireless transceiver circuitry. The wireless transceiver circuitry may include transmitter circuitry configured to transmit wireless communications signals using the antenna(s) and receiver circuitry configured to receive wireless communications signals using the antenna(s). 
     The wireless circuitry of device  7 . 3 - 10  may be formed from one or more integrated circuits, power amplifier circuitry, low-noise input amplifiers, passive RF components, one or more antennas, transmission lines, and other circuitry for handling RF wireless signals. The wireless circuitry may include radio-frequency transceiver circuitry for handling various radio-frequency communications bands. For example, the wireless circuitry of device  7 . 3 - 10  may include wireless local area network (WLAN) and wireless personal area network (WPAN) transceiver circuitry. This transceiver circuitry may handle 2.4 GHz and 5 GHz bands for WiFi® (IEEE 802.11) communications and other WLAN communications and the 2.4 GHz Bluetooth® communications band or other WPAN bands and may sometimes be referred to herein as WLAN/WPAN transceiver circuitry or local transceiver circuitry. 
     The wireless circuitry of device  7 . 3 - 10  may use remote wireless circuitry such as cellular telephone transceiver circuitry for handling wireless communications in frequency ranges (communications bands) such as a cellular low band (LB) from 600 to 960 MHz, a cellular low-midband (LMB) from 1410 to 1510 MHz, a cellular midband (MB) from 1710 to 2170 MHz, a cellular high band (HB) from 2300 to 2700 MHZ, a cellular ultra-high band (UHB) from 3300 to 5000 MHz, or other communications bands between 600 MHz and 5000 MHz. If desired, the cellular telephone transceiver circuitry may support 5G communications using a low band at 600-850 MHz, a mid-band at 2.5-3.7 GHZ, and a high band at 25-39 GHz. Wireless communications may also be provided using other frequency ranges (e.g., frequencies above 100 MHz, above 1 GHz, 1-30 GHz, 100 Mhz-300 GHz, 24 GHz, less than 300 GHz, less than 100 GHz, 10-300 GHz or other mm-wave frequencies, and/or other suitable frequencies). WLAN/WPAN transceiver circuitry and/or cellular transceiver circuitry may handle voice data and non-voice data. 
     If desired, the antennas and other wireless circuitry of device  7 . 3 - 10  may include satellite navigation system circuitry such as Global Positioning System (GPS) receiver circuitry for receiving GPS signals at 1575 MHz or for handling other satellite positioning data (e.g., GLONASS signals at 1609 MHz). Satellite navigation system signals are received from a constellation of satellites orbiting the earth. Wireless circuitry in device  7 . 3 - 10  can include circuitry for other short-range (local) and long-range (remote) wireless links if desired. For example, wireless circuitry in device  7 . 3 - 10  may be provided to receive television and radio signals, paging signals, near field communications (NFC) signals at 13.56 MHz or other suitable NFC frequencies, ultrawideband (UWB) signals (e.g., UWB signals from 6-8.5 GHZ, UWB signals from 3.5-9 GHZ, etc.). Wireless circuitry in device  7 . 3 - 10  may also include antennas and transceiver for handling sensing applications (e.g., radar). If desired, antennas may be provided in arrays (e.g., phased antenna arrays) that support beam steering. These arrangements and other arrangements may be used in supporting wireless communications, wireless sensing, wireless location services, wireless power, and other wireless operations. 
     The wireless circuitry of device  7 . 3 - 10  may include antennas that are formed using any suitable antenna types. For example, the antennas of device  7 . 3 - 10  may include antennas with resonating elements that are formed from slot antenna structures, loop antenna structures, patch antenna structures, stacked patch antenna structures, antenna structures having parasitic elements, inverted-F antenna structures, planar inverted-F antenna structures, helical antenna structures, monopole antennas, dipole antenna structures, Yagi (Yagi-Uda) antenna structures, surface integrated waveguide structures, coils, hybrids of these designs, etc. If desired, one or more of the antennas may be cavity-backed antennas. 
     Different types of antennas may be used for different bands and combinations of bands. For example, one type of antenna may be used in forming a local wireless link antenna whereas another type of antenna is used in forming a remote wireless link antenna. If desired, space may be conserved within device  7 . 3 - 10  by using a single antenna to handle two or more different communications bands. For example, a single antenna in device  7 . 3 - 10  may be used to handle communications in a WiFi® or Bluetooth® communication band while also handling communications at one or more cellular telephone frequencies. In some configurations, some cellular telephone communications (e.g., low-band and mid-band communications) may be handled using a first antenna (e.g., an inverted-F antenna), whereas other communications (e.g., high-band cellular communications) may be handled using one or more phased antenna arrays (e.g., multiple linear patch antenna arrays each of which is mounted in a different orientation and each of which has a different angle of view so that a desired amount of angular coverage is achieved). 
     To provide antenna structures in device  7 . 3 - 10  with the ability to cover different frequencies of interest, one or more of the antennas of device  7 . 3 - 10  may be provided with circuitry such as filter circuitry (e.g., one or more passive filters and/or one or more tunable filter circuits). Discrete components such as capacitors, inductors, and resistors may be incorporated into the filter circuitry. Capacitive structures, inductive structures, and resistive structures may also be formed from patterned metal structures (e.g., part of an antenna). If desired, antenna(s) in device  7 . 3 - 10  may be provided with adjustable circuits such as tunable components that tune the antenna over communications (frequency) bands of interest. The tunable components may be part of a tunable filter or tunable impedance matching network, may be part of an antenna resonating element, may span a gap between an antenna resonating element and antenna ground, etc. 
     Radio-frequency transmission line paths may be used to convey antenna signals between the radio-frequency transceiver circuitry of device  7 . 3 - 10  and the antenna(s) of device  7 . 3 - 10 . These paths may include one or more radio-frequency transmission lines (sometimes referred to herein as transmission lines). Radio-frequency transmission line paths may each include a positive signal conductor and a ground signal conductor. Transmission lines in device  7 . 3 - 10  may include coaxial cable transmission lines, stripline transmission lines, microstrip transmission lines, edge-coupled microstrip transmission lines, edge-coupled stripline transmission lines, transmission lines formed from waveguide structures (e.g., coplanar waveguides or grounded coplanar waveguides), combinations of these types of transmission lines and/or other transmission line structures. 
     If desired, matching networks may be used to help match impedances in the wireless circuitry of device  7 . 3 - 10 . A matching network may, for example, include components such as inductors, resistors, and capacitors configured to match the impedance of an antenna to the impedance of an associated radio-frequency transmission line path that is used in coupling the antenna to a transceiver. Matching network components may be provided as discrete components (e.g., surface mount technology components) or may be formed from housing structures, printed circuit board structures, traces on plastic supports, etc. Components such as these may also be used in forming antenna filter circuitry and may be tunable and/or fixed components. 
     Radio-frequency transmission line paths may be coupled to antenna feed structures associated with antennas in device  7 . 3 - 10 . As an example, an antenna in device  7 . 3 - 10  such as an inverted-F antenna, a planar inverted-F antenna, a patch antenna, a loop antenna, or other antenna may have an antenna feed with a positive antenna feed terminal and a ground antenna feed terminal. The positive antenna feed terminal may be coupled to an antenna resonating (radiating) element within the antenna. The ground antenna feed terminal may be coupled to an antenna ground in the antenna. The positive feed terminal may be coupled to a positive signal line in a transmission line and the ground feed terminal may be coupled to a ground signal line in the transmission line. 
     Other types of antenna feed arrangements may be used if desired. For example, an antenna may be fed using multiple feeds each coupled to a respective port of a transceiver over a corresponding transmission line. If desired, a given transmission line signal conductor may be coupled to multiple locations on an antenna and/or switches may be interposed within the paths between a transceiver and the feed terminals of an antenna. 
       FIGS.  7 . 3 - 2    is a diagram of illustrative wireless communications circuitry for device  7 . 3 - 10 . As shown in  FIGS.  7 . 3 - 2   , the wireless circuitry includes radio-frequency transceiver  7 . 3 - 60 , which is coupled to antenna  7 . 3 - 40  by transmission line  7 . 3 - 62 . Antenna  7 . 3 - 40  may have an antenna resonating element  52  (sometimes referred to as an antenna resonating structure or antenna resonator) and antenna ground  7 . 3 - 50 . Antenna resonating element  52  may be formed from any suitable antenna resonating element structures. In the example of  FIGS.  7 . 3 - 2   , antenna resonating element  52  is an inverted-F antenna resonating element having resonating element arm  7 . 3 - 56 , which is coupled to ground  7 . 3 - 50  by return path  7 . 3 - 54  and which is fed using antenna feed  7 . 3 - 58 . Feed  7 . 3 - 58  has positive and ground feed terminals coupled respectively to positive and ground signal lines in transmission line  7 . 3 - 62 . Conductive structures making up antenna  7 . 3 - 40  may be formed from thin-film metal traces on printed circuits (e.g., flexible printed circuits formed from sheets of polyimide or other flexible polymer substrates). If desired, the conductive structures making up antenna  7 . 3 - 40  (e.g., ground structures for antenna  7 . 3 - 40 ) may include conductive structural members such as portions of a housing for device  7 . 3 - 10  (e.g., a metal chassis and/or other internal and/or external frame structures, metal housing walls, metal component support brackets, and/or other conductive housing structures), and/or other structures in device  7 . 3 - 10  that are formed from metal and/or other conductive material. 
     Antennas may be mounted within device  7 . 3 - 10  using mounting brackets, using biasing structures that press antenna components against housing structures, using adhesive, using screws and other fasteners, using press-fit connections, using solder, welds, conductive adhesive, and/or other conductive attachment mechanisms, using one or more frames, carriers, and/or other internal support structures, and/or other mounting arrangements. In an illustrative configuration, flexible printed circuit antenna  7 . 3 - 40  has compound curvature and is attached to an overlapping dielectric member such as display cover layer  7 . 3 - 12 CG and/or member  7 . 3 - 13  that has an opposing surface of matching compound curvature. By matching the compound curvature of the substrate of antenna  7 . 3 - 40  to the compound curvature of an associated overlapping dielectric layer, antenna  7 . 3 - 40  may be configured to fit within the potentially tight confines of device  7 . 3 - 10  without adversely affecting the shape and appearance of device  7 . 3 - 10 . As an example, by matching the compound curvature of the substrate of antenna  7 . 3 - 40  to the compound curvature of an overlapping dielectric structure such as member  7 . 3 - 13 , antenna  7 . 3 - 40  can be attached to the inner surface or outer surface of member  7 . 3 - 13  with adhesive. Display cover layer  7 . 3 - 12 CG may then be mounted on device  7 . 3 - 10  so that edge portion E of layer  7 . 3 - 12 CG overlaps member  7 . 3 - 13  and antenna  7 . 3 - 40 . 
     Consider, as an example, the illustrative antenna structures of  FIGS.  7 . 3 - 3   . As shown in  FIGS.  7 . 3 - 3   , antenna  7 . 3 - 40  may be formed from antenna substrate  7 . 3 - 64 . Substrate  7 . 3 - 64  may be formed from a flexible printed circuit with a surface of compound curvature (sometimes referred to as a non-developable surface). Antenna substrate  7 . 3 - 64  contains metal traces such as metal trace  7 . 3 - 66  (e.g., a patterned thin-film metal layer). In an illustrative configuration, substrate  7 . 3 - 64  has an upper layer (e.g., an upper polyimide layer or other sheet of polymer) and a lower layer (e.g., a lower polyimide layer or other sheet of polymer) and trace  7 . 3 - 66  is formed from a patterned thin-film metal layer that is between the upper and lower layers. Trace  7 . 3 - 66  is patterned to form antenna resonating element  52  ( FIGS.  7 . 3 - 2   ) and/or other antenna structures. Antenna  7 . 3 - 40  of  FIGS.  7 . 3 - 3    is formed from a planar sheet of printed circuit substrate material that was contoured in a contouring tool to produce a desired three-dimensional shape with compound curvature. At least some portions of the curved surface of substrate  7 . 3 - 64  may be characterized by a radius of curvature R of 4 mm to 250 mm, 8 mm to 200 mm, 10 mm to 150 mm, at least 5 mm, at least 12 mm, at least 16 mm, at least 20 mm, at least 30 mm, less than 200 mm, less than 100 mm, less than 75 mm, less than 55 mm, less than 35 mm, and/or other suitable amount of curvature. 
     After forming a flexible printed circuit antenna with compound curvature of the type shown in  FIGS.  7 . 3 - 3   , this compound curvature flexible printed circuit antenna may be attached to the surface of a dielectric support structure in device  7 . 3 - 10 . In an illustrative arrangement, the compound curvature flexible printed circuit antenna is attached to the inner surface of dielectric member  7 . 3 - 13  using a layer of adhesive.  FIGS.  7 . 3 - 4    is a cross-sectional side view of an illustrative vacuum lamination tool that may be used in attaching antenna  7 . 3 - 40  to member  7 . 3 - 13 . As shown in  FIGS.  7 . 3 - 4   , tool  7 . 3 - 70  may have movable upper and lower dies such as upper die  7 . 3 - 82  with concave surface  7 . 3 - 72  and lower die  7 . 3 - 74  with convex surface  7 . 3 - 76 . Surfaces  7 . 3 - 72  and  7 . 3 - 76  may be characterized by compound curvature (e.g., compound curvature that matches the compound curvature of the inner and outer surfaces of member  7 . 3 - 13  and that matches the compound curvature of the inner and outer surfaces of flexible printed circuit antenna  7 . 3 - 40 ). Prior to lamination, a layer of adhesive such as adhesive  7 . 3 - 15  may be suspended between member  7 . 3 - 13  and antenna  7 . 3 - 40 . During lamination, tool  7 . 3 - 70  may use vacuum enclosure  7 . 3 - 80  to produce a vacuum while member  30  is pressed against antenna  7 . 3 - 40  by moving die  7 . 3 - 82  towards die  7 . 3 - 74 . While pressure is applied between member  7 . 3 - 13  and antenna  7 . 3 - 40  in this way, dies  7 . 3 - 82  and  7 . 3 - 74  may optionally apply heat to facilitate lamination. The presence of vacuum helps prevent air bubbles from forming as adhesive  7 . 3 - 15  is compressed between member  30  and antenna  7 . 3 - 40 . 
       FIGS.  7 . 3 - 5    is a cross-sectional side view of member  7 . 3 - 13  (e.g., layer  7 . 3 - 13 - 2  of  FIG.  7 . 3 - 1    or other dielectric antenna support structure) following lamination in tool  7 . 3 - 70  to attach antenna  7 . 3 - 40  to member  7 . 3 - 13  with adhesive. In general, antenna  7 . 3 - 40  may be attached to an inner or outer surface of a supporting member and this attachment surface may have convex or concave curvature. In the example of  FIGS.  7 . 3 - 5   , member  7 . 3 - 13  has an inwardly facing concave surface of compound curvature and the outwardly facing surface of antenna  7 . 3 - 40  has matching compound curvature. 
     After antenna  7 . 3 - 40  is attached to a shroud or other dielectric member (e.g., member  7 . 3 - 13  of  FIG.  7 . 3 - 1    or other member) using adhesive  7 . 3 - 15 , the shroud or other dielectric member may be attached to other portions of housing  7 . 3 - 12  (e.g., using screws or other fasteners, using adhesive, etc.). In arrangements in which member  7 . 3 - 13  is separate from cover layer  7 . 3 - 12 CG, the attachment of antenna  7 . 3 - 40  to member  7 . 3 - 13  may help preserve the ability of cover layer  7 . 3 - 12 CG to be removed (e.g., to permit rework or repair of device  7 . 3 - 10 ). 
       FIGS.  7 . 3 - 6    is a perspective view of antenna  7 . 3 - 40  on member  7 . 3 - 13  taken from the outside of device  7 . 3 - 10  with cover layer  7 . 3 - 12 CG removed. As shown in the example of  FIGS.  7 . 3 - 6   , antenna  7 . 3 - 40  may be mounted to the underside (inner surface) of member  7 . 3 - 13 . The outwardly facing surface of member  7 . 3 - 13  in  FIGS.  7 . 3 - 6    is convex. The opposing inwardly facing surface of member  7 . 3 - 13  in  FIGS.  7 . 3 - 6    is concave (e.g., the surface of member  7 . 3 - 13  on the far side of member  7 . 3 - 13  of  FIGS.  7 . 3 - 6    is concave). Antenna  7 . 3 - 40  may have an outwardly facing convex surface that is attached to the concave inwardly facing surface of member  7 . 3 - 13 . 
     One or more metal structures in device  7 . 3 - 10  such as metal structure  7 . 3 - 90  (e.g., a metal chassis or other metal housing structure) may serve as antenna ground  7 . 3 - 50  of  FIG.  7 . 3 - 2   . Member  7 . 3 - 13  may have openings  7 . 3 - 92  through which leg portions or other protruding portions of antenna  7 . 3 - 40  may pass. In the example of  FIGS.  7 . 3 - 6   , protruding portion  7 . 3 - 40 - 1  of antenna  7 . 3 - 40  has a return path metal trace (forming return path  7 . 3 - 54  of  FIGS.  7 . 3 - 2   ) that is shorted to metal structure  7 . 3 - 90  using metal fastener  7 . 3 - 94 . Protruding portion  7 . 3 - 40 - 2  of antenna  7 . 3 - 40  may include a metal trace forming a positive feed terminal. A cable or other transmission line (see, e.g., transmission line  7 . 3 - 62  of  FIGS.  7 . 3 - 2   ) may be coupled to connector  7 . 3 - 96 . Connector  7 . 3 - 96  may have a positive terminal coupled to the positive feed terminal and may have a negative terminal that is shorted to metal structure  7 . 3 - 90  (e.g., via metal fastener  7 . 3 - 98 ). Conductive adhesive, solder, welded connections, and/or other conductive connections may be used in attaching the metal trace of antenna  7 . 3 - 40  to metal structure  7 . 3 - 80  and connector  7 . 3 - 96 , if desired. The use of fasteners  7 . 3 - 94  and  7 . 3 - 98  (e.g., screws) is illustrative. Following installation of member  7 . 3 - 13  and antenna  7 . 3 - 40  into device  7 . 3 - 10  (e.g., by attaching member  7 . 3 - 13  to housing  7 . 3 - 12  and attaching antenna  7 . 3 - 40  to structure  7 . 3 - 90 ), cover layer  7 . 3 - 12 CG may be mounted to the front of housing  7 . 3 - 12 , thereby covering member  7 . 3 - 13  and antenna  7 . 3 - 40  as shown in  FIGS.  7 . 3 - 1   . 
     VIII: Bent MLB 
       FIG.  8 - 0    illustrates a view of an example of an HMD device  8 - 100 . The HMD device  8 - 100  can include a logic board system  8 - 102  including one or more printed circuit boards, processors, other computing components, memory components, circuitry, and so forth mounted to one or more components of the HMD  8 - 100 . 
       FIG.  8 - 1    illustrates a plan view of an example of a logic board  8 - 102 , which can include a main logic board of the HMD  8 - 100  shown in  FIG.  8 - 0   . In at least one example, the logic board  8 - 102  includes a first portion  8 - 104  and a second portion  8 - 106  joined together at a transition portion  8 - 108 . As shown in the top view of  FIG.  8 - 23   , in at least one example, the first portion  8 - 104  of the MLB  8 - 102  can be disposed at an angle θ relative to the second portion  8 - 106  of the MLB  8 - 102 . In at least one example, the angle θ can disposed the first and second portions  8 - 104 ,  8 - 106  of the MLB  8 - 102  to accommodate the curvature of a user&#39;s face. For example, the lateral (left-to-right) direction of the user&#39;s face is curved and the HMD  8 - 100  can be generally curved to accommodate the user&#39;s face. This curvature can form a thin profile against the user&#39;s face and minimize the moment arm and thus the torque applied to the user&#39;s face from the HMD  8 - 100 , including components within the HMD  8 - 100 . 
     In order to accommodate the curvature of the user&#39;s face, not only is the outside or external surfaces of the HMD  8 - 100  curved, as shown in  FIG.  8 - 0   , but internal components of the HMD  8 - 100  can be similarly curved to fit tightly and compactly within the HMD  8 - 100  to minimize bulk and size of the device. For example, in at least one example, the angle θ of the first portion  8 - 104  relative to the second portion  8 - 106  can be between about 5-degrees and about 60-degrees to form a bent MLB  8 - 102 . In one example, the angle θ can be between about 5-degrees and about 60-degrees. In one example, the angle θ can be between about 10-degrees and about 50-degrees. In one example, the angle θ can be between about 12-degrees and about 45-degrees. In one example, the angle θ can be about 15-degrees. 
     As shown in the close up top view of  FIG.  8 - 24   , the transition portion  8 - 108 , which can be referred to as the bend portion  8 - 108 , can be thinner than the first and second portion  8 - 104 ,  8 - 106 , as shown. In at least one example, the MLB  8 - 102  can be manufactured and assembled, including the application of adhesive tapes and other components, while the MLB  8 - 102  is not bent. That is, in its resting state, the MLB  8 - 102  can be planar such that the first portion  8 - 104  is not disposed at an angle relative to the second portion  8 - 106 . Then, when the MLB  8 - 102  is disposed within the HMD  8 - 100  during assembly/manufacture of the HMD  8 - 100 , the MLB  8 - 102  can be bent at the transition/bend portion  8 - 108  as shown and fixed in position within the HMD  8 - 100 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  8 - 1 - 8 - 3    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  8 - 4 - 8 - 5    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  8 - 4 - 8 - 5    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  8 - 1 - 8 - 3   . 
       FIG.  8 - 4    illustrates a cross-sectional view of another example of a MLB  8 - 202  including a first portion  8 - 204  and a second portion  8 - 206  with a transition portion  8 - 208  disposed between the first portion  8 - 204  and the second portion  8 - 206 . In the illustrated example of  FIG.  8 - 4   , the MLB  8 - 202  is not bent at the transition portion  8 - 208 , for example when the MLB  8 - 202  has not yet been bent for assembly within the HMD  8 - 100 . In one example, the transition portion  8 - 208  is thinner than the first and second portion  8 - 204 ,  8 - 206  such that when the first and second portions  8 - 204 ,  8 - 206  are urged toward each other, the MLB  8 - 202  naturally bends at the transition portion  8 - 208 . 
     In at least one example, the transition portion  8 - 208  includes a number of layers  8 - 201  including conductive layers, such as copper conductive layers, and insulation layers, including pre-preg insulation layers disposed between the conductive layers. The thicknesses of the layers for the first, second, and transition portions  8 - 204 ,  8 - 206 ,  8 - 208  of the MLB  8 - 202  can be between about 0.006 mm and about 0.053 mm. The number of layers, types of layers, and thicknesses of each of those layers can vary in other examples. 
     In at least one example, the transition portion  8 - 208  can include an upper combined coverlay-adhesive layer  8 - 203 . In at least one example, the transition portion  8 - 208  can include a lower sliver shield layer  8 - 205  below a lower combined coverlay-adhesive layer  8 - 207 . The copper and pre-preg layers can be disposed between the coverlay-adhesive layers  8 - 203 ,  8 - 207 . The MLB  8 - 202  can include a core layer  8 - 209  disposed between adjacent copper layers and extending from the first portion  8 - 204 , through the transition portion  8 - 208 , on through the second portion  8 - 206  as a single core layer  8 - 209 . Likewise, any of the other layers noted herein can extend from the first portion  8 - 204 , through the transition portion  8 - 208 , on through the second portion  8 - 206  as single continuous layers that cross the transition portion  8 - 208 . In one example, the transition portion  8 - 208  is not a separate MLB portion disposed between separate, unconnected other portions. Rather, the transition portion  8 - 208  can be an integral part of the MLB  8 - 202  and include the same or similar layers of material electronically connecting or otherwise connecting the first and second portion  8 - 204 ,  8 - 206  thereof. 
     In at least one example, one or more core and/or conductive material layers extends continuously from the first portion  8 - 204  to the second portion  8 - 206  through the transition portion  8 - 208 . In at least one example, one or more of these layers can form a thermal/heat connection between the first portion  8 - 204  and the second portion  8 - 206 . In at least one example, there is no need for connectors between the first portion  8 - 204  and the second portion  8 - 208 . In at least one example, the MLB  8 - 202  can include aluminum plating for solder-ability. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  8 - 4    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  8 - 1 - 8 - 3  and  8 - 5    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  8 - 1 - 8 - 3  and  8 - 5    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  8 - 4   . 
       FIG.  8 - 5    illustrates a top perspective view of a subassembly of the HMD  8 - 100  including an MLB  8 - 302  and first and second fans  8 - 310   a ,  8 - 310   b  secured to the MLB  8 - 302 . In at least one example the first fan  8 - 310   a  is secured to or adjacent the first portion  8 - 304  of the MLB  8 - 302  and the second fan  8 - 310   b  is secured to or adjacent the second portion  8 - 306  of the MLB  8 - 302 . In such an example, the fans  8 - 310   a - b  can be disposed at the same angle relative to one another as the first and second portions  8 - 304 ,  8 - 306  of the MLB  8 - 302  are disposed relative to one another. In this way, the MLB  8 - 302  as well as the fans  310   a - b  can be disposed compactly within the curved HMD  8 - 100  device to minimize device volume. 
     In addition, the first and second fans  8 - 310   a - b  can be secured to the MLB  8 - 302  such that thermally conductive housings of the fans  8 - 310   a - b  can be thermally coupled to one or more heat-generating components of the MLB  8 - 302 . In one example, the fans  8 - 310   a - b  can be secured to the first and second portions  8 - 304 ,  8 - 306  of the MLB  8 - 302  such that the housings of the fans  8 - 310   a - b  form at least part of one or more electromagnetic interference shields for components of the MLB  8 - 302 . More details regarding the EMI shielding and thermal conductivity of the fans  8 - 310   a - b  with relation to the MLB are given elsewhere herein. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  8 - 5    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  8 - 1 - 8 - 4    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  8 - 1 - 8 - 5  and  8 - 5    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  8 - 5   . 
     IX: Thermals 
       FIGS.  9 . 0 - 1    illustrates a view of an HMD  100  including a thermal management system  102 . The thermal management system  102  is described in more detail here in section IX. 
     9.1: Air Deflector for a Cooling System in a Head-Mounted Device 
     Head-mounted devices are an attractive technology for providing an immersive user experience. For example, head-mounted devices are gaining increased popularity for providing VR, AR, and MR experiences for applications such as gaming, movies, or simulations for professional training, among other potential applications. 
     Head-mounted devices can employ a wearable device housing that is secured to a user&#39;s head, and various electronic components within the housing, such as displays, integrated circuits, memory, audio devices, or electronic circuitry. As with other electronic devices, head-mounted devices can employ a cooling system based on circulation of air to maintain electronic components at desirable operating temperatures. The cooling system can also be used to cool the user&#39;s face from heat build-up inside the head-mounted device. 
     Maintaining efficient operation without unduly detracting from the user experience is a challenging task for head-mounted devices. The shape of the head-mounted device or layout of internal components can lead to a tortuous flow path for the cooling system. The proximity of the air flow path to the user&#39;s head can create undesired effects that detract from the user experience, such as excessive noise that interferes with the audio of the device in a noticeable manner. Some head-mounted devices may employ movable components that can interrupt the air flow path, such as adjustable optics that can be moved to account for a given user&#39;s interpupillary distance (IPD). IPD is defined as the distance between the centers of the pupils of a user&#39;s eyes. This adjustability can in turn make it difficult to design a cooling system in a given device that is suitable for different users. 
     According to some embodiments disclosed herein, a cooling system for a head-mounted device may employ an air deflector designed to affect a flow of air within the head-mounted device. The air deflector may be positioned in an air flow path extending through the housing of the head-mounted device, and can be designed to reduce the turbulence of air in the cooling system. For example, the air deflector can be positioned between a surface of an internal component and an incoming stream of air, at a reduced angle relative to the surface of the internal component so as to create smooth or more laminar flow over or across the component. The air deflector can be mounted to a movable component, such as an adjustable display assembly, so as to affect the flow of air as the moveable component is adjusted for particular users in a manner that results in a partial occlusion of the air flow path by the movable component. The air deflector can be configured to pivot or otherwise move to account for changes in the incident angle of air resulting from changes in the position of movable components. The air deflector can include or be coupled to additional thermal structures to enhance heat transfer affects resulting from the flow of air over the air deflector. For example, the air deflector can include an integrated heat sink and/or be coupled to heat-generating components via a thermally conductive interface material to enhance dissipation of heat from such components. 
     These and other embodiments are discussed below with reference to  FIGS.  9 . 1 - 1    through  FIGS.  9 . 1 - 9   . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these Figures is for explanatory purposes only and should not be construed as limiting. 
       FIGS.  9 . 1 - 1    illustrates an example of a head-mounted device  9 . 1 - 100  secured to a head  9 . 1 - 20  of a user  9 . 1 - 10 . As seen in  FIGS.  9 . 1 - 1   , the head-mounted device  9 . 1 - 100  can include a housing  9 . 1 - 110  that is securable to the user&#39;s head  9 . 1 - 20  via a securement element  9 . 1 - 150 . The securement element  9 . 1 - 150  can include a band, a strap, a rim, temples of a glasses frame, or any other suitable mechanism that serves to secure and retain the housing  9 . 1 - 110  on the head  9 . 1 - 20  of the user  9 . 1 - 10 . The securement element  9 . 1 - 150  can be an integral part of the housing  9 . 1 - 110  or be implemented as a separate component attached thereto. The housing  9 . 1 - 110  can further include or be coupled to one or more nose pads that serve to rest the housing  9 . 1 - 110  on the nose of the user  9 . 1 - 10 . 
     The housing  9 . 1 - 110  can enclose and support various functional components therein, such as integrated circuits, memory devices, processors, electronic circuitry, input/output devices, or other electronic components. In  FIGS.  9 . 1 - 1   , housing  9 . 1 - 110  is shown as containing a display  9 . 1 - 120 , a controller  9 . 1 - 130 , and an air circulation device  9 . 1 - 140  therein. The display  9 . 1 - 120  can be positioned in front of the eyes of the user  9 . 1 - 10  to provide information within the user&#39;s field of view. The air circulation device  9 . 1 - 140  can urge air through the housing  9 . 1 - 110  and over components such as the display  9 . 1 - 120  to cool such components. The controller  9 . 1 - 130  can be configured to control operation of one or more components, such as the display  9 . 1 - 120  and/or air circulation device  9 . 1 - 140 . 
     The display  9 . 1 - 120  can transmit light from a physical environment for viewing by the user  9 . 1 - 10 . For example, the display  9 . 1 - 120  can include optical elements, such as lenses for vision correction. The display  9 . 1 - 120  can be configured to present information in addition to (e.g., overlaid with) the physical environment viewed by the user. Alternatively, the display  9 . 1 - 120  can be configured to provide information to the exclusion of the physical environment. In either case, the display  9 . 1 - 120  can be configured to present graphics to, for example, present a computer-generated reality environment to the user  9 . 1 - 10 . 
     A physical environment refers to a physical world that people can sense and/or interact with without aid of electronic systems. Physical environments, such as a physical park, include physical articles, such as physical trees, physical buildings, and physical people. People can directly sense and/or interact with the physical environment, such as through sight, touch, hearing, taste, and smell. 
     In contrast, a computer-generated reality (CGR) environment refers to a wholly or partially simulated environment that people sense and/or interact with via an electronic system. In CGR, a subset of a person&#39;s physical motions, or representations thereof, are tracked, and, in response, one or more characteristics of one or more virtual objects simulated in the CGR environment are adjusted in a manner that comports with at least one law of physics. For example, a CGR system may detect a person&#39;s head turning and, in response, adjust graphical content and an acoustic field presented to the person in a manner similar to how such views and sounds would change in a physical environment. In some situations, (e.g., for accessibility reasons), adjustments to characteristic(s) of virtual object(s) in a CGR environment may be made in response to representations of physical motions (e.g., vocal commands). 
     A person may sense and/or interact with a CGR object using any one of their senses, including sight, sound, touch, taste, and smell. For example, a person may sense and/or interact with audio objects that create 3D or spatial audio environment that provides the perception of point audio sources in 3D space. In another example, audio objects may enable audio transparency, which selectively incorporates ambient sounds from the physical environment with or without computer-generated audio. In some CGR environments, a person may sense and/or interact only with audio objects. 
     Examples of CGR include virtual reality and mixed reality. 
     A virtual reality (VR) environment refers to a simulated environment that is designed to be based entirely on computer-generated sensory inputs for one or more senses. A VR environment comprises a plurality of virtual objects with which a person may sense and/or interact. For example, computer-generated imagery of trees, buildings, and avatars representing people are examples of virtual objects. A person may sense and/or interact with virtual objects in the VR environment through a simulation of the person&#39;s presence within the computer-generated environment, and/or through a simulation of a subset of the person&#39;s physical movements within the computer-generated environment. 
     In contrast to a VR environment, which is designed to be based entirely on computer-generated sensory inputs, a mixed reality (MR) environment refers to a simulated environment that is designed to incorporate sensory inputs from the physical environment, or a representation thereof, in addition to including computer-generated sensory inputs (e.g., virtual objects). On a virtuality continuum, a mixed reality environment is anywhere between, but not including, a wholly physical environment at one end and virtual reality environment at the other end. 
     In some MR environments, computer-generated sensory inputs may respond to changes in sensory inputs from the physical environment. Also, some electronic systems for presenting an MR environment may track location and/or orientation with respect to the physical environment to enable virtual objects to interact with real objects (that is, physical articles from the physical environment or representations thereof). For example, a system may account for movements so that a virtual tree appears stationery with respect to the physical ground. 
     Different forms of head-mounted devices that enable a person to sense and/or interact with various CGR environments. Examples include smart glasses, helmets, visors, or goggles. A head-mounted device may have one or more speaker(s) and an integrated opaque display. Alternatively, a head-mounted system may be configured to accept an external opaque display (e.g., a smartphone). The head-mounted device may incorporate one or more imaging sensors to capture images or video of the physical environment, and/or one or more microphones to capture audio of the physical environment. Rather than an opaque display, a head-mounted device may have a transparent or translucent display. The transparent or translucent display may have a medium through which light representative of images is directed to a person&#39;s eyes. The display may utilize digital light projection, OLEDs, LEDs, uLEDs, liquid crystal on silicon, laser scanning light source, or any combination of these technologies. The medium may be an optical waveguide, a hologram medium, an optical combiner, an optical reflector, or any combination thereof. In one embodiment, the transparent or translucent display may be configured to become opaque selectively. Projection-based systems may employ retinal projection technology that projects graphical images onto a person&#39;s retina. Projection systems also may be configured to project virtual objects into the physical environment, for example, as a hologram or on a physical surface. 
       FIGS.  9 . 1 - 2    shows an example of the head-mounted device  9 . 1 - 100  in front view. As seen in  FIGS.  9 . 1 - 2   , the display  9 . 1 - 120  ( FIGS.  9 . 1 - 1   ) can include a first display assembly  9 . 1 - 121   a  and a second display assembly  9 . 1 - 121   b , which collectively form a pair of display assemblies corresponding to the two eyes of a user. Each of the display assemblies may include any appropriate combination of electronic and optical elements to present graphical information to the user. For example, each display assembly may include a display layer having an array of electronically controlled pixels that can provide a visual output. The display assembly may further include optical elements, such as lenses, mirrors, etc., and/or a gaze tracking device, to facilitate generation of an enhanced computer generated reality that is responsive to a gaze and/or pose of the user. 
     The pair of display assemblies can be mounted to the housing  9 . 1 - 110  and separated by a distance  9 . 1 - 215 . The distance  9 . 1 - 215  between the pair of display assemblies can designed to correspond to the IPD of a user. The distance  9 . 1 - 215  can be adjustable to account for different IPDs of different users that may wear the head-mounted device  9 . 1 - 100 . For example, either or both of the display assemblies may be movably mounted to the housing  9 . 1 - 110  to permit the display assemblies to move or translate laterally to make the distance  9 . 1 - 215  larger or smaller. Any type of manual or automatic mechanism may be used to permit the distance  9 . 1 - 215  between the display assemblies to be an adjustable distance. For example, the display assemblies can be mounted to the housing via slidable tracks or guides that permit manual or electronically actuated movement of one or more of the display assemblies to adjust the distance  9 . 1 - 215 . 
     As seen in  FIGS.  9 . 1 - 2   , the air circulation device  9 . 1 - 140  can be positioned in or otherwise mounted to the housing  9 . 1 - 110  so as to urge a flow of air through an interior space  9 . 1 - 225  of the housing  9 . 1 - 110 . The housing  9 . 1 - 110  can include a port that permits fluid communication between the interior space  9 . 1 - 225  and an environment external to the housing  9 . 1 - 110 , to create a flow path of air in the housing  9 . 1 - 110 . In  FIGS.  9 . 1 - 2   , the housing is shown with a pair of inlet ports  9 . 1 - 240  at a bottom side thereof, and an outlet port  9 . 1 - 250  at a top side thereof, which creates air flow paths  9 . 1 - 275  extending from the inlet ports  9 . 1 - 240  to the outlet port  9 . 1 - 250 . Each of the ports can include a vent, screen, hole, porous membrane, and/or other fluidic opening that permits fluid communication thereacross. However, it is contemplated that the housing  9 . 1 - 110  can generally include any suitable number inlet ports and outlet ports in any suitable locations with respect to the housing to permit a flow of air therein. The air circulation device  9 . 1 - 140  can be implemented as a fan that is configured to draw air into the inlet port(s)  9 . 1 - 240  and urge air out of the outlet port  9 . 1 - 250 . However, any suitable number of fans or other air circulation devices can be included to urge movement of air. 
     The air flow path  9 . 1 - 275  can extend over or across components such as heat-generating electronic components mounted within the housing. For example, the pair of display assemblies may include heat-generating display layers, and the air circulation device  9 . 1 - 140  may be configured to generate a flow of air so that the air flow path  9 . 1 - 275  extends over each of the display assemblies  9 . 1 - 121   a  and  9 . 1 - 121   b  to cool the heat-generating layers by dissipating heat therefrom. Alternatively, or in combination, the air circulation device  9 . 1 - 140  may be configured to circulate air over other electronic components, such as integrated circuit chips, other input/output devices, or the like, or across the user&#39;s face. 
       FIGS.  9 . 1 - 3    shows an example of a display assembly  9 . 1 - 121 , and an air flow path  9 . 1 - 275  extending over surfaces of the display assembly. The display assembly  9 . 1 - 121  can be one of a pair of display assemblies like in the example shown in  FIGS.  9 . 1 - 2   , where each of the first and second display assemblies  9 . 1 - 121   a  and  9 . 1 - 121   b  can be configured similarly to the display assembly  9 . 1 - 121 . As seen in  FIGS.  9 . 1 - 3   , the display assembly  9 . 1 - 121  can include a display layer  9 . 1 - 314 , a heat sink  9 . 1 - 342 , a circuit board  9 . 1 - 370 , one or more components  9 . 1 - 386  on the circuit board, and an enclosure  9 . 1 - 393  that serves to enclose and support the foregoing components. 
     As shown in  FIGS.  9 . 1 - 3   , the display assembly  9 . 1 - 121  can have a front side for viewing images and a back side opposite the front side. The display layer  9 . 1 - 314  can include operative components of the display that form images capable of being viewed by the user from the front side thereof. The display layer  9 . 1 - 314  can, for example, include any suitable operational display panel having an array of electronically controlled pixels that can provide a visual output, such as an OLED, uLED, or LCD panel. The display assembly  9 . 1 - 121  can further include other optional components, which can support specialized display functions for providing an immersive head-mounted display. For example, the display assembly  9 . 1 - 121  can include as gaze-tracking devices or eye trackers (e.g., positioned beside the display layer  9 . 1 - 314 ), and/or optics (e.g., positioned in front of the display layer). The optics can be configured to help optically adjust and correctly project the image based content being displayed by the display layer  9 . 1 - 314  for close up viewing. The optics can include one or more lenses, mirrors, or other optical elements. 
     In the example shown in  FIGS.  9 . 1 - 3   , the air flow path  9 . 1 - 275  passes over the display assembly  9 . 1 - 121  across a back side thereof, so as to dissipate heat generated from the display layer  9 . 1 - 314  through the back side. To facilitate dissipation of heat, a heat sink  9 . 1 - 342  can be positioned behind the back side of the display layer  9 . 1 - 314 . The heat sink  9 . 1 - 342  can include a plurality of fins  9 . 1 - 377  positioned in the air flow path  9 . 1 - 275  so as to increase a surface area of the back surface that is exposed to the flow of air. The heat sink  9 . 1 - 342  can be thermally coupled to the back side or back surface of the display layer  9 . 1 - 314  via a thermal interface  9 . 1 - 361 , such as a thermally conductive adhesive or other appropriate thermally conductive material, to enhance heat transfer (e.g., conduction) from the display layer through the heat sink and to the stream of air. 
     As seen in  FIGS.  9 . 1 - 3   , the display assembly  9 . 1 - 121  can further include other structures, such as a circuit board  9 . 1 - 370  (e.g., a flexible or rigid printed circuit board) on the back side of the display assembly. The circuit board  9 . 1 - 370  can have one or more components  9 . 1 - 386  mounted thereon. The components  9 . 1 - 386  can be, for example, passive or active electronic components surface mounted to the circuit board  9 . 1 - 370 , such as integrated circuit chips, resistors, capacitors, or other structures that can protrude from the surface of the circuit board  9 . 1 - 370 . 
     The component(s)  9 . 1 - 386 , and/or or other structures of the display assembly, can partially impede or obstruct the free flow of air and have a tendency to increase a turbulence of the air in the flow path. For example,  FIGS.  9 . 1 - 3    shows an example in which increased impedance caused by the presence of components  9 . 1 - 386  causes more turbulent flow, which can degrade the efficiency or user experience as described above. 
       FIGS.  9 . 1 - 4    shows another example of the display assembly  9 . 1 - 121 . The example shown in  FIGS.  9 . 1 - 4    employs a similar structure to  FIGS.  9 . 1 - 3   , but additionally includes an air deflector  9 . 1 - 400  positioned in the air flow path  9 . 1 - 275 . The air deflector  9 . 1 - 400  is a structure that can be mounted to surfaces within the head-mounted device to reduce a turbulence of air passing through the head-mounted device and across the air deflector. The air deflector  9 . 1 - 400  can have a surface designed to create less turbulent, more laminar flow for air that is incident on the surface. For example, the air deflector  9 . 1 - 400  can provide a smoother surface or lower angle with respect to an incoming stream of air generated by the air circulation device  9 . 1 - 140  (e.g.,  FIGS.  9 . 1 - 2   ), compared to structures in the head-mounted device that the incoming stream of air would otherwise contact if the air deflector were not present. The air deflector  9 . 1 - 400  can be a rigid component made of any appropriate material, such as plastic, ceramic, or metal. In some embodiments, the air deflector  9 . 1 - 400  can be configured as a dedicated wall structure that is mounted to internal structures or inserted into an interior space of the housing solely to affect the properties of the flow of air incident through the interior space, without providing other mechanical or electrical functions. 
     In the example shown in  FIGS.  9 . 1 - 4   , the air deflector  9 . 1 - 400  is mounted to the back side of the display assembly  9 . 1 - 121 . The air deflector  9 . 1 - 400  is mounted on and attached to the circuit board  9 . 1 - 370 , and extends at least partially over the component(s)  9 . 1 - 386  so as to at least partially shield the components  9 . 1 - 386  from the incoming air in the air flow path  9 . 1 - 275 . The air deflector  9 . 1 - 400  can, for example, be positioned down-stream from the heat sink  9 . 1 - 342  with respect to the air flow path  9 . 1 - 275 . Compared to the surfaces of the components  9 . 1 - 386  in the absence of the air deflector  9 . 1 - 400 , the surface of the air deflector that is in the flow path and positioned to receive an incident stream of air can have a smoother surface with fewer bends or steps. Accordingly, the air deflector  9 . 1 - 400  can be configured to make the air flow path  9 . 1 - 275  less tortuous. 
     Although the air deflector  9 . 1 - 400  is shown mounted to the circuit board  9 . 1 - 370  on the back side of the display assembly  9 . 1 - 121 , it is contemplated that the air deflector  9 . 1 - 400  can be mounted in any other desired location within the housing of the head-mounted device in which reduced turbulence is desired. For example, the air deflector  9 . 1 - 400  can be mounted to the heat-sink or another surface on the back side of the display assembly  9 . 1 - 121 , another non-back side surface of the display assembly, or another internal component within the housing of the head-mounted device. 
       FIGS.  9 . 1 - 5    illustrates another example of the air deflector  9 . 1 - 400 . In the example shown in  FIGS.  9 . 1 - 5   , the air deflector  9 . 1 - 400  is configured similar to the example shown in  FIGS.  9 . 1 - 4   , but also includes an integral heat sink so that the air deflector  9 . 1 - 400  can further dissipate heat from the components  9 . 1 - 386  that are shielded by the air deflector  9 . 1 - 400 , where such components can be heat-generating electronic components. The surface of the air deflector  9 . 1 - 400  that receives the incident air can include multiple fins  9 . 1 - 477 , which increase a surface area of the surface receiving incident air. To maintain sufficiently laminar flow, the fins  9 . 1 - 477  can, for example, be configured as longitudinal fins that extend in the direction of air flow, or as a series of aligned pins that are arranged in rows extending along the direction of air flow, among other possible structural arrangements. To facilitate heat transfer, the air deflector  9 . 1 - 400  can be made of a material having a sufficiently high thermal conductivity, such as copper or aluminum. The air deflector  9 . 1 - 400  can be coupled to one or several of the components  9 . 1 - 386 . To further enhance the ability of the air deflector  9 . 1 - 400  to dissipate heat, the air deflector  9 . 1 - 400  can be thermally coupled to such components via a thermally conductive interface  9 . 1 - 461 , such as a conductive adhesive or other appropriate thermally conductive material. 
       FIGS.  9 . 1 - 6    illustrates another example of the air deflector  9 . 1 - 400 . The air deflector  9 . 1 - 400  as shown in  FIGS.  9 . 1 - 6    can be configured similar to the examples of  FIGS.  9 . 1 - 4    or  FIGS.  9 . 1 - 5   , except that in  FIGS.  9 . 1 - 6   , the air deflector  9 . 1 - 400  is movably mounted to a surface (in this case, a surface of the circuit board  9 . 1 - 370 ), rather than non-movably or fixedly mounted to the surface like in the previous examples. The movable mounting can permit the air deflector  9 . 1 - 400  to have an adjustable angle with respect to the incoming stream of air in the air flow path  9 . 1 - 275 . This can be useful to, for example, allow the adjustable angle to be optimized for reduced turbulence in various positions of the air deflector when the position of the air deflector with respect to the flow path or housing is otherwise moved. When the air deflector  9 . 1 - 400  is mounted to a display assembly that is movable adjust the distance  9 . 1 - 215  (e.g.,  FIGS.  9 . 1 - 2   ), the air deflector  9 . 1 - 400  can be configured to compensate for its changed position by also moving with respect to the display assembly. For example, the air deflector  9 . 1 - 400  can be configured to move or rotate relative to the display assembly in response to or otherwise in accordance with movement of the display assembly relative to the housing. The movement of the air deflector  9 . 1 - 400  can be achieved using, for example, a piezo electric actuator or other actuator, and/or a mechanical linkage that synchronizes movement of the display assembly to rotation of the air deflector  9 . 1 - 400 . In the example shown in  FIGS.  9 . 1 - 6   , the air deflector  9 . 1 - 400  is pivotally mounted to the surface of the display assembly  9 . 1 - 121 . The actuator can be configured to rotate the air deflector  9 . 1 - 400  about pivot point  9 . 1 - 603 , based on movement of the display assembly  9 . 1 - 121  relative to the housing or based on changes to the distance  9 . 1 - 215  between the pair of display assemblies, so as to adjust an angle of incidence of air onto the surface of the air deflector  9 . 1 - 400  to account for a new position of the air deflector with respect to the flow path extending through the housing. 
       FIGS.  9 . 1 - 7  through  9 . 1 - 8    show an example of how an air deflector  9 . 1 - 400  can reduce turbulence of air in a head-mounted device.  FIGS.  9 . 1 - 7  through  9 . 1 - 8    show examples of arrangement without and with an air deflector, respectively. 
       FIGS.  9 . 1 - 7    shows an arrangement having a component  9 . 1 - 386  mounted in a flow path and positioned to receive an incoming stream  9 . 1 - 735  of air in the flow path (e.g., similar to  FIGS.  9 . 1 - 3   ). The component  9 . 1 - 386  has a surface  9 . 1 - 759  positioned in the flow path to receive the incoming stream  9 . 1 - 735  of air thereon. The incoming stream  9 . 1 - 735  is incident on the surface  9 . 1 - 759 , and forms an angle  9 . 1 - 0  with respect to the surface  9 . 1 - 759 . In this example, the angle θ between the incoming stream  9 . 1 - 735  and the surface of the component  9 . 1 - 386  is approximately 90 degrees. Stated another way, the angle of incidence of the incoming stream  9 . 1 - 735  is approximately zero, wherein the angle of incidence is defined by the angle between the incoming stream and the normal to the incident surface. The large angle between the incoming stream and the incidence surface, or equivalently the low incident angle, causes a dramatic change in the current of air that tends to create turbulent patterns as the incoming stream impinges on the component, and then continues to flow along a tortuous flow path around the component  9 . 1 - 386 . 
       FIGS.  9 . 1 - 8    shows an identical arrangement to  FIGS.  9 . 1 - 7   , except that air deflector  9 . 1 - 400  is mounted in the flow path to deflect, at least partially, the incoming stream  9 . 1 - 735  of air away from incidence onto the surface  9 . 1 - 759  of the component  9 . 1 - 386  (e.g., similar to  FIGS.  9 . 1 - 4   ). The incoming stream  9 . 1 - 735  is directed towards the surface  9 . 1 - 759  of the component  9 . 1 - 386  at the same angle  9 . 1 - 0  as shown in  FIGS.  9 . 1 - 7   . This is represented in  FIGS.  9 . 1 - 8    by the dashed arrow, which illustrates what the path of the incoming stream  9 . 1 - 735  would be if the air deflector  9 . 1 - 400  were not present. However, due to the presence of air deflector  9 . 1 - 400 , the incoming stream  9 . 1 - 735  is wholly or partially deflected away from incidence onto the surface  9 . 1 - 759  of the component  9 . 1 - 386 . The air deflector  9 . 1 - 400  has a surface  9 . 1 - 859  positioned in the flow path to receive the incoming stream  9 . 1 - 735  of air thereon, and the surface  9 . 1 - 859  of the air deflector  9 . 1 - 400  forms an angle  9 . 1 -φ relative to the incoming stream  9 . 1 - 735  that is less than the angle  9 . 1 -θ. Stated another way, the angle of incidence of the incoming stream  9 . 1 - 735  onto the surface  9 . 1 - 859  of the air deflector is greater than what the angle of incidence onto the surface  9 . 1 - 759  of the component  9 . 1 - 386  would be if the air deflector were not present. As a result of such configuration, the air that further propagates downstream after incidence onto the air deflector  9 . 1 - 400  is deflected to a less drastic degree compared to if the air deflector  9 . 1 - 400  were not present and the air were to be incident unobstructed onto the component  9 . 1 - 386 . As a result, the air follows a less tortuous flow path, which can beneficially reduce noise within the device for a given flow velocity and/or improve efficiency of the cooling system. 
     The air deflector  9 . 1 - 400  can be configured as any suitable wall structure that forms the desired, turbulence-reducing angle, with respect to the incoming stream  9 . 1 - 735  of air. Although the wall is shown in  FIGS.  9 . 1 - 8    as having a straight geometry, in various embodiments the wall can, for example, have a straight, a bent, or curved geometry. As shown in  FIGS.  9 . 1 - 8   , the air deflector  9 . 1 - 400  and the component  9 . 1 - 386  can both be mounted to the same common surface  9 . 1 - 801 , which can be, for example, any suitable surface of the display assembly. As shown in  FIGS.  9 . 1 - 8   , the surface  9 . 1 - 859  (air-receiving surface) of the air deflector can further form an obtuse angle with respect to the surface  9 . 1 - 801  onto which the air deflector  9 . 1 - 400  is mounted, which can be useful for reducing turbulence in cases where the incoming stream  9 . 1 - 735  of air propagates in a direction parallel to the surface  9 . 1 - 801 . 
     Components of the head-mounted device can be operably connected to provide the performance described herein.  FIGS.  9 . 1 - 9    shows a simplified block diagram of an example of the head-mounted device  9 . 1 - 100 . 
     As shown in  FIGS.  9 . 1 - 9   , the head-mounted device  9 . 1 - 100  can include a controller  9 . 1 - 130  with one or more processing units that include or are configured to access a memory  9 . 1 - 918  having instructions stored thereon. The instructions or computer programs may be configured to perform one or more of the operations or functions described with respect to the head-mounted device  9 . 1 - 100 . The controller  9 . 1 - 130  can be implemented as any electronic device capable of processing, receiving, or transmitting data or instructions. For example, the controller  9 . 1 - 130  may include one or more of: a microprocessor, a central processing unit (CPU), an application-specific integrated circuit (ASIC), a digital signal processor (DSP), or combinations of such devices. As described herein, the term “processor” is meant to encompass a single processor or processing unit, multiple processors, multiple processing units, or other suitably configured computing element or elements. 
     The memory  9 . 1 - 918  can store electronic data that can be used by the head-mounted device  9 . 1 - 100 . For example, the memory  9 . 1 - 918  can store electrical data or content such as, for example, audio and video files, documents and applications, device settings and user preferences, timing and control signals or data for the various modules, data structures or databases, and so on. The memory  9 . 1 - 918  can be configured as any type of memory. By way of example only, the memory  9 . 1 - 918  can be implemented as random access memory, read-only memory, Flash memory, removable memory, or other types of storage elements, or combinations of such devices. 
     The head-mounted device  9 . 1 - 100  can further include a display  9 . 1 - 120  for displaying visual information for a user. The display  9 . 1 - 120  can provide visual (e.g., image or video) output, and can include a pair of display assemblies as described herein. The display  9 . 1 - 120  can be or include an opaque, transparent, and/or translucent display. The display  9 . 1 - 120  may have a transparent or translucent medium through which light representative of images is directed to a user&#39;s eyes. The display  9 . 1 - 120  may utilize digital light projection, OLEDs, LEDs, uLEDs, liquid crystal on silicon, laser scanning light source, or any combination of these technologies. The medium may be an optical waveguide, a hologram medium, an optical combiner, an optical reflector, or any combination thereof. In some embodiments, the transparent or translucent display may be configured to become opaque selectively. Projection-based systems may employ retinal projection technology that projects graphical images onto a person&#39;s retina. Projection systems also may be configured to project virtual objects into the physical environment, for example, as a hologram or on a physical surface. The head-mounted device  9 . 1 - 100  can include optics configured to help optically adjust and correctly project the image based content being displayed by the display  9 . 1 - 120  for close up viewing. The optics can include one or more lenses, mirrors, or other optical devices. 
     In some embodiments, controller  9 . 1 - 130  can receive user inputs from controls  9 . 1 - 908  and execute operations in response to the inputs. For example, controller  9 . 1 - 130  can be configured to receive sound from the microphone  9 . 1 - 930 . In response to receiving the sound, controller  9 . 1 - 130  can run the voice recognition module to identify voice commands. 
     Head-mounted device  9 . 1 - 100  can include a battery  9 . 1 - 920 , which can charge and/or power components of the head-mounted device  9 . 1 - 100 . The battery  9 . 1 - 920  can also charge and/or power components connected to the head-mounted device  9 . 1 - 100 , such as a portable electronic device  9 . 1 - 902 . 
     Head-mounted device  9 . 1 - 100  can include the air circulation device  9 . 1 - 140  for cooling down components of the head-mounted device  9 . 1 - 100 . The head-mounted device  9 . 1 - 100  can further include an air deflector  9 . 1 - 400  disposed in an air flow path and configured to receive a stream of air generated by the air circulation device  9 . 1 - 140 , as further described herein. The air deflector  9 . 1 - 400  can optionally be movable by an actuator  9 . 1 - 949 , as further described herein. The controller  9 . 1 - 130  can be configured to operate the actuator  9 . 1 - 949  to move or rotate the air deflector based on inputs from a user and/or adjustments to assemblies of the display  9 . 1 - 120 . 
     The head-mounted device  9 . 1 - 100  can include an input/output component  9 . 1 - 926 , which can include any suitable component for connecting head-mounted device  9 . 1 - 100  to other devices. Suitable components can include, for example, audio/video jacks, data connectors, or any additional or alternative input/output components. 
     The head-mounted device  9 . 1 - 100  can include communications circuitry  9 . 1 - 928  for communicating with one or more servers or other external devices  9 . 1 - 90  using any suitable communications protocol. For example, communications circuitry  9 . 1 - 928  can support Wi-Fi (e.g., a 802.11 protocol), Ethernet, Bluetooth, high frequency systems (e.g., 900 MHZ, 2.4 GHz, and 5.6 GHz communication systems), infrared, TCP/IP (e.g., any of the protocols used in each of the TCP/IP layers), HTTP, BitTorrent, FTP, RTP, RTSP, SSH, any other communications protocol, or any combination thereof. Communications circuitry  9 . 1 - 928  can also include an antenna for transmitting and receiving electromagnetic signals. 
     The head-mounted device  9 . 1 - 100  can include audio devices such as a microphone  9 . 1 - 930  and/or speaker  9 . 1 - 912 . The microphone  9 . 1 - 930  can be configured to detect sounds from a user and/or environment. The microphone  9 . 1 - 930  can be operably connected to the controller  9 . 1 - 130  for detection of sound levels and communication of detections for further processing. The speaker  9 . 1 - 212  can be configured to emit sounds to a user and/or environment. The speaker  9 . 1 - 212  can be operably connected to the controller  9 . 1 - 130  for control of speaker output, including sound levels and/or other sound characteristics. 
     The head-mounted device  9 . 1 - 100  can optionally connect to a portable electronic device  9 . 1 - 902 , which can provide certain functions. For the sake of brevity, the portable electronic device  9 . 1 - 902  will not be described in detail in  FIGS.  9 . 1 - 9   . It should be appreciated, however, that the portable electronic device  9 . 1 - 902  may be embodied in a variety of forms including a variety of features, all or some of which can be utilized by the head-mounted device  9 . 1 - 100  (e.g., input/output, controls, processing, battery, etc.). The portable electronic device  9 . 1 - 902  can be configured to receive cooling from operation of air circulation device  9 . 1 - 140 . The portable electronic device  9 . 1 - 902  can provide a handheld form factor (e.g., small portable electronic device which is light weight, fits in a pocket, etc.). Although not limited to these, examples include media players, phones (including smart phones), PDAs, computers, and the like. The portable electronic device  9 . 1 - 902  may include a screen  9 . 1 - 913  for presenting the graphical portion of the media to the user. The screen  9 . 1 - 913  can be utilized as the primary screen of the head-mounted device  9 . 1 - 100 . 
     The head-mounted device  9 . 1 - 100  can include a dock  9 . 1 - 906  operative to receive the portable electronic device  9 . 1 - 902 . The dock  9 . 1 - 906  can include a connector (e.g., Lightning, USB, FireWire, power, DVI, etc.), which can be plugged into a complementary connector of the portable electronic device  9 . 1 - 902 . The dock  9 . 1 - 906  may include features for helping to align the connectors during engagement and for physically coupling the portable electronic device  9 . 1 - 902  to the head-mounted device  9 . 1 - 100 . For example, the dock  9 . 1 - 906  may define a cavity for placement of the portable electronic device  9 . 1 - 902 . The dock  9 . 1 - 906  may also include retaining features for securing portable electronic device  9 . 1 - 902  within the cavity. The connector on the dock  9 . 1 - 906  can function as a communication interface between the portable electronic device  9 . 1 - 902  and the head-mounted device  9 . 1 - 100 . 
     The head-mounted device  9 . 1 - 100  can include one or more other sensors. Such sensors can be configured to sense substantially any type of characteristic such as, but not limited to, images, pressure, light, touch, force, temperature, position, motion, and so on. For example, the sensor can be a photodetector, a temperature sensor, a light or optical sensor, an atmospheric pressure sensor, a humidity sensor, a magnet, a gyroscope, an accelerometer, a chemical sensor, an ozone sensor, a particulate count sensor, and so on. By further example, the sensor can be a bio-sensor for tracking biometric characteristics, such as health and activity metrics. Other user sensors can perform facial feature detection, facial movement detection, facial recognition, eye tracking, user mood detection, user emotion detection, voice detection, etc. Sensors can include a camera which can capture image based content of the outside world. 
     9.2: Fan with Debris Mitigation 
     Head-mountable devices, such as head-mountable displays, headsets, visors, smartglasses, head-up display, etc., can perform a range of functions that are managed by the components (e.g., sensors, circuitry, and other hardware) included with the wearable device. The head-mountable device can provide a user experience that is immersive or otherwise natural so the user can easily focus on enjoying the experience without being distracted by the mechanisms of the head-mountable device. 
     Components of a head-mountable device can generate heat during operation. The performance of electronic devices are often limited by their ability to effectively dissipate the heat generated by computing and other workloads. Excessive heat for long durations of time can damage the components of the head-mountable device and cause discomfort to the user. Heat can be mitigated in a number of ways, including with active mechanisms (e.g., fans, blowers, air movers, and the like) that are integrated into the head-mountable device. In order to dissipate heat effectively within a small form factor, active cooling is sometimes used. Active cooling refers to a thermal architecture wherein heat is dissipated via forced convection. 
     An active cooling device (e.g., fans, blowers, air movers, and the like) can generate a flow of air from an inlet to an outlet thereof. Such inlets and outlets can define a pathway that receives air from and/or delivers air to an environment external to the head-mountable device. However, by providing such exposure to an external environment, the fan is susceptible to intrusion by particles or other debris from the external environment. In particular, when the fan is not in operation, such particles can collect and become lodged between the parts of the fan. In such a condition, the fan may encounter difficultly resuming operation due to the introduction of particles between moving parts of the fan. In particular, particles lodged between the stationary parts (i.e., fan housing) and moving parts (i.e., rotor, impeller, etc.) can cause the fan to stall. 
     Systems of the present disclosure can provide fans that mitigate the intrusion of particles and other debris. Fans can include a protrusion that creates a tortuous pathway to direct incoming particles away from sensitive regions. Fans can include openings to allow particles to exit the fan. Fans can include a variable spacing between the stationary parts (i.e., fan housing) and moving parts (i.e., rotor, impeller, etc.) to avoid collection of particles. Fans can include an adhesive pad that collects and retains particles at a location that does not interfere with operation of the impeller. 
     These and other embodiments are discussed below with reference to  FIGS.  9 . 2 - 1  through  9 . 2 - 13   . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these Figures is for explanatory purposes only and should not be construed as limiting. 
     According to some embodiments, for example as shown in  FIGS.  9 . 2 - 1   , a head-mountable device  9 . 2 - 100  includes a frame  9 . 2 - 110  that is worn on a head of a user. The frame  9 . 2 - 110  can be positioned in front of the eyes of a user to provide information within a field of view of the user. The frame  9 . 2 - 110  can provide nose pads or another feature to rest on a user&#39;s nose. The frame  9 . 2 - 110  can be supported on a user&#39;s head with the head engager  9 . 2 - 120 . The head engager  9 . 2 - 120  can wrap or extend along opposing sides of a user&#39;s head. The head engager  9 . 2 - 120  can include earpieces for wrapping around or otherwise engaging or resting on a user&#39;s ears. It will be appreciated that other configurations can be applied for securing the head-mountable device  9 . 2 - 100  to a user&#39;s head. For example, one or more bands, straps, belts, caps, hats, or other components can be used in addition to or in place of the illustrated components of the head-mountable device  9 . 2 - 100 . By further example, the head engager  9 . 2 - 120  can include multiple components to engage a user&#39;s head. 
     The frame  9 . 2 - 110  can provide structure around a peripheral region thereof to support any internal components of the frame  9 . 2 - 110  in their assembled position. For example, the frame  9 . 2 - 110  can enclose and support various internal components (including for example integrated circuit chips, processors, memory devices, cameras, displays, lenses, and other circuitry) to provide computing and functional operations for the head-mountable device  9 . 2 - 100 , as discussed further herein. Any number of components can be included within and/or on the frame  9 . 2 - 110  and/or the head engager  9 . 2 - 120 . 
     The frame  9 . 2 - 110  can include and/or support one or more cameras  9 . 2 - 130 . The cameras  9 . 2 - 130  can be positioned on or near an outer side of the frame  9 . 2 - 110  to capture images of views external to the head-mountable device  9 . 2 - 100 . The captured images can be used for display to the user or stored for any other purpose. 
     The head-mountable device can be provided with displays that provide visual output for viewing by a user wearing the head-mountable device. As further shown in  FIGS.  9 . 2 - 1   , one or more displays  9 . 2 - 140  can be positioned on an inner side  9 . 2 - 124  of the head-mountable device  9 . 2 - 100 , for example within an eye chamber  9 . 2 - 126 . For example, a pair of displays  9 . 2 - 140  can be provided, where each display  9 . 2 - 140  is movably positioned to be within the field of view of each of a user&#39;s two eyes. Each display  9 . 2 - 140  can be adjusted to align with a corresponding eye of the user. For example, each display  9 . 2 - 140  can be moved along one or more axes until a center of each display  9 . 2 - 140  is aligned with a center of the corresponding eye. 
     A display  9 . 2 - 140  can transmit light from a physical environment (e.g., as captured by a camera) for viewing by the user. Such a display  9 . 2 - 140  can include optical properties, such as lenses for vision correction based on incoming light from the physical environment. Additionally or alternatively, a display  9 . 2 - 140  can provide information as a display within a field of view of the user. Such information can be provided to the exclusion of a view of a physical environment or in addition to (e.g., overlaid with) a physical environment. 
     A physical environment relates to a physical world that people can sense and/or interact with without necessarily requiring the aid of an electronic device. A computer-generated reality environment relates to a wholly or partially simulated environment that people sense and/or interact with the assistance of an electronic device. Examples of computer-generated reality include mixed reality and virtual reality. Examples of mixed realities can include augmented reality and augmented virtuality. Some examples of electronic devices that enable a person to sense and/or interact with various computer-generated reality environments include head-mountable systems, projection-based systems, heads-up displays (HUDs), vehicle windshields having integrated display capability, windows having integrated display capability, displays formed as lenses designed to be placed on a person&#39;s eyes (e.g., similar to contact lenses), headphones/earphones, speaker arrays, input systems (e.g., wearable or handheld controllers with or without haptic feedback), smartphones, tablets, and desktop/laptop computers. A head-mountable device can have an integrated opaque display, have a transparent or translucent display, or be configured to accept an external opaque display (e.g., smartphone). 
     Referring again to  FIGS.  9 . 2 - 1   , the head-mountable device can be provided with one or more flow channels that extend through at least a portion of the frame thereof to provide cooling to components of the head-mountable device. As shown in  FIGS.  9 . 2 - 1   , the flow channels can include and/or be connected to a system inlet  9 . 2 - 112 . The system inlet  9 . 2 - 112  can provide airflow directly to the fan  9 . 2 - 200  and/or one or more components of the head-mountable device  9 . 2 - 100 , such as a circuit component  9 . 2 - 150 . The air received by the system inlet  9 . 2 - 112  can be directed to a system outlet  9 . 2 - 116  and/or other outlets for exhaust out of the head-mountable device  9 . 2 - 100  by the fan  9 . 2 - 200 . 
     While the system inlet  9 . 2 - 112  is depicted at a bottom portion of the frame  9 . 2 - 110  and the system outlet  9 . 2 - 116  is depicted at a top portion of the frame  9 . 2 - 110 , it will be recognized that inlets, outlets, and flow channels there between can be positioned at any portion of the head-mountable device  9 . 2 - 100 . The system outlet  9 . 2 - 116  can be provided at a location that will allow exiting air to exhaust to an environment that is not disruptive to the user. For example, the system outlet  9 . 2 - 116  can be provided at a location and in and orientation that directs hot air away from the user. Multiple flow channels can be interconnected, such that multiple inlets and/or multiple outlets are connected to each other. 
     One or more fans  9 . 2 - 200  can be operated to provide cooling to one or more circuit components  9 . 2 - 150  of the head-mountable device  9 . 2 - 100 . The circuit component  9 . 2 - 150  can be an electrical component that generates heat during operation. The circuit component  9 . 2 - 150  can be a component of a circuit board  9 . 2 - 152 . The circuit component  9 . 2 - 150  can be operably and structurally coupled to the circuit board  9 . 2 - 152 . A portion of the fan  9 . 2 - 200  can be thermally connected to the circuit component  9 . 2 - 150 . 
     The fan  9 . 2 - 200  can receive a flow of air from a system inlet  9 . 2 - 112  and direct the flow of air to a system outlet  9 . 2 - 116 . For example, the fan  9 . 2 - 200 , the circuit component  9 . 2 - 150 , and/or the circuit board  9 . 2 - 152  can be positioned within an internal chamber  9 . 2 - 118  (e.g., plenum chamber) of the head-mountable device  9 . 2 - 100 . 
     While several components are shown within the frame  9 . 2 - 110 , it will be understood that some or all of these components can be located anywhere within or on the head-mountable device  9 . 2 - 100 . For example, one or more of these components can be positioned within the head engager  9 . 2 - 120  of the head-mountable device  9 . 2 - 100 . 
     Referring now to  FIGS.  9 . 2 - 2   , the fan can provide features for moving air across surfaces for dissipating heat. As shown in  FIGS.  9 . 2 - 2   , the fan  9 . 2 - 200  can include a fan housing  9 . 2 - 208  that includes a cover  9 . 2 - 210  over an impeller  9 . 2 - 220 . The cover  9 . 2 - 210  can form a fan inlet  9 . 2 - 212 , for example through a top surface thereof. The cover  9 . 2 - 210  can include or be connected to an exhaust duct  9 . 2 - 290  that forms a fan outlet  9 . 2 - 292  of the fan  9 . 2 - 200 . 
     The impeller  9 . 2 - 220  of the fan  9 . 2 - 200  can be positioned to receive air through the fan inlet  9 . 2 - 212  and direct the air to the fan outlet  9 . 2 - 292 . The fan  9 . 2 - 200  can include a motor to drive rotation of the impeller  9 . 2 - 220 . For example, the fan  9 . 2 - 200  can include a stator and an impeller  9 . 2 - 220  configured to rotate about the stator. The impeller  9 . 2 - 220  can include multiple blades  9 . 2 - 224  that extend radially outwardly away from the central hub of the impeller. As the impeller  9 . 2 - 220  rotates, the blades  9 . 2 - 224  receive air from the fan inlet  9 . 2 - 212  and direct the air radially outwardly and toward the fan outlet  9 . 2 - 292 . The impeller  9 . 2 - 220  can be stabilized by one or more bearings between the impeller  9 . 2 - 220  and the stator. The bearing can include a fluid hydrodynamic bearing, for example with oil or another fluid between the impeller  9 . 2 - 220  and the stator. It will be understood that other types of bearings are contemplated, including mechanical bearings, journal bearings, plain bearings, ball bearings, and the like. The bearing can provide radial and/or axial support to the impeller  9 . 2 - 220  as it rotates about the stator. 
     The impeller  9 . 2 - 220  can direct air or another gas within, against, or across one or more components of the fan  9 . 2 - 200 . The fan  9 . 2 - 200  can be operated based on one or more operating parameters that are controllable during use. The operating parameters can be determined, at least in part, based on a demand for cooling (e.g., based on a temperature of one or more components). The operating parameters can be further determined based on acceptable sound levels and characteristics to be produced by the fan  9 . 2 - 200  and along the flow channel. 
     Referring now to  FIGS.  9 . 2 - 3  and  9 . 2 - 4   , the fan can be thermally connected to components to be cooled. As shown in  FIGS.  9 . 2 - 3   , the fan housing  9 . 2 - 208  can further include a base plate  9 . 2 - 230 . The cover  9 . 2 - 210  and the base plate  9 . 2 - 230  of the fan housing  9 . 2 - 208  can, together, define an interior space  9 . 2 - 280  there between. The impeller  9 . 2 - 220  can be positioned within the interior space  9 . 2 - 280  and mounted to the base plate  9 . 2 - 230 . As such, the base plate  9 . 2 - 230  provides structural support to the impeller  9 . 2 - 220  for stable operation (e.g., during rotation). 
     A heat sink  9 . 2 - 240  can provide one or more fins  9 . 2 - 242  between the interior space  9 . 2 - 280  and the fan outlet  9 . 2 - 292  formed by the exhaust duct  9 . 2 - 290 . The heat sink  9 . 2 - 240  is thermally connected to the circuit component  9 . 2 - 150 , which is operably and structurally coupled to the circuit board  9 . 2 - 152 . The heat sink  9 . 2 - 240  can be thermally connected to the circuit component  9 . 2 - 150  by a direct connection (e.g., no intervening structure) or by a thermal interface  9 . 2 - 160 . For example, a thermal paste or other thermally conductive material can be provided to thermally and/or structurally connect the circuit component  9 . 2 - 150  to the heat sink  9 . 2 - 240 . 
     The base plate  9 . 2 - 230 , the heat sink  9 . 2 - 240 , and/or the fins  9 . 2 - 242  can be of a metal or other material having high thermal conductivity. The material can provide high rigidity and strength to provide support to components mounted to the base plate  9 . 2 - 230  and to securely mount to other components (e.g., the frame) of the head-mountable device. 
     The cover  9 . 2 - 210  can be of a material that provides protection to the impeller  9 . 2 - 220  and any other components in the interior space. The material can be plastic, metal, and/or another material. The cover  9 . 2 - 210  can be a monolithic, unitary, and/or unibody structure, rather than an assembly of parts. 
     As shown in  FIGS.  9 . 2 - 3    and  FIGS.  9 . 2 - 4   , the fan  9 . 2 - 200  can generate a flow of air from an inlet (e.g., system inlet  9 . 2 - 112  and/or fan inlet  9 . 2 - 212 ) to an outlet (e.g., system outlet  9 . 2 - 116  and/or fan outlet  9 . 2 - 292 ). It will be understood that the system outlet  9 . 2 - 116  can be directly connected to the fan outlet  9 . 2 - 292 , such that all air directed through the fan outlet  9 . 2 - 292  can be further directed to the system outlet  9 . 2 - 116 . Such inlets and outlets can define a pathway that receives air from and/or delivers air to an environment external to the head-mountable device. During operation of the fan  9 . 2 - 200 , the air flow can be maintained such that any particles or other debris travelling with the air flow moves through the fan  9 . 2 - 200  without remaining therein. Furthermore, the movement of the impeller  9 . 2 - 220  can disrupt and propel any resting particles until they are ejected from the fan  9 . 2 - 200 . 
     However, as shown in  FIGS.  9 . 2 - 5   , by providing such exposure to an external environment, the fan  9 . 2 - 200  is susceptible to intrusion by particles or other debris from the external environment. In particular, when the fan  9 . 2 - 200  is not in operation (e.g., when the impeller  9 . 2 - 220  is not rotating), particles  9 . 2 - 2  can collect and become lodged between the moving and stationary parts of the fan  9 . 2 - 200 . For example, the particles  9 . 2 - 2  can enter through the system inlet  9 . 2 - 112  and into the fan inlet  9 . 2 - 212  and/or through the system outlet  9 . 2 - 116  and into the fan outlet  9 . 2 - 292  and become lodged between the stationary parts (i.e., fan housing, base plate, etc.) and moving parts (i.e., rotor, impeller, etc.) of the fan  9 . 2 - 200 . In such a condition, the fan  9 . 2 - 200  may encounter difficultly resuming operation due to the introduction of particles between moving and stationary parts of the fan  9 . 2 - 200 . In particular, particles  9 . 2 - 2  lodged between the impeller  9 . 2 - 220  and the base plate  9 . 2 - 230  can cause the impeller  9 . 2 - 220  to stall. 
     It can be desirable to provide features that mitigate the collection of such particles while the fan  9 . 2 - 200  is not in operation without reducing the effectiveness of the fan while it is in operation. 
     Referring now to  FIGS.  9 . 2 - 5   , a fan can include features that create a tortuous pathway to prevent the incoming particles from entering the sensitive regions. For example, as shown in  FIGS.  9 . 2 - 5   , a protrusion  9 . 2 - 232  can extend from the base plate  9 . 2 - 230  to block and/or redirect a pathway of an incoming particle  9 . 2 - 2 . For example, the protrusion  9 . 2 - 232  can be positioned between at least a portion of the impeller  9 . 2 - 220  and the fan outlet  9 . 2 - 292 . The protrusion  9 . 2 - 232  can extend toward the cover  9 . 2 - 210  while allowing an air flow pathway over the protrusion between the impeller  9 . 2 - 220  and the fan outlet  9 . 2 - 292 . 
     The impeller  9 . 2 - 220  can be formed with a support disk  9 . 2 - 228  on one side of the blades  9 . 2 - 224 . Where a support disk  9 . 2 - 228  is provided, such as is shown in  FIGS.  9 . 2 - 5   , the protrusion  9 . 2 - 232  can extend from the base plate  9 . 2 - 230  to at least a height of a portion of the support disk  9 . 2 - 228 . Where blades  9 . 2 - 224  are provided, the protrusion  9 . 2 - 232  can extend from the base plate  9 . 2 - 230  to at least a height of one or more of the blades  9 . 2 - 224 . As such, the protrusion  9 . 2 - 232  can shield and/or occupy a space between the base plate  9 . 2 - 230  and the support disk  9 . 2 - 228 . It will be understood that a tortuous pathway around the protrusion  9 . 2 - 232  can connect the fan outlet  9 . 2 - 292  to the space between the base plate  9 . 2 - 230  and the support disk  9 . 2 - 228 . However, the protrusion can be formed such that incoming particles  9 . 2 - 2  are instead deflected to above the support disk  9 . 2 - 228 , so that they do not lodge between the base plate  9 . 2 - 230  and the support disk  9 . 2 - 228 . 
     In some embodiments, as further shown in  FIGS.  9 . 2 - 5   , the protrusion  9 . 2 - 232  can be formed as an annular ring or closed loop. Such a closed loop can surround a periphery of the impeller  9 . 2 - 220 . For example, the annular ring can encircle the support disk  9 . 2 - 228  and/or a portion of each of the blades  9 . 2 - 224 . 
     In some embodiments, as further shown in  FIGS.  9 . 2 - 5   , the protrusion  9 . 2 - 232  defines a surface  9 . 2 - 286  facing away from the impeller  9 . 2 - 220 . This surface  9 . 2 - 286  can be that on which the particle  9 . 2 - 2  is incident upon entry through the fan outlet  9 . 2 - 292 . The surface  9 . 2 - 286  can be formed to direct the particle away from the space between the base plate  9 . 2 - 230  and the impeller  9 . 2 - 220 . For example, the surface can form an angle that is oblique with respect to the base plate  9 . 2 - 230  to form a ramp. 
     Referring now to  FIGS.  9 . 2 - 6   , the protrusion  9 . 2 - 232  can overlap with at least a portion of the impeller  9 . 2 - 220 . For example, the blades  9 . 2 - 224  can be positioned between the cover  9 . 2 - 210  and the base plate  9 . 2 - 230 , and each of the blades can form a notch  9 . 2 - 288 . The protrusion  9 . 2 - 232  extends from the base plate  9 . 2 - 230  and into at least some of the notches  9 . 2 - 288 . As the impeller  9 . 2 - 220  rotates, the notches  9 . 2 - 288  pass over the protrusion  9 . 2 - 232  without physical contact. 
     Where a support disk  9 . 2 - 228  of the impeller  9 . 2 - 220  is provided, such as is shown in  FIGS.  9 . 2 - 6   , the protrusion  9 . 2 - 232  can extend from the base plate  9 . 2 - 230  to at least a height of a portion of the support disk  9 . 2 - 228 . In some embodiments, as further shown in  FIGS.  9 . 2 - 6   , the protrusion  9 . 2 - 232  can be formed as an annular ring. For example, the annular ring can encircle the support disk  9 . 2 - 228  without encircling the blades  9 . 2 - 224 , but instead protruding towards the blades  9 . 2 - 224  and into the notches  9 . 2 - 288 . As such, the protrusion  9 . 2 - 232  can block a space between the base plate  9 . 2 - 230  and the support disk  9 . 2 - 228 . It will be understood that a tortuous pathway around the protrusion  9 . 2 - 232  can connect the fan outlet  9 . 2 - 292  to the space between the base plate  9 . 2 - 230  and the support disk  9 . 2 - 228 . However, the protrusion can be formed such that incoming particles are instead deflected to above the support disk  9 . 2 - 228 , so that they do not lodge between the base plate  9 . 2 - 230  and the support disk  9 . 2 - 228 . 
     It will be understood that any protrusion  9 . 2 - 232  disclosed herein need not form an annular ring and that a variety of other shapes are contemplated, such as an arc, a flat wall, and the like. It will be understood that any number of protrusions  9 . 2 - 232  can be provided. For example, both the protrusion of  FIGS.  9 . 2 - 5    (i.e., overlapping the impeller  9 . 2 - 220 ) and the protrusion of  FIGS.  9 . 2 - 6    (i.e., surrounding the impeller  9 . 2 - 220 ) can be provided in combination. 
     In some embodiments, a distance between any protrusion  9 . 2 - 232  disclosed herein and the impeller  9 . 2 - 220  (e.g., the support disk  9 . 2 - 228  and/or the blades  9 . 2 - 224 ) can be smaller than a distance between the base plate  9 . 2 - 230  and the impeller  9 . 2 - 220  (e.g., the support disk  9 . 2 - 228 ). The smaller distance between the protrusion  9 . 2 - 232  and the impeller  9 . 2 - 220  can serve to block particles having a dimension that is greater than such a distance. As such, only particles smaller than such a distance can pass the protrusion  9 . 2 - 232 . When such small particles are between the base plate  9 . 2 - 230  and the impeller  9 . 2 - 220 , the particles will not be large enough to extend to both the base plate  9 . 2 - 230  and the impeller  9 . 2 - 220  and will thereby be unable to lodge therein or arrest operation of the impeller  9 . 2 - 220 . 
     Referring now to  FIGS.  9 . 2 - 7   , the base plate can have features that facilitate particle blocking and promote impeller operation. For example, as shown in  FIGS.  9 . 2 - 7   , the base plate  9 . 2 - 230  can have a variable profile at different portions thereof and with respect to the impeller  9 . 2 - 220 . For example, the base plate can form a base plate central portion  9 . 2 - 262  and one or more base plate peripheral portions  9 . 2 - 260 . The impeller  9 . 2 - 220  can form an impeller central portion  9 . 2 - 252  and one or more impeller peripheral portions  9 . 2 - 250 . A distance  9 . 2 - 266  between the base plate central portion  9 . 2 - 262  and the impeller central portion  9 . 2 - 252  is greater than a distance  9 . 2 - 264  between the base plate peripheral portion  9 . 2 - 260  and the impeller peripheral portion  9 . 2 - 250 . The smaller distance  9 . 2 - 264  between the base plate central portion  9 . 2 - 262  and the impeller central portion  9 . 2 - 252  can serve to block particles having a dimension that is greater than the distance  9 . 2 - 264 . As such, only particles smaller than such a distance can pass the gap defined by the distance  9 . 2 - 264 . When such small particles are between the base plate central portion  9 . 2 - 262  and the impeller central portion  9 . 2 - 252 , the particles will not be large enough to span the distance  9 . 2 - 266  to contact both the base plate central portion  9 . 2 - 262  and the impeller central portion  9 . 2 - 252  and will thereby be unable to lodge therein or arrest operation of the impeller  9 . 2 - 220 . It will be understood that a number of variations can be provided, including any number of different distances and/or transitions there between. 
     Referring now to  FIGS.  9 . 2 - 8   , the base plate can have features that facilitate particle evacuation and/or capture. For example, as shown in  FIGS.  9 . 2 - 8   , the base plate  9 . 2 - 230  can define one or more openings  9 . 2 - 270  that extend entirely through the base plate  9 . 2 - 230 . The impeller  9 . 2 - 220  can be positioned to overlap at least some of the openings  9 . 2 - 270  of the base plate  9 . 2 - 230 . When particles are introduced between the base plate  9 . 2 - 230  and the impeller  9 . 2 - 220 , the particles can be provided with an exit from such a region through the openings  9 . 2 - 270 . The openings  9 . 2 - 270  can be connected to an external environment or a collection chamber for releasing and/or capturing the particles. It will be understood that any number of openings  9 . 2 - 270  can be provided in any arrangement. It will be further understood that the openings  9 . 2 - 270  can have any size and/or dimension to facilitate particle removal. It will be understood that openings can be provided at any location, including other than the base plate  9 . 2 - 230 . 
     Referring now to  FIGS.  9 . 2 - 9  and  9 . 2 - 10   , a fan can include a retention mechanism that captures particles. For example, as shown in  FIGS.  9 . 2 - 9   , a pad  9 . 2 - 272  can be provided on a side of the base plate  9 . 2 - 230  facing and/or covering the openings  9 . 2 - 270 . In such an embodiment, the openings  9 . 2 - 270  can be the same or similar to those described elsewhere herein. As shown in  FIGS.  9 . 2 - 10   , the pad  9 . 2 - 272  can be positioned on a side of the openings that is opposite the impeller  9 . 2 - 220 , such as on an outer side of the base plate  9 . 2 - 230 . Rather than releasing the particles that pass through the openings  9 . 2 - 270 , the pad  9 . 2 - 272  can capture the particles and retain them away from the impeller  9 . 2 - 220 . 
     Once the particles are removed from the interior space  9 . 2 - 280  containing the impeller  9 . 2 - 220 , they can be retained at a location that does not interfere with the operation of the impeller  9 . 2 - 220 . For example, the pad  9 . 2 - 272  can provide a surface that retains particles upon contact of the particle with the pad  9 . 2 - 272 . For example, the pad  9 . 2 - 272  can include an adhesive (e.g., pressure-sensitive adhesive). As used herein, an adhesive can include any material that has adhesion properties and/or stickiness, such as a polymer, glue, cement, paste, laminate, and/or other material that bonds to particles upon contact therewith. By further example, the pad  9 . 2 - 272  can include an uncured or partially cured substance that is exposed to the interior space  9 . 2 - 280  and/or the openings  9 . 2 - 270 . In some embodiments, the fan  9 . 2 - 200  can include one or more other retention mechanisms, such as an operable electrode or other surface that is configured to be electrically charged to attract particles. Additionally or alternatively, the fan  9 . 2 - 200  can include a filter and/or a mechanically actuated container for selectively containing particles that migrate out of the interior space  9 . 2 - 280  and/or through the openings  9 . 2 - 270 . 
     Referring now to  FIGS.  9 . 2 - 11  and  9 . 2 - 12   , a fan can provide multiple features that provide particle mitigation properties. For example, the fan  9 . 2 - 200  of  FIGS.  9 . 2 - 11  and  9 . 2 - 12    includes one or more protrusions  9 . 2 - 232  extending from the base plate  9 . 2 - 230  to block and/or redirect a pathway of an incoming particle. Such protrusion(s)  9 . 2 - 232  can include any one or more features described herein with respect to the fan of  FIG.  9 . 2 - 5  or  9 . 2 - 6   . 
     By further example, the fan  9 . 2 - 200  of  FIGS.  9 . 2 - 11  and  9 . 2 - 12    includes a base plate  9 . 2 - 230  having a variable profile at different portions thereof and with respect to the impeller  9 . 2 - 220 , such as with a base plate central portion  9 . 2 - 262  and one or more base plate peripheral portions  9 . 2 - 260 . Along with features of the impeller  9 . 2 - 220 , the base plate  9 . 2 - 230  can define different distances there between, as described with respect to the fan of  FIGS.  9 . 2 - 7   . 
     By further example, the fan  9 . 2 - 200  of  FIGS.  9 . 2 - 11  and  9 . 2 - 12    includes a base plate  9 . 2 - 230  that defines one or more openings  9 . 2 - 270  extending entirely through the base plate  9 . 2 - 230 . Such openings  9 . 2 - 270  can include any one or more features described herein with respect to the fan of  FIGS.  9 . 2 - 8   . 
     By further example, the fan  9 . 2 - 200  of  FIGS.  9 . 2 - 11  and  9 . 2 - 12    includes a pad  9 . 2 - 272  on an outer side of the base plate  9 . 2 - 230  to capture particles from the interior space  9 . 2 - 280 . The pad  9 . 2 - 272  can include any one or more features described herein with respect to the fan of  FIGS.  9 . 2 - 9  and  9 . 2 - 10   . 
     It will be understood that any number of features described herein can be combined in a single fan to provide particle mitigation. It will be further understood that no one feature is necessary to provide effective particle mitigation to a fan. 
     Referring now to  FIGS.  9 . 2 - 13   , components of the head-mountable device can be operably connected to provide the performance described herein.  FIGS.  9 . 2 - 13    shows a simplified block diagram of an illustrative head-mountable device  9 . 2 - 100  in accordance with one embodiment of the invention. It will be appreciated that components described herein can be provided on either or both of a frame and/or a head engager of the head-mountable device  9 . 2 - 100 . It will be understood that additional components, different components, or fewer components than those illustrated may be utilized within the scope of the subject disclosure. 
     As shown in  FIGS.  9 . 2 - 13   , the head-mountable device  9 . 2 - 100  can include a controller  9 . 2 - 180  with one or more processing units that include or are configured to access a memory  9 . 2 - 182  having instructions stored thereon. The instructions or computer programs may be configured to perform one or more of the operations or functions described with respect to the head-mountable device  9 . 2 - 100 . The controller  9 . 2 - 180  can be implemented as any electronic device capable of processing, receiving, or transmitting data or instructions. For example, the controller  9 . 2 - 180  may include one or more of: a microprocessor, a central processing unit (CPU), an application-specific integrated circuit (ASIC), a digital signal processor (DSP), or combinations of such devices. As described herein, the term “processor” is meant to encompass a single processor or processing unit, multiple processors, multiple processing units, or other suitably configured computing element or elements. 
     The memory  9 . 2 - 182  can store electronic data that can be used by the head-mountable device  9 . 2 - 100 . For example, the memory  9 . 2 - 182  can store electrical data or content such as, for example, audio and video files, documents and applications, device settings and user preferences, timing and control signals or data for the various modules, data structures or databases, and so on. The memory  9 . 2 - 182  can be configured as any type of memory. By way of example only, the memory  9 . 2 - 182  can be implemented as random access memory, read-only memory, Flash memory, removable memory, or other types of storage elements, or combinations of such devices. 
     The head-mountable device  9 . 2 - 100  can further include a display  9 . 2 - 140  for displaying visual information for a user. The display  9 . 2 - 140  can provide visual (e.g., image or video) output. The display  9 . 2 - 140  can be or include an opaque, transparent, and/or translucent display. The display  9 . 2 - 140  may have a transparent or translucent medium through which light representative of images is directed to a user&#39;s eyes. The display  9 . 2 - 140  may utilize digital light projection, OLEDs, LEDs, uLEDs, liquid crystal on silicon, laser scanning light source, or any combination of these technologies. The medium may be an optical waveguide, a hologram medium, an optical combiner, an optical reflector, or any combination thereof. In one embodiment, the transparent or translucent display may be configured to become opaque selectively. Projection-based systems may employ retinal projection technology that projects graphical images onto a person&#39;s retina. Projection systems also may be configured to project virtual objects into the physical environment, for example, as a hologram or on a physical surface. The head-mountable device  9 . 2 - 100  can include an optical subassembly  9 . 2 - 214  configured to help optically adjust and correctly project the image-based content being displayed by the display  9 . 2 - 140  for close up viewing. The optical subassembly  9 . 2 - 214  can include one or more lenses, mirrors, or other optical devices. 
     The head-mountable device  9 . 2 - 100  can include the fan  9 . 2 - 200  and/or any other suitable component for cooling down components of the head-mountable device  9 . 2 - 100 . Suitable components can include, for example, impellers, pipes for transferring heat, vents, apertures, holes, any other component suitable for distributing and diffusing heat, or any combination thereof. The fan  9 . 2 - 200  may also or instead be manufactured from materials selected for heat dissipation properties. For example, a housing of the head-mountable device  9 . 2 - 100  may be configured to distribute heat away from components thereof and/or the user. 
     The head-mountable device  9 . 2 - 100  can include a battery  9 . 2 - 184 , which can charge and/or power components of the head-mountable device  9 . 2 - 100 . The battery  9 . 2 - 184  can also charge and/or power components connected to the head-mountable device  9 . 2 - 100 . 
     The head-mountable device  9 . 2 - 100  can include an input/output component  9 . 2 - 186 , which can include any suitable component for connecting head-mountable device  9 . 2 - 100  to other devices. Suitable components can include, for example, audio/video jacks, data connectors, or any additional or alternative input/output components. The input/output component  9 . 2 - 186  can include buttons, keys, or another feature that can act as a keyboard for operation by the user. 
     The head-mountable device  9 . 2 - 100  can include the microphone  9 . 2 - 188  as described herein. The microphone  9 . 2 - 188  can be operably connected to the controller  9 . 2 - 180  for detection of sound levels and communication of detections for further processing, as described further herein. 
     The head-mountable device  9 . 2 - 100  can include the speakers  9 . 2 - 190  as described herein. The speakers  9 . 2 - 190  can be operably connected to the controller  9 . 2 - 180  for control of speaker output, including sound levels, as described further herein. 
     The head-mountable device  9 . 2 - 100  can include one or more other sensors. Such sensors can be configured to sense substantially any type of characteristic such as, but not limited to, images, pressure, light, touch, force, temperature, position, motion, and so on. For example, the sensor can be a photodetector, a temperature sensor, a light or optical sensor, an atmospheric pressure sensor, a humidity sensor, a magnet, a gyroscope, an accelerometer, a chemical sensor, an ozone sensor, a particulate count sensor, and so on. By further example, the sensor can be a bio-sensor for tracking biometric characteristics, such as health and activity metrics. Other user sensors can perform facial feature detection, facial movement detection, facial recognition, eye tracking, user mood detection, user emotion detection, voice detection, etc. Sensors can include a camera which can capture image based content of the outside world. 
     The head-mountable device  9 . 2 - 100  can include communications circuitry  9 . 2 - 192  for communicating with one or more servers or other devices using any suitable communications protocol. For example, communications circuitry  9 . 2 - 192  ca support Wi-Fi (e.g., a 802.11 protocol), Ethernet, Bluetooth, high frequency systems (e.g., 900 MHz, 2.4 GHz, and 5.6 GHz communication systems), infrared, TCP/IP (e.g., any of the protocols used in each of the TCP/IP layers), HTTP, BitTorrent, FTP, RTP, RTSP, SSH, any other communications protocol, or any combination thereof. Communications circuitry  9 . 2 - 192  can also include an antenna for transmitting and receiving electromagnetic signals. 
     While various embodiments and aspects of the present disclosure are illustrated with respect to a head-mountable device, it will be appreciated that the subject technology can encompass and be applied to other devices. For example, a noise mitigation system in accordance with embodiments disclosed herein can be included with an electronic device that generates heat during operation. Such an electronic device can be or include a desktop computing device, a laptop-computing device, a display, a television, a portable device, a phone, a tablet computing device, a mobile computing device, a wearable device, a watch, and/or a digital media player. Such devices can include an impeller and flow channels to facilitate cooling as described herein. 
     Accordingly, embodiments of the present disclosure provide a head-mountable device that provides a fan that effectively manages heat while also mitigating the intrusion of particles and other debris. Fans can include a protrusion that creates a tortuous pathway to direct incoming particles away from sensitive regions. Fans can include openings to allow particles to exit the impeller. Fans can include a variable spacing between the impeller and the base plate to avoid collection of particles. Fans can include an adhesive pad that collects and retains particles at a location that does not interfere with operation of the impeller. 
     9.3: Ventilation 
       FIGS.  9 . 3 - 1    illustrates an example of a head-mountable display (HMD)  9 . 3 - 100 . The HMD  9 . 3 - 100  can include a ventilation system  9 . 3 - 102  for controlling airflow through the HMD  9 . 3 - 100  and cooling various components of the HMD  9 . 3 - 100  described herein. In at least one example, the ventilation system  9 . 3 - 102  can be configured to cool one or more heat generating components or subsystems of the HMD  9 . 3 - 100 , including, for example, the optical modules  9 . 3 - 108   a ,  9 . 3 - 108   b  and/or one or more processors or other heat generating components of the logic board  9 . 3 - 110 . The ventilation system  9 . 3 - 102  can include a frame assembly  9 . 3 - 106 , which can include a multi-part chassis, and a fan assembly  9 . 3 - 104 . The fan assembly  9 . 3 - 104  can be coupled to the frame assembly  9 . 3 - 106  and the frame assembly  9 . 3 - 106  such that the fan assembly  9 . 3 - 104  is disposed between the frame assembly  9 . 3 - 106  and the logic board  9 . 3 - 110 . 
     In at least one example, when the HMD  9 . 3 - 100  is assembled, the optical modules  9 . 3 - 108   a ,  9 . 3 - 108   b  can be coupled to the frame assembly  9 . 3 - 106  such that the optical modules  9 . 3 - 108   a ,  9 . 3 - 108   b  are each aligned with an aperture defined by the frame assembly  9 . 3 - 106  (illustrated in subsequent figures, including  FIGS.  9 . 3 - 2  and  9 . 3 - 11   ). In this way, in at least one example, the optical modules  9 . 3 - 108   a ,  9 . 3 - 108   b  can be in fluid communication with the fan assembly  9 . 3 - 104  through the frame assembly  9 . 3 - 106  disposed between the optical modules  9 . 3 - 108   a ,  9 . 3 - 108   b  and the fan assembly  9 . 3 - 104 . In at least one example, one or the optical modules  9 . 3 - 108   a ,  9 . 3 - 108   b  can each include one or more display screens (illustrated in subsequent figures, for example  FIGS.  9 . 3 - 11   ) or other heat-generating electronic components. For example, the first optical module  9 . 3 - 108   a  can include a first display screen and the second optical module  9 . 3 - 108   b  can include a second display screen, both of which can be oriented toward the user&#39;s eyes when donning the HMD  9 . 3 - 100 . 
       FIGS.  9 . 3 - 2    illustrates an example of a ventilation system  9 . 3 - 202  in an assembled configuration. In at least one example, the ventilation system  9 . 3 - 202  includes a first fan  9 . 3 - 112   a  and a second fan  9 . 3 - 112   b  coupled with a frame assembly  9 . 3 - 206  (e.g., the chassis). In at least one example, the frame assembly  9 . 3 - 206  can include an outer frame  9 . 3 - 215  and an inner frame  9 . 3 - 214  coupled to the outer frame  9 . 3 - 215 . The frame assembly  9 . 3 - 206  can be a structural frame assembly. The outer frame  9 . 3 - 215  can be a structural outer frame and define an exterior surface of the HMD  9 . 3 - 100  and the inner frame  9 . 3 - 214  can be and inner structural frame disposed within an internal volume  9 . 3 - 226  defined by the outer frame  9 . 3 - 215 . As noted above with reference to  FIGS.  9 . 3 - 1   , an optical module including a display screen (not shown in  FIGS.  9 . 3 - 2   ) can be coupled to the frame assembly  9 . 3 - 206 , for example coupled to the inner frame  9 . 3 - 214 , and aligned with a first aperture  9 . 3 - 216   a  or the second aperture  9 . 3 - 216   b . In one example, a first optical module with a first screen can be aligned with the first aperture  9 . 3 - 216   a  and a second optical module with a second screen can be aligned with the second aperture  9 . 3 - 216   b.    
     In at least one example, the inner frame  9 . 3 - 214  of the frame assembly  9 . 3 - 206  of the head mountable display device can be coupled to the outer frame  9 . 3 - 215  at discrete touch points  9 . 3 - 218  to minimize contact and thermal transfer between the inner frame  9 . 3 - 214  and the outer frame  9 . 3 - 215 . Two discrete touch points  9 . 3 - 218  are labeled and shown in  FIGS.  9 . 3 - 2    and are given as non-limiting examples. More or less than the touch points  9 . 3 - 218  shown can be included at various positions or locations of the inner frame  9 . 3 - 214  and outer frame  9 . 3 - 215 . The discrete touch points  9 . 3 - 218  where the inner frame  9 . 3 - 214  is coupled to the outer frame  9 . 3 - 215  can be a part of a first set of discrete touch points such that the inner frame  9 . 3 - 214  is thermally isolated from the outer frame  9 . 3 - 215  via the first set of discrete touch points. The inner frame  9 . 3 - 214  can be referred to as an internal frame and the outer frame  9 . 3 - 215  can be referred to as an external frame. 
     In at least one example, each touch-point  9 . 3 - 218  of the first set of discrete touch points can include an insulating member  9 . 3 - 220  disposed between the inner frame  9 . 3 - 214  and the outer frame  9 . 3 - 215 . In this way, the inner frame  9 . 3 - 214  can be thermally isolated from the outer frame  9 . 3 - 215  via a first set of insulating members, including the insulating members  9 . 3 - 220  shown in  FIGS.  9 . 3 - 2   , disposed between the inner frame  9 . 3 - 214  and the outer frame  9 . 3 - 215  at the first set of discrete touch points, with at least one insulating member  9 . 3 - 220  of the first set of insulating members disposed at each discrete touch point  9 . 3 - 218  of the first set of discrete touch points. 
     In at least one example, the outer frame  9 . 3 - 215  can include a first material and the inner frame  9 . 3 - 214  can include a second material different than the first material. The first material of the outer frame  9 . 3 - 215  can include aesthetically pleasing materials, including aluminum, other metals, plastics, composite materials, and so forth. In at least one example, the second material of the inner frame  9 . 3 - 214  can include a material stronger than the first material of the outer frame  9 . 3 - 215 . In one example, the second material can include a magnesium alloy. Other examples of the second material can include other metals stiffer and/or stronger than the first material, including, metal alloys, carbon fiber, composite materials and alloys, and so forth. 
     The external/outer frame  9 . 3 - 215  can define the internal volume  9 . 3 - 226  as well as a first opening  9 . 3 - 122  (shown in the front, perspective view of the frame assembly  9 . 3 - 106  of  FIGS.  9 . 3 - 1   ) and a second opening  9 . 3 - 224  opposite the first opening  9 . 3 - 122 . In at least one example, the outer frame  9 . 3 - 215  can define one or more intake ports  9 . 3 - 228  between the first opening  9 . 3 - 122  and the second opening  9 . 3 - 224 . The intake ports  9 . 3 - 228  can include one or more apertures defined by the outer frame  9 . 3 - 215  through which the internal volume  9 . 3 - 226  can be in fluid communication with an external environment. 
     In at least one example, the external/outer frame  9 . 3 - 215  can define one or more exhaust ports  9 . 3 - 230  between the first opening  9 . 3 - 122  and the second opening  9 . 3 - 224 . The exhaust ports  9 . 3 - 230  can include one or more apertures defined by the outer frame  9 . 3 - 215  through which the internal volume  9 . 3 - 226  can be in fluid communication with the external environment. In at least one example, one or more of the exhaust ports  9 . 3 - 230  can be centrally located between the first and second apertures  9 . 3 - 216   a - b  of the internal frame  9 . 3 - 214 . 
     In at least one example, the fan assembly  9 . 3 - 204  of the HMD  9 . 3 - 100  can include a first fan  9 . 3 - 212   a  disposed in the internal volume  9 . 3 - 226  and a second fan  9 . 3 - 212   b  disposed in the internal volume  9 . 3 - 226 . The first fan  9 . 3 - 212   a  can be aligned with the first aperture  9 . 3 - 216   a  and the second fan  9 . 3 - 212   b  can be aligned with the second aperture  9 . 3 - 216   b . In general, the term “aligned” as used herein can refer to a relative position with two components adjacent or near one another. For example, the first fan  9 . 3 - 212   a  can be aligned with the first aperture  9 . 3 - 216   a  such that an air intake feature or opening of the first fan  9 . 3 - 212   a  is positioned or configured to receive air into the first fan  9 . 3 - 212   a  through the first aperture  9 . 3 - 216   a . In at least one example, the first fan  9 . 3 - 212   a  can be “aligned” with the first aperture  9 . 3 - 216   a  such that an axis of rotation of a fan blade assembly  9 . 3 - 217   a  of the first fan  9 . 3 - 212   a  can pass through the first aperture  9 . 3 - 216   a . In one example, the axis of rotation of the fan blade assembly  9 . 3 - 217   a  can be parallel, aligned with, or close in proximity and angle with a central, normal axis of the first aperture  9 . 3 - 216   a  defined by the inner frame  9 . 3 - 214 . Thus, the term “aligned” is used herein to denote the arrangement of the fans  9 . 3 - 212   a - b  and respective apertures  9 . 3 - 216   a - b  as shown in  FIGS.  9 . 3 - 2   . The same use of the term “aligned” can be used to describe the relative position of the second fan  9 . 3 - 212   b  with the second aperture  9 . 3 - 216   a , as illustrated in  FIG.  9 . 3 - 2   . 
     In at least one example, the first fan  9 . 3 - 212   a  and the second fan  9 . 3 - 212   b  are configured to draw air into the internal volume  9 . 3 - 226  through the intake port  9 . 3 - 228  and push the air out from the internal volume  9 . 3 - 226  through the exhaust port  9 . 3 - 230 . As noted above, the first and second fans  9 . 3 - 212   a ,  9 . 3 - 212   b  can be aligned with the first and second apertures  9 . 3 - 216   a ,  9 . 3 - 216   b  of the inner frame  9 . 3 - 214 , respectively, to draw air through the apertures  9 . 3 - 216   a - b  and into the fans  9 . 3 - 212   a - b  via fan inlets  9 . 3 - 232   a ,  9 . 3 - 232   b , respectively. The first fan  9 . 3 - 212   a  can include a housing  9 . 3 - 234   a  defining the fan inlet  9 . 3 - 232   a  through which air can enter the first fan  9 . 3 - 212   a  before being blown through the exhaust port  9 . 3 - 230 . Likewise, the second fan  9 . 3 - 212   b  can include a housing  9 . 3 - 234   b  defining the fan inlet  9 . 3 - 232   b  through which air can enter the second fan  9 . 3 - 212   b  before being blown through the exhaust port  9 . 3 - 230 . The fan inlets  9 . 3 - 232   a - b  can be adjacent to and/or aligned with the apertures  9 . 3 - 216   a - b , respectively, such that air flows into the internal volume  9 . 3 - 226  through the intake ports  9 . 3 - 228  then through the apertures  9 . 3 - 216   a - b  and then through the fan inlets  9 . 3 - 232   a - b . The fans  9 . 3 - 212   a - b  are configured to blow the air out from the internal volume  9 . 3 - 226  through the exhaust ports  9 . 3 - 230 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  9 . 3 - 2    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  9 . 3 - 3 - 9 . 3 - 11    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  9 . 3 - 3 - 9 . 3 - 11    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  9 . 3 - 2   . 
       FIGS.  9 . 3 - 3    illustrates an example of a fan  9 . 3 - 312  including a housing  9 . 3 - 334 . The housing  9 . 3 - 334  can include a first housing shell  9 . 3 - 336  and a second housing shell  9 . 3 - 338  coupled to the first housing shell  9 . 3 - 336 . The first and second housing shells  9 . 3 - 336 ,  9 . 3 - 338  can define an internal fan volume  9 . 3 - 340 . The first and second housing shells  9 . 3 - 336 ,  9 . 3 - 338  can also be referred to as first and second housings, respectively. In at least one example, a plurality of fan fins or blades  9 . 3 - 342  are disposed in the internal fan volume  9 . 3 - 340  and configured or assembled to rotate about an axis of rotation  9 . 3 - 344 . This axis of rotation  9 . 3 - 344  can be the same axis of rotations discussed above with reference to  FIGS.  9 . 3 - 2    and which is aligned to pass through the aperture  9 . 3 - 216   a  or  9 . 3 - 216   b  shown in  FIGS.  9 . 3 - 2   . In at least one example, the first housing shell  9 . 3 - 336  and the second housing shell  9 . 3 - 338  can define a fan outlet  9 . 3 - 346  through which air is pushed by the blades  9 . 3 - 342  when rotated. In at least one example, the second housing shell  9 . 3 - 338  can define a fan inlet  9 . 3 - 332  through which the blades  9 . 3 - 342  are configured to draw air into the internal fan volume  9 . 3 - 340 . 
     In at least one example, the fan  9 . 3 - 312  can be coupled to a logic board assembly  9 . 3 - 310  including one or more printed circuit boards (PCBs) and electronic components. In particular, the first housing shell  9 . 3 - 336  can be thermally coupled with or in direct thermal or mechanical contact with a heat generating component  9 . 3 - 348  of the logic board assembly  9 . 3 - 310 . In at least one example, the first housing shell  9 . 3 - 336  can be formed of or include a thermally conductive material, such as a metal, such that heat generated by the heat generating component  9 . 3 - 348  is conductively transferred through the first housing shell  9 . 3 - 336  and air blowing through the internal fan volume  9 . 3 - 340  can convectively cool the first housing shell  9 . 3 - 336 . In this way, heat generated by the heat generating component  9 . 3 - 348  is carried out through the fan outlet  9 . 3 - 346 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  9 . 3 - 3    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  9 . 3 - 2  and  9 . 3 - 4 - 9 . 3 - 11    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  9 . 3 - 2  and  9 . 3 - 4 - 9 . 3 - 11    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  9 . 3 - 3   . 
       FIGS.  9 . 3 - 4    illustrates a side, cross-sectional view of a fan  9 . 3 - 412  including a housing  9 . 3 - 434 . The housing  9 . 3 - 434  can include a first housing shell  9 . 3 - 436  and a second housing shell  9 . 3 - 438 , which can also be referred to as first and second housings, respectively, defining an internal fan volume  9 . 3 - 440 . In at least one example, a plurality of fan fins or blades  9 . 3 - 442  are disposed in the internal fan volume  9 . 3 - 440  and configured or assembled to rotate about an axis of rotation  9 . 3 - 444 . This axis of rotation  9 . 3 - 444  can be the same axis of rotations discussed above with reference to  FIGS.  9 . 3 - 2    and which is aligned to pass through the aperture  9 . 3 - 216   a  or  9 . 3 - 216   b  shown in  FIGS.  9 . 3 - 2   . In at least one example, the first housing shell  9 . 3 - 436  and the second housing shell  9 . 3 - 438  can define a fan outlet  9 . 3 - 446  through which air is pushed by the blades  9 . 3 - 442  when rotated. In at least one example, the second housing shell  9 . 3 - 438  can define a fan inlet (not shown in the side view of  FIGS.  9 . 3 - 4   ) through which the blades  9 . 3 - 442  are configured to draw air into the internal fan volume  9 . 3 - 440 . 
     In at least one example, the fan  9 . 3 - 412  can be coupled to a logic board assembly or printed circuit board (PCB)  9 . 3 - 410  including one or more electronic components. In particular, the first housing shell  9 . 3 - 436  can be thermally coupled with or in direct thermal or mechanical contact with a heat generating component  9 . 3 - 448  of the PCB  9 . 3 - 410 . In at least one example, the first housing shell  9 . 3 - 436  can be formed of or include a thermally conductive material, such as a metal, such that heat generated by the heat generating component  9 . 3 - 448  is conductively transferred through the first housing shell  9 . 3 - 436  and air blowing through the internal fan volume  9 . 3 - 440  can convectively cool the first housing shell  9 . 3 - 436 . In this way, heat generated by the heat generating component  9 . 3 - 448  is carried out through the fan outlet  9 . 3 - 446 . 
     In at least one example, the first housing shell  9 . 3 - 436  can be thermally coupled directly with the heat generating component  9 . 3 - 448  via a thermal paste or adhesive  9 . 3 - 450  disposed between the heat generating component  9 . 3 - 448  and the first housing shell  9 . 3 - 436 . In this way, the first housing shell  9 . 3 - 436  can be a heat sink disposed in the internal volume of an electronic device, such as the HMD  9 . 3 - 100  shown in  FIGS.  9 . 3 - 1   . The housing  9 . 3 - 434  of the fan  9 . 3 - 412  can include the metal heat sink. As noted above and shown in  FIGS.  9 . 3 - 2   , multiple fans  9 . 3 - 212   a - b  can be disposed in the HMD  9 . 3 - 100  such that the fan  9 . 3 - 412  shown in  FIGS.  9 . 3 - 4   , which can also serve as and thus be referred to as a heat sink for the heat generating component  9 . 3 - 448 , can be one of multiple fans including a second fan and heat sink of an HMD or other electronic device. 
     In at least one example, the system or device shown in  FIGS.  9 . 3 - 4    can include an electrically conductive fence  9 . 3 - 452  coupled to the PCB  9 . 3 - 410  and surrounding a perimeter of the electronic/heat generating component  9 . 3 - 448 . As noted above, the fan  9 . 3 - 412  or fan assembly, can include the housing  9 . 3 - 434  having the first housing shell  9 . 3 - 436  coupled to the fence  9 . 3 - 452  and disposed over the electronic/heat generating component  9 . 3 - 448 . The first housing shell  9 . 3 - 436  and the fence  9 . 3 - 452  can form an electromagnetic interference (EMI) shield around the electronic/heat generating component  9 . 3 - 448  or any other electronic component coupled to or part of the PCB  9 . 3 - 410 . 
     In at least one embodiment, the fence  9 . 3 - 452 , which can also be referred to as an EMI fence  9 . 3 - 452 , can include an EMI foam  9 . 3 - 456  and a metal portion  9 . 3 - 454  disposed between the PCB  9 . 3 - 410  and the first housing shell  9 . 3 - 436  as shown, with the EMI foam  9 . 3 - 456  disposed between the metal fence  9 . 3 - 452  or fence portion/sidewall  9 . 3 - 454  and the second housing shell  9 . 3 - 436 . The metal fence or fence portion  9 . 3 - 454  can be disposed between the EMI foam  9 . 3 - 456  and the PCB  9 . 3 - 410 . In at least one example, housing  9 . 3 - 434 , including the second housing shell  9 . 3 - 436 , can be coupled to the metal EMI fence  9 . 3 - 452  or metal portion/sidewall  9 . 3 - 454  via the EMI foam  9 . 3 - 456 . In at least one example, the fence  9 . 3 - 452  or sidewall  9 . 3 - 454  can extend from the second housing shell  9 . 3 - 436  of the housing  9 . 3 - 434  of the fan  9 . 3 - 412 . 
     In at least one example, the first housing shell  9 . 3 - 436  defines or is disposed in a first major plane  9 . 3 - 458  parallel to the PCB  9 . 3 - 410 , or parallel to a second major plane  9 . 3 - 460  in which the PCB  9 . 3 - 410  is disposed or which the PCB  9 . 3 - 410  defines. In at least one example, the second housing shell  9 . 3 - 436  defines or includes a planar portion  9 . 3 - 464  disposed parallel to the PCB  9 . 3 - 410 , for example parallel to the second major plane  9 . 3 - 460 , and the heat generating component  9 . 3 - 448  is coupled to the planar portion  9 . 3 - 464 . 
     The fan  9 . 3 - 412  shown in  FIGS.  9 . 3 - 4    can thus be part of an EMI shield assembly protecting the various component of the PCB or electronic components coupled thereto, including the heat generating component  9 . 3 - 448 . The metal housing  9 . 3 - 434 , and in particular the second housing shell  9 . 3 - 436  thereof, can form a shield volume  9 . 3 - 462  in which the heat generating component and other components associated with the PCB  9 . 3 - 410  can be disposed. The components within the shield assembly can include one or more antennas or other components sensitive to electromagnetic waves. In at least one example, the heat generating component  9 . 3 - 448  can include one or more processors or processing units. The EMI shielding assembly can also be referred to as a shield can and the fan  9 . 3 - 412  can be a part of a convective cooling EMI shield can that simultaneously acts to cool various components associated with the PCB  9 . 3 - 410  and shield those components from electromagnetic interference. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  9 . 3 - 4    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  9 . 3 - 2 - 9 . 3 - 3  and  9 . 3 - 5 - 9 . 3 - 11    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  9 . 3 - 2 - 9 . 3 - 3  and  9 . 3 - 5 - 9 . 3 - 11    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  9 . 3 - 4   . 
       FIGS.  9 . 3 - 5    illustrates a plan view of a fan  9 . 3 - 512  having a first side or housing shell  9 . 3 - 536 . The housing shell  9 . 3 - 536  can define a fan inlet  9 . 3 - 532  and a plurality of fan blades  9 . 3 - 542  can be arranged and configured to draw air into the fan  9 . 3 - 512  when rotated. The housing shell  9 . 3 - 536  can also at least partially define a fan outlet  9 . 3 - 546  through which air is expelled from the fan when the blades  9 . 3 - 542  rotate. 
     In at least one example, the housing or housing shell  9 . 3 - 536  can include one or more discrete connection features or points  9 . 3 - 566 . These discrete connection features or points  9 . 3 - 566  can be used to couple the fan  9 . 3 - 512  to one or more other components of an electronic device, including the HMD  9 . 3 - 100  shown in  9 . 3 - 100 , by providing material features through which connectors or connector assemblies can extend or couple to one or more frames, chassis, or other structural components of the HMD  9 . 3 - 100 . In at least one example, the discrete connection features or points  9 . 3 - 566  can provide reduce surface area contacting other structural components, relative to a total area or perimeter of the housing shell  9 . 3 - 536 , such that heat transfer from the fan  9 . 3 - 512  (as pulled from the heat-generating components with which the fan can be contacting as shown in  FIGS.  9 . 3 - 4    and described above) to structural components with which a user may come into contact is limited. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  9 . 3 - 5    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  9 . 3 - 2 - 9 . 3 - 4  and  9 . 3 - 6 - 9 . 3 - 11    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  9 . 3 - 2 - 9 . 3 - 4  and  9 . 3 - 6 - 9 . 3 - 11    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  9 . 3 - 2   . 
       FIGS.  9 . 3 - 6    illustrates a plan view of a fan  9 . 3 - 612  having a second side or housing shell  9 . 3 - 638 . The housing shell  9 . 3 - 638  can at least partially define a fan outlet  9 . 3 - 646  through which air is expelled from the fan during operation. 
     In at least one example, the housing or housing shell  9 . 3 - 638  can include one or more discrete connection features or points  9 . 3 - 666 . These discrete connection features or points  9 . 3 - 666  can be used to couple the fan  9 . 3 - 612  to one or more other components of an electronic device, including the HMD  9 . 3 - 100  shown in  9 . 3 - 100 , by providing material features through which connectors or connector assemblies can extend or couple to one or more frames, chassis, or other structural components of the HMD  9 . 3 - 100 . In at least one example, the discrete connection features or points  9 . 3 - 666  can provide reduce surface area contacting other structural components, relative to a total area or perimeter of the housing shell  9 . 3 - 638 , such that heat transfer from the fan  9 . 3 - 612  (as pulled from the heat-generating components with which the fan can be contacting as shown in  FIGS.  9 . 3 - 4    and described above) to structural components with which a user may come into contact is limited. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  9 . 3 - 6    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  9 . 3 - 2 - 9 . 3 - 5  and  9 . 3 - 7 - 9 . 3 - 11    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  9 . 3 - 2 - 9 . 3 - 5  and  9 . 3 - 7 - 9 . 3 - 11    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  9 . 3 - 6   . 
       FIGS.  9 . 3 - 7    is an exploded perspective view of a fan  9 . 3 - 712  that can be similar to the fans  9 . 3 - 512 ,  9 . 3 - 612  shown in  FIGS.  9 . 3 - 5  and  9 . 3 - 6   , respectively. The fan  9 . 3 - 712  can include a housing including a first housing shell  9 . 3 - 736  and an opposing second housing shell  9 . 3 - 738  defining an internal volume in which a fan blade assembly including a plurality of fan blades  9 . 3 - 742  are disposed. The first or second housing shells  9 . 3 - 736 ,  9 . 3 - 738  can include or be coupled to one or more outlet fins  9 . 3 - 768  to direct air out through an outlet of the fan  9 . 3 - 712  defined by the first and/or second housing shells  9 . 3 - 736 ,  9 . 3 - 738 . 
     In at least one example, the first housing shell  9 . 3 - 736  can include or be coupled to one or more discrete connection features or points  9 . 3 - 766  similar to those discrete connection features or points  9 . 3 - 566 ,  9 . 3 - 666  described above with reference to  FIGS.  9 . 3 - 5  and  9 . 3 - 6   , respectively. In at least one example, the first housing shell  9 . 3 - 736  can define a fan inlet  9 . 3 - 732  through which air is pulled into an internal volume of the fan  9 . 3 - 712 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  9 . 3 - 7    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  9 . 3 - 2 - 9 . 3 - 6  and  9 . 3 - 8 - 9 . 3 - 11    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  9 . 3 - 2 - 9 . 3 - 6  and  9 . 3 - 8 - 9 . 3 - 11    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  9 . 3 - 7   . 
       FIGS.  9 . 3 - 8    illustrates a perspective view of a fan assembly  9 . 3 - 804  including a first fan  9 . 3 - 812   a  and a second fan  9 . 3 - 812   b . Each fan  9 . 3 - 812   a - b  can include respective housings  9 . 3 - 836   a - b . The housings  9 . 3 - 836   a - b  can be similar to the first housing shell  9 . 3 - 736  shown in  FIGS.  9 . 3 - 7    and the first housing shell  9 . 3 - 436  shown in  FIGS.  9 . 3 - 4    and contacting the heat generating component  9 . 3 - 448 . In at least one example, the housings  9 . 3 - 836   a - b , or housing shells, can include respective etched portions  9 . 3 - 870   a  and  9 . 3 - 870   b . The etched portions  9 . 3 - 870   a  and  9 . 3 - 870   b  can correspond in position and shape to the EMI fence  9 . 3 - 452  shown in  FIGS.  9 . 3 - 4    such that the EMI fence  9 . 3 - 452  is coupled or in contact with the housings  9 . 3 - 836   a - b  of the fans  9 . 3 - 812   a - b  at the etched portions  9 . 3 - 870   a  and  9 . 3 - 870   b , respectively. The etched portions  9 . 3 - 870   a - b  can be formed using chemical etching, laser etching, or other etching processes used to prepare the surface of the housings  9 . 3 - 836   a - b  for conductive contact with the EMI fence  9 . 3 - 452 . In at least one example, the etched portions  9 . 3 - 870   a - b  are better suited than other portions or surfaces of the housings  9 . 3 - 836   a - b  to make contact with the EMI fence  9 . 3 - 452  so that an effective EMI shield is formed as noted above with reference to at least  FIGS.  9 . 3 - 4   . 
     In at least one example, as shown in  FIGS.  9 . 3 - 8   , the first fan  9 . 3 - 812   a  can be disposed in or define a first major plane  9 . 3 - 872   a  and the second fan  9 . 3 - 812   a  can be disposed in or define a second major plane  9 . 3 - 872   b . The term “major plane” when referring to major planes  9 . 3 - 872   a - b  can include a plane that intersects or define a cross-sectional area of each fan  9 . 3 - 812   a - b  parallel to a planar surface of the first or second housing shells of the respective housings  9 . 3 - 836   a - b . A cross-sectional areas of the fans  9 . 3 - 812   a - b  disposed in the major planes  9 . 3 - 872   a - b  are greater than cross-sectional areas of the fans  9 . 3 - 812   a - b  disposed in planes orthogonal to the major planes  9 . 3 - 872   a - b . In at least one example, the fans  9 . 3 - 812   a - b  include blades rotating about an axis or rotation normal to the major planes  9 . 3 - 872   a - b . These axes of rotation can be similar to the axis of rotation  9 . 3 - 444  shown in  FIGS.  9 . 3 - 4    and the major planes  9 . 3 - 872   a - b  can be similar to and/or parallel to the plane  9 . 3 - 458  shown in  FIGS.  9 . 3 - 4   . 
     In at least one example, the first major plane  9 . 3 - 872   a  and the second major plane  9 . 3 - 872   b  are disposed at an angle θ relative to one another such that the first and second major planes  9 . 3 - 872   a - b  are non-parallel with one another. The angle θ can accommodate a curvature of one or more other components of the HMD  9 . 3 - 100  shown in  FIGS.  9 . 3 - 1   , including the frame, front cover and display screen, and so forth. This curvature, and the angle θ of the major planes  9 . 3 - 872   a - b  relative to one another can be designed to accommodate and complements the curvature of a user&#39;s face, including relative angles of left and right facial features and brow/forehead/cheek curvatures. The angle θ can dispose the fans  9 . 3 - 812   a - b  such that the fans  9 . 3 - 812   a - b  can fit compactly within the HMD  9 . 3 - 100  and a general curvature thereof as the fans are aligned with apertures of the internal frame and various MLBs and other components disposed relative to one another at the same or similar angle θ. 
     In at least one example, the fan assembly  9 . 3 - 804  can include a graphite bridge between the fans  9 . 3 - 812   a - b . In at least one example, the fan assembly  9 . 3 - 804  can include tape disposed between bodies of the fans  9 . 3 - 812   a - b  for heat transfer equalization between the fans  9 . 3 - 812   a - b.    
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  9 . 3 - 8    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  9 . 3 - 2 - 9 . 3 - 7  and  9 . 3 - 9 - 9 . 3 - 11    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  9 . 3 - 2 - 9 . 3 - 7  and  9 . 3 - 9 - 9 . 3 - 11    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  9 . 3 - 8   . 
       FIGS.  9 . 3 - 9    illustrates a partially assembled view of an MLB  9 . 3 - 910  including one or more heat generating components  9 . 3 - 948   a - b  surrounded by respective EMI fences  9 . 3 - 952   a - b . A fan  9 . 3 - 912  is illustrated in dotted lines to indicate a position of the fan overlaying the MLB  9 . 3 - 910  and coupled with the first EMI fence  9 . 3 - 952  forming a first EMI fence about the component  9 . 3 - 948  and other components illustrated on the MLB  9 . 3 - 910 . The fan  9 . 3 - 912  is transparent to illustrate component otherwise visually blocked in the view of  FIGS.  9 . 3 - 9   . 
     In at least one example, a barrier  9 . 3 - 974  can be coupled to the MLB  9 . 3 - 910  surrounding the heat generating component  9 . 3 - 948   b  with which the housing of a fan can be thermally coupled, as shown and described with reference to  FIGS.  9 . 3 - 4   . The barrier can extend from the MLB  9 . 3 - 910  to contact a housing or other surface of the fan  9 . 3 - 912  disposed against the heat generating component  9 . 3 - 948  to define a containment volume in which the heat generating component  9 . 3 - 948   b  is disposed. In at least one example, the barrier  9 . 3 - 974  is configured to contain a thermal paste, such as the thermal paste  9 . 3 - 450  shown in  FIGS.  9 . 3 - 4   , thermally coupling the heat generating component  9 . 3 - 948  and the fan contacting the heat generating component  9 . 3 - 948   b , which is not shown but can be similar to the fan  9 . 3 - 912  shown in contact with the heat generating component  9 . 3 - 948   a . That is, during assembly, as the fan is pressed toward or against the heat generating component  9 . 3 - 948   b , the thermal paste disposed there between can expand outward but the barrier  9 . 3 - 974  contains the thermal paste and prevents it from contacting or contaminating other component of or on the MLB  9 . 3 - 910  outside the barrier  9 . 3 - 974 . In at least one example, the barrier  9 . 3 - 974  can include foam, silicone gel, other compressible materials, or other materials suitable for forming the barrier  9 . 3 - 974  as described above. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  9 . 3 - 9    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  9 . 3 - 2 - 9 . 3 - 8  and  9 . 3 - 10 - 9 . 3 - 11    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  9 . 3 - 2 - 9 . 3 - 8  and  9 . 3 - 10 - 9 . 3 - 11    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  9 . 3 - 9   . 
       FIGS.  9 . 3 - 10    illustrates a partial perspective view of an example of a fan  9 . 3 - 1012  coupled to an inner frame  9 . 3 - 1014  of an HMD device, which can be similar to the HMD  9 . 3 - 100  shown in  FIGS.  9 . 3 - 1   . In the example of  FIGS.  9 . 3 - 10   , the housing or housing shell  9 . 3 - 1036  of the fan  9 . 3 - 1012  can be coupled to the inner frame  9 . 3 - 1014  at a discrete touch point  9 . 3 - 1018  of a set of discrete touch points that minimize or reduce thermal transfer from the fan  9 . 3 - 1012  to the inner frame  9 . 3 - 1014 . In at least one example, the touch point  9 . 3 - 1018  can be an assembly including one or more insulating members or insulators  9 . 3 - 1020   a  and  9 . 3 - 1020   b  disposed between the housing shell  9 . 3 - 1036  of the fan  9 . 3 - 1012  and the inner frame  9 . 3 - 1014 . 
     In such an example, the internal/inner frame  9 . 3 - 1014  can be thermally isolated from the housing/housing shell  9 . 3 - 1036 . In at least one example, the touch point  9 . 3 - 1018  or touch point assembly can include an interface between the housing or housing shell  9 . 3 - 1036  of the fan  9 . 3 - 1012  including the insulator(s)  9 . 3 - 1020   a - b  thermally isolating the fan  9 . 3 - 1012  from the internal frame  9 . 3 - 1014 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  9 . 3 - 10    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  9 . 3 - 2 - 9 . 3 - 9  and  9 . 3 - 11    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  9 . 3 - 2 - 9 . 3 - 9  and  9 . 3 - 11    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  9 . 3 - 10   . 
       FIGS.  9 . 3 - 11    illustrates a side, cross-sectional view of an example of an HMD  9 . 3 - 1100  including an outer frame  9 . 3 - 1115  defining a first opening  9 . 3 - 1122  and a second opening  9 . 3 - 1124  and an internal volume  9 . 3 - 1126 . The outer frame  9 . 3 - 1115  can define an external surface of the HMD  9 . 3 - 1100  and be thus referred to as an external frame. In at least one example, a front cover assembly  9 . 3 - 1176  can be disposed in or across the first opening  9 . 3 - 1122  and a curtain assembly  9 . 3 - 1178  can occlude the second opening  9 . 3 - 1124 . In at least one example, the curtain assembly  9 . 3 - 1178  can include an elastic layer  9 . 3 - 1180  defining an external surface and an air-impermeable layer  9 . 3 - 1182  defining the internal volume  9 . 3 - 1126 . 
     In at least one example, the external frame  9 . 3 - 1115  can define an intake port  9 . 3 - 1128  disposed between the first and second openings  9 . 3 - 1122 ,  9 . 3 - 1124  and an exhaust port  9 . 3 - 1130  disposed between the first and second openings  9 . 3 - 1122 ,  9 . 3 - 1124 . In particular, the external frame can include an upper sidewall  9 . 3 - 1113  defining the exhaust port  9 . 3 - 1130  and a lower sidewall  9 . 3 - 1119  defining the inlet port  9 . 3 - 1128 . The lower sidewall  9 . 3 - 1119  can be disposed opposite the upper sidewall  9 . 3 - 1113 . 
     In at least one example, the HMD  9 . 3 - 1100  includes an internal or inner frame  9 . 3 - 1114  disposed in the internal volume  9 . 3 - 1126  and coupled to the external frame  9 . 3 - 1115 . The inner frame  9 . 3 - 1114  can define an aperture  9 . 3 - 1116 . The HMD  9 . 3 - 1100  can also include a fan  9 . 3 - 1112  disposed in the internal volume  9 . 3 - 1126 . The fan can include a housing defining an internal fan volume in which a fan blade assembly  9 . 3 - 1117  is disposed. In at least one example, the housing of the fan  9 . 3 - 1112  can include a first housing shell  9 . 3 - 1136  and a second housing shell  9 . 3 - 1138 . In one example, the second housing shell  9 . 3 - 1138  can define a fan inlet  9 . 3 - 1132  and the first and second housing shells  9 . 3 - 1136 ,  9 . 3 - 1138  can together define a fan outlet  9 . 3 - 1146 . 
     In at least one example, the HMD  9 . 3 - 1100  can include an MLB or PCB  9 . 3 - 1110  disposed in the internal volume  9 . 3 - 1126 . The HMD  9 . 3 - 1100  can also include one or more heat generating components  9 . 3 - 1148  coupled to the MLB  9 . 3 - 1110 . In at least one example, the first housing shell  9 . 3 - 1136  can be thermally coupled or in direct contact with the PCB  9 . 3 - 1110  and in particular with the heat generating component  9 . 3 - 1148 . In one or more examples, the second housing shell  9 . 3 - 1138  can be coupled to the inner frame  9 . 3 - 1114  and the fan inlet  9 . 3 - 1132  can be aligned with the aperture  9 . 3 - 1116  defined by the inner frame  9 . 3 - 1114  as shown. The second housing shell  9 . 3 - 1138  defining the fan inlet  9 . 3 - 1132  can face the display assembly, including the display screen  9 . 3 - 1188  of the optical module  9 . 3 - 1108 . 
     In at least one example, the HMD  9 . 3 - 1100  can include an optical module  9 . 3 - 1108  disposed in the internal volume  9 . 3 - 1126  and connected to the inner frame  9 . 3 - 1114 . In one example, the optical module  9 . 3 - 1108  is coupled to the inner frame  9 . 3 - 1114  via a bracket  9 . 3 - 1184 . In one example, the optical module  9 . 3 - 1108  can include a display assembly having a housing  9 . 3 - 1186 , a display screen  9 . 3 - 1188 , and a lens  9 . 3 - 1190  aligned with the display screen  9 . 3 - 1188 . The fan  9 . 3 - 1112  can be disposed between the display assembly of the optical module  9 . 3 - 1108  and the PCB  9 . 3 - 1110 . As shown, in at least one example, the curtain assembly  9 . 3 - 1178  can be coupled to the outer frame  9 . 3 - 1115  and occlude the second opening  9 . 3 - 1124  between the outer frame  9 . 3 - 1115  and the lens  9 . 3 - 1190 . 
     In at least one example, the fan  9 . 3 - 1112  is configured to draw air (represented by dotted arrows in  FIGS.  9 . 3 - 11   ) from an external environment into the internal volume  9 . 3 - 1126  through the intake port  9 . 3 - 1128  and push the air out from the internal volume through the exhaust port  9 . 3 - 1130 . In at least one example, the fan  9 . 3 - 1112  can be an air mover configured to draw air into the internal volume  9 . 3 - 1126  through the intake port  9 . 3 - 1128  and push the air out of the internal volume  9 . 3 - 1126  through the exhaust port  9 . 3 - 1130 . 
     In at least one example, the air inlet port  9 . 3 - 1128 , the fan inlet  9 . 3 - 1132 , the fan outlet  9 . 3 - 1146 , and the air exhaust port  9 . 3 - 1130  can define an airflow path (as indicated by the dotted arrows in  FIGS.  9 . 3 - 11   ) with the air inlet port  9 . 3 - 1128  upstream from the fan inlet  9 . 3 - 1132 , the fan inlet  9 . 3 - 1132  upstream from the fan outlet  9 . 3 - 1146  and the fan outlet  9 . 3 - 1148  upstream from the air exhaust port  9 . 3 - 1130 . The airflow path can thus be defined between the display assembly of the optical module  9 . 3 - 1108  and the fan inlet  9 . 3 - 1132 . In at least one example, the internal frame  9 . 3 - 1114  can be coupled to the external frame  9 . 3 - 1115  such that the internal frame  9 . 3 - 1114  is disposed between the display assembly, including the display screen  9 . 3 - 1188 , and the fan  9 . 3 - 1112  defining the fan inlet  9 . 3 - 1132  aligned with the aperture  9 . 3 - 1116  (or “opening”). In at least one example, the fan  9 . 3 - 1112  is coupled to the internal frame  9 . 3 - 1114  such that the internal frame  9 . 3 - 1114  is disposed between the fan  9 . 3 - 1112  and the optical module  9 . 3 - 1108  and the fan  9 . 3 - 1112  is disposed between the internal frame  9 . 3 - 1114  and the PCB  9 . 3 - 1110 , including the heat generating component  9 . 3 - 1148  thermally coupled to the fan  9 . 3 - 1112 . In at least one example, the heat generating component  9 . 3 - 1148  can include a processor or multiple processors and components of a processing unit. 
     In this way, the fan  9 . 3 - 1112  is configured to convectively cool the display assembly, including the display screen  9 . 3 - 1188  of the optical module  9 . 3 - 1108  via the air flowing in the airflow path. The fan  9 . 3 - 1112  can also be configured to conductively cool the processor  9 . 3 - 1148  via direct contact with the housing or first housing shell  9 . 3 - 1136  of the housing, as shown. The air moving through the fan  9 . 3 - 1112  via the fan blade assembly  9 . 3 - 1117  can convectively cool the first housing shell  9 . 3 - 1136  such that the processor  9 . 3 - 1148  is indirectly convectively cooled by the fan  9 . 3 - 1112 . In at least one example, the processor  9 . 3 - 1148  is disposed outside the airflow path. In one example, the metal housing shell  9 . 3 - 1136  is disposed between the processor  9 . 3 - 1148  and the airflow path. 
     The external/outer frame  9 . 3 - 1115  can be referred to as a housing or an outer housing. In at least one example, the fan  9 . 3 - 1112  is configured to draw air through the inlet port  9 . 3 - 1128 , through the aperture  9 . 3 - 1116  of the inner frame  9 . 3 - 1114  downstream from the inlet port  9 . 3 - 1128 , through the fan inlet  9 . 3 - 1132  downstream from the aperture  9 . 3 - 1116 , and through the exhaust port  9 . 3 - 1130  downstream from the fan inlet  9 . 3 - 1132 . The aperture/opening  9 . 3 - 1116  defined by the internal frame  9 . 3 - 1114  can be aligned with the display screen  9 . 3 - 1188  and the fan  9 . 3 - 1112  can be disposed between the display screen  9 . 3 - 1188  and the processor  9 . 3 - 1148 . The fan  9 . 3 - 1112  can be aligned with the aperture. In at least one example, the fan outlet  9 . 3 - 1146  is positioned, oriented, and otherwise shaped or configured to direct the air away from the processor  9 . 3 - 1148  and the PCB  9 . 3 - 1110  in general as the fan  9 . 3 - 1112  pushed the air out through the exhaust port  9 . 3 - 1130  from the internal volume  9 . 3 - 1126  to the external environment. The fan outlet  9 . 3 - 1146  is configured to direct air toward the exhaust port  9 . 3 - 1130 . 
     In at least one example, the inner frame  9 . 3 - 1114  can define a first side  9 . 3 - 1192  and a second side  9 . 3 - 1194  opposite the first side  9 . 3 - 1192 . In at least one example, the optical module  9 . 3 - 1108 , including the display screen  9 . 3 - 1188  of the display assembly can be coupled to the first side  9 . 3 - 1192  of the inner frame  9 . 3 - 1114  and be aligned with the aperture  9 . 3 - 1116  defined by the inner frame  9 . 3 - 1114 . In at least one example, the fan  9 . 3 - 1112  can be coupled to the second side  9 . 3 - 1194  of the inner frame  9 . 3 - 1114  and be aligned with the aperture  9 . 3 - 1116 . 
     In the example of the HMD  9 . 3 - 1100  shown in  FIGS.  9 . 3 - 11   , only one of the fan  9 . 3 - 1112 , optical module  9 . 3 - 1108 , aperture  9 . 3 - 1116 , MLB/PCB  9 . 3 - 1110 , and heat generating component  9 . 3 - 1148  are shown based on the cross-sectional side view of the HMD  9 . 3 - 1100 . However, it will be understood that examples of the HMD  9 . 3 - 1100  can include multiple of these components, including two fans, two optical modules, two apertures, two MLB/PCB components, and two heat generating components as illustrated in examples of other HMD devices and components illustrated in  FIGS.  9 . 3 - 1 - 9 . 3 - 10   . Any of the features described with reference to these components in  FIGS.  9 . 3 - 11    can be applied to the second of any of these components included in the HMD  9 . 3 - 1100 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  9 . 3 - 11    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  9 . 3 - 2 - 9 . 3 - 10    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  9 . 3 - 2 - 9 . 3 - 10    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  9 . 3 - 11   . 
     X: Chassis 
       FIGS.  10 . 0 - 1    illustrates a view of an example of an HMD  10 - 100  including a structural frame assembly  10 - 102 . The structural frame assembly  10 - 102  is described in more detail here in section X. 
       FIG.  10 - 1    illustrates a view of a head mountable device (HMD)  10 - 100  similar to examples described herein. The HMD  10 - 100  can include a structural frame assembly  10 - 102  referred to as a chassis or frame assembly including gone or more frame components or sub-frames coupled together. The structural frame assembly  10 - 102  can be configured to provide mechanical strength and support to one or more other components of the HMD  10 - 100  coupled thereto. Other components are described herein and can include optical modules, fan assemblies, printed circuit boards, processors and other electronic computing components, sensor, displays, screens, and any other component disclosed herein. 
     The term “structural” as used herein can include components adding to the structural rigidity, durability, shape, and strength of the HMD  10 - 100 . As shown and noted elsewhere with reference to other figures, the frame assembly  10 - 102  can define two or more apertures, for example first and second apertures  10 - 104   a ,  10 - 104   b , which align in position with corresponding fan assemblies  10 - 106   a ,  10 - 106   b , which each include a fan. The apertures  10 - 104   a ,  10 - 104   b  can also correspond in position, when assembled, to corresponding optical modules  10 - 108   a ,  10 - 108   b , which can each include a display screen. 
       FIG.  10 - 2    shows a partially assembled view of the HMD  10 - 100  shown in  FIG.  10 - 1   , including a frame assembly  10 - 202  defining first and second apertures  10 - 204   a ,  10 - 204   b . In at least one example, the frame assembly  10 - 202  can include an intermediate frame  10 - 210  defining the apertures  10 - 204   a - b . In at least one example, an optical mounting bracket  10 - 211  can be coupled to the intermediate frame  10 - 210  between the apertures  10 - 204   a ,  10 - 204   b . The display screens of the optical modules  10 - 108   a - b  shown in  FIG.  10 - 1    can be coupled to first and second cantilevered guide rods  10 - 212   a ,  10 - 212   b  of the mounting bracket  10 - 211 . The optical modules  10 - 108   a - b  are not shown in  FIG.  10 - 2    in order to illustrate the intermediate frame  10 - 210 , the mounting bracket  10 - 211 , and the cantilevered guide rods  10 - 212   a - b , but when assembled, the optical modules  10 - 108   a - b  can be coupled to respective guide rods  10 - 212   a - b  such that the first optical module  10 - 108   a  is aligned in position with the first aperture  10 - 204   a  and the second optical module  10 - 108   b  is aligned with the second aperture  10 - 204   b.    
     The optical mounting bracket  10 - 211  can also be referred to as a mounting frame or an optical mounting frame where respective display assemblies of the optical modules  10 - 108   a - b  can slidably engage the guide rods  10 - 212   a - b . In at least one example, the mounting bracket  10 - 211  can include first and second cantilever arms  10 - 214   a ,  10 - 214   b  from which the respective guide rods  10 - 212   a ,  10 - 212   b  extend. In this way, one or more optical modules, assemblies, display assemblies, display screens, and so forth can be coupled to the mounting bracket  10 - 211  to align with respective apertures  10 - 204   a ,  10 - 204   b . In this way, the fans of the fan assemblies  10 - 216   a ,  10 - 216   b  coupled to the intermediate frame  10 - 210  and aligned with the apertures  10 - 204   a - b  as shown, can convectively cool the displays or display assemblies coupled to the guide rods  10 - 212   a - b  through the apertures  10 - 204   a - b . The intermediate frame  10 - 210  can be disposed between the mounting bracket  10 - 211  and any display assemblies coupled thereto such that the display assemblies are mounted to a first side of the intermediate frame  10 - 210  and the fan assemblies  10 - 216   a - b  are mounted to a second side of the intermediate frame  10 - 210  opposite the first side. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  10 - 1  and  10 - 2    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  10 - 3 - 10 - 6    described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  10 - 3 - 10 - 6    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  10 - 1  and  10 - 2   . 
       FIG.  10 - 3    shows a perspective view of a sub-assembly of an HMD similar to the HMD  10 - 100  of  FIG.  10 - 1   . The sub-assembly shown in  FIGS.  9 . 3 - 3    includes a frame assembly  10 - 302 . The frame assembly  10 - 302  can be referred to as a chassis or a chassis assembly and include an outer frame  10 - 318  and an intermediate frame  10 - 310 . The outer frame  10 - 318  can include a first material and define an internal volume  10 - 322 . In at least one example, the intermediate frame  10 - 310  can be disposed in the internal volume  10 - 322  and be coupled to the external frame  10 - 318 . The intermediate frame  10 - 310  can include a second material different than the first material. The intermediate frame  10 - 310  can define a first aperture  10 - 304   a  and a second aperture  10 - 304   b.    
     In at least one example, the frame assembly  10 - 302  can also include an inner frame  10 - 324  coupled to the intermediate frame  10 - 310 . In at least one example, the inner frame  10 - 324  can include at third material. In one example, the third material can be different than the first and second materials of the outer/external frame  10 - 318  and the intermediate frame  10 - 310 , respectively. In at least one example, the second material of the intermediate frame  10 - 310  is stronger than the first material of the outer frame  10 - 318 . In at least one example, the third material of the inner frame  10 - 324  is stiffer than the first material of the outer frame  10 - 318  and the second material of the intermediate frame  10 - 310 . In at least one example, the inner frame  10 - 324  is stiffer due to its geometry and/or size. 
     In at least one example, the outer frame  10 - 318  can define an outer visible surface of the HMD  10 - 100  and the internal volume  10 - 322 . In at least one example, the intermediate frame  10 - 310  can be coupled to the outer frame  10 - 318  and define a first aperture  10 - 304   a  and a second aperture  10 - 304   b . In at least one example, the inner frame  10 - 324  can be coupled to the intermediate frame  10 - 310  between the first aperture  10 - 304   a  and the second aperture  10 - 304   b  on a first side  10 - 326  of the intermediate frame  10 - 324 . 
     The first material of the outer frame  10 - 318  can include any number of materials providing structure to the HMD  10 - 100  and can include aesthetically pleasing materials defining the external, visible surface of the HMD  10 - 100 . In at least one example, the first material can include aluminum, titanium, other metals, ceramics, plastics, composites, carbon fibers, and so forth. In at least one example, the second material of the intermediate frame  10 - 318  can include materials stronger than the first material. Examples of the second material can include magnesium and magnesium alloys. In at least one example the third material of the inner frame can include carbon fiber. 
     Accordingly, the first material of the outer frame  10 - 318  can be aesthetically pleasing and light-weight as well as color-customizable and so forth while the second material of the intermediate frame  10 - 310  can include a stronger, machine-able material for structural integrity and durability. The intermediate frame  10 - 310  can also thus be machined to include features compatible with other components of the HMD  10 - 100  coupling thereto. Also, the third material of the inner frame  10 - 324  can be stiffer to provide added structural strength and integrity in the center of the structure between the apertures  10 - 304   a - b  to reduce deformation of the frame assembly  10 - 302  as a whole in case of user drop events and so forth. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  10 - 3    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  10 - 1 - 10 - 2  and  10 - 4 ,  10 - 6    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  10 - 1 ,  10 - 2  and  10 - 4 ,  10 - 6    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  10 - 3   . 
       FIG.  10 - 4    in at least one example, such as the frame assembly  10 - 402  shown in  FIG.  10 - 4   , the frame assembly  10 - 402  can include an outer frame  10 - 418  and an inner frame  10 - 410 . The frame assembly  10 - 402  can be referred to as a chassis or a chassis assembly and include an outer frame  10 - 418  and an intermediate frame  10 - 410  without any inner frame coupled to the intermediate frame  10 - 410  as shown in the example of  FIG.  10 - 3    with the inner frame  10 - 324  coupled to the intermediate frame  10 - 324 . Referring back to  FIG.  10 - 4   , the outer frame  10 - 418  can include a first material and define an internal volume  10 - 422 . In at least one example, the intermediate frame  10 - 410  can be disposed in the internal volume  10 - 422  and be coupled to the external frame  10 - 418 . The intermediate frame  10 - 410  can include a second material different than the first material. The intermediate frame  10 - 410  can define a first aperture  10 - 404   a  and a second aperture  10 - 404   b.    
     In at least one example, the outer frame  10 - 418  can define an outer visible surface of the HMD  100  and the internal volume  10 - 422 . In at least one example, the intermediate frame  10 - 410  can be coupled to the outer frame  10 - 418  and define a first aperture  10 - 404   a  and a second aperture  10 - 404   b.    
     The first material of the outer frame  10 - 418  can include any number of materials providing structure to the HMD  10 - 100  and can include aesthetically pleasing materials defining the external, visible surface of the HMD  10 - 100 . In at least one example, the first material can include aluminum, titanium, other metals, ceramics, plastics, composites, carbon fibers, and so forth. In at least one example, the second material of the intermediate frame  10 - 418  can include materials stronger than the first material. Examples of the second material can include magnesium and magnesium alloys. 
     Accordingly, the first material of the outer frame  10 - 418  can be aesthetically pleasing and light-weight as well as color-customizable and so forth while the second material of the intermediate frame  10 - 410  can include a stronger, machine-able material for structural integrity and durability. The intermediate frame  10 - 410  can also thus be machined to include features compatible with other components of the HMD  10 - 100  coupling thereto. 
     In at least one example, the intermediate frame  10 - 410  can be coupled to the outer frame  10 - 418  at discrete touch points  10 - 428  which limit the heat transfer between the intermediate frame  10 - 410  and the external/outer frame  10 - 418 . In at least one example, the intermediate frame  10 - 410  is not coupled continuously around an outer edge thereof to the external/outer frame  10 - 418 . Rather, a set of discrete touch points  10 - 428  are the only physical contact between the intermediate frame  10 - 410  and the outer frame  10 - 418 . In this way, heat generated by components of the HMD  10 - 100  and transferred to the intermediate frame  10 - 410 , for example by heat generating components coupled to the intermediate frame  10 - 410 , does not substantially transfer to the user-facing, external surface defining outer frame  10 - 418  with which the user can make contact. In this way, the external/outer frame  10 - 418  can act as a thermal barrier between the user and the intermediate frame  10 - 410  and any heat generating components coupled thereto. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  10 - 4    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  10 - 1 ,  10 - 3  and  10 - 5 ,  10 - 6    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  10 - 1 ,  10 - 3  and  10 - 5 ,  10 - 6    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  10 - 4   . 
       FIG.  10 - 5    illustrates a plan view of an example of a frame assembly  10 - 402  for an HMD device including an outer frame  10 - 518 , an intermediate frame  10 - 510  coupled to the outer frame  10 - 518  and an inner frame  10 - 524  coupled to the intermediate frame  10 - 510 . In at least one example, the intermediate frame  10 - 510  can define two apertures  10 - 504   a ,  10 - 504   b  and the inner frame  10 - 524  can be coupled to the inner frame  10 - 510  centrally between the apertures  10 - 504   a - b.    
     In at least one example, the inner frame  10 - 524  can be coupled mechanically to the intermediate frame  10 - 510  via one or more fasteners or fastener assemblies. In at least one example, the inner frame  10 - 524  can be coupled to the inner frame  10 - 510  via an adhesive such as glue or epoxy. In at least one example, the inner frame  10 - 524  is glued to the intermediate frame  10 - 510  along substantially an entire surface area of the inner frame  10 - 524  facing the intermediate frame  10 - 510 . In this way, mechanical stresses are transferred from the intermediate frame  10 - 510  to substantially all of the inner frame  10 - 524  to structurally support and stiffen the intermediate frame  10 - 510  and thus the whole frame assembly  10 - 502 . In at least one example, the outer frame  10 - 518  surrounds a periphery of the intermediate frame  10 - 510 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  10 - 5    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  10 - 1 ,  10 - 4  and  10 - 6    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  10 - 1 ,  10 - 4  and  10 - 6    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  10 - 5   . 
       FIG.  10 - 6    shows a close-up, cutaway view of an example of a touch point  10 - 628  where an intermediate frame  10 - 610  is coupled to an outer frame  10 - 618 . In at least one example, the intermediate frame  10 - 610  can be coupled to the outer frame  10 - 618  at discrete connection or touch points  10 - 628  which limit the heat transfer between the intermediate frame  10 - 610  and the external/outer frame  10 - 618 . In at least one example, the intermediate frame  10 - 610  is not coupled continuously around an outer edge thereof to the external/outer frame  10 - 618 . Rather, a set of discrete touch points  10 - 628  are the only physical contact between the intermediate frame  10 - 610  and the outer frame  9 . 3 - 618 . In this way, heat generated by components of the HMD  10 - 100  and transferred to the intermediate frame  10 - 610 , for example by heat generating components coupled to the intermediate frame  10 - 610 , does not substantially transfer to the user-facing, external surface defining outer frame  10 - 618  with which the user can make contact. In this way, the external/outer frame  10 - 618  can act as a thermal barrier between the user and the intermediate frame  10 - 610  and any heat generating components coupled thereto. 
     In at least one example, each of the discrete touch points  10 - 628  where the intermediate frame  10 - 610  couples to the external/outer frame  10 - 618  includes a fastener  10 - 630  and one or more insulating members, for example a first insulating member  10 - 632  disposed between the fastener  10 - 630  and the intermediate frame  10 - 610  and a second insulating member  10 - 634  disposed between the fastener  10 - 630  and the outer frame  10 - 618 . These insulating members  10 - 632 ,  10 - 634  can be referred to as insulating interfaces or can be a part of an insulating interface to reduce heat transfer between the intermediate frame  10 - 610  and the outer frame  10 - 618 . In at least one example, the insulating members  10 - 632 ,  10 - 634  can be O-rings. In at least one example, the insulating members  10 - 632 ,  10 - 634  of the insulating interfaces at the touch points  10 - 628  can include insulating materials such as plastics, rubbers, foams, or other insulating, non-conductive materials. In at least one example, the fastener  10 - 630  can include an insulating material, such as a plastic, rubber, foam, or other non-conductive material preventing heat transfer between the intermediate frame  10 - 610  and the outer frame  10 - 618 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  10 - 6    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  10 - 1 ,  10 - 5    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  10 - 1 ,  10 - 5    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  10 - 6   . 
     XI: Optical Module 
       FIG.  11 - 1    illustrates a view of an example of an HMD  11 - 100  including an optical module system  11 - 102 . The optical module system  11 - 102  is described in more detail here in section XI. 
     11.1: IPD Adjust 
       FIGS.  11 . 1 - 1    illustrates a rear perspective view of a display assembly  11 . 1 - 102  of an HMD  11 . 1 - 100  with one or more components removed, including a curtain assembly, in order to illustrate various internal components of the HMD  11 . 1 - 100 .  FIG.  1    illustrates the display assembly  11 . 1 - 102 , which can include a first display module  11 . 1 - 104   a  and a second display module  11 . 1 - 104   b . The first display module  11 . 1 - 104   a  can be slidably engaged/coupled with a first adjustment mechanism  11 . 1 - 106   a  and the second optical module  11 . 1 - 104   b  can be slidably engaged/coupled with a second adjustment mechanism  11 . 1 - 106   b . The first and second adjustment mechanisms  11 . 1 - 106   a - b  can include first and second respective guide-rods  11 . 1 - 108   a - b  and motors  11 . 1 - 110   a - b . The first and second guide-rods can be similar such that the description of one applies to the other, with the first guide-rod  11 . 1 - 108   a  extending a first direction away from a bracket  11 . 1 - 112  and the second guide-rod  11 . 1 - 108   b  extending a second direction away from the bracket  11 . 1 - 112  opposite the first direction. 
     Likewise, the first and second motors  11 . 1 - 110   a - b  can be similar such that the description of one applies to the other, with the first motor  110   a  mechanically engaging the first optical module  11 . 1 - 104   a  to adjust the position thereof via the first guide-rod  11 . 1 - 108   a  and the second motor  11 . 1 - 110   b  mechanically engaging the second optical module  11 . 1 - 104   b  to adjust the position thereof via the second guide-rod  11 . 1 - 108   b . In at least one example, the HMD  11 . 1 - 100  also includes a depressible and/or rotatable button  11 . 1 - 114  electrically coupled to the first and second motors  11 . 1 - 110   a - b . The button  11 . 1 - 114  can electrically communicate with the first and second motors  11 . 1 - 110   a - b  via a processor or other circuitry components to cause the first and second motors  11 . 1 - 110   a - b  to activate and cause the first and second optical modules  11 . 1 - 104   a - b , respectively, to change position relative to one another. 
     In at least one example, the first and second optical modules  11 . 1 - 104   a - b  can include respective display screens configured to project light toward the user&#39;s eyes when donning the HMD  11 . 1 - 100 . In at least one example, the user can manipulate (i.e., depress and/or rotate) the button  11 . 1 - 114  to activate a positional adjustment of the optical modules  11 . 1 - 104   a - b  to match the inter-pupillary distance of the user&#39;s eyes. The optical modules  11 . 1 - 104   a - b  can also include one or more cameras or other sensors/sensor systems for imaging and measuring the IPD of the user such that the optical modules  11 . 1 - 104   a - b  can be adjusted to match the IPD. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  11 . 1 - 1    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in any other figures shown and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to any other figure shown and described herein, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  11 . 1 - 1   . 
     11.1.1: Crown 
       FIG.  11 . 1   . 1 - 1  illustrates a rear perspective view of an inter-pupillary distance (IPD) adjustment system  11 . 1 . 1 - 102  including first and second optical modules  11 . 1 . 1 - 104   a - b  slidably engaging/coupled to respective guide-rods  11 . 1 . 1 - 108   a - b  and motors  11 . 1 . 1 - 110   a - b  of left and right adjustment subsystems  11 . 1 . 1 - 106   a - b . The IPD adjustment system  11 . 1 . 1 - 102  can be coupled to a bracket  11 . 1 . 1 - 112  and include a button  11 . 1 . 1 - 114  in electrical communication with the motors  11 . 1 . 1 - 110   a - b . In at least one example, the button  11 . 1 . 1 - 114  can electrically communicate with the first and second motors  11 . 1 . 1 - 110   a - b  via a processor or other circuitry components to cause the first and second motors  11 . 1 . 1 - 110   a - b  to activate and cause the first and second optical modules  11 . 1 . 1 - 104   a - b , respectively, to change position relative to one another. 
     In at least one example, the first and second optical modules  11 . 1 . 1 - 104   a - b  can include respective display screens configured to project light toward the user&#39;s eyes when donning the HMD  11 . 1 . 1 - 100 . In at least one example, the user can manipulate (i.e., depress and/or rotate) the button  11 . 1 . 1 - 114  to activate a positional adjustment of the optical modules  11 . 1 . 1 - 104   a - b  to match the inter-pupillary distance of the user&#39;s eyes. The optical modules  11 . 1 . 1 - 104   a - b  can also include one or more cameras or other sensors/sensor systems for imaging and measuring the IPD of the user such that the optical modules  11 . 1 . 1 - 104   a - b  can be adjusted to match the IPD. 
     In one example, the user can manipulate the button  11 . 1 . 1 - 114  to cause an automatic positional adjustment of the first and second optical modules  11 . 1 . 1 - 104   a - b . In one example, the user can manipulate the button  11 . 1 . 1 - 114  to cause a manual adjustment such that the optical modules  11 . 1 . 1 - 104   a - b  move further or closer away, for example when the user rotates the button  11 . 1 . 1 - 114  one way or the other, until the user visually matches her/his own IPD. In one example, the manual adjustment is electronically communicated via one or more circuits and power for the movements of the optical modules  11 . 1 . 1 - 104   a - b  via the motors  11 . 1 . 1 - 110   a - b  is provided by an electrical power source. In one example, the adjustment and movement of the optical modules  11 . 1 . 1 - 104   a - b  via a manipulation of the button  11 . 1 . 1 - 114  is mechanically actuated via the movement of the button  11 . 1 . 1 - 114 . 
     In at least one example, the IPD adjustment system, including the button  11 . 1 . 1 - 114 , motors, guide-rods, optical modules  11 . 1 . 1 - 104   a - b , encoders, sensors, controllers, and/or other IPD adjustment components described herein, can include an automatic back-off functionality. In such an example, when the user pushes the button  11 . 1 . 1 - 104   a - b  to adjust the optical modules  11 . 1 . 1 - 104   a - b , the optical modules  11 . 1 . 1 - 104   a - b  can move closer together, for example toward a user&#39;s nose. At a predetermined/set position of the optical modules  11 . 1 . 1 - 104   a - b  during IPD adjustment, the predetermined position being detected by the encoders or otherwise determined by the controller controlling the IPD adjustment motors, the controller can cause the optical modules  11 . 1 . 1 - 104   a - b  to reverse direction and move away from one another, either after the user has released the button  11 . 1 . 1 - 114  and/or even if the user has not released the button  11 . 1 . 1 - 114 . In this way, the automatic back-off operation of the optical modules  11 . 1 . 1 - 104   a - b  during IPD adjustments can prevent the optical modules  11 . 1 . 1 - 104   a - b  from contacting or running into any facial features of the user or unintended contact against any other components of the HMD. 
     In at least one example, the automatic back-off function of the IPD adjust system can cause the optical modules  11 . 1 . 1 - 104   a - b  to back off or reverse direction so the optical modules  11 . 1 . 1 - 104   a - b  move away from one another a set distance away from where the unintended contact or facial feature interference position occurs. In at least one example, the automatic back-off function of the IPD adjust system can cause the optical modules  11 . 1 . 1 - 104   a - b  to back off or reverse direction so the optical modules  11 . 1 . 1 - 104   a - b  move away from one another to a set position predetermined to be a position where the optical modules  11 . 1 . 1 - 104   a - b  are not in any contact with user facial features or any unintended contact with other HMD components. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  11 . 1   . 1 - 1  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in any other figures shown and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to any other figure shown and described herein, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  11 . 1   . 1 - 1 . 
     11.1.1.1: Adjustment Mechanism for Head-Mounted Display 
     For improved performance, head-mounted displays require the alignment of optical components such as lenses, projection units, displays, screens, etc. to unique facial features among a variety of users in order to promote comfort, fit, performance, etc. During the alignment process, optical components can be physically repositioned in respect to other components within a head-mounted display. As packaging space is constrained within head-mounted displays, physical repositioning mechanisms should be small in size with tightly controlled motion, for example, small electro-mechanical actuators. Though small in size, the actuators should be able to withstand dynamic events and properly function after experiencing these dynamic events. Dynamic events can include shaking, jostling, dropping, impacting, or otherwise handling the head-mounted display in a manner that leads to forces being exerted upon optical components or subcomponents within the electro-mechanical actuators. 
     In an example where an actuator used to reposition optical components employs a lead screw, locking-release mechanisms that allow disengagement or slip along the lead screw can be used in conjunction with dampening mechanisms that allow controlled sliding and/or stopping along a guide rail to prevent damage both to the actuator and to the optical components that undergo motion through use of the actuator. The locking-release and dampening mechanisms can include motor-driven, lead-screw system designs that leverage linear spring-loaded nut assemblies, torsional spring-loaded plates, drop-detection-based lock engagement or disengagement, and optical-component motion dampening during dynamic events. Detailed descriptions of these and other applicable mechanisms that improve performance of physical repositioning within a head-mounted display are described herein. 
     In a first aspect, a head-mounted display includes an optical assembly and an actuator. The actuator includes a movement mechanism configured to adjust a position of an optical component within the optical assembly, a locking-release mechanism configured to modify operation of the movement mechanism upon detection of a dynamic event, and a dampening mechanism configured to control positional changes of the optical component during the dynamic event. 
     In the first aspect, the movement mechanism can be configured to adjust the position of the optical component within the optical assembly to a pre-dynamic event position in accordance with detection of an end of the dynamic event. The movement mechanism can comprise a lead screw and a nut assembly, the nut assembly configured to translate along the lead screw based on rotation of the lead screw. The nut assembly can comprise separable first and second portions, and the locking-release mechanism can include a spring that linearly biases one of the first and second portions of the nut assembly against the lead screw. The modified operation of the movement mechanism can include one of the first and second portions of the nut assembly being disengaged from the lead screw. The dampening mechanism can comprise an electromagnetic component configured to generate attraction between the nut assembly and the lead screw during the dynamic event. The movement mechanism can comprise a lead screw and a plate configured to translate along the lead screw based on rotation of the lead screw. The locking-release mechanism can include a spring that torsionally biases the plate against the lead screw. Modified operation of the movement mechanism can include the plate being disengaged from the lead screw. The dampening mechanism can comprise an electromagnetic component configured to generate attraction between the plate and the lead screw during the dynamic event. The dampening mechanism can comprise an electromagnetic component configured to repel the movement mechanism from end portions of the actuator during the dynamic event. The dampening mechanism can comprise a spring or cable configured to restrict motion of the movement mechanism during the dynamic event. The first aspect may include any combination of the features described in this paragraph. 
     In a second aspect, a method includes detecting, using a sensor in a head-mounted display, a start of a dynamic event. In accordance with detection of the start of the dynamic event and using a locking-release mechanism, the method includes modifying operation of a movement mechanism. The movement mechanism is configured to adjust a position of an optical component within an optical assembly of the head-mounted display. The method includes detecting, using the sensor in the head-mounted display, an end of the dynamic event. In accordance with detection of the end of the dynamic event and using the movement mechanism, the method includes adjusting the position of the optical component within the optical assembly to a pre-dynamic event position. 
     In the second aspect, and in accordance with detection of the start of the dynamic event and using a dampening mechanism, the method can include controlling positional changes of the movement mechanism during the dynamic event. The dampening mechanism can comprise an electromagnetic component configured to repel components within the movement mechanism during the dynamic event. The dampening mechanism can comprise an electromagnetic component configured to cause attraction between components within the movement mechanism during the dynamic event. The dampening mechanism can comprise a spring or cable configured to restrict motion of the movement mechanism during the dynamic event. The movement mechanism can comprise a lead screw and a nut assembly nut assembly configured to translate along the lead screw based on rotation of the lead screw. The nut assembly can comprise separable first and second portions. The locking-release mechanism can include a spring that linearly biases one of the first and second portions of the nut assembly against the lead screw. Modified operation of the movement mechanism can include one of the first and second portions of the nut assembly being disengaged from the lead screw. The movement mechanism can comprise a lead screw and a plate configured to translate along the lead screw based on rotation of the lead screw. The locking-release mechanism can include a spring that torsionally biases the plate against the lead screw. Modified operation of the movement mechanism can include the plate being disengaged from the lead screw. The second aspect may include any combination of the features described in this paragraph. 
     In a third aspect, an actuator includes a movement mechanism configured to adjust a position of an optical component within an optical assembly of a head-mounted display. The movement mechanism includes a lead screw and a threaded component. The threaded component is configured to translate along the lead screw based on rotation of the lead screw. The actuator also includes a locking-release mechanism configured to modify operation of the movement mechanism upon detection of a dynamic event. The locking-release mechanism includes a spring configured to bias the threaded component against the lead screw. Modified operation of the movement mechanism includes the threaded component being disengaged from the lead screw. The actuator also includes a dampening mechanism configured to control positional changes of the movement mechanism during the dynamic event. 
     In the third aspect, the dampening mechanism can comprise an electromagnetic component configured to generate attraction between the threaded component and the lead screw during the dynamic event. The dampening mechanism can comprise an electromagnetic component configured to repel the movement mechanism from end portions of the actuator during the dynamic event. The third aspect may include any combination of the features described in this paragraph. 
       FIG.  11 . 1   . 1 . 1 - 1  is a top view of a head-mounted display  11 . 1 . 1 . 1 - 100  that includes a display assembly  11 . 1 . 1 . 1 - 102  and a head support  11 . 1 . 1 . 1 - 104 . The display assembly  11 . 1 . 1 . 1 - 102  can include a housing  11 . 1 . 1 . 1 - 106  and various sub-assemblies and electronics therein, such as one or more optical assemblies  11 . 1 . 1 . 1 - 108 ,  11 . 1 . 1 . 1 - 110 , sensors  11 . 1 . 1 . 1 - 112  (e.g., detecting gaze and/or user status), power electronics  11 . 1 . 1 . 1 - 114 , and actuators  11 . 1 . 1 . 1 - 116 . Various portions or subcomponents of the optical assemblies  11 . 1 . 1 . 1 - 108 ,  11 . 1 . 1 . 1 - 110  (e.g., such as lenses, projection units, screens, or displays) can be repositionable using the one or more actuators  11 . 1 . 1 . 1 - 116 , details of which are described further herein. The optical assemblies  11 . 1 . 1 . 1 - 108 ,  11 . 1 . 1 . 1 - 110 , sensors  11 . 1 . 1 . 1 - 112 , power electronics  11 . 1 . 1 . 1 - 114 , and actuators  11 . 1 . 1 . 1 - 116  are depicted schematically and in dashed lines to illustrate being hidden from view and disposed within the display assembly  11 . 1 . 1 . 1 - 102 . 
     The head support  11 . 1 . 1 . 1 - 104  is coupled to left and right sides of the display assembly  11 . 1 . 1 . 1 - 102  (e.g., to the housing  11 . 1 . 1 . 1 - 106 ), extending rearward of the display assembly  11 . 1 . 1 . 1 - 102  and between the left and right sides thereof. When the head-mounted display  11 . 1 . 1 . 1 - 100  is worn on a head of a user, the display assembly  11 . 1 . 1 . 1 - 102  extends across a front of the head of the user (i.e., the face of the user), while the head support  11 . 1 . 1 . 1 - 104  extends rearward along left and right sides of the head of the user and across a rear of the head of the user. Thus, the display assembly  11 . 1 . 1 . 1 - 102  and the head support  11 . 1 . 1 . 1 - 104  cooperatively extend around the head of the user. The head support  11 . 1 . 1 . 1 - 104  includes a band  11 . 1 . 1 . 1 - 118  and one or more circumferential adjustment mechanisms  11 . 1 . 1 . 1 - 120  (e.g., two as shown on the left and right sides) 
     The band  11 . 1 . 1 . 1 - 118  forms a primary portion of the head support  11 . 1 . 1 . 1 - 104 , which engages the head of the user to support the head-mounted display  11 . 1 . 1 . 1 - 100  worn on the head of the user. Each of the circumferential adjustment mechanisms  11 . 1 . 1 . 1 - 120  removably couples the head support  11 . 1 . 1 . 1 - 104  to the display assembly  11 . 1 . 1 . 1 - 102  and allows the head support  11 . 1 . 1 . 1 - 104  to change length overall by changing the length between the band  11 . 1 . 1 . 1 - 118  and the circumferential adjustment mechanisms  11 . 1 . 1 . 1 - 120 . Though two circumferential adjustment mechanisms  11 . 1 . 1 . 1 - 120  are shown as acting from opposing sides of the head support  11 . 1 . 1 . 1 - 104 , the head support  11 . 1 . 1 . 1 - 104  may alternatively employ a single adjustment mechanism. 
     The band  11 . 1 . 1 . 1 - 118  can be elastic, flexible, or otherwise deformable to accommodate different shapes of heads of the users. The band  11 . 1 . 1 . 1 - 118  can be formed of or otherwise include an elastic or silicone material. The band  11 . 1 . 1 . 1 - 118  may also be configured to transfer electrical power from a power source (e.g., an external battery) to the display assembly  11 . 1 . 1 . 1 - 102 . For example, the band  11 . 1 . 1 . 1 - 118  may include a flexible circuit  11 . 1 . 1 . 1 - 122  extending from a power connector  11 . 1 . 1 . 1 - 124 , through the circumferential adjustment mechanism  11 . 1 . 1 . 1 - 120 , to a corresponding power connector (not shown) of the display assembly  11 . 1 . 1 . 1 - 102 . The flexible circuit  11 . 1 . 1 . 1 - 122  (hidden and illustrated in dash-dot lines) may be embedded in the elastic or silicone material of the band  11 . 1 . 1 . 1 - 118  or otherwise hidden from view by the material of the band  11 . 1 . 1 . 1 - 118 . In this manner, the band  11 . 1 . 1 . 1 - 118  may supply electrical current to the display assembly  11 . 1 . 1 . 1 - 102  inclusive of the optical assemblies  11 . 1 . 1 . 1 - 108 ,  11 . 1 . 1 . 1 - 110 , sensors  11 . 1 . 1 . 1 - 112 , power electronics  11 . 1 . 1 . 1 - 114 , and actuators  11 . 1 . 1 . 1 - 116 . 
       FIGS.  11 . 1   . 1 . 1 - 2 A and  11 . 1 . 1 . 1 - 2 B are detail and partially-exploded sectional views of an actuator  11 . 1 . 1 . 1 - 200 . The partially-exploded sectional view of  FIG.  11 . 1   . 1 . 1 - 2 B is representative of the location indicated in  FIG.  11 . 1   . 1 . 1 - 2 A. The actuator  11 . 1 . 1 . 1 - 200  may serve as the actuator  11 . 1 . 1 . 1 - 116  disposed in the head-mounted display  11 . 1 . 1 . 1 - 100  of  FIG.  11 . 1   . 1 . 1 - 1 . 
     The actuator  11 . 1 . 1 . 1 - 200  includes a movement mechanism formed by a motor  11 . 1 . 1 . 1 - 202 , a lead screw  11 . 1 . 1 . 1 - 204 , a guide rail  11 . 1 . 1 . 1 - 206 , and a nut assembly  11 . 1 . 1 . 1 - 208 . The nut assembly  11 . 1 . 1 . 1 - 208  is configured to translate along the lead screw  11 . 1 . 1 . 1 - 204  based on rotation of the lead screw  11 . 1 . 1 . 1 - 204  and thread engagement between the nut assembly  11 . 1 . 1 . 1 - 208  and the lead screw  11 . 1 . 1 . 1 - 204 . For example, the lead screw  11 . 1 . 1 . 1 - 204  can be rotationally driven by the motor  11 . 1 . 1 . 1 - 202  to cause translation of the nut assembly  11 . 1 . 1 . 1 - 208 . In this manner, the movement mechanism is configured to adjust a position of an optical component (e.g., a lens, a display, a projector, a screen, a lens and a display, etc., not shown) within an optical assembly (e.g., within the optical assemblies  11 . 1 . 1 . 1 - 108 ,  11 . 1 . 1 . 1 - 110  within the head-mounted display  11 . 1 . 1 . 1 - 100 ). The threads on the lead screw  11 . 1 . 1 . 1 - 204 , the threads on the nut assembly  11 . 1 . 1 . 1 - 208 , or both, can be coated to provide a longer life for the actuator  11 . 1 . 1 . 1 - 200  and to reduce friction and noise associated with operation of the actuator  11 . 1 . 1 . 1 - 200 . 
     Positional adjustment of optical component(s) can be achieved using the described movement mechanism, for example, by coupling the optical component(s) that require(s) repositioning to a portion of the nut assembly  11 . 1 . 1 . 1 - 208 , though the coupling is not shown. The movement mechanism can be used to modify optical component spacing along a focal axis or modify interpupillary distance, for example, between optical assemblies within the same head-mounted display. The nut assembly  11 . 1 . 1 . 1 - 208  can be designed to avoid unintended rotation, for example, by use of the guide rail  11 . 1 . 1 . 1 - 206  extending parallel to the lead screw  11 . 1 . 1 . 1 - 204 , though other anti-rotation features in the actuator  11 . 1 . 1 . 1 - 200  are also possible. When the guide rail  11 . 1 . 1 . 1 - 206  is used as shown in  FIGS.  11 . 1   . 1 . 1 - 2 A and  11 . 1 . 1 . 1 - 2 B, the nut assembly  11 . 1 . 1 . 1 - 208  translates along both the lead screw  11 . 1 . 1 . 1 - 204  and the guide rail  11 . 1 . 1 . 1 - 206  based on rotation of the lead screw  11 . 1 . 1 . 1 - 204  as driven by the motor  11 . 1 . 1 . 1 - 202 . 
     The actuator  11 . 1 . 1 . 1 - 200  also includes a locking-release mechanism formed by separable halves or portions  11 . 1 . 1 . 1 - 210 ,  11 . 1 . 1 . 1 - 212  of the nut assembly  11 . 1 . 1 . 1 - 208  and one or more springs  11 . 1 . 1 . 1 - 214  that can linearly bias the portion  11 . 1 . 1 . 1 - 212  against the lead screw  11 . 1 . 1 . 1 - 204 , against the guide rail  11 . 1 . 1 . 1 - 206 , and against the portion  11 . 1 . 1 . 1 - 210  of the nut assembly  11 . 1 . 1 . 1 - 208  as is best seen in the partially-exploded sectional view of  FIG.  11 . 1   . 1 . 1 - 2 B. In this example, the nut assembly  11 . 1 . 1 . 1 - 208  has a split-nut configuration with the portion  11 . 1 . 1 . 1 - 212  of the nut assembly  11 . 1 . 1 . 1 - 208  configured to translate away from the portion  11 . 1 . 1 . 1 - 210  of the nut assembly  11 . 1 . 1 . 1 - 208  when a load experienced is sufficient to overcome a load generated by the springs  11 . 1 . 1 . 1 - 214  against the portion  11 . 1 . 1 . 1 - 212  of the nut assembly  11 . 1 . 1 . 1 - 208 . Sufficient loads of this type can occur during dynamic events (e.g., shaking, jostling, dropping, or impacting a head-mounted display that includes the actuator  11 . 1 . 1 . 1 - 200 ). 
     During a dynamic event that generates sufficient force, the threads on the lead screw  11 . 1 . 1 . 1 - 204  and the threads on the portions  11 . 1 . 1 . 1 - 210 ,  11 . 1 . 1 . 1 - 212  can be disengaged, and the nut assembly  11 . 1 . 1 . 1 - 208  becomes free to slide along the lead screw  11 . 1 . 1 . 1 - 204  and the guide rail  11 . 1 . 1 . 1 - 206 , thus limiting or avoiding damage to the nut assembly  11 . 1 . 1 . 1 - 208  and/or to the lead screw  11 . 1 . 1 . 1 - 204  during the dynamic event based on operation of the described locking-release mechanism. After a dynamic event, when sufficient loads are no longer experienced by the actuator  11 . 1 . 1 . 1 - 200 , the threads on the lead screw  11 . 1 . 1 . 1 - 204  and the threads on the portions  11 . 1 . 1 . 1 - 210 ,  11 . 1 . 1 . 1 - 212  of the nut assembly  11 . 1 . 1 . 1 - 208  can become reengaged, and the nut assembly  11 . 1 . 1 . 1 - 208  can once again be driven along the lead screw  11 . 1 . 1 . 1 - 204  by the motor. 
     Upon reengagement, the nut assembly  11 . 1 . 1 . 1 - 208  may be in a different position along the lead screw  11 . 1 . 1 . 1 - 204  than prior to or during the dynamic event. In this case, the movement mechanism may be configured to adjust the position of the nut assembly  11 . 1 . 1 . 1 - 208 , and in turn, any optical components coupled or otherwise connected to the nut assembly  11 . 1 . 1 . 1 - 208 , to a pre-dynamic event position, that is, to a position that was documented for the nut assembly  11 . 1 . 1 . 1 - 208  in respect to the lead screw  11 . 1 . 1 . 1 - 204  prior to the dynamic event. Though both portions  11 . 1 . 1 . 1 - 210 ,  11 . 1 . 1 . 1 - 212  of the nut assembly  11 . 1 . 1 . 1 - 208  are shown as being threaded at the interface to the lead screw  11 . 1 . 1 . 1 - 204 , in some examples, only the portion  11 . 1 . 1 . 1 - 212  may include threads without impacting operation of the actuator  11 . 1 . 1 . 1 - 200 . 
     To effectively implement the locking-release mechanism of the actuator  11 . 1 . 1 . 1 - 200 , a position sensor (not shown) can be used to determine a position of the nut assembly  11 . 1 . 1 . 1 - 208  in respect to the lead screw  11 . 1 . 1 . 1 - 204  before, during, and after a dynamic event. An accelerometer, inertial measurement unit, or any other sensor (not shown) can be used to determine a beginning, duration, and end of the dynamic event. In another example, a magnetic encoder system can include a sensor that determines a landing position of the portions  11 . 1 . 1 . 1 - 210 ,  11 . 1 . 1 . 1 - 212  of the nut assembly  11 . 1 . 1 . 1 - 208  in respect to the lead screw  11 . 1 . 1 . 1 - 204  (or other portion of the actuator  11 . 1 . 1 . 1 - 200 ), and a controller can perform a recalibration, repositioning the portions  11 . 1 . 1 . 1 - 210 ,  11 . 1 . 1 . 1 - 212  of the nut assembly  11 . 1 . 1 . 1 - 208  to pre-dynamic event positions along the lead screw  11 . 1 . 1 . 1 - 204  after the dynamic event. 
       FIGS.  11 . 1   . 1 . 1 - 3 A and  11 . 1 . 1 . 1 - 3 B are detail and partially-exploded sectional views of another actuator  11 . 1 . 1 . 1 - 300 . The partially-exploded sectional view of  FIG.  11 . 1   . 1 . 1 - 3 B is representative of the location indicated in  FIG.  11 . 1   . 1 . 1 - 3 A. The actuator  11 . 1 . 1 . 1 - 300  may serve as the actuator  11 . 1 . 1 . 1 - 116  disposed in the head-mounted display  11 . 1 . 1 . 1 - 100  of  FIG.  11 . 1   . 1 . 1 - 1  and may be similar to the actuator  11 . 1 . 1 . 1 - 200  described in respect to  FIGS.  11 . 1   . 1 . 1 - 2 A and  11 . 1 . 1 . 1 - 2 B. Differences between the actuator  11 . 1 . 1 . 1 - 300  and the actuator  11 . 1 . 1 . 1 - 200  are highlighted herein. 
     The actuator  11 . 1 . 1 . 1 - 300  includes a movement mechanism formed by a motor  11 . 1 . 1 . 1 - 302 , a lead screw  11 . 1 . 1 . 1 - 304 , a guide rail  11 . 1 . 1 . 1 - 306 , and a nut assembly  11 . 1 . 1 . 1 - 308 . The nut assembly  11 . 1 . 1 . 1 - 308  is configured to translate along the lead screw  11 . 1 . 1 . 1 - 304  and the guide rail  11 . 1 . 1 . 1 - 306  based on rotation of the lead screw  11 . 1 . 1 . 1 - 304  and thread engagement between the nut assembly  11 . 1 . 1 . 1 - 308  and the lead screw  11 . 1 . 1 . 1 - 304 . The nut assembly  11 . 1 . 1 . 1 - 308  can be designed to avoid unintended rotation, for example, by use of the guide rail  11 . 1 . 1 . 1 - 306  extending parallel to the lead screw  11 . 1 . 1 . 1 - 304 , though other anti-rotation features in the actuator  11 . 1 . 1 . 1 - 300  are also possible. 
     The actuator  11 . 1 . 1 . 1 - 300  also includes a locking-release mechanism formed by separable portions  11 . 1 . 1 . 1 - 310 ,  11 . 1 . 1 . 1 - 311 ,  11 . 1 . 1 . 1 - 312  of the nut assembly  11 . 1 . 1 . 1 - 308  and one or more springs  11 . 1 . 1 . 1 - 314  that can linearly bias the portion  11 . 1 . 1 . 1 - 312  against the guide rail  11 . 1 . 1 . 1 - 306  and the central-most portion  11 . 1 . 1 . 1 - 311  of the nut assembly  11 . 1 . 1 . 1 - 308 . In turn, the portion  11 . 1 . 1 . 1 - 311  can be linearly biased against the lead screw  11 . 1 . 1 . 1 - 304  and the portion  11 . 1 . 1 . 1 - 310  of the nut assembly  11 . 1 . 1 . 1 - 308  as is best seen in the partially-exploded sectional view of  FIG.  11 . 1   . 1 . 1 - 3 B. In this example, the nut assembly  11 . 1 . 1 . 1 - 308  has three portions  11 . 1 . 1 . 1 - 310 ,  11 . 1 . 1 . 1 - 311 ,  11 . 1 . 1 . 1 - 312 , with the portions  11 . 1 . 1 . 1 - 311 ,  11 . 1 . 1 . 1 - 312  of the nut assembly  11 . 1 . 1 . 1 - 208  configured to translate away and become disengaged from the portion  11 . 1 . 1 . 1 - 310  of the nut assembly  11 . 1 . 1 . 1 - 308  when a load experienced is sufficient to overcome a load generated by the springs  11 . 1 . 1 . 1 - 314  against the portion  11 . 1 . 1 . 1 - 312  of the nut assembly  11 . 1 . 1 . 1 - 208 . Sufficient loads of this type can occur during dynamic events (e.g., shaking, jostling, dropping, or impacting a head-mounted display that includes the actuator  11 . 1 . 1 . 1 - 300 ). 
     The actuator  11 . 1 . 1 . 1 - 300  of  FIGS.  11 . 1   . 1 . 1 - 3 A and  11 . 1 . 1 . 1 - 3 B differs from the actuator  11 . 1 . 1 . 1 - 200  of  FIGS.  11 . 1   . 1 . 1 - 2 A and  11 . 1 . 1 . 1 - 2 B in that the nut assembly  11 . 1 . 1 . 1 - 308  is used in place of the nut assembly  11 . 1 . 1 . 1 - 208 . The actuator  11 . 1 . 1 . 1 - 300  of  FIGS.  11 . 1   . 1 . 1 - 3 A and  11 . 1 . 1 . 1 - 3 B may be useful in vertically tight packaging spaces, that is, the actuator  11 . 1 . 1 . 1 - 300  may be shorter in height than the actuator  11 . 1 . 1 . 1 - 200  due to the construction of the nut assemblies  11 . 1 . 1 . 1 - 208 ,  11 . 1 . 1 . 1 - 308 . In comparison, the actuator  11 . 1 . 1 . 1 - 200  of  FIGS.  11 . 1   . 1 . 1 - 2 A and  11 . 1 . 1 . 1 - 2 B may be useful in horizontally tight packaging spaces, that is, the actuator  11 . 1 . 1 . 1 - 200  may be narrower in width than the actuator  11 . 1 . 1 . 1 - 300  due to the construction of the nut assemblies  11 . 1 . 1 . 1 - 208 ,  11 . 1 . 1 . 1 - 308 . 
       FIGS.  11 . 1   . 1 . 1 - 4 A and  11 . 1 . 1 . 1 - 4 B are detail and partially-exploded sectional views of another actuator  11 . 1 . 1 . 1 - 400 . The partially-exploded sectional view of  FIG.  11 . 1   . 1 . 1 - 4 B is representative of the location indicated in  FIG.  11 . 1   . 1 . 1 - 4 A. The actuator  11 . 1 . 1 . 1 - 400  may serve as the actuator  11 . 1 . 1 . 1 - 116  disposed in the head-mounted display  11 . 1 . 1 . 1 - 100  of  FIG.  11 . 1   . 1 . 1 - 1  and may be similar to the actuators  11 . 1 . 1 . 1 - 200 ,  11 . 1 . 1 . 1 - 300  described in respect to  FIGS.  11 . 1   . 1 . 1 - 2 A,  11 . 1 . 1 . 1 - 2 B,  11 . 1 . 1 . 1 - 3 A, and  11 . 1 . 1 . 1 - 3 B. Differences between the actuator  11 . 1 . 1 . 1 - 400  and the actuators  11 . 1 . 1 . 1 - 200 ,  11 . 1 . 1 . 1 - 300  are highlighted herein. 
     The actuator  11 . 1 . 1 . 1 - 400  includes a movement mechanism formed by a motor  11 . 1 . 1 . 1 - 402 , a lead screw  11 . 1 . 1 . 1 - 404 , a guide rail  11 . 1 . 1 . 1 - 406 , and a torsion assembly  11 . 1 . 1 . 1 - 408 . The torsion assembly  11 . 1 . 1 . 1 - 408  is configured to translate along the lead screw  11 . 1 . 1 . 1 - 404  and the guide rail  11 . 1 . 1 . 1 - 406  based on rotation of the lead screw  11 . 1 . 1 . 1 - 404  and thread engagement between the torsion assembly  11 . 1 . 1 . 1 - 408  and the lead screw  11 . 1 . 1 . 1 - 404 . The torsion assembly  11 . 1 . 1 . 1 - 408  can be designed to avoid unintended rotation, for example, by use of the guide rail  11 . 1 . 1 . 1 - 406  extending through the torsion assembly  11 . 1 . 1 . 1 - 408 , though other anti-rotation features in the actuator  11 . 1 . 1 . 1 - 400  are also possible. 
     The torsion assembly  11 . 1 . 1 . 1 - 408  includes a plate  11 . 1 . 1 . 1 - 410 , a support  11 . 1 . 1 . 1 - 412 , and a torsional spring  11 . 1 . 1 . 1 - 414  that can torsionally bias the plate  11 . 1 . 1 . 1 - 410  against the lead screw  11 . 1 . 1 . 1 - 404  to form a locking-release mechanism of the actuator  11 . 1 . 1 . 1 - 400 . In this example, the torsional spring  11 . 1 . 1 . 1 - 414  acts against both the plate  11 . 1 . 1 . 1 - 410  and the support  11 . 1 . 1 . 1 - 412  such that the plate  11 . 1 . 1 . 1 - 410  is configured to rotate away and become disengaged from the lead screw  11 . 1 . 1 . 1 - 404  when a load experienced is sufficient to overcome a load generated by the spring  11 . 1 . 1 . 1 - 414  against the plate  11 . 1 . 1 . 1 - 410  and the support  11 . 1 . 1 . 1 - 412 . Sufficient loads of this type can occur during dynamic events (e.g., shaking, jostling, dropping, or impacting a head-mounted display that includes the actuator  11 . 1 . 1 . 1 - 400 ). 
     The actuator  11 . 1 . 1 . 1 - 400  of  FIGS.  11 . 1   . 1 . 1 - 4 A and  11 . 1 . 1 . 1 - 4 B differs from the actuators  11 . 1 . 1 . 1 - 200 ,  11 . 1 . 1 . 1 - 300  of  FIGS.  11 . 1   . 1 . 1 - 2 A,  11 . 1 . 1 . 1 - 2 B,  11 . 1 . 1 . 1 - 3 A, and  11 . 1 . 1 . 1 - 3 B in that the torsion assembly  11 . 1 . 1 . 1 - 408  with the rotatable plate  11 . 1 . 1 . 1 - 410  is used in place of the nut assemblies  11 . 1 . 1 . 1 - 208 ,  11 . 1 . 1 . 1 - 308 . Use of the actuator  11 . 1 . 1 . 1 - 400  of  FIGS.  11 . 1   . 1 . 1 - 4 A and  11 . 1 . 1 . 1 - 4 B can support unique positional relationships between the lead screw  11 . 1 . 1 . 1 - 404  and the guide rail  11 . 1 . 1 . 1 - 406  since these components can be both vertically and horizontally offset while remaining generally parallel. Further, there is less physical engagement between the plate  11 . 1 . 1 . 1 - 410  and the lead screw  11 . 1 . 1 . 1 - 404  than is present between the nut assemblies  11 . 1 . 1 . 1 - 208 ,  11 . 1 . 1 . 1 - 308  and the lead screws  11 . 1 . 1 . 1 - 204 ,  11 . 1 . 1 . 1 - 304 , reducing for example, friction-based and wear-based failure potential. 
     The actuators  11 . 1 . 1 . 1 - 200 ,  11 . 1 . 1 . 1 - 300 ,  11 . 1 . 1 . 1 - 400  described in respect to  FIGS.  11 . 1   . 1 . 1 - 2 A,  11 . 1 . 1 . 1 - 2 B,  11 . 1 . 1 . 1 - 3 A,  11 . 1 . 1 . 1 - 3 B,  11 . 1 . 1 . 1 - 4 A, and  11 . 1 . 1 . 1 - 4 B include split-nut and torsional-type movement mechanisms using lead screws  11 . 1 . 1 . 1 - 204 ,  11 . 1 . 1 . 1 - 304 ,  11 . 1 . 1 . 1 - 404 . Additional designs for movement mechanisms can include living-hinge nut assemblies and back-driveable lead screws (not shown). For example, a living-hinge nut assembly can be separated along a central axis with coupled top and bottom edges such that the sides of the nut assembly disengage from the lead screw under sufficient force while the top and bottom edges remain coupled. In another example, a back-driveable lead screw can be controlled to reduce forces experienced during a dynamic event by enabling back-driving during the dynamic event. In addition, movement mechanisms may use rack-and-pinion, pneumatic, hydraulic, piezo-actuated, and electromagnetic designs (not shown) to achieve positioning and repositioning of optical components within optical assemblies. 
     A consequence of using the locking-release mechanisms described in respect to  FIGS.  11 . 1   . 1 . 1 - 2 A,  11 . 1 . 1 . 1 - 2 B,  11 . 1 . 1 . 1 - 3 A,  11 . 1 . 1 . 1 - 3 B,  11 . 1 . 1 . 1 - 4 A, and  11 . 1 . 1 . 1 - 4 B is that the nut assemblies  11 . 1 . 1 . 1 - 208 ,  11 . 1 . 1 . 1 - 308  and torsion assembly  11 . 1 . 1 . 1 - 408  can become generally free-sliding along the guide rails  11 . 1 . 1 . 1 - 206 ,  11 . 1 . 1 . 1 - 306 ,  11 . 1 . 1 . 1 - 406  during a dynamic event sufficient to disengage the nut assemblies  11 . 1 . 1 . 1 - 208 ,  11 . 1 . 1 . 1 - 308  and torsion assembly  11 . 1 . 1 . 1 - 408  from the lead screws  11 . 1 . 1 . 1 - 204 ,  11 . 1 . 1 . 1 - 304 ,  11 . 1 . 1 . 1 - 404 . To avoid damage to optical components having motion controlled by the actuators  11 . 1 . 1 . 1 - 200 ,  11 . 1 . 1 . 1 - 200 ,  11 . 1 . 1 . 1 - 400 , various dampening mechanisms can be implemented to control positional changes of the otherwise free-sliding optical components during dynamic events. 
       FIGS.  11 . 1   . 1 . 1 - 5 A and  11 . 1 . 1 . 1 - 5 B are detail views of magnetic dampening mechanisms for an actuator  11 . 1 . 1 . 1 - 500  similar to the actuators  11 . 1 . 1 . 1 - 200 ,  11 . 1 . 1 . 1 - 300 ,  11 . 1 . 1 . 1 - 400  of  FIGS.  11 . 1   . 1 . 1 - 2 A,  11 . 1 . 1 . 1 - 2 B,  11 . 1 . 1 . 1 - 3 A,  11 . 1 . 1 . 1 - 3 B,  11 . 1 . 1 . 1 - 4 A, and  11 . 1 . 1 . 1 - 4 B. The actuator  11 . 1 . 1 . 1 - 500  may serve as the actuator  11 . 1 . 1 . 1 - 116  disposed in the head-mounted display  11 . 1 . 1 . 1 - 100  of  FIG.  11 . 1   . 1 . 1 - 1 . The actuator  11 . 1 . 1 . 1 - 500  includes a movement mechanism formed by a motor  11 . 1 . 1 . 1 - 502 , a lead screw  11 . 1 . 1 . 1 - 504 , a guide rail  11 . 1 . 1 . 1 - 506 , and a nut assembly  11 . 1 . 1 . 1 - 508 . The nut assembly  11 . 1 . 1 . 1 - 508  is configured to translate along the lead screw  11 . 1 . 1 . 1 - 504  and the guide rail  11 . 1 . 1 . 1 - 506  based on rotation of the lead screw  11 . 1 . 1 . 1 - 504  and thread engagement between the nut assembly  11 . 1 . 1 . 1 - 508  and the lead screw  11 . 1 . 1 . 1 - 504 . 
     Locking-release mechanisms similar to those described in reference to  FIGS.  11 . 1   . 1 . 1 - 2 A,  11 . 1 . 1 . 1 - 2 B,  11 . 1 . 1 . 1 - 3 A, and  11 . 1 . 1 . 1 - 3 B, can cause the nut assembly  11 . 1 . 1 . 1 - 508  to become disengaged from the lead screw  11 . 1 . 1 . 1 - 504 , for example, during a dynamic event. To minimize free sliding of the nut assembly  11 . 1 . 1 . 1 - 508 , the actuator  11 . 1 . 1 . 1 - 500  can include an electromagnetic dampening mechanism formed by an electromagnetic component  11 . 1 . 1 . 1 - 516  disposed within or forming a portion of the nut assembly  11 . 1 . 1 . 1 - 508  as shown in dotted line in  FIGS.  11 . 1   . 1 . 1 - 5 A and  11 . 1 . 1 . 1 - 5 B. 
     Referring to  FIG.  11 . 1   . 1 . 1 - 5 A, the electromagnetic dampening mechanism can include electromagnetic components  11 . 1 . 1 . 1 - 518 ,  11 . 1 . 1 . 1 - 520  disposed at ends of the guide rail  11 . 1 . 1 . 1 - 506  as shown in dotted line for hidden representation. In this embodiment, the electromagnetic components  11 . 1 . 1 . 1 - 518 ,  11 . 1 . 1 . 1 - 520  can be controlled to actuate and repel the electromagnetic component  11 . 1 . 1 . 1 - 516  during a dynamic event, thus serving as cushions or bumpers to slow or prohibit the nut assembly  11 . 1 . 1 . 1 - 508  from impacting the ends of the guide rail  11 . 1 . 1 . 1 - 506  when sliding. In other embodiments, the nut assembly  11 . 1 . 1 . 1 - 508  may include portions formed of ferritic or other material susceptible to magnetic repulsion, and the nut assembly  11 . 1 . 1 . 1 - 508  may be repelled by the electromagnetic components  11 . 1 . 1 . 1 - 518 ,  11 . 1 . 1 . 1 - 520  during dynamic events. 
     Referring to  FIG.  11 . 1   . 1 . 1 - 5 B, the electromagnetic dampening mechanism can include electromagnetic component  11 . 1 . 1 . 1 - 522  that runs the length of the guide rail  11 . 1 . 1 . 1 - 506  as shown in dotted line. In this embodiment, the electromagnetic component  11 . 1 . 1 . 1 - 522  can be controlled to actuate and attract the electromagnetic component  11 . 1 . 1 . 1 - 516  during a dynamic event, thus serving to slow motion of the nut assembly  11 . 1 . 1 . 1 - 508  during the dynamic event due to attraction between the electromagnetic components  11 . 1 . 1 . 1 - 516 ,  11 . 1 . 1 . 1 - 522 . In other embodiments, the guide rail  11 . 1 . 1 . 1 - 506  may be formed of ferritic or other material susceptible to magnetic attraction, and the electromagnetic component  11 . 1 . 1 . 1 - 516  may be attracted to the guide rail  11 . 1 . 1 . 1 - 506  during dynamic events. 
     Other magnetic dampening mechanisms are also possible. For example, the electromagnetic components  11 . 1 . 1 . 1 - 516 ,  11 . 1 . 1 . 1 - 518 ,  11 . 1 . 1 . 1 - 520 ,  11 . 1 . 1 . 1 - 522  may be designed to repel in non-powered states and attract in powered states (or vice-versa), and some of the electromagnetic components  11 . 1 . 1 . 1 - 516 ,  11 . 1 . 1 . 1 - 518 ,  11 . 1 . 1 . 1 - 520 ,  11 . 1 . 1 . 1 - 522  may be replaced with non-powered ferritic or other magnetically-susceptible materials without changing the overall function of the dampening mechanism. 
       FIGS.  11 . 1   . 1 . 1 - 6 A and  11 . 1 . 1 . 1 - 6 B are detail views of mechanical dampening mechanisms for an actuator  11 . 1 . 1 . 1 - 600  similar to the actuators  11 . 1 . 1 . 1 - 200 ,  11 . 1 . 1 . 1 - 300 ,  11 . 1 . 1 . 1 - 500  of  FIGS.  11 . 1   . 1 . 1 - 2 A,  11 . 1 . 1 . 1 - 2 B,  11 . 1 . 1 . 1 - 3 A,  11 . 1 . 1 . 1 - 3 B,  11 . 1 . 1 . 1 - 5 A, and  11 . 1 . 1 . 1 - 5 B. The actuator  11 . 1 . 1 . 1 - 600  may serve as the actuator  11 . 1 . 1 . 1 - 116  disposed in the head-mounted display  11 . 1 . 1 . 1 - 100  of  FIG.  11 . 1   . 1 . 1 - 1 . The actuator  11 . 1 . 1 . 1 - 600  includes a movement mechanism formed by a motor  11 . 1 . 1 . 1 - 602 , a lead screw  11 . 1 . 1 . 1 - 604 , a guide rail  11 . 1 . 1 . 1 - 606 , and a nut assembly  11 . 1 . 1 . 1 - 608 . The nut assembly  11 . 1 . 1 . 1 - 608  is configured to translate along the lead screw  11 . 1 . 1 . 1 - 604  and the guide rail  11 . 1 . 1 . 1 - 606  based on rotation of the lead screw  11 . 1 . 1 . 1 - 604  and thread engagement between the nut assembly  11 . 1 . 1 . 1 - 608  and the lead screw  11 . 1 . 1 . 1 - 604 . 
     Locking-release mechanisms similar to those described in reference to  FIGS.  11 . 1   . 1 . 1 - 2 A,  11 . 1 . 1 . 1 - 2 B,  11 . 1 . 1 . 1 - 3 A, and  11 . 1 . 1 . 1 - 3 B can cause the nut assembly  11 . 1 . 1 . 1 - 608  to become disengaged from the lead screw  11 . 1 . 1 . 1 - 604 , for example, during a dynamic event. To minimize free sliding of the nut assembly  11 . 1 . 1 . 1 - 608 , the actuator  11 . 1 . 1 . 1 - 600  can include a mechanical dampening mechanism such as those described in reference to  FIGS.  11 . 1   . 1 . 1 - 6 A and  11 . 1 . 1 . 1 - 6 B. 
     Referring to  FIG.  11 . 1   . 1 . 1 - 6 A, the mechanical dampening mechanism can include one or more cables  11 . 1 . 1 . 1 - 626 ,  11 . 1 . 1 . 1 - 628  (two are shown in solid and dotted line) disposed proximate to ends of the lead screw  11 . 1 . 1 . 1 - 604  and the guide rail  11 . 1 . 1 . 1 - 606 . In this embodiment, the cables  11 . 1 . 1 . 1 - 626 ,  11 . 1 . 1 . 1 - 628  can either wind or unwind during a dynamic event, serving to slow or counteract motion of the nut assembly  11 . 1 . 1 . 1 - 608  to avoid having the nut assembly  11 . 1 . 1 . 1 - 608  impact the ends of the lead screw  11 . 1 . 1 . 1 - 604  and guide rail  11 . 1 . 1 . 1 - 606  when sliding. For example, the cables  11 . 1 . 1 . 1 - 626 ,  11 . 1 . 1 . 1 - 628  can be engaged under high torque conditions, or the cables  11 . 1 . 1 . 1 - 626 ,  11 . 1 . 1 . 1 - 628  can be designed with a predetermined amount of unwinding or stretching that occurs before stopping movement of the nut assembly  11 . 1 . 1 . 1 - 608 . 
     Referring to  FIG.  11 . 1   . 1 . 1 - 6 B, the mechanical dampening mechanism can include one or more springs  11 . 1 . 1 . 1 - 630 ,  11 . 1 . 1 . 1 - 632  (two are shown) disposed proximate to ends of the lead screw  11 . 1 . 1 . 1 - 604  and the guide rail  11 . 1 . 1 . 1 - 606 . In this embodiment, the springs  11 . 1 . 1 . 1 - 630 ,  11 . 1 . 1 . 1 - 632  can compress or expand during a dynamic event, serving to slow or counteract motion of the nut assembly  11 . 1 . 1 . 1 - 608  to avoid having the nut assembly  11 . 1 . 1 . 1 - 608  impact the ends of the lead screw  11 . 1 . 1 . 1 - 604  and guide rail  11 . 1 . 1 . 1 - 606  when sliding. In another example, the springs  11 . 1 . 1 . 1 - 630 ,  11 . 1 . 1 . 1 - 632  can be coupled to bumpers or electromagnetic components in order to combine mechanical and electromagnetic dampening means (not shown). 
       FIG.  11 . 1   . 1 . 1 - 7  is a flowchart depicting a process  11 . 1 . 1 . 1 - 700  of operation for an actuator similar to the actuators  11 . 1 . 1 . 1 - 200 ,  11 . 1 . 1 . 1 - 300 ,  11 . 1 . 1 . 1 - 400 ,  11 . 1 . 1 . 1 - 500 ,  11 . 1 . 1 . 1 - 600  of  FIGS.  11 . 1   . 1 . 1 - 2 A,  11 . 1 . 1 . 1 - 2 B,  11 . 1 . 1 . 1 - 3 A,  11 . 1 . 1 . 1 - 3 B,  11 . 1 . 1 . 1 - 4 A,  11 . 1 . 1 . 1 - 4 B,  11 . 1 . 1 . 1 - 5 A,  11 . 1 . 1 . 1 - 5 B,  11 . 1 . 1 . 1 - 6 A, and  11 . 1 . 1 . 1 - 6 B disposed within a head-mounted display similar to the head-mounted display  11 . 1 . 1 . 1 - 100  of  FIG.  11 . 1   . 1 . 1 - 1 . 
     At  11 . 1 . 1 . 1 - 710 , the process  11 . 1 . 1 . 1 - 700  includes detecting a start of a dynamic event, for example, using a sensor such as a position sensor, an accelerometer, an inertial measurement unit, or other sensor within or associated with the head-mounted display. The dynamic event can include events such as shaking, jostling, dropping, impacting, or otherwise handling the head-mounted display in a manner that leads to forces above a disengagement threshold being exerted upon components of the actuator and optical components of an optical assembly. 
     At  11 . 1 . 1 . 1 - 720 , the process  11 . 1 . 1 . 1 - 700  includes activating a locking-release mechanism such as the locking-release mechanisms described in reference to the actuators  11 . 1 . 1 . 1 - 200 ,  11 . 1 . 1 . 1 - 300 ,  11 . 1 . 1 . 1 - 400 . The locking-release mechanism can be activated in accordance with detection of the start of the dynamic event, for example, based on or associated with detection of the start of the dynamic event. The locking-release mechanism can modify operation of the movement mechanism within the associated actuator. The movement mechanism can include a lead screw and a nut assembly or a torsion assembly. Modifying operation of the movement mechanism can include disengaging the nut assembly (e.g., the nut assemblies  11 . 1 . 1 . 1 - 208 ,  11 . 1 . 1 . 1 - 308 ) or the torsion assembly (e.g., the torsion assembly  11 . 1 . 1 . 1 - 408 ) from the lead screw (e.g., the lead screws  11 . 1 . 1 . 1 - 204 ,  11 . 1 . 1 . 1 - 304 ,  11 . 1 . 1 . 1 - 404 ). 
     At  11 . 1 . 1 . 1 - 730 , shown in dotted line to represent its optional nature, the process  11 . 1 . 1 . 1 - 700  includes activating a dampening mechanism such as the dampening mechanisms described in reference to the actuators  11 . 1 . 1 . 1 - 500 ,  11 . 1 . 1 . 1 - 600 . The dampening mechanism can be activated in accordance with detection of the start of the dynamic event. In another example, the dampening mechanism can be activated in accordance with detected motion of the movement mechanism that surpasses a predetermined threshold speed or distance, indicating that a dynamic event is ongoing. The dampening mechanism can include electromagnetic components susceptible to attraction or repulsion or mechanical components such as springs and cables. 
     At  11 . 1 . 1 . 1 - 740 , the process  11 . 1 . 1 . 1 - 700  includes detecting an end of the dynamic event, for example, using a sensor such as a position sensor, an accelerometer, an inertial measurement unit, or other sensor within or associated with the head-mounted display. The end of the dynamic event can also be detected based on detecting arrested motion of components within a movement mechanism that were previously in motion during the dynamic event. 
     At  11 . 1 . 1 . 1 - 750 , shown in dotted line to represent its optional nature, the process  11 . 1 . 1 . 1 - 700  includes adjusting a position of an optical component within an optical assembly to a pre-dynamic event position. The movement mechanism can be used to move, for example, a nut assembly or a torsion assembly within an actuator from its current post-dynamic event position to the pre-dynamic event position. Movement of the optical component is tied to movement of the movement mechanism in this example. Repositioning supports improved function of the overall head-mounted display as the user will not be required to readjust optical components within optical assemblies after a dynamic event. 
       FIG.  11 . 1   . 1 . 1 - 8  shows an example of a hardware configuration for a controller  11 . 1 . 1 . 1 - 800  that may be used to implement portions of the head-mounted display  11 . 1 . 1 . 1 - 100 . In the illustrated example, the controller  11 . 1 . 1 . 1 - 800  includes a processor  11 . 1 . 1 . 1 - 802 , a memory device  11 . 1 . 1 . 1 - 804 , a storage device  11 . 1 . 1 . 1 - 806 , one or more input devices  11 . 1 . 1 . 1 - 808 , and one or more output devices  11 . 1 . 1 . 1 - 810 . These components may be interconnected by hardware such as a bus  11 . 1 . 1 . 1 - 812  that allows communication between the components. 
     The processor  11 . 1 . 1 . 1 - 802  may be a conventional device such as a central processing unit and is operable to execute computer program instructions and perform operations described by the computer program instructions. The memory device  11 . 1 . 1 . 1 - 804  may be a volatile, high-speed, short-term information storage device such as a random-access memory module. The storage device  11 . 1 . 1 . 1 - 806  may be a non-volatile information storage device such as a hard drive or a solid-state drive. The input devices  11 . 1 . 1 . 1 - 808  may include sensors and/or any type of human-machine interface, such as buttons, switches, a keyboard, a mouse, a touchscreen input device, a gestural input device, or an audio input device. The output devices  11 . 1 . 1 . 1 - 810  may include any type of device operable to provide an indication to a user regarding an operating state, such as a display screen, a light-control panel, or an audio output. 
     11.1.1.2: Crown Input and Feedback for Head-Mountable Devices 
     It can be desirable to provide a mechanism for a user to provide inputs to a head-mountable device to facilitate user interaction with the head-mountable device. It can be further desirable to provide a mechanism for providing feedback to the user. Such feedback can be provided in the form of haptic feedback delivered to the user. However, haptic feedback can feel unpleasant when applied across an entire device that is mounted on a head of the user. Where the user is providing tactile inputs by contacting an input member with another portion of the body, such as a finger or hand, the haptic feedback can be locally applied to that portion of the user&#39;s body, so that the haptic feedback is delivered in a way that is effective and pleasant to the user. 
     Systems of the present disclosure can provide a head-mountable device with a crown module with an input system that allows a user to provide inputs by rotating or otherwise applying torque to a crown of the crown module. The head-mountable device can interpret the rotation and/or torque as a user input. The crown module can further include a feedback system that provides localized haptic feedback at the crown. The haptic feedback can be effectively perceived by the user at the crown without causing the entire head-mountable device to vibrate against the head and/or face of the user. 
     According to some embodiments, for example as shown in  FIG.  11 . 1   . 1 . 2 - 1 , a head-mountable device  11 . 1 . 1 . 2 - 100  includes a frame  11 . 1 . 1 . 2 - 110  that is worn on a head of a user. The frame  11 . 1 . 1 . 2 - 110  can be positioned in front of the eyes of a user to provide information within a field of view of the user. The frame  11 . 1 . 1 . 2 - 110  can provide nose pads or another feature to rest on a user&#39;s nose. The frame  11 . 1 . 1 . 2 - 110  can be supported on a user&#39;s head with the securement element  11 . 1 . 1 . 2 - 120 . The securement element  11 . 1 . 1 . 2 - 120  can wrap or extend along opposing sides of a user&#39;s head. The securement element  11 . 1 . 1 . 2 - 120  can include earpieces for wrapping around or otherwise engaging or resting on a user&#39;s ears. It will be appreciated that other configurations can be applied for securing the head-mountable device  11 . 1 . 1 . 2 - 100  to a user&#39;s head. For example, one or more bands, straps, belts, caps, hats, or other components can be used in addition to or in place of the illustrated components of the head-mountable device  11 . 1 . 1 . 2 - 100 . By further example, the securement element  11 . 1 . 1 . 2 - 120  can include multiple components to engage a user&#39;s head. 
     The frame  11 . 1 . 1 . 2 - 110  can provide structure around a peripheral region thereof to support any internal components of the frame  11 . 1 . 1 . 2 - 110  in their assembled position. For example, the frame  11 . 1 . 1 . 2 - 110  can enclose and support various internal components (including for example integrated circuit chips, processors, memory devices and other circuitry) to provide computing and functional operations for the head-mountable device  11 . 1 . 1 . 2 - 100 , as discussed further herein. Any number of components can be included within and/or on the frame  11 . 1 . 1 . 2 - 110  and/or the securement element  11 . 1 . 1 . 2 - 120 . 
     The frame  11 . 1 . 1 . 2 - 110  can include and/or support one or more cameras  11 . 1 . 1 . 2 - 50 . The cameras  11 . 1 . 1 . 2 - 50  can be positioned on or near an outer side  11 . 1 . 1 . 2 - 112  of the frame  11 . 1 . 1 . 2 - 110  to capture images of views external to the head-mountable device  11 . 1 . 1 . 2 - 100 . As used herein, an outer side  11 . 1 . 1 . 2 - 112  of a portion of a head-mountable device is a side that faces away from the user and/or towards an external environment. The captured images can be used for display to the user or stored for any other purpose. 
     The head-mountable device can be provided with one or more display screens  11 . 1 . 1 . 2 - 90  that provide visual output for viewing by a user wearing the head-mountable device. As shown in  FIG.  11 . 1   . 1 . 2 - 1 , one or more optical modules containing display screens  11 . 1 . 1 . 2 - 90  can be positioned on an inner side  11 . 1 . 1 . 2 - 114  of the frame  11 . 1 . 1 . 2 - 110 . As used herein, an inner side of a portion of a head-mountable device is a side that faces toward the user and/or away from the external environment. For example, a pair of optical modules can be provided, where each optical module is movably positioned to be within the field of view of each of a user&#39;s two eyes. Each optical module can be adjusted to align with a corresponding eye of the user. Movement of each of the optical modules can match movement of a corresponding camera  11 . 1 . 1 . 2 - 50 . Accordingly, the optical module is able to accurately reproduce, simulate, or augment a view based on a view captured by the camera  11 . 1 . 1 . 2 - 50  with an alignment that corresponds to the view that the user would have naturally without the head-mountable device  11 . 1 . 1 . 2 - 100 . 
     A display screen  11 . 1 . 1 . 2 - 90  can transmit light from a physical environment (e.g., as captured by a camera) for viewing by the user. Such a display screen can include optical properties, such as lenses for vision correction based on incoming light from the physical environment. Additionally or alternatively, a display screen  11 . 1 . 1 . 2 - 90  can provide information as a display within a field of view of the user. Such information can be provided to the exclusion of a view of a physical environment or in addition to (e.g., overlaid with) a physical environment. 
     As further shown in  FIG.  11 . 1   . 1 . 2 - 1 , the head-mountable device  11 . 1 . 1 . 2 - 100  can include a crown module  11 . 1 . 1 . 2 - 200  that received input from a user and provides feedback to the user. The crown module  11 . 1 . 1 . 2 - 200  can be provided on exterior surface of the head-mountable device  11 . 1 . 1 . 2 - 100 , such as on the frame  11 . 1 . 1 . 2 - 110 . As shown in  FIG.  11 . 1   . 1 . 2 - 1 , the crown module  11 . 1 . 1 . 2 - 200  can be provided on a lateral side  11 . 1 . 1 . 2 - 116  that is defined by and outwardly facing surface between the outer side  11 . 1 . 1 . 2 - 112  and the inner side  11 . 1 . 1 . 2 - 114  of the frame  11 . 1 . 1 . 2 - 110 . It will be understood that the crown module  11 . 1 . 1 . 2 - 200  can be provided at any portion of the head-mountable device  11 . 1 . 1 . 2 - 100 , including any portion of the frame  11 . 1 . 1 . 2 - 110  (e.g., outer side  11 . 1 . 1 . 2 - 112  or inner side  11 . 1 . 1 . 2 - 114 ) and/or on the securement element  11 . 1 . 1 . 2 - 120 . It will be further understood that multiple crown modules  11 . 1 . 1 . 2 - 200  can be provided by the head-mountable device  11 . 1 . 1 . 2 - 100 . For example, separate crown modules  11 . 1 . 1 . 2 - 200  can be provided on a same side or opposing sides of the head-mountable device  11 . 1 . 1 . 2 - 100 . 
     Referring now to  FIG.  11 . 1   . 1 . 2 - 2 , the crown module  11 . 1 . 1 . 2 - 200  can be provided as a self-contained component that is connected both to other portions of the head-mountable device  11 . 1 . 1 . 2 - 100 . For example, the frame  11 . 1 . 1 . 2 - 110  or another portion of the head-mountable device  11 . 1 . 1 . 2 - 100  can provide a recess  11 . 1 . 1 . 2 - 118  into which the crown module  11 . 1 . 1 . 2 - 200  can be inserted. By providing the crown module  11 . 1 . 1 . 2 - 200  as a self-contained component, the crown module  11 . 1 . 1 . 2 - 200  can be sealed so that its internal components are protected from an external environment. 
     The crown module  11 . 1 . 1 . 2 - 200  can include one or more attachment elements configured to facilitate mechanical coupling or connection of the crown module  11 . 1 . 1 . 2 - 200  and the frame  11 . 1 . 1 . 2 - 110  by engaging complementary attachment elements of the frame  11 . 1 . 1 . 2 - 110  (e.g., within the recess  11 . 1 . 1 . 2 - 118 ). The attachment elements can include protrusions, grooves, locks, latches, snaps, screws, clasps, threads, magnets, and/or pins can be included on the crown module  11 . 1 . 1 . 2 - 200  and/or the frame  11 . 1 . 1 . 2 - 110  for securely attaching the crown module  11 . 1 . 1 . 2 - 200  to the frame  11 . 1 . 1 . 2 - 110 . 
     The crown module  11 . 1 . 1 . 2 - 200  and the frame  11 . 1 . 1 . 2 - 110  can each include one or more communication interfaces that facilitate a communication link between the crown module  11 . 1 . 1 . 2 - 200  and the frame  11 . 1 . 1 . 2 - 110  (e.g., a controller within the frame  11 . 1 . 1 . 2 - 110 ). The communication interfaces can include one or more of a variety of features, such as electrical connectors, pogo pins, conductive surfaces, wireless receivers/transmitters, and/or inductive coupling features (e.g., coils) for communicably coupling the components of the frame  11 . 1 . 1 . 2 - 110  and the crown module  11 . 1 . 1 . 2 - 200 . 
     Referring now to  FIG.  11 . 1   . 1 . 2 - 3 , the crown module  11 . 1 . 1 . 2 - 200  can provide an input system  11 . 1 . 1 . 2 - 230  that facilitates receiving input by a user and a feedback system  11 . 1 . 1 . 2 - 240  that provides feedback to the user. For the purposes of the following description, the described crown module  11 . 1 . 1 . 2 - 200  is one example of that shown and discussed above with respect to  FIGS.  11 . 1   . 1 . 2 - 1  and  11 . 1 . 1 . 2 - 2 . However, certain features of the crown module  11 . 1 . 1 . 2 - 200 , including the external surface geometry, may be simplified or vary with respect to aspects of the crown module  11 . 1 . 1 . 2 - 200  discussed above. 
     As shown in  FIG.  11 . 1   . 1 . 2 - 3 , the crown module  11 . 1 . 1 . 2 - 200  can include a housing  11 . 1 . 1 . 2 - 210  that defines at least a portion of an outer periphery of the crown module  11 . 1 . 1 . 2 - 200  and contains internal components thereof. A crown  11 . 1 . 1 . 2 - 222  can be provided at an exterior portion of the housing  11 . 1 . 1 . 2 - 210 . For example, the crown  11 . 1 . 1 . 2 - 222  can protrude from a surface of the housing  11 . 1 . 1 . 2 - 210  to be accessible by user. The crown  11 . 1 . 1 . 2 - 222  can be connected to a shaft  11 . 1 . 1 . 2 - 220  that extends within the housing  11 . 1 . 1 . 2 - 210 . The crown  11 . 1 . 1 . 2 - 222  and/or the shaft  11 . 1 . 1 . 2 - 220  can be supported relative to the housing  11 . 1 . 1 . 2 - 210  by one or more bearings  11 . 1 . 1 . 2 - 236  that facilitates rotation and/or translation of the crown  11 . 1 . 1 . 2 - 222  and/or the shaft  11 . 1 . 1 . 2 - 220  relative to the housing  11 . 1 . 1 . 2 - 210 . 
     The housing  11 . 1 . 1 . 2 - 210  can define a first chamber  11 . 1 . 1 . 2 - 232  that is sealed from an external environment. Components of the input system  11 . 1 . 1 . 2 - 230  can be positioned within the first chamber  11 . 1 . 1 . 2 - 232 . As such, the components of the input system  11 . 1 . 1 . 2 - 230  can be protected from ingress of fluids and/or particles that would interfere with operation of the input system  11 . 1 . 1 . 2 - 230 . The first chamber  11 . 1 . 1 . 2 - 232  can be defined at least in part by one or more seal members (e.g., O-rings) that move with the shaft  11 . 1 . 1 . 2 - 220  within the first chamber  11 . 1 . 1 . 2 - 232  of the housing  11 . 1 . 1 . 2 - 210 . 
     The housing  11 . 1 . 1 . 2 - 210  can define a second chamber  11 . 1 . 1 . 2 - 242  that is also sealed from an external environment. Components of the feedback system  11 . 1 . 1 . 2 - 240  can be positioned within the second chamber  11 . 1 . 1 . 2 - 242 . As such, the components of the feedback system  11 . 1 . 1 . 2 - 240  can be protected from ingress of fluids and/or particles that would interfere with operation of the feedback system  11 . 1 . 1 . 2 - 240 . At least one component (e.g., connector  11 . 1 . 1 . 2 - 250 ) of the feedback system  11 . 1 . 1 . 2 - 240  (e.g., in the second chamber  11 . 1 . 1 . 2 - 242 ) can be connected to the shaft  11 . 1 . 1 . 2 - 220  or another component of the input system  11 . 1 . 1 . 2 - 230  (e.g., in the first chamber  11 . 1 . 1 . 2 - 232 ). The second chamber  11 . 1 . 1 . 2 - 242  and the first chamber  11 . 1 . 1 . 2 - 232  can optionally be connected to form a single continuous chamber. 
     The crown  11 . 1 . 1 . 2 - 222  and/or the shaft  11 . 1 . 1 . 2 - 220  can be supported by one or more bearings  11 . 1 . 1 . 2 - 236  that facilitates rotation and/or translation of the crown  11 . 1 . 1 . 2 - 222  and/or the shaft  11 . 1 . 1 . 2 - 220  relative to the housing  11 . 1 . 1 . 2 - 210 . The connector  11 . 1 . 1 . 2 - 250  can be supported by one or more bearings  11 . 1 . 1 . 2 - 246  that facilitates translation of the connector  11 . 1 . 1 . 2 - 250  with the crown  11 . 1 . 1 . 2 - 222  and/or the shaft  11 . 1 . 1 . 2 - 220 . The bearings  11 . 1 . 1 . 2 - 246  can optionally allow rotation of the connector  11 . 1 . 1 . 2 - 250  relative to the crown  11 . 1 . 1 . 2 - 222  and/or the shaft  11 . 1 . 1 . 2 - 220  while coupling the connector  11 . 1 . 1 . 2 - 250  to the shaft  11 . 1 . 1 . 2 - 220  for translation in unison. 
     In some embodiments, the crown  11 . 1 . 1 . 2 - 222  may be used to accept rotary input from the user, which may be used to control aspects of the head-mountable device. The crown  11 . 1 . 1 . 2 - 222  may be knurled or otherwise textured to improve grip with the user&#39;s finger and/or thumb. In some embodiments, a crown  11 . 1 . 1 . 2 - 222  may be turned by the user to scroll a display or select from a range of values. In other embodiments, the crown  11 . 1 . 1 . 2 - 222  may be rotated to move a cursor or other type of selection mechanism from a first displayed location to a second displayed location in order to select an icon or move the selection mechanism between various icons that are output on the display. The crown may also be used to control the volume of a speaker, the brightness of the display screen, visual output of the head-mountable device, or control other hardware settings. 
     In some embodiments, an optical encoder may be used to detect the rotational motion of the crown about an axis. More specifically, the example provided below with respect to  FIG.  11 . 1   . 1 . 2 - 3  may use an optical encoder to detect rotational movement, rotational direction and/or rotational speed of a component of the electronic device. Once the rotational movement, rotational direction and/or rotational speed have been determined, this information may be used to output or change information and images that are presented on a display or user interface of the head-mountable device. 
     As shown in the example embodiment of  FIG.  11 . 1   . 1 . 2 - 3 , the optical encoder of the present disclosure includes a light source  11 . 1 . 1 . 2 - 270 , an optical sensor  11 . 1 . 1 . 2 - 272  (e.g., photodiode and/or photodiode array), and a shaft  11 . 1 . 1 . 2 - 220 . In some embodiments, the optical encoder of the present disclosure can utilize an encoding pattern  11 . 1 . 1 . 2 - 262  disposed directly on the shaft  11 . 1 . 1 . 2 - 220 . For example, the encoding pattern  11 . 1 . 1 . 2 - 262  can include a number of light and dark markings or stripes that are axially disposed along the shaft  11 . 1 . 1 . 2 - 220 . Each stripe or combination of stripes on the shaft  11 . 1 . 1 . 2 - 220  may be used to identify a position of the shaft  11 . 1 . 1 . 2 - 220 . For example, as light is emitted from the light source  11 . 1 . 1 . 2 - 270  and reflected off of the shaft  11 . 1 . 1 . 2 - 220  into the optical sensor  11 . 1 . 1 . 2 - 272 , a position, rotation, rotation direction and rotation speed of the shaft  11 . 1 . 1 . 2 - 220  may be determined. Once the rotation direction and speed are determined, this information may be used to output or change information or images that are presented on the display or user interface of the head-mountable device. 
     In other embodiments, the shape or form of the shaft  11 . 1 . 1 . 2 - 220  of the encoder may be used to determine a position, rotation, rotation direction and rotation speed of the shaft  11 . 1 . 1 . 2 - 220 . For example, the shaft  11 . 1 . 1 . 2 - 220  may be fluted or have a number of channels that cause the light to be reflected in a number of different directions. Accordingly, a diffractive pattern may be used to determine the rotation, rotation direction and rotation speed of the shaft  11 . 1 . 1 . 2 - 220 . 
     As shown in  FIG.  11 . 1   . 1 . 2 - 3 , a crown assembly may be provided partially within the housing  11 . 1 . 1 . 2 - 210  of the crown module  11 . 1 . 1 . 2 - 200  and may be formed from a crown  11 . 1 . 1 . 2 - 222  disposed at the end of a shaft  11 . 1 . 1 . 2 - 220 . As discussed above, the crown module  11 . 1 . 1 . 2 - 200  includes an optical encoder that includes a shaft  11 . 1 . 1 . 2 - 220 , a light source  11 . 1 . 1 . 2 - 270 , and an optical sensor  11 . 1 . 1 . 2 - 272 . Although an optical sensor is specifically mentioned, embodiments disclosed herein may use various types of sensors that are arranged in various configurations for detecting the movement described herein. For example, the movement of the shaft  11 . 1 . 1 . 2 - 220  may be detected by an image sensor, a light sensor such as a CMOS light sensor or imager, a photovoltaic cell or system, photo resistive component, a laser scanner and the like. 
     The optical encoder may produce an encoder output that is used to determine positional data of the crown  11 . 1 . 1 . 2 - 222 . In particular, the optical encoder may produce an output that is used to detect that movement of the crown  11 . 1 . 1 . 2 - 222  including the direction of the movement, speed of the movement and so on. The movement may be rotational movement (e.g., about the axis  11 . 1 . 1 . 2 - 290 ), translational movement (e.g., along or parallel to the axis  11 . 1 . 1 . 2 - 290 ), angular movement (e.g., tilt relative to the axis  11 . 1 . 1 . 2 - 290 ), and so on. The optical encoder may also be used to detect the degree of the change of rotation of the crown  11 . 1 . 1 . 2 - 222  and/or the angle of rotation of the crown  11 . 1 . 1 . 2 - 222  as well as the speed and the direction of the rotation of the crown  11 . 1 . 1 . 2 - 222 . 
     The crown  11 . 1 . 1 . 2 - 222  can be coupled to and/or monolithically formed with the shaft  11 . 1 . 1 . 2 - 220 . In some cases, the shaft  11 . 1 . 1 . 2 - 220  and crown  11 . 1 . 1 . 2 - 222  may be formed as a single piece. As the shaft  11 . 1 . 1 . 2 - 220  is coupled to, or is otherwise a part of the crown  11 . 1 . 1 . 2 - 222 , as the crown  11 . 1 . 1 . 2 - 222  rotates or moves in a particular direction and at a particular speed, the shaft  11 . 1 . 1 . 2 - 220  also rotates or moves in the same direction and with the same speed. 
     As further shown in  FIG.  11 . 1   . 1 . 2 - 3 , the feedback system  11 . 1 . 1 . 2 - 240  can include mechanisms that facilitate haptic feedback. A feedback system can be implemented as any suitable device configured to provide force feedback, vibratory feedback, tactile sensations, and the like. For example, in one embodiment, the feedback system may be implemented as a linear actuator configured to provide a punctuated haptic feedback, such as a tap or a knock. 
     According to some embodiments, the feedback system  11 . 1 . 1 . 2 - 240  can include a magnetic element  11 . 1 . 1 . 2 - 252 . The magnetic element  11 . 1 . 1 . 2 - 252  can be coupled to the shaft  11 . 1 . 1 . 2 - 220  (e.g., via a connector  11 . 1 . 1 . 2 - 250 ) such that the magnetic element  11 . 1 . 1 . 2 - 252  moves along the axis  11 . 1 . 1 . 2 - 290  relative to the housing  11 . 1 . 1 . 2 - 210  along with the shaft  11 . 1 . 1 . 2 - 220 . For example, the magnetic element  11 . 1 . 1 . 2 - 252  can be directly or indirectly (e.g., via the connector  11 . 1 . 1 . 2 - 250  and/or the intermediate bearing  11 . 1 . 1 . 2 - 246 ) connected to the shaft  11 . 1 . 1 . 2 - 220 . 
     The magnetic element  11 . 1 . 1 . 2 - 252  can include a temporary magnet of a soft magnetic material or a permanent magnet of a hard magnetic material. As used herein, “magnet” can include a magnet of a hard magnetic material and/or a magnet of a soft magnetic material. Hard magnetic materials include materials that retain their magnetism even after the removal of an applied magnetic field. Magnets that include hard magnetic material can form permanent magnets. Hard magnetic materials include neodymium (NdFeB), iron-neodymium, iron-boron, cobalt-samarium, iron-chromium-cobalt, and combinations or alloys thereof. Soft magnetic materials include materials that are responsive to magnetic fields, but do not retain their magnetism after removal of an applied magnetic field. Magnets that include soft magnetic material can form temporary magnets. Soft magnetic materials include iron, iron-cobalt, iron-silicon, steel, stainless steel, iron-aluminum-silicon, nickel-iron, ferrites, and combinations or alloys thereof. It will be recognized that “hard magnetic” and “soft magnetic” does not necessarily relate to the rigidity of the materials. 
     The feedback system  11 . 1 . 1 . 2 - 240  can further include a magnetic field generator to induce a magnetic field in the magnetic element  11 . 1 . 1 . 2 - 252 . For example, one or more coils  11 . 1 . 1 . 2 - 244  can be positioned on one or more sides of the magnetic element  11 . 1 . 1 . 2 - 252 . The coils  11 . 1 . 1 . 2 - 244  can include one or more helical windings in one or more layers. It will be recognized that any number of windings and arrangements of the coil can be provided to induce a magnetic field. 
     It will be recognized that various arrangements and alterations to the above description can be implemented to provide haptic feedback. For example, the magnetic element  11 . 1 . 1 . 2 - 252  can be exchanged with the coils  11 . 1 . 1 . 2 - 244  such that the magnetic element  11 . 1 . 1 . 2 - 252  is moveable with the housing  11 . 1 . 1 . 2 - 210  and the coils  11 . 1 . 1 . 2 - 244  are moveable with the shaft  11 . 1 . 1 . 2 - 220 . The magnetic element  11 . 1 . 1 . 2 - 252  can have a variety of shapes and sizes. Multiple magnetic elements can be provided. These and other designs can be implemented to facilitate an induced magnetic field and magnetic forces between the magnetic elements. 
     As shown in  FIG.  11 . 1   . 1 . 2 - 3 , the coils  11 . 1 . 1 . 2 - 244  are operated to induce a magnetic field near the magnetic element  11 . 1 . 1 . 2 - 252 . When the coils  11 . 1 . 1 . 2 - 244  are activated with an electric current, the causes the magnetic element  11 . 1 . 1 . 2 - 252  to move under the influence of a magnetic force. For example, where the magnetic element  11 . 1 . 1 . 2 - 252  is a temporary magnet of a soft magnetic material, the magnetic field can cause the magnetic domains of the magnetic element  11 . 1 . 1 . 2 - 252  to align with the magnetic field. The magnetic element  11 . 1 . 1 . 2 - 252  will then be attracted toward a direction based on the activated coils  11 . 1 . 1 . 2 - 244 . Additionally or alternatively, the magnetic element  11 . 1 . 1 . 2 - 252  can be a permanent magnet of a hard magnetic material. Based on the alignment (i.e., polarity) of such a permanent magnet, the magnetic field causes the magnetic element  11 . 1 . 1 . 2 - 252  to attract toward or repel away from one or more coils  11 . 1 . 1 . 2 - 244  when activated. 
     As the magnetic element  11 . 1 . 1 . 2 - 252  moves (e.g., along the axis  11 . 1 . 1 . 2 - 290 ), the shaft  11 . 1 . 1 . 2 - 220  moves relative to the housing  11 . 1 . 1 . 2 - 210  (e.g., along the axis  11 . 1 . 1 . 2 - 290 ). As described above, the magnetic element  11 . 1 . 1 . 2 - 252  is connected to the shaft  11 . 1 . 1 . 2 - 220  to move with the shaft  11 . 1 . 1 . 2 - 220  and the coils  11 . 1 . 1 . 2 - 244  are connected to the housing  11 . 1 . 1 . 2 - 210  to move with the housing  11 . 1 . 1 . 2 - 210 . As such, magnetic forces between the magnetic element  11 . 1 . 1 . 2 - 252  and the coils  11 . 1 . 1 . 2 - 244  are transmitted to the shaft  11 . 1 . 1 . 2 - 220  and the housing  11 . 1 . 1 . 2 - 210  to cause relative movement there between. 
     The haptic feedback can include movement of the shaft  11 . 1 . 1 . 2 - 220  relative to the housing and along the axis  11 . 1 . 1 . 2 - 290  of the crown module  11 . 1 . 1 . 2 - 200 . For example, the magnetic element  11 . 1 . 1 . 2 - 252  can be aligned along the axis  11 . 1 . 1 . 2 - 290  of the crown module  11 . 1 . 1 . 2 - 200 . Movement from haptic feedback can be along the same axis  11 . 1 . 1 . 2 - 290  about which the crown  11 . 1 . 1 . 2 - 222  and the shaft  11 . 1 . 1 . 2 - 220  rotate. Additionally or alternatively, movement from haptic feedback can be along another axis or in multiple axes and directions. 
     In use, the coils  11 . 1 . 1 . 2 - 244  can be operated to provide haptic feedback while the user is operating (e.g., contacting and/or rotating) the crown  11 . 1 . 1 . 2 - 222 . The haptic feedback can be provided based on a variety of conditions and parameters. For example, the haptic feedback can be controlled by providing an electric current to the coils  11 . 1 . 1 . 2 - 244 . The induced and corresponding magnetic force between the magnetic element  11 . 1 . 1 . 2 - 252  and the coils  11 . 1 . 1 . 2 - 244  is based on the current in the coils  11 . 1 . 1 . 2 - 244 . As such, the current can have a duration, amplitude, frequency, waveform, duty cycle, or other parameters as desired for a desired and corresponding haptic feedback. 
     For example, the shaft  11 . 1 . 1 . 2 - 220  can be made to vibrate by applying a control signal to the coils  11 . 1 . 1 . 2 - 244 . The control signal may be a wave having a predetermined amplitude and/or frequency. When the control signal is applied, the induced magnetic field causes the shaft  11 . 1 . 1 . 2 - 220  to vibrate at the frequency of the control signal. The frequency can be in a range between 10 Hz and 5,000 Hz, 50 Hz and 1,000 Hz, or 100 Hz and 500 Hz. The frequency of the control signal may be adjusted to alter the rate of movement of the shaft  11 . 1 . 1 . 2 - 220  if a certain vibration is desired. The amplitude of the control signal may be correlated to the magnitude of movement of the shaft  11 . 1 . 1 . 2 - 220 , and may be adjusted to alter the intensity of the vibration. 
     The feedback system  11 . 1 . 1 . 2 - 240  can provide haptic feedback to a user by moving the shaft  11 . 1 . 1 . 2 - 220  of the crown module  11 . 1 . 1 . 2 - 200  relative to the housing  11 . 1 . 1 . 2 - 210 . In contrast to haptic feedback applied directly to the housing  11 . 1 . 1 . 2 - 210  and/or other portions of the head-mountable device, haptic feedback provided at the shaft  11 . 1 . 1 . 2 - 220  more directly provides sensations relating to the shaft  11 . 1 . 1 . 2 - 220 . For example, haptic feedback can be provided while the user is operating (e.g., contacting and/or rotating) the crown  11 . 1 . 1 . 2 - 222 . As the shaft  11 . 1 . 1 . 2 - 220  and the crown  11 . 1 . 1 . 2 - 222  are moved (e.g., vibrated) relative to the housing  11 . 1 . 1 . 2 - 210 , the rest of the head-mountable device can remain stationary, so that the haptic feedback is not felt by the user at other locations of contact. By further example, while the user is wearing the head-mountable device, the haptic feedback can nonetheless be localized to the crown  11 . 1 . 1 . 2 - 222  so that the user feels the haptic feedback only at that location. 
     The feedback system  11 . 1 . 1 . 2 - 240  can provide haptic feedback based on operation of the crown module  11 . 1 . 1 . 2 - 200 . For example, haptic feedback can be provided while the crown  11 . 1 . 1 . 2 - 222  and/or the shaft  11 . 1 . 1 . 2 - 220  are rotated by the user. Incremental and/or periodic haptic feedback can be provided based on the rotation performed by the user. By further example, the haptic feedback can be provided at a speed that corresponds to the speed of rotation performed by the user. As such, the haptic feedback can provide confirmation to the user relating to the input that is received by the user. 
     The feedback system  11 . 1 . 1 . 2 - 240  can provide haptic feedback based on activities performed by the head-mountable device. For example, the haptic feedback can correspond to visual information that is output to the user by the head-mountable device. By further example, visual information can be modified by use or operation of the crown, and haptic feedback can be provided to indicate how the user can interact with the visual information. For example, the user can rotate the crown in one or both of two directions to cause the head-mountable device to perform certain actions. Such rotation be performed to control the volume of a speaker, the brightness of the display screen, visual output of the head-mountable device, optical settings of an optical subassembly, or control other hardware settings. Rotation can be performed to scroll through a list or other set of items visually displayed by the head-mountable device. 
     While a first type of haptic feedback can be provided as the user rotates the crown, a second type of haptic feedback can be provided to indicate how the user can interact with the head-mountable device and/or limitations regarding the user input. For example, as the user scrolls through a list displayed by the head-mountable device, a first type of haptic feedback can be provided based on the user input (e.g., speed of rotation, etc.). By further example, as the user reaches the end of a list, a second type of haptic feedback can be provided to indicate that the user has reached the end of the list. Additionally or alternatively, different types of feedback can be provided in this way for other actions, such as zooming in on or out from an image, changing volume settings, changing display brightness, and the like. 
     The feedback system  11 . 1 . 1 . 2 - 240  can provide haptic feedback for one or more other purposes. According to some embodiments, the haptic feedback can notify the user based on a message, alert, or alarm. Such notifications can be accompanied by other feedback, including tactile, auditory, and/or visual feedback on the crown module  11 . 1 . 1 . 2 - 200  and/or the external device. According to some embodiments, the haptic feedback can provide confirmation that a user selection (e.g., made with the crown module  11 . 1 . 1 . 2 - 200 ) has been received by the head-mountable device and/or an external device. According to some embodiments, the haptic feedback can inform the user regarding status or operation of the head-mountable device and/or an external device. 
     Referring now to  FIGS.  11 . 1   . 1 . 2 - 4 - 11 . 1 . 1 . 2 - 7 , a crown module  11 . 1 . 1 . 2 - 300  can be provided for receiving input from a user and providing feedback to the user. The crown module  11 . 1 . 1 . 2 - 300  can be provided with the head-mountable device as an alternative to and/or in addition to the crown module  11 . 1 . 1 . 2 - 200 . For example, the crown module  11 . 1 . 1 . 2 - 300  can be a self-contained module that is assembled with other components of the head-mountable device as described herein with respect to the crown module  11 . 1 . 1 . 2 - 200 . 
     As shown in  FIG.  11 . 1   . 1 . 2 - 4 , the crown module  11 . 1 . 1 . 2 - 300  can include a housing  11 . 1 . 1 . 2 - 310  that defines at least a portion of an outer periphery of the crown module  11 . 1 . 1 . 2 - 300  and contains internal components thereof. A crown  11 . 1 . 1 . 2 - 322  can be provided at an exterior portion of the housing  11 . 1 . 1 . 2 - 310 . For example, the crown  11 . 1 . 1 . 2 - 322  can protrude from a surface of the housing  11 . 1 . 1 . 2 - 310  to be accessible by user. The crown  11 . 1 . 1 . 2 - 322  can be connected to a shaft  11 . 1 . 1 . 2 - 320  that extends within the housing  11 . 1 . 1 . 2 - 310 . The crown  11 . 1 . 1 . 2 - 322  and/or the shaft  11 . 1 . 1 . 2 - 320  can be supported relative to the housing  11 . 1 . 1 . 2 - 310  by one or more bearings  11 . 1 . 1 . 2 - 336 . 
     The housing  11 . 1 . 1 . 2 - 310  can define a chamber  11 . 1 . 1 . 2 - 332  that is sealed from an external environment. As such, the components of the crown module  11 . 1 . 1 . 2 - 300  can be protected from ingress of fluids and/or particles that would interfere with operation thereof. 
     The crown module  11 . 1 . 1 . 2 - 300  can include a torque sensor  11 . 1 . 1 . 2 - 330  configured to detect torque that is applied by a user to the crown  11 . 1 . 1 . 2 - 322  and transferred to the shaft  11 . 1 . 1 . 2 - 320 . For example, the user can apply a torque to the crown  11 . 1 . 1 . 2 - 322  by urging the crown  11 . 1 . 1 . 2 - 322  to rotate about the axis  11 . 1 . 1 . 2 - 390 . It will be understood that such a torque may not result in significant rotation about the axis  11 . 1 . 1 . 2 - 390 . For example, the crown  11 . 1 . 1 . 2 - 322  and/or the shaft  11 . 1 . 1 . 2 - 320  can be coupled to the housing  11 . 1 . 1 . 2 - 310  such that no significant rotation is achieved. Despite this coupling, a torque can be applied to the crown  11 . 1 . 1 . 2 - 322  and transferred to the shaft  11 . 1 . 1 . 2 - 320 . As the shaft  11 . 1 . 1 . 2 - 320  is subjected to such a torque, the torque sensor  11 . 1 . 1 . 2 - 330  can detect the torque and interpret the torque as an input from the user. Mechanisms for detecting torque are described further herein. 
     In some embodiments, the crown  11 . 1 . 1 . 2 - 322  may be used to accept torque input from the user, which may be used to control aspects of the head-mountable device. The crown  11 . 1 . 1 . 2 - 322  may be knurled or otherwise textured to improve grip with the user&#39;s finger and/or thumb. In some embodiments, the crown  11 . 1 . 1 . 2 - 322  may be operated to provide inputs such as those described with respect to the crown module  11 . 1 . 1 . 2 - 200 . For example, the crown  11 . 1 . 1 . 2 - 322  may be torqued by the user to scroll a display or select from a range of values. In other embodiments, the crown  11 . 1 . 1 . 2 - 322  may be torqued to move a cursor or other type of selection mechanism from a first displayed location to a second displayed location in order to select an icon or move the selection mechanism between various icons that are output on the display. The crown may also be used to control the volume of a speaker, the brightness of the display screen, visual output of the head-mountable device, or control other hardware settings. 
     As further shown in  FIG.  11 . 1   . 1 . 2 - 4 , the crown module  11 . 1 . 1 . 2 - 300  can be provided with a feedback system  11 . 1 . 1 . 2 - 350  that includes mechanisms that facilitate haptic feedback. According to some embodiments, the feedback system  11 . 1 . 1 . 2 - 350  can be the same or similar to the feedback system  11 . 1 . 1 . 2 - 240  of the crown module  11 . 1 . 1 . 2 - 200 . For example, the feedback system  11 . 1 . 1 . 2 - 350  can be coupled to the shaft  11 . 1 . 1 . 2 - 320  such that a component of the feedback system  11 . 1 . 1 . 2 - 350  moves along the axis  11 . 1 . 1 . 2 - 390  relative to the housing  11 . 1 . 1 . 2 - 310  along with the shaft  11 . 1 . 1 . 2 - 320 . Additionally or alternatively, the feedback system  11 . 1 . 1 . 2 - 350  can include or be connected to motors, hydraulic actuators, pneumatic actuators, magnetic actuators, piezoelectric actuators, electroactive materials (e.g., polymers), stepper motors, shape-memory alloys, and/or the like for providing mechanical movement as haptic feedback. 
     As shown in  FIGS.  11 . 1   . 1 . 2 - 5  and  11 . 1 . 1 . 2 - 6 , the torque sensor  11 . 1 . 1 . 2 - 330  can include multiple strain gauges  11 . 1 . 1 . 2 - 330 A,  11 . 1 . 1 . 2 - 330 B,  11 . 1 . 1 . 2 - 330 C, and  11 . 1 . 1 . 2 - 330 D. The strain gauges of the torque sensor  11 . 1 . 1 . 2 - 330  can operate as a resistive sensor formed from a material that exhibits a change in electrical resistance (e.g., conductance) in response to a dimensional change such as compression, tension, or force. The strain gauges can each be a compliant material that exhibits at least one electrical property that is variable in response to deformation, deflection, or shearing of the electrode. The strain gauges may be formed from a piezoelectric, piezoresistive, resistive, or other strain-sensitive materials. 
     As further shown in  FIGS.  11 . 1   . 1 . 2 - 5  and  11 . 1 . 1 . 2 - 6 , the strain gauges  11 . 1 . 1 . 2 - 330 A,  11 . 1 . 1 . 2 - 330 B,  11 . 1 . 1 . 2 - 330 C, and  11 . 1 . 1 . 2 - 330 D can be distributed at different locations on the shaft  11 . 1 . 1 . 2 - 320 . For example, as shown in  FIG.  11 . 1   . 1 . 2 - 5 , some of the strain gauges can be placed on a first radial side of the shaft  11 . 1 . 1 . 2 - 320 , and other strain gauges can be placed on a second radial side of the shaft  11 . 1 . 1 . 2 - 320 . By further example, as shown in  FIG.  11 . 1   . 1 . 2 - 6 , some of the strain gauges can be placed in a first orientation relative to an axis of the shaft  11 . 1 . 1 . 2 - 320 , and other strain gauges can be placed in a second orientation relative to the axis of the shaft  11 . 1 . 1 . 2 - 320 . In such different orientations, an applied torque will cause some of the strain gauges to be placed in compression whereas another strain gauge is placed into tension (e.g., tensile strain) in response to the torque. 
     As shown in  FIG.  11 . 1   . 1 . 2 - 7 , the strain gauges  11 . 1 . 1 . 2 - 330 A,  11 . 1 . 1 . 2 - 330 B,  11 . 1 . 1 . 2 - 330 C, and  11 . 1 . 1 . 2 - 330 D can be coupled to each other in an electrical circuit  11 . 1 . 1 . 2 - 360 . The electrical circuit can be configured to monitor one or more electrical properties (e.g., resistance, capacitance, accumulated charge, inductance, and so on) of the strain gauges for changes. The electrical circuit  11 . 1 . 1 . 2 - 360  then quantifies these changes, which may be used to estimate the applied torque. For example, the electrical circuit  11 . 1 . 1 . 2 - 360  can include a Wheatstone bridge arrangement to measure an electrical resistance by balancing two legs of a bridge circuit with opposing pairs of strain gauges. An applied voltage can be compared to a measured voltage within the Wheatstone bridge of the electrical circuit  11 . 1 . 1 . 2 - 360  to determine a combined effect of torque on the multiple strain gauges  11 . 1 . 1 . 2 - 330 A,  11 . 1 . 1 . 2 - 330 B,  11 . 1 . 1 . 2 - 330 C, and  11 . 1 . 1 . 2 - 330 D. 
     The multiple strain gauges  11 . 1 . 1 . 2 - 330 A,  11 . 1 . 1 . 2 - 330 B,  11 . 1 . 1 . 2 - 330 C, and  11 . 1 . 1 . 2 - 330 D can be connected to approximate a magnitude of torsional strain (e.g., torque) experienced by the shaft  11 . 1 . 1 . 2 - 320 . In some embodiments, a magnitude of strain can be obtained by measuring a common property (e.g., parallel and/or series resistance) and/or a differential property (e.g., voltage division) of the multiple strain gauges. For example, differential property estimates can be combined with or compared to common property estimates. By further example, the differential property estimate and common property estimate can be combined by unweighted or weighted averaging. In some embodiments, the maximum or minimum of the two estimates can be used. In some embodiments, other methods of combining or deciding between the two estimates can be used. 
     Once the resistance of each strain gauge is obtained via calculation or measurement, each can be compared to a known baseline resistance value in order to determine whether the strain gauges are experiencing tension or compression. In other words, when the force-sensitive structure experiences a reaction force, it may deform, causing one or more strain gauges to either expand (e.g., tension) or contract (e.g., compression), which can cause the resistance thereof to change in a mathematically predictable manner. In some cases, the resistance or other electrical property of a strain gauge is measured as a relative value, which may factor environmental effects, such as temperature and/or residual or static strain. 
     For certain materials, resistance can change linearly with compression or tension. For other materials, resistance can change following a known curve in response to compression or tension. Accordingly, depending upon the material selected for the strain gauges, and the position of the strain gauges on the shaft  11 . 1 . 1 . 2 - 320 , a particular resistance and/or measured voltage can be correlated to a particular amount of strain experienced by a particular strain gauge, which in turn can itself be correlated to an amount of force applied to the force-sensitive structure, which in turn can be correlated to an amount of torque applied to the crown. 
     Referring now to  FIG.  11 . 1   . 1 . 2 - 8 , components of the head-mountable device can be operably connected to provide the performance described herein.  FIG.  11 . 1   . 1 . 2 - 8  shows a simplified block diagram of an illustrative head-mountable device  11 . 1 . 1 . 2 - 100  in accordance with one embodiment of the invention. It will be appreciated that components described herein can be provided on either or both of a frame and/or a securement element of the head-mountable device  11 . 1 . 1 . 2 - 100 . 
     As shown in  FIG.  11 . 1   . 1 . 2 - 8 , the head-mountable device  11 . 1 . 1 . 2 - 100  can include a controller  11 . 1 . 1 . 2 - 70  with one or more processing units that include or are configured to access a memory  11 . 1 . 1 . 2 - 18  having instructions stored thereon. The instructions or computer programs may be configured to perform one or more of the operations or functions described with respect to the head-mountable device  11 . 1 . 1 . 2 - 100 . The controller  11 . 1 . 1 . 2 - 70  can be implemented as any electronic device capable of processing, receiving, or transmitting data or instructions. For example, the controller  11 . 1 . 1 . 2 - 70  may include one or more of: a microprocessor, a central processing unit (CPU), an application-specific integrated circuit (ASIC), a digital signal processor (DSP), or combinations of such devices. As described herein, the term “processor” is meant to encompass a single processor or processing unit, multiple processors, multiple processing units, or other suitably configured computing element or elements. 
     The memory  11 . 1 . 1 . 2 - 18  can store electronic data that can be used by the head-mountable device  11 . 1 . 1 . 2 - 100 . For example, the memory  11 . 1 . 1 . 2 - 18  can store electrical data or content such as, for example, audio and video files, documents and applications, device settings and user preferences, timing and control signals or data for the various modules, data structures or databases, and so on. The memory  11 . 1 . 1 . 2 - 18  can be configured as any type of memory. By way of example only, the memory  11 . 1 . 1 . 2 - 18  can be implemented as random access memory, read-only memory, Flash memory, removable memory, or other types of storage elements, or combinations of such devices. 
     The head-mountable device  11 . 1 . 1 . 2 - 100  can include the camera  11 . 1 . 1 . 2 - 50  for capturing a view of an environment external to the head-mountable device  11 . 1 . 1 . 2 - 100 . The camera  11 . 1 . 1 . 2 - 50  can include an optical sensor, such as a photodiode or a photodiode array. Additionally or alternatively, the camera  11 . 1 . 1 . 2 - 50  can include one or more of various types of optical sensors that are arranged in various configurations for detecting user inputs described herein. The camera  11 . 1 . 1 . 2 - 50  may be configured to capture an image of a scene or subject located within a field of view of the camera  11 . 1 . 1 . 2 - 50 . The image may be stored in a digital file in accordance with any one of a number of digital formats. In some embodiments, the head-mountable device  11 . 1 . 1 . 2 - 100  includes a camera, which includes an image sensor formed from a charge-coupled device (CCD) and/or a complementary metal-oxide-semiconductor (CMOS) device, a photovoltaic cell, a photo resistive component, a laser scanner, and the like. It will be recognized that a camera can include other motion sensing devices. 
     The head-mountable device  11 . 1 . 1 . 2 - 100  can include a battery  11 . 1 . 1 . 2 - 20 , which can charge and/or power components of the head-mountable device  11 . 1 . 1 . 2 - 100 . 
     The head-mountable device  11 . 1 . 1 . 2 - 100  can include one or more other input/output components  11 . 1 . 1 . 2 - 26 , which can include any suitable component for connecting head-mountable device  11 . 1 . 1 . 2 - 100  to other devices. Suitable components can include, for example, audio/video jacks, data connectors, and/or any additional or alternative input/output components. The input/output components  11 . 1 . 1 . 2 - 26  can include a microphone. The microphone can be operably connected to the controller  11 . 1 . 1 . 2 - 70  for receiving audio input, including voice commands from the user. The input/output components  11 . 1 . 1 . 2 - 26  can include one or more speakers. The speakers can be operably connected to the controller  11 . 1 . 1 . 2 - 70  for control of speaker output, including sound levels. 
     The head-mountable device  11 . 1 . 1 . 2 - 100  can include communications circuitry  11 . 1 . 1 . 2 - 28  for communicating with one or more servers or other devices using any suitable communications protocol. For example, the communications circuitry  11 . 1 . 1 . 2 - 28  can support Wi-Fi (e.g., a 802.11 protocol), Ethernet, Bluetooth, high frequency systems (e.g., 900 MHZ, 2.4 GHz, and 5.6 GHz communication systems), infrared, TCP/IP (e.g., any of the protocols used in each of the TCP/IP layers), HTTP, BitTorrent, FTP, RTP, RTSP, SSH, any other communications protocol, or any combination thereof. The communications circuitry  11 . 1 . 1 . 2 - 28  can also include an antenna for transmitting and receiving electromagnetic signals. 
     The head-mountable device  11 . 1 . 1 . 2 - 100  can further include an optical module for displaying visual information for a user, including the display screen  11 . 1 . 1 . 2 - 90  and/or an optical subassembly  11 . 1 . 1 . 2 - 14 . The head-mountable device  11 . 1 . 1 . 2 - 100  can include an optical subassembly  11 . 1 . 1 . 2 - 14  configured to help optically adjust and correctly project the image based content being displayed by the display  11 . 1 . 1 . 2 - 90  for close up viewing. The optical subassembly  11 . 1 . 1 . 2 - 14  can include one or more lenses, mirrors, or other optical devices. 
     The head-mountable device  11 . 1 . 1 . 2 - 100  can optionally connect to a portable electronic device  11 . 1 . 1 . 2 - 22 , which can provide certain functions. The portable electronic device  11 . 1 . 1 . 2 - 22  can provide a handheld form factor (e.g., small portable electronic device which is light weight, fits in a pocket, etc.). Although not limited to these, examples include media players, phones (including smart phones), PDAs, watches, computers, and the like. The portable electronic device may include a screen for presenting the graphical portion of the media to the user. The portable electronic device can provide processing capabilities by communicating with the controller  11 . 1 . 1 . 2 - 70  of the head-mountable device  11 . 1 . 1 . 2 - 100 . The portable electronic device can provide actuation to dislodge particles from components of the head-mountable device  11 . 1 . 1 . 2 - 100  by operating a haptic feedback element of the portable electronic device. 
     The head-mountable device  11 . 1 . 1 . 2 - 100  can include a dock  11 . 1 . 1 . 2 - 32  operative to receive the portable electronic device  11 . 1 . 1 . 2 - 22 . The dock  11 . 1 . 1 . 2 - 32  can include a connector (e.g., Lightning, USB, FireWire, power, DVI, etc.), which can be plugged into a complementary connector of the portable electronic device  11 . 1 . 1 . 2 - 22 . The dock  11 . 1 . 1 . 2 - 32  may include features for helping to align the connectors during engagement and for physically coupling the portable electronic device  11 . 1 . 1 . 2 - 22  to the head-mountable device  11 . 1 . 1 . 2 - 100 . For example, the dock  11 . 1 . 1 . 2 - 32  may define a cavity for placement of the portable electronic device  11 . 1 . 1 . 2 - 22 . The dock  11 . 1 . 1 . 2 - 32  may also include retaining features for securing portable electronic device  11 . 1 . 1 . 2 - 22  within the cavity. The connector on the dock  11 . 1 . 1 . 2 - 32  can function as a communication interface between the portable electronic device  11 . 1 . 1 . 2 - 22  and the head-mountable device  11 . 1 . 1 . 2 - 100 . 
     The head-mountable device  11 . 1 . 1 . 2 - 100  can include one or more other sensors  11 . 1 . 1 . 2 - 40 . Such sensors can be configured to sense substantially any type of characteristic such as, but not limited to, images, pressure, light, touch, force, temperature, position, motion, and so on. For example, the sensor can be a photodetector, a temperature sensor, a light or optical sensor, an atmospheric pressure sensor, a humidity sensor, a magnet, a gyroscope, an accelerometer, a chemical sensor, an ozone sensor, a particulate count sensor, and so on. By further example, the sensor can be a bio-sensor for tracking biometric characteristics, such as health and activity metrics. Other user sensors can perform facial feature detection, facial movement detection, facial recognition, eye tracking, user mood detection, user emotion detection, voice detection, etc. 
     The head-mountable device  11 . 1 . 1 . 2 - 100  can include one or more sensors (e.g., eye sensor) for tracking features of the user wearing the head-mountable device  11 . 1 . 1 . 2 - 100 . For example, such sensors can perform facial feature detection, facial movement detection, facial recognition, eye tracking, user mood detection, user emotion detection, voice detection, etc. For example, an eye sensor can optically capture a view of an eye (e.g., pupil) and determine a direction of a gaze of the user. Such eye tracking may be used to determine a location and/or direction of interest. Detection and/or amplification of sound can then be focused if it is received from sources at such a location and/or along such a direction. 
     While some embodiments of touch-based input devices disclosed herein relate to head-mountable devices, it will be appreciated that the subject technology can encompass and be applied to other devices. For example, an input device (e.g., crown module) in accordance with embodiments disclosed herein can include a phone, a tablet computing device, a mobile computing device, a watch, a laptop computing device, a mouse, a game controller, a remote control, a digital media player, a stylus, and/or any other electronic device. Further, the external device can be any device that interacts with a touch-based input device. For example, an external device in accordance with embodiments disclosed herein can include a tablet, a phone, a laptop computing device, a desktop computing device, a wearable device, a mobile computing device, a tablet computing device, a display, a television, a phone, a digital media player, and/or any other electronic device. 
     Accordingly, embodiments of the present disclosure provide a head-mountable device with a crown module having an input system that allows a user to provide inputs by rotating or otherwise applying torque to a crown of the crown module. The head-mountable device can interpret the rotation and/or torque as a user input. The crown module can further include a feedback system that provides localized haptic feedback at the crown. The haptic feedback can be effectively perceived by the user at the crown without causing the entire head-mountable device to vibrate against the head and/or face of the user. 
     Various examples of aspects of the disclosure are described below as clauses for convenience. These are provided as examples, and do not limit the subject technology. 
     Clause A: a head-mountable device comprising: a housing; a crown positioned at least partially outside the housing; a shaft positioned within the housing and connected to the crown such that the shaft rotates with the crown and about an axis; a sensor for detecting rotation of the shaft; a magnetic element coupled to the shaft such that the magnetic element moves along the axis with the shaft; and a coil coupled to the housing and configured to induce a magnetic field in the magnetic element, such that, when the coil is activated, the magnetic element provides haptic feedback by moving the shaft and the crown relative to the housing. 
     Clause B: a head-mountable device comprising: a housing; a crown positioned at least partially outside the housing; a shaft positioned within the housing and connected to the crown such that a torque applied to the crown is transferred to the shaft; a sensor for detecting the torque transferred to the shaft; and a haptic feedback device coupled to the housing and configured to provide haptic feedback by moving the shaft and the crown relative to the housing. 
     Clause C: a crown module for a head-mountable device, the crown module comprising: a housing configured to be coupled to a frame of the head-mountable device; a crown positioned at least partially outside the housing; a shaft positioned within a sealed chamber of the housing and connected to the crown such that the shaft rotates with the crown and about an axis; a sensor for detecting rotation of the shaft and positioned within the sealed chamber; and a haptic feedback device positioned within the sealed chamber of the housing and configured to provide haptic feedback by moving the shaft and the crown along the axis and relative to the housing. 
     One or more of the above clauses can include one or more of the features described below. It is noted that any of the following clauses may be combined in any combination with each other, and placed into a respective independent clause, e.g., clause A, B, or C. 
     Clause 1: a frame; a display on an inner side of the frame; a camera on an outer side of the frame; a speaker; and a microphone. 
     Clause 2: the shaft is positioned within a sealed chamber of the housing that contains the sensor. 
     Clause 3: a pair of seal members each sealingly engaging an inner surface of the housing and an outer surface of the shaft, wherein the sensor is positioned axially between the pair of seal members. 
     Clause 4: the sensor is an optical sensor and the shaft comprises a visual feature for detection by the optical sensor. 
     Clause 5: the magnetic element and the coil are positioned within a sealed chamber of the housing. 
     Clause 6: the magnetic element is coupled to the shaft such that the magnetic element is configured to rotate independently of the shaft. 
     Clause 7: the coil is a first coil on a first axial side of the magnetic element; and the head-mountable device comprises a second coil on a second axial side of the magnetic element. 
     Clause 8: a controller configured to operate the coil in response to a detection by the sensor that the shaft is rotating. 
     Clause 9: the torque is about an axis and the haptic feedback comprises movement along the axis. 
     Clause 10: the sensor comprises multiple strain gauges. 
     Clause 11: the sensor comprises multiple strain gauges electrically connected to each other in a Wheatstone bridge arrangement. 
     Clause 12: at least two of the multiple strain gauges have different orientations relative to a longitudinal axis of the shaft. 
     Clause 13: the haptic feedback device comprises an actuator configured to expand toward and contract away from the shaft. 
     Clause 14: a controller configured to operate the haptic feedback device in response to a detection by the sensor that the torque is applied to the crown. 
     Clause 15: a pair of seal members each sealingly engaging an inner surface of the housing and an outer surface of the shaft, wherein the sensor is positioned axially between the pair of seal members. 
     Clause 16: the sensor is an optical sensor and the shaft comprises a visual feature for detection by the optical sensor. 
     Clause 17: the haptic feedback device comprises: a magnetic element coupled to the shaft such that the magnetic element moves along the axis with the shaft; and a coil coupled to the housing and configured to induce a magnetic field in the magnetic element, such that, when the coil is activated, the magnetic element provides haptic feedback by moving the shaft and the crown relative to the housing. 
     11.1.2: Wishbone and Mustache 
       FIG.  11 . 1   . 2 - 1  illustrates a front perspective view of a portion of an HMD  11 . 1 . 2 - 100 , including an outer structural frame  11 . 1 . 2 - 102  and an inner or intermediate structural frame  11 . 1 . 2 - 104  defining first and second apertures  11 . 1 . 2 - 106   a ,  11 . 1 . 2 - 106   b . The apertures  11 . 1 . 2 - 106   a - b  are shown in dotted lines in  FIG.  11 . 1   . 2 - 1  because a view of the apertures  11 . 1 . 2 - 106   a - b  can be blocked by one or more other components of the HMD  11 . 1 . 2 - 100  coupled to the inner frame  11 . 1 . 2 - 104  and/or the outer frame  11 . 1 . 2 - 102 , as shown. In at least one example, the HMD  11 . 1 . 2 - 100  can include a first mounting bracket  11 . 1 . 2 - 108  coupled to the inner frame  11 . 1 . 2 - 104 . In at least one example, the mounting bracket  11 . 1 . 2 - 108  is coupled to the inner frame  11 . 1 . 2 - 104  between the first and second apertures  11 . 1 . 2 - 106   a - b.    
     The mounting bracket  11 . 1 . 2 - 108  can include a middle or central portion  11 . 1 . 2 - 109  coupled to the inner frame  11 . 1 . 2 - 104 . In some examples, the middle or central portion  11 . 1 . 2 - 109  may not be the geometric middle or center of the bracket  11 . 1 . 2 - 108 . Rather, the middle/central portion  11 . 1 . 2 - 109  can be disposed between first and second cantilevered extension arms extending away from the middle portion  11 . 1 . 2 - 109 . In at least one example, the mounting bracket  108  includes a first cantilever arm  11 . 1 . 2 - 112  and a second cantilever arm  11 . 1 . 2 - 114  extending away from the middle portion  11 . 1 . 2 - 109  of the mount bracket  11 . 1 . 2 - 108  coupled to the inner frame  11 . 1 . 2 - 104 . 
     As shown in  FIG.  11 . 1   . 2 - 1 , the outer frame  11 . 1 . 2 - 102  can define a curved geometry on a lower side thereof to accommodate a user&#39;s nose when the user dons the HMD  11 . 1 . 2 - 100 . The curved geometry can be referred to as a nose bridge  11 . 1 . 2 - 111  and be centrally located on a lower side of the HMD  11 . 1 . 2 - 100  as shown. In at least one example, the mounting bracket  11 . 1 . 2 - 108  can be connected to the inner frame  11 . 1 . 2 - 104  between the apertures  11 . 1 . 2 - 106   a - b  such that the cantilevered arms  11 . 1 . 2 - 112 ,  11 . 1 . 2 - 114  extend downward and laterally outward away from the middle portion  11 . 1 . 2 - 109  to compliment the nose bridge  11 . 1 . 2 - 111  geometry of the outer frame  11 . 1 . 2 - 102 . In this way, the mounting bracket  11 . 1 . 2 - 108  is configured to accommodate the user&#39;s nose as noted above. The nose bridge  11 . 1 . 2 - 111  geometry accommodates the nose in that the nose bridge  11 . 1 . 2 - 111  provides a curvature that curves with, above, over, and around the user&#39;s nose for comfort and fit. 
     The first cantilever arm  11 . 1 . 2 - 112  can extend away from the middle portion  11 . 1 . 2 - 109  of the mounting bracket  11 . 1 . 2 - 108  in a first direction and the second cantilever arm  11 . 1 . 2 - 114  can extend away from the middle portion  11 . 1 . 2 - 109  of the mounting bracket  11 . 1 . 2 - 10  in a second direction opposite the first direction. The first and second cantilever arms  11 . 1 . 2 - 112 ,  11 . 1 . 2 - 114  are referred to as “cantilevered” or “cantilever” arms because each arm  11 . 1 . 2 - 112 ,  11 . 1 . 2 - 114 , includes a distal free end  11 . 1 . 2 - 116 ,  11 . 1 . 2 - 118 , respectively, which are free of affixation from the inner and outer frames  11 . 1 . 2 - 102 ,  11 . 1 . 2 - 104 . In this way, the arms  11 . 1 . 2 - 112 ,  11 . 1 . 2 - 114  are cantilevered from the middle portion  11 . 1 . 2 - 109 , which can be connected to the inner frame  11 . 1 . 2 - 104 , with distal ends  11 . 1 . 2 - 102 ,  11 . 1 . 2 - 104  unattached. 
     In at least one example, the HMD  11 . 1 . 2 - 100  can include one or more components coupled to the mounting bracket  11 . 1 . 2 - 108 . In one example, the components include a plurality of sensors  11 . 1 . 2 - 110   a - f . Each sensor of the plurality of sensors  11 . 1 . 2 - 110   a - f  can include various types of sensors, including cameras, IR sensors, and so forth. In some examples, one or more of the sensors  11 . 1 . 2 - 110   a - f  can be used for object recognition in three-dimensional space such that it is important to maintain a precise relative position of two or more of the plurality of sensors  11 . 1 . 2 - 110   a - f . The cantilevered nature of the mounting bracket  11 . 1 . 2 - 108  can protect the sensors  11 . 1 . 2 - 110   a - f  from damage and altered positioning in the case of accidental drops by the user. Because the sensors  11 . 1 . 2 - 110   a - f  are cantilevered on the arms  11 . 1 . 2 - 112 ,  11 . 1 . 2 - 114  of the mounting bracket  11 . 1 . 2 - 108 , stresses and deformations of the inner and/or outer frames  11 . 1 . 2 - 104 ,  11 . 1 . 2 - 102  are not transferred to the cantilevered arms  11 . 1 . 2 - 112 ,  11 . 1 . 2 - 114  and thus do not affect the relative positioning of the sensors  11 . 1 . 2 - 110   a - f  coupled/mounted to the mounting bracket  11 . 1 . 2 - 108 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  11 . 1   . 2 - 1  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  11 . 1   . 2 - 2 - 11 . 1 . 2 - 5  and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  11 . 1   . 2 - 2 - 11 . 1 . 2 - 5  can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  11 . 1   . 2 - 1 . 
       FIG.  11 . 1   . 2 - 2  illustrates a perspective view of an example of a mounting bracket  11 . 1 . 2 - 208  including a middle portion  11 . 1 . 2 - 209  and first and second arms  11 . 1 . 2 - 212 ,  11 . 1 . 2 - 214  extending from the middle portion  11 . 1 . 2 - 209  to form a nose bridge  11 . 1 . 2 - 211  geometry. One or more components including sensors  11 . 1 . 2 - 210   a - f  can be coupled to the mounting bracket  11 . 1 . 2 - 208 , including to the cantilever arms  11 . 1 . 2 - 212 ,  11 . 1 . 2 - 214 . In at least one example, the material of the mounting bracket  11 . 1 . 2 - 208  can include a strong, durable, and stiff material and/or geometry to maintain a relative position between sensors  11 . 1 . 2 - 210   a - f  during drop events or other forces causing deformations to the frame to which the mounting bracket  11 . 1 . 2 - 209  is coupled. The material of the mounting bracket  11 . 1 . 2 - 208  can include metals such as aluminum, steel including stainless steel, magnesium or magnesium alloys, hard plastics, ceramics, composite materials, carbon fiber, or any combination thereof. 
     In at least one example, a first sensor, including any of the sensors  11 . 1 . 2 - 210   a - c  can be coupled to the first cantilever arm  11 . 1 . 2 - 212  and a second sensor, including any of the sensors  11 . 1 . 2 - 210   e - f  can be coupled to the second cantilever arm  11 . 1 . 2 - 214 . In at least one example, the first sensor can include a visual sensor such as a camera or an infrared sensor. In at least one example, the second sensor can include a visual sensor such as a camera or an infrared sensor. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  11 . 1   . 2 - 2  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  11 . 1   . 2 - 1  and  11 . 1 . 2 - 3 - 11 . 1 . 2 - 5  and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  11 . 1   . 2 - 1  and  11 . 1 . 2 - 3 - 11 . 1 . 2 - 5  can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  11 . 1   . 2 - 2 . 
       FIG.  11 . 1   . 2 - 3  illustrates a perspective view of a partially assembled HMD  11 . 1 . 2 - 300  including an outer frame  11 . 1 . 2 - 302 , an inner frame  11 . 1 . 2 - 304  coupled to the outer frame  11 . 1 . 2 - 302 , and a second mounting bracket  11 . 1 . 2 - 320  coupled to the inner frame  11 . 1 . 2 - 304  between first and second apertures  11 . 1 . 2 - 306   a - b  defined by the inner frame  11 . 1 . 2 - 304 . In at least one example, the second mounting bracket  11 . 1 . 2 - 320  can include first and second cantilevered mounting arms  11 . 1 . 2 - 322 ,  11 . 1 . 2 - 324  extending in opposite directions from one another from a middle portion  11 . 1 . 2 - 326  of the mounting bracket  11 . 1 . 2 - 320 . The first and second cantilever arms  11 . 1 . 2 - 322 ,  11 . 1 . 2 - 324  can be free of affixation to any other component or frame of the HMD  11 . 1 . 2 - 300  similar to the cantilever arms  11 . 1 . 2 - 112 ,  11 . 1 . 2 - 114  of the mounting bracket  11 . 1 . 2 - 108  shown in  FIG.  11 . 1   . 2 - 1 . In at least one example, the mounting bracket  11 . 1 . 2 - 320  can be coupled to the inner frame  11 . 1 . 2 - 304  at the middle portion  11 . 1 . 2 - 326  of the mounting bracket  11 . 1 . 2 - 320 . 
     In at least one example, the first cantilever arm  11 . 1 . 2 - 322  can extend from the middle portion  11 . 1 . 2 - 326  in a first direction and the second cantilever arm  11 . 1 . 2 - 324  can extend from the middle portion  11 . 1 . 2 - 326  in a second direction opposite the first direction. In at least one example, a first guide rod  11 . 1 . 2 - 328  can be coupled to the first cantilever arm  11 . 1 . 2 - 322  and extend in the first direction and a second guide rod  11 . 1 . 2 - 330  can be coupled to the second cantilever arm  11 . 1 . 2 - 324  and extend in the second direction opposite the first direction. 
     In at least one example, first and second optical modules (not shown in  FIG.  11 . 1   . 2 - 3 ) can be slidably/adjustably coupled to the mounting bracket  11 . 1 . 2 - 320  via the first and second guide rods  11 . 1 . 2 - 328 ,  11 . 1 . 2 - 330 . The guide rods  11 . 1 . 2 - 328 ,  11 . 1 . 2 - 330  can include distal ends  11 . 1 . 2 - 332 ,  11 . 1 . 2 - 334 , respectively, which are opposite respective proximal ends  11 . 1 . 2 - 333 ,  11 . 1 . 2 - 335 . The distal ends  11 . 1 . 2 - 332 ,  11 . 1 . 2 - 334  can be free of affixation to any other component of frame  11 . 1 . 2 - 302 ,  11 . 1 . 2 - 304  of the HMD  11 . 1 . 2 - 300 . In this way, the guide rods  11 . 1 . 2 - 328 ,  11 . 1 . 2 - 330  are cantilevered with free ends floating. In one or more examples, as noted above, one or more optical modules, including display screens, sensors, structural components, and electronic components can be slidably coupled to the inner frame  11 . 1 . 2 - 304  via the guide rods  11 . 1 . 2 - 328 ,  11 . 1 . 2 - 330  and cantilever arms  11 . 1 . 2 - 322 ,  11 . 1 . 2 - 324  of the mounting bracket  11 . 1 . 2 - 320 . The sensors, display screens, and other components of the optical module may be such that they operate most effectively when a relative distance between and position of those coupled to the first guide rod  11 . 1 . 2 - 328  and the second guide rod  11 . 1 . 2 - 330  are maintained, even during drop events and when other structural components, including the outer and/or inner frames  11 . 1 . 2 - 302 ,  11 . 1 . 2 - 304  are inadvertently deformed or bent. 
     Along these lines, the cantilevered nature of the mounting bracket  11 . 1 . 2 - 320  can protect the optical modules and components thereof from damage and altered positioning in the case of accidental drops by the user. Because the optical modules cantilevered on the guide rods  11 . 1 . 2 - 328 ,  11 . 1 . 2 - 330  and the cantilever arms  11 . 1 . 2 - 322 ,  11 . 1 . 2 - 324  of the mounting bracket  11 . 1 . 2 - 320 , stresses and deformations of the inner and/or outer frames  11 . 1 . 2 - 304 ,  11 . 1 . 2 - 302  are not transferred to the cantilevered arms  11 . 1 . 2 - 322 ,  11 . 1 . 2 - 323  and/or the guide rods  11 . 1 . 2 - 328 ,  11 . 1 . 2 - 330  and thus do not affect the relative positioning of the optical modules slidably coupled/mounted to the mounting bracket  11 . 1 . 2 - 320 . Along these lines, the material of the mounting bracket  11 . 1 . 2 - 320  can include metals such as aluminum, steel including stainless steel, magnesium or magnesium alloys, hard plastics, ceramics, composite materials, carbon fiber, or any combination thereof. In at least one example, the guide rods  11 . 1 . 2 - 328 ,  11 . 1 . 2 - 330  can include low weight, low friction materials. For example, the guide rods  11 . 1 . 2 - 328 ,  11 . 1 . 2 - 330  can include carbon fiber laminate materials. The guide rods  11 . 1 . 2 - 328 ,  11 . 1 . 2 - 330  can be formed as hollow tubes to decrease weight. 
     In at least one example, the first and second cantilever arms  11 . 1 . 2 - 322  and  11 . 1 . 2 - 324  and/or the first and second guide-rods  11 . 1 . 2 - 328  and  11 . 1 . 2 - 330  extending therefrom, respectively, can be disposed at an angle relative to one another. The angle can be such that the bracket  11 . 1 . 2 - 326  and associated components, including guide-rods  11 . 1 . 2 - 328  and  11 . 1 . 2 - 330 , cantilever arms  11 . 1 . 2 - 322  and  11 . 1 . 2 - 324 , and display units connected to the guide-rods  11 . 1 . 2 - 328  and  11 . 1 . 2 - 330  are curved and/or oriented to complement the curvature of the user&#39;s face. In at least one example, the angle can be between about 5-degrees and about 25-degrees. In one example, the angle can be between about 10-degrees and about 20-degrees, for example about 15-degrees. In at least one example, the bracket  11 . 1 . 2 - 326  can be made of magnesium and carbon fiber panels on a rear side thereof to add stiffness. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  11 . 1   . 2 - 3  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  11 . 1   . 2 - 1 - 11 . 1 . 2 - 2  and  11 . 1 . 2 - 4 - 11 . 1 . 2 - 5  and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  11 . 1   . 2 - 1 - 11 . 1 . 2 - 2  and  11 . 1 . 2 - 4 - 11 . 1 . 2 - 5  can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  11 . 1   . 2 - 3 . 
       FIG.  11 . 1   . 2 - 4  illustrates a portion of an HMD  11 . 1 . 2 - 400  including an optical module  11 . 1 . 2 - 436  slidably engaged to a mounting bracket via a guide rod  11 . 1 . 2 - 428 , which can be similar to the guide rod  11 . 1 . 2 - 328  shown in  FIG.  11 . 1   . 2 - 3  and described above. The optical module  11 . 1 . 2 - 436  can include a display assembly  11 . 1 . 2 - 438  including a display screen  11 . 1 . 2 - 440 , all of which can be slidably engaged with the guide rod  11 . 1 . 2 - 428  as shown. As noted above, the components and parts shown and described in  FIGS.  11 . 1   . 2 - 1 - 11 . 1 . 2 - 3  can be one of a set of components, including a set of two optical modules as part of a single HMD  11 . 1 . 2 - 400  such that the optical module  11 . 1 . 2 - 428  shown in  FIG.  11 . 1   . 2 - 4  is one of two optical modules with the second optical module slidably engaged/coupled with a guide rod extending in an opposite direction to that of the guide rod  11 . 1 . 2 - 428  shown from a mounting bracket. Likewise, the display assembly  11 . 1 . 2 - 438  and the display screen  11 . 1 . 2 - 440  can be one of a set of two display assemblies and display screens within the single HMD  11 . 1 . 2 - 400 . 
     In at least one example, the optical module  11 . 1 . 2 - 436  can include a micro-OLED display coupled to a heat sink via and adhesive, such as a pressure sensitive adhesive or other adhesive. In at least one example, the optical module  11 . 1 . 2 - 436  can include a silicone backplane. The heat sink can include magnesium and have no fins. In one example, a drop or more of structural adhesive can be applied during assembly/manufacturing to the center of the back of the display of the optical module  11 . 1 . 2 - 436  to lock the display in with the heat sink. The adhesive can include epoxy. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  11 . 1   . 2 - 4  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  11 . 1   . 2 - 1 - 11 . 1 . 2 - 3  and  11 . 1 . 2 - 5  and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  11 . 1   . 2 - 1 - 11 . 1 . 2 - 3  and  11 . 1 . 2 - 5  can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  11 . 1   . 2 - 4 . 
       FIG.  11 . 1   . 2 - 5  illustrates an example of an HMD  11 . 1 . 2 - 500  including a first, outer structural frame  11 . 1 . 2 - 502  defining an internal volume  11 . 1 . 2 - 542 . A front cover  11 . 1 . 2 - 541  and a rear cover  11 . 1 . 2 - 543  disposed opposite the front cover  11 . 1 . 2 - 541  can also define the internal volume  11 . 1 . 2 - 542 . In at least one example, the HMD  11 . 1 . 2 - 500  can also include a second, inner structural frame  11 . 1 . 2 - 504  disposed in the internal volume  11 . 1 . 2 - 542  and coupled to the outer frame  11 . 1 . 2 - 502 . The inner frame  11 . 1 . 2 - 502  can define a first side  11 . 1 . 2 - 503  and a second side  11 . 1 . 2 - 505  opposite the first side  11 . 1 . 2 - 503 . 
     In at least one example, the HMD  11 . 1 . 2 - 500  can include a first mounting bracket  11 . 1 . 2 - 508  coupled to the first side  11 . 1 . 2 - 503  of the inner frame  11 . 1 . 2 - 504  and a second mounting bracket  11 . 1 . 2 - 520  coupled to the second side  11 . 1 . 2 - 505  of the inner frame  11 . 1 . 2 - 504 . In at least one example, the first mounting bracket  11 . 1 . 2 - 508  can be similar to the first mounting brackets  11 . 1 . 2 - 108 ,  11 . 1 . 2 - 208  shown in  FIGS.  11 . 1   . 2 - 1  and  11 . 1 . 2 - 2  and described above. In at least one example, the second mounting bracket  11 . 1 . 2 - 520  can be similar to the other second mounting bracket  11 . 1 . 2 - 320  described above and shown in  FIG.  11 . 1   . 2 - 3 . 
     In at least one example, one or more sensors  11 . 1 . 2 - 510   a ,  11 . 1 . 2 - 510   b  can be mounted or coupled to a cantilever arm or middle portion of the first mounting bracket  11 . 1 . 2 - 508 . In at least one example, an optical module  11 . 1 . 2 - 536  can be slidably engaged/coupled to the second mounting bracket  11 . 1 . 2 - 520 . The optical module  11 . 1 . 2 - 536  can include a display assembly including a display screen  11 . 1 . 2 - 540  and one or more sensors  11 . 1 . 2 - 544   a ,  11 . 1 . 2 - 544   b . In at least one example, at least one of the sensors  11 . 1 . 2 - 544   a - b  can include a visual sensor such as a camera. In at least one example, the first and second sensors  11 . 1 . 2 - 544   a - b  are both cameras. 
     As oriented in the view of  FIG.  11 . 1   . 2 - 5 , the sensors  11 . 1 . 2 - 510   a - b  coupled to the first mounting bracket  11 . 1 . 2 - 508  can be pointed in a forward direction and the sensors  11 . 1 . 2 - 544   a - b  and/or display screen  11 . 1 . 2 - 540  of the optical module  11 . 1 . 2 - 536  can be pointed in a rearward direction opposite the forward direction. In at least one example, the sensors  11 . 1 . 2 - 510   a - b  coupled to the first mounting bracket  11 . 1 . 2 - 508  can include image capturing sensors or other sensors configured to object recognition and detection in front of or around the HMD  11 . 1 . 2 - 500 . The detected, sensed, or captured images and objects can be displayed by the display screen  11 . 1 . 2 - 536 , which can be configured to project the images and light rearward toward a user&#39;s eyes when the user dons the HMD  11 . 1 . 2 - 500 . 
     In at least one example, the HMD  11 . 1 . 2 - 500  can also include a controller electronically coupled to the sensors  11 . 1 . 2 - 510   a - b  and optical module  11 . 1 . 2 - 536 , including the display screen  11 . 1 . 2 - 540  and sensors  11 . 1 . 2 - 544   a - b . The controller can be configured to capture an image via one or more of the sensors  11 . 1 . 2 - 510   a - b  coupled to the first mounting bracket  11 . 1 . 2 - 508  and display the image on the display screen  11 . 1 . 2 - 540  of the optical module  11 . 1 . 2 - 536  coupled to the second mounting bracket  11 . 1 . 2 - 520 . In order to project an accurate representation of the captured objects in real space to the user on the display screen  11 . 1 . 2 - 540 , the relative position, distance, angles, and orientation between the sensors  11 . 1 . 2 - 510   a - b  and the display screen  11 . 1 . 2 - 540  can be maintained even during drop events or when the outer/inner frames  11 . 1 . 2 - 502 ,  11 . 1 . 2 - 504  are deformed or bumped. The first mounting bracket  11 . 1 . 2 - 508  can be cantilevered and centrally coupled to the inner frame  11 . 1 . 2 - 504  at the first side  11 . 1 . 2 - 503  thereof as noted above and the second mounting bracket  11 . 1 . 2 - 520  can be cantilevered and centrally coupled to the inner frame  11 . 1 . 2 - 504  at the second side  11 . 1 . 2 - 505  thereof. In this way, the positions, orientations, angles, and distances between sensors  11 . 1 . 2 - 510   a - b  of the first mounting bracket  11 . 1 . 2 - 508  and the sensors  11 . 1 . 2 - 544   a - b  and display screen  11 . 1 . 2 - 540  of the second mounting bracket  11 . 1 . 2 - 520  can be maintained. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  11 . 1   . 2 - 5  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  11 . 1   . 2 - 1 - 11 . 1 . 2 - 4  and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  11 . 1   . 2 - 1 - 11 . 1 . 2 - 4  can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  11 . 1   . 2 - 5 . 
     11.1.3: Upper Guide Rod System 
       FIG.  11 . 1   . 3 - 1  illustrates a rear perspective view of a display assembly  11 . 1 . 3 - 102  of an HMD  11 . 1 . 3 - 100  with one or more components removed, including a curtain assembly, in order to illustrate various internal components of the HMD  11 . 1 . 3 - 100 .  FIG.  11 . 1   . 3 - 1  illustrates the display assembly  11 . 1 . 3 - 102 , which can include a first display module  11 . 1 . 3 - 104   a  and a second display module  11 . 1 . 3 - 104   b . The first display module  11 . 1 . 3 - 104   a  can be slidably engaged/coupled with a first adjustment mechanism  11 . 1 . 3 - 106   a  and the second optical module  11 . 1 . 3 - 104   b  can be slidably engaged/coupled with a second adjustment mechanism  11 . 1 . 3 - 106   b . The first and second adjustment mechanisms  11 . 1 . 3 - 106   a - b  can include first and second respective guide-rods  11 . 1 . 3 - 108   a - b  and motors  11 . 1 . 3 - 110   a - b . The first and second guide-rods can be similar such that the description of one applies to the other, with the first guide-rod  11 . 1 . 3 - 108   a  extending a first direction away from a bracket  11 . 1 . 3 - 112  and the second guide-rod  11 . 1 . 3 - 108   b  extending a second direction away from the bracket  11 . 1 . 3 - 112  opposite the first direction. 
     Likewise, the first and second motors  11 . 1 . 3 - 110   a - b  can be similar such that the description of one applies to the other, with the first motor  11 . 1 . 3 - 110   a  mechanically engaging the first optical module  11 . 1 . 3 - 104   a  to adjust the position thereof via the first guide-rod  11 . 1 . 3 - 108   a  and the second motor  11 . 1 . 3 - 110   b  mechanically engaging the second optical module  11 . 1 . 3 - 104   b  to adjust the position thereof via the second guide-rod  11 . 1 . 3 - 108   b . In at least one example, the HMD  11 . 1 . 3 - 100  also includes a depressible and/or rotatable button  11 . 1 . 3 - 114  electrically coupled to the first and second motors  11 . 1 . 3 - 110   a - b . The button  11 . 1 . 3 - 114  can electrically communicate with the first and second motors  11 . 1 . 3 - 110   a - b  via a processor or other circuitry components to cause the first and second motors  11 . 1 . 3 - 110   a - b  to activate and cause the first and second optical modules  11 . 1 . 3 - 104   a - b , respectively, to change position relative to one another. 
     In at least one example, the first and second optical modules  11 . 1 . 3 - 104   a - b  can include respective display screens configured to project light toward the user&#39;s eyes when donning the HMD  11 . 1 . 3 - 100 . In at least one example, the user can manipulate (i.e., depress and/or rotate) the button  11 . 1 . 3 - 114  to activate a positional adjustment of the optical modules  11 . 1 . 3 - 104   a - b  to match the inter-pupillary distance of the user&#39;s eyes. The optical modules  11 . 1 . 3 - 104   a - b  can also include one or more cameras or other sensors/sensor systems for imaging and measuring the IPD of the user such that the optical modules  11 . 1 . 3 - 104   a - b  can be adjusted to match the IPD. 
     The first and second optical modules  11 . 1 . 3 - 104   a - b  can be mechanically and slidably coupled to the first and second guide-rods  11 . 1 . 3 - 108   a - b , respectively. In at least one example, the first optical module  11 . 1 . 3 - 104   a  can include an aperture, through-hole, or channel engaged with the guide-rod  11 . 1 . 3 - 108   a  to hold the optical module  11 . 1 . 3 - 104   a  in position axially with the guide-rod  11 . 1 . 3 - 108   a  as the optical module  11 . 1 . 3 - 104   a  slides along the guide-rod  11 . 1 . 3 - 108   a  during IPD adjustments. The same can be included in the second optical module  11 . 1 . 3 - 104   b . In some examples, the optical modules  11 . 1 . 3 - 104   a - b  can include a guide-rod slidably engaged with respective channels of the adjustment mechanism  11 . 1 . 3 - 102 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  11 . 1   . 3 - 1  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in any other figures shown and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to any other figure shown and described herein, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  11 . 1   . 3 - 1 . 
       FIG.  11 . 1   . 3 - 2  illustrates a close up view of an example of an HMD  11 . 1 . 3 - 200  and an adjustment mechanism  11 . 1 . 3 - 202  similar to those shown in  FIG.  11 . 1   . 3 - 1  but with the optical module removed to more clearly illustrate the upper guide-rod  11 . 1 . 3 - 208 . The guide rod  11 . 1 . 3 - 208  can extend at or near or above a top portion or side of the optical module and guide the optical module back and forth axially along the guide-rod  11 . 1 . 3 - 208  during IPD adjustments described above. In at least one example, the guide-rod  11 . 1 . 3 - 208  extends from a central bracket  11 . 1 . 3 - 212  secured to a frame or chassis  11 . 1 . 3 - 216  of the HMD  11 . 1 . 3 - 200 . In one example, the guide-rod  11 . 1 . 3 - 208  is cantilevered such that a first, proximal end  11 . 1 . 3 - 220  of the guide-rod  11 . 1 . 3 - 208  is secured to the bracket  11 . 1 . 3 - 212  and a second, distal end  11 . 1 . 3 - 222  opposite the first end  11 . 1 . 3 - 220  is free of affixation to any other structure. In this way, the guide-rod  11 . 1 . 3 - 208  can be cantilevered away from the bracket  11 . 1 . 3 - 212 . 
     In at least one example, the adjustment mechanism  11 . 1 . 3 - 202  also includes a stop  11 . 1 . 3 - 218  positioned at or near the distal end  11 . 1 . 3 - 222  of the guide-rod  11 . 1 . 3 - 208 . In at least one example, the stop  11 . 1 . 3 - 218  can include a cavity or channel in which the distal end of the guide-rod  11 . 1 . 3 - 208  extends. However, in at least one example, no contact is made between the guide-rod  11 . 1 . 3 - 208  and the stop  11 . 1 . 3 - 218  such that the guide-rod  11 . 1 . 3 - 208  is cantilevered as noted above. The stop  11 . 1 . 3 - 218  can be configured to prevent the optical module (not shown) from travelling too far distally along the guide-rod  11 . 1 . 3 - 208  and beyond the distal end  11 . 1 . 3 - 222  thereof during IPD adjustment. The stop  11 . 1 . 3 - 218  can also serve to prevent too much deformation of the cantilevered guide-rod  11 . 1 . 3 - 208  during unintended drop events or other forces that would cause a deformation of the guide-rod  11 . 1 . 3 - 208  and a movement of the distal end  11 . 1 . 3 - 222  of the guide-rod  11 . 1 . 3 - 208 . In at least one example, the distal end  11 . 1 . 3 - 222  of the guide-rod  11 . 1 . 3 - 208  extends into or is a least partially surrounded by the stop  11 . 1 . 3 - 218  without making contact during normal use of the HMD  11 . 1 . 3 - 200 . If unintentionally moved or deformed elastically, the distal end  11 . 1 . 3 - 222  or portion of the guide-rod  11 . 1 . 3 - 208  can make contact with the stop  11 . 1 . 3 - 218  such that the stop  11 . 1 . 3 - 218  prevents too much deformation of the guide-rod  11 . 1 . 3 - 208 , thus preventing plastic deformation and damage to the guide-rod  11 . 1 . 3 - 208 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  11 . 1   . 3 - 1 - 11 . 1 . 3 - 2  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in any other figures shown and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to any other figure shown and described herein, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  11 . 1   . 3 - 1 - 11 . 1 . 3 - 2 . 
       FIG.  11 . 1   . 3 - 3  shows another view of an example of an adjustment system  11 . 1 . 3 - 302  similar to those shown in  FIGS.  11 . 1   . 3 - 1  and  11 . 1 . 3 - 2  but with a first guide-rod  11 . 1 . 3 - 308   a  and a second guide-rod  11 . 1 . 3 - 308   b  shown extending in opposite directions from a central bracket  11 . 1 . 3 - 312 . In the illustrated example, the adjustment mechanism  11 . 1 . 3 - 302  also includes a second stop  11 . 1 . 3 - 318   a  associated with the distal end of the first guide-rod  11 . 1 . 3 - 308   a , which can be similar in form and function to the stop  11 . 1 . 3 - 218  described above with reference to  FIG.  11 . 1   . 3 - 2 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  11 . 1   . 3 - 3  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in any other figures shown and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to any other figure shown and described herein, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  11 . 1   . 3 - 3 . 
     11.1.3.1: Motors 
       FIG.  1    illustrates a rear perspective view of a display assembly  11 . 1 . 3 . 1 - 102  of an HMD  11 . 1 . 3 . 1 - 100  with one or more components removed, including a curtain assembly, in order to illustrate various internal components of the HMD  11 . 1 . 3 . 1 - 100 .  FIG.  11 . 1   . 3 . 1 - 1  illustrates the display assembly  11 . 1 . 3 . 1 - 102 , which can include a first display module  11 . 1 . 3 . 1 - 104   a  and a second display module  11 . 1 . 3 . 1 - 104   b . The first display module  11 . 1 . 3 . 1 - 104   a  can be slidably engaged/coupled with a first adjustment mechanism  11 . 1 . 3 . 1 - 106   a  and the second optical module  11 . 1 . 3 . 1 - 104   b  can be slidably engaged/coupled with a second adjustment mechanism  11 . 1 . 3 . 1 - 106   b . The first and second adjustment mechanisms  11 . 1 . 3 . 1 - 106   a - b  can include first and second respective guide-rods  11 . 1 . 3 . 1 - 108   a - b  and motors  11 . 1 . 3 . 1 - 110   a - b . The first and second guide-rods can be similar such that the description of one applies to the other, with the first guide-rod  11 . 1 . 3 . 1 - 108   a  extending a first direction away from a bracket  11 . 1 . 3 . 1 - 112  and the second guide-rod  11 . 1 . 3 . 1 - 108   b  extending a second direction away from the bracket  11 . 1 . 3 . 1 - 112  opposite the first direction. 
     Likewise, the first and second motors  11 . 1 . 3 . 1 - 110   a - b  can be similar such that the description of one applies to the other, with the first motor  11 . 1 . 3 . 1 - 110   a  mechanically engaging the first optical module  11 . 1 . 3 . 1 - 104   a  to adjust the position thereof via the first guide-rod  11 . 1 . 3 . 1 - 108   a  and the second motor  11 . 1 . 3 . 1 - 110   b  mechanically engaging the second optical module  11 . 1 . 3 . 1 - 104   b  to adjust the position thereof via the second guide-rod  11 . 1 . 3 . 1 - 108   b . In at least one example, the HMD  11 . 1 . 3 . 1 - 100  also includes a depressible and/or rotatable button  11 . 1 . 3 . 1 - 114  electrically coupled to the first and second motors  11 . 1 . 3 . 1 - 110   a - b . The button  11 . 1 . 3 . 1 - 114  can electrically communicate with the first and second motors  11 . 1 . 3 . 1 - 110   a - b  via a processor or other circuitry components to cause the first and second motors  11 . 1 . 3 . 1 - 110   a - b  to activate and cause the first and second optical modules  11 . 1 . 3 . 1 - 104   a - b , respectively, to change position relative to one another. 
     In at least one example, the motors  11 . 1 . 3 . 1 - 110   a - b  are configured to operate the optical modules  11 . 1 . 3 . 1 - 104   a - b  inward and outward relative to the bracket  11 . 1 . 3 . 1 - 112  and thus relative to the user&#39;s nose. As such, in at least one example, the motors  11 . 1 . 3 . 1 - 110   a - b  operate under MS1 safety standards and the button  11 . 1 . 3 . 1 - 114  operating/instigating the motors  11 . 1 . 3 . 1 - 110   a - b  includes a “dead-man&#39;s button” so the motors  11 . 1 . 3 . 1 - 110   a - b  are only operable when the button  11 . 1 . 3 . 1 - 110   a - b  is manipulated, e.g. depressed and/or rotated. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  11 . 1   . 3 . 1 - 1  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in any other figures shown and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to any other figure shown and described herein, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  11 . 1   . 3 . 1 - 1 . 
       FIG.  11 . 1   . 3 . 1 - 2  illustrates a perspective view of an example of a motor  11 . 1 . 3 . 1 - 210  similar to those  11 . 1 . 3 . 1 - 110   a - b  shown in  FIG.  11 . 1   . 3 . 1 - 1 .  FIG.  11 . 1   . 3 . 1 - 3  illustrates a cross-sectional view thereof to illustrate internal components of the motor  11 . 1 . 3 . 1 - 210 . In at least one example, the motor  11 . 1 . 3 . 1 - 210  can include a gear box and breakaway, spring-loaded clutches in a nut (e.g., “split nut”). The split nut can be configured to prevent forces in a drop event from transferring through the motor  11 . 1 . 3 . 1 - 210 . In at least one example, during use, the motor  11 . 1 . 3 . 1 - 210  can be operated in resonant valleys to reduce noise. 
     In at least one example, the motor can include one or more dust covers (not shown) surrounding the housing to prevent dust and other debris from contaminating the internal volume of the motor  11 . 1 . 3 . 1 - 210  where the lead screw and other components are disposed. In at least one example, the dust covers can include tape wraps. In at least one example, the motor  11 . 1 . 3 . 1 - 11 . 1 . 3 . 1 - 210  can include a split screw/nut, as noted above, to reduce noise and form a breakaway component to release the optical module driven by the motor  11 . 1 . 3 . 1 - 210  such that forces transferred from the motor  11 . 1 . 3 . 1 - 210  to the optical module are minimized during high impact scenarios such as a drop event. 
     In at least one example, the motor  11 . 1 . 3 . 1 - 210  can include a gear reduction mechanism or assembly, such as a planetary gear reduction assembly. In at least one example, the motor  11 . 1 . 3 . 1 - 210  can include a clutch to tune the force to a certain level to release down length. In at least one example, the motor  11 . 1 . 3 . 1 - 210  can include pre-loaded ball bearings to reduce play in the motor  11 . 1 . 3 . 1 - 210 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  11 . 1   . 3 . 1 - 2  and  11 . 1 . 3 . 1 - 3  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in any other figures shown and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to any other figure shown and described herein, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  11 . 1   . 3 . 1 - 2  and  11 . 1 . 3 . 1 - 3 . 
     11.1.3.1.1: Electronic Devices with Optical Module Positioning Systems 
     A head-mounted device may have optical modules that present images to a user&#39;s eyes. Each optical module may have a lens barrel with a display and a lens that presents an image from the display to a corresponding eye box. 
     To accommodate users with different interpupillary distances, the optical modules may be slidably coupled to guide members such as guide rods. Actuators may slide the optical modules towards or away from each other along the guide rods, thereby accommodating different interpupillary distances. 
     The guide rods may be formed from fiber-reinforced composite tubes with one or more end caps that are fastened to a frame in the head-mounted device. A common end cap may, if desired, be used to join a pair of guide rods. End caps may be formed as separate pieces that are attached to the ends of the fiber composite tubes or other guide rod structures and/or may be integral portions of the fiber composite tubes or other guide rod structures. 
     The guide rods may include a left guide rod or left pair of guide rods slidably engaged with a left optical module and a right guide rod or right pair of guide rods slidably engaged with a right optical module. Left and right guide rods may be angled at a non-zero angle with respect to each other to help guide the optical modules parallel to the surface of a user&#39;s face. 
     The tubes of the guide rods may be partly or completely filled with cores to add strength. Low-friction coatings such as metal coatings may be applied to the tubes and on corresponding inner surfaces of the optical module structures that receive the tubes. 
     An electronic device such as a head-mounted device may have a front face that faces away from a user&#39;s head and may have an opposing rear face that faces the user&#39;s head. Optical modules at the rear face may be used to provide images to a user&#39;s eyes. The positions of the optical modules may be adjusted to accommodate different user interpupillary distances. The head-mounted device may have actuators and optical module guide structures to allow the optical module positions to be adjusted. 
     A top view of an illustrative head-mounted device is shown in  FIG.  11 . 1   . 3 . 1 . 1 - 1 . As shown in  FIG.  11 . 1   . 3 . 1 . 1 - 1 , head-mounted devices such as electronic device  11 . 1 . 3 . 1 . 1 - 10  may have head-mounted support structures such as housing  11 . 1 . 3 . 1 . 1 - 12 . Housing  11 . 1 . 3 . 1 . 1 - 12  may include portions (e.g., head-mounted support structures  11 . 1 . 3 . 1 . 1 - 12 T) to allow device  11 . 1 . 3 . 1 . 1 - 10  to be worn on a user&#39;s head. Support structures  11 . 1 . 3 . 1 . 1 - 12 T may be formed from fabric, polymer, metal, and/or other material. Support structures  11 . 1 . 3 . 1 . 1 - 12 T may form a strap or other head-mounted support structures to help support device  11 . 1 . 3 . 1 . 1 - 10  on a user&#39;s head. A main support structure (e.g., main housing portion  11 . 1 . 3 . 1 . 1 - 12 M) of housing  11 . 1 . 3 . 1 . 1 - 12  may support electronic components such as displays  11 . 1 . 3 . 1 . 1 - 14 . 
     Main housing portion  11 . 1 . 3 . 1 . 1 - 12 M may include housing structures formed from metal, polymer, glass, ceramic, and/or other material. For example, housing portion  11 . 1 . 3 . 1 . 1 - 12 M may have housing walls on front face F and housing walls on adjacent top, bottom, left, and right side faces that are formed from rigid polymer or other rigid support structures and these rigid walls may optionally be covered with electrical components, fabric, leather, or other soft materials, etc. Housing portion  11 . 1 . 3 . 1 . 1 - 12 M may also have internal support structures such as a frame and/or structures that perform multiple functions such as controlling airflow while providing structural support. The walls of housing portion  11 . 1 . 3 . 1 . 1 - 12 M may enclose internal components  11 . 1 . 3 . 1 . 1 - 38  in interior region  11 . 1 . 3 . 1 . 1 - 34  of device  11 . 1 . 3 . 1 . 1 - 10  and may separate interior region  11 . 1 . 3 . 1 . 1 - 34  from the environment surrounding device  11 . 1 . 3 . 1 . 1 - 10  (exterior region  11 . 1 . 3 . 1 . 1 - 36 ). Internal components  11 . 1 . 3 . 1 . 1 - 38  may include integrated circuits, actuators, batteries, sensors, and/or other circuits and structures for device  11 . 1 . 3 . 1 . 1 - 10 . Housing  11 . 1 . 3 . 1 . 1 - 12  may be configured to be worn on a head of a user and may form glasses, a hat, a helmet, goggles, and/or other head-mounted device. Configurations in which housing  11 . 1 . 3 . 1 . 1 - 12  forms goggles may sometimes be described herein as an example. 
     Front face F of housing  11 . 1 . 3 . 1 . 1 - 12  may face outwardly away from a user&#39;s head and face. Opposing rear face R of housing  11 . 1 . 3 . 1 . 1 - 12  may face the user. Portions of housing  11 . 1 . 3 . 1 . 1 - 12  (e.g., portions of main housing  11 . 1 . 3 . 1 . 1 - 12 M) on rear face R may form a cover such as cover  11 . 1 . 3 . 1 . 1 - 12 C (sometimes referred to as a curtain). The presence of cover  11 . 1 . 3 . 1 . 1 - 12 C on rear face R may help hide internal housing structures, internal components  11 . 1 . 3 . 1 . 1 - 38 , and other structures in interior region  11 . 1 . 3 . 1 . 1 - 34  from view by a user. 
     Device  11 . 1 . 3 . 1 . 1 - 10  may have left and right optical modules  11 . 1 . 3 . 1 . 1 - 40 . Optical modules  11 . 1 . 3 . 1 . 1 - 40  support electrical and optical components such as light-emitting components and lenses and may therefore sometimes be referred to as optical assemblies, optical systems, optical component support structures, lens and display support structures, electrical component support structures, or housing structures. Each optical module may include a respective display  11 . 1 . 3 . 1 . 1 - 14 , lens  11 . 1 . 3 . 1 . 1 - 30 , and support structure such as lens barrel  11 . 1 . 3 . 1 . 1 - 32 . Lens barrel  11 . 1 . 3 . 1 . 1 - 32 , which may sometimes be referred to as lens support structures, optical component support structures, optical module support structures, or optical module portions, may include hollow cylindrical structures with open ends or other supporting structures to house displays  11 . 1 . 3 . 1 . 1 - 14  and lenses  11 . 1 . 3 . 1 . 1 - 30 . Lens barrels  11 . 1 . 3 . 1 . 1 - 32  may, for example, include a left lens barrel that supports a left display  11 . 1 . 3 . 1 . 1 - 14  and left lens  11 . 1 . 3 . 1 . 1 - 30  and a right lens barrel that supports a right display  11 . 1 . 3 . 1 . 1 - 14  and right lens  11 . 1 . 3 . 1 . 1 - 30 . 
     Displays  11 . 1 . 3 . 1 . 1 - 14  may include arrays of pixels or other display devices to produce images. Displays  11 . 1 . 3 . 1 . 1 - 14  may, for example, include organic light-emitting diode pixels formed on substrates with thin-film circuitry and/or formed on semiconductor substrates, pixels formed from crystalline semiconductor dies, liquid crystal display pixels, scanning display devices, and/or other display devices for producing images. 
     Lenses  11 . 1 . 3 . 1 . 1 - 30  may include one or more lens elements for providing image light from displays  11 . 1 . 3 . 1 . 1 - 14  to respective eyes boxes  11 . 1 . 3 . 1 . 1 - 13 . Lenses may be implemented using refractive glass lens elements, using mirror lens structures (catadioptric lenses), using Fresnel lenses, using holographic lenses, and/or other lens systems. 
     When a user&#39;s eyes are located in eye boxes  11 . 1 . 3 . 1 . 1 - 13 , displays (display panels)  11 . 1 . 3 . 1 . 1 - 14  operate together to form a display for device  11 . 1 . 3 . 1 . 1 - 10  (e.g., the images provided by respective left and right optical modules  11 . 1 . 3 . 1 . 1 - 40  may be viewed by the user&#39;s eyes in eye boxes  11 . 1 . 3 . 1 . 1 - 13  so that a stereoscopic image is created for the user). The left image from the left optical module fuses with the right image from a right optical module while the display is viewed by the user. 
     It may be desirable to monitor the user&#39;s eyes while the user&#39;s eyes are located in eye boxes  11 . 1 . 3 . 1 . 1 - 13 . For example, it may be desirable to use a camera to capture images of the user&#39;s irises (or other portions of the user&#39;s eyes) for user authentication. It may also be desirable to monitor the direction of the user&#39;s gaze. Gaze tracking information may be used as a form of user input and/or may be used to determine where, within an image, image content resolution should be locally enhanced in a foveated imaging system. To ensure that device  11 . 1 . 3 . 1 . 1 - 10  can capture satisfactory eye images while a user&#39;s eyes are located in eye boxes  11 . 1 . 3 . 1 . 1 - 13 , each optical module  11 . 1 . 3 . 1 . 1 - 40  may be provided with a camera such as camera  11 . 1 . 3 . 1 . 1 - 42  and one or more light sources such as light-emitting diodes  11 . 1 . 3 . 1 . 1 - 44  or other light-emitting devices such as lasers, lamps, etc. Cameras  11 . 1 . 3 . 1 . 1 - 42  and light-emitting diodes  11 . 1 . 3 . 1 . 1 - 44  may operate at any suitable wavelengths (visible, infrared, and/or ultraviolet). As an example, diodes  11 . 1 . 3 . 1 . 1 - 44  may emit infrared light that is invisible (or nearly invisible) to the user. This allows eye monitoring operations to be performed continuously without interfering with the user&#39;s ability to view images on displays  11 . 1 . 3 . 1 . 1 - 14 . 
     Not all users have the same interpupillary distance IPD. To provide device  11 . 1 . 3 . 1 . 1 - 10  with the ability to adjust the interpupillary spacing between modules  11 . 1 . 3 . 1 . 1 - 40  along lateral dimension X and thereby adjust the spacing IPD between eye boxes  11 . 1 . 3 . 1 . 1 - 13  to accommodate different user interpupillary distances, device  11 . 1 . 3 . 1 . 1 - 10  may be provided with optical module positioning systems in housing  11 . 1 . 3 . 1 . 1 - 12 . The positioning systems may have guide members and actuators  11 . 1 . 3 . 1 . 1 - 43  that are used to position optical modules  11 . 1 . 3 . 1 . 1 - 40  with respect to each other. 
     Actuators  11 . 1 . 3 . 1 . 1 - 43  can be manually controlled and/or computer-controlled actuators (e.g., computer-controlled motors) for moving lens barrels  11 . 1 . 3 . 1 . 1 - 32  relative to each other. Information on the locations of the user&#39;s eyes may be gathered using, for example, cameras  11 . 1 . 3 . 1 . 1 - 42 . The locations of eye boxes  11 . 1 . 3 . 1 . 1 - 13  can then be adjusted accordingly. 
     As shown in the rear view of device  11 . 1 . 3 . 1 . 1 - 10  of  FIG.  11 . 1   . 3 . 1 . 1 - 2 , cover  11 . 1 . 3 . 1 . 1 - 12 C may cover rear face R while leaving lenses  11 . 1 . 3 . 1 . 1 - 30  of optical modules  11 . 1 . 3 . 1 . 1 - 40  uncovered (e.g., cover  11 . 1 . 3 . 1 . 1 - 12 C may have openings that are aligned with and receive modules  11 . 1 . 3 . 1 . 1 - 40 ). As modules  11 . 1 . 3 . 1 . 1 - 40  are moved relative to each other along dimension X to accommodate different interpupillary distances for different users, modules  11 . 1 . 3 . 1 . 1 - 40  move relative to fixed housing structures such as the walls of main portion  11 . 1 . 3 . 1 . 1 - 12 M and move relative to each other. 
     A schematic diagram of an illustrative electronic device such as a head-mounted device or other wearable device is shown in  FIG.  11 . 1   . 3 . 1 . 1 - 3 . Device  11 . 1 . 3 . 1 . 1 - 10  of  FIG.  11 . 1   . 3 . 1 . 1 - 3  may be operated as a stand-alone device and/or the resources of device  11 . 1 . 3 . 1 . 1 - 10  may be used to communicate with external electronic equipment. As an example, communications circuitry in device  11 . 1 . 3 . 1 . 1 - 10  may be used to transmit user input information, sensor information, and/or other information to external electronic devices (e.g., wirelessly or via wired connections). Each of these external devices may include components of the type shown by device  11 . 1 . 3 . 1 . 1 - 10  of  FIG.  11 . 1   . 3 . 1 . 1 - 3 . 
     As shown in  FIG.  11 . 1   . 3 . 1 . 1 - 3 , a head-mounted device such as device  11 . 1 . 3 . 1 . 1 - 10  may include control circuitry  11 . 1 . 3 . 1 . 1 - 20 . Control circuitry  11 . 1 . 3 . 1 . 1 - 20  may include storage and processing circuitry for supporting the operation of device  11 . 1 . 3 . 1 . 1 - 10 . The storage and processing circuitry may include storage such as nonvolatile memory (e.g., flash memory or other electrically-programmable-read-only memory configured to form a solid state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Processing circuitry in control circuitry  11 . 1 . 3 . 1 . 1 - 20  may be used to gather input from sensors and other input devices and may be used to control output devices. The processing circuitry may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors and other wireless communications circuits, power management units, audio chips, application specific integrated circuits, etc. During operation, control circuitry  11 . 1 . 3 . 1 . 1 - 20  may use display(s)  11 . 1 . 3 . 1 . 1 - 14  and other output devices in providing a user with visual output and other output. 
     To support communications between device  11 . 1 . 3 . 1 . 1 - 10  and external equipment, control circuitry  11 . 1 . 3 . 1 . 1 - 20  may communicate using communications circuitry  11 . 1 . 3 . 1 . 1 - 22 . Circuitry  11 . 1 . 3 . 1 . 1 - 22  may include antennas, radio-frequency transceiver circuitry, and other wireless communications circuitry and/or wired communications circuitry. Circuitry  11 . 1 . 3 . 1 . 1 - 22 , which may sometimes be referred to as control circuitry and/or control and communications circuitry, may support bidirectional wireless communications between device  11 . 1 . 3 . 1 . 1 - 10  and external equipment (e.g., a companion device such as a computer, cellular telephone, or other electronic device, an accessory such as a point device, computer stylus, or other input device, speakers or other output devices, etc.) over a wireless link. For example, circuitry  11 . 1 . 3 . 1 . 1 - 22  may include radio-frequency transceiver circuitry such as wireless local area network transceiver circuitry configured to support communications over a wireless local area network link, near-field communications transceiver circuitry configured to support communications over a near-field communications link, cellular telephone transceiver circuitry configured to support communications over a cellular telephone link, or transceiver circuitry configured to support communications over any other suitable wired or wireless communications link. Wireless communications may, for example, be supported over a Bluetooth® link, a WiFi® link, a wireless link operating at a frequency between 10 GHz and 400 GHz, a 60 GHz link, or other millimeter wave link, a cellular telephone link, or other wireless communications link. Device  11 . 1 . 3 . 1 . 1 - 10  may, if desired, include power circuits for transmitting and/or receiving wired and/or wireless power and may include batteries or other energy storage devices. For example, device  11 . 1 . 3 . 1 . 1 - 10  may include a coil and rectifier to receive wireless power that is provided to circuitry in device  11 . 1 . 3 . 1 . 1 - 10 . 
     Device  11 . 1 . 3 . 1 . 1 - 10  may include input-output devices such as devices  11 . 1 . 3 . 1 . 1 - 24 . Input-output devices  11 . 1 . 3 . 1 . 1 - 24  may be used in gathering user input, in gathering information on the environment surrounding the user, and/or in providing a user with output. Devices  11 . 1 . 3 . 1 . 1 - 24  may include one or more displays such as display(s)  11 . 1 . 3 . 1 . 1 - 14 . Display(s)  11 . 1 . 3 . 1 . 1 - 14  may include one or more display devices such as organic light-emitting diode display panels (panels with organic light-emitting diode pixels formed on polymer substrates or silicon substrates that contain pixel control circuitry), liquid crystal display panels, microelectromechanical systems displays (e.g., two-dimensional mirror arrays or scanning mirror display devices), display panels having pixel arrays formed from crystalline semiconductor light-emitting diode dies (sometimes referred to as microLEDs), and/or other display devices. 
     Sensors  11 . 1 . 3 . 1 . 1 - 16  in input-output devices  11 . 1 . 3 . 1 . 1 - 24  may include force sensors (e.g., strain gauges, capacitive force sensors, resistive force sensors, etc.), audio sensors such as microphones, touch and/or proximity sensors such as capacitive sensors such as a touch sensor that forms a button, trackpad, or other input device), and other sensors. If desired, sensors  11 . 1 . 3 . 1 . 1 - 16  may include optical sensors such as optical sensors that emit and detect light, ultrasonic sensors, optical touch sensors, optical proximity sensors, and/or other touch sensors and/or proximity sensors, monochromatic and color ambient light sensors, image sensors, fingerprint sensors, iris scanning sensors, retinal scanning sensors, and other biometric sensors, temperature sensors, sensors for measuring three-dimensional non-contact gestures (“air gestures”), pressure sensors, sensors for detecting position, orientation, and/or motion (e.g., accelerometers, magnetic sensors such as compass sensors, gyroscopes, and/or inertial measurement units that contain some or all of these sensors), health sensors such as blood oxygen sensors, heart rate sensors, blood flow sensors, and/or other health sensors, radio-frequency sensors, depth sensors (e.g., structured light sensors and/or depth sensors based on stereo imaging devices that capture three-dimensional images), optical sensors such as self-mixing sensors and light detection and ranging (LIDAR) sensors that gather time-of-flight measurements, humidity sensors, moisture sensors, gaze tracking sensors, electromyography sensors to sense muscle activation, facial sensors, and/or other sensors. In some arrangements, device  11 . 1 . 3 . 1 . 1 - 10  may use sensors  11 . 1 . 3 . 1 . 1 - 16  and/or other input-output devices to gather user input. For example, buttons may be used to gather button press input, touch sensors overlapping displays can be used for gathering user touch screen input, touch pads may be used in gathering touch input, microphones may be used for gathering audio input (e.g., voice commands), accelerometers may be used in monitoring when a finger contacts an input surface and may therefore be used to gather finger press input, etc. 
     If desired, electronic device  11 . 1 . 3 . 1 . 1 - 10  may include additional components (see, e.g., other devices  11 . 1 . 3 . 1 . 1 - 18  in input-output devices  11 . 1 . 3 . 1 . 1 - 24 ). The additional components may include haptic output devices, actuators for moving movable housing structures, audio output devices such as speakers, light-emitting diodes for status indicators, light sources such as light-emitting diodes that illuminate portions of a housing and/or display structure, other optical output devices, and/or other circuitry for gathering input and/or providing output. Device  11 . 1 . 3 . 1 . 1 - 10  may also include a battery or other energy storage device, connector ports for supporting wired communication with ancillary equipment and for receiving wired power, and other circuitry. 
     A rear view of device  11 . 1 . 3 . 1 . 1 - 10  is shown in  FIG.  11 . 1   . 3 . 1 . 1 - 4 . In the example of  FIG.  11 . 1   . 3 . 1 . 1 - 4 , cover  11 . 1 . 3 . 1 . 1 - 12 C has been removed to expose internal housing structures such as frame  11 . 1 . 3 . 1 . 1 - 12 FC. Frame  11 . 1 . 3 . 1 . 1 - 12 FC may be formed from polymer support structures, support structurers formed from carbon-fiber composite and/or other fiber-composite materials, metal support structures, glass housing structures, and/or other support structures for main housing portion  11 . 1 . 3 . 1 . 1 - 12 M. Portions of frame  11 . 1 . 3 . 1 . 1 - 12 FC that run horizontally (along the upper and lower edges of device  11 . 1 . 3 . 1 . 1 - 10  parallel to the X axis of  FIG.  11 . 1   . 3 . 1 . 1 - 4 ) may be joined by vertically extending edge portions of frame  11 . 1 . 3 . 1 . 1 - 12 FC (see, e.g., frame edge portions  11 . 1 . 3 . 1 . 1 - 12 FC-E) along the left and right sides of device  11 . 1 . 3 . 1 . 1 - 10  and may be joined in the center of device  11 . 1 . 3 . 1 . 1 - 10  by a vertically extending nose bridge portion (see, e.g., frame center portion  11 . 1 . 3 . 1 . 1 - 12 FC-M). 
     Optical modules  11 . 1 . 3 . 1 . 1 - 40  may be guided using optical module guide structures such as optical module guide rods  11 . 1 . 3 . 1 . 1 - 50 . Guide rods  11 . 1 . 3 . 1 . 1 - 50  may extend horizontally across device  11 . 1 . 3 . 1 . 1 - 10  (e.g., parallel to the X axis of  FIG.  11 . 1   . 3 . 1 . 1 - 4 ). In the example of  FIG.  11 . 1   . 3 . 1 . 1 - 4 , guide rods  11 . 1 . 3 . 1 . 1 - 50  include an upper set of left and right guide rods at upper position  11 . 1 . 3 . 1 . 1 - 52  and a lower set of left and right guide rods at lower position  11 . 1 . 3 . 1 . 1 - 54 . There may be more guide rods or fewer guide rods in device  11 . 1 . 3 . 1 . 1 - 10 , if desired. Fasteners  11 . 1 . 3 . 1 . 1 - 56  (e.g., screws) or other attachment structures (e.g., adhesive, welds, etc.) may be used to attach guide rods  11 . 1 . 3 . 1 . 1 - 50  to frame  11 . 1 . 3 . 1 . 1 - 12 FC. The fasteners may, as an example, attach the ends of rods  11 . 1 . 3 . 1 . 1 - 50  that are located on the left of device  11 . 1 . 3 . 1 . 1 - 10  between left frame edge portion  11 . 1 . 3 . 1 . 1 - 12 FC-E and frame center portion  11 . 1 . 3 . 1 . 1 - 12 FC-M and may attach the ends of rods  11 . 1 . 3 . 1 . 1 - 50  that are located on the right of device  11 . 1 . 3 . 1 . 1 - 10  between right frame edge portion  11 . 1 . 3 . 1 . 1 - 12 FC-E and frame center portion  11 . 1 . 3 . 1 . 1 - 12 FC-M. Guide rods  11 . 1 . 3 . 1 . 1 - 50  may also be attached to the frame or to an intermediary subframe. Guide rods may be fixed to a frame or subframe at both ends (as illustrated) or each guide rod may only be attached to the frame at a single end. For example, each guide rod may only be fixed to the frame at a central portion of the frame. 
     Each optical module may have portions that slidably engage guide rods  11 . 1 . 3 . 1 . 1 - 50 . For example, each optical module  11 . 1 . 3 . 1 . 1 - 40  may have an upper guide rod engagement portion (sometimes referred to as a hanger or hanger portion) such as optical module portion  11 . 1 . 3 . 1 . 1 - 40 H that receives and engages a respective guide rod  11 . 1 . 3 . 1 . 1 - 50  at upper guide rod position  11 . 1 . 3 . 1 . 1 - 52 . Each optical module  11 . 1 . 3 . 1 . 1 - 40  may also have a lower guide rod engagement portion (sometimes referred to as a toe or toe portion) such as optical module portion  11 . 1 . 3 . 1 . 1 - 40 T that receives and engages a respective guide rod  11 . 1 . 3 . 1 . 1 - 50  at lower guide rod position  11 . 1 . 3 . 1 . 1 - 54 . Portion  11 . 1 . 3 . 1 . 1 - 40 T may be an integral portion of lens barrel  11 . 1 . 3 . 1 . 1 - 32  or other support structure for optical module  11 . 1 . 3 . 1 . 1 - 40  or may be formed from one or more separate structures attached to lens barrel  11 . 1 . 3 . 1 . 1 - 32 . Portion  11 . 1 . 3 . 1 . 1 - 40 H may be an integral portion of lens barrel  11 . 1 . 3 . 1 . 1 - 32  or may, as shown in  FIG.  11 . 1   . 3 . 1 . 1 - 4 , be formed from one or more separate optical module structures that are attached to lens barrel  11 . 1 . 3 . 1 . 1 - 32  by fasteners  11 . 1 . 3 . 1 . 1 - 60  (e.g., screws). If desired, guide rods  11 . 1 . 3 . 1 . 1 - 50  may be provided with structures that serve as stops to prevent over-travel of optical modules  11 . 1 . 3 . 1 . 1 - 40 . As an example, each of guide rods  11 . 1 . 3 . 1 . 1 - 50  may have one or more stop structures such as plug  11 . 1 . 3 . 1 . 1 - 57 . During sliding movement of optical module  11 . 1 . 3 . 1 . 1 - 40 , excessive movement will be prevented when portion  11 . 1 . 3 . 1 . 1 - 40 H contacts plug  11 . 1 . 3 . 1 . 1 - 57  and is thereby stopped by plug  11 . 1 . 3 . 1 . 1 - 57 . Optical module sliding motion stop structures such as plug  11 . 1 . 3 . 1 . 1 - 57  may be attached to guide rods  11 . 1 . 3 . 1 . 1 - 50  by screwing, by welding, using adhesive, or using other attachment structures. Plug  11 . 1 . 3 . 1 . 1 - 57  may be a separate structure from rods  11 . 1 . 3 . 1 . 1 - 50  or such structures may be formed as part of guide rods  11 . 1 . 3 . 1 . 1 - 50  (e.g., a metal guide rod or other guide rod may be machined or otherwise formed into a shape that includes an integral sliding motion stop structure). 
     Guide rods  11 . 1 . 3 . 1 . 1 - 50  may have circular cross-sectional shapes (when viewed in the Y-Z plane of  FIG.  11 . 1   . 3 . 1 . 1 - 4 ) or may have other suitable cross-sectional shape. The portions of optical module  11 . 1 . 3 . 1 . 1 - 40  that receive guide rods  11 . 1 . 3 . 1 . 1 - 50  may have corresponding mating shapes (e.g., full or partial circular openings with inner diameters corresponding to the outer diameters of guide rods  11 . 1 . 3 . 1 . 1 - 50 ). To prevent sticking, the inner surfaces of the optical module guide rod openings and/or the outer surfaces of guide rods  11 . 1 . 3 . 1 . 1 - 50  may be provided with low stick surfaces (e.g., using low-stick coatings, lubricant such as grease, etc.). 
     Actuators  11 . 1 . 3 . 1 . 1 - 43  may have associated threaded members such as threaded actuator rods  11 . 1 . 3 . 1 . 1 - 62 . Optical module portions  11 . 1 . 3 . 1 . 1 - 40 H may have corresponding threaded nuts  11 . 1 . 3 . 1 . 1 - 64  or other threaded portions that receive threaded actuator rods  11 . 1 . 3 . 1 . 1 - 62 . During operation, actuators  11 . 1 . 3 . 1 . 1 - 43  may rotate threaded actuator rods  11 . 1 . 3 . 1 . 1 - 62  about actuator rod rotational axes  11 . 1 . 3 . 1 . 1 - 66 , thereby moving optical modules  11 . 1 . 3 . 1 . 1 - 40  outwardly (away from each other) in directions  11 . 1 . 3 . 1 . 1 - 68  or inwardly (towards each other) in directions  11 . 1 . 3 . 1 . 1 - 70  as desired to adjust the positions of optical modules  11 . 1 . 3 . 1 . 1 - 40  relative to each other (e.g., to adjust the lens-center-to-lens-center spacing of the left and right lenses in device  11 . 1 . 3 . 1 . 1 - 10  to accommodate different interpupillary distances for different users). 
       FIG.  11 . 1   . 3 . 1 . 1 - 5  is an end view of an illustrative portion of optical module  11 . 1 . 3 . 1 . 1 - 40  such as portion  11 . 1 . 3 . 1 . 1 - 40 H of  FIG.  11 . 1   . 3 . 1 . 1 - 4 . As shown in  FIG.  11 . 1   . 3 . 1 . 1 - 5 , optical module portion  11 . 1 . 3 . 1 . 1 - 40 H may have a first opening such as opening  11 . 1 . 3 . 1 . 1 - 72  (e.g., a threaded opening in nut  11 . 1 . 3 . 1 . 1 - 64  or other portion of optical module portion  11 . 1 . 3 . 1 . 1 - 40 H). Opening  72  may receive threaded rod  11 . 1 . 3 . 1 . 1 - 62 . 
     Optical module portion  11 . 1 . 3 . 1 . 1 - 40 H may also have a second opening such as opening  11 . 1 . 3 . 1 . 1 - 76 . Opening  11 . 1 . 3 . 1 . 1 - 76  may receive a guide rod (e.g., a guide rod located at upper position  11 . 1 . 3 . 1 . 1 - 52 ). If desired, optical module portion  11 . 1 . 3 . 1 . 1 - 40 T may have an opening such as opening  11 . 1 . 3 . 1 . 1 - 76  to receive a guide rod (e.g., a guide rod located at lower position  11 . 1 . 3 . 1 . 1 - 54 ). As shown in  FIG.  11 . 1   . 3 . 1 . 1 - 5 , the inner surfaces of opening  11 . 1 . 3 . 1 . 1 - 76  may have one or more layers of material that form low-friction coating  11 . 1 . 3 . 1 . 1 - 78 . In an illustrative configuration, optical module  11 . 1 . 3 . 1 . 1 - 40  (e.g., optical module portion  11 . 1 . 3 . 1 . 1 - 40 H, lens barrel  11 . 1 . 3 . 1 . 1 - 32 , and/or optical module portion  11 . 1 . 3 . 1 . 1 - 40 T) are formed from metal (e.g., aluminum) and coating  11 . 1 . 3 . 1 . 1 - 78  is formed from one or more deposited (e.g., electrodeposited) metal layers (e.g., nickel, etc.). Configurations in which modules  11 . 1 . 3 . 1 . 1 - 40  are formed from polymer coated with metal and/or in which modules  11 . 1 . 3 . 1 . 1 - 40  are formed from other materials may be used, if desired. 
     Guide rods  11 . 1 . 3 . 1 . 1 - 52  may be formed from elongated guide member structures such as tubes. The tubes may be cylindrical tubes or may be tubes of other suitable shapes (e.g., tubes with rectangular cross-sectional shapes, etc.). Guide rods  11 . 1 . 3 . 1 . 1 - 52  may be formed from tubes that are completed hollow, that are partly hollow and partly filled with cores, or tubes that are completely filled with filler material (e.g., rods that are formed from tubes entirely filled with a core material that differs in composition, density, method of fabrication, or other respects from the material of the tubes such as a composite core material formed from fibers or other structures embedded in polymer, rods that are formed from a single material such as solid cylindrical rods, solid composite rods of fiber-composite material such as solid cylindrical composite rods, rods formed from other composites, solid rods of metal or polymer, etc.). The use of tubes that are at least partly hollow may help save weight and thereby help enhance the comfort of a user wearing device  11 . 1 . 3 . 1 . 1 - 10 . 
       FIG.  11 . 1   . 3 . 1 . 1 - 6  is a cross-sectional view of an illustrative guide rod. As shown in  FIG.  11 . 1   . 3 . 1 . 1 - 6 , guide rod  11 . 1 . 3 . 1 . 1 - 50  may include guide rod tube  11 . 1 . 3 . 1 . 1 - 82  and a guide rod end member such as end cap  11 . 1 . 3 . 1 . 1 - 86 . Guide rod tube  11 . 1 . 3 . 1 . 1 - 82  may be hollow and may be characterized by cylindrical walls surrounding interior  84 . In the example of  FIG.  11 . 1   . 3 . 1 . 1 - 6 , tube  11 . 1 . 3 . 1 . 1 - 82  extends along longitudinal axis  11 . 1 . 3 . 1 . 1 - 80  and is rotationally symmetric around axis  11 . 1 . 3 . 1 . 1 - 80  (e.g., tube  11 . 1 . 3 . 1 . 1 - 82  is cylindrical). End cap  11 . 1 . 3 . 1 . 1 - 86  may be attached to the end of tube  11 . 1 . 3 . 1 . 1 - 82  using adhesive, fasteners, welds, by molding polymer end cap material over the end of tube  11 . 1 . 3 . 1 . 1 - 82 , and/or other attachment mechanisms. In an illustrative configuration, end cap  11 . 1 . 3 . 1 . 1 - 86  may have a first opening such as opening  11 . 1 . 3 . 1 . 1 - 88  (e.g., a through-hole opening) to receive a first fastener (e.g., a first screw) and may have a second opening such as opening  11 . 1 . 3 . 1 . 1 - 90  (e.g., a through-hole opening) to receive a second fastener (e.g., a second screw). The first screw may be used to attach end cap  11 . 1 . 3 . 1 . 1 - 86  to the end of tube  11 . 1 . 3 . 1 . 1 - 82  and the second screw may be used to attach end cap  11 . 1 . 3 . 1 . 1 - 86  to main housing portion  11 . 1 . 3 . 1 . 1 - 12 M (e.g., frame  11 . 1 . 3 . 1 . 1 - 12 FC). Other numbers of end cap openings and/or different features (e.g., threads) in end cap  11 . 1 . 3 . 1 . 1 - 86  for attaching end cap  11 . 1 . 3 . 1 . 1 - 86  to the end of tube  11 . 1 . 3 . 1 . 1 - 82  may be used, if desired. 
       FIG.  11 . 1   . 3 . 1 . 1 - 7  is a side view of guide rod  11 . 1 . 3 . 1 . 1 - 50  of  FIG.  11 . 1   . 3 . 1 . 1 - 7  following insertion of end cap  11 . 1 . 3 . 1 . 1 - 86  into the end of tube  11 . 1 . 3 . 1 . 1 - 82 . The portion of end cap  11 . 1 . 3 . 1 . 1 - 86  that is inserted into tube  11 . 1 . 3 . 1 . 1 - 82  may have an outer diameter that corresponds to the inner diameter of tube  11 . 1 . 3 . 1 . 1 - 82 . 
     As shown in the cross-sectional view of  FIG.  11 . 1   . 3 . 1 . 1 - 8 , fasteners  11 . 1 . 3 . 1 . 1 - 92  (e.g., screws) may be received by opening  11 . 1 . 3 . 1 . 1 - 88  (to attach end cap  11 . 1 . 3 . 1 . 1 - 86  to tube  11 . 1 . 3 . 1 . 1 - 82 ) and opening  11 . 1 . 3 . 1 . 1 - 90  (to attach end cap  11 . 1 . 3 . 1 . 1 - 86  and the rest of guide rod  11 . 1 . 3 . 1 . 1 - 50  to a housing structure such as frame  11 . 1 . 3 . 1 . 1 - 12 FC (e.g., by screwing a threaded end of the fastener passing through opening  11 . 1 . 3 . 1 . 1 - 90  into threaded frame opening  11 . 1 . 3 . 1 . 1 - 94 . If desired, end cap  11 . 1 . 3 . 1 . 1 - 86  may have a flattened surface portion such as planar surface  96  that rests against a corresponding planar portion of frame  11 . 1 . 3 . 1 . 1 - 12 FC when end cap  11 . 1 . 3 . 1 . 1 - 86  is attached to frame  11 . 1 . 3 . 1 . 1 - 12 FC. The presence of mating planar surfaces may help prevent undesired rotation of guide rod  11 . 1 . 3 . 1 . 1 - 50  about axis  11 . 1 . 3 . 1 . 1 - 80 . 
     As shown in  FIG.  11 . 1   . 3 . 1 . 1 - 4 , device  11 . 1 . 3 . 1 . 1 - 10  may have left and right guide rods (e.g., upper left and right guide rods and lower left and right guide rods). Left and right guide rods  11 . 1 . 3 . 1 . 1 - 50  may have separate end caps  11 . 1 . 3 . 1 . 1 - 86  each with a respective opening  90 , as shown in  FIG.  11 . 1   . 3 . 1 . 1 - 9 , or left and right guide rods  11 . 1 . 3 . 1 . 1 - 50  may share a common end cap  11 . 1 . 3 . 1 . 1 - 86  with one or more openings  90 , as shown in  FIG.  11 . 1   . 3 . 1 . 1 - 10 . A shared end cap may be straight (e.g., so that axes  11 . 1 . 3 . 1 . 1 - 80  of the left and right guide rods are parallel and aligned with each other as shown by straight end cap  11 . 1 . 3 . 1 . 1 - 86  of  FIG.  11 . 1   . 3 . 1 . 1 - 11 ) or a shared end cap may be bent (e.g., so that axes  11 . 1 . 3 . 1 . 1 - 80  of the left and right guide rods are not parallel and instead are angled at a non-zero angle A with respect to each other as shown by bent end cap  11 . 1 . 3 . 1 . 1 - 86  of  FIG.  11 . 1   . 3 . 1 . 1 - 12 ). Bent end cap structures such as the bent end cap of  FIG.  11 . 1   . 3 . 1 . 1 - 12  may be formed from single pieces of metal (or other material) or may be formed from two members (e.g., two metal members) joined by a welded joint (e.g., a laser weld), an adhesive joint, or other bond  11 . 1 . 3 . 1 . 1 - 98 . The use of separate or common end cap structures to orient the left and right guide rods  11 . 1 . 3 . 1 . 1 - 50  at a non-zero angle A with respect to each other (e.g., an angle A of at least 3°, at least 6°, at least 9°, at least 15°, at least 20°, at least 30°, at least 40°, less than 50°, less than 45°, less than 35°, less than 25°, and/or less than) 15° may help orient the left and right optical modules of device  11 . 1 . 3 . 1 . 1 - 10  in front of the left and right sides of the user&#39;s face (which tend to angle away from each other slightly). Left and right guide rods may also be made from a single tube. The single tube may be straight (e.g., left and right guide rods may be formed from respective left and right portions of a single straight tube) or the left and right guide rods may be formed from respective left and right portions of a single tube that has been molded or bent so that the left and right guide rods are oriented at a non-zero angle A with respect to each other. 
     If desired, rods  11 . 1 . 3 . 1 . 1 - 50  may be attached to housing  11 . 1 . 3 . 1 . 1 - 12 M (e.g., frame  11 . 1 . 3 . 1 . 1 - 12 FC) using press-fit connections between rods  11 . 1 . 3 . 1 . 1 - 50  and housing  11 . 1 . 3 . 1 . 1 - 12 M, using shrink-fit connections between rods  11 . 1 . 3 . 1 . 1 - 50  and housing  11 . 1 . 3 . 1 . 1 - 12 M, and/or using other attachment mechanisms such as gluing (e.g., gluing rods  11 . 1 . 3 . 1 . 1 - 50  to housing  11 . 1 . 3 . 1 . 1 - 12 M). In some arrangements, some or all of end caps  11 . 1 . 3 . 1 . 1 - 86  may be omitted (e.g., to help reduce weight). For example, housing  11 . 1 . 3 . 1 . 1 - 12 M (e.g., frame  11 . 1 . 3 . 1 . 1 - 12 FC) may include portions that are configured to be received within the cylindrical hollow interior of rods  11 . 1 . 3 . 1 . 1 - 50  at the ends of rods  11 . 1 . 3 . 1 . 1 - 50  (e.g., housing  11 . 1 . 3 . 1 . 1 - 12 M may have integral housing portions with the shapes of end caps  11 . 1 . 3 . 1 . 1 - 84  that attach to the insides of rods  11 . 1 . 3 . 1 . 1 - 50  using a friction fit from a press-fit or shrink-fit connection and/or using adhesive), housing  11 . 1 . 3 . 1 . 1 - 12 M may include portions that are configured to form cylindrical openings or other shapes (e.g., clamp shapes) that receive the ends of rods  11 . 1 . 3 . 1 . 1 - 50  (e.g., rods  11 . 1 . 3 . 1 . 1 - 50  may be inserted within openings in housing  11 . 1 . 3 . 1 . 1 - 12 M to attach to the outer surfaces of the ends of rods  11 . 1 . 3 . 1 . 1 - 50  to housing  11 . 1 . 3 . 1 . 1 - 12 M with a friction fit and/or adhesive), and/or other housing structures (e.g., portions of housing  11 . 1 . 3 . 1 . 1 - 12 M such as portions of frame  11 . 1 . 3 . 1 . 1 - 12 FC) that mount rods  11 . 1 . 3 . 1 . 1 - 50  to housing  11 . 1 . 3 . 1 . 1 - 12 M directly. 
     If desired, some or all of the interior of each guide rod tube may be filled with supporting material. Consider, as an example, guide rod  11 . 1 . 3 . 1 . 1 - 50  of  FIG.  11 . 1   . 3 . 1 . 1 - 13 . As shown in  FIG.  11 . 1   . 3 . 1 . 1 - 13 , each guide rod  11 . 1 . 3 . 1 . 1 - 50  in device  11 . 1 . 3 . 1 . 1 - 10  may have a pair of end caps  11 . 1 . 3 . 1 . 1 - 86  (one at each of the opposing ends of guide rod tube  11 . 1 . 3 . 1 . 1 - 82 ). At one or more locations along the length of tube  11 . 1 . 3 . 1 . 1 - 82  such as illustrative location  11 . 1 . 3 . 1 . 1 - 100 , tube  11 . 1 . 3 . 1 . 1 - 82  may be provided with a solid supportive core such as core  11 . 1 . 3 . 1 . 1 - 102  (e.g., a polymer core such as a cylindrical rod-shaped polymer foam core, a gel core, low-density thermoplastic polymer, or other low-density core of polymer, a metal core, a wood core, or a core formed from other supportive structures). Location  11 . 1 . 3 . 1 . 1 - 100  may correspond to the center portion of tube  11 . 1 . 3 . 1 . 1 - 82  over which openings in optical module  11 . 1 . 3 . 1 . 1 - 40  such as opening  11 . 1 . 3 . 1 . 1 - 76  travel (as an example). Core  11 . 1 . 3 . 1 . 1 - 102  may be shear-coupled to the inner surface of tube  11 . 1 . 3 . 1 . 1 - 82  and may help provide guide tube  11 . 1 . 3 . 1 . 1 - 50  with bending strength and torsional strength. 
     Tube  11 . 1 . 3 . 1 . 1 - 82  may be formed from metal, polymer, and/or fiber-composite material such as carbon-fiber material, fiberglass material (e.g., glass-fiber-reinforced structural polymer), other fiber-reinforced polymer, etc. The use of fiber-composite tubes may help reduce the weight of rods  11 . 1 . 3 . 1 . 1 - 50 . 
     Consider, as an example, tube  11 . 1 . 3 . 1 . 1 - 82  of  FIGS.  11 . 1   . 3 . 1 . 1 - 14  and  11 . 1 . 3 . 1 . 1 - 15 . As shown in the side view of  FIG.  11 . 1   . 3 . 1 . 1 - 14  and the cross-sectional end view of  FIG.  11 . 1   . 3 . 1 . 1 - 15 , fiber-composite tube  11 . 1 . 3 . 1 . 1 - 82  of  FIGS.  11 . 1   . 3 . 1 . 1 - 14  and  11 . 1 . 3 . 1 . 1 - 15  may have fibers  11 . 1 . 3 . 1 . 1 - 104  embedded in polymer  11 . 1 . 3 . 1 . 1 - 106 . Fibers  11 . 1 . 3 . 1 . 1 - 104  may be carbon fibers, glass fibers, or other strands of material for enhancing the strength of polymer  11 . 1 . 3 . 1 . 1 - 106 . Polymer  11 . 1 . 3 . 1 . 1 - 106 , which may sometimes be referred to as binder or resin, may be epoxy, polyether ether ketone (PEEK), a thermoset polymer, a thermoplastic polymer, and/or other polymer material. Materials such as PEEK may exhibit satisfactory wear properties and a low coefficient of friction. Other polymers may be used for forming binder for tube  11 . 1 . 3 . 1 . 1 - 82  if desired. 
     Fibers  11 . 1 . 3 . 1 . 1 - 104  may extend in one or more different directions. For example, fibers  11 . 1 . 3 . 1 . 1 - 104  may include fibers that extend longitudinally (parallel to tube longitudinal axis  11 . 1 . 3 . 1 . 1 - 80 ), that wrap around the circumference of tube  84  (e.g., about axis  11 . 1 . 3 . 1 . 1 - 80 ), and/or that have angled orientations (e.g., +/−45°) relative to axis  11 . 1 . 3 . 1 . 1 - 80 . These different types of fiber may be formed in a single layer of fibers or multiple layers of fiber may overlap in a stack. The stack of fiber layers may wrap around tube  11 . 1 . 3 . 1 . 1 - 82  and may optionally be covered with a low-friction coating. 
     Consider, as an example, the fiber-composite guide rod tube in the cross-sectional side view of  FIG.  11 . 1   . 3 . 1 . 1 - 16 . In the example of  FIG.  11 . 1   . 3 . 1 . 1 - 16 , guide rod  11 . 1 . 3 . 1 . 1 - 50  includes tube  11 . 1 . 3 . 1 . 1 - 82  and end cap  11 . 1 . 3 . 1 . 1 - 86 . End cap  11 . 1 . 3 . 1 . 1 - 86  has a hollow tube shape with hollow interior region  11 . 1 . 3 . 1 . 1 - 110 . The outer diameter of end cap  11 . 1 . 3 . 1 . 1 - 86  (which includes a hollow portion in the example of  FIG.  11 . 1   . 3 . 1 . 1 - 16 ) corresponds to the inner diameter of tube  11 . 1 . 3 . 1 . 1 - 82 . A layer of optional adhesive  11 . 1 . 3 . 1 . 1 - 112  may be used to help attach end cap  11 . 1 . 3 . 1 . 1 - 86  to tube  11 . 1 . 3 . 1 . 1 - 82 . 
     Tube  11 . 1 . 3 . 1 . 1 - 82  is hollow and surrounds interior region  11 . 1 . 3 . 1 . 1 - 84 . Tube  11 . 1 . 3 . 1 . 1 - 82  has a hollow cylindrical fiber-composite tube portion formed (in the example of  FIG.  11 . 1   . 3 . 1 . 1 - 16 ) from fiber-composite layers  11 . 1 . 3 . 1 . 1 - 114  covered with low-friction coating  11 . 1 . 3 . 1 . 1 - 116 . Layers  11 . 1 . 3 . 1 . 1 - 114  form a hollow cylindrical fiber-composite tube surrounding interior  11 . 1 . 3 . 1 . 1 - 84 . There may be any suitable number N of fiber layers in the tube formed from layers  11 . 1 . 3 . 1 . 1 - 114  (e.g., N may be at least one, at least three, at least four, at least six, less than ten, less than eight, less than seven, 3-8, etc.). Fiber composite layers  11 . 1 . 3 . 1 . 1 - 114  in the example of  FIG.  11 . 1   . 3 . 1 . 1 - 16  include six fiber-composite layers: L1, L2, L3, L4, L5, and L6. These layers may have uniaxially aligned fibers and may have fibers that are oriented at 0°, 90°, 0°, 0°, 90°, and 0°, respectively relative to axis  11 . 1 . 3 . 1 . 1 - 80 . Configurations in which layers  11 . 1 . 3 . 1 . 1 - 114  include fibers oriented at +/−45° or other angles may also be used. Fibers oriented along the length of tube  11 . 1 . 3 . 1 . 1 - 82  may enhance bend strength. Fibers that wrap around tube  11 . 1 . 3 . 1 . 1 - 82  perpendicular to axis  11 . 1 . 3 . 1 . 1 - 80  may enhance tube strength. Fibers oriented at +/−45° may enhance torsional rigidity. If desired, different portions along the length of tube  11 . 1 . 3 . 1 . 1 - 82  may have different fiber orientations and/or different number of fiber layers. 
     Coating layer  11 . 1 . 3 . 1 . 1 - 116  may, if desired, be formed from one or more metal layers. As an example, first (inner) metal layer C1 may be a nickel cobalt layer that has a thickness of 50 microns or other suitable thickness and second (outer) metal layer C2 may be an electroless nickel layer that is deposited on top of the nickel cobalt layer and has a thickness of 50-400 microns thick, 100-200 microns thick, or other suitable thickness. Layer C1 may serve as an adhesion promotion layer. The outer surface of layers  11 . 1 . 3 . 1 . 1 - 114  may be etched prior to coating layers  11 . 1 . 3 . 1 . 1 - 114  with layer C1 to enhance adhesion of layer C1 to layers  11 . 1 . 3 . 1 . 1 - 114 . Layer C1 may interlock with epoxy (or other polymer) in layers  11 . 1 . 3 . 1 . 1 - 114  and may enhance adhesion of layer C2. Layer C2 may help provide tube  11 . 1 . 3 . 1 . 1 - 82  with low friction as tube  11 . 1 . 3 . 1 . 1 - 82  moves back and forth within an opening in portion  11 . 1 . 3 . 1 . 1 - 40 H or  11 . 1 . 3 . 1 . 1 - 40 T of optical module  11 . 1 . 3 . 1 . 1 - 40  (see, e.g., opening  11 . 1 . 3 . 1 . 1 - 76  of  FIG.  11 . 1   . 3 . 1 . 1 - 5 , which may have a nickel coating or other low-friction coating  11 . 1 . 3 . 1 . 1 - 78 ). The thickness of layer C2 may help enhance the strength of tube  11 . 1 . 3 . 1 . 1 - 82  (e.g., bending strength). 
     By using guide rods  11 . 1 . 3 . 1 . 1 - 50 , the lateral positions of modules  11 . 1 . 3 . 1 . 1 - 40  in device  11 . 1 . 3 . 1 . 1 - 10  may be adjusted to accommodate different user interpupillary distances. Optical module position adjustments may be automated or manual. The optical positioning system(s) of device  11 . 1 . 3 . 1 . 1 - 10  may use guide structures such as guide rods  11 . 1 . 3 . 1 . 1 - 50  to allow optical modules  11 . 1 . 3 . 1 . 1 - 40  to move along a desired axis. The guide rods and the mounting structures used to attach the guide rods to housing  11 . 1 . 3 . 1 . 1 - 12 M (e.g., frame  11 . 1 . 3 . 1 . 1 - 12 FC) may be sufficiently rigid and strong to resist deformation and misalignment in the event that device  11 . 1 . 3 . 1 . 1 - 10  is inadvertently dropped. 
     To reduce the burden on actuators  11 . 1 . 3 . 1 . 1 - 43  as actuators  11 . 1 . 3 . 1 . 1 - 43  rotate threaded rods  11 . 1 . 3 . 1 . 1 - 62  to move modules  11 . 1 . 3 . 1 . 1 - 40  along guide rods  11 . 1 . 3 . 1 . 1 - 50 , guide rods  11 . 1 . 3 . 1 . 1 - 50  may be slidably coupled to modules  11 . 1 . 3 . 1 . 1 - 40  using low-friction structures. These low friction structures may include using low-friction coating materials such as nickel. The coating layer(s) may be polished (e.g., using a centerless grinding tool) and/or otherwise finished to help reduce friction. If desired, the use of nickel coating material may be omitted (e.g., when finishing rods  11 . 1 . 3 . 1 . 1 - 50  using burnishing, grinding, or polishing to provide a low-friction surface). The weight of guide rods  11 . 1 . 3 . 1 . 1 - 50 , which may affect user comfort, may be reduced by using fiber-composite materials or other light materials in forming guide rod tubes  11 . 1 . 3 . 1 . 1 - 82 . 
     An example of a finishing process that may be used to help reduce friction between modules  11 . 1 . 3 . 1 . 1 - 40  and rods  11 . 1 . 3 . 1 . 1 - 50  is superfinishing. Superfinishing is a microfinishing technique that can be used to enhance the surface finish of an item while also enhancing the accuracy of the contours of the item (e.g., enhancing the accuracy of the desired shapes of rods  11 . 1 . 3 . 1 . 1 - 50  such as enhancing the cylindricity of rods  11 . 1 . 3 . 1 . 1 - 50  and/or the accuracy of the desired shapes for the mating portions of modules  11 . 1 . 3 . 1 . 1 - 40 ). With superfinishing, small amounts of surface material (e.g., 1-2 microns) are removed by superfinishing equipment using abrasive. The surface of a superfinished item may be less smoothly polished than when the item is finished using smooth polishing equipment (e.g., there may be residual cross-hatched microscratches on the surface of a superfinished item due to oscillations and/or other movements of the abrasive and rotations of the item during finishing). By superfinishing or otherwise treating (e.g., by burnishing, grinding, polishing, etc.) one or more surfaces of parts that slide relative to each other (e.g., the surface of rods  11 . 1 . 3 . 1 . 1 - 50 ), wear may be decreased and smooth sliding operations may be ensured (with or without using coatings such as nickel coating layers). 
     Fiber-composite tubes may include multiple layers of fiber-composite material (e.g., carbon fiber layers with different fiber orientations). The fiber orientations used in the fiber-composite layers may be selected to enhance bending strength, hoop strength (resistance to tube crushing), and/or torsional strength. End caps  11 . 1 . 3 . 1 . 1 - 86  may have solid portions and/or hollow portions and may be formed from one or more metals, polymer, fiber-composite material, etc. 
     Low-friction coatings for tubes  11 . 1 . 3 . 1 . 1 - 82  (see, e.g., coating layers  11 . 1 . 3 . 1 . 1 - 116  of  FIG.  11 . 1   . 3 . 1 . 1 - 16 ) may be formed from metals such as nickel, nickel cobalt, nickel iron, cobalt, chrome (e.g., a top coat of chrome to serve as a hard coat that reduces friction), and/or other low-friction durable (low-wear) coatings. 
     If desired, the fibers in tubes  11 . 1 . 3 . 1 . 1 - 82  may have different fiber orientations (layups) at different portions of tubes  11 . 1 . 3 . 1 . 1 - 82  (e.g., bending strength may be enhanced with fibers that run the length of tubes  11 . 1 . 3 . 1 . 1 - 82 , torsional rigidity may be enhanced by fibers oriented +/−45° with respect to axis  11 . 1 . 3 . 1 . 1 - 80 , and these fibers may be present along the entire length of tubes  11 . 1 . 3 . 1 . 1 - 82  or only parts of tubes  11 . 1 . 3 . 1 . 1 - 82 ), and/or crush/hoop strength may be enhanced using fibers that wrap around axis  11 . 1 . 3 . 1 . 1 - 80  (e.g., particularly at the ends of tubes  11 . 1 . 3 . 1 . 1 - 82  where tubes  11 . 1 . 3 . 1 . 1 - 82  are being attached to end caps  11 . 1 . 3 . 1 . 1 - 86 ). 
     Tubes  11 . 1 . 3 . 1 . 1 - 82  may, if desired, be provided with strength-enhancing members such as overmolded polymer strengthening members, bonded polymer and/or metal pieces, etc. The surface of tube  11 . 1 . 3 . 1 . 1 - 82  may be treated using acid, laser ablation, primer, sand blasting, and/or other treatments to enhance adhesion prior to overmolding operations. 
     If desired, tubes  11 . 1 . 3 . 1 . 1 - 82  may be provided with tapered portions. As shown in FIG.  11 . 1 . 3 . 1 . 1 - 17 , for example, tube  11 . 1 . 3 . 1 . 1 - 82  may have tapered portions  11 . 1 . 3 . 1 . 1 - 82 T that are formed by grinding and/or machining the end of tube  11 . 1 . 3 . 1 . 1 - 82 . By forming a tapered outer surface formed at the end of tube  11 . 1 . 3 . 1 . 1 - 82 , tube  11 . 1 . 3 . 1 . 1 - 82  may be precisely aligned with a mating tapered portion of housing  11 . 1 . 3 . 1 . 1 - 12  (e.g., a tapered opening in a portion of main housing portion  11 . 1 . 3 . 1 . 1 - 12 M, a portion of frame  11 . 1 . 3 . 1 . 1 - 12 FC, or other tapered portion of housing  11 . 1 . 3 . 1 . 1 - 12  that is machined or otherwise formed in the housing). Tapered tube  11 . 1 . 3 . 1 . 1 - 82  of  FIG.  11 . 1   . 3 . 1 . 1 - 17  may be retained within the corresponding tapered opening of housing  11 . 1 . 3 . 1 . 1 - 12  using a press-fit connection, using adhesive, using fasteners, and/or using other suitable attachment mechanisms. In the example of  FIG.  11 . 1   . 3 . 1 . 1 - 17 , tube  11 . 1 . 3 . 1 . 1 - 82  has been provided with insert  11 . 1 . 3 . 1 . 1 - 120 . Insert  11 . 1 . 3 . 1 . 1 - 120  may be a solid cylindrical member or a tube that is configured to be received within the end of the hollow center of tube  11 . 1 . 3 . 1 . 1 - 82 . Adhesive  111 . 1 . 3 . 1 . 1 - 22  or other attachment mechanisms may be used to help attach insert  11 . 1 . 3 . 1 . 1 - 120  to tube  11 . 1 . 3 . 1 . 1 - 82 . Insert  11 . 1 . 3 . 1 . 1 - 120  may be formed from metal or other suitable materials and may have threads  11 . 1 . 3 . 1 . 1 - 124  that mate with corresponding threads in housing  11 . 1 . 3 . 1 . 1 - 12 . These threads provide axial retention for tube  11 . 1 . 3 . 1 . 1 - 82  (e.g., threads  11 . 1 . 3 . 1 . 1 - 124  help hold the tapered end of tube  11 . 1 . 3 . 1 . 1 - 82  into the corresponding tapered opening of housing  11 . 1 . 3 . 1 . 1 - 12 ). Adhesive may, if desired, be used to lock threads  11 . 1 . 3 . 1 . 1 - 124  in place, may be used to help attach the outer surface of tapered portion  11 . 1 . 3 . 1 . 1 - 82 T to housing  11 . 1 . 3 . 1 . 1 - 12 , and/or may otherwise be used to help reinforce the joint between tube  11 . 1 . 3 . 1 . 1 - 82  and housing  11 . 1 . 3 . 1 . 1 - 12 . 
     11.1.3.1.2: Electronic Device with Lens Positioning Sensing 
     To accommodate users with different interpupillary distances, the left and right lens assemblies may be moved towards or away from each other. A user may supply the interpupillary distance of the user to the head-mounted device, an image sensor or other device may be used in measuring the interpupillary distance to provide to the head-mounted device, and/or gaze tracking sensors in the head-mounted device may measure the interpupillary distance of the user while the head-mounted device is being worn on the head of the user. Other sensing arrangements may be used to measure lens assembly positions relative to the user&#39;s nose, if desired. 
     To prevent excessive pressure on the surface of the user&#39;s nose, force sensors can be used determine how much pressure is applied to the user&#39;s nose with the lenses as the lens-to-lens spacing is changed. Control circuitry in the head-mounted device may adjust the left and right lenses to match the user&#39;s interpupillary distance, unless the lenses apply too much pressure to the user&#39;s nose (e.g., the pressure measured by the force sensors exceeds a threshold). In some situations, the left and right lenses may be spaced so that the lens-to-lens spacing between the left and right lenses matches the user&#39;s interpupillary distance. In other situations, the lens-to-lens spacing between the left and right lenses will be slightly larger than the user&#39;s interpupillary distance to ensure that the lenses do not press excessively against the user&#39;s nose. Sensor circuitry such as force sensing circuitry may be used to provide the control circuitry with real-time feedback on the pressure applied by the lenses to the user&#39;s nose, thereby ensuring that the positions of the left and right lenses are adjusted satisfactorily. 
     A schematic diagram of an illustrative system having an electronic device with sensor circuitry that ensures satisfactory placement of lenses relative to a user&#39;s facial features is shown in  FIG.  11 . 1   . 3 . 1 . 2 - 1 . As shown in  FIG.  11 . 1   . 3 . 1 . 2 - 1 , system  11 . 1 . 3 . 1 . 2 - 8  may include one or more electronic devices such as electronic device  11 . 1 . 3 . 1 . 2 - 10 . The electronic devices of system  11 . 1 . 3 . 1 . 2 - 8  may include computers, cellular telephones, head-mounted devices, wristwatch devices, and other electronic devices. Configurations in which electronic device  11 . 1 . 3 . 1 . 2 - 10  is a head-mounted device are sometimes described herein as an example. 
     As shown in  FIG.  11 . 1   . 3 . 1 . 2 - 1 , electronic devices such as electronic device  11 . 1 . 3 . 1 . 2 - 10  may have control circuitry  11 . 1 . 3 . 1 . 2 - 12 . Control circuitry  11 . 1 . 3 . 1 . 2 - 12  may include storage and processing circuitry for controlling the operation of device  11 . 1 . 3 . 1 . 2 - 10 . Circuitry  11 . 1 . 3 . 1 . 2 - 12  may include storage such as hard disk drive storage, nonvolatile memory (e.g., electrically-programmable-read-only memory configured to form a solid-state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Processing circuitry in control circuitry  11 . 1 . 3 . 1 . 2 - 12  may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors, power management units, audio chips, graphics processing units, application specific integrated circuits, and other integrated circuits. Software code may be stored on storage in circuitry  11 . 1 . 3 . 1 . 2 - 12  and run on processing circuitry in circuitry  11 . 1 . 3 . 1 . 2 - 12  to implement control operations for device  11 . 1 . 3 . 1 . 2 - 10  (e.g., data gathering operations, operations involved in processing three-dimensional facial image data, operations involving the adjustment of components using control signals, etc.). Control circuitry  11 . 1 . 3 . 1 . 2 - 12  may include wired and wireless communications circuitry. For example, control circuitry  11 . 1 . 3 . 1 . 2 - 12  may include radio-frequency transceiver circuitry such as cellular telephone transceiver circuitry, wireless local area network (WiFi®) transceiver circuitry, millimeter wave transceiver circuitry, and/or other wireless communications circuitry. 
     During operation, the communications circuitry of the devices in system  11 . 1 . 3 . 1 . 2 - 8  (e.g., the communications circuitry of control circuitry  11 . 1 . 3 . 1 . 2 - 12  of device  11 . 1 . 3 . 1 . 2 - 10 ), may be used to support communication between the electronic devices. For example, one electronic device may transmit video and/or audio data to another electronic device in system  11 . 1 . 3 . 1 . 2 - 8 . Electronic devices in system  11 . 1 . 3 . 1 . 2 - 8  may use wired and/or wireless communications circuitry to communicate through one or more communications networks (e.g., the internet, local area networks, etc.). The communications circuitry may be used to allow data to be received by device  11 . 1 . 3 . 1 . 2 - 10  from external equipment (e.g., a tethered computer, a portable device such as a handheld device or laptop computer, online computing equipment such as a remote server or other remote computing equipment, or other electrical equipment) and/or to provide data to external equipment. 
     Device  11 . 1 . 3 . 1 . 2 - 10  may include input-output devices  11 . 1 . 3 . 1 . 2 - 22 . Input-output devices  11 . 1 . 3 . 1 . 2 - 22  may be used to allow a user to provide device  11 . 1 . 3 . 1 . 2 - 10  with user input. Input-output devices  11 . 1 . 3 . 1 . 2 - 22  may also be used to gather information on the environment in which device  11 . 1 . 3 . 1 . 2 - 10  is operating. Output components in devices  11 . 1 . 3 . 1 . 2 - 22  may allow device  11 . 1 . 3 . 1 . 2 - 10  to provide a user with output and may be used to communicate with external electrical equipment. 
     As shown in  FIG.  11 . 1   . 3 . 1 . 2 - 1 , input-output devices  11 . 1 . 3 . 1 . 2 - 22  may include one or more displays such as display  11 . 1 . 3 . 1 . 2 - 14 . In some configurations, display  11 . 1 . 3 . 1 . 2 - 14  of device  11 . 1 . 3 . 1 . 2 - 10  includes left and right display panels (sometimes referred to as left and right portions of display  11 . 1 . 3 . 1 . 2 - 14  and/or left and right displays) that are in alignment with the user&#39;s left and right eyes and are viewable through left and right lens assemblies, respectively. In other configurations, display  11 . 1 . 3 . 1 . 2 - 14  includes a single display panel that extends across both eyes. 
     Display  11 . 1 . 3 . 1 . 2 - 14  may be used to display images. The visual content that is displayed on display  11 . 1 . 3 . 1 . 2 - 14  may be viewed by a user of device  11 . 1 . 3 . 1 . 2 - 10 . Displays in device  11 . 1 . 3 . 1 . 2 - 10  such as display  11 . 1 . 3 . 1 . 2 - 14  may be organic light-emitting diode displays or other displays based on arrays of light-emitting diodes, liquid crystal displays, liquid-crystal-on-silicon displays, projectors or displays based on projecting light beams on a surface directly or indirectly through specialized optics (e.g., digital micromirror devices), electrophoretic displays, plasma displays, electrowetting displays, microLED displays, or any other suitable displays. 
     Display  11 . 1 . 3 . 1 . 2 - 14  may present computer-generated content such as virtual reality content and mixed reality content to a user. Virtual reality content may be displayed in the absence of real-world content. Mixed reality content, which may sometimes be referred to as augmented reality content, may include computer-generated images that are overlaid on real-world images. The real-world images may be captured by a camera (e.g., a forward-facing camera) and merged with overlaid computer-generated content or an optical coupling system may be used to allow computer-generated content to be overlaid on top of real-world images. As an example, a pair of mixed reality glasses or other augmented reality head-mounted display may include a display device that provides images to a user through a beam splitter, prism, holographic coupler, or other optical coupler. Configurations in which display  11 . 1 . 3 . 1 . 2 - 14  is used to display virtual reality content to a user through lenses are described herein as an example. 
     Input-output devices  11 . 1 . 3 . 1 . 2 - 22  may include sensors  11 . 1 . 3 . 1 . 2 - 16 . Sensors  11 . 1 . 3 . 1 . 2 - 16  may include, for example, three-dimensional sensors (e.g., three-dimensional image sensors such as structured light sensors that emit beams of light and that use two-dimensional digital image sensors to gather image data for three-dimensional images from light spots that are produced when a target is illuminated by the beams of light, binocular three-dimensional image sensors that gather three-dimensional images using two or more cameras in a binocular imaging arrangement, three-dimensional LIDAR (light detection and ranging) sensors, three-dimensional radio-frequency sensors, or other sensors that gather three-dimensional image data), cameras (e.g., infrared and/or visible digital image sensors), gaze tracking sensors (e.g., a gaze tracking system based on an image sensor and, if desired, a light source that emits one or more beams of light that are tracked using the image sensor after reflecting from a user&#39;s eyes), touch sensors, buttons, force sensors, sensors such as contact sensors based on switches, gas sensors, pressure sensors, moisture sensors, magnetic sensors, audio sensors (microphones), ambient light sensors, microphones for gathering voice commands and other audio input, sensors that are configured to gather information on motion, position, and/or orientation (e.g., accelerometers, gyroscopes, compasses, and/or inertial measurement units that include all of these sensors or a subset of one or two of these sensors), fingerprint sensors and other biometric sensors, optical position sensors (optical encoders), and/or other position sensors such as linear position sensors, and/or other sensors. As shown in  FIG.  11 . 1   . 3 . 1 . 2 - 1 , sensors  11 . 1 . 3 . 1 . 2 - 16  may include sensing circuitry (sensor circuitry) that is configured to measure the pressure applied between objects in system  11 . 1 . 3 . 1 . 2 - 8 . The sensing circuitry may include one or more sensors such as one or more force sensors  11 . 1 . 3 . 1 . 2 - 20 . Sensing circuitry such as force sensors  11 . 1 . 3 . 1 . 2 - 20  may, for example, be used to sense an amount of pressure applied by lens assemblies in device  11 . 1 . 3 . 1 . 2 - 10  to a user&#39;s nose. 
     User input and other information may be gathered using sensors and other input devices in input-output devices  11 . 1 . 3 . 1 . 2 - 22 . If desired, input-output devices  11 . 1 . 3 . 1 . 2 - 22  may include other devices  11 . 1 . 3 . 1 . 2 - 24  such as haptic output devices (e.g., vibrating components), light-emitting diodes and other light sources, speakers such as ear speakers for producing audio output, and other electrical components. Device  11 . 1 . 3 . 1 . 2 - 10  may include circuits for receiving wireless power, circuits for transmitting power wirelessly to other devices, batteries and other energy storage devices (e.g., capacitors), joysticks, buttons, and/or other components. 
     Electronic device  11 . 1 . 3 . 1 . 2 - 10  may have housing structures (e.g., housing walls, straps, etc.), as shown by illustrative support structures  11 . 1 . 3 . 1 . 2 - 26  of  FIG.  11 . 1   . 3 . 1 . 2 - 1 . In configurations in which electronic device  11 . 1 . 3 . 1 . 2 - 10  is a head-mounted device (e.g., a pair of glasses, goggles, a helmet, a hat, a headband, etc.), support structures  11 . 1 . 3 . 1 . 2 - 26  may include head-mounted support structures (e.g., a helmet housing, head straps, temples in a pair of eyeglasses, goggle housing structures, and/or other head-mounted structures). The head-mounted support structures may be configured to be worn on a head of a user during operation of device  11 . 1 . 3 . 1 . 2 - 10  and may support display(s)  11 . 1 . 3 . 1 . 2 - 14 , sensors  11 . 1 . 3 . 1 . 2 - 16 , other components  11 . 1 . 3 . 1 . 2 - 24 , other input-output devices  11 . 1 . 3 . 1 . 2 - 22 , and control circuitry  11 . 1 . 3 . 1 . 2 - 12 . 
       FIG.  11 . 1   . 3 . 1 . 2 - 2  is a top view of electronic device  11 . 1 . 3 . 1 . 2 - 10  in an illustrative configuration in which electronic device  11 . 1 . 3 . 1 . 2 - 10  is a head-mounted device. As shown in  FIG.  11 . 1   . 3 . 1 . 2 - 2 , electronic device  11 . 1 . 3 . 1 . 2 - 10  may include support structures (see, e.g., support structures  11 . 1 . 3 . 1 . 2 - 26  of  FIG.  11 . 1   . 3 . 1 . 2 - 1 ) that are used in housing the components of device  11 . 1 . 3 . 1 . 2 - 10  and mounting device  11 . 1 . 3 . 1 . 2 - 10  onto a user&#39;s head. These support structures may include, for example, structures that form housing walls and other structures for main unit  11 . 1 . 3 . 1 . 2 - 26 - 2  (e.g., exterior housing walls, lens assembly structures, etc.) and straps or other supplemental support structures such as structures  11 . 1 . 3 . 1 . 2 - 26 - 1  that help to hold main unit  11 . 1 . 3 . 1 . 2 - 26 - 2  on a user&#39;s face so that the user&#39;s eyes are located within eye boxes  11 . 1 . 3 . 1 . 2 - 60 . 
     Display  11 . 1 . 3 . 1 . 2 - 14  may include left and right display panels (e.g., left and right pixel arrays, sometimes referred to as left and right displays or left and right display portions) that are mounted respectively in left and right display modules  11 . 1 . 3 . 1 . 2 - 70  corresponding respectively to a user&#39;s left eye (and left eye box  11 . 1 . 3 . 1 . 2 - 60 ) and right eye (and right eye box). Modules  11 . 1 . 3 . 1 . 2 - 70 , which may sometimes be referred to as lens support structures, lens assemblies, lens housings, or lens and display housings, may be individually positioned relative to the housing wall structures of main unit  11 . 1 . 3 . 1 . 2 - 26 - 2  and relative to the user&#39;s eyes using positioning circuitry such as respective left and right positioners  11 . 1 . 3 . 1 . 2 - 58 . Positioners  11 . 1 . 3 . 1 . 2 - 58  may include stepper motors, piezoelectric actuators, motors, linear electromagnetic actuators, and/or other electronic components for adjusting lens assembly positions. Positioners  11 . 1 . 3 . 1 . 2 - 58  may be controlled by control circuitry  11 . 1 . 3 . 1 . 2 - 12  during operation of device  11 . 1 . 3 . 1 . 2 - 10 . For example, positioners  11 . 1 . 3 . 1 . 2 - 58  may be used to adjust the spacing between modules  11 . 1 . 3 . 1 . 2 - 70  (and therefore the lens-to-lens spacing between the left and right lenses of modules  11 . 1 . 3 . 1 . 2 - 70 ) to match the interpupillary distance (IPD) of a user&#39;s eyes. This allows the user to view the left and right display portions of display  11 . 1 . 3 . 1 . 2 - 14  in the left and right lens modules. In some cases, however, the lenses may apply excess pressure to the user&#39;s nose if adjusted to the user&#39;s IPD. Therefore, sensors may be incorporated into device  11 . 1 . 3 . 1 . 2 - 10  to monitor the pressure on the user&#39;s nose. An illustrative arrangement that includes force sensors to monitor the pressure applied to the user&#39;s nose is shown in  FIG.  11 . 1   . 3 . 1 . 2 - 3 . 
     As shown in  FIG.  11 . 1   . 3 . 1 . 2 - 3 , lens assembly  11 . 1 . 3 . 1 . 2 - 70  (e.g., one of the two lens modules  11 . 1 . 3 . 1 . 2 - 70  shown in  FIG.  11 . 1   . 3 . 1 . 2 - 2 ) may be adjacent to the user&#39;s nose  11 . 1 . 3 . 1 . 2 - 40  when device  11 . 1 . 3 . 1 . 2 - 10  is being worn on a user&#39;s head. Only one of the two lens modules is shown in  FIG.  11 . 1   . 3 . 1 . 2 - 3  for simplicity, but the other lens assembly  11 . 1 . 3 . 1 . 2 - 70  may have the same structure. 
     To ensure that display  11 . 1 . 3 . 1 . 2 - 14  is viewable by the user when the user&#39;s eyes are located in eye boxes  11 . 1 . 3 . 1 . 2 - 60  ( FIG.  11 . 1   . 3 . 1 . 2 - 2 ), control circuitry  11 . 1 . 3 . 1 . 2 - 12  may attempt to align lens centers LC with the centers PC of the user&#39;s eyes. At the same time, control circuitry  11 . 1 . 3 . 1 . 2 - 12  may use sensor circuitry such as force sensors  11 . 1 . 3 . 1 . 2 - 20  to monitor the pressure applied to nose  11 . 1 . 3 . 1 . 2 - 40  by lens modules  11 . 1 . 3 . 1 . 2 - 70  to ensure that lens modules  11 . 1 . 3 . 1 . 2 - 70  do not press excessively on nose  11 . 1 . 3 . 1 . 2 - 40  and cause discomfort. 
     In scenarios in which the user&#39;s nose is small, there may be ample room available to align lens centers LC with eye centers PC. In scenarios in which the user&#39;s nose is larger, control circuitry  11 . 1 . 3 . 1 . 2 - 12  may position modules  11 . 1 . 3 . 1 . 2 - 70  as shown in  FIG.  11 . 1   . 3 . 1 . 2 - 3 . For example, a distance between lens modules  11 . 1 . 3 . 1 . 2 - 70  (the lens-to-lens spacing) may be larger than would be desired for perfect alignment of lens centers LC with eye centers PC. The use of this wider lens-to-lens spacing helps ensure that lens modules  11 . 1 . 3 . 1 . 2 - 70  will not exert more inward force on nose  11 . 1 . 3 . 1 . 2 - 40  than would be comfortable to a user, while still allowing satisfactory viewing of content on display  11 . 1 . 3 . 1 . 2 - 14  through lenses  11 . 1 . 3 . 1 . 2 - 72 . Lens modules  11 . 1 . 3 . 1 . 2 - 70  may be placed at a non-zero distance (gap) from the side surfaces of nose  11 . 1 . 3 . 1 . 2 - 40  or may be spaced apart from the side surfaces of nose  11 . 1 . 3 . 1 . 2 - 40  by a predetermined gap. A user may select which of these options is most comfortable to the user and/or a default setting may be supplied to control circuitry  11 . 1 . 3 . 1 . 2 - 12 . 
     In operation, positioners  11 . 1 . 3 . 1 . 2 - 58  may move lens modules  11 . 1 . 3 . 1 . 2 - 70  ( FIG.  11 . 1   . 3 . 1 . 2 - 2 ) toward nose  11 . 1 . 3 . 1 . 2 - 40 , attempting to align the lens center LC of each lens with eye centers PC. Sensors may be incorporated into device  11 . 1 . 3 . 1 . 2 - 10  to ensure that excess pressure is not applied to nose  11 . 1 . 3 . 1 . 2 - 40  by lens modules  11 . 1 . 3 . 1 . 2 - 70 . In general, any desired sensor circuitry may be used to measure the pressure on nose  11 . 1 . 3 . 1 . 2 - 40 . In one example, each lens module  11 . 1 . 3 . 1 . 2 - 70  may have one or more force sensors  11 . 1 . 3 . 1 . 2 - 20 . 
     Force sensor  11 . 1 . 3 . 1 . 2 - 20  may be incorporated between nasal flap  11 . 1 . 3 . 1 . 2 - 29  and each lens assembly  11 . 1 . 3 . 1 . 2 - 70 . Nasal flap  11 . 1 . 3 . 1 . 2 - 29  may be a fabric, polymer, or other material component that allows for a comfortable fit for a user of device  11 . 1 . 3 . 1 . 2 - 10 . For example, nasal flap  11 . 1 . 3 . 1 . 2 - 29  may be interposed between each lens assembly  11 . 1 . 3 . 1 . 2 - 70  and nose  11 . 1 . 3 . 1 . 2 - 40 . In some embodiments, nasal flap  11 . 1 . 3 . 1 . 2 - 29  may extend along both sides and over the top of nose  11 . 1 . 3 . 1 . 2 - 40  (e.g., over at least a portion of a bridge of nose  11 . 1 . 3 . 1 . 2 - 40 ). However, this is merely illustrative. Nasal flap  11 . 1 . 3 . 1 . 2 - 29  may have two separate portion, one between left lens assembly  11 . 1 . 3 . 1 . 2 - 70  and nose  11 . 1 . 3 . 1 . 2 - 40  and another between right lens assembly  11 . 1 . 3 . 1 . 2 - 70  and nose  11 . 1 . 3 . 1 . 2 - 40 , or may be omitted from device  11 . 1 . 3 . 1 . 2 - 10  if desired. 
     In embodiments in which nasal flap  11 . 1 . 3 . 1 . 2 - 29  is included in device  11 . 1 . 3 . 1 . 2 - 10 , force sensors  11 . 1 . 3 . 1 . 2 - 20  may be incorporated between each lens assembly  11 . 1 . 3 . 1 . 2 - 70  (i.e., left lens assembly  11 . 1 . 3 . 1 . 2 - 70  and right lens assembly  11 . 1 . 3 . 1 . 2 - 70 ) and nasal flap  11 . 1 . 3 . 1 . 2 - 29 . However, this location of force sensors  11 . 1 . 3 . 1 . 2 - 20  is merely illustrative. In general, force sensors  11 . 1 . 3 . 1 . 2 - 20  may be included in any desired location within device  11 . 1 . 3 . 1 . 2 - 10 . For example, force sensors  11 . 1 . 3 . 1 . 2 - 20  may be formed between nasal flap  11 . 1 . 3 . 1 . 2 - 29  and nose  11 . 1 . 3 . 1 . 2 - 40  as shown by position  11 . 1 . 3 . 1 . 2 - 20 ′, may be formed within nasal flap  11 . 1 . 3 . 1 . 2 - 29  or be integral with nasal flap  11 . 1 . 3 . 1 . 2 - 29  as shown by position  11 . 1 . 3 . 1 . 2 - 20 ″, or may be formed within lens assembly  11 . 1 . 3 . 1 . 2 - 70  as shown by position  11 . 1 . 3 . 1 . 2 - 20 ″. 
     Although  FIG.  11 . 1   . 3 . 1 . 2 - 3  shows a single force sensor  11 . 1 . 3 . 1 . 2 - 20  between nose  11 . 1 . 3 . 1 . 2 - 40  and lens assembly  11 . 1 . 3 . 1 . 2 - 70 , this is merely illustrative. Device  11 . 1 . 3 . 1 . 2 - 10  may have one force sensor  11 . 1 . 3 . 1 . 2 - 20  between nose  11 . 1 . 3 . 1 . 2 - 40  and each lens assembly  11 . 1 . 3 . 1 . 2 - 70 , may have multiple force sensors between nose  11 . 1 . 3 . 1 . 2 - 40  and each lens assembly  11 . 1 . 3 . 1 . 2 - 70 , may have one single force sensor between nose  11 . 1 . 3 . 1 . 2 - 40  and one lens assembly  11 . 1 . 3 . 1 . 2 - 70 , or may have any other desired arrangement of force sensor(s)  11 . 1 . 3 . 1 . 2 - 20 . 
     Regardless of where force sensors  11 . 1 . 3 . 1 . 2 - 20  are formed within device  11 . 1 . 3 . 1 . 2 - 10 , force sensors  11 . 1 . 3 . 1 . 2 - 20  may monitor the amount of force applied to nose  11 . 1 . 3 . 1 . 2 - 40  by lens modules  11 . 1 . 3 . 1 . 2 - 70  to ensure that excess pressure is not applied to nose  11 . 1 . 3 . 1 . 2 - 40 . Force sensors  11 . 1 . 3 . 1 . 2 - 20  may continuously monitor the applied force, or may flag control circuitry  11 . 1 . 3 . 1 . 2 - 12  when the applied force surpasses a threshold. Force sensors  11 . 1 . 3 . 1 . 2 - 20  may be any desired type of sensor that monitors the amount of force/pressure applied to nose  11 . 1 . 3 . 1 . 2 - 40 . Some examples of force sensors  11 . 1 . 3 . 1 . 2 - 20  are shown in  FIGS.  11 . 1   . 3 . 1 . 2 - 4 A- 11 . 1 . 3 . 1 . 2 - 4 C. 
     In some examples, force sensor(s)  11 . 1 . 3 . 1 . 2 - 20  may be direct force sensors, such as force sensor  11 . 1 . 3 . 1 . 2 - 20  of  FIG.  11 . 1   . 3 . 1 . 2 - 4 A. Direct force sensor  11 . 1 . 3 . 1 . 2 - 20  of  FIG.  11 . 1   . 3 . 1 . 2 - 4 A may be a pressure sensitive resistor and may include electrodes formed form interdigitated finger traces  11 . 1 . 3 . 1 . 2 - 27 . In particular, when pressure is applied to direct force sensor  11 . 1 . 3 . 1 . 2 - 20 , the resistance may be determined due to a change in distance between portions of interdigitated finger traces  11 . 1 . 3 . 1 . 2 - 27 , and circuitry, such as control circuitry  11 . 1 . 3 . 1 . 2 - 12 , may determine the force applied to direct force sensor  11 . 1 . 3 . 1 . 2 - 20 . If direct force sensor  11 . 1 . 3 . 1 . 2 - 20  is placed in any of the possible locations shown in  FIG.  11 . 1   . 3 . 1 . 2 - 3 , or otherwise placed between a user&#39;s nose and lens assembly  11 . 1 . 3 . 1 . 2 - 70 , direct force sensor  11 . 1 . 3 . 1 . 2 - 20  may be used to determine the force that lens assembly  11 . 1 . 3 . 1 . 2 - 70  applies to nose  11 . 1 . 3 . 1 . 2 - 40  as it is moved toward nose  11 . 1 . 3 . 1 . 2 - 40 . Direct force sensor  11 . 1 . 3 . 1 . 2 - 20  may be used to continuously measure the pressure against nose  11 . 1 . 3 . 1 . 2 - 40 , or may be used as a thresholding sensor (i.e., control circuitry  11 . 1 . 3 . 1 . 2 - 12  may determine that the resistance of direct force sensor  11 . 1 . 3 . 1 . 2 - 20  is over a threshold and that the force on nose  11 . 1 . 3 . 1 . 2 - 40  is therefore too great). 
     In other examples, force sensor(s)  11 . 1 . 3 . 1 . 2 - 20  may be formed from conductive strands, yarn, or fibers and incorporated into a fabric in device  11 . 1 . 3 . 1 . 2 - 10 . For example, force sensor(s)  11 . 1 . 3 . 1 . 2 - 20  may be formed form smart fabrics or other fabrics that include conductive strands. The conductive strands may be arranged to form a force sensor. An example of this arrangement is shown in  FIG.  11 . 1   . 3 . 1 . 2 - 4 B. 
     As shown in  FIG.  11 . 1   . 3 . 1 . 2 - 4 B, force sensor  11 . 1 . 3 . 1 . 2 - 20  may be incorporated into fabric  11 . 1 . 3 . 1 . 2 - 31 . In particular, fabric  11 . 1 . 3 . 1 . 2 - 31  may include conductive strands  11 . 1 . 3 . 1 . 2 - 28  and non-conductive strands  11 . 1 . 3 . 1 . 2 - 33 . Conductive strands  11 . 1 . 3 . 1 . 2 - 28  may be arranged to form a force sensor. For example, control circuitry, such as control circuitry  11 . 1 . 3 . 1 . 2 - 12 , may measure capacitance or resistance changes between conductive strands  11 . 1 . 3 . 1 . 2 - 28 . Because the capacitance/resistance will change as the distance between conductive strands  11 . 1 . 3 . 1 . 2 - 28  changes, the capacitance/resistance measurements are indicative of the amount of force on fabric  11 . 1 . 3 . 1 . 2 - 31 . Conductive strands may be formed from a metal, such as silver or copper, which may optionally be coated onto a polymer or fabric strand. 
     Although conductive strands  11 . 1 . 3 . 1 . 2 - 28  are shown as straight strands in  FIG.  11 . 1   . 3 . 1 . 2 - 4 B, this is merely illustrative. In some embodiments, conductive strands  11 . 1 . 3 . 1 . 2 - 28  may be serpentine or have other desired shapes, which may allow fabric  11 . 1 . 3 . 1 . 2 - 31  to have improved flexibility. 
     Non-conductive strands  11 . 1 . 3 . 1 . 2 - 33  may be interspersed between at least some of conductive strands  11 . 1 . 3 . 1 . 2 - 28 , if desired. As shown in  FIG.  11 . 1   . 3 . 1 . 2 - 4 B, every other strand may be a non-conductive strand. However, this is merely illustrative. Any desired number of conductive and non-conductive strands may be incorporate into fabric  11 . 1 . 3 . 1 . 2 - 31  to form force sensor  11 . 1 . 3 . 1 . 2 - 20 . 
     Fabric  11 . 1 . 3 . 1 . 2 - 31  may form some or all of nasal flap  11 . 1 . 3 . 1 . 2 - 29 , or may otherwise cover nasal flap  11 . 1 . 3 . 1 . 2 - 29 . Because nasal flap  11 . 1 . 3 . 1 . 2 - 29  is between nose  11 . 1 . 3 . 1 . 2 - 40  and lens assembly  11 . 1 . 3 . 1 . 2 - 70  when device  11 . 1 . 3 . 1 . 2 - 10  is worn by a user ( FIG.  11 . 1   . 3 . 1 . 2 - 2 ), force sensor  11 . 1 . 3 . 1 . 2 - 20  may indicate the amount of force applied to nose  11 . 1 . 3 . 1 . 2 - 40  by lens assembly  11 . 1 . 3 . 1 . 2 - 70 . Any desired number of force sensors  11 . 1 . 3 . 1 . 2 - 20  may be incorporated into nasal flap  11 . 1 . 3 . 1 . 2 - 29  in this way. 
     Alternatively or additionally, fabric  11 . 1 . 3 . 1 . 2 - 31  may be otherwise incorporated into device  11 . 1 . 3 . 1 . 2 - 10 . For example, fabric  11 . 1 . 3 . 1 . 2 - 31  may form a curtain fabric that is incorporated into device  11 . 1 . 3 . 1 . 2 - 10  between support structure  11 . 1 . 3 . 1 . 2 - 26 - 2  and lenses  11 . 1 . 3 . 1 . 2 - 70  ( FIG.  11 . 1   . 3 . 1 . 2 - 2 ) to hide internal components. At least a portion of the curtain fabric having force sensor(s)  11 . 1 . 3 . 1 . 2 - 20  may be between lens assembly  11 . 1 . 3 . 1 . 2 - 70  and nose  11 . 1 . 3 . 1 . 2 - 40  (e.g., between lens assembly  11 . 1 . 3 . 1 . 2 - 70  and nasal flap  11 . 1 . 3 . 1 . 2 - 29 ) and may therefore measure the force applied to nose  11 . 1 . 3 . 1 . 2 - 40  by lens assembly  11 . 1 . 3 . 1 . 2 - 70 . In general, however, fabric  11 . 1 . 3 . 1 . 2 - 31  may be incorporated into device  11 . 1 . 3 . 1 . 2 - 10  into any desired manner. Any desired number of force sensors  11 . 1 . 3 . 1 . 2 - 20  may be incorporated into fabric  11 . 1 . 3 . 1 . 2 - 31 . 
     In other examples, force sensor(s)  11 . 1 . 3 . 1 . 2 - 20  may be incorporated into an interior region of nasal flap  11 . 1 . 3 . 1 . 2 - 29 . An example of an arrangement in which a force sensor is located in the interior of nasal flap  11 . 1 . 3 . 1 . 2 - 29  is shown in  FIG.  11 . 1   . 3 . 1 . 2 - 4 C. 
     As shown in  FIG.  11 . 1   . 3 . 1 . 2 - 4 C, nasal flap  11 . 1 . 3 . 1 . 2 - 29  may have peripheral region  11 . 1 . 3 . 1 . 2 - 29 A that surrounds inner region  11 . 1 . 3 . 1 . 2 - 29 B (also referred to as a cavity herein). Peripheral region  11 . 1 . 3 . 1 . 2 - 29 A may be formed from polymer, rubber, or any other desired material. Inner region  11 . 1 . 3 . 1 . 2 - 29 B may be filled with air, gas, liquid, or any other desired substance. Force sensor  11 . 1 . 3 . 1 . 2 - 20  may be located in inner region  11 . 1 . 3 . 1 . 2 - 29 B and may produce a force measurement in response to increased pressure in inner region  11 . 1 . 3 . 1 . 2 - 29 B. For example, as lens assembly  11 . 1 . 3 . 1 . 2 - 70  squeezes nasal flap  11 . 1 . 3 . 1 . 2 - 29  against nose  11 . 1 . 3 . 1 . 2 - 40  ( FIG.  11 . 1   . 3 . 1 . 2 - 2 ), the pressure of the air, gas, liquid, or other material within nasal flap  11 . 1 . 3 . 1 . 2 - 29  may increase, increasing the pressure on force sensor  11 . 1 . 3 . 1 . 2 - 20 . In some examples, force sensor  11 . 1 . 3 . 1 . 2 - 20  may be a barometric pressure sensor that measures the increased pressure within inner region  11 . 1 . 3 . 1 . 2 - 29 B when lens assembly  11 . 1 . 3 . 1 . 2 - 70  is moved against nose  11 . 1 . 3 . 1 . 2 - 40 . Therefore, force sensor  11 . 1 . 3 . 1 . 2 - 20  may produce a force measurement indicative of the force applied to the user&#39;s nose by lens assembly  11 . 1 . 3 . 1 . 2 - 70 . 
     Although  FIG.  11 . 1   . 3 . 1 . 2 - 4 C shows one side or portion of nasal flap  11 . 1 . 3 . 1 . 2 - 29 , this is merely illustrative. Any desired number of force sensors  11 . 1 . 3 . 1 . 2 - 20  may be incorporated into inner region  11 . 1 . 3 . 1 . 2 - 29 B of nasal flap  11 . 1 . 3 . 1 . 2 - 29  on one or both sides of the user&#39;s nose. Additionally, although  FIG.  11 . 1   . 3 . 1 . 2 - 4 C shows force sensor  11 . 1 . 3 . 1 . 2 - 20  implemented as a pressure sensor in nasal flap  11 . 1 . 3 . 1 . 2 - 29 , force sensor  11 . 1 . 3 . 1 . 2 - 20  may be a pressure sensor anywhere between nose  11 . 1 . 3 . 1 . 2 - 40  and lens assembly  11 . 1 . 3 . 1 . 2 - 70  when worn by a user. For example, force sensor  11 . 1 . 3 . 1 . 2 - 20  may be implemented as a pressure sensor within an edge portion of lens assembly  11 . 1 . 3 . 1 . 2 - 70  (i.e., in position  11 . 1 . 3 . 1 . 2 - 20 ″ of  FIG.  11 . 1   . 3 . 1 . 2 - 3 ), if desired. 
     As an alternative to the sensors shown in  FIGS.  11 . 1   . 3 . 1 . 2 - 4 A- 11 . 1 . 3 . 1 . 2 - 4 C, force sensor  11 . 1 . 3 . 1 . 2 - 20  may be implemented to measure the deflection of nasal flap  11 . 1 . 3 . 1 . 2 - 29  relative to lens assembly  11 . 1 . 3 . 1 . 2 - 70 . An example of this type of force sensor is shown in  FIG.  11 . 1   . 3 . 1 . 2 - 5 . 
     As shown in  FIG.  11 . 1   . 3 . 1 . 2 - 5 , force sensor  11 . 1 . 3 . 1 . 2 - 20  may be mounted to (or within) a portion of lens assembly  11 . 1 . 3 . 1 . 2 - 70 . Force sensor  11 . 1 . 3 . 1 . 2 - 20  may be a sensor that measures the proximity of nasal flap  11 . 1 . 3 . 1 . 2 - 29 , such as a capacitive proximity sensor, a resistive proximity sensor, an optical proximity sensor, an ultrasonic proximity sensor, or any other desired type of proximity sensor. Optionally, magnet  11 . 1 . 3 . 1 . 2 - 32  may be embedded within (or mounted to) nasal flap  11 . 1 . 3 . 1 . 2 - 29 , and force sensor  11 . 1 . 3 . 1 . 2 - 20  may be implemented as a Hall Effect sensor. In this way, the Hall Effect sensor may determine the proximity of magnet  11 . 1 . 3 . 1 . 2 - 32  and therefore the proximity of nasal flap  11 . 1 . 3 . 1 . 2 - 29 . By measuring the proximity of nasal flap  11 . 1 . 3 . 1 . 2 - 29  (i.e., the amount by which nasal flap  11 . 1 . 3 . 1 . 2 - 29  has moved), the proximity sensor may provide an output that is indicative of the amount of force applied to the user&#39;s nose by lens assembly  11 . 1 . 3 . 1 . 2 - 70 . 
     Although the previous embodiments have included a dedicated force sensor  11 . 1 . 3 . 1 . 2 - 20  to ensure that excessive force is not applied to a user&#39;s nose, this is merely illustrative. Device  11 . 1 . 3 . 1 . 2 - 10  may use other sensors (such as sensors  11 . 1 . 3 . 1 . 2 - 16 ) to determine if the force applied to nose  11 . 1 . 3 . 1 . 2 - 40  exceeds a threshold. An example of this arrangement is shown in  FIG.  11 . 1   . 3 . 1 . 2 - 6 . 
     As shown in  FIG.  11 . 1   . 3 . 1 . 2 - 6 , one or more flexible members  11 . 1 . 3 . 1 . 2 - 34  may extend from lens assembly  11 . 1 . 3 . 1 . 2 - 70 . Flexible members  11 . 1 . 3 . 1 . 2 - 34  may be formed from rubber, polymer, fabric, or any other desired flexible material. Device  11 . 1 . 3 . 1 . 2 - 10  may also include light-emitting component  11 . 1 . 3 . 1 . 2 - 36  and light-detecting component  11 . 1 . 3 . 1 . 2 - 42 , which may be used in gaze tracking or other desired operations. For example, light-emitting component  11 . 1 . 3 . 1 . 2 - 36  may be an infrared light-emitting component and light-detecting component  11 . 1 . 3 . 1 . 2 - 42  may be an infrared light-detecting component. To determine the gaze of a user of device  11 . 1 . 3 . 1 . 2 - 10 , infrared light-emitting component  11 . 1 . 3 . 1 . 2 - 36  may emit light toward the eye of the user, and the reflections from the user&#39;s eye may be detected by infrared light-detecting component  11 . 1 . 3 . 1 . 2 - 42 . These reflections may indicate the direction of the user&#39;s gaze. In general, however, light-emitting component  11 . 1 . 3 . 1 . 2 - 36  and light-detecting component  11 . 1 . 3 . 1 . 2 - 42  may be any desired components and may operate in any desired wavelength. 
     When assembly  11 . 1 . 3 . 1 . 2 - 70  moves toward nasal flap  11 . 1 . 3 . 1 . 2 - 29  (and therefore nose  11 . 1 . 3 . 1 . 2 - 40 ), it will eventually contact and push against nasal flap  11 . 1 . 3 . 1 . 2 - 29  and nose  11 . 1 . 3 . 1 . 2 - 40 , causing flexible members  11 . 1 . 3 . 1 . 2 - 34  to be moved in direction  11 . 1 . 3 . 1 . 2 - 38 . If flexible members  11 . 1 . 3 . 1 . 2 - 34  move far enough (i.e., the amount of force applied to nose  11 . 1 . 3 . 1 . 2 - 40  exceeds or meets a threshold), flexible members  11 . 1 . 3 . 1 . 2 - 34  may block light-emitting component  11 . 1 . 3 . 1 . 2 - 36 . As a result, light-detecting component  11 . 1 . 3 . 1 . 2 - 42  may stop detecting light emitted by light-emitting component  11 . 1 . 3 . 1 . 2 - 36 . Based on the changed signal of light-detecting component  11 . 1 . 3 . 1 . 2 - 42 , control circuitry  11 . 1 . 3 . 1 . 2 - 12  may stop positioners  11 . 1 . 3 . 1 . 2 - 58  from moving lens assembly  11 . 1 . 3 . 1 . 2 - 70  further toward the user&#39;s nose. 
     Although flexible members  11 . 1 . 3 . 1 . 2 - 34  are shown as covering light-emitting component  11 . 1 . 3 . 1 . 2 - 36 , this is merely illustrative. Flexible members  11 . 1 . 3 . 1 . 2 - 34  may cover light-detecting component  11 . 1 . 3 . 1 . 2 - 42 , or may merely move between light-emitting component  11 . 1 . 3 . 1 . 2 - 36  and light-detecting component  11 . 1 . 3 . 1 . 2 - 42 . Additionally, any desired number of flexible members  11 . 1 . 3 . 1 . 2 - 34  may be used. 
     If desired, the position of lens modules  11 . 1 . 3 . 1 . 2 - 70  relative to the corresponding surfaces of nose  11 . 1 . 3 . 1 . 2 - 40  may be measured using feedback from motors in positioners  11 . 1 . 3 . 1 . 2 - 58  as lens modules  11 . 1 . 3 . 1 . 2 - 70  are moved into contact with the surfaces of nose  11 . 1 . 3 . 1 . 2 - 40 . An illustrative control circuit for a positioner such as positioner  11 . 1 . 3 . 1 . 2 - 58  is shown in  FIG.  11 . 1   . 3 . 1 . 2 - 7 . Control circuitry  11 . 1 . 3 . 1 . 2 - 12  ( FIG.  11 . 1   . 3 . 1 . 2 - 1 ) may include a motor controller such as controller  11 . 1 . 3 . 1 . 2 - 80 . Controller  11 . 1 . 3 . 1 . 2 - 80  may drive motor  11 . 1 . 3 . 1 . 2 - 86  in a positioner  11 . 1 . 3 . 1 . 2 - 58  to move an associated lens module  11 . 1 . 3 . 1 . 2 - 70  by suppling a power supply voltage Vin to motor  11 . 1 . 3 . 1 . 2 - 86  using path  11 . 1 . 3 . 1 . 2 - 84 . While voltage Vin is being supplied to motor  11 . 1 . 3 . 1 . 2 - 86 , controller  11 . 1 . 3 . 1 . 2 - 80  of control circuitry  11 . 1 . 3 . 1 . 2 - 12  monitors the resulting current flow (current I) through path  11 . 1 . 3 . 1 . 2 - 84  using sensor circuit  82  (e.g., a current sensing resistor with a corresponding analog-to-digital converter circuit, etc.). Power supply voltage Vin may remain relatively constant while motor  11 . 1 . 3 . 1 . 2 - 86  moves lens assembly  11 . 1 . 3 . 1 . 2 - 70 . Positioner  11 . 1 . 3 . 1 . 2 - 58  may initially be used to position an edge of lens assembly  11 . 1 . 3 . 1 . 2 - 70  at a location that is distant from nose  11 . 1 . 3 . 1 . 2 - 40 . Control circuitry  11 . 1 . 3 . 1 . 2 - 12  may then direct positioner  11 . 1 . 3 . 1 . 2 - 58  to move lens assembly  11 . 1 . 3 . 1 . 2 - 70  toward nose  11 . 1 . 3 . 1 . 2 - 40 . Controller  11 . 1 . 3 . 1 . 2 - 80  of control circuitry  11 . 1 . 3 . 1 . 2 - 12  may monitor the current I that flows through path  11 . 1 . 3 . 1 . 2 - 84  and I sensed by sensor  82 . When lens assembly  11 . 1 . 3 . 1 . 2 - 70  is pressed against the side of nose  11 . 1 . 3 . 1 . 2 - 40 , current I will increase. When current I surpasses a desired threshold (that is related to the force applied to nose  11 . 1 . 3 . 1 . 2 - 40 ), control circuitry  11 . 1 . 3 . 1 . 2 - 12  may stop positioner  11 . 1 . 3 . 1 . 2 - 58  from applying more force to nose  11 . 1 . 3 . 1 . 2 - 40  with lens assembly  11 . 1 . 3 . 1 . 2 - 70 . 
     Illustrative operations involved in operating device  11 . 1 . 3 . 1 . 2 - 10  in system  11 . 1 . 3 . 1 . 2 - 8  are shown in  FIG.  11 . 1   . 3 . 1 . 2 - 8 . 
     During the operations of block  11 . 1 . 3 . 1 . 2 - 100 , information on the distance between the user&#39;s eyes (interpupillary distance IPD, sometimes referred to as pupillary distance) may be gathered. With one illustrative arrangement, device  11 . 1 . 3 . 1 . 2 - 10  or other equipment in system  11 . 1 . 3 . 1 . 2 - 8  gathers the user&#39;s interpupillary distance from the user by prompting the user to type the interpupillary distance into a data entry box on display  11 . 1 . 3 . 1 . 2 - 14  or a display in other equipment in system  11 . 1 . 3 . 1 . 2 - 8 . The user may also supply the user&#39;s interpupillary distance using voice input or other user input arrangements. With another illustrative arrangement, a sensor in device  11 . 1 . 3 . 1 . 2 - 10  or other a sensor in a stand-alone computer, portable device, or other equipment in system  11 . 1 . 3 . 1 . 2 - 8  may measure the user&#39;s interpupillary distance. For example, a sensor such as a two-dimensional or three-dimensional image sensor may gather an image of the user&#39;s face to measure the value of interpupillary distance IPD. After the measurement of the interpupillary distance has been made, the interpupillary distance may be provided to device  11 . 1 . 3 . 1 . 2 - 10  (e.g., over a wired or wireless communications paths). If desired, gaze trackers may measure the locations of the centers of the user&#39;s eyes PD and thereby determine IPD from direct measurement as a user is wearing device  11 . 1 . 3 . 1 . 2 - 10  on the user&#39;s head. 
     After gathering interpupillary distance IPD, control circuitry  11 . 1 . 3 . 1 . 2 - 12  of device  11 . 1 . 3 . 1 . 2 - 10  may, during the operations of block  11 . 1 . 3 . 1 . 2 - 102 , use positioners  11 . 1 . 3 . 1 . 2 - 58  to adjust the lens-to-lens spacing between lens centers LC so that this distance matches interpupillary distance IPD and so that the centers of lenses  11 . 1 . 3 . 1 . 2 - 72  are aligned with respective eye centers PC. While positioners  11 . 1 . 3 . 1 . 2 - 58  are moving lens modules  11 . 1 . 3 . 1 . 2 - 70  and lenses  11 . 1 . 3 . 1 . 2 - 72  (e.g., while the lens-to-lens spacing is being reduced to move modules  11 . 1 . 3 . 1 . 2 - 70  towards adjacent surfaces of the user&#39;s nose), control circuitry  11 . 1 . 3 . 1 . 2 - 12  uses force sensing circuitry (e.g., force sensor(s)  11 . 1 . 3 . 1 . 2 - 20 ) to monitor the force applied by lens modules  11 . 1 . 3 . 1 . 2 - 70  on nose  11 . 1 . 3 . 1 . 2 - 40 ). In some situations, the user&#39;s nose  11 . 1 . 3 . 1 . 2 - 40  may prevent lenses  11 . 1 . 3 . 1 . 2 - 72  from being brought sufficiently close to each other to allow the lens-to-lens spacing to exactly match IPD without creating a risk of discomfort for the user. In other words, force sensor(s)  11 . 1 . 3 . 1 . 2 - 20  may indicate that too much force is being applied to nose  11 . 1 . 3 . 1 . 2 - 40  by lens modules  11 . 1 . 3 . 1 . 2 - 70 , or that the force being applied to nose  11 . 1 . 3 . 1 . 2 - 40  by lens modules  11 . 1 . 3 . 1 . 2 - 70  has reached a threshold. Control circuitry  11 . 1 . 3 . 1 . 2 - 12  may then stop positioners  11 . 1 . 3 . 1 . 2 - 58  from moving lens modules  11 . 1 . 3 . 1 . 2 - 70  further toward nose  11 . 1 . 3 . 1 . 2 - 40 . If desired, positioners  11 . 1 . 3 . 1 . 2 - 58  may move lens modules  11 . 1 . 3 . 1 . 2 - 70  off of nose  11 . 1 . 3 . 1 . 2 - 40  by a desired gap (e.g., a gap G of at least 0.1 mm, at least 0.2 mm, at least 1 mm, at least 2 mm, less than 5 mm, or other suitable spacing). 
     Following the positioning of modules  11 . 1 . 3 . 1 . 2 - 70  at desired locations relative to nose  11 . 1 . 3 . 1 . 2 - 40  to ensure user comfort while wearing device  11 . 1 . 3 . 1 . 2 - 10 , control circuitry  11 . 1 . 3 . 1 . 2 - 12  may use display  11 . 1 . 3 . 1 . 2 - 14  to present visual content to the user through lenses  11 . 1 . 3 . 1 . 2 - 72  (block  11 . 1 . 3 . 1 . 2 - 104 ). 
     11.1.3.2: Sensors/Encoders 
       FIG.  11 . 1   . 3 . 2 - 1  illustrates a front, right, perspective view of a portion of an HMD  11 . 1 . 3 . 2 - 100  including first and second optical modules  11 . 1 . 3 . 2 - 102   a ,  11 . 1 . 3 . 2 - 102   b . The first and second optical modules  11 . 1 . 3 . 2 - 102   a - b  can be secured to an adjustment mechanism configured to alter the position of the optical modules  11 . 1 . 3 . 2 - 102   a - b  relative to other components of the HMD  11 . 1 . 3 . 2 - 100 , for example relative to the bracket  11 . 1 . 3 . 2 - 108 . For example, the first optical module  11 . 1 . 3 . 2 - 102   a  can include follower  11 . 1 . 3 . 2 - 104  slidably engaged with a guide-rod  11 . 1 . 3 . 2 - 106  via a channel of the follower  11 . 1 . 3 . 2 - 104  such that the optical module  11 . 1 . 3 . 2 - 102   a  can be adjusted laterally in the axial direction of the guide-rod  11 . 1 . 3 . 2 - 106 . In some examples, the optical modules  11 . 1 . 3 . 2 - 102   a - b  can include a guide-rod slidably engaged with a follower mechanism or some other sliding track system. The illustrated example is not limiting. 
     In at least one example, a motor (not shown) can be activated to move and adjust the position of the optical module  11 . 1 . 3 . 2 - 102   a  along the guide rod  11 . 1 . 3 . 2 - 106 . In order to determine the position of the optical module  11 . 1 . 3 . 2 - 102   a  relative to the second optical module  11 . 1 . 3 . 2 - 102   b  or relative to other components of the HMD  11 . 1 . 3 . 2 - 100 , the HMD  11 . 1 . 3 . 2 - 100  can include an encoder  11 . 1 . 3 . 2 - 109 . In one example, the encoder  11 . 1 . 3 . 2 - 109  is not on or disposed with the motor. Rather, in at least one example, such as the example shown in  FIG.  11 . 1   . 3 . 2 - 1 , the encoder  11 . 1 . 3 . 2 - 109  can be disposed outside the motor and secured in to the frame or bracket  11 . 1 . 3 . 2 - 108  of the HMD  11 . 1 . 3 . 2 - 100 . In one example, the encoder  109  can be secured to the frame and/or bracket  108  of the HMD  11 . 1 . 3 . 2 - 100  directly. In one example, the encoder assembly  11 . 1 . 3 . 2 - 109  can be secured to the frame and/or bracket  11 . 1 . 3 . 2 - 108  of the HMD  11 . 1 . 3 . 2 - 100  indirectly via an encoder bracket. 
     In one example, the encoder  11 . 1 . 3 . 2 - 109  assembly can include a magnet  11 . 1 . 3 . 2 - 110  having a master strip  11 . 1 . 3 . 2 - 113  and a Nonius strip  11 . 1 . 3 . 2 - 111 , each including magnetic pole strips  11 . 1 . 3 . 2 - 112 , forming adjacent magnetic pole pairs, for interacting with a ferromagnetic or magnetic chip component translatable with the optical module when the optical module  11 . 1 . 3 . 2 - 102   a  is moved along the guide-rod  11 . 1 . 3 . 2 - 106 . In at least one example, the magnetic pole pairs  11 . 1 . 3 . 2 - 112  of the Nonius strip  11 . 1 . 3 . 2 - 111  are phase shifted relative to the master strip  11 . 1 . 3 . 2 - 113  such that the master strip  11 . 1 . 3 . 2 - 113  can be used for local position detection and the Nonius strip  11 . 1 . 3 . 2 - 111  can be used for determining which pole pair is correlated to the detected position. In this way, the magnet  11 . 1 . 3 . 2 - 110  of the encoder  11 . 1 . 3 . 2 - 109  can serve as part of an absolute encoder for detecting position across the whole range of movement of the optical module  11 . 1 . 3 . 2 - 102   a.    
     In at least one example, the encoder  11 . 1 . 3 . 2 - 109  is a non-contact encoder. In one example, the encoder  11 . 1 . 3 . 2 - 109  is a Hall-effect encoder. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  11 . 1   . 3 . 2 - 1  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in any other figures shown and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to any other figure shown and described herein, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  11 . 1   . 3 . 2 - 1 . 
       FIG.  11 . 1   . 3 . 2 - 2  illustrates a top, left, perspective view of a portion of an HMD  11 . 1 . 3 . 2 - 200  including an optical module  11 . 1 . 3 . 2 - 202  having a follower  11 . 1 . 3 . 2 - 204  slidably engaged with a guide-rod  11 . 1 . 3 . 2 - 206 . The optical module  11 . 1 . 3 . 2 - 202  can be slid along the guide-rod  11 . 1 . 3 . 2 - 206  via a motor (not shown). In at least one example, the HMD  11 . 1 . 3 . 2 - 200  can include an encoder assembly  11 . 1 . 3 . 2 - 209 . The encoder assembly  11 . 1 . 3 . 2 - 209  can include a magnet  11 . 1 . 3 . 2 - 210  fixed in position to the HMD  11 . 1 . 3 . 2 - 200 , for example to the chassis, frame, or bracket  11 . 1 . 3 . 2 - 208  of the HMD  11 . 1 . 3 . 2 - 200 . In at least one example, the encoder assembly  11 . 1 . 3 . 2 - 209  can include a chip  11 . 1 . 3 . 2 - 216  or chip assembly including a housing and a chip configured to slide relative to the magnet  11 . 1 . 3 . 2 - 210  along with the optical module  11 . 1 . 3 . 2 - 202 . The chip  11 . 1 . 3 . 2 - 216  can be secured with the optical module  11 . 1 . 3 . 2 - 202  via a flexure bracket  11 . 1 . 3 . 2 - 214  secured to the chip  11 . 1 . 3 . 2 - 216  (or chip assembly  11 . 1 . 3 . 2 - 216 ). In at least one example, the flexure bracket  11 . 1 . 3 . 2 - 214  is fixed to the optical module  11 . 1 . 3 . 2 - 202  at a first end  11 . 1 . 3 . 2 - 218  and biased against the magnet  11 . 1 . 3 . 2 - 210  at a second end  11 . 1 . 3 . 2 - 222 . In one example, the flexure bracket  11 . 1 . 3 . 2 - 214  biases the chip  11 . 1 . 3 . 2 - 216  against the magnet  11 . 1 . 3 . 2 - 210  at the second end  11 . 1 . 3 . 2 - 222  thereof so the chip  11 . 1 . 3 . 2 - 216  is disposed adjacent to the magnet  11 . 1 . 3 . 2 - 210  and configured to slide relative to the magnet  11 . 1 . 3 . 2 - 210  when the optical module  11 . 1 . 3 . 2 - 202  is moved via the motor (not shown). 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  11 . 1   . 3 . 2 - 2  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in any other figures shown and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to any other figure shown and described herein, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  11 . 1   . 3 . 2 - 2 . 
       FIG.  11 . 1   . 3 . 2 - 3  illustrates an example of an encoder assembly  11 . 1 . 3 . 2 - 309  including a bracket  11 . 1 . 3 . 2 - 314  biasing a chip  11 . 1 . 3 . 2 - 316  against a magnet  11 . 1 . 3 . 2 - 310 . In at least one example, the bracket  11 . 1 . 3 . 2 - 314  includes a first end/portion  11 . 1 . 3 . 2 - 318  secured to an optical module of an HMD device and a second end  11 . 1 . 3 . 2 - 322  opposite the first In at least one example, the flexure bracket  11 . 1 . 3 . 2 - 314  is fixed to the chip  11 . 1 . 3 . 2 - 316  via a secondary flexure bracket  11 . 1 . 3 . 2 - 320  to correct for the angle of the magnet  11 . 1 . 3 . 2 - 310  during assembly of the encoder assembly  11 . 1 . 3 . 2 - 309 . The secondary flexure bracket  11 . 1 . 3 . 2 - 320  can extend from a distal end  11 . 1 . 3 . 2 - 322  of a primary flexure bracket  11 . 1 . 3 . 2 - 317  which is coupled to a proximal end/portion  11 . 1 . 3 . 2 - 318  configured to be fixed to an optical module of an HMD device. 
     In at least one example, the primary and/or secondary flexure brackets  11 . 1 . 3 . 2 - 317 ,  11 . 1 . 3 . 2 - 320 , as well as the proximal portion  11 . 1 . 3 . 2 - 318 , can be formed of a plastic such as Ultum. In one example, the encoder magnet  11 . 1 . 3 . 2 - 310  can include a low friction tape, for example a Teflon tape. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  11 . 1   . 3 . 2 - 3  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in any other figures shown and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to any other figure shown and described herein, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  11 . 1   . 3 . 2 - 3 . 
     11.1.3.2.1: Sensor Assembly 
     Systems of the present disclosure can provide a head-mountable device that securely supports sensors that are isolated from an external environment within a sealed container and orientations over time and avoids applying excessive strain on the sensors, thereby protecting them from harm. The sensor can be mounted on a flex circuit or circuit board, which is rigidly secured within an enclosure, including for example a plate and a case. The case and the plate can be sealed together with the flex circuit extending there through (e.g., at an opening formed by sealing glue) to operably connected to another component. 
     Accordingly, the sensor assemblies described herein facilitate effective performance of sensors mounted therein. The sensor assemblies provides high robustness during the entirety of the service life of the head-mountable device without placing strain on the sensors themselves. The seal of the enclosure can isolate the sensor from external influences by preventing ingress of elements through the case and plate. The sensor assemblies further enable the mounting of sensors on a flex circuit, where desired. 
     These and other embodiments are discussed below with reference to  FIGS.  11 . 1   . 3 . 2 . 1 - 1  through  11 . 1 . 3 . 2 . 1 - 5 . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these Figures is for explanatory purposes only and should not be construed as limiting. 
     According to some embodiments, for example as shown in  FIG.  11 . 1   . 3 . 2 . 1 - 1 , a head-mountable device  11 . 1 . 3 . 2 . 1 - 100  includes a frame  11 . 1 . 3 . 2 . 1 - 110  that is worn on a head of a user. The frame  11 . 1 . 3 . 2 . 1 - 110  can be positioned in front of the eyes of a user to provide information within a field of view of the user. The frame  11 . 1 . 3 . 2 . 1 - 110  can provide nose pads or another feature to rest on a user&#39;s nose. The frame  11 . 1 . 3 . 2 . 1 - 110  can be supported on a user&#39;s head with the head engager  11 . 1 . 3 . 2 . 1 - 120 . The head engager  11 . 1 . 3 . 2 . 1 - 120  can wrap or extend along opposing sides of a user&#39;s head. The head engager  11 . 1 . 3 . 2 . 1 - 120  can include earpieces for wrapping around or otherwise engaging or resting on a user&#39;s ears. It will be appreciated that other configurations can be applied for securing the head-mountable device  11 . 1 . 3 . 2 . 1 - 100  to a user&#39;s head. For example, one or more bands, straps, belts, caps, hats, or other components can be used in addition to or in place of the illustrated components of the head-mountable device  11 . 1 . 3 . 2 . 1 - 100 . By further example, the head engager  11 . 1 . 3 . 2 . 1 - 120  can include multiple components to engage a user&#39;s head. 
     The frame  11 . 1 . 3 . 2 . 1 - 110  can provide structure around a peripheral region thereof to support any internal components of the frame  11 . 1 . 3 . 2 . 1 - 110  in their assembled position. For example, the frame  11 . 1 . 3 . 2 . 1 - 110  can enclose and support various internal components (including for example integrated circuit chips, processors, memory devices and other circuitry) to provide computing and functional operations for the head-mountable device  11 . 1 . 3 . 2 . 1 - 100 , as discussed further herein. While several components are shown within the frame  11 . 1 . 3 . 2 . 1 - 110 , it will be understood that some or all of these components can be located anywhere within or on the head-mountable device  11 . 1 . 3 . 2 . 1 - 100 . For example, one or more of these components can be positioned within the head engager  11 . 1 . 3 . 2 . 1 - 120  of the head-mountable device  11 . 1 . 3 . 2 . 1 - 100 . 
     The frame  11 . 1 . 3 . 2 . 1 - 110  can include and/or support one or more cameras  11 . 1 . 3 . 2 . 1 - 130 . The cameras  11 . 1 . 3 . 2 . 1 - 130  can be positioned on or near an outer side  11 . 1 . 3 . 2 . 1 - 122  of the frame  11 . 1 . 3 . 2 . 1 - 110  to capture images of views external to the head-mountable device  11 . 1 . 3 . 2 . 1 - 100 . As used herein, an outer side of a portion of a head-mountable device is a side that faces away from the user and/or towards an external environment. The captured images can be used for display to the user or stored for any other purpose. Each of the cameras  11 . 1 . 3 . 2 . 1 - 130  can be movable along the outer side  11 . 1 . 3 . 2 . 1 - 122 . For example, a track or other guide can be provided for facilitating movement of the camera  11 . 1 . 3 . 2 . 1 - 130  therein. 
     The head-mountable device  11 . 1 . 3 . 2 . 1 - 100  can include displays  11 . 1 . 3 . 2 . 1 - 140  that provide visual output for viewing by a user wearing the head-mountable device  11 . 1 . 3 . 2 . 1 - 100 . One or more displays  11 . 1 . 3 . 2 . 1 - 140  can be positioned on or near an inner side  11 . 1 . 3 . 2 . 1 - 124  of the frame  11 . 1 . 3 . 2 . 1 - 110 . As used herein, an inner side  11 . 1 . 3 . 2 . 1 - 124  of a portion of a head-mountable device is a side that faces toward the user and/or away from the external environment. 
     A display  11 . 1 . 3 . 2 . 1 - 140  can transmit light from a physical environment (e.g., as captured by a camera) for viewing by the user. Such a display  11 . 1 . 3 . 2 . 1 - 140  can include optical properties, such as lenses for vision correction based on incoming light from the physical environment. Additionally or alternatively, a display  11 . 1 . 3 . 2 . 1 - 140  can provide information as a display within a field of view of the user. Such information can be provided to the exclusion of a view of a physical environment or in addition to (e.g., overlaid with) a physical environment. 
     The head-mountable device  11 . 1 . 3 . 2 . 1 - 100  can include one or more sensors  11 . 1 . 3 . 2 . 1 - 70 . The sensors  11 . 1 . 3 . 2 . 1 - 70  can be mounted to the frame  11 . 1 . 3 . 2 . 1 - 110  in a secure manner, as described further herein. Any number and type of sensors can be provided. For example, one or more of the sensors  11 . 1 . 3 . 2 . 1 - 70  can be or include an inertial measurement unit (“IMU”) that provides information regarding a characteristic of the head-mountable device  11 . 1 . 3 . 2 . 1 - 100 , such as inertial angles thereof. For example, the IMU can include a six-degrees of freedom IMU that calculates the position, velocity, and/or acceleration of the head-mountable device  11 . 1 . 3 . 2 . 1 - 100  based on six degrees of freedom (x, y, z, θ x , θ y , and θ z ). The IMU can include one or more of an accelerometer, a gyroscope, and/or a magnetometer. Additionally or alternatively, the sensors  11 . 1 . 3 . 2 . 1 - 70  can detect motion characteristics of the head-mountable device  11 . 1 . 3 . 2 . 1 - 100  with one or more other motion sensors, such as an accelerometer, a gyroscope, a global positioning sensor, a tilt sensor, and so on for detecting movement and acceleration of the head-mountable device  11 . 1 . 3 . 2 . 1 - 100 . The sensors  11 . 1 . 3 . 2 . 1 - 70  can provide data to a controller for processing. Such data can influence other operations of the head-mountable device  11 . 1 . 3 . 2 . 1 - 100 , such as the information provided on the display  11 . 1 . 3 . 2 . 1 - 140 . 
     It will be understood that the sensors  11 . 1 . 3 . 2 . 1 - 70  (e.g., IMUs) and associated assemblies can be coupled to and/or integrated with one or more other components of the head-mountable device  11 . 1 . 3 . 2 . 1 - 100 . For example, one or more sensors  11 . 1 . 3 . 2 . 1 - 70  can be coupled to components that move relative to the frame  11 . 1 . 3 . 2 . 1 - 110  or other components of the head-mountable device  11 . 1 . 3 . 2 . 1 - 100 . By further example, one or more sensors  11 . 1 . 3 . 2 . 1 - 70  can be coupled to one or more displays  11 . 1 . 3 . 2 . 1 - 140 , the frame  11 . 1 . 3 . 2 . 1 - 110 , and/or other components  11 . 1 . 3 . 2 . 1 - 160  of the head-mountable device  11 . 1 . 3 . 2 . 1 - 100  (e.g., controllers, input/output devices, and the like). Accordingly, by operating multiple sensors  11 . 1 . 3 . 2 . 1 - 70 , the absolute and/or relative position and/or orientation of each of the associated components can be determined. For example, one or more (e.g., a pair of) displays  11 . 1 . 3 . 2 . 1 - 140  can move relative to each other and/or the frame  11 . 1 . 3 . 2 . 1 - 110  to be aligned with the eyes of the user, and sensors  11 . 1 . 3 . 2 . 1 - 70  coupled to each of the displays  11 . 1 . 3 . 2 . 1 - 140  can track such movement. By further example, the sensors  11 . 1 . 3 . 2 . 1 - 70  of separate head-mountable devices  11 . 1 . 3 . 2 . 1 - 100  (e.g., worn by different users) can share information so that each of the head-mountable devices  11 . 1 . 3 . 2 . 1 - 100  can determine the absolute and/or relative position and/or orientation of the other and optionally output corresponding information (e.g., visual information via displays  11 . 1 . 3 . 2 . 1 - 140 ) so the users can perceive each other&#39;s presence, for example in a shared CGR environment. 
     Solid state motion sensors, such as IMUs, can be affected by strain, temperature, and humidity. Over time, exposure to these influences can result in a wider and less predictable operating range over the life of the IMU. For example, devices relying on absolute measurement of orientation and/or acceleration using MEMS IMUs may require bounded changes in bias, axis orthogonality, and cross-axis sensitivity. Typically, these sensors are surface mounted on a rigid PCB and care is taken to choose PCB locations that minimize strain and temperature shift. However, for some more demanding applications this is not sufficient to extract all performance from the sensor. 
     Referring now to  FIGS.  11 . 1   . 3 . 2 . 1 - 2 - 11 . 1 . 3 . 2 . 1 - 4 , a sensor assembly can be provided to securely support one or more sensors of a head-mountable device while simultaneously protecting the sensors from exposure to an external environment as well as forces applied to the assembly. It will be understood that the assembly described herein can be one of multiple assemblies and can include one or more other components of the head-mountable device. It will be further understood that a sensor assembly can be integrated into the head-mountable device and/or provided as a module thereto. 
     As shown in  FIG.  11 . 1   . 3 . 2 . 1 - 2 , a sensor assembly  11 . 1 . 3 . 2 . 1 - 10  can include a sensor  11 . 1 . 3 . 2 . 1 - 70  (e.g., IMU) housed within a protective case. A top case  11 . 1 . 3 . 2 . 1 - 20  of the sensor assembly  11 . 1 . 3 . 2 . 1 - 10  can be securely coupled to the bottom plate  11 . 1 . 3 . 2 . 1 - 112  or another component of the head-mountable device. For example, the bottom plate  11 . 1 . 3 . 2 . 1 - 112  can be part of, integrated with, and/or coupled to a frame and/or another component of the head-mountable device. 
     The sensor  11 . 1 . 3 . 2 . 1 - 70  can be coupled to a flex circuit  11 . 1 . 3 . 2 . 1 - 60 . For example, the sensor  11 . 1 . 3 . 2 . 1 - 70  can be directly mounted to a surface of the flex circuit  11 . 1 . 3 . 2 . 1 - 60 . Additionally or alternatively, the sensor  11 . 1 . 3 . 2 . 1 - 70  can be mounted to board that is directly or indirectly coupled to the flex circuit  11 . 1 . 3 . 2 . 1 - 60 . The flex circuit  11 . 1 . 3 . 2 . 1 - 60  can, in turn, directly or indirectly couple the sensor  11 . 1 . 3 . 2 . 1 - 70  and/or the board to other components of the sensor assembly  11 . 1 . 3 . 2 . 1 - 10 , such as the top case  11 . 1 . 3 . 2 . 1 - 20  and/or the bottom plate  11 . 1 . 3 . 2 . 1 - 112 . 
     The sensor  11 . 1 . 3 . 2 . 1 - 70  can be implemented as an integrated circuit, such as one or more of an industry standard integrated circuit, an application-specific integrated circuit (ASIC), an application-specific standard product (ASSP), and the like. The sensor  11 . 1 . 3 . 2 . 1 - 70  can have a size and shape that is accommodated by the housing structure of the assembly and provides the desired performance characteristics. For example, the sensor  11 . 1 . 3 . 2 . 1 - 70  can be integrated with a wafer and/or board. 
     The one or more sensors  11 . 1 . 3 . 2 . 1 - 70  of the sensor assembly  11 . 1 . 3 . 2 . 1 - 10  can be mounted to the flex circuit  11 . 1 . 3 . 2 . 1 - 60  that operably connects the sensors  11 . 1 . 3 . 2 . 1 - 70  of the sensor assembly  11 . 1 . 3 . 2 . 1 - 10  to each other and/or other components. As used herein, “flexible circuit” or “flex circuit” is a structure that includes a conductive layer, an insulation layer, and optionally a substrate layer. A flex circuit can be provided in electrical communication with at least one electrode, terminal, and/or connector. A flex circuit forms circuitry that includes a pattern of conductors of the conductive layer typically in the form of pads, which are typically formed on a surface of an insulating material of the insulation layer. Such circuitry is typically metallic, such as of a copper or copper alloy. In general, a flex circuit is thin, having a total thickness of from about 1 mm to about 30 mm. A flex circuit is generally flexible, such that it can conform to contours of other components. A flex circuit may be any suitable size and constructed in any suitable shape. For example, the size of a flex circuit may be determined by the power requirements of the components connected thereto, the conductivity of the flex circuit, the distance between operably connected components, or any other suitable criteria. It will be understood that, additionally or alternatively, a cable or other conductive material surrounded by an insulating layer can be provided. Such a cable can be connected via a connector and/or a hot bar and/or wire bond to one or more boards and/or other electronic components. 
     Providing operable connections to and from the sensors  11 . 1 . 3 . 2 . 1 - 70  via a flex circuit  11 . 1 . 3 . 2 . 1 - 60  can facilitate such connections while occupying little space within the head-mountable device. Additionally, a flex circuit  11 . 1 . 3 . 2 . 1 - 60  can conform and bend around other components of the head-mountable device. These features can help the head-mountable device maintain a low weight and small size. 
     The flex circuit  11 . 1 . 3 . 2 . 1 - 60  can include multiple segments that have different characteristics. For example, a first segment  11 . 1 . 3 . 2 . 1 - 62  of the flex circuit  11 . 1 . 3 . 2 . 1 - 60  can support the sensor  11 . 1 . 3 . 2 . 1 - 70 . The first segment  11 . 1 . 3 . 2 . 1 - 62  can defined a terminal and portion of the flex circuit  11 . 1 . 3 . 2 . 1 - 60  for residing within the enclosure of the sensor assembly  11 . 1 . 3 . 2 . 1 - 10 . The flex circuit  11 . 1 . 3 . 2 . 1 - 60  can further include a second segment  11 . 1 . 3 . 2 . 1 - 64  that extends from the first segment  11 . 1 . 3 . 2 . 1 - 62  and/or within the enclosure. The second segment  11 . 1 . 3 . 2 . 1 - 64  can provide strain relief features, such as a shape and/or size that promotes flexibility, bending, and/or stretching. For example, as shown in  FIG.  11 . 1   . 3 . 2 . 1 - 2 , the second segment  11 . 1 . 3 . 2 . 1 - 64  can include an S-curve, a serpentine shape, an undulating shape, and/or another non-linear shape. With such a shape, the second segment  11 . 1 . 3 . 2 . 1 - 64  can easily alter it shape upon application of tension and/or other forces along the length of the flex circuit  11 . 1 . 3 . 2 . 1 - 60 . For example, tension and/or other forces applied at a third segment  11 . 1 . 3 . 2 . 1 - 66  of the flex circuit  11 . 1 . 3 . 2 . 1 - 60  that is away from the sensor  11 . 1 . 3 . 2 . 1 - 70  (e.g., within or outside the enclosure) can be absorbed by adjustments along the second segment  11 . 1 . 3 . 2 . 1 - 64 , such that such tension and/or other forces are not transmitted to the sensor  11 . 1 . 3 . 2 . 1 - 70 . Such adjustments can include an adjustment to the effective length of the second segment  11 . 1 . 3 . 2 . 1 - 64  (e.g., between the first segment  11 . 1 . 3 . 2 . 1 - 62  and the third segment  11 . 1 . 3 . 2 . 1 - 66 ), adjustments to the curvature along the second segment  11 . 1 . 3 . 2 . 1 - 64 , and the like. By further example, the second segment  11 . 1 . 3 . 2 . 1 - 64  can alter its shape along and/or about one or more axes extending through the second segment  11 . 1 . 3 . 2 . 1 - 64 . As such, the sensor  11 . 1 . 3 . 2 . 1 - 70  can maintain a consistent position and/or orientation despite the application of external forces. 
     A base plate  11 . 1 . 3 . 2 . 1 - 80  can provide support to the sensor  11 . 1 . 3 . 2 . 1 - 70  and the flex circuit  11 . 1 . 3 . 2 . 1 - 60 . For example, the base plate  11 . 1 . 3 . 2 . 1 - 80  can be disposed adjacent to the sensor  11 . 1 . 3 . 2 . 1 - 70  on a side of the flex circuit  11 . 1 . 3 . 2 . 1 - 60  that is opposite the sensor  11 . 1 . 3 . 2 . 1 - 70 . Additionally or alternatively, the flex circuit  11 . 1 . 3 . 2 . 1 - 60  can be or include a circuit board that provides a rigid substrate with electrical connections between components and/or to other components. Where the flex circuit  11 . 1 . 3 . 2 . 1 - 60  includes or forms a rigid circuit board, the base plate  11 . 1 . 3 . 2 . 1 - 80  can optionally be omitted. 
     The sensor assembly  11 . 1 . 3 . 2 . 1 - 10  can further include a mounting adhesive  11 . 1 . 3 . 2 . 1 - 50  disposed between the base plate  11 . 1 . 3 . 2 . 1 - 80  and the bottom plate  11 . 1 . 3 . 2 . 1 - 112 . The mounting adhesive  11 . 1 . 3 . 2 . 1 - 50  can bond opposing surfaces of the base plate  11 . 1 . 3 . 2 . 1 - 80  and the bottom plate  11 . 1 . 3 . 2 . 1 - 112  to each other. The mounting adhesive  11 . 1 . 3 . 2 . 1 - 50  can have a coefficient of thermal expansion that is complementary to the coefficients of thermal expansion of the base plate  11 . 1 . 3 . 2 . 1 - 80  and/or the bottom plate  11 . 1 . 3 . 2 . 1 - 112  to minimize strain on the sensor  11 . 1 . 3 . 2 . 1 - 70 . As used herein, a coefficient of thermal expansion can refer to either a linear coefficient of thermal expansion and/or a volumetric coefficient of thermal expansion. As used herein, any two coefficients of thermal expansion are “complementary” when one is within ±20%, ±15%, ±10%, ±9%, ±8%, ±7%, ±6%, ±5%, ±4%, ±3%, ±2%, or ±1% of the other, inclusive of any intermediate value there between. For example, the mounting adhesive  11 . 1 . 3 . 2 . 1 - 50  can have a coefficient of thermal expansion that is within ±10%, ±9%, ±8%, ±7%, ±6%, ±5%, ±4%, ±3%, ±2%, or ±1% of the coefficient of thermal expansion of the base plate  11 . 1 . 3 . 2 . 1 - 80  and/or the bottom plate  11 . 1 . 3 . 2 . 1 - 112 . 
     The top case  11 . 1 . 3 . 2 . 1 - 20  defines an opening  11 . 1 . 3 . 2 . 1 - 28  to permit electrical communication with the sensor  11 . 1 . 3 . 2 . 1 - 70 , for example, at a side portion  11 . 1 . 3 . 2 . 1 - 22  of the top case  11 . 1 . 3 . 2 . 1 - 20 . For example, the flex circuit  11 . 1 . 3 . 2 . 1 - 60  (e.g., at the third segment  11 . 1 . 3 . 2 . 1 - 66  thereof) can extend through the opening  11 . 1 . 3 . 2 . 1 - 28 , so that a connector  11 . 1 . 3 . 2 . 1 - 68  at a terminal end of the flex circuit  11 . 1 . 3 . 2 . 1 - 60  (e.g., at an end opposite the sensor  11 . 1 . 3 . 2 . 1 - 70  and the first segment  11 . 1 . 3 . 2 . 1 - 62 ) can connect to another component. The connector  11 . 1 . 3 . 2 . 1 - 68  coupled to the flex circuit  11 . 1 . 3 . 2 . 1 - 60  can be in electrical or other operably communication with the sensor  11 . 1 . 3 . 2 . 1 - 70 . As such, the connector  11 . 1 . 3 . 2 . 1 - 68  can operably connect the sensor  11 . 1 . 3 . 2 . 1 - 70  to another component, such as a controller of the head-mountable device. 
     The sensor assembly  11 . 1 . 3 . 2 . 1 - 10  further includes a sealing member  11 . 1 . 3 . 2 . 1 - 52  disposed within the opening  11 . 1 . 3 . 2 . 1 - 28  to seal the enclosure from an external environment and permit electrical communication with the sensor  11 . 1 . 3 . 2 . 1 - 70  along the flex circuit  11 . 1 . 3 . 2 . 1 - 60 . For example, the sealing member  11 . 1 . 3 . 2 . 1 - 52  can entirely surround a portion of the flex circuit  11 . 1 . 3 . 2 . 1 - 60  (e.g., along the third segment  11 . 1 . 3 . 2 . 1 - 66 ). The flex circuit  11 . 1 . 3 . 2 . 1 - 60  can extend entirely through the sealing member  11 . 1 . 3 . 2 . 1 - 52  to extend to either side thereof, such as to the interior and exterior of the enclosure. The sealing member  11 . 1 . 3 . 2 . 1 - 52  can include an adhesive, silicone, and/or other sealing member. The sealing member  11 . 1 . 3 . 2 . 1 - 52  can be provided as a liquid that solidifies (e.g., cures) during assembly. 
     As shown in  FIG.  11 . 1   . 3 . 2 . 1 - 3 , the sensor assembly  11 . 1 . 3 . 2 . 1 - 10  can form an enclosure  11 . 1 . 3 . 2 . 1 - 12  to house the sensor  11 . 1 . 3 . 2 . 1 - 70  (e.g., IMU). For example, the top case  11 . 1 . 3 . 2 . 1 - 20 , the bottom plate  11 . 1 . 3 . 2 . 1 - 112 , and the sealing member  11 . 1 . 3 . 2 . 1 - 52  can form a sealed enclosure  11 . 1 . 3 . 2 . 1 - 12  that surrounds a sealed volume. The sensor  11 . 1 . 3 . 2 . 1 - 70 , the base plate  11 . 1 . 3 . 2 . 1 - 80 , and a portion of the flex circuit  11 . 1 . 3 . 2 . 1 - 60  can be contained within the sealed volume. 
     To form the enclosure  11 . 1 . 3 . 2 . 1 - 12 , the top case  11 . 1 . 3 . 2 . 1 - 20  can be coupled to the bottom plate  11 . 1 . 3 . 2 . 1 - 112  in a sealing manner. For example, the top case  11 . 1 . 3 . 2 . 1 - 20  can be welded (e.g., laser welded) to the bottom plate  11 . 1 . 3 . 2 . 1 - 112 . Other types of couplings are contemplated, such as bonding, adhesion, and the like. The coupling can maintain the opening  11 . 1 . 3 . 2 . 1 - 28 , for example at a side portion  11 . 1 . 3 . 2 . 1 - 22  of the top case  11 . 1 . 3 . 2 . 1 - 20 . 
     The flex circuit  11 . 1 . 3 . 2 . 1 - 60 , including the connector  11 . 1 . 3 . 2 . 1 - 68 , can be provided through the opening  11 . 1 . 3 . 2 . 1 - 28 . The sealing member  11 . 1 . 3 . 2 . 1 - 52  can be formed about a portion of the flex circuit  11 . 1 . 3 . 2 . 1 - 60  that extends through the opening  11 . 1 . 3 . 2 . 1 - 28 . The sealing member  11 . 1 . 3 . 2 . 1 - 52  can extend from the top case  11 . 1 . 3 . 2 . 1 - 20  to the bottom plate  11 . 1 . 3 . 2 . 1 - 112 . As such, the sealing member  11 . 1 . 3 . 2 . 1 - 52  can fill the only opening  11 . 1 . 3 . 2 . 1 - 28  that provides fluid communication between the interior of the enclosure  11 . 1 . 3 . 2 . 1 - 12  and an external environment (e.g., exterior to the enclosure  11 . 1 . 3 . 2 . 1 - 12 ). 
     As further shown in  FIG.  11 . 1   . 3 . 2 . 1 - 3 , the opening  11 . 1 . 3 . 2 . 1 - 28  can be defined on a side portion  11 . 1 . 3 . 2 . 1 - 22  of the top case  11 . 1 . 3 . 2 . 1 - 20 . It will be understood that the opening  11 . 1 . 3 . 2 . 1 - 28  can be provided at any portion of the top case  11 . 1 . 3 . 2 . 1 - 20 , as described further herein. Regardless of its location, the opening  11 . 1 . 3 . 2 . 1 - 28  can be filled with the sealing member  11 . 1 . 3 . 2 . 1 - 52  and around the flex circuit  11 . 1 . 3 . 2 . 1 - 60 . 
     By coupling the top case  11 . 1 . 3 . 2 . 1 - 20  to the bottom plate  11 . 1 . 3 . 2 . 1 - 112  and by filling the opening  11 . 1 . 3 . 2 . 1 - 28  with the sealing member  11 . 1 . 3 . 2 . 1 - 52 , the enclosure  11 . 1 . 3 . 2 . 1 - 12  can provide a sealed volume for the sensor  11 . 1 . 3 . 2 . 1 - 70 . As used herein, and enclosure, volume, or other space that is “sealed” is at least partially and/or selectively isolated from an external environment. For example, the top case  11 . 1 . 3 . 2 . 1 - 20  and the bottom plate  11 . 1 . 3 . 2 . 1 - 112  can be impermeable, such that they form a barrier against ingress and/or egress of matter there through. The sealing member  11 . 1 . 3 . 2 . 1 - 52  can also form a barrier. It will be understood that one or more barriers can be selectively permeable while still maintaining a sealed volume and/or enclosure. For example, the sealing member  11 . 1 . 3 . 2 . 1 - 52  can be permeable to one or more gases and/or substances on an atomic level. However, the sealing member  11 . 1 . 3 . 2 . 1 - 52  can be impermeable to (e.g., preventing ingress or egress of) molecules, compounds, and/or liquids. In some embodiments, the enclosure can be hermetically or “airtight” sealed, however other seals are contemplated. 
     As shown in  FIG.  11 . 1   . 3 . 2 . 1 - 4 , the connector  11 . 1 . 3 . 2 . 1 - 68  can extend through other portions of the top case  11 . 1 . 3 . 2 . 1 - 20 , such as an upper portion  11 . 1 . 3 . 2 . 1 - 24  of the top case  11 . 1 . 3 . 2 . 1 - 20 . The upper portion  11 . 1 . 3 . 2 . 1 - 24  can be a portion of the top case that is opposite the bottom plate  11 . 1 . 3 . 2 . 1 - 112 . One or more openings  11 . 1 . 3 . 2 . 1 - 28  can be defined on the upper portion  11 . 1 . 3 . 2 . 1 - 24  of the top case  11 . 1 . 3 . 2 . 1 - 20 , and corresponding connectors  11 . 1 . 3 . 2 . 1 - 68  can extend through the openings  11 . 1 . 3 . 2 . 1 - 28 . Outside of the enclosure  11 . 1 . 3 . 2 . 1 - 12 , a mating connector  11 . 1 . 3 . 2 . 1 - 90  can be operably coupled to the connectors  11 . 1 . 3 . 2 . 1 - 68 . For example, the interface between the mating connector  11 . 1 . 3 . 2 . 1 - 90  and the connector(s)  11 . 1 . 3 . 2 . 1 - 68  can include one or more electrodes, contact plates, wires, pogo pins, and the like. 
     Where the connectors  11 . 1 . 3 . 2 . 1 - 68  extend through the opening  11 . 1 . 3 . 2 . 1 - 28 , one or more sealing members  11 . 1 . 3 . 2 . 1 - 52  can be formed to fill the openings  11 . 1 . 3 . 2 . 1 - 28  and surround corresponding portions of the connectors  11 . 1 . 3 . 2 . 1 - 68 . Where the openings are formed in the upper portion  11 . 1 . 3 . 2 . 1 - 24  of the top case  11 . 1 . 3 . 2 . 1 - 20 , the sealing members  11 . 1 . 3 . 2 . 1 - 52  need not engage the bottom plate  11 . 1 . 3 . 2 . 1 - 112 . As such, the top case  11 . 1 . 3 . 2 . 1 - 20  and the bottom plate  11 . 1 . 3 . 2 . 1 - 112  can be coupled with a continuous seal, such as a weld. 
     It will be understood that the opening(s)  11 . 1 . 3 . 2 . 1 - 28  can be provided at any portion of the top case  11 . 1 . 3 . 2 . 1 - 20  and/or the bottom plate  11 . 1 . 3 . 2 . 1 - 112 . Regardless of its location, the opening  11 . 1 . 3 . 2 . 1 - 28  can be filled with the sealing member  11 . 1 . 3 . 2 . 1 - 52  and around the flex circuit  11 . 1 . 3 . 2 . 1 - 60  and/or connector(s)  11 . 1 . 3 . 2 . 1 - 68 . 
     Sensor assemblies of the present disclosure can provide predictable performance over the life of the head-mountable device by isolating the sensor from strain and humidity changes. This can enables absolute orientation measurements between multiple sensors over the life of the head-mountable device, without requiring repeated calibration. By providing each sensor (e.g., IMU) in a consistent position and orientation, any one or multiple sensors can be dedicated as a reference sensor for the entire head-mountable device. 
     Referring now to  FIG.  11 . 1   . 3 . 2 . 1 - 5 , components of the head-mountable device can be operably connected to provide the performance described herein.  FIG.  11 . 1   . 3 . 2 . 1 - 5  shows a simplified block diagram of an illustrative head-mountable device  11 . 1 . 3 . 2 . 1 - 100  in accordance with one embodiment of the invention. It will be appreciated that components described herein can be provided on either or both of a frame and/or a head engager of the head-mountable device  11 . 1 . 3 . 2 . 1 - 100 . It will be understood that additional components, different components, or fewer components than those illustrated may be utilized within the scope of the subject disclosure. 
     As shown in  FIG.  11 . 1   . 3 . 2 . 1 - 5 , the head-mountable device  11 . 1 . 3 . 2 . 1 - 100  can include a controller  11 . 1 . 3 . 2 . 1 - 180  (e.g., control circuitry) with one or more processing units that include or are configured to access a memory  11 . 1 . 3 . 2 . 1 - 182  having instructions stored thereon. The instructions or computer programs may be configured to perform one or more of the operations or functions described with respect to the head-mountable device  11 . 1 . 3 . 2 . 1 - 100 . The controller  11 . 1 . 3 . 2 . 1 - 180  can be implemented as any electronic device capable of processing, receiving, or transmitting data or instructions. For example, the controller  11 . 1 . 3 . 2 . 1 - 180  may include one or more of: a microprocessor, a central processing unit (CPU), an application-specific integrated circuit (ASIC), a digital signal processor (DSP), or combinations of such devices. As described herein, the term “processor” is meant to encompass a single processor or processing unit, multiple processors, multiple processing units, or other suitably configured computing element or elements. 
     The memory  11 . 1 . 3 . 2 . 1 - 182  can store electronic data that can be used by the head-mountable device  11 . 1 . 3 . 2 . 1 - 100 . The memory  11 . 1 . 3 . 2 . 1 - 182  can include the memory of the flex circuit  11 . 1 . 3 . 2 . 1 - 170  described herein. For example, the memory  11 . 1 . 3 . 2 . 1 - 182  can store electrical data or content such as, for example, audio and video files, documents and applications, device settings and user preferences, timing and control signals or data for the various modules, data structures or databases, and so on. The memory  11 . 1 . 3 . 2 . 1 - 182  can be configured as any type of memory. By way of example only, the memory  11 . 1 . 3 . 2 . 1 - 182  can be implemented as random access memory, read-only memory, Flash memory, removable memory, or other types of storage elements, or combinations of such devices. 
     The head-mountable device  11 . 1 . 3 . 2 . 1 - 100  can further include a display  11 . 1 . 3 . 2 . 1 - 140  for displaying visual information for a user. The display  11 . 1 . 3 . 2 . 1 - 140  can provide visual (e.g., image or video) output. The display  11 . 1 . 3 . 2 . 1 - 140  can be or include an opaque, transparent, and/or translucent display. The display  11 . 1 . 3 . 2 . 1 - 140  may have a transparent or translucent medium through which light representative of images is directed to a user&#39;s eyes. The display  11 . 1 . 3 . 2 . 1 - 140  may utilize digital light projection, OLEDs, LEDs, uLEDs, liquid crystal on silicon, laser scanning light source, or any combination of these technologies. The medium may be an optical waveguide, a hologram medium, an optical combiner, an optical reflector, or any combination thereof. In one embodiment, the transparent or translucent display may be configured to become opaque selectively. Projection-based systems may employ retinal projection technology that projects graphical images onto a person&#39;s retina. Projection systems also may be configured to project virtual objects into the physical environment, for example, as a hologram or on a physical surface. The head-mountable device  11 . 1 . 3 . 2 . 1 - 100  can include an optical subassembly configured to help optically adjust and correctly project the image-based content being displayed by the display  11 . 1 . 3 . 2 . 1 - 140  for close up viewing. The optical subassembly can include one or more lenses, mirrors, or other optical devices. 
     The head-mountable device  11 . 1 . 3 . 2 . 1 - 100  can include one or more sensors  11 . 1 . 3 . 2 . 1 - 70 , such as the sensors of a sensor assembly, as described herein. The head-mountable device  11 . 1 . 3 . 2 . 1 - 100  can include one or more other sensors, including sensors of a sealed enclosure assembly and/or other sensors. Such sensors can be configured to sense substantially any type of characteristic such as, but not limited to, images, pressure, light, touch, force, temperature, position, motion, and so on. For example, the sensor can be a photodetector, a temperature sensor, a light or optical sensor, an atmospheric pressure sensor, a humidity sensor, a magnet, a gyroscope, an accelerometer, a chemical sensor, an ozone sensor, a particulate count sensor, and so on. By further example, the sensor can be a bio-sensor for tracking biometric characteristics, such as health and activity metrics. Other user sensors can perform facial feature detection, facial movement detection, facial recognition, eye tracking, user mood detection, user emotion detection, voice detection, etc. Sensors can include a camera which can capture image based content of the outside world. 
     The head-mountable device  11 . 1 . 3 . 2 . 1 - 100  can include an input/output component  11 . 1 . 3 . 2 . 1 - 186 , which can include any suitable component for connecting head-mountable device  11 . 1 . 3 . 2 . 1 - 100  to other devices. Suitable components can include, for example, audio/video jacks, data connectors, or any additional or alternative input/output components. The input/output component  11 . 1 . 3 . 2 . 1 - 186  can include buttons, keys, or another feature that can act as a keyboard for operation by the user. 
     The head-mountable device  11 . 1 . 3 . 2 . 1 - 100  can include the microphone  11 . 1 . 3 . 2 . 1 - 188  as described herein. The microphone  11 . 1 . 3 . 2 . 1 - 188  can be operably connected to the controller  11 . 1 . 3 . 2 . 1 - 180  for detection of sound levels and communication of detections for further processing, as described further herein. 
     The head-mountable device  11 . 1 . 3 . 2 . 1 - 100  can include the speakers  11 . 1 . 3 . 2 . 1 - 190  as described herein. The speakers  11 . 1 . 3 . 2 . 1 - 190  can be operably connected to the controller  11 . 1 . 3 . 2 . 1 - 180  for control of speaker output, including sound levels, as described further herein. 
     The head-mountable device  11 . 1 . 3 . 2 . 1 - 100  can include communications circuitry  11 . 1 . 3 . 2 . 1 - 192  for communicating with one or more servers or other devices using any suitable communications protocol. For example, communications circuitry  11 . 1 . 3 . 2 . 1 - 192  can support Wi-Fi (e.g., a 802.11 protocol), Ethernet, Bluetooth, high frequency systems (e.g., 900 MHz, 2.4 GHz, and 5.6 GHz communication systems), infrared, TCP/IP (e.g., any of the protocols used in each of the TCP/IP layers), HTTP, BitTorrent, FTP, RTP, RTSP, SSH, any other communications protocol, or any combination thereof. Communications circuitry  11 . 1 . 3 . 2 . 1 - 192  can also include an antenna for transmitting and receiving electromagnetic signals. 
     The head-mountable device  11 . 1 . 3 . 2 . 1 - 100  can include a battery, which can charge and/or power components of the head-mountable device  11 . 1 . 3 . 2 . 1 - 100 . The battery can also charge and/or power components connected to the head-mountable device  11 . 1 . 3 . 2 . 1 - 100 . 
     While various embodiments and aspects of the present disclosure are illustrated with respect to a head-mountable device, it will be appreciated that the subject technology can encompass and be applied to other devices. For example, a sensor assembly in accordance with embodiments disclosed herein can be included with an electronic device that is moved while in use. Such an electronic device can be or include a desktop computing device, a laptop-computing device, a display, a television, a portable device, a phone, a tablet computing device, a mobile computing device, a wearable device, a watch, and/or a digital media player. 
     Accordingly, embodiments of the present disclosure provide a head-mountable device that securely supports sensors in a manner that maintains their positions and orientations over time and avoids applying excessive strain on the sensors, thereby protecting them from harm. Accordingly, the sensor assemblies described herein facilitate effective performance of sensors mounted therein. The sensor assemblies provides high robustness during the entirety of the service life of the head-mountable device without placing strain on the sensors themselves. The seal of the enclosure can isolate the sensor from external influences by preventing ingress of elements through the case and plate. The sensor assemblies further enable the mounting of sensors on a flex circuit, where desired. 
     Various examples of aspects of the disclosure are described below as clauses for convenience. These are provided as examples, and do not limit the subject technology. 
     Clause A: a sensor assembly for a head-mountable device, the sensor assembly comprising: a flex circuit; an inertial measurement unit coupled to the flex circuit; a base plate disposed adjacent to the inertial measurement unit on a side of the flex circuit that is opposite the inertial measurement unit; and an enclosure disposed around the inertial measurement unit, the base plate, and a portion of the flex circuit, wherein the enclosure comprises: a top case; and a bottom plate coupled to the top case, wherein the bottom plate and the top case are positioned to define a sealed volume containing the inertial measurement unit, the base plate, and the portion of the flex circuit. 
     Clause B: a sensor assembly for a head-mountable device, the sensor assembly comprising: a flex circuit; an inertial measurement unit coupled to the flex circuit; an enclosure disposed around the inertial measurement unit and a first portion of the flex circuit, wherein the enclosure defines an opening, wherein a second portion of the flex circuit extends through the opening to permit electrical communication with the inertial measurement unit; and a sealing member disposed within the opening and around the second portion of the flex circuit to seal the enclosure from an external environment. 
     Clause C: a sensor assembly for a head-mountable device, the sensor assembly comprising: a flex circuit; an inertial measurement unit coupled to the flex circuit; a connector in electrical communication with the inertial measurement unit and coupled to the flex circuit; an enclosure disposed around the inertial measurement unit and the flex circuit, wherein the enclosure defines an opening, wherein the connector extends through the opening to permit electrical communication with the inertial measurement unit; and a sealing member disposed within the opening and around the connector to seal the enclosure from an external environment. 
     One or more of the above clauses can include one or more of the features described below. It is noted that any of the following clauses may be combined in any combination with each other, and placed into a respective independent clause, e.g., clause A, B, or C. 
     Clause 1: the inertial measurement unit comprises an accelerometer, a gyroscope, or a magnetometer. 
     Clause 2: the top case defines an opening to permit electrical communication with the inertial measurement unit. 
     Clause 3: the flex circuit extends through the opening in the top case. 
     Clause 4: a connector coupled to the flex circuit and in electrical communication with the inertial measurement unit, wherein the connector extends through the opening in the top case. 
     Clause 5: the opening is defined on a side portion of the top case. 
     Clause 6: the opening is defined on an upper portion of the top case. 
     Clause 7: a sealing member disposed within the opening to seal the enclosure from an external environment and permit electrical communication with the inertial measurement unit. 
     Clause 8: the sealing member comprises a sealing adhesive or silicone. 
     Clause 9: a mounting adhesive disposed between the base plate and the bottom plate, wherein the mounting adhesive has a coefficient of thermal expansion that is complementary to a coefficient of thermal expansion of the top case and the bottom plate to minimize strain on the inertial measurement unit. 
     Clause 10: the inertial measurement unit is mounted to the flex circuit extending entirely through the enclosure. 
     Clause 11: the first portion of the flex circuit within the enclosure defines an S-curve portion. 
     Clause 12: the connector comprises a plurality of pins extending through the sealing member. 
     11.1.3.2.2: Electronic Devices with Movable Optical Assemblies 
     The optical assemblies may have gaze trackers. The gaze trackers may be used to make interpupillary distance measurements and eye relief measurements. Adjustments to the positions of the optical assemblies may be made by the motors based on the interpupillary distance measurements and eye relief measurements. For example, no position adjustments may be made unless a measured eye relief measurement exceeds an eye relief threshold and the measured interpupillary distance is lower than an interpupillary distance threshold. If the measured eye relief measurement is sufficiently large and the measured interpupillary distance is sufficiently small, adjustments to the positions of the optical assemblies may be made in which, for a given measured eye relief, a smaller measured interpupillary distance results in a larger adjustment to an optical assembly position than a larger measured interpupillary distance. 
       FIG.  11 . 1   . 3 . 2 . 2 - 1  is a schematic diagram of an illustrative electronic device of the type that may include movable components. Device  11 . 1 . 3 . 2 . 2 - 10  of  FIG.  11 . 1   . 3 . 2 . 2 - 1  may be a head-mounted device (e.g., goggles, glasses, a helmet, and/or other head-mounted device), a cellular telephone, a tablet computer, a laptop computer, a wristwatch, a peripheral device (sometimes referred to as a peripheral) such as a pair of headphones, or other electronic equipment. In an illustrative configuration, device  11 . 1 . 3 . 2 . 2 - 10  is a head-mounted device such as a pair of goggles (sometimes referred to as virtual reality goggles, mixed reality goggles, augmented reality glasses, etc.). 
     As shown in the illustrative top view of device  11 . 1 . 3 . 2 . 2 - 10  of  FIG.  11 . 1   . 3 . 2 . 2 - 1 , device  11 . 1 . 3 . 2 . 2 - 10  may have a housing such as housing  11 . 1 . 3 . 2 . 2 - 12  (sometimes referred to as a head-mounted support structure or head-mounted support). Housing  11 . 1 . 3 . 2 . 2 - 12  may include a main portion such as portion  11 . 1 . 3 . 2 . 2 - 12 M (sometimes referred to as a main unit or head-mounted unit) and other head-mounted support structures such as head strap  11 . 1 . 3 . 2 . 2 - 12 T. When housing  11 . 1 . 3 . 2 . 2 - 12  is being worn on the head of a user, the front of housing  11 . 1 . 3 . 2 . 2 - 12  may face outwardly away from the user, the rear of housing  11 . 1 . 3 . 2 . 2 - 12  may face towards the user, and the user&#39;s eyes may be located in eye boxes  11 . 1 . 3 . 2 . 2 - 36 . 
     Device  11 . 1 . 3 . 2 . 2 - 10  may have electrical and optical components that are used in displaying images to eye boxes  11 . 1 . 3 . 2 . 2 - 36  when device  11 . 1 . 3 . 2 . 2 - 10  is being worn. These components may include left and right optical assemblies  11 . 1 . 3 . 2 . 2 - 20  (sometimes referred to as optical modules). Each optical assembly  11 . 1 . 3 . 2 . 2 - 20  may have an optical assembly support  11 . 1 . 3 . 2 . 2 - 38  (sometimes referred to as a lens barrel or optical module support) and guide rails  11 . 1 . 3 . 2 . 2 - 22  along which optical assemblies  11 . 1 . 3 . 2 . 2 - 20  may slide to adjust optical-assembly-to-optical-assembly separation to accommodate different user interpupillary distances. 
     Each assembly  11 . 1 . 3 . 2 . 2 - 20  may have a display  11 . 1 . 3 . 2 . 2 - 32  that has an array of pixels for displaying images and a lens  11 . 1 . 3 . 2 . 2 - 34 . Display  11 . 1 . 3 . 2 . 2 - 32  and lens  11 . 1 . 3 . 2 . 2 - 34  of each assembly  11 . 1 . 3 . 2 . 2 - 20  may be coupled to and supported by support  11 . 1 . 3 . 2 . 2 - 38 . During operation, images displayed by displays  11 . 1 . 3 . 2 . 2 - 32  may be presented to eye boxes  11 . 1 . 3 . 2 . 2 - 36  through lenses  11 . 1 . 3 . 2 . 2 - 34  for viewing by a user. Each optical assembly  11 . 1 . 3 . 2 . 2 - 20  may also have a gaze tracker  11 . 1 . 3 . 2 . 2 - 50 . Gaze trackers  11 . 1 . 3 . 2 . 2 - 50  may each include one or more light sources (e.g., infrared light-emitting diodes that provide flood illumination and glints for eye tracking) and an associated camera (e.g., an infrared camera). Using gaze trackers  11 . 1 . 3 . 2 . 2 - 50 , which may sometimes be referred to as gaze tracking systems or gaze tracking sensors, device  11 . 1 . 3 . 2 . 2 - 10  can gather data on a user&#39;s eyes located in eye boxes  11 . 1 . 3 . 2 . 2 - 36 . As an example, the direction in which a user&#39;s eyes are pointing (sometimes referred to as a user&#39;s point of gaze or direction of view) may be measured. Biometric information such as iris scan information may also be gathered. In addition, gaze trackers  11 . 1 . 3 . 2 . 2 - 50 , may be used to measure the location of a user&#39;s eyes relative to device  11 . 1 . 3 . 2 . 2 - 10  and thereby measure the eye relief of the user&#39;s eyes (e.g., the distance between the lenses of device  11 . 1 . 3 . 2 . 2 - 10  and the eyes) and the separation between the user&#39;s left and right eyes (sometimes referred to as the user&#39;s interpupillary distance). If desired, gaze trackers  11 . 1 . 3 . 2 . 2 - 50  (e.g., the cameras of trackers  11 . 1 . 3 . 2 . 2 - 50 ) may capture images of the skin of the user&#39;s face surrounding the user&#39;s eyes (e.g., to measure whether this skin is loose or taut). 
     Each optical assembly may have magnets, clips, and/or other engagement features to allow removable vision correction lenses (sometimes referred to as prescription lenses) to be removably attached to assemblies  11 . 1 . 3 . 2 . 2 - 20  in alignment with lenses  11 . 1 . 3 . 2 . 2 - 34  (see, e.g., illustrative optional vision correction lenses  11 . 1 . 3 . 2 . 2 - 51 ). Lenses  11 . 1 . 3 . 2 . 2 - 51  may have magnets that are sensed by sensors  11 . 1 . 3 . 2 . 2 - 53  (e.g., magnetic sensors in assemblies  11 . 1 . 3 . 2 . 2 - 20 ) or sensors  11 . 1 . 3 . 2 . 2 - 53  may be optical sensors, switches, or other sensors configured to gather other information indicating when lenses  11 . 1 . 3 . 2 . 2 - 51  are present. 
     Housing  11 . 1 . 3 . 2 . 2 - 12  may have a flexible curtain (sometimes referred to as a flexible rear housing wall or fabric housing wall) such as curtain  11 . 1 . 3 . 2 . 2 - 12 R on the rear of device  11 . 1 . 3 . 2 . 2 - 10  facing eye boxes  11 . 1 . 3 . 2 . 2 - 36 . Curtain  11 . 1 . 3 . 2 . 2 - 12 R has openings that receive assemblies  11 . 1 . 3 . 2 . 2 - 20 . The edges of curtain  11 . 1 . 3 . 2 . 2 - 12 R that surround each support  11 . 1 . 3 . 2 . 2 - 38  may be coupled to that support  11 . 1 . 3 . 2 . 2 - 38 . The outer peripheral edge of curtain  11 . 1 . 3 . 2 . 2 - 12 R may be attached to rigid housing walls forming an outer shell portion of main housing  11 . 1 . 3 . 2 . 2 - 12 M. 
     The walls of housing  11 . 1 . 3 . 2 . 2 - 12  may separate interior region  11 . 1 . 3 . 2 . 2 - 28  within device  11 . 1 . 3 . 2 . 2 - 10  from exterior region  11 . 1 . 3 . 2 . 2 - 30  surrounding device  11 . 1 . 3 . 2 . 2 - 10 . 
     Inner ends  11 . 1 . 3 . 2 . 2 - 24  of guide rails  11 . 1 . 3 . 2 . 2 - 22  may be attached to central housing portion  11 . 1 . 3 . 2 . 2 - 12 C. Opposing outer ends  11 . 1 . 3 . 2 . 2 - 26  may, in an illustrative configuration, be unsupported (e.g., the outer end portions of rails  11 . 1 . 3 . 2 . 2 - 22  may not directly contact housing  11 . 1 . 3 . 2 . 2 - 12 , so that these ends float in interior region  11 . 1 . 3 . 2 . 2 - 28  with respect to housing  11 . 1 . 3 . 2 . 2 - 12 ). 
     Device  11 . 1 . 3 . 2 . 2 - 10  may include control circuitry and other components such as component  11 . 1 . 3 . 2 . 2 - 40 . The control circuitry may include storage, processing circuitry formed from one or more microprocessors and/or other circuits. To support communications between device  11 . 1 . 3 . 2 . 2 - 10  and external equipment, the control circuitry may include wireless communications circuitry. Components  11 . 1 . 3 . 2 . 2 - 40  may include sensors such as such as force sensors (e.g., strain gauges, capacitive force sensors, resistive force sensors, etc.), audio sensors such as microphones, touch and/or proximity sensors such as capacitive sensors, optical sensors such as optical sensors that emit and detect light, ultrasonic sensors, and/or other touch sensors and/or proximity sensors, monochromatic and color ambient light sensors, image sensors, sensors for detecting position, orientation, and/or motion (e.g., accelerometers, magnetic sensors such as compass sensors, gyroscopes, and/or sensors such as inertial measurement units that contain some or all of these sensors), radio-frequency sensors, depth sensors (e.g., structured light sensors and/or depth sensors based on stereo imaging devices), optical sensors such as self-mixing sensors and light detection and ranging (LIDAR) sensors that gather time-of-flight measurements, humidity sensors, moisture sensors, visual inertial odometry sensors, gaze tracking sensors, and/or other sensors. In some arrangements, devices  11 . 1 . 3 . 2 . 2 - 10  may use sensors to gather user input (e.g., button press input, touch input, etc.). Sensors may also be used in gathering environmental motion (e.g., device motion measurements, temperature measurements, ambient light readings, etc.) and/or may be used in measuring user activities and/or attributes (e.g., point-of-gaze, eye relief, interpupillary distance, etc.). If desired, position sensors such as encoders (e.g., optical encoders, magnetic encoders, etc.) may measure the position and therefore the movement (e.g., the velocity, acceleration, etc.) of optical assemblies  11 . 1 . 3 . 2 . 2 - 20  along rails  11 . 1 . 3 . 2 . 2 - 22 . 
       FIG.  11 . 1   . 3 . 2 . 2 - 2  is a rear view of an illustrative portion of device  11 . 1 . 3 . 2 . 2 - 10  (e.g., an inside left portion). Device  11 . 1 . 3 . 2 . 2 - 10  may have left and right actuators (e.g., motors) such as motor  11 . 1 . 3 . 2 . 2 - 48  that are used to rotate an elongated threaded shaft such as screw  11 . 1 . 3 . 2 . 2 - 44 . Nut  11 . 1 . 3 . 2 . 2 - 46  has threads that engage the threads on screw  11 . 1 . 3 . 2 . 2 - 44 . As the motor  11 . 1 . 3 . 2 . 2 - 48  on each side of device  11 . 1 . 3 . 2 . 2 - 10  is turned, a corresponding nut  11 . 1 . 3 . 2 . 2 - 46  is driven in the +X or −X direction (in accordance with whether screw  11 . 1 . 3 . 2 . 2 - 44  is being rotated clockwise or counterclockwise). In turn, this moves the optical assembly  11 . 1 . 3 . 2 . 2 - 20  on that side of device  11 . 1 . 3 . 2 . 2 - 10  in the +X or −X direction along its optical assembly guide rail  11 . 1 . 3 . 2 . 2 - 22 . If desired, the left and right motors  11 . 1 . 3 . 2 . 2 - 48  may be adjusted independently, so that optical assemblies  11 . 1 . 3 . 2 . 2 - 20  on the left and right of device  11 . 1 . 3 . 2 . 2 - 10  may be moved independently. 
     Each assembly  11 . 1 . 3 . 2 . 2 - 20  (e.g., each support  11 . 1 . 3 . 2 . 2 - 38  of  FIG.  11 . 1   . 3 . 2 . 2 - 1 ) may have portions that receive a corresponding rail  11 . 1 . 3 . 2 . 2 - 22  and that guide that assembly  11 . 1 . 3 . 2 . 2 - 20  along that rail  11 . 1 . 3 . 2 . 2 - 22 . By controlling the activity of motors  11 . 1 . 3 . 2 . 2 - 48  in tandem or individually, the spacing between the left and right optical assemblies of device  11 . 1 . 3 . 2 . 2 - 10  can be adjusted to accommodate the interpupillary distance of different users. For example, if gaze trackers  11 . 1 . 3 . 2 . 2 - 50  determine that a user has closely spaced eyes, assemblies  11 . 1 . 3 . 2 . 2 - 20  may be moved inwardly (towards each other) and if a user has widely spaced eyes, assemblies  11 . 1 . 3 . 2 . 2 - 20  may be moved outwardly (away from each other). By matching the spacing between optical assemblies  11 . 1 . 3 . 2 . 2 - 20  to the measured interpupillary distance of a user, the user may satisfactorily view visual content being presented by the displays of the optical assemblies. 
     The position and therefore the movement of each optical assembly may be monitored using one or more sensors. In the illustrative configuration of  FIG.  11 . 1   . 3 . 2 . 2 - 2 , motor  11 . 1 . 3 . 2 . 2 - 48  (e.g., the motor on the left side of device  11 . 1 . 3 . 2 . 2 - 10 ) and optical module  11 . 1 . 3 . 2 . 2 - 20  have been provided with an optical assembly position sensor based on a magnetic encoder. The encoder includes magnetic strip  11 . 1 . 3 . 2 . 2 - 54  and magnetic sensor  11 . 1 . 3 . 2 . 2 - 52 . Magnetic sensor  11 . 1 . 3 . 2 . 2 - 52  may be a Hall Effect sensor or other suitable magnetic sensor that is configured to move with optical assembly  11 . 1 . 3 . 2 . 2 - 20 . Magnetic strip  11 . 1 . 3 . 2 . 2 - 54 , which may be affixed to housing  11 . 1 . 3 . 2 . 2 - 12 , has a series of magnet poles (e.g., north and south poles) that extend along the X dimension parallel to guide rail  11 . 1 . 3 . 2 . 2 - 22  and parallel to the X-axis position adjustment direction associated with optical assembly  11 . 1 . 3 . 2 . 2 - 20 . As optical assembly  11 . 1 . 3 . 2 . 2 - 20  is moved along guide rail  11 . 1 . 3 . 2 . 2 - 22 , the magnetic encoder (e.g., sensor  11 . 1 . 3 . 2 . 2 - 52 ) measures the change in magnetic field resulting as magnetic sensor  11 . 1 . 3 . 2 . 2 - 52  passes by different magnets in strip  11 . 1 . 3 . 2 . 2 - 54 . In this way, optical assembly  11 . 1 . 3 . 2 . 2 - 20  is provided with a position reference (e.g., by counting north and south magnetic poles in strip  11 . 1 . 3 . 2 . 2 - 54 ). The position measurements made with the position sensor may reveal attributes of the motion of optical assembly  11 . 1 . 3 . 2 . 2 - 20  such as the velocity of optical assembly  11 . 1 . 3 . 2 . 2 - 20  and, if desired, the acceleration and deceleration of assembly  11 . 1 . 3 . 2 . 2 - 20 . Accordingly, the magnetic encoder (or other position sensor) associated with each optical assembly  11 . 1 . 3 . 2 . 2 - 30  may be used by device  11 . 1 . 3 . 2 . 2 - 10  to measure the location of that optical assembly so that the spacing between optical assemblies  11 . 1 . 3 . 2 . 2 - 20  can be satisfactorily adjusted (e.g., to help ensure that the spacing between optical assemblies  11 . 1 . 3 . 2 . 2 - 20  has been adjusted to match or nearly match the user&#39;s interpupillary distance). The position readings from the optical assembly position sensors (e.g., the magnetic encoders) may also be used to determine the velocities of the optical assemblies as they are moved. 
     Optical assembly velocity information and/or other information from the optical assembly position sensors (e.g., linear position sensors formed from the magnetic encoders) may be used in monitoring whether the optical assemblies have slowed down their movement due to contact between the optical assemblies and the nose of a user. Consider, as an example, the arrangement of  FIG.  11 . 1   . 3 . 2 . 2 - 3 .  FIG.  11 . 1   . 3 . 2 . 2 - 3  is a rear view of a central nose-bridge portion NB of device  11 . 1 . 3 . 2 . 2 - 10 . As shown in  FIG.  11 . 1   . 3 . 2 . 2 - 3 , nose bridge portion NB of housing portion  11 . 1 . 3 . 2 . 2 - 12 C has a nose-shaped recess  11 . 1 . 3 . 2 . 2 - 60 , which is configured to receive the user&#39;s nose (see, e.g., illustrative nose surface  11 . 1 . 3 . 2 . 2 - 62 ). In some situations (e.g., when a user has a wide interpupillary distance), optical assemblies  11 . 1 . 3 . 2 . 2 - 20  reside at a non-zero distance G from surface  11 . 1 . 3 . 2 . 2 - 62  (sometimes referred to as gap G) after motors  11 . 1 . 3 . 2 . 2 - 48  adjusted the positions of assemblies  11 . 1 . 3 . 2 . 2 - 20  to match the user&#39;s interpupillary distance. In other situations, such as when a user has a large eye relief and small interpupillary distance, motors  11 . 1 . 3 . 2 . 2 - 48  may move optical assemblies inward until curtain  11 . 1 . 3 . 2 . 2 - 12 R ( FIG.  11 . 1   . 3 . 2 . 2 - 1 ) and optical assemblies  11 . 1 . 3 . 2 . 2 - 20  contact the left and right sides of nose surface  11 . 1 . 3 . 2 . 2 - 62 . 
     When optical assemblies  11 . 1 . 3 . 2 . 2 - 20  contact the left and right sides of nose surface of  11 . 1 . 3 . 2 . 2 - 62 , motors  11 . 1 . 3 . 2 . 2 - 48  will encounter resistance to further lateral movement of the optical assemblies along the X axis. This will cause optical assemblies  11 . 1 . 3 . 2 . 2 - 20  to move more slowly. The optical assembly position sensors will sense the reduction in the velocity of the optical assemblies. In this way, device  11 . 1 . 3 . 2 . 2 - 10  is informed that optical assemblies  11 . 1 . 3 . 2 . 2 - 20  are contacting and pressing against nose surface  11 . 1 . 3 . 2 . 2 - 62 . To ensure that device  11 . 1 . 3 . 2 . 2 - 10  is comfortable as device  11 . 1 . 3 . 2 . 2 - 10  is being worn on the head of the user, the position of the optical assemblies may, in response to this detected nose contact, be adjusted outward, away from nose surface  11 . 1 . 3 . 2 . 2 - 62  (e.g., by 1-3 mm or other suitable amount). This outward nudge in the positions of the optical assemblies may be made even if the final separation between the optical assemblies is slightly larger than the user&#39;s measured interpupillary distance. 
     Following outward adjustment of optical assemblies  11 . 1 . 3 . 2 . 2 - 20 , a non-zero gap G may be created between optical assemblies  11 . 1 . 3 . 2 . 2 - 20  and corresponding side portions of the user&#39;s nose (e.g., adjacent portions of nose surface  11 . 1 . 3 . 2 . 2 - 62 ) and/or inward pressure imposed on the sides of the user&#39;s nose by optical assemblies  11 . 1 . 3 . 2 . 2 - 20  can be reduced to enhance comfort. By monitoring the optical assembly position sensors during optical assembly position adjustment with motors  11 . 1 . 3 . 2 . 2 - 48 , device  11 . 1 . 3 . 2 . 2 - 10  can identify a location for assemblies  11 . 1 . 3 . 2 . 2 - 20  in which the left and right assemblies  11 . 1 . 3 . 2 . 2 - 20  are separated by distance that is matched as closely as possible to the user&#39;s measured interpupillary distance while ensuring satisfactory comfort for the user. 
     Another way in which the position of optical assemblies  11 . 1 . 3 . 2 . 2 - 20  can be adjusted satisfactorily involves the use of eye relief measurements from gaze trackers  11 . 1 . 3 . 2 . 2 - 50 . When device  11 . 1 . 3 . 2 . 2 - 10  is placed onto the head of the user, gaze trackers  11 . 1 . 3 . 2 . 2 - 50  may measure the user&#39;s interpupillary distance and may measure the user&#39;s eye relief. Motors  11 . 1 . 3 . 2 . 2 - 48  may then adjust the positions of optical assemblies  11 . 1 . 3 . 2 . 2 - 20  based on the measured eye relief and measured interpupillary distance of the user. In an illustrative configuration, the positions of optical assemblies  11 . 1 . 3 . 2 . 2 - 20  maybe offset (e.g., nudged outwards from the location where the spacing between assemblies  11 . 1 . 3 . 2 . 2 - 20  matches the measured interpupillary distance of the user) by an amount that varies depending on both measured interpupillary distance and measured eye relief. 
     This type of approach is illustrated in the graph of  FIG.  11 . 1   . 3 . 2 . 2 - 4 . In the graph of  FIG.  11 . 1   . 3 . 2 . 2 - 4 , lateral offset ΔX represents the distance by which an optical assembly is adjusted outwardly (beyond the nominal position in which optical assembly separation matches measured interpupillary distance) to enhance fit. There are three illustrative curves in the graph of  FIG.  11 . 1   . 3 . 2 . 2 - 4 . Curve  11 . 1 . 3 . 2 . 2 - 64  corresponds to a measured interpupillary distance IPDA, curve  11 . 1 . 3 . 2 . 2 - 66  corresponds to a measured interpupillary distance IPDB, which is larger than interpupillary distance IPDA, and curve  11 . 1 . 3 . 2 . 2 - 68  corresponds to a measured interpupillary distance IPDC, which is larger than interpupillary distance IPDA and larger than interpupillary distance IPDB. The values of IPDA, IPDB, and IPDC may be in the range of 52 to 74 mm or any other suitable range. In practice, device  11 . 1 . 3 . 2 . 2 - 10  may store a family of curves (e.g., empirically determined curves) for any suitable number of measured interpupillary distances and/or may represent the relationships embodied in curves such as curves  11 . 1 . 3 . 2 . 2 - 64 ,  11 . 1 . 3 . 2 . 2 - 66 , and  11 . 1 . 3 . 2 . 2 - 68  using tables, functions, and/or other data structures. 
     As the graph of a  FIG.  11 . 1   . 3 . 2 . 2 - 4  illustrates, if the measured eye relief a user falls below a predetermined threshold (e.g., ERTH in the example of  FIG.  11 . 1   . 3 . 2 . 2 - 4 ), the user is unlikely to experience pressure from optical assemblies  11 . 1 . 3 . 2 . 2 - 20  on the sides of the user&#39;s nose. Accordingly, for measured user eye relief values below eye relief threshold ERTH, optical assemblies  11 . 1 . 3 . 2 . 2 - 20  may be separated along the X axis by a distance that matches the measured interpupillary distance of the user (e.g., there is no need to use a non-zero value of ΔX to adjust the position of optical assemblies  11 . 1 . 3 . 2 . 2 - 20  outwardly after they have reached the appropriate spacing to match the user&#39;s interpupillary distance). The value of ΔX is therefore zero for users with measured eye relief values of less than ERTH (which may be, for example, a value between 18 mm and 22 mm or other suitable eye relief threshold value). 
     If however, the measured eye relief of a user exceeds eye relief threshold ERTH, it may be beneficial for certain users to increase the spacing between optical assemblies  11 . 1 . 3 . 2 . 2 - 20  by nudging each optical assembly outwardly by an amount ΔX. As an example, consider users with measured interpupillary distances of IPDA. For this class of user, it may be beneficial to adjust the spacing of optical assemblies  11 . 1 . 3 . 2 . 2 - 20  outwardly by ΔX values that follow curve  11 . 1 . 3 . 2 . 2 - 64 . As shown by curve  11 . 1 . 3 . 2 . 2 - 64 , ΔX may be zero for measured eye relief values of less than ERTH, whereas for measured eye relief values exceeding ERTH, ΔX may rise progressively as a function of measured eye relief (e.g., up to a maximum ΔX value at a maximum measured eye relief value of ERMX, which may be, for example, 25 mm or other suitable value). Users with larger interpupillary distances, such as a measured interpupillary distance of IPDB, may benefit from a less aggressive outward increase in optical assembly spacing, as shown by curve  11 . 1 . 3 . 2 . 2 - 66  in which the value of ΔX rises more slowly as a function of increasing measured eye relief over ERTH than curve  11 . 1 . 3 . 2 . 2 - 64 . 
     An interpupillary distance threshold may be present above which it may not be desirable to make any ΔX adjustments for a user, regardless of their measured eye relief. When, for example, a user has a large measured interpupillary distance (e.g., a measured interpupillary distance of IPDC, which exceeds the interpupillary distance threshold), there will generally not be a comfort benefit in increasing optical assembly spacing. As a result, the recommended outward adjustment in optical assembly position ΔX as a function of measured eye relief for users with these larger measured interpupillary distances follows curve  11 . 1 . 3 . 2 . 2 - 68  (e.g., ΔX remains at zero, even for a measured eye relief of ERMX). With this approach, only at measured interpupillary distances below the interpupillary distance threshold will outward adjustments in optical assembly position be used. 
     The graph of  FIG.  11 . 1   . 3 . 2 . 2 - 4  illustrates how motors  11 . 1 . 3 . 2 . 2 - 48  may place optical assemblies  11 . 1 . 3 . 2 . 2 - 20  at positions that vary from the measured positions of the user&#39;s eyes. In the example of  FIG.  11 . 1   . 3 . 2 . 2 - 4 , measured eye relief and interpupillary distance values were are in determining satisfactory positions for assemblies  11 . 1 . 3 . 2 . 2 - 20 . In particular, outward adjustments ΔX in the positions of optical assemblies  11 . 1 . 3 . 2 . 2 - 20  were recommended as a function of the interpupillary distance and eye relief values measured for a user with gaze trackers  11 . 1 . 3 . 2 . 2 - 50 . If desired, additional factors may be taken into consideration in making optical assembly adjustments such as these (e.g., additional factors that may be used in combination with or as an alternative to using interpupillary distance measurements and eye relief measurements) and/or other suitable action may be taken based on data gathered with gaze trackers  11 . 1 . 3 . 2 . 2 - 50  and/or other sensors in device  11 . 1 . 3 . 2 . 2 - 10 . 
       FIG.  11 . 1   . 3 . 2 . 2 - 5  is a flow chart illustrating operations involved in using motors  11 . 1 . 3 . 2 . 2 - 48  and other components in device  11 . 1 . 3 . 2 . 2 - 10  to make adjustments to device  11 . 1 . 3 . 2 . 2 - 10  as a function of data gathered with gaze trackers  11 . 1 . 3 . 2 . 2 - 50  and/or other sensors. 
     During the operations of block  11 . 1 . 3 . 2 . 2 - 70 , device  11 . 1 . 3 . 2 . 2 - 10  may gather data. The gathered data may include, for example, measurements obtained by gaze trackers  11 . 1 . 3 . 2 . 2 - 50 . These measurements may include, for example, the measured interpupillary distance of a user wearing device  11 . 1 . 3 . 2 . 2 - 10 , the measured eye relief of a user wearing device  11 . 1 . 3 . 2 . 2 - 10 , the measured skin tautness around the eyes of a user wearing device  11 . 1 . 3 . 2 . 2 - 10 , and/or other gaze tracker measurements. The measurements of block  11 . 1 . 3 . 2 . 2 - 70  may also include measurements with the position sensors (e.g., the magnetic encoders) of optical assemblies  11 . 1 . 3 . 2 . 2 - 20 . For example, when a user dons device  11 . 1 . 3 . 2 . 2 - 10 , motors  11 . 1 . 3 . 2 . 2 - 48  may automatically start to move optical assemblies  11 . 1 . 3 . 2 . 2 - 20  to positions associated with the user&#39;s measured interpupillary distance from gaze trackers  11 . 1 . 3 . 2 . 2 - 50 . During this initial movement or during movement in response to a user input command or other movement, the position sensors may be used to monitor the velocities of assemblies  11 . 1 . 3 . 2 . 2 - 20 . In response to detected slowing of the speed of inward movement of assemblies  11 . 1 . 3 . 2 . 2 - 20 , device  11 . 1 . 3 . 2 . 2 - 10  can conclude that assemblies  11 . 1 . 3 . 2 . 2 - 20  are beginning to exert pressure on the sides of the user&#39;s nose (e.g., nose surface  11 . 1 . 3 . 2 . 2 - 62 ). The locations associated with the measured reduction in optical assembly velocity are another form of measurement data that may be gathered during block  11 . 1 . 3 . 2 . 2 - 70 . 
     Further information that can be gathered during the operations of block  11 . 1 . 3 . 2 . 2 - 70  relates to the status of vision correction lenses  11 . 1 . 3 . 2 . 2 - 51  on device  11 . 1 . 3 . 2 . 2 - 10 . Vision correction lenses  11 . 1 . 3 . 2 . 2 - 51  may contain magnets that produce a magnetic field. Device  11 . 1 . 3 . 2 . 2 - 10  (e.g., assemblies  11 . 1 . 3 . 2 . 2 - 20 ) may have vision correction lens sensors such as magnetic sensors  11 . 1 . 3 . 2 . 2 - 53  that determine whether or not lenses  11 . 1 . 3 . 2 . 2 - 51  are present by monitoring for the presence of the magnetic fields produced by lenses  11 . 1 . 3 . 2 . 2 - 51 . In response to detection of the magnetic fields from lenses  11 . 1 . 3 . 2 . 2 - 51  with sensors  11 . 1 . 3 . 2 . 2 - 53 , it can be concluded that lenses  11 . 1 . 3 . 2 . 2 - 51  are present. 
     During the operations of block  11 . 1 . 3 . 2 . 2 - 72 , device  11 . 1 . 3 . 2 . 2 - 10  can take action based on the data gathered at block  11 . 1 . 3 . 2 . 2 - 70 . As an example, the positions of optical assemblies  11 . 1 . 3 . 2 . 2 - 20  may be adjusted using motors  11 . 1 . 3 . 2 . 2 - 48 . In some configurations, the positions of optical assemblies  11 . 1 . 3 . 2 . 2 - 20  may be nudged outwards by an amount ΔX determined from measured interpupillary distance and eye relief values, as described in connection with FIG.  11 . 1 . 3 . 2 . 2 - 4 . If desired, optical assemblies  11 . 1 . 3 . 2 . 2 - 20  may, in some scenarios, be moved inward slightly (e.g., assemblies  11 . 1 . 3 . 2 . 2 - 20  may be moved towards each other such that assemblies  11 . 1 . 3 . 2 . 2 - 20  are closer than dictated by the measured value of a user&#39;s interpupillary distance to increase nasal field-of-view overlap when doing so presents a low risk of optical assembly nose surface contact or when any potential nose contact can be detected with an optical module position sensor or other sensor). To reduce wrinkles in rear curtain  11 . 1 . 3 . 2 . 2 - 12 R, optical assemblies  11 . 1 . 3 . 2 . 2 - 20  may be moved inwards or outwards slightly to tighten curtain  11 . 1 . 3 . 2 . 2 - 12 R. Optical assembly movement with motors  11 . 1 . 3 . 2 . 2 - 48  may also be used to provide a user with an alert (e.g., a haptic alert indicating that an incoming message has been received and/or another condition has been determined to be present). In some arrangements, gaze sensor measurements from trackers  11 . 1 . 3 . 2 . 2 - 50  during block  11 . 1 . 3 . 2 . 2 - 70  can be used to determine whether the user&#39;s skin surrounding the user&#39;s eyes is taut. In response to detecting an increase in skin tautness as assemblies  11 . 1 . 3 . 2 . 2 - 20  are moved (e.g., as assemblies  11 . 1 . 3 . 2 . 2 - 20  are being moved towards each other), it can be concluded that assemblies  11 . 1 . 3 . 2 . 2 - 20  are pressing or are about to press on nose surface  11 . 1 . 3 . 2 . 2 - 62 , so further movement inward of assemblies  11 . 1 . 3 . 2 . 2 - 20  can be halted and/or assemblies  11 . 1 . 3 . 2 . 2 - 20  may be nudged outwardly to compensate. The positions of optical assemblies  11 . 1 . 3 . 2 . 2 - 20  may also be adjusted in response to detection that vision correction lenses  11 . 1 . 3 . 2 . 2 - 51  are present or are not present. The presence of lenses  11 . 1 . 3 . 2 . 2 - 51  may, as an example, increase or decrease the risk of nose contact by assemblies  11 . 1 . 3 . 2 . 2 - 20 , so information on the presence of lenses  11 . 1 . 3 . 2 . 2 - 51  (and, if desired, the prescription associated with lenses  11 . 1 . 3 . 2 . 2 - 51 ) may be taken into account when using motors  11 . 1 . 3 . 2 . 2 - 48  to adjust the position of assemblies  11 . 1 . 3 . 2 . 2 - 20 . In general, these types of adjustments and/or other suitable actions may be taken during the operations of block  11 . 1 . 3 . 2 . 2 - 72  in response to gaze tracker measurement data and/or other data measured during the operations of block  11 . 1 . 3 . 2 . 2 - 70 . 
     11.1.3.2.3: Electronic Devices with Movable Optical Assemblies 
     The force with which the motors move the optical assemblies towards a central nose bridge portion of the device and therefore towards nose surfaces located at the nose bridge portion may be limited. The force may be limited using a clutch such as a magnetic clutch or a physical clutch based on structures that decouple from each other to limit the force. The force may also be limited by monitoring the force and halting the motors in response to detection of a given amount of force. Sensor measurements and electrical motor load measurements may be used in measuring the force. If desired, motor operation may be controlled by a user-operated button. The direction of permitted optical assembly movement when accommodating different interpupillary distances may also be controlled. 
       FIG.  11 . 1   . 3 . 2 . 3 - 1  is a schematic diagram of an illustrative electronic device of the type that may include movable optical assemblies to accommodate different interpupillary distances. Device  11 . 1 . 3 . 2 . 3 - 10  of  FIG.  11 . 1   . 3 . 2 . 3 - 1  may be a head-mounted device (e.g., goggles, glasses, a helmet, and/or other head-mounted device. In an illustrative configuration, device  11 . 1 . 3 . 2 . 3 - 10  is a head-mounted device such as a pair of goggles (sometimes referred to as virtual reality goggles, mixed reality goggles, augmented reality glasses, etc.). 
     As shown in the illustrative cross-sectional top view of device  11 . 1 . 3 . 2 . 3 - 10  of  FIG.  11 . 1   . 3 . 2 . 3 - 1 , device  11 . 1 . 3 . 2 . 3 - 10  may have a housing such as housing  11 . 1 . 3 . 2 . 3 - 12  (sometimes referred to as a head-mounted support structure, head-mounted housing, or head-mounted support). Housing  11 . 1 . 3 . 2 . 3 - 12  may include a front portion such as front portion  11 . 1 . 3 . 2 . 3 - 12 F and a rear portion such as rear portion  11 . 1 . 3 . 2 . 3 - 12 R. When device  11 . 1 . 3 . 2 . 3 - 10  is worn on the head of a user, rear portion  11 . 1 . 3 . 2 . 3 - 12 R rests against the face of the user and helps block stray light from reaching the eyes of the user and nose bridge portion NB of housing  11 . 1 . 3 . 2 . 3 - 12  rests on the nose of the user. 
     Main portion  11 . 1 . 3 . 2 . 3 - 12 M of housing  11 . 1 . 3 . 2 . 3 - 12  may be attached to head strap  11 . 1 . 3 . 2 . 3 - 12 T. Head strap  11 . 1 . 3 . 2 . 3 - 12 T may be used to help mount main portion  11 . 1 . 3 . 2 . 3 - 12  on the head and face of a user. Main portion  11 . 1 . 3 . 2 . 3 - 12 M may have a rigid shell formed from housing walls of polymer, glass, metal, and/or other materials. When housing  11 . 1 . 3 . 2 . 3 - 12  is being worn on the head of a user, the front of housing  11 . 1 . 3 . 2 . 3 - 12  may face outwardly away from the user, the rear of housing  11 . 1 . 3 . 2 . 3 - 12  (and rear portion  11 . 1 . 3 . 2 . 3 - 12 R) may face towards the user. In this configuration, rear portion  11 . 1 . 3 . 2 . 3 - 12 R may face the user&#39;s eyes located in eye boxes  11 . 1 . 3 . 2 . 3 - 36 . 
     Device  11 . 1 . 3 . 2 . 3 - 10  may have electrical and optical components that are used in displaying images to eye boxes  11 . 1 . 3 . 2 . 3 - 36  when device  11 . 1 . 3 . 2 . 3 - 10  is being worn. These components may include left and right optical assemblies  11 . 1 . 3 . 2 . 3 - 20  (sometimes referred to as optical modules). Each optical assembly  11 . 1 . 3 . 2 . 3 - 20  may have an optical assembly support  11 . 1 . 3 . 2 . 3 - 38  (sometimes referred to as a lens barrel, optical module support, or support structure) and guide rails  11 . 1 . 3 . 2 . 3 - 22  along which optical assemblies  11 . 1 . 3 . 2 . 3 - 20  may slide to adjust optical-assembly-to-optical-assembly separation to accommodate different user interpupillary distances. 
     Each assembly  11 . 1 . 3 . 2 . 3 - 20  may have a display  11 . 1 . 3 . 2 . 3 - 32  that has an array of pixels for displaying images and a lens  11 . 1 . 3 . 2 . 3 - 34 . Lens  11 . 1 . 3 . 2 . 3 - 34  may optionally have a removable vision correction lens for correcting user vision defects (e.g., refractive errors such as nearsightedness, farsightedness, and/or astigmatism). In each assembly  11 . 1 . 3 . 2 . 3 - 20 , display  11 . 1 . 3 . 2 . 3 - 32  and lens  11 . 1 . 3 . 2 . 3 - 34  may be coupled to and supported by support  11 . 1 . 3 . 2 . 3 - 38 . During operation, images displayed by displays  11 . 1 . 3 . 2 . 3 - 32  may be presented to eye boxes  11 . 1 . 3 . 2 . 3 - 36  through lenses  11 . 1 . 3 . 2 . 3 - 34  for viewing by the user. 
     Rear portion  11 . 1 . 3 . 2 . 3 - 12 R may include flexible structures (e.g., a flexible polymer layer, a flexible fabric layer, etc.) so that portion  11 . 1 . 3 . 2 . 3 - 12 R can stretch to accommodate movement of supports  11 . 1 . 3 . 2 . 3 - 38  toward and away from each other to accommodate different user interpupillary distances. 
     The walls of housing  11 . 1 . 3 . 2 . 3 - 12  may separate interior region  11 . 1 . 3 . 2 . 3 - 28  within device  11 . 1 . 3 . 2 . 3 - 10  from exterior region  11 . 1 . 3 . 2 . 3 - 30  surrounding device  11 . 1 . 3 . 2 . 3 - 10 . In interior region  11 . 1 . 3 . 2 . 3 - 28 , optical assemblies  11 . 1 . 3 . 2 . 3 - 20  may be mounted on guide rails  11 . 1 . 3 . 2 . 3 - 22 . Guide rails  11 . 1 . 3 . 2 . 3 - 22  may be attached to central housing portion  11 . 1 . 3 . 2 . 3 - 12 C. If desired, the outer ends of guide rails  11 . 1 . 3 . 2 . 3 - 22  may be unsupported (e.g., the outer end portions of rails  11 . 1 . 3 . 2 . 3 - 22  may not directly contact housing  11 . 1 . 3 . 2 . 3 - 12 , so that these ends float in interior region  11 . 1 . 3 . 2 . 3 - 28  with respect to housing  11 . 1 . 3 . 2 . 3 - 12 ). 
     Device  11 . 1 . 3 . 2 . 3 - 10  may include control circuitry and other components such as components  11 . 1 . 3 . 2 . 3 - 40 . The control circuitry may include storage, processing circuitry formed from one or more microprocessors and/or other circuits. To support communications between device  11 . 1 . 3 . 2 . 3 - 10  and external equipment, the control circuitry may include wireless communications circuitry. Components  11 . 1 . 3 . 2 . 3 - 40  may include sensors such as such as force sensors (e.g., strain gauges, capacitive force sensors, resistive force sensors, etc.), audio sensors such as microphones, touch and/or proximity sensors such as capacitive sensors, optical sensors such as optical sensors that emit and detect light, ultrasonic sensors, and/or other touch sensors and/or proximity sensors, monochromatic and color ambient light sensors, image sensors, sensors for detecting position, orientation, and/or motion (e.g., accelerometers, magnetic sensors such as compass sensors, gyroscopes, and/or sensors such as inertial measurement units that contain some or all of these sensors), radio-frequency sensors, depth sensors (e.g., structured light sensors and/or depth sensors based on stereo imaging devices), optical sensors such as self-mixing sensors and light detection and ranging (LIDAR) sensors that gather time-of-flight measurements, humidity sensors, moisture sensors, visual inertial odometry sensors, current sensors, voltage sensors, and/or other sensors. In some arrangements, devices  11 . 1 . 3 . 2 . 3 - 10  may use sensors to gather user input (e.g., button press input, touch input, etc.). Sensors may also be used in gathering environmental motion (e.g., device motion measurements, temperature measurements, ambient light readings, etc.). 
     Optical assemblies  11 . 1 . 3 . 2 . 3 - 20  may have gaze trackers  11 . 1 . 3 . 2 . 3 - 62  (sometimes referred to as gaze tracker sensors). Gaze trackers  11 . 1 . 3 . 2 . 3 - 62 , which may operate through lenses  11 . 1 . 3 . 2 . 3 - 34 , may include one or more light sources such as infrared light-emitting diodes that emit infrared light to illuminate the eyes of a user in eye boxes  11 . 1 . 3 . 2 . 3 - 36 . Gaze trackers  11 . 1 . 3 . 2 . 3 - 62  also include infrared cameras for capturing images of the user&#39;s eyes and measuring reflections (glints) of infrared light from each of the infrared light sources. By processing these eye images, gaze trackers  11 . 1 . 3 . 2 . 3 - 62  may track the user&#39;s eyes and determine the point-of-gaze of the user. Gaze trackers  11 . 1 . 3 . 2 . 3 - 62  may also measure the locations of the user&#39;s eyes (e.g., the user&#39;s eye relief and the user&#39;s interpupillary distance). 
     To accommodate users with different interpupillary distances (eye-to-eye spacings), the spacing between the left and right optical assemblies  11 . 1 . 3 . 2 . 3 - 20  in device  11 . 1 . 3 . 2 . 3 - 10  can be adjusted (e.g., to match or nearly match the user&#39;s measured interpupillary distance). Device  11 . 1 . 3 . 2 . 3 - 10  may have left and right actuators (e.g., motors) such as motors  11 . 1 . 3 . 2 . 3 - 48 . Each motor  11 . 1 . 3 . 2 . 3 - 48  may be used to rotate an elongated threaded shaft (screw) such as shaft  11 . 1 . 3 . 2 . 3 - 44 . A nut  11 . 1 . 3 . 2 . 3 - 46  is provided on each shaft  11 . 1 . 3 . 2 . 3 - 44 . The nut has threads that engage the threads on that shaft  11 . 1 . 3 . 2 . 3 - 44 . When a shaft is rotated, the nut on the shaft is driven in the +X or −X direction (in accordance with whether the shaft is being rotated clockwise or counterclockwise). In turn, this moves the optical assembly  11 . 1 . 3 . 2 . 3 - 20  that is attached to the nut in the +X or −X direction along its optical assembly guide rail  11 . 1 . 3 . 2 . 3 - 22 . Each assembly  11 . 1 . 3 . 2 . 3 - 20  (e.g., support  11 . 1 . 3 . 2 . 3 - 38 ) may have portions that receive one of guide rails  11 . 1 . 3 . 2 . 3 - 22  so that the assembly is guided along the guide rail. By controlling the activity of motors  11 . 1 . 3 . 2 . 3 - 48 , the spacing between the left and right optical assemblies of device  11 . 1 . 3 . 2 . 3 - 10  can be adjusted to accommodate the interpupillary distance of different users. For example, if a user has closely spaced eyes, assemblies  11 . 1 . 3 . 2 . 3 - 20  may be moved inwardly (towards each other and towards nose bridge portion NB of housing  11 . 1 . 3 . 2 . 3 - 12 ) and if a user has widely spaced eyes, assemblies  11 . 1 . 3 . 2 . 3 - 20  may be moved outwardly (away from each other). 
     When device  11 . 1 . 3 . 2 . 3 - 10  is being worn by a user, the user&#39;s head is located in region  11 . 1 . 3 . 2 . 3 - 68 . The presence of the user&#39;s head (and therefore a determination of whether device  11 . 1 . 3 . 2 . 3 - 10  is being worn or is unworn) may be made using one or more sensors (e.g., gaze trackers  11 . 1 . 3 . 2 . 3 - 62 , which may detect the presence of the eyes of the user in eye boxes  11 . 1 . 3 . 2 . 3 - 36 , rear-facing sensors such as sensor  11 . 1 . 3 . 2 . 3 - 66  on main housing  11 . 1 . 3 . 2 . 3 - 12 M, head-facing sensors mounted on strap  11 . 1 . 3 . 2 . 3 - 12 T such as sensor  11 . 1 . 3 . 2 . 3 - 64 , and/or other head presence sensors. These sensors may include cameras, light sensors (e.g., visible light or infrared sensors that measure when ambient light levels have dropped due to shadowing by the head of a user), proximity sensors (e.g., sensors that emit light such as infrared light and that measure corresponding reflected light from a user&#39;s head with an infrared light sensor, capacitive proximity sensors, ultrasonic acoustic proximity sensors, etc.), switches and/or other force-sensing sensors that detect head pressure when a user&#39;s head is present, and/or other head presence sensors. 
     When device  11 . 1 . 3 . 2 . 3 - 10  is being worn and a user&#39;s head is present in region  11 . 1 . 3 . 2 . 3 - 68 , the nose of the user will be present under nose bridge portion NB of housing  11 . 1 . 3 . 2 . 3 - 12 . When optical assemblies  11 . 1 . 3 . 2 . 3 - 20  are moved towards each other so that assemblies  11 . 1 . 3 . 2 . 3 - 20  are spaced apart by an amount that matches or nearly matches the user&#39;s interpupillary distance, inner side surfaces  11 . 1 . 3 . 2 . 3 - 60  of support structures  11 . 1 . 3 . 2 . 3 - 38  in assemblies  11 . 1 . 3 . 2 . 3 - 20  will move toward opposing outer side surfaces  11 . 1 . 3 . 2 . 3 - 61  of the user&#39;s nose. With sufficient inward movement of assemblies  11 . 1 . 3 . 2 . 3 - 20 , surfaces  11 . 1 . 3 . 2 . 3 - 60  may contact and press against nose surfaces  11 . 1 . 3 . 2 . 3 - 61 . As a result, an outward force on assemblies  11 . 1 . 3 . 2 . 3 - 20  is created by nose surfaces  11 . 1 . 3 . 2 . 3 - 61 . To avoid discomfort that might arise if the user&#39;s nose is pressed against by more than a desired amount, device  11 . 1 . 3 . 2 . 3 - 10  may be provided with features to limit inward nose pressure (e.g., to limit inward force by assemblies  11 . 1 . 3 . 2 . 3 - 20 ). 
     With an illustrative embodiment, whenever device  11 . 1 . 3 . 2 . 3 - 10  is mounted on the head of a user, motors  11 . 1 . 3 . 2 . 3 - 48  may only be permitted to move optical assemblies  11 . 1 . 3 . 2 . 3 - 20  away from each other and not towards each other. This ensures that surfaces  11 . 1 . 3 . 2 . 3 - 60  will never move towards each other while the user&#39;s nose is present, so that the user&#39;s nose will never be pressed excessively by moving surfaces  11 . 1 . 3 . 2 . 3 - 60 . 
     The operation of device  11 . 1 . 3 . 2 . 3 - 10  in this type of arrangement is illustrated in the flow chart of  FIG.  11 . 1   . 3 . 2 . 3 - 2 . 
     During the operations of block  11 . 1 . 3 . 2 . 3 - 80 , device  11 . 1 . 3 . 2 . 3 - 10  may be powered up. Device  11 . 1 . 3 . 2 . 3 - 10  may, for example, be powered up in response to detection of a user button press on button  11 . 1 . 3 . 2 . 3 - 70 . During power up operations, a power supply supplies power (e.g., a power supply voltage) to the control circuits, sensors, displays, and other components  11 . 1 . 3 . 2 . 3 - 40  of device  11 . 1 . 3 . 2 . 3 - 10 . 
     During the operations of block  11 . 1 . 3 . 2 . 3 - 82 , motors  11 . 1 . 3 . 2 . 3 - 48 , in response to detection of the power-up condition (e.g., in response to detecting the presence of the power supply voltage), may move optical assemblies  11 . 1 . 3 . 2 . 3 - 20  away from each other. Gaze trackers  11 . 1 . 3 . 2 . 3 - 62  may, in response to detection of the power up condition (e.g., in response to the power supply voltage), capture images of the user&#39;s eyes in eye boxes  11 . 1 . 3 . 2 . 3 - 36  and may use this information in determining a target separation between optical assemblies  11 . 1 . 3 . 2 . 3 - 20  (e.g., gaze trackers  11 . 1 . 3 . 2 . 3 - 62  may measure the user&#39;s interpupillary distance and/or other eye characteristics such as the user&#39;s eye relief and may use the user&#39;s measured interpupillary distance and/or other eye characteristics in establishing a target separation for optical assemblies  11 . 1 . 3 . 2 . 3 - 20 ). During the operations of block  11 . 1 . 3 . 2 . 3 - 82 , motors  11 . 1 . 3 . 2 . 3 - 48  may move optical assemblies  11 . 1 . 3 . 2 . 3 - 20  apart until the target separation (target positions) for optical assemblies  11 . 1 . 3 . 2 . 3 - 20  is reached. 
     During the operations of block  11 . 1 . 3 . 2 . 3 - 84 , after motors  11 . 1 . 3 . 2 . 3 - 48  have placed optical assemblies  11 . 1 . 3 . 2 . 3 - 20  into their desired positions, further movement of assemblies  11 . 1 . 3 . 2 . 3 - 20  may be halted and device  11 . 1 . 3 . 2 . 3 - 10  may be used to present images to eye boxes  11 . 1 . 3 . 2 . 3 - 36  for viewing by the user. 
     After the user has finished viewing content with device  11 . 1 . 3 . 2 . 3 - 10 , device  11 . 1 . 3 . 2 . 3 - 10  may be powered down for storage. In an illustrative scenario, a button press on button  11 . 1 . 3 . 2 . 3 - 70  or other input is used to instruct device  11 . 1 . 3 . 2 . 3 - 10  to shut down. Optical assemblies  11 . 1 . 3 . 2 . 3 - 20  may be maintained in their current position while device  11 . 1 . 3 . 2 . 3 - 10  is powered down or can be moved towards each other in preparation for subsequent outward movements (see, e.g., block  11 . 1 . 3 . 2 . 3 - 82 ). When moving optical assemblies  11 . 1 . 3 . 2 . 3 - 20  towards each other (at block  11 . 1 . 3 . 2 . 3 - 86  or another time such as during initial power up operations), a head-presence sensor may be used to detect whether the user&#39;s head is present in region  11 . 1 . 3 . 2 . 3 - 68 . The head-presence sensor may, as an example, be used to confirm that the user&#39;s head is not present whenever optical assemblies  11 . 1 . 3 . 2 . 3 - 82  are being moved towards each other. For example, during the operations of block  11 . 1 . 3 . 2 . 3 - 86 , motors  11 . 1 . 3 . 2 . 3 - 48  may move optical assemblies  11 . 1 . 3 . 2 . 3 - 20  towards each other in response to the detected user power down command (e.g., the button press input on button  11 . 1 . 3 . 2 . 3 - 70 ) provided that no head is being detected by the head-presence sensor. 
     With another illustrative embodiment, which is illustrated in the flow chart of  FIG.  11 . 1   . 3 . 2 . 3 - 3 , motors  11 . 1 . 3 . 2 . 3 - 48  may be configured to only move optical assemblies toward each other when a user actively permits movement of optical assemblies  11 . 1 . 3 . 2 . 3 - 20 . As an example, a user input device such as button  11 . 1 . 3 . 2 . 3 - 70  may be mounted on housing  11 . 1 . 3 . 2 . 3 - 12  (e.g., on an external surface of housing  11 . 1 . 3 . 2 . 3 - 12 M or other external surface of housing  11 . 1 . 3 . 2 . 3 - 12 ). Motors  11 . 1 . 3 . 2 . 3 - 48  may be configured to inhibit movement of assemblies  11 . 1 . 3 . 2 . 3 - 20  (e.g., motors  11 . 1 . 3 . 2 . 3 - 48  may remain stationary) whenever button  11 . 1 . 3 . 2 . 3 - 70  is not being pressed. In response to detecting the pressing and continual holding of button  11 . 1 . 3 . 2 . 3 - 70  by a user, motors  11 . 1 . 3 . 2 . 3 - 48  may move optical assemblies  11 . 1 . 3 . 2 . 3 - 20  to adjust the spacing between assemblies  11 . 1 . 3 . 2 . 3 - 20 . Motors  11 . 1 . 3 . 2 . 3 - 48  may, for example, move optical assemblies  11 . 1 . 3 . 2 . 3 - 20  from an initial configuration in which optical assemblies  11 . 1 . 3 . 2 . 3 - 20  are spaced apart by their maximum spacing or other wide spacing to a target configuration in which optical assemblies  11 . 1 . 3 . 2 . 3 - 20  are separated by a distance equal to or nearly equal to the measured interpupillary distance associated with the user&#39;s eyes (e.g., assemblies  11 . 1 . 3 . 2 . 3 - 20  may be moved toward their target positions as measured by gaze trackers  11 . 1 . 3 . 2 . 3 - 62 ). 
     As shown in  FIG.  11 . 1   . 3 . 2 . 3 - 3 , device  11 . 1 . 3 . 2 . 3 - 10  may be powered up during the operations of block  11 . 1 . 3 . 2 . 3 - 90 . Following power-up operations, when device  11 . 1 . 3 . 2 . 3 - 10  is being worn on the head of the user, gaze trackers  11 . 1 . 3 . 2 . 3 - 62  may optionally measure the user&#39;s interpupillary distance and/or other eye characteristics of the user&#39;s eyes. These measurements may establish a desired separation (e.g., target positions) for optical assemblies  11 . 1 . 3 . 2 . 3 - 20 . 
     During the operations of block  11 . 1 . 3 . 2 . 3 - 92 , a user has an opportunity to press button  11 . 1 . 3 . 2 . 3 - 70 . When no button press input is detected, motors  11 . 1 . 3 . 2 . 3 - 48  remain stationary, so that optical assemblies  11 . 1 . 3 . 2 . 3 - 20  do not move. When button press input is detected, motors  11 . 1 . 3 . 2 . 3 - 48  are allowed to move to adjust the separation between assemblies  11 . 1 . 3 . 2 . 3 - 20 . As an example, motors  11 . 1 . 3 . 2 . 3 - 48  may move assemblies  11 . 1 . 3 . 2 . 3 - 20  inwardly towards their target positions. During movement of assemblies  11 . 1 . 3 . 2 . 3 - 20 , motors  11 . 1 . 3 . 2 . 3 - 48  may be halted in response to any detected user release of button  11 . 1 . 3 . 2 . 3 - 70 . In this way, the positions of assemblies  11 . 1 . 3 . 2 . 3 - 20  will be adjusted so long as button  11 . 1 . 3 . 2 . 3 - 70  is being pressed, but will stop in response to release of button  11 . 1 . 3 . 2 . 3 - 70  (e.g., when the user desires to prevent movement of assemblies  11 . 1 . 3 . 2 . 3 - 20  that could press against nose surfaces  11 . 1 . 3 . 2 . 3 - 62 ). The user in this scenario remains in continuous control of assemblies  11 . 1 . 3 . 2 . 3 - 20 . 
     Once the desired positions of assemblies  11 . 1 . 3 . 2 . 3 - 20  have been reached, device  11 . 1 . 3 . 2 . 3 - 10  may be used to view images while being worn by the user. Motors  11 . 1 . 3 . 2 . 3 - 48  may stop automatically when the target positions measured by gaze trackers  11 . 1 . 3 . 2 . 3 - 62  are reached or motors  11 . 1 . 3 . 2 . 3 - 48  may stop when the user releases button  11 . 1 . 3 . 2 . 3 - 70 . If desired, the direction of movement of assemblies  11 . 1 . 3 . 2 . 3 - 20  may be controlled by providing device  11 . 1 . 3 . 2 . 3 - 10  with two of buttons  11 . 1 . 3 . 2 . 3 - 70  (e.g., an inward movement button and an outward movement button) or by providing a first button  11 . 1 . 3 . 2 . 3 - 70  to control movement (e.g., a go/stop button) and a second button (e.g., a slider with two positions) that is used to choose between inward and outward movement settings. Arrangements in which device  11 . 1 . 3 . 2 . 3 - 10  has non-button user input devices such as microphones for gather voice commands, touch screen displays, and/or other user input devices may also be used in controlling movement of motors  11 . 1 . 3 . 2 . 3 - 48 . 
     Clutches may be used to limit the amount of inward force that is applied by optical assemblies  11 . 1 . 3 . 2 . 3 - 20  when assemblies  11 . 1 . 3 . 2 . 3 - 20  are moved towards nose surfaces  11 . 1 . 3 . 2 . 3 - 61  by motors  11 . 1 . 3 . 2 . 3 - 48 . 
     In the example of  FIG.  11 . 1   . 3 . 2 . 3 - 4 , a split nut arrangement is used for nut  11 . 1 . 3 . 2 . 3 - 46 , forming a mechanical clutch (sometimes referred to as a force limiter). As shown in  FIG.  11 . 1   . 3 . 2 . 3 - 4 , nut  11 . 1 . 3 . 2 . 3 - 46  may be moved (parallel to the X axis) by rotating threaded shaft  11 . 1 . 3 . 2 . 3 - 44 . Nut  11 . 1 . 3 . 2 . 3 - 46  may have a split such as split  11 . 1 . 3 . 2 . 3 - 91 . When excess force is experienced (e.g., when assembly  11 . 1 . 3 . 2 . 3 - 20  starts to press against nose surface  11 . 1 . 3 . 2 . 3 - 61 ), nut  11 . 1 . 3 . 2 . 3 - 46  will be forced apart in directions  11 . 1 . 3 . 2 . 3 - 93  and will slide over the threads on shaft  11 . 1 . 3 . 2 . 3 - 44  rather than being driven inwardly by the interaction between the threads on shaft  11 . 1 . 3 . 2 . 3 - 44  and the corresponding threads in nut  11 . 1 . 3 . 2 . 3 - 46 . In this way, excess force is released and the clutch provided by split  11 . 1 . 3 . 2 . 3 - 91  helps prevent more than a desired amount of force from being applied to the user&#39;s nose. The split nut configuration of  FIG.  11 . 1   . 3 . 2 . 3 - 4  therefore serves as a release mechanism that limits pressure from assembly  11 . 1 . 3 . 2 . 3 - 20  on nose surface  11 . 1 . 3 . 2 . 3 - 61 . 
     Illustrative magnetic clutches are shown in  FIGS.  11 . 1   . 3 . 2 . 3 - 5  and  11 . 1 . 3 . 2 . 3 - 6 . In the magnetic clutch arrangement of  FIG.  11 . 1   . 3 . 2 . 3 - 5 , a magnet  11 . 1 . 3 . 2 . 3 - 95  is attached to nut  11 . 1 . 3 . 2 . 3 - 46  and a magnet  11 . 1 . 3 . 2 . 3 - 96 , which is attracted to magnet  11 . 1 . 3 . 2 . 3 - 95 , is attached to support  11 . 1 . 3 . 2 . 3 - 38 . Optional structures  11 . 1 . 3 . 2 . 3 - 98  (e.g., non-magnetic coatings and/or protruding structures that establish an air gap between magnets  11 . 1 . 3 . 2 . 3 - 95  and  11 . 1 . 3 . 2 . 3 - 96 ) may be used to adjust the holding force established by the attraction between magnets  11 . 1 . 3 . 2 . 3 - 95  and  11 . 1 . 3 . 2 . 3 - 96 . During normal operations, magnets  11 . 1 . 3 . 2 . 3 - 95  and  11 . 1 . 3 . 2 . 3 - 96  are magnetically coupled to each other, so that optical assembly support  11 . 1 . 3 . 2 . 3 - 38  is moved back and forth along the length of shaft  11 . 1 . 3 . 2 . 3 - 44  (e.g., parallel to the X axis in the example of  FIG.  11 . 1   . 3 . 2 . 3 - 5 ), thereby adjusting the spacing between assemblies  11 . 1 . 3 . 2 . 3 - 20 . If, however, inward movement of assemblies  11 . 1 . 3 . 2 . 3 - 20  is resisted due to contact between surfaces  11 . 1 . 3 . 2 . 3 - 60  and nose surfaces  11 . 1 . 3 . 2 . 3 - 61 , the holding force of the magnetic clutch will be exceeded, magnets  11 . 1 . 3 . 2 . 3 - 95  and  11 . 1 . 3 . 2 . 3 - 96  will separate, and this decoupling of magnets  11 . 1 . 3 . 2 . 3 - 95  and  11 . 1 . 3 . 2 . 3 - 96  will prevent further inward movement of assemblies  11 . 1 . 3 . 2 . 3 - 20 . 
     In the example of  FIG.  11 . 1   . 3 . 2 . 3 - 6 , two rows of magnets are provided that are configured so that their exposed poles oppose each other. With this type of arrangement, the poles of magnets  11 . 1 . 3 . 2 . 3 - 95 ′ attract the poles of magnets  11 . 1 . 3 . 2 . 3 - 96 ′, so that during normal operations, nut  11 . 1 . 3 . 2 . 3 - 46  is magnetically coupled to support  11 . 1 . 3 . 2 . 3 - 38  and is able to move support  11 . 1 . 3 . 2 . 3 - 38  and thereby adjust the position of the optical assembly associated with support  11 . 1 . 3 . 2 . 3 - 38 . In the event that surface  11 . 1 . 3 . 2 . 3 - 60  of support  11 . 1 . 3 . 2 . 3 - 38  presses against nose surface  11 . 1 . 3 . 2 . 3 - 61 , the magnetic holding ability of the magnetic clutch of  FIG.  11 . 1   . 3 . 2 . 3 - 6  will be exceeded, magnets  11 . 1 . 3 . 2 . 3 - 95 ′ and  11 . 1 . 3 . 2 . 3 - 96 ′ will decouple and will slide past each other, and additional inward movement of assemblies  11 . 1 . 3 . 2 . 3 - 20  towards the nose of the user will be prevented. 
       FIG.  11 . 1   . 3 . 2 . 3 - 7  is a diagram showing how interlocking physical features may be used to implement a mechanical version of the magnetic clutch of  FIG.  11 . 1   . 3 . 2 . 3 - 6 . In the example of  FIG.  11 . 1   . 3 . 2 . 3 - 7 , pin  11 . 1 . 3 . 2 . 3 - 100  is coupled to nut  11 . 1 . 3 . 2 . 3 - 46  and protrusions  11 . 1 . 3 . 2 . 3 - 102  (sometimes referred to as snap features, locking structures, or a releasable pin lock), which are coupled to support  11 . 1 . 3 . 2 . 3 - 38 . During normal operations, pin  11 . 1 . 3 . 2 . 3 - 100  will not bend and will transfer force along the X direction from nut  11 . 1 . 3 . 2 . 3 - 46  to support  11 . 1 . 3 . 2 . 3 - 38  to move assembly  11 . 1 . 3 . 2 . 3 - 20 . If excess force is generated (e.g., when assembly  11 . 1 . 3 . 2 . 3 - 20  contacts nose surface  11 . 1 . 3 . 2 . 3 - 61 ), pin  11 . 1 . 3 . 2 . 3 - 100  will bend and slip past protrusions  11 . 1 . 3 . 2 . 3 - 102  in the pin lock, thereby decoupling nut  11 . 1 . 3 . 2 . 3 - 46  from support  11 . 1 . 3 . 2 . 3 - 38  and preventing further movement of assemblies  11 . 1 . 3 . 2 . 3 - 20  towards the user&#39;s nose. 
       FIGS.  11 . 1   . 3 . 2 . 3 - 8 ,  11 . 1 . 3 . 2 . 3 - 9 , and  11 . 1 . 3 . 2 . 3 - 10  show illustrative mechanical clutch designs that may be used to implement mechanical version of the magnetic clutch of  FIG.  11 . 1   . 3 . 2 . 3 - 5 . In the arrangement of  FIG.  11 . 1   . 3 . 2 . 3 - 8 , pin  11 . 1 . 3 . 2 . 3 - 108  is coupled to nut  11 . 1 . 3 . 2 . 3 - 46  and interlocks with a releasable pin lock formed from snap  11 . 1 . 3 . 2 . 3 - 110  on support  11 . 1 . 3 . 2 . 3 - 38 . When excess force is applied, snap  11 . 1 . 3 . 2 . 3 - 110  deforms and pin  11 . 1 . 3 . 2 . 3 - 108  escapes from snap  11 . 1 . 3 . 2 . 3 - 110  to release the clutch. In the arrangement of  FIG.  11 . 1   . 3 . 2 . 3 - 9 , the downward protrusion of snap  11 . 1 . 3 . 2 . 3 - 110  of  FIG.  11 . 1   . 3 . 2 . 3 - 8  has been replaced with protrusion  11 . 1 . 3 . 2 . 3 - 112  to form a releasable pin lock. In the example of  FIG.  11 . 1   . 3 . 2 . 3 - 10 , a releasable pin lock is formed from spring  11 . 1 . 3 . 2 . 3 - 114  and movable ball  11 . 1 . 3 . 2 . 3 - 116  instead of protrusion  11 . 1 . 3 . 2 . 3 - 112  of  FIG.  11 . 1   . 3 . 2 . 3 - 9 . 
     As shown in  FIG.  11 . 1   . 3 . 2 . 3 - 11 , force-sensitive components (sometimes referred to as switches or force sensors) may be used in detecting when more than a desired amount of force is being applied by nut  11 . 1 . 3 . 2 . 3 - 46  to support  11 . 1 . 3 . 2 . 3 - 38 . Motors  11 . 1 . 3 . 2 . 3 - 48  are configured to deactivate in response to detecting this high amount of force (e.g., a force exceeding a predetermined threshold), thereby preventing excess nose pressure from assemblies  11 . 1 . 3 . 2 . 3 - 20 . In the illustrative arrangement of  FIG.  11 . 1   . 3 . 2 . 3 - 11 , a protruding structure such as pin  11 . 1 . 3 . 2 . 3 - 122  is attached to nut  11 . 1 . 3 . 2 . 3 - 46 . Pin  11 . 1 . 3 . 2 . 3 - 122  extends between first and second switches  11 . 1 . 3 . 2 . 3 - 120  (or other force sensors such as strain gauges, etc.). Each switch  11 . 1 . 3 . 2 . 3 - 120  in the example of  FIG.  11 . 1   . 3 . 2 . 3 - 11  has a stationary switch body  11 . 1 . 3 . 2 . 3 - 124  within which a movable plunger  11 . 1 . 3 . 2 . 3 - 126  is mounted. As nut  11 . 1 . 3 . 2 . 3 - 46  is moved back and forth along the X axis by rotation of shaft  11 . 1 . 3 . 2 . 3 - 44  from motors  11 . 1 . 3 . 2 . 3 - 48 , pin  11 . 1 . 3 . 2 . 3 - 122  pushes against plungers  11 . 1 . 3 . 2 . 3 - 126  (e.g., the plunger in the right-hand switch  11 . 1 . 3 . 2 . 3 - 120  when pushing support  11 . 1 . 3 . 2 . 3 - 38  to the right and the plunger in the left-hand switch  11 . 1 . 3 . 2 . 3 - 120  when pushing support  11 . 1 . 3 . 2 . 3 - 38  to the left). Plungers  11 . 1 . 3 . 2 . 3 - 126  are not pressed significantly into bodies  11 . 1 . 3 . 2 . 3 - 124  so long as a threshold amount of force on the plungers  11 . 1 . 3 . 2 . 3 - 126  is not exceeded. The interaction between pin  11 . 1 . 3 . 2 . 3 - 122  and switches  11 . 1 . 3 . 2 . 3 - 120  therefore allows lateral force to be transferred from nut  11 . 1 . 3 . 2 . 3 - 46  to support  11 . 1 . 3 . 2 . 3 - 38  to slide support  11 . 1 . 3 . 2 . 3 - 38  along rails  11 . 1 . 3 . 2 . 3 - 22  parallel to the X axis. If, however, assembly  11 . 1 . 3 . 2 . 3 - 20  contacts nose surface  11 . 1 . 3 . 2 . 3 - 61 , support  11 . 1 . 3 . 2 . 3 - 38  will resist further movement and pin  11 . 1 . 3 . 2 . 3 - 122  will push against the plunger  11 . 1 . 3 . 2 . 3 - 126  that is in contact with pin  11 . 1 . 3 . 2 . 3 - 122  with more than the threshold amount of force. In response to detecting a change in the state of switch  11 . 1 . 3 . 2 . 3 - 120  due to this elevated force, motors  11 . 1 . 3 . 2 . 3 - 48  are e halted, thereby preventing more than a desired amount of force from being applied to nose surface  11 . 1 . 3 . 2 . 3 - 61 . 
     In the example of  FIG.  11 . 1   . 3 . 2 . 3 - 12 , a torque-sensitive switch mechanism (switch  11 . 1 . 3 . 2 . 3 - 130 , sometimes referred to as a torque sensor or torque-sensitive switch) is used to couple rotating shaft portion  11 . 1 . 3 . 2 . 3 - 44 - 2  of shaft  11 . 1 . 3 . 2 . 3 - 44  to rotating shaft portion  11 . 1 . 3 . 2 . 3 - 44 - 1  of shaft  11 . 1 . 3 . 2 . 3 - 44 . Nut  11 . 1 . 3 . 2 . 3 - 46  may be coupled to portion  11 . 1 . 3 . 2 . 3 - 44 - 1  and may be used to move support  11 . 1 . 3 . 2 . 3 - 38  parallel to shaft  11 . 1 . 3 . 2 . 3 - 44 . Motor  11 . 1 . 3 . 2 . 3 - 48  may rotate shaft portion  11 . 1 . 3 . 2 . 3 - 44 - 2  to move nut  11 . 1 . 3 . 2 . 3 - 46 . Switch  11 . 1 . 3 . 2 . 3 - 130  may have a first switch portion  11 . 1 . 3 . 2 . 3 - 130 - 1  and a second switch portion  11 . 1 . 3 . 2 . 3 - 130 - 2 . When less than a threshold amount of torque is applied to portion  11 . 1 . 3 . 2 . 3 - 130 - 2  by shaft portion  11 . 1 . 3 . 2 . 3 - 44 - 2 , portion  11 . 1 . 3 . 2 . 3 - 130 - 2  transfers this applied torque to portion  11 . 1 . 3 . 2 . 3 - 130 - 1 , which, in turn, transfers this applied torque to portion  11 . 1 . 3 . 2 . 3 - 44 - 1 . The rotation of portion  11 . 1 . 3 . 2 . 3 - 44 - 1  moves nut  11 . 1 . 3 . 2 . 3 - 46  laterally to adjust the lateral position of support  11 . 1 . 3 . 2 . 3 - 38  and assembly  11 . 1 . 3 . 2 . 3 - 20 . In the event that support  11 . 1 . 3 . 2 . 3 - 38  contacts nose surface  11 . 1 . 3 . 2 . 3 - 61 , support  11 . 1 . 3 . 2 . 3 - 38  will resist further lateral motion by nut  11 . 1 . 3 . 2 . 3 - 46 . This will create a rise in torque that is detected by switch  11 . 1 . 3 . 2 . 3 - 130 . In response, motor  11 . 1 . 3 . 2 . 3 - 48  halts its motion. By stopping motors  11 . 1 . 3 . 2 . 3 - 48  in response to detecting an amount of torque exceeding a predetermined torque threshold, motors  11 . 1 . 3 . 2 . 3 - 48  can be prevented from applying more force than desired to nose surfaces  11 . 1 . 3 . 2 . 3 - 61 . 
     If desired, the load on motors  11 . 1 . 3 . 2 . 3 - 48  can be monitored electronically, so that motors  11 . 1 . 3 . 2 . 3 - 48  can be halted if more than a desired motor load is encountered. An illustrative motor control circuit is shown in  FIG.  11 . 1   . 3 . 2 . 3 - 13 . As shown in  FIG.  11 . 1   . 3 . 2 . 3 - 13 , motor  11 . 1 . 3 . 2 . 3 - 48  may be driven using a drive circuit such as driver  11 . 1 . 3 . 2 . 3 - 140 . Driver  11 . 1 . 3 . 2 . 3 - 140  may apply any suitable drive signal to motor  11 . 1 . 3 . 2 . 3 - 48  (e.g., a sinusoidal motor drive current, etc.). Current sensor  11 . 1 . 3 . 2 . 3 - 144  may measure the current Imeas that flows through motor  11 . 1 . 3 . 2 . 3 - 48 . Voltage sensor  11 . 1 . 3 . 2 . 3 - 142  may measure the voltage Vmeas across the terminals of motor  11 . 1 . 3 . 2 . 3 - 48 . The counterelectromotive force (“back EMF”) that is produced when operating motor  11 . 1 . 3 . 2 . 3 - 48  is equal to (Imeas*Rm−Vmeas), where Rm is the known motor resistance associated with motor  11 . 1 . 3 . 2 . 3 - 48 . During operation of motor  11 . 1 . 3 . 2 . 3 - 48 , back EMF may be calculated in real time and the phase difference between Imeas and the back EMF may be monitored to determine the motor load being experienced by motor  11 . 1 . 3 . 2 . 3 - 48  (e.g., motor  11 . 1 . 3 . 2 . 3 - 48  may be determined to be unloaded when the measured phase difference is 90°, may be determined to be fully loaded when the measured phase difference is 0°, and may be determined to be experiencing an intermediate amount of load when the measured phase difference is between 0° and 90°). Motors  11 . 1 . 3 . 2 . 3 - 48  may be configured to halt operation in response to a determination that the motor load has exceeded a desired amount (e.g., a determination that the measured phase difference between Imeas and back EMF is less than a predetermined threshold phase shift value such as less than 85° or less than 40°). 
     If desired, motor  11 . 1 . 3 . 2 . 3 - 48  may be supplied with a rotatory encoder, as shown in the example of  FIG.  11 . 1   . 3 . 2 . 3 - 14 . In this example, magnet  11 . 1 . 3 . 2 . 3 - 150  is attached to shaft  11 . 1 . 3 . 2 . 3 - 44  and magnetic sensor  11 . 1 . 3 . 2 . 3 - 152  is used to monitor the magnetic field produced by magnet  11 . 1 . 3 . 2 . 3 - 150 . Sensor  11 . 1 . 3 . 2 . 3 - 152  and magnet  11 . 1 . 3 . 2 . 3 - 152  therefore serve as an encoder that measures the rotation of shaft  11 . 1 . 3 . 2 . 3 - 44  and therefore the rotation of the rotor in motor  11 . 1 . 3 . 2 . 3 - 48 . With this arrangement, the phase shift between the angle of the rotor and the drive current (e.g., Imeas) may be used as a measure of motor load or (in an arrangement in which motor  11 . 1 . 3 . 2 . 3 - 48  is being controlled by establishing a phase of 0° between rotor angle and drive current) motor load may be determined by monitoring Imeas and comparing Imeas to a threshold (as examples). When motor load is not elevated (e.g., below a predetermined threshold), motors  11 . 1 . 3 . 2 . 3 - 48  may rotate normally to move nut  11 . 1 . 3 . 2 . 3 - 46  along shaft  11 . 1 . 3 . 2 . 3 - 44  and thereby move optical assemblies  11 . 1 . 3 . 2 . 3 - 20 . When optical assemblies  11 . 1 . 3 . 2 . 3 - 20  contact nose surfaces  11 . 1 . 3 . 2 . 3 - 61 , motor load will increase beyond a threshold amount. In response, motors  11 . 1 . 3 . 2 . 3 - 48  may be halted, thereby preventing excess pressure on nose surfaces  11 . 1 . 3 . 2 . 3 - 61 . 
     Another illustrative technique for electronically monitoring motor load involves the use of a linear encoder of the type shown in  FIG.  11 . 1   . 3 . 2 . 3 - 15 . As shown in  FIG.  11 . 1   . 3 . 2 . 3 - 15 , encoder  11 . 1 . 3 . 2 . 3 - 156  may include a strip of magnets  11 . 1 . 3 . 2 . 3 - 158  and a corresponding magnetic sensor  11 . 1 . 3 . 2 . 3 - 160 . Sensor  11 . 1 . 3 . 2 . 3 - 160  may be attached to nut  11 . 1 . 3 . 2 . 3 - 46 . Magnets  11 . 1 . 3 . 2 . 3 - 158  may be attached to a stationary support surface (e.g., a portion of housing  11 . 1 . 3 . 2 . 3 - 12 ). When motor  11 . 1 . 3 . 2 . 3 - 48  rotates shaft  11 . 1 . 3 . 2 . 3 - 44 , nut  11 . 1 . 3 . 2 . 3 - 46  is moved parallel to shaft  11 . 1 . 3 . 2 . 3 - 44 . This causes sensor  11 . 1 . 3 . 2 . 3 - 160  to move past magnets  11 . 1 . 3 . 2 . 3 - 158  while measuring the magnetic fields from magnets  11 . 1 . 3 . 2 . 3 - 158 . By monitoring the measured changes in magnetic field strength due to movement of sensor  11 . 1 . 3 . 2 . 3 - 160  past magnets  11 . 1 . 3 . 2 . 3 - 158 , motors  11 . 1 . 3 . 2 . 3 - 48  can determine the speed of nut  11 . 1 . 3 . 2 . 3 - 46  and can determine when nut  11 . 1 . 3 . 2 . 3 - 46  has stopped and motors  11 . 1 . 3 . 2 . 3 - 48  have stalled (e.g., when applied current to motors  11 . 1 . 3 . 2 . 3 - 48  is failing to cause lateral motion in nuts  11 . 1 . 3 . 2 . 3 - 46 ). 
     The graph of  FIG.  11 . 1   . 3 . 2 . 3 - 16  illustrates how stall detection may be used in measuring the force experienced by assemblies  11 . 1 . 3 . 2 . 3 - 20  when motors  11 . 1 . 3 . 2 . 3 - 48  contact nose surfaces  11 . 1 . 3 . 2 . 3 - 61 . Solid line  11 . 1 . 3 . 2 . 3 - 160  represent an increasing amount of force F of the type that may be experienced by optical assemblies  11 . 1 . 3 . 2 . 3 - 20  (e.g., support  11 . 1 . 3 . 2 . 3 - 38  and nut  11 . 1 . 3 . 2 . 3 - 46 ) as optical assemblies  11 . 1 . 3 . 2 . 3 - 20  come into contact with nose surfaces  11 . 1 . 3 . 2 . 3 - 61  and begin to feel resistance (back pressure) from nose surfaces  11 . 1 . 3 . 2 . 3 - 61 . Current Im may be applied to motors  11 . 1 . 3 . 2 . 3 - 48  by driver  11 . 1 . 3 . 2 . 3 - 140  ( FIG.  11 . 1   . 3 . 2 . 3 - 13 ) in a pattern that follows dashed line  11 . 1 . 3 . 2 . 3 - 162 , which represents the amount of resulting motor force produced by current Im. 
     In the example of  FIG.  11 . 1   . 3 . 2 . 3 - 16 , motor  11 . 1 . 3 . 2 . 3 - 48  periodically stalls at stall points  11 . 1 . 3 . 2 . 3 - 170 . This is because the amount of forward force produced by the current Im applied to the motor is equaled by the back pressure (reverse force F) produced as nose surface  11 . 1 . 3 . 2 . 3 - 61  resists further forward motion by optical assembly  11 . 1 . 3 . 2 . 3 - 20 . By using an encoder such as encoder  11 . 1 . 3 . 2 . 3 - 156  of  FIG.  11 . 1   . 3 . 2 . 3 - 15 , the motion of nut  11 . 1 . 3 . 2 . 3 - 46  and therefore the movement of support  11 . 1 . 3 . 2 . 3 - 38  and optical assembly  11 . 1 . 3 . 2 . 3 - 20  can be monitored. Whenever no motion is detected in response to a particular value of applied current Im, it can be concluded that optical assembly  11 . 1 . 3 . 2 . 3 - 20  has halted (e.g., motor  11 . 1 . 3 . 2 . 3 - 48  has stalled at one of stall points  11 . 1 . 3 . 2 . 3 - 170 ). Motor  11 . 1 . 3 . 2 . 3 - 48  may be a stepper motor that is controlled by application of sets of pulses (e.g.,  10  pulses at a time). Following detection of a stall (e.g., when it is detected that motor  11 . 1 . 3 . 2 . 3 - 48  moved assembly  11 . 1 . 3 . 2 . 3 - 20  less than expected after a given set of current pulses was applied), the magnitude of applied current Im may, as shown in  FIG.  11 . 1   . 3 . 2 . 3 - 16 , initially increase (to ensure further movement) and then decrease until the force being produced by motor  11 . 1 . 3 . 2 . 3 - 48  matches the reverse force due to nose surface contact. Each time a stall point  11 . 1 . 3 . 2 . 3 - 170  is reached in this way, the amount of current Im may be evaluated. With this approach, the magnitude of the current applied to motor  11 . 1 . 3 . 2 . 3 - 48  closely tracks the increase in force F due to nose contact so that the magnitude of the current can be used as a measure of pressure on nose surface  11 . 1 . 3 . 2 . 3 - 61 . Motors  11 . 1 . 3 . 2 . 3 - 48  may be configured to halt (e.g., no more current will be applied) in response to the value of Im exceeding a predetermined threshold value. 
     The flow chart of  FIG.  11 . 1   . 3 . 2 . 3 - 17  shows illustrative operations that may be involved in operating device  11 . 1 . 3 . 2 . 3 - 10  in an arrangement in which electrical monitoring of motor load is used to determine when to halt motor operation and thereby prevent more than a desired amount of force being applied by optical assemblies  11 . 1 . 3 . 2 . 3 - 20  on nose surfaces  11 . 1 . 3 . 2 . 3 - 61 . 
     During the operations of block  11 . 1 . 3 . 2 . 3 - 200 , device  11 . 1 . 3 . 2 . 3 - 10  may be powered up (e.g., in response to a detected button press or other activity). 
     Once device  11 . 1 . 3 . 2 . 3 - 10  has powered up, gaze trackers  11 . 1 . 3 . 2 . 3 - 62  may measure the separation between the user&#39;s eyes (user interpupillary distance) and other eye characteristics to determine target positions for optical assemblies  11 . 1 . 3 . 2 . 3 - 20 . Motors  11 . 1 . 3 . 2 . 3 - 48  may then rotates shafts  11 . 1 . 3 . 2 . 3 - 44  to move optical assemblies  11 . 1 . 3 . 2 . 3 - 20  towards the target positions (e.g., by moving assemblies  11 . 1 . 3 . 2 . 3 - 20  towards nose bridge portion NB of housing  11 . 1 . 3 . 2 . 3 - 12 ). During motor operation, motor load may be electrically monitored (e.g., using back EMF measurements, using encoder output, using measurements of applied current, and/or using other measurements of the types described in connection with  FIGS.  11 . 1   . 3 . 2 . 3 - 13 ,  11 . 1 . 3 . 2 . 3 - 14 ,  11 . 1 . 3 . 2 . 3 - 15 , and  11 . 1 . 3 . 2 . 3 - 16 ). 
     If optical assemblies  11 . 1 . 3 . 2 . 3 - 20  contact nose surfaces  11 . 1 . 3 . 2 . 3 - 61 , nose surfaces  11 . 1 . 3 . 2 . 3 - 61  will produce a force against optical assemblies  11 . 1 . 3 . 2 . 3 - 20  that tends to resist further movement. Motors  11 . 1 . 3 . 2 . 3 - 48  are configured to halt operation in response to detection of more than a desired amount of motor load (see, e.g., block  11 . 1 . 3 . 2 . 3 - 204 ). At this point, device  11 . 1 . 3 . 2 . 3 - 10  may be operated normally and used in presenting images to the user&#39;s eyes in eye boxes  11 . 1 . 3 . 2 . 3 - 36 . 
     Following use of device  11 . 1 . 3 . 2 . 3 - 10 , device  11 . 1 . 3 . 2 . 3 - 10  can be powered down (see, e.g., the operations of block  11 . 1 . 3 . 2 . 3 - 206 ). Device  11 . 1 . 3 . 2 . 3 - 10  may, as an example, be powered down in response to detection of a user button press or other activities. 
     11.1.3.3: Hard Stops 
       FIG.  11 . 1   . 3 . 3 - 1  illustrates a portion of an example of an HMD  11 . 1 . 3 . 3 - 100  including a frame  11 . 1 . 3 . 3 - 102  and an optical module  11 . 1 . 3 . 3 - 104  adjustably connected to the frame  11 . 1 . 3 . 3 - 102 . The HMD  11 . 1 . 3 . 3 - 100  can also include a stop bracket  11 . 1 . 3 . 3 - 106  coupled to the frame adjacent the optical module  11 . 1 . 3 . 3 - 104 . In at least one example, the stop bracket  11 . 1 . 3 . 3 - 106  can include a first hard-stop feature  11 . 1 . 3 . 3 - 108   a  and a second hard stop feature  11 . 1 . 3 . 3 - 108   b  extending/protruding therefrom to form a stop surface against which the optical module  11 . 1 . 3 . 3 - 104  can contact in cases where the optical module  11 . 1 . 3 . 3 - 104  travels or is adjusted too far laterally during IPD adjustments described elsewhere herein. 
     In particular, the optical module  11 . 1 . 3 . 3 - 104  can include a housing  11 . 1 . 3 . 3 - 110  configured to contact the stop bracket  11 . 1 . 3 . 3 - 106 , or the tabs  11 . 1 . 3 . 3 - 108   a - b  thereof, in a drop event or, for example, when a user adjusts or causes the optical module  11 . 1 . 3 . 3 - 104  to move too far laterally during the IPD adjustment. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  11 . 1   . 3 . 3 - 1  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in any other figures shown and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to any other figure shown and described herein, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  11 . 1   . 3 . 3 - 1 . 
       FIG.  11 . 1   . 3 . 3 - 2  illustrates another view of an example of an HMD  11 . 1 . 3 . 3 - 200  including an optical module  11 . 1 . 3 . 3 - 204  having a housing  11 . 1 . 3 . 3 - 210  and a stop bracket  11 . 1 . 3 . 3 - 206 , including a tab  11 . 1 . 3 . 3 - 208 , secured to a frame  11 . 1 . 3 . 3 - 202 . The HMD  11 . 1 . 3 . 3 - 200  can also include an adjustment mechanism  11 . 1 . 3 . 3 - 203  for the optical module  11 . 1 . 3 . 3 - 204  of an HMD  11 . 1 . 3 . 3 - 200 . In at least one example, the adjustment mechanism  11 . 1 . 3 . 3 - 203  also includes a stop  11 . 1 . 3 . 3 - 218  positioned at or near a distal end of a guide-rod  11 . 1 . 3 . 3 - 212 . The guide rod  11 . 1 . 3 . 3 - 212  can be disposed at least partially within a follower  11 . 1 . 3 . 3 - 214  of the optical module  11 . 1 . 3 . 3 - 204  such that the optical module  11 . 1 . 3 . 3 - 204  can be translated laterally along the guide rod  11 . 1 . 3 . 3 - 212  during adjustment for user IPD. 
     In at least one example, the stop  11 . 1 . 3 . 3 - 218  can include a cavity or channel in which the distal end of the guide-rod  11 . 1 . 3 . 3 - 212  extends. However, in at least one example, no contact is made between the guide-rod  11 . 1 . 3 . 3 - 212  and the stop  11 . 1 . 3 . 3 - 218  such that the guide-rod  11 . 1 . 3 . 3 - 212  is cantilevered. The stop  11 . 1 . 3 . 3 - 218  can be configured to prevent the optical module  11 . 1 . 3 . 3 - 204  from travelling too far distally along the guide-rod  11 . 1 . 3 . 3 - 212  and beyond the distal end thereof during IPD adjustment. The stop  11 . 1 . 3 . 3 - 218  can also serve to prevent too much deformation of the cantilevered guide-rod  11 . 1 . 3 . 3 - 212  during unintended drop events or other forces that would cause a deformation of the guide-rod  11 . 1 . 3 . 3 - 212  and a movement of the distal end of the guide-rod  11 . 1 . 3 . 3 - 212 . In at least one example, the distal end of the guide-rod  11 . 1 . 3 . 3 - 212  extends into or is a least partially surrounded by the stop  11 . 1 . 3 . 3 - 218  without making contact during normal use of the HMD  11 . 1 . 3 . 3 - 200 . If unintentionally moved or deformed elastically, the distal end or portion of the guide-rod  11 . 1 . 3 . 3 - 212  can make contact with the stop  11 . 1 . 3 . 3 - 218  such that the stop  11 . 1 . 3 . 3 - 218  prevents too much deformation of the guide-rod  11 . 1 . 3 . 3 - 212 , thus preventing plastic deformation and damage to the guide-rod  11 . 1 . 3 . 3 - 212 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  11 . 1   . 3 . 3 - 2  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in any other figures shown and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to any other figure shown and described herein, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  11 . 1   . 3 . 3 - 2 . 
     11.1.3.4: Upper Biasing Members 
       FIG.  11 . 1   . 3 . 4 - 1  illustrates an example of an adjustment mechanism  11 . 1 . 3 . 4 - 102  of an HMD  11 . 1 . 3 . 3 - 100  including a follower  11 . 1 . 3 . 3 - 104  of an optical module slidably engaged with a guide-rod  11 . 1 . 3 . 4 - 106 . The adjustment mechanism  11 . 1 . 3 . 4 - 102  can also include an encoder  11 . 1 . 3 . 4 - 110  for position sensing as the optical module is adjusted/moved along the guide-rod  11 . 1 . 3 . 4 - 106  for IPD adjustments. 
     In at least one example, the adjustment mechanism  11 . 1 . 3 . 4 - 102  can also include a biasing element  11 . 1 . 3 . 4 - 108  biasing the guide-rod  11 . 1 . 3 . 4 - 106  within the follower  11 . 1 . 3 . 4 - 104  to reduce slack and play between the follower  11 . 1 . 3 . 3 - 104  and the guide-rod  11 . 1 . 3 . 4 - 106 , either during adjustments or when stationary. In at least one example, the biasing element  11 . 1 . 3 . 4 - 108  includes an elongate spring finger fixed to the follower  11 . 1 . 3 . 3 - 104  or other optical module component at fixation points  11 . 1 . 3 . 4 - 112  and a distal end  11 . 1 . 3 . 4 - 114  biased toward the guide-rod  11 . 1 . 3 . 3 - 106  and extending through an aperture  11 . 1 . 3 . 4 - 116  of the follower  11 . 1 . 3 . 4 - 104  to contact the guide-rod  11 . 1 . 3 . 4 - 106  through the aperture  11 . 1 . 3 . 4 - 116 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  11 . 1   . 3 . 4 - 1  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in any other figures shown and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to any other figure shown and described herein, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  11 . 1   . 3 . 4 - 1 . 
       FIG.  11 . 1   . 3 . 4 - 2  illustrates a portion of another example of an adjustment mechanism  11 . 1 . 3 . 4 - 202  including a guide-rod  11 . 1 . 3 . 4 - 206  and a biasing member  11 . 1 . 3 . 4 - 208 . The follower is not illustrated in  FIG.  11 . 1   . 3 . 4 - 2  for illustrative purposes but the biasing element  11 . 1 . 3 . 4 - 208  can be fixed to a follower at the fixation point  11 . 1 . 3 . 4 - 212 . The distal end  11 . 1 . 3 . 4 - 214  of the biasing element  11 . 1 . 3 . 4 - 208  can include a distal spring finger  11 . 1 . 3 . 4 - 218  or other feature biased toward and contacting the guide-rod in order to bias the guide-rod  11 . 1 . 3 . 4 - 206  within the follower (to which the biasing element  11 . 1 . 3 . 4 - 208  is fixed) to remove slack and play between the follower and the guide-rod  11 . 1 . 3 . 4 - 206 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  11 . 1   . 3 . 4 - 2  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in any other figures shown and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to any other figure shown and described herein, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  11 . 1   . 3 . 4 - 2 . 
     11.1.4: Lower Guide Rod System 
     11.1.4.1: Electronic Devices with Biased Guide Rails 
     An electronic device may have input-output devices for gathering input and providing output. These devices may include optical components such as cameras, displays, and lenses. 
     A top view of an illustrative electronic device is shown in  FIG.  11 . 1   . 4 . 1 - 1 . Electronic device  11 . 1 . 4 . 1 - 10  of  FIG.  11 . 1   . 4 . 1 - 1  may be a head-mounted device or other suitable device. As shown in  FIG.  11 . 1   . 4 . 1 - 1 , device  11 . 1 . 4 . 1 - 10  may have a housing such as housing  11 . 1 . 4 . 1 - 12 . Housing  11 . 1 . 4 . 1 - 12 , which may sometimes be referred to as a housing wall, external housing, housing structures, enclosure, or case, may be formed from materials such as polymer, glass, metal, crystalline materials such as sapphire, ceramic, fabric, foam, wood, other materials, and/or combinations of these materials. 
     Device  11 . 1 . 4 . 1 - 10  may have any suitable shape. Housing  11 . 1 . 4 . 1 - 12  may, for example, be configured to form a head-mounted housing in the shape of a pair of goggles (e.g., goggles having optional head straps such as straps  11 . 1 . 4 . 1 - 12 T, a nose bridge portion in nose bridge region NB that is configured to fit over a user&#39;s nose and help support housing  11 . 1 . 4 . 1 - 12  on the user&#39;s nose, etc.) and/or other head-mounted structures. Housing  11 . 1 . 4 . 1 - 12  may separate interior region  11 . 1 . 4 . 1 - 26  from exterior region  11 . 1 . 4 . 1 - 28 . Housing  11 . 1 . 4 . 1 - 12  may include portions such as front portion (front wall)  11 . 1 . 4 . 1 - 12 F on front face F of device  11 . 1 . 4 . 1 - 10 , rear portion (rear wall)  11 . 1 . 4 . 1 - 12 R on opposing rear face R of device  11 . 1 . 4 . 1 - 10 , and sidewall portions such as sidewall portions  11 . 1 . 4 . 1 - 12 W on sides W that extend between front portion  11 . 1 . 4 . 1 - 12 F and rear portion  11 . 1 . 4 . 1 - 12 R, so that housing  11 . 1 . 4 . 1 - 12  encloses interior region  11 . 1 . 4 . 1 - 26 . 
     Electrical and optical components may be mounted within housing  11 . 1 . 4 . 1 - 12  (e.g., in interior region  11 . 1 . 4 . 1 - 26 ). As an example, housing  11 . 1 . 4 . 1 - 12  may have optical components in interior region  11 . 1 . 4 . 1 - 26  such as displays  11 . 1 . 4 . 1 - 14  and lenses  11 . 1 . 4 . 1 - 38 . Displays  11 . 1 . 4 . 1 - 14  and lenses  11 . 1 . 4 . 1 - 38  may be mounted in optical modules  11 . 1 . 4 . 1 - 30  (sometimes referred to as lens barrels, display and lens support structures, etc.). Images from displays  11 . 1 . 4 . 1 - 14  may be viewable from eye boxes  11 . 1 . 4 . 1 - 36  through lenses  11 . 1 . 4 . 1 - 38 . A left display and left lens in a left optical module  11 . 1 . 4 . 1 - 30  may be used to present a left-eye image to a user&#39;s left eye in a left eye box  11 . 1 . 4 . 1 - 36  and a right display and right lens in a right optical module  11 . 1 . 4 . 1 - 30  may be used to present a right-eye image to a user&#39;s right eye in right eye box  11 . 1 . 4 . 1 - 36 . Manual adjustment mechanisms and/or electrically adjustable actuators  11 . 1 . 4 . 1 - 13  (e.g., motors or other electrically adjustable positioners) may be used to move optical modules  11 . 1 . 4 . 1 - 30  horizontally across the front of the user&#39;s face (e.g., to adjust distance D between modules  11 . 1 . 4 . 1 - 30  along a direction parallel to the X-axis or nearly parallel to the X-axis of  FIG.  11 . 1   . 4 . 1 - 1 ). Optical modules  11 . 1 . 4 . 1 - 30  may, for example, be moved closer to each other or farther apart from each other as needed to accommodate different user interpupillary distances. 
     A schematic diagram of an illustrative electronic device is shown in  FIG.  11 . 1   . 4 . 1 - 2 . Device  11 . 1 . 4 . 1 - 10  of  FIG.  11 . 1   . 4 . 1 - 2  may be operated as a stand-alone device and/or the resources of device  11 . 1 . 4 . 1 - 10  may be used to communicate with external electronic equipment. As an example, communications circuitry in device  11 . 1 . 4 . 1 - 10  may be used to transmit user input information, sensor information, and/or other information to external electronic devices (e.g., wirelessly or via wired connections) and/or may be used to receive such information from external electronic devices. Each of these external devices may include components of the type shown by device  11 . 1 . 4 . 1 - 10  of  FIG.  11 . 1   . 4 . 1 - 2 . 
     As shown in  FIG.  11 . 1   . 4 . 1 - 2 , electronic device  11 . 1 . 4 . 1 - 10  may include control circuitry  11 . 1 . 4 . 1 - 20 . Control circuitry  11 . 1 . 4 . 1 - 20  may include storage and processing circuitry for supporting the operation of device  11 . 1 . 4 . 1 - 10 . The storage and processing circuitry may include storage such as nonvolatile memory (e.g., flash memory or other electrically-programmable-read-only memory configured to form a solid-state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Processing circuitry in control circuitry  11 . 1 . 4 . 1 - 20  may be used to gather input from sensors (e.g., cameras) and other input devices and may be used to control output devices. The processing circuitry may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors and other wireless communications circuits, power management units, audio chips, application specific integrated circuits, etc. During operation, control circuitry  11 . 1 . 4 . 1 - 20  may use display(s)  11 . 1 . 4 . 1 - 14  and other output devices in providing a user with visual output and other output. 
     To support communications between device  11 . 1 . 4 . 1 - 10  and external equipment, control circuitry  11 . 1 . 4 . 1 - 20  may communicate using communications circuitry  11 . 1 . 4 . 1 - 22 . Circuitry  11 . 1 . 4 . 1 - 22  may include antennas, radio-frequency transceiver circuitry, and other wireless communications circuitry and/or wired communications circuitry. Circuitry  11 . 1 . 4 . 1 - 22 , which may sometimes be referred to as control circuitry and/or control and communications circuitry, may support bidirectional wireless communications between device  11 . 1 . 4 . 1 - 10  and external equipment (e.g., a companion device such as a computer, cellular telephone, or other electronic device, an accessory such as a point device, computer stylus, or other input device, speakers or other output devices, etc.) over a wireless link. For example, circuitry  11 . 1 . 4 . 1 - 22  may include radio-frequency transceiver circuitry such as wireless local area network transceiver circuitry configured to support communications over a wireless local area network link, near-field communications transceiver circuitry configured to support communications over a near-field communications link, cellular telephone transceiver circuitry configured to support communications over a cellular telephone link, or transceiver circuitry configured to support communications over any other suitable wired or wireless communications link. Wireless communications may, for example, be supported over a Bluetooth® link, a WiFi® link, a wireless link operating at a frequency between 10 GHz and 400 GHz, a 60 GHz link, or other millimeter wave link, a cellular telephone link, or other wireless communications link. Device  11 . 1 . 4 . 1 - 10  may, if desired, include power circuits for transmitting and/or receiving wired and/or wireless power and may include batteries or other energy storage devices. For example, device  11 . 1 . 4 . 1 - 10  may include a coil and rectifier to receive wireless power that is provided to circuitry in device  11 . 1 . 4 . 1 - 10 . 
     Device  11 . 1 . 4 . 1 - 10  may include input-output devices such as devices  11 . 1 . 4 . 1 - 24 . Electronic components such as input-output devices  11 . 1 . 4 . 1 - 24  may be used in gathering user input, in gathering information on the environment surrounding the user, and/or in providing a user with output. 
     Devices  11 . 1 . 4 . 1 - 24  may include one or more displays such as display(s)  11 . 1 . 4 . 1 - 14 . Display(s)  11 . 1 . 4 . 1 - 14  may include one or more display devices such as organic light-emitting diode display panels (panels with organic light-emitting diode pixels formed on polymer substrates or silicon substrates that contain pixel control circuitry), liquid crystal display panels, microelectromechanical systems displays (e.g., two-dimensional mirror arrays or scanning mirror display devices), display panels having pixel arrays formed from crystalline semiconductor light-emitting diode dies (sometimes referred to as microLEDs), and/or other display devices. 
     Devices  11 . 1 . 4 . 1 - 24  may also include cameras  11 . 1 . 4 . 1 - 34 . Cameras  11 . 1 . 4 . 1 - 34  may include visible light cameras, infrared cameras, and/or cameras that are sensitive at multiple wavelengths, may include three-dimensional camera systems such as depth sensors (e.g., structured light sensors and/or depth sensors based on stereo imaging devices that capture three-dimensional images), may include time-of-flight cameras, and/or may include other cameras. Cameras  11 . 1 . 4 . 1 - 34  may face toward the user of device  11 . 1 . 4 . 1 - 10  and/or away from the user of device  11 . 1 . 4 . 1 - 10 . 
     Sensors  11 . 1 . 4 . 1 - 16  in input-output devices  11 . 1 . 4 . 1 - 24  may include force sensors (e.g., strain gauges, capacitive force sensors, resistive force sensors, etc.), audio sensors such as microphones, touch and/or proximity sensors such as capacitive sensors such as a touch sensor that forms a button, trackpad, or other input device), and other sensors. If desired, sensors  11 . 1 . 4 . 1 - 16  may include optical sensors such as optical sensors that emit and detect light, ultrasonic sensors, optical touch sensors, optical proximity sensors, and/or other touch sensors and/or proximity sensors, monochromatic and color ambient light sensors, fingerprint sensors, iris scanning sensors, retinal scanning sensors, and other biometric sensors, temperature sensors, sensors for measuring three-dimensional non-contact gestures (“air gestures”), pressure sensors, sensors for detecting position, orientation, and/or motion (e.g., accelerometers, magnetic sensors such as compass sensors, gyroscopes, and/or inertial measurement units that contain some or all of these sensors), health sensors such as blood oxygen sensors, heart rate sensors, blood flow sensors, and/or other health sensors, radio-frequency sensors, optical sensors such as self-mixing sensors and light detection and ranging (LIDAR) sensors, humidity sensors, moisture sensors, gaze tracking sensors, electromyography sensors to sense muscle activation, facial sensors, interferometric sensors, time-of-flight sensors, magnetic sensors, resistive sensors, distance sensors, angle sensors, and/or other sensors. In some arrangements, device  11 . 1 . 4 . 1 - 10  may use sensors  11 . 1 . 4 . 1 - 16  and/or other input-output devices  11 . 1 . 4 . 1 - 24  to gather user input. For example, input-output devices  11 . 1 . 4 . 1 - 24  such as buttons may be used to gather button press input, touch sensors overlapping displays can be used for gathering user touch screen input, touch pads may be used in gathering touch input, microphones may be used for gathering audio input (e.g., voice commands), accelerometers may be used in monitoring when a finger contacts an input surface and may therefore be used to gather finger press input, etc. 
     Input-output devices  11 . 1 . 4 . 1 - 24  may include optical components such as depth sensors (e.g., structured light sensors or other sensors that gather three-dimensional image data), optical proximity sensors, ambient light sensors (e.g., color ambient light sensors), optical time-of-flight sensors and other sensors  11 . 1 . 4 . 1 - 16  that are sensitive to visible and/or infrared light and that may emit visible and/or infrared light (e.g., devices  11 . 1 . 4 . 1 - 24  may contain optical sensors that emit and/or detect light). For example, a visible-light image sensor in a camera may have a visible light flash or an associated infrared flood illuminator to provide illumination while the image sensor captures a two-dimensional and/or three-dimensional image. An infrared camera such as an infrared structured light camera that captures three-dimensional infrared images may have an infrared flood illuminator that emits infrared flood illumination and/or may have a dot projector the emits an array of infrared light beams. Infrared proximity sensors may emit infrared light and detect the infrared light after the infrared light has reflected from a target object. 
     If desired, electronic device  11 . 1 . 4 . 1 - 10  may include additional components (see, e.g., other devices  11 . 1 . 4 . 1 - 18  in input-output devices  11 . 1 . 4 . 1 - 24 ). The additional components may include haptic output devices, actuators for moving movable structures in device  11 . 1 . 4 . 1 - 10 , audio output devices such as speakers, light-emitting diodes for status indicators, light sources such as light-emitting diodes that illuminate portions of a housing and/or display structure, other optical output devices, and/or other circuitry for gathering input and/or providing output. Device  11 . 1 . 4 . 1 - 10  may also include a battery or other energy storage device, connector ports for supporting wired communication with ancillary equipment and for receiving wired power, and other circuitry. 
     To help maintain desired alignment between optical modules  11 . 1 . 4 . 1 - 30  as optical modules  11 . 1 . 4 . 1 - 30  are moved by actuators  11 . 1 . 4 . 1 - 13  ( FIG.  11 . 1   . 4 . 1 - 1 ), optical modules  11 . 1 . 4 . 1 - 30  may be mounted on optical module guiding structures such as guide rails or other elongated support members. This type of arrangement is shown in the top view of device  11 . 1 . 4 . 1 - 10  of  FIG.  11 . 1   . 4 . 1 - 3 . As shown in  FIG.  11 . 1   . 4 . 1 - 3 , optical modules  11 . 1 . 4 . 1 - 30  may be slidably coupled to guide rails  11 . 1 . 4 . 1 - 44  to allow modules  11 . 1 . 4 . 1 - 30  to move horizontally (e.g., laterally along the X-axis to accommodate different user interpupillary distances). 
     Guide rails  11 . 1 . 4 . 1 - 44  may have circular cross-sectional shapes (e.g., guide rails  11 . 1 . 4 . 1 - 44  may be cylindrical rods) or may have other cross-sectional shapes. Guide rods  11 . 1 . 4 . 1 - 44  may be formed from metal, polymer, and/or other materials. Hollow and/or solid members may be used in forming guide rods  11 . 1 . 4 . 1 - 44 . To help reduce friction between guide rods  11 . 1 . 4 . 1 - 44  and optical modules  11 . 1 . 4 . 1 - 30 , guide rods  11 . 1 . 4 . 1 - 44  and/or mating portions of modules  11 . 1 . 4 . 1 - 30  may, if desired, be provided with a low-friction coating (e.g., nickel, etc.). 
     Guide rails  11 . 1 . 4 . 1 - 44  may span the width of housing  11 . 1 . 4 . 1 - 12 . There may be left and right guide rails  11 . 1 . 4 . 1 - 44  in device  11 . 1 . 4 . 1 - 10  that are joined at a housing support structure aligned with nose bridge portion NB or left and right guide rails  11 . 1 . 4 . 1 - 44  may be formed as integral portions of a single guide rail member that extends across housing  11 . 1 . 4 . 1 - 12 . Rails  11 . 1 . 4 . 1 - 44  may be straight or may, if desired, have a slight bend at nose bridge portion NB (e.g., to rake the left and right sides of the guide rails backwards slightly to conform to the shape of a user&#39;s face). As shown in the rear view of device  11 . 1 . 4 . 1 - 10  of  FIG.  11 . 1   . 4 . 1 - 4 , there may be upper and lower guide rails  11 . 1 . 4 . 1 - 44  on the left and right of device  11 . 1 . 4 . 1 - 10  such as upper guide rail  11 . 1 . 4 . 1 - 44 T and lower guide rail  11 . 1 . 4 . 1 - 44 . Arrangements with fewer guide rails or more guide rails may be used, if desired. 
       FIG.  11 . 1   . 4 . 1 - 5  is a side view of an illustrative optical module  11 . 1 . 4 . 1 - 30  mounted on guide rails  11 . 1 . 4 . 1 - 44 . In the example of  FIG.  11 . 1   . 4 . 1 - 5 , optical module  11 . 1 . 4 . 1 - 30  has an upper portion such as portion  11 . 1 . 4 . 1 - 30 T and a lower portion such as portion  11 . 1 . 4 . 1 - 30 B. Portions  11 . 1 . 4 . 1 - 30 T and/or  11 . 1 . 4 . 1 - 30 B may be integrally formed with main supporting structure  11 . 1 . 4 . 1 - 30 M of the lens barrel structures and/or other support structures of optical module  11 . 1 . 4 . 1 - 30  and/or may be separate members that are coupled (e.g., using welds, fasteners, adhesive, etc.) to main supporting structure  11 . 1 . 4 . 1 - 30 M. Lens  11 . 1 . 4 . 1 - 38  may be aligned with display  11 . 1 . 4 . 1 - 14  so that an image on display  11 . 1 . 4 . 1 - 14  may be viewed through lens  11 . 1 . 4 . 1 - 38  from eye box  11 . 1 . 4 . 1 - 36 . 
     As shown in  FIG.  11 . 1   . 4 . 1 - 5 , optical module  11 . 1 . 4 . 1 - 30  may have portions that receive and couple to guide rails  11 . 1 . 4 . 1 - 44  while allowing optical module  11 . 1 . 4 . 1 - 30  to slide along guide rails  11 . 1 . 4 . 1 - 44 . For example, upper portion  11 . 1 . 4 . 1 - 30 T may have a guide rail opening (optical module opening)  11 . 1 . 4 . 1 - 50  such as opening  11 . 1 . 4 . 1 - 50 T that receives upper guide rail  11 . 1 . 4 . 1 - 44 T and lower portion  11 . 1 . 4 . 1 - 30 B may have a guide rail opening such as opening  11 . 1 . 4 . 1 - 50 B that receives lower guide rail  11 . 1 . 4 . 1 - 44 B. Openings  11 . 1 . 4 . 1 - 50 T and  11 . 1 . 4 . 1 - 50 B may by cylindrical openings with circular cross-sectional shapes that receive the cylindrical members forming rails  11 . 1 . 4 . 1 - 44 T and  11 . 1 . 4 . 1 - 44 B and/or may have other shapes that partly or fully surround rails  11 . 1 . 4 . 1 - 44 T and  11 . 1 . 4 . 1 - 44 B. 
     To prevent rails  11 . 1 . 4 . 1 - 44  from becoming stuck in guide rail openings  11 . 1 . 4 . 1 - 50  of optical module  11 . 1 . 4 . 1 - 30 , the inner diameter of optical module openings may be slightly larger (e.g., by 2-50 microns, at least 5 microns, less than 100 microns, or other suitable amount) than the outer diameter of rails  11 . 1 . 4 . 1 - 44 . To prevent excess motion, which could lead to misalignment of optical modules, device  11 . 1 . 4 . 1 - 10  may be provided with guide rail biasing systems. The guide rail biasing systems may have movable biasing members (e.g., pins, plates, spherical members, hemispherical members, etc.) and biasing elements that apply force to the biasing members. The biasing elements may be, for example, springs such as coil springs, leaf springs, and/or other spring members, may be compressed polymer (e.g., elastomeric material, foam, etc.), may be magnets, and/or may be other biasing components that can be used to bias the biasing members in a desired direction. 
     Using the guide rail biasing systems, guide rails  11 . 1 . 4 . 1 - 44  may be pushed towards desired positions within openings  11 . 1 . 4 . 1 - 50  to help remove undesired play between guide rails  11 . 1 . 4 . 1 - 44  and openings  11 . 1 . 4 . 1 - 50 . As an example, guide rails  11 . 1 . 4 . 1 - 44  may be pressed against the inner surfaces of openings  11 . 1 . 4 . 1 - 50  (e.g., at a location on the side of openings  11 . 1 . 4 . 1 - 50  that faces eye boxes  11 . 1 . 4 . 1 - 36  rather than the opposing side of openings  11 . 1 . 4 . 1 - 50  that faces outwardly away from the user) and/or may be pressed against a structure mounted within openings  11 . 1 . 4 . 1 - 50  such as a pin or other support member. 
     Consider, as an example, the cross-sectional side view of optical module portion  11 . 1 . 4 . 1 - 30 B of  FIG.  11 . 1   . 4 . 1 - 6 A. As shown in  FIG.  11 . 1   . 4 . 1 - 6 A, portion  11 . 1 . 4 . 1 - 30 B may have an opening such as opening  11 . 1 . 4 . 1 - 50 B that receives lower guide rail  11 . 1 . 4 . 1 - 44 B. Lower guide rail biasing system  11 . 1 . 4 . 1 - 52 B may have a lower guide rail biasing element such as lower guide rail biasing element  11 . 1 . 4 . 1 - 54 B (e.g., a spring) and a corresponding movable biasing system member such as biasing member  11 . 1 . 4 . 1 - 56 B (e.g., a pin with a hemispherical head) and/or system  11 . 1 . 4 . 1 - 52 B may be formed from other biasing structures (e.g., a leaf spring, compressed foam, etc.). Under pressure from biasing element  11 . 1 . 4 . 1 - 54 B, member  11 . 1 . 4 . 1 - 56 B may press in direction  11 . 1 . 4 . 1 - 58  against an adjacent surface of guide rail  11 . 1 . 4 . 1 - 44 B (e.g., the side of rail  11 . 1 . 4 . 1 - 44 B facing biasing system  11 . 1 . 4 . 1 - 52 B and facing away from the user). Pin  11 . 1 . 4 . 1 - 60  may be mounted on an opposing side of opening  11 . 1 . 4 . 1 - 50 B. As a result of the force imposed on guide rail  11 . 1 . 4 . 1 - 44 B in direction  11 . 1 . 4 . 1 - 58 , guide rail  11 . 1 . 4 . 1 - 44 B may bear against member  11 . 1 . 4 . 1 - 56  (or, if desired, the inner surface of opening  11 . 1 . 4 . 1 - 50 B) at contact location  11 . 1 . 4 . 1 - 62  (e.g., a location on the surface of opening  11 . 1 . 4 . 1 - 50 B facing the user and eye boxes). This creates a well-defined location for guide rail  11 . 1 . 4 . 1 - 44 B relative to the structures of optical module  11 . 1 . 4 . 1 - 30  and helps prevent rail  11 . 1 . 4 . 1 - 44 B from moving excessively in the Y and/or Z dimensions within opening  11 . 1 . 4 . 1 - 50 B, thereby helping to ensure that optical module  11 . 1 . 4 . 1 - 30  is aligned satisfactorily with respect to eye box  11 . 1 . 4 . 1 - 30 . Additional friction and resistance to sliding of optical module  11 . 1 . 4 . 1 - 30  along the X axis is created by the use of biasing systems such as biasing system  11 . 1 . 4 . 1 - 52 B of  FIG.  11 . 1   . 4 . 1 - 6 A, but when it is desired to move optical modules  11 . 1 . 4 . 1 - 30  with respect to each other to adjust their spacing to accommodate a user interpupillary distance, sufficient force along the X dimension can be applied to overcome this friction. 
     If desired, spring  11 . 1 . 4 . 1 - 54 B and biasing element  11 . 1 . 4 . 1 - 56 B may be located on opposite sides of guide rail  11 . 1 . 4 . 1 - 44 B. Consider, as an example, the illustrative configuration of biasing system  11 . 1 . 4 . 1 - 52 B that is shown in  FIG.  11 . 1   . 4 . 1 - 6 B. In this configuration, biasing system  11 . 1 . 4 . 1 - 52 B has a biasing member  11 . 1 . 4 . 1 - 57  with a portion that forms biasing element  11 . 1 . 4 . 1 - 56 B. Biasing member  11 . 1 . 4 . 1 - 57  may be attached to portion  11 . 1 . 4 . 1 - 30 B of optical module  11 . 1 . 4 . 1 - 30  using an attachment mechanism such as screw  11 . 1 . 4 . 1 - 67  or other fastener. Screw  11 . 1 . 4 . 1 - 67  may have a threaded shaft or other structure that is fixedly attached to portion  11 . 1 . 4 . 1 - 30 B. The shaft of screw  11 . 1 . 4 . 1 - 67  may be received within a slot in member  11 . 1 . 4 . 1 - 57 . The slot may extend parallel to the Y axis of  FIG.  11 . 1   . 4 . 1 - 6 B to allow biasing member  11 . 1 . 4 . 1 - 57  to slide back and forth parallel to the Y axis. 
     A biasing element such as spring  11 . 1 . 4 . 1 - 54 B (e.g., a compression spring or other biasing spring) may be used to bias member  11 . 1 . 4 . 1 - 57  in the −Y direction. Spring  11 . 1 . 4 . 1 - 54 B may have a first end that presses against a surface of member  11 . 1 . 4 . 1 - 57  such as surface  11 . 1 . 4 . 1 - 61  and an opposing second end that presses against a surface of portion  11 . 1 . 4 . 1 - 30 B such as surface  11 . 1 . 4 . 1 - 63 . When member  11 . 1 . 4 . 1 - 57  is moved in direction  11 . 1 . 4 . 1 - 65 , spring  11 . 1 . 4 . 1 - 59  is compressed. Spring  11 . 1 . 4 . 1 - 59  thereafter presses against surface  11 . 1 . 4 . 1 - 61  and biases the portion of member  11 . 1 . 4 . 1 - 57  that forms element  11 . 1 . 4 . 1 - 54 B in direction  11 . 1 . 4 . 1 - 58  relative to portion  11 . 1 . 4 . 1 - 30 B. This causes member  11 . 1 . 4 . 1 - 57  to contact rail  11 . 1 . 4 . 1 - 44 B at contact location  11 . 1 . 4 . 1 - 69  and to thereby bias the opposing side of rail  11 . 1 . 4 . 1 - 44 B against pin  11 . 1 . 4 . 1 - 60  at contact location  11 . 1 . 4 . 1 - 62 . 
     Member  11 . 1 . 4 . 1 - 57  may be formed from one or more materials. For example, member  11 . 1 . 4 . 1 - 57  may be formed from a metal (e.g., aluminum, stainless steel, etc.). The metal may be covered with a hard low-friction coating such as an electroless nickel coating to enhance wear resistance. As another example, member  11 . 1 . 4 . 1 - 57  may be formed from a polymer (e.g., a low-friction composite polymer filled with carbon fibers, fiberglass fibers, or other fibers). The low-friction composite polymer may be formed from a polymer such as PEEK (polyether ether ketone) or other polymer (e.g., a polymer that may be shaped by a molding process such as injection molding). 
     The examples of  FIGS.  11 . 1   . 4 . 1 - 6 A and  11 . 1 . 4 . 1 - 6 B demonstrate how lower guide rail  11 . 1 . 4 . 1 - 44 B may be provided with an associated guide rail biasing system (illustrative system  11 . 1 . 4 . 1 - 52 B). If desired, upper guide rails  11 . 1 . 4 . 1 - 44 T may also be provided with guide rail biasing systems. As shown in the cross-sectional side view of upper portion  11 . 1 . 4 . 1 - 30 T of optical module  11 . 1 . 4 . 1 - 30 , optical module  11 . 1 . 4 . 1 - 30  may have biasing systems  11 . 1 . 4 . 1 - 52 T- 1  and  11 . 1 . 4 . 1 - 52 T- 2 . System  11 . 1 . 4 . 1 - 52 T- 1  may have biasing element  11 . 1 . 4 . 1 - 54 T- 1  for forcing movable biasing member  11 . 1 . 4 . 1 - 56 T- 1  against an adjacent surface of guide rail  11 . 1 . 4 . 1 - 44 T in direction  11 . 1 . 4 . 1 - 66  (or may have other biasing structures such as a leaf spring, compressed foam, etc.). System  11 . 1 . 4 . 1 - 52 T- 2  may have biasing element  11 . 1 . 4 . 1 - 54 T- 2  for forcing movable biasing member  11 . 1 . 4 . 1 - 56 T- 2  against an adjacent surface of guide rail  11 . 1 . 4 . 1 - 44  (or may have other biasing structures such as a leaf spring, compressed foam, etc.). Using systems  11 . 1 . 4 . 1 - 52 T- 1  and  11 . 1 . 4 . 1 - 52 T- 2 , guide rail  11 . 1 . 4 . 1 - 44 T may be biased leftward so that the surface of guide rail  11 . 1 . 4 . 1 - 44 T contacts the inner surface of opening  11 . 1 . 4 . 1 - 50 T at nominal biasing location  11 . 1 . 4 . 1 - 68  (e.g., a location on the surface of opening  11 . 1 . 4 . 1 - 50 T that faces the user and the eye boxes associated with device  11 . 1 . 4 . 1 - 10 ). The force of gravity tends to pull downwardly (in the −Z direction of  FIG.  11 . 1   . 4 . 1 - 7 ) on guide rail  11 . 1 . 4 . 1 - 44 T. To compensate for the force of gravity and thereby ensure that location  11 . 1 . 4 . 1 - 68  is located on the left side of guide rail  11 . 1 . 4 . 1 - 44 T (facing the user and eye box  11 . 1 . 4 . 1 - 36 ) as shown in  FIG.  11 . 1   . 4 . 1 - 7 , biasing element  11 . 1 . 4 . 1 - 54 T- 1  may be stronger than biasing element  11 . 1 . 4 . 1 - 54 T- 2 . 
     During operation of device  11 . 1 . 4 . 1 - 10 , a user&#39;s face and/or other external objects may impose forces on optical modules  11 . 1 . 4 . 1 - 30 . This gives rise to a potential for three different types of misalignment between the left and right optical modules in device  11 . 1 . 4 . 1 - 10 . 
     Rotation of optical modules  11 . 1 . 4 . 1 - 30  about the X axis, which may sometimes be referred to as splay, may cause a first type of misalignment in which one image appears at a different height than the other image. For example, splay may rotate the left eye image from the left optical module up relative to the left eye box while rotating the right eye image from the right optical module down relative to the right eye box. 
     Rotation of optical modules  11 . 1 . 4 . 1 - 30  about the Y axis, which may sometimes be referred to as image rotation, may produce a second type of misalignment in which the left and/or right images from modules  11 . 1 . 4 . 1 - 30  to appear tilted relative to the horizon. For example, the left eye image may be rotated counterclockwise and the right eye image may be rotated clockwise. 
     Another type of misalignment that may arise between optical modules  11 . 1 . 4 . 1 - 30  relates to rotation of one or both of optical modules  11 . 1 . 4 . 1 - 30  about the Z axis. This type of misalignment, which may sometimes be referred to as vergence, may be characterized by situations in which the left and/or right optical module points too far to the left or right in the X-Y plane. 
     All of these types of misalignment are preferably maintained at low levels (e.g., below +/−0.5°, below +/−0.4°, below +/−0.3°, or below +/−0.2°, as examples). In some situations, splay is the most undesired type of misalignment for users, so minimization of splay may be particular helpful in enhancing user comfort. 
     The use of guide rail biasing systems such as the illustrative biasing systems of  FIGS.  11 . 1   . 4 . 1 - 6 A,  11 . 1 . 4 . 1 - 6 B, and  11 . 1 . 4 . 1 - 7  may help minimize optical module misalignment. When biasing systems  11 . 1 . 4 . 1 - 52 T- 1  and  11 . 1 . 4 . 1 - 52 T- 2  perform satisfactorily, the left optical module will be biased against upper rail  11 . 1 . 4 . 1 - 44 T at a location such as location  11 . 1 . 4 . 1 - 68  of  FIG.  11 . 1   . 4 . 1 - 7  and will be biased against lower rail  11 . 1 . 4 . 1 - 44 B at a location such as location  11 . 1 . 4 . 1 - 62  of  FIG.  11 . 1   . 4 . 1 - 6 A or  FIG.  11 . 1   . 4 . 1 - 6 B, whereas the right optical module will likewise be biased against upper rail  11 . 1 . 4 . 1 - 44 T at a location such as location  11 . 1 . 4 . 1 - 68  of  FIG.  11 . 1   . 4 . 1 - 7  and will be biased against lower rail  11 . 1 . 4 . 1 - 44 B at a location such as location  11 . 1 . 4 . 1 - 62  of  FIG.  11 . 1   . 4 . 1 - 6 A or  FIG.  11 . 1   . 4 . 1 - 6 B. In this case, both the left and right optical modules will have the same position relative to guide rails  11 . 1 . 4 . 1 - 44 T and  11 . 1 . 4 . 1 - 44 B and there will be no splay. A potential for splay may arise when stress from a drop event or other unexpected excessive force causes a biasing system to fail. 
     Consider, as an example, a worse case splay scenario in which biasing system  11 . 1 . 4 . 1 - 52 T- 2  in the left optical module fails due to excessive force and in which biasing system  11 . 1 . 4 . 1 - 52 T- 1  in the right optical module fails due to excessive force. Although this type of asymmetric failure changes the biasing locations of the left and right optical modules, the biasing arrangement of  FIG.  11 . 1   . 4 . 1 - 7  helps prevent undesired splay from arising. Failure of biasing system  11 . 1 . 4 . 1 - 52 T- 2  in the left optical module will cause guide rail  11 . 1 . 4 . 1 - 44 T in the left optical module to be positioned against the inner surface of opening  11 . 1 . 4 . 1 - 50 T at location  11 . 1 . 4 . 1 - 72 , whereas failure of biasing system  11 . 1 . 4 . 1 - 52 T- 1  in the right optical module will cause guide rail  11 . 1 . 4 . 1 - 44 T in the right optical module to be positioned against the inner surface of opening  11 . 1 . 4 . 1 - 50 T at a different location such as location  11 . 1 . 4 . 1 - 70 . 
     In the example of  FIG.  11 . 1   . 4 . 1 - 7 , system  11 . 1 . 4 . 1 - 52 T- 2  is located at an angle A=+45° relative to the Y axis and system  11 . 1 . 4 . 1 - 52 T- 1  is located at an angle A=−45° relative to the Y axis. When systems  11 . 1 . 4 . 1 - 52 T- 2  and  11 . 1 . 4 . 1 - 52 T- 1  are operating satisfactorily, guide rail  11 . 1 . 4 . 1 - 44 T will therefore press against the inner side wall of opening  11 . 1 . 4 . 1 - 50 T at location  11 . 1 . 4 . 1 - 68 . When opening  11 . 1 . 4 . 1 - 50 T in the left optical module contacts rail  11 . 1 . 4 . 1 - 44 T at location  11 . 1 . 4 . 1 - 72  while opening  11 . 1 . 4 . 1 - 50 T in the right optical module contacts rail  11 . 1 . 4 . 1 - 44 T at location  11 . 1 . 4 . 1 - 70 , both the left and right optical modules will be positioned more to the right (in the orientation of  FIG.  11 . 1   . 4 . 1 - 7 ) than when in their desired nominal biased position. Lower guide rod  11 . 1 . 4 . 1 - 44 B is biased satisfactorily at location  11 . 1 . 4 . 1 - 62  with respect to lower optical module portion  11 . 1 . 4 . 1 - 30 B for both the left and right optical modules (in this example). As a result, both the left and right optical modules will tip forward slightly (rotating a small amount about the X axis), under outward pressure from the user&#39;s face. Although both the left and right optical modules tilt forward in this way, the amount of tilt of the left optical module, which is dictated by the lateral displacement of rail  11 . 1 . 4 . 1 - 44 T experienced when rail  11 . 1 . 4 . 1 - 44 T contacts opening  11 . 1 . 4 . 1 - 50 T at position  11 . 1 . 4 . 1 - 72 , is equal to the amount of tilt of the right optical module, which is dictated by the lateral displacement of rail  11 . 1 . 4 . 1 - 44 T experienced when rail  11 . 1 . 4 . 1 - 44 T contacts opening  11 . 1 . 4 . 1 - 50 T at position  11 . 1 . 4 . 1 - 70 . No splay will therefore result. Because the configuration of the biasing systems of  FIG.  11 . 1   . 4 . 1 - 7  tends to prevent splay from arising, this configuration may sometimes be referred to as a splay-optimized or splay-immune biasing scheme. Other biasing schemes may be used, if desired (e.g., schemes for the upper guide rail that have more biasing systems per optical module, schemes with fewer biasing systems per optical module, schemes in which the biasing systems press in different directions such as the +Z direction, −Y direction, etc.). The configuration of  FIG.  11 . 1   . 4 . 1 - 7  is illustrative. 
       FIGS.  11 . 1   . 4 . 1 - 8  and  11 . 1 . 4 . 1 - 9  show how a kinematic mounting scheme may be used to couple guide rails  11 . 1 . 4 . 1 - 44  and optical modules  11 . 1 . 4 . 1 - 30 . As shown in  FIG.  11 . 1   . 4 . 1 - 8 , upper guide rails  11 . 1 . 4 . 1 - 44 T may be biased in the −Y direction by biasing system  11 . 1 . 4 . 1 - 70 . Biasing system  11 . 1 . 4 . 1 - 70  may include biasing element  11 . 1 . 4 . 1 - 72  and movable biasing member  11 . 1 . 4 . 1 - 74  that contacts an adjacent portion of guide rail  11 . 1 . 4 . 1 - 44 T and pushes guide rail  11 . 1 . 4 . 1 - 44 T in direction  11 . 1 . 4 . 1 - 78  or may include other biasing structures (e.g., a leaf spring, compressed foam, etc.). Biasing system  11 . 1 . 4 . 1 - 80  may have a biasing element such as biasing element  11 . 1 . 4 . 1 - 82  that pushes biasing member  11 . 1 . 4 . 1 - 84  (e.g., a cylindrical pin) in direction  11 . 1 . 4 . 1 - 86  until motion of member  11 . 1 . 4 . 1 - 84  is stopped by stop structures  11 . 1 . 4 . 1 - 88 . Biasing system  11 . 1 . 4 . 1 - 90  may have a biasing element such as biasing element  11 . 1 . 4 . 1 - 92  that pushes biasing member  11 . 1 . 4 . 1 - 94  in direction  11 . 1 . 4 . 1 - 96  until motion of member  11 . 1 . 4 . 1 - 94  is stopped by stop structures  11 . 1 . 4 . 1 - 88 . In this configuration, guide rail  11 . 1 . 4 . 1 - 44 T is biased into contact with member  11 . 1 . 4 . 1 - 84  at known location  11 . 1 . 4 . 1 - 100  and into contact with member  11 . 1 . 4 . 1 - 94  at known location  11 . 1 . 4 . 1 - 102 . Lower guide rail  11 . 1 . 4 . 1 - 44 B may be biased to known location  11 . 1 . 4 . 1 - 62  using a biasing system such as system  11 . 1 . 4 . 1 - 52 B of  FIG.  11 . 1   . 4 . 1 - 6 A or  FIG.  11 . 1   . 4 . 1 - 6 B. 
       FIG.  11 . 1   . 4 . 1 - 9  is a rear view of optical module  11 . 1 . 4 . 1 - 30  with this type of kinematic rail mounting arrangement. As shown in  FIG.  11 . 1   . 4 . 1 - 9 , there may be two of biasing systems  11 . 1 . 4 . 1 - 80  in portion  11 . 1 . 4 . 1 - 30 T of module  11 . 1 . 4 . 1 - 30  and two of biasing systems  11 . 1 . 4 . 1 - 90  in portion  11 . 1 . 4 . 1 - 30 T of module  11 . 1 . 4 . 1 - 30  in addition to two of biasing systems  11 . 1 . 4 . 1 - 70 . This establishes a first pair of known contact locations towards the right end of rail  11 . 1 . 4 . 1 - 44 T (e.g., a first location  11 . 1 . 4 . 1 - 100  and a first location  11 . 1 . 4 . 1 - 102 ) and, establishes a second pair of known contact locations at a different location along the length of rail  11 . 1 . 4 . 1 - 44 T such as towards the left end of rail  11 . 1 . 4 . 1 - 44 T (e.g., a second location  11 . 1 . 4 . 1 - 100  and a second location  11 . 1 . 4 . 1 - 102 ). In lower portion  11 . 1 . 4 . 1 - 30 B, the location of optical module relative to guide rail  11 . 1 . 4 . 1 - 44 B is established at known location  11 . 1 . 4 . 1 - 62 . Determining these five known locations on optical module  11 . 1 . 4 . 1 - 30  where rails  11 . 1 . 4 . 1 - 44  contact module  11 . 1 . 4 . 1 - 30  (e.g., two locations  11 . 1 . 4 . 1 - 100  in upper portion  11 . 1 . 4 . 1 - 30 T, two locations  11 . 1 . 4 . 1 - 102  in upper portion  11 . 1 . 4 . 1 - 30 T, and one location  11 . 1 . 4 . 1 - 62  in lower portion  11 . 1 . 4 . 1 - 30 B), helps constrain five of the six degrees of freedom for motion of module  11 . 1 . 4 . 1 - 30  relative to rails  11 . 1 . 4 . 1 - 44  and the other support structures of device  11 . 1 . 4 . 1 - 10 . The sixth degree of freedom (motion along the X axis) is not constrained except by operation of actuator  11 . 1 . 4 . 1 - 13 , so that actuator  11 . 1 . 4 . 1 - 13  may be used to adjust the X-axis location of module  11 . 1 . 4 . 1 - 30  to accommodate different user interpupillary distances. 
     If desired, control circuitry  11 . 1 . 4 . 1 - 20  may use one or more sensors  11 . 1 . 4 . 1 - 16  to monitor the location of optical modules  11 . 1 . 4 . 1 - 30 . If misalignment is detected, corresponding action can be taken. For example, positioners may be adjusted to correct for the misalignment, image data for a display and/or camera may be warped to compensate for misalignment, alerts may be provided to the user and/or others, etc. 
     An illustrative optical module sensor system is shown in  FIGS.  11 . 1   . 4 . 1 - 10  and  11 . 1 . 4 . 1 - 11 .  FIG.  11 . 1   . 4 . 1 - 10  is a perspective view of an illustrative guide rail position sensor. Sensor  11 . 1 . 4 . 1 - 120  of  FIG.  11 . 1   . 4 . 1 - 10  has a biasing member such as leaf spring member  11 . 1 . 4 . 1 - 122  with a protruding portion such as portion  11 . 1 . 4 . 1 - 124 . Strain gauge  11 . 1 . 4 . 1 - 126  may be coupled to member  11 . 1 . 4 . 1 - 122  and may be monitored by control circuitry  11 . 1 . 4 . 1 - 20  to detect bending in member  11 . 1 . 4 . 1 - 122 . As shown in  FIG.  11 . 1   . 4 . 1 - 11 , sensor  11 . 1 . 4 . 1 - 120  of  FIG.  11 . 1   . 4 . 1 - 10  may be attached to optical module  11 . 1 . 4 . 1 - 30  with a fastener such as screw  11 . 1 . 4 . 1 - 128  or other attachment mechanism so that protruding portion  11 . 1 . 4 . 1 - 124  protrudes into opening  11 . 1 . 4 . 1 - 50  and into contact an adjacent surface of guide rail  11 . 1 . 4 . 1 - 44 . Changes in the position of guide rail  11 . 1 . 4 . 1 - 44  will result in changes in the detected strain by sensor  11 . 1 . 4 . 1 - 120 , so sensor  11 . 1 . 4 . 1 - 120  can monitor the position of guide rail  11 . 1 . 4 . 1 - 44  relative to optical module  11 . 1 . 4 . 1 - 30  along axis  11 . 1 . 4 . 1 - 130 . If desired, additional sensors such as sensor  11 . 1 . 4 . 1 - 120  may be located at additional positions about rail  11 . 1 . 4 . 1 - 44  (see, e.g., illustrative orthogonal position  11 . 1 . 4 . 1 - 120 ′ of  FIG.  11 . 1   . 4 . 1 - 11 , which allows position measurements along axis  11 . 1 . 4 . 1 - 132 ). 
     11.1.4.2: Lower Guide Rods 
       FIG.  11 . 1   . 4 . 2 - 1  illustrates a subassembly of an HMD  11 . 1 . 4 . 2 - 100  including a display bracket  11 . 1 . 4 . 2 - 106  having upper arms  11 . 1 . 4 . 2 - 110   a ,  11 . 1 . 4 . 2 - 110   b  extending outward in opposite directions and lower arms  11 . 1 . 4 . 2 - 112   a ,  11 . 1 . 4 . 2 - 112   b  extending in opposite directions, respectively. The subassembly can include a first optical module  11 . 1 . 4 . 2 - 108   a  slidably coupled to the display bracket  11 . 1 . 4 . 2 - 106  at a first upper guide-rod  11 . 1 . 4 . 2 - 102   a  and a first lower guide-rod  11 . 1 . 4 . 2 - 104   a  as well as a second optical module  11 . 1 . 4 . 2 - 108   b  slidably coupled to the display bracket  11 . 1 . 4 . 2 - 106  at a second upper guide rod  11 . 1 . 4 . 2 - 102   b  and a second lower guide-rod  11 . 1 . 4 . 2 - 104   b . The first and second optical modules  11 . 1 . 4 . 2 - 108   a - b  can be adjusted laterally relative to one another and/or the display bracket  11 . 1 . 4 . 2 - 106  along the guide-rods  11 . 1 . 4 . 2 - 102   a - b ,  11 . 1 . 4 . 2 - 104   a - b  during IPD adjustments. The first and second upper guide-rods  11 . 1 . 4 . 2 - 102   a - b  can be part of an upper guide-rod assembly  11 . 1 . 4 . 2 - 102  and the first and second lower guide-rods  11 . 1 . 4 . 2 - 104   a - b  can be part of a lower guide-rod assembly  11 . 1 . 4 . 2 - 104 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  11 . 1   . 4 . 2 - 1  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in any other figures shown and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to any other figure shown and described herein, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  11 . 1   . 4 . 2 - 1 . 
     11.1.4.2.1: Electrical Contacts 
       FIG.  11 . 1   . 4 . 2 . 1 - 1  illustrates a lower guide-rod system  11 . 1 . 4 . 2 . 1 - 102  of an HMD  11 . 1 . 4 . 2 . 1 - 100  including a lower guide-rod  11 . 1 . 4 . 2 . 1 - 106  configured to guide the movement of an optical module  11 . 1 . 4 . 2 . 1 - 104  In at least one example, the lower guide-rod system  11 . 1 . 4 . 2 . 1 - 102  also includes a stop  11 . 1 . 4 . 2 . 1 - 108  similar to other stops described herein, for protecting the cantilevered guide-rod  11 . 1 . 4 . 2 . 1 - 106 , which does not make contact with the stop  11 . 1 . 4 . 2 . 1 - 108  during normal use but through which the distal end of the guide-rod  11 . 1 . 4 . 2 . 1 - 106  may extend to contact the stop  11 . 1 . 4 . 2 . 1 - 108  in a drop event or other circumstance causing a deflection of the distal end of the guide-rod  11 . 1 . 4 . 2 . 1 - 106 . In at least one example, stop  11 . 1 . 4 . 2 . 1 - 108  can also prevent the optical module  11 . 1 . 4 . 2 . 1 - 104  from travelling too far along the guide-rod  11 . 1 . 4 . 2 . 1 - 106 . 
     In at least one example, the lower guide-rod system  11 . 1 . 4 . 2 . 1 - 102  can also include an electrical contact  11 . 1 . 4 . 2 . 1 - 110  biased against the guide-rod  11 . 1 . 4 . 2 . 1 - 106 . The biased electrical contact  11 . 1 . 4 . 2 . 1 - 110  can thus maintain contact with the guide-rod  11 . 1 . 4 . 2 . 1 - 106  during operation, including movements, vibrations, and other disturbances in the position of the guide-rod  11 . 1 . 4 . 2 . 1 - 106  during use. The biased electrical contact  11 . 1 . 4 . 2 . 1 - 110  can maintain electrical contact for electrical grounding purposes and for forming other electrical pathways between the guide-rod  11 . 1 . 4 . 2 . 1 - 106  and other components of the HMD  11 . 1 . 4 . 2 . 1 - 100 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  11 . 1   . 4 . 2 . 1 - 1  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in any other figures shown and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to any other figure shown and described herein, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  11 . 1   . 4 . 2 . 1 - 1 . 
     11.1.4.2.2: Biasing Members 
       FIG.  11 . 1   . 4 . 2 . 2 - 1  illustrates a perspective view of an example of an adjustment mechanism or assembly  11 . 1 . 4 . 2 . 2 - 100  including a lower guide-rod  11 . 1 . 4 . 2 . 2 - 102  and a stop  11 . 1 . 4 . 2 . 2 - 104 . The adjustment assembly  11 . 1 . 4 . 2 . 2 - 100  can also include a spring  11 . 1 . 4 . 2 . 2 - 106  biasing the guide-rod  11 . 1 . 4 . 2 . 2 - 106  passing through the stop  11 . 1 . 4 . 2 . 2 - 104 . The spring  11 . 1 . 4 . 2 . 2 - 106  can press the guide-rod  11 . 1 . 4 . 2 . 2 - 106  against the stop  11 . 1 . 4 . 2 . 2 - 104  to remove any slack or play between the guide-rod  11 . 1 . 4 . 2 . 2 - 102  and the stop  11 . 1 . 4 . 2 . 2 - 104  during use while allowing some flexure of the guide-rod  11 . 1 . 4 . 2 . 2 - 102 . In at least one example, the spring  11 . 1 . 4 . 2 . 2 - 106  can be a conductive biasing member maintaining contact with the guide-rod  11 . 1 . 4 . 2 . 2 - 102  for electrical grounding or providing other electrical pathways between the guide-rod  11 . 1 . 4 . 2 . 2 - 102  and one or more other components of the HMD. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  11 . 1   . 4 . 2 . 2 - 1  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in any other figures shown and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to any other figure shown and described herein, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  11 . 1   . 4 . 2 . 2 - 1 . 
     11.2: Barrels and Baskets 
     11.2.1: Lens Mounting Systems 
     The lenses may be mounted in a head-mounted device using support structures that help minimize lens stress. In an illustrative embodiment, the lenses are mounted in rigid lens supports such as metal lens barrels using lens mounts based on flexures. The flexures help prevent stress from being applied to the lenses even when the electronic devices are subjected to changes in operating temperature that cause the lenses to expand and contract. 
       FIG.  11 . 2   . 1 - 1  is a schematic diagram of an illustrative electronic device of the type that may include lenses mounted with flexures. Device  11 . 2 . 1 - 10  of  FIG.  11 . 2   . 1 - 1  may be a head-mounted device (e.g., goggles, glasses, a helmet, and/or other head-mounted device. In an illustrative configuration, device  11 . 2 . 1 - 10  is a head-mounted device such as a pair of goggles (sometimes referred to as virtual reality goggles, mixed reality goggles, augmented reality glasses, etc.). 
     As shown in the illustrative cross-sectional top view of device  11 . 2 . 1 - 10  of  FIG.  11 . 2   . 1 - 1 , device  11 . 2 . 1 - 10  may have a housing such as housing  11 . 2 . 1 - 12  (sometimes referred to as a head-mounted support structure, head-mounted housing, or head-mounted support). Housing  11 . 2 . 1 - 12  may include a front portion such as front portion  11 . 2 . 1 - 12 F and a rear portion such as rear portion  11 . 2 . 1 - 12 R. When device  11 . 2 . 1 - 10  is worn on the head of a user, rear portion  11 . 2 . 1 - 12 R rests against the face of the user and helps block stray light from reaching the eyes of the user. 
     Main portion  11 . 2 . 1 - 12 M of housing  11 . 2 . 1 - 12  may be attached to head strap  11 . 2 . 1 - 12 T. Head strap  11 . 2 . 1 - 12 T may be used to help secure main portion  11 . 2 . 1 - 12  on the head and face of a user. Main portion  11 . 2 . 1 - 12 M may have a rigid shell formed from housing walls of polymer, glass, metal, and/or other materials. When housing  11 . 2 . 1 - 12  is being worn on the head of a user, the front of housing  11 . 2 . 1 - 12  may face outwardly away from the user and the rear of housing  11 . 2 . 1 - 12  (and rear portion  11 . 2 . 1 - 12 R) may face towards the user. In this configuration, rear portion  11 . 2 . 1 - 12 R may face the user&#39;s eyes located in eye boxes  11 . 2 . 1 - 36 . 
     Device  11 . 2 . 1 - 10  may have electrical and optical components that are used in displaying images to eye boxes  11 . 2 . 1 - 36  when device  11 . 2 . 1 - 10  is being worn. These components may include left and right optical assemblies  11 . 2 . 1 - 20  (sometimes referred to as optical modules). Each optical assembly  11 . 2 . 1 - 20  may have an optical assembly support  11 . 2 . 1 - 38  (sometimes referred to as a lens barrel, optical module support, or support structure). Supports  11 . 2 . 1 - 38  may have hollow tubular shapes or other suitable shapes. Optical assemblies  11 . 2 . 1 - 20  may slide laterally along guide rails to adjust optical-assembly-to-optical-assembly separation to accommodate different user interpupillary distances. Rear portion  11 . 2 . 1 - 12 R may include flexible structures (e.g., a flexible polymer layer, a flexible fabric layer, etc.) so that portion  11 . 2 . 1 - 12 R can stretch to accommodate movement of assemblies  11 . 2 . 1 - 20 . 
     Each assembly  11 . 2 . 1 - 20  may have a display  11 . 2 . 1 - 32  coupled to one end of support  11 . 2 . 1 - 38  and a lens mounted to an opposing end of support  11 . 2 . 1 - 38 . Display  11 . 2 . 1 - 32  has an array of pixels for displaying images. Lens  11 . 2 . 1 - 34  may optionally have a removable vision correction lens for correcting user vision defects (e.g., refractive errors such as nearsightedness, farsightedness, and/or astigmatism). During operation, images displayed by displays  11 . 2 . 1 - 32  may be presented to eye boxes  11 . 2 . 1 - 36  through lenses  11 . 2 . 1 - 34  for viewing by the user. 
     To help satisfy design constraints (e.g., low weight, compact size, wide field of view, high resolution, etc.), lenses  11 . 2 . 1 - 34  may be catadioptric lenses. A catadioptric lens incorporates optical components such as polarizers and wave plates to create a folded optical path that can help reduce lens size. The use of these polarization-sensitive optical components may, however, make lenses  11 . 2 . 1 - 34  sensitive to performance degradation due to stress-induced birefringence effects. Lenses  11 . 2 . 1 - 34  may, as an example, include lens elements formed from polymers (for example, COC polymer (cyclic olefin copolymer) or other suitable polymers) that help minimize lens weight, but these polymers may exhibit birefringence when subjected to excess stress. 
     Satisfactory operation for lenses  11 . 2 . 1 - 34  may be achieved by mounting lenses  11 . 2 . 1 - 34  using lens mounts that help isolate lenses  11 . 2 . 1 - 34  from sources of stress. In an illustrative configuration, supports  11 . 2 . 1 - 38  may be formed from strong rigid materials such as metal (e.g., aluminum, etc.). When operating device  11 . 2 . 1 - 10  over wide temperature ranges (e.g., 0-50° C.), there is a risk that expansion and contraction of materials in assemblies  11 . 2 . 1 - 20  and, more particularly, mismatches in the coefficient of thermal expansion between the polymer of lenses  11 . 2 . 1 - 34  and the metal of supports  11 . 2 . 1 - 38 , can give rise to unwanted stress in lenses  11 . 2 . 1 - 34 , leading to unwanted stress-induced birefringence in lenses  11 . 2 . 1 - 34 . Unwanted temperature-change-induced stress may be avoided by mounting lenses  11 . 2 . 1 - 34  within supports  11 . 2 . 1 - 38  using lens mounts based on flexures. 
       FIG.  11 . 2   . 1 - 2  is a front view of an illustrative lens for device  11 . 2 . 1 - 10 . A shown in  FIG.  11 . 2   . 1 - 2 , lens  11 . 2 . 1 - 34  may be surrounded by flexible lens mount  11 . 2 . 1 - 40 . Lens mount  11 . 2 . 1 - 40  may include one or more flexures. In an illustrative embodiment, lens mount  11 . 2 . 1 - 40  contains a ring-shaped flexure that runs around the entire perimeter of lens  11 . 2 . 1 - 34 . Arrangements in which mount  11 . 2 . 1 - 40  includes a number of separate segments (see, e.g., illustrative segment SG) each of which has a separate strip-shaped flexure segment or in which mount  11 . 2 . 1 - 40  includes discrete flexures such as illustrative discrete flexure DL (e.g., a set of three discrete flexures attached to the periphery of lens  11 . 2 . 1 - 34  at three evenly spaced locations along the periphery) may also be used. Support  11 . 2 . 1 - 38  may be attached to mount  11 . 2 . 1 - 40  along the outer peripheral edge of mount  11 . 2 . 1 - 40  or support  11 . 2 . 1 - 38  may be attached to mount  11 . 2 . 1 - 40  where support  11 . 2 . 1 - 38  overlaps mount  11 . 2 . 1 - 40  (see, e.g.,  FIG.  11 . 2   . 1 - 3 ). 
       FIG.  11 . 2   . 1 - 3  is a cross-sectional side view of an illustrative peripheral portion of lens  11 . 2 . 1 - 38  and mount  11 . 2 . 1 - 40  of  FIG.  11 . 2   . 1 - 2  taken along line  11 . 2 . 1 - 42  of  FIG.  11 . 2   . 1 - 2  and viewed in direction  11 . 2 . 1 - 44 . As shown in  FIG.  11 . 2   . 1 - 3 , support (lens barrel)  11 . 2 . 1 - 38  may be overlapped by mount  11 . 2 . 1 - 40 . Mount  11 . 2 . 1 - 40  may be formed from polymer, metal, or other material that is shaped to form a flexure. In the example of  FIG.  11 . 2   . 1 - 3 , mount  11 . 2 . 1 - 40  is a ring-shaped flexure with a U-shaped cross section that runs along the ring-shaped peripheral edge of lens  11 . 2 . 1 - 34 . First portion  11 . 2 . 1 - 40 - 1  of the flexure is attached to the peripheral edge of lens  11 . 2 . 1 - 34  with a first ring of adhesive  11 . 2 . 1 - 50  and second portion  11 . 2 . 1 - 40 - 2  of the flexure is attached to support  11 . 2 . 1 - 38  with a second ring of adhesive  11 . 2 . 1 - 52 . Bend  11 . 2 . 1 - 40 - 3  runs along mount  11 . 2 . 1 - 40  between portions  11 . 2 . 1 - 40 - 1  and  11 . 2 . 1 - 4 - 2  and allows mount  11 . 2 . 1 - 40  to flex. For example, portion  11 . 2 . 1 - 40 - 1  may move radially outwardly towards portion  11 . 2 . 1 - 40 - 2  when lens  11 . 2 . 1 - 34  expands radially due to an increase in operating temperature. The metal of support  11 . 2 . 1 - 38  has a significantly lower coefficient of thermal expansion than the polymer of lens  11 . 2 . 1 - 34 , but is separated by an air gap from the edge of lens  11 . 2 . 1 - 34 , so that lens  11 . 2 . 1 - 34  can expand radially outwardly without contacting support  11 . 2 . 1 - 38  and is therefore not stressed due to pressure from contact with support  11 . 2 . 1 - 38 . 
     Mount  11 . 2 . 1 - 40  may, if desired, be formed from a material such as polyetherimide (PEI) or other rigid polymer that exhibits a high yield strength and a high elongation-to-failure value. The elastic modulus of the material forming mount  11 . 2 . 1 - 40  may be, for example, at least 1.5 GPa, at least 2 GPa, or at least 3 GPa, as examples. The high strength and high rigidity of the material forming mount  11 . 2 . 1 - 40  helps mount  11 . 2 . 1 - 40  accurately hold lens  11 . 2 . 1 - 34  in place. The material of mount  11 . 2 . 1 - 40  may exhibit a yield strength (e.g., at least 30 MPa, at least 70 MPa, or at least 100 MPa), so that mount  11 . 2 . 1 - 40  exhibits a good yield-strength-to-stiffness ratio. To provide mount  11 . 2 . 1 - 40  with satisfactory durability, it may be desirable for the material of mount to have an elongation-to-failure value of at least 25%, at least 35%, or at least 40%. To ensure satisfactory attachment of mount  11 . 2 . 1 - 40  with adhesive  11 . 2 . 1 - 52  and  11 . 2 . 1 - 50 , the material of mount  11 . 2 . 1 - 40  preferably also exhibits satisfactory compatibility (bond strength) with adhesives  11 . 2 . 1 - 52  and  11 . 2 . 1 - 50 . Adhesives  11 . 2 . 1 - 52  and  11 . 2 . 1 - 50  may be, for example, epoxy, so the material of mount  11 . 2 . 1 - 40  preferably forms satisfactory bonds with epoxy. The coefficient of thermal expansion of the material of mount  11 . 2 . 1 - 40  may be matched (e.g., within 20%, within 10%, within 5%, or within 2%) to the coefficient of thermal expansion of the material of lens  11 . 2 . 1 - 34 . In this way, lens stress can be avoided that might otherwise be imposed by mount  11 . 2 . 1 - 40  on lens  11 . 2 . 1 - 34  if, for example, the perimeter of mount  11 . 2 . 1 - 40  were to expand less quickly than the perimeter of lens  11 . 2 . 1 - 34  as a function of increasing operating temperature and thereby constrain the expansion of lens  11 . 2 . 1 - 34 . Other types of materials may be used in forming mount  11 . 2 . 1 - 40 , if desired. The use of polyetherimide that is matched to the polymer of lens  11 . 2 . 1 - 34  in its coefficient of thermal expansion and that exhibits high yield strength and high elongation-to-failure values is illustrative. 
     If desired, thermally induced lens stress may be minimized using a reverse flange adhesive attachment arrangement of the type shown in  FIG.  11 . 2   . 1 - 4 . With this type of arrangement, lens  11 . 2 . 1 - 34  may be provided with a milled flange that is bonded to a surface of support  11 . 2 . 1 - 38  that faces radially outward (away from lens  11 . 2 . 1 - 34 ). As shown in  FIG.  11 . 2   . 1 - 4 , adhesive  11 . 2 . 1 - 50  (e.g., a soft adhesive) may be used to attach outwardly facing surface  11 . 2 . 1 - 53  of support  11 . 2 . 1 - 38  to opposing inwardly faced milled flange surface  11 . 2 . 1 - 54  of lens  11 . 2 . 1 - 34 . An air gap such as air gap  11 . 2 . 1 - 56  is preferably present between outer peripheral edge surface  11 . 2 . 1 - 58  of lens  11 . 2 . 1 - 34  and opposing inwardly facing surface  11 . 2 . 1 - 60  of support  11 . 2 . 1 - 38 , thereby ensuring that the periphery of lens  11 . 2 . 1 - 34  (surface  11 . 2 . 1 - 58 ) will not contact support  11 . 2 . 1 - 38  and will therefore not receive stress from support  11 . 2 . 1 - 38  even as lens  11 . 2 . 1 - 34  expands radially outward as a function of increasing operating temperature. 
     If desired a set of discretely located flexures such as the illustrative flexures of  FIGS.  11 . 2   . 1 - 5  and  11 . 2 . 1 - 6  may also be used in supporting lens  11 . 2 . 1 - 34  (e.g., at three points along the perimeter of lens  11 . 2 . 1 - 34 ). In the examples of  FIGS.  11 . 2   . 1 - 5  and  11 . 2 . 1 - 6 , lens  11 . 2 . 1 - 34  is viewed from the rear of device  11 . 2 . 1 - 10  (e.g., along the Z axis). One side of illustrative flexures  11 . 2 . 1 - 64  and  11 . 2 . 1 - 66  may be attached to the periphery of lens  11 . 2 . 1 - 34  using adhesive  11 . 2 . 1 - 68  and another side of flexures  11 . 2 . 1 - 64  and  11 . 2 . 1 - 66  may be attached to support  11 . 2 . 1 - 38  (e.g., using adhesive, screws or other fasteners, etc.). During fluctuations in operating temperature, lens  11 . 2 . 1 - 34  may expand radially outward and may contract radially inward. With the flexure arrangements of  FIGS.  11 . 2   . 1 - 5  and  11 . 2 . 1 - 6 , flexures  11 . 2 . 1 - 64  and  11 . 2 . 1 - 66  may radially flex to accommodate the lens expansion and contraction, while accurately maintaining a desired mounting location for lens  11 . 2 . 1 - 34  within support  11 . 2 . 1 - 38 . In arrangements in with flexures are located at discrete locations around the periphery of lens  11 . 2 . 1 - 34  (e.g., three discrete locations), sealing structures (e.g., ring-shaped elastomeric boots) may be used to help seal the periphery of lens  11 . 2 . 1 - 34  against support  11 . 2 . 1 - 38  and thereby prevent dust and moisture ingress into assembly  11 . 2 . 1 - 20 . As with the ring-shaped flexure of  FIGS.  11 . 2   . 1 - 2  and  11 . 2 . 1 - 3 , the discrete flexures of  FIGS.  11 . 2   . 1 - 5  and  11 . 2 . 1 - 6  may be used to maintain lens  11 . 2 . 1 - 34  in a satisfactory position (without significant lateral shifting in X and Y position and without significant tilt away from the Z axis) while flexing to accommodate expansion and contraction due to temperature changes. The strength of mount  11 . 2 . 1 - 40  may help assemblies  11 . 2 . 1 - 20  resist plastic deformation and other damage during drop events and other undesired impacts leading to excessive stress. 
       FIGS.  11 . 2   . 1 - 7 ,  11 . 2 . 1 - 8 ,  11 . 2 . 1 - 9 , and  11 . 2 . 1 - 10  are illustrative cross-sectional side views of illustrative flexures for mount  11 . 2 . 1 - 40 . The flexures of  FIGS.  11 . 2   . 1 - 7 ,  11 . 2 . 1 - 8 ,  11 . 2 . 1 - 9 , and  11 . 2 . 1 - 10  may extend in a continuous ring around the periphery of lens  11 . 2 . 1 - 34 , may be formed from a number of segments that extend around lens  11 . 2 . 1 - 34 , or may be formed from discretely located flexures (as shown, for example, in  FIGS.  11 . 2   . 1 - 5  and  11 . 2 . 1 - 6 ). In the arrangements of  FIGS.  11 . 2   . 1 - 7 ,  11 . 2 . 1 - 8 , and  11 . 2 . 1 - 9 , mount  11 . 2 . 1 - 40  has a U-shaped cross-sectional shape. 
     In the example of  FIG.  11 . 2   . 1 - 7 , screw  11 . 2 . 1 - 70  attaches mount (flexure)  11 . 2 . 1 - 40  to support  11 . 2 . 1 - 38  and adhesive  11 . 2 . 1 - 78  attaches mount  11 . 2 . 1 - 40  to lens  11 . 2 . 1 - 34 . When using a screw such as screw  11 . 2 . 1 - 70  to attach mount  11 . 2 . 1 - 40  to support  11 . 2 . 1 - 38  instead of adhesive, a separate seal such as seal  11 . 2 . 1 - 72  may be used to seal lens  11 . 2 . 1 - 34  to support  11 . 2 . 1 - 38  and thereby help prevent moisture and dust ingress into the interior of assembly  11 . 2 . 1 - 20 . Seal  11 . 2 . 1 - 72  may be an O-ring formed from an elastomeric gasket material, may be a ring of adhesive (e.g., pressure sensitive adhesive or other soft sealing adhesive), or may be other sealant. If desired, adhesive  11 . 2 . 1 - 76  may be provided at the interface between mount  11 . 2 . 1 - 40  and support  11 . 2 . 1 - 38  (e.g., epoxy or other rigid adhesive) and may be used both to attach mount  11 . 2 . 1 - 40  to support  11 . 2 . 1 - 38  and to seal mount  11 . 2 . 1 - 40  to support  11 . 2 . 1 - 38 . The use of screws such as screw  11 . 2 . 1 - 70  of  FIG.  11 . 2   . 1 - 7  to attach mount  11 . 2 . 1 - 40  to support  11 . 2 . 1 - 38  is illustrative. 
     The illustrative configurations of  FIGS.  11 . 2   . 1 - 8 ,  11 . 2 . 1 - 9 , and  11 . 2 . 1 - 10  may also involve the use of screws  11 . 2 . 1 - 70 , adhesive  11 . 2 . 1 - 76 , and/or seal  11 . 2 . 1 - 72  to attach and seal mount  11 . 2 . 1 - 40  to support  11 . 2 . 1 - 38 . In the example of  FIG.  11 . 2   . 1 - 8 , mount  11 . 2 . 1 - 40  protrudes inwardly under the outer edge of lens  11 . 2 . 1 - 34 . In the example of  FIG.  11 . 2   . 1 - 9 , screw  11 . 2 . 1 - 70  extends radially inward towards the center of lens  11 . 2 . 1 - 34 . In the example of  FIG.  11 . 2   . 1 - 10 , the flexure formed by mount  11 . 2 . 1 - 40  does not have a central bend, but rather relies on its length to ensure that sufficient flexure bending is available to accommodate radial outward expansion of lens  11 . 2 . 1 - 34  as operating temperatures rise. 
     The diagram of  FIG.  11 . 2   . 1 - 11  shows how adhesive  11 . 2 . 1 - 78  may be dispensed to attach mount  11 . 2 . 1 - 40  to the outer edge of lens  11 . 2 . 1 - 34 . In the example of  FIG.  11 . 2   . 1 - 11 , lens  11 . 2 . 1 - 34  and mount  11 . 2 . 1 - 40  have been flipped upside down relative to the orientation of  FIG.  11 . 2   . 1 - 3 . As shown in  FIG.  11 . 2   . 1 - 11 , mount  11 . 2 . 1 - 40  may have a chamfer such as chamfer  11 . 2 . 1 - 84  that locally widens the gap between lens  11 . 2 . 1 - 34  and mount  11 . 2 . 1 - 40  to facilitate the introduction of liquid adhesive into this gap. Adhesive may be dispensed in liquid form in direction  11 . 2 . 1 - 82  from dispenser  11 . 2 . 1 - 80  and may then be drawn into the gap between mount  11 . 2 . 1 - 40  and lens  11 . 2 . 1 - 34  by capillary action. After the liquid adhesive material has been drawn into the gap, the adhesive may be cured to form adhesive  11 . 2 . 1 - 78  (e.g., using ultraviolet light curing, curing using a two-part adhesive arrangement, etc.). To help control the downward progress of adhesive  11 . 2 . 1 - 78  while adhesive  11 . 2 . 1 - 78  is liquid, mount  11 . 2 . 1 - 40  may be provided with a stop feature such as step  11 . 2 . 1 - 86 . The presence of step  11 . 2 . 1 - 86  on the inwardly facing surface of mount  11 . 2 . 1 - 40  creases a surface tension break that stops the downward flow of adhesive  11 . 2 . 1 - 78  when flowing into the joint between lens  11 . 2 . 1 - 34  and mount  11 . 2 . 1 - 40 . In this way, the dispensing of adhesive  11 . 2 . 1 - 78  may be limited to a desired bonding area between mount  11 . 2 . 1 - 40  and lens  11 . 2 . 1 - 34 . 
     11.3: Rear-Facing Cameras 
     11.3.1: Optical Module for Head-Mounted Device 
     One aspect of the disclosure is an optical module for a head-mounted device that is configured to present content to a user. The optical module includes an optical module housing assembly, a display assembly, and an eye camera. The optical module housing assembly has a first end and a second end. The lens is connected to the optical module housing assembly and positioned at the first end of the optical module housing assembly. The display assembly is connected to the optical module housing assembly and is positioned at the second end of the optical module housing assembly. The display assembly is configured to cause the content to be displayed to the user through the lens. The eye camera is connected to the optical module housing assembly and is positioned at the second end of the optical module housing assembly. The eye camera is configured to obtain images through the lens. 
     In some implementations of the optical module, the optical module housing assembly includes a first portion that is connected to a second portion, and the lens is retained between the first portion and the second portion. In some implementations of the optical module, projections are defined on the lens and channels are defined on the first portion of the optical module housing assembly such that the projections are located in the channels and engage the first portion of the optical module housing assembly within the channels to secure the lens relative to the optical module housing assembly and restrain movement of the lens relative to the optical module housing assembly. In some implementations of the optical module, the lens and the display assembly are connected to the optical module housing assembly in a side-by-side arrangement. In some implementations of the optical module, the optical module housing assembly defines an internal space between the lens and the display assembly. 
     In some implementations of the optical module, the optical module also includes a vent port that allows air to travel between the internal space and an outside environment, and a filter element that restrains foreign particles from entering the internal space. In some implementations of the optical module, the optical module also includes a dust trap that is located in the internal space and is configured to retain foreign particles. 
     In some implementations of the optical module, the optical module also includes a fiducial marker that is formed on the lens and is visible in images obtained by the eye camera for use in calibration. In some implementations of the optical module, the lens is a catadioptric lens. In some implementations of the optical module, the lens is a part of a catadioptric optical system. 
     Another aspect of the disclosure is an optical module for a head-mounted device that is configured to present content to a user. The optical module includes an optical module housing assembly that defines an internal space, a lens that is connected to the optical module housing assembly, a display assembly that is connected to the optical module housing assembly. The display assembly is configured to cause the content to be displayed to the user through the lens. An infrared emitter is located between the lens and the display assembly in the internal space of the optical module housing assembly. The infrared emitter is configured to emit infrared radiation through the lens. 
     In some implementations of the optical module, the infrared emitter includes a flexible circuit and emissive components that are connected to the flexible circuit and are configured to emit infrared radiation. In some implementations of the optical module, wherein the emissive components are arranged in an array around an optical axis of the optical module housing assembly. In some implementations of the optical module, the flexible circuit extends through an electrical port that is formed through the optical module housing assembly and a sealing element is formed on the flexible circuit and is engaged with the optical module housing assembly at the electrical port. In some implementations of the optical module, the optical module housing assembly defines an optical pathway opening that is adjacent to the display assembly and is configured to allow light to pass from the display assembly to the lens, a base surface that extends around the optical pathway opening, wherein the infrared emitter is located on the base surface, and a peripheral wall that is located outward from the base surface. 
     In some implementations of the optical module, the optical module also includes an eye camera that is configured to obtain images that show reflected portions of the infrared radiation that is emitted by the infrared emitter. In some implementations of the optical module, the eye camera is connected to the optical module housing assembly and is configured to obtain the images through the lens. In some implementations of the optical module, the lens is a catadioptric lens. In some implementations of the optical module, the lens is a part of a catadioptric optical system. 
     Another aspect of the disclosure is a head-mounted device that is configured to present content to a user. The head-mounted device includes a housing, a first optical module that is located in the housing, and a second optical module that is located in the housing. An interpupillary distance adjustment assembly supports the first optical module and the second optical module with respect to the housing to allow adjustment of a distance between the first optical module and the second optical module. The head-mounted device also includes a first front-facing camera that is connected to the first optical module and is movable in unison with the first optical module by the interpupillary distance adjustment assembly, and a second front-facing camera that is connected to the second optical module and is movable in unison with the second optical module by the interpupillary distance adjustment assembly. Adjustment of the distance between the first optical module and the second optical module by the interpupillary distance adjustment assembly also adjusts a distance between the first front-facing camera and the second front-facing camera. 
     In some implementations of the head-mounted device, the housing includes one or more optically transmissive panels through which the first front-facing camera and the second front-facing camera may obtain images of an environment. 
     In some implementations of the head-mounted device, an optical axis of the first front-facing camera is aligned with an optical axis of the first optical module and an optical axis of the second front-facing camera is aligned with an optical axis of the second optical module. 
     In some implementations of the head-mounted device, the first front-facing camera is connected in a fixed relationship with respect to the first optical module, and the second front-facing camera is connected in a fixed relationship with respect to the second optical module. 
     In some implementations of the head-mounted device, the interpupillary distance adjustment assembly maintains a first spacing between an optical axis of the first optical module and an optical axis of the second optical module generally equal to a second spacing between an optical axis of the first front-facing camera and an optical axis of the second front facing camera during adjustment of the distance between the first optical module and the second optical module. 
       FIG.  11 . 3   . 1 - 1  is a block diagram that shows an example of a hardware configuration for a head-mounted device  11 . 3 . 1 - 100 . The head-mounted device  11 . 3 . 1 - 100  is intended to be worn on the head of a user and includes components that are configured to display content to the user. Components that are included in the head-mounted device  11 . 3 . 1 - 100  may be configured to track motion of parts of the user&#39;s body, such as the user&#39;s head and hands. Motion tracking information that is obtained by components of the head-mounted device can be utilized as inputs that control aspects of the generation and display of the content to the user, so that the content displayed to the user can be part of a CGR experience in which the user is able to view and interact with virtual environments and virtual objects. In the illustrated example, the head-mounted device  11 . 3 . 1 - 100  includes a device housing  11 . 3 . 1 - 102 , a face seal  11 . 3 . 1 - 104 , a support structure  11 . 3 . 1 - 106 , a processor  11 . 3 . 1 - 108 , a memory  11 . 3 . 1 - 110 , a storage device  11 . 3 . 1 - 112 , a communications device  11 . 3 . 1 - 114 , sensors  11 . 3 . 1 - 116 , a power source  11 . 3 . 1 - 118 , and optical modules  11 . 3 . 1 - 120 . The head-mounted device  11 . 3 . 1 - 100  includes two of the optical modules  11 . 3 . 1 - 120 , to display content to the user&#39;s eyes. The optical modules  11 . 3 . 1 - 120  may each include an optical module housing  11 . 3 . 1 - 122 , a display assembly  11 . 3 . 1 - 124 , and a lens assembly  11 . 3 . 1 - 126 . 
     The device housing  11 . 3 . 1 - 102  is a structure that supports various other components that are included in the head-mounted device. The device housing  11 . 3 . 1 - 102  may be an enclosed structure such that certain components of the head-mounted device  11 . 3 . 1 - 100  are contained within the device housing  11 . 3 . 1 - 102  and thereby protected from damage. 
     The face seal  11 . 3 . 1 - 104  is connected to the device housing  11 . 3 . 1 - 102  and is located at areas around a periphery of the device housing  11 . 3 . 1 - 102  where contact with the user&#39;s face is likely. The face seal  11 . 3 . 1 - 104  functions to conform to portions of the user&#39;s face to allow the support structure  11 . 3 . 1 - 106  to be tensioned to an extent that will restrain motion of the device housing  11 . 3 . 1 - 102  with respect to the user&#39;s head. The face seal  11 . 3 . 1 - 104  may also function to reduce the amount of light from the physical environment around the user that reaches the user&#39;s eyes. The face seal  11 . 3 . 1 - 104  may contact areas of the user&#39;s face, such as the user&#39;s forehead, temples, and cheeks. The face seal  11 . 3 . 1 - 104  may be formed from a compressible material, such as open-cell foam or closed cell foam. 
     The support structure  11 . 3 . 1 - 106  is connected to the device housing  11 . 3 . 1 - 102 . The support structure  11 . 3 . 1 - 106  is a component or collection of components that function to secure the device housing  11 . 3 . 1 - 102  in place with respect to the user&#39;s head so that the device housing  11 . 3 . 1 - 102  is restrained from moving with respect to the user&#39;s head and maintains a comfortable position during use. The support structure  11 . 3 . 1 - 106  can be implemented using rigid structures, elastic flexible straps, or inelastic flexible straps. 
     The processor  11 . 3 . 1 - 108  is a device that is operable to execute computer program instructions and is operable to perform operations that are described by the computer program instructions. The processor  11 . 3 . 1 - 108  may be implemented using a conventional device, such as a central processing unit, and provided with computer-executable instructions that cause the processor  11 . 3 . 1 - 108  to perform specific functions. The processor  11 . 3 . 1 - 108  may be a special-purpose processor (e.g., an application-specific integrated circuit or a field-programmable gate array) that implements a limited set of functions. The memory  11 . 3 . 1 - 110  may be a volatile, high-speed, short-term information storage device such as a random-access memory module. 
     The storage device  11 . 3 . 1 - 112  is intended to allow for long term storage of computer program instructions and other data. Examples of suitable devices for use as the storage device  11 . 3 . 1 - 112  include non-volatile information storage devices of various types, such as a flash memory module, a hard drive, or a solid-state drive. 
     The communications device  11 . 3 . 1 - 114  supports wired or wireless communications with other devices. Any suitable wired or wireless communications protocol may be used. 
     The sensors  11 . 3 . 1 - 116  are components that are incorporated in the head-mounted device  11 . 3 . 1 - 100  to provide inputs to the processor  11 . 3 . 1 - 108  for use in generating CGR content. The sensors  11 . 3 . 1 - 116  include components that facilitate motion tracking (e.g., head tracking and optionally handheld controller tracking in six degrees of freedom). The sensors  11 . 3 . 1 - 116  may also include additional sensors that are used by the device to generate and/or enhance the user&#39;s experience in any way. The sensors  11 . 3 . 1 - 116  may include conventional components such as cameras, infrared cameras, infrared emitters, depth cameras, structured-light sensing devices, accelerometers, gyroscopes, and magnetometers. The sensors  11 . 3 . 1 - 116  may also include biometric sensors that are operable to physical or physiological features of a person, for example, for use in user identification and authorization. Biometric sensors may include fingerprint scanners, retinal scanners, and face scanners (e.g., two-dimensional and three-dimensional scanning components operable to obtain image and/or three-dimensional surface representations). Other types of devices can be incorporated in the sensors  11 . 3 . 1 - 116 . The information that is generated by the sensors  11 . 3 . 1 - 116  is provided to other components of the head-mounted device  11 . 3 . 1 - 100 , such as the processor  11 . 3 . 1 - 108 , as inputs. 
     The power source  11 . 3 . 1 - 118  supplies electrical power to components of the head-mounted device  11 . 3 . 1 - 100 . In some implementations, the power source  11 . 3 . 1 - 118  is a wired connection to electrical power. In some implementations, the power source  11 . 3 . 1 - 118  may include a battery of any suitable type, such as a rechargeable battery. In implementations that include a battery, the head-mounted device  11 . 3 . 1 - 100  may include components that facilitate wired or wireless recharging. 
     In some implementations of the head-mounted device  11 . 3 . 1 - 100 , some or all of these components may be included in a separate device that is removable. For example, any or all of the processor  11 . 3 . 1 - 108 , the memory  11 . 3 . 1 - 110 , and/or the storage device  11 . 3 . 1 - 112 , the communications device  11 . 3 . 1 - 114 , and the sensors  11 . 3 . 1 - 116  may be incorporated in a device such as a smart phone that is connected (e.g., by docking) to the other portions of the head-mounted device  11 . 3 . 1 - 100 . 
     In some implementations of the head-mounted device  11 . 3 . 1 - 100 , the processor  11 . 3 . 1 - 108 , the memory  11 . 3 . 1 - 110 , and/or the storage device  11 . 3 . 1 - 112  are omitted, and the corresponding functions are performed by an external device that communicates with the head-mounted device  11 . 3 . 1 - 100 . In such an implementation, the head-mounted device  11 . 3 . 1 - 100  may include components that support a data transfer connection with the external device using a wired connection or a wireless connection that is established using the communications device  11 . 3 . 1 - 114 . 
     The components that are included in the optical modules support the function of displaying content to the user in a manner that supports CGR experiences. The optical modules  11 . 3 . 1 - 120  are each assemblies that include multiple components, which include the optical module housing  11 . 3 . 1 - 122 , the display assembly  11 . 3 . 1 - 124 , and the lens assembly  11 . 3 . 1 - 126 , as will be described further herein. 
     Other components may also be included in each of the optical modules. Although not illustrated in  FIGS.  11 . 3   . 1 - 2 - 3 , the optical modules  11 . 3 . 1 - 120  may be supported by adjustment assemblies that allow the position of the optical modules  11 . 3 . 1 - 120  to be adjusted. As an example, the optical modules  11 . 3 . 1 - 120  may each be supported by an interpupillary distance adjustment mechanism that allows the optical modules  11 . 3 . 1 - 120  to slide laterally toward or away from each other. As another example, the optical modules  11 . 3 . 1 - 120  may be supported by an eye relief distance adjustment mechanism that allows adjustment of the distance between the optical modules  11 . 3 . 1 - 120  and the user&#39;s eyes. 
       FIG.  11 . 3   . 1 - 2  is a top view illustration that shows the head-mounted device  11 . 3 . 1 - 100 , including the device housing  11 . 3 . 1 - 102 , the face seal  11 . 3 . 1 - 104 , and the support structure  11 . 3 . 1 - 106 .  FIG.  11 . 3   . 1 - 3  is a rear view illustration taken along line A-A of  FIG.  11 . 3   . 1 - 2 . In the illustrated example, the device housing  11 . 3 . 1 - 102  is a generally rectangular structure having a width that is selected to be similar to the width of the head of a typical person, and a height selected so as to extend approximately from the forehead to the base of the nose of a typical person. This configuration is an example, and other shapes and sizes may be used. 
     An eye chamber  11 . 3 . 1 - 328  is defined by the device housing  11 . 3 . 1 - 102  and is bordered by the face seal  11 . 3 . 1 - 104  at its outer periphery. The eye chamber  11 . 3 . 1 - 328  is open to the exterior of the head-mounted device  11 . 3 . 1 - 100  to allow the user&#39;s face to be positioned adjacent to the eye chamber  11 . 3 . 1 - 328 , which is otherwise enclosed by the device housing  11 . 3 . 1 - 102 . The face seal  11 . 3 . 1 - 104  may extend around part of all of the periphery of the device housing  11 . 3 . 1 - 102  adjacent to the eye chamber  11 . 3 . 1 - 328 . The face seal  11 . 3 . 1 - 104  may function to exclude some of the light from the environment around the head-mounted device  11 . 3 . 1 - 100  from entering the eye-chamber  11 . 3 . 1 - 328  and reaching the user&#39;s eyes. 
     In the illustrated example, the support structure  11 . 3 . 1 - 106  is a headband type device that is connected to left and right lateral sides of the device housing  11 . 3 . 1 - 102  and is intended to extend around the user&#39;s head. Other configurations may be used for the support structure  11 . 3 . 1 - 106 , such as a halo-type configuration in which the device housing  11 . 3 . 1 - 102  is supported by a structure that is connected to a top portion of the device housing  11 . 3 . 1 - 102 , engages the user&#39;s forehead above the device housing  11 . 3 . 1 - 102 , and extends around the user&#39;s head, or a mohawk-type configuration in which a structure extends over the user&#39;s head. Although not illustrated, the support structure  11 . 3 . 1 - 106  may include passive or active adjustment components, which may be mechanical or electromechanical, that allow portions of the support structure  11 . 3 . 1 - 106  to expand and contract to adjust the fit of the support structure  11 . 3 . 1 - 106  with respect to the user&#39;s head. 
     The optical modules  11 . 3 . 1 - 120  are located in the device housing  11 . 3 . 1 - 102  and extend outward into the eye chamber  11 . 3 . 1 - 328 . Portions of the optical modules  11 . 3 . 1 - 120  are located in the eye chamber  11 . 3 . 1 - 328  so that the user can see the content that is displayed by the optical modules  11 . 3 . 1 - 120 . The optical modules  11 . 3 . 1 - 120  are located within the eye chamber  11 . 3 . 1 - 328  at locations that are intended to be adjacent to the user&#39;s eyes. As an example, the head-mounted device  11 . 3 . 1 - 100  may be configured to position portions of the lens assemblies  11 . 3 . 1 - 126  of the optical modules  11 . 3 . 1 - 120  approximately 15 millimeters from the user&#39;s eyes. 
       FIG.  11 . 3   . 1 - 4  is a perspective view illustration that shows one of the optical modules  11 . 3 . 1 - 120 , including the optical module housing  11 . 3 . 1 - 122 , the display assembly  11 . 3 . 1 - 124 , and the lens assembly  11 . 3 . 1 - 126 . The display assembly  11 . 3 . 1 - 124  and the lens assembly  11 . 3 . 1 - 126  are each connected to the optical module housing  11 . 3 . 1 - 122 . In the illustrated example, the lens assembly  11 . 3 . 1 - 126  is positioned at a front end of the optical module  11 . 3 . 1 - 120 , and the display assembly  11 . 3 . 1 - 124  is positioned at a rear end of the optical module  11 . 3 . 1 - 120 . The optical module housing  11 . 3 . 1 - 122  defines an internal space between the display assembly  11 . 3 . 1 - 124  and the lens assembly  11 . 3 . 1 - 126  to allow light to travel from the display assembly  11 . 3 . 1 - 124  to the lens assembly  11 . 3 . 1 - 126  within an environment that is sealed and protected from external contaminants while protecting sensitive components from damage. 
     The display assembly  11 . 3 . 1 - 124  includes a display screen that is configured to display content, such as images, according to signals received from the processor  11 . 3 . 1 - 108  and/or from external devices using the communications device  11 . 3 . 1 - 114  in order to output CGR content to the user. As an example, the display assembly  11 . 3 . 1 - 124  may output still images and/or video images in response to received signals. The display assembly  11 . 3 . 1 - 124  may include, as examples, an LED screen, an LCD screen, an OLED screen, a micro LED screen, or a micro OLED screen. 
     The lens assembly  11 . 3 . 1 - 126  includes one or more lenses that direct light to the user&#39;s eyes in a manner that allows viewing of CGR content. In some implementations, the lens assembly  11 . 3 . 1 - 126  is a catadioptric optical system that utilizes both reflection and refraction in order to achieve desired optical properties in a small package size. Reflection, in some implementations, may be achieved by internal reflection at boundaries between material layers of a single lens. Thus, in some implementations, the lens assembly  11 . 3 . 1 - 126  may be implemented using a single multi-layered catadioptric lens. 
     The lens assembly  11 . 3 . 1 - 126  may be positioned partially within the optical module housing  11 . 3 . 1 - 122 . As will be explained further herein, the optical module housing  11 . 3 . 1 - 122  may include two or more components that are configured to retain the lens assembly in a desired position and orientation. 
       FIG.  11 . 3   . 1 - 5  is an exploded side view diagram showing components of an optical module  11 . 3 . 1 - 520  according to a first example.  FIG.  11 . 3   . 1 - 5  is a schematic view intended to show the positional relationships between various features and does not include specific structural details of the components of the optical module  11 . 3 . 1 - 520 . The optical module  11 . 3 . 1 - 520  can be implemented in the context of a head-mounted display (e.g., the head-mounted device  11 . 3 . 1 - 100 ) and may be implemented according to the description of the optical module  11 . 3 . 1 - 120  and the further description herein. The optical module  11 . 3 . 1 - 520  includes an optical module housing assembly  11 . 3 . 1 - 522 , a display assembly  11 . 3 . 1 - 524 , a lens  11 . 3 . 1 - 526 , an eye camera  11 . 3 . 1 - 530 , and an infrared emitter  11 . 3 . 1 - 532 . As will be described further herein, these components are arranged along an optical axis  11 . 3 . 1 - 521  of the optical module  11 . 3 . 1 - 520  such that images generated using the display assembly are projected to the user along the optical axis  11 . 3 . 1 - 521 . 
     Although the lens  11 . 3 . 1 - 526  is described as a single element herein, it should be understood that the lens  11 . 3 . 1 - 526  may be part of an assembly of optical elements or may be an assembly of optical elements, as described with respect to the lens assembly  11 . 3 . 1 - 126 . Thus, for example the lens  11 . 3 . 1 - 526  may be a catadioptric lens or the lens  11 . 3 . 1 - 526  may be part of a catadioptric optical system. 
     The optical module housing assembly  11 . 3 . 1 - 522  may include multiple parts that are connected to each other. In the illustrated example, the optical module housing assembly  11 . 3 . 1 - 522  includes a housing body  11 . 3 . 1 - 534  and a retainer  11 . 3 . 1 - 536 . The housing body  11 . 3 . 1 - 534  is configured to be connected to other structures within the housing of a head-mounted display (e.g., in the device housing  11 . 3 . 1 - 102  of the head-mounted device  11 . 3 . 1 - 100 ). The housing body  11 . 3 . 1 - 534  is also provides a structure to which other components of the optical module  11 . 3 . 1 - 520  may be attached, including the display assembly  11 . 3 . 1 - 524 , the eye camera  11 . 3 . 1 - 530  and the infrared emitter  11 . 3 . 1 - 532 . The primary portions of the optical module housing assembly  11 . 3 . 1 - 522 , such as the housing body  11 . 3 . 1 - 534  and the retainer  11 . 3 . 1 - 536 , may be made from a rigid material, such as plastic or aluminum. The optical module housing assembly  11 . 3 . 1 - 522  is arranged around the optical axis  11 . 3 . 1 - 521 , and both visible light and infrared radiation may be incident on surfaces of the optical module housing assembly  11 . 3 . 1 - 522 . For this reason, portions of the optical module housing assembly  11 . 3 . 1 - 522  may be coated with materials (e.g., paints or other coating materials) that exhibit low reflectance of both visible and infrared wavelengths of electromagnetic radiation. 
     The retainer  11 . 3 . 1 - 536  is connected to an outer (e.g., user-facing) end of the housing body  11 . 3 . 1 - 534  of the optical module  11 . 3 . 1 - 520 . As examples, the retainer  11 . 3 . 1 - 536  may be connected to the housing body  11 . 3 . 1 - 534  by fasteners or by an adhesive. The retainer  11 . 3 . 1 - 536  and the housing body  11 . 3 . 1 - 534  of the optical module housing assembly  11 . 3 . 1 - 522  are configured such that the lens  11 . 3 . 1 - 526  is retained between the retainer  11 . 3 . 1 - 536  and the housing body  11 . 3 . 1 - 534 , as will be explained further herein. The retainer  11 . 3 . 1 - 536  and the housing body  11 . 3 . 1 - 534  have ring-like configurations along the optical axis  11 . 3 . 1 - 521  to allow light from the display assembly  11 . 3 . 1 - 524  to pass through the lens  11 . 3 . 1 - 526  and toward the user. 
     The display assembly  11 . 3 . 1 - 524  includes a seal  11 . 3 . 1 - 538 , a bezel  11 . 3 . 1 - 540 , a display module  11 . 3 . 1 - 542 , a thermal interface  11 . 3 . 1 - 544 , and a heat sink  11 . 3 . 1 - 546 . The display assembly  11 . 3 . 1 - 524  is connected to the optical module housing assembly  11 . 3 . 1 - 522 . As an example, the display assembly  11 . 3 . 1 - 524  may be connected to the optical module housing assembly  11 . 3 . 1 - 522  by screws or other fasteners that allow disassembly of the display assembly  11 . 3 . 1 - 524  from the optical module housing assembly  11 . 3 . 1 - 522  (e.g., to allow for inspection and/or repair). The seal  11 . 3 . 1 - 538  is a sealing material of any suitable type that is configured to prevent foreign particle (e.g., dust) intrusion at the interface of the display assembly  11 . 3 . 1 - 524  with the optical module housing assembly  11 . 3 . 1 - 522 . The bezel  11 . 3 . 1 - 540  is a structural component that supports the display module  11 . 3 . 1 - 542  and protects it from damage. As an example, bezel  11 . 3 . 1 - 540  may be connected to the heat sink  11 . 3 . 1 - 546  (e.g., by screws of other fasteners) to capture the display module  11 . 3 . 1 - 542  and the heat sink  11 . 3 . 1 - 546 . The seal  11 . 3 . 1 - 538  may be engaged with the bezel  11 . 3 . 1 - 540  and the optical module housing assembly  11 . 3 . 1 - 522  to seal the interface between them. 
     The seal  11 . 3 . 1 - 538  and the bezel  11 . 3 . 1 - 540  have a ring-like configuration with central openings along the optical axis  11 . 3 . 1 - 521  in order to avoid blocking light emission from the display module  11 . 3 . 1 - 542  toward the lens  11 . 3 . 1 - 526 . 
     The display module  11 . 3 . 1 - 542  includes a display screen that displays images (e.g., by emitting light using a grid of light-emitting elements to define a picture). The display module  11 . 3 . 1 - 542  may be implemented using any suitable display technology, including light-emitting diode-based display technologies, organic light-emitting diode-based display technologies, and micro light-emitting diode-based display technologies. In some implementations, a layer of cover glass is attached (e.g., by laminating) to the display surface of the display module  11 . 3 . 1 - 542  to provide strength, to serve as a mounting feature, and to serve as a sealing interface. 
     The thermal interface  11 . 3 . 1 - 544  is a thermally conductive and electrically non-conductive material that is located between the display module  11 . 3 . 1 - 542  and the heat sink  11 . 3 . 1 - 546  to promote heat transfer from the display module  11 . 3 . 1 - 542  to the heat sink  11 . 3 . 1 - 546 . The thermal interface  11 . 3 . 1 - 544  is a compliant material that is able to fill in gaps that would otherwise be present between the display module  11 . 3 . 1 - 542  and the heat sink  11 . 3 . 1 - 546 , and which would reduce the efficiency of heat transfer. As an example, the thermal interface may be dispensable thermal gel that is applied to the display module  11 . 3 . 1 - 542  or the heat sink  11 . 3 . 1 - 546 . A reworkable material may be used for the thermal interface  11 . 3 . 1 - 544 , such as a material that is applied by room-temperature vulcanization. 
     The heat sink  11 . 3 . 1 - 546  is a rigid structure (e.g., formed from metal) that readily conducts heat and is configured to release heat to the ambient environment. As an example, the heat sink  11 . 3 . 1 - 546  may incorporate structures that increase surface area, such as fins, to promote heat dissipation, and/or may include features that conduct heat away from heat-generating components (e.g., the display module  11 . 3 . 1 - 542 ), such as a heat pipe. 
       FIG.  11 . 3   . 1 - 6  is a front view illustration that shows the lens  11 . 3 . 1 - 526  according to an example, and  FIG.  11 . 3   . 1 - 7  is a cross-section view illustration taken along line B-B of  FIG.  11 . 3   . 1 - 6  showing the lens  11 . 3 . 1 - 526 . The lens  11 . 3 . 1 - 526  is an optical element (or combination of multiple optical elements, e.g., multiple lenses) that is configured to refract and/or reflect light that is incident on the lens  11 . 3 . 1 - 526 . In the illustrated example, the lens  11 . 3 . 1 - 526  is formed from molded transparent plastic, by glass may be used. Surface configurations that cause refraction and/or reflection of light (e.g., convexity and concavity) are not shown in the figures for simplicity and clarity, and these features may be defined as needed for desired performance of the optical system. 
     The lens  11 . 3 . 1 - 526  includes a lens body  11 . 3 . 1 - 648  and projections  11 . 3 . 1 - 649  that extend outward from the lens body  11 . 3 . 1 - 648 . The lens body  11 . 3 . 1 - 648  extends from an outer surface  11 . 3 . 1 - 750  (oriented toward the user) to an inner surface  11 . 3 . 1 - 751  (oriented toward the display assembly  11 . 3 . 1 - 524 . The lens body  11 . 3 . 1 - 648  will typically have a width (or range of widths) that is greater than the height of the lens body  11 . 3 . 1 - 648  as measured along the optical axis  11 . 3 . 1 - 521  of the optical module  11 . 3 . 1 - 520 . The lens body  11 . 3 . 1 - 648  may be formed in any shape (as viewed from an end along the optical axis  11 . 3 . 1 - 521 ), such as generally cylindrical, oval, rounded rectangle, or irregular. The projections  11 . 3 . 1 - 649  may have a height (in the direction of the optical axis  11 . 3 . 1 - 521 ) that is less than the height of the lens body  11 . 3 . 1 - 648 , such as 10 percent to 50 percent of the height of the lens body  11 . 3 . 1 - 648 . As will be explained herein, the projections  11 . 3 . 1 - 649  facilitate alignment and retention of the lens  11 . 3 . 1 - 526  relative to the optical module housing assembly  11 . 3 . 1 - 522 . 
     In the illustrated example, a peripheral wall of the lens body  11 . 3 . 1 - 648  extends from the outer surface  11 . 3 . 1 - 750  to the inner surface  11 . 3 . 1 - 751  without tapering, so that the peripheral wall is generally in alignment with the optical axis  11 . 3 . 1 - 521  and the outer surface  11 . 3 . 1 - 750  and the inner surface  11 . 3 . 1 - 751  are generally the same in shape and size (e.g., except for minor deviations such as the projections  11 . 3 . 1 - 649 ). In other implementations, the peripheral wall of the lens body  11 . 3 . 1 - 648  may be tapered. For example, the peripheral wall of the lens body  11 . 3 . 1 - 648  may be tapered progressively away from the optical axis  11 . 3 . 1 - 521  in a direction of travel extending from the outer surface  11 . 3 . 1 - 750  to the inner surface  11 . 3 . 1 - 751 , so that that the size of the outer surface  11 . 3 . 1 - 750  is smaller than the size of the inner surface  11 . 3 . 1 - 751 . 
       FIG.  11 . 3   . 1 - 8  is a front view illustration that shows the housing body  11 . 3 . 1 - 534  of the optical module housing assembly  11 . 3 . 1 - 522 , and  FIG.  11 . 3   . 1 - 9  is a cross-section view illustration taken along line C-C of  FIG.  11 . 3   . 1 - 8  showing the housing body  11 . 3 . 1 - 534 . The housing body  11 . 3 . 1 - 534  includes a base portion  11 . 3 . 1 - 852 , an optical pathway opening  11 . 3 . 1 - 853  that is formed through the base portion  11 . 3 . 1 - 852 , a peripheral wall  11 . 3 . 1 - 854  that extends around the optical pathway opening  11 . 3 . 1 - 853 . 
     The base portion  11 . 3 . 1 - 852  extends generally perpendicular to the optical axis  11 . 3 . 1 - 521  of the optical module  11 . 3 . 1 - 520 . The base portion  11 . 3 . 1 - 852  may incorporate features that allow attachment of other components to the optical module housing assembly  11 . 3 . 1 - 522 . As one example, the display assembly  11 . 3 . 1 - 524  may be attached to the base portion  11 . 3 . 1 - 852  (e.g., by fasteners or adhesives). As another example, the eye camera  11 . 3 . 1 - 530  may be attached to the base portion  11 . 3 . 1 - 852  (e.g., by fasteners or adhesives). 
     The peripheral wall  11 . 3 . 1 - 854  extends outward from the base portion  11 . 3 . 1 - 852  in a direction that is generally toward the user and generally aligned with the optical axis  11 . 3 . 1 - 521  of the optical module  11 . 3 . 1 - 520 . As viewed along the optical axis  11 . 3 . 1 - 521 , the shape and size of the peripheral wall  11 . 3 . 1 - 854  is similar to that of the outer periphery of the lens  11 . 3 . 1 - 526 , since the peripheral wall  11 . 3 . 1 - 854  is part of the structure that supports and retains the lens  11 . 3 . 1 - 526 , as will be described further herein. A vent port  11 . 3 . 1 - 855  is formed through the peripheral wall  11 . 3 . 1 - 854  and may extend, for example, between inner and outer surfaces of the peripheral wall  11 . 3 . 1 - 854  in a direction that is generally perpendicular to the optical axis  11 . 3 . 1 - 521  of the optical module  11 . 3 . 1 - 520 . An electrical port  11 . 3 . 1 - 856  is formed through the peripheral wall  11 . 3 . 1 - 854  and may extend, for example, between inner and outer surfaces of the peripheral wall  11 . 3 . 1 - 854  in a direction that is generally perpendicular to the optical axis  11 . 3 . 1 - 521  of the optical module  11 . 3 . 1 - 520 . 
     A base surface  11 . 3 . 1 - 857  is defined on the base portion  11 . 3 . 1 - 852  and is located inward from the peripheral wall  11 . 3 . 1 - 854 . The base surface  11 . 3 . 1 - 857  is adjacent to and extends around the optical pathway opening  11 . 3 . 1 - 853 , which is an opening that is defined by the housing body  11 . 3 . 1 - 534  to allow light to travel from the display assembly  11 . 3 . 1 - 524  to the lens  11 . 3 . 1 - 526 . A camera opening  11 . 3 . 1 - 858  is formed through the base surface  11 . 3 . 1 - 857  and is adjacent to, but separate from, the optical pathway opening  11 . 3 . 1 - 853 . The camera opening  11 . 3 . 1 - 858  extends through the base surface  11 . 3 . 1 - 857  in a direction that is generally toward the user. As examples the camera opening  11 . 3 . 1 - 858  may extend through the base surface  11 . 3 . 1 - 857  in a direction that is generally aligned with the optical axis  11 . 3 . 1 - 521  of the optical module  11 . 3 . 1 - 520 , or within 45 degrees of parallel to the optical axis  11 . 3 . 1 - 521  of the optical module  11 . 3 . 1 - 520 . 
       FIG.  11 . 3   . 1 - 10  is a front view illustration that shows the retainer  11 . 3 . 1 - 536  of the optical module housing assembly  11 . 3 . 1 - 522 , and  FIG.  11 . 3   . 1 - 11  is a cross-section view illustration taken along line D-D of  FIG.  11 . 3   . 1 - 10  showing the retainer  11 . 3 . 1 - 536 . The retainer  11 . 3 . 1 - 536  includes a peripheral wall  11 . 3 . 1 - 1060  that extends around an optical pathway opening  11 . 3 . 1 - 1061 . The peripheral wall  11 . 3 . 1 - 1060  includes an upper inner periphery portion  11 . 3 . 1 - 1062  that borders and extends around the optical pathway opening  11 . 3 . 1 - 1061 . The upper inner periphery portion  11 . 3 . 1 - 1062  is configured to receive the lens body  11 . 3 . 1 - 648  of the lens  11 . 3 . 1 - 526 . Channels  11 . 3 . 1 - 1063  are formed in the upper inner periphery portion  11 . 3 . 1 - 1062  and are open to the optical pathway opening  11 . 3 . 1 - 1061 . The size and position of the channels  11 . 3 . 1 - 1063  corresponds to the size and position of the projections  11 . 3 . 1 - 649  of the lens  11 . 3 . 1 - 526  such that the projections  11 . 3 . 1 - 649  can be received in the channels  11 . 3 . 1 - 1063  to secure the lens  11 . 3 . 1 - 526  relative to the housing body  11 . 3 . 1 - 534  and restrain relative movement. The peripheral wall  11 . 3 . 1 - 1060  includes a lower inner periphery portion  11 . 3 . 1 - 1064  that borders and extends around the optical pathway opening  11 . 3 . 1 - 1061 . The lower inner periphery portion  11 . 3 . 1 - 1064  is configured for connection to the peripheral wall  11 . 3 . 1 - 854  of the housing body  11 . 3 . 1 - 534 . 
       FIG.  11 . 3   . 1 - 12  is a front view illustration that shows the infrared emitter  11 . 3 . 1 - 532 . The infrared emitter  11 . 3 . 1 - 532  includes a flexible circuit  11 . 3 . 1 - 1266 , emissive components  11 . 3 . 1 - 1267 , an electrical connector  11 . 3 . 1 - 1268 , and a sealing element  11 . 3 . 1 - 1269 . The flexible circuit  11 . 3 . 1 - 1266  is a flexible substrate that has electrical conductors formed on it. The flexible substrate may be nonconductive polymer film. The electrical conductors may be conductive traces formed from copper. As an example, the flexible circuit  11 . 3 . 1 - 1266  may be formed by multiple layers of nonconductive polymer film with conductive traces formed between adjacent layers of the film. As will be explained further herein, the shape of the flexible circuit  11 . 3 . 1 - 1266  may be arranged such that is conforms to the shape of a portion of the optical module housing assembly  11 . 3 . 1 - 522  such that the infrared emitter may be located in or connected to the optical module housing assembly  11 . 3 . 1 - 522 . In the illustrated example, the flexible circuit  11 . 3 . 1 - 1266  has a c-shaped configured that allows the flexible circuit  11 . 3 . 1 - 1266  to extend around the optical axis  11 . 3 . 1 - 521  of the optical module  11 . 3 . 1 - 520  so that the emissive components  11 . 3 . 1 - 1267  may be arranged around the optical axis  11 . 3 . 1 - 521  in an array without blocking the optical path (pathway along which light may travel) between the display assembly  11 . 3 . 1 - 524  and the lens  11 . 3 . 1 - 526  of the optical module  11 . 3 . 1 - 520 . 
     The emissive components  11 . 3 . 1 - 1267  are components that are configured to emit infrared radiation within one or more wavelength bands. The infrared radiation that is emitted by the emissive components  11 . 3 . 1 - 1267  and reflected by the user&#39;s eye may be imaged by the eye camera  11 . 3 . 1 - 530  for use in imaging tasks. 
     The emissive components  11 . 3 . 1 - 1267  may be for example, infrared light emitting diodes. In one implementation, the emissive components  11 . 3 . 1 - 1267  include a first group of components that are configured to emit infrared radiation in a first wavelength band and a second group of components that are configured to emit infrared radiation in a second wavelength band. The first and second wavelength bands may correspond to different imaging tasks. As an example, the first wavelength band may be configured for use in biometric identification by iris scanner (e.g., a wavelength band including 850 nanometers), and the second wavelength band may be configured for use in eye gaze direction tracking (e.g., a wavelength band including 940 nanometers). 
     The electrical connector  11 . 3 . 1 - 1268  of the infrared emitter  11 . 3 . 1 - 532  is a standard component of any suitable type that allows connection to other components to provide electrical power and, optionally, operating commands, to the infrared emitter  11 . 3 . 1 - 532 . The sealing element  11 . 3 . 1 - 1269  is formed on the flexible circuit  11 . 3 . 1 - 1266  between the electrical connector  11 . 3 . 1 - 1268  and the emissive components  11 . 3 . 1 - 1267 . As best seen in  FIG.  11 . 3   . 1 - 13 , which is a cross-section view illustration showing the flexible circuit and a portion of the peripheral wall  11 . 3 . 1 - 854  of the housing body  11 . 3 . 1 - 534  of the optical module housing assembly  11 . 3 . 1 - 522 , the flexible circuit  11 . 3 . 1 - 1266  extends through and is surrounded by the sealing element  11 . 3 . 1 - 1269 . The sealing element  11 . 3 . 1 - 1269  is formed from a resilient flexible material that is configured to engage a portion of the optical module housing assembly  11 . 3 . 1 - 522  to allow the flexible circuit  11 . 3 . 1 - 1266  to exit the interior of the optical module housing assembly  11 . 3 . 1 - 522  without providing a pathway along which foreign particles (e.g., dust particles) may enter the interior of the optical module housing assembly  11 . 3 . 1 - 522 . As an example, the sealing element  11 . 3 . 1 - 1269  may be formed from silicone that is overmolded onto the flexible circuit  11 . 3 . 1 - 1266  such that the flexible circuit extends through the sealing element  11 . 3 . 1 - 1269 . In the illustrated example, the flexible circuit  11 . 3 . 1 - 1266  extends through the electrical port  11 . 3 . 1 - 856  of the housing body  11 . 3 . 1 - 534  such that the sealing element  11 . 3 . 1 - 1269  is located in the electrical port  11 . 3 . 1 - 856  and is engaged with the housing body  11 . 3 . 1 - 534  ad the electrical port  11 . 3 . 1 - 856  to define a seal and occupy the electrical port  11 . 3 . 1 - 856  to prevent entry of foreign particles. 
       FIG.  11 . 3   . 1 - 14  is a cross section view illustration that shows the optical module  11 . 3 . 1 - 520 . 
     The lens  11 . 3 . 1 - 526  is disposed between the housing body  11 . 3 . 1 - 534  and the retainer  11 . 3 . 1 - 536 . The housing body  11 . 3 . 1 - 534  is connected to the retainer  11 . 3 . 1 - 536  such that the lens  11 . 3 . 1 - 526  is located between the housing body  11 . 3 . 1 - 534  and the retainer  11 . 3 . 1 - 536 . Thus, the housing body  11 . 3 . 1 - 534  and the retainer  11 . 3 . 1 - 536  engage the lens  11 . 3 . 1 - 526  such that the lens  11 . 3 . 1 - 526  is restrained from moving relative to the housing body  11 . 3 . 1 - 534  and the retainer  11 . 3 . 1 - 536 . To protect the lens  11 . 3 . 1 - 526  from damage (e.g., if the head-mounted device  11 . 3 . 1 - 100  is dropped), a layer of adhesive may be present between the lens  11 . 3 . 1 - 526  and portions of the housing body  11 . 3 . 1 - 534  and/or the retainer  11 . 3 . 1 - 536 . The adhesive that is used for this purposes is strong to secure the lens  11 . 3 . 1 - 526  in a desired alignment and is flexible and elastic to cushion the lens  11 . 3 . 1 - 526  in the event of vibration or impact shock, and to allow the lens  11 . 3 . 1 - 526  to return to its original position. 
     The vent port  11 . 3 . 1 - 855  is formed through the peripheral wall  11 . 3 . 1 - 854  of the housing body  11 . 3 . 1 - 534  and allows air to enter and exit an internal space  11 . 3 . 1 - 1470  of the optical module  11 . 3 . 1 - 520 . The internal space  11 . 3 . 1 - 1470  is defined within the optical module housing assembly  11 . 3 . 1 - 522  by the housing body  11 . 3 . 1 - 534  and the retainer  11 . 3 . 1 - 536  and between the lens  11 . 3 . 1 - 526  and the display assembly  11 . 3 . 1 - 524 . The internal space  11 . 3 . 1 - 1470  is sealed from the outside environment except at the vent port  11 . 3 . 1 - 855 . The vent port  11 . 3 . 1 - 885  is a passage that allows air to travel between the internal space  11 . 3 . 1 - 1470  and the outside environment that is located around the optical module  11 . 3 . 1 - 520 . By allowing air to enter and exit the internal space  11 . 3 . 1 - 1470 , air pressure within the internal space  11 . 3 . 1 - 1470  remains at or near ambient (e.g., outside the optical module  11 . 3 . 1 - 520 ) air pressure. To exclude foreign particles from the internal space  11 . 3 . 1 - 1470 , a filter element  11 . 3 . 1 - 1471  is connected to the vent port  11 . 3 . 1 - 885  such that any air that passes through the vent port  11 . 3 . 1 - 855  must pass through filter element  11 . 3 . 1 - 1471 . The filter element  11 . 3 . 1 - 1471  is configured to restrain foreign particles from entering the internal space through the vent port  11 . 3 . 1 - 885  (e.g., by preventing entry of foreign particles that are larger than a pore size of the filter material). As examples, the filter element  11 . 3 . 1 - 1471  may be located in or on the vent port  11 . 3 . 1 - 855 . The filter element  11 . 3 . 1 - 1471  has a small pore size that is intended to exclude small particles (e.g., dust particles) from the internal space  11 . 3 . 1 - 1470 . As one example, the filter element  11 . 3 . 1 - 1471  may be formed from a polytetrafluoroethylene (PTFE) filter material. To capture particles that are present inside the internal space  11 . 3 . 1 - 1470 , a dust trap  11 . 3 . 1 - 1472  may be located in the internal space  11 . 3 . 1 - 1470 , for example, connected to an internal surface of the housing body  11 . 3 . 1 - 534 . The dust trap  11 . 3 . 1 - 1472  is configured to retain foreign particles on its surface, so that the foreign particles do not instead settle on surfaces where they may cause an optical aberration. As an example, the dust trap  11 . 3 . 1 - 1472  may be an adhesive element, such as a sheet coated in adhesive material, to which airborne particles that are inside the internal space  11 . 3 . 1 - 1470  may become affixed, which prevents the particles from attaching to the display assembly  11 . 3 . 1 - 524 , or the lens  11 . 3 . 1 - 526 , which could cause optical aberrations that are perceptible to the user (e.g., a visual artifact similar to a dead pixel). 
     The eye camera  11 . 3 . 1 - 530  is a still image camera or video camera that is configured to obtain images. When in use, the images that are obtained by the eye camera  11 . 3 . 1 - 530  include a visual representation of part of or all of the user&#39;s eye, so that the obtained images may be used for biometric identification (e.g., verifying the identity of the user based on an image of the user&#39;s eye) and gaze tracking. In the implementations that are discussed herein, the eye camera  11 . 3 . 1 - 530  is sensitive to infrared light (i.e., electromagnetic radiation in the infrared portion of the electromagnetic spectrum). Thus, the eye camera  11 . 3 . 1 - 530  may be configured to obtain images that show reflected portions of the infrared radiation that is emitted by the infrared emitter  11 . 3 . 1 - 532 , and these reflected portions of infrared radiation, as represented in the images, are useful for observing and identifying features of the user&#39;s eye, which may be done using a machine vision-based system that is implemented in software that is executed by the head-mounted device  11 . 3 . 1 - 100 . In alternative implementations, the eye camera  11 . 3 . 1 - 530  may instead by implemented using a visible spectrum camera or may be supplemented using the visible spectrum camera in addition to an infrared spectrum camera. 
     The eye camera  11 . 3 . 1 - 530  is connected to the housing body  11 . 3 . 1 - 534  of the optical module housing assembly  11 . 3 . 1 - 522  adjacent to the camera opening  11 . 3 . 1 - 858  of the housing body  11 . 3 . 1 - 534  such that an optical axis  11 . 3 . 1 - 1431  of the eye camera  11 . 3 . 1 - 530  extends through the camera opening  11 . 3 . 1 - 858 . In the illustrated example, the eye camera  11 . 3 . 1 - 530  is oriented such that an optical axis  11 . 3 . 1 - 1431  of the eye camera  11 . 3 . 1 - 530  is substantially aligned with the optical axis  11 . 3 . 1 - 521  of the optical module  11 . 3 . 1 - 520 . However, the eye camera  11 . 3 . 1 - 530  is positioned near an outer periphery of the lens  11 . 3 . 1 - 526  and is therefore offset and outward from the optical axis  11 . 3 . 1 - 521  of the optical module  11 . 3 . 1 - 520 . Thus, the housing body  11 . 3 . 1 - 534  and/or the eye camera  11 . 3 . 1 - 530  may be configured (e.g., by an inclined mounting surface) such that the optical axis  11 . 3 . 1 - 1431  of the eye camera  11 . 3 . 1 - 530  is angled toward the optical axis  11 . 3 . 1 - 521  of the optical module  11 . 3 . 1 - 520 , as shown in  FIG.  11 . 3   . 1 - 15 , which is a cross-section view illustration that shows the optical module  11 . 3 . 1 - 520  according to an alternative implementation. 
     Returning to  FIG.  11 . 3   . 1 - 14 , in some implementations, a fiducial marker  11 . 3 . 1 - 1465  may be formed on the lens  11 . 3 . 1 - 526 . The fiducial marker  11 . 3 . 1 - 1465  is any manner of marking that can be perceived and located in images obtained by the eye camera  11 . 3 . 1 - 530 . The fiducial marker  11 . 3 . 1 - 1465  is visible in images obtained by the eye camera  11 . 3 . 1 - 530  for use in calibration. The head-mounted device  11 . 3 . 1 - 100  is calibrated to account for manufacturing conditions, user attributes, and/or other factors that may cause visual aberrations. During an initial calibration, the position of the fiducial marker  11 . 3 . 1 - 1465  is determined and stored. The lens  11 . 3 . 1 - 526  may shift with respect to other components, such as the optical module housing assembly  11 . 3 . 1 - 522 , for example, if the head-mounted device  11 . 3 . 1 - 100  is dropped. The changed position of the lens  11 . 3 . 1 - 526  can be identified by comparing the position of the lens  11 . 3 . 1 - 526  in images obtained by the eye camera  11 . 3 . 1 - 530  with the position of the lens  11 . 3 . 1 - 526  in the images that was obtained at the time of calibration. In response to determining that the lens position has changed, calibration is performed again to address any visual aberrations that may have resulted from the shift in position of the lens  11 . 3 . 1 - 526 . 
     The infrared emitter  11 . 3 . 1 - 532  is located on the base surface  11 . 3 . 1 - 857  of the housing body  11 . 3 . 1 - 534  and extends around the optical axis  11 . 3 . 1 - 521  within the internal space  11 . 3 . 1 - 1470  that is defined within the optical module housing assembly  11 . 3 . 1 - 522  by the housing body  11 . 3 . 1 - 534  and the retainer  11 . 3 . 1 - 536  and between the lens  11 . 3 . 1 - 526  and the display assembly  11 . 3 . 1 - 524 . The display assembly  11 . 3 . 1 - 524  is connected to the housing body  11 . 3 . 1 - 534  of optical module housing assembly  11 . 3 . 1 - 522  adjacent to the optical pathway opening  11 . 3 . 1 - 853  of the housing body  11 . 3 . 1 - 534 . 
     In one implementation, the optical module  11 . 3 . 1 - 520  includes the optical module housing assembly  11 . 3 . 1 - 522 , the display assembly  11 . 3 . 1 - 524 , the lens  11 . 3 . 1 - 526 , and the eye camera  11 . 3 . 1 - 530 . The lens  11 . 3 . 1 - 526  is positioned at a first end of the optical module housing assembly  11 . 3 . 1 - 522 , the display assembly and the eye camera  11 . 3 . 1 - 530  are positioned at a second end of the optical module housing assembly  11 . 3 . 1 - 522 , and the internal space  11 . 3 . 1 - 1470  is defined within the optical module housing assembly  11 . 3 . 1 - 522  between the first end and the second end. The lens  11 . 3 . 1 - 526  is positioned such that it is able to obtain images of the user&#39;s eye through the lens  11 . 3 . 1 - 526 . The lens  11 . 3 . 1 - 526  may be connected to the optical module housing assembly  11 . 3 . 1 - 522  such that it is positioned adjacent to the display assembly  11 . 3 . 1 - 524 , such as in a side-by-side arrangement with respect to the display assembly  11 . 3 . 1 - 524 . 
     In one implementation, the optical module  11 . 3 . 1 - 520  includes the optical module housing assembly  11 . 3 . 1 - 522 , the display assembly  11 . 3 . 1 - 524 , the lens  11 . 3 . 1 - 526 , and the infrared emitter  11 . 3 . 1 - 532 . The lens  11 . 3 . 1 - 526  is positioned at a first end of the optical module housing assembly  11 . 3 . 1 - 522 , the display assembly is positioned at a second end of the optical module housing assembly  11 . 3 . 1 - 522 , and the internal space  11 . 3 . 1 - 1470  is defined within the optical module housing assembly  11 . 3 . 1 - 522  between the first end and the second end. The infrared emitter  11 . 3 . 1 - 532  is positioned in the internal space  11 . 3 . 1 - 1470  between the lens  11 . 3 . 1 - 526  and the display assembly  11 . 3 . 1 - 524 . The infrared emitter  11 . 3 . 1 - 532  is positioned such that is able to project infrared radiation onto the user&#39;s eye through the lens  11 . 3 . 1 - 526 . The optical module  11 . 3 . 1 - 520  also includes the eye camera  11 . 3 . 1 - 530 , which is connected to the optical module housing assembly  11 . 3 . 1 - 522  such that the infrared emitter  11 . 3 . 1 - 532  is positioned between (e.g., along the optical axis  11 . 3 . 1 - 521 ) the eye camera  11 . 3 . 1 - 530  and the lens  11 . 3 . 1 - 526 . 
     The lens  11 . 3 . 1 - 526  may be connected to the optical module housing assembly  11 . 3 . 1 - 522  such that it is positioned adjacent to the display assembly  11 . 3 . 1 - 524 , such as in a side-by-side arrangement with respect to the display assembly  11 . 3 . 1 - 524 . 
     In the implementation shown in  FIG.  11 . 3   . 1 - 14 , the infrared emitter  11 . 3 . 1 - 532  is located on the base surface  11 . 3 . 1 - 857  of the housing body  11 . 3 . 1 - 534  in the internal space  11 . 3 . 1 - 1470 .  FIG.  11 . 3   . 1 - 16  is a cross-section view illustration that shows the optical module  11 . 3 . 1 - 520  according to an alternative implementation in which the infrared emitter  11 . 3 . 1 - 532  is located outside of the housing body  11 . 3 . 1 - 534  of the optical module housing assembly  11 . 3 . 1 - 522 . In this implementation, the infrared emitter  11 . 3 . 1 - 532  is connected (e.g., by an adhesive) to an exterior surface of the housing body  11 . 3 . 1 - 534  such that it is positioned adjacent to and extends around the display assembly  11 . 3 . 1 - 524 . 
     An infrared-transmissive panel  11 . 3 . 1 - 1673  is formed in the housing body  11 . 3 . 1 - 534  to allow infrared radiation that is emitted by the infrared emitter  11 . 3 . 1 - 532  to travel through the optical module housing assembly  11 . 3 . 1 - 522  and the lens  11 . 3 . 1 - 526 . The infrared-transmissive panel  11 . 3 . 1 - 1673  is formed from a material that allows infrared radiation to pass through it without significant losses. As examples, the infrared-transmissive panel  11 . 3 . 1 - 1673  may be formed from glass or from an infrared transmissive plastic. In the illustrated example, the infrared-transmissive panel  11 . 3 . 1 - 1673  extends through an aperture that is formed through the base surface  11 . 3 . 1 - 857 . The infrared-transmissive panel  11 . 3 . 1 - 1673  may be a single panel that extends along the base surface  11 . 3 . 1 - 857  adjacent to all of the emissive components  11 . 3 . 1 - 1267  of the infrared emitter  11 . 3 . 1 - 532 , or may be multiple panels that extend through separate apertures that are formed through the base surface  11 . 3 . 1 - 857  adjacent to individual ones of the emissive components  11 . 3 . 1 - 1267 . In some implementations, the infrared-transmissive panel  11 . 3 . 1 - 1673  may be omitted in favor of forming part or all of the optical module housing assembly  11 . 3 . 1 - 522  (e.g., the housing body  11 . 3 . 1 - 534 ) from an infrared-transmissive material. 
     In the examples shown in  FIGS.  11 . 3   . 1 - 14 - 16 , the optical module  11 . 3 . 1 - 120  is shown as including a single eye camera, which is represented by the eye camera  11 . 3 . 1 - 530 . The optical module  11 . 3 . 1 - 120  could instead include more than one eye camera (e.g., two eye cameras), with each of the eye cameras being configured to obtain images showing infrared radiation that is reflected from the eye of the user. The eye cameras are located at different locations (e.g., opposite lateral sides of the eye of the user) and may be oriented at different angular orientations. The images output by multiple eye cameras may provide a more complete view of the eye of the user. 
       FIG.  11 . 3   . 1 - 17  is a side-view illustration that shows the display module  11 . 3 . 1 - 542  according to an implementation. The display module  11 . 3 . 1 - 542  includes a silicon wafer  11 . 3 . 1 - 1775  and a display element layer  11 . 3 . 1 - 1776  (e.g., an organic light-emitting diode layer) that is located on the silicon wafer  11 . 3 . 1 - 1775 . The display element layer  11 . 3 . 1 - 1776  may be covered by a glass layer  11 . 3 . 1 - 1777 . A display connector  11 . 3 . 1 - 1778  includes a first portion  11 . 3 . 1 - 1779  and a second portion  11 . 3 . 1 - 1780 . The first portion  11 . 3 . 1 - 1779  of the display connector  11 . 3 . 1 - 1778  is a flexible connector (e.g., a two-layer flexible connector) that is connected to silicon wafer  11 . 3 . 1 - 1775  by an electrical connection  11 . 3 . 1 - 1781  that connects individual conductors formed on the silicon wafer  11 . 3 . 1 - 1775  with individual conductors formed on the first portion  11 . 3 . 1 - 1779  of the display connector  11 . 3 . 1 - 1778 . As an example, the electrical connection  11 . 3 . 1 - 1781  may include an anisotropic film that bonds the display connector  11 . 3 . 1 - 1778  to the silicon wafer  11 . 3 . 1 - 1775  while allowing electrical communication. 
     The second portion  11 . 3 . 1 - 1780  of the display connector  11 . 3 . 1 - 1778  is a multi-layer (e.g., six layer) flexible connector, of the type commonly referred to as a “rigid flex” connector. The second portion  11 . 3 . 1 - 1780  may include a cavity  11 . 3 . 1 - 1782  that is defined by removal of one or more of the layers of the multi-layer structure of the second portion  11 . 3 . 1 - 1780 . A driver integrated circuit  11 . 3 . 1 - 1783  is located in the cavity  11 . 3 . 1 - 1782  in order to protect the driver integrated circuit  11 . 3 . 1 - 1783 . The function of the driver integrated circuit  11 . 3 . 1 - 1783  is to receive display signals in a first format (e.g., encoded or multiplexed) and interpret the signals into a second format that is usable by the display element layer  11 . 3 . 1 - 1776  of the display module  11 . 3 . 1 - 542  to output images. A connector  11 . 3 . 1 - 1784  (e.g., a micro-coaxial connector) may be located on and electrically connected to the second portion  11 . 3 . 1 - 1780  of the display connector  11 . 3 . 1 - 1778  in order to connect the display module  11 . 3 . 1 - 542  to other components (e.g., to a computing device that provides content to be displayed). 
       FIG.  11 . 3   . 1 - 18  is a top-view illustration that shows interpupillary distance adjustment mechanisms  11 . 3 . 1 - 1885  that each support one of the optical modules  11 . 3 . 1 - 520  (i.e., left and right optical modules) with respect to the device housing  11 . 3 . 1 - 102 . The interpupillary distance adjustment mechanisms  11 . 3 . 1 - 1885  are an example of an interpupillary distance adjustment assembly that is configured to adjust a distance between the optical modules  11 . 3 . 1 - 520  that display content to the left eye and the right eye of the user, in order to match the spacing between the optical modules  11 . 3 . 1 - 520  with the spacing between the user&#39;s eyes. 
     The optical modules  11 . 3 . 1 - 520  may be supported such that the optical axis  11 . 3 . 1 - 521  of each of the optical modules  11 . 3 . 1 - 520  extends generally in a front-to-back direction of the device housing  11 . 3 . 1 - 102 . The interpupillary distance adjustment mechanisms  11 . 3 . 1 - 1885  include support rods  11 . 3 . 1 - 1886  and actuator assemblies  11 . 3 . 1 - 1887  that are configured to cause movement of the optical modules  11 . 3 . 1 - 520  along the support rods  11 . 3 . 1 - 1886  in response to a control signal. The actuator assemblies  11 . 3 . 1 - 1887  may include conventional motion control components such as electric motors that are connected to the optical modules  11 . 3 . 1 - 520  by components such as lead screws or belts to cause movement. Mounting brackets  11 . 3 . 1 - 1888  may be connected to the optical modules  11 . 3 . 1 - 520  such that the support rods  11 . 3 . 1 - 1886  are connected to the mounting brackets  11 . 3 . 1 - 1888 , such as by extending through apertures  11 . 3 . 1 - 1889  that are formed through the mounting brackets  11 . 3 . 1 - 1888 . The interpupillary distance adjustment mechanisms  11 . 3 . 1 - 1885  may also include biasing elements such as springs that are engaged with the mounting brackets  11 . 3 . 1 - 1888  to reduce or eliminate unintended motion of the mounting brackets  11 . 3 . 1 - 1888  and/or the optical modules  11 . 3 . 1 - 520  with respect to the support rods  11 . 3 . 1 - 1886 . The support rods  11 . 3 . 1 - 1886  may be angled relative to a lateral (e.g., side-to-side) dimension of the device housing  11 . 3 . 1 - 102  such that they move toward the user as they move outward. As an example, the support rods may be angled by five degrees relative to the lateral dimension. 
       FIG.  11 . 3   . 1 - 19  is a side view illustration that shows one of the interpupillary distance adjustment mechanisms  11 . 3 . 1 - 1885 . The support rods  11 . 3 . 1 - 1886  may include upper and lower support rods for each of the optical modules that support the optical modules  11 . 3 . 1 - 520  such that the optical axis  11 . 3 . 1 - 521  of each optical module  11 . 3 . 1 - 520  is angled slightly downward, such as by five degrees. Springs  11 . 3 . 1 - 1990  (e.g., leaf springs) may be seated in the apertures  11 . 3 . 1 - 1889  of the mounting brackets  11 . 3 . 1 - 1888  and located forward from the support rods  11 . 3 . 1 - 1886  to bias the optical modules  11 . 3 . 1 - 520  toward the user. 
       FIG.  11 . 3   . 1 - 20  is a top-view cross-section illustration that shows front-facing cameras  11 . 3 . 1 - 2091  that are supported by each of the optical modules  11 . 3 . 1 - 520 . Openings or optically-transmissive panels  11 . 3 . 1 - 2092  (e.g., clear plastic) are included in the device housing  11 . 3 . 1 - 102  such that the front-facing cameras  11 . 3 . 1 - 2091  are able to obtain images of the surrounding environment through the optically-transmissive panels  11 . 3 . 1 - 2092 . A single panel or separate panels may be used for the optically-transmissive panels  11 . 3 . 1 - 2092 , and as such the device housing  11 . 3 . 1 - 102  may include one or more of the optically-transmissive panels  11 . 3 . 1 - 2092 . Thus, the device housing  11 . 3 . 1 - 102  may include one or more of the optically-transmissive panels  11 . 3 . 1 - 2092  through which the front-facing cameras may obtain images of an environment from a point of view that simulates the point of view of the user. 
     The front-facing cameras  11 . 3 . 1 - 2091  may be connected to and supported by a corresponding one of the optical modules  11 . 3 . 1 - 520 . The front-facing cameras  11 . 3 . 1 - 2091  may be positioned such that they are located on and substantially aligned with the optical axis  11 . 3 . 1 - 521  of a corresponding one of the optical modules  11 . 3 . 1 - 520  (e.g., the optical axes of the front-facing cameras  11 . 3 . 1 - 2091  may be substantially aligned with the optical axes of the optical modules  11 . 3 . 1 - 520 ). The front-facing cameras  11 . 3 . 1 - 2091  are oriented away from the user and are supported such that they are moved by the interpupillary distance adjustment mechanisms  11 . 3 . 1 - 1885 . Accordingly, when the user adjusts the interpupillary distance between the optical modules  11 . 3 . 1 - 520 , the distance between the front-facing cameras  11 . 3 . 1 - 2091  is also adjusted. Thus, images from the front-facing cameras  11 . 3 . 1 - 2091 , when displayed to the user, have been captured at the user&#39;s own interpupillary distance and therefore are presented more accurately in stereo vision. Thus, in some implementations, the optical axis of a first one of the front-facing cameras  11 . 3 . 1 - 2091  is aligned with an optical axis of a first one of the optical modules  11 . 3 . 1 - 520  and an optical axis of a second one of the front-facing cameras  11 . 3 . 1 - 2091  is aligned with an optical axis of a second one of the optical modules  11 . 3 . 1 - 520 . Thus, in some implementations, a first one of the front-facing cameras  11 . 3 . 1 - 2091  is connected in a fixed relationship with respect to a first one of the optical modules  11 . 3 . 1 - 520 , and a second one of the front-facing cameras  11 . 3 . 1 - 2091  is connected in a fixed relationship with respect to a second one of the optical modules  11 . 3 . 1 - 520 . Thus, in some implementations, the interpupillary distance adjustment mechanisms a first spacing between an optical axis of a first one of the optical modules  11 . 3 . 1 - 520  and an optical axis of a second one of the optical modules  11 . 3 . 1 - 520  generally equal to a second spacing between an optical axis of a first one of the front-facing cameras  11 . 3 . 1 - 2091  and an optical axis of a second one of the front facing cameras  11 . 3 . 1 - 2091  during adjustment of the distance between the optical modules  11 . 3 . 1 - 520 . 
       FIG.  11 . 3   . 1 - 21  is an illustration that shows connection of the eye camera  11 . 3 . 1 - 530  and the infrared emitter  11 . 3 . 1 - 532  to a computing device  11 . 3 . 1 - 2193  by an optical module jumper board  11 . 3 . 1 - 2194 . The computing device  11 . 3 . 1 - 2193  may be, for example, a computing device that incorporates the processor  11 . 3 . 1 - 108  of the head-mounted device  11 . 3 . 1 - 100 . The optical module jumper board  11 . 3 . 1 - 2194  has a data connection to the computing device  11 . 3 . 1 - 2193  over which signals and data to and from the eye camera  11 . 3 . 1 - 530  and the infrared emitter  11 . 3 . 1 - 532  are transmitted. The optical module jumper board  11 . 3 . 1 - 2194  also has separate data connections to each of the eye camera  11 . 3 . 1 - 530  and the infrared emitter  11 . 3 . 1 - 532 . Additional components could be included in the optical module  11 . 3 . 1 - 520  and connected to the optical module jumper board  11 . 3 . 1 - 2194  by additional separate connections. The optical module jumper board  11 . 3 . 1 - 2194  may be mounted to the optical module  11 . 3 . 1 - 520 , and therefore, moves in unison with the optical module  11 . 3 . 1 - 520  during interpupillary distance adjustment. As a result, the number and size of electrical connections that are made to components that are not mounted to the optical module (e.g., the computing device  11 . 3 . 1 - 2193 ) is decreased. The optical module jumper board  11 . 3 . 1 - 2194  may be, as examples, a rigid flex circuit board, a flexible circuit board, or a printed component board. 
     11.3.2: Cameras and Leds 
       FIG.  11 . 3   . 2 - 1  illustrates an example of an optical module  11 . 3 . 2 - 100  for use in an electronic device such as an HMD, including HDM devices described herein. As shown in one or more other examples described herein, the optical module  11 . 3 . 2 - 100  can be one of two optical modules within an HMD, with each optical module aligned to project light toward a user&#39;s eye. In this way, a first optical module can project light via a display screen toward a user&#39;s first eye and a second optical module of the same device can project light via another display screen toward the user&#39;s second eye. 
     In at least one example, the optical module  11 . 3 . 2 - 100  can include an optical frame or housing  11 . 3 . 2 - 102 , which can also be referred to as a barrel or optical module barrel. The optical module  11 . 3 . 2 - 100  can also include a display  11 . 3 . 2 - 104 , including a display screen or multiple display screens, coupled to the housing  11 . 3 . 2 - 102 . The display  11 . 3 . 2 - 104  can be coupled to the housing  11 . 3 . 2 - 102  such that the display  11 . 3 . 2 - 104  is configured to project light toward the eye of a user when the HMD of which the display module  11 . 3 . 2 - 100  is a part is donned during use. In at least one example, the housing  11 . 3 . 2 - 102  can surround the display  11 . 3 . 2 - 104  and provide connection features for coupling other components of optical modules described herein. 
     In one example, the optical module  11 . 3 . 2 - 100  can include one or more cameras  11 . 3 . 2 - 106  coupled to the housing  11 . 3 . 2 - 102 . The camera  11 . 3 . 2 - 106  can be positioned relative to the display  11 . 3 . 2 - 104  and housing  11 . 3 . 2 - 102  such that the camera  11 . 3 . 2 - 106  is configured to capture one or more images of the user&#39;s eye during use. In at least one example, the optical module  11 . 3 . 2 - 100  can also include a light strip  11 . 3 . 2 - 108  surrounding the display  11 . 3 . 2 - 104 . In one example, the light strip  11 . 3 . 2 - 108  is disposed between the display  11 . 3 . 2 - 104  and the camera  11 . 3 . 2 - 106 . The light strip  11 . 3 . 2 - 108  can include a plurality of lights  11 . 3 . 2 - 110 . The plurality of lights can include one or more light emitting diodes (LEDs) or other lights configured to project light toward the user&#39;s eye when the HMD is donned. The individual lights  11 . 3 . 2 - 110  of the light strip  11 . 3 . 2 - 108  can be spaced about the strip  11 . 3 . 2 - 108  and thus spaced about the display  11 . 3 . 2 - 104  uniformly or non-uniformly at various locations on the strip  11 . 3 . 2 - 108  and around the display  11 . 3 . 2 - 104 . 
     In at least one example, the housing  11 . 3 . 2 - 102  defines a viewing opening  11 . 3 . 2 - 101  through which the user can view the display  11 . 3 . 2 - 104  when the HMD device is donned. In at least one example, the LEDs are configured and arranged to emit light through the viewing opening  11 . 3 . 2 - 101  and onto the user&#39;s eye. In one example, the camera  11 . 3 . 2 - 106  is configured to capture one or more images of the user&#39;s eye through the viewing opening  11 . 3 . 2 - 101 . 
     As noted above, each of the components and features of the optical module  11 . 3 . 2 - 100  shown in  FIG.  11 . 3   . 2 - 1  can be replicated in another (e.g., second) optical module disposed with the HMD to interact (e.g., project light and capture images) of another eye of the user. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  11 . 3   . 2 - 1  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  11 . 3   . 2 - 2 - 11 . 3 . 2 - 5  and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  11 . 3   . 2 - 2 - 11 . 3 . 2 - 5  can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  11 . 3   . 2 - 1 . 
       FIG.  11 . 3   . 2 - 2  illustrates a cross-sectional view of an example of an optical module  11 . 3 . 2 - 200  including a housing  11 . 3 . 2 - 202 , display assembly  11 . 3 . 2 - 204  coupled to the housing  11 . 3 . 2 - 202 , and a lens  11 . 3 . 2 - 216  coupled to the housing  11 . 3 . 2 - 202 . In at least one example, the housing  11 . 3 . 2 - 202  defines a first aperture or channel  11 . 3 . 2 - 212  and a second aperture or channel  11 . 3 . 2 - 214 . The channels  11 . 3 . 2 - 212 ,  11 . 3 . 2 - 214  can be configured to slidably engage respective rails or guide rods of an HMD device to allow the optical module  11 . 3 . 2 - 200  to adjust in position relative to the user&#39;s eyes for match the user&#39;s interpapillary distance (IPD). The housing  11 . 3 . 2 - 202  can slidably engage the guide rods to secure the optical module  11 . 3 . 2 - 200  in place within the HMD. 
     In at least one example, the optical module  11 . 3 . 2 - 200  can also include a lens  11 . 3 . 2 - 216  coupled to the housing  11 . 3 . 2 - 202  and disposed between the display assembly  11 . 3 . 2 - 204  and the user&#39;s eyes when the HMD is donned. The lens  11 . 3 . 2 - 216  can be configured to direct light from the display assembly  11 . 3 . 2 - 204  to the user&#39;s eye. In at least one example, the lens  11 . 3 . 2 - 216  can be a part of a lens assembly including a corrective lens removably attached to the optical module  11 . 3 . 2 - 200 . In at least one example, the lens  11 . 3 . 2 - 216  is disposed over the light strip  11 . 3 . 2 - 208  and the one or more eye-tracking cameras  11 . 3 . 2 - 206  such that the camera  11 . 3 . 2 - 206  is configured to capture images of the user&#39;s eye through the lens  11 . 3 . 2 - 216  and the light strip  11 . 3 . 2 - 208  includes lights configured to project light through the lens  11 . 3 . 2 - 216  to the users&#39; eye during use. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  11 . 3   . 2 - 2  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  11 . 3   . 2 - 1  and  11 . 3 . 2 - 3 - 11 . 3 . 2 - 5  and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  11 . 3   . 2 - 1  and  11 . 3 . 2 - 3 - 11 . 3 . 2 - 5  can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  11 . 3   . 2 - 2 . 
       FIG.  11 . 3   . 2 - 3  illustrates a close-up cutaway view of an example of an optical module  11 . 3 . 2 - 304  for use in HMD devices described herein. The illustrated example includes a display assembly  11 . 3 . 2 - 304 , a lens  11 . 3 . 2 - 316 , and a light strip  11 . 3 . 2 - 308  including a plurality of lights  11 . 3 . 2 - 310  coupled to the optical module barrel/housing  11 . 3 . 2 - 302 . In at least one example, the light strip  11 . 3 . 2 - 308  is secured to the housing  11 . 3 . 2 - 302  via an adhesive  11 . 3 . 2 - 318  strip or portion. The adhesive  11 . 3 . 2 - 318  can include a pressure sensitive adhesive, epoxy, or other adhesive. The display assembly  11 . 3 . 2 - 304  can include a display housing  11 . 3 . 2 - 319  coupling various layers, including display layers and covers of the display assembly  11 . 3 . 2 - 304  to the housing  11 . 3 . 2 - 302 . In at least one example, the display housing  11 . 3 . 2 - 319  of the display assembly  11 . 3 . 2 - 304  can be coupled to the optical module housing  11 . 3 . 2 - 302  via a dust seal component  11 . 3 . 2 - 320 . The dust seal component  11 . 3 . 2 - 320  can be configured to prevent dust and debris from the air and/or external environment from infiltrating the display assembly  11 . 3 . 2 - 304  and lens  11 . 3 . 2 - 316  and spaces there between. In at least one example, the dust seal component  11 . 3 . 2 - 320  can include an adhesive. In one example, the dust seal component  11 . 3 . 2 - 320  can include a foam. In one example, the dust seal component  11 . 3 . 2 - 320  can include a material block or portion disposed between the display assembly housing  11 . 3 . 2 - 319  and the optical module housing  11 . 3 . 2 - 302 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  11 . 3   . 2 - 3  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  11 . 3   . 2 - 1 - 11 . 3 . 2 - 2  and  11 . 3 . 2 - 4 - 11 . 3 . 2 - 5  and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  11 . 3   . 2 - 1 - 11 . 3 . 2 - 2  and  11 . 3 . 2 - 4 - 11 . 3 . 2 - 5  can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  11 . 3   . 2 - 3 . 
       FIG.  11 . 3   . 2 - 4  illustrates a plan view of a portion of an example of an optical module  11 . 3 . 2 - 400  for use in HMD devices described herein. The optical module  11 . 3 . 2 - 400  can include a structural frame or housing  11 . 3 . 2 - 402  defining a viewing opening  11 . 3 . 2 - 401  and a display screen  11 . 3 . 2 - 404  coupled to the housing  11 . 3 . 2 - 402 . The display screen  11 . 3 . 2 - 404  can be a part of a multi-component/layered display assembly. The display screen  11 . 3 . 2 - 404  can include or define an inner edge  11 . 3 . 2 - 422 , an outer edge  11 . 3 . 2 - 424  opposite the inner edge  11 . 3 . 2 - 422 , a lower edge  11 . 3 . 2 - 426  extending between the inner edge  11 . 3 . 2 - 422  and the outer edge  11 . 3 . 2 - 424  and an upper edge  11 . 3 . 2 - 428  opposite the lower edge  11 . 3 . 2 - 426  and also extending between the inner edge  11 . 3 . 2 - 422  and the outer edge  11 . 3 . 2 - 424 . The optical module  11 . 3 . 2 - 400  can also include a first camera  11 . 3 . 2 - 406   a  and a second camera  11 . 3 . 2 - 406   b  coupled to the housing  11 . 3 . 2 - 402 . 
     In at least one example, the first and second cameras  11 . 3 . 2 - 406   a - b  can be positioned to capture images and visualize the user&#39;s pupil without being obstructed in view by other facial features of the user, including eyelids and eyelashes, cheeks protrusion, and the like. Accordingly, in at least one example the first camera  11 . 3 . 2 - 406   a  can be disposed adjacent the inner edge  11 . 3 . 2 - 422  and the second camera  11 . 3 . 2 - 406   b  can be disposed adjacent the lower edge  11 . 3 . 2 - 426 . In one example, the first camera  11 . 3 . 2 - 406   a  can be disposed closer to the lower edge  11 . 3 . 2 - 426  than the upper edge  11 . 3 . 2 - 428 . In one example, the second camera  11 . 3 . 2 - 406   b  can be disposed closer to the outer edge  11 . 3 . 2 - 424  than the inner edge  11 . 3 . 2 - 422 . 
     In at least one example, the optical module  11 . 3 . 2 - 400  can include an illumination strip  11 . 3 . 2 - 408  disposed on the housing  11 . 3 . 2 - 402  between the first and second cameras  11 . 3 . 2 - 406   a - b  and the display screen  11 . 3 . 2 - 404 . The illumination strip  11 . 3 . 2 - 408  can be disposed around a periphery of the display screen  11 . 3 . 2 - 404 . In at least one example, the illumination strip  11 . 3 . 2 - 408  can include a plurality of lights  11 . 3 . 2 - 410 , which can include one or more types of lights or light sources, such as LEDs. 
     As noted above with reference to  FIG.  11 . 3   . 2 - 1 , the optical modules described herein can be one of two optical modules of an HMD, each optical module configured to align and project light onto one eye of the user. Accordingly, the inner edge  11 . 3 . 2 - 422 , outer edge  11 . 3 . 2 - 424 , lower edge  11 . 3 . 2 - 426 , and upper edge  11 . 3 . 2 - 428  of the display screen  11 . 3 . 2 - 404  shown in  FIG.  11 . 3   . 2 - 4  can each be one of a set of inner edges, outer edges, lower edges, and upper edges of two adjacent display screens within an HMD. In such an example, the inner edges of the adjacent display screens of adjacent optical modules within a single HMD device can be disposed between respective outer edges. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  11 . 3   . 2 - 4  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  11 . 3   . 2 - 1 - 11 . 3 . 2 - 3  and  11 . 3 . 2 - 5  and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  11 . 3   . 2 - 1 - 11 . 3 . 2 - 3  and  11 . 3 . 2 - 5  can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  11 . 3   . 2 - 4 . 
     In the examples of optical modules shown and described with reference to  FIGS.  11 . 3   . 2 - 1 - 511 . 3 . 2 —and elsewhere herein, a controller of the HMD can be electrically coupled to the first and second cameras  11 . 3 . 2 - 406   a ,  11 . 3 . 2 - 406   b  as well as to the plurality of lights  11 . 3 . 2 - 410  of the illumination strip  11 . 3 . 2 - 408 . The controller can be configured to determine a gaze direction of an eye of the user by analyzing a pattern of light reflected off the eye and projected by the plurality of lights  11 . 3 . 2 - 410 . In at least one example, determining the gaze direction can include assigning a first weight to a first pattern of reflected light from the plurality of lights  11 . 3 . 2 - 410  and captured by the first camera  11 . 3 . 2 - 410   a  and assigning a second weight to a second pattern of reflected light from the plurality of lights  11 . 3 . 2 - 410  and captured by the second camera  11 . 3 . 2 - 410   b . Then, the controller can determine a weighted average pattern from the first pattern and the second pattern. The first weight and the second weight can be set based on a position of the display screen relative to the user&#39;s eye. 
     The position of the display screen  11 . 3 . 2 - 404  relative to the eye of the user can be determined by a first image capture by the first camera  11 . 3 . 2 - 406   a  and a second image captured by the second camera  11 . 3 . 2 - 406   b . In at least one example, the controller can be configured to change the first weight and the second weight as the position of the display screen  11 . 3 . 2 - 404  relative to the eye changes. That is, as the user moves the gaze direction of the eye such that the position of the pupil relative to the display screen  11 . 3 . 2 - 404  and the first and second cameras  11 . 3 . 2 - 406   a - b  changes, the weight given to images taken by the first and second cameras  11 . 3 . 2 - 406   a - b  can be changed to rely more heavily on the camera  11 . 3 . 2 - 406   a - b  positioned for the best determination of gaze direction based on the new position of the pupil. In addition, changes to the overall position of the cameras  11 . 3 . 2 - 406   a - b  and screen  11 . 3 . 2 - 404  relative to the user&#39;s eye, for example when donning and using the HMD device as the device slightly shifts around or is adjusted for comfort by the user over time during use, the weights can be changed to optimize gaze direction measurement and determination. Other factors, such as optical module adjustment for IPD adjustments and so forth can also affect the weight given to each camera  11 . 3 . 2 - 406   a - b  during gaze detection determination. 
       FIG.  11 . 3   . 2 - 5  illustrates a partial cutaway view of an example of an optical module  11 . 3 . 2 - 500  including a display assembly  11 . 3 . 2 - 504  and a camera  11 . 3 . 2 - 506  mounted on or coupled with a housing  11 . 3 . 2 - 502 . The display assembly  11 . 3 . 2 - 504  can include a display screen disposed within, parallel to, or defining a first major plane  11 . 3 . 2 - 530 . The major plane  11 . 3 . 2 - 530  can define a first normal axis  11 . 3 . 2 - 532  perpendicular to the major plane  11 . 3 . 2 - 530 . The display screen of the display assembly  11 . 3 . 2 - 504  is configured to direct light in a direction parallel to the first normal axis  11 . 3 . 2 - 532 . The camera  11 . 3 . 2 - 506  can include a lens  11 . 3 . 2 - 534  disposed within, parallel to, or defining a second major plane  11 . 3 . 2 - 536  defining a second normal axis  11 . 3 . 2 - 538  perpendicular to the second major plane  11 . 3 . 2 - 536 . 
     In at least one example, the first normal axis  11 . 3 . 2 - 532  is disposed at a first angle relative to the second normal axis  11 . 3 . 2 - 538  such that the first normal axis  11 . 3 . 2 - 532  and the second normal axis  11 . 3 . 2 - 528  are non-parallel. The first camera  406   a  of  FIG.  11 . 3   . 2 - 4  illustrates a similar first lens  11 . 3 . 2 - 434  and the second camera  11 . 3 . 2 - 406   b  includes a second lens  11 . 3 . 2 - 440 . In at least one example, the second lens  11 . 3 . 2 - 440  can define a normal axis disposed at a second angle relative to the first normal axis  11 . 3 . 2 - 532 . The first angle can be different than the second angle. The normal axis defined by the second lens  11 . 3 . 2 - 440  of the second camera  11 . 3 . 2 - 406   b  can also be non-parallel, or disposed at an angle relative to, the second normal axis  11 . 3 . 2 - 538  of the other camera  11 . 3 . 2 - 406   a  (or  11 . 3 . 2 - 506  illustrated in  FIG.  11 . 3   . 2 - 5 ). The first angle can be different than the second angle relative to the first major plane  530  defined by the display screen of the display assembly  11 . 3 . 2 - 504 . The angles of the cameras and lenses thereof can be configured to provide unobstructed and direct visualization of the user&#39;s pupil to capture images and track the user&#39;s gaze. 
     In at least one example, the cameras, including the camera  11 . 3 . 2 - 506  illustrated in  FIG.  11 . 3   . 2 - 5  are disposed between the display assembly  11 . 3 . 2 - 504  and the viewing opening  11 . 3 . 2 - 501  defined by the housing  11 . 3 . 2 - 502  and/or the HMD frame. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  11 . 3   . 2 - 5  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  11 . 3   . 2 - 1 - 11 . 3 . 2 - 4  and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  11 . 3   . 2 - 1 - 11 . 3 . 2 - 4  can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  11 . 3   . 2 - 5 . 
     11.4: Display 
     11.4.1: Display System with Interchangeable Lens 
       FIG.  11 . 4   . 1 - 0  illustrates a view of an HMD  11 . 4 . 1 - 100  including a display assembly  11 . 4 . 1 - 102 . The display assembly  11 . 4 . 1 - 102  is described here in more detail in sections  11 . 4 . 1 - 11 . 4 . 3 . 
     Different users may have different refractive errors of their eyes, such that different users require different corrective lenses. Disclosed herein are implementations of display systems. In one implementation, a display system includes a display, a removable lens assembly, and a lens detection sensor. The removable lens is removably coupleable to the display. The lens detection sensor detects the removable lens assembly coupled to the display. The display system may further include a head-mounted display unit that includes the display and the lens detection sensor. The display system may determine lens information from the removable lens with the lens detection sensor, and may provide an indicator of the removable lens according to the lens information. 
     In another implementation, a method is provided for operating a head-mounted display unit, which includes identifying a removable lens coupled to a display module of the head-mounted display unit, and providing an indication according to the identifying of the removable lens. The indication may be a configuration indication that differs between the removable lens and another removable lens coupleable to the display module. The method may further include identifying a user, while the indication may be a compatibility indication of compatibility between the removable lens and the user. 
     In another implementation, a display system includes a head-mounted display unit and a removable lens assembly. The head-mounted display unit includes a display module. The removable lens assembly includes a lens element and a frame coupled to the lens element. The removable lens assembly may be removably to the display module in a single orientation with a magnetic attachment features coupled to the frame and surrounding the lens element. The removable lens assembly may be removably coupleable to the display module with an interference fit, for example, with the removably lens include a compliant annular protrusion that receives a lens mount of the display module axially therein. The removable lens assembly may be removably coupleable to the display module with sprung latch mechanisms. 
     In an implementation, a display system includes a head-mounted display unit and a removable lens assembly. The head mounted display unit includes a display module and a corresponding mechanical coupling feature. The removable lens assembly includes a corrective lens element, a frame coupled to the corrective lens element, and a mechanical coupling feature coupled to the frame. The mechanical coupling feature is receivable by the corresponding mechanical coupling feature to removably couple the removable lens assembly to the head-mounted display unit. 
     The mechanical coupling feature may engage the corresponding mechanical coupling feature to prevent movement of the removable lens assembly relative to the head-mounted display unit transverse to, along, and/or about an optical axis of the corrective lens element. The removable lens assembly may further include a magnetic coupling feature spaced part from the mechanical coupling feature and by which the removable lens assembly is magnetically coupleable to the head-mounted display unit. The display module may include a lens mount that includes the corresponding mechanical coupling feature, and the removable lens assembly may be removably coupleable to the display module. 
     Disclosed herein are embodiments of display systems that include a head-mounted display unit and one or more interchangeable lenses. The interchangeable lenses may be configured according to characteristics of different eyes of different users (e.g., according to an eyeglass prescription). For example, one pair of interchangeable lenses may be associated with (e.g., prescribed for) one user, while another pair of interchangeable. Lenses may be associated with another user. The interchangeable lenses are mountable to the head-mounted display unit in an interchangeable manner such that different lenses may be mounted to the head-mounted display unit to accommodate different users with different eye characteristics. The interchangeable lenses may be mounted to the head-mounted display unit in various different manners. The display system may also identify the interchangeable lenses mounted to the head-mounted display unit and/or identify the user and perform various operations in response thereto. 
     Referring to  FIGS.  11 . 4   . 1 - 1  and  11 . 4 . 1 - 2 , a display system  11 . 4 . 1 - 100  generally includes a head-mounted display unit  11 . 4 . 1 - 110  and one or more removable lens assemblies  11 . 4 . 1 - 120 . The head-mounted display unit  11 . 4 . 1 - 110  generally includes a housing  11 . 4 . 1 - 112 , a head support  11 . 4 . 1 - 114 , one or more display modules  11 . 4 . 1 - 116  (e.g., two; one for each eye), and various other electronics (e.g., controller or computing device, communications interface, audio input and/or output devices, sensors, battery or other power electronics; see also  FIG.  11 . 4   . 1 - 12 ). The display system  11 . 4 . 1 - 100  is configured to provide a computer-generated reality (e.g., virtual reality or mixed reality) in which case the display system  11 . 4 . 1 - 100  may be considered a computer-generated reality system or computer-generated reality display system, a virtual reality system or a virtual reality display system, or a mixed reality display system or mixed reality system. The terms computer-generated reality, virtual reality, and mixed reality are discussed in further detail below. 
     The housing  11 . 4 . 1 - 112  contains or is otherwise coupled to the one or more display modules  11 . 4 . 1 - 116  and the other electronics. The housing  11 . 4 . 1 - 112  may also include various compliant components coupled thereto for engaging the face of the user to be supported thereon (not shown). The head support  11 . 4 . 1 - 114  is coupled to the housing  11 . 4 . 1 - 112  to support the head-mounted display unit  11 . 4 . 1 - 110  on a head of a user with the one or more display modules  11 . 4 . 1 - 116  in suitable positions relative to eyes of the user. As shown, the head support  11 . 4 . 1 - 114  may be configured as a strap or band that is configured to extend around the head of the user. The display module  11 . 4 . 1 - 116  is coupled to the housing  11 . 4 . 1 - 112  in a fixed position (as shown). 
     Referring to  FIGS.  11 . 4   . 1 - 3 A- 3 B, the display module  11 . 4 . 1 - 116  generally includes a display  11 . 4 . 1 - 316   a , a primary lens  11 . 4 . 1 - 316   b , and a lens mount  11 . 4 . 1 - 316   c . The display  11 . 4 . 1 - 316   a  is configured to display graphics to the user. The display  11 . 4 . 1 - 316   a  may, for example, be a display screen, such as a liquid crystal display (LCD), organic light-emitting diode (OLED) display, or other suitable display. The display  11 . 4 . 1 - 316   a  may be coupled to a circuit board or other support. 
     The primary lens  11 . 4 . 1 - 316   b  is coupled to the display  11 . 4 . 1 - 316   a . The primary lens  11 . 4 . 1 - 116   b  is positioned between the display  11 . 4 . 1 - 116   a  and the eye of the user and refracts light emitted from the display  11 . 4 . 1 - 316   a  before reaching the eye of the user. The primary lens  11 . 4 . 1 - 316   b  may be omitted depending on configurations of the display  11 . 4 . 1 - 316   a  and/or the removable lens assemblies  11 . 4 . 1 - 120 . 
     The lens mount  11 . 4 . 1 - 316   c  is configured to couple to and support the removable lens assembly  11 . 4 . 1 - 120  between the display  11 . 4 . 1 - 316   a  and the eye of the user (i.e., with the primary lens  11 . 4 . 1 - 316   b  being between the display  11 . 4 . 1 - 316   a  and the removable lens assembly  11 . 4 . 1 - 120 ). As shown, the lens mount  11 . 4 . 1 - 316   c  may additionally couple to and support the primary lens  11 . 4 . 1 - 316   b  relative to the display  11 . 4 . 1 - 316   a , such as by being coupled to the circuit board or other support of the display  11 . 4 . 1 - 316   a . The lens mount  11 . 4 . 1 - 316   c  may surround the primary lens  11 . 4 . 1 - 316   b  (e.g., forming a bezel or barrel structure of the primary lens  11 . 4 . 1 - 316   b ). 
     The removable lens assembly  11 . 4 . 1 - 120  generally includes a lens element  11 . 4 . 1 - 330  and a lens frame  11 . 4 . 1 - 340 . The lens frame  11 . 4 . 1 - 340  is coupled to the lens element  11 . 4 . 1 - 330  and is configured to removably couple to the lens mount  11 . 4 . 1 - 316   c  for supporting the lens element  11 . 4 . 1 - 330  in a predetermined position on to the display module  11 . 4 . 1 - 116  (e.g., relative to the display  11 . 4 . 1 - 316   a  and/or the primary lens  11 . 4 . 1 - 316   b ). The lens frame  11 . 4 . 1 - 340  is, for example, coupled to an outer peripheral surface of the lens element  11 . 4 . 1 - 330 . Variations of the lens mount  11 . 4 . 1 - 316   c  and interaction with the lens frame  11 . 4 . 1 - 340  are discussed in further detail below. The removable lens assembly  11 . 4 . 1 - 120  may also be referred to as an interchangeable lens assembly, an interchangeable lens, a removable lens, or a lens. 
     The lens element  11 . 4 . 1 - 330  may be a corrective lens, for example, to address refractive errors of the eye of the user, such as myopia, hypermetropia, and/or astigmatism. The removable lens assembly  11 . 4 . 1 - 120  and the lens element  11 . 4 . 1 - 330  may be associated with a particular user by having particular lens characteristics to address the particular refractive errors of the eye of the particular user (e.g., eye characteristics). Such lens characteristics and eye characteristics may each include sphere, cylinder, axis, and/or other parameters conventionally used to define eyeglass prescriptions or refractive errors of eyes of one or more users. 
     The lens characteristics of the lens element  11 . 4 . 1 - 330  and, thereby the removable lens assembly  11 . 4 . 1 - 120 , may also include a center (e.g., optical axis) that is to be aligned with a pupil of the user. Therefore, the removable lens assembly  11 . 4 . 1 - 120  may be mounted to the lens mount  11 . 4 . 1 - 316   c  in a predetermined position (i.e., up/down, left/right, and rotational position) to ensure proper spatial positioning of the lens element  11 . 4 . 1 - 330  relative to the eye of the user. 
     The lens element  11 . 4 . 1 - 330  may, instead, be a non-corrective lens that protects the primary lens  11 . 4 . 1 - 316   b  from contact by the user and/or debris, which might otherwise interfere with the user viewing the display  11 . 4 . 1 - 316   a  (e.g., scratching or obstructing the view). For example, one pair of removable lens assemblies  11 . 4 . 1 - 120  may include lens elements  11 . 4 . 1 - 330  that are corrective for use by a particular user, while another pair of removable lens assemblies  11 . 4 . 1 - 120  may include lens elements  11 . 4 . 1 - 330  that are non-corrective for use by other users (e.g., users not requiring corrective lenses). 
     Referring to  FIGS.  11 . 4   . 1 - 4 - 11 . 4 . 1 - 6 , an outer periphery of the removable lens assembly  11 . 4 . 1 - 120  may have different shapes. As shown in  FIG.  11 . 4   . 1 - 4 , the removable lens assembly  11 . 4 . 1 - 120  has an outer periphery that is formed by the lens frame  11 . 4 . 1 - 340  and that is ovular (i.e., having curved left and right ends; as shown). The outer periphery may instead be circular (see lens frame  11 . 4 . 1 - 540  of lens assembly  11 . 4 . 1 - 520  in  FIG.  11 . 4   . 1 - 5 ) or may be rectangular (see lens frame  11 . 4 . 1 - 640  of lens assembly  11 . 4 . 1 - 620  in  FIG.  11 . 4   . 1 - 6 ). As discussed in further detail below, those lens assemblies having outer peripheries that are rotationally symmetric (e.g., being ovular, circular, rectangular, or other suitable shape) may be further configured to couple to the lens mount  11 . 4 . 1 - 316   c  in different rotational positions about the optical axis of the lens element  11 . 4 . 1 - 330  for use with the display module  11 . 4 . 1 - 116 . Alternatively, the removable lens assemblies  11 . 4 . 1 - 120  may have outer peripheries that are rotationally asymmetric, such that the lens mount  11 . 4 . 1 - 316   c  is coupleable in only one position for use with the display module  11 . 4 . 1 - 116 . 
     Referring again to  FIG.  11 . 4   . 1 - 3 A and  FIG.  11 . 4   . 1 - 4 , light is emitted by the display  11 . 4 . 1 - 316   a  and passes through the primary lens  11 . 4 . 1 - 316   b  and then through the lens element  11 . 4 . 1 - 330  before reaching the eye of the user. For example, light emitted from outer edges  11 . 4 . 1 - 360   a  of the display  11 . 4 . 1 - 316   a  may generally follow an outer light path  11 . 4 . 1 - 360  (e.g., a ray trace), whereby the light emitted from the display  11 . 4 . 1 - 316   a  is refracted by the primary lens  11 . 4 . 1 - 316   b  and is subsequently refracted by the lens element  11 . 4 . 1 - 330 . The outer light path  11 . 4 . 1 - 360  passes through the primary lens  11 . 4 . 1 - 316   b  generally a passage location  11 . 4 . 1 - 360   b . The outer light path  11 . 4 . 1 - 360  enters an entry side  11 . 4 . 1 - 330   a  of the lens element  11 . 4 . 1 - 330  at entry point  11 . 4 . 1 - 360   c  and exits an exit side  11 . 4 . 1 - 330   b  of the light path at an entry point  11 . 4 . 1 - 360   c . Moving through the lens element  11 . 4 . 1 - 330  from the entry side  11 . 4 . 1 - 330   a  to the exit side  11 . 4 . 1 - 330   b , the outer light path  11 . 4 . 1 - 360  narrows, so as to focus toward the eye of the user. 
     Areas of the lens element  11 . 4 . 1 - 330  outside of the entry point  11 . 4 . 1 - 360   c  and the exit point  11 . 4 . 1 - 360   d  (i.e., radially outward toward edges of the lens element  11 . 4 . 1 - 330 ) do not function to refract light toward the eye of the user and may be considered non-functional (e.g., non-refractive). Such non-functional areas of the lens element  11 . 4 . 1 - 330  may be removed and/or blocked by the lens frame  11 . 4 . 1 - 340 , thereby permitting different structural configurations of the lens frame  11 . 4 . 1 - 340 . As shown in  FIG.  11 . 4   . 1 - 4 , the functional area of the lens element  11 . 4 . 1 - 330  may, as shown, be smaller than a functional area of the primary lens  11 . 4 . 1 - 316   b  (e.g., smaller in area, width, and/or height than the functional the passage location  11 . 4 . 1 - 360   b  on the entry side  11 . 4 . 1 - 330   a  at the entry point  11 . 4 . 1 - 360   c  and/or on the exit side  11 . 4 . 1 - 330   b  at the exit point  11 . 4 . 1 - 360   d ). The functional area of the lens element  11 . 4 . 1 - 330  may also, as shown, be smaller than the display  11 . 4 . 1 - 316   a  (e.g., smaller in area, width and/or height than the light emitting of the display  11 . 4 . 1 - 316   a  on the entry side  11 . 4 . 1 - 330   a  at the entry point  11 . 4 . 1 - 360   c  and/or on the exit side  11 . 4 . 1 - 330   b  at the exit point  11 . 4 . 1 - 360   d ). The functional area of the primary lens  11 . 4 . 1 - 316   b  may, as shown, be larger than the display  11 . 4 . 1 - 316   a  (e.g., larger in area, width, and/or height than a light emitting area of the display  11 . 4 . 1 - 316   a ). Additionally, the functional areas of the primary lens  11 . 4 . 1 - 316   b  and the lens element  11 . 4 . 1 - 330  may allow for any eye camera  11 . 4 . 1 - 319  of the head-mounted display unit  11 . 4 . 1 - 110  (e.g., of the display module  11 . 4 . 1 - 116 ) to observe the eye of the user (e.g., light moving in a direction opposite that emitted by the display  11 . 4 . 1 - 316   a ). 
     Still referring to  FIGS.  11 . 4   . 1 - 3 A- 3 B, an outer periphery of the lens element  11 . 4 . 1 - 330  is coupled to an inner periphery of the lens frame  11 . 4 . 1 - 340 , for example, being engaged thereby and/or adhered thereto. The outer periphery of the lens element  11 . 4 . 1 - 330  and the inner periphery of the lens frame  11 . 4 . 1 - 340  have corresponding shapes allowing coupling therebetween, such as being straight (e.g., having the same cross-sectional shape moving axially) or tapering (e.g., widening moving toward the display module  11 . 4 . 1 - 116 ). The outer periphery of the lens element  11 . 4 . 1 - 330  may have generally the same size as the primary lens  11 . 4 . 1 - 316   b  (e.g., having generally the same area, width, and/or height), which defines non-functional area of the lens element  11 . 4 . 1 - 330 . 
     Referring to  FIGS.  11 . 4   . 1 - 7 - 11 . 4 . 1 - 9  and as referenced above, the non-functional area of the lens element  11 . 4 . 1 - 330  may be reduced with the lens element  11 . 4 . 1 - 330  being smaller than the primary lens  11 . 4 . 1 - 316   b.    
     As shown in  FIG.  11 . 4   . 1 - 7 , a removable lens assembly  11 . 4 . 1 - 720  is a variation of the removable lens assembly  11 . 4 . 1 - 120  and generally includes a lens element  11 . 4 . 1 - 730  and a lens frame  11 . 4 . 1 - 740 . The lens element  11 . 4 . 1 - 730  is a variation of the lens element  11 . 4 . 1 - 330  differing by being smaller, such as by being smaller than the primary lens  11 . 4 . 1 - 316   b  (e.g., smaller in area, width, and/or height). The lens frame  11 . 4 . 1 - 740  is a variation of the lens frame  11 . 4 . 1 - 340  differing by extending radially inward further than the lens frame  11 . 4 . 1 - 740 , such as by having an inner periphery that is smaller than the primary lens  11 . 4 . 1 - 316   b  and/or the lens mount  11 . 4 . 1 - 316   c  (e.g., being smaller in area, width, and/or height). 
     Referring to  FIG.  11 . 4   . 1 - 8 , a removable lens assembly  11 . 4 . 1 - 820  is a variation of the removable lens assembly  11 . 4 . 1 - 120  and generally includes a lens element  11 . 4 . 1 - 830  and a lens frame  11 . 4 . 1 - 840 . The lens element  11 . 4 . 1 - 830  is a variation of the lens element  11 . 4 . 1 - 830  differing by having edges that are beveled (e.g., tapered) at an angle or curvature that more closely conforms to the angle at which the outer light path  11 . 4 . 1 - 360  (e.g., the angle of refraction) passes through the lens element  11 . 4 . 1 - 830 . For example, an outer peripheral surface of the lens element  11 . 4 . 1 - 830  may extend at an angle measured relative to the optical axis of the lens element that is greater than zero, such as between 15 and 75 degrees, such as between 30 and 60 degrees). Edges of the outer peripheral surface on entry and exit sides of the lens element  11 . 4 . 1 - 830  may be smaller than peripheral dimensions of the primary lens  11 . 4 . 1 - 316   b  (e.g., being smaller in area, width, and/or height). The edge of the peripheral surface on the exit side of the lens element  11 . 4 . 1 - 830  may be smaller than the edge of the peripheral surface on the entry side thereof (e.g., reducing in size moving away from the primary lens  11 . 4 . 1 - 316   b  and toward the eye of the user). The lens frame  11 . 4 . 1 - 840  is a variation of the lens frame  11 . 4 . 1 - 340  by extending radially inward further than the lens frame  11 . 4 . 1 - 340 , such as by having an inner periphery that is smaller than the primary lens  11 . 4 . 1 - 316   b  on one or both of entry and exit sides of the lens element  11 . 4 . 1 - 830 . The lens frame  11 . 4 . 1 - 840  is coupled to the outer peripheral surface of the lens element  11 . 4 . 1 - 830  and may follow or otherwise accommodate the shape of the peripheral surface of the lens element  11 . 4 . 1 - 830  (e.g., having a matching bevel or contour). 
     Referring to  FIG.  11 . 4   . 1 - 9  a removable lens assembly  11 . 4 . 1 - 920  is a variation of the removable lens assembly  11 . 4 . 1 - 120  and generally includes a lens element  11 . 4 . 1 - 930  and a lens frame  11 . 4 . 1 - 940 . The lens element  11 . 4 . 1 - 930  is a variation of the lens element  11 . 4 . 1 - 330  differing by having non-functional areas of the exit side (e.g., or other concave side) forming a forward surface, which may be planar, that is coupled to the lens frame  11 . 4 . 1 - 940  and surrounds functional areas of the lens element  11 . 4 . 1 - 930 . The lens frame  11 . 4 . 1 - 940  is a variation of the lens frame  11 . 4 . 1 - 340  by being coupled to the forward surface of the exit side  11 . 4 . 1 - 930   b  of the lens element  11 . 4 . 1 - 930 , and protruding inward from the lens mount  11 . 4 . 1 - 316   c.    
     Referring again to  FIGS.  11 . 4   . 1 - 3 - 11 . 4 . 1 - 4 , the lens mount  11 . 4 . 1 - 316   c  and the removable lens assembly  11 . 4 . 1 - 120  are cooperatively configured for the removable lens assembly  11 . 4 . 1 - 120  to removably couple to the lens mount  11 . 4 . 1 - 316   c  to be supported thereby. As shown, the lens mount  11 . 4 . 1 - 316   c  and the removable lens assembly  11 . 4 . 1 - 120  include cooperative coupling features  11 . 4 . 1 - 316   d ,  11 . 4 . 1 - 322 , respectively. The cooperative coupling features  11 . 4 . 1 - 316   d ,  11 . 4 . 1 - 322  are magnetic and may be referred to as magnetic coupling features. The magnetic coupling features  11 . 4 . 1 - 316   d ,  11 . 4 . 1 - 322  are arranged at corresponding positions of the lens mount  11 . 4 . 1 - 316   c  and the lens frame  11 . 4 . 1 - 340  of the removable lens assembly  11 . 4 . 1 - 120  for the lens mount  11 . 4 . 1 - 316   c  to support the removable lens assembly  11 . 4 . 1 - 120  in the one or more predetermined positions (i.e., vertically, laterally, and rotationally) at which the lens element  11 . 4 . 1 - 330  is properly aligned with the eye of the user. 
     For each pair of the corresponding magnetic coupling features  11 . 4 . 1 - 316   d ,  11 . 4 . 1 - 322 , one may be a permanent magnet, while the other the other is an attractor element (e.g., an attractor plate made of a ferromagnetic material) or another permanent magnet of suitable orientation. For example, the magnetic coupling features  11 . 4 . 1 - 316   d  of the lens mount  11 . 4 . 1 - 316   c  may be attractor plates, while the magnetic coupling features  11 . 4 . 1 - 322  of the removable lens assembly  11 . 4 . 1 - 120  are permanent magnets. 
     The magnetic coupling features  11 . 4 . 1 - 316   d ,  11 . 4 . 1 - 322  are provided in suitable size, number, and location for the lens mount  11 . 4 . 1 - 316   c  to adequately support the removable lens assembly  11 . 4 . 1 - 120  during use, while still allowing the removable lens assembly  11 . 4 . 1 - 120  to be easily removed by the user. For example, as shown, the lens mount  11 . 4 . 1 - 316   c  and the removable lens assembly  11 . 4 . 1 - 120  may include six pairs of the magnetic coupling features  11 . 4 . 1 - 316   d ,  11 . 4 . 1 - 322 , more or less. 
     The removable lens assembly  11 . 4 . 1 - 120  and the lens mount  11 . 4 . 1 - 316   c  may be configured for the removable lens assembly  11 . 4 . 1 - 120  to be mounted to the lens mount  11 . 4 . 1 - 316   c  in only one predetermined position. For example, the lens element  11 . 4 . 1 - 330  may have an optical axis that is to be positioned off-center relative to the lens mount  11 . 4 . 1 - 316   c , thereby requiring the removable lens assembly  11 . 4 . 1 - 120  to be coupled to the lens mount  11 . 4 . 1 - 316   c  in one predetermined position. If the removable lens assembly  11 . 4 . 1 - 120  were instead coupled the lens mount  11 . 4 . 1 - 316   c  in another position rotated therefrom, the optical axis would be moved relative to the lens mount  11 . 4 . 1 - 316   c  and, thereby, relative to the display  11 . 4 . 1 - 316   a  and the eye of the user, resulting in reduced image quality perceived by the user. 
     The magnetic coupling features  11 . 4 . 1 - 316   d ,  11 . 4 . 1 - 322  may be configured to prevent incorrect positioning of the removable lens assembly  11 . 4 . 1 - 120  relative to the lens mount  11 . 4 . 1 - 316   c . For example, as shown in  FIG.  11 . 4   . 1 - 4  with the magnetic coupling features  11 . 4 . 1 - 322  illustrated in dashed lines in  FIG.  11 . 4   . 1 - 4  (e.g., being embedded, hidden, or otherwise coupled to the lens frame  11 . 4 . 1 - 340 ), the magnetic coupling features  11 . 4 . 1 - 316   d ,  11 . 4 . 1 - 322  may be positioned and/or distributed asymmetrically to prevent alignment of corresponding pairs of the magnetic coupling features  11 . 4 . 1 - 316   d ,  11 . 4 . 1 - 322  and, thereby, prevent coupling of the removable lens assembly  11 . 4 . 1 - 120  to the lens mount  11 . 4 . 1 - 316   c  in another position (e.g., rotated by 180 degrees). Instead or additionally, the magnetic characteristics of the magnetic coupling features  11 . 4 . 1 - 316   d ,  11 . 4 . 1 - 322  (e.g., orientation and whether being an attractor plate or a permanent magnetic) may prevent magnetic coupling of those magnetic coupling features  11 . 4 . 1 - 316   d  that do not correspond to each other in another position (e.g., rotated orientation). For example, if the removable lens assembly  11 . 4 . 1 - 120  were rotated by 180 degrees, the magnetic coupling features  11 . 4 . 1 - 316   d ,  11 . 4 . 1 - 322  that are aligned but do not correspond to each other may not attract each other (e.g., two attractor plates) or may repel each other (e.g., permanent magnets with common poles positioned adjacent each other). 
     The lens mount  11 . 4 . 1 - 316   c  and the removable lens assembly  11 . 4 . 1 - 120  may be further configured to mechanically prevent misalignment thereof. For example, the lens mount  11 . 4 . 1 - 316   c  and the removable lens assembly  11 . 4 . 1 - 120  may further include mechanical engagement features  11 . 4 . 1 - 316   e ,  11 . 4 . 1 - 324 , respectively. The mechanical engagement features  11 . 4 . 1 - 316   e ,  11 . 4 . 1 - 324  mechanically engage each other to ensure proper positioning and/or alignment between the lens mount  11 . 4 . 1 - 316   c  and the removable lens assembly  11 . 4 . 1 - 120 . The mechanical engagement features  11 . 4 . 1 - 316   e ,  11 . 4 . 1 - 324  may further engage each other to further prevent relative movement of the removable lens assembly  11 . 4 . 1 - 120  to the lens mount  11 . 4 . 1 - 316   c  (e.g., in vertical and left-to-right directions relative to the optical axis, while the magnetic coupling features  11 . 4 . 1 - 316   d ,  11 . 4 . 1 - 322  prevent movement in along the optical axis). 
     The mechanical engagement features  11 . 4 . 1 - 316   e ,  11 . 4 . 1 - 324  may, for example, be a recess and a protrusion, respectively, that is received by the recess, or vice versa. If the removable lens assembly  11 . 4 . 1 - 120  were arranged in different position relative to the lens mount  11 . 4 . 1 - 316   c , the protrusion would instead engage another surface of the lens mount  11 . 4 . 1 - 316   c  to prevent mating of the removable lens assembly  11 . 4 . 1 - 120  thereto. Alternatively, the opposed axially-facing (e.g., mating) surfaces of the display module  11 . 4 . 1 - 116  (e.g., the lens mount  11 . 4 . 1 - 316   c ) and the removable lens assembly  11 . 4 . 1 - 120  (e.g., of the lens frame  11 . 4 . 1 - 340 ) may have three-dimensional contours that prevent misalignment of the removable lens assemble  11 . 4 . 1 - 120  relative to the lens mount  11 . 4 . 1 - 316   c.    
     Instead of having only one mounting position, the removable lens assembly  11 . 4 . 1 - 120  and the lens mount  11 . 4 . 1 - 316   c  may be configured for the removable lens assembly  11 . 4 . 1 - 120  to be mounted to the lens mount  11 . 4 . 1 - 316   c  in more than one position. For example, the lens frame  11 . 4 . 1 - 340  and the lens mount  11 . 4 . 1 - 316   c , the coupling features, and/or the mechanical alignment features may be cooperatively configured for the removable lens assembly  11 . 4 . 1 - 120  to couple to the lens mount  11 . 4 . 1 - 316   c  in two predetermined positions (e.g., by being two-fold rotationally symmetric) or more (e.g., three, or four) and to not couple to the lens mount  11 . 4 . 1 - 316   c  in other positions. 
     Referring to  FIG.  11 . 4   . 1 - 10 A- 11 . 4 . 1 - 11 B, variations of the removable lens assembly  11 . 4 . 1 - 120  and the lens mount  11 . 4 . 1 - 316   c  are configured for mechanically coupling the removable lens assembly  11 . 4 . 1 - 120  to the lens mount  11 . 4 . 1 - 316   c.    
     As shown in  FIGS.  11 . 4   . 1 - 10 A- 11 . 4 . 1 - 10 B, a display module  11 . 4 . 1 - 1016  includes a lens mount  11 . 4 . 1 - 1016   c  to which removable lens assembly  11 . 4 . 1 - 1020  is removably coupleable with interfitting structures (e.g., forming an interference fit). The removable lens assembly  11 . 4 . 1 - 1020  includes the lens element  11 . 4 . 1 - 330  and a lens frame  11 . 4 . 1 - 1040 , which may be coupled to each other and otherwise configured in the manners described previously (see, e.g.,  FIGS.  11 . 4   . 1 - 3 A- 11 . 4 . 1 - 9 ). The lens frame  11 . 4 . 1 - 1040  includes a barrel  11 . 4 . 1 - 1042  having an annular protrusion  11 . 4 . 1 - 1044  (e.g., a lip or circumferential lip). The barrel  11 . 4 . 1 - 1042  extends axially rearward (e.g., of the lens element  11 . 4 . 1 - 330 ), while the annular protrusion  11 . 4 . 1 - 1044  protrudes radially inward therefrom. The lens frame  11 . 4 . 1 - 1040  may further define an annular recess  11 . 4 . 1 - 1046  (e.g., a circumferential channel; not labeled) extending radially outward from the annular protrusion  11 . 4 . 1 - 1044  and positioned axially outward thereof (i.e., away from the display  11 . 4 . 1 - 316   a ). The annular protrusion  11 . 4 . 1 - 1044  and the annular recess  11 . 4 . 1 - 1046  extend circumferentially around the axis of the lens element  11 . 4 . 1 - 330  entirely or substantially entirely (e.g., 80% or more). 
     The lens mount  11 . 4 . 1 - 1016   c  includes a barrel  11 . 4 . 1 - 1016   d  having an annular protrusion  11 . 4 . 1 - 1016   e . The barrel  11 . 4 . 1 - 1016   d  extends axially forward, while the annular protrusion  11 . 4 . 1 - 1016   e  protrudes radially outward therefrom. The annular protrusion  11 . 4 . 1 - 1016   e  further defines an annular recess  11 . 4 . 1 - 1016   f  (e.g., a circumferential channel) positioned axially forward and extending radially inward thereof. The annular protrusion  11 . 4 . 1 - 1016   e  and the annular recess  1016   f  defined thereby extend circumferentially around the axis of the lens element  11 . 4 . 1 - 330  entirely or substantially entirely. 
     The barrel  11 . 4 . 1 - 1042  of the lens frame  11 . 4 . 1 - 1040  is configured to receive the barrel  11 . 4 . 1 - 1016   d  of the lens mount  11 . 4 . 1 - 1016   c  therein, so as to removably couple the removable lens assembly  11 . 4 . 1 - 1020  to the lens mount  11 . 4 . 1 - 1016   c . The annular protrusion  11 . 4 . 1 - 1044  of the lens frame  11 . 4 . 1 - 1040  is received by and retained in the annular recess  11 . 4 . 1 - 1016   f  of the lens mount  11 . 4 . 1 - 1016   c , while the annular protrusion  11 . 4 . 1 - 1016   e  of the lens mount  11 . 4 . 1 - 1016   c  is received by and retained in the annular recess  11 . 4 . 1 - 1046  of the lens frame  11 . 4 . 1 - 1040 . 
     An inner periphery of the annular protrusion  11 . 4 . 1 - 1044  of the lens frame  11 . 4 . 1 - 1040  is smaller than an outer periphery of the annular protrusion  11 . 4 . 1 - 1016   e  of the lens mount  11 . 4 . 1 - 1016   c , such that axial surfaces of the annular protrusions  11 . 4 . 1 - 1044 ,  11 . 4 . 1 - 1016   e  overlap each other radially and engage each other axially to prevent unintended decoupling of the removable lens assembly  11 . 4 . 1 - 1020  from the display module  11 . 4 . 1 - 1016 . The annular protrusion  11 . 4 . 1 - 1044 , as may be the barrel  11 . 4 . 1 - 1042 , of the lens frame  11 . 4 . 1 - 1040  is formed of a compliant material, such as rubber or other polymer, that allows the inner periphery of the annular protrusion  11 . 4 . 1 - 1044  to expand and stretch over the annular protrusion  11 . 4 . 1 - 1016   e  of the lens mount  11 . 4 . 1 - 1016   c . The lens frame  11 . 4 . 1 - 1040  being formed of, or otherwise including, the compliant material forming the annular protrusion  11 . 4 . 1 - 1044  may also provide a soft material that may incidentally contact or otherwise engage the face of the user. Alternatively, the barrel  11 . 4 . 1 - 1042  and/or the annular protrusion  11 . 4 . 1 - 1044  may form a flexure. In a still further alternative, the barrel  11 . 4 . 1 - 1042  and the annular protrusion  11 . 4 . 1 - 1044  of the removable lens assembly  11 . 4 . 1 - 1020  are rigid, while the barrel  11 . 4 . 1 - 1016   d  and/or the annular protrusion  11 . 4 . 1 - 1016   e  of the display module  11 . 4 . 1 - 1016  are compliant or form a flexure. 
     While the lens frame  11 . 4 . 1 - 1040  and the lens mount  11 . 4 . 1 - 1016   c  have been shown and described with the lens mount  11 . 4 . 1 - 1016   c  (e.g., the barrel  11 . 4 . 1 - 1016   d  and the annular protrusion  11 . 4 . 1 - 1016   e ) being received within the lens frame  11 . 4 . 1 - 1040 , the lens frame  11 . 4 . 1 - 1040  and the lens mount  11 . 4 . 1 - 1016   c  may be arranged in an opposite manner with the lens frame  11 . 4 . 1 - 1040  receiving the lens mount  11 . 4 . 1 - 1016   c.    
     As shown in  FIGS.  11 . 4   . 1 - 11 A- 11 . 4 . 1 - 11 B, a display module  11 . 4 . 1 - 1116  includes a lens mount  11 . 4 . 1 - 1116   c  to which a removable lens assembly  11 . 4 . 1 - 1120  is removably coupled with sprung mechanisms. The removable lens assembly  11 . 4 . 1 - 1120  includes retractable latches  11 . 4 . 1 - 1122  (e.g., latch members) that are normally sprung radially outward for receipt into receptacles  11 . 4 . 1 - 1116   d  of the lens mount  11 . 4 . 1 - 1116   c  corresponding thereto. For example, the retractable latches  11 . 4 . 1 - 1122  extend axially rearward from a rear axial surface of a lens frame  11 . 4 . 1 - 1140  and include inner ends  11 . 4 . 1 - 1124  (e.g., hooked or outwardly protruding ends) that extend radially outward therefrom (e.g., hooked ends). When the retractable latches  11 . 4 . 1 - 1122  are retracted (e.g., by a user pressing on exposed ends of the protrusions), the retractable latches  11 . 4 . 1 - 1122  are axially insertable into or removable from the receptacles  11 . 4 . 1 - 1116   d  in the forward axial surface of the lens mount  11 . 4 . 1 - 1116   c  corresponding thereto. The inner ends of the retractable latches  11 . 4 . 1 - 1122  are received behind lips  11 . 4 . 1 - 1116   e  of the receptacles  11 . 4 . 1 - 1116   d . Springs (shown schematically; not labeled) bias the retractable latches  11 . 4 . 1 - 1122  radially outward upon release by the user, such that the inner ends of the retractable latches  11 . 4 . 1 - 1122  are arranged behind and engage the lips  11 . 4 . 1 - 1116   e  of the receptacles  11 . 4 . 1 - 1116   d  to retain the removable lens assembly  11 . 4 . 1 - 1120  on the lens mount  11 . 4 . 1 - 1116   c . While the retractable latches  11 . 4 . 1 - 1122  have been shown and described as being part of the removable lens assembly  11 . 4 . 1 - 1120 , retractable latches may instead be provided on the display module  11 . 4 . 1 - 1116  for releasably engaging the removable lens assembly  11 . 4 . 1 - 1120  in a similar manner (e.g., engaging receptacles in an outer peripheral surface of the lens frame  11 . 4 . 1 - 1140 . 
     Referring to  FIGS.  11 . 4   . 1 - 12 A- 11 . 4 . 1 - 12 B, in still further examples, variations of the removable lens assembly  11 . 4 . 1 - 120  may be coupled to the lens mount  11 . 4 . 1 - 1216   c  in other manners. For example, a removable lens assembly  11 . 4 . 1 - 1220  and the lens mount  11 . 4 . 1 - 1216   c  may include corresponding mating features  11 . 4 . 1 - 1222 ,  11 . 4 . 1 - 1223  (e.g., tabs and detents or protrusions) that allow for coupling of the removable lens assembly  11 . 4 . 1 - 1220  to the lens mount  11 . 4 . 1 - 1216   c  with one or more of an axial motion  11 . 4 . 1 - 1222   a , a vertical motion  11 . 4 . 1 - 1222   b , or a turning motion  11 . 4 . 1 - 1222   c . The axial motion  11 . 4 . 1 - 1222   a , the vertical motion  11 . 4 . 1 - 1222   b , or the turning motion  11 . 4 . 1 - 1222   c  (e.g., a quarter turn) cause engagement and coupling of the removable lens assembly  11 . 4 . 1 - 1220  to the lens mount  11 . 4 . 1 - 1216   c  via the corresponding mating features  11 . 4 . 1 - 1222 ,  11 . 4 . 1 - 1223 . 
     Referring additionally to  FIG.  11 . 4   . 1 - 12 C, a removable lens assembly  11 . 4 . 1 - 1220 ′ is coupled to a display unit  11 . 4 . 1 - 1216 ′ via a friction or interference fit. In particular, the removable lens assembly  11 . 4 . 1 - 1220  includes a lens frame  11 . 4 . 1 - 1240 ′ that is received by a lens mount  11 . 4 . 1 - 1216   c ′ of the display unit  11 . 4 . 1 - 1216 ′, so as to be retained thereby via friction (e.g., with the lens frame  11 . 4 . 1 - 1240 ′ being compressed radially inward by the lens mount  11 . 4 . 1 - 1216   c ′). 
     Referring additionally to  FIG.  11 . 4   . 1 - 12 D, the removable lens assembly  11 . 4 . 1 - 1220 ′ is coupled to the display unit  11 . 4 . 1 - 1216 ′ via the retractable pins  11 . 4 . 1 - 1216   d . For example, the display unit  11 . 4 . 1 - 1216 ′ includes the lens mount  11 . 4 . 1 - 1216   c ′ that receives the lens frame  11 . 4 . 1 - 1240 ′ of the removable lens assembly  11 . 4 . 1 - 1220 ′ therein and the retractable pins  11 . 4 . 1 - 1216   d , which are selectively extended into corresponding recesses of the lens frame  11 . 4 . 1 - 1240 ′. For example, the pins  11 . 4 . 1 - 1216   d ′ may be spring biased radially inward, so as to be received by the lens frame  11 . 4 . 1 - 1240 ′. 
     Referring to  FIGS.  11 . 4   . 1 - 13 A- 11 . 4 . 1 - 13 F, in a still further example, a removable lens assembly  11 . 4 . 1 - 1320  includes a mechanical coupling feature  11 . 4 . 1 - 1322  (e.g., a male component, or protrusion), which may be referred to as a lens mechanical coupling feature  11 . 4 . 1 - 1322 , while a display module  11 . 4 . 1 - 1310  includes a corresponding mechanical coupling feature  11 . 4 . 1 - 1312  (e.g., a female component, recess, or receptacle), which may be referred to as a display mechanical coupling feature  11 . 4 . 1 - 1312 . The lens mechanical coupling feature  11 . 4 . 1 - 1322  and the display mechanical coupling feature  11 . 4 . 1 - 1312  mechanically mate with each other to mechanically couple the removable lens assembly  11 . 4 . 1 - 1320  to the display module  11 . 4 . 1 - 1310 . For example, as discussed in further detail below, the lens mechanical coupling feature  11 . 4 . 1 - 1322  and the display mechanical coupling feature  11 . 4 . 1 - 1312  may prevent movement of the removable lens assembly  11 . 4 . 1 - 1320  relative to the display module  11 . 4 . 1 - 1310  in directions generally perpendicular to, generally parallel with, and/or about an optical axis of the removable lens assembly  11 . 4 . 1 - 1320 . 
     The removable lens assembly  11 . 4 . 1 - 1320  may further include one or more magnetic coupling features  11 . 4 . 1 - 1324 , which may be referred to as lens magnetic coupling features  11 . 4 . 1 - 1324 , while the display module  11 . 4 . 1 - 1310  includes one or more corresponding magnetic coupling features  11 . 4 . 1 - 1314 , which may be referred to as display magnetic coupling features  11 . 4 . 1 - 1314 . The lens magnetic coupling feature  11 . 4 . 1 - 1324  and the display magnetic coupling feature  11 . 4 . 1 - 1314  magnetically couple to each other to magnetically couple the removeable lens assembly  11 . 4 . 1 - 1320  to the display module  11 . 4 . 1 - 1310 . For example, as discussed in further detail below, the lens magnetic coupling feature  11 . 4 . 1 - 1324  and the display magnetic coupling feature  11 . 4 . 1 - 1314  may prevent movement of the removable lens assembly  11 . 4 . 1 - 1320  relative to the display module  11 . 4 . 1 - 1310  in directions generally parallel with the optical axis of the removable lens assembly  11 . 4 . 1 - 1320 . 
     The lens mechanical coupling feature  11 . 4 . 1 - 1322  of the removable lens assembly  11 . 4 . 1 - 1320  may be arranged on a first side thereof (e.g., left, right, upper, lower, inner, or outer), while the one or more lens magnetic coupling features  11 . 4 . 1 - 1324  may be arranged on a second side thereof, which may be spaced apart from and/or opposite the first side (e.g., right, left, lower, upper, outer, or inner, respectively, such as with the lens element  11 . 4 . 1 - 330  being positioned therebetween). The lens mechanical coupling feature  11 . 4 . 1 - 1322  may be formed by (e.g., be integrally-formed with) a lens frame  11 . 4 . 1 - 340 , while the one or more lens magnetic coupling features  11 . 4 . 1 - 1324  are coupled to the lens frame  11 . 4 . 1 - 340 . The lens magnetic coupling features  11 . 4 . 1 - 1324  may be or include one or more permanent magnets and/or an attractor material (e.g., ferromagnetic material or component). 
     The display module  11 . 4 . 1 - 1310  includes a lens mount  11 . 4 . 1 - 1311  having the lens mechanical coupling feature  11 . 4 . 1 - 1322  and the one or more lens magnetic coupling features  11 . 4 . 1 - 1324 . As discussed in further detail below, the display mechanical coupling feature  11 . 4 . 1 - 1312  of the lens mount  11 . 4 . 1 - 1311  is configured to receive the lens mechanical coupling feature  11 . 4 . 1 - 1322  therein. The one or more display magnetic coupling features  11 . 4 . 1 - 1314  of the lens mount  11 . 4 . 1 - 1311  magnetically couple to the lens magnetic coupling features  11 . 4 . 1 - 1324  (e.g., being a permanent magnet of opposite polarity or being an attractor material). 
     As referenced above, the lens mechanical coupling feature  11 . 4 . 1 - 1322  of the removable lens assembly  11 . 4 . 1 - 1320  and the display mechanical coupling feature  11 . 4 . 1 - 1312  of the display module  11 . 4 . 1 - 1310  are configured to mate with each other and, in particular, with the lens mechanical coupling feature  11 . 4 . 1 - 1322  being received by the display mechanical coupling feature  11 . 4 . 1 - 1312 . When received thereby, the lens mechanical coupling feature  11 . 4 . 1 - 1322  engages the display mechanical coupling feature  11 . 4 . 1 - 1312  to prevent relative movement therebetween. For example, the lens mechanical coupling feature  11 . 4 . 1 - 1322  engages the display mechanical coupling feature  11 . 4 . 1 - 1312  to prevent relative shearing movement between the removable lens assembly  11 . 4 . 1 - 1320  and the display module  11 . 4 . 1 - 1310 , such as upward movement, lateral movement (e.g., in a direction generally perpendicular to an optical axis of the removable lens assembly  11 . 4 . 1 - 1320 ), and/or rotational movement of the removable lens assembly  11 . 4 . 1 - 1320  relative to the display module  11 . 4 . 1 - 1310  (e.g., generally about the optical axis of the removable lens assembly  11 . 4 . 1 - 1320 ). 
     To prevent relative upward movement, an upwardly-facing surface of the lens mechanical coupling feature  11 . 4 . 1 - 1322  engages a downwardly facing surface of the display mechanical coupling feature  11 . 4 . 1 - 1312 . To prevent relative downward movement, a downwardly-facing surface of the lens mechanical coupling feature  11 . 4 . 1 - 1322  engages an upwardly-facing surface of the display mechanical coupling feature  11 . 4 . 1 - 1312 . For example, upper and lower ends of the lens mechanical coupling feature  11 . 4 . 1 - 1322  engaging respective upper and lower ends of the display mechanical coupling feature  11 . 4 . 1 - 1312  (see  FIGS.  11 . 4   . 1 - 13 A and  11 . 4 . 1 - 13 B). Alternatively, upward- and downward-facing surfaces at intermediate heights of the lens mechanical coupling feature  11 . 4 . 1 - 1322  and the display mechanical coupling feature  11 . 4 . 1 - 1312  engage each other to prevent upward and downward relative movement. 
     To prevent relative lateral outward movement (e.g., away from the nose of the user), a laterally outward-facing surface of the lens mechanical coupling feature  11 . 4 . 1 - 1322  engages a laterally inwardly-facing surface of the display mechanical coupling feature  11 . 4 . 1 - 1312  (e.g., a leftward facing surface and a rightward facing surface, respectively, for a left removable lens assembly  11 . 4 . 1 - 1320 ). To prevent relative lateral inward movement (e.g., toward the nose of the user), a laterally inward-facing surface of the lens mechanical coupling feature  11 . 4 . 1 - 1322  engages a laterally outward-facing surface of the display mechanical coupling feature  11 . 4 . 1 - 1312  (e.g., a rightward facing surface and a leftward facing surface, respectively, for a left removable lens assembly  11 . 4 . 1 - 1320 ), as is shown in  FIG.  11 . 4   . 1 - 13 F. For example, as is illustrated, the lens mechanical coupling feature  11 . 4 . 1 - 1322  and the display mechanical coupling feature  11 . 4 . 1 - 1312  have curved shapes with outward and inward sides of the lens mechanical coupling feature  11 . 4 . 1 - 1322  (e.g., concave and convex surfaces) engaging respective outward and inward sides of the display mechanical coupling feature  11 . 4 . 1 - 1312  (e.g., convex and concave surfaces, respectively). 
     To prevent relative rotational movement, the lens mechanical coupling feature  11 . 4 . 1 - 1322 , in spaced apart locations on opposite sides thereof (e.g., two on each side), engages the display mechanical coupling feature  11 . 4 . 1 - 1312  on opposite sides thereof. For example, as a clockwise torque is applied to the removable lens assembly  11 . 4 . 1 - 1320 , upper inner surfaces and lower outer surfaces of the lens mechanical coupling feature  11 . 4 . 1 - 1322  and the display mechanical coupling feature  11 . 4 . 1 - 1312  engage each other to constrain (e.g., prevent) relative clockwise rotation therebetween. As a counterclockwise torque is applied to the removable lens assembly  11 . 4 . 1 - 1320 , upper outer surface and lower inner surfaces of the lens mechanical coupling feature  11 . 4 . 1 - 1322  and the display mechanical coupling feature  11 . 4 . 1 - 1312  engage each other to constrain (e.g., prevent) relative counterclockwise rotation therebetween. 
     The lens mechanical coupling feature  11 . 4 . 1 - 1322  and the display mechanical coupling feature  11 . 4 . 1 - 1312  may additionally be configured to prevent normal movement (e.g., in the general direction of the optical axis) away from the display module  11 . 4 . 1 - 1310 . For example, the display mechanical coupling feature  11 . 4 . 1 - 1312  may include a forward-facing surface (e.g., an undercut or toe-in feature) that engages a rearward facing surface of the lens mechanical coupling feature  11 . 4 . 1 - 1322 . 
     Referring to  FIG.  11 . 4   . 1 - 13 E, to couple the removable lens assembly  11 . 4 . 1 - 1320  to the display module  11 . 4 . 1 - 1310 , the lens mechanical coupling feature  11 . 4 . 1 - 1322  is first inserted into the display mechanical coupling feature  11 . 4 . 1 - 1312 , and the removable lens assembly  11 . 4 . 1 - 1320  is rotated to bring the magnetic coupling features  11 . 4 . 1 - 1324 ,  11 . 4 . 1 - 1314  closer to each other to magnetically couple, as discussed in further detail below. 
     As referenced above, the lens magnetic coupling features  11 . 4 . 1 - 1324  of the removable lens assembly  11 . 4 . 1 - 1320  and the display magnetic coupling features  11 . 4 . 1 - 1314  of the display module  11 . 4 . 1 - 1310  couple to each other magnetically. The lens magnetic coupling features  11 . 4 . 1 - 1324  and the display magnetic coupling features  11 . 4 . 1 - 1314  are arranged at corresponding positions and with corresponding magnetic properties (e.g., being opposite polarity, or one being a ferromagnetic material and the other a permanent magnetic). The lens magnetic coupling features  11 . 4 . 1 - 1324  and the display magnetic coupling features  11 . 4 . 1 - 1314  couple to each other magnetically with force therebetween applied generally in the direction of the optical axis. The lens magnetic coupling features  11 . 4 . 1 - 1324  and the display magnetic coupling features  11 . 4 . 1 - 1314  are also spaced apart from the pivot axis defined by the display mechanical coupling feature  11 . 4 . 1 - 1312 , such that the magnetic coupling for therebetween prevent rotation about the pivot axis defined the display mechanical coupling feature  11 . 4 . 1 - 1312 . For example, as shown, the lens magnetic coupling features  11 . 4 . 1 - 1324  and the display magnetic coupling features  11 . 4 . 1 - 1314  are positioned generally opposite the lens mechanical coupling feature  11 . 4 . 1 - 1322  and the display mechanical coupling feature  11 . 4 . 1 - 1312 , respectively. 
     Variations of the removable lens assembly  11 . 4 . 1 - 1320  and the display module  11 . 4 . 1 - 1310  are contemplated. For example, the lens mechanical coupling feature  11 . 4 . 1 - 1322  and the display mechanical coupling feature  11 . 4 . 1 - 1312  may have different shapes (e.g., being straight), may be provided in different numbers (e.g., two or more sets thereof, such as pins and apertures, at spaced apart locations to prevent movement as described above), and/or may be provided at different locations (e.g., at upper, lower, or inner sides thereof). Furthermore, the display mechanical coupling feature  11 . 4 . 1 - 1312  may be provided by a different part of the head-mounted display unit  11 . 4 . 1 - 110 , such as to the housing  11 . 4 . 1 - 112  (e.g., a curtain or other structure surrounding the display module  11 . 4 . 1 - 1310 ). Further, instead of the magnetic coupling features  11 . 4 . 1 - 1324 ,  11 . 4 . 1 - 1314 , the removable lens assembly  11 . 4 . 1 - 1320  and the display module  11 . 4 . 1 - 1310  may instead or additionally include mechanical coupling features (e.g., interfitting structures or latches as described above). 
     It should be noted that, for each of the manners for coupling described above with respect to  FIGS.  11 . 4   . 1 - 3 A- 3 B and  11 . 4 . 1 - 10 A- 11 . 4 . 1 - 13 F, the lens mount may be coupled to another portion of the head-mounted display unit  11 . 4 . 1 - 110 , such as the housing  11 . 4 . 1 - 112  (e.g., chassis), as opposed to the display module  11 . 4 . 1 - 116  or variations thereof. 
     Referring to  FIGS.  11 . 4   . 1 - 14 A- 11 . 4 . 1 - 16 , the display system  11 . 4 . 1 - 100  and the removable lens assemblies  11 . 4 . 1 - 120  are cooperatively configured for the display system  11 . 4 . 1 - 100  to detect the removable lens assembly  11 . 4 . 1 - 120  coupled to the head-mounted display unit  11 . 4 . 1 - 110 . For example, the head-mounted display unit  11 . 4 . 1 - 110  includes one or more lens detection sensors  11 . 4 . 1 - 1418   a  for detecting the one or more removable lens assemblies  11 . 4 . 1 - 120  coupled thereto. In detecting the removable lens assembly  11 . 4 . 1 - 120 , the display system  11 . 4 . 1 - 100  may determine whether or not any of the removable lens assemblies  11 . 4 . 1 - 120  is coupled to the display module  11 . 4 . 1 - 116  (e.g., using a proximity sensor, image recognition, and/or mechanical switch), whether the removable lens assembly  11 . 4 . 1 - 120  is properly coupled (e.g., using a proximity sensor, image recognition, or hall sensors with magnets coupled to the removable lens assembly  120 ), and/or other information about the removable lens assembly  11 . 4 . 1 - 120  (e.g., corrective or not, serial or model number, associated user, and/or lens characteristics using other types of lens detection sensors  11 . 4 . 1 - 1418   a  discussed below) to support other functionality. The display system  11 . 4 . 1 - 100  may provide an indication of whether or not the removable lens assembly  11 . 4 . 1 - 120  is coupled to the display module  11 . 4 . 1 - 116 , whether the removable lens assembly  11 . 4 . 1 - 120  is properly position relative to the display module  11 . 4 . 1 - 116 , and/or other indications (e.g., of the lens configuration and/or lens compatibility as discussed below). 
     Each removable lens assembly  11 . 4 . 1 - 120  may further include a lens tag  11 . 4 . 1 - 1428  that is in communication with or otherwise readable by the one or more lens detection sensors  11 . 4 . 1 - 1418   a . According to detection of the removable lens assembly  11 . 4 . 1 - 120 , the display system  11 . 4 . 1 - 100  may perform one or more subsequent operations, which may include changing output of the display module  11 . 4 . 1 - 116 , providing an indicator of the removable lens assembly  11 . 4 . 1 - 120 , and/or providing an indicator of compatibility of the removable lens assembly  11 . 4 . 1 - 120  with a user. The display system also includes a controller  11 . 4 . 1 - 1450  that controls operation of the head-mounted display unit  11 . 4 . 1 - 110 , for example, to detect the removable lens assemblies  11 . 4 . 1 - 120  coupled thereto and/or perform subsequent operations in response thereto. The controller  11 . 4 . 1 - 1450  may be wholly or partially physically incorporated into the head-mounted display unit  11 . 4 . 1 - 110  (as indicated by dash-lines) or may be wholly or partially provided physically separate therefrom (e.g., in an external computing device). An example hardware configuration for the controller  11 . 4 . 1 - 1450  is discussed below with reference to  FIG.  11 . 4   . 1 - 14 . 
     In detecting the removable lens assembly  11 . 4 . 1 - 120 , the display system  11 . 4 . 1 - 100  may identify or determine information about the removable lens assembly  11 . 4 . 1 - 120 , which may be referred to as lens information. The lens information may include an identifier and/or technical characteristics of the removable lens assembly  11 . 4 . 1 - 120 . The identifier may be of the removable lens assembly  11 . 4 . 1 - 120  itself (e.g., a serial number or a model number; referred to as a lens identifier), a user associated with the lens (e.g., a user name or user ID; referred to as an associated user identifier), and/or other identifying information (e.g., manufacturer, date; referred to as manufacturing information). The technical characteristics may include refractive characteristics, such as sphere, cylinder, and/or axis parameters and/or an optical axis location of the removable lens assembly  11 . 4 . 1 - 120 , which may be referred to as lens characteristics, lens characteristic information, or prescription information. The lens information determined by the display system  11 . 4 . 1 - 100  may then be used by the display system  11 . 4 . 1 - 100  to perform the subsequent operations referenced above and described in further detail below. 
     The display system  11 . 4 . 1 - 100  may determine the lens information of the removable lens assembly  11 . 4 . 1 - 120  actively coupled (e.g., being coupled or currently coupled) to the head-mounted display unit  11 . 4 . 1 - 110  in various different manners and with different devices. The display system  11 . 4 . 1 - 100  may determine the lens information from the removable lens assembly  11 . 4 . 1 - 120 , such as with the lens detection sensor  11 . 4 . 1 - 1418   a  from the lens tag  11 . 4 . 1 - 1428 . In a first example, the lens tag  11 . 4 . 1 - 1428  is an electronic device that stores or otherwise electrically provides the lens information. For example, the lens tag  11 . 4 . 1 - 1428  each removable lens assembly  11 . 4 . 1 - 120  may be an electrically-erasable programmable read-only memory device (EEPROM), a radio frequency identification device (RFID), or an internal resistance device (e.g., an electrical resistance or resistor). The lens detection sensor  11 . 4 . 1 - 1418   a  is a corresponding electrical device or system configured to receive and/or otherwise receive the lens information from the lens tag  11 . 4 . 1 - 1428 , such as by being in wireless or physical communication with the EEPROM, to read a signal of the RFID, and/or to measure or otherwise determine a resistance signature of the resistance device. 
     In a second example, the lens tag  11 . 4 . 1 - 1428  has a magnetic signature. The removable lens assembly  11 . 4 . 1 - 120  includes various magnets (e.g., the magnetic coupling features  11 . 4 . 1 - 322 ) or other magnets, which may provide a magnetic signature by having varied strength and/or position and which are detected by the lens detection sensor  11 . 4 . 1 - 1418   a , which may include one or more hall sensors or other magnetic sensors that sense the magnetic field of the magnets. The combination of magnetic fields detected by the lens detection sensor  11 . 4 . 1 - 1418   a  provide the lens information (e.g., a lens identifier of a particular magnetic signature). Hall sensors may instead or additionally be used to determine whether the removable lens assembly  11 . 4 . 1 - 120  is properly coupled to the 
     In a third example, the lens tag  11 . 4 . 1 - 1428  is an optical marking that displays or otherwise communicates the lens information. For example, the lens tag  11 . 4 . 1 - 1428  may be an optical or infrared (IR) marking on the removable lens assembly  11 . 4 . 1 - 120 , such as a quick response code (QR Code), a bar code, or alphanumeric characters. The lens detection sensor  11 . 4 . 1 - 1418   a  of the head-mounted display unit  11 . 4 . 1 - 110  is a corresponding optical reading device, such as a camera or a combined illuminator/sensor device (e.g., a QR Code scanner), which may be the eye camera  11 . 4 . 1 - 319  that also observes the eye as described previously. In one particular example, the lens tag  11 . 4 . 1 - 1428  is an infrared bar code. 
     The display system  11 . 4 . 1 - 100  may determine the lens information required for one or more further operations directly from the removable lens assembly  11 . 4 . 1 - 120  itself. Instead or additionally, the display system  11 . 4 . 1 - 100  may determine additional lens information from other sources for performing subsequent operations. For example, the display system  11 . 4 . 1 - 100  may receive only initial lens information (e.g., the lens identifier or the associated user identifier) from the removable lens assembly  11 . 4 . 1 - 120 , while other lens information (e.g., the associated user identifier and/or lens characteristics) are stored in association therewith by the display system  11 . 4 . 1 - 100  (e.g., by a storage of the controller  11 . 4 . 1 - 1450 ). Thus, by determining initial lens information from the removable lens assembly  11 . 4 . 1 - 120 , the display system  11 . 4 . 1 - 100  may determine (e.g., retrieve) other lens information to support subsequent operations. 
     The lens information of the removable lens assembly  11 . 4 . 1 - 120  coupled to the display module  11 . 4 . 1 - 116  may be used to control various functionality of the display system  11 . 4 . 1 - 100 , which may include operating the display  11 . 4 . 1 - 316   a , providing an indication to the user of the removable lens assemblies  11 . 4 . 1 - 120  coupled thereto, and/or determining compatibility of the removable lens assemblies  11 . 4 . 1 - 120  with the user. Operation of the display  11 . 4 . 1 - 316   a  may be performed according to the lens information of the removable lens assembly  11 . 4 . 1 - 120  coupled to the display module  11 . 4 . 1 - 116 , such that operation of the display  11 . 4 . 1 - 316   a  may differ between one of the removable lens assemblies  11 . 4 . 1 - 120  having one set of lens characteristics and another of the removable lens assemblies  11 . 4 . 1 - 120  having another set of lens characteristics (e.g., different brightness and/or pixel output to account for different characteristics of the optical system formed by the primary lens  11 . 4 . 1 - 316   b  and the lens elements  11 . 4 . 1 - 330  or different lens characteristics). 
     Instead or additionally, the lens information of the removable lens assembly  11 . 4 . 1 - 120  coupled to the display module  11 . 4 . 1 - 116  may be used to provide an indication to the user or potential users of the removable lens assembly  11 . 4 . 1 - 120  coupled thereto, which may be referred to as a lens configuration indication. The lens configuration indication differs for different ones of the removable lens assemblies  11 . 4 . 1 - 120 , for example, those removable lens assemblies  11 . 4 . 1 - 120  associated with different users. A first lens configuration indication is provided for one of the removable lens assemblies  11 . 4 . 1 - 120  (or one pair of the removable lens assemblies  11 . 4 . 1 - 120  associated with each other), while a second, different lens configuration indication is provided another of the removable lens assemblies  11 . 4 . 1 - 120  (or another pair of the removable lens assemblies  11 . 4 . 1 - 120  associated with each other). The display indication (e.g., or output instruction) may be stored in association with the lens information (e.g., the lens identifier, associated user identifier, or lens characteristics), for example, by the display system  11 . 4 . 1 - 100  and retrieved upon determining the lens identifier. The configuration indication may be provided irrespective of the user (e.g., without detecting or identifying the user). 
     The lens configuration indication may be provided as, for example, an external light indicator, a display indicator, or an audible indictor. The external light indicator is provided with an external visual indicator  11 . 4 . 1 - 1418   b  (e.g., a light-emitting diode) of the head-mounted display unit  11 . 4 . 1 - 110 , which is visible by potential users prior to wearing the head-mounted display unit  11 . 4 . 1 - 110  and may differ, for example, by color (e.g., red vs. blue to differentiate between first and second users). The display indicator is provided by the display  11 . 4 . 1 - 316   a  and is viewable by the user or potential users through the removable lens assembly  11 . 4 . 1 - 120 . The display indicator may for example, be a color, symbol, or set of characters that differs for different users and is identifiable even for potential users having low acuity with the removable lens assembly  11 . 4 . 1 - 120  (e.g., for which the display  11 . 4 . 1 - 316   a  may appear blurry). The audible indicator is provided by an audio output device  11 . 4 . 1 - 1418   c  of or associated with the head-mounted display unit  11 . 4 . 1 - 110  (e.g., a speaker or headphones) and may be audible to potential users prior to or after wearing the head-mounted display unit  11 . 4 . 1 - 110 . The audible indicator may, for example, be spoken words (e.g., identifying the user associated with the removable lens assembly  11 . 4 . 1 - 120 ) or tones that for different users associated with the removable lens assemblies  11 . 4 . 1 - 120 . 
     Referring to  FIGS.  11 . 4   . 1 - 14 A and  11 . 4 . 1 - 14 B, a method  11 . 4 . 1 - 1400  is provided for indicating a lens configuration, which generally includes a first operation  11 . 4 . 1 - 1410  of identifying a removable lens assembly  11 . 4 . 1 - 120  coupled to a display module  11 . 4 . 1 - 116 , a second operation  11 . 4 . 1 - 1420  of retrieving a configuration indicator instruction, and a third operation  11 . 4 . 1 - 1430  of providing a configuration indication according to the instruction. The first operation  11 . 4 . 1 - 1410  of identifying the interchangeable lens assembly is performed with the lens detection sensor  11 . 4 . 1 - 1418   a  and the controller  11 . 4 . 1 - 1450  determining lens information from the lens tag  11 . 4 . 1 - 1428  of the one or more removable lens assembly  11 . 4 . 1 - 120  coupled to the one or more display modules  11 . 4 . 1 - 116  of the head-mounted display unit  11 . 4 . 1 - 110 . The second operation  11 . 4 . 1 - 1420  of retrieving a configuration indication instruction is performed, for example, by the controller  11 . 4 . 1 - 1450  retrieving from a storage the instruction associated with the lens information. The third operation  11 . 4 . 1 - 1430  of providing the configuration indication is provided, for example, by the display module  11 . 4 . 1 - 116 , the external visual indicator  11 . 4 . 1 - 1418   b , and/or the audio output device  11 . 4 . 1 - 1418   c , as operated by the controller  11 . 4 . 1 - 1450 . 
     Referring to  FIGS.  11 . 4   . 1 - 14 A- 11 . 4 . 1 - 16 , instead or additionally, the display system  11 . 4 . 1 - 100  may use the lens information of the removable lens assembly  11 . 4 . 1 - 120  coupled thereto to determine compatibility with the user and accordingly provide a compatibility indicator to the user (e.g., only of incompatibility is determined). The display system  11 . 4 . 1 - 100 , in addition to determining the lens information of the removable lens assembly  11 . 4 . 1 - 120  coupled to the head-mounted display unit  11 . 4 . 1 - 110 , identifies the current user, and compares user information with the lens information to determine compatibility. 
     The display system  11 . 4 . 1 - 100  may identify the user in various manners, for example, with biometric sensing, receipt of user credentials, and/or authentication by another device. To identify the user via biometrics, the display system  11 . 4 . 1 - 100  may include a biometric sensor  11 . 4 . 1 - 1418   d  to identify the user with biometrics (e.g., facial recognition, fingerprint recognition, voice recognition, iris recognition, and/or recognition of other biometric parameters, such as ear geometry, bone conduction or density characteristics, forehead features, or skin). The biometric sensor  11 . 4 . 1 - 1418   d  is, for example, coupled physically coupled to the head-mounted display unit. For example, in the case of identifying the user with iris detection, the biometric sensor  11 . 4 . 1 - 1418   d  may be the eye camera  11 . 4 . 1 - 319  described previously and which may be the same sensor for identifying the removable lens assembly  11 . 4 . 1 - 120 . To identify the user via user credentials, the display system  11 . 4 . 1 - 100  receives user credentials (e.g., a username and password) from an input device of or associated with the display system  11 . 4 . 1 - 100  (e.g., a microphone of the head-mounted display unit, or an external device  11 . 4 . 1 - 1460  in communication with the controller  11 . 4 . 1 - 1450  or the head-mounted display unit  11 . 4 . 1 - 110 ). To identify the user by authentication from another device, the external device  11 . 4 . 1 - 1460  may authenticate the user (e.g., via biometrics or credentials) and communicate such authentication to the controller  11 . 4 . 1 - 1450 . The external device  11 . 4 . 1 - 1460  may be considered part of the display system  11 . 4 . 1 - 100  but may function independent thereof (e.g., a smartphone). 
     The display system  11 . 4 . 1 - 100  stores or otherwise receives user information. The user information may include a user identifier (e.g., a username or user number) stored in association with an associated lens identifier (e.g., a serial number or model number of one or more of the removable lens assemblies  11 . 4 . 1 - 120  associated with the user) and/or eye characteristic information (e.g., sphere, cylinder, and/or axis parameters; referred to herein as eye characteristics). 
     The display system  11 . 4 . 1 - 100  may receive the user information during an initializing process in which the user information is associated with the lens information. During the initializing process, the user information may be input by the user or received from another source (e.g., the external device  11 . 4 . 1 - 1460 ). The lens information may be obtained from the removable lens assemblies  11 . 4 . 1 - 120  when coupled to the display module  11 . 4 . 1 - 116  as described above (e.g., electronically and/or optically), or be input by the user (e.g., inputting the serial or model number of the removable lens assembly  11 . 4 . 1 - 120 ) or received from another source (e.g., the external device  11 . 4 . 1 - 1460 ). 
     Referring to  FIG.  11 . 4   . 1 - 15 , to determine compatibility, the display system  11 . 4 . 1 - 100  compares the lens information  11 . 4 . 1 - 1510  to the user information  11 . 4 . 1 - 1520 , for example, using a compatibility determiner  11 . 4 . 1 - 1500 . For example, or more of: (a) the lens information  11 . 4 . 1 - 1510  of the lens identifier  11 . 4 . 1 - 1512  is compared to the user information  11 . 4 . 1 - 1520  of the associated lens identifier  11 . 4 . 1 - 1522 , (b) the lens information  11 . 4 . 1 - 1510  of the associated user identifier  11 . 4 . 1 - 1514  is compared to the user information  11 . 4 . 1 - 1520  of the user identifier  11 . 4 . 1 - 1524 ), or (c) the lens information  11 . 4 . 1 - 1510  of the lens characteristics  11 . 4 . 1 - 1516  is compared to the user information  11 . 4 . 1 - 1520  of the eye characteristics  11 . 4 . 1 - 1526 . The display system  11 . 4 . 1 - 100  determines the one or more removable lens assemblies  11 . 4 . 1 - 120  to be compatible if the lens information matches (e.g., equals) the user information, or incompatible if not matching. 
     If determined compatible, the compatibility indication may provide a positive indication (e.g., that the removable lens assembly  11 . 4 . 1 - 120  is compatible with the user) or be omitted (e.g., proceeding to normal operation). If determined incompatible, the compatibility indication is negative (e.g., indicating that the removable lens assembly  11 . 4 . 1 - 120  is not compatibility with the user). Such a negative indication may, for example, include an icon that graphically indicates incompatibility, textual instructions or information, and/or audible instructions or information. 
     Referring to  FIG.  11 . 4   . 1 - 16 , a method  11 . 4 . 1 - 1600  is provided for operating the display system  11 . 4 . 1 - 100  to determine and indicate compatibility of the removable lens assembly  11 . 4 . 1 - 120  with the user. The method includes a first operation  11 . 4 . 1 - 1610  of identifying the one or more removable lens assemblies  11 . 4 . 1 - 120  coupled to the one or more display modules  11 . 4 . 1 - 116 , a second operation  11 . 4 . 1 - 1620  of identifying the user, a third operation  11 . 4 . 1 - 1630  of comparing the lens information to the user information, and a fourth operation  11 . 4 . 1 - 1640  of indicating compatibility. The first operation  11 . 4 . 1 - 1610  of identifying the one or more removable lens assemblies  11 . 4 . 1 - 120  may be performed by the one or more lens detection sensors  11 . 4 . 1 - 1418   a  in cooperation with the controller  11 . 4 . 1 - 1450 . The first operation  11 . 4 . 1 - 1610  may also include determining lens information. The second operation  11 . 4 . 1 - 1620  of identifying the user may be performed biometrically (e.g., cooperatively by the biometric sensor  11 . 4 . 1 - 1418   d  in cooperation with controller  11 . 4 . 1 - 1450 ), by receiving credentials from the user, and/or by user authentication with the external device  11 . 4 . 1 - 1460 . The second operation  11 . 4 . 1 - 1620  may also include determining user information. The third operation  11 . 4 . 1 - 1630  of determining lens compatibility is performed by comparing the lens information to the user information, for example, with the controller  11 . 4 . 1 - 1450  according to the compatibility determiner  11 . 4 . 1 - 1500 . The fourth operation  11 . 4 . 1 - 1640  of indicating compatibility is performed, for example, with the head-mounted display unit  11 . 4 . 1 - 110  as operated by the controller  11 . 4 . 1 - 1450 . If compatible, a positive indication is provided, which may be usual operation of the display system  11 . 4 . 1 - 100 . If incompatible, a negative indication is provided reflecting incompatibility (e.g., by the head-mounted display unit  11 . 4 . 1 - 110  as operated by the controller  11 . 4 . 1 - 1450 ). 
     Referring to  FIG.  11 . 4   . 1 - 17 , an example hardware configuration of the controller  11 . 4 . 1 - 1450  is shown. The controller  11 . 4 . 1 - 1450  is a computing device, which may generally include a processing device  11 . 4 . 1 - 1710  (e.g., a processor or CPU), a memory  11 . 4 . 1 - 1720  (e.g., a volatile short-term memory, such as a random-access memory module), a storage  11 . 4 . 1 - 1730  (e.g., a long-term storage device, such as a hard disk or solid state drive), a communications interface  11 . 4 . 1 - 1740  by which the controller sends and/or receives signals to and/or from other components (e.g., the display module  11 . 4 . 1 - 116  and various sensors), and a bus  11 . 4 . 1 - 1750  by which the other components of the controller  11 . 4 . 1 - 1450  communicate with each other. The controller  11 . 4 . 1 - 1450  may be physically connected to the head-mounted display unit  11 . 4 . 1 - 110  or otherwise be in communication therewith. The display system  11 . 4 . 1 - 100  may include additional controllers  11 . 4 . 1 - 1450  or components thereof. The controller  11 . 4 . 1 - 1450  may include software programming (e.g., stored by the storage  11 . 4 . 1 - 1730 ) that includes instructions executable by the processing device  11 . 4 . 1 - 1710  to perform the methods and operations described herein. 
     11.4.2: Electronic Device System with Supplemental Lenses 
     Some users of head-mounted devices have visual defects such as myopia, hyperopia, astigmatism, or presbyopia. It can be challenging to ensure that an optical system in a head-mounted device operates satisfactorily for these users. If care is not taken, it may be difficult or impossible for a user with visual defects to focus properly on content that is being displayed. 
     A head-mounted device may have a display that displays content for a user. Head-mounted support structures in the device may support the device on the head of the user. A non-removable lens system may be supported by the head-mounted support structures and may be used to present content on the display to eye boxes. The user may view the content when the user&#39;s eyes are located in the eye boxes. 
     The head-mounted support structures may be configured to receive a removable supplemental lens system in alignment with the non-removable lens system. Magnetic coupling structures or other engagement structures may be used to removably couple the supplemental lens system to the head-mounted device. 
     The removable supplemental lens system may be used to adjust the lens characteristics of the non-removable lens system and thereby customize the head-mounted display to accommodate the user&#39;s vision. For example, the removable supplemental lens system may include supplemental left and right lenses that align with and supplemental corresponding left and right lenses in the non-removable lens system and that include astigmatic lens characteristics and other characteristics that allow the head-mounted device to be used by a user with astigmatism or other visual defects. 
     Removable supplemental lens systems associated with different users may be configured to operate with a shared head-mounted device. To ensure that the head-mounted device can customize its operation for each user, the removable supplemental lens system of each user may be provided with information that identifies the user associated with that removable supplemental lens system and other stored information. This information be stored in the removable supplemental lens system using bar codes or other optically readable patterns, programmable memory, or other data storage. A memory reader, gaze tracking system, or other sensor may be used in retrieving the stored information. 
     The information that is stored in the removable supplemental lens system may include information on the optical characteristics of the removable supplemental lens system, user information such as the user&#39;s eyeglass prescription, and/or other information. When the removable supplemental lens system is installed in the head-mounted device, control circuitry in the head-mounted device may retrieve the stored information and may take appropriate action. For example, the control circuitry may use lens positioners to adjust lens spacing and other operating parameters of the lens systems based on the user&#39;s interpupillary distance and other information on the user. 
     A head-mounted device may contain a display formed from one or more display panels (displays) for displaying visual content to a user. A lens system may be used to allow the user to focus on the display and view the visual content. To ensure that a wide range of users are able to clearly focus on the display and view visual content, the head-mounted device may receive removable supplemental lenses. The supplemental lenses may address the visual defects of users that are not otherwise addressed by the lens system. For example, a user with astigmatism may have supplemental lenses that correct for astigmatism. When this user desires to view content with the head-mounted device, the supplemental lenses may be installed within the head-mounted device to help correct for the user&#39;s astigmatism. With one illustrative arrangement, the supplemental lenses may be coupled to the head-mounted support structures using magnets or other removable fasteners that attach the supplemental lenses to non-removable lenses in the device. 
     If desired, removable supplemental lenses may be provided with the ability to store information that is subsequently retrieved and used by the head-mounted device. This information, which may sometimes be referred to as supplemental lens information or stored information may include lens power information and other information that is specific to the supplemental lenses and/or user information for a user associated with the lenses (e.g., a username, eyeglasses prescription information, etc.). Storage circuitry such as programmable read-only memory may be used to store the supplemental lens information and/or the supplemental lens information may be stored in other ways (e.g., using bar codes, text on the supplemental lenses, other patterned optically readable information, etc.). During operation, after a user has installed the supplemental lenses in a head-mounted device, the head-mounted device may retrieve and use the supplemental lens information. For example, information on the optical characteristics of the lens may be used by the head-mounted device to help correct for optical aberrations in the lenses (e.g., spherical aberration, chromatic aberration, pincushion distortion, barrel distortion, etc.), information on the user may be used to adjust the interpupillary distance of lenses or may be used to present the user with a login screen that has been prepopulated with the user&#39;s username, etc. 
     A schematic diagram of an illustrative system that may use supplemental head-mounted display lenses is shown in  FIG.  11 . 4   . 2 - 1 . As shown in  FIG.  11 . 4   . 2 - 1 , system  11 . 4 . 2 - 8  may include one or more electronic devices such as electronic device  11 . 4 . 2 - 10 . The electronic devices of system  11 . 4 . 2 - 8  may include computers, cellular telephones, head-mounted devices, wristwatch devices, and other electronic devices. Configurations in which electronic device  11 . 4 . 2 - 10  is a head-mounted device are sometimes described herein as an example. 
     As shown in  FIG.  11 . 4   . 2 - 1 , electronic devices such as electronic device  11 . 4 . 2 - 10  may have control circuitry  11 . 4 . 2 - 12 . Control circuitry  11 . 4 . 2 - 12  may include storage and processing circuitry for controlling the operation of device  11 . 4 . 2 - 10 . Circuitry  11 . 4 . 2 - 12  may include storage such as hard disk drive storage, nonvolatile memory (e.g., electrically-programmable-read-only memory configured to form a solid-state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Processing circuitry in control circuitry  11 . 4 . 2 - 12  may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors, power management units, audio chips, graphics processing units, application specific integrated circuits, and other integrated circuits. Software code may be stored on storage in circuitry  11 . 4 . 2 - 12  and run on processing circuitry in circuitry  11 . 4 . 2 - 12  to implement control operations for device  11 . 4 . 2 - 10  (e.g., data gathering operations, operations involved in processing three-dimensional facial image data, operations involving the adjustment of components using control signals, etc.). Control circuitry  11 . 4 . 2 - 12  may include wired and wireless communications circuitry. For example, control circuitry  11 . 4 . 2 - 12  may include radio-frequency transceiver circuitry such as cellular telephone transceiver circuitry, wireless local area network (WiFi®) transceiver circuitry, millimeter wave transceiver circuitry, and/or other wireless communications circuitry. 
     During operation, the communications circuitry of the devices in system  11 . 4 . 2 - 8  (e.g., the communications circuitry of control circuitry  11 . 4 . 2 - 12  of device  11 . 4 . 2 - 10 ), may be used to support communication between the electronic devices. For example, one electronic device may transmit video and/or audio data to another electronic device in system  11 . 4 . 2 - 8 . Electronic devices in system  11 . 4 . 2 - 8  may use wired and/or wireless communications circuitry to communicate through one or more communications networks (e.g., the internet, local area networks, etc.). The communications circuitry may be used to allow data to be received by device  11 . 4 . 2 - 10  from external equipment (e.g., a tethered computer, a portable device such as a handheld device or laptop computer, online computing equipment such as a remote server or other remote computing equipment, or other electrical equipment) and/or to provide data to external equipment. 
     Device  11 . 4 . 2 - 10  may include input-output devices  11 . 4 . 2 - 22 . Input-output devices  11 . 4 . 2 - 22  may be used to allow a user to provide device  11 . 4 . 2 - 10  with user input. Input-output devices  11 . 4 . 2 - 22  may also be used to gather information on the environment in which device  11 . 4 . 2 - 10  is operating. Output components in devices  11 . 4 . 2 - 22  may allow device  11 . 4 . 2 - 10  to provide a user with output and may be used to communicate with external electrical equipment. 
     As shown in  FIG.  11 . 4   . 2 - 1 , input-output devices  11 . 4 . 2 - 22  may include one or more displays such as display(s)  11 . 4 . 2 - 14 . In some configurations, display  11 . 4 . 2 - 14  of device  11 . 4 . 2 - 10  includes left and right display panels in alignment with the user&#39;s left and right eyes, respectively. In other configurations, display  11 . 4 . 2 - 14  includes a single display panel that extends across both eyes. 
     Display  11 . 4 . 2 - 14  may be used to display images. The visual content that is displayed on display  11 . 4 . 2 - 14  may be viewed by a user of device  11 . 4 . 2 - 10 . Displays in device  11 . 4 . 2 - 10  such as display  11 . 4 . 2 - 14  may be organic light-emitting diode displays or other displays based on arrays of light-emitting diodes, liquid crystal displays, liquid-crystal-on-silicon displays, projectors or displays based on projecting light beams on a surface directly or indirectly through specialized optics (e.g., digital micromirror devices), electrophoretic displays, plasma displays, electrowetting displays, or any other suitable displays. 
     Display  11 . 4 . 2 - 14  may present computer-generated content such as virtual reality content and mixed reality content to a user. Virtual reality content may be displayed in the absence of real-world content. Mixed reality content, which may sometimes be referred to as augmented reality content, may include computer-generated images that are overlaid on real-world images. The real-world images may be captured by a camera (e.g., a forward-facing camera) and merged with overlaid computer-generated content or an optical coupling system may be used to allow computer-generated content to be overlaid on top of real-world images. As an example, a pair of mixed reality glasses or other augmented reality head-mounted display may include a display device that provides images to a user through a beam splitter, prism, holographic coupler, or other optical coupler. Configurations in which display  11 . 4 . 2 - 14  is used to display virtual reality content to a user through lenses are described herein as an example. 
     Input-output circuitry  11 . 4 . 2 - 22  may include sensors  11 . 4 . 2 - 16 . Sensors  11 . 4 . 2 - 16  may include, for example, three-dimensional sensors (e.g., three-dimensional image sensors such as structured light sensors that emit beams of light and that use two-dimensional digital image sensors to gather image data for three-dimensional images from light spots that are produced when a target is illuminated by the beams of light, binocular three-dimensional image sensors that gather three-dimensional images using two or more cameras in a binocular imaging arrangement, three-dimensional LIDAR (light detection and ranging) sensors, three-dimensional radio-frequency sensors, or other sensors that gather three-dimensional image data), cameras (e.g., infrared and/or visible digital image sensors), gaze tracking sensors (e.g., a gaze tracking system based on an image sensor and, if desired, a light source that emits one or more beams of light that are tracked using the image sensor after reflecting from a user&#39;s eyes), touch sensors, buttons, capacitive proximity sensors, light-based (optical) proximity sensors, other proximity sensors, force sensors, sensors such as contact sensors based on switches, gas sensors, pressure sensors, moisture sensors, magnetic sensors, audio sensors (microphones), ambient light sensors, microphones for gathering voice commands and other audio input, sensors that are configured to gather information on motion, position, and/or orientation (e.g., accelerometers, gyroscopes, compasses, and/or inertial measurement units that include all of these sensors or a subset of one or two of these sensors), and/or other sensors. 
     User input and other information may be gathered using sensors and other input devices in input-output devices  11 . 4 . 2 - 22 . If desired, input-output devices  11 . 4 . 2 - 22  may include other devices  11 . 4 . 2 - 24  such as haptic output devices (e.g., vibrating components), light-emitting diodes and other light sources, speakers such as ear speakers for producing audio output, and other electrical components. Device  11 . 4 . 2 - 10  may include circuits for receiving wireless power, circuits for transmitting power wirelessly to other devices, batteries and other energy storage devices (e.g., capacitors), joysticks, buttons, and/or other components. 
     Electronic device  11 . 4 . 2 - 10  may have housing structures (e.g., housing walls, straps, etc.), as shown by illustrative support structures  11 . 4 . 2 - 26  of  FIG.  11 . 4   . 2 - 1 . In configurations in which electronic device  11 . 4 . 2 - 10  is a head-mounted device (e.g., a pair of glasses, goggles, a helmet, a hat, etc.), support structures  11 . 4 . 2 - 26  may include head-mounted support structures (e.g., a helmet housing, head straps, temples in a pair of eyeglasses, goggle housing structures, and/or other head-mounted structures). The head-mounted support structures may be configured to be worn on a head of a user during operation of device  11 . 4 . 2 - 10  and may support display(s)  11 . 4 . 2 - 14 , sensors  11 . 4 . 2 - 16 , other components  11 . 4 . 2 - 24 , other input-output devices  11 . 4 . 2 - 22 , and control circuitry  11 . 4 . 2 - 12 . 
       FIG.  11 . 4   . 2 - 2  is a top view of electronic device  11 . 4 . 2 - 10  in an illustrative configuration in which electronic device  11 . 4 . 2 - 10  is a head-mounted device. As shown in  FIG.  11 . 4   . 2 - 2 , electronic device  11 . 4 . 2 - 10  may include support structures (see, e.g., support structures  11 . 4 . 2 - 26  of  FIG.  11 . 4   . 2 - 1 ) that are used in housing the components of device  11 . 4 . 2 - 10  and mounting device  11 . 4 . 2 - 10  onto a user&#39;s head. These support structures may include, for example, structures that form housing walls and other structures for main unit  11 . 4 . 2 - 26 - 2  and straps or other supplemental support structures such as structures  11 . 4 . 2 - 26 - 1  that help to hold main unit  11 . 4 . 2 - 26 - 2  on a user&#39;s face so that the user&#39;s eyes are located within eye boxes  11 . 4 . 2 - 60 . 
     Display  11 . 4 . 2 - 14  may include left and right display panels (e.g., left and right pixel arrays, sometimes referred to as left and right displays) that are mounted respectively in left and right display modules  11 . 4 . 2 - 70  corresponding respectively to a user&#39;s left eye (and left eye box  11 . 4 . 2 - 60 ) and right eye (and right eye box). Modules  11 . 4 . 2 - 70 , which may sometimes be referred to as lens support structures or lens housings, may be individually positioned relative to the housing wall structures of main unit  11 . 4 . 2 - 26 - 2  and relative to the user&#39;s eyes using respective left and right positioners  11 . 4 . 2 - 58  (e.g., stepper motors, piezoelectric actuators, motors, linear electromagnetic actuators, and/or other electronic components for adjusting position). Positioners  11 . 4 . 2 - 58  may be controlled by control circuitry  11 . 4 . 2 - 12  during operation of device  11 . 4 . 2 - 10 . For example, positioners  11 . 4 . 2 - 58  may be used to adjust the spacing between modules  11 . 4 . 2 - 70  (and therefore the lens-to-lens spacing between the lenses of modules  11 . 4 . 2 - 70 ) to match the interpupillary distance IPD of a user&#39;s eyes. 
     Device  11 . 4 . 2 - 10  may include a non-removable lens system (sometimes referred to as a permanent lens system or fixed lens system) to which the user of device  11 . 4 . 2 - 10  may attach removable supplemental lenses to accommodate the user&#39;s vision (astigmatism, etc.). When the removable supplemental lenses are absent, a user may use the non-removable lens system. Users with particular visual defects (e.g., astigmatism), may benefit from the attachment of the removable supplemental lenses to the non-removable lenses. When the supplemental lenses are coupled to device  11 . 4 . 2 - 10  in alignment with the non-removable lenses, the supplemental lenses and non-removable lenses operate together to form a combined lens system with combined optical properties determined by both the non-removable lenses and the supplemental lenses. 
     As shown in  FIG.  11 . 4   . 2 - 2 , for example, each lens module  11 . 4 . 2 - 70  may include a non-removable lens  11 . 4 . 2 - 72 . Lenses  11 . 4 . 2 - 72  may be Fresnel lenses, single-element or multiple-element lenses formed from clear materials such as glass or plastic, mirror lenses, catadioptric lenses, and/or lenses of other types. The position of lenses  11 . 4 . 2 - 72  along the X axis may be adjusted so that lenses  11 . 4 . 2 - 72  are separated by a distance that matches the user&#39;s interpupillary distance (distance IPD). The position of lenses  11 . 4 . 2 - 72  along the Z axis may be adjusted to help a user focus on display  11 . 4 . 2 - 14 . 
     The amount of travel available to lenses  11 . 4 . 2 - 72  may be limited and/or lenses  11 . 4 . 2 - 72  may have attributes that make it difficult for lenses  11 . 4 . 2 - 72  alone to be used to view content on display  11 . 4 . 2 - 14  by all users. For example, a user may have astigmatism or may be strongly nearsighted or farsighted. In situations such as these, lenses  11 . 4 . 2 - 72  when used alone may not be able to correct for the user&#39;s vision. To ensure that the user&#39;s vision is fully corrected, supplemental lenses  11 . 4 . 2 - 76  may be coupled to lenses  11 . 4 . 2 - 72  to provide further adjustments. For example, if a user is astigmatic, supplemental lenses  11 . 4 . 2 - 76  may be configured to compensate for the astigmatism of the user. If there are multiple users associated with device  11 . 4 . 2 - 10 , each of the multiple users may potentially have a different respective set of supplemental lenses  11 . 4 . 2 - 76 . Lenses  11 . 4 . 2 - 76  may be joined together to form a supplemental lens system based on a single supplemental lens module or may be housed in separate lens modules that form a multi-lens-module supplemental lens system. 
     As shown in  FIG.  11 . 4   . 2 - 2 , for example, supplemental lenses  11 . 4 . 2 - 76  may be mounted in supplemental lens modules (sometimes referred to as supplemental lens support structures or housings) such as supplemental lens modules  11 . 4 . 2 - 74 . Lens modules  11 . 4 . 2 - 74  may be removably coupled to support structures  11 . 4 . 2 - 26  by using coupling structures  11 . 4 . 2 - 78  in non-removable lens modules  11 . 4 . 2 - 70  and corresponding coupling structures that mate with coupling structures  11 . 4 . 2 - 78  such as coupling structures  11 . 4 . 2 - 80  in removable lens modules  11 . 4 . 2 - 74  to couple lens modules  11 . 4 . 2 - 74  to lens modules  11 . 4 . 2 - 70 . Coupling structures  11 . 4 . 2 - 78  and  11 . 4 . 2 - 80  may be formed from magnetic couplers (e.g., magnets and/or magnetic structures such as iron bars), clips, hook-and-loop fasteners, snaps, screws and other threaded fasteners, temporary adhesive, spring-loaded engagement structures, and/or other coupling mechanisms. 
     Device  11 . 4 . 2 - 10  may include a gaze detection system that is configured to monitor a user&#39;s eyes in eye boxes  11 . 4 . 2 - 60 . The gaze detection system may, as an example, use optical eye monitoring components to monitor the user&#39;s direction of gaze as the user is using device  11 . 4 . 2 - 10 . With one illustrative configuration, a gaze tracker such a gaze tracker  11 . 4 . 2 - 86  may be located in each lens module  11 . 4 . 2 - 70 . In this position, the gaze tracker  11 . 4 . 2 - 86  in each lens module  11 . 4 . 2 - 70  may monitor a user&#39;s eye in a respective eye box  11 . 4 . 2 - 60  through a respective lens  11 . 4 . 2 - 72  and a respective mating supplemental lens  11 . 4 . 2 - 76 . Gaze trackers  11 . 4 . 2 - 86  may be placed at peripheral portions of lens modules  11 . 4 . 2 - 70  so that gaze trackers  11 . 4 . 2 - 86  do not block the user&#39;s view of display  11 . 4 . 2 - 14  through removable lenses  11 . 4 . 2 - 76  and non-removable lenses  11 . 4 . 2 - 72 . 
     If desired, the removable lens system of  FIG.  11 . 4   . 2 - 2  (e.g., one or both of lens modules  11 . 4 . 2 - 74  and/or a single removable lens holder that is configured to support both of removable lenses  11 . 4 . 2 - 76 ) may be provided with electronic storage such as storage  11 . 4 . 2 - 84 . Storage  11 . 4 . 2 - 84  may be programmable read-only memory (e.g., electrically programmable read-only memory, memory formed from laser-programmed fuses, and/or other programmable memory), may be formed from a pattern of metal traces and/or resistive elements (e.g., to encode information in the form of a pattern of conductive paths, a pattern of resistors and/or their associated resistance values, and/or other circuitry that is configured to serve as storage that stores data in the removable lenses for electrical retrieval by device  11 . 4 . 2 - 10 ). Storage  11 . 4 . 2 - 84 , which may be formed as part of the removable lens system (e.g., as part of one or more of modules  11 . 4 . 2 - 74  and associated removable lenses  11 . 4 . 2 - 76 ), may be used in storing lens information, user information, and/or other information electrically. When the removable lens system is coupled to the non-removable lens system (e.g., one or both lens modules  11 . 4 . 2 - 70  such as storage  11 . 4 . 2 - 84 ), electrically stored information reader  11 . 4 . 2 - 82  may retrieve the data stored in storage  11 . 4 . 2 - 84 . Information reader  11 . 4 . 2 - 82  may be, for example, an electrically programmed read-only memory reader, a memory reader that reads laser-programmed storage or other programmable memory, circuitry that electrically measures storage  11 . 4 . 2 - 84  (e.g., to detect a pattern of conductive traces and/or resistive elements that are being used to store data), and/or other reader circuitry. 
     Information such as lens information, user information, and other information may also be stored using non-electrical storage. This storage may include, for example, bar codes, text, patterns of dots, and/or other information that can be detected optically. Optical data reading operations may be performed using infrared and/or visible cameras or other equipment (e.g., bar code scanning equipment) that reads optically encoded information. As an example, optical data reading equipment that reads data on the removable lens by imaging the removable lens such as infrared digital image sensors in gaze tracking sensors  11 . 4 . 2 - 86  and/or visible-light digital image sensors may be used in reading bar codes, text, and/or other optically encoded information. 
     Gaze trackers  11 . 4 . 2 - 86  (sometimes referred to as gaze monitoring systems, gaze tracking systems, eye monitoring systems, etc.) may monitor the eyes of the user in eye boxes  11 . 4 . 2 - 60  during operation of device  11 . 4 . 2 - 10 . Gaze trackers  11 . 4 . 2 - 86  may include light sources and light-detecting components such as image sensors. Each gaze tracker  11 . 4 . 2 - 86  may, as an example, include a light source such as a light-emitting diode light source or laser light source that produces an array of light beams (e.g., infrared light beams that do not interfere with the vision of the user). Blanket (flood) illumination at infrared and/or visible wavelengths may also be provided. Each gaze tracker  11 . 4 . 2 - 86  may also include a respective infrared image sensor (and/or visible-light image sensor) that faces a user&#39;s eye in a respective eye box  11 . 4 . 2 - 60  and capture images of the user&#39;s eye through lenses  11 . 4 . 2 - 72  and lenses  11 . 4 . 2 - 76  while the user&#39;s eye is being illuminated with the light beams from the light source of that gaze tracker. During normal operation, control circuitry  11 . 4 . 2 - 20  may use gaze trackers  11 . 4 . 2 - 86  to track a user&#39;s gaze (e.g., so that images being displayed on displays  11 . 4 . 2 - 14  can be adjusted based on the direction of the user&#39;s gaze, etc.). When it is desired to read information in the removable lens system that has been optically stored (e.g., by encoding the information in the form of a bar code, text, etc.), one or more image sensors such as one or more infrared image sensors in one or more gaze trackers  11 . 4 . 2 - 86  may optically read the information from the supplemental lens system (e.g., from one or more lens modules  11 . 4 . 2 - 74 ). 
       FIG.  11 . 4   . 2 - 3  is a front view of an illustrative removable lens module showing arrangements that may be used to store information in the lens module. As shown in  FIG.  11 . 4   . 2 - 3 , lens module  11 . 4 . 2 - 74  may include lens support structures  11 . 4 . 2 - 74 H. Support structures  11 . 4 . 2 - 74 H may be formed from polymer, metal, and/or other materials and may surround or otherwise support lens  11 . 4 . 2 - 76  to form lens module  11 . 4 . 2 - 74 . Coupling structures  11 . 4 . 2 - 80  may be supported by support structures  11 . 4 . 2 - 74 H, so that coupling structures  11 . 4 . 2 - 80  hold lens module  11 . 4 . 2 - 74  to lens module  11 . 4 . 2 - 70  and support structures  11 . 4 . 2 - 26  when coupling structures  11 . 4 . 2 - 80  are coupled to mating coupling structures  11 . 4 . 2 - 78  in lens module  11 . 4 . 2 - 70 . 
     Lens information, user information, and other information can be stored in removable lens module  11 . 4 . 2 - 74  using one or more approaches. With one illustrative arrangement, electrical storage  11 . 4 . 2 - 84  is included in lens module  11 . 4 . 2 - 74 . Data may be stored in electrical storage  11 . 4 . 2 - 84 . When removable lens module  11 . 4 . 2 - 74  is coupled to non-removable lens module  11 . 4 . 2 - 70 , reader  11 . 4 . 2 - 82  may couple to electrical storage  11 . 4 . 2 - 84  and may retrieve the stored data from electrical storage  11 . 4 . 2 - 84 . 
     With another illustrative arrangement, a bar code such as bar code  11 . 4 . 2 - 92  may be included in lens module  11 . 4 . 2 - 74 . Bar code  11 . 4 . 2 - 92  may, as an example, be an infrared bar code that is formed from an ink that is visibly transparent while being opaque to infrared light (e.g., infrared light at the wavelength emitted by an infrared light source in gaze tracker  11 . 4 . 2 - 86 ) or other bar code structure that is transparent to visible light and opaque at infrared wavelengths. With this type of arrangement, bar code  11 . 4 . 2 - 92  will be transparent to the user and will not interfere with the user&#39;s viewing of content on display  11 . 4 . 2 - 14 , even though bar code  11 . 4 . 2 - 92  is formed on lens  11 . 4 . 2 - 76 . At the same time, gaze tracker  11 . 4 . 2 - 86  is pointed towards lens  11 . 4 . 2 - 76  so that gaze tracker  11 . 4 . 2 - 86  may capture an image of bar code  11 . 4 . 2 - 92  at infrared wavelengths. 
     Bar codes, text information, or other visible-light (and/or infrared light) markings may be formed from laser markings, patterned ink, patterned metal coatings, and/or other markings formed in regions such as region  11 . 4 . 2 - 90 . Visible-light-encoded information of this type (e.g., information stored optically) may be read by a visible light image sensor in lens module  11 . 4 . 2 - 70  or an image sensor elsewhere in device  11 . 4 . 2 - 10  (e.g., a visible light image sensor in sensors  11 . 4 . 2 - 16 ). In some situations, visible-light-encoded information such as text, icons, or other optically readable patterns may contain information that is recognizable to a user (e.g., text that a user can read such as the user&#39;s name or user name, an icon that a user can recognize, etc.). User recognizable information may be used to label lens modules  11 . 4 . 2 - 74  so that lens modules that belong to different users are not mixed up. An image sensor in device  11 . 4 . 2 - 10  (e.g., an image sensor in gaze tracker  11 . 4 . 2 - 87  and/or other image sensors) may optically read text, icons, or other patterned structures on the removable lens system that are also recognizable to the user and/or the image sensor may read patterns such as bar codes and other patterns that do not contain user-recognizable text or icons. 
     In some situations, it may be desirable to encrypt the information that is being stored in removable lens module  11 . 4 . 2 - 74 . Stored information may be encrypted using a cryptographic key. Control circuitry  11 . 4 . 2 - 12  can decrypt the encrypted information using a corresponding cryptographic key suitable for decryption (which may be the same as the encrypting key or may be related to the encrypting key). Control circuitry  11 . 4 . 2 - 12  can obtain the key from the user (e.g., using input-output devices  11 . 4 . 2 - 22 ). If desired, control circuitry  11 . 4 . 2 - 12  can obtain the key from a remote server (e.g., an online service). A user password, username, or other information may, if desired, be gathered from the user and/or storage in device  11 . 4 . 2 - 10  and may be used by control circuitry  11 . 4 . 2 - 12  in obtaining the key from the online service. In some arrangements, keys may be stored in control circuitry  11 . 4 . 2 - 12  for use in decrypting encrypted information stored in lens module  11 . 4 . 2 - 74  (e.g., when device  11 . 4 . 2 - 10  is only used by the members of a single household). These illustrative arrangements and/or other arrangements for encrypting and decrypting the information stored in removable lens module  11 . 4 . 2 - 74  may be used to enhance privacy for stored user information. 
     Illustrative steps involved in using system  11 . 4 . 2 - 8  are shown in  FIG.  11 . 4   . 2 - 4 . 
     During the operations of block  11 . 4 . 2 - 100 , a user who desires to use device  11 . 4 . 2 - 10  but who has specific visual defects (astigmatism, etc.) may attach a removable lens system that is associated with that user to device  11 . 4 . 2 - 10 . During attachment of the removable lens system, removable lens support structures such as lens modules  11 . 4 . 2 - 74  may be aligned with non-removable lens support structures such as lens modules  11 . 4 . 2 - 70 , so that lenses  11 . 4 . 2 - 76  are aligned with corresponding lenses  11 . 4 . 2 - 72 . Coupling structures  11 . 4 . 2 - 82  and  11 . 4 . 2 - 84  may be used to hold lens modules  11 . 4 . 2 - 74  and  11 . 4 . 2 - 72  together. The presence of lenses  11 . 4 . 2 - 76  in alignment with lenses  11 . 4 . 2 - 72  allows the optical properties of lenses  11 . 4 . 2 - 76  to modify the optical properties of lenses  11 . 4 . 2 - 72  (e.g., to increase or decrease lens power, to add lens asymmetry to compensate for user astigmatism, etc.). In this way, the optical system of device  11 . 4 . 2 - 10  may be tailored to the user&#39;s vision. 
     During the operations of block  11 . 4 . 2 - 102 , control circuitry  11 . 4 . 2 - 12  of head-mounted device  11 . 4 . 2 - 10  may read information that has been stored in the removable lens system (e.g., using gaze trackers  11 . 4 . 2 - 86 , a visible-light image sensor, a memory reader such as reader  11 . 4 . 2 - 82 , etc.). The information that is read may be stored using a bar code, text, other optically readable patterns (e.g., patterned ink, patterned metal, or other patterned structures), and/or using electrical data storage arrangements (see, e.g., bar code  11 . 4 . 2 - 92 , visible information  11 . 4 . 2 - 90 , and electrically stored information in storage  11 . 4 . 2 - 84  of  FIG.  11 . 4   . 2 - 3 ). 
     In general, any suitable data may be stored in the removable lens system. This information may include, for example, information regarding the user that is associated with the removable lens system such as the user&#39;s eyeglass prescription (e.g., the spherical component of the user&#39;s prescription to be handled by spherical lens power, the astigmatic component of the user&#39;s prescription to be handled by an aspherical lens power, and/or the interpupillary distance of the user&#39;s prescription), the date of the prescription, the expiration date of the prescription, the user&#39;s username and other user credentials, manufacturing information such as the date of manufacture of the removable lens system, the name of the manufacturer, a serial number, a lot number, and a model name or other model type information, lens materials for lenses  11 . 4 . 2 - 76 , lens powers (e.g., spherical lens power and/or aspherical lens power) and/or other optical characteristics of lenses  11 . 4 . 2 - 76  such as known aberrations that can be later corrected using compensating image warping operations performed by control circuitry  11 . 4 . 2 - 12  during video playback, and/or other information. 
     After reading this information from the removable lens system during the operations of block  11 . 4 . 2 - 102 , control circuitry  11 . 4 . 2 - 12  may, if desired, make adjustments to the components in device  11 . 4 . 2 - 10 . These adjustments may be made based on real-time measurements from gaze trackers  11 . 4 . 2 - 86  and/or based on the stored data obtained from the removable lens system. For example, control circuitry  11 . 4 . 2 - 12  may use positioners  11 . 4 . 2 - 58  to adjust lens-to-lens spacing based on real-time measurements of the user&#39;s interpupillary distance determined from the positions of the user&#39;s pupils measured with gaze trackers  11 . 4 . 2 - 86  and/or based on stored interpupillary distance information for the user that is obtained from removable lens modules  11 . 4 . 2 - 74 . Control circuitry  11 . 4 . 2 - 12  may also adjust the positions of the lenses in device  11 . 4 . 2 - 10  along the Z axis of  FIG.  11 . 4   . 2 - 2  to place lenses  11 . 4 . 2 - 72  and associated supplemental lenses  11 . 4 . 2 - 76  in a satisfactory location to allow the user to view content on displays  11 . 4 . 2 - 14 . These focusing adjustments with lenses  11 . 4 . 2 - 72  and  11 . 4 . 2 - 76  may be made based on the user&#39;s prescription, the powers of lenses  11 . 4 . 2 - 76 , and other information obtained using the stored information in lenses  11 . 4 . 2 - 76 . In arrangements in which lenses  11 . 4 . 2 - 72  have adjustable power (e.g., when lenses  11 . 4 . 2 - 72  are tunable lenses), control circuitry  11 . 4 . 2 - 12  can adjust lenses  11 . 4 . 2 - 72  to implement the spherical component of the user&#39;s prescription. The asymmetric component of a user&#39;s prescription may be handled using the asymmetric portion of supplemental lenses  11 . 4 . 2 - 76  (as an example). 
     After making adjustments to device  11 . 4 . 2 - 10  based on information from the supplemental lens system formed from lenses  11 . 4 . 2 - 76 , a user may place device  11 . 4 . 2 - 10  on the user&#39;s head. 
     During the operations of block  11 . 4 . 2 - 104 , control circuitry  11 . 4 . 2 - 12  may make further adjustments to the components in device  11 . 4 . 2 - 10 . For example, control circuitry  11 . 4 . 2 - 12  may use positioners  11 . 4 . 2 - 58  to adjust the lens-to-lens separation of left and right lenses  11 . 4 . 2 - 72  (and left and right lenses  11 . 4 . 2 - 76 ) based on information from sensors in device  11 . 4 . 2 - 10  (e.g., based on a measured interpupillary distance gathered using gaze trackers  11 . 4 . 2 - 86 ). 
     During the operations of block  11 . 4 . 2 - 104 , an iris scanner, fingerprint sensor, input-output devices that gather username and password information, or other components may be used in authenticating the user. The user may, for example, be directed to supply a username and password so that control circuitry  11 . 4 . 2 - 12  can establish a communications link with an online service that provides media content (block  11 . 4 . 2 - 108 ). If the user is not authenticated by control circuitry  11 . 4 . 2 - 12  (e.g., if the user′ username and password or other credentials (biometric information, etc.) are not authenticated, control circuitry  11 . 4 . 2 - 12  can, during the operations of block  11 . 4 . 2 - 110 , prevent the user from accessing the online service, selectively block content, and/or take other suitable action. If desired, the user may be allowed to override the authentication process at block  11 . 4 . 2 - 110  and/or additional authentication techniques may be used in scenarios in which the user fails authentication during the operations block  11 . 4 . 2 - 106 . 
     In some configurations of device  11 . 4 . 2 - 10 , lenses  11 . 4 . 2 - 76  may carry user information (e.g., user game preferences, user movie preferences, etc.). In these configurations, control circuitry  11 . 4 . 2 - 12  can refrain from decoding this information until the user has been authenticated during the operations of block  11 . 4 . 2 - 106  and an appropriate decryption key has been obtained (e.g., the user&#39;s password). 
     In some embodiments, user information is stored by control circuitry  11 . 4 . 2 - 12 . The user information may include information on a user&#39;s eye glasses prescription (e.g., a required optical prescription strength). The information stored in lenses  11 . 4 . 2 - 76  may include information on the optical prescription strength of lenses  11 . 4 . 2 - 76 . After determining the user of device  11 . 4 . 2 - 10  and retrieving the user&#39;s stored prescription from control circuitry  11 . 4 . 2 - 12 , control circuitry  11 . 4 . 2 - 12  may compare this retrieved user prescription information with the prescription of lenses  11 . 4 . 2 - 76  to determine if there is a satisfactory match. In the event that the prescription of lenses  11 . 4 . 2 - 76  does not match the user&#39;s prescription, control circuitry  11 . 4 . 2 - 12  may use display  11 . 4 . 2 - 14  and/or other output circuitry to issue a visible and/or audible alert for the user and may, if desired, decline to display content on display  11 . 4 . 2 - 14 . 
     In scenarios in which optical aberration information is included in the information stored in lenses  11 . 4 . 2 - 76  (e.g., information indicating that lenses  11 . 4 . 2 - 76  have a particular amount of barrel distortion, pincushion distortion, spherical aberration, chromatic aberration, etc.), control circuitry  11 . 4 . 2 - 12  may, during the playback operations of block  11 . 4 . 2 - 108 , apply geometric transformations or other video processing to the content being displayed on display  11 . 4 . 2 - 14  that compensates for this distortion. In this way, a user may view content that is free or nearly free of aberrations. If, as an example, lenses  11 . 4 . 2 - 76  exhibit pincushion distortion of a particular amount, information on this amount of pincushion distortion can be stored in lenses  11 . 4 . 2 - 76 . During playback operations, control circuitry  11 . 4 . 2 - 12  can apply compensating (barrel) image warping to the content for display  11 . 4 . 2 - 14  to ensure that the displayed content exhibits low distortion and aberrations, thereby enhancing the content viewing experience for the user. In general, control circuitry  11 . 4 . 2 - 12  may apply any image processing correction that helps enhance image quality and the user&#39;s viewing experience. For example, corrections can be applied to correct for chromatic aberration, spherical aberration, etc. 
     In accordance with an embodiment, a system is provided that includes a head-mounted support structure, a display coupled to the head-mounted support structure, control circuitry configured to supply content using the display, a non-removable lens coupled to the head-mounted support structure that is configured to present the content from the display in an eye box, and a removable supplemental lens removably coupled to head-mounted support structure between the non-removable lens and the eye box, the removable supplemental lens is configured to store information that is retrieved by the control circuitry. 
     In accordance with another embodiment, the system includes a gaze tracking system having a light emitter and an infrared image sensor, the removable supplemental lens has a bar code that stores the information, the bar code is opaque at infrared wavelengths and transparent at visible wavelengths, and the image sensor is configured to read the bar code while the removable supplemental lens is removably coupled to the head-mounted support structure. 
     In accordance with another embodiment, the bar code includes infrared-opaque-and-visible-light-transparent ink on a clear lens element in the removable supplemental lens. 
     In accordance with another embodiment, the information includes information selected from the group consisting of an eyeglass prescription, an eyeglass prescription date of expiration, a username, a manufacturing date, a manufacturer, a serial number, a manufacturing lot number, a model name, a model type, and information on optical characteristics of the removable supplemental lens. 
     In accordance with another embodiment, the system includes a data reading device coupled to the head-mounted support structure, the control circuitry is configured to use the data reading device to obtain the information from the removable supplemental lens. 
     In accordance with another embodiment, the information includes an eyeglasses prescription. 
     In accordance with another embodiment, the removable supplemental lens includes memory and the data reading device is configured to obtain the information from the memory. 
     In accordance with another embodiment, the removable supplemental lens includes text and the data reading device is configured to obtain the information by optically reading the text. 
     In accordance with another embodiment, the removable supplemental lens includes circuitry configured to store data and the data reading device is configured to obtain the information from the circuitry. 
     In accordance with another embodiment, the removable supplemental lens includes programmable read only memory, the data reading device is configured to obtain the information from the programmable read only memory, and the information includes a lens characteristics of the removable supplemental lens. 
     In accordance with another embodiment, the lens characteristic includes a lens power associated with the removable supplemental lens. 
     In accordance with another embodiment, the lens characteristic includes an aspherical lens power associated with the removable supplemental lens. 
     In accordance with another embodiment, the lens characteristic includes an optical aberration associated with the removable supplemental lens. 
     In accordance with another embodiment, the control circuitry is configured to obtain the information when the removable supplemental lens is coupled to the head-mounted support structure. 
     In accordance with another embodiment, the information includes information selected from the group consisting of a username, a serial number, optical characteristics of the removable supplemental lens, and eyeglass prescription information. 
     In accordance with an embodiment, a head-mounted device, is provided that includes head-mounted support structures, a display configured to display content, left and right positioners, a non-removable lens system having left and right lenses that are positioned respectively by the left and right positioners, a removable lens system with removable supplemental left and right lenses that are removably coupled to the head-mounted support structures in alignment respectively with the right and left lenses, and control circuitry configured to obtain information stored in the removable lens system. 
     In accordance with another embodiment, the information includes an interpupillary distance for a user and the control circuitry is configured to use the left and right positioners to adjust a lens-center-to-lens-center spacing associated with the left and right lenses to match the interpupillary distance. 
     In accordance with another embodiment, the head-mounted device includes an optically readable pattern on the removable lens system that stores the information. 
     In accordance with another embodiment, the head-mounted device includes an image sensor that optically reads the optically readable pattern to retrieve the information from the removable lens system. 
     In accordance with another embodiment, the image sensor includes an infrared image sensor in a gaze tracking system. 
     In accordance with another embodiment, the information includes a user&#39;s eyeglass prescription and the control circuitry is configured to adjust the left and right positioners based on the user&#39;s eyeglass prescription. 
     In accordance with an embodiment, an electronic device configured to operate with a removable supplemental lens system is provided that includes a display that displays content, control circuitry, a non-removable lens system configured to allow the display content to be viewed from eye boxes, a removable supplemental lens system, and head-mounted support structures that support the display and that support the non-removable lens system between the eye boxes and the display, the removable supplemental lens system is configured to be supported between the non-removable lenses and the eye boxes, the removable supplemental lens system is configured to store information, and the control circuitry is configured to obtain the information from the removable supplemental lens system when the removable supplemental lens system is between the non-removable lenses and the eye boxes. 
     In accordance with another embodiment, the electronic device includes lens positioners, the control circuitry is configured to use the lens positioners to adjust a lens-to-lens spacing associated with left and right lenses in the non-removable lens system based on the information obtained from the removable supplemental lens system. 
     In accordance with another embodiment, the information includes an optical prescription associated with the removable supplemental lens system and the control circuitry is configured to issue an alert to a user in response to determining that there is a mismatch between the user&#39;s prescription and the optical prescription associated with the removable supplemental lens system. 
     In accordance with another embodiment, the information includes distortion information specifying an optical distortion associated with the supplemental lens system and the control circuitry is configured to apply a compensating correction to the content being displayed on the display based on the distortion information. 
     The foregoing is merely illustrative and various modifications can be made to the described embodiments. The foregoing embodiments may be implemented individually or in any combination. 
     11.4.3: RX Lenses 
       FIG.  11 . 4   . 3 - 1  illustrates an optical assembly  11 . 4 . 3 - 100  for use in an HMD device such as those described herein. The optical assembly  11 . 4 . 3 - 100  can be configured to project light toward the user&#39;s eyes when donning the HMD. In at least one example, the optical assembly  11 . 4 . 3 - 100  can include a removably attachable prescription (“Rx”) lens  11 . 4 . 3 - 102 . The Rx lens  11 . 4 . 3 - 102  can be removably coupled to a rim  11 . 4 . 3 - 104 . The rim  11 . 4 . 3 - 104  can be fixed to an outer housing  11 . 4 . 3 - 106  of the optical assembly  11 . 4 . 3 - 106 . The Rx lens  11 . 4 . 3 - 102  can be swappable to accommodate the vision correction needs of various users of the HMD. The optical assembly  11 . 4 . 3 - 100  can also include a display, such as a display screen (not shown), configured to project light toward the user&#39;s eye(s) through the Rx lens  11 . 4 . 3 - 102 . 
       FIG.  11 . 4   . 3 - 2  illustrates the optical assembly  11 . 4 . 3 - 100  with the Rx lens removed to illustrate another lens  11 . 4 . 3 - 108  of the optical assembly. The fixed lens  11 . 4 . 3 - 108  shown in  FIG.  11 . 4   . 3 - 2  can be disposed below/behind the Rx lens  11 . 4 . 3 - 102  shown in  FIG.  11 . 4   . 3 - 1  (e.g., with the Rx lens  11 . 4 . 3 - 102  disposed between the fixed lens  11 . 4 . 3 - 108  and the user&#39;s eye  11 . 4 . 3 - 108  during use) and substantially un-removably fixed to the optical assembly  11 . 4 . 3 - 100 , for example to the rim  11 . 4 . 3 - 104  of the optical assembly  11 . 4 . 3 - 100 . 
       FIG.  11 . 4   . 3 - 3  illustrates another example of the optical assembly with the Rx lens  11 . 4 . 3 - 102 , rim  11 . 4 . 3 - 104 , and housing  11 . 4 . 3 - 106  removed to illustrate an array of magnets  11 . 4 . 3 - 110  disposed peripherally to the lens  11 . 4 . 3 - 108  and the Rx lens  11 . 4 . 3 - 102 . The magnets  11 . 4 . 3 - 110  can be embedded in the rim or otherwise fixed in the optical assembly  11 . 4 . 3 - 100  as shown to provide magnetic securement of the Rx lens  11 . 4 . 3 - 102 , which can also include a complimentary array of magnets to removably attach the Rx lens  11 . 4 . 3 - 102  to the optical assembly  11 . 4 . 3 - 100  over the fixed lens  11 . 4 . 3 - 108 . 
     The magnet array  11 . 4 . 3 - 110  can be used for macro-alignment of the Rx lens  11 . 4 . 3 - 102  to the assembly  11 . 4 . 3 - 100  to ensure the Rx lens  11 . 4 . 3 - 102  is rotated correctly relative to the fixed lens  11 . 4 . 3 - 108  and the rest of the assembly  11 . 4 . 3 - 100 , including the rim  11 . 4 . 3 - 104  and the housing  11 . 4 . 3 - 106 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  11 . 4   . 3 - 1 - 11 . 4 . 3 - 3  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in any other figures shown and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to any other figure shown and described herein, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  11 . 4   . 3 - 1 - 11 . 4 . 3 - 3 . 
       FIG.  11 . 4   . 3 - 4  illustrates an example of an optical assembly  11 . 4 . 3 - 200  configured for receiving a removably attachable Rx lens.  FIG.  11 . 4   . 3 - 4  shows the assembly  11 . 4 . 3 - 200  as assembled as well as an exploded view thereof. The two magnet arrays  11 . 4 . 3 - 210  and  11 . 4 . 3 - 212  are also shown. The assembly  11 . 4 . 3 - 200  can include a housing  11 . 4 . 3 - 206 , rim  11 . 4 . 3 - 204 , a first magnet array  11 . 4 . 3 - 210 , and a second magnet array  11 . 4 . 3 - 212 . In at least one example, the Rx lens (not shown) can be coupled with the rim  11 . 4 . 3 - 204  such that the Rx lens and the rim  11 . 4 . 3 - 204  are removably secured to the housing  11 . 4 . 3 - 206 . The second magnet array  11 . 4 . 3 - 212  can be embedded or otherwise secured with the rim  11 . 4 . 3 - 204  such that the second magnet array  11 . 4 . 3 - 212  magnetically couples to the first magnet array  11 . 4 . 3 - 210  to removably secure the Rx lens and the rim  11 . 4 . 3 - 204  to the housing  11 . 4 . 3 - 206 . 
     In at least one example, the magnet arrays  11 . 4 . 3 - 210 ,  11 . 4 . 3 - 212 , either those fixed to the rim  11 . 4 . 3 - 204  or those fixed to the Rx lens, can be mapped when the lenses are installed by the user to ensure proper lens alignment with the system/assembly  11 . 4 . 3 - 200  in order to ensure the correct lens (Rx lens) is being installed. The Rx lens itself can be mapped (e.g., automatically mapped by imaging components such as rear-facing cameras, including gaze tracking cameras . . . etc.) upon user install to ensure the correct lens is being secured by the user. 
       FIG.  11 . 4   . 3 - 5  illustrates left and right magnet arrays  11 . 4 . 3 - 310   a ,  11 . 4 . 3 - 310   b , respectively, which can be used for attaching left and right Rx lenses, respectively. In at least one example, the left and right arrays  11 . 4 . 3 - 310   a - b  can include individual magnets with alternating polarities, where the polarities of the left magnet array  11 . 4 . 3 - 310   a  are opposite relative to the corresponding magnets of the right magnet array  11 . 4 . 3 - 310   b . The polarities of the individual magnets of the left and right arrays  11 . 4 . 3 - 310   a - b  can be flipped about a line of symmetry  11 . 4 . 3 - 314  between the user&#39;s eyes and/or between the left and right arrays  11 . 4 . 3 - 310   a - b.    
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  11 . 4   . 3 - 4  and  11 . 4 . 3 - 5  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in any other figures shown and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to any other figure shown and described herein, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  11 . 4   . 3 - 4  and  11 . 4 . 3 - 5 . 
       FIGS.  11 . 4   . 3 - 6  and  11 . 4 . 3 - 7  illustrate a perspective view and a side/cross-sectional view of an example of an Rx lens  11 . 4 . 3 - 402  configured to be removably attached to an optical assembly/module of an HMD device. Depending on the prescription of the Rx lens  11 . 4 . 3 - 402 , one or more proximal, perimeter edges of the lens  11 . 4 . 3 - 402  may be disposed proud of the housing or rim of the optical assembly. In at least one example, the proximal, peripheral edge  11 . 4 . 3 - 416  of the lens  11 . 4 . 3 - 402  can include a rounded and/or chamfered geometry to reduce sharp or abrupt edge features. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  11 . 4   . 3 - 6  and  11 . 4 . 3 - 7  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in any other figures shown and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to any other figure shown and described herein, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  11 . 4   . 3 - 6  and  11 . 4 . 3 - 7 . 
       FIG.  11 . 4   . 3 - 8  illustrates an example of an Rx lens assembly including an Rx lens  11 . 4 . 3 - 502  secured to a rim  11 . 4 . 3 - 504  and a magnet array  11 . 4 . 3 - 512  embedded in the rim  11 . 4 . 3 - 504 . In at least one example, the magnets of the magnet array  11 . 4 . 3 - 512  can be molded into the rim  11 . 4 . 3 - 504 . In at least one example, the assembly includes a notched feature  11 . 4 . 3 - 520  where a protruding feature  11 . 4 . 3 - 524  of the lens  11 . 4 . 3 - 502  extends into a recess  11 . 4 . 3 - 522  of the rim  11 . 4 . 3 - 504  to form a mechanical fit between the lens  11 . 4 . 3 - 502  and the rim  11 . 4 . 3 - 504 . In at least one example, the notched feature  11 . 4 . 3 - 520  includes a protrusion of the rim  11 . 4 . 3 - 504  extending into a cavity of the lens  11 . 4 . 3 - 502 . In at least one example, the rim  11 . 4 . 3 - 504  is removably securable to the housing of an optical module via the magnets of the magnet array  11 . 4 . 3 - 512  and the lens  11 . 4 . 3 - 502  is removably securable to the rim  11 . 4 . 3 - 504  via the notched feature  502 . In addition, in at least one example, the optical assemblies described herein can include multiple notch assemblies similar to the notch assembly  11 . 4 . 3 - 520  shown in  FIG.  8    between the lens  11 . 4 . 3 - 502  and the rim  11 . 4 . 3 - 504 , including one or more notch features securing the rim  11 . 4 . 3 - 504  to a housing or other component of the optical assembly, and/or a notch assembly securing the optical assembly to other component of the HMD. In at least one example, the magnets of the magnet array  11 . 4 . 3 - 512  can vary in size to achieve desired attraction forces where the magnets of the magnet array  11 . 4 . 3 - 512  include a shaved or chamfered corner to accommodate the lens edges adjacent the magnets. 
     In at least one example, the assembly can include one or more opaque films or inks applied to the proximal, peripheral edges  11 . 4 . 3 - 526 ,  11 . 4 . 3 - 528  of the lens  11 . 4 . 3 - 502  and the rim  11 . 4 . 3 - 504 , respectively, to minimize edge glares and unsightly light features at the peripheral edge of the assembly. In at least one example, as shown in  FIG.  11 . 4   . 3 - 8 , the proximal, peripheral edge  11 . 4 . 3 - 526  of the lens  11 . 4 . 3 - 502  can extend beyond the proximal, peripheral edge  11 . 4 . 3 - 528  of the rim  11 . 4 . 3 - 504 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  11 . 4   . 3 - 8  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in any other figures shown and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to any other figure shown and described herein, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  11 . 4   . 3 - 8 . 
       FIG.  11 . 4   . 3 - 9  illustrates another example of a removable Rx lens assembly including an Rx lens  11 . 4 . 3 - 602  secured to a rim  11 . 4 . 3 - 604  at a notch feature  11 . 4 . 3 - 620 . The assembly can also include one or more magnets of a magnet array  11 . 4 . 3 - 612  embedded within, e.g., molded with, the rim  11 . 4 . 3 - 604 . In the illustrated example of  FIG.  11 . 4   . 3 - 9 , the proximal, peripheral edge  11 . 4 . 3 - 628  of the rim  11 . 4 . 3 - 604  extends beyond the proximal, peripheral edge  11 . 4 . 3 - 626  of the lens  11 . 4 . 3 - 602 . 
       FIG.  11 . 4   . 3 - 10  illustrates another example of a removable Rx lens assembly including an Rx lens  11 . 4 . 3 - 702  secured to a rim  11 . 4 . 3 - 704  at a notch feature  11 . 4 . 3 - 720 . The assembly can also include one or more magnets of a magnet array  11 . 4 . 3 - 712  embedded within, e.g., molded with, the rim  11 . 4 . 3 - 704 . In the illustrated example of  FIG.  11 . 4   . 3 - 10 , the proximal, peripheral edge  11 . 4 . 3 - 728  of the rim  11 . 4 . 3 - 704  extends beyond the proximal, peripheral edge  11 . 4 . 3 - 726  of the lens  11 . 4 . 3 - 702 . In the illustrated example of  FIG.  11 . 4   . 3 - 10 , a film or ink  11 . 4 . 3 - 718  can be applied to the edge  11 . 4 . 3 - 728  of the rim  11 . 4 . 3 - 702  to reduce glare and edge/lensing effects around a periphery of the assembly. 
       FIG.  11 . 4   . 3 - 11  illustrates another example of a removable Rx lens assembly including an Rx lens  11 . 4 . 3 - 802  secured to a rim  11 . 4 . 3 - 804  at a notch feature  11 . 4 . 3 - 820 . The assembly can also include one or more magnets of a magnet array  11 . 4 . 3 - 812  embedded within, e.g., molded with, the rim  11 . 4 . 3 - 804 . In the illustrated example of  FIG.  11 . 4   . 3 - 11 , the proximal, peripheral edge  11 . 4 . 3 - 828  of the rim  11 . 4 . 3 - 804  can be substantially flush with the proximal, peripheral edge  11 . 4 . 3 - 826  of the lens  802 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  11 . 4   . 3 - 8 - 11 . 4 . 3 - 11  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in any other figures shown and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to any other figure shown and described herein, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  11 . 4   . 3 - 8 - 11 . 4 . 3 - 11 . 
     In at least one example of the present disclosure, the lenses of HMD devices disclosed herein, including the Rx lenses disclosed herein, can include multiple layers or lenses stacked together as layers. In at least one example, an HMD lens stack can include three or more lenses or layers of lenses stacked together to form a single lens. In at least one example, the lens stack can include four or more lenses. In at least one example, a four-lens stack can include a first lens, a second lens coupled to the first lens, a third lens coupled to the second with the second lens between the first lens and the third lens, and a fourth lens coupled to the third lens with the third lens between the fourth lens and the second lens. In such an example, the fourth lens can be an Rx lens customized to the prescription of the user. In one example, the lens stack can include a first, second, and third lens with the second lens disposed between the third lens and the first lens. In such an example, the third lens can include the Rx lens customized to the user&#39;s prescription. In such examples, e.g., the third lens of a three stack lens being the Rx lens or the fourth lens of a four-stack lens being Rx lens, the other lenses can be standard lenses of the HMD across other SKUs or HMD devices. The Rx lens of the stacks can be permanently fixed to the other lenses or lens layers of the stack or the Rx lenses can be removably coupled thereto to allow replacement. 
     In at least one example, any of the lenses, including Rx lenses, of HMD devices described herein, can include identification features recognizable by the HMD device, including via the various cameras, sensors, and controllers of the optical systems of the HMD, to recognize and verify the correct lens is attached for a given user. In one example, lenses described herein can include radio-frequency identification features detectable by the HMD. In one example, the lenses can include IR visible barcodes detectable by IR sensors/cameras of the HMD. In one example, the lenses can include QR codes or other codes or visual identification features which the HMD can detect and compare against server-based data or other data to verify the correct lenses for given users and/or for lens calibration purposes. In at least one example, the sensor system of the optical module of the HMD, including the cameras, sensors, controllers/processors, can be used to detect a distortion of light through an Rx lens and match the distortion to a certain prescription and/or user associated with the prescription to verify the correct lens is being used for the given user. 
     In at least one example of the present disclosure, any of the lenses described herein, including Rx lenses described herein, can be formed to accommodate a prescription based on a visual/focal distance provided by the display/lens system of the HMD to the user, which can be determined based on a traditional prescription of a user based on infinity and 60 cm standard prescription distances. In one example, if a user provides a standard prescription, for example a prescription associated with a visual distance of infinity and 60 cm, or other distances common in standard prescriptions, the Rx lenses of the HMD can be made based on that prescription to be a prescription for a visual distance of 1-meter. 1-meter is used as a non-limiting example of a focal distance of the optical display system of the HMD. Other distances can be used to match the effective focal distance provided by the HMD, if that effective focal distance displayed by the HMD is a distance other than 1-meter. 
     XII: Curtain 
       FIGS.  12 . 0 - 1    illustrates a view of an HMD  12 . 0 - 100  including a curtain assembly  12 . 0 - 102 . The curtain assembly  12 . 0 - 102  is described in more detail here in section XII. 
     12.1: Electronic Devices with Stretchable Fabric Covers 
     Electronic devices such as head-mounted electronic devices may include displays for presenting images to users. To accommodate variations in the interpupillary distances associated with different users, a head-mounted device may have left-eye and right-eye optical modules that move with respect to each other. Each optical module may include a display device for producing an image and an associated optical component such as a lens for providing the image to an associated eye box in which an eye of the user is located for viewing the image. The optical modules, which may sometimes be referred to as optical systems, display systems, lens systems, lens and display assemblies, etc., may each have a support structure such as a lens barrel that supports a respective display and lens. 
     Actuators may be used to position the lens barrels within the housing of a head-mounted device. To hide the actuators and other electrical components such as integrated circuits, batteries, sensors, etc. and to hide potentially unsightly internal housing structures from view, the rear of a head-mounted device that faces the user may be provided with a cosmetic covering. Openings in the cosmetic covering may receive the lens barrels of the optical modules. The cosmetic covering may be configured to accommodate movement in the positions of the optical modules for different interpupillary distances. 
     The covering may be a stretchable fabric cover having first strands that form a warp knit layer with diamond-shaped openings and second strands that zig-zag back and forth within the diamond-shaped openings. The first strands may be elastic and may provide stretch in the warp direction, while the second strands may be covered strands that provide stretch in the weft direction. The covered strands may have a drawn textured yarn covering that forms a fuzzy and opaque layer for hiding electrical components. 
     If desired, the cover may be a jacquard fabric with different zones of stretch, opacity, and/or other characteristics. For example, the cover may have a nose bridge region interposed between eye regions. The nose bridge region may have greater opacity and less stretch than the eye regions. Arrangements in which the cover is formed from three-dimensional fabric having cylindrical portions that move relative to one another in response to optical module movements may also be used. 
     An electronic device such as a head-mounted device may have a front face that faces away from a user&#39;s head and may have an opposing rear face that faces the user&#39;s head. Optical modules on the rear face may be used to provide images to a user&#39;s eyes. The positions of the optical modules may be adjusted to accommodate different user interpupillary distances. Internal device structures may be hidden from view by the user by covering the rear face of the device with a curtain. The curtain, which may sometimes be referred to as a cover, covering structure, rear housing cover, rear housing wall, rear housing structure, cosmetic covering, etc., may help block potentially unsightly internal structures from view, while accommodating movement of the optical modules. 
     A top view of an illustrative head-mounted device with a curtain is shown in  FIGS.  12 . 1 - 1   . As shown in  FIGS.  12 . 1 - 1   , head-mounted devices such as electronic device  12 . 1 - 10  may have head-mounted support structures such as housing  12 . 1 - 12 . Housing  12 . 1 - 12  may include portions (e.g., support structures  12 . 1 - 12 T) to allow device  12 . 1 - 10  to be worn on a user&#39;s head. Support structures  12 . 1 - 12 T may be formed from fabric, polymer, metal, and/or other material. Support structures  12 . 1 - 12 T may form a strap or other head-mounted support structures that help support device  12 . 1 - 10  on a user&#39;s head. A main support structure (e.g., main housing portion  12 . 1 - 12 M) of housing  12 . 1 - 12  may support electronic components such as displays  12 . 1 - 14 . Main housing portion  12 . 1 - 12 M may include housing structures formed from metal, polymer, glass, ceramic, and/or other material. For example, housing portion  12 . 1 - 12 M may have housing walls on front face F and housing walls on adjacent top, bottom, left, and right side faces that are formed from rigid polymer or other rigid support structures and these rigid walls may optionally be covered with electrical components, fabric, leather, or other soft materials, etc. The walls of housing portion  12 . 1 - 12 M may enclose internal components  12 . 1 - 38  in interior region  12 . 1 - 34  of device  12 . 1 - 10  and may separate interior region  12 . 1 - 34  from the environment surrounding device  12 . 1 - 10  (exterior region  12 . 1 - 36 ). Internal components  12 . 1 - 38  may include integrated circuits, actuators, batteries, sensors, and/or other circuits and structures for device  12 . 1 - 10 . Housing  12 . 1 - 12  may be configured to be worn on a head of a user and may form glasses, a hat, a helmet, goggles, and/or other head-mounted device. Configurations in which housing  12 . 1 - 12  forms goggles may sometimes be described herein as an example. 
     Front face F of housing  12 . 1 - 12  may face outwardly away from a user&#39;s head and face. Opposing rear face R of housing  12 . 1 - 12  may face the user. Portions of housing  12 . 1 - 12  (e.g., portions of main housing  12 . 1 - 12 M) on rear face R may form a cover such as curtain  12 . 1 - 12 C. In an illustrative configuration, curtain  12 . 1 - 12 C includes a fabric layer that separates interior region  12 . 1 - 34  from the exterior region to the rear of device  12 . 1 - 10 . Other structures may be used in forming curtain  12 . 1 - 12 C, if desired. The presence of curtain  12 . 1 - 12 C on rear face R may help hide internal housing structures, internal components  12 . 1 - 38 , and other structures in interior region  12 . 1 - 34  from view by a user. 
     Device  12 . 1 - 10  may have left and right optical modules  12 . 1 - 40  (sometimes referred to as optical assemblies). Each optical module may include a respective display  12 . 1 - 14 , lens  12 . 1 - 30 , and support structure  12 . 1 - 32 . Support structures  12 . 1 - 32 , which may sometimes be referred to as lens barrels or optical module support structures, may include hollow cylindrical structures with open ends or other supporting structures to house displays  12 . 1 - 14  and lenses  12 . 1 - 30 . Support structures  12 . 1 - 32  may, for example, include a left lens barrel that supports a left display  12 . 1 - 14  and left lens  12 . 1 - 30  and a right lens barrel that supports a right display  12 . 1 - 14  and right lens  12 . 1 - 30 . Displays  12 . 1 - 14  may include arrays of pixels or other display devices to produce images. Displays  12 . 1 - 14  may, for example, include organic light-emitting diode pixels formed on substrates with thin-film circuitry and/or formed on semiconductor substrates, pixels formed from crystalline semiconductor dies, liquid crystal display pixels, scanning display devices, and/or other display devices for producing images. Lenses  12 . 1 - 30  may include one or more lens elements for providing image light from displays  12 . 1 - 14  to respective eyes boxes  12 . 1 - 13 . Lenses may be implemented using refractive glass lens elements, using mirror lens structures (catadioptric lenses), using holographic lenses, and/or other lens systems. When a user&#39;s eyes are located in eye boxes  12 . 1 - 13 , displays (display panels)  12 . 1 - 14  operate together to form a display for device  12 . 1 - 10  (e.g., the images provided by respective left and right optical modules  12 . 1 - 40  may be viewed by the user&#39;s eyes in eye boxes  12 . 1 - 13  so that a stereoscopic image is created for the user). The left image from the left optical module fuses with the right image from a right optical module while the display is viewed by the user. 
     Not all users have the same interpupillary distance P. To provide device  12 . 1 - 10  with the ability to adjust the interpupillary spacing between modules  12 . 1 - 40  along lateral dimension X and thereby adjust the spacing P between eye boxes  12 . 1 - 13  to accommodate different user interpupillary distances, device  12 . 1 - 10  may be provided with actuators  12 . 1 - 42 . Actuators  12 . 1 - 42  can be manually controlled and/or computer-controlled actuators (e.g., computer-controlled motors) for moving support structures  12 . 1 - 32  relative to each other. 
     As shown in  FIGS.  12 . 1 - 2   , curtain  12 . 1 - 12 C may cover rear face R while leaving lenses  12 . 1 - 30  of optical modules  12 . 1 - 40  uncovered (e.g., curtain  12 . 1 - 12 C may have openings that are aligned with and receive modules  12 . 1 - 40 ). As modules  12 . 1 - 40  are moved relative to each other along dimension X to accommodate different interpupillary distances for different users, modules  12 . 1 - 40  move relative to fixed housing structures such as the walls of main portion  12 . 1 - 12 M and move relative to each other. To prevent undesired wrinkling and buckling of curtain  12 . 1 - 12 C as optical modules  12 . 1 - 40  are moved relative to rigid portions of housing  12 . 1 - 12 M and relative to each other, a fabric layer or other cover layer in curtain  12 . 1 - 12 C may be configured to slide, stretch, open/close, and/or otherwise adjust to accommodate optical module movement. 
     A schematic diagram of an illustrative electronic device such as a head-mounted device or other wearable device is shown in  FIGS.  12 . 1 - 3   . Device  12 . 1 - 10  of  FIGS.  12 . 1 - 3    may be operated as a stand-alone device and/or the resources of device  12 . 1 - 10  may be used to communicate with external electronic equipment. As an example, communications circuitry in device  12 . 1 - 10  may be used to transmit user input information, sensor information, and/or other information to external electronic devices (e.g., wirelessly or via wired connections). Each of these external devices may include components of the type shown by device  12 . 1 - 10  of  FIGS.  12 . 1 - 3   . 
     As shown in  FIGS.  12 . 1 - 3   , a head-mounted device such as device  12 . 1 - 10  may include control circuitry  12 . 1 - 20 . Control circuitry  12 . 1 - 20  may include storage and processing circuitry for supporting the operation of device  12 . 1 - 10 . The storage and processing circuitry may include storage such as nonvolatile memory (e.g., flash memory or other electrically-programmable-read-only memory configured to form a solid state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Processing circuitry in control circuitry  12 . 1 - 20  may be used to gather input from sensors and other input devices and may be used to control output devices. The processing circuitry may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors and other wireless communications circuits, power management units, audio chips, application specific integrated circuits, etc. During operation, control circuitry  12 . 1 - 20  may use display(s)  12 . 1 - 14  and other output devices in providing a user with visual output and other output. 
     To support communications between device  12 . 1 - 10  and external equipment, control circuitry  12 . 1 - 20  may communicate using communications circuitry  12 . 1 - 22 . Circuitry  12 . 1 - 22  may include antennas, radio-frequency transceiver circuitry, and other wireless communications circuitry and/or wired communications circuitry. Circuitry  12 . 1 - 22 , which may sometimes be referred to as control circuitry and/or control and communications circuitry, may support bidirectional wireless communications between device  12 . 1 - 10  and external equipment (e.g., a companion device such as a computer, cellular telephone, or other electronic device, an accessory such as a point device, computer stylus, or other input device, speakers or other output devices, etc.) over a wireless link. For example, circuitry  12 . 1 - 22  may include radio-frequency transceiver circuitry such as wireless local area network transceiver circuitry configured to support communications over a wireless local area network link, near-field communications transceiver circuitry configured to support communications over a near-field communications link, cellular telephone transceiver circuitry configured to support communications over a cellular telephone link, or transceiver circuitry configured to support communications over any other suitable wired or wireless communications link. Wireless communications may, for example, be supported over a Bluetooth® link, a WiFi® link, a wireless link operating at a frequency between 10 GHz and 400 GHz, a 60 GHz link, or other millimeter wave link, a cellular telephone link, or other wireless communications link. Device  12 . 1 - 10  may, if desired, include power circuits for transmitting and/or receiving wired and/or wireless power and may include batteries or other energy storage devices. For example, device  12 . 1 - 10  may include a coil and rectifier to receive wireless power that is provided to circuitry in device  12 . 1 - 10 . 
     Device  12 . 1 - 10  may include input-output devices such as devices  12 . 1 - 24 . Input-output devices  12 . 1 - 24  may be used in gathering user input, in gathering information on the environment surrounding the user, and/or in providing a user with output. Devices  12 . 1 - 24  may include one or more displays such as display(s)  12 . 1 - 14 . Display(s)  12 . 1 - 14  may include one or more display devices such as organic light-emitting diode display panels (panels with organic light-emitting diode pixels formed on polymer substrates or silicon substrates that contain pixel control circuitry), liquid crystal display panels, microelectromechanical systems displays (e.g., two-dimensional mirror arrays or scanning mirror display devices), display panels having pixel arrays formed from crystalline semiconductor light-emitting diode dies (sometimes referred to as microLEDs), and/or other display devices. 
     Sensors  12 . 1 - 16  in input-output devices  12 . 1 - 24  may include force sensors (e.g., strain gauges, capacitive force sensors, resistive force sensors, etc.), audio sensors such as microphones, touch and/or proximity sensors such as capacitive sensors such as a touch sensor that forms a button, trackpad, or other input device), and other sensors. If desired, sensors  12 . 1 - 16  may include optical sensors such as optical sensors that emit and detect light, ultrasonic sensors, optical touch sensors, optical proximity sensors, and/or other touch sensors and/or proximity sensors, monochromatic and color ambient light sensors, image sensors, fingerprint sensors, iris scanning sensors, retinal scanning sensors, and other biometric sensors, temperature sensors, sensors for measuring three-dimensional non-contact gestures (“air gestures”), pressure sensors, sensors for detecting position, orientation, and/or motion (e.g., accelerometers, magnetic sensors such as compass sensors, gyroscopes, and/or inertial measurement units that contain some or all of these sensors), health sensors such as blood oxygen sensors, heart rate sensors, blood flow sensors, and/or other health sensors, radio-frequency sensors, depth sensors (e.g., structured light sensors and/or depth sensors based on stereo imaging devices that capture three-dimensional images), optical sensors such as self-mixing sensors and light detection and ranging (LIDAR) sensors that gather time-of-flight measurements, humidity sensors, moisture sensors, gaze tracking sensors, electromyography sensors to sense muscle activation, facial sensors, and/or other sensors. In some arrangements, device  12 . 1 - 10  may use sensors  12 . 1 - 16  and/or other input-output devices to gather user input. For example, buttons may be used to gather button press input, touch sensors overlapping displays can be used for gathering user touch screen input, touch pads may be used in gathering touch input, microphones may be used for gathering audio input, accelerometers may be used in monitoring when a finger contacts an input surface and may therefore be used to gather finger press input, etc. 
     If desired, electronic device  12 . 1 - 10  may include additional components (see, e.g., other devices  12 . 1 - 18  in input-output devices  12 . 1 - 24 ). The additional components may include haptic output devices, actuators for moving movable housing structures, audio output devices such as speakers, light-emitting diodes for status indicators, light sources such as light-emitting diodes that illuminate portions of a housing and/or display structure, other optical output devices, and/or other circuitry for gathering input and/or providing output. Device  12 . 1 - 10  may also include a battery or other energy storage device, connector ports for supporting wired communication with ancillary equipment and for receiving wired power, and other circuitry. 
       FIGS.  12 . 1 - 4  and  12 . 1 - 5    are top views of device  12 . 1 - 10  showing how the optical modules of device  12 . 1 - 10  move with respect to each other along lateral dimension X to accommodate different interpupillary distances P (the distance between a user&#39;s left and right eyes). In the example of  FIGS.  12 . 1 - 4   , left optical module  12 . 1 - 40 L and right optical module  12 . 1 - 40 R have been moved towards each other to accommodate a small interpupillary distance. In the example of  FIGS.  12 . 1 - 5   , left optical module  12 . 1 - 40 L and right optical module  12 . 1 - 40 R have been moved away from each other to accommodate a large interpupillary distance. 
     Curtain  12 . 1 - 12 C has edge portions such as left portion  12 . 1 - 12 C-L between left housing wall  12 . 1 - 12 M-L and left optical module  12 . 1 - 40 L and right portion  12 . 1 - 12 C-R between right housing wall  12 . 1 - 12 M-R and right optical module  12 . 1 - 40 R. Middle portion  12 . 1 - 12 C-M of curtain  12 . 1 - 12 C extends between left optical module  12 . 1 - 40 L and right optical module  12 . 1 - 40 R. In the configuration of  FIGS.  12 . 1 - 4   , optical modules  12 . 1 - 40 L and  12 . 1 - 40 R are relatively close to each other, so middle portion  12 . 1 - 12 C-M is relatively small and portions  12 . 1 - 12 C-L and  12 . 1 - 12 C-R are relatively large. In the configuration of  FIGS.  12 . 1 - 5   , optical modules  12 . 1 - 40 L and  12 . 1 - 40 R are relatively far from each other, so left portion  12 . 1 - 12 C-L and right portion  12 . 1 - 12 C-R are shorter along lateral dimension X and middle portion  12 . 1 - 12 C-M has been enlarged relative to the configuration of  FIGS.  12 . 1 - 4   . 
     To help accommodate differences in size for curtain  12 . 1 - 12 C (e.g., length changes for portions of curtain  12 . 1 - 12 C along lateral dimension X), curtain  12 . 1 - 12 C may include a cover layer such as cover layer  12 . 1 - 12 CC formed from a stretchable material such as fabric. Cover layer  12 . 1 - 12 CC may be supported by a rigid frame. The fabric of cover layer  12 . 1 - 12 CC may be provided with a peripheral elastic band that helps allow the fabric to slide relative to the frame while being retained securely on the frame, thereby further helping curtain  12 . 1 - 12 C to be dynamically adjusted without exhibiting undesired buckling and wrinkling. Cover layer  12 . 1 - 12 CC may have left and right openings to receive respective left and right optical modules  12 . 1 - 40 L and  12 . 1 - 40 R. 
     Cover layer  12 . 1 - 12 CC may be formed from fabric having the desired opacity (e.g., sufficiently opaque to hide electronic components in device  12 . 1 - 10 ), stretch (e.g., cover  12 . 1 - 12 CC may be configured to stretch by at least 50%, at least 75%, at least 100%, or at least 150% of its length to allow dynamic expansion and contraction of cover  12 . 1 - 12 CC as optical modules  12 . 1 - 40 L and  12 . 1 - 40 R are adjusted, as an example), modulus of elasticity (e.g., the elastic modulus of cover  12 . 1 - 12 CC may be sufficiently low so that less electrical power is required for actuators  12 . 1 - 42  to stretch cover  12 . 1 - 12 CC), durability (e.g., cover  12 . 1 - 12 CC may be configured to maintain dimensional stability over long term use by avoiding wrinkling after being exposed to skin sebum and/or other chemicals, repeated stretching, etc.), cosmetic appeal, and/or other suitable characteristics. 
       FIGS.  12 . 1 - 6    is a front view of an illustrative cover layer for curtain  12 . 1 - 12 C. As shown in  FIGS.  12 . 1 - 6   , cover layer  12 . 1 - 12 CC may include fabric formed from fabric such as fabric  12 . 1 - 62 . Fabric  12 . 1 - 62  may be warp knit fabric, weft knit fabric, or other knit fabric (e.g., to promote stretchiness), may be woven fabric, braided fabric, non-woven fabric, etc. If desired, a layer of stretchy plastic may be attached to fabric  12 . 1 - 62  and/or an elastomeric polymer layer may be used in place of some or all of fabric  12 . 1 - 62  in forming layer  12 . 1 - 12 CC. Fabric  12 . 1 - 62  may be formed from interlaced (intertwined) strands of material such as polymer strands, strands of cotton or other natural material, synthetic material, and/or other materials. The strands in fabric  12 . 1 - 62  may include monofilament strands and/or multi-filament strands. In an illustrative configuration, some of the strands of fabric  12 . 1 - 62  may be selected to provide fabric  12 . 1 - 62  with strength, some of strands in fabric  12 . 1 - 62  may be formed from elastomeric material that enhances the ability of fabric  12 . 1 - 62  to stretch (and that has a lower elastic modulus than the strands that provide fabric  12 . 1 - 62  with strength). Examples of stretchable strand materials include elastomeric materials such as silicone, spandex, and thermoplastic polyurethane (TPU). Examples of strength-enhancing strand materials include polyester, nylon, etc. Other materials may be used in forming the strands in fabric  12 . 1 - 62 , if desired. 
     Strands in fabric  12 . 1 - 62  may be dyed at the fabric level (e.g., after strands are knit, woven, or otherwise interlaced to form fabric  12 . 1 - 62 ), may be dyed at the strand or yarn level (e.g., after material is made into monofilament or multifilament strands), and/or may be dyed at the polymerization level (e.g., before material is made into monofilament or multifilament strands). Dyeing yarn may cause some shrinkage and should therefore be taken into account when determining the appropriate amount of stretch for fabric  12 . 1 - 62 . 
     In the example of  FIGS.  12 . 1 - 6   , fabric  12 . 1 - 62  includes warp knit strands  12 . 1 - 44  and zig-zag strands  12 . 1 - 46 . Warp knit strands  12 . 1 - 44  may be warp knit strands that form a pattern of openings such as diamond-shaped openings  12 . 1 - 64 . If desired, openings  12 . 1 - 64  may have other shapes. The use of diamond shapes for openings  12 . 1 - 64  is merely illustrative. Warp knit strands  12 . 1 - 44  may be formed from stretchable materials such as silicone, spandex, thermoplastic polyurethane (TPU), and or other stretchable materials, or warp strands  12 . 1 - 44  may be formed from strength-enhancing strand materials such as polyester, nylon, etc. Other materials may be used in forming strands  12 . 1 - 44 , if desired. 
     Zig-zag strands  12 . 1 - 46  may be interlaced with warp knit strands  12 . 1 - 44  and may zig-zag back and forth (e.g., back in forth in the weft or x-direction) multiple times within each diamond-shaped opening  12 . 1 - 64 . Zig-zag strands  12 . 1 - 46  (sometimes referred to as running strands, accordion strands, etc.) may be formed from stretchable materials such as silicone, spandex, thermoplastic polyurethane (TPU), and or other stretchable materials, or zig-zag strands  12 . 1 - 46  may be formed from strength-enhancing strand materials such as polyester, nylon, etc. Other materials may be used in forming strands  12 . 1 - 46 , if desired. Strands  12 . 1 - 46 , may, if desired, be covered strands that include a covering. The covering may be a different material than the core. For example, the core may be a stretchable material such as spandex and the covering may be a less-stretchable material such as polyester or polyethylene terephthalate. If desired, the covering may be relatively “fuzzy” (e.g., may have a frayed or otherwise not smooth texture) to increase opacity of cover  12 . 1 - 12 CC. 
     In some arrangements, warp knits strands  12 . 1 - 44  and zig-zag strands  12 . 1 - 46  are formed in the same layer (e.g., formed in single layer). In other arrangements, warp knit strands  12 . 1 - 44  may be formed in front of or behind zig-zag strands  12 . 1 - 46  (e.g., warp knit strands  12 . 1 - 44  may be formed in a first layer and zig-zag strands  12 . 1 - 46  may be formed in a second layer that overlaps the first layer). In one illustrative arrangement, zig-zag strands  12 . 1 - 46  are positioned on the side closer to the user&#39;s face and warp strands  12 . 1 - 44  are positioned on the side closer to the electrical components in device  12 . 1 - 10 . In configurations where zig-zag strands  12 . 1 - 46  include a covering (e.g., a covering of polyester, polyethylene terephthalate, and/or other suitable material), placing zig-zag strands  12 . 1 - 46  on the user-facing side of cover  12 . 1 - 12 CC may help protect the stretchable strands in fabric  12 . 1 - 62  (e.g., strands  12 . 1 - 44  and/or the core of strands  12 . 1 - 46 ) from facial chemicals such as sebum that may otherwise cause wear to elastic strands in fabric  12 . 1 - 62 . If desired, warp knit strands  12 . 1 - 44  may be smooth strands and zig-zag strands  12 . 1 - 46  may be textured fuzzy strands. 
     The use of warp knit strands  12 . 1 - 44  and zig-zag strands  12 . 1 - 46  may provide cover  12 . 1 - 12 CC with different features. For example, warp knit strands  12 . 1 - 44  extend in the warp direction, thereby promoting shrinkage and increasing stretch in the warp direction. In contrast, zig-zag strands  12 . 1 - 46  may zig-zag in the weft direction, thereby promoting shrinkage and increasing stretch in the weft direction. For example, when fabric  12 . 1 - 62  is in an unstretched state, each diamond-shaped opening  12 . 1 - 64  has dimension D 1  in the vertical (warp) direction (e.g., parallel to the y-axis of  FIGS.  12 . 1 - 6   ) and dimension D 2  in the horizontal (weft) direction (e.g., parallel to the x-axis of  FIGS.  12 . 1 - 6   ). When fabric  12 . 1 - 62  is in a stretched state, as shown in  FIGS.  12 . 1 - 7   , each diamond-shaped opening  12 . 1 - 64  has dimension D 3  in the vertical (warp) direction (e.g., parallel to the y-axis of  FIGS.  12 . 1 - 7   ) and dimension D 4  in the horizontal (weft) direction (e.g., parallel to the x-axis of  FIGS.  12 . 1 - 7   ). Dimension D 3  may be larger than dimension D 1 , and dimension D 4  may be larger than dimension D 2 . In other words, openings  12 . 1 - 64  may open and expand when cover  12 . 1 - 12 CC is stretched. Warp knit strands  12 . 1 - 44  stretch in the warp direction, thereby allowing the opening and closing of openings  12 . 1 - 64 , whereas zig-zag strands  12 . 1 - 44  stretch in the weft direction, thereby providing fabric  12 . 1 - 62  with four-way stretch capabilities. The opacity provided by the fuzzy covering on zig-zag strands  12 . 1 - 46  may also serve to hide electrical components from view when openings  12 . 1 - 64  are expanded. 
     Another factor that may affect the stretchability of fabric  12 . 1 - 62  of cover  12 . 1 - 12 CC is heat setting. In arrangements where fabric  12 . 1 - 62  is heat set, fabric  12 . 1 - 62  may be stretched and heat set to remove wrinkles. This may, however, reduce some of the stretch in fabric  12 . 1 - 62 . If desired, fabric  12 . 1 - 62  of cover  12 . 1 - 12 CC may be produced without any heat setting (e.g., may only undergo washing, dyeing, and drying) to retain the desired stretchability of fabric  12 . 1 - 62 . 
       FIGS.  12 . 1 - 8    is a side view of an illustrative strand  12 . 1 - 44  that may be used to form warp knit strands  12 . 1 - 44  of fabric  12 . 1 - 62 . In the example of  FIGS.  12 . 1 - 8   , strand  12 . 1 - 44  may be a single strand of material such as spandex, silicone, thermoplastic polyurethane (TPU), and/or other stretchable material, or strand  12 . 1 - 44  may be a single strand of strength-enhancing strand materials such as polyester, nylon, etc. In one illustrative configuration, strand  12 . 1 - 44  is a single strand of spandex having 20 denier, 40 denier, 30 denier, between 15 and 60 denier, between 20 and 50 denier, greater than 40 denier, less than 40 denier, etc. This is merely illustrative. If desired, strand  12 . 1 - 44  may be formed from other suitable materials and/or may have other denier values. 
       FIGS.  12 . 1 - 9    is a side view of an illustrative strand  12 . 1 - 46  that may be used to form zig-zag strands  12 . 1 - 46  of fabric  12 . 1 - 62 . In the example of  FIGS.  12 . 1 - 9   , strand  12 . 1 - 46  may include a core such as core  12 . 1 - 48 . Core  12 . 1 - 48  may be an elastic core formed from one or more strands of material such as spandex, silicone, thermoplastic polyurethane (TPU), and/or other stretchable material, or core  12 . 1 - 48  may be one or more strands of strength-enhancing strand materials such as polyester, nylon, etc. In one illustrative configuration, core  12 . 1 - 48  is an elastic core formed from a single strand of spandex having 20 denier, 40 denier, 30 denier, between 15 and 60 denier, between 20 and 50 denier, greater than 40 denier, less than 40 denier, etc. The denier value of core  12 . 1 - 48  of zig-zag strand  12 . 1 - 46  may, for example, be less than the denier value of warp knit strands  12 . 1 - 44 . This is merely illustrative. If desired, strand  12 . 1 - 46  may be formed from other suitable materials and/or may have other denier values. 
     As shown in  FIGS.  12 . 1 - 9   , core  12 . 1 - 48  of strand  12 . 1 - 46  may be covered with one or more covering strands  12 . 1 - 50 . Covering strands  12 . 1 - 50  may be formed from strength-enhancing strand materials such as polyester, nylon, etc., or covering strands  12 . 1 - 50  may be formed from elastic material such as spandex, silicone, thermoplastic polyurethane (TPU), and/or other stretchable material. Cover  12 . 1 - 50  may have any suitable denier value such as 30 denier, 20 denier, 40 denier, between 15 and 60 denier, between 20 and 50 denier, greater than 40 denier, less than 40 denier, etc. The denier value of cover  12 . 1 - 50  of zig-zag strand  12 . 1 - 46  may, for example, be greater than the denier value of core  12 . 1 - 48  of strands  12 . 1 - 46 . Covering strand  12 . 1 - 50  may be a single filament covering or may include multiple filaments such as  12 . 1 - 24  filaments,  12 . 1 - 30  filaments,  12 . 1 - 20  filaments, more than 12.1-20 filaments, less than 12.1-20 filaments, etc. Covering strand  12 . 1 - 50  may be textured, non-textured, bright, semi-dull, etc. In one illustrative configuration, covering strand  12 . 1 - 50  is a drawn textured yarn of polyester that is simultaneously twisted and drawn to provide a textured, fuzzy covering over core  12 . 1 - 48 . Because polyester is less sensitive to facial chemicals such as sebum than spandex, covering core  12 . 1 - 48  with a fuzzy yarn such as strand  12 . 1 - 50  ensures that most of the surface facing the user is polyester and thus more durable throughout continued use. The use of drawn textured yarn for covering  12 . 1 - 50  may also increase the opacity of cover  12 . 1 - 12 CC to help hide electronic components even when cover  12 . 1 - 12 CC and curtain  12 . 1 - 12 C are stretched and openings  12 . 1 - 64  expand. 
     If desired, covering strand  12 . 1 - 50  may be formed from a conductive material. Using conductive strands for covering  12 . 1 - 50  may provide a conductive layer on fabric  12 . 1 - 62  in which conductive covering strands  12 . 1 - 50  are in contact with one another across the surface of the fabric. In other configurations, strand  12 . 1 - 46  may be a polycarbonate or braided cord with an electrically conductive core. Arrangements in which strand  12 . 1 - 46  includes a multi-component yarn with different polymers joined together within each filament may also be used. When exposed to heat, the different polymers may shrink to different degrees, thereby producing a smooth helical crimp. This may provide strands  12 . 1 - 46  with texture while still allowing strands  12 . 1 - 46  to maintain durable stretch and recovery. 
     If desired, cover  12 . 1 - 12 CC may have regions with different levels of stretch, opacity, and/or other characteristics. This type of arrangement is illustrated in  FIGS.  12 . 1 - 10   . As shown in  FIGS.  12 . 1 - 10   , cover  12 . 1 - 12 CC may have regions with different fabric characteristics such as eye regions  12 . 1 - 54  and nose region  12 . 1 - 56 . Eye regions  12 . 1 - 54  may surround left and right openings such as lens barrel openings  12 . 1 - 52  for receiving respective left and right lenses  12 . 1 - 30  of respective left and right optical modules  12 . 1 - 40 . Nose region  12 . 1 - 56  may be interposed between eye regions  12 . 1 - 54  and may be configured to overlap the nose bridge of the user when device  12 . 1 - 10  is mounted on the user&#39;s head. 
     If desired, fabric  12 . 1 - 62  of cover  12 . 1 - 12 CC may be a jacquard knit fabric that allows for different zones of fabric to have different features such as different fabric construction, different strand material(s), different stretch capabilities, different opacity levels, etc. For example, nose region  12 . 1 - 56  may be constructed to have a higher opacity and/or lower stretch capability than eye regions  12 . 1 - 54 . For example, to reduce stretch in nose region  12 . 1 - 56  relative to eye regions  12 . 1 - 54 , zig-zag strands  12 . 1 - 46  may be omitted from fabric  12 . 1 - 62  in region  12 . 1 - 56  and may only be present in eye regions  12 . 1 - 54 . Additionally or alternatively, strands in nose region  12 . 1 - 56  may be mostly or entirely formed from less-stretchable material such as polyester instead of spandex. More textured yarns may be present in nose region  12 . 1 - 56  than in eye regions  12 . 1 - 54  to increase opacity of nose region  12 . 1 - 56  relative to eye regions  12 . 1 - 54 , if desired. 
       FIGS.  12 . 1 - 11    is a perspective view of an illustrative example in which cover  12 . 1 - 12 CC is formed from three-dimensional fabric  12 . 1 - 62 . Three-dimensional fabric  12 . 1 - 62  of  FIGS.  12 . 1 - 11    may have a fabric construction similar to a pair of pants, where fabric  12 . 1 - 62  forms left and right cylindrical portions around openings  12 . 1 - 52 . Instead of being mostly flat in the z-direction, fabric  12 . 1 - 62  may have a non-zero dimension P 1  in the z-direction, may have a dimension P 2  in the x-direction, with openings  12 . 1 - 52  having a diameter P 3  and separated by a distance P 4 . With this type of configuration, movement of optical modules  12 . 1 - 40  may be accommodated by moving the “legs” of fabric  12 . 1 - 62  around openings  12 . 1 - 52  towards or away from one another (e.g., moving the left and right cylindrical portions of fabric  12 . 1 - 62  relative to one another), which involves less stretching of fabric  12 . 1 - 62  and instead involves moving an entire piece of fabric  12 . 1 - 62  in a given direction. The use of three-dimensional fabric  12 . 1 - 62  of  FIGS.  12 . 1 - 11    is merely illustrative. If desired, cover  12 . 1 - 12 CC may be formed from a mostly flat fabric  12 . 1 - 62 . 
     In accordance with an embodiment, a fabric cover for a head-mounted device having first and second lenses is provided that includes first strands that form a warp knit layer having diamond-shaped openings, in which the warp knit layer is configured to stretch in a warp direction; second strands that are interlaced with the first strands and that zig-zag back and forth within the diamond-shaped openings, in which the second strands are configured to stretch in a weft direction; and first and second openings for receiving the respective first and second lenses. 
     In accordance with another embodiment, the first strands include elastic strands. 
     In accordance with another embodiment, the second strands include covered strands. 
     In accordance with another embodiment, the covered strands each include an elastic core and a covering strand. 
     In accordance with another embodiment, the covering strand includes a drawn textured yarn that increases an opacity within the diamond-shaped openings. 
     In accordance with another embodiment, the drawn textured yarn includes polyester. 
     In accordance with another embodiment, the elastic strands each have a first denier value and the elastic core has a second denier value that is less than the first denier value. 
     In accordance with another embodiment, at least some of the first strands and second strands are dyed at a polymerization level. 
     In accordance with another embodiment, at least some of the first strands and second strands are dyed at a fabric level. 
     In accordance with another embodiment, the fabric cover also includes a nose bridge region interposed between the first and second openings, in which the nose bridge region has higher opacity and lower stretch than other portions of the fabric cover. 
     In accordance with an embodiment, a head-mounted device is provided that includes a housing separating an interior region from an exterior region that surrounds the housing, first and second lenses in the housing that are configured to provide images respectively to first and second eye boxes, and a fabric cover configured to block the interior region from view, in which the fabric cover includes: first and second cover openings that are respectively aligned with the first and second lenses; smooth strands that stretch in a first direction; and textured strands that stretch in a second direction. 
     In accordance with another embodiment, the textured strands include an elastic core and a fuzzy covering. 
     In accordance with another embodiment, the smooth strands include warp knit strands. 
     In accordance with another embodiment, the warp knit strands have a higher denier value than the elastic core. 
     In accordance with another embodiment, the elastic core is less than 40 denier. 
     In accordance with another embodiment, the smooth strands form a pattern of openings and the textured strands zig-zig back and forth within the openings. 
     In accordance with another embodiment, the openings include diamond-shaped openings. 
     In accordance with another embodiment, the first direction is a warp direction and the second direction is a weft direction. 
     In accordance with an embodiment, a head-mounted device is provided that includes a housing, left and right lenses supported by the housing, in which the left lens is configured to provide a left image to a left eye box, in which the right lens is configured to provide a right image to a right eye box, and in which the left and right lenses are configured to move relative to each other, and a three-dimensional fabric cover that has a left cylindrical portion that is aligned with the left lens and a right cylindrical portion that is aligned with the right lens, in which the three-dimensional fabric cover is configured to block an interior region of the housing from view and in which the left and right cylindrical portions are configured to move relative to one another in response to movement of the right and left lenses to accommodate different interpupillary distances. 
     In accordance with another embodiment, the three-dimensional fabric cover includes a nose bridge portion coupled between the first and second cylindrical portions. 
     The foregoing is merely illustrative and various modifications can be made to the described embodiments. The foregoing embodiments may be implemented individually or in any combination. 
     12.2: Curtain Assembly 
       FIGS.  12 . 2 - 1    illustrates a view of an example of an HMD  12 . 2 - 100  including a frame assembly  12 . 2 - 102  and first and second optical modules  12 . 2 - 104   a ,  12 . 2 - 104   b  configured to be coupled to the frame assembly  12 . 2 - 102 . In at least one example, the frame assembly  12 . 2 - 102  defines a rearward facing viewing opening through which the user donning the HMD  12 . 2 - 100  can view light projected from display screens of the optical module  12 . 2 - 104   a - b . The frame assembly  12 . 2 - 102  of the HMD  12 . 2 - 100  can define first and second apertures aligned with the first and second optical modules  12 . 2 - 104   a - b  as shown in  FIGS.  12 . 2 - 1    and described elsewhere herein with reference to other figures. 
     In at least one example, the optical modules  12 . 2 - 104   a - b  can include optical components and assemblies such as display screens, lenses, and so forth. The HMD  12 . 2 - 100  can also include a curtain assembly  12 . 2 - 106  coupled to the frame assembly  12 . 2 - 102  and extending across the viewing opening between the frame assembly  12 . 2 - 102  and the optical modules  12 . 2 - 104   a - b , for example between the frame assembly  12 . 2 - 102  and the lenses of the optical modules  12 . 2 - 104   a - b . The curtain assembly can include multiple components, parts, and layers, to manage airflow through the HMD  12 . 2 - 100  due to cooling systems having fans blowing air through an internal volume of the HMD  100 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  12 . 2 - 1    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  12 . 2 - 2 - 12 . 2 - 13    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  12 . 2 - 2 - 12 . 2 - 13    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  12 . 2 - 1   . 
       FIGS.  12 . 2 - 2    shows a rear, perspective view of an HMD  12 . 2 - 200  including a frame assembly  12 . 2 - 202  defining an external surface of the device, an internal volume  12 . 2 - 208 , and a rear-facing viewing opening  12 . 2 - 210 , which can also be referred to as an aperture or viewing aperture. The HMD  12 . 2 - 200  also includes a display module  12 . 2 - 204  including a lens  12 . 2 - 218  through which the user can view the light projected from a display screen of the optical module  12 . 2 - 204  disposed in the internal volume  12 . 2 - 208 . In at least one example, the HMD  12 . 2 - 200  also includes a curtain assembly  12 . 2 - 206  occluding the viewing opening  12 . 2 - 210  between the lens  12 . 2 - 218  and the frame assembly  12 . 2 - 202 . 
     In at least one example, the HMD  12 . 2 - 200  can include one or more motors mechanically coupled to the optical module  12 . 2 - 204  to alter a position of the optical module  12 . 2 - 204 , including the lens  12 . 2 - 218 , relative to the frame assembly  12 . 2 - 202 . Such motors are illustrated in other figures and described elsewhere herein. These motors can be used to adjust the optical module(s)  12 . 2 - 204  of the HMD  12 . 2 - 200  to match the interpupillary distance defined by the user&#39;s eyes. In at least one example, the curtain assembly  12 . 2 - 206  can be elastically deformable or stretchy to accommodate the different possible positions of the optical module  12 . 2 - 204 , including the lens  12 . 2 - 218 , such that the curtain assembly occludes the viewing aperture/opening  12 . 2 - 210  between the frame assembly  12 . 2 - 202  and the optical module(s)  12 . 2 - 204  or lens(es)  12 . 2 - 218  thereof regardless of the position or movement of the optical module  12 . 2 - 204  and lens  12 . 2 - 218 . 
     The HMD  12 . 2 - 200  shown in  FIGS.  12 . 2 - 2    includes a first optical module  12 . 2 - 204  configured to align with a user&#39;s left eye but does not show a second optical module configured to align with a user&#39;s right eye so that the internal volume  12 . 2 - 208  and fan  12 . 2 - 212  can be seen. The HMD  12 . 2 - 200  can include one or more fans  12 . 2 - 212  in the internal volume. The HMD  12 . 2 - 200  can include a second fan, not seen, aligned with the optical module  12 . 2 - 204  shown. The frame assembly  12 . 2 - 202  can define one or more intake ports  12 . 2 - 214  and one or more exhaust ports  12 . 2 - 216  on an opposing side of the frame assembly  12 . 2 - 202  relative to the intake ports  12 . 2 - 214 . The fan  12 . 2 - 212  can be configured to draw air into the internal volume  12 . 2 - 208  through the intake ports  12 . 2 - 214  and exhaust the air from the internal volume  12 . 2 - 208  out through the exhaust ports  12 . 2 - 216 . 
     The fan  12 . 2 - 212  can be configured to draw air in and out of the internal volume  12 . 2 - 208  to cool various heat-generating components within the HMD  12 . 2 - 200 . Such components can include processors, display screens of the optical module(s)  12 . 2 - 204 , sensors, and so forth. In such an example, the curtain assembly  12 . 2 - 206  can be configured to prevent the flow of air from reaching the user&#39;s eyes when donning the HMD  12 . 2 - 200  such that the airflow through the HMD  12 . 2 - 200  does not dry the eyes out and cause discomfort. 
     Accordingly, in order serve the dual purpose of preventing airflow from reaching the user&#39;s eyes and maintaining an elastic, aesthetically pleasing barrier occluding the viewing aperture/opening  12 . 2 - 210 , the curtain assembly  12 . 2 - 206  can include a first layer defining an external surface of the HMD  12 . 2 - 200  and a second layer defining the internal volume  12 . 2 - 208 , with the first layer being elastic and the second layer being non-elastic and/or air-impermeable. In one example, the second layer can be semi-elastic, or have some elasticity, but remain air-impermeable when stretched or deformed. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  12 . 2 - 2    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  12 . 2 - 1  and  12 . 2 - 3 - 12 . 2 - 13    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  12 . 2 - 1  and  12 . 2 - 3 - 12 . 2 - 13    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  12 . 2 - 2   . 
       FIGS.  12 . 2 - 3    shows a rear, plan view of an example of an HMD  12 . 2 - 300  similar to the HMD  12 . 2 - 200  shown in  FIGS.  12 . 2 - 2   , including a frame  12 . 2 - 302  defining an internal volume  12 . 2 - 308 , viewing opening  12 . 2 - 310 , intake port(s)  12 . 2 - 314 , and exhaust port(s)  12 . 2 - 316 . The HMD  12 . 2 - 300  can also include first and second fans  12 . 2 - 312   a ,  12 . 2 - 312   b  disposed in the internal volume  12 . 2 - 308  and drawing air through the internal volume  12 . 2 - 308  via the intake ports  12 . 2 - 314  and exhaust ports  12 . 2 - 316 .  FIGS.  12 . 2 - 3    does not include optical modules in order to visualize the internal volume  12 . 2 - 308  and fans  12 . 2 - 312   a - b  aligned with the apertures and optical module positions. The HMD  12 . 2 - 300  can also include a curtain assembly  12 . 2 - 306  similar to the curtain assembly  12 . 2 - 206  illustrated in  FIGS.  12 . 2 - 2   , where the curtain assembly  12 . 2 - 306  occludes the viewing opening  12 . 2 - 310  between the frame  12 . 2 - 302  and the optical modules. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  12 . 2 - 3    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  12 . 2 - 1 - 12 . 2 - 2  and  12 . 2 - 4 - 12 . 2 - 13    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  12 . 2 - 1 - 12 . 2 - 2  and  12 . 2 - 4 - 12 . 2 - 13    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  12 . 2 - 3   . 
       FIGS.  12 . 2 - 4    illustrates a side, cross-sectional view of an example of and HMD  12 . 2 - 400  donned by a user  12 . 2 - 401 . The HMD  12 . 2 - 400  can include a frame, also referred to as a housing  12 . 2 - 402 , defining an internal volume  12 . 2 - 408  and a fan assembly  12 . 2 - 412  disposed in the internal volume  12 . 2 - 408 . The housing  12 . 2 - 402  can define an intake port  12 . 2 - 414  and an exhaust port  12 . 2 - 416  and the fan assembly  12 . 2 - 412  can be configured to draw air  12 . 2 - 424  through the intake port  12 . 2 - 414  into the internal volume and push the air  12 . 2 - 424  out from the internal volume  12 . 2 - 408  through the exhaust port  12 . 2 - 416 . 
     In at least one example, the HMD  12 . 2 - 400  can include an optical module  12 . 2 - 404  directing light from a display screen toward the user&#39;s eye and a light seal  12 . 2 - 420  extending around a perimeter of the HMD  12 . 2 - 400  between the housing  12 . 2 - 402  and the user&#39;s face when the user  12 . 2 - 401  dons the HMD  12 . 2 - 400  as shown. In at least one example, the HMD  12 . 2 - 400  also includes a curtain assembly  12 . 2 - 406  extending between the housing  12 . 2 - 402  and the optical module  12 . 2 - 404 . The curtain assembly  12 . 2 - 406  can be configured to block the air  12 . 2 - 424  from entering a space  12 . 2 - 422  between the eyes/face of the user  12 . 2 - 401 , the space being defined by the user  12 . 2 - 401 , the light seal  12 . 2 - 420 , the optical module  12 . 2 - 404 , and the curtain assembly  12 . 2 - 406 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  12 . 2 - 4    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  12 . 2 - 1 - 12 . 2 - 3  and  12 . 2 - 5 - 12 . 2 - 13    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  12 . 2 - 1 - 12 . 2 - 3  and  12 . 2 - 5 - 12 . 2 - 13    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  12 . 2 - 4   . 
       FIGS.  12 . 2 - 5    illustrates a perspective view of a curtain assembly  12 . 2 - 506  for use in HMD devices described herein. The curtain assembly  12 . 2 - 506  can define first and second apertures  12 . 2 - 526   a  and  12 . 2 - 526   b . As noted above, examples of curtain assemblies can include multiple layers, including a first elastic layer and a second non-elastic, air-impermeable layer. For reference,  FIGS.  12 . 2 - 3    shows the user facing side of a curtain assembly  12 . 2 - 306  including the first, elastic layer  12 . 2 - 334 . The view of  FIGS.  12 . 2 - 5    shows an internal side or surface of the curtain assembly  12 . 2 - 506  including a second, internal air-impermeable, layer  12 . 2 - 528  defining an internal volume of an HMD. 
     In at least one example, the second layer can extend between curtain trim rings  12 . 2 - 530   a  and  12 . 2 - 530   b  and a structural curtain backplate  12 . 2 - 532 . The rings  12 . 2 - 530   a - b  and backplate  12 . 2 - 532  can be made of more rigid, structural material including plastics, metals, and so forth, to give structure and shape to the first and second layers of the curtain assembly  12 . 2 - 506  attached thereto. 
     In at least one example, the layers of the curtain assembly  12 . 2 - 506 , including the second layer  12 . 2 - 528  shown in  FIGS.  12 . 2 - 5   , can be coupled around a first perimeter edge  12 . 2 - 542  thereof to the backplate  12 . 2 - 532  and to the rings  12 . 2 - 530   a - b  and/or one or more optical assemblies at a second and third inner edges  12 . 2 - 544  of the layers forming or defining the apertures  12 . 2 - 526   a - b , respectively. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  12 . 2 - 5    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  12 . 2 - 1 - 12 . 2 - 4  and  12 . 2 - 6 - 12 . 2 - 13    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  12 . 2 - 1 - 12 . 2 - 4  and  12 . 2 - 6 - 12 . 2 - 13    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  12 . 2 - 5   . 
       FIGS.  12 . 2 - 6    shows a view of a curtain assembly  12 . 2 - 606 , which can be similar to the assembled curtain assembly  12 . 2 - 506  shown in  FIGS.  12 . 2 - 5   . The view of  FIGS.  12 . 2 - 6    illustrates a curtain assembly  12 . 2 - 606  including a structural backplate  12 . 2 - 632 , trim rings  12 . 2 - 630 , a second air-impermeable layer  12 . 2 - 628 , and a first elastic layer  12 . 2 - 634 . The curtain assembly  12 . 2 - 606  can also include first adhesive portions or layers  12 . 2 - 636  disposed between and coupling the second layer  12 . 2 - 628  to the backplate  12 . 2 - 632  and the trim rings  12 . 2 - 630  and second adhesive portions or layers  12 . 2 - 638  disposed between and coupling the second layer  12 . 2 - 628  and the first layer  12 . 2 - 634 . The curtain assembly can also include additional stiffener components or assemblies  12 . 2 - 640  coupled to the first layer  12 . 2 - 634 . 
     In at least one example, the first and second layers  12 . 2 - 628 ,  12 . 2 - 634  are coupled together and/or to the backplate  12 . 2 - 632  and/or stiffener components  12 . 2 - 640  around a first perimeter edge of the layers  12 . 2 - 628 ,  12 . 2 - 634 . The layers  12 . 2 - 628 ,  12 . 2 - 634  can also be coupled together at second edges defining the apertures at the optical modules and/or trim rings  12 . 2 - 630 . In at least one example, the layers  12 . 2 - 628 ,  12 . 2 - 634  are fixed in position relative to one another at these edges only and are free of affixation to one another between the edges. That is, the layers  12 . 2 - 628 ,  12 . 2 - 634  can extend between the first and second edges (i.e., perimeter and aperture edges) free of affixation to one another. In this way, the layers  12 . 2 - 628 ,  12 . 2 - 634  of the curtain assembly  12 . 2 - 606  are fixed relative to each other at the edges  12 . 2 - 542 ,  12 . 2 - 544  but free to move relative to one another between the edges  12 . 2 - 542 ,  12 . 2 - 544 . 
     In at least one example, the first, elastic layer  12 . 2 - 634  includes a woven fabric material/textile. In at least one example, the first, elastic layer  634  includes an elastic polymer material. In at least one example, the first, elastic layer  12 . 2 - 634  includes natural woven fibers. In at least one example, the first, elastic layer  12 . 2 - 634  includes synthetic woven fibers. In at least one example, the second, air-impermeable and/or non-elastic layer  12 . 2 - 628  includes plastic, or other non-elastic material configured to prevent air from passing through the curtain assembly  12 . 2 - 606 . 
     In at least one example, the first layer  12 . 2 - 634  of the curtain assembly  12 . 2 - 606  extends between the lens and frame of an HMD, as described above, in tension. As such, as the lens is moved relative to the housing/frame of the HMD, when certain portions or areas of the first layer  12 . 2 - 634  are reduced in size or span a short distance, the portion or area of the layer  12 . 2 - 634  remains in tension to avoid wrinkling or folding during use. In at least one example, the second layer  628  of the curtain assembly  12 . 2 - 606  extends between the lens and frame of an HMD, as described above, in compression such that certain areas or portions of the second layer  12 . 2 - 628  can expand to cover a larger distance or area as the lens connected to the second edge  12 . 2 - 544  moves. 
     Along these lines, in at least one example, the first and second layers  12 . 2 - 634 ,  12 . 2 - 628  occupy the same area between the lenses and the frame of an HMD, as described above, but have different surface areas. That is, in at least one example, the first layer  12 . 2 - 634  can include a first surface area and the second layer  12 . 2 - 628  can include a second surface greater than the first surface area while the first and second layers  12 . 2 - 634 ,  12 . 2 - 628  extend across the same area defined between the frame/housing and the optical modules/lenses of an HMD, as shown in other figures. In this way, as the lenses are moved relative to the frame, the second layer  12 . 2 - 628 , which is non-elastic, can accommodate the movement without tearing or being damaged and the first layer  12 . 2 - 634  can accommodate the movement while staying taught. 
     In at least one example, the second layer  12 . 2 - 632  can include one more materials, including but not limited to, acrylic copolymers, silicone, TPU silicone blends, TPU, nitrile rubber, to achieve the impermeability as well as low crinkle noises as the optical modules are adjusted. In at least one example, the second layer  12 . 2 - 632  can be thermoformed to include dimples. 
     In at least one example, the curtain assembly  12 . 2 - 606  can prevent dust and chemical ingress into the HMD device and reliability performance can be enhanced due to the curtain assembly  12 . 2 - 606 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  12 . 2 - 6    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  12 . 2 - 1 - 12 . 2 - 5  and  12 . 2 - 7 - 12 . 2 - 13    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  12 . 2 - 1 - 12 . 2 - 5  and  12 . 2 - 7 - 12 . 2 - 13    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  12 . 2 - 6   . 
       FIGS.  12 . 2 - 7    illustrates a plan view of an example of a curtain assembly  12 . 2 - 706  including first and second layers  12 . 2 - 734 ,  12 . 2 - 728 , respectively, coupled to a frame or backplate  12 . 2 - 732  at an outer perimeter edge  12 . 2 - 742  and extending to inner edges  12 . 2 - 744   a ,  12 . 2 - 744   b  coupled to trim rings  12 . 2 - 730   a ,  12 . 2 - 730   b  and/or optical modules  12 . 2 - 704   a ,  12 . 2 - 704   b , respectively. In at least one example, the first layer can extend to cover substantially all of an area between the backplate  12 . 2 - 732  and trim rings  12 . 2 - 744   a - b  coupling the layers  12 . 2 - 734 ,  12 . 2 - 728  to the optical modules  12 . 2 - 704   a - b . In the plan view of  FIGS.  12 . 2 - 7   , the second layer  12 . 2 - 728  overlies the first layer  12 . 2 - 734  such that much of the first layer  12 . 2 - 734  is not visible. However, as noted above, the first layer  12 . 2 - 734  can occlude substantially all of the aperture/opening between the lenses of the optical modules  12 . 2 - 704   a - b  and the frame or backplate  12 . 2 - 732 . 
     In at least one example, the second layer  12 . 2 - 728  can extend between the frame or backplate  12 . 2 - 732  and the lenses of the optical modules  12 . 2 - 704   a - b  and/or the trim rings  12 . 2 - 730   a - b  to define laterally positioned gaps  12 . 2 - 746   a  and  12 . 2 - 746   b  where the second layer  12 . 2 - 728  does not extend between the frame/backplate  12 . 2 - 732  and the lenses/optical modules  12 . 2 - 704   a - b . In at least one example, the second layer  12 . 2 - 728  covers between about 70% and about 90% of the area between the frame/backplate  12 . 2 - 732  and the optical modules  12 . 2 - 704   a - b /trim rings  12 . 2 - 730   a - b . In some examples, the second layer  12 . 2 - 728  covers between about 75% and about 90% of the area, for examples between about 80% and about 85% of the area. The gaps  12 . 2 - 746   a  and  12 . 2 - 746   b  can also allow for some airflow through the curtain assembly  12 . 2 - 706  and to the user&#39;s eyes, which can keep the face and eye area of the user cool as well as provide de-fogging airflow for the screens of the displays and lenses of the HMD. 
     The curtain assembly  12 . 2 - 706  can define a middle portion  12 . 2 - 748  extending between the first lens or optical module  12 . 2 - 704   a  and the second lens or optical module  12 . 2 - 704   b . The curtain assembly  12 . 2 - 706  can also define a first side portion  12 . 2 - 750  extending between the first lens or optical module  12 . 2 - 704   a  and the frame/backplate  12 . 2 - 732  and a second side portion  12 . 2 - 752  extending between the second lens or optical module  12 . 2 - 704   b  and the frame/backplate  12 . 2 - 732 . In such an example, the gaps  12 . 2 - 746   a - b  can be disposed or positioned at the first and second side portions  12 . 2 - 750 ,  12 . 2 - 752 , respectively. As such, as the lenses/optical modules  12 . 2 - 704   a - b  are moved laterally back and forth in the direction indicated by arrows  12 . 2 - 754 , the absence of material of the second layer  12 . 2 - 728  at the gaps  12 . 2 - 746   a - b  can reduce forces opposing the static positioning of the optical modules  12 . 2 - 704   a - b  and/or opposing the forces of motors configured to move the optical modules  12 . 2 - 704   a - b  relative to the frame/backplate  12 . 2 - 732 , as described above. 
     In at least one example, the curtain assembly  12 . 2 - 706  can bias the optical modules against the guide-rods to remove slack or backlash between the components of the IPD adjustment system. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  12 . 2 - 7    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  12 . 2 - 1 - 12 . 2 - 6  and  12 . 2 - 8 - 12 . 2 - 13    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  12 . 2 - 1 - 12 . 2 - 6  and  12 . 2 - 8 - 12 . 2 - 13    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  12 . 2 - 7   . 
       FIGS.  12 . 2 - 8    illustrates a partial view of an example of a curtain assembly  12 . 2 - 806  including first and second layers  12 . 2 - 834 ,  12 . 2 - 828  coupled to an optical module  12 . 2 - 804 . In particular,  FIGS.  12 . 2 - 8    shows the first and second layers  12 . 2 - 834 ,  12 . 2 - 828  coupled to the housing  12 . 2 - 856  of the optical module  12 . 2 - 804  coupled to a lens  12 . 2 - 818 . In at least one example, the layers  12 . 2 - 834 ,  12 . 2 - 828  can be pressed or coupled between the housing  12 . 2 - 856  and a curtain trim ring  12 . 2 - 830  as shown. In at least one example, the layers  12 . 2 - 834 ,  12 . 2 - 828  can be adhered to the ring  12 . 2 - 830  and the housing  12 . 2 - 856  of the optical module  12 . 2 - 804  via opposing adhesive portions  12 . 2 - 860   a ,  12 . 2 - 860   b . In the illustrated example, the layers  12 . 2 - 834 ,  12 . 2 - 828  can curve around the ring  12 . 2 - 830  and around between the ring  12 . 2 - 830  and housing  12 . 2 - 856  such that the ring  12 . 2 - 830  is hidden from view from the perspective of the user. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  12 . 2 - 8    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  12 . 2 - 1 - 12 . 2 - 7  and  12 . 2 - 9 - 12 . 2 - 13    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  12 . 2 - 1 - 12 . 2 - 7  and  12 . 2 - 9 - 12 . 2 - 13    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  12 . 2 - 8   . 
       FIGS.  12 . 2 - 9 - 12 . 2 - 13    shown partial, cross-sectional views of examples of curtain assemblies.  FIGS.  12 . 2 - 9 - 12 . 2 - 13    illustrate various examples of how various parts of the curtain assemblies can be coupled together. For example,  FIGS.  12 . 2 - 9    illustrates first and second layers  12 . 2 - 934 ,  12 . 2 - 928  of a curtain assembly  12 . 2 - 906  can be coupled to the backplate  12 . 2 - 932  with various adhesive layers and glue portions. In one example, a first adhesive layer  12 . 2 - 960   a  is disposed between and coupling a stiffener  12 . 2 - 940  and the first layer  12 . 2 - 934 . The first layer  12 . 2 - 934  can include an end abutting or adjacent a sidewall  12 . 2 - 933  of the backplate  12 . 2 - 932 . A second adhesive layer  12 . 2 - 960   b  can be disposed between and coupling the second layer  12 . 2 - 928  and the first layer  12 . 2 - 934 . A third adhesive layer  12 . 2 - 960   c  can be disposed between and couple the second layer  12 . 2 - 928  and the backplate  12 . 2 - 932 . A glue portion  12 . 2 - 961  can be disposed between and couple the first layer  12 . 2 - 934  and the backplate  12 . 2 - 932  and extend between an end of the second layer  12 . 2 - 928  and the sidewall  12 . 2 - 933 . 
       FIGS.  12 . 2 - 10    illustrates an example of a curtain assembly  12 . 2 - 1006  including a first adhesive layer between and coupling a stiffener  12 . 2 - 1040  to the first layer  12 . 2 - 1034 , a second adhesive layer  12 . 2 - 1060   b  between and coupling the first layer  12 . 2 - 1034  and the second layer  12 . 2 - 1028 , and a third adhesive layer  12 . 2 - 1060   c  between and coupling the second layer  12 . 2 - 1028  and the backplate  12 . 2 - 1032 . In one example, a glue portion  12 . 2 - 1061  can couple the stiffener  12 . 2 - 1040 , first layer  12 . 2 - 1034 , and the second layer  12 . 2 - 1028 , as well as the various adhesive layers  12 . 2 - 1060   a - c , to the backplate  12 . 2 - 1032  at the ends of the various components and layers between the sidewall  12 . 2 - 1033  and the layers  12 . 2 - 1034 ,  12 . 2 - 1028 . In at least one example, a space remains between the glue portion  12 . 2 - 1061  and a sidewall  12 . 2 - 1033  of the backplate  12 . 2 - 1032 . 
       FIGS.  12 . 2 - 11    illustrates an example of a curtain assembly  12 . 2 - 1106  including a first adhesive layer  12 . 2 - 1160   a  between and coupling a stiffener  12 . 2 - 1140  and a first layer  12 . 2 - 1134 , a second adhesive layer  12 . 2 - 1160   b  between and coupling the first layer  12 . 2 - 1134  and the second layer  12 . 2 - 1128 , and a third adhesive layer  12 . 2 - 1160   c  between and coupling the second layer  12 . 2 - 1128  and the backplate  12 . 2 - 1132 . An end of the first layer  12 . 2 - 1134  can abut or be disposed adjacent to a sidewall  12 . 2 - 1133  of the backplate  12 . 2 - 1132  and a glue portion  12 . 2 - 1161  can be disposed between an end of the second layer  12 . 2 - 1128  and the sidewall  12 . 2 - 1133 . The glue portion  12 . 2 - 1161  can be disposed between and couple the first layer  12 . 2 - 1134  and the backplate  12 . 2 - 1132 . In at least one example, the backplate  12 . 2 - 1132  can define an aperture  12 . 2 - 1162  through which the glue portion  12 . 2 - 1162  can be dispensed and at least partially disposed, as shown. 
       FIGS.  12 . 2 - 12    illustrates an example of a curtain assembly  12 . 2 - 1206  including a first adhesive layer  12 . 2 - 1260   a  disposed between and coupling a stiffener  12 . 2 - 1240  to a first layer  12 . 2 - 1234 , a second adhesive layer  12 . 2 - 1260   b  disposed between and coupling the first layer  12 . 2 - 1234  and the second layer  12 . 2 - 1228 , and a third adhesive layer disposed between and coupling the second layer  12 . 2 - 1228  and the backplate  12 . 2 - 1232 . In the illustrates example of  FIGS.  12 . 2 - 12   , the first layer  12 . 2 - 1234  can curve around and be coupled to the backplate  12 . 2 - 1232  via a fourth adhesive layer  12 . 2 - 1260   d  such that a portion of the backplate is disposed between opposing portions of the first layer  1234  within the curved geometry thereof. 
       FIGS.  12 . 2 - 13    illustrates an example of a curtain assembly  12 . 2 - 1306  including a first adhesive layer between and coupling a stiffener  12 . 2 - 1340  to the first layer  12 . 2 - 1334 , a second adhesive layer  12 . 2 - 1360   b  between and coupling the first layer  12 . 2 - 1334  and the second layer  12 . 2 - 1328 , and a third adhesive layer  12 . 2 - 1360   c  between and coupling the second layer  12 . 2 - 1328  and the backplate  12 . 2 - 1332 . In one example, a glue portion  12 . 2 - 1361  can couple the stiffener  12 . 2 - 1340 , first layer  12 . 2 - 1334 , and the second layer  12 . 2 - 1328 , as well as the various adhesive layers  12 . 2 - 1360   a - c , to the backplate  12 . 2 - 1332  at the end of the various components and layers between the sidewall  12 . 2 - 1333  and the layers  12 . 2 - 1334 ,  12 . 2 - 1328 . In at least one example, the glue portion  12 . 2 - 1361  can extend substantially all the way between the ends of the layers  12 . 2 - 1334 ,  12 . 2 - 1328  and a sidewall  12 . 2 - 1333  of the backplate  12 . 2 - 1332 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  12 . 2 - 9 - 12 . 2 - 13    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in  FIGS.  12 . 2 - 1 - 12 . 2 - 8    and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to  FIGS.  12 . 2 - 1 - 12 . 2 - 8    can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  12 . 2 - 9 - 12 . 2 - 13   . 
     XIII: Light Seal 
       FIGS.  13 . 0 - 1    illustrates a view of an HMD  13 . 0 - 100  including a light seal  13 . 0 - 102 . The light seal  13 . 0 - 102  is described in more detail here in section XIII. 
       FIGS.  13 . 0 - 2 A  illustrates a front perspective view of a device seal  13 . 0 - 202  according to one embodiment.  FIGS.  13 . 0 - 2 B  illustrates a bottom-rear perspective view of the device seal  13 . 0 - 202 .  FIGS.  13 . 0 - 2 C  illustrates a rear view of the device seal  13 . 0 - 202 . The device seal  13 . 0 - 202  illustrated in  FIGS.  13 . 0 - 2 A  through  FIGS.  13 . 0 - 2 C  can be substantially similar to, including some or all of the features of, the device seals or light seals described herein. 
     In some examples, the device seal  13 . 0 - 202  can be constructed and designed as depicted in  FIGS.  13 . 0 - 2 A  through  FIGS.  13 . 0 - 2 C . The device seal  13 . 0 - 202  can include a facial interface  13 . 0 - 216  to contact a wearer&#39;s face. The facial interface  13 . 0 - 216  can include a compressible material, such as a foam, that is compliant and comfortable against the user&#39;s face. 
     The device seal  13 . 0 - 202  can include a cover  13 . 0 - 214  extending from the facial interface  13 . 0 - 216  and defining an aperture through which the user can view a display of the HMD. The cover  13 . 0 - 214  can help to block light from the outside environment. The device seal  13 . 0 - 202  can include a nose relief  13 . 0 - 215  positioned, sized, and shaped to accommodate for the user&#39;s nose. While other designs and configurations are envisions, those depicted in  FIGS.  13 . 0 - 2 A  through  FIGS.  13 . 0 - 2 C  provide the user with exceptional user experience and engagement with the HMD. 
     13.1: Electronic Devices with Covering Structures 
     Electronic devices such as head-mounted devices are configured to be worn on a head of a user. A head-mounted device may have left and right optical systems for presenting images to a user&#39;s left and right eyes. Not all users have the same physical distance separating their eyes. To accommodate differences in interpupillary distance between different users, a head-mounted device may have a mechanism for adjusting the positions of the left and right optical systems. 
     An electronic device such as a head-mounted device may have a front face that faces away from a user&#39;s head and may have an opposing rear face that faces the user&#39;s head. Optical modules on the rear face may be used to provide images to a user&#39;s eyes. The positions of the optical modules may be adjusted to accommodate different user interpupillary distances. Internal device structures may be hidden from view by the user by covering the rear face of the device with a curtain. The curtain, which may sometimes be referred to as a cover, covering structure, rear housing cover, rear housing wall, rear housing structure, cosmetic covering, etc., may help block potentially unsightly internal structures from view, while accommodating movement of the optical modules. 
     A top view of an illustrative head-mounted device with a curtain is shown in  FIGS.  13 . 1 - 1   . As shown in  FIGS.  13 . 1 - 1   , head-mounted devices such as electronic device  13 . 1 - 10  may have head-mounted support structures such as housing  13 . 1 - 12 . Housing  13 . 1 - 12  may include portions (e.g., support structures  13 . 1 - 12 T) to allow device  13 . 1 - 10  to be worn on a user&#39;s head. Support structures  13 . 1 - 12 T may be formed from fabric, polymer, metal, and/or other material. Support structures  13 . 1 - 12 T may form a strap or other head-mounted support structures that help support device  13 . 1 - 10  on a user&#39;s head. A main support structure (e.g., main housing portion  13 . 1 - 12 M) of housing  13 . 1 - 12  may support electronic components such as displays  13 . 1 - 14 . Main housing portion  13 . 1 - 12 M may include housing structures formed from metal, polymer, glass, ceramic, and/or other material. For example, housing portion  13 . 1 - 12 M may have housing walls on front face F and housing walls on adjacent top, bottom, left, and right side faces that are formed from rigid polymer or other rigid support structures and these rigid walls may optionally be covered with electrical components, fabric, leather, or other soft materials, etc. The walls of housing portion  13 . 1 - 12 M may enclose internal components  13 . 1 - 38  in interior region  13 . 1 - 34  of device  13 . 1 - 10  and may separate interior region  13 . 1 - 34  from the environment surrounding device  13 . 1 - 10  (exterior region  13 . 1 - 36 ). Internal components  13 . 1 - 38  may include integrated circuits, actuators, batteries, sensors, and/or other circuits and structures for device  13 . 1 - 10 . Housing  13 . 1 - 12  may be configured to be worn on a head of a user and may form glasses, a hat, a helmet, goggles, and/or other head-mounted device. Configurations in which housing  13 . 1 - 12  forms goggles may sometimes be described herein as an example. 
     Front face F of housing  13 . 1 - 12  may face outwardly away from a user&#39;s head and face. Opposing rear face R of housing  13 . 1 - 12  may face the user. Portions of housing  13 . 1 - 12  (e.g., portions of main housing  13 . 1 - 12 M) on rear face R may form a cover such as curtain  13 . 1 - 12 C. In an illustrative configuration, curtain  13 . 1 - 12 C includes a fabric layer that separates interior region  13 . 1 - 34  from the exterior region to the rear of device  13 . 1 - 10 . Other structures may be used in forming curtain  13 . 1 - 12 C, if desired. The presence of curtain  13 . 1 - 12 C on rear face R may help hide internal housing structures, internal components  13 . 1 - 38 , and other structures in interior region  13 . 1 - 34  from view by a user. 
     Device  13 . 1 - 10  may have left and right optical modules  13 . 1 - 40 . Each optical module may include a respective display  13 . 1 - 14 , lens  13 . 1 - 30 , and support structure  13 . 1 - 32 . Support structures  13 . 1 - 32 , which may sometimes be referred to as lens barrels or optical module support structures, may include hollow cylindrical structures with open ends or other supporting structures to house displays  13 . 1 - 14  and lenses  13 . 1 - 30 . Support structures  13 . 1 - 32  may, for example, include a left lens barrel that supports a left display  13 . 1 - 14  and left lens  13 . 1 - 30  and a right lens barrel that supports a right display  13 . 1 - 14  and right lens  13 . 1 - 30 . Displays  13 . 1 - 14  may include arrays of pixels or other display devices to produce images. Displays  13 . 1 - 14  may, for example, include organic light-emitting diode pixels formed on substrates with thin-film circuitry and/or formed on semiconductor substrates, pixels formed from crystalline semiconductor dies, liquid crystal display pixels, scanning display devices, and/or other display devices for producing images. Lenses  13 . 1 - 30  may include one or more lens elements for providing image light from displays  13 . 1 - 14  to respective eyes boxes  13 . 1 - 13 . Lenses may be implemented using refractive glass lens elements, using mirror lens structures (catadioptric lenses), using holographic lenses, and/or other lens systems. When a user&#39;s eyes are located in eye boxes  13 . 1 - 13 , displays (display panels)  13 . 1 - 14  operate together to form a display for device  13 . 1 - 10  (e.g., the images provided by respective left and right optical modules  13 . 1 - 40  may be viewed by the user&#39;s eyes in eye boxes  13 . 1 - 13  so that a stereoscopic image is created for the user). The left image from the left optical module fuses with the right image from a right optical module while the display is viewed by the user. 
     Not all users have the same interpupillary distance IPD. To provide device  13 . 1 - 10  with the ability to adjust the interpupillary spacing between modules  13 . 1 - 40  along lateral dimension X and thereby adjust the spacing IPD between eye boxes  13 . 1 - 13  to accommodate different user interpupillary distances, device  13 . 1 - 10  may be provided with actuators  13 . 1 - 42 . Actuators  13 . 1 - 42  can be manually controlled and/or computer-controlled actuators (e.g., computer-controlled motors) for moving support structures  13 . 1 - 32  relative to each other. 
     As shown in  FIGS.  13 . 1 - 2   , curtain  13 . 1 - 12 C may cover rear face F while leaving lenses  13 . 1 - 30  of optical modules  13 . 1 - 40  uncovered (e.g., curtain  13 . 1 - 12 C may have openings that are aligned with and receive modules  13 . 1 - 40 ). As modules  13 . 1 - 40  are moved relative to each other along dimension X to accommodate different interpupillary distances for different users, modules  13 . 1 - 40  move relative to fixed housing structures such as the walls of main portion  13 . 1 - 12 M and move relative to each other. To prevent undesired wrinkling and buckling of curtain  13 . 1 - 12 C as optical modules  13 . 1 - 40  are moved relative to rigid portions of housing  13 . 1 - 12 M and relative to each other, a fabric layer or other cover layer in curtain  13 . 1 - 12 C may be configured to slide, stretch, open/close, and/or otherwise adjust to accommodate optical module movement. 
     A schematic diagram of an illustrative electronic device such as a head-mounted device or other wearable device is shown in  FIGS.  13 . 1 - 3   . Device  13 . 1 - 10  of  FIGS.  13 . 1 - 3    may be operated as a stand-alone device and/or the resources of device  13 . 1 - 10  may be used to communicate with external electronic equipment. As an example, communications circuitry in device  13 . 1 - 10  may be used to transmit user input information, sensor information, and/or other information to external electronic devices (e.g., wirelessly or via wired connections). Each of these external devices may include components of the type shown by device  13 . 1 - 10  of  FIGS.  13 . 1 - 3   . 
     As shown in  FIGS.  13 . 1 - 3   , a head-mounted device such as device  13 . 1 - 10  may include control circuitry  13 . 1 - 20 . Control circuitry  13 . 1 - 20  may include storage and processing circuitry for supporting the operation of device  13 . 1 - 10 . The storage and processing circuitry may include storage such as nonvolatile memory (e.g., flash memory or other electrically-programmable-read-only memory configured to form a solid state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Processing circuitry in control circuitry  13 . 1 - 20  may be used to gather input from sensors and other input devices and may be used to control output devices. The processing circuitry may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors and other wireless communications circuits, power management units, audio chips, application specific integrated circuits, etc. During operation, control circuitry  13 . 1 - 20  may use display(s)  13 . 1 - 14  and other output devices in providing a user with visual output and other output. 
     To support communications between device  13 . 1 - 10  and external equipment, control circuitry  13 . 1 - 20  may communicate using communications circuitry  13 . 1 - 22 . Circuitry  13 . 1 - 22  may include antennas, radio-frequency transceiver circuitry, and other wireless communications circuitry and/or wired communications circuitry. Circuitry  13 . 1 - 22 , which may sometimes be referred to as control circuitry and/or control and communications circuitry, may support bidirectional wireless communications between device  13 . 1 - 10  and external equipment (e.g., a companion device such as a computer, cellular telephone, or other electronic device, an accessory such as a point device, computer stylus, or other input device, speakers or other output devices, etc.) over a wireless link. For example, circuitry  13 . 1 - 22  may include radio-frequency transceiver circuitry such as wireless local area network transceiver circuitry configured to support communications over a wireless local area network link, near-field communications transceiver circuitry configured to support communications over a near-field communications link, cellular telephone transceiver circuitry configured to support communications over a cellular telephone link, or transceiver circuitry configured to support communications over any other suitable wired or wireless communications link. Wireless communications may, for example, be supported over a Bluetooth® link, a WiFi® link, a wireless link operating at a frequency between 10 GHz and 400 GHz, a 60 GHz link, or other millimeter wave link, a cellular telephone link, or other wireless communications link. Device  13 . 1 - 10  may, if desired, include power circuits for transmitting and/or receiving wired and/or wireless power and may include batteries or other energy storage devices. For example, device  13 . 1 - 10  may include a coil and rectifier to receive wireless power that is provided to circuitry in device  13 . 1 - 10 . 
     Device  13 . 1 - 10  may include input-output devices such as devices  13 . 1 - 24 . Input-output devices  13 . 1 - 24  may be used in gathering user input, in gathering information on the environment surrounding the user, and/or in providing a user with output. Devices  13 . 1 - 24  may include one or more displays such as display(s)  13 . 1 - 14 . Display(s)  13 . 1 - 14  may include one or more display devices such as organic light-emitting diode display panels (panels with organic light-emitting diode pixels formed on polymer substrates or silicon substrates that contain pixel control circuitry), liquid crystal display panels, microelectromechanical systems displays (e.g., two-dimensional mirror arrays or scanning mirror display devices), display panels having pixel arrays formed from crystalline semiconductor light-emitting diode dies (sometimes referred to as microLEDs), and/or other display devices. 
     Sensors  13 . 1 - 16  in input-output devices  13 . 1 - 24  may include force sensors (e.g., strain gauges, capacitive force sensors, resistive force sensors, etc.), audio sensors such as microphones, touch and/or proximity sensors such as capacitive sensors such as a touch sensor that forms a button, trackpad, or other input device), and other sensors. If desired, sensors  13 . 1 - 16  may include optical sensors such as optical sensors that emit and detect light, ultrasonic sensors, optical touch sensors, optical proximity sensors, and/or other touch sensors and/or proximity sensors, monochromatic and color ambient light sensors, image sensors, fingerprint sensors, iris scanning sensors, retinal scanning sensors, and other biometric sensors, temperature sensors, sensors for measuring three-dimensional non-contact gestures (“air gestures”), pressure sensors, sensors for detecting position, orientation, and/or motion (e.g., accelerometers, magnetic sensors such as compass sensors, gyroscopes, and/or inertial measurement units that contain some or all of these sensors), health sensors such as blood oxygen sensors, heart rate sensors, blood flow sensors, and/or other health sensors, radio-frequency sensors, depth sensors (e.g., structured light sensors and/or depth sensors based on stereo imaging devices that capture three-dimensional images), optical sensors such as self-mixing sensors and light detection and ranging (LIDAR) sensors that gather time-of-flight measurements, humidity sensors, moisture sensors, gaze tracking sensors, electromyography sensors to sense muscle activation, facial sensors, and/or other sensors. In some arrangements, device  13 . 1 - 10  may use sensors  13 . 1 - 16  and/or other input-output devices to gather user input. For example, buttons may be used to gather button press input, touch sensors overlapping displays can be used for gathering user touch screen input, touch pads may be used in gathering touch input, microphones may be used for gathering audio input, accelerometers may be used in monitoring when a finger contacts an input surface and may therefore be used to gather finger press input, etc. 
     If desired, electronic device  13 . 1 - 10  may include additional components (see, e.g., other devices  13 . 1 - 18  in input-output devices  13 . 1 - 24 ). The additional components may include haptic output devices, actuators for moving movable housing structures, audio output devices such as speakers, light-emitting diodes for status indicators, light sources such as light-emitting diodes that illuminate portions of a housing and/or display structure, other optical output devices, and/or other circuitry for gathering input and/or providing output. Device  13 . 1 - 10  may also include a battery or other energy storage device, connector ports for supporting wired communication with ancillary equipment and for receiving wired power, and other circuitry. 
       FIGS.  13 . 1 - 4  and  13 . 1 - 5    are top views of device  13 . 1 - 10  showing how the optical modules of device  13 . 1 - 10  move with respect to each other along lateral dimension X to accommodate different interpupillary distances IPD (the distance between a user&#39;s left and right eyes). In the example of  FIGS.  13 . 1 - 4   , left optical module  13 . 1 - 40 L and right optical module  13 . 1 - 40 R have been moved towards each other to accommodate a small interpupillary distance. In the example of  FIGS.  13 . 1 - 5   , left optical module  13 . 1 - 40 L and right optical module  13 . 1 - 40 R have been moved away from each other to accommodate a large interpupillary distance. 
     Curtain  13 . 1 - 12 C has edge portions such as left portion  13 . 1 - 12 C-L between left housing wall  13 . 1 - 12 M-L and left optical module  13 . 1 - 40 L and right portion  13 . 1 - 12 C-R between right housing wall  13 . 1 - 12 M-R and right optical module  13 . 1 - 40 R. Middle potion  13 . 1 - 12 C-M of curtain  13 . 1 - 12 C extends between left optical module  13 . 1 - 40 L and right optical module  13 . 1 - 40 R. In the configuration of  FIGS.  13 . 1 - 4   , optical modules  13 . 1 - 40 L and  13 . 1 - 40 R are relatively close to each other, so middle portion  13 . 1 - 12 C-M is relatively small and portions  13 . 1 - 12 C-L and  13 . 1 - 12 C-R are relatively large. In the configuration of  FIGS.  13 . 1 - 5   , optical modules  13 . 1 - 40 L and  13 . 1 - 40 R are relatively far from each other, so left portion  13 . 1 - 12 C-L and right portion  13 . 1 - 12 C-R are shorter along lateral dimension X and middle portion  13 . 1 - 12 C-M has been enlarged relative to the configuration of  FIGS.  13 . 1 - 4   . 
     To help accommodate differences in size for curtain  13 . 1 - 12 C (e.g., length changes for portions of curtain  13 . 1 - 12 M along lateral dimension X), curtain  13 . 1 - 12 C may include a cover layer formed from a stretchable material such as fabric. The cover layer may be supported by a rigid frame. The fabric may be provided with a peripheral elastic band that helps allow the fabric to slide relative to the frame while being retained securely on the frame, thereby further helping curtain  13 . 1 - 12 M to be dynamically adjusted without exhibiting undesired buckling and wrinkling. 
       FIGS.  13 . 1 - 6    is a side view of device  13 . 1 - 10  showing how device  13 . 1 - 10  may, if desired, include cooling features to help cool display  13 . 1 - 14  and other internal components  13 . 1 - 38 . As shown in  FIGS.  13 . 1 - 6   , for example, device  13 . 1 - 10  may have a fan such as fan  13 . 1 - 50 . Fan  13 . 1 - 50  may be mounted in housing  13 . 1 - 12  (e.g., a fan housing in housing  13 . 1 - 12 , etc.) in a configuration that allows fan  13 . 1 - 50  to expel air from housing  13 . 1 - 12 . To allow cool air to move past the face of the user while cooling electrical components in interior region  13 . 1 - 34  such as internal components  13 . 1 - 38  and display  13 . 1 - 14 , curtain  13 . 1 - 12 C may be permeable to air. Curtain  13 . 1 - 12 C may, as an example, have one or more air-flow-promotion openings. In some examples, curtain  13 . 1 - 12 C is formed from knit or woven fabric that has a sufficiently loose knit or weave to allow air to flow through the interstitial gaps between adjacent strands of material in the fabric (e.g., between warp and weft strands). Openings may also be formed by laser cutting and/or other opening formation techniques. In examples in which curtain  13 . 1 - 12 C is permeable to air, air may flow into device  13 . 1 - 10  through curtain  13 . 1 - 12 C, may flow past display  13 . 1 - 14  and other internal components  13 . 1 - 38 , and may exit interior region  13 . 1 - 34  through fan  13 . 1 - 50 . Air may flow in this way to cool device  13 . 1 - 10  while device  13 . 1 - 10  is being worn on a head of a user. If desired, housing  13 . 1 - 12  may have additional openings (e.g., slot shaped openings on an upper wall, side wall, and/or lower wall) that provide additional pathways for air flow. 
     To allow a cover layer in curtain  13 . 1 - 12 C to slide back and forth during adjustments to the positions of modules  13 . 1 - 40 , curtain  13 . 1 - 12 C may be provided with a rigid frame that supports the cover layer without excessively restricting lateral motion of the cover layer.  FIGS.  13 . 1 - 7    is an exploded perspective view of a left-hand portion of an illustrative curtain. As shown in  FIGS.  13 . 1 - 7   , curtain  13 . 1 - 12 C may include frame  13 . 1 - 12 CF and cover layer  13 . 1 - 12 CC. Cover layer  13 . 1 - 12 CC may have left and right openings such as lens barrel opening  13 . 1 - 52  to receive respective left and right optical modules  13 . 1 - 40 . Frame  13 . 1 - 54  may have corresponding left and right openings such as illustrative opening  13 . 1 - 54 . Opening  13 . 1 - 54  may be larger than opening  13 . 1 - 52  to accommodate lateral movement of module  13 . 1 - 40 . When mounted in device  13 . 1 - 10 , cover layer  13 . 1 - 12 CC may be attached to frame  13 . 1 - 12 CF so that cover layer  13 . 1 - 12 CC can stretch and/or slide relative to frame  13 . 1 - 12 CF. 
     If desired, cover layer  13 . 1 - 12 CC may include multiple layers of material. As shown in the top view of  FIGS.  13 . 1 - 8   , for example, cover layer  13 . 1 - 12 CC may, include outer layer  13 . 1 - 56  and inner layer  13 . 1 - 58 . Outer layer  13 . 1 - 56  may be, for example, a fabric layer. Inner layer  13 . 1 - 58  may be, for example, a layer of fabric formed form polymer strands that have been thermally formed into an accordion shape. In this type of arrangement, outer layer  13 . 1 - 56  may be a fabric with a desired visual appearance, whereas inner layer  13 . 1 - 58  may be a fabric or other layer that provides cover layer  13 . 1 - 12 CC with a desired opacity to help block internal device components from view. Layer  13 . 1 - 12 CC may be attached to module  13 . 1 - 40  using attachment structures  13 . 1 - 60 . Structures  13 . 1 - 60  may include retention rings, screws and other fasteners, clips, and other structures that mechanically attach layer  13 . 1 - 12 CC to support structures  13 . 1 - 32  of module  13 . 1 - 40  and/or may include adhesive for attaching layer  13 . 1 - 12 CC to module  13 . 1 - 40 . 
     If desired, layer  13 . 1 - 12 CC may be provided with a peripheral elastic band. This type of arrangement is shown in  FIGS.  13 . 1 - 9   . As shown in  FIGS.  13 . 1 - 9   , cover layer  13 . 1 - 12 CC may have fabric  13 . 1 - 62  and peripheral elastic band  13 . 1 - 64 . Fabric  13 . 1 - 62  may be warp knit fabric, weft knit fabric, or other knit fabric (e.g., to promote stretchiness), may be woven fabric, braided fabric, non-woven fabric, etc. If desired, a layer of stretchy plastic may be attached to fabric  13 . 1 - 62  and/or an elastomeric polymer layer may be used in place of some or all of fabric  13 . 1 - 62  in forming layer  13 . 1 - 12 CC. Fabric  13 . 1 - 62  may be formed from interlaced (intertwined) strands of material such as polymer strands, strands of cotton or other natural material, synthetic material, and/or other materials. The strands in fabric  13 . 1 - 62  may include monofilament strands and/or multi-filament strands. In some examples, some of the strands of fabric  13 . 1 - 62  may be selected to provide fabric  13 . 1 - 62  with strength, whereas other strands in fabric  13 . 1 - 62  may be formed from elastomeric material that enhances the ability of fabric  13 . 1 - 62  to stretch (and that has a lower elastic modulus than the strands that provide fabric  13 . 1 - 62  with strength). Examples of stretchable strand materials include elastomeric materials such as silicone and thermoplastic polyurethane (TPU). Examples of strength-enhancing strand materials include polyester, nylon, etc. Elastic band  13 . 1 - 64  may be formed from a strand of elastomeric material such as a strand of silicone or thermoplastic polyurethane or other stretchable material. Other materials may be used in forming the strands in fabric  13 . 1 - 62 , if desired. 
     Elastic band  13 . 1 - 64  may be attached to along the outer edge of fabric  13 . 1 - 62  by sewing, by knitting band  13 . 1 - 64  into a knit fabric, using adhesive, using crimped connections or other fasteners, and/or by otherwise attaching band  13 . 1 - 64  to the periphery of fabric  13 . 1 - 62 . If desired, band  13 . 1 - 64  may be formed from warp strands and weft strands in a woven fabric (see, e.g., band  13 . 1 - 64  of fabric  13 . 1 - 62  of  FIGS.  13 . 1 - 10   ). 
     The fabric or other material forming cover layer  13 . 1 - 12 CC may be stretchable. As shown in  FIGS.  13 . 1 - 11   , for example, layer  13 . 1 - 12 CC may be configured to be stretched without damage from a first shape characterized by length  13 . 1 -L 1  along dimension X to a second larger shape characterized by length  13 . 1 -L 2  along dimension X. The amount of stretching ( 13 . 1 -L 2 / 13 . 1 -L 1 ) that layer  13 . 1 - 12 CC may accommodate may be, for example, at least 50%, at least 75%, at least 100%, or at least 150%. 
     When attaching cover layer  13 . 1 - 12 CC to frame  13 . 1 - 12 CC, band  13 . 1 - 64  may fit over the outside of frame  13 . 1 - 12 CC. Band  13 . 1 - 64  may then tug inwardly on the portions of cover layer  13 . 1 - 12 CC that overlap the edges of the frame. This, in turn, will help to tension the main portion of layer  13 . 1 - 12 CC outwardly (e.g., in lateral dimensions X and Y), thereby ensuring that cover layer  13 . 1 - 12 CC will remain taut. At the same time, there may be at least some allowed lateral slippage of layer  13 . 1 - 12 CC back and forth as needed to accommodate changes in the positions of modules  13 . 1 - 40 . 
     An example shape for frame  13 . 1 - 12 CF of curtain  13 . 1 - 12 C is shown in  FIGS.  13 . 1 - 12   . As shown in  FIGS.  13 . 1 - 12   , frame  13 . 1 - 12 CF may have left and right openings  13 . 1 - 54  to overlap the desired range of positions achievable by modules  13 . 1 - 40 . Frame  13 . 1 - 12 CF may have an outer ring-shaped portion  13 . 1 - 66  bridged in the portion of frame  13 . 1 - 12 CF that overlaps the user&#39;s nose by bridging middle portion  13 . 1 - 68 . Openings  13 . 1 - 54  may be rectangular, oval, teardrop shaped, circular, and/or may have other suitable shapes. 
       FIGS.  13 . 1 - 13    shows how elastic band  13 . 1 - 64  may help provide fabric  13 . 1 - 62  of cover  13 . 1 - 12 CC with the ability to slide laterally relative to frame to accommodate movement in optical modules  13 . 1 - 40  while helping to keep fabric  13 . 1 - 62  taut. Elastic band  13 . 1 - 64  is normally in a stretched state. As a result, band  13 . 1 - 64  attempts to contract and, in doing so, tends to pull fabric  13 . 1 - 62  around frame  13 . 1 - 12 CF in direction  13 . 1 - 74 . On the inner side of frame  13 . 1 - 12 CF, the tightening force from band  13 . 1 - 64  is therefore normally pulling fabric  13 . 1 - 62  in direction  13 . 1 - 76 , whereas on the opposing outer (rear-facing) side of frame  13 . 1 - 12 CF, the tightening force of band  13 . 1 - 64  tends to pull fabric  13 . 1 - 62  towards the periphery of frame  13 . 1 - 12 CF in direction  13 . 1 - 78 . The presence of band  13 . 1 - 64  therefore helps tighten fabric  13 . 1 - 62  and prevent wrinkles in cover layer  13 . 1 - 12 CC. 
     When it is desired to move optical module  13 . 1 - 40 , fabric  13 . 1 - 62  can slide back and forth over frame  13 . 1 - 12 CF as needed. Consider, as an example, a scenario in which module  13 . 1 - 40  is moved in direction  13 . 1 - 70 . This pulls fabric  13 . 1 - 62  on the outer side of frame  13 . 1 - 12 CF (e.g., frame portion  13 . 1 - 66 ) in direction  13 . 1 - 80 . On the inner side of frame  13 . 1 - 12 CF, the edge of fabric  13 . 1 - 62  is pulled in direction  13 . 1 - 72 , causing band  13 . 1 - 64  to stretch and expand slightly (e.g., so that band  13 . 1 - 64  moves to position  13 . 1 - 64 ′). Due to the pull on fabric  13 . 1 - 62  in direction  13 . 1 - 80 , fabric  13 . 1 - 62  slides around frame  13 . 1 - 12 CF and across the outer surface of frame  13 . 1 - 12 CF in direction  13 . 1 - 82 , thereby helping to accommodate movement of module  13 . 1 - 40  without wrinkling cover layer  13 . 1 - 12 CC. 
     To facilitate sliding movement of cover  13 . 1 - 12 CC around the edges of frame  13 . 1 - 12 CF in this way, at least the left and right edges of cover  13 . 1 - 12 CC (and adjacent portions along the upper and lower edges of cover  13 . 1 - 12 CC) may not be fixedly attached to frame  13 . 1 - 12 CF or housing  13 . 1 - 12 . An illustrative configuration for mounting curtain  13 . 1 - 12 C within housing portion  13 . 1 - 12 M of housing  13 . 1 - 12  of device  13 . 1 - 10  is shown in  FIGS.  13 . 1 - 14   . In the example of  FIGS.  13 . 1 - 14   , housing portion  13 . 1 - 12 M has a central support such as housing structure  13 . 1 - 84 . Curtain  13 . 1 - 12 C may be fixedly attached to structure  13 . 1 - 84  using attachment mechanism  13 . 1 - 86 . Mechanism  13 . 1 - 86  may include, for example, glue and/or mechanical structures that grip fabric  13 . 1 - 62  and/or other portions of curtain  13 . 1 - 12 C to hold curtain  13 . 1 - 12 C firmly in place within device  13 . 1 - 10 . Mechanism  13 . 1 - 86  may, if desired, include engagement structures (e.g., snap features) that allow curtain  13 . 1 - 12 C to be removed and replaced with another curtain. When mounted in device  13 . 1 - 10 , however, mechanism  13 . 1 - 86  will hold curtain  13 . 1 - 12 C securely. 
     As shown in  FIGS.  13 . 1 - 14   , device  13 . 1 - 10  may, if desired, have a ring-shaped opaque light seal such as light seal  13 . 1 - 90 . Light seal  13 . 1 - 90  may be configure to be removable (e.g. so that light seal  13 . 1 - 90  may be replaced when worn). Foam or other soft materials may be used in forming light seal  13 . 1 - 90 . 
     The attachment structures used in mechanism  13 . 1 - 86  of  FIGS.  13 . 1 - 14    may or may not permit fabric  13 . 1 - 62  to slide freely with respect to frame  13 . 1 - 12 CF. To permit fabric  13 . 1 - 62  to slide freely with respect to frame  13 . 1 - 12 CF of curtain  13 . 1 - 12 C elsewhere in curtain  13 . 1 - 12 C (e.g., at the left and right edges of curtain  13 . 1 - 12 C and at other portions of curtain  13 . 1 - 12 C away from attachment mechanism  13 . 1 - 86 ), curtain  13 . 1 - 12 C may float with respect to housing  13 . 1 - 12 M (except at attachment mechanism  13 . 1 - 86 ). As an example, the opposing ends of curtain  13 . 1 - 12 C at the left and right edges of curtain  13 . 1 - 12 C may be separated from nearby portions of housing portion  13 . 1 - 12 M by air gaps  13 . 1 - 88 . This prevents the fabric of layer  13 . 1 - 12 CC from becoming caught between frame  13 . 1 - 12 CF and housing  13 . 1 - 12 .  FIGS.  13 . 1 - 15    shows how attachment structures  13 . 1 - 86  may, if desired, be used to couple a bridging central portion of frame  13 . 1 - 12 CF such as portion  13 . 1 - 68  to housing structure  13 . 1 - 84  in the center of frame  13 . 1 - 12 C (e.g., along the upper and lower edges of curtain  13 . 1 - 12 C). 
       FIGS.  13 . 1 - 16    is a cross-sectional view of curtain  13 . 1 - 12 C showing how attachment mechanism  13 . 1 - 86  for attaching curtain  13 . 1 - 12  to housing structure  13 . 1 - 84  may include trim member  13 . 1 - 94 . Adhesive  13 . 1 - 92  may be used to attach fabric  13 . 1 - 62  and frame  13 . 1 - 12 CF to trim member  13 . 1 - 94  (e.g., in the vicinity of attachment mechanism  13 . 1 - 86  in the areas of curtain  13 . 1 - 12 C that overlap bridging portion  13 . 1 - 86  of frame  13 . 1 - 12 CF). Trim member  13 . 1 - 94  and housing structure  13 . 1 - 84  may have mating engagement structures. For example, trim member  13 . 1 - 94  may have a snap such as snap  13 . 1 - 96  that mates with a corresponding hook such as hook  13 . 1 - 98  on housing structure  13 . 1 - 84 . When it is desired to snap curtain  13 . 1 - 12 C in place, curtain  13 . 1 - 12 C may be pressed into housing  13 . 1 - 12 M in the −Z direction. If desired, curtain  13 . 1 - 12 C may be removed by disengaging (e.g., unsnapping) the engagement structures (e.g., when it is desired to remove or replace curtain  13 . 1 - 12 C). 
     If desired, curtain  13 . 1 - 12 C may be used in equipment other than devices  13 . 1 - 10 . Consider, as an example, the arrangement of  FIGS.  13 . 1 - 17   . As shown in  FIGS.  13 . 1 - 17   , apparatus  13 . 1 - 100  may have a movable member such as movable member  13 . 1 - 102 . Apparatus  13 . 1 - 100  may be, for example, a joystick or automobile stick shift and movable member  13 . 1 - 102  may be a movable shaft. Movable member  13 . 1 - 102  may move in one or more directions  13 . 1 - 104 . Curtain  13 . 1 - 12 C may have a cover layer  13 . 1 - 12 CC formed from fabric  13 . 1 - 62  or other covering material and may have peripheral elastic band  13 . 1 - 64 . Cover layer  13 . 1 - 12 CC may be mounted over a frame such as frame  13 . 1 - 12 CF. This allows cover layer  13 . 1 - 12 CC to slip and/or stretch during movement of member  13 . 1 - 102  to help avoid wrinkling in curtain  13 . 1 - 12 C. 
     13.2: Device with a Removable Cushion 
     A head-mounted device may include a head-mounted support structure that allows the device to be worn on the head of a user. The head-mounted device may have displays that are supported by the head-mounted support structure for presenting a user with visual content. The head-mounted device may also have sensors such as front-facing cameras and other sensors for gathering information on the environment surrounding the device. 
     The head-mounted device may include a removable cushion that is configured to be attached to the head-mounted support structure. The removable cushion may contact the user&#39;s face when the head-mounted device is worn by the user. There are many advantages to using a removable cushion of this type in the head-mounted device. Because the cushion contacts the user&#39;s face during operation, the cushion may tend to become dirty over time. If the cushion is not removable, it may be difficult to clean the cushion. However, the removable cushion is easy to remove from the head-mounted device to wash. The removable cushion is therefore easy to keep clean during repeated uses. 
     In addition to making the cushion easier to clean, the removability of the cushion makes the cushion easier to replace. If the cushion becomes worn down over time or damaged, the removable cushion may simply be replaced with a new removable cushion. This extends the lifetime of the head-mounted device. 
     The removable cushion may also be customizable. If the cushion is not removable, the properties of the cushion in the head-mounted device are fixed. With a removable cushion, an optimal removable cushion out of multiple options may be selected by a user. For example, the user may select a removable cushion with a desired color, softness, and dimensions. In this way, the user may select a custom removable cushion that is the most comfortable for them. 
       FIGS.  13 . 2 - 1    is a top view of a head-mounted electronic device  13 . 2 - 10 . As shown in  FIGS.  13 . 2 - 1   , head-mounted device  13 . 2 - 10  may include housing  13 . 2 - 12 . Housing  13 . 2 - 12  is configured to be worn on a user&#39;s head and may sometimes be referred to as a head-mounted housing or head-mounted support structure. Housing  13 . 2 - 12  may have curved head-shaped surfaces, a nose-bridge portion such as portion NB that is configured to rest on a user&#39;s nose when device  13 . 2 - 10  is on a user&#39;s head, a strap  13 . 2 - 12 T for supporting device  13 . 2 - 10  on the user&#39;s head, and/or may have other features that allow device  13 . 2 - 10  to be worn by a user. Housing  13 . 2 - 12  may have walls or other structures that separate an interior region of device  13 . 2 - 10 , such as interior region  13 . 2 - 42 , from an exterior region surrounding device  13 . 2 - 10 , such as exterior region  13 . 2 - 44 . Electrical components  13 . 2 - 40  (e.g., integrated circuits, sensors, control circuitry, light-emitting diodes, lasers, and other light-emitting devices, other control circuits and input-output devices, etc.) may be mounted on printed circuits and/or other structures within device  13 . 2 - 10  (e.g., in interior region  13 . 2 - 42 ). 
     To present a user with images for viewing from eye boxes such as eye box  13 . 2 - 34 , device  13 . 2 - 10  may include rear-facing displays in optical modules  13 . 2 - 16  (sometimes referred to as optical assemblies, etc.). There may be, for example, a left rear-facing display in left optical module  13 . 2 - 16 L for presenting an image through a left lens to a user&#39;s left eye in a left eye box and a right rear-facing display in right optical module  13 . 2 - 16 R for presenting an image through a right lens to a user&#39;s right eye in a right eye box. 
     The user&#39;s eyes are located in eye boxes  13 . 2 - 34  at rear side R of device  13 . 2 - 10  when inwardly facing surface  13 . 2 - 18  of housing  13 . 2 - 12  rests against the outer surface of the user&#39;s face. On rear side R, housing  13 . 2 - 12  may have cushioned structures (sometimes referred to as light seal structures) to enhance user comfort as surface  13 . 2 - 18  rests against the user&#39;s face. 
     Device  13 . 2 - 10  may have forward-facing components such has forward-facing cameras  13 . 2 - 20  on front side F that face outwardly away from the user. Cameras  13 . 2 - 20  may generally be oriented in the +Y direction of  FIGS.  13 . 2 - 1   . If desired, the left-hand camera may face slightly to the left and the right-hand camera may face slightly to the right to enhance the overall coverage of cameras  13 . 2 - 20 . During operation, images captured by cameras  13 . 2 - 20  (sometimes referred to as pass-through video and/or pass-through images) and/or computer-generated content such as text, graphics, etc. may be displayed for the user by displays in modules  13 . 2 - 16 . 
       FIGS.  13 . 2 - 2    is a cross-sectional top view of an optical module  13 . 2 - 16 . As shown in  FIGS.  13 . 2 - 2   , optical module  13 . 2 - 16  may include display  13 . 2 - 14  and lens  13 . 2 - 22  mounted in optical module housing  13 . 2 - 24 . Display  13 . 2 - 14  presents images in eye box  13 . 2 - 34 . 
       FIGS.  13 . 2 - 3 A  is a cross-sectional top view of a head-mounted device  13 . 2 - 10  with a removable cushion  13 . 2 - 60  in its unattached (removed) state. As shown in  FIGS.  13 . 2 - 3 A , head-mounted support structure  13 . 2 - 12  may include a rigid front structure  13 . 2 - 52  (sometimes referred to as front portion, rigid structure, etc.) at the front side F of the head-mounted device  13 . 2 - 10 . Head-mounted support structure  13 . 2 - 12  additionally includes support posts  13 . 2 - 54  (sometimes referred to as rigid support posts, rigid sidewall portions, side portions, sidewall portions, support structures, etc.). The support structures  13 . 2 - 54  extend from the front portion  13 . 2 - 52  towards the rear side R. The support structures  13 . 2 - 54  may couple the front portion  13 . 2 - 52  to a flexible rear portion  13 . 2 - 56  (sometimes referred to as flexible structure, flexible ring, flexible light seal structure, etc.) of the head-mounted support structure  13 . 2 - 12 . Interior  13 . 2 - 42  (with associated optical modules  13 . 2 - 16 L/ 13 . 2 - 16 R and other electronic components) is formed in the volume between front structure  13 . 2 - 52  and rear structure  13 . 2 - 56 . 
     In some examples, a sidewall extending from front structure  13 . 2 - 52  in the −Y-direction may be formed continuously around the perimeter of the front structure  13 . 2 - 52 . In some examples, to minimize the weight of the head-mounted support structure  13 . 2 - 12 , discrete support posts  13 . 2 - 54  may be dispersed around the perimeter of the front structure  13 . 2 - 52 . The support posts are each coupled between the rigid front structure  13 . 2 - 52  and the flexible rear structure  13 . 2 - 56 . Air-filled gaps (or gaps filled with another desired light filler material) may separate adjacent support posts  13 . 2 - 54 . 
     Front structure  13 . 2 - 52  may be an opaque structure that extends across the entire front side of device  13 . 2 - 10 . Alternatively, front structure  13 . 2 - 52  may be a ring-shaped structure that extends in a ring around the front side of device  13 . 2 - 10 . Flexible rear structure  13 . 2 - 56  may be a ring-shaped structure that extends in a ring around some or all of the rear side of device  13 . 2 - 10 . Flexible rear structure  13 . 2 - 56  may be ring-shaped (with a central opening) to allow the user to view optical modules  13 . 2 - 16  while device  13 . 2 - 10  is worn by the user. 
     Head-mounted support structure  13 . 2 - 12  may optionally be covered on one or more sides by a textile layer  13 . 2 - 64  (sometimes referred to as fabric layer, etc.). Textile layer  13 . 2 - 64  may be formed from an opaque or light-shielding material (e.g., black yarn) or may formed from an underlying material coated with an opaque or light-shielding material (e.g., black dye or ink). The textile layer  13 . 2 - 64  may be formed by a woven fabric or a nonwoven fabric. As examples, the textile layer  13 . 2 - 64  may be formed from any suitable type of fabric such as knit fabric, woven fabric, braided fabric, etc. 
     In addition to head-mounted support structure  13 . 2 - 12 , the head-mounted device includes a removable cushion  13 . 2 - 60  (sometimes referred to as a removable cushion member, removable foam, removable foam member, cushion, foam, etc.). Removable cushion  13 . 2 - 60  includes a cushion member  13 . 2 - 58  (sometimes referred to as cushion, cushion structure, foam member, foam structure, foam, etc.) that is optionally covered by a textile layer  13 . 2 - 62 . Cushion member  13 . 2 - 58  may be formed from foam or another desired compressible material. Textile layer  13 . 2 - 62  may be formed from an opaque or light-shielding material (e.g., black yarn) or may formed from an underlying material coated with an opaque or light-shielding material (e.g., black dye or ink). The textile layer  13 . 2 - 62  may be formed by a woven fabric or a nonwoven fabric. As examples, the textile layer  13 . 2 - 62  may be formed from any suitable type of fabric such as knit fabric, woven fabric, braided fabric, etc. 
     The removable cushion may be attached to flexible structure  13 . 2 - 56  of head-mounted support structure  13 . 2 - 12 .  FIGS.  13 . 2 - 3 B  is a cross-sectional top view of a head-mounted device  13 . 2 - 10  with a removable cushion  13 . 2 - 60  attached to an associated head-mounted support structure  13 . 2 - 12 . As shown, when attached, the removable cushion  13 . 2 - 60  may be adjacent to flexible rear structure  13 . 2 - 56 . Flexible structure  13 . 2 - 56  and removable cushion  13 . 2 - 60  may combine to form light seal structures  13 . 2 - 66  for the head-mounted device  13 . 2 - 10 . Flexible structure  13 . 2 - 56  may therefore sometimes be referred to as a light seal structure, flexible light seal structure, non-removable light seal structure, etc. Removable cushion  13 . 2 - 60  may sometimes be referred to as a light seal structure, cushioning light seal structure, removable light seal structure, etc. 
     When head-mounted device  13 . 2 - 10  is worn on the head of a user, the light seal structures  13 . 2 - 66  may conform to the user&#39;s face. To provide an immersive experience to the viewer and preserve high contrast in the display viewed by the user during operation, it may be desirable to block ambient light from reaching the user&#39;s eyes. This ensures the only light viewed by the user is from the display in the head-mounted device  13 . 2 - 10 . Light seal structures  13 . 2 - 66  may be sufficiently flexible and/or compressible to conform to a user&#39;s face during operation, preventing stray light from entering the head-mounted device  13 . 2 - 10 . 
     Flexible light seal structure  13 . 2 - 56  may be formed from a flexible plastic structure. The flexible plastic structure may be sufficiently flexible to conform to the shape of a user&#39;s face when worn by the user. However, the flexible plastic structure may have sufficient rigidity to hold its shape while being worn (e.g., and does not bias towards or away from the user&#39;s face while being worn). Flexible light seal structure  13 . 2 - 56  may not be compressible (e.g., may be less compressible than cushion  13 . 2 - 60 ). Removable cushion  13 . 2 - 60  may also be sufficiently flexible to conform to the shape of a user&#39;s face when worn by the user. Cushion  13 . 2 - 60  may be compressible. The compressible cushion may be pressed against the user&#39;s face to form a tight light seal while still remaining comfortable for the user. 
     There are many advantages to using a removable cushion as shown in  FIGS.  13 . 2 - 3 A and  13 . 2 - 3 B  in the head-mounted device  13 . 2 - 10  (as opposed to a permanently fixed cushion). Because the cushion  13 . 2 - 60  contacts the user&#39;s face during operation, the cushion  13 . 2 - 60  may tend to become dirty over time. If the cushion  13 . 2 - 60  is not removable, it may be difficult to clean the cushion  13 . 2 - 60 . However, the removable cushion  13 . 2 - 60  is easy to remove from the head-mounted device  13 . 2 - 10  to wash. The removable cushion  13 . 2 - 60  therefore is easy to keep clean during repeated uses. The removable cushion  13 . 2 - 60  may be formed using materials that will not be damaged by water to promote easy cleaning. The removable cushion  13 . 2 - 60  may be formed using materials that dry quickly so that the removable cushion  13 . 2 - 60  is ready for use quickly after cleaning. 
     In addition to making the cushion  13 . 2 - 60  easier to clean, the removability of the cushion  13 . 2 - 60  makes the cushion  13 . 2 - 60  easier to replace. If the cushion  13 . 2 - 60  becomes worn down over time or damaged, the removable cushion  13 . 2 - 60  may simply be replaced with a new removable cushion  13 . 2 - 60 . This extends the lifetime of the head-mounted device  13 . 2 - 10 . 
     The removable cushion  13 . 2 - 60  may also be customizable. If the cushion  13 . 2 - 60  is not removable, the properties of the cushion in the head-mounted device  13 . 2 - 10  are fixed. With a removable cushion  13 . 2 - 60 , an optimal removable cushion  13 . 2 - 60  out of multiple options may be selected by a user. For example, the user may select a removable cushion  13 . 2 - 60  with a desired color and/or dimensions. In this way, the user may select a custom removable cushion  13 . 2 - 60  that is the most comfortable for them. 
     There are numerous dimensions of the removable cushion  13 . 2 - 60  that may be customized for different users. One such dimension is the cushion&#39;s thickness  13 . 2 - 92  (e.g., the dimension parallel to the Y-direction) as shown in  FIGS.  13 . 2 - 3 B . Some users may prefer a thicker cushion  13 . 2 - 60  for maximum comfort whereas other users may prefer a thinner cushion  13 . 2 - 60  for maximum comfort. 
     The cushion thickness  13 . 2 - 92  may also impact the eye relief of the optical system. The eye relief is defined as the distance  13 . 2 - 94  between an optical module and a respective eye box that views that optical module. The thickness  13 . 2 - 92  of the removable cushion  13 . 2 - 60  therefore influences the eye relief distance  13 . 2 - 94 . A thicker removable cushion  13 . 2 - 60  will result in a larger associated eye relief distance whereas a thinner removable cushion  13 . 2 - 60  will result in a smaller associated eye relief distance. 
     In some examples, depending on the characteristics of a user&#39;s eye (e.g., a user&#39;s glasses prescription), a specific eye relief may be preferable. For example, a first user may not require glasses. For the first user, a first eye relief distance may be optimal. A second user may have a glasses prescription. The optical module may be customized for the user&#39;s glasses prescription (e.g., by adding removable lenses to the optical module). In this scenario, for the second user, a second eye relief distance that is different than the first eye relief distance may be optimal. In this situation, the first user may use a first removable cushion having a first thickness (that produces the first eye relief distance) whereas the second user may use a second removable cushion having a second thickness that is different than the first thickness (and that produces the second eye relief distance). 
     Thickness  13 . 2 - 92  may be any desired magnitude (e.g., between 1.5 millimeters and 2.5 millimeters, between 2.5 millimeters and 3.5 millimeters, between 3.5 millimeters and 4.5 millimeters, between 1 millimeter and 5 millimeters, between 1 millimeter and 10 millimeters, etc.). 
     The removable cushion selected by a user may therefore have a thickness  13 . 2 - 92  that is optimized for their comfort and/or optimizes eye relief distance for the system. 
     Instead of (or in addition to) customizing the thickness (or other dimensions) of the removable cushion, a user may customize the material of the removable cushion. For example, some users may prefer a softer foam in the removable cushion whereas other users may prefer a harder foam in the removable cushion. Different options for the removable cushions may therefore have the same dimensions but be formed from different materials. 
       FIGS.  13 . 2 - 4    is a perspective view of head-mounted support structure  13 . 2 - 12  (without the removable cushion  13 . 2 - 60 ). As shown, front structure  13 . 2 - 52  covers the entire front side of the head-mounted device  13 . 2 - 10  (e.g., front structure  13 . 2 - 52  does not have a central opening). In contrast, flexible rear structure  13 . 2 - 56  is ring-shaped with a central opening (e.g., the flexible rear structure  13 . 2 - 56  defines a central opening). The user may view optical modules  13 . 2 - 16  through the central opening while using the head-mounted device. As shown in  FIGS.  13 . 2 - 4   , support posts  13 . 2 - 54  connect the flexible rear structure  13 . 2 - 56  to rigid front structure  13 . 2 - 52 . Interior volume  13 . 2 - 42  (with associated optical modules  13 . 2 - 16  and other electronic components) is formed between front structure  13 . 2 - 52  and rear structure  13 . 2 - 56 . Any desired number of support structures may be included in the device. The support structures may be distributed around the periphery of the device to ensure that the central opening with optical modules  13 . 2 - 16  is not blocked. 
     Each support structure may optionally be attached to the flexible structure  13 . 2 - 56  using a pivoting portion with a spherical joint. The spherical joint may allow for the flexible structure  13 . 2 - 56  to be pressed against the user&#39;s face at any desired angle, allowing the flexible structure  13 . 2 - 56  to conform to the user&#39;s face as much as possible (promoting a tight light seal). 
     It should be noted that head-mounted support structure  13 . 2 - 12  in  FIGS.  13 . 2 - 4    may also be covered by textile layer  13 . 2 - 64 , similar to as shown and discussed in connection with  FIGS.  13 . 2 - 3 A . The textile layer is omitted from  FIGS.  13 . 2 - 4    so as to not obfuscate the drawing. 
       FIGS.  13 . 2 - 5 A  is a rear view of flexible structure  13 . 2 - 56 . As shown, flexible structure  13 . 2 - 56  may have a ring-shape that extends partially or completely in a ring around a central opening  13 . 2 - 70 . The flexible structure  13 . 2 - 56  may have a gap along the lower edge to accommodate the nose of the user during operation. If desired, an optional nasal-region-mounted structure  13 . 2 - 72  that is configured to rest on the nose of a user may bridge the gap between the first and second ends of the flexible structure. Structure  13 . 2 - 72  may conform to the facial topology of the user around the nasal region and block light from entering the head-mounted device during operation. Alternatively, nasal-region-mounted structure  13 . 2 - 72  may be omitted and flexible structure  13 . 2 - 56  may form a complete ring around the central opening such that a portion of the flexible structure  13 . 2 - 56  rests on the nose of the user during operation of the head-mounted device  13 . 2 - 10 . 
     The flexible structure  13 . 2 - 56  be attached to support posts  13 . 2 - 54  at various points around the flexible structure.  FIGS.  13 . 2 - 5 A  shows an example where four support posts  13 . 2 - 54  are coupled to the flexible structure. As previously mentioned, each support post may be coupled to the flexible structure with a spherical joint to accommodate the different curvatures of different users&#39; faces. 
       FIGS.  13 . 2 - 5 B  is a rear view of removable cushion  13 . 2 - 60 . As shown, removable cushion  13 . 2 - 60  may have a ring-shape that extends partially or completely in a ring around a central opening  13 . 2 - 74 . The cushion  13 . 2 - 60  may have a gap along the lower edge to accommodate the nose of the user during operation. If desired, an optional nasal-region-mounted structure  13 . 2 - 76  that is configured to rest on the nose of a user may bridge the gap between the first and second ends of the removable cushion. Structure  13 . 2 - 76  may conform to the facial topology of the user around the nasal region and block light from entering the head-mounted device during operation. Alternatively, nasal-region-mounted structure  13 . 2 - 76  may be omitted and cushion  13 . 2 - 60  may form a complete ring around the central opening such that a portion of the cushion  13 . 2 - 60  rests on the nose of the user during operation of the head-mounted device. 
     Although it is generally desirable for cushion  13 . 2 - 60  to be compressible (for user comfort and a tight light seal), cushion  13 . 2 - 60  may include one or more high-rigidity portions  13 . 2 - 60 - 1 . The high-rigidity portions  13 . 2 - 60 - 1  may provide sufficient structural integrity to prevent cushion  13 . 2 - 60  from being totally compressed. This may, for example, prevent a user&#39;s face from striking internal electronic components of the head-mounted device and/or flexible structure  13 . 2 - 56  during an impact event. High-rigidity portions  13 . 2 - 60 - 1  are distributed across cushion  13 . 2 - 60 . Low-rigidity portions  13 . 2 - 60 - 2  are interposed in the remaining areas of the cushion (e.g., between each adjacent pair of high-rigidity portions  13 . 2 - 60 - 1 ). 
     The high-rigidity portions  13 . 2 - 60 - 1  have a greater rigidity and a lower compressibility than low-rigidity portions  13 . 2 - 60 - 2 . The high-rigidity portions  13 . 2 - 60 - 1  may be formed from a different material than low-rigidity portions  13 . 2 - 60 - 2 . In other words, cushion structure  13 . 2 - 58  (see  FIGS.  13 . 2 - 3 A and  13 . 2 - 3 B ) includes high-rigidity portions  13 . 2 - 60 - 1  formed from a first material and low-rigidity portions  13 . 2 - 60 - 2  formed from a second material that is different than the first material. As another example, the entire cushion structure  13 . 2 - 58  may be formed from the same base material. However, high-rigidity portions  13 . 2 - 60 - 1  may include an additive (e.g., a second material) that increases rigidity relative to low-rigidity portions  13 . 2 - 60 - 2 . 
     As shown in  FIGS.  13 . 2 - 5 B , the cushion  13 . 2 - 60  may have approximately the same footprint as flexible rear structure  13 . 2 - 56  from  FIGS.  13 . 2 - 5 A  (when viewed from the rear). Additionally, high-rigidity portions  13 . 2 - 60 - 1  may be positioned to overlap support posts  13 . 2 - 54  (in the Y-direction) when the removable cushion  13 . 2 - 60  is attached to flexible rear structure  13 . 2 - 56 . In other words, each high-rigidity portion  13 . 2 - 60 - 1  is aligned (in the Y-direction) with a respective portion of flexible structure  13 . 2 - 56  that is attached to a support post. Each high-rigidity portion  13 . 2 - 60 - 1  is therefore aligned (in the Y-direction) with a respective support post. This type of arrangement may ensure that the high-rigidity portions  13 . 2 - 60 - 1  provide the target structural integrity for cushion  13 . 2 - 60  while minimizing detectability to the user. The presence of support posts  13 . 2 - 54  make flexible structure  13 . 2 - 56  more rigid in these locations. Therefore, positioning high-rigidity portions  13 . 2 - 60 - 1  in the same location consolidates the high-rigidity portions. 
       FIGS.  13 . 2 - 5 B  shows another dimension  13 . 2 - 96  of the removable cushion that may be customized. Dimension  13 . 2 - 96  may be referred to as the height (e.g., in the Z-direction) of the cushion along an upper edge of the cushion. The upper edge of the cushion may be adjacent to the user&#39;s forehead during use and may therefore sometimes be referred to as a forehead portion of the cushion. Some users may prefer a larger upper-edge height  13 . 2 - 96  than others for maximum comfort. A user may therefore select a removable cushion  13 . 2 - 60  that has an optimal upper-hedge height  13 . 2 - 96  for their needs. Height  13 . 2 - 96  may be greater than 3 millimeters, greater than 5 millimeters, greater than 10 millimeters, greater than 15 millimeters, greater than 20 millimeters, greater than 30 millimeters, greater than 50 millimeters, less than 3 millimeters, less than 5 millimeters, less than 10 millimeters, less than 15 millimeters, less than 20 millimeters, less than 30 millimeters, less than 50 millimeters, between 5 millimeters and 30 millimeters, etc. 
     Dimension  13 . 2 - 98  (as shown in  FIGS.  13 . 2 - 5 B ) may also be customized. Dimension  13 . 2 - 98  may be referred to as the total height (e.g., in the Z-direction) of the cushion. The total height may influence factors such as the separation between the upper edge of the cushion and a user&#39;s eyebrow. Some users may prefer greater separation between the upper edge of the cushion and the user&#39;s eyebrow than others for maximum comfort. A user may therefore select a removable cushion  13 . 2 - 60  that has an optimal height  13 . 2 - 98  for their needs. Height  13 . 2 - 98  may be greater than 5 centimeters, greater than 8 centimeters, greater than 10 centimeters, greater than 15 centimeters, greater than 20 centimeters, less than 8 centimeters, less than 10 centimeters, less than 15 centimeters, less than 20 centimeters, between 5 centimeters and 15 centimeters, etc. 
     Head-mounted support structure  13 . 2 - 12  and/or removable cushion  13 . 2 - 60  may include attachment structures that are used to secure removable cushion  13 . 2 - 60  to the head-mounted support structure.  FIGS.  13 . 2 - 6 A  is a rear view of flexible rear structure  13 . 2 - 56  (of head-mounted support structure  13 . 2 - 12 ) showing how the flexible rear structure may include one or more primary attachment structures  13 . 2 - 82 P and, optionally, one or more auxiliary attachment structures  13 . 2 - 82 A.  FIGS.  13 . 2 - 6 B  is a rear view of removable cushion  13 . 2 - 60  showing how the removable cushion may include one or more primary attachment structures  13 . 2 - 84 P and, optionally, one or more auxiliary attachment structures  13 . 2 - 84 A. 
     Each primary attachment structure  13 . 2 - 82 P in flexible rear structure  13 . 2 - 56  may be configured to attach to a corresponding primary attachment structure  13 . 2 - 84 P in removable cushion  13 . 2 - 60 . Similarly, each auxiliary attachment structure  13 . 2 - 82 A in flexible rear structure  13 . 2 - 56  may be configured to attach to a corresponding auxiliary attachment structure  13 . 2 - 84 A in removable cushion  13 . 2 - 60 . As shown in  FIGS.  13 . 2 - 6 A , each primary attachment structure  13 . 2 - 82 P in flexible rear structure  13 . 2 - 56  may align in the Y-direction with a corresponding primary attachment structure  13 . 2 - 84 P in removable cushion  13 . 2 - 60  and each auxiliary attachment structure  13 . 2 - 82 A in flexible rear structure  13 . 2 - 56  may align in the Y-direction with a corresponding auxiliary attachment structure  13 . 2 - 84 A in removable cushion  13 . 2 - 60 . 
     A wide range of attachment structures may be used for attachment structures  13 . 2 - 82 P,  13 . 2 - 84 P,  13 . 2 - 82 A, and  13 . 2 - 84 A. The attachment structures may include protrusions, recesses, grooves, posts, magnets, flexible fabric, hooks, loops, snaps, buttons, suction cups, a draw string, a zipper, adhesive (e.g., tape), a flexible band, or any other desired type of attachment structure. 
     As some specific examples of possible attachment structures, primary attachment structures  13 . 2 - 82 P on flexible structure  13 . 2 - 56  may be protrusions that are configured to mate (interlock) with corresponding recesses ( 13 . 2 - 84 P) on cushion  13 . 2 - 60 . Primary attachment structures  13 . 2 - 82 P on flexible structure  13 . 2 - 56  may be magnets (e.g., permanent magnets) that are configured to magnetically couple with corresponding magnets ( 13 . 2 - 84 P) on cushion  13 . 2 - 60 . Primary attachment structures  13 . 2 - 82 P and  13 . 2 - 84 P may form hook-and-loop fasteners (e.g., with hooks on structures  13 . 2 - 56  and loops on cushion  13 . 2 - 60  or vice versa). Primary attachment structures  13 . 2 - 82 P and  13 . 2 - 84 P may form snap fasteners. The first halves of the snaps may be formed on structures  13 . 2 - 56  and may have recesses and the second halves of the snaps may be formed on cushion  13 . 2 - 60  and may have protrusions with grooves that snap into place when pressed into the recesses (or vice versa). As yet another example, primary attachment structures  13 . 2 - 82 P on flexible structure  13 . 2 - 56  may be grooves that are configured to mate (interlock) with corresponding posts ( 13 . 2 - 84 P) on cushion  13 . 2 - 60 . As yet another example, primary attachment structures  13 . 2 - 82 P on flexible structure  13 . 2 - 56  may be posts. Cushion  13 . 2 - 60  may have a layer (ring) of flexible fabric that is configured to be stretched around the posts. As yet another example, rear flexible structure  13 . 2 - 56  may include an attached fabric channel. The removable cushion is then placed in the fabric channel during operation. 
     The specific example of a locations for primary attachment structures  13 . 2 - 82 P and  13 . 2 - 84 P in  FIGS.  13 . 2 - 6 A and  13 . 2 - 6 B  are merely illustrative. In general, primary attachment structures may be included at any desired locations. 
     The primary attachment structures may be the primary mechanism by which the removable cushion is secured onto the head-mounted support structure. However, in order to provide additional attachment strength and/or to ensure a tight light seal, auxiliary attachment structures  13 . 2 - 82 A and  13 . 2 - 84 A may also be included. As shown in  FIGS.  13 . 2 - 6 B , auxiliary attachment structures  13 . 2 - 82 A and  13 . 2 - 84 A may be included along the left and right edges of the rear flexible structure and removable cushion. The rear flexible structure and removable cushion may commonly have high degrees of curvature in these areas when the head-mounted device is in operation (due to the curvature of the user&#39;s face). These high degrees of curvature may make the cushion and rear flexible structure susceptible to light leakage (e.g., through a gap between the cushion and rear flexible structure when the cushion and rear flexible structure are bent to conform to the user&#39;s face). The auxiliary attachment structures  13 . 2 - 82 A and  13 . 2 - 84 A in these high-risk areas may ensure a tight seal around the entire perimeter of the removable cushion and the rear flexible structure. The specific example of a locations for auxiliary attachment structures  13 . 2 - 82 A and  13 . 2 - 84 A in  FIGS.  13 . 2 - 6 A and  13 . 2 - 6 B  are merely illustrative. In general, auxiliary attachment structures may be included at any desired locations (e.g., adjacent/between any desired primary attachment structures). 
     Auxiliary attachment structures  13 . 2 - 82 A and  13 . 2 - 84 A may form hook-and-loop fasteners (e.g., with hooks on structures  13 . 2 - 56  and loops on cushion  13 . 2 - 60  or vice versa). Auxiliary attachment structures  13 . 2 - 82 A and  13 . 2 - 84 A may alternatively form snap fasteners. The first halves of the snaps may be formed on structures  13 . 2 - 56  and may have recesses and the second halves of the snaps may be formed on cushion  13 . 2 - 60  and may have protrusions with grooves that snap into place when pressed into the recesses (or vice versa). In general, any of the aforementioned attachment structures may be used for the auxiliary attachment structures. 
     If desired, multiple types of attachment structures may be used at each attachment point to ensure a secure attachment between the removable cushion  13 . 2 - 60  and the head-mounted support structure  13 . 2 - 12 .  FIGS.  13 . 2 - 7    is a cross-sectional top view of an illustrative head-mounted device showing how protrusions, recesses, and magnets may be used to secure the removable cushion  13 . 2 - 60  to head-mounted support structure  13 . 2 - 12 . As shown in  FIG.  13 . 2 - 7   , protrusions  13 . 2 - 82 P- 2  extend from flexible rear structure  13 . 2 - 56  in the −Y-direction (e.g., towards the removable cushion  13 . 2 - 60 ). Removable cushion  13 . 2 - 60  has corresponding recesses  13 . 2 - 84 P- 2 . Each recess  13 . 2 - 84 P- 2  may mate with a corresponding protrusion  13 . 2 - 82 P- 2 . 
     In addition to the protrusions and recesses, magnets are also used as attachment structures for the head-mounted device. As shown in  FIGS.  13 . 2 - 7   , magnets  13 . 2 - 82 P- 1  may be embedded in protrusions  13 . 2 - 82 P- 2 . A single magnet may be embedded in each protrusion or multiple magnets may be embedded in each protrusion. A corresponding magnet  13 . 2 - 84 P- 1  may be formed in cushion structure  13 . 2 - 58  adjacent to each respective recess  13 . 2 - 84 P- 2 . When removable structure  13 . 2 - 60  is attached to the head-mounted support structure, protrusions  13 . 2 - 82 P- 2  extend into and mate with recesses  13 . 2 - 84 P- 2 . When the protrusions  13 . 2 - 82 P- 2  and recesses  13 . 2 - 84 P- 2  are mated, magnets  13 . 2 - 82 P- 1  will also be magnetically coupled to magnets  13 . 2 - 84 P- 1 . This provides additional security for the removable cushion attachment and prevents gravity from causing the removable cushion to fall off of the head-mounted support structure. 
     The example of  FIGS.  13 . 2 - 7    is merely illustrative. In addition to the primary attachment structures of protrusions, recesses, and magnets, auxiliary attachment structures such as snaps or hook-and-loop fasteners may also be included to secure the removable cushion to head-mounted support structure  13 . 2 - 12  and ensure a tight light seal. 
     In some examples, protrusions  13 . 2 - 82 P- 2  may be formed integrally with rear flexible structure  13 . 2 - 56  or may be formed from a separate material that is attached to rear flexible structure  13 . 2 - 56 . Alternatively, in the arrangement of  FIGS.  13 . 2 - 7   , the protrusions  13 . 2 - 82 P- 2  are formed integrally with support posts  13 . 2 - 54 . In other words, portions of support posts  13 . 2 - 54  extend through openings in rear flexible structure  13 . 2 - 56  to form attachment structures  13 . 2 - 82 P- 2 . The support posts  13 . 2 - 54  may also extend through openings in textile layer  13 . 2 - 64  (which covers head-mounted support structure  13 . 2 - 12 ). In this away, the support post protrusions  13 . 2 - 82 P- 2  serve as a visual indicator to a user that the head-mounted support structure is not yet attached to a removable cushion. The support post protrusions  13 . 2 - 82 P- 2  also provide recognizable alignment markers that allow the user to easily align and attach the removable cushion to the head-mounted support structure. 
       FIGS.  13 . 2 - 8    is a rear view of an illustrative removable cushion  13 . 2 - 60 . As shown in  FIGS.  13 . 2 - 8   , the removable cushion may have one or more incorporated hinge structures  13 . 2 - 86 . The hinge structures  13 . 2 - 86  may be formed by a highly flexible portion of the cushion member. Alternatively, the foldable portions  13 . 2 - 86  may be formed by a post that is formed separately from the cushion member. Hinge structures  13 . 2 - 86  allow for rotation of portions of the cushion member relative to other portions of the cushion member. Each hinge structure  13 . 2 - 86  may allow for rotation of the cushion member around a respective bend axis  13 . 2 - 88 . The presence of hinge structures  13 . 2 - 86  may therefore allow for the cushion member to be folded to have a smaller footprint than in the unfolded state, increasing the portability of the cushion member. 
     In one possible arrangement, each hinge structure  13 . 2 - 86  may be formed by a post that allows rotation of adjacent cushion portions relative to each other. The post may optionally double as an attachment structure for attaching the removable cushion to the head-mounted support structure. For example, the head-mounted support structure may have a groove and the post (which defines a hinge structure for the cushion) slides into the groove to attach the removable cushion to the head-mounted support structure. 
     The example of two hinge structures being included in the removable cushion is merely illustrative. In general, any desired number of hinge structures may be included in the removable cushion. 
       FIGS.  13 . 2 - 9    is a schematic diagram of a system  13 . 2 - 200  that includes a head-mounted support structure  13 . 2 - 12  (with optical modules  13 . 2 - 16 L/ 13 . 2 - 16 R and other associated electronics at its interior) and numerous removable cushions  13 . 2 - 60 . As previously shown and discussed, one of the removable cushions may be attached to the head-mounted support structure to form a head-mounted device  13 . 2 - 10  with an attached cushion. 
     System  13 . 2 - 200  may include any desired number of removable cushions  13 . 2 - 60 - 1  through  13 . 2 - 60 -N having different properties. As previously discussed, the varying properties of the removable cushions may include color, thickness, upper-edge height, total height, any other desired dimension, the softness of the material, or any other desired characteristic. 
     As specific examples, a user may select a removable cushion having a desired color out of many color options. A user may select a removable cushion having a thickness that optimizes the optical properties (e.g., eye relief) of the head-mounted device. A user may select a removable cushion having a desired upper-edge height to maximize their comfort when wearing the head-mounted device. 
     System  13 . 2 - 200  may include any desired number removable cushions, each with a unique set of properties. As one example, a user may purchase a head-mounted device and receive the head-mounted support structure and multiple removable cushions having various sizes (e.g., thicknesses and/or total heights). The multiple removable cushions may include a small removable cushion, a medium removable cushion that is larger in at least one dimension than the small removable cushion, and a large removable cushion that is larger than the medium removable cushion in at least one dimension. The user may attach their preferred removable cushion to the head-mounted support structure before using the head-mounted device. 
     As another example, a user may select their preferred removable cushion (e.g., preferred color, size, etc.) when purchasing a head-mounted device. The user receives their preferred removable cushion in addition to the head-mounted support structure and may attach their preferred removable cushion to the head-mounted support structure before using the head-mounted device. The removable cushion may then repeatedly be removed and washed during the lifetime of the head-mounted device. 
     In accordance with some examples, a head-mounted device is provided that includes a head-mounted support structure including a rigid structure, a flexible structure, and support structures that couple the flexible structure to the rigid structure, the flexible structure at least partially defines a central opening, left and right optical assemblies configured to display images, the left and right optical assemblies are coupled to the head-mounted support structure, and a removable cushion that is configured to be selectively attached to the flexible structure, the removable cushion at least partially surrounds the central opening. 
     In accordance with some examples, the left and right optical assemblies are viewable through the central opening, the head-mounted support structure is covered by a first textile layer, the removable cushion is covered by a second textile layer, the removable cushion has first portions having a first rigidity and second portions having a second rigidity, the removable cushion has recesses and first magnets, at least one of the first magnets is adjacent to each recess, and the head-mounted support structure includes protrusions at the flexible structure that extend away from the rigid structure, each protrusion is configured to mate with a respective recess of the removable cushion when the removable cushion is attached to the flexible structure, and second magnets, at least one of the second magnets is positioned in each protrusion and the first magnets are configured to magnetically couple to the second magnets when the removable cushion is attached to the flexible structure. 
     In some examples, the left and right optical assemblies are viewable through the central opening. In some examples the head-mounted support structure is covered by a textile layer. In some examples, the support structures are configured to protrude past the flexible structure and through openings in the textile layer. In some examples, the removable cushion has recesses that are configured to receive the protruding portions of the support structures when the removable cushion is attached to the flexible structure. In some examples, first magnets are formed in the removable cushion adjacent to the recesses, second magnets are formed in protruding portions of the support structures, and the first magnets are configured to magnetically couple to the second magnets when the removable cushion is attached to the flexible structure. 
     In accordance with some examples, the flexible structure includes a first auxiliary attachment structure between first and second protruding portions, the removable cushion includes a second auxiliary attachment structure between first and second recesses, and the first auxiliary attachment structure is configured to couple to the second auxiliary attachment structure when the removable cushion is attached to the flexible structure. In some examples, the first and second auxiliary attachment structures include a hook-and-loop fastener. In some examples, the removable cushion includes a cushion that is covered by a textile layer. 
     In accordance with some examples, first magnets are formed in the removable cushion, second magnets are formed in the head-mounted support structure, and the first magnets are configured to magnetically couple to the second magnets when the removable cushion is attached to the flexible structure. In some examples, the removable cushion has first portions having a first rigidity and second portions having a second rigidity that is lower than the first rigidity. In some examples, each one of the first portions is aligned with a respective support structure. In some examples, the removable cushion includes a hinge structure that allows a first portion of the removable cushion to rotate relative to a second portion of the removable cushion. 
     In accordance with some examples, a head-mounted device is provided that includes a head-mounted support structure including a flexible portion that is configured to conform to a face when the head-mounted support structure is worn, a rigid portion that is separated from the flexible portion by a gap, side portions that bridge the gap between the rigid portion and the flexible portion, and protrusions at the flexible portion that extend away from the rigid portion, and a first set of magnets, each one of the first set of magnets is positioned in a respective one of the protrusions, and a removable cushion that is configured to be selectively attached to the flexible portion, the removable cushion is configured to conform to and directly contact the face when the head-mounted support structure is worn while the removable cushion is attached and the removable cushion includes recesses that are configured to mate with the protrusions of the head-mounted support structure, and a second set of magnets that is configured to magnetically couple to the first set of magnets. In some examples, the head-mounted support structure is covered by a textile layer and the protrusions extend through openings in the textile layer. In some examples, the protrusions are formed by the side portions. In some examples, the head-mounted device includes left and right optical assemblies configured to display images, the left and right optical assemblies are formed in an interior volume of the head-mounted support structure. 
     In accordance with some examples, a system is provided that includes a head-mounted support structure, left and right optical assemblies configured to display images, the left and right optical assemblies are coupled to the head-mounted support structure, and a plurality of removable cushions, each one of the removable cushions is configured to be selectively attached to the head-mounted support structure and each one of the plurality of removable cushions has a property that is unique relative to that property in the remaining removable cushions. In some examples, the property includes a property selected from the group consisting of a color, a thickness, a cushion material, a total height, and an upper-edge height. 
     13.3: Electronic Devices with Light-Blocking Fabrics 
     An electronic device, such as a head-mounted device, may have a front face that faces away from a user&#39;s head and may have an opposing rear face that faces the user&#39;s head. The head-mounted device may include a main housing portion with displays that display images and optical modules through which the images are viewable from eye boxes. A face frame that is coupled to the main housing portion may form a light seal around the eye boxes. The light seal may include one or more fabric layers such as knit fabric layers. The light seal may include inner and outer fabric layers, a face portion that rests against the user&#39;s face, and a nose bridge portion that accommodates the user&#39;s nose. The outer fabric layer may be a seamless tube of knit fabric that forms an outermost layer of the light seal. The inner fabric layer may be a light-blocking fabric that lines the inner surface of the seamless tube of knit fabric. The knit fabric may have a modified bird&#39;s eye pattern that incorporates knit stitches and tuck stitches. The tuck stitches may provide the fabric with additional lengthwise stretch so that the light seal fabric does not buckle when the head-mounted device is placed on the user&#39;s head. The tuck stitches also allow for strands of different colors to be visible on the outer surface of the light seal. 
     The light-blocking fabric may include a dark-colored weft knit layer facing the eye box, a light-colored weft knit layer facing the seamless tube of knit fabric, and a middle layer joining the light-colored weft knit layer and the dark-colored weft knit layer. The light-colored weft knit layer may match a color of the seamless tube of knit fabric. The dark-colored weft knit layer may ensure sufficient opacity without being visible through the seamless tube of knit fabric. 
     A top view of an illustrative head-mounted device that may include a fabric light seal is shown in  FIGS.  13 . 3 - 1   . As shown in  FIGS.  13 . 3 - 1   , head-mounted devices such as electronic device  13 . 3 - 10  may have head-mounted support structures such as housing  13 . 3 - 12 . Housing  13 . 3 - 12  may include portions (e.g., support structures  13 . 3 - 12 T) to allow device  13 . 3 - 10  to be worn on a user&#39;s head. Support structures  13 . 3 - 12 T (sometimes referred to as temple housing structures or temple housing portions) may be formed from fabric, polymer, metal, and/or other material. Support structures  13 . 3 - 12 T may form a strap or other head-mounted support structures that help support device  13 . 3 - 10  on a user&#39;s head. Some or all of temple housing portions  13 . 3 - 12 T may overlap a user&#39;s temples when device  13 . 3 - 10  is worn on the user&#39;s head. A main support structure (e.g., main housing portion  13 . 3 - 12 M) of housing  13 . 3 - 12  may support electronic components such as displays  13 . 3 - 14 . Main housing portion  13 . 3 - 12 M may include housing structures formed from metal, polymer, glass, ceramic, and/or other material. For example, housing portion  13 . 3 - 12 M may have housing walls on front face F and housing walls on adjacent top, bottom, left, and right side faces that are formed from rigid polymer or other rigid support structures and these rigid walls may optionally be covered with electrical components, fabric, leather, or other soft materials, etc. The walls of housing portion  13 . 3 - 12 M may enclose internal components  13 . 3 - 38  in interior region  13 . 3 - 34  of device  13 . 3 - 10  and may separate interior region  13 . 3 - 34  from the environment surrounding device  13 . 3 - 10  (exterior region  13 . 3 - 36 ). Internal components  13 . 3 - 38  may include integrated circuits, actuators, batteries, sensors, and/or other circuits and structures for device  13 . 3 - 10 . Housing  13 . 3 - 12  may be configured to be worn on a head of a user and may form glasses, a hat, a helmet, goggles, and/or other head-mounted device. Configurations in which housing  13 . 3 - 12  forms goggles may sometimes be described herein as an example. 
     Front face F of housing  13 . 3 - 12  may face outwardly away from a user&#39;s head and face. Opposing rear face R of housing  13 . 3 - 12  may face the user. Portions of housing  13 . 3 - 12  (e.g., portions of main housing  13 . 3 - 12 M) on rear face R may form a cover such as curtain  13 . 3 - 12 C. In some examples, curtain  13 . 3 - 12 C includes a fabric layer that separates interior region  13 . 3 - 34  from the exterior region to the rear of device  13 . 3 - 10 . Other structures may be used in forming curtain  13 . 3 - 12 C, if desired. The presence of curtain  13 . 3 - 12 C on rear face R may help hide internal housing structures, internal components  13 . 3 - 38 , and other structures in interior region  13 . 3 - 34  from view by a user. 
     Device  13 . 3 - 10  may have left and right optical modules  13 . 3 - 40 . Each optical module may include a respective display  13 . 3 - 14 , lens  13 . 3 - 30 , and support structure  13 . 3 - 32 . Support structures  13 . 3 - 32 , which may sometimes be referred to as lens barrels or optical module support structures, may include hollow cylindrical structures with open ends or other supporting structures to house displays  13 . 3 - 14  and lenses  13 . 3 - 30 . Support structures  13 . 3 - 32  may, for example, include a left lens barrel that supports a left display  13 . 3 - 14  and left lens  13 . 3 - 30  and a right lens barrel that supports a right display  13 . 3 - 14  and right lens  13 . 3 - 30 . Displays  13 . 3 - 14  may include arrays of pixels or other display devices to produce images. Displays  13 . 3 - 14  may, for example, include organic light-emitting diode pixels formed on substrates with thin-film circuitry and/or formed on semiconductor substrates, pixels formed from crystalline semiconductor dies, liquid crystal display pixels, scanning display devices, and/or other display devices for producing images. Lenses  13 . 3 - 30  may include one or more lens elements for providing image light from displays  13 . 3 - 14  to respective eyes boxes  13 . 3 - 13 . Lenses may be implemented using refractive glass lens elements, using mirror lens structures (catadioptric lenses), using holographic lenses, and/or other lens systems. When a user&#39;s eyes are located in eye boxes  13 . 3 - 13 , displays (display panels)  13 . 3 - 14  operate together to form a display for device  13 . 3 - 10  (e.g., the images provided by respective left and right optical modules  13 . 3 - 40  may be viewed by the user&#39;s eyes in eye boxes  13 . 3 - 13  so that a stereoscopic image is created for the user). The left image from the left optical module fuses with the right image from a right optical module while the display is viewed by the user. 
     Not all users have the same interpupillary distance P. To provide device  13 . 3 - 10  with the ability to adjust the interpupillary spacing between modules  13 . 3 - 40  along lateral dimension X and thereby adjust the spacing P between eye boxes  13 . 3 - 13  to accommodate different user interpupillary distances, device  13 . 3 - 10  may be provided with one or more actuators  13 . 3 - 42 . Actuators  13 . 3 - 42  can be manually controlled and/or computer-controlled actuators (e.g., computer-controlled motors) for moving support structures  13 . 3 - 32  relative to each other. 
     As shown in  FIGS.  13 . 3 - 2   , curtain  13 . 3 - 12 C may cover rear face F while leaving lenses  13 . 3 - 30  of optical modules  13 . 3 - 40  uncovered (e.g., curtain  13 . 3 - 12 C may have openings that are aligned with and receive modules  13 . 3 - 40 ). As modules  13 . 3 - 40  are moved relative to each other along dimension X to accommodate different interpupillary distances for different users, modules  13 . 3 - 40  move relative to fixed housing structures such as the walls of main portion  13 . 3 - 12 M and move relative to each other. To prevent undesired wrinkling and buckling of curtain  13 . 3 - 12 C as optical modules  13 . 3 - 40  are moved relative to rigid portions of housing  13 . 3 - 12 M and relative to each other, a fabric layer or other cover layer in curtain  13 . 3 - 12 C may be configured to slide, stretch, open/close, and/or otherwise adjust to accommodate optical module movement. 
     A schematic diagram of an illustrative electronic device such as a head-mounted device or other wearable device is shown in  FIGS.  13 . 3 - 3   . Device  13 . 3 - 10  of  FIGS.  13 . 3 - 3    may be operated as a stand-alone device and/or the resources of device  13 . 3 - 10  may be used to communicate with external electronic equipment. As an example, communications circuitry in device  13 . 3 - 10  may be used to transmit user input information, sensor information, and/or other information to external electronic devices (e.g., wirelessly or via wired connections). Each of these external devices may include components of the type shown by device  13 . 3 - 10  of  FIGS.  13 . 3 - 3   . 
     As shown in  FIGS.  13 . 3 - 3   , a head-mounted device such as device  13 . 3 - 10  may include control circuitry  13 . 3 - 20 . Control circuitry  13 . 3 - 20  may include storage and processing circuitry for supporting the operation of device  13 . 3 - 10 . The storage and processing circuitry may include storage such as nonvolatile memory (e.g., flash memory or other electrically-programmable-read-only memory configured to form a solid state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Processing circuitry in control circuitry  13 . 3 - 20  may be used to gather input from sensors and other input devices and may be used to control output devices. The processing circuitry may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors and other wireless communications circuits, power management units, audio chips, application specific integrated circuits, etc. During operation, control circuitry  13 . 3 - 20  may use display(s)  13 . 3 - 14  and other output devices in providing a user with visual output and other output. 
     To support communications between device  13 . 3 - 10  and external equipment, control circuitry  13 . 3 - 20  may communicate using communications circuitry  13 . 3 - 22 . Circuitry  13 . 3 - 22  may include antennas, radio-frequency transceiver circuitry, and other wireless communications circuitry and/or wired communications circuitry. Circuitry  13 . 3 - 22 , which may sometimes be referred to as control circuitry and/or control and communications circuitry, may support bidirectional wireless communications between device  13 . 3 - 10  and external equipment (e.g., a companion device such as a computer, cellular telephone, or other electronic device, an accessory such as a point device, computer stylus, or other input device, speakers or other output devices, etc.) over a wireless link. For example, circuitry  13 . 3 - 22  may include radio-frequency transceiver circuitry such as wireless local area network transceiver circuitry configured to support communications over a wireless local area network link, near-field communications transceiver circuitry configured to support communications over a near-field communications link, cellular telephone transceiver circuitry configured to support communications over a cellular telephone link, or transceiver circuitry configured to support communications over any other suitable wired or wireless communications link. Wireless communications may, for example, be supported over a Bluetooth® link, a WiFi® link, a wireless link operating at a frequency between 10 GHz and 400 GHz, a 60 GHz link, or other millimeter wave link, a cellular telephone link, or other wireless communications link. Device  13 . 3 - 10  may, if desired, include power circuits for transmitting and/or receiving wired and/or wireless power and may include batteries or other energy storage devices. For example, device  13 . 3 - 10  may include a coil and rectifier to receive wireless power that is provided to circuitry in device  13 . 3 - 10 . 
     Device  13 . 3 - 10  may include input-output devices such as devices  13 . 3 - 24 . Input-output devices  13 . 3 - 24  may be used in gathering user input, in gathering information on the environment surrounding the user, and/or in providing a user with output. Devices  13 . 3 - 24  may include one or more displays such as display(s)  13 . 3 - 14 . Display(s)  13 . 3 - 14  may include one or more display devices such as organic light-emitting diode display panels (panels with organic light-emitting diode pixels formed on polymer substrates or silicon substrates that contain pixel control circuitry), liquid crystal display panels, microelectromechanical systems displays (e.g., two-dimensional mirror arrays or scanning mirror display devices), display panels having pixel arrays formed from crystalline semiconductor light-emitting diode dies (sometimes referred to as microLEDs), and/or other display devices. 
     Sensors  13 . 3 - 16  in input-output devices  13 . 3 - 24  may include force sensors (e.g., strain gauges, capacitive force sensors, resistive force sensors, etc.), audio sensors such as microphones, touch and/or proximity sensors such as capacitive sensors such as a touch sensor that forms a button, trackpad, or other input device), and other sensors. If desired, sensors  13 . 3 - 16  may include optical sensors such as optical sensors that emit and detect light, ultrasonic sensors, optical touch sensors, optical proximity sensors, and/or other touch sensors and/or proximity sensors, monochromatic and color ambient light sensors, image sensors, fingerprint sensors, iris scanning sensors, retinal scanning sensors, and other biometric sensors, temperature sensors, sensors for measuring three-dimensional non-contact gestures (“air gestures”), pressure sensors, sensors for detecting position, orientation, and/or motion (e.g., accelerometers, magnetic sensors such as compass sensors, gyroscopes, and/or inertial measurement units that contain some or all of these sensors), health sensors such as blood oxygen sensors, heart rate sensors, blood flow sensors, and/or other health sensors, radio-frequency sensors, depth sensors (e.g., structured light sensors and/or depth sensors based on stereo imaging devices that capture three-dimensional images), optical sensors such as self-mixing sensors and light detection and ranging (LIDAR) sensors that gather time-of-flight measurements, humidity sensors, moisture sensors, gaze tracking sensors, electromyography sensors to sense muscle activation, facial sensors, and/or other sensors. In some arrangements, device  13 . 3 - 10  may use sensors  13 . 3 - 16  and/or other input-output devices to gather user input. For example, buttons may be used to gather button press input, touch sensors overlapping displays can be used for gathering user touch screen input, touch pads may be used in gathering touch input, microphones may be used for gathering audio input, accelerometers may be used in monitoring when a finger contacts an input surface and may therefore be used to gather finger press input, etc. 
     If desired, electronic device  13 . 3 - 10  may include additional components (see, e.g., other devices  13 . 3 - 18  in input-output devices  13 . 3 - 24 ). The additional components may include haptic output devices, actuators for moving movable housing structures, audio output devices such as speakers, light-emitting diodes for status indicators, light sources such as light-emitting diodes that illuminate portions of a housing and/or display structure, other optical output devices, and/or other circuitry for gathering input and/or providing output. Device  13 . 3 - 10  may also include a battery or other energy storage device, connector ports for supporting wired communication with ancillary equipment and for receiving wired power, and other circuitry. 
       FIGS.  13 . 3 - 4    is a perspective view of device  13 . 3 - 10  showing how a face frame may form a light seal around eye boxes  13 . 3 - 13  ( FIGS.  13 . 3 - 1   ) to help prevent outside light from leaking into the viewing area of head-mounted device  13 . 3 - 10 . As shown in  FIGS.  13 . 3 - 4   , device  13 . 3 - 10  may include main housing portion  13 . 3 - 12 M which is configured to be mounted on a user&#39;s head. To help block outside light (e.g., ambient light in the user&#39;s environment that is not emitted by displays  13 . 3 - 14  of device  13 . 3 - 10 ) from entering the viewing area of head-mounted device  13 . 3 - 10  where eye boxes  13 . 3 - 13  are located, a light seal such as light seal  13 . 3 - 52  may be formed between main housing portion  13 . 3 - 12 M and the user&#39;s face. For example, light seal  13 . 3 - 52  may extend between main housing portion  13 . 3 - 12 M and temple housing portions  13 . 3 - 12 T ( FIGS.  13 . 3 - 1   ) to help prevent light from entering any gaps between device  13 . 3 - 10  and the user&#39;s face. 
     Light seal  13 . 3 - 52  (sometimes referred to as face frame  13 . 3 - 52 ) may include one or more rigid structures such as a rigid internal frame member or other stiff structure and one or more flexible materials such as fabric, foam, polymer, or other suitable materials. For example, light seal  13 . 3 - 52  may include a ring-shaped or horseshoe-shaped frame that surrounds eye boxes  13 . 3 - 13  ( FIGS.  13 . 3 - 1   ) and that is covered by one or more layers of fabric. As shown in  FIGS.  13 . 3 - 4   , light seal  13 . 3 - 52  may include one or more different fabric layers such as outer fabric layer  13 . 3 - 72 , inner fabric layer  13 . 3 - 58 , nose bridge fabric  13 . 3 - 60 , and face fabric  13 . 3 - 56 . Face fabric  13 . 3 - 56  may rest against the user&#39;s face when device  13 . 3 - 10  is worn on the user&#39;s head. Face fabric  13 . 3 - 56  may include one or more layers of foam covered in one or more layers of fabric (e.g., a warp knit fabric, a weft knit fabric, a spacer fabric, a woven fabric, and/or any other suitable fabric). Nose bridge fabric  13 . 3 - 60  may be formed from a stretchable textile to accommodate different nose shapes. 
     Outer fabric layer  13 . 3 - 72  may be a seamless tube of fabric that loops around the optical axes A of lenses  13 . 3 - 30  of optical modules  13 . 3 - 40 . The optical axis A of each lens  13 . 3 - 30  extends parallel to the Y-direction of  FIGS.  13 . 3 - 4   . The lengthwise direction L of outer fabric layer  13 . 3 - 72  may extend parallel to the optical axes A of lens  13 . 3 - 30  (and thus parallel to the Y-axis of  FIGS.  13 . 3 - 4   ), whereas the widthwise direction W of outer fabric layer  13 . 3 - 72  may extend perpendicular to the optical axes A of lens  13 . 3 - 30  of optical modules  13 . 3 - 40 . In a weft knit fabric, the rows of loops (referred to as courses) extend in the width direction W, while the columns of loops (referred to as wales) extend in the length direction L. 
     Outer fabric layer  13 . 3 - 72  may form an outermost surface of device  13 . 3 - 10 , if desired. Outer layer  13 . 3 - 72  may be formed from fabric such as knit fabric (e.g., warp knit fabric, weft knit fabric, etc.), woven fabric, spacer fabric (e.g., inner and outer knit layers separated by a gap and joined by a spacer layer such as a monofilament strand), braided fabric, and/or any other suitable fabric. In some examples, outer fabric layer  13 . 3 - 72  is a stretchable, seamless tube of weft knit fabric having a bird&#39;s eye pattern or other suitable two-color pattern (as an example). Arrangements in which outer layer  13 . 3 - 72  is formed from non-fabric materials such as polymer, silicone, or elastomer may also be used. Arrangements in which outer fabric layer  13 . 4 - 72  of light seal  13 . 4 - 52  is formed from a stretchable fabric are sometimes described herein as an example. 
     If desired, outer fabric layer  13 . 3 - 72  may be formed from weft knit fabric with a bird&#39;s eye pattern that uses two types of strands such as strands with a first property (e.g., a first color, material, fuzziness, denier, elasticity, etc.) and strands with a second property (e.g., a second color, material, fuzziness, denier, elasticity, etc.). The two types of strands may be visible on the outer surface of outer fabric layer  13 . 3 - 72  to provide the outer surface of outer fabric layer  13 . 3 - 72  with color variation, material variation, texture variation, etc. For example, the color variation on the outer surface of outer fabric layer  13 . 3 - 72  may form a checker pattern, a stripe pattern, a diamond pattern, a grid pattern, a dot pattern, or any other suitable pattern alternating between a first color and a second color (e.g., black and white, white and grey, grey and black, etc.). 
     If care is not taken, outer fabric layer  13 . 3 - 72  may not have sufficient stretch in the length direction L, which in turn may cause outer fabric layer  13 . 3 - 72  to buckle (e.g., wrinkle) when face seal  13 . 3 - 52  is pressed against the user&#39;s face. To avoid buckling in outer fabric layer  13 . 3 - 72 , outer fabric layer  13 . 3 - 72  may incorporate one or more tuck stitches. The tuck stitches may be used in place of miss stitches (sometimes referred to as float stitches) to add stretch in length direction L. Tuck stitches allow outer fabric layer  13 . 3 - 72  to have a bird&#39;s eye pattern (to allow for color variation across the outer surface of outer fabric layer  13 . 3 - 72 ) while providing additional stretch in lengthwise direction L of outer fabric layer  13 . 3 - 72 . 
     Inner fabric layer  13 . 3 - 58  may be a light-blocking fabric that lines the interior surface of outer fabric layer  13 . 3 - 72  of light seal  13 . 3 - 52 . Inner fabric layer  13 . 3 - 58  may be formed from one or more opaque (e.g., black) fabric layers. Inner fabric layer  13 . 3 - 58  may include one or more layers of knit fabric, warp knit fabric, weft knit fabric, woven fabric, spacer fabric, braided fabric, and/or any other suitable type of fabric. It may be desirable to use dark-colored fabric for inner fabric layer  13 . 3 - 58  to help keep the viewing area around eye boxes  13 . 3 - 13  sufficiently dark while the user is viewing images on displays  13 . 3 - 14  of device  13 . 3 - 10 . If care is not taken, however, the dark color of inner fabric layer  13 . 3 - 58  may be visible through outer fabric layer  13 . 3 - 72 . If outer fabric layer  13 . 3 - 72  is a light color such as white or gray and inner fabric layer  13 . 3 - 58  is a dark color such as black, for example, the black color of inner fabric layer  13 . 3 - 58  may be visible through openings or gaps between strands in outer fabric layer  13 . 3 - 72 , which may be visually distracting. If inner fabric layer  13 . 3 - 58  is instead made from a light-colored fabric such as white or gray, inner fabric layer  13 . 3 - 58  may not be as visible through outer fabric layer  13 . 3 - 72 , but the opacity of inner fabric layer  13 . 3 - 58  may be reduced, which can lead to an unsatisfactory viewing experience if care is not taken. 
     To prevent inner fabric layer  13 . 3 - 58  from being overly visible through outer fabric layer  13 . 3 - 72  without compromising opacity, inner fabric layer  13 . 3 - 58  may include one or more dark-colored inner fabric layers on the viewing side (e.g., facing eye boxes  13 . 3 - 13  and lenses  13 . 3 - 30  of optical modules  13 . 3 - 40 ) and one or more light-colored outer fabric layers on the non-viewing side (e.g., facing outer fabric layer  13 . 3 - 72 ). The dark-colored inner fabric layer of inner fabric layer  13 . 3 - 58  may be black, dark gray, or other suitable dark color, whereas the light-colored outer fabric layer of inner fabric layer  13 . 3 - 58  may be white, gray, light gray, cream, off-white, or other suitable light color. The dark inner layer and light outer layer of inner fabric layer  13 . 3 - 58  may be weft knit layers that are coupled by a middle spacer layer, if desired. For example, a spacer layer formed from a multifilament or monofilament strand may be used to join the dark inner layer and light outer layer of inner fabric layer  13 . 3 - 58 . The spacer layer may be a drawn textured yarn with a low denier value such as 20D or other suitable denier value to ensure that the spacer layer has sufficient density to block outside light from entering the viewing area. If desired, strands that make up inner fabric layer  13 . 3 - 58  may be extruded, solution-dyed strands with high texture and high stretch, thereby increasing the shrinkage and opacity of inner fabric layer  13 . 3 - 58 . 
     A knitting machine or other equipment may be used in forming fabric for device  13 . 3 - 10  such as inner fabric layer  13 . 3 - 58 .  FIGS.  13 . 3 - 5 A  is a schematic diagram of an illustrative knitting system. As shown in  FIGS.  13 . 3 - 5 A , strand source  13 . 3 - 66  in knitting system  13 . 3 - 64  may be used in supplying strands  13 . 3 - 68  to guide and needle structures  13 . 3 - 70 . Structures  13 . 3 - 70  may include strand guide structures (e.g., a system of movable guide bars with eyelets that guide strands  13 . 3 - 68 ) and needle systems (e.g., needle guide systems that guide sets of individually adjustable needles so that the needles may interact with the strands dispensed by the guide bars). During operations, a controller may control electrically adjustable positioners in system  13 . 3 - 64  to manipulate the positions of guide bars and needles in system  13 . 3 - 64  and thereby knit strands  13 . 3 - 68  into fabric  13 . 3 - 58 . Take down  13 . 3 - 74  (e.g., a pair of mating rollers or other equipment forming a takedown system) may be used to gather fabric  13 . 3 - 58  that is produced during knitting. 
     A knitting machine or other equipment may be used in forming fabric for device  13 . 4 - 10  such as outer fabric layer  13 . 4 - 72 .  FIGS.  13 . 3 - 5 B  is a schematic diagram of an illustrative knitting system. As shown in  FIGS.  13 . 3 - 5 B , strand source  13 . 4 - 66  in knitting system  13 . 4 - 64  may be used in supplying strands  13 . 4 - 68  to guide and needle structures  13 . 4 - 70 . Structures  13 . 4 - 70  may include strand guide structures (e.g., a system of movable guide bars with eyelets that guide strands  13 . 4 - 68 ) and needle systems (e.g., needle guide systems that guide sets of individually adjustable needles so that the needles may interact with the strands dispensed by the guide bars). During operations, a controller may control electrically adjustable positioners in system  13 . 4 - 64  to manipulate the positions of guide bars and needles in system  13 . 4 - 64  and thereby knit strands  13 . 4 - 68  into fabric  13 . 4 - 72 . Take down  13 . 4 - 74  (e.g., a pair of mating rollers or other equipment forming a takedown system) may be used to gather fabric  13 . 4 - 72  that is produced during knitting. 
       FIGS.  13 . 3 - 6    is a diagram of a layer of a weft knit fabric that may be included in inner fabric layer  13 . 3 - 58 . Knit fabric such as fabric  13 . 3 - 58  may be made up of courses  13 . 3 - 78  (e.g., rows of loops formed by strands  13 . 3 - 68 ) and wales  13 . 3 - 76  (e.g., columns of loops formed by strands  13 . 3 - 68 ). In a weft knit fabric of the type shown in  FIGS.  13 . 3 - 6    (sometimes referred to as a flat knit fabric), strands  13 . 3 - 68  form loops that extend horizontally across the fabric. An illustrative strand  13 . 3 - 68 ′ among strands  13 . 3 - 68  has been highlighted to show the horizontal path taken by each strand  13 . 3 - 68  in fabric  13 . 3 - 72 . In contrast, a warp knit fabric includes wales  13 . 3 - 76  formed from strands  13 . 3 - 68  that follow zig-zag paths vertically down the fabric. 
     A cross-sectional side view of a portion of light seal  13 . 3 - 52  of device  13 . 3 - 10  is illustrated in  FIGS.  13 . 3 - 7   . As shown in  FIGS.  13 . 3 - 7   , light seal  13 . 3 - 52  may include inner fabric layer  13 . 3 - 58  and outer fabric layer  13 . 3 - 72 . Outer fabric layer  13 . 3 - 72  may form an outermost surface of light seal  13 . 3 - 52  and may be viewed in direction  13 . 3 - 82  by an outside viewer such as viewer  13 . 3 - 80 . Inner fabric layer  13 . 3 - 58  may line the inner surface of outer fabric layer  13 . 3 - 72  and may face towards eye boxes such as eye box  13 . 3 - 13 . When device  13 . 3 - 10  is placed on a user&#39;s head, the user&#39;s eyes may be located in eye boxes such as eye box  13 . 3 - 13 . 
     Inner fabric layer  13 . 3 - 58  may have inner and outer fabric layers such as light-colored outer fabric layer  13 . 3 - 58 L and dark-colored inner fabric layer  13 . 3 - 58 D coupled by a spacer layer such as spacer layer  13 . 3 - 58 M. Light-colored outer fabric layer  13 . 3 - 58 L may be white, off-white, cream, gray, light gray, or other suitable light color. Light-colored outer fabric layer  13 . 3 - 58 L may, for example, match a color of outer fabric layer  13 . 3 - 72 . Dark-colored inner fabric layer  13 . 3 - 58 D may be black, dark gray, dark blue, or other suitable dark color. Dark-colored inner fabric layer  13 . 3 - 58 D and light-colored outer fabric layer  13 . 3 - 58 L may be weft knit layers (e.g., of the type shown in  FIGS.  13 . 3 - 6   ), and may be formed from strands  13 . 3 - 68  (e.g., drawn textured yarns) having a denier value of 40D (or two-ply 20D strands) or other suitable denier value. 
     Spacer layer  13 . 3 - 58 M (sometimes referred to as middle layer, etc.) may be formed from one or more multifilament strands  13 . 3 - 68  with a low denier value such as 20D, 25D, or other suitable denier value. Spacer layer  13 . 3 - 58 M may be formed from black strands, white strands, gray strands, or strands of any other suitable color. By selecting strands  13 . 3 - 68  for spacer layer  13 . 3 - 58 M that have half the denier value of strands  13 . 3 - 68  of light-colored inner fabric layer  13 . 3 - 58 L and dark-colored fabric layer  13 . 3 - 58 D, fabric  13 . 3 - 58  may appear truly white (or other suitable light color) when viewed in direction  13 . 3 - 82 ) and may appear truly black (or other suitable dark color) when viewed from eye box  13 . 3 - 13 ), regardless of the color of spacer layer  13 . 3 - 58 M. 
     Strands in light-colored inner fabric layer  13 . 3 - 58 L, dark-colored outer fabric layer  13 . 3 - 58 D, and spacer layer  13 . 3 - 58 M may be formed from materials such as nylon, polyester, elastic material such as spandex, or other suitable materials. In general, nylon is bulkier than polyester due to less intermingling points and intermingling intensity among individual filaments. If polyester strands are used, the polyester may be modified to reduce intermingling between filaments such that the filaments in each polyester strand spread out more and thus block more light. 
     Inner fabric layer  13 . 3 - 58  may be formed using a circular knitting machine. The circular knitting machine may have a high gauge such as 80 gauge or other suitable gauge to ensure a higher number of strands per inch in the fabric. The higher number of strands per inch in fabric  13 . 3 - 58 , the higher the opacity will be. For example, fabric  13 . 3 - 58  may have more than 80 strands per inch, more than 90 strands per inch, more than 100 strands per inch, less than 100 strands per inch, or other suitable density of strands  13 . 3 - 68 . Additionally, fabric  13 . 3 - 58  may include stretchable strands (e.g., strands formed from spandex or other elastic material) to add shrinkage to fabric  13 . 3 - 58  and further increase the opacity of light seal  13 . 3 - 52 . Fabric  13 . 3 - 58  may be heat set at a low heat setting (or not heat set at all) to help maintain opacity and stretch. 
       FIGS.  13 . 3 - 8    is a perspective view of inner fabric layer  13 . 3 - 58 . As shown in  FIGS.  13 . 3 - 8   , inner fabric layer  13 . 3 - 58  may include dark-colored inner fabric layer  13 . 3 - 58 D facing eye box  13 . 3 - 13  and light-colored outer fabric layer  13 . 3 - 58 L facing outside viewers such as viewer  13 . 3 - 80  viewing light seal  13 . 3 - 52  in direction  13 . 3 - 82 . Spacer layer  13 . 3 - 58 M may include one or more multifilament strands such as multifilament strand  13 . 3 - 68 M coupled between dark-colored inner fabric layer  13 . 3 - 58 D and light-colored outer fabric layer  13 . 3 - 58 L. Spacer layer  13 . 3 - 58 M may be a low denier yarn (e.g., 20D, 25D, or other suitable denier value) to increase fabric density and minimize the gap G between dark-colored inner fabric layer  13 . 3 - 58 D and light-colored outer fabric layer  13 . 3 - 58 L. 
     Strands  13 . 3 - 68 D of dark-colored inner fabric layer  13 . 3 - 58 D and strands  13 . 3 - 68 L of light-colored outer fabric layer  13 . 3 - 58 L may have twice the denier value of spacer strand  13 . 3 - 68 M (e.g., strands  13 . 3 - 68 M may have a denier value of 20D, whereas strands  13 . 3 - 68 D and  13 . 3 - 68 L may have a denier value of 40D or may be two-ply strands with each ply having a denier value of 20D) such that fabric  13 . 3 - 58  appears truly white (or other light color) when viewed in direction  13 . 3 - 82  by viewer  13 . 3 - 80  and truly black (or other dark color) when viewed from eye box  13 . 3 - 13 . In general, strands  13 . 3 - 68 L,  13 . 3 - 68 D, and  13 . 3 - 68 M may be formed from any suitable material, such as nylon, polyester, and spandex or other elastic material, etc. If desired, strands  13 . 3 - 68 D and  13 . 3 - 68 L may be two-ply strands that each include an elastic strand having a denier value of 20D and a polyester strand having a denier value of 20D. 
       FIGS.  13 . 3 - 9    is a cross-sectional side view of light seal  13 . 3 - 52  in an example in which outer fabric layer  13 . 3 - 72  has been omitted. With this type of arrangement, light-colored outer fabric layer  13 . 3 - 58 L may form the outermost surface of light seal  13 . 3 - 52  and may be viewed directly by viewer  13 . 3 - 80  in direction  13 . 3 - 82 . Due to the presence of middle layer  13 . 3 - 58 M, dark-colored fabric  13 . 3 - 58 D may not be viewable through light-colored outer fabric layer  13 . 3 - 58 L. In other words, fabric layer  13 . 3 - 58  may appear truly white (or other light color) when viewed in direction  13 . 3 - 82  and truly black (or other dark color) when viewed from eye box  13 . 3 - 13 . 
     13.4: Electronic Devices with Stretchable Fabrics 
       FIGS.  13 . 4 - 6 ,  13 . 4 - 7 , and  13 . 4 - 8    show illustrative types of stitches that may be incorporated into fabric  13 . 4 - 72  to adjust the amount of stretch in the fabric. 
     In the example of  FIGS.  13 . 4 - 6   , fabric  13 . 4 - 72  is a weft knit fabric made up of courses  13 . 4 - 78  (e.g., rows of loops formed by strands  13 . 4 - 68 ) and wales  13 . 4 - 76  (e.g., columns of loops formed by strands  13 . 4 - 68 ). In a weft knit fabric of the type shown in  FIGS.  13 . 4 - 6    (sometimes referred to as a flat knit fabric), strands  13 . 4 - 68  form loops that extend horizontally across the fabric. An illustrative strand  13 . 4 - 68 ′ among strands  13 . 4 - 68  has been highlighted to show the horizontal path taken by each strand  13 . 4 - 68  in fabric  13 . 4 - 72 . In contrast, a warp knit fabric includes wales  13 . 4 - 76  formed from strands  13 . 4 - 68  that follow zig-zag paths vertically down the fabric. 
     The example of  FIGS.  13 . 4 - 6    shows courses (rows)  13 . 4 - 78  that are made up entirely of knit stitches. This is sometimes referred to as a plain jersey or single jersey fabric. If desired, color variations may be incorporated into fabric  13 . 4 - 72  by incorporating miss stitches and/or float stitches. A miss stitch is illustrated in  FIGS.  13 . 4 - 7   . 
     As shown in  FIGS.  13 . 4 - 7   , fabric  13 . 4 - 72  may include courses  13 . 4 - 78  such as first course  13 . 4 - 78 - 1 , second course  13 . 4 - 78 - 2 , third course  13 . 4 - 78 - 3 , and fourth course  13 . 4 - 78 - 4 . First course  13 . 4 - 78 - 1 , third course  13 . 4 - 78 - 3 , and fourth course  13 . 4 - 78 - 4  each include a series of knit stitches  13 . 4 - 80 . The legs of each knit stitch loop are connected to the head of the previous loop in the previous course. Second course  13 . 4 - 78 - 2  incorporates miss stitch  13 . 4 - 84 . To create a miss stitch such as miss stitch  13 . 4 - 84  during knitting, a given strand is not collected by a needle, which causes the strand to float behind the needle while remaining connected to the loops on either side of miss stitch  13 . 4 - 84 . If the strands of courses  13 . 4 - 78  include different colors, miss stitch  13 . 4 - 84  will create a dual color pattern on each side of the fabric. 
     If desired, outer fabric layer  13 . 4 - 72  may have a bird&#39;s eye pattern that incorporates alternating knit stitches  13 . 4 - 80  and miss stitches  13 . 4 - 84 . For example, outer fabric layer  13 . 4 - 72  may have a four row repeat pattern. In the first and second rows, the odd-numbered wales  13 . 4 - 76  may be knit stitches  13 . 4 - 80 , while the even-numbered wales  13 . 4 - 76  may be tuck stitches  13 . 4 - 84 . In the third and fourth rows, the odd-numbered wales  13 . 4 - 76  may be tuck stitches  13 . 4 - 84 , while the even-numbered wales  13 . 4 - 76  maybe knit stitches  13 . 4 - 80 . This pattern may repeat to create a two-color bird&#39;s eye pattern on the outer surface of outer fabric layer  13 . 4 - 72 . 
     As shown in  FIGS.  13 . 4 - 7   , miss stitches  13 . 4 - 84  extend in the width direction W of fabric  13 . 4 - 72  and float horizontally across the fabric. If additional stretch is desired in lengthwise direction L of fabric  13 . 4 - 72 , some or all of the miss stitches  13 . 4 - 84  may be replaced by tuck stitches. This type of arrangement is illustrated in  FIGS.  13 . 4 - 8   . 
     As shown in  FIGS.  13 . 4 - 8   , fabric  13 . 4 - 72  may include courses  13 . 4 - 78  such as first course  13 . 4 - 78 - 1 , second course  13 . 4 - 78 - 2 , and third course  13 . 4 - 78 - 3 . First course  13 . 4 - 78 - 1 , second course  13 . 4 - 78 - 2 , and third course  13 . 4 - 78 - 3  may each include alternating knit stitches  13 . 4 - 80  and tuck stitches  13 . 4 - 82 . Tuck stitches  13 . 4 - 82  may be created by placing two strands  13 . 4 - 68  on a single needle, thereby tucking the extra strand  13 . 4 - 68  behind the first strand  13 . 4 - 68 . The legs of tuck stitch  13 . 4 - 82  are not connected to the head of the previous loop. For example, as shown in  FIGS.  13 . 4 - 8   , the legs of each tuck stitch  13 . 4 - 82  in course  13 . 4 - 78 - 3  are not connected to head  13 . 4 - 84  of the previous loop in course  13 . 4 - 78 - 2 . 
     If desired, outer fabric layer  13 . 4 - 72  may have a bird&#39;s eye pattern that incorporates alternating knit stitches  13 . 4 - 80  and tuck stitches  13 . 4 - 82 . For example, outer fabric layer  13 . 4 - 72  may have a four-row-repeat-pattern of the type shown in  FIGS.  13 . 4 - 9   . 
     As shown in  FIGS.  13 . 4 - 9   , the odd numbered wales of the first course may be tuck stitches  13 . 4 - 82  (represented as a “V” in  FIGS.  13 . 4 - 9   ), whereas the even numbered wales of the first course may be knit stitches  13 . 4 - 80  (represented as an “O” in  FIGS.  13 . 4 - 9   ). The second course may be all knit stitches  13 . 4 - 80 . The odd numbered wales of the third course may be knit stitches  13 . 4 - 80 , whereas the even numbered wales of the second course may be tuck stitches  13 . 4 - 82 . The fourth course may be all knit stitches  13 . 4 - 80 . This pattern may repeat to create a two-color modified bird&#39;s eye pattern on the outer surface of outer fabric layer  13 . 4 - 72 . 
     The incorporation of tuck stitches  13 . 4 - 82  provides additional stretch in the lengthwise direction L because tuck stitches  13 . 4 - 82  have strand segments that extend in the L direction to form the tuck stitch. The longer stitch length in length direction L from incorporating one or more tuck stitches  13 . 4 - 82  allows the seamless tube of fabric  13 . 4 - 72  to stretch and shrink in direction L without buckling or wrinkling when device  13 . 4 - 10  is placed on the user&#39;s head and light seal  13 . 4 - 52  is pressed against the user&#39;s face. If desired, strands  13 . 4 - 68  of fabric  13 . 4 - 72  may include strands of first and second colors to create a two-color pattern on the outside of light seal  13 . 4 - 52 . This is merely illustrative, however. If desired, more than two colors may be used or strands  13 . 4 - 68  may have other distinct properties (e.g., different deniers, different amounts of fuzziness, different textures, different diameters, different materials, etc.). 
     If desired, fabric  13 . 4 - 72  may be placed in boiling water after fabric  13 . 4 - 72  is formed. The post-processing step of boiling fabric  13 . 4 - 72  may cause additional shrinkage in the length direction L of fabric  13 . 4 - 72 , which in turn reduces the tendency of light seal  13 . 4 - 52  to buckle when pressed against the user&#39;s face. The boiling step may, for example, result in more shrinkage in length direction L of fabric  13 . 4 - 72  than in width direction W of fabric  13 . 4 - 72 . Additionally, the increased stretch in the length direction L of modified bird&#39;s eye fabric  13 . 4 - 72  results in greater frame mobility and deflection (e.g., light seal  13 . 4 - 52  may be more mobile and deflect more easily when covered by fabric  13 . 4 - 72  having increased lengthwise stretch), which can significantly increase user comfort. 
     The pattern of  FIGS.  13 . 4 - 8  and  13 . 4 - 9    is merely illustrative. If desired, fabric  13 . 4 - 72  may include both miss stitches  13 . 4 - 84  and tuck stitches  13 . 4 - 82 , depending on the amount of stretch desired in length direction L. For example, if more or maximum stretch in lengthwise direction L is desired, fabric  13 . 4 - 72  may be free of miss stitches  13 . 4 - 84  and may follow the four-row-repeat-pattern of  FIGS.  13 . 4 - 9   . 
     13.5: Contactless Sensors for a Head-Mountable Device 
     The following disclosure relates to a facial interface in a head-mountable device. More particularly, the present embodiments relate to a facial interface that contains sensorially transparent materials for head-mountable devices used for AR/VR experiences. These facial interfaces can enable sensors to interact with a user through sensorially transparent materials. As used herein, the term “sensorially transparent materials” include materials that allow for the transfer of sensor signals therethrough. 
     In one example, the head-mountable device of the present disclosure includes a display and a light seal portion (hereafter “light seal”). Light seals enable a user to experience a light shielded environment, where outside ambient light and possibly other environmental items, are blocked form the user field of view. The shielded environment allows for better user interaction and a more immersive experience. The light seal, as a facial interface, can be customized to a user&#39;s facial profile such that the light seal physically interacts with the user&#39;s face to fit snugly on or around the forehead, eyes, nose, and other features or bones, such as the maxilla regions, that can vary user to user. Additionally, a light seal can include components connecting the display to the facial interface, such as a webbing, housing or a frame positioned between the display and facial interface. 
     Conventional light seals of conventional head-mountable devices are passive and do not include a facial interface with sensorially transparent materials. Indeed, passive light seals create a light shielded environment, but do not include active component integration to enable contactless readings from sensors embedded in a facial interface. Therefore, conventional light seals do not provide contactless readings of a user via sensorially transparent materials and sensors. 
     By contrast, a light seal of the present disclosure includes a facial interface with sensorially transparent materials for active component integration. A light seal with active components has advantages over a traditional passive light seal. A light seal with sensorially transparent materials can include active components that can monitor user responses without direct contact, lending to improved user comfort while wearing the head-mountable device. Sensors configured in this contactless manner can also avoid biological ingress from user skin (e.g., lotion, makeup, sunscreen, etc.). A head-mountable device that monitors such user responses can also create a highly customized user experience (unlike the sensors of conventional head-mountable devices that are “blind” to the user experience). 
     Sensors can be important for creating a customized user experience. An active light seal can contain sensors to measure a user&#39;s response or engagement via indicators, such as core body temperature, sweat, heart rate, electrical signals from the heart (e.g., ECG, EKG, EXG, etc.), brain activity (e.g., EEG signals, frontal lobe activity), etc. Additionally, the sensor data can be used as feedback data, for example, to monitor user fatigue or obtain activity-specific metrics. 
     Sensors of the present disclosure can be implemented on or within a facial interface in myriad different ways. For example, a sensor can be oriented towards a user and positioned on a facial interface surface opposite of the surface that contacts a user. In another example, a sensor can be oriented towards a user and embedded inside the facial interface. In these or other examples, the sensor can include a field of view that projects towards a user and through at least a portion of the facial interface. Thus, such portions of the facial interface can be sensorially transparent to allow the sensor to obtain a sensor reading through at least a portion of the facial interface. 
     Sensors can also be implemented in different ways for different facial interfaces. That is, the head-mountable device of the present disclosure can implement a facial interface with a base layer and an interchangeable layer. The interchangeable layer can be exchanged or swapped out for a different interchangeable layer. In some examples, the different interchangeable layer can correspond to a different user activity, such as a yoga activity versus a movie-watching activity. In certain implementations, the yoga interchangeable layer can include a different sensor arrangement than the movie-watching interchangeable layer (e.g., for obtaining different, activity-specific metrics). 
     These and other embodiments are discussed below with reference to  FIGS.  13 . 5 - 1 - 13 . 5 - 7 B . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these figures is for explanatory purposes only and should not be construed as limiting. Furthermore, as used herein, a system, a method, an article, a component, a feature, or a sub-feature comprising at least one of a first option, a second option, or a third option should be understood as referring to a system, a method, an article, a component, a feature, or a sub-feature that can include one of each listed option (e.g., only one of the first option, only one of the second option, or only one of the third option), multiple of a single listed option (e.g., two or more of the first option), two options simultaneously (e.g., one of the first option and one of the second option), or combination thereof (e.g., two of the first option and one of the second option). 
       FIGS.  13 . 5 - 1    illustrates a top view profile of a head-mountable device  13 . 5 - 100  worn on a user head. The head-mountable device  13 . 5 - 100  can include a display  13 . 5 - 102  (e.g., one or more optical lenses or display screens in front of the eyes of the user). The display  13 . 5 - 102  can include a display for presenting an augmented reality visualization, a virtual reality visualization, or other suitable visualization. 
     The head-mountable device  13 . 5 - 100  also includes a facial interface  13 . 5 - 103  and a sensor  13 . 5 - 108  positioned (e.g., attached to or embedded within) on the facial interface  13 . 5 - 103 . As used herein, the terms “facial interface” or “engagement interface” refer to a portion of the head mountable device  13 . 5 - 100  that engages a user face via direct contact. In particular, a facial interface includes portions of the head-mountable device  13 . 5 - 100  that conform to (e.g., compress against) regions of a user face. To illustrate, a facial interface can include a pliant (or semi-pliant) facetrack that spans the forehead, wraps around the eyes, contacts the zygoma and maxilla regions of the face, and bridges the nose. In addition, a facial interface can include various components forming a structure, webbing, cover, fabric, or frame of a head-mountable device disposed between the display  13 . 5 - 102  and the user skin. In particular implementations, a facial interface can include a seal (e.g., a light seal, environment seal, dust seal, air seal, etc.). It will be appreciated that the term “seal” can include partial seals or inhibitors, in addition to complete seals (e.g., a partial light seal where some ambient light is blocked and a complete light seal where all ambient light is blocked when the head-mountable device is donned). 
     In addition, the term “sensor” refers to one or more different sensing devices, such as a camera or imaging device, temperature device, oxygen device, movement device, brain activity device, sweat gland activity device, breathing activity device, muscle contraction device, etc. Some particular examples of sensors include an electrooculography sensor, electrocardiogra sensor, EKG sensor, heart rate variability sensor, blood volume pulse sensor, SpO2 sensor, compact pressure sensor, electromyography sensor, core-body temperature sensor, galvanic skin sensor, accelerometer, gyroscope, magnetometer, inclinometer, barometer, infrared sensor, global positioning system sensor, etc. 
     In one example, the head mountable device  13 . 5 - 100  includes a sensor controller  13 . 5 - 104 . The sensor controller  13 . 5 - 104  can include a processor (e.g., a system on chip, integrated circuit, driver, microcontroller, application processor, crossover processor, etc.). Further the sensor controller  13 . 5 - 104  can include one or more memory devices (e.g., individual nonvolatile memory, processor-embedded nonvolatile memory, random access memory, memory integrated circuits, DRAM chips, stacked memory modules, storage devices, memory partitions, etc.). In certain implementations, the sensor controller  13 . 5 - 104  is positioned within one or both arms  13 . 5 - 105 ,  13 . 5 - 106  of the head-mountable device  13 . 5 - 100  (e.g., for integration with an HMD processor/memory component). In alternative implementations, the sensor controller  13 . 5 - 104  is physically integrated within the sensors  13 . 5 - 108  themselves. 
     The sensor controller  13 . 5 - 104  can perform myriad different functions. For example, the memory device can store computer-executable instructions that, when executed by the processor, cause the sensor controller  13 . 5 - 104  to receive sensor data from the sensors  13 . 5 - 108  and transmit a signal based on the sensor data. For instance, the sensor controller  13 . 5 - 104  can transmit a sensor signal to the display  13 . 5 - 102 . In response to the sensor signal, the display  13 . 5 - 102  can power off, present a digital notification (e.g., user-generated notification, push notification, context-generated notification, system-generated notification, smart notification, etc.), or render at least one of an avatar or an avatar emotional response. As used herein, the term “avatar” is a visual representation of a person for use in digital context, such as with the head mountable device  13 . 5 - 100 . An avatar can include animated characters, animals, objects, emojis, etc. that can depict human emotion (e.g., as detected via the sensors  13 . 5 - 108  of the head-mountable device  13 . 5 - 100 ). The depiction of human emotion through an avatar constitutes an avatar emotional response. 
     Additionally shown in  FIGS.  13 . 5 - 1   , the head-mountable device  13 . 5 - 100  includes one or more arms  13 . 5 - 105 ,  13 . 5 - 106 . The arms  13 . 5 - 105 ,  13 . 5 - 106  are connected to the display  13 . 5 - 102  and extend distally toward the rear of the head. The arms  13 . 5 - 105 ,  13 . 5 - 106  are configured to secure the display  13 . 5 - 102  in a position relative to the head (e.g., such that the display  13 . 5 - 102  is maintained in front of a user&#39;s eyes). For example, the arms  13 . 5 - 105 ,  13 . 5 - 106  extend over the user&#39;s ears  13 . 5 - 107 . In certain examples, the arms  13 . 5 - 105 ,  13 . 5 - 106  rest on the user&#39;s ears  13 . 5 - 107  to secure the head-mountable device  13 . 5 - 100  via friction between the arms  13 . 5 - 105 ,  13 . 5 - 106  and the head. Additionally, or alternatively, the arms  13 . 5 - 105 ,  13 . 5 - 106  can rest against the head. For example, the arms  13 . 5 - 105 ,  13 . 5 - 106  can apply opposing pressures to the sides of the head to secure the head-mountable device  13 . 5 - 100  to the head. Optionally, the arms  13 . 5 - 105 ,  13 . 5 - 106  can be connected to each other via a strap (shown in dashed lines) that can compress the head-mountable device  13 . 5 - 100  against the head). 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 5 - 1    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 5 - 1   . 
       FIGS.  13 . 5 - 2 A- 13 . 5 - 2 B  respectively illustrate a side view and front view profiles of an example of the head mountable device  13 . 5 - 100 . As discussed above, the head mountable device  13 . 5 - 100  includes the display  13 . 5 - 102 , the facial interface  13 . 5 - 103 , the sensor controller  13 . 5 - 104 , and the sensors  13 . 5 - 108 . Indeed, at least one sensor  13 . 5 - 108  is positioned on or within the facial interface  13 . 5 - 103 . Additionally, the facial interface  13 . 5 - 103  can wrap around eyes  13 . 5 - 201  and bridge a nose  13 . 5 - 202  of a user. The head-mountable device  13 . 5 - 100  can also include connections  13 . 5 - 206  that movably constrain the display  13 . 5 - 102  and the facial interface  13 . 5 - 103  (e.g., at forehead and cheek regions of a user face). Examples of the connections  13 . 5 - 206  include a pivot connection, spring connection, etc. 
     The sensors  13 . 5 - 108  can be positioned in myriad different configurations. In one example, at least one of the sensors  13 . 5 - 108  is positioned in a pliant region  13 . 5 - 212  of the facial interface  13 . 5 - 103  between the connections  13 . 5 - 206 . A “pliant region” refers to the portion(s) of the facial interface  13 . 5 - 103  disposed between the connections  13 . 5 - 206 , where the facial interface  13 . 5 - 103  is more flexible and conformable. In certain implementations, one or more of the sensors  13 . 5 - 108  is positioned in a middle portion of the pliant region  13 . 5 - 212  (approximately equidistant from the connections  13 . 5 - 206 ). By positioning one or more of the sensors  13 . 5 - 108  within the pliant region  13 . 5 - 212 , pressure points felt by the user can be mitigated. Additionally, or alternatively, the sensors  13 . 5 - 108  can be positioned in certain configurations, depending on the desired location (on the user) to be sensed (e.g., a forehead region, an eye region, a nasal region, etc.). 
     Further, the term “forehead region” refers to the anatomical area of a human head between the eyes and scalp. Additionally, the term “nasal region” refers to the anatomical area of a human nose. 
     Also shown in  FIGS.  13 . 5 - 2 A- 13 . 5 - 2 B , the head-mountable device  13 . 5 - 100  can include a power source  13 . 5 - 203 . In some examples, the power source  13 . 5 - 203  can include one or more electrochemical cells with connections for powering electrical devices. For example, in some examples, the power source  13 . 5 - 203  includes a lithium-ion battery, alkaline battery, carbon zinc battery, lead-acid battery, nickel-cadmium battery, nickel-metal hydride battery, etc. It will therefore be appreciated that the power source  13 . 5 - 203  can be dispensable or rechargeable, as may be desired. In certain implementations, the power source  13 . 5 - 203  is connected to the sensor controller  13 . 5 - 104  via one or more electrical connections. In some examples, though not required, the power source  13 . 5 - 203  is mounted to the sensor controller  13 . 5 - 104 . 
     The head-mountable device  13 . 5 - 100  can also include an interface  13 . 5 - 210 , which can be electromechanical or wireless. The interface  13 . 5 - 210  can communicatively couple the sensors  13 . 5 - 108  to at least one of the power source  13 . 5 - 203 , the sensor controller  13 . 5 - 104 , or an HMD processor/memory component (not shown). 
     In some examples, the sensor  13 . 5 - 108  can connect to the sensor controller  13 . 5 - 104  (or the HMD processor/memory component, not shown) via certain wireless communication protocols, such as via a wireless local area network protocol, wireless area network protocol, wireless personal area network protocol, wide area protocol, etc. Some particular examples of wireless communication via such protocols include a Wi-Fi based communication, mesh network communication, BlueTooth® communication, near-field communication, low-energy communication, Zigbee communication, Z-wave communication, and 6LoWPAN communication. In a particular implementation, the sensor  13 . 5 - 108  is communicatively coupled to the sensor controller  13 . 5 - 104  (or the HMD processor/memory component, not shown via a wireless 60 GHz frequency. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 5 - 2 A- 13 . 5 - 2 B  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 5 - 2 A- 13 . 5 - 2 B . 
     As discussed above, sensors can be disposed on or within the facial interface of the present disclosure. In accordance with one or more such examples,  FIGS.  13 . 5 - 3    illustrates one of the sensors  13 . 5 - 108  positioned on a surface of the facial interface  13 . 5 - 103 . Specifically, as shown in  FIGS.  13 . 5 - 3   , the sensor  13 . 5 - 108  is positioned on a first surface  13 . 5 - 318  oriented towards a user side  13 . 5 - 324  (and away from a display side  13 . 5 - 326 ). Additionally, the sensor  13 . 5 - 108  includes a field of view  13 . 5 - 320  that crosses through an interface material  13 . 5 - 301  from the first surface  13 . 5 - 318  to a second surface  13 . 5 - 322  opposing the first surface  13 . 5 - 318 . Thus, the portion of the interface material  13 . 5 - 301  corresponding to the field of view  13 . 5 - 320  between the first surface  13 . 5 - 318  and the second surface  13 . 5 - 322  can be sensorially transparent-constituting a sensorially transparent window through the interface material  13 . 5 - 301 . 
     The terms “sensorially transparent” refers to a type of material penetrable by a sensor measurement signal without substantial loss to the quality or accuracy of the sensor measurement signal (where “substantial” means greater than about 5%, about 10%, about 25%, about 50%, or greater than 50% discrepancy from a ground truth signal). For example, a sensorially transparent material can allow a heart rate sensor to accurately detect electrical, magnetic, or audio heart data indicative of a heart palpation, heartbeat, heart rhythm, etc. despite the sensorially transparent material being disposed between the heart rate sensor and the user. The sensor measurement signal is therefore a wireless signal to and/or from a sensor, where the wireless signal comprises wavelike properties (e.g., frequency, amplitude, etc.) that allow the wireless signal to propagate through the sensorially transparent material. 
     Relatedly, the term “sensorially transparent window” refers to the portion of the interface material  13 . 5 - 301  that is sensorially transparent. In some examples, the sensorially transparent window includes an entirety of the interface material  13 . 5 - 301 . In other examples, the sensorially transparent window includes at least a portion of the interface material  13 . 5 - 301  for the field of view  13 . 5 - 320 . In these or other examples, the sensorially transparent window can be sized and shaped according to the field of view  13 . 5 - 320 . 
     The interface material  13 . 5 - 301  composes or defines (at least in part) the facial interface  13 . 5 - 103 . The interface material  13 . 5 - 301  can include the first surface  13 . 5 - 318  and the second surface  13 . 5 - 322  opposing the first surface  13 . 5 - 318 , as illustrated in at least  FIGS.  13 . 5 - 3   . In at least some examples, the second surface  13 . 5 - 322  is configured to contact user skin. 
     Additionally, the interface material  13 . 5 - 301  can include at least one of foam, gel, or fabric. The interface material  13 . 5 - 301  can likewise include a combination of foam (e.g., polyurethane foam cushion, cotton foam), gel (e.g., silicone, polyurethane, etc.), or fabric (e.g., cotton, leather, leatherette, etc.). For example, the interface material  13 . 5 - 301  can include multiple different layers (e.g., an outer leatherette layer forming the second surface  13 . 5 - 322  and a foam layer underneath forming the first surface  13 . 5 - 318 ). The combination described is merely exemplary and other embodiments, materials, configurations and/or combinations are contemplated herein. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 5 - 3    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 5 - 3   . 
     Other sensor arrangements are also within the scope of the present disclosure. In accordance with one or more examples,  FIGS.  13 . 5 - 4    illustrates the facial interface  13 . 5 - 103  which can include sensorially transparent windows through which sensor measurement signals to or from the sensors  13 . 5 - 108   a ,  13 . 5 - 108   b  can pass. The sensors  13 . 5 - 108   a ,  13 . 5 - 108   b  can detect physiological or biological changes of the user&#39;s body through corresponding fields of view  13 . 5 - 320   a ,  13 . 5 - 320   b . For example, a head-mountable device  13 . 5 - 100  can detect changes to a user body temperature or a heat profile via an infrared sensor. Other biological sensors can be added or substituted, as may be desired. 
     In particular,  FIGS.  13 . 5 - 4    shows the sensor  13 . 5 - 108   a  in a same or similar position as described above in relation to  FIGS.  13 . 5 - 3   . The sensor  13 . 5 - 108   a  is positioned as a contactless sensor on the first surface  13 . 5 - 318  (i.e., on the display side  13 . 5 - 326 ). In addition,  FIGS.  13 . 5 - 4    shows the facial interface  13 . 5 - 103  includes the sensor  13 . 5 - 108   b  disposed between the first surface  13 . 5 - 318  and the second surface  13 . 5 - 322 . That is, the sensor  13 . 5 - 320   b  is embedded within the facial interface  13 . 5 - 103  such that the sensor  13 . 5 - 320   b  is invisible and inaccessible from the second surface  13 . 5 - 322  (i.e., the skin-facing surface). By being offset from the second surface  13 . 5 - 322 , the sensor  13 . 5 - 108   b  is also a contactless sensor. 
     In one or more examples, the sensors  13 . 5 - 108   a ,  13 . 5 - 108   b  are the same type of sensors (albeit positioned differently). In other examples, the sensors  13 . 5 - 108   a ,  13 . 5 - 108   b  are different types of sensors. Similarly, the sensors  13 . 5 - 108   a ,  13 . 5 - 108   b  can have the same field of view or a different field of view, as may be desired. Alternative embodiments can also include the same or different sensors with alternative fields of view. For instance, the fields of view  13 . 5 - 320   a ,  13 . 5 - 320   b  can be oriented or angled towards a particular location along the second surface  13 . 5 - 322  (e.g., for measuring a particular location on the user). Additionally, or alternatively, the fields of view  13 . 5 - 320   a ,  13 . 5 - 320   b  can intersect, overlap, and/or include mutually exclusive measurement regions. 
     One of ordinary skill in the art will appreciate that the sensor depth for the sensor  13 . 5 - 320   b  can vary any distance from the first surface  13 . 5 - 318  to the second surface  13 . 5 - 322 . An example of sensor depth variation is shown where the sensor  13 . 5 - 108   a  is on the first surface  13 . 5 - 318  of the interface material  13 . 5 - 301 , which increases the sensor field of view  13 . 5 - 320   a . Similarly, sensor  13 . 5 - 108   b  is a distance from the first surface  13 . 5 - 318 , being disposed within the interface material  13 . 5 - 301 , and therefore closer to the second surface  13 . 5 - 322  than the sensor  13 . 5 - 108   a . In some instances, this closer positioning of the sensor  13 . 5 - 108   b  to the second surface  13 . 5 - 322  can correspondingly reduce the field of view  13 . 5 - 320   b . This is only one example variation of sensor depth, as a multitude of sensors can be disposed on the first surface  13 . 5 - 318  of the interface material  13 . 5 - 301 , or between the first surface  13 . 5 - 318  and the second surface  13 . 5 - 322 , within the interface material  13 . 5 - 301 . In one example, the second surface  13 . 5 - 322  abuts a forehead region or a nasal region of a user head when the head-mountable device  13 . 5 - 100  is donned. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 5 - 4    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 5 - 4   . 
       FIGS.  13 . 5 - 5    illustrates yet another example of the facial interface  13 . 5 - 103 , which can include multiple sensors embedded within the interface material  13 . 5 - 301 .  FIGS.  13 . 5 - 5    also illustrates none of the sensors  13 . 5 - 108   a ,  13 . 5 - 108   b  are positioned on the first surface  13 . 5 - 318 . In this example, the sensors  13 . 5 - 108   a ,  13 . 5 - 108   b  can be wirelessly coupled to a power source and/or an HMD memory/processor component. Indeed, the sensors  13 . 5 - 108   a ,  13 . 5 - 108   b  can be powered by an inductive coil running through the head-mountable device  13 . 5 - 100  (e.g., adjacent to the first surface  13 . 5 - 318 ). Similarly, the sensors  13 . 5 - 108   a ,  13 . 5 - 108   b  can be communicatively coupled to an HMD memory/processor component via a wireless communication protocol (e.g., for sending sensor data/sensor signals or receiving sensor feedback). 
     As further shown in  FIGS.  13 . 5 - 5   , sensor  13 . 5 - 108   a  and sensor  13 . 5 - 108   b  are disposed within the interface material  13 . 5 - 301 . Sensors  13 . 5 - 108   a ,  13 . 5 - 108   b  can be positionally modified to different locations (e.g., laterally, or depth-wise) than illustrated in  FIGS.  13 . 5 - 5   . Similarly, the sensor  13 . 5 - 108   a  can vary in type and configuration, transmission power, shape, and size from the sensor  13 . 5 - 108   b  (as indicated above). The field of view  13 . 5 - 320   a  of the sensor  13 . 5 - 108   a  can vary from the field of view  13 . 5 - 320   b  of  13 . 5 - 108   b . In another example, the fields of view  13 . 5 - 320   a ,  13 . 5 - 320   b  can be the same configuration (also indicated above). 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 5 - 5    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 5 - 5   . 
       FIGS.  13 . 5 - 6 A and  13 . 5 - 6 B  illustrate another example of the facial interface  13 . 5 - 103  comprising multiple layers of interface material. In a particular example shown in  FIGS.  13 . 5 - 6 A- 13 . 5 - 6 B , the interface material includes a base layer  13 . 5 - 624  and an interchangeable layer  13 . 5 - 626 . The base layer  13 . 5 - 624  is affixed to the head-mountable device  13 . 5 - 100 —thereby forming a permanent portion of the facial interface  13 . 5 - 103 . By contrast, the interchangeable layer  13 . 5 - 626  is removably attached to the base layer  13 . 5 - 624  (e.g., via fasteners  13 . 5 - 628   a ,  13 . 5 - 628   b ) such that the interchangeable layer  13 . 5 - 626  can be swapped out for a different interchangeable layer (e.g., that supports or is designated for a different user activity). 
     The fasteners  13 . 5 - 628   a ,  13 . 5 - 628   b  can include myriad different fasteners. For example, the fasteners  13 . 5 - 628   a ,  13 . 5 - 628   b  can include brooches, buttons, buckles, clasps, eyelets, fabric ties, frog closures, grommets, hook and eyes, laces, loop fasteners, pins, poppers, press studs, snap fasteners, toggles, hook and loop Velcro® tape, zippers, etc. Additionally, rather than fasteners, temporary adhesives (e.g., tape, glue, tack, etc.) can be implement. It will be appreciated that more than two fasteners can be utilized. Additionally, only a single fastener may be implanted in certain instances. 
     In one example,  FIGS.  13 . 5 - 6 A  shows a configuration where the interchangeable layer  13 . 5 - 626  of the facial interface  13 . 5 - 103  can be removably attached to the base layer  13 . 5 - 624  via the fasteners  13 . 5 - 628   a ,  13 . 5 - 628   b . The interchangeable layer  13 . 5 - 626  can include sensor  13 . 5 - 108   a  with a field of view  13 . 5 - 320   a . The sensor  13 . 5 - 108   a  can be a wireless biometric sensor, which can include a temperature sensor, a respiration sensor, a heart activity sensor, or a brain activity sensor. Additionally or alternatively, the sensor  13 . 5 - 108   a  can include a sensor indicative of certain biological responses (e.g., a stress response). The sensor  13 . 5 - 108   a  can be disposed such that the sensor field of view  13 . 5 - 320   a , when oriented toward a region of the face, can detect characteristics unique to the user. In a particular example, the sensor  13 . 5 - 108   a  can include an infrared sensor capable of detecting user characteristics, such as body temperature or changes to body temperature. 
     In another example,  FIGS.  13 . 5 - 6 B  shows a configuration where an additional interchangeable layer  13 . 5 - 630  of the facial interface  13 . 5 - 103  can be removably attached to the base layer  13 . 5 - 624 . The additional interchangeable layer  13 . 5 - 630  can be removably attached to the base layer  13 . 5 - 624  via the fasteners  13 . 5 - 628   a ,  13 . 5 - 628   b , as similarly described above. Different from the interchangeable layer  13 . 5 - 626 , the additional interchangeable layer  13 . 5 - 630  can include an additional sensor  13 . 5 - 108   b  with an additional field of view  13 . 5 - 320   b . Here, the sensor  13 . 5 - 108   b  and corresponding field of view differs from the sensor  13 . 5 - 108   a . In this manner, facial interface  13 . 5 - 103  can be compatible with myriad different interchangeable layers as may be desired for different user activities (or different types of users, such as kids or adults). 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 5 - 6 A- 13 . 5 - 6 B  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 5 - 6 A- 13 . 5 - 6 B . 
       FIGS.  13 . 5 - 7 A  illustrates a perspective view of an electronic device  13 . 5 - 700  including a wearable display  13 . 5 - 702 , an engagement interface  13 . 5 - 726 , and a contactless sensor  13 . 5 - 714  coupled to the engagement interface  13 . 5 - 726 . In one or more examples, the electronic device  13 . 5 - 700  is the same as or similar to the head-mountable device  13 . 5 - 100  described above. For example, the engagement interface  13 . 5 - 726  can be the same as or similar to the facial interface  13 . 5 - 103  described above. Similarly, the wearable display  13 . 5 - 702  can be the same as or similar to the display  13 . 5 - 102  described above. 
     The engagement interface  13 . 5 - 726  is adjustable for different size, shapes, and contours of facial features. For example, the engagement interface  13 . 5 - 726  can flexibly conform to a user face via connections  13 . 5 - 628   a - 13 . 5 - 628   d  (e.g., that are the same as or similar to the connections  13 . 5 - 206  described above). To illustrate, the connections  13 . 5 - 628   a - 13 . 5 - 628   d  each include pivot connections. However, the connections  13 . 5 - 628   a - 13 . 5 - 628   d  can be adjusted to include different types of connections (e.g., foam hard stops, leaf springs, compliant mechanisms, etc.). 
     In another example, the engagement interface  13 . 5 - 726  includes an interfacing material  13 . 5 - 727 , wherein the contactless sensor  13 . 5 - 714   a  can be invisible and inaccessible through a skin-facing surface of the interfacing material  13 . 5 - 727 . The contactless sensor  13 . 5 - 714   a  can be oriented toward the first facial region  13 . 5 - 725   a , such a user&#39;s forehead, when the device is worn. 
     In another example, the electronic device includes additional contactless sensors  13 . 5 - 714   b ,  13 . 5 - 714   c  coupled to the engagement interface  13 . 5 - 726 . The additional contactless sensors  13 . 5 - 714   b ,  13 . 5 - 714   c  can be mounted on (or within) an engagement interface  13 . 5 - 732  (e.g., a nose piece). The additional contactless sensors  13 . 5 - 714   b ,  13 . 5 - 714   c  can be oriented toward a second facial region  13 . 5 - 730 , such as a user&#39;s nose, different from the first facial region  13 . 5 - 725   a  when the device is worn. In this manner, the contactless sensors  13 . 5 - 714   a - 13 . 5 - 714   c  can be oriented away from the wearable display  13 . 5 - 702  (and instead oriented towards a user head or skin, not shown). 
       FIGS.  13 . 5 - 7 B  shows a perspective exploded view of the electronic device  13 . 5 - 700  with sensors  13 . 5 - 714   a ,  13 . 5 - 714   b , and  13 . 5 - 714   c . In one example, the sensors can be removably attached to the engagement interfaces  13 . 5 - 726 ,  13 . 5 - 732  (e.g., for swapping out with different sensors). In other examples, the sensors are permanently affixed to the engagement interfaces  13 . 5 - 726 ,  13 . 5 - 732 . Additionally, the sensors  13 . 5 - 714   a - 13 . 5 - 714   c  can be disposed on the engagement interfaces  13 . 5 - 726 ,  13 . 5 - 732  such that the sensors are positioned over corresponding sensorially transparent windows  13 . 5 - 734   a - 13 . 5 - 734   c . The sensorially transparent windows  13 . 5 - 734   a ,  13 . 5 - 734   b , and  13 . 5 - 734   c  allow sensor measurement signals to pass to and from the sensors  13 . 5 - 714   a - 13 . 5 - 714   c.    
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 5 - 7 A- 13 . 5 - 7 B  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 5 - 7 A- 13 . 5 - 7 B . 
     13.6: Integrated Health Sensors 
     The following disclosure relates to a facial interface of a head-mountable device. More particularly, the present embodiments relate to a facial interface of a head-mountable device including health sensors used for acquiring biometric information of the user, and performing actions in response to the detected biometric information. These facial interfaces can enable health sensors to interact with a user to collect biometric information. In some examples, a processor causes a component of the HMD to perform an action in response to the biometric information collected by the sensor. As used herein “an action” or “perform an action” can refer to any electrical or mechanical act the causes a change in one or more components of the HMD. For example, the action can include displaying a notification for the user, or activating or deactivating one or more components. In some examples, a sensor can collect biometric information of a user who is exercising. The biometric information can be related to temperature, respiration, stress response, heart activity, or brain activity or any other pertinent data. Further, in response to the collected data, they system can perform an action (e.g., notify the user of the collected data. 
     Head-mountable devices equipped with sensors are utilized for various purposes that detect limited user feedback, such as movement or positioning feedback, providing limited information in response to the user. For example, a user may have an increase in heartrate or brain activity while performing an action or movement. Conventional head-mountable devices are not suitably equipped to capture all the necessary biometric information generated during use. 
     In contrast, the head-mountable devices of the present disclosure include a facial interface that can be integrated or incorporated with sensors configured to collect user data, such as biometric information, including heartrate, respiration, brain activity, etc. By capturing the user data, the facial interface sensors can provide improved and increased feedback to the user. A head-mountable device with sensors monitoring user biometric or health feedback can create highly customized user experiences, unlike the sensors of conventional head-mountable devices that are unable to consider, react to, or further enhance the user experience. 
     Sensors positioned on the facial interface are important to create a customized user experience. The head-mountable device of the present disclosure can contain sensors to measure a user&#39;s response or engagement via indicators, such as heart rate, electrical signals from the heart (e.g., ECG, EKG, EXG, etc.), brain activity (e.g., EEG signals, frontal lobe activity), core body temperature, etc. Additionally, the sensor data can be used as feedback data, for example, to monitor user fatigue, facial expressions, or obtain activity-specific metrics. 
     The head-mountable device can have different sensors implemented in different ways. For example, the head-mountable device of the present disclosure can implement sensors removably attached to the facial interface. In some examples, the removably attached sensors correspond to a different user activity, such as exercise, learning activities, health, clinical settings, etc. In certain implementations, the exercise sensors can include a different sensor, such as a heart rate monitor sensor, whereas the learning activity sensor may include a brain activity sensor. The sensors can be implemented in different applications and arrangement/combinations for obtaining different, activity biometric readings. 
     These and other embodiments are discussed below with reference to  FIGS.  13 . 6 - 1 - 13 . 6 - 6   . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these Figures is for explanatory purposes only and should not be construed as limiting. Furthermore, as used herein, a system, a method, an article, a component, a feature, or a sub-feature comprising at least one of a first option, a second option, or a third option should be understood as referring to a system, a method, an article, a component, a feature, or a sub-feature that can include one of each listed option (e.g., only one of the first option, only one of the second option, or only one of the third option), multiple of a single listed option (e.g., two or more of the first option), two options simultaneously (e.g., one of the first option and one of the second option), or combination thereof (e.g., two of the first option and one of the second option). 
       FIGS.  13 . 6 - 1    illustrates a block diagram of a head-mountable device  13 . 6 - 100  including a frame  13 . 6 - 116 , a display  13 . 6 - 108 , a facial interface  13 . 6 - 112 , and a support or retention band  13 . 6 - 123 . The display  13 . 6 - 108  can include one or more optical lenses or display screens in front of the eyes of a user. The display  13 . 6 - 108  can include a display or display unit for presenting an augmented reality visualization, a virtual reality visualization, or other suitable visualization to a user. Additionally, the display  13 . 6 - 108  can be positioned in or on the frame  13 . 6 - 116 . Similarly, a facial interface  13 . 6 - 112  can be connected to the frame  13 . 6 - 116 . In some examples, the frame  13 . 6 - 116  can be a housing for the display  13 . 6 - 108 . Alternatively, a separate housing can form an exterior portion of the head-mountable device  13 . 6 - 100 . 
     The facial interface  13 . 6 - 112  can include one or more sensors  13 . 6 - 124 , support attachments  13 . 6 - 132 , display attachments  13 . 6 - 128 , and feedback or output modules  13 . 6 - 136 . The sensors  13 . 6 - 124  can be removably attached to the facial interface  13 . 6 - 112 . As used herein, the terms “facial interface”, “engagement interface”, or “light seal” refer to a portion of the head mountable device  13 . 6 - 100  that engages (i.e., contacts or conforms to) a user&#39;s face. In particular, the facial interface  13 . 6 - 112  can include portions of a head-mountable device that conform or press against regions of a user&#39;s face. The facial interface  13 . 6 - 112  can be positioned between the display  13 . 6 - 108  and the user&#39;s face. In some examples, the facial interface  13 . 6 - 112  can include a pliant (or semi-pliant) facetrack or lumen that spans the forehead, wraps around the eyes, contacts other regions of the face (etc., zygoma and maxilla regions), and bridges the nose. 
     In addition, the facial interface  13 . 6 - 112  can include various components forming a frame, structure, or webbing of a head mountable device disposed between the display  13 . 6 - 108  and the user&#39;s skin. In particular implementations, the facial interface  13 . 6 - 112  can include a seal (e.g., a light seal, environment seal, dust seal, air seal, etc.). It will be appreciated that the term “seal” can include partial seals or inhibitors, in addition to complete seals (e.g., a partial light seal where come ambient light is blocked and a complete light seal where all ambient light is blocked when the head-mountable device  13 . 6 - 100  is donned). The facial interface  13 . 6 - 112  can be removably attached to the frame  13 . 6 - 116  and in electrical communication with the display  13 . 6 - 108 . 
     The sensor  13 . 6 - 124  of the facial interface  13 . 6 - 112  can collect biometric information, such as the user&#39;s vital sign (including body temperature, pulse data, respiration data, and blood pressure). The sensor  13 . 6 - 124  can generate a signal based on the collected user information and transmit the signal to a processor that can cause an output  13 . 6 - 136  to perform an action in response to the signal (i.e., in response to the biometric information collected by the sensor  13 . 6 - 124 . For example, a user may perform a rigorous activity, such as lifting weights or working out while wearing the HMD  13 . 6 - 100 . During such activity, the user&#39;s heart rate or other vital signs may be elevated or changed and is detectable by the sensor  13 . 6 - 124 . The sensor  13 . 6 - 124  can generate one or more signals based on the received input. The sensor  13 . 6 - 124  can transmit the signal to one or more components of the HMD  13 . 6 - 100  (e.g., to the display  13 . 6 - 108 , to the output  13 . 6 - 136 ). The display  13 . 6 - 108 , being in electrical communication with the facial interface  13 . 6 - 112 , can receive electrical communication and provide feedback to the user related to their biometric readings (e.g., visual feedback, audio feedback, haptic feedback, etc.). Feedback can include determining when the user needs to take a break, or when the difficulty of an activity needs to be lowered or raised, the output can include scheduling various activities for the user or recommending adjustment of the HMD. 
     As used herein, the term “sensor” refers to one or more different sensing devices, such as a camera or imaging device, temperature device, oxygen device, movement device, brain activity device, sweat gland activity device, breathing activity device, muscle contraction device, etc. As used herein, the term “biometric,” “biometric information,” or “biometric feature” can refer to vital signs, including heart rate, pulse, respiration rate, respiration amplitude, or any other health related data. The term “biometric,” “biometric information,” or “biometric feature” can also refer to biological measurements or physical characteristics, such as facial expressions or emoting. In some examples, the sensor can detect or sense biometric features including features of the autonomic nervous system. Some particular examples of sensors include an electrooculography (EOG) sensor, electrocardiogramhy (ECG or EKG) sensor, photoplethysmography (PPG) sensor heart rate sensor, hear rate variability sensor, blood volume pulse sensor, oxygen saturation (SpO2) sensor, compact pressure sensor, electromyography (EMG) sensor, core-body temperature sensor, galvanic skin response (GSR) sensor, functional near-infrared spectroscopy (FNIR) sensor, non-contact passive infrared (IR) sensor, accelerometer, gyroscope, magnetometer, inclinometer, barometer, infrared sensor, global positioning system sensor, etc. Additional sensor examples can include, contact microphones (e.g., pressure-based MEMS), bioelectrical activity sensors, UV exposure sensors, or particle sensors. In some examples, certain sensors can be used to assess stress and emotion. The HMD can then provide feedback or output related to the detected stress and emotion. In some examples, the sensors can operate through coin cell battery or Bluetooth connectivity. In some examples, the sensors are powered by a primary battery of the HMD. 
     The sensors described herein can allow for observations of the autonomic nervous system (ANS), to observe relaxation and stress indicators, mental health, medical treatments, etc. Using the sensors, physicians and care takers could have live feedback of biometrics. Use cases can include fitness settings, user content, workplace, telepresence, clinical, education, training, pain, therapy, etc. In some examples, the sensors can be used to capture facial expressions. This is particularly relevant given the user&#39;s face is covered by HMD. For example, the HMD could use MEMS or motion tracking sensors to detect facial expressions 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 6 - 1    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 6 - 1   . 
       FIGS.  13 . 6 - 2    illustrates a head-mountable device  13 . 6 - 200 . The head-mountable device  13 . 6 - 200  can be substantially similar to, including some or all of the features of, the head-mountable devices described herein, such as head-mountable device  13 . 6 - 100 . In some examples, the head-mountable device  13 . 6 - 200  includes a controller  13 . 6 - 213  (e.g., sensor controller). While the controller  13 . 6 - 213  is illustrated as being positioned in the support  13 . 6 - 223 , the position of the controller  13 . 6 - 213  is not limited to the support  13 . 6 - 223 , but can be positioned in/on the facial interface  13 . 6 - 212  or in the HMD display housing. The controller  13 . 6 - 213  can include a processor and a memory device storing computer-executable instructions that, when executed by the processor, cause the controller to receive biometric data from one or more sensors  13 . 6 - 224 , transmit a signal based on the sensor data, and generate a signal to cause a component to perform an action in response to the signal. 
     The controller  13 . 6 - 213  can include one or more processors (e.g., a system on chip, integrated circuit, driver, microcontroller, application processor, crossover processor, etc.). Further, the controller  13 . 6 - 213  can include one or more memory devices (e.g., individual nonvolatile memory, processor-embedded nonvolatile memory, random access memory, memory integrated circuits, DRAM chips, stacked memory modules, storage devices, memory partitions, etc.). In certain implementations, the controller  13 . 6 - 213  is communicatively coupled to a power source (e.g., a battery). 
     In some examples, the controller  13 . 6 - 213  stores sensor data received from the sensor(s)  13 . 6 - 224  in the memory. The controller  13 . 6 - 213  can receive and/or transmit signals based on sensor data. For example, as will be described below, the controller  13 . 6 - 213 , by way of the processor and memory, can transmit a signal to the display  13 . 6 - 208  based on the sensor data (e.g., causing the display  13 . 6 - 208  or the head-mountable device  13 . 6 - 200  to perform an action, such as present a certain message, power off, react to biometric feedback, etc. 
     The controller  13 . 6 - 213  can perform any number of different functions. For example, the memory device can store computer-executable instructions that, when executed by the processor, cause the controller to receive sensor data from the sensors  13 . 6 - 224  and transmit a signal based on the sensor data. For instance, the controller  13 . 6 - 213  can transmit a sensor signal to a display  13 . 6 - 208 . In response to the controller  13 . 6 - 213  the display  13 . 6 - 208  can perform a wide variety of actions, including power off or power on, react to a user generated facial expression, present a digital notification (e.g., user-generated notification, push notification, context-generated notification, system-generated notification, smart notification, etc.). In some examples, the memory device storing computer-executable instructions that, when executed by the processor, cause the controller  13 . 6 - 213  to receive biometric data from the sensor(s)  13 . 6 - 224 , transmit a signal based on the sensor data, and perform an action in response to the signal. 
     As illustrated, the head-mountable device (e.g., wearable electronic device)  13 . 6 - 200  can include the display  13 . 6 - 208 , the facial interface  13 . 6 - 212 , and the sensor(s)  13 . 6 - 224  coupled to the engagement interface  13 . 6 - 212 . The sensors  13 . 6 - 224  can be embedded, encapsulated, deposited, adhered, or otherwise attached to the facial interface  13 . 6 - 212 . The sensors  13 . 6 - 224  can be non-contact sensors or can directly contact or touch the user&#39;s head. The sensor(s)  13 . 6 - 224  can be configured to detect a biometric feature of the user  13 . 6 - 220 . The sensor(s)  13 . 6 - 224 , upon detecting a signal from the biometric feature, can transmit the signal to a component of the head-mountable device  13 . 6 - 200  causing the head-mountable device  13 . 6 - 200  to perform an action. For example, the head-mountable device  13 . 6 - 200  can change shape (i.e., tighten or loosen), move, vibrate, rotate, recalibrate, or reposition in response to the sensor signal of the biometric feature. 
     The head-mountable device  13 . 6 - 200  can include a support, headband, or retention band  13 . 6 - 223  connected to the display  13 . 6 - 208  and/or frame  13 . 6 - 216 . The retention band  13 . 6 - 223  is configured to secure the display  13 . 6 - 108  and/or frame  13 . 6 - 216  relative to the user&#39;s head  13 . 6 - 220  (e.g., such that the display  13 . 6 - 208  is maintained in front of a user&#39;s eyes). The retention band  13 . 6 - 223  can be constructed from elastic material, inelastic material, or a combination of elastic and inelastic material. The retention band  13 . 6 - 223  is adjustable such that the retention band  13 . 6 - 223  conforms to the various shapes and sizes of a user&#39;s head  13 . 6 - 220 . In some examples, the retention band  13 . 6 - 223  secures the head-mountable device  13 . 6 - 200  via friction between the user&#39;s head  13 . 6 - 220  and the retention band  13 . 6 - 223 . In some examples, the retention band  13 . 6 - 223  elastically secures the head-mountable device  13 . 6 - 200  to the user&#39;s head  13 . 6 - 220 . In some examples, the retention band  13 . 6 - 223  is coupled to a ratchet system or mechanism securing the head-mountable device  13 . 6 - 200  to the user&#39;s head  13 . 6 - 220 . In some examples, the retention band  13 . 6 - 223  is disposed above or on an ear  13 . 6 - 221  of the user  13 . 6 - 220 , supporting the head-mountable device  13 . 6 - 200 . 
     In some examples, the housing or frame  13 . 6 - 216  of the head-mountable display  13 . 6 - 200  is connected via a connector  13 . 6 - 215  to the facial interface  13 . 6 - 212 , the retention band  13 . 6 - 223  being connected to the frame  13 . 6 - 216  and/or the facial interface  13 . 6 - 212 , securing the head-mountable device  13 . 6 - 200  to the user&#39;s head  13 . 6 - 220  above or over the ears  13 . 6 - 221  of the user. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 6 - 2    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 6 - 2   . 
       FIGS.  13 . 6 - 3    shows a rear view of a head-mountable device  13 . 6 - 300 . The head-mountable device  13 . 6 - 300  can be substantially similar to, including some or all of the features of, the head-mountable devices described herein, such as head-mountable device  13 . 6 - 100  and  13 . 6 - 200 . The head-mountable device  13 . 6 - 300  can include a facial interface (e.g., a facial engagement feature)  13 . 6 - 312  and a retention band  13 . 6 - 323 . The head-mountable device  13 . 6 - 300  further includes connectors  13 . 6 - 332   a ,  13 . 6 - 332   b  (collectively referred to as connector(s)  13 . 6 - 332 ) connecting the retention band  13 . 6 - 323  and the facial engagement interface  13 . 6 - 312 . In some examples, the facial interface  13 . 6 - 312  is adjustable, accommodating to a user&#39;s head in a comfortable and secure way that shields the user from ambient light. 
     The connector  13 . 6 - 332  connecting the retention band  13 . 6 - 323  to the facial engagement interface  13 . 6 - 312  includes a first connector side  13 . 6 - 332   a  attached to the retention band  13 . 6 - 323  and a second connector side  13 . 6 - 332   b  attached to the facial engagement feature  13 . 6 - 312 . In addition to forming a mechanical attachment between the retention band  13 . 6 - 323  and the facial interface  13 . 6 - 312 , the connector  13 . 6 - 332  can include various types of electrical connections, such as a pogo-pin connection, or other connection types to electrically connect the retention band  13 . 6 - 323  with the facial interface  13 . 6 - 312 . When mated, the first connector side  13 . 6 - 332   a  and the second connector side  13 . 6 - 332   b  mate (e.g., mechanically mate, magnetically mate via a magnet, electrically mate, etc.) such that the connectors  13 . 6 - 332  are in mechanical contact and electrical contact. When the first connector side  13 . 6 - 332   a  and the second connector side  13 . 6 - 332   b  are connected in this manner, electrical signals can be relayed through the from the retention band  13 . 6 - 323  to the facial engagement interface  13 . 6 - 312 , and vice versa. That is, when connected, the facial engagement interface  13 . 6 - 312  can provide data and/or power to the retention band  13 . 6 - 323 , for example to a sensor or electrical component  13 . 6 - 324   c  of the retention band. Likewise, when connected via connector  13 . 6 - 332 , the retention band  13 . 6 - 323  can provide data and/or power to the facial interface  13 . 6 - 312 , for example, to the sensors  13 . 6 - 324   a  and/or  13 . 6 - 324   b . In some examples, the connectors  13 . 6 - 332   a ,  13 . 6 - 332   b  can magnetically mate or couple one to the other, transmitting information via a wireless communication connection and receiving power from a wireless power source (e.g., an inductive charger). 
     The head-mountable device  13 . 6 - 300  includes one or more sensor(s)  13 . 6 - 324   a ,  13 . 6 - 324   b ,  13 . 6 - 324   c  (collectively “sensors  13 . 6 - 324 ”). In some examples, the sensors  13 . 6 - 324  can be in contact (indirect or direct) with a user. For example, the sensors be positioned to receive input from a user&#39;s forehead, cheek, nose, temple region, back of the head, or at any location on the user&#39;s head/face. In some examples, the sensors  13 . 6 - 324  can be in indirect contact with a user. For example, the sensors  13 . 6 - 324  may detect a user&#39;s biometric features via infrared, IR, optics, imaging, or other means of detecting biometric features in an indirect or contactless way. The sensors can be a biometric or health sensor, such as any of those described above. 
     In some examples, the sensors  13 . 6 - 324  include a first sensor  13 . 6 - 324   a  oriented towards a first facial region (e.g., the forehead) and coupled to the engagement interface  13 . 6 - 312 , a second sensor  13 . 6 - 324   b  oriented towards a second facial region (e.g., a nasal region) and coupled to the engagement interface  13 . 6 - 312 , and a third sensor or sensor set  13 . 6 - 324   c  coupled to the retention band  13 . 6 - 323  and configured to contact a side or back of the user&#39;s head. When wearing the head-mountable device  13 . 6 - 300  the first sensor  13 . 6 - 324   a  orients towards the first facial region is different in orientation and location from the second sensor  13 . 6 - 324  which orients toward the second facial region. It will be appreciated by one skilled in the art that other configurations are contemplated herein, and the above example is for illustrative purposes only. 
     The sensors  13 . 6 - 324  can be located at various locations of the head-mountable device  13 . 6 - 300  and can differ in function or sensing capacity depending on where the sensor is located. For example, the sensors  13 . 6 - 324   b  are positioned on the engagement interface  13 . 6 - 312  proximate a nose of the user and may sense a user&#39;s respiratory biometrics providing a user with feedback indicating oxygen saturation (e.g., SPO 2 ). In some examples, the sensor  13 . 6 - 324   a  is positioned on the engagement interface  13 . 6 - 312  contacting a forehead of a user. In some examples, the sensor  13 . 6 - 324   a  detects pulse, facial expression, brain activity or stress response while performing an activity. In some examples, the sensor  13 . 6 - 324   c  is positioned on the retention band  13 . 6 - 323 , adjacent the temple(s) of a user or at another location where the retention band  13 . 6 - 323  is contacting the user. In some examples, a sensor fusion exists in which one or more of the sensors  13 . 6 - 324  coordinate and communicate with each other and/or with other sensors on the HMD  13 . 6 - 300  to collectively gather user data. 
     In some examples, forehead sensors, such as sensor  13 . 6 - 324   a , can be used for brain imaging and observations of the central nervous system. While a single forehead sensor  13 . 6 - 324   a  is shown in  FIGS.  13 . 6 - 3 B , it will be understood that multiple sensors or a sensor array can be located on the facial interface  13 . 6 - 312  to contact, or be proximate to, the user&#39;s head. In some examples, the sensors can be configured to perform EEG (electroencephalography) or functional near-infrared spectroscopy (FNIR). 
     In some examples, the position of the forehead sensors  13 . 6 - 324   a  can be tuned or tailored to sense brain areas related to language, learning, memory, comprehension, sleep, stress, pain, attention, fear, discomfort, etc. In some examples, the facial interface  13 . 6 - 312  can be integrated with a sensor array that includes one or more transmitters that emit a signal. The transmitter(s) can be positioned a certain distance away from one or more detectors that sense the emitted signal. Based on the received signals by the detectors, a processor can infer or determine certain brain activity. In some examples, the sensors  13 . 6 - 324   a  can form a brain-computer interface (BCI), such as a non-invasive neural interface. In some examples, the integrated sensor array can be used to detect the parieto-frontal network of the brain. 
     In some examples, the sensors  13 . 6 - 324   a ,  13 . 6 - 324   c  can be used to perform EEG detections. The sensors  13 . 6 - 324   a ,  13 . 6 - 324   c  can measure the electrical activity in the cerebral cortex (the outer layer of the brain). The sensors  13 . 6 - 324   a ,  13 . 6 - 324   c  can include electrodes that are placed on a user&#39;s head, then the electrodes can non-invasively detect brainwaves from the subject. The EEG sensors  13 . 6 - 324   a ,  13 . 6 - 324   c  can record up to several thousands of snapshots of the electrical activity generated in the brain every second. The recorded brainwaves can be sent to amplifiers, then to a local processor, a remote electronic device, or the cloud to process the data. 
     In some examples, the sensors  13 . 6 - 324   a ,  13 . 6 - 324   c  can be used to perform functional near-infrared spectroscopy (FNIR). The sensors  13 . 6 - 324   a ,  13 . 6 - 324   c  can use low levels of non-ionizing light to record changes in cerebral blood flow in the brain through optical sensors placed on the surface of the head. 
     The sensors described herein can perform both passive and active EEG. The passive electrode signals can be sent to a remote processor to analyze. Amplification of the active EEG signals can be performed locally at the site to eliminate coexistence and interference that may be present with passive signal. 
     The HMD can include an output or feedback module that provides feedback based on the detections from the sensors. The feedback from the system can include displaying visualizations to the user. For example, the feedback can include breathing visualizations, visualization that relate to the user&#39;s attention, promptings or suggestions to the user to take a break or change an activity, or make other recommendations or notifications to the user. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 6 - 3    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 6 - 3   . 
       FIGS.  13 . 6 - 4    shows a side or cross-sectional view of portion of a facial interface  13 . 6 - 412  wherein a sensor(s)  13 . 6 - 424  is embedded in the facial interface  13 . 6 - 412 . The facial interface  13 . 6 - 412  includes a first side or surface  13 . 6 - 407  facing the display side  13 . 6 - 429  (i.e., toward the HMD display) and a second side or surface  13 . 6 - 409  facing toward a user side  13 . 6 - 427 . In some examples, the first surface  13 . 6 - 407  attaches to a frame of the HMD. In some examples, the second surface  13 . 6 - 409  directly contacts or touches the user&#39;s head when the HMD is donned or being worn by the user. 
     The facial interface  13 . 6 - 412  can include one or more sensors  13 . 6 - 424   a ,  13 . 6 - 424   b ,  13 . 6 - 424   c  (collectively “sensors  13 . 6 - 424 ). The sensor  13 . 6 - 424   a  can be positioned on the first surface  13 . 6 - 407  of the facial interface  13 . 6 - 412 . In some examples, the first surface  13 . 6 - 409  can an interior and the second surface  13 . 6 - 409  can be an exterior of the facial interface  13 . 6 - 412 . Thus, the sensor  13 . 6 - 424   a  can be positioned on an interior of the facial interface  13 . 6 - 412 . In some examples, the facial interface  13 . 6 - 412  can include a sensor  13 . 6 - 424   b  is embedded, encapsulated, or otherwise surrounded by the facial interface  13 . 6 - 412 . The facial interface  13 . 6 - 412  can include, a sensor  13 . 6 - 424   c  that positioned such that a portion of the sensor  13 . 6 - 424   c  is exposed through the second surface  13 . 6 - 409 . In other words, the sensor  13 . 6 - 424   c  can at least partially define the second surface  13 . 6 - 409  and can directly contact or touch the user&#39;s skin. In some examples, the sensor  13 . 6 - 424   c  can be partially surrounded or embedded in the facial interface  13 . 6 - 412 . In some examples, the sensor  13 . 6 - 424   c  is external to the facial interface  13 . 6 - 412 , positioned on the exterior  13 . 6 - 409  of the facial interface  13 . 6 - 412 . 
     The sensors  13 . 6 - 424   a  and  13 . 6 - 424   b  can have corresponding sensor areas  13 . 6 - 425   a ,  13 . 6 - 425   b , respectively. The sensor areas  13 . 6 - 425   a ,  13 . 6 - 425   b  can represent a field of view or cone of influence that is transparent to a signal emitted by the sensors  13 . 6 - 424   a ,  13 . 6 - 424   b . In some examples, the field of view of the sensors  13 . 6 - 424  can be approximately 50 degrees. The sensors  13 . 6 - 424   a ,  13 . 6 - 424   b  can detect physiological, biological, and/or biometric changes of the user&#39;s body through corresponding sensor areas  13 . 6 - 425   a ,  13 . 6 - 425   b . For example, the sensors  13 . 6 - 424   a ,  13 . 6 - 424   b  can detect changes to a user through the material of the facial interface  13 . 6 - 412 . Other sensors may be added or substituted, as may be desired. In some examples, the sensor areas  13 . 6 - 425   a ,  13 . 6 - 425   b  can comprises a different material than the rest of the facial interface  13 . 6 - 412 . For example, the sensor areas  13 . 6 - 425   a ,  13 . 6 - 425   b  can be transparent to certain signals from the sensors  13 . 6 - 424   a ,  13 . 6 - 424   b  or from the user, while the remainder of the facial interface  13 . 6 - 412  is not transparent or transmissive to such signals. 
     Sensor  13 . 6 - 424   b  is a distance from the first surface  13 . 6 - 407 , disposed with in the facial interface  13 . 6 - 412 , and therefore closer to the second surface  13 . 6 - 409  than the sensor  13 . 6 - 424   a . In some instances, this closer positioning of the sensor  13 . 6 - 424   b  to the second surface  13 . 6 - 409  can correspondingly reduce the field of view  13 . 6 - 425   b . In some examples, the sensor  13 . 6 - 424   c  may be flush with the second surface  13 . 6 - 409  and/or directly contacting a user&#39;s face and/or skin. This is only one example of sensor depth variation, as a plurality of sensors can be disposed on the first surface  13 . 6 - 407  of the facial interface  13 . 6 - 412 , or between the first surface  13 . 6 - 407  and the second surface  13 . 6 - 409  of the facial interface  13 . 6 - 412 . In some examples, the second surface  13 . 6 - 409  (e.g., user side  13 . 6 - 427 ) abuts a forehead region or nasal region of a user head when the head-mountable device  13 . 6 - 100  is donned. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 6 - 4    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 6 - 4   . 
       FIGS.  13 . 6 - 5    shows a front perspective view of a facial interface  13 . 6 - 512 . The facial interface  13 . 6 - 512  can be substantially similar to, including some or all of the features of, the facial interface described herein, such as facial interfaces  13 . 6 - 112 ,  13 . 6 - 212 ,  13 . 6 - 312 , and  13 . 6 - 412 . In some examples, the facial interface  13 . 6 - 512  can include a framed or clamshell design. In some examples, various components and/or circuitry can be discretely positioned within the volume of the clamshell design. The facial interface  13 . 6 - 512  can include a first side  13 . 6 - 540   a  and a second side  13 . 6 - 540   b . The first side  13 . 6 - 540   a  can be proximate the user when donned. In some examples, the first side  13 . 6 - 540   a  can contact the user&#39;s face. The second side  13 . 6 - 540   b  can be positioned between the first side  13 . 6 - 540   a  and a display of the HMD. In some examples, the second side  13 . 6 - 540   b  is attached to the display housing. 
     The first side  13 . 6 - 540   a  can include a connector  13 . 6 - 532  configured to attach the facial interface  13 . 6 - 512  to a component, such as a retention band. The connector  13 . 6 - 532  can be mechanical and/or electrical. In some examples, the connector  13 . 6 - 532  is electrically connected to a sensor  13 . 6 - 524   c . The first side  13 . 6 - 540   a  can configured to contact a face of a user. The first side  13 . 6 - 540   a  can include sensors  13 . 6 - 524   a ,  13 . 6 - 524   b ,  13 . 6 - 524   c ,  13 . 6 - 524   d  that can be configured to detect a biometric feature(s) of the user. In some examples, the facial interface includes a nasal support  13 . 6 - 539 . The nasal support  13 . 6 - 539  can be configured to contact the user&#39;s nose. The nasal support  13 . 6 - 539  can include at least one sensor  13 . 6 - 524   b.    
     In some examples, the facial interface  13 . 6 - 512  can be interchangeable with a different facial interface having different sensors or structure. For example, a user may exercise with the head-mountable device  13 . 6 - 100  using a first facial interface with specific sensors for exercising. The same user then may game or perform a leisure activity using the head-mountable device  13 . 6 - 100  fitted with a different facial interface with different sensors or structure for those activities. This is only one example and one of ordinary skill in the art will appreciate other examples are contemplated herein. 
     In some examples, the sensors  13 . 6 - 524  are removably attached to the facial interface  13 . 6 - 512 . For example, a user may wish to replace or add certain sensors to the facial interface  13 . 6 - 512 . A user can selectively attach or remove a sensor  13 . 6 - 524  and replace it with a different sensor depending on the activity of the user or purpose of the sensor. In this way, the sensors can accommodate the needs or demands of a user for a specific activity requiring specific sensors without needing to replace the entire HMD or facial interface  13 . 6 - 512 . 
     In some examples, the facial interface  13 . 6 - 512  includes a connector  13 . 6 - 536 . The connector  13 . 6 - 536  can be electrically connected one or more of the sensors  13 . 6 - 524  via a wired or wireless connection  13 . 6 - 543 . In some examples, the facial interface  13 . 6 - 512  is in electrical communication with a display via the wired connection link  13 . 6 - 543  and/or the connector  13 . 6 - 536 , such as, a braided electrical cable, a pogo connection, or other type of wired connection. In some examples, the facial interface  13 . 6 - 512  can be in electrical communication with the display  13 . 6 - 208  via a wireless connection (e.g., low-power Bluetooth BLE connection). In some examples, data or signals from the sensors  13 . 6 - 524  are transmitted through the link  13 . 6 - 543  to the connector  13 . 6 - 536 , where they are further transmitted to the HMD itself to be processed. Thus, in some examples, the facial interface  13 . 6 - 512  serves as an interconnect between the retention band and the display unit. In some examples, the retention band can mate directly into the display unit, bypassing the facial interface  13 . 6 - 612 . In some examples, the facial interface  13 . 6 - 512  can include an Orion style connection (e.g., 3 contact pads and 3 contact pins). 
     In some examples, the facial interface  13 . 6 - 512  includes a bridging feature  13 . 6 - 541  structurally connecting the first side  13 . 6 - 540   a  and the second side  13 . 6 - 540   b . In some examples, the bridging features  13 . 6 - 541  can provide a conduit for wires or electronics, such as the link  13 . 6 - 543 . In some examples, the bridging feature  13 . 6 - 541  is flexible permitting the facial interface first side  13 . 6 - 540   a  to move, pivot, or change shape relative to the facial interface second side  13 . 6 - 540   b . In some examples, the bridging feature  13 . 6 - 541  is rigid maintaining the facial interface first side  13 . 6 - 540   a  unmoved relative to the facial interface second side  13 . 6 - 540   b.    
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 6 - 5    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 6 - 5   . 
       FIGS.  13 . 6 - 6    illustrates a perspective view of select components of a head-mountable device  13 . 6 - 600 . The HMD  13 . 6 - 600  can be substantially similar to, including some or all of the features of, the HMDs described herein. The HMD  13 . 6 - 600  can include chassis or a frame  13 . 6 - 609  that can at least partially house a display, and a facial interface  13 . 6 - 612  that can contact a user&#39;s face. In some examples, the HMD  13 . 6 - 600  changes shape in response to a signal from one or more sensor(s)  13 . 6 - 624   a ,  13 . 6 - 624   b ,  13 . 6 - 624   c  (collectively “sensors  13 . 6 - 624 ”). The frame  13 . 6 - 609  can be connected via a connector(s)  13 . 6 - 615  to the facial interface  13 . 6 - 612 . In some examples, the connectors  13 . 6 - 615  are electrical and/or mechanical connectors configured to establish electrical and/or mechanical connections between the frame  13 . 6 - 609  and the facial interface  13 . 6 - 612 . In some examples, the connectors  13 . 6 - 615  are articulable or movable, such that the connectors  13 . 6 - 615  can move to change a shape of the HMD  13 . 6 - 600 . In some examples, the HMD  13 . 6 - 600  can include strain gauges or point load pressure sensors that can be used on posts  13 . 6 - 615  to act as pressure sensors to extrapolate retention band tension. The system can then guide the user to adjust for proper fitting of the HMD (e.g., prompt to tighten or loosen or change an angle of the band. In some examples, the display can illustrate for the user how to adjust band up/down based on the detected tension. In some examples, semiconductor(s) can be embedded in the facial interface (e.g., in the foam of the facial interface). 
     In some examples, the facial interface  13 . 6 - 612  can be integrated with one or more feedback modules or outputs  13 . 6 - 650 ,  13 . 6 - 653 . The sensors  13 . 6 - 624  can receive biometric input from a user. Based on the input detected by the sensors  13 . 6 - 624 , the outputs  13 . 6 - 650 ,  13 . 6 - 653  can perform some action. The outputs  13 . 6 - 650 ,  13 . 6 - 653  can include LEDs, haptics, speakers, motors, displays, or any other feedback devices that creates an output in response to the signals of biometric information from the sensors  13 . 6 - 624 . For example, one or more of the sensors  13 . 6 - 624  can detect when the HMD is donned, for example through capacitive or pressure sensors. The sensors  13 . 6 - 624  can then generate a signal to be analyzed by a processor. The processor can then cause a first feedback output  13 . 6 - 650 , such as an LED to emit a certain color of light indicating that the head-mountable device  13 . 6 - 600  is donned. In some examples, the second feedback output  13 . 6 - 653  can be a feedback output such as a haptic feedback output alerting a user of a notification (e.g., message notification, text notification, email notification, etc.) In some examples, the output  13 . 6 - 653  can be a display visible by the user and which can extend the peripheral view of the user. It should be appreciated by one of normal skill in the art that the above example is one example and other examples of feedback outputs are contemplated herein. 
     In some examples, the chassis or frame  13 . 6 - 609  can at least partially define an exterior surface of the HMD  13 . 6 - 600 . In some examples, health sensors  13 . 6 - 616  can be mounted, attached, or otherwise integrated into/onto the chassis  13 . 6 - 609 . While two health sensors  13 . 6 - 616  are illustrated, it will be understood that one, two, or more than two sensors can be used and are contemplated by the present disclosure. The health sensors  13 . 6 - 616  can be substantially similar to, including some or all of the features of, the sensor described herein (e.g., sensors  13 . 6 - 624 ). In some examples, the health sensors  13 . 6 - 616  can be non-contact sensors (i.e., the health sensors  13 . 6 - 616  can collect the intended data without needing to be in direct physical contact with the user. The health sensors  13 . 6 - 616  can be positioned on a bottom surface of the chassis  13 . 6 - 609 . The health sensors  616  can at least partially define an exterior surface of the HMD  13 . 6 - 600 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 6 - 6    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 6 - 6   . 
     13.7: Health Sensing Retention Band 
     According to some aspects of the present disclosure, a head-mountable device can include a housing, a display positioned in the housing, a processor positioned in the housing, and a retention band connected to the housing, the retention band including a sensor to monitor a brain activity of a user. 
     In some examples, the retention band includes a sensor array positionable adjacent a back of a head of the user when the head-mountable device is worn by the user. The sensor array can detect brain activity of the user. The sensor can transmit a signal to the processor, the signal based on the biometric information. The processor can analyze the signal and cause the head-mountable device to perform an action in response to the analysis of the signal. The action can be at least one of providing a visual feedback, providing an audio feedback, or providing a haptic feedback. 
     In some examples, the retention band is removably attached to the housing. The sensor can be removably attached to the retention band. The retention band can be in electrical communication with the display. The retention band can be pivotably attached to the housing. 
     According to some aspects, a headband for a head-mountable device can include a first end to attach to the head-mountable device, a second end to attach to the head-mountable device, and a sensor positioned between the first end and the second end. The sensor can detect brain activity of a user, and generate a signal based on the detected brain activity. 
     In some examples, the sensor can be embedded in the headband. The headband can include an area that is transparent to the signal emitted by the sensor. The headband can be articulable relative to the head-mountable device. The headband can include an expandable surface area. The signal can include a display command signal. 
     According to some aspects, a wearable electronic device can include a display, a retention band attached to the display, the retention band having a first orientation and a second orientation, and a sensor connected to the retention band. The sensor can detect a brain activity of a user and produce a signal based on the brain activity. A processor can perform a first analysis of the signal in response to the retention band being in the first orientation, and can perform a second analysis of the signal in response to the retention band being in the second orientation. 
     In some examples, the wearable electronic device can perform an action in response to the signal. The wearable electronic device can include a head-mountable device. The retention band can be adjustable. The sensor can perform at least one of functional near-infrared spectroscopy or electroencephalography. 
     In some examples, the retention band is movable between a first position and a second position. The sensor can be oriented toward a first brain region in the first position, and a second brain region in the second position when the wearable electronic device is worn by the user. The retention band can produce at least one of visual, audio, or haptic feedback in response to the signal from the sensor. The sensor can be a first sensor. The wearable device can further include a second sensor connected to the retention band. The second sensor can collect vital signs. 
     The present disclosure includes details of a retention band for a head-mountable device that is integrated with health sensors. The integrated retention band can enable both optical and electrical neural imaging of the user&#39;s head. In some examples, the sensor-integrated headband can be used for brain imaging. For example, the sensors can be configured to perform functional near-infrared spectroscopy (FNIR). In some examples, the sensors can be used to perform EEG (electroencephalography). 
     The retention band can be adjustable or articulable to position the sensors at desired locations on the user&#39;s head. By having the ability to adjust the retention band, the sensors can observe specific areas of the brain. For example, the position of the sensors can be tuned or tailored to look at brain areas related to language, learning, memory, comprehension, sleep, stress, pain, attention, fear, discomfort, etc. 
     In some examples, the retention band can be integrated with a sensor array that includes one or more transmitters that emit a signal. The transmitter(s) can be positioned a certain distance away from one or more detectors that sense the emitted signal. Based on the received signals by the detectors, a processor can infer or determine certain brain activity. In some examples, the integrated sensors on the retention band form a brain-computer interface (BCI), such as a non-invasive neural interface. In some examples, the integrated sensor array can be used to detect the parieto-frontal network of the brain. 
     The HMD can include an output or feedback module that provides feedback based on the detections from the sensors. The feedback from the system can include displaying visualizations to the user. For example, the feedback can include breathing visualizations, visualization that relate to the user&#39;s attention, promptings or suggestions to the user to take a break or change an activity, or make other recommendations or notifications to the user. 
     These and other embodiments are discussed below with reference to  FIGS.  13 . 7 - 1 - 13 . 7 - 6   . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these Figures is for explanatory purposes only and should not be construed as limiting. Furthermore, as used herein, a system, a method, an article, a component, a feature, or a sub-feature comprising at least one of a first option, a second option, or a third option should be understood as referring to a system, a method, an article, a component, a feature, or a sub-feature that can include one of each listed option (e.g., only one of the first option, only one of the second option, or only one of the third option), multiple of a single listed option (e.g., two or more of the first option), two options simultaneously (e.g., one of the first option and one of the second option), or combination thereof (e.g., two of the first option and one of the second option). 
       FIGS.  13 . 7 - 1    shows a schematic block diagram of a head-mountable device  13 . 7 - 100  including a display  13 . 7 - 104 , a retention band  13 . 7 - 108 , a memory  13 . 7 - 106 , and a processor  13 . 7 - 120 . The display  13 . 7 - 104  can include one or more optical lenses or display screens in front of the eyes of a user. The display  13 . 7 - 104  can present an augmented reality visualization, a virtual reality visualization, or other suitable visualization to a user. Additionally, the display  13 . 7 - 104  can be positioned in or on a housing or frame that supports the display  13 . 7 - 104  and additional electronic components. The display  13 . 7 - 104  in combination with the frame or the housing and other electronic components supported by the housing can be referred to as the display unit  13 . 7 - 104  and the display  13 . 7 - 104 . The retention band  13 . 7 - 108  can be connected to the frame or housing of the display  13 . 7 - 104 . 
     In some examples, the retention band  13 . 7 - 108  can include a support, strap, headband, belt, arms, or any other attachment mechanism capable of supporting the HMD  13 . 7 - 100  on the user&#39;s head. In some examples, the retention band  13 . 7 - 108  can include a pliant (or semi-pliant) material or fabric that wraps partially around the user&#39;s head. 
     The retention band  13 . 7 - 108  can be made from a flexible material and can securely and snuggly fit around a head of the user. The retention band  13 . 7 - 108  can be made from a woven fabric, leather, polymer, or any other material compatible to be integrated with the sensors  13 . 7 - 112  or having electromagnetic transmissive properties. In some examples, the retention band  13 . 7 - 108  can be made from silicone or thermoplastic polyurethane (TPU). In some examples, the retention band  13 . 7 - 108  can be made from compression molded materials, such as rubber. The sensors  13 . 7 - 112  can be integrated into the compression molded materials. In some examples, the retention band  13 . 7 - 108  is semi-rigid. For example, the retention band  13 . 7 - 108  can be rigid where the sensors  13 . 7 - 112  are located. The sensors  13 . 7 - 112  can be flexible or deformable relative to each other such that the sensor array can move, flex, or stretch with the retention band  13 . 7 - 108 . The retention band  13 . 7 - 108  can include rigid sections where electronic components are located. 
     The retention band  13 . 7 - 108  can include two ends, straps or sidebands  13 . 7 - 108   a  that connect to the display  13 . 7 - 104 . The retention band  13 . 7 - 108  can include a middle section  13 . 7 - 108   b  that is positioned between the ends  13 . 7 - 108   a  of the retention band  13 . 7 - 108 . The middle section  13 . 7 - 108   b  can configured to contact the back of the user&#39;s head, while the sidebands  13 . 7 - 108   a  can be considered to contact the sides of the user&#39;s head. The retention band  13 . 7 - 108  can include pivots  13 . 7 - 115 . The pivots  13 . 7 - 115  can be located between the display  13 . 7 - 104  and the sidebands  13 . 7 - 108   a  such that the entire retention band  13 . 7 - 108  can pivot or rotate relative to the display  13 . 7 - 104 . In some examples, the pivots  13 . 7 - 115  can be positioned between the sidebands  13 . 7 - 108   a  and the middle section  13 . 7 - 108   b  to enable the middle section  13 . 7 - 108   b  to pivot or rotate relative to the sidebands  13 . 7 - 108   b  and display  13 . 7 - 104 . Further detail of the pivots will be provided below with regard to  FIGS.  13 . 7 - 4 A- 13 . 7 - 4 C . 
     The retention band  13 . 7 - 108  can include one or more sensors  13 . 7 - 112 . It will be understood that throughout the disclosure, reference to “sensors” can refer to one or more sensors. The phrase “sensors” allows for, but does not necessarily require, multiple sensors. In some examples, the sensors  13 . 7 - 112  can be removably attached to the retention band  13 . 7 - 108 . The retention band  13 . 7 - 108  can be removably attached to the display  13 . 7 - 104  and in electrical communication with the display  13 . 7 - 104 . In other words, the sensors  13 . 7 - 112  can be in electrical communication with the display  13 . 7 - 104 . 
     The sensor  13 . 7 - 112  of the retention band  13 . 7 - 108  can collect biometric information, such as brain activity and vital signs (including body temperature, pulse data, respiration data, and blood pressure). The sensor  13 . 7 - 112  can generate a signal based on the collected user information and transmit the signal to the processor  13 . 7 - 120  to be analyzed. In some examples, the memory  13 . 7 - 106  can include programmed instructions responsive to the signals from the sensors  13 . 7 - 112 . In some examples, the instructions can cause an output or feedback module to perform an action in response to the signal (i.e., in response to the biometric information collected by the sensor  13 . 7 - 112 ). In some examples, the retention band  13 . 7 - 108  itself provides feedback (e.g., through LEDs, tactile or haptic feedback, changing its shape, color, etc.) based on the collected biometric information. 
     For example, a user may be in a meditation or mental awareness session that is facilitated by the HMD  13 . 7 - 100 . During the activity, the user&#39;s brain waves or other biometric may change in a way that is detectable by the sensors  13 . 7 - 112 . The sensors  13 . 7 - 112  can generate one or more signals based on the received input. The sensors  13 . 7 - 112  can transmit the signals to the processor  13 . 7 - 120 , which may cause an action by one or more components of the HMD  13 . 7 - 100  (e.g., to the display  13 . 7 - 104  to provide a visual feedback to the user). The display  13 . 7 - 104 , being in electrical communication with the retention band  13 . 7 - 108 , can receive electrical communication and provide feedback to the user related to their biometric readings (e.g., visual feedback, audio feedback, haptic feedback, etc.). Feedback can include determining when the user needs to take a break, or when the difficulty of an activity needs to be lowered or raised, the output can include scheduling various activities for the user or recommending adjustment of the HMD  13 . 7 - 100 . 
     The sensors  13 . 7 - 112  can include a variety of different sensing devices. The sensors  13 . 7 - 112  can include, but are not limited to, brain activity sensors, a camera or imaging device, temperature device, oxygen device, movement device, brain activity device, sweat gland activity device, breathing activity device, muscle contraction device, etc. In some examples, the sensor can detect or sense biometric features including features of the autonomic nervous system. 
     Some particular examples of sensors include an electrooculography (EOG) sensor, electrocardiogramhy (ECG or EKG) sensor, EEG (electroencephalography), photoplethysmography (PPG) sensor heart rate sensor, hear rate variability sensor, blood volume pulse sensor, oxygen saturation (SpO2) sensor, compact pressure sensor, electromyography (EMG) sensor, core-body temperature sensor, galvanic skin response (GSR) sensor, functional near-infrared spectroscopy (FNIR) sensor, functional magnetic infrared imaging (FMRI) sensor, non-contact passive infrared (IR) sensor, accelerometer, gyroscope, magnetometer, inclinometer, barometer, infrared sensor, global positioning system sensor, etc. 
     In some examples, the sensors  13 . 7 - 112  can include, contact microphones (e.g., pressure-based MEMS), bioelectrical activity sensors, UV exposure sensors, or particle sensors. In some examples, certain sensors can be used to assess stress and emotion. In some examples, the sensors can operate through coin cell battery or Bluetooth connectivity. In some examples, the sensors are powered by a primary battery of the HMD. 
     The sensors  13 . 7 - 112  described herein can allow for observations of the autonomic nervous system (ANS), to observe relaxation and stress indicators, mental health, medical treatments, etc. Using the sensors, physicians and care takers could have live feedback of biometrics. Use cases can include fitness settings, user content, workplace, telepresence, clinical, education, training, pain, therapy, etc. In some examples, the sensors can be used to capture facial expressions. This is particularly relevant given the user&#39;s face is covered by HMD. For example, the HMD could use MEMS or motion tracking sensors to detect facial expressions. 
     In some examples, the sensors  13 . 7 - 112  can be used to perform EEG detections. The sensors  13 . 7 - 112  can measure the electrical activity in the cerebral cortex (the outer layer of the brain). The sensors  13 . 7 - 112  can include electrodes that are placed on a participant&#39;s head, then the electrodes can non-invasively detect brainwaves from the subject. The EEG sensors  13 . 7 - 112  can record up to several thousands of snapshots of the electrical activity generated in the brain every second. The recorded brainwaves can be sent to amplifiers, then to the processor  13 . 7 - 120 , a remote electronic device, or the cloud to process the data. 
     In some examples, the sensors  13 . 7 - 112  can be used to perform functional near-infrared spectroscopy (FNIR). The sensors  13 . 7 - 112  can use low levels of non-ionizing light to record changes in cerebral blood flow in the brain through the optical sensors  13 . 7 - 112  placed on the surface of the scalp. The signals can be recorded via flexible fiber optic cables. 
     The HMD  13 . 7 - 100  can be a wearable device can include electronic components that are communicatively coupled to each other and to the sensors  13 . 7 - 112  via a wired or wireless communications link. The communications link can be a physical connection, such as an electrical wire, or can be a wireless connection, such as Bluetooth, Wi-Fi, proximity sensors, etc. In some examples, the HMD  13 . 7 - 100  can be communicatively coupled to a companion electronic device, such as a remote, or a personal computing device such as a smart phone, a smart watch, a laptop, a tablet, an HMD, or any other form of electronic device. As described in further detail below, the signals from the sensors  13 . 7 - 112  can influence the HMD  13 . 7 - 100 . For example, the sensors  13 . 7 - 112  can influence the visual information, content, style, frequency, and operation of the content provided by the sensors  13 . 7 - 212 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 7 - 1    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 7 - 1   . 
       FIGS.  13 . 7 - 2    shows a top view of a head-mountable device (HMD)  13 . 7 - 200 . The HMD  13 . 7 - 200  can be substantially similar to, and can include some or all of the features of, the head-mountable devices described herein, such as HMD  13 . 7 - 100 . The HMD  13 . 7 - 200  can include a display unit  13 . 7 - 204  and a retention band  13 . 7 - 208 . The display unit  13 . 7 - 204  can include electronic components, such as a display, processor, memory, controller, battery, and other circuitry and electronics. The display unit  13 . 7 - 204  can include a frame or housing structurally supporting the electronic components. In some examples, the display  13 . 7 - 204  includes an opaque, translucent, transparent, or semi-transparent screen, including any number lenses, for presenting visual data. 
     The frame can at least partially border one or more edges of the display. In some examples, the frame can be configured to contact a user&#39;s head or face. In some examples, the frame blocks outside light and limits the peripheral view of the user. Various components of the display unit  13 . 7 - 204  can be housed within the frame. For example, the hardware and electronics which allow functionality of the HMD can be housed within the frame. In some examples, the frame can include attachment mounts for connecting the display unit  13 . 7 - 204  to the retention band  13 . 7 - 208 . For example, the frame can include mechanical attachments, magnetic attachments, or any other suitable connector to attach the retention band  13 . 7 - 208  to the display unit  13 . 7 - 204 . 
     The HMD  13 . 7 - 200  can be worn on the user&#39;s head such that the display unit  13 . 7 - 204  is positioned over the user&#39;s face and disposed over one or both of the user&#39;s eyes. In some examples, the retention band  13 . 7 - 208  can be positioned against (i.e., contacting or pressing against) the side and/or back of a user&#39;s head when worn. In some examples, the retention band  13 . 7 - 208  can be at least partially positioned above the user&#39;s ear or ears. In some examples, the retention band  13 . 7 - 208  can be positioned adjacent to the user&#39;s ear or ears. The display unit  13 . 7 - 204  and the retention band  13 . 7 - 208  can form a loop defining an aperture  13 . 7 - 205  for the user&#39;s head. It should be understood, however, that this configuration is just one example of how the components of an HMD  13 . 7 - 200  can be arranged, and that in some examples, a different number of connector straps and/or retention bands can be included. 
     In some examples, the retention band  13 . 7 - 208  includes a first sensor array  13 . 7 - 212   a  including one or more sensors. The sensor array  13 . 7 - 212   a  can be positioned on the back of the retention band  13 . 7 - 208 , located to contact or be proximate the back of the user&#39;s head when the HMD  13 . 7 - 200  is donned by the user. In some examples, the retention band  13 . 7 - 208  includes a second sensor array  13 . 7 - 212   b  including one or more sensors. The second sensor array  13 . 7 - 212   b  can be positioned on a side strap or arm of the retention band  13 . 7 - 208 , located to contact or be proximate the side of the user&#39;s head when the HMD  13 . 7 - 200  is donned by the user. In some examples, the first sensor array  13 . 7 - 212   a  and the second sensor array  13 . 7 - 212   b  are separate and distinct, both in form and functionality. The first sensor array  13 . 7 - 212   a  can include different sensors than the second sensor array  13 . 7 - 212   b.    
     In some examples, the first sensor array  13 . 7 - 212   a  can translate or move along the retention band  13 . 7 - 208 . For examples, the first sensor array  13 . 7 - 212   a  can slide from a first position on the retention band  13 . 7 - 208  (e.g., the back of the band) to a second position on the retention band  13 . 7 - 208  (e.g., the side strap). Translation of the sensor array  13 . 7 - 212   a  can be manually or automatically actuated. In the manner, the sensor array  13 . 7 - 212   a  can move to monitor different regions of the user&#39;s head. 
     In some examples, the first sensor array  13 . 7 - 212   a  and the second sensor array  13 . 7 - 212   b  (collectively referred to as “sensors  13 . 7 - 212 ”) include the same sensors and are considered part of a single array of sensors. The sensors  13 . 7 - 212  can be positioned on an interior surface  13 . 7 - 207  of the retention band  13 . 7 - 208 . In some examples, the sensors  13 . 7 - 212  are positioned in the retention band  13 . 7 - 208  (i.e., between the interior surface  13 . 7 - 207  and the exterior surface  13 . 7 - 209 . In some examples, the sensors  13 . 7 - 212  can span substantially across an entirety of a length of the retention band  13 . 7 - 208 . As will be discussed in greater detail herein, the sensors  13 . 7 - 212  can be integrated with the retention band  13 . 7 - 208  in a variety of ways. 
     In some examples, the sensors  13 . 7 - 212  can be used to perform EEG detections. The sensors  13 . 7 - 212  can measure the electrical activity in the cerebral cortex (the outer layer of the brain). The sensors  13 . 7 - 212  can include electrodes that are placed on a participant&#39;s head, then the electrodes can non-invasively detect brainwaves from the subject. The EEG sensors  13 . 7 - 212  can record up to several thousands of snapshots of the electrical activity generated in the brain every second. The recorded brainwaves can be sent to amplifiers, then to the HMD computer, a remote electronic device, or the cloud to process the data. 
     In some examples, the sensors  13 . 7 - 212  can be used to perform functional near-infrared spectroscopy (FNIR). The sensors  13 . 7 - 212  can use low levels of non-ionizing light to record changes in cerebral blood flow in the brain through the optical sensors  13 . 7 - 212  placed on the surface of the scalp. The signals can be recorded via flexible fiber optic cables. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 7 - 2    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 7 - 2   . Further details regarding the integration of sensors into an HMD retention band are described below with reference to  FIGS.  13 . 7 - 3   . 
       FIGS.  13 . 7 - 3    shows a cross-sectional side view of a head-mountable device (HMD)  13 . 7 - 300 . The HMD  13 . 7 - 300  can be substantially similar to, and can include some or all of the features of, the head-mountable devices described herein, such as HMDs  13 . 7 - 100  and  13 . 7 - 200 . In some examples, sensor(s)  13 . 7 - 312  positioned in/on the retention band  13 . 7 - 308  can be operatively coupled to one or more electronic components in the display unit  13 . 7 - 304 . For example, the sensors  13 . 7 - 312  can be electrically coupled a power source, such as a battery  13 . 7 - 335  that is disposed within or on the housing of the display unit  13 . 7 - 304  via an electrical connection  13 . 7 - 311 . The sensor  13 . 7 - 312  can be communicatively coupled with a processor  13 . 7 - 320  or other controller that is housed within the head-mountable device  13 . 7 - 300 . The battery  13 . 7 - 335  and/or the processor  13 . 7 - 320  can be located with the housing of the display unit  13 . 7 - 304 , located within the retention band  13 . 7 - 108 , disposed exterior to the housing of the display unit  13 . 7 - 304  and the retention band  13 . 7 - 108 , or combinations thereof. The sensor  13 . 7 - 312  can generate and transmit signals reflective of the biometrics of the user to the processor  13 . 7 - 320 . The processor  13 . 7 - 320  can then cause commands or instructions to be issued. 
     Although the sensors  13 . 7 - 312  are shown as being connected to the battery  13 . 7 - 335  and the processor  13 . 7 - 320  through a wired connection  13 . 7 - 311 , in some examples, the sensor  13 . 7 - 312  can wirelessly receive data and/or power from the battery  13 . 7 - 335  and/or processor  13 . 7 - 320  by any desired method or technology. In some examples, the sensors  13 . 7 - 312  can be communicatively coupled to an external device that is not located on the HMD  13 . 7 - 300 . 
     Further, although the components of the wearable electronic device  13 . 7 - 300  are shown as being connected to one another at certain locations, it should be understood that any of the components of the HMD  13 . 7 - 300  can be electrically and/or mechanically connected to one or more of any of the other components of the HMD  13 . 7 - 300 , in any manner and location, as desired. 
     In some examples, the retention band  13 . 7 - 308  can include other operational or functional components in addition to the sensors  13 . 7 - 312 . For example, the retention band  13 . 7 - 108  can include a battery  13 . 7 - 335  that can be in electrical communication with the display unit  13 . 7 - 304  and/or the sensors  13 . 7 - 312 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 7 - 3    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 7 - 3   . 
       FIGS.  13 . 7 - 4 A  shows a rear perspective view of a retention band  13 . 7 - 408 . The retention band  13 . 7 - 408  can be substantially similar to, and can include some or all of the features of, the retention bands described herein, such as retention bands  13 . 7 - 108 ,  13 . 7 - 208 , and  13 . 7 - 308 . While  FIGS.  13 . 7 - 4 A  illustrates the sensors  13 . 7 - 412  being visible on an exterior of the retention band  13 . 7 - 408 , it will be understood that in some examples, the sensors  13 . 7 - 412  are concealed or hidden behind a cover or layer of the retention band  13 . 7 - 408 . The sensors  13 . 7 - 412  can be configured to directly contact or touch the user&#39;s head. In some examples, the sensors  13 . 7 - 412  are non-contact sensors. In some examples, the retention band  13 . 7 - 408  is made up of multiple components. For example, the retention band  13 . 7 - 408  can include sidebands or arms  13 . 7 - 408   a  and a middle section or back  13 . 7 - 408   b  (collectively retention band  13 . 7 - 408 ). The back  13 . 7 - 408   b  can be connected at each end to the arms  13 . 7 - 408   a  via pivots  13 . 7 - 415 . 
     In some examples, the sensors  13 . 7 - 412  can be interconnected (i.e., electrically connected) via wires  13 . 7 - 411 . The wires  13 . 7 - 411  can further define a communications link between the display unit and the retention band  13 . 7 - 408 . The sensors  13 . 7 - 412  can be disposed substantially across the entirety of the back  13 . 7 - 408   b  of the retention band  13 . 7 - 408 . In some examples, the sensors  13 . 7 - 412  can be intentionally positioned to align with specific portions of the user&#39;s head when donned. As shown in greater detail with reference to  FIGS.  13 . 7 - 4 B and  13 . 7 - 4 C , the retention band  13 . 7 - 408  can be articulable. By adjusting, articulating, or moving the position of the retention band  13 . 7 - 408  relative to the HMD, the sensors  13 . 7 - 412  can be positioned at various regions of the user&#39;s head. 
     In some examples, the sensors  13 . 7 - 412  can include a grounding sensor. For example, one or more of the sensors  13 . 7 - 412  can be a grounding sensor that is used as a reference sensor. Detections from the sensors  13 . 7 - 412  can be compared against the grounding reference sensor to analyze the detections. In some examples, the sensors  13 . 7 - 412  can included rounded edges and/or elastomeric or soft material that is transparent to the signal produced by the sensors  13 . 7 - 412 . The rounded edges and soft material can help to reduce discomfort when the sensors  13 . 7 - 412  are contacting the user&#39;s head. In some examples, the geometry of the sensors  13 . 7 - 412  can be specifically designed to assist in parting the user&#39;s hair to ensure efficient contact against the user&#39;s head. In some examples, the sensors move (e.g., vibrate, rotate or oscillate) to aid in proper contact against the user&#39;s skull. 
     In some examples, the retention band  13 . 7 - 408  is a specialized band that can be interchanged with other headbands that are customized for different uses. For example, there can be individual custom retention bands for education, sports, industrial applications, health, learning application, training application, etc. Each different retention band can be integrated with sensors specific for its needs. 
     In some examples, the retention band  13 . 7 - 408  can be detachable or removable from the HMD. In other words, the HMD can include a standard headband that is replaced or modified with the retention band  13 . 7 - 408 . In some examples, the retention band  13 . 7 - 408  can be an add-on component that is added to the HMD when desired or needed. The retention band  13 . 7 - 408  can include electrical/mechanical attachment mechanisms to attach to the HMD. For example, the retention band  13 . 7 - 408  can include attachment mechanisms  13 . 7 - 418  that can electrically, mechanically, and or magnetically connect the retention band  13 . 7 - 408  to an HMD. In some examples, the retention band wirelessly connects to the HMD to transmit data. The retention band  13 . 7 - 408  can include its own power source and processing capabilities. 
     In some examples, the retention band  13 . 7 - 408  can be expandable to reveal additional sensors  13 . 7 - 412 . For example, the retention band  13 . 7 - 408  can fold or overlap itself to reduce a surface area of the retention band  13 . 7 - 408 . Then when desired, the retention band  13 . 7 - 408  can be stretched, unfolded, or expanded, revealing additional sensors that were previously covered. 
       FIGS.  13 . 7 - 4 B  shows a side view of the retention band  13 . 7 - 408  rotated upward. As illustrated, the back  13 . 7 - 408   b  of the retention band  13 . 7 - 408  can rotate relative to the side arms  13 . 7 - 408   a  via the pivots  13 . 7 - 415 . In some examples, the entire retention band  13 . 7 - 408  can rotate relative to a housing or frame of the HMD (i.e., the pivots  13 . 7 - 415  can be located on the HMD housing). Movement of the retention band  13 . 7 - 408  can be manual or automated. In some examples, the user is prompted, guided, or instructed to move the retention band  13 . 7 - 408  into a certain position or configuration. For example, the system may prompt the user to adjust the retention band  13 . 7 - 408  to optimize the ability of the sensors  13 . 7 - 412  to detect the desired biometric data. In this configuration the sensors  13 . 7 - 412  would be positioned at or near the top of the user&#39;s head. 
     In some examples, the HMD include multiple retention bands  13 . 7 - 408 . For example, the HMD can include a back-of-the-head band that wrap around the back of the user&#39;s head, and an over-the-head band that wraps over the top of the user&#39;s head. Each of the bands can include the sensors described herein. In some examples, the retention band  13 . 7 - 408  can be a bifurcated strap with sensors on each bifurcated section to increase the coverage area of the sensors over the head. 
       FIGS.  13 . 7 - 4 C  shows a side view of the retention band  13 . 7 - 408  rotated downward. As illustrated, the back  13 . 7 - 408   b  of the retention band  13 . 7 - 408  can rotate relative to the side arms  13 . 7 - 408   a  via the pivots  13 . 7 - 415 . In some examples, the entire retention band  13 . 7 - 408  can rotate relative to a housing or frame of the HMD (i.e., the side straps  13 . 7 - 408   a  also rotate and the pivots  13 . 7 - 415  are located on the HMD housing). Movement of the retention band  13 . 7 - 408  can be manual or automated. In some examples, the user is prompted, guided, or instructed to move the retention band  13 . 7 - 408  into a certain position or configuration. For example, the system may prompt the user to adjust the retention band  13 . 7 - 408  downward to optimize the ability of the sensors  13 . 7 - 412  to detect the desired biometric data. In this configuration the sensors  13 . 7 - 412  would be positioned at or near the bottom of the user&#39;s head, near the top of the neck. In this configuration, the sensors  13 . 7 - 412  can be used to detect information about neck muscles. In some examples, the location or position of the retention band  13 . 7 - 408  can determine what type of analysis is performed by the processor. For example, the processor can interpret a signal differently based on the detected position of the retention band  13 . 7 - 408  relative to the HMD and/or user when the signal was acquired. In some examples, the signal analysis changes based on the region of the brain being monitored by the sensors  13 . 7 - 412 . In some examples, operation of the sensors themselves changes based on the position of the retention band  13 . 7 - 408 . For example, different sensors can be used when monitoring different areas of the brain, or different sensor emission/detections can be used based on the location of the sensors relative to the user. According to one example, a position detecting IMU, sensor, encoder, or the like can be positioned in the retention band  13 . 7 - 408  to detect the relative position of the band and generate a signal indicative of the position. The processor can then use that signal to determine the type of analysis to be performed on the data collected from the sensors  13 . 7 - 412 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 7 - 4 A- 13 . 7 - 4 C  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 7 - 4 A- 13 . 7 - 4 C . 
       FIGS.  13 . 7 - 5    illustrates a perspective exploded view of an HMD  13 . 7 - 500 . The HMD  13 . 7 - 500  can be substantially similar to, and can include some or all of the features of, any of the HMDs described herein, such as HMD  13 . 7 - 100 ,  13 . 7 - 200 ,  13 . 7 - 300 , and  13 . 7 - 500 . The HMD  13 . 7 - 500  can include a display unit  13 . 7 - 504  and a retention band  13 . 7 - 508 . The retention band  13 . 7 - 508  can include an inner portion  13 . 7 - 507  that contacts a user&#39;s head when worn, and an outer portion  13 . 7 - 509  at least partially defining an exterior surface of the wearable device  13 . 7 - 500 . The outer portion  13 . 7 - 509  can be integrally formed with the inner portion  13 . 7 - 507 . In other words, the inner portion  13 . 7 - 507  and the outer portion  13 . 7 - 509  can be made from a single piece of material (shown separated here for simplicity). In some examples, the outer portion  13 . 7 - 509  and the inner portion  13 . 7 - 507  can be coupled together, for example using adhesive, thread, or other coupling techniques. 
     In some examples, the HMD  13 . 7 - 500  can include a first sensor array  13 . 7 - 512   a  and a second sensor array  13 . 7 - 512   b  (collectively referred to as “sensors  13 . 7 - 512 ”). In some examples, the inner portion  13 . 7 - 507  can include a material that is transmissive to the sensors  13 . 7 - 512 . The sensors  13 . 7 - 512  can be positioned between the inner portion  13 . 7 - 507  and the outer portion  13 . 7 - 509 , such that the sensors  13 . 7 - 512  are positioned within the retention band  13 . 7 - 508 . In some examples, the inner portion  13 . 7 - 507  of the retention band  13 . 7 - 508  can include a perforated section adjacent the sensors  13 . 7 - 512 . The perforated section can include holes, gaps, or apertures formed in the material of the inner portion  13 . 7 - 507 . 
     The holes in the inner portion  13 . 7 - 507  can correspond to locations of the sensors  13 . 7 - 512  and can allow signals from the sensors, or the sensors themselves to pass through the holes. In other words, the sensors  13 . 7 - 512  can be exposed and accessible at the inner portion  13 . 7 - 507 , for example, to contact the user&#39;s head. In some examples, the sensors  13 . 7 - 512  are embedded or sandwiched between the inner portion  13 . 7 - 507  and the outer portion  13 . 7 - 509  of the retention band  13 . 7 - 508 . In some examples, the sensors  13 . 7 - 512  can be position adjacent a sensorially transparent or transmissive window  13 . 7 - 530  that allows for the transmission and reception of signals between the user&#39;s head and the sensors  13 . 7 - 512 . The sensors  13 . 7 - 512  can be electrically coupled to an electronic component  13 . 7 - 520  in the display unit  13 . 7 - 504 , such as a processor or battery, via one or more electrical connections  13 . 7 - 511 . 
     In some examples, the sensor arrays  13 . 7 - 512  can flexible and capable of bending or curving to according to a shape of the retention band  13 . 7 - 508  on the user&#39;s head. In some examples, the sensors  13 . 7 - 512  include flex cables and/or optical fibers or fiber optic cables that are woven into the fabric of the retention band  13 . 7 - 508 . In some examples, the retention band  13 . 7 - 508  can include conductive fibers woven into the fabric of the retention band  13 . 7 - 508 . 
     The retention band  13 . 7 - 508  can be made from a flexible material and can securely and snuggly fit around a head of the user. The retention band  13 . 7 - 508  can be made from a woven fabric, leather, polymer, or any other material compatible with micro-perforations or having electromagnetic transmissive properties. In some examples, the retention band  13 . 7 - 508  can be made from silicone or thermoplastic polyurethane (TPU). In some examples, the retention band  13 . 7 - 508  can be made from compression molded materials, such as rubber. The sensors  13 . 7 - 512  can be integrated into the compression molded materials. In some examples, the retention band  13 . 7 - 508  is semi-rigid. For example, the retention band  13 . 7 - 508  can be rigid where the sensors  13 . 7 - 512  are located. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 7 - 5    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 7 - 5   . 
       FIGS.  13 . 7 - 6    shows a side or cross-sectional view of portion of a retention band  13 . 7 - 608 . The retention band  13 . 7 - 608  can be substantially similar to, and can include some or all of the features of, the retention bands described herein, such as retention band  13 . 7 - 108 ,  13 . 7 - 208 ,  13 . 7 - 308 ,  13 . 7 - 408 , and  13 . 7 - 508 . The retention band  13 . 7 - 608  can include one or more sensors  13 . 7 - 612   a ,  13 . 7 - 612   b , and  13 . 7 - 612   c  (collectively referred to as “sensors  13 . 7 - 612 ”). The sensors  13 . 7 - 612  can be partially or entirely embedded or encapsulated in the retention band  13 . 7 - 608 . The retention band  13 . 7 - 608  can include a first, interior surface  13 . 7 - 607  facing the user&#39;s head when donned, and a second, exterior surface  13 . 7 - 609  facing away from the user and defining an exterior of the retention band  13 . 7 - 608 . In some examples, the first surface  13 . 7 - 607  directly contacts or touches the user&#39;s head when the HMD is donned or being worn by the user. 
     The sensor  13 . 7 - 612   a  can be positioned on the exterior surface  13 . 7 - 609  of the retention band  13 . 7 - 608 . In some examples, the first surface  13 . 7 - 607  can define an interior and the second surface  13 . 7 - 609  can be an exterior of the retention band  13 . 7 - 608 . Thus, the sensor  13 . 7 - 612   a  can be positioned on an exterior of the retention band  13 . 7 - 608 . In some examples, the retention band  13 . 7 - 608  can include a sensor  13 . 7 - 612   b  that is embedded, encapsulated, or otherwise surrounded by the retention band  13 . 7 - 608 . The retention band  13 . 7 - 608  can include, a sensor  13 . 7 - 612   c  that is positioned such that a portion of the sensor  13 . 7 - 612   c  is exposed through the inner surface  13 . 7 - 607 . In other words, the sensor  13 . 7 - 612   c  can at least partially define an interior of the retention band  13 . 7 - 608  and can directly contact or touch the user&#39;s head. In some examples, the sensor  13 . 7 - 612   c  can be partially surrounded or embedded in the retention band  13 . 7 - 608 . In some examples, the sensor  13 . 7 - 612   c  is externally attached to the retention band  13 . 7 - 608 , positioned on the interior of the retention band  13 . 7 - 608 . 
     The sensors  13 . 7 - 612   a  and  13 . 7 - 612   b  can have corresponding sensor areas  13 . 7 - 625   a ,  13 . 7 - 625   b , respectively. The sensor areas  13 . 7 - 625   a ,  13 . 7 - 625   b  can represent a field of view or cone of influence that is transparent to a signal emitted by the sensors  13 . 7 - 612   a ,  13 . 7 - 612   b . In some examples, the field of view of the sensors  13 . 7 - 612  can be approximately 50 degrees. The sensors  13 . 7 - 612   a ,  13 . 7 - 612   b  can detect physiological, biological, and/or biometric changes of the user&#39;s body through corresponding sensor areas  13 . 7 - 625   a ,  13 . 7 - 625   b . For example, the sensors  13 . 7 - 612   a ,  13 . 7 - 612   b  can detect changes to a user through the material of the retention band  13 . 7 - 608 . Other sensors may be added or substituted, as may be desired. In some examples, the sensor areas  13 . 7 - 625   a ,  13 . 7 - 625   b  can include a different material than the rest of the retention band  13 . 7 - 608 . For example, the sensor areas  13 . 7 - 625   a ,  13 . 7 - 625   b  can be transparent to certain signals from the sensors  13 . 7 - 612   a ,  13 . 7 - 612   b  or from the user, while the remainder of the retention band  13 . 7 - 608  is not transparent or transmissive to such signals. 
     In some instances, the closer positioning of sensor  13 . 7 - 612   b  to the first surface  13 . 7 - 607  than the sensor  13 . 7 - 612   a  can correspondingly reduce the field of view  13 . 7 - 625   b . In some examples, the sensor  13 . 7 - 612   c  may be flush with the inner surface  13 . 7 - 607  and/or directly contacting a user&#39;s head. This is only one example of sensor depth variation, as a plurality of sensors can be disposed on the first surface  13 . 7 - 607  of the retention band  13 . 7 - 608 , or between the first surface  13 . 7 - 607  and the second surface  13 . 7 - 609  of the retention band  13 . 7 - 608 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 7 - 6    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 7 - 6   . 
     13.8: Conductive Fabric Architecture 
     In some examples, a head mountable device includes a display, a housing at least partially surrounding the display, a facial interface attached to the housing, and a cover positioned between the housing and the facial interface, the cover including a conductive fabric. 
     In some examples of the head-mountable device, the facial interface directly contacts a face of a user, the cover blocks ambient light, the conductive fabric elastically deforms in response to a movement of the cover, and an electrical property of the conductive fabric detectably changes in response to the movement. 
     In additional examples, the conductive fabric is elastic. Additionally, the conductive fabric can be interwoven with the cover. During use, the geometry of the conductive fabric can change in response to a user input. In other examples, the conductive fabric is configured to detect a change in a distance between the housing and the facial interface. In yet other examples, the conductive fabric forms a serpentine pattern. In some instances, the conductive fabric includes an array of parallel conductive threads arranged perpendicular to a direction of deformation of the cover, and a distance between two of the parallel conductive threads changes in response to a facial movement of a user. The conductive fabric can include a user input member. 
     In some examples, a wearable electronic device can include a display, a frame at least partially surrounding the display, and a light-blocking material attached to the frame, the light-blocking material including a conductive fabric. 
     In some examples the conductive fabric elastically deforms in response to a facial movement of a user. Additionally, a capacitance of the conductive fabric can detectably change in response to a deformation of the conductive fabric. In some examples, the conductive fabric includes or acts as a switch. In other examples, the conductive fabric electrically connects a first electronic component and a second electronic component. In other examples the conductive fabric directly contacts a user&#39;s skin. In some instances the conductive fabric includes a touch-sensitive input member. 
     In other embodiments a facial interface for a head-mountable device can include a facial contact, a conductive component incorporated into the facial contact, the conductive component configured to generate a signal based on a change in a capacitance of the conductive component, and a processor connected to the conductive component. 
     In some examples the facial contact is positioned adjacent a nasal region when the head-mountable device is worn by a user. In other examples, the conductive component is configured to detect a facial expression. 
     The following embodiments relate to a light seal of a head-mountable device including conductive fabric. As used herein “conductive fabric” can refer to the material or fabric into/onto which conductive elements are incorporated, or can also refer to the conductive elements themselves, separate from the material or fabric with which they are integrated. The conductive fabric can act as sensors, input members, and electrical interconnects to enable sensors and other electronic components to interact with one another. An input member can be any button, switch, capacitive surface, dial, slider, or other physically activated input system that can be used to provide an indication to a processor of a user&#39;s intent. 
     Conventional head-mountable devices equipped with sensors are utilized for various purposes that detect limited user feedback, such as movement or positioning feedback, providing limited information in response to the user. These conventional sensors and electrical interconnects are often located in non-ideal and burdensome locations, taking up much needed space and unable to seamlessly integrated with the existing HMD architecture. 
     By contrast, the head-mountable device of the present disclosure includes a light seal with conduct fabric that collects user information, captures changes in user motion, such as facial movements, and can receive user input, such as touch input. The conductive fabric can accomplish this while being unobtrusively integrated with the light seal. A head-mountable device with such conductive fabrics accomplishes increased functionality without adding burdensome components or structures. 
     There are a number of different uses for “smart” conductive fabric described herein. For example, the disclosed conductive fabrics can be used for: detecting respiration patterns and amplitudes, receiving user input (such as touch/force inputs and playback controls), detecting facial expressions, detecting tension or force on a user&#39;s face from the facial interface and/or headband support, raising or lowering the temperature of the light seal, moisture detection, and low power “on-head” detection of when the HMD is being handled or worn by the user. 
     These and other embodiments are discussed below with reference to  FIGS.  13 . 8 - 1 A- 13 . 8 - 7   . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these Figures is for explanatory purposes only and should not be construed as limiting. Furthermore, as used herein, a system, a method, an article, a component, a feature, or a sub-feature comprising at least one of a first option, a second option, or a third option should be understood as referring to a system, a method, an article, a component, a feature, or a sub-feature that can include one of each listed option (e.g., only one of the first option, only one of the second option, or only one of the third option), multiple of a single listed option (e.g., two or more of the first option), two options simultaneously (e.g., one of the first option and one of the second option), or combination thereof (e.g., two of the first option and one of the second option). 
       FIGS.  13 . 8 - 1 A  illustrates a block diagram of a head-mountable device  13 . 8 - 100  including a frame  13 . 8 - 104 , a display  13 . 8 - 105 , a support  13 . 8 - 109 , and a light seal  13 . 8 - 112 . The display  13 . 8 - 105  can include one or more optical lenses or display screens in front of the eyes of a user. The display  13 . 8 - 105  can include a display for presenting an augmented reality visualization, a virtual reality visualization, or other suitable visualization. Additionally, the display  13 . 8 - 105  can be positioned at least partially in or on the frame  13 . 8 - 104 . Similarly, the light seal  13 . 8 - 112  can be connected to the frame  13 . 8 - 104 . In some examples, the light seal  13 . 8 - 112  includes the frame  13 . 8 - 104  (i.e., the frame  13 . 8 - 104  is part of the light seal  13 . 8 - 112 ). 
     The light seal  13 . 8 - 112  can include electrical components (e.g., sensors  13 . 8 - 120 ), a cover  13 . 8 - 113 , a facial interface  13 . 8 - 108 , and conductive fabric  13 . 8 - 116 . The frame  13 . 8 - 104  can be a housing of the display  13 . 8 - 105 . Further, the frame  13 . 8 - 104  can also be considered to be a part of or separate from the light seal  13 . 8 - 112 . As used herein, the term “light seal” can refer to a portion of the head mountable device  13 . 8 - 100  that engages or shields a user&#39;s face. In particular, the light seal  13 . 8 - 112  includes portions (e.g., the facial interface) that conform to, contact, or press against regions of the user&#39;s face. 
     The light seal  13 . 8 - 112  can include a pliant (or semi-pliant) facetrack or face engagement component that spans the forehead, wraps around the eyes, contacts other regions of the face (e.g., zygoma and maxilla regions), and bridges the nose. In addition, the light seal  13 . 8 - 112  can include various components forming a frame, structure, or webbing of a head-mountable device disposed between the display  13 . 8 - 105  and the user&#39;s skin, such as the cover  13 . 8 - 113 . The cover  13 . 8 - 113  can include a seal, environment seal, dust seal, air seal, etc. that is positioned between the gap between the display  13 . 8 - 105  and the user&#39;s face. The cover  13 . 8 - 113  can be a woven fabric that is non-rigid or deformable. The cover  13 . 8 - 113  can be elastically deformable. In some examples, the cover  13 . 8 - 113  can be a plastic, rubber, or polymer material. In some examples, the cover  13 . 8 - 113  can be rigid. 
     The cover  13 . 8 - 113  can form an eye-box through which the user can view the display  13 . 8 - 105 . It will be appreciated that the term “seal” can include partial seals or inhibitors, in addition to complete seals (e.g., a partial light seal where come ambient light is blocked and a complete light seal where all ambient light is blocked when the head-mountable device  13 . 8 - 100  is donned). 
     The light seal  13 . 8 - 112  can be removeably attached to the frame  13 . 8 - 104  and in electrical communication with the display  13 . 8 - 105 . The light seal  13 . 8 - 112  can include an electrical component  13 . 8 - 120 , such as a sensor  13 . 8 - 120 . The sensor  13 . 8 - 120  can collect user or environmental data, such as biometric information. The sensor  13 . 8 - 120  can transmit signals to the HMD, and more particularly to the display  13 . 8 - 105 . The sensor  13 . 8 - 120  can transmit signals to an output configured to perform an action in response to the information collected by the sensor  13 . 8 - 120 . 
     The light seal  13 . 8 - 112  can include conductive fabric  13 . 8 - 116 . In some examples, the conductive fabric  13 . 8 - 116  can include highly elastic copper threads that can compress and stretch while still maintaining electrical connectivity. The conductive fabric  13 . 8 - 116  can be elastically deformable (i.e., capable of temporary change in length, volume, or shape). In some examples, the conductive fabric  13 . 8 - 116  can include electrically conductive carbon fibers. In some examples, the conductive fabric  13 . 8 - 116  can collect biometric information of the user. More specifically, the biometric information of the user can manifest itself in movement of the user&#39;s face (e.g., nasal movements indicative of respiration or breathing patterns). These movement can cause a change in the conductive fabric (i.e., stretching or compressing). The degree or amount to which the conductive fabric is moved by the user&#39;s facial movements can be detectable by the conductive fabric and can be used to generate signals that can be analyzed by a processor of the HMD  13 . 8 - 100 . As described in greater detail below, the conductive fabric can be a thread, line, wire, plate, or any other structure that is capable of conducting electricity. The conductive fabric  13 . 8 - 116  can be integrated with the light seal  13 . 8 - 112 . For example, the conductive fabric  13 . 8 - 116  can be embedded, interwoven, or encapsulated with the cover  13 . 8 - 113  or facial interface  13 . 8 - 108 . 
     The display  13 . 8 - 105 , being in electrical communication with the light seal  13 . 8 - 112 , and more specifically with the conductive fabric  13 . 8 - 116 , can receive electrical communication and provide feedback to the user related to the readings of the readings of the conductive fabric  13 . 8 - 116  (e.g. visual feedback, audio feedback, haptic feedback, etc.). 
     As used herein, the term “sensor” refers to one or more different sensing devices, such as a camera or imaging device, temperature device, oxygen device, movement device, brain activity device, sweat gland activity device, breathing activity device, muscle contraction device, etc. In some examples, the sensor can sense biometric features including features of the autonomic nervous system. Some particular examples of sensors include an electrooculography sensor, electrocardiogramansor, EKG sensor, hear rate variability sensor, blood volume pulse sensor, SpO2 sensor, compact pressure sensor, electromyography sensor, core-body temperature sensor, galvanic skin sensor, accelerometer, gyroscope, magnetometer, inclinometer, barometer, infrared sensor, global positioning system sensor, etc. Additional sensor examples can include, contact microphones (e.g., press based MEMS), bioelectrical activity sensors, UV exposure sensors, or particle sensors. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 8 - 1 A  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 8 - 1 A . 
       FIGS.  13 . 8 - 1 B  shows a top partial view of a head-mountable device  13 . 8 - 100 . The head-mountable device  13 . 8 - 100  of  FIGS.  13 . 8 - 1    can be substantially similar to, including some or all of the features of, the head-mountable device  13 . 8 - 100  described in  FIGS.  13 . 8 - 1 A . The HMD  13 . 8 - 100  can include a display (also referred to as a display unit, or housing)  13 . 8 - 105  and a retention band  13 . 8 - 109 . The display  13 . 8 - 105  can include any number of internal electronic components  13 . 8 - 107 . The HMD  13 . 8 - 100  can include a frame  13 . 8 - 104  (which can also be referred to as a housing) attached to the display  13 . 8 - 105 . In some examples, the display  13 . 8 - 105  includes an opaque, translucent, transparent, or semi-transparent screen, including any number lenses, for presenting visual data. The frame  13 . 8 - 104  can at least partially border one or more edges of the display  13 . 8 - 105 . The frame  13 . 8 - 104  can be attached to a cover  13 . 8 - 128  at one end of the cover  13 . 8 - 128 . At an opposite end, the cover  13 . 8 - 128  can form, or be attached to, a facial interface  13 . 8 - 108 . In some examples, the frame  13 . 8 - 104 , cover  13 . 8 - 113 , and facial interface  13 . 8 - 108  can together form the light seal  13 . 8 - 112 . It will be understood, however, that the light seal  13 . 8 - 112  can include fewer or more components that those listed or shown. 
     The HMD  13 . 8 - 100  can be worn on a user&#39;s head  13 . 8 - 102  such that the display  13 . 8 - 105  is positioned over the user&#39;s face and disposed over one or both of the user&#39;s eyes. The display  13 . 8 - 105  can be connected to the retention band  13 . 8 - 109  and/or the light seal  13 . 8 - 112 . In some examples, the retention band  13 . 8 - 109  can be positioned against the side of a user&#39;s head  13 . 8 - 102  and in contact therewith. In some examples, the retention band  13 . 8 - 109  can be at least partially positioned above the user&#39;s ear or ears. In some examples, the retention band  13 . 8 - 109  can be positioned adjacent to the user&#39;s ear or ears. The retention band  13 . 8 - 109  can extend around the user&#39;s head  13 . 8 - 102 . In this way, the display  13 . 8 - 105  and the retention band  13 . 8 - 109  can form a loop that can retain the wearable electronic device  13 . 8 - 100  on the user&#39;s head  13 . 8 - 102 . It should be understood, however, that this configuration is just one example of how the components of a modular wearable electronic device  13 . 8 - 100  can be arranged, and that in some examples, a different number of connector straps and/or retention bands can be included. Although the particular component  13 . 8 - 100  can be referred to as an HMD, it should be understood that the terms wearable device, wearable electronic device, HMD, HMD device, and/or HMD system can be used to refer to any wearable device, including smart glasses. 
     In some examples, the frame  13 . 8 - 104  is attached to a facial interface  13 . 8 - 108 . The facial interface  13 . 8 - 108  can contact a user&#39;s head and/or face. In some examples, the cover  13 . 8 - 113  can be a light blocking component that extends between the frame  13 . 8 - 104  and the facial interface  13 . 8 - 108 . The light blocking component can cover or surround a perimeter of the frame  13 . 8 - 104  and/or the facial interface  13 . 8 - 108 . 
     The cover  13 . 8 - 113  can be a cloth, fabric, woven material, plastic, rubber, or any other suitable opaque or semi-opaque material. In some examples, the cover  13 . 8 - 113  is flexible, having the ability to repeatedly stretch, compress, and deform. The cover  13 . 8 - 113  can be elastically or in-elastically deformable. The facial interface  13 . 8 - 108  in combination with the cover  13 . 8 - 113  can block outside light and limits the peripheral view of the user  13 . 8 - 102 . In some examples, the cover  13 . 8 - 113  and the facial interface  13 . 8 - 108  is the same or a unitary component. As will be discussed in greater detail below, the light seal  13 . 8 - 112  can include a conductive fabric that is able to serve multiple functions and provide added benefits and functionality to the HMD  13 . 8 - 100 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 8 - 1 B  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 8 - 1 B . 
       FIGS.  13 . 8 - 2    shows a bottom perspective view of select components of a light seal  13 . 8 - 212 . The light seal  13 . 8 - 212  can be substantially similar to, including some or all of the features of, the light seals described herein, such as light seal  13 . 8 - 112 . The light seal  13 . 8 - 212  can be implemented on an HMD, such as HMD  13 . 8 - 100 . 
     The light seal  13 . 8 - 212  can be integrated with conductive fabrics  13 . 8 - 216   a ,  13 . 8 - 216   b ,  13 . 8 - 216   c ,  13 . 8 - 216   d , and  13 . 8 - 216   e  (collectively referred to as conductive fabric  13 . 8 - 216 ). The conductive fabric  13 . 8 - 216  or thread can be positioned at various locations on the light seal  13 . 8 - 212 . For example, the conductive fabric  13 . 8 - 216  can be positioned on or in the cover (e.g., embedded, encapsulated, or interwoven into the cover) 
     In some examples, the conductive fabric  13 . 8 - 216  can be positioned on the facial interface  13 . 8 - 208  of the light seal  13 . 8 - 212 . In one example, the conductive fabric  13 . 8 - 216  can be in direct contact with a user, for example, touching a user&#39;s forehead, cheek, nose, temple region, back of the head, or at any location where the HMD contacts the user. 
     The light seal  13 . 8 - 212  can include a nasal region  13 . 8 - 218  that is configured to contact a user&#39;s nose. The nasal region  13 . 8 - 218  can be considered part of the facial interface  13 . 8 - 208  and/or the cover  13 . 8 - 213 , or can be its own separate section of the light seal  13 . 8 - 212 . The nasal region  13 . 8 - 218  can include two sections corresponding to each side of a user&#39;s nose. As illustrated in  FIGS.  13 . 8 - 2   , conductive threads  13 . 8 - 216   a  can be positioned in the nasal region  13 . 8 - 218 . In some examples, the conductive thread  13 . 8 - 216   a  is a single continuous wire that spans the nasal region  13 . 8 - 218  (i.e., both sides of the nose and across the bridge of the nose). In some examples, each side of the nasal region  13 . 8 - 218  of the light seal  13 . 8 - 212  can include separate or distinct conductive threads  13 . 8 - 216   a.    
     The conductive thread  13 . 8 - 216   a  can be intentionally located to detect movements of the user&#39;s nose. For example, when a user breathes, the nose can move a certain amount based on the intensity or amplitude of the breath. This movement can in turn cause the nasal region  13 . 8 - 218  of the light seal  13 . 8 - 212  to change shape or move (e.g., compress or stretch). The conductive thread  13 . 8 - 216   a  can be ideally positioned such that as the nasal region  13 . 8 - 218  moves in response to nasal movements. The conductive threads  13 . 8 - 216  are likewise stretched or compressed by nasal movements. 
     The movement of the conductive thread  13 . 8 - 216   a  can result in a detectable change in the conductivity and/or capacitance of the conductive threads  13 . 8 - 216 , which can be used to infer respiration or facial expression data. As used herein “detectable” can refer to a change in a signal generated by the conductive fabric that is able to be recognized and analyzed by a processor. In some examples, the conductive threads  13 . 8 - 216  can directly contact the user&#39;s nose and can be impacted by the capacitance of the user&#39;s nose. In some examples, the conductive threads  13 . 8 - 216   a  are embedded, encapsulated, or interwoven into the material of the nasal region (i.e., the fabric of the cover  13 . 8 - 213  and/or the facial interface  13 . 8 - 208 ) such that the conductive threads  13 . 8 - 216   a  do not directly contact the user&#39;s skin, but still respond to movements of the user&#39;s face relative to the light seal  13 . 8 - 212 . 
     The light seal  13 . 8 - 212  can include a forehead region that is configured to contact a user&#39;s forehead when the HMD is donned by the user. The forehead region can be impacted (e.g., moved by) movements of the user&#39;s forehead and/or eyebrows. As illustrated in  FIGS.  13 . 8 - 2   , conductive threads  13 . 8 - 216   d  can be positioned in the forehead region of the cover  13 . 8 - 213 . In some examples, the conductive thread  13 . 8 - 216   d  is a single continuous wire that spans a substantial portion of the forehead region. In some examples, the forehead region of the cover  13 . 8 - 213  includes multiple conductive threads  13 . 8 - 216   d  (e.g., one over each eyebrow). 
     The conductive thread  13 . 8 - 216   d  can be intentionally located to be able to detect movements of the user&#39;s forehead and/or eyebrows. For example, a user&#39;s facial expression can be inferred from the position and movements of the forehead. This movement can in turn result in movements of the light seal  13 . 8 - 212 . The conductive thread  13 . 8 - 216   d  can be ideally positioned such that as the forehead moves, the conductive thread  13 . 8 - 216   d  are likewise stretched or compressed. This movement of the conductive thread  13 . 8 - 216   d  can result in a detectable change in an electrical property (e.g., conductivity and/or capacitance) of the conductive thread  13 . 8 - 216   d , which can be used to infer biometric information, such as facial expressions. In some examples, the conductive threads  13 . 8 - 216   d  can directly contact the user&#39;s skin and can be excited by the capacitance of the user&#39;s skin. In some examples, the conductive threads  13 . 8 - 216   d  are embedded, encapsulated, or interwoven into the material of the nasal region (i.e., the fabric of the cover  13 . 8 - 213  and/or the facial interface  13 . 8 - 208 ) such that the conductive threads  13 . 8 - 216   d  do not directly contact the user&#39;s skin, but still respond to movements of the user&#39;s face relative to the light seal  13 . 8 - 212 . 
     The light seal  13 . 8 - 212  can include a facial interface  13 . 8 - 208  that is configured to directly contact and conform against the user&#39;s face when the HMD is donned by the user. The facial interface  13 . 8 - 208  can be a distinct components from the cover  13 . 8 - 213  or can be a single unitary piece of the cover  13 . 8 - 213 . For example, the facial interface  13 . 8 - 208  can include foam that is attached to the cover  13 . 8 - 213 . The facial interface can be impacted (e.g., moved by) movements of the user&#39;s face. As illustrated in  FIGS.  13 . 8 - 2   , conductive threads  13 . 8 - 216   e  can be positioned in/on the facial interface  13 . 8 - 208 . 
     The conductive thread  13 . 8 - 216   e  can be intentionally located to be able to detect movements of the user&#39;s face. Movement of the user&#39;s face can transfer to movement of the facial interface  13 . 8 - 208  and movements of the conductive thread  13 . 8 - 216   e , resulting in a detectable change in an electrical property (e.g., conductivity and/or capacitance) of the conductive thread  13 . 8 - 216   e . In some examples, the conductive threads  13 . 8 - 216   e  can directly contact the user&#39;s skin and can be excited by the capacitance of the user&#39;s skin. In some examples, the conductive threads  13 . 8 - 216   e  are embedded, encapsulated, or interwoven into the material of the facial interface such that the conductive threads  13 . 8 - 216   e  do not directly contact the user&#39;s skin, but still respond to movements of the user&#39;s face relative to the light seal  13 . 8 - 212 . In some examples, the conductive fabric  13 . 8 - 216   e  can be interwoven into the facial interface  13 . 8 - 208  while still being exposed and capable of contacting the user&#39;s face. 
     The light seal  13 . 8 - 212  can include one or more electronic components  13 . 8 - 220 . The electronic component  13 . 8 - 220  can be any form of module, sensor, unit, input, or output that performs a function of the light seal  13 . 8 - 212  and/or the HMD. For example, the component  13 . 8 - 220  can be a battery, processor, haptic actuator, LED, display, speaker, biometric sensor, etc. 
     The component  13 . 8 - 220  can be positioned at various locations on the light seal  13 . 8 - 212 . The component  13 . 8 - 220  can be positioned on or in the cover (e.g., embedded, encapsulated, or interwoven into the cover). In some examples, the component  13 . 8 - 220  can be positioned on the facial interface  13 . 8 - 208  or the frame of the light seal  13 . 8 - 212 . In one example, the component  13 . 8 - 220  can be in direct contact with a user, for example, touching a user&#39;s forehead, cheek, nose, temple region, back of the head, or at any location where light seal  13 . 8 - 212  contacts the user&#39;s head. 
     In some examples, a conductive fabric  13 . 8 - 216   c  can be electrically connected to the component  13 . 8 - 220 . The conductive fabric  13 . 8 - 216   c  can be an interconnect to electrically connect the components with another component of the HMD. In some examples, the component  13 . 8 - 220  is a processor configured to analyze the signals transmitted/generated by the conductive thread  13 . 8 - 216   c.    
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 8 - 2    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 8 - 2   . 
       FIGS.  13 . 8 - 3    shows a top view of a head-mountable device (HMD)  13 . 8 - 300 . The HMD  13 . 8 - 300  can be substantially similar to, including some or all of the features of, the devices described herein, such as HMD  13 . 8 - 100  and light seal  13 . 8 - 212 . In some examples, the HMD  13 . 8 - 300  can include an electronic module  13 . 8 - 307 . The electronic module  13 . 8 - 307  can be positioned on or housed in the display unit  13 . 8 - 305 . The electronic module  13 . 8 - 307  can be a battery, a processor, a display, a camera, or any other electronic component. In some examples the electronic module  13 . 8 - 307  can be attached to the frame  13 . 8 - 304 . The electronic module  13 . 8 - 307  can be in electrical communication with an electronic component  13 . 8 - 320  attached to the light seal  13 . 8 - 312 . 
     In some examples, the electronic component  13 . 8 - 307  and the electronic module  13 . 8 - 320  can be electrically connected by a conductive fabric  13 . 8 - 316   a  that runs through the cover  13 . 8 - 313  of the light seal  13 . 8 - 312 . In some examples, the conductive fabric  13 . 8 - 316   a  runs through or across the headband or support  13 . 8 - 309 . In some examples, the conductive fabric  13 . 8 - 316   a  runs through or across the frame  13 . 8 - 304 . In this manner, the conductive fabric  13 . 8 - 316   a  serves as an interconnect (i.e., electrical connection) between the electronic component  13 . 8 - 320  on the light seal  13 . 8 - 312  and the electronic module  13 . 8 - 307 . In some examples, the detections of the conductive fabrics  13 . 8 - 216  can be combined with other sensors on-board or remote from the HMD  13 . 8 - 300  to gather data is a sensor fusion manner. For example, the conductive threads in combination with user facing cameras can be used for facial expression detection. 
     In some examples, the HMD  13 . 8 - 300  can include a conductive fabric  13 . 8 - 316   b  incorporated onto and/or into the support  13 . 8 - 309  of the HMD  13 . 8 - 300 . The support  13 . 8 - 309  can be a headband having flexible sections. The conductive fabric  13 . 8 - 316   b  can be configured to stretch or compress in response to the headband  13 . 8 - 309  stretching or compressing. In some examples, the conductive fabric  13 . 8 - 316   b  can generate signals indicative of the tightness or position of the headband. In some examples, a user can be notified based on the signals generated by the conductive fabric  13 . 8 - 316   b . The headband  13 . 8 - 309  can be automatically adjusted based on the signals generated by the conductive fabric  13 . 8 - 316   b.    
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 8 - 3    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 8 - 3   . 
       FIGS.  13 . 8 - 4 A  shows a conductive fabric  13 . 8 - 416  in a neutral or rest state. In some examples, the conductive fabric  13 . 8 - 416  is positioned in a serpentine pattern (curving, undulating, or winding in a repeated pattern). As illustrated in  FIGS.  13 . 8 - 4 A , in a rest or neutral state (i.e., when no external forces are acting on the conductive fabric  13 . 8 - 416 , the nodes or peaks of the conductive fabric  13 . 8 - 416  can be a distance d 1  from each other. The distance d 1  can be visualized as the wavelength or distance between parallel sections of the conductive fabric  13 . 8 - 416 . 
     By monitoring the electrical current passing through the conductive fabric, a processor can determine the amount and direction of strain the fabric is experiencing. In some examples, the conductive fabric  13 . 8 - 416  arranged in a serpentine pattern can be excited using a 100-140 kHz sine or square wave to generate a capacitive field. The system can then monitor for changes in the alternating current (AC) component over time (e.g., to determine respiration data). In some examples, a frequency sweep is performed to acquire a frequency/phase dependent signal. 
     In some examples, the geometry or shape of conductive threads  13 . 8 - 416  can be tuned depending on what is being monitored and the required sensitivity levels. In some examples, the system primarily monitors the change in capacitance of the conductive threads  13 . 8 - 416 . 
       FIGS.  13 . 8 - 4 B  shows the conductive fabric  13 . 8 - 416  in a compressed state. The compressed state may be the result of the light seal (such as the cover and/or facial interface) being acted upon by the user. In the compressed state, the distance separating the peaks or the parallel sections of the conductive fabric is a distance d 2 . The distance d 2  can be less than the distance d 1 . As a result, the conductive properties (e.g., capacitance) of the conductive fabric can change in a detectable manner. Thus, it can be determined that the conductive fabric  13 . 8 - 416  is compressed and even the degree to which the conductive fabric  13 . 8 - 416  is compressed. From this, biometric information, facial expression, breathing patterns, user input, and other user information can be determined. 
       FIGS.  13 . 8 - 4 C  shows the conductive fabric  13 . 8 - 416  in a stretched state. The stretched state may be the result of the light seal (such as the cover and/or facial interface) being acted upon by the user. In the stretched state, the distance separating the peaks or the parallel sections of the conductive fabric is a distance d 3 . The distance d 3  can be greater than the rest distance d 1 . As a result, the conductive properties of the conductive fabric change in a detectable manner as the area and geometry of the conductive fabric  13 . 8 - 416  is modified. Thus, it can be determined that the conductive fabric  13 . 8 - 416  is stretched and even the degree to which the conductive fabric  13 . 8 - 416  is stretched. From this, biometric information, facial expression, breathing patterns, user input, and other user information can be determined. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 8 - 4 A- 13 . 8 - 4 C  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 8 - 4 A- 13 . 8 - 4 C . 
       FIGS.  13 . 8 - 5 A  shows a conductive component  13 . 8 - 516   a  on an exterior  13 . 8 - 517  of a cover  13 . 8 - 513 . The conductive component  13 . 8 - 516   a  can be substantially similar to, including some or all of the features of, the conductive fabrics described herein, such as conductive fabric  13 . 8 - 216 ,  13 . 8 - 316 , and  13 . 8 - 416 . As described herein, a light seal can include a cover  13 . 8 - 513  that extends between the HMD and the user&#39;s face. The cover  13 . 8 - 513  can define a hollow, aperture, or opening that forms an internal volume or eye-box. The cover  13 . 8 - 513  can therefore include an internal or interior surface  13 . 8 - 515  that defines the internal volume, and an exterior surface  13 . 8 - 517  that defines an exterior of the light seal. 
     As illustrated in  FIGS.  13 . 8 - 5 A , the conductive component  13 . 8 - 516   a , can be positioned on the exterior surface  13 . 8 - 517 . In some examples, the conductive component  13 . 8 - 516   a  at least partially defines the exterior of the light seal. In some examples, the conductive component  13 . 8 - 516   a  is accessible to the user from the exterior of the HMD. For example, the conductive component  13 . 8 - 516   a  can be responsive to a user&#39;s touch or deformation of the conductive component  13 . 8 - 516   a . In this manner, a user who is wearing the HMD can control or provide input by touch (e.g., tapping or sliding a finger) or physically manipulation (e.g., deforming) the conductive component  13 . 8 - 516   a  with their hand. The conductive component  13 . 8 - 516   a  can be attached to the cover  13 . 8 - 513  using any suitable methods, including but not limited to adhesion, molding, mechanical fasteners, friction, weaving, magnets, etc. 
       FIGS.  13 . 8 - 5 B  shows a conductive component  13 . 8 - 516   b  interwoven into the cover  13 . 8 - 513 . The conductive component  13 . 8 - 516   b  can be substantially similar to, including some or all of the features of, the conductive fabrics described herein, such as conductive fabric  13 . 8 - 216 ,  13 . 8 - 316 ,  13 . 8 - 416 , and  13 . 8 - 516   a . As described herein, a light seal can include a cover  13 . 8 - 513  that extends between the HMD and the user&#39;s face. The cover  13 . 8 - 513  can be made from a fabric or material having a thickness within which the conductive component  13 . 8 - 516   b  can be deposited. 
     As illustrated in  FIGS.  13 . 8 - 5 B , the conductive component  13 . 8 - 516   b , can be positioned in (e.g., surrounded, encapsulated, embedded, interwoven, etc.) the cover  13 . 8 - 513 . In some examples, the conductive component  13 . 8 - 516   b  at least partially defines the exterior of the light seal. In some examples, at least a portion of the conductive component  13 . 8 - 516   b  is accessible to the user from the exterior of the HMD, even while being woven into the cover  13 . 8 - 513 . For example, the conductive component  13 . 8 - 516   b  can be responsive to a user&#39;s touch or deformation of the conductive component  13 . 8 - 516   b . In this manner, a user who is wearing the HMD can control or provide input by touch or physically manipulation the conductive component  13 . 8 - 516   b  (e.g., with their hand). The conductive component  13 . 8 - 516   b  can be attached to the cover  13 . 8 - 513  using any suitable methods, including but not limited to adhesion, molding, mechanical fasteners, friction, weaving, magnets, etc. Further, it will be understood that as the cover  13 . 8 - 513  is deformed (e.g., by movements of the user&#39;s face) the embedded or interwoven conductive component  13 . 8 - 516   b  can likewise be deformed. This deformation change the conductive properties of the conductive component  13 . 8 - 516   b  is a detectable manner, which can then cause the HMD to perform an action. 
       FIGS.  13 . 8 - 5 C  shows a conductive component  13 . 8 - 516   c  on an interior  13 . 8 - 515  of the cover  13 . 8 - 513 . The conductive component  13 . 8 - 516   c  can be substantially similar to, including some or all of the features of, the conductive fabrics described herein, such as conductive fabric  13 . 8 - 216 ,  13 . 8 - 316 ,  13 . 8 - 416 ,  13 . 8 - 516   a , and  13 . 8 - 516   b . As described herein, a light seal can include a cover  13 . 8 - 513  that extends between the HMD and the user&#39;s face. The cover  13 . 8 - 513  can define a hollow, aperture, or opening that forms an internal volume or eye-box. The cover  13 . 8 - 513  can therefore include an internal or interior surface  13 . 8 - 515  that defines the internal volume, and an exterior surface  13 . 8 - 517  that defines an exterior of the light seal. 
     As illustrated in  FIGS.  13 . 8 - 5 C , the conductive component  13 . 8 - 516   c , can be positioned on the interior surface  13 . 8 - 515 . In some examples, the conductive component  13 . 8 - 516   c  at least partially defines the interior surface of the light seal. In some examples, the conductive component  13 . 8 - 516   c  is accessible to the face of the user from the interior of the light seal. For example, the conductive component  13 . 8 - 516   c  can be responsive to a user&#39;s facial touch or movements. The conductive component  13 . 8 - 516   c  can be attached to the cover  13 . 8 - 513  using any suitable methods, including but not limited to adhesion, molding, mechanical fasteners, friction, weaving, magnets, etc. 
       FIGS.  13 . 8 - 5 D  shows free-floating conductive components. In some examples, the light seal includes a continuous conductive plate  13 . 8 - 519  free-floating (i.e., not anchored) in/on the cover  13 . 8 - 513  in a position that cover, overlays, overlaps, or is adjacent to conductors  13 . 8 - 521  deposited in the cover  13 . 8 - 513 . In this manner, the deposited conductors  13 . 8 - 521  can freely move and still provide signals representative of the deformations of the cover  13 . 8 - 513 , even if not galvanically connected to the plate  13 . 8 - 519 . It will be understood that a light seal can include multiple conductive components, conductive fabrics, and plates. For example, a cover  13 . 8 - 513  can include one or more of the conductive component arrangements described with reference to  FIGS.  13 . 8 - 5 A- 13 . 8 - 5 D . In some examples, the conductive components includes metal material that is deposited (e.g., using laser direct structuring (LDS) methods) onto fabric of the cover and/or facial interface. In some examples, the conductive components could be a combination of rigid and flexible conductive materials. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 8 - 5 A- 13 . 8 - 5 D  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures, including in any of  FIGS.  13 . 8 - 5 A- 13 . 8 - 5 D , described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 8 - 5 A- 13 . 8 - 5 D . 
       FIGS.  13 . 8 - 6    shows a side perspective view of a light seal  13 . 8 - 612 . The light seal  13 . 8 - 612  can be substantially similar to, including some or all of the features of, any of the light seals described herein, such as light seal  13 . 8 - 112 ,  13 . 8 - 212 , and  13 . 8 - 312 . The light seal  13 . 8 - 612  can include a frame  13 . 8 - 604  configured to attached to a display unit, a facial interface  13 . 8 - 608  configured to contact and conform to a user&#39;s face, and a cover  13 . 8 - 613  extending between the frame  13 . 8 - 604  and the facial interface  13 . 8 - 608  and establishing a light-blocking component or shielding. 
     The cover  13 . 8 - 613  can include a first conductive fabric  13 . 8 - 616   a  that is connected to the cover  13 . 8 - 613  in a fashion described herein. The first conductive fabric  13 . 8 - 616   a  can include a wire or thread arranged in a generally serpentine pattern or in a zig-zag pattern. In some examples, the first conductive fabric  13 . 8 - 616   a  can include a geometry that is responsive to (i.e., prone to move) force in the vertical direction (as oriented in  FIGS.  13 . 8 - 6   ). 
     The configuration of the conductive fabric  13 . 8 - 616   a  can be ideal for receiving user input. For example, the user can squeeze or pinch the cover  13 . 8 - 613  along the vertical axis to provide input to the system. In some examples, the conductive fabric  13 . 8 - 616   a  can be configured to detect an inward force, orthogonal to the surface plane of the side of the cover  13 . 8 - 613 . For example, the conductive fabric  13 . 8 - 616   a  can detect the user pushing the cover inward with their finger (e.g., toward the user&#39;s temple). 
     In some examples, the cover  13 . 8 - 613  can include a second conductive fabric  13 . 8 - 616   b  that is connected to the cover  13 . 8 - 613  in a fashion described herein. The second conductive fabric  13 . 8 - 616   b  can be oriented substantially perpendicular to the first conductive fabric  13 . 8 - 616   a . In other words, a longitudinal axis of the first conductive fabric  13 . 8 - 616   a  can be perpendicular to the longitudinal axis of the second conductive fabric  13 . 8 - 616   b . It will be understood that the light seal  13 . 8 - 612  can include one of or both the first conductive fabric  13 . 8 - 616   a  and the second conductive fabric  13 . 8 - 616   b . The conductive fabric  13 . 8 - 616   b  can include a wire or thread arranged in a generally serpentine pattern or in a zig-zag pattern. In some examples, the conductive fabric  13 . 8 - 616   b  can include a geometry that is responsive to a force in the horizontal direction (as oriented in  FIGS.  13 . 8 - 6   ). 
     The configuration of the conductive fabric  13 . 8 - 616   b  can be ideal for receiving user input. For example, the user can squeeze or pinch the cover  13 . 8 - 613  along the horizontal axis to provide input to the system. In some examples, the conductive fabric  13 . 8 - 616   b  is sensitive to changes in the distance between the frame  13 . 8 - 604  and the facial interface  13 . 8 - 608 . A user can provide input by pressing the display unit or frame  13 . 8 - 604  toward the facial interface  13 . 8 - 608 . In some examples, the conductive fabric  13 . 8 - 616   b  can be configured to detect an inward force, orthogonal to the surface plane of the side of the cover  13 . 8 - 613 . For example, the conductive fabric  13 . 8 - 616   b  can detect the user pushing the cover inward with their finger (e.g., toward the user&#39;s temple). In some examples, the light seal  13 . 8 - 612  can include multiple serpentine patterned conductive threads that run parallel to one another. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 8 - 6    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 8 - 6   . 
       FIGS.  13 . 8 - 7    shows a bottom perspective view of a light seal  13 . 8 - 712 . The light seal  13 . 8 - 712  can be substantially similar to, including some or all of the features of, any of the light seals described herein, such as light seal  13 . 8 - 112 ,  13 . 8 - 212 ,  13 . 8 - 312 , and  13 . 8 - 612 . The light seal  13 . 8 - 712  can include a cover  13 . 8 - 713  and a first and second nasal sections  13 . 8 - 718   a  and  13 . 8 - 718   b , respectively (collectively referred to as nasal sections  13 . 8 - 718 ). In some examples, the light seal can include an array of conductive threads  13 . 8 - 716   a  and  13 . 8 - 716   b  (collectively referred to as the array of conductive threads  13 . 8 - 716 ). The array of conductive thread  13 . 8 - 716  can be arranged parallel to one another such that changes in a distance separating the parallel threads is detectable and can be used to infer nasal movements and respiration data, such as instantaneous estimation of breath amplitude. The parallel conductive threads  13 . 8 - 716  can be arranged perpendicular to the direction of travel or deformation of the nasal regions  13 . 8 - 718 . 
     For example, it can be seen in  FIGS.  13 . 8 - 7    that the distances separating the first array of conductive threads  13 . 8 - 716   a  is less than the distances between the threads of the second array of conductive threads  13 . 8 - 716   b . It can therefore be determined (e.g., by a processor) that the first nasal section  13 . 8 - 718   a  is being compressed and/or the second nasal section  13 . 8 - 718   b  is being stretched. 
     It will be understood that location of the array of conductive threads  13 . 8 - 716  is not limited to the nasal sections  13 . 8 - 718 . For example, any of the conductive fabrics described herein can instead be arranged as a parallel array of conductive threads. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 8 - 7    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 8 - 7   . 
     13.9: Facial Interface Having Integrated Health Sensors 
     The following disclosure relates to a facial interface of a head-mountable device. More particularly, the present embodiments relate to a facial interface of a head-mountable device including a pressure sensor assembly used for acquiring biometric information. It will be understood the head-mountable device (HMD) in the present disclosure can apply to AR/VR head sets and also to smart computer glasses. In some examples, a processor causes a component of the HMD to perform an action in response to the biometric information collected by the sensor(s). As used herein “an action” or “perform an action” can refer to any electrical or mechanical act causing a change in one or more of the components of the HMD. For example, an action can include displaying a notification for the user and/or activating or deactivating one or more components. 
     In some examples, a sensor can collect biometric information from the nose and/or forehead of the user wearing the device, including when they are performing an activity, like exercising. The biometric information can be related to pulse, respiration, facial expression, heart rate, blood pressure, changes in the cartilage, chewing, or any other pertinent data. In some examples, a combination of sensors (sensor fusion) can be used to determine the biometric information. For example, a facial expression can be determined using both the pressure-based MEMS sensor described herein as well as an internal facing camera of the HMD. Further, in response to the collected data, the system can perform an action, for example, notifying the user of the collected data. 
     Head-mountable devices equipped with sensors are utilized for various purposes that detect user feedback, such as positioning or movement feedback, providing limited information in response to a user. For example, a user may have an increase in respiration or heart rate while performing a movement or action. Conventional head-mountable devices are not appropriately equipped to capture all the desired biometric information. 
     In contrast, the head-mountable devices of the present disclosure include a facial interface that can be incorporated or integrated with sensor(s), such as a pressure sensor assembly, configured to collect user data, such as biometric information, including pulse, respiration, facial expressions, heart rate, blood pressure, etc. By capturing the user data, the pressure sensor assembly of the facial interface can provide increased and improved feedback to the user. A head-mountable device with sensors monitoring user biometrics or health feedback creates a highly customized user experience, unlike the sensors of a conventional head-mountable device, which are unable to consider the user experience to such a degree. 
     Pressure sensor assemblies positioned on the facial interface can be used to create a customized user experience. The head-mountable device of the present disclosure can include pressure sensors to measure a user&#39;s response or engagement via indicators, such as pulse, respiration, facial expressions, heart rate, blood pressure, etc. Additionally, the sensor data can be used to monitor user fatigue, facial expressions, change in a user state, or obtain activity-specific metrics. 
     The head-mountable device can include sensors that are integrated on/in the facial interface in a variety of different ways. For example, the head-mountable device of the present disclosure can implement a first facial interface with a first sensor assembly being interchangeable, along with a second facial interface with a second sensor assembly. In some examples, the removeably attached sensor assembly can correspond to a different user activity, such as exercise, health, clinical settings, learnings activities, etc. 
     These and other embodiments are discussed below with reference to  FIGS.  13 . 9 - 1  through  13 . 9 - 6   . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these Figures is for explanatory purposes only and should not be construed as limiting. Furthermore, as used herein, a system, a method, an article, a component, a feature, or a sub-feature comprising at least one of a first option, a second option, or a third option should be understood as referring to a system, a method, an article, a component, a feature, or a sub-feature that can include one of each listed option (e.g., only one of the first option, only one of the second option, or only one of the third option), multiple of a single listed option (e.g., two or more of the first option), two options simultaneously (e.g., one of the first option and one of the second option), or combination thereof (e.g., two of the first option and one of the second option). 
       FIGS.  13 . 9 - 1    illustrates a block diagram of a head-mountable device  13 . 9 - 100  including a frame or housing  13 . 9 - 116 , a display  13 . 9 - 108 , a facial interface  13 . 9 - 112 , and a support  13 . 9 - 123 . The display  13 . 9 - 108  can include one or more optical lenses or display screens in front of the eyes of a user. The display  13 . 9 - 108  can include a screen or display unit for presenting an augmented reality visualization, a virtual reality visualization, or other suitable visualization to a user. Additionally, the display  13 . 9 - 108  can be positioned in or on the housing  13 . 9 - 116 . Similarly, a facial interface  13 . 9 - 112  can be connected to the housing  13 . 9 - 116 . In some examples, the housing  13 . 9 - 116  can support or house a variety of electronic components, including the display  13 . 9 - 108 . 
     The facial interface  13 . 9 - 112  can include one or more sensors (e.g., pressure sensors)  13 . 9 - 124 , support attachments  13 . 9 - 132 , display attachments  13 . 9 - 128 , and feedback or output modules  13 . 9 - 136 . The sensors  13 . 9 - 124  can be removably attached to the facial interface  13 . 9 - 112 . As used herein, the terms “facial interface,” “engagement interface,” or “light seal” refer to a portion of the head mountable device  13 . 9 - 100  that engages (i.e., contacts or conforms to) a user&#39;s face. 
     In particular, the facial interface  13 . 9 - 112  can include portions of a head-mountable device that conform to, or press against, regions of a user&#39;s face. The facial interface  13 . 9 - 112  can be positioned between the display  13 . 9 - 108  or frame  13 . 9 - 116  and the user&#39;s face. In some examples, the facial interface  13 . 9 - 112  can include a pliant (or semi-pliant) facetrack or lumen that spans the forehead, wraps around the eyes, contacts other regions of the face (e.g., zygoma and maxilla regions), and bridges the nose. 
     In addition, the facial interface  13 . 9 - 112  can include various components forming a frame, structure, housing, or webbing of the head mountable device  13 . 9 - 100  disposed between the display  13 . 9 - 108  and the user&#39;s skin. In some examples, the facial interface  13 . 9 - 112  can include a seal (e.g., a light seal, environment seal, dust seal, air seal, etc.). As used herein, the term “seal” can include partial seals or inhibitors, in addition to complete seals (e.g., a partial light seal where some ambient light is blocked and a complete light seal where all ambient light is blocked when the head-mountable device  13 . 9 - 100  is donned). The facial interface  13 . 9 - 112  can be removably attached to the housing  13 . 9 - 116  and can be in electrical communication with the display  13 . 9 - 108 . 
     The sensor(s)  13 . 9 - 124  of the facial interface  13 . 9 - 112  can collect biometric information, such as the user&#39;s vital sign (including pulse data, respiration data, and blood pressure). The pressure sensor assembly  13 . 9 - 124  can generate a signal based on the collected user information and transmit the signal to a processor that can cause an output  13 . 9 - 136  to perform an action in response to the signal (i.e., in response to the biometric information collected). For example, a user can perform a rigorous activity, such as lifting weights or working out while wearing the HMD  13 . 9 - 100 . During such activity, the user&#39;s heart rate or other vital signs can elevate or change, being detectable by the sensor  13 . 9 - 124 . 
     The sensor  13 . 9 - 124  can generate one or more signals based on the received input. The sensor  13 . 9 - 124  can transmit the signal to one or more components of the HMD  13 . 9 - 100  (e.g., to the display  13 . 9 - 108  and/or the output  13 . 9 - 136 ). The display  13 . 9 - 108 , being in electrical communication with the facial interface  13 . 9 - 112 , can receive electrical communication and provide feedback to the user related to their biometric readings (e.g., visual feedback, audio feedback, haptic feedback, etc.). Feedback can include determining when a user needs to take a break, or when the difficulty of an activity needs to be lowered or raised, the output  13 . 9 - 136  can include scheduling various activities for the user or recommending adjustment of the HMD  13 . 9 - 100 . 
     As used herein, the term “sensor,” “sensors,” “pressure sensor assembly,” or “sensor assembly” can refer to one or more sensing devices, such as a MEMS pressure sensor. In some examples, the HMD  13 . 9 - 100  includes a pressure sensing assembly having a deformable pad that deflects in response to a movement of a user&#39;s face, creating an increase or decrease in internal pressure of a pressure channel (e.g., a fluid volume connecting a deformable pad and a pressure sensors in fluid communication with the deformable pad) that changes in response to the deflection. The change in internal pressure caused by a movement in the user&#39;s face can be detected by a pressure sensor connected fluidly connected to the channel and attached to the deformable pad. 
     In some examples, the sensor is capable of detecting a deflection of the deformable pad as small as 10 microns. In some examples, the sensor is capable of detecting a deflection of the deformable pad that is less than as 10 microns. In some examples, the sensor is capable of detecting a deflection of the deformable pad as small as 20 microns. In some examples, the sensor is capable of detecting a deflection of the deformable pad as small as 50 microns. In some examples, the sensor is capable of detecting a deflection of the deformable pad as small as 100 microns. In some examples, the sensor is capable of detecting a deflection of the deformable pad greater than 100 microns. In some examples, the sensor is capable of detecting a deflection of the deformable pad that is approximately 100 nanometers or less. 
     The term “pressure sensor” or “sensor” used within the context of this application can refer to a sensor capable of sensing pressure, such as a compact based microphone (e.g., a MEMS sensor) with a sensitivity of about 2.5 millibar. In some examples, the pressure-based MEMS sensor can have a sensitivity of about 250 Pascals. In some examples, the pressure-based MEMS sensor can include an electrical circuit, such as a Winston bridge, to detect nano-metric deflections of the deformable pad. A pressure sensor or a sensor used for this application can be sensitive enough to detect a pulse, respiration, or other pressure related feedback related to a user. 
     In some instances, the change in pressure detected in a pressure sensor or sensor can generate an electrical signal capable of being interpreted by a controller/processor as a facial expression. In some examples, the pressure assembly is tuned to capture a specific pressure range applicable to a certain activity requiring other sensitivity values including approximately 1 millibar, 1.5 millibar, 3 millibar, or 4.5 millibar. When the pressure sensor is paired with another pressure sensor, a rate can be captured, such as blood volume, pulse rate, or other rates applicable and interpretable from the cardiovascular system. 
     In some examples, certain sensors can be used to assess stress and emotion through changes in pressures via direct contact with a user&#39;s face. The HMD can then provide feedback or output related to the detected stress and emotion. In some examples, the sensors can operate through coin cell battery or Bluetooth connectivity. In some examples, the sensors are powered by a primary battery of the HMD. 
     The sensors described herein can allow for observations of the cardiovascular system and respiration, to observe relaxation and stress indicators, mental health, medical treatments, etc. Using the disclosed sensors integrated on the facial interface, physicians and care takers could have live feedback of biometrics. Use cases can include fitness settings, user content, workplace, telepresence, clinical, education, training, pain, therapy, etc. In some examples, the sensors can be used to capture facial expressions. This is particularly relevant given the user&#39;s face is covered by the HMD when worn. For example, the HMD could include a micro electromechanical system (MEMS) to detect changes in blood vessel diameter, blood pressure, pulse, heat rate, respiration, and detecting facial expressions. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 9 - 1    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 9 - 1   . 
       FIGS.  13 . 9 - 2 A  illustrates a head-mountable device  13 . 9 - 200 . The head-mountable device  13 . 9 - 200  can be substantially similar to, including some or all of the features of, the head-mountable devices described herein, such as head-mountable device  13 . 9 - 100 . In some examples, the head-mountable device  13 . 9 - 200  includes a controller  13 . 9 - 213  (e.g., a sensor controller). While the controller  13 . 9 - 213  is illustrated as being positioned in a support (e.g., a support arm)  13 . 9 - 223 , the position of the controller  13 . 9 - 213  is not limited to the support  13 . 9 - 223 , and the controller  13 . 9 - 213  can be positioned in/on the facial interface  13 . 9 - 212  or in the HMD display housing. 
     The controller  13 . 9 - 213  can include a processor and a memory device storing computer-executable instructions that, when executed by the processor, cause the controller to receive biometric data from one or more pressure sensor assemblies  13 . 9 - 224 , transmit a signal based on the sensor data, and generate a signal to cause a component to perform an action in response to the received signal. 
     The controller  13 . 9 - 213  can include one or more processors (e.g., a system on chip, integrated circuit, driver, microcontroller, application processor, crossover processor, etc.). Further, the controller  13 . 9 - 213  can include one or more memory devices (e.g., individual nonvolatile memory, processor-embedded nonvolatile memory, random access memory, memory integrated circuits, DRAM chips, stacked memory modules, storage devices, memory partitions, etc.). In certain implementations, the controller  13 . 9 - 213  is communicatively coupled to a power source (e.g., a battery). 
     In some examples, the controller  13 . 9 - 213  stores sensor data received from the sensor assembly  13 . 9 - 224  in the memory. The controller  13 . 9 - 213  can receive and/or transmit signals based on sensor data. For example, as will be described below, the controller  13 . 9 - 213 , by way of the processor and memory, can transmit a signal to the display  13 . 9 - 208  based on the sensor data from the pressure sensor (e.g., causing the display  13 . 9 - 208  or the head-mountable device  13 . 9 - 200  to perform an action, such as present a certain message, power off, react to biometric feedback, etc.). 
     The controller  13 . 9 - 213  can perform any number of different functions. For example, the memory device can store computer-executable instructions that, when executed by the processor, cause the controller to receive sensor data from the sensor assemblies  13 . 9 - 224  and transmit a signal based on the sensor data. For instance, the controller  13 . 9 - 213  can transmit a sensor signal to a display  13 . 9 - 208 . 
     In response to the controller  13 . 9 - 213  the display  13 . 9 - 208  can perform a wide variety of actions, including power off or power on, react to a user generated facial expression, present a digital notification (e.g., a user-generated notification, a push notification, a context-generated notification, a system-generated notification, a smart notification, etc.). In some examples, the memory device can store computer-executable instructions that, when executed by the processor, cause the controller  13 . 9 - 213  to receive biometric data from the sensor assemblies  13 . 9 - 224 , transmit a signal based on the sensor data, and perform an action in response to the signal. 
     As illustrated, the head-mountable device (e.g., a wearable electronic device)  13 . 9 - 200  can include the display  13 . 9 - 208 , the facial interface  13 . 9 - 212 , and the pressure sensor(s)  13 . 9 - 224  coupled to the facial interface  13 . 9 - 212 . The pressure sensor assembly  13 . 9 - 224  can be embedded, encapsulated, deposited, adhered, or otherwise attached to the facial interface  13 . 9 - 212 . A portion of the pressure sensor assembly  13 . 9 - 224 , such as a deformable pad configured to contact a user&#39;s face, can deflect in response to a movement of a user&#39;s face. 
     The pressure sensor assembly  13 . 9 - 224  can be configured to detect a biometric feature of the user  13 . 9 - 220 . The pressure sensor assembly  13 . 9 - 224 , upon detecting a signal from the biometric feature, can transmit the signal to a component of the head-mountable device  13 . 9 - 200  causing the head-mountable device  13 . 9 - 200  to perform an action. For example, the head-mountable device  13 . 9 - 200  can change shape (i.e., tighten or loosen), move, vibrate, rotate, recalibrate, or reposition in response to the sensor signal of the biometric feature. 
     The head-mountable device  13 . 9 - 200  can include a headband, a retention band, a strap, or a support  13 . 9 - 223  connected to the display  13 . 9 - 208  and/or the frame  13 . 9 - 216 . The support  13 . 9 - 223  can secure the display  13 . 9 - 108  and/or housing  13 . 9 - 216  relative to the user&#39;s head  13 . 9 - 220  (e.g., such that the display  13 . 9 - 208  is maintained in front of a user&#39;s eyes). The support  13 . 9 - 223  can be constructed from elastic material, inelastic material, or a combination of elastic and inelastic material. 
     The support  13 . 9 - 223  can be adjustable such that the support  13 . 9 - 223  conforms to the various shapes and sizes of a user&#39;s head  13 . 9 - 220 . In some examples, the support  13 . 9 - 223  secures the head-mountable device  13 . 9 - 200  via friction between the user&#39;s head  13 . 9 - 220  and the retention band  13 . 9 - 223 . In some examples, the support  13 . 9 - 223  elastically secures the head-mountable device  13 . 9 - 200  to the user&#39;s head  13 . 9 - 220 . In some examples, the support  13 . 9 - 223  is coupled to a ratchet system or mechanism securing the head-mountable device  13 . 9 - 200  to the user&#39;s head  13 . 9 - 220 . In some examples, the support  13 . 9 - 223  is disposed above or on an ear  13 . 9 - 221  of the user&#39;s head  13 . 9 - 220 , supporting the head-mountable device  13 . 9 - 200 . 
     In some examples, the housing or frame  13 . 9 - 216  of the head-mountable display  13 . 9 - 200  is connected via a connector  13 . 9 - 215  to the facial interface  13 . 9 - 212 , the retention band  13 . 9 - 223  being connected to the frame  13 . 9 - 216  and/or the facial interface  13 . 9 - 212 , securing the head-mountable device  13 . 9 - 200  to the user&#39;s head  13 . 9 - 220  above or over the ears  13 . 9 - 221  of the user. 
       FIGS.  13 . 9 - 2 B  shows an example facial interface  13 . 9 - 212  including one or more deformable pads  13 . 9 - 225   a  and  13 . 9 - 225   b  (collectively referred to as “deformable pads  13 . 9 - 225 ”) disposed on the facial interface  13 . 9 - 212 . The deformable pads  13 . 9 - 225  can include thin membranes or sheets. The deformable pads  13 . 9 - 225  can be inflatable membranes or pillows. While multiple deformable pads  13 . 9 - 225  are shown, it will be understood that one deformable pad  13 . 9 - 225  and sensor  13 . 9 - 224  can be used to achieve the same or similar results. The deformable pads  13 . 9 - 225  can be in fluid communication with one or more pressure sensors  13 . 9 - 224   a  and  13 . 9 - 224   b  (collectively referred to as “pressure sensors  13 . 9 - 224 ”). 
     It will be noted that because of the use of deformable pad  13 . 9 - 225 , the pressure sensors  13 . 9 - 225  do not need to be in direct contact with the user&#39;s skin. This allows the sensors  13 . 9 - 225  to be more securely housed within the facial interface  13 . 9 - 212  where motion on the user&#39;s skin can be transferred through various materials of the facial interface  13 . 9 - 212  to the deformable pad  13 . 9 - 225 . The deformable pads  13 . 9 - 225  can be located at any location of the facial interface  13 . 9 - 212  that enables them to contact a face of a user. In some examples, the deformable pads  13 . 9 - 225   a  are located at/on a nose of a user when the head-mountable device is worn. The deformable pads  13 . 9 - 225   a  can be in fluid communication with the pressure sensors  13 . 9 - 224   a . The pressure sensors  13 . 9 - 224   a  can detect changes in pressure caused by a movement in the deformable pads  13 . 9 - 225   a.    
     The movement in the deformable pads  13 . 9 - 225   a  can be caused by a movement of the user&#39;s face. For example, a user may breathe through his/her nose, deflecting the deformable pads  13 . 9 - 225   a , and thereby a change in internal pressure of the pressure sensor assembly. The pressure sensors  13 . 9 - 224  can detect the change in pressure and transmit a signal based on the change to the head-mountable device, such as the head-mountable device  13 . 9 - 200  shown in  FIGS.  13 . 9 - 2 A , via an electrical signal. In some examples the change in pressure results in a piezoelectric reaction, a change in resistance, or a diaphragm deflection to generate the signal. 
     Similarly, the deformable pads  13 . 9 - 225   b  can be located near a forehead region of a user&#39;s face when the facial interface  13 . 9 - 212  is donned. The deformable pads  13 . 9 - 225   b  can be in fluid communication with the pressure sensors  13 . 9 - 224   b . The pressure sensors  13 . 9 - 224   b  can detect changes in pressure caused by a movement in the deformable pads  13 . 9 - 225   b  in response to a movement of a user&#39;s face. For example, a user may perform an activity, such as making a facial expression or exercising, increasing the user&#39;s blood pressure, and thereby causing the detectable forehead vasculature to enlarge. 
     As the forehead vasculature enlarges, deformable pads  13 . 9 - 225   b  deflect, causing a change in internal pressure to the pressure assembly. The pressure sensors  13 . 9 - 224   b  detects the change in pressure and communicates the pressure change by generating an electrical signal in response to the pressure change, which signal can then be transmitted to the head-mountable device. Likewise, a change in the user&#39;s facial expression can cause a deflection of the deformable pads  13 . 9 - 225   b  which can be detected by the pressure sensors  13 . 9 - 224   b . The pressure sensors  13 . 9 - 224   b  then generate an electrical signal in response to the detected deflection, which signal can then be analyzed by a processor to infer a characteristic of the user&#39;s facial expression. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 9 - 2 A and  13 . 9 - 2 B  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 9 - 2 A and  13 . 9 - 2 B . 
       FIGS.  13 . 9 - 3    shows a rear perspective view of a head-mountable device  13 . 9 - 300 . The head-mountable device  13 . 9 - 300  can be substantially similar to, and can include some or all of the features of, the head-mountable devices described herein, such as head-mountable device  13 . 9 - 100  and  13 . 9 - 200 . The head-mountable device  13 . 9 - 300  shown in  FIGS.  13 . 9 - 3    includes a support  13 . 9 - 323 , a facial interface  13 . 9 - 312 , overmolded portions  13 . 9 - 313   a  and  13 . 9 - 313   b  (collectively referred to as “overmolded portions  13 . 9 - 313 ”) connected to the facial interface  13 . 9 - 312 , deformable pads  13 . 9 - 325 , channels  13 . 9 - 326 , and sensors  13 . 9 - 324 . 
     The channel  13 . 9 - 326  allows for fluid communication between the deformable pad  13 . 9 - 325  and the sensor  13 . 9 - 324 . As described in further detail with regards to  FIGS.  13 . 9 - 4 A and  13 . 9 - 4 B , the channel  13 . 9 - 326  can define an internal volume. In some examples, the channel  13 . 9 - 326  defines a support arm for the deformable pad  13 . 9 - 325 . The channel  13 . 9 - 326  can be adjustable such that the position of the deformable pad  13 . 9 - 325  relative to the user&#39;s nose can be modified for varied applications or users. For example, one user may have specific nose shape, and another user may have a substantially different nose shape. The channel  13 . 9 - 326  can be adjusted, such that the nose shape can be accommodated in a comfortable and precise manner. 
     In some examples, the deformable pads  13 . 9 - 325  can be adjustable. For example, the deformable pads  13 . 9 - 325  can move or pivot relative to the facial interface  13 . 9 - 312 . The deformable pads  13 . 9 - 325  can change their pitch, yaw, or roll in order to establish a better fit and comfort on the user&#39;s face. In some examples, the stiffness or rigidity of the deformable pads  13 . 9 - 325  can be adjusted or tuned based on needs and preferences. The stiffness of the deformable pads  13 . 9 - 325  can be adjusted based on the pressure in the channels  13 . 9 - 326 . In some examples, the deformable pads  13 . 9 - 325  are automatically adjusted based on the user or application. Further details regarding deformable pads are provided below with reference to  FIGS.  13 . 9 - 4 A and  13 . 9 - 4 B . 
     In some examples, a user may perform a rigorous activity, requiring more rigid contact of the deformable pad  13 . 9 - 325 . The deformable pad  13 . 9 - 325  can be replaced with a more rigid deformable pad. In some examples, the deformable pad  13 . 9 - 325  can be replaced with a less rigid deformable pad to accommodate other certain activities, such as test taking or learning activities. 
     The overmolded portion  13 . 9 - 313  can include portions contacting the user&#39;s face. For example, overmolded portion  13 . 9 - 313   b  can contact the maxilla and/or zygoma facial regions, creating a comfortable conforming fit to a user&#39;s face. Similarly, the overmolded portion  13 . 9 - 313   a  can contact a user&#39;s forehead conforming to each user&#39;s forehead unique attributes, providing a comfortable customized fit for an individual user. In some examples, the overmolded portion  13 . 9 - 313  can be considered part of the facial interface  13 . 9 - 312 . The overmolded portions  13 . 9 - 313  can secure the sensor assembly to the facial interface  13 . 9 - 312 , creating an esthetically pleasing head-mountable display. In some examples, the overmolded portions  13 . 9 - 313  can create or define the channels  13 . 9 - 326 . In some examples, the overmolded portions  13 . 9 - 313  can form a housing to house or conceal the sensors  13 . 9 - 324  and/or channels  13 . 9 - 326 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 9 - 3    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 9 - 3   . 
       FIGS.  13 . 9 - 4 A  shows a perspective view of a portion of a facial interface  13 . 9 - 412 . The facial interface  13 . 9 - 412  can be substantially similar to, and can include some or all of the features of, the facial interfaces described herein, such as facial interfaces  13 . 9 - 112 ,  13 . 9 - 212 , and  13 . 9 - 312 . The facial interface  13 . 9 - 412  can include an arm or channel  13 . 9 - 426  that defines an internal volume  13 . 9 - 428 . The internal volume  13 . 9 - 428  can establish a fluid communication between a deformable pad  13 . 9 - 425  and a sensor  13 . 9 - 424 . In some examples, the internal volume includes air or some other gas. In some examples, the internal volume  13 . 9 - 428  include a liquid or pneumatic fluid. The sensor  13 . 9 - 424  can be a pressure-based MEMS sensor  13 . 9 - 424 . The sensor  13 . 9 - 424  can be disposed within the arm  13 . 9 - 426  of the facial interface  13 . 9 - 412 . In some examples, the sensor  13 . 9 - 424  is positioned on an exterior of the arm  13 . 9 - 426 . 
     The deformable pad  13 . 9 - 425  can include an inflatable member that can flex or deform in response to an external force. The deformable pad  13 . 9 - 425  can be removably attached to the arm  13 . 9 - 426  via a connector  13 . 9 - 433 . The connector  13 . 9 - 433  can any suitable mechanical or magnetic attachment mechanisms. In some examples, the connector  13 . 9 - 433  can include a barbed configuration capable of removably securing the deformable pad  13 . 9 - 425  to the arm  13 . 9 - 426 . The deformable pad  13 . 9 - 425  can include a female connector  13 . 9 - 435  to receive a male connector  13 . 9 - 433  on the arm  13 . 9 - 426 . In some examples, the connector  13 . 9 - 433  can be threaded such that the deformable pad  13 . 9 - 425  is threaded onto the connector  13 . 9 - 433 . 
     The term “barbed” used within the context of this application can define one end of a component connecting to another end of a different component. For example, the deformable pad can connect to the facial interface  13 . 9 - 412  via a barbed connection. The connection can be physical and contain egressed portions on one component and ingress portions on another components. These egressed portions and ingress portions can form a threaded fit, snap fit, compressive fit, or other fit that can produce fluid communication between the deformable pad  13 . 9 - 425  and the sensor  13 . 9 - 424 . 
     As shown in  FIGS.  13 . 9 - 4 B , in some examples, the connector  13 . 9 - 433  can establish a substantially air-tight seal between the deformable pad  13 . 9 - 425  and the internal volume  13 . 9 - 428  of the channel. In some examples, the gasket, O-ring, or other type of seal is positioned around the connector  13 . 9 - 433  to improve the seal. In some examples, a known and predetermined amount of air is permitted to escape the internal volume  13 . 9 - 428  as the deformable pad  13 . 9 - 425  moves. By controlling the amount of escaped air, the pressure sensor is still able to deduce facial movement. 
     The deformable pad  13 . 9 - 425  can be include an elastomeric material. The deformable pad  13 . 9 - 425  can include a soft material that is comfortable on the user&#39;s skin. The deformable pad  13 . 9 - 425  can include, but is not limited to, any of the following: Natural Rubber, Polyisoprene, Butyl Rubber (IIR, Isobutene-isoprene), Chloroprene (CR, Neoprene®), Ethylene Propylene Diene (EPDM), Fluorocarbon (FMK, Viton®), Fluorosilicone (FSI), Nitrile Butadiene (NBR), Saturated Nitrile (HNBR), Silicone Rubber (SI, Gum and Liquid), Styrene Butadiene (SBR), and Urethane (PU, Polyurethane). 
     In some examples, the deformable pad can include a compressible elastomer such as a silicon material that is durable and not likely to wear down. The deformable pad can be soft and resilient to moisture, such as sweat. In some examples, the deformable pad is overmolded or can be a stand-alone piece that is attached to the facial interface. 
     Further, the deformable pad  13 . 9 - 425  can be tuned for specific needs or users. For example, one deformable pad can be constructed of one elastomeric material for one application or user, and another deformable pad can include elastomeric material designed for another application or user. In some examples, the deformable pad  13 . 9 - 425  can be shaped or manufactured to increase or decrease sensitivity of the deformable pad  13 . 9 - 425  for a certain activity. In some examples, the deformable pad  13 . 9 - 425  can be constructed of a combination of materials, such as two or more types of elastomeric materials. 
     An example operation of the sensor module can occur when a user, wearing the HMD, has facial movement. A facial movement can include movement in the nose, forehead, cheeks, or any other portion of the face/head. Facial movements can be indicative of respiration, pulse, and facial expressions. Because the deformable pad  13 . 9 - 425  is in close proximity or touching the user, the facial movement can push or deflect the deformable pad  13 . 9 - 425  a certain amount. The deflection of the deformable pad  13 . 9 - 425  can cause a temporary increase in the pressure within the channel  13 . 9 - 428 . The change in pressure can be detected by the sensor  13 . 9 - 424 , which transmits a signal, based on the pressure fluctuation to be processed (e.g., by a processor on board the HMD). In some examples, the sensor  13 . 9 - 424  can be integrated into other components of the HMD (e.g., integrated onto the headband of the HMD). 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 9 - 4 A and  13 . 9 - 4 B  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 9 - 4 A and  13 . 9 - 4 B . 
       FIGS.  13 . 9 - 5 A  shows a rear view of a facial interface  13 . 9 - 512 . The facial interface  13 . 9 - 512  can be substantially similar to, and can include some or all of the features of, the facial interfaces described herein, such as facial interfaces  13 . 9 - 112 ,  13 . 9 - 212 ,  13 . 9 - 312 , and  13 . 9 - 412 . In some examples, the facial interface  13 . 9 - 512  can include a pressure sensor located in a forehead-contacting region of the facial interface  13 . 9 - 512 . The pressure sensor can include a deformable pad (e.g., inflatable member)  13 . 9 - 525 . In some examples, the deformable pad is overmolded to the facial interface  13 . 9 - 512 , sealing the deformable pad  13 . 9 - 525  and creating a pressure chamber wherein facial movements of a user cause the deformable pad  13 . 9 - 525  to deflect or deform. The deflection of the deformable pad  13 . 9 - 525  creates a change in pressure within the pressure chamber. The change in pressure can be detectible to the pressure sensor which can then generate a signal to be analyzed or interpreted by a processor/controller. 
     As illustrated in  FIGS.  13 . 9 - 5 A , the deformable pad  13 . 9 - 525  can include an elongated shape. The elongated shape of the deformable pad  13 . 9 - 525  can span across a substantial portion of the user&#39;s forehead when the user is wearing the HMD. The elongated horizontal shape of the deformable pad  13 . 9 - 525  can be predetermined to cover a maximum amount of veins/vessels in the forehead. In some examples, the deformable pad  13 . 9 - 525  can be centered on the facial interface  13 . 9 - 512 , and therefore positioned to contact a center of the user&#39;s forehead. However, other positioned are also possible. In some examples, the deformable pad  13 . 9 - 525  is off-center from the center or middle of the facial interface  13 . 9 - 512 . In some examples, the facial interface  13 . 9 - 512  wraps partially around the user&#39;s head, such that the deformable pad  13 . 9 - 525  can be positioned proximate a temple of the user when the HMD is donned. 
       FIGS.  13 . 9 - 5 B  shows a rear view of the facial interface  13 . 9 - 512  having a plurality of deformable pads  13 . 9 - 525   a ,  13 . 9 - 525   b ,  13 . 9 - 525   c ,  13 . 9 - 525   d  (collectively referred to as “deformable pads  13 . 9 - 525 ”). The deformable pads  13 . 9 - 525  can each deflect to cause a change in pressure. In some examples, the facial interface  13 . 9 - 512  includes a plurality of pressure chambers. The pressure chambers can be isolated and distinct or can be in fluid communication with each other. The pressure within the channel(s) can be equalized. In some examples, each deformable pad  13 . 9 - 525  can be fluidically connected to a single sensor. 
     In some examples, the system include multiple sensors, each of which is fluidically connected to one or more of the deformable pads  13 . 9 - 525 . The increased number of deformable pads  13 . 9 - 525  can increase accuracy and sensitivity for a head-mountable device used in combination with the facial interface  13 . 9 - 512 . In some examples, a combination of the deformable pads  13 . 9 - 525  can be used to detect or track motions along the user&#39;s forehead. For example, by using at least pad  13 . 9 - 525   a  and pad  13 . 9 - 525   b , a vertical signal or input can be monitored or tracked across the user&#39;s forehead. Likewise, using at least pad  13 . 9 - 525   c  and pad  13 . 9 - 525   d , a horizontal signal or input can be tracked as it traverses horizontally across a user&#39;s forehead. 
     In some examples, the combination of pads  13 . 9 - 525  can detect a user&#39;s pulse transit time (e.g., the time it takes a pulse wave to travel between two pulse sites). For example, deformable pad  13 . 9 - 525   a  and deformable pad  13 . 9 - 525   b  can be placed over the supratrochlear artery (e.g. the arterial supply to the forehead and the anterior part of the scalp) measuring the pulse transit time from deformable pad  13 . 9 - 525   a  to deformable pad  13 . 9 - 525   b . Similarly, any artery or combination thereof can be used to measure arterial functions, such as pulse transit times in a vertical, horizontal, or combinatory manner. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 9 - 5 A and  13 . 9 - 5 B  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures, can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 9 - 5 A and  13 . 9 - 5 B . 
       FIGS.  13 . 9 - 6    illustrates a side cross-sectional view of a portion of a sensor module of a facial interface  13 . 9 - 612 . The facial interface  13 . 9 - 612  can be substantially similar to, and can include some or all of the features of, the facial interfaces described herein, such as facial interface  13 . 9 - 112 ,  13 . 9 - 212 ,  13 . 9 - 312 ,  13 . 9 - 412 , and  13 . 9 - 512 . The sensor module of  FIG.  13 . 9 - 6    includes a deformable pad  13 . 9 - 625 , a contact portion  13 . 9 - 627  configured to contact a user&#39;s head, and a sensor  13 . 9 - 624 . 
     A channel  13 . 9 - 628  can define an internal volume that places the deformable pad  13 . 9 - 625  and the sensor  13 . 9 - 624  in fluid communication. In some examples, the deformable pad  13 . 9 - 625  can deform in response to facial movements. The deformable pad  13 . 9 - 625  can be constructed of a compressible elastomer. The facial interface  13 . 9 - 612  can include an overmolded portion  13 . 9 - 627 . The overmolded portion  13 . 9 - 627  can contact a user&#39;s face and can provide a comfortable point of contact to the user&#39;s forehead, nose, around the user&#39;s eyes, and other contact regions the overmolded portion contacts the user&#39;s face. 
     In some examples, the overmolded portion  13 . 9 - 627  is formed of a soft elastomer that is resilient to sweat/perspiration from a user. As illustrated, a pressure-based MEMS sensor  13 . 9 - 624  can be embedded or housed within the facial interface  13 . 9 - 612 . The MEMS sensor can have dimension of approximately 2 mm by 2 mm.  FIGS.  13 . 9 - 6    illustrates the deformable pad  13 . 9 - 625  being proud from (extending or protruding from) the facial interface  13 . 9 - 612 . However, in some examples, the deformable pad  13 . 9 - 625  can be flush with the facial interface  13 . 9 - 612 , specifically flush with the section of the facial interface  13 . 9 - 612  that abuts or contacts the forehead of the user. 
     An example operation of the sensor module can occur when a user, wearing the HMD, has facial movement. A facial movement can include movement in the nose, forehead, cheeks, or any other portion of the face/head. Facial movements can be indicative of respiration, pulse, and facial expressions. Because the deformable pad  13 . 9 - 625  is in close proximity or touching the user, the facial movement can push or deflect the deformable pad  13 . 9 - 625  a certain amount. 
     The deflection of the deformable pad  13 . 9 - 625  causes a temporary increase in the pressure within the channel  13 . 9 - 628 . The change in pressure can be detected by the sensor  13 . 9 - 624 , which transmits a signal, based on the pressure fluctuation to be processed (e.g., by a processor on board the HMD). In some examples, the sensor  13 . 9 - 624  can be integrated into other components of the HMD (e.g., integrated onto the headband of the HMD). 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 9 - 6    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 9 - 6   . 
     13.10: Touch Sensitive Input Surface 
     The following disclosure relates to a head-mountable device. More particularly, the present embodiments relate to a light seal of a head-mountable device including manual touch sensitive input surfaces for a user donning the head-mountable device to provide commands to the head-mountable device. As used herein the term “conductive fabric” can refer to the material or fabric into/onto which conductive elements are incorporated, or can refer to the conductive elements themselves, separate from the material or fabric with which they are integrated. The conductive fabric can include a plurality of conductive fibers. The conductive fabric can act as sensors, input members, and electrical interconnects to enable sensors and other electronic components to interact with one another. A manual input sensor can be a physically activated input system that can be used to provide an indication to a processor of a user&#39;s intent. 
     The head-mountable device of the present disclosure includes a manual input sensor that utilizes a touch sensitive surface that receives user input, such as touch input. The touch sensitive surface can accomplish this while being unobtrusively integrated with the light seal. A head-mountable device with such touch sensitive surfaces adds another way for a user to provide input or commands to a head-mountable device and adding functionality without adding burdensome components or structures. 
     These and other embodiments are discussed below with reference to  FIGS.  13 . 10 - 1 A through  13 . 10 - 8 B . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these Figures is for explanatory purposes only and should not be construed as limiting. Furthermore, as used herein, a system, a method, an article, a component, a feature, or a sub-feature including at least one of a first option, a second option, or a third option should be understood as referring to a system, a method, an article, a component, a feature, or a sub-feature that can include one of each listed option (e.g., only one of the first option, only one of the second option, or only one of the third option), multiple of a single listed option (e.g., two or more of the first option), two options simultaneously (e.g., one of the first option and one of the second option), or combination thereof (e.g., two of the first option and one of the second option). 
       FIGS.  13 . 10 - 1 A  illustrates a block diagram of a head-mountable device (“HMD”)  13 . 10 - 100  including a frame  13 . 10 - 104 , a display  13 . 10 - 105 , a support (e.g., a retention band  13 . 10 - 109 ), and a light seal or cover  13 . 10 - 112 . The display  13 . 10 - 105  can include one or more optical lenses or display screens in front of the eyes of a user. The display  13 . 10 - 105  can include a display for presenting an augmented reality visualization, a virtual reality visualization, or other suitable visualization. Additionally, the frame  13 . 10 - 104  can at least partially surround the display  13 . 10 - 105 . Similarly, the light seal  13 . 10 - 112  can be connected to the frame  13 . 10 - 104 . In some examples, the light seal  13 . 10 - 112  includes the frame  13 . 10 - 104  (i.e., the frame  13 . 10 - 104  is part of the light seal  13 . 10 - 112 ). 
     The light seal  13 . 10 - 112  can include electrical components (e.g., sensors  13 . 10 - 120 ), a cover  13 . 10 - 113 , a facial interface  13 . 10 - 108 , and a conductive fabric  13 . 10 - 116 . The frame  13 . 10 - 104  can be a housing of the display  13 . 10 - 105 . Further, the frame  13 . 10 - 104  can also be considered to be a part of or separate from the light seal  13 . 10 - 112 . As used herein, the term “light seal” can refer to a portion of the HMD  13 . 10 - 100  that engages or shields a user&#39;s face. In particular, the light seal  13 . 10 - 112  includes portions (e.g., the facial interface) that conform to, contact, or press against regions of the user&#39;s face. 
     The light seal  13 . 10 - 112  includes a body with an inner surface and an outer surface. The outer surface is exposed to the external environment and the inner surface is disposed within the space formed by the HMD  13 . 10 - 100  and the user&#39;s face. The light seal can include a pliant (or semi-pliant) facetrack or face engagement component that spans the forehead, wraps around the eyes, contacts other regions of the face (e.g., zygoma and maxilla regions), and bridges the nose. In addition, the light seal  13 . 10 - 112  can include various components forming a frame, structure, or webbing of a head-mountable device disposed between the display  13 . 10 - 105  and the user&#39;s skin, such as the cover  13 . 10 - 113 . The cover  13 . 10 - 113  can include a seal, environment seal, dust seal, air seal, etc. that is positioned between the gap between the display  13 . 10 - 105  and the user&#39;s face. The cover  13 . 10 - 113  can be a woven fabric that is non-rigid or deformable. The cover  13 . 10 - 113  can be elastically deformable. In some examples, the cover  13 . 10 - 113  can be a plastic, rubber, or polymer material. In some examples, the cover  13 . 10 - 113  can be rigid. 
     The cover  13 . 10 - 113  can form an eye-box through which the user can view the display  13 . 10 - 105 . It will be appreciated that the term “seal” can include partial seals or inhibitors, in addition to complete seals (e.g., a partial light seal where come ambient light is blocked and a complete light seal where all ambient light is blocked when the HMD  13 . 10 - 100  is donned). 
     The light seal  13 . 10 - 112  can have a removable fastener that enables the light seal to be removably attachable to the frame  13 . 10 - 104  and in electrical communication with the display  13 . 10 - 105 . The light seal  13 . 10 - 112  can include an electrical component, such as a sensor  13 . 10 - 120 . The sensor  13 . 10 - 120  can collect user or environmental data, such as biometric information. The sensor  13 . 10 - 120  may receive input from the user, such as the user physically touching the sensor to input or command to the HMD  13 . 10 - 100  as an indication of the user&#39;s intent. The sensor  13 . 10 - 120  can transmit signals to the HMD  13 . 10 - 100 , and more particularly to the display  13 . 10 - 105 . The sensor  13 . 10 - 120  can transmit signals to an output configured to perform an action in response to the information collected by the sensor  13 . 10 - 120 . 
     The light seal  13 . 10 - 112  can include conductive fabric  13 . 10 - 116 . In some examples, the conductive fabric  13 . 10 - 116  can include highly elastic copper threads that can compress and stretch while still maintaining electrical connectivity. The conductive fabric  13 . 10 - 116  can be elastically deformable (i.e., capable of temporary change in length, volume, or shape). In some examples, the conductive fabric  13 . 10 - 116  can include electrically conductive carbon fibers. In some examples, the conductive fabric  13 . 10 - 116  can collect input from the user by the user engaging the conductive fabric through touch, such as with a finger or fingers. These movements can cause a change in the conductive fabric  13 . 10 - 116  (i.e., stretching or compressing). The degree or amount to which the conductive fabric  13 . 10 - 116  is moved by the user&#39;s finger can be detectable by the conductive fabric  13 . 10 - 116  and can be used to generate signals that can be analyzed by a processor of the HMD  13 . 10 - 100 . As described in greater detail below, the conductive fabric  13 . 10 - 116  can be a thread, line, wire, plate, or any other structure that is capable of conducting electricity. The conductive fabric  13 . 10 - 116  can be integrated with the light seal  13 . 10 - 112 . For example, the conductive fabric  13 . 10 - 116  can be embedded, interwoven, or encapsulated with the cover  13 . 10 - 113  or facial interface  13 . 10 - 108 . 
     The display  13 . 10 - 105 , being in electrical communication with the light seal  13 . 10 - 112 , and more specifically with the conductive fabric  13 . 10 - 116 , can receive electrical communication and provide feedback to the user related to the readings of the conductive fabric  13 . 10 - 116  (e.g., visual feedback, audio feedback, haptic feedback, etc.). 
     As used herein, the term “sensor” refers to one or more different sensing devices, such as a camera or imaging device, strain gauge, capacitance sensors, proximity sensors, touch sensitive sensors, accelerometers, and the like. Additional sensors can include a temperature sensor, oxygen sensor, movement sensor, brain activity sensor, sweat gland activity sensor, breathing activity sensor, muscle contraction sensor, etc. In some examples, the sensor can sense biometric features including features of the autonomic nervous system. Some examples of sensors include an electrooculography sensor, electrocardiogramsor, EKG sensor, hear rate variability sensor, blood volume pulse sensor, SpO2 sensor, compact pressure sensor, electromyography sensor, core-body temperature sensor, galvanic skin sensor, accelerometer, gyroscope, magnetometer, inclinometer, barometer, infrared sensor, global positioning system sensor, etc. Additional sensor examples can include, contact microphones (e.g., press-based MEMS), bioelectrical activity sensors, UV exposure sensors, or particle sensors. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 10 - 1 A  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 10 - 1 A . 
       FIGS.  13 . 10 - 1 B  shows a top partial view of an HMD  13 . 10 - 100 . The HMD  13 . 10 - 100  of  FIGS.  13 . 10 - 1 B  can be substantially similar to, including some or all of the features of, the HMD  13 . 10 - 100  described in  FIGS.  13 . 10 - 1 A . The HMD  13 . 10 - 100  can include a display (also referred to as a display unit)  13 . 10 - 105 . The display  13 . 10 - 105  can include any number of internal electronic components  13 . 10 - 107 . The HMD  13 . 10 - 100  can include a frame  13 . 10 - 104  (which can also be referred to as a housing) attached to the display  13 . 10 - 105 . In some examples, the display  13 . 10 - 105  includes an opaque, translucent, transparent, or semi-transparent screen, including any number lenses, for presenting visual data. The frame  13 . 10 - 104  can at least partially border one or more edges of the display  13 . 10 - 105 . The frame  13 . 10 - 104  can be attached to a cover  13 . 10 - 128  at one end of the cover  13 . 10 - 128 . At an opposite end, the cover  13 . 10 - 128  can form, or be attached to, a facial interface  13 . 10 - 108 . In some examples, the frame  13 . 10 - 104 , cover  13 . 10 - 113 , and facial interface  13 . 10 - 108  can together form the light seal  13 . 10 - 112 . It will be understood, however, that the light seal  13 . 10 - 112  can include fewer or more components that those listed or shown. 
     The HMD  13 . 10 - 100  can be worn on a user&#39;s head  13 . 10 - 20  such that the display  13 . 10 - 105  is positioned over the user&#39;s face and disposed over one or both of the user&#39;s eyes. The HMD  13 . 10 - 100  can further include a retention band  13 . 10 - 109 . The retention band  13 . 10 - 109  can secure the HMD  13 . 10 - 100  to the user&#39;s head  13 . 10 - 20  during use so that they user can enjoy the experience provided by the HMD  13 . 10 - 100 . The display  13 . 10 - 105  can be connected to the retention band  13 . 10 - 109  and/or the light seal  13 . 10 - 112 . In some examples, the retention band  13 . 10 - 109  can be positioned against the side of a user&#39;s head  13 . 10 - 20  and in contact therewith. In some examples, the retention band  13 . 10 - 109  can be at least partially positioned above the user&#39;s ear or ears. In some examples, the retention band  13 . 10 - 109  can be positioned adjacent to the user&#39;s ear or ears. The retention band  13 . 10 - 109  can extend around the user&#39;s head  13 . 10 - 20 . In this way, the display  13 . 10 - 105  and the retention band  13 . 10 - 109  can form a loop that can retain the HMD  13 . 10 - 100  on the user&#39;s head  13 . 10 - 20 . It should be understood, however, that this configuration is just one example of how the components of the HMD  13 . 10 - 100  can be arranged, and that in some examples, a different number of connector straps and/or retention bands can be included. Although the HMD  13 . 10 - 100  is referred to as an HMD, it should be understood that the terms wearable device, wearable electronic device, head-mountable device, HMD, HMD device, and/or HMD system can be used to refer to any wearable device, including smart glasses. 
     In some examples, the frame  13 . 10 - 104  is attached to a facial interface  13 . 10 - 108 . The facial interface  13 . 10 - 108  can contact a user&#39;s head  13 . 10 - 20  and/or face. In some examples, the cover  13 . 10 - 113  can be a light blocking component that extends between the frame  13 . 10 - 104  and the facial interface  13 . 10 - 108 . The light blocking component can cover or surround a perimeter of the frame  13 . 10 - 104  and/or the facial interface  13 . 10 - 108 . 
     The cover  13 . 10 - 113  can be a cloth, fabric, woven material, plastic, rubber, or any other suitable opaque or semi-opaque material. In some examples, the cover  13 . 10 - 113  is flexible, having the ability to repeatedly stretch, compress, and deform. The cover  13 . 10 - 113  can be elastically or in-elastically deformable. The facial interface  13 . 10 - 108  in combination with the cover  13 . 10 - 113  can block outside light and limits the peripheral view of the user. In some examples, the cover  13 . 10 - 113  and the facial interface  13 . 10 - 108  is the same or a unitary component. As will be discussed in greater detail below, the light seal  13 . 10 - 112  can include the conductive fabric  13 . 10 - 116  that is able to serve multiple functions and provide added benefits and functionality to the HMD  13 . 10 - 100 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 10 - 1 B  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 10 - 1 B . 
       FIGS.  13 . 10 - 2    shows a bottom perspective view of select components of a light seal  13 . 10 - 212 . The light seal  13 . 10 - 212  can be substantially similar to, including some or all of the features of, the light seals described herein, such as light seal  13 . 10 - 112 . The light seal  13 . 10 - 212  can be implemented on an HMD, such as HMD  13 . 10 - 100 . 
     The light seal  13 . 10 - 212  can be integrated with conductive fabrics  13 . 10 - 216   a ,  13 . 10 - 216   b  (collectively referred to as conductive fabric  13 . 10 - 216 ). The conductive fabric  13 . 10 - 216  or thread can be positioned at various locations on the light seal  13 . 10 - 212 . For example, the conductive fabric  13 . 10 - 216  can be positioned on or in the cover  13 . 10 - 213  (e.g., embedded, encapsulated, or interwoven into the cover). 
     In some examples, the conductive fabric  13 . 10 - 216  can be positioned on the facial interface  13 . 10 - 208  of the light seal  13 . 10 - 212 . In one example, the conductive fabric  13 . 10 - 216  can be in direct contact with a user, for example, touching a user&#39;s forehead, cheek, nose, temple region, back of the head, or at any location where the HMD contacts the user. In some examples, the conductive fabrics  13 . 10 - 216  is disposed on an outer surface of the light seal  13 . 10 - 212 . 
     In the illustrated example of  FIGS.  13 . 10 - 2   , conductive fabrics  13 . 10 - 216   a  are disposed on a left side of the light seal and conductive fabrics  13 . 10 - 216   b  are disposed on a right side of the light seal  13 . 10 - 212 . The conductive fabrics  13 . 10 - 216   a  can be disposed in a specific touch sensitive surface  13 . 10 - 218   a  and conductive fabrics  13 . 10 - 216   b  can be disposed in a touch sensitive surface  13 . 10 - 218   b . The touch sensitive surfaces  13 . 10 - 218   a ,  13 . 10 - 218   b  can be manual input sensors to receive input from the user by manually or physically touching the touch sensitive surfaces  13 . 10 - 218   a ,  13 . 10 - 218   b  to provide input and/or commands to the HMD  13 . 10 - 100 . However, the conductive fabric  13 . 10 - 216  can be located in a variety of different locations of the light seal  13 . 10 - 212 . 
       FIGS.  13 . 10 - 3 A  shows a top view of an HMD  13 . 10 - 300 . The HMD  13 . 10 - 300  can be substantially similar to, including some or all of the features of, the devices described herein, such as the HMD  13 . 10 - 100  and the light seal  13 . 10 - 212 . In some examples, the HMD  13 . 10 - 300  can include an electronic component (e.g., a sensor  13 . 10 - 320 ). The electronic component can be positioned on or housed in the display unit  13 . 10 - 305 . The electronic component can be a battery, a processor, a display, a camera, or any other electronic component. In some examples, the electronic module can be attached to the frame  13 . 10 - 304 . 
     In some examples, the electronic component (e.g., the sensor  13 . 10 - 320 ) can be electrically connected by a conductive fabric  13 . 10 - 316  that runs through the cover  13 . 10 - 313  of the light seal  13 . 10 - 312 . In some examples, the conductive fabric  13 . 10 - 316  runs through or across a retention band  13 . 10 - 309 . In some examples, the conductive fabric  13 . 10 - 316  runs through or across the frame  13 . 10 - 304 . In this manner, the conductive fabric  13 . 10 - 316  serves as an interconnect (i.e., an electrical connection) between the sensor  13 . 10 - 320  on the light seal  13 . 10 - 312 . In some examples, the detections of the conductive fabrics  13 . 10 - 316  can be combined with other sensors on-board or remote from the HMD  13 . 10 - 300  to gather data is a sensor fusion manner. In the illustrated example, the light seal  13 . 10 - 312  includes a conductive fabric  13 . 10 - 316   a  disposed on a left side of the light seal  13 . 10 - 312  and a conductive fabric  13 . 10 - 316   b  disposed on a right side of the light seal  13 . 10 - 312 . The conductive fabric  13 . 10 - 316   a  can be disposed in a specific touch sensitive surface  13 . 10 - 318   a  and the conductive fabric  13 . 10 - 316   b  can be disposed in a specific touch surface  13 . 10 - 318   b . These specific touch sensitive surfaces  13 . 10 - 318   a ,  13 . 10 - 318   b  are configured to allow a user to provide input to the HMD  13 . 10 - 100  by touching the specific touch sensitive surfaces  13 . 10 - 318   a ,  13 . 10 - 318   b.    
     In the illustrated example, the light seal  13 . 10 - 312  includes two touch sensitive surfaces  13 . 10 - 318   a ,  13 . 10 - 318   b , however, the present disclosure is not so limited and can include more or less than two touch sensitive surfaces. In some examples, the entire light seal  13 . 10 - 312  can be a touch sensitive surface that allows the user to input information or commands to the HMD  13 . 10 - 300  from any location on the light seal  13 . 10 - 312 . In some examples, there can be a distinct number of touch sensitive surfaces to enable the user to input information or commands into the HMD  13 . 10 - 300 . In some examples, each distinct touch sensitive surface can be directed to a specific type of input or functionality. For example, one touch sensitive surface can be for volume control, another touch sensitive input can be for scrolling, and the like. The HMD  13 . 10 - 300  can include an indicator that directs the user to the location of the distinct touch sensitive surfaces on the outer surface of the light seal  13 . 10 - 312 . 
     In some examples, the HMD  13 . 10 - 300  can include conductive fabrics  13 . 10 - 316   c ,  13 . 10 - 316   d  incorporated onto and/or into the retention band  13 . 10 - 309  of the HMD  13 . 10 - 300 . The retention band  13 . 10 - 309  can be a headband having flexible sections. The conductive fabrics  13 . 10 - 316   c ,  13 . 10 - 316   d  can be configured to stretch or compress in response to the retention band  13 . 10 - 309  stretching or compressing. In some examples, the conductive fabrics  13 . 10 - 316   c ,  13 . 10 - 316   d  can generate signals indicative of the tightness or position of the retention band  13 . 10 - 309 . In some examples, a user can be notified based on the signals generated by the conductive fabrics  13 . 10 - 316   c ,  13 . 10 - 316   d . The retention band  13 . 10 - 309  can be automatically adjusted based on the signals generated by the conductive fabrics  13 . 10 - 316   c ,  13 . 10 - 316   d . In the illustrated example, the retention band  13 . 10 - 309  include the conductive fabric  13 . 10 - 316   c  disposed on a left side of the retention band  13 . 10 - 309  and the conductive fabric  13 . 10 - 316   d  disposed on a right side of the retention band  13 . 10 - 309  and can be connected to electrical components  13 . 10 - 320  on the frame  13 . 10 - 304  of the HMD  13 . 10 - 300 . 
       FIGS.  13 . 10 - 3 B  illustrates a top view of a user interacting with the touch sensitive surface  13 . 10 - 318   b . For ease of illustration, the touch sensitive surface  13 . 10 - 318   b  depicts a single conductive fiber but can includes a plurality of conductive fibers. In some examples, the conductive fabric  13 . 10 - 316   b  is woven into a strain gauge pattern and the conductive fabric  13 . 10 - 316   b  is electrically connected to an electrical component (e.g., a sensor  13 . 10 - 320 ) of the HMD  13 . 10 - 300 . The user&#39;s engagement with the touch sensitive surface  13 . 10 - 318   b  can be interpreted as input to provide commands to the HMD  13 . 10 - 300 . 
     In some examples, the conductive fabric  13 . 10 - 316   b  is physically connected to the sensor  13 . 10 - 320 . The sensor  13 . 10 - 320  can be a strain gauge and the conductive fabric  13 . 10 - 316   b  is physically coupled to the strain gauge  13 . 10 - 320 . The strain gauge  13 . 10 - 320  can measure the displacement of the conductive fabric  13 . 10 - 316   b  and interpret or transform the displacement of the conductive fabric  13 . 10 - 316   b  into input for the HMD  13 . 10 - 300 . 
     In some examples, the touch sensitive surface  13 . 10 - 318   b  includes yarn that is physically connected to the strain gauge  13 . 10 - 320 . All the yarn disposed within a perimeter of the touch sensitive surface  13 . 10 - 318   b  can be physically connected to the strain gauge  13 . 10 - 320 . The touch sensitive surface  13 . 10 - 318   b  leverages the elastic nature of the yarn. In other words, as the user presses or touches the yarn of the touch sensitive surface  13 . 10 - 318   b , the strain is transferred to the strain gauge  13 . 10 - 320  that is disposed away from the touch sensitive surface  13 . 10 - 318   b  and the strain gauge can measure the displacement of the yarn and interpreted the displacement as input for the HMD  13 . 10 - 300 . In some examples, the touch sensitive surface  13 . 10 - 318  can include both conductive fabric  13 . 10 - 316   b  and yarn. 
       FIGS.  13 . 10 - 3 B  illustrates the user pushing or poking the specific touch sensitive surface  13 . 10 - 318   b  with a finger  13 . 10 - 10 , which displaces the conductive fabric  13 . 10 - 316   b  and transfers the displacement to the strain gauge  13 . 10 - 320 , which interprets the displacement as a command or input to the HMD  13 . 10 - 300 . However, the user can use a number of different gestures with their finger  13 . 10 - 10  to provide input to the HMD  13 . 10 - 300 . As discuss above, the user can press (e.g., poke) against the touch sensitive surface  13 . 10 - 318   b . The user can also slide their finger  13 . 10 - 10  along the touch sensitive surface  13 . 10 - 318   b . The user can stretch the touch sensitive surface  13 . 10 - 318   b  by using two fingers to stretch the conductive fabric  13 . 10 - 316   b  between the two fingers or push the conductive fabric  13 . 10 - 316   b  closer together. This gesture can be interpreted as zooming in or zooming out. The user can also pinch the conductive fabric  13 . 10 - 316 . This gesture can be interpreted as taking a picture. In some examples, the number of fingers used in the gesture can indicate a different type of input. For example, two fingers swiping can be interpreted by the system as a different input relative to a single finger swiping, or three fingers swiping. 
     The strain gauge  13 . 10 - 320  can be incorporated into the frame  13 . 10 - 304  of the HMD  13 . 10 - 300 . In some examples, the strain gauge  13 . 10 - 320  can be disposed on the frame  13 . 10 - 304  of the HMD  13 . 10 - 300 . In some examples, the strain gauge  13 . 10 - 320  can be disposed within the frame  13 . 10 - 304 . For example, the frame  13 . 10 - 304  can includes a slot in which the strain gauge  13 . 10 - 320  is disposed and the yarn and/or conductive fabric  13 . 10 - 316   b  can enter to physically connect to the strain gauge  13 . 10 - 320 . Accordingly, the strain gauge  13 . 10 - 320  is shielded from environmental conditions, such as sweat, fluid, debris, and the like. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 10 - 3    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 10 - 3   . 
       FIGS.  13 . 10 - 4    illustrates a touch sensitive surface  13 . 10 - 418  on a light seal  13 . 10 - 412  of an HMD  13 . 10 - 400 . The HMD  13 . 10 - 400  can be substantially similar to, including some or all of the features of, the devices described herein, such as the HMDs  13 . 10 - 100 ,  13 . 10 - 300  and the light seal  13 . 10 - 212 . The touch sensitive surface  13 . 10 - 418  can include a distinct perimeter  13 . 10 - 419 . In the illustrated example, the perimeter  13 . 10 - 419  of the touch sensitive surface  13 . 10 - 418  is a circle, however, the perimeter  13 . 10 - 419  can include a plurality of different shapes. For example, the perimeter  13 . 10 - 419  can be semi-circular, ovoid, rectangular, trapezoidal, polygonal, triangular, and a variety of different shapes. As discuss above, the touch sensitive surface  13 . 10 - 418  can include a conductive fabric with conductive fibers (not shown) and/or yarn that can be connected to a strain gauge. 
     The touch sensitive surface  13 . 10 - 418  can include an indicator to alert or guide the user to the touch sensitive surface  13 . 10 - 418 . For example, when the user is donning the HMD  13 . 10 - 400 , the user cannot see the outer surface of the light seal  13 . 10 - 412  or the touch sensitive surface  13 . 10 - 418  on the light seal  13 . 10 - 412  as the HMD  13 . 10 - 400  is intentionally blocking their view outside of light seal  13 . 10 - 412  and HMD  13 . 10 - 400 . Therefore, when the user is donning the HMD  13 . 10 - 400 , the user attempts to engage with the touch sensitive surface  13 . 10 - 418  blindly. The indicator can provide feedback to help to user locate the touch sensitive surface  13 . 10 - 418  in a variety of different ways so that the user can input commands to the HMD  13 . 10 - 400  using the touch sensitive surface  13 . 10 - 418 . 
     In the illustrated example of  FIGS.  13 . 10 - 4   , the indicator can use tactile feedback to guide the finger  13 . 10 - 10  of the user to the touch sensitive surface  13 . 10 - 418 . For example, a plurality of projections  13 . 10 - 417  or bumps can project outward from the surface of a light seal  13 . 10 - 412  along the perimeter  13 . 10 - 419  of the touch sensitive surface. Therefore, when the user slides their finger  13 . 10 - 10  over the projections  13 . 10 - 417 , the user&#39;s feels a tactile sensation on the tip of their finger  13 . 10 - 10 . These tactile sensations can orient and alert the user to the perimeter of the touch sensitive surface  13 . 10 - 418 . In other words, as the finger  13 . 10 - 10  of the user approaches the touch sensitive surface  13 . 10 - 418  the projections  13 . 10 - 417  provide tactile feedback to the user to know that they have arrived at the touch sensitive surface  13 . 10 - 418 . The projections  13 . 10 - 417  can be spaced along the perimeter  13 . 10 - 419  of the touch sensitive surface  13 . 10 - 418 . The space between adjacent projections  13 . 10 - 417  may be less than the width a fingertip, so that user can feel it. In some examples, the projection  13 . 10 - 417  may be a continuous projection that extends along the entire perimeter  13 . 10 - 419  of the touch sensitive surface  13 . 10 - 418 . In this manner, the user knows when they are approaching the perimeter  13 . 10 - 419  of the touch sensitive surface  13 . 10 - 418 , either from the outside of the touch sensitive surface  13 . 10 - 418  or from the inside of the touch sensitive surface  13 . 10 - 418 . 
     The projections  13 . 10 - 417  can have a variety of different shapes. In the illustrated example, the projections  13 . 10 - 417  have a circular shape. However, the scope of the present disclosure incorporates a variety of different shapes for the projections, such a rectangular, triangular, polygonal, and the like. 
     In some examples, the projections  13 . 10 - 417  can be positioned in patterns to help orient the user. For example, the bottom portion of the perimeter  13 . 10 - 419  can have a different pattern than the rest of the perimeter  13 . 10 - 419 . For example, the bottom portion of the perimeter can have a cluster of three projections  13 . 10 - 419  that are closer to each other to indicate to the user that they are approaching the bottom portion of the perimeter  13 . 10 - 419  of the touch sensitive surface  13 . 10 - 418 . Similarly, a top portion of the perimeter  13 . 10 - 419  can have a similar pattern as the bottom portion of a different pattern. The same goes for a left portion of the perimeter  13 . 10 - 419  and a right portion of the perimeter  13 . 10 - 419 . 
     In some examples, the tactile feedback can be dynamic. In other words, in some examples the projections  13 . 10 - 417  can be felt, and in other examples the projections  13 . 10 - 417  cannot be felt. For example, in situations where the touch sensitive surface  13 . 10 - 418  is turned on and active, the projections  13 . 10 - 417  can be felt and in situations where the touch sensitive surface  13 . 10 - 418  is turned off and not active the projections  13 . 10 - 417  cannot be felt. In some examples, the projections  13 . 10 - 417  can be part of an inflatable bladder that inflates to make the projections  13 . 10 - 417  project outward from the outer surface of the light seal  13 . 10 - 412  and deflate to make the projections  13 . 10 - 417  retract back into the light seal  13 . 10 - 412 . In some other examples, the projections  13 . 10 - 417  can be selectively activated by a solenoid or other electrically actuated system that extends the projections  13 . 10 - 417  outward from the outer surface of the light seal  13 . 10 - 412  when the touch sensitive surface  13 . 10 - 418  is active, and retracts the projections  13 . 10 - 417  when the touch sensitive surface  13 . 10 - 418  is not active. As discussed below in more detail, the touch sensitive surface  13 . 10 - 418  can become active when a position sensor detects a near touch. When a near touch is detected, the touch sensitive surface  13 . 10 - 418  can be turned on and the projections  13 . 10 - 417  can be activated and extended. 
     In other examples, the indicator can include visual feedback that alerts or guides the finger  13 . 10 - 10  of the user to the touch sensitive surface  13 . 10 - 418 . Visual feedback can be displayed on a display  13 . 10 - 405  of the HMD  13 . 10 - 400  to the user. The HMD  13 . 10 - 400  can include a sensor, such as a camera, that is disposed on the frame  13 . 10 - 404  of the HMD  13 . 10 - 400 . The sensor can capture images or video of the touch sensitive surface  13 . 10 - 418  on the outer surface of the light seal  13 . 10 - 412 . Accordingly, the display  13 . 10 - 405  can display the images or video from the sensor directly to the user on the display  13 . 10 - 405  to show the user where their finger  13 . 10 - 10  is in relation to the touch sensitive surface  13 . 10 - 418  to help the user find and utilize the touch sensitive surface  13 . 10 - 418 . 
     In another example, the display  13 . 10 - 405  can merely display animations or visual cues to the user to orient the finger  13 . 10 - 10  of the user toward the touch sensitive surface  13 . 10 - 418 . For example, the display  13 . 10 - 405  can display to the user an animation, rather than a real-time video or image of how close the finger  13 . 10 - 10  of the user is relative to the touch sensitive surface  13 . 10 - 418 . The animations or visual cues may be display in a corner on a peripheral of the display  13 . 10 - 405  so that the animations or visual cues are not overlap over the central area of the display  13 . 10 - 405 . 
     In another example, an inner surface of the light seal  13 . 10 - 412  can include a plurality of light emitting diodes. The light emitting diodes can be used to provide feedback to the user to guide the user to the touch sensitive surface  13 . 10 - 418 . In some examples, the light emitting diodes can outline the touch sensitive surface  13 . 10 - 418  on the inner surface of the light seal  13 . 10 - 412 . When the light emitting diodes light up, the user can see the relative location of the touch sensitive surface  13 . 10 - 418 , albeit, from the inside of the light seal  13 . 10 - 412 . In some examples, the light emitting diodes on the inner surface of the light seal  13 . 10 - 412  may light up to show the user the current location of the finger  13 . 10 - 10  of the user from the inside of the light seal so that user can see the approximate location of their finger  13 . 10 - 10  relative to the touch sensitive surface  13 . 10 - 418 . 
     In another example, the color of the light emitting diodes can change color based on the location of the finger  13 . 10 - 10  of the user relative to the location of the touch sensitive surface  13 . 10 - 418 . For example, the light emitting diode can transition from a red color to a yellow color to a green color based on the location of the finger  13 . 10 - 10 . The light emitting diode can be red when the finger  13 . 10 - 10  of is not near the touch sensitive surface  13 . 10 - 418 , transition to a yellow color as the finger  13 . 10 - 10  get closer to the perimeter  13 . 10 - 419  of the touch sensitive surface  13 . 10 - 418  and turn green when the finger  13 . 10 - 10  engages the touch sensitive surface  13 . 10 - 418 . 
     In some examples, the indicator can include haptic feedback. The light seal  13 . 10 - 412  can include a haptic engine  13 . 10 - 434  that is electrically connected to processor  13 . 10 - 430  via a wire  13 . 10 - 432 . The haptic engine is configured to provide haptic feedback (e.g., vibration) to the user when their finger  13 . 10 - 10  approaches the perimeter  13 . 10 - 419  of the touch sensitive surface  13 . 10 - 418 . The vibration of the haptic engine  13 . 10 - 434  can orient and alert the user to the perimeter  13 . 10 - 419  of the touch sensitive surface  13 . 10 - 418 . In other words, as the finger  13 . 10 - 10  of the user approaches the touch sensitive surface  13 . 10 - 418  the haptic engine  13 . 10 - 434  vibrates to provide feedback to the user to know that they have arrived at the touch sensitive surface  13 . 10 - 418 . In this manner, the user knows when they are approaching the perimeter  13 . 10 - 419  of the touch sensitive surface  13 . 10 - 418 , either from the outside of the touch sensitive surface  13 . 10 - 418  or from the inside of the touch sensitive surface  13 . 10 - 418 . 
     The processor  13 . 10 - 430  can also perform a variety of different functions, including communicating with each of the feedback mechanisms discussed above. The processor  13 . 10 - 430  can function as a triggering device that can turn on and turn off the touch sensitive surface. In an attempt to conserve power, the touch sensitive surface  13 . 10 - 418  can be turned on for a user to provide input to the HMD  13 . 10 - 400  and turned off when the touch sensitive surface  13 . 10 - 418  is not in use. The processor  13 . 10 - 430  can be in communication with a number of different sensors. 
     The processor  13 . 10 - 430  can be in communication with a position sensor that detects a near touch, which is when the finger  13 . 10 - 10  or hand of the user is within a predetermined distance of the touch sensitive surface  13 . 10 - 418 . For example, the processor  13 . 10 - 430  can be in communication with a plurality of proximity sensors that are disposed around the perimeter  13 . 10 - 419  of touch sensitive surface  13 . 10 - 418  or disposed within the touch sensitive surface  13 . 10 - 418 . The proximity sensor can detect a near touch. Accordingly, when the proximity sensor senses a near touch, the processor  13 . 10 - 430  triggers the touch sensitive surface  13 . 10 - 418  to turn on so that the user can engage with the touch sensitive surface  13 . 10 - 418 . In other words, the processor  13 . 10 - 430  can turn on the touch sensitive surface  13 . 10 - 418  when the sensor detects a near touch. 
     In another example, the processor  13 . 10 - 430  can be in communication with a camera that captures images and/or video of the touch sensitive surface  13 . 10 - 418 . The camera monitors the touch sensitive surface  13 . 10 - 418  on the outer surface of the light seal  13 . 10 - 412  and captures images and/or video of the touch sensitive surface  13 . 10 - 418 . When the camera detects a near touch, the processor  13 . 10 - 430  determines that the user is about to touch the touch sensitive surface  13 . 10 - 418  and triggers the touch sensitive surface  13 . 10 - 418  to turn on so that the user can engage with the touch sensitive surface  13 . 10 - 418 . 
     The HMD  13 . 10 - 400  can use a number of different position sensors, such as proximity sensors, cameras, capacitance sensors, or any other type of sensor to detect a near touch (e.g., the approach of the user&#39;s finger, hand, etc. to trigger turning on the touch sensitive surface  13 . 10 - 418 ). These systems can be used to avoid having environmental factors, such as fluid, water, humidity, sweat, accidently engaging with the touch sensitive surface  13 . 10 - 418  and triggering input to the HMD  13 . 10 - 400  when the user was not intending to. 
       FIGS.  13 . 10 - 5  through  13 . 10 - 8 B  illustrate additional ways in which the user can provide input for the HMD in a variety of different manners.  FIGS.  13 . 10 - 5    illustrates an HMD  13 . 10 - 500  with a frame  13 . 10 - 504 . The frame  13 . 10 - 504  of the HMD  13 . 10 - 500  is coupled to the light seal  13 . 10 - 512 . The light seal  13 . 10 - 512  includes a touch sensitive surface  13 . 10 - 518  that is disposed on an outer surface of a light seal  13 . 10 - 512 . In the illustrated example, the touch sensitive surface  13 . 10 - 518  can include a plurality of conductive fabrics  13 . 10 - 516  organized (e.g., weaved) in a specific pattern. In the illustrated example, the conduct fabric  13 . 10 - 516  is organized in a matrix pattern. When the conductive fabric  13 . 10 - 516  is organized in this pattern, the conductive fabric  13 . 10 - 516  can track the movement of the finger  13 . 10 - 10  along the touch sensitive surface  13 . 10 - 518 . The conductive fabrics  13 . 10 - 516  can be spaced apart a predetermined distance such that environmental factors can not adversely affect input to the touch sensitive surface  13 . 10 - 518 . 
       FIGS.  13 . 10 - 6    illustrates an HMD  13 . 10 - 600  with a frame  13 . 10 - 604 . The frame  13 . 10 - 604  of the HMD  13 . 10 - 600  is connected to the light seal  13 . 10 - 612 . The light seal  13 . 10 - 612  includes a touch sensitive surface  13 . 10 - 618  that is disposed on an outer surface of a light seal  13 . 10 - 612 . In the illustrated example, the touch sensitive surface  13 . 10 - 618  includes a plurality of planes  13 . 10 - 616  that are woven into the light seal  13 . 10 - 612  and are spaced vertically from each other. The planes  13 . 10 - 616  can get a capacitance reading to track the movement of the finger  13 . 10 - 10  of the user. In other words, the finger  13 . 10 - 10  can do a swiping motion or gesture in a vertical direction and the planes  13 . 10 - 616  can track the movement. This gesture can be used for volume control or scrolling. In some examples, the planes  13 . 10 - 616  can be spaced horizontally form each other and the user can do a swiping motion or gesture in a horizontal direction. 
       FIGS.  13 . 10 - 7    illustrates an HMD  13 . 10 - 700  with a frame  13 . 10 - 704 . The frame  13 . 10 - 704  can include a sensor  13 . 10 - 720 . The sensor  13 . 10 - 720  can be an accelerometer that can detect vibrations in the frame  13 . 10 - 704 . Accordingly, the user can tap the frame  13 . 10 - 704  of the HMD  13 . 10 - 700  with their finger  13 . 10 - 10 . Tapping on the frame  13 . 10 - 704  can interpreted as input from the user to the HMD  13 . 10 - 700 . 
       FIGS.  13 . 10 - 8 A and  13 . 10 - 8 B  illustrate an HMD  13 . 10 - 800  with a frame  13 . 10 - 804 . The frame  13 . 10 - 804  can include a sensor  13 . 10 - 820 . The sensor  13 . 10 - 820  can be a strain gauge that can detect mechanical deflection of the frame  13 . 10 - 804 .  FIGS.  13 . 10 - 8 A  illustrates the frame  13 . 10 - 804  in a natural or unconstrained state.  FIGS.  13 . 10 - 8 B  illustrates the frame  13 . 10 - 804  in constrained state in which the user is squeezing a top and a bottom of the frame  13 . 10 - 804  toward each other, thereby mechanically deflecting the frame  13 . 10 - 804 . The deflection of the frame  13 . 10 - 804  can be interpreted as input to the HMD  13 . 10 - 800 . For example, squeezing the HMD  13 . 10 - 800  as illustrated can be interpreted as a command to take a picture of the display of the HMD  13 . 10 - 800 . 
     13.11: Face Engaging Structures 
     The following disclosure relates to wearable electronic devices (e.g., head-mountable device). More particularly, the present embodiments relate to connections between a display and a facial interface of a head-mountable device. Additionally, the present embodiments relate to lightweight variations of a facial interface. 
     When donning a head-mountable device or during use, the position and weight of the head-mountable device can affect the quality of user experience. Indeed, conventional head-mountable devices can be heavy due to the mass of materials (or combination of materials) used in manufacture. The additional weight can apply excess pressure to a user&#39;s face causing discomfort and pressure points at certain regions of the face. This user discomfort can degrade the overall AR/VR experience. 
     Exacerbating the weight factor, facial regions that contact a head-mountable device can have unique facial characteristics, such as variations of cranial width or length or variations in facial bones (e.g., frontal bone, zygoma, maxilla, etc.). Thus, these areas of high facial variability can undergo acute pressure or lend to faster user exhaustion. 
     Connections of the present disclosure address these and/or other issues of conventional head-mountable devices by providing dynamic adjustment. Indeed, connections of the present disclosure can dynamically adjust to the facial variability of users to provide a comfortable user experience. Additionally, connections (and associated connector positioning) of the present disclosure can better distribute applied loads across the user face for improved comfort compared to conventional head-mountable device. These connections can range from mechanical-based connectors providing more mechanical ranges of motion to organic connectors providing more organic ranges of motion. 
     To illustrate, the connectors may be positioned at various locations (e.g., maxilla, zygoma, frontal bone, etc.). The connectors can include various types of joints (e.g., pivot joint, soft joint, flexure joint, spring joint, etc.). The connectors can, in combination, provide a comfortable pressure distribution. 
     Additionally, or alternatively to connectors, a head-mountable device of the present disclosure includes facial interface with a flexible (or flexure) portion. The flexure portion of the facial interface can include an elastomer material capable of recovering to an original shape after being flexed or stretched. In certain instances, the flexure region includes a particular pattern (e.g., a serpentine pattern) that can naturally flex, move, and conform to a user face. 
     Furthermore, embodiments described below provide examples of adjustment mechanisms, connectors, and facial interfaces of head-mountable devices that are lighter and more comfortable than conventional head-mountable devices. To illustrate, a head-mountable device can include a display frame with relief cutouts (e.g., lozenge-shaped thru-holes, cored-out portions, dimples, etc.). The relief cutouts can reduce the weight of the head-mountable device while maintaining structural integrity of the head-mountable device. 
     In another example, the head-mountable device includes a stiffener member that allows for a reduced amount of implemented material (and hence, weight savings). The stiffener member can be positioned between the display frame and the facial interface. The stiffener member can also be implemented as an additional shot of material in manufacturing. Additionally, or alternatively, the stiffener member can be overmolded to strengthen certain areas (e.g., at connection sites). 
     These and other embodiments are discussed below with reference to  FIGS.  13 . 11 - 1 - 13 . 11 - 13   . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these Figures is for explanatory purposes only and should not be construed as limiting. Furthermore, as used herein, a system, a method, an article, a component, a feature, or a sub-feature including at least one of a first option, a second option, or a third option should be understood as referring to a system, a method, an article, a component, a feature, or a sub-feature that can include one of each listed option (e.g., only one of the first option, only one of the second option, or only one of the third option), multiple of a single listed option (e.g., two or more of the first option), two options simultaneously (e.g., one of the first option and one of the second option), or combination thereof (e.g., two of the first option and one of the second option). 
       FIGS.  13 . 11 - 1    illustrates a top view of a head-mountable device  100  worn on a user head  13 . 11 - 101 . The head mountable device can include a display  13 . 11 - 102  (e.g., one or more optical lenses or display screens in front of the eyes of the user). The display  13 . 11 - 102  can include a display for presenting a virtual reality visualization, an augmented reality visualization, or other suitable visualization. 
     The head-mountable device  13 . 11 - 100  also includes a facial interface  13 . 11 - 104 . As used herein, the term “facial interface” refers to a portion of the head-mountable device  13 . 11 - 100  that engages a user face via direct contact. In particular, a facial interface includes portions of the head-mountable device  13 . 11 - 100  that conform to (e.g., compress against) regions of the user face. For example, a facial interface may include a pliant (or semi-pliant) face track that spans the forehead, wraps around the eyes, contacts the zygoma and maxilla regions of the face, and bridges the nose. Furthermore, a facial interface can include various components forming a structure, webbing, cover, fabric, or frame of a head-mountable device disposed between the display  13 . 11 - 102  and the user skin. In particular implementations, a facial interface can include a seal (e.g., a light seal, environment seal, dust seal, air seal, etc.). It will be appreciated that the term “seal” can include partial seals or inhibitors, in addition to complete seals (e.g., a partial light seal where some ambient light is blocked and a complete light seal where all ambient light is blocked when the head-mountable device is donned). 
     In addition, the head-mountable device  13 . 11 - 100  includes connector(s)  13 . 11 - 106 . As used herein, the terms “connector” or “joint” refer to a joining between the display  13 . 11 - 102  and facial interface  13 . 11 - 104 . In some examples, a connector allows the facial interface  13 . 11 - 104  to translate or rotate relative to the display  13 . 11 - 102  via the connector. In other examples, the connector allows the facial interface  13 . 11 - 104  to both translate and rotate relative to the facial interface  13 . 11 - 104 . For example, the connector(s)  13 . 11 - 106 , when acted on can translate the facial interface  13 . 11 - 104  toward or away from the display  13 . 11 - 102  (e.g., in a linear fashion). In another example, the connection(s)  13 . 11 - 106  when acted on can rotate the facial interface  13 . 11 - 104  up or down relative to the display  13 . 11 - 102  (e.g., in an angular fashion). In particular implementations, the connector(s)  13 . 11 - 106  moveably constrain the facial interface  13 . 11 - 104  to the display  13 . 11 - 102  at a forehead location, a zygoma location, or a maxilla location. 
     In this manner, the connection(s)  13 . 11 - 106  can movably constrain the display  13 . 11 - 102  relative to the facial interface  13 . 11 - 104 . As used herein, the term “movably constrain” refers to the type of connection that can dynamically move (e.g., translate or rotate), yet retain control over a particular element&#39;s movement or position. For example, to “movably constrain” means the connection(s)  13 . 11 - 106  can bound movement of the display  13 . 11 - 102  within two degrees of freedom (e.g., along a horizontal plane and along an additional plane non-planar with the horizontal plane) relative to the facial interface  13 . 11 - 104 . 
     As used herein, the term “forehead region” refers to an area of a human face between the eyes and the scalp of a human. Additionally, the term “maxilla region” refers to an area of a human face corresponding to the zygomatic bone structure of a human. Similarly, the term “maxilla region” refers to an area of a human face corresponding to the maxilla bone structure of a human. Further, the term “temple region” refers to an area of a human face between a respective eye and ear on a particular side of a face (e.g., between cheek bones and a forehead region). It will be appreciated that the foregoing regions can correspond to particular structure of the head-mountable device  13 . 11 - 100 . However, such structure of the head-mountable device  13 . 11 - 100  is not dependent on a face or a user. 
     The connector(s)  13 . 11 - 106  allow the facial interface  13 . 11 - 104  to conform freely to a wide range of facial topographies, thereby allowing the facial interface  13 . 11 - 104  to pivot and flex relative to the display  13 . 11 - 102 . The connector(s)  13 . 11 - 106  can also distribute loads (e.g., forces exerted from different facial topographies or compression from the strap  13 . 11 - 103 ) evenly on a user&#39;s face. The connector(s)  13 . 11 - 106  can include one or more joints (e.g., pivot joint, soft joint, flexure joint, spring joint, etc.) that allow (or actively provide) translation or rotation of the facial interface  13 . 11 - 104  relative to the display  13 . 11 - 102 . 
     Furthermore, locations of the connector(s)  13 . 11 - 106  can be modified or tuned, as can the number of connections. For example, the location and or number of the connector(s)  13 . 11 - 106  can correlate to an amount of force or pressure exerted on the user at any one datum (e.g., forehead region, maxilla region, zygoma region, etc.) In other instances, the location and/or the number of connector(s)  13 . 11 - 106  can correspond to rigidity (or rigidity variances) between the connector(s)  13 . 11 - 106 . 
     Additionally shown in  FIGS.  13 . 11 - 1   , the head-mountable device  13 . 11 - 100  includes one or more arms  13 . 11 - 108 ,  13 . 11 - 110 . The arms  13 . 11 - 108 ,  13 . 11 - 110  are connected to the display  13 . 11 - 102  and extend distally toward the rear of the head. The arms  13 . 11 - 108 ,  13 . 11 - 110  are configured to secure the display in a position relative to the user head  13 . 11 - 101  (e.g., such that the display  13 . 11 - 102  is maintained in front of a user&#39;s eyes). For example, the arms  13 . 11 - 108 ,  13 . 11 - 110  extend over the user&#39;s ears  13 . 11 - 112 . In certain examples, the arms  13 . 11 - 108 ,  13 . 11 - 110  rest on the user&#39;s ears  13 . 11 - 112  to secure the head-mountable device  13 . 11 - 100  via friction between the arms  13 . 11 - 108 ,  13 . 11 - 110  and the user head  13 . 11 - 101 . For example, the arms  13 . 11 - 108 ,  13 . 11 - 110  can apply opposing pressures to the sides of the user head  13 . 11 - 101  to secure the head-mountable device  13 . 11 - 100  to the user head  13 . 11 - 101 . Optionally, the arms  13 . 11 - 108 ,  13 . 11 - 110  can be connected to each other via a strap  13 . 11 - 103  (shown in the dashed lines) that can compress the head-mountable device  13 . 11 - 100  against the user head  13 . 11 - 101 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 11 - 1    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 11 - 1   . 
       FIGS.  13 . 11 - 2 A- 13 . 11 - 2 B  respectively illustrate side and front view profiles of an example of the head-mountable device  13 . 11 - 100 . As discussed above, the head-mountable device  13 . 11 - 100  includes the display  13 . 11 - 102 , the facial interface  13 . 11 - 104 , and the connector(s)  13 . 11 - 106 . In particular, as shown in  FIGS.  13 . 11 - 2 A- 13 . 11 - 2 B , the facial interface  13 . 11 - 104  can indeed wrap around the eyes  13 . 11 - 201 , bridge the nose  13 . 11 - 202 , span the forehead  13 . 11 - 203 , and contact a maxilla facial region  13 . 11 - 204  and a zygoma facial region  13 . 11 - 205 . 
     Additionally shown in  FIGS.  13 . 11 - 2 B  are the locations of the connector(s)  13 . 11 - 106 . In particular examples, the connector(s)  13 . 11 - 106  are located at the forehead  13 . 11 - 203 , the maxilla facial region  13 . 11 - 204 , and the zygoma facial region  13 . 11 - 205 . Other locations of the connector(s)  13 . 11 - 106  are herein contemplated. However, the connector(s)  13 . 11 - 106  in at least these positions can provide a dynamic, yet stable connection between the display  13 . 11 - 102  and the facial interface  13 . 11 - 104 . 
     It will be appreciated that the connector(s)  13 . 11 - 106  at the different locations can be the same, or in certain cases, different. For instance, the connector(s)  13 . 11 - 106  can include a first joint (e.g., socket joint, soft joint, molded hinge joint, butterfly flexure joint, cam pivot joint, cross axis pivot joint, etc.) positioned at the forehead  13 . 11 - 203 . In addition, the connector(s)  13 . 11 - 106  can include a second joint, different from the first joint, such as a pivot joint, elastomer spring joint, soft joint, single ball joint, etc. Indeed, different arrangements and types of the connector(s)  13 . 11 - 106  can be implemented to provide a particular force profile, amount of rigidity, etc. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 11 - 2 A- 13 . 11 - 2 B  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 11 - 2 A- 13 . 11 - 2 B . 
     As previously discussed, head-mountable device  13 . 11 - 100  can include connections at various locations. The forehead location of the connector(s) will now be discussed below. 
       FIGS.  13 . 11 - 3 A  illustrates an example of the connection(s)  13 . 11 - 106  located at the forehead  13 . 11 - 203 . The connector(s)  13 . 11 - 106  can translate and/or rotate the facial interface  13 . 11 - 104  relative to the display  13 . 11 - 102  via the connector(s)  13 . 11 - 106 . The connector(s)  13 . 11 - 106  can vary in mobility (e.g., ranging from mechanical to organic), as will be discussed below. Specifically,  FIGS.  13 . 11 - 3 B- 13 . 11 - 3 E  illustrate a few examples of the connector(s)  13 . 11 - 106 , namely connectors  13 . 11 - 300 - 13 . 11 - 306 , respectively. 
       FIGS.  13 . 11 - 3 B  illustrates the connector  13 . 11 - 300 . The connector  13 . 11 - 300  includes a single degree of freedom. In particular, the connector  13 . 11 - 300  can include a pivot connection that rotatably connects the display  13 . 11 - 102  to the facial interface  13 . 11 - 104 . For instance, the facial interface  13 . 11 - 104  can laterally pivot relative to the display  13 . 11 - 102 , where the display  13 . 11 - 102  includes a pivot point. In these or other examples, the connector  13 . 11 - 300  limits the amount of motion (e.g., to certain angular thresholds relative to the pivot point). 
       FIGS.  13 . 11 - 3 C  illustrates the connector  13 . 11 - 302 . The connector  13 . 11 - 302  includes one degree of freedom plus translation. In certain implementations, the connector  13 . 11 - 302  includes a cam pivot. The connector  13 . 11 - 302  can rotationally pivot similar to the connector  13 . 11 - 300 . In addition, the connector  13 . 11 - 302  can slide laterally such that the pivot point can also move. In this manner, the connector  13 . 11 - 302  can slidably and rotationally connect the display  13 . 11 - 102  to the facial interface  13 . 11 - 104 . With more free range of motion, the connector  13 . 11 - 302  allows more organic motion than the connector  13 . 11 - 300 . 
       FIGS.  13 . 11 - 3 D  illustrates the connector  13 . 11 - 304 . The connector  13 . 11 - 304  includes a cross-axis pivot joint connector (e.g., butterfly flexure). The connector  13 . 11 - 304  therefore includes a cross-axis design allowing each contact point on the facial interface  13 . 11 - 104  to move independently in a plane (e.g., x-y plane). The thickness of the cross-axis pivot joint can vary in flexure dependent on the thickness of a cross-axis member. For example, increasing the thickness (e.g., cross-sectional area) or decreasing the length of the cross-axis member(s) increases the flexural rigidity of the cross-axis pivot joint. Inversely, decreasing the thickness or increasing the length of the cross-axis member(s) decreases the flexural rigidity of the cross-axis pivot joint. For instance, a cross-axis member with a larger cross-sectional area while maintaining the length of the cross-axis member(s) increases the amount of compression force required to deflect the cross-axis member(s). In addition, the flexural rigidity can be altered by varying the material type or durometer. 
       FIGS.  13 . 11 - 3 E  illustrates the connector  13 . 11 - 306 . The connector  13 . 11 - 306  includes a flexure joint connector (e.g., a compliant mechanism capable of storing strain energy in one or more deformable/elastic members). The connector  13 . 11 - 306  can include a symmetrical design. In addition, the connector  13 . 11 - 306  can include a predetermined amount of stiffness in one or more directions, and a predetermined amount of flexibility in one or more different directions. The topology or architecture of the flexure joint can coincide with desired rigidity/flexibility properties in particular directions for providing mobility and/or support of the facial interface  13 . 11 - 104  relative to the display  13 . 11 - 102 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 11 - 3 A- 13 . 11 - 3 E  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 11 - 3 A- 13 . 11 - 3 E . 
       FIGS.  13 . 11 - 4 A  illustrates the connector(s)  13 . 11 - 106  located at the zygoma facial region  13 . 11 - 205 . The connector(s)  13 . 11 - 106  can moveably constrain the facial interface  13 . 11 - 104  to the display  13 . 11 - 102  in the same or similar ways as discussed above. The connector(s)  13 . 11 - 106  can include a variety of different connectors. Examples of the connector(s)  13 . 11 - 106 , namely connectors  13 . 11 - 400 - 13 . 11 - 414 , are respectively discussed below in relation to  FIGS.  13 . 11 - 4 B- 13 . 11 - 4 E . 
       FIGS.  13 . 11 - 4 B  illustrates the connector  13 . 11 - 400 . The connector  13 . 11 - 400  can include a pivot joint (e.g., a single ball joint). The connector  13 . 11 - 400  includes a male portion and a female portion. The male portion is positioned on the display  13 . 11 - 102 , and the female portion is positioned on the facial interface  13 . 11 - 104 . When the male and female portions of the connector  13 . 11 - 400  are joined, the facial interface  13 . 11 - 104  can move relative to the display  13 . 11 - 102 . In particular, the connector  13 . 11 - 400  can allow for spherical motion between the facial interface  13 . 11 - 104  and the display  13 . 11 - 102  of about 5 degrees, about 10 degrees, about 25 degrees, about 50 degrees, about 70 degrees, or more. 
       FIGS.  13 . 11 - 4 C  illustrates the connector  13 . 11 - 402 . The connector  13 . 11 - 402  can include a spring joint connector (e.g., metal spring, elastomer spring, plastic spring, composite spring, etc.) located at the zygomatic location. The connector  13 . 11 - 402  can translate and rotate (e.g., to accommodate cheek or eye movement, or a user&#39;s particular facial features). The connector  13 . 11 - 402  can be rigidly attached to the display  13 . 11 - 102 , while being rotatably and translatably connected to the facial interface  13 . 11 - 104 . 
       FIGS.  13 . 11 - 4 D- 13 . 11 - 4 E  illustrate the connectors  13 . 11 - 406 - 13 . 11 - 408 , respectively. The connectors  13 . 11 - 406 - 13 . 11 - 408  include soft joints with compressible portions  13 . 11 - 407 . The term “soft joint” references a joint that can compress and decompress. For example, a soft joint includes datums or hardstops, between which the soft joint can compress. The connector  13 . 11 - 406  includes two compressible portions  13 . 11 - 407 , while the connector  13 . 11 - 408  includes a single compressible portion. The compressible portions  13 . 11 - 407  can be disposed between rigid portions of the connectors  13 . 11 - 406 ,  13 . 11 - 408 . In addition, the positioning of the compressible portions  13 . 11 - 407  within the connectors  13 . 11 - 406 ,  13 . 11 - 408  can provide corresponding mobility in a predetermined direction. For instance, the connector  13 . 11 - 406  with two of the compressible portions  13 . 11 - 407  can include mobility (e.g., compressibility) in a first direction and mobility in a second direction different than the first direction. The connector  13 . 11 - 408  includes mobility in at least the first direction by virtue of a single compressible portion. 
     In one example, the compressible portions  13 . 11 - 407  include a foam material. The foam material can be adhered (e.g., glued, pasted, melted, etc.) in between portions of the connectors  13 . 11 - 406 ,  13 . 11 - 408 . Additionally, or alternatively, the compressible portions  13 . 11 - 407  can include materials with properties that impart flexibility, softness, compressibility, deformability, etc. Examples of such material can include silicone, polymers, elastomers, hydrogels, etc. In addition, the compressible portions  13 . 11 - 407  can include molded material, printed material, cast material, etc. 
       FIGS.  13 . 11 - 4 F- 13 . 11 - 4 G  respectively illustrate cross-sectional views of the connectors  13 . 11 - 410 ,  13 . 11 - 412 . The connector  13 . 11 - 410  includes a socket connector. The connector  13 . 11 - 412  includes a mini-socket connector. The connectors  13 . 11 - 410 ,  13 . 11 - 412  are similar to the connector  13 . 11 - 400 —each including a male and female portion. The female portion (i.e., socket) corresponds to the facial interface  13 . 11 - 104 , and the male portion (i.e., pin and ball) corresponds to the display  13 . 11 - 102 . In particular, however, the connectors  13 . 11 - 410 ,  13 . 11 - 412  include a pivot center located at a specific place. For instance, the connector  13 . 11 - 410  includes a pivot center  13 . 11 - 411  located on the surface of the facial interface  13 . 11 - 104 . By contrast, the connector  13 . 11 - 412  includes a pivot center  13 . 11 - 413  that is positionally offset from the surface of the facial interface  13 . 11 - 104  (e.g., by about 0.5 mm to about 5 mm, or about 1.4 mm). In at least some examples, these variations of a pivot center can provide different rotational mobility. 
       FIGS.  13 . 11 - 4 H  illustrates the connector  13 . 11 - 414 . The connector  13 . 11 - 414  is the same as the connector  13 . 11 - 306  discussed above in relation to  FIGS.  13 . 11 - 3 E . Indeed, connector  13 . 11 - 414  can include a flexure joint providing compliant-mechanism based mobility between the facial interface  13 . 11 - 104  and the display  13 . 11 - 102 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 11 - 4 A- 13 . 11 - 4 H  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 11 - 4 A- 13 . 11 - 4 H . 
     As briefly indicated above, examples of a head-mountable device disclosed herein include a facial interface with one or more flexure portions that conform to myriad facial profiles and contours, thereby evenly distributing pressure and providing a comfortable user experience. These flexure portions can, via connectors described above, allow the facial interface  13 . 11 - 104  to more easily translate or rotate relative to the display  13 . 11 - 102 . In accordance with one or more such examples,  FIGS.  13 . 11 - 5 A- 13 . 11 - 5 G  illustrate example facial interfaces with flexure portions. 
       FIGS.  13 . 11 - 5 A  illustrates a perspective view of a head-mountable device  13 . 11 - 500  including the display  13 . 11 - 102 , a facial interface  13 . 11 - 502 , and the connector(s)  13 . 11 - 106 . The facial interface  13 . 11 - 502  includes a flexure portion  13 . 11 - 504 . 
     As used herein, the term “flexure portion” refers to a section of a facial interface that is independently movable from other sections of the facial interface. In particular examples, a flexure portion is movable, bendable, flexible, deformable, translatable, rotatable (i.e., twistable), etc. relative to one or more other sections of the facial interface  13 . 11 - 104 . A flexure portion can include various patterns, relief cutouts, slits, holes, etc. defined by the facial interface facial interface  13 . 11 - 104  to impart the desired flexibility. In certain implementations, a flexure portion includes a serpentine pattern (e.g., winds, turns, curvature, folds, etc.). Additionally, a flexure portion can include a same material as other non-flexure (e.g., solid) sections of the facial interface  13 . 11 - 104 . In other examples, the flexure portion includes a different material than other non-flexure sections of the facial interface  13 . 11 - 104 . 
     As will be described below in relation to  FIGS.  13 . 11 - 5 B- 13 . 11 - 5 G , the flexure portion  13 . 11 - 504  can be positioned at various locations (e.g., top corner region, bottom corner region, forehead region, temple region, etc.). In particular, the various flexure portions will be described positionally in relation to connectors  13 . 11 - 106   a - 13 . 11 - 106   f . The connectors  13 . 11 - 106   a ,  13 . 11 - 106   b  correspond to forehead connectors for engaging a forehead of a face. The connectors  13 . 11 - 106   c ,  13 . 11 - 106   d  correspond to zygoma connectors for engaging a zygoma region of a face. The connectors  13 . 11 - 106   e ,  13 . 11 - 106   f  correspond to maxilla connectors for engaging a maxilla region of a face. 
       FIGS.  13 . 11 - 5 B  illustrates a facial interface  13 . 11 - 506 . The facial interface  13 . 11 - 506  is devoid of a flexure portion. That is, each section between the connectors  13 . 11 - 106   a - 106   f  includes a solid section. 
     By contrast,  FIGS.  13 . 11 - 5 C  illustrates a facial interface  13 . 11 - 508  with flexure portions  13 . 11 - 510   a - 13 . 11 - 510   b  positioned a temple region of a face. Indeed, the flexure portions  13 . 11 - 510   a - 13 . 11 - 510   b  are respectively positioned in between the forehead connectors (i.e., the connectors  13 . 11 - 106   a ,  13 . 11 - 106   b ) and the zygoma connectors (i.e., the connectors  13 . 11 - 106   c ,  13 . 11 - 106   d ). The flexure portions  13 . 11 - 510   a - 13 . 11 - 510   b  can span an entire section of the facial interface  13 . 11 - 508  between the forehead connectors and the zygoma connectors. In other implementations, the flexure portions  13 . 11 - 510   a - 13 . 11 - 510   b  can span a partial section of the facial interface  13 . 11 - 508  between the forehead connectors and the zygoma connectors. For instance, the flexure portions  13 . 11 - 510   a - 13 . 11 - 510   b  may stop at a connector support area that encompasses a threshold distance around one or more of the connectors  13 . 11 - 106   a - 106   d , thereby leaving a more supportive region underneath and immediately adjacent to the connectors  13 . 11 - 106   a - 13 . 11 - 106   d.    
     In  FIGS.  13 . 11 - 5 C , solid sections—devoid of flexure portions—of the facial interface  13 . 11 - 508  also exist. In particular, the forehead region between the connectors  13 . 11 - 106   a ,  13 . 11 - 106   b  includes a solid section. Similarly, the bottom corner region between the zygoma connectors and the maxilla connectors are solid sections. 
       FIGS.  13 . 11 - 5 D  illustrates a facial interface  13 . 11 - 512  with flexure portions  13 . 11 - 514   a ,  13 . 11 - 514   b  positioned at the temple region, and flexure portions  13 . 11 - 514   c ,  13 . 11 - 514   d  positioned at a bottom corner region. In particular, the flexure portions  13 . 11 - 514   a ,  13 . 11 - 514   b  are similarly positioned between the forehead connectors (i.e., the connectors  13 . 11 - 106   a ,  13 . 11 - 106   b ) and the zygoma connectors (i.e., the connectors  13 . 11 - 106   c ,  13 . 11 - 106   d ). In addition, the flexure portions  13 . 11 - 514   c ,  13 . 11 - 514   d  are respectively positioned between the zygoma connectors and the maxilla connectors (i.e., the connectors  13 . 11 - 106   e ,  13 . 11 - 106   f ).  FIG.  13 . 11 - 5 D  also shows a solid section in the forehead region between the connectors  13 . 11 - 106   a ,  13 . 11 - 106   b.    
       FIGS.  13 . 11 - 5 E  illustrates a facial interface  13 . 11 - 516  with flexure portions  13 . 11 - 518   a - 13 . 11 - 518   e . The flexure portions  13 . 11 - 518   a ,  13 . 11 - 518   c  are respectively positioned at temple regions of a face between the forehead connectors (i.e., the connectors  13 . 11 - 106   a ,  13 . 11 - 106   b ) and the zygoma connectors (i.e., the connectors  13 . 11 - 106   c ,  13 . 11 - 106   d ). The flexure portion  13 . 11 - 518   b  is positioned at a forehead region of a face (i.e., between the connectors  13 . 11 - 106   a ,  13 . 11 - 106   b ). The flexure portions  13 . 11 - 518   d ,  13 . 11 - 518   e  are positioned at a bottom corner region between the zygoma connectors and the maxilla connectors (i.e., the connectors  13 . 11 - 106   e ,  13 . 11 - 106   f ). Further shown in  FIGS.  13 . 11 - 5 E , the only portions of the facial interface  13 . 11 - 516  devoid of a flexure portion includes the regions at the joints or connectors themselves (and a surrounding radius). 
       FIGS.  13 . 11 - 5 F  illustrates a facial interface  13 . 11 - 520  with flexure portions positioned at the forehead region and a bottom temple region (as a temple flare). In particular, the facial interface  13 . 11 - 520  includes the flexure portion  13 . 11 - 522   a  positioned at a forehead region of a face between the connectors  13 . 11 - 106   a ,  13 . 11 - 106   b . In addition, the facial interface  13 . 11 - 520  includes flexure portions  13 . 11 - 522   b ,  13 . 11 - 522   c  respectively positioned between the forehead connectors and the zygoma connectors, and adjacent to the zygoma connectors (i.e., connectors  13 . 11 - 106   c ,  13 . 11 - 106   d ). In  FIGS.  13 . 11 - 5 F , solid regions devoid of flexure portions also include portions of the forehead region, portions of the temple region, and all of the bottom corner region between the zygoma and maxilla connectors. 
       FIGS.  13 . 11 - 5 G  illustrates a facial interface  13 . 11 - 524  with flexure portions  13 . 11 - 526   a - 13 . 11 - 526   e . The flexure portions  13 . 11 - 526   a ,  13 . 11 - 526   e  are respectively positioned between the forehead connectors and the zygoma connectors, and adjacent to the zygoma connectors (i.e., connectors  13 . 11 - 106   c ,  13 . 11 - 106   d ). The flexure portions  13 . 11 - 526   b ,  13 . 11 - 526   d  are respectively positioned between the forehead connectors and the zygoma connectors, and adjacent to the forehead connectors (i.e., connectors  13 . 11 - 106   a ,  13 . 11 - 106   b ). In addition, the flexure portion  13 . 11 - 526   c  is positioned at a forehead region of a face between the connectors  13 . 11 - 106   a ,  13 . 11 - 106   b . In  FIGS.  13 . 11 - 5 G , solid regions devoid of flexure portions include portions of the temple region and all of the bottom corner region between the zygoma and maxilla connectors. 
     It will be appreciated that the foregoing examples of flexure portions can be implemented to provide varying degrees (and places) of flexibility and conformability. In this manner, the head-mountable device of the present disclosure can provide myriad different forms of flexibility as may be desired. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 11 - 5 A- 13 . 11 - 5 G  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 11 - 5 A- 13 . 11 - 5 G . 
     The head-mountable device of the present disclosure can be manufactured in myriad different ways. Some methods or manufacturing processes can provide various advantages (e.g., decreased material consumption, weight savings, manufacturing friendliness, etc.). The following description in relation to  FIGS.  13 . 11 - 6 A- 12    describes manufacturing and/or associated apparatus aspects. 
       FIGS.  13 . 11 - 6 A- 13 . 11 - 6 B  illustrate an example assembly  13 . 11 - 600  of a head-mountable device implementing a double shot of material in accordance with one or more examples of the present disclosure. As shown, the assembly  13 . 11 - 600  includes the connectors  13 . 11 - 106   a - 13 . 11 - 106   f  described above. In addition, the assembly  13 . 11 - 600  includes a facial interface  13 . 11 - 602  having flexure portions  13 . 11 - 604 . The flexure portions  13 . 11 - 604  may be the same as, or similar to the flexure portions  13 . 11 - 510   a ,  13 . 11 - 510   b  described above in relation to  FIGS.  13 . 11 - 5 C . 
     In particular implementations, the assembly  13 . 11 - 600  in  FIGS.  13 . 11 - 6 A  includes only a first shot of material. In some examples, the first shot of material includes an injection molded material. For instance, the first shot of material includes a more rigid material, such as at least one of Pebax, TPU, TPE, PP, ABS, etc. In other examples, the first shot of material is less rigid (and more flexible). 
     Additionally shown in  FIGS.  13 . 11 - 6 A , the assembly  13 . 11 - 600  can include tooling inserts  13 . 11 - 606 . The tooling inserts  13 . 11 - 606  can be positioned at locations configured to receive a second shot of material. The tooling inserts  13 . 11 - 606  can be positioned at various locations along the facial interface  13 . 11 - 602 . In particular implementations, however, the tooling inserts  13 . 11 - 606  are positioned at the forehead region between the connectors  13 . 11 - 106   a ,  13 . 11 - 106   b , and at temple regions between the forehead connectors and the zygoma connectors (i.e., between the connectors  13 . 11 - 106   a ,  13 . 11 - 106   b  and the connectors  13 . 11 - 106   c ,  13 . 11 - 106   d ). 
     In  FIGS.  13 . 11 - 6 B , the assembly  13 . 11 - 600  includes a second shot of material applied to the tooling inserts  13 . 11 - 606 . In one or more examples, the second shot of material includes an overmolded, flexible material. For instance, the second shot of material includes an elastomer material (e.g., Neoprene, EVA, PVC, Acrylonitrile Butadiene Rubber, Ethylene Propylene, Fluorelastomer, Silicone, etc.). Thus, in some examples, the first shot of material and the second shot of material can be different. In these or other examples, the assembly  13 . 11 - 600  implementing double shots of material can provide various advantages, such as strategically positioned support, improved flexibility, etc. 
     In alternative examples, more than two shots of material can be implemented. Further, in some alternative examples, only a single shot of material is implemented. For instance, the assembly  13 . 11 - 600  can include a single, lightweight material (e.g., all aluminum). 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 11 - 6 A- 13 . 11 - 6 B  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 11 - 6 A- 13 . 11 - 6 B . 
     Other variations of combined materials are also herein contemplated. For example, some head-mountable devices of the present disclosure include a display frame of the display  13 . 11 - 102 , where the display frame includes a combination of frame shells forming a box-type frame enclosure. In accordance with one or more such examples,  FIGS.  13 . 11 - 7 A- 13 . 11 - 7 B  illustrate an example display  13 . 11 - 700  of a head-mountable device. In particular,  FIGS.  13 . 11 - 7 A- 13 . 11 - 7 B  respectively illustrate assembled and exploded views of the display  13 . 11 - 700 . 
     As shown, the display  13 . 11 - 700  includes a first frame shell  13 . 11 - 702  and a second frame shell  13 . 11 - 704 . The first frame shell  13 . 11 - 702  includes a pocket  13 . 11 - 706  defined by the surface of the first frame shell  13 . 11 - 702 . The pocket  13 . 11 - 706  can include one or more recesses, cutouts, etc. In certain implementations, the pocket  13 . 11 - 706  is defined by tooling surfaces (e.g., for injection molding, casting, etc.). The pocket  13 . 11 - 706  can additionally include webbing, such as a mold webbing, (indicated in dashed lines) that sections off or separates different portions of the pocket  13 . 11 - 706 . 
     In some examples, the first frame shell  13 . 11 - 702  and the second frame shell  13 . 11 - 704  include different materials (albeit not required). For instance, the first frame shell  13 . 11 - 702  can include a plastic material, and the second frame shell  13 . 11 - 704  can include a metal material. In this manner, the second frame shell  13 . 11 - 704  can be combined with the first frame shell  13 . 11 - 702  to increase a stiffness or rigidity across the display  13 . 11 - 700 . Indeed, the second frame shell  13 . 11 - 704  as a stiffener member can provide structural support, thereby stabilizing the display frame and increasing resistance to stresses (e.g., torsional stress, compressive stress, tension stress, etc.). 
     The first frame shell  13 . 11 - 702  and the second frame shell  13 . 11 - 704  can be formed or assembled in a variety of different ways. In some examples, the first frame shell  13 . 11 - 702  and the second frame shell  13 . 11 - 704  can be manufactured using molding techniques (e.g., injection molding, casting, etc.). In certain implementations, the first frame shell  13 . 11 - 702  includes a first-shot mold, and the second frame shell  13 . 11 - 704  includes a second-shot mold. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 11 - 7 A- 13 . 11 - 7 B  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 11 - 7 A- 13 . 11 - 7 B . 
     As discussed above, displays (specifically display frames) can include a combination of materials. Additionally, or alternatively, display frames have one or more a relief cutouts. In accordance with one or more such examples,  FIGS.  13 . 11 - 8 A- 13 . 11 - 8 B  illustrate an example display  13 . 11 - 800 . 
     As shown, the display  13 . 11 - 800  includes relief cutouts  13 . 11 - 802  defined by the surface of the display frame. As used herein, the term “relief cutouts” refers to thru-holes, dimples, slits, slots, core-outs, recesses, etc. In addition, a relief cutout provides weight advantages over a conventional head-mountable device while maintaining the structural integrity (e.g., deflection integrity, stress integrity, etc.) of the display frame. 
     Here, the relief cutouts  13 . 11 - 802  include thru-holes in the display frame. The thru-holes can be sized and shaped in myriad different ways. In some examples, the thru-holes are rectangular, square, triangular, circular, etc. In other examples, the thru-holes are lozenge-shaped (e.g., cylindrical, pill-shaped) thru-holes. The relief cutouts  13 . 11 - 802  also include a spacing that can be optimized or tuned (as may be desired). Indeed, the relief cutouts  13 . 11 - 802  can be spaced apart to satisfy (e.g., meet or exceed) a threshold force or stress/strain profile of the display  13 . 11 - 800 . Additionally, or alternatively, the relief cutouts  13 . 11 - 802  can be spaced apart to provide a particular amount of flexibility or rigidity. In certain implementations, the relief cutouts  13 . 11 - 802  are omitted from certain areas (e.g., at connector support areas corresponding to the connector(s)  13 . 11 - 106 ). Thus, some connector support areas are devoid of the relief cutouts  13 . 11 - 802 . 
     Additionally, as shown in  FIGS.  13 . 11 - 8 B , the display  13 . 11 - 800  includes a support structure  13 . 11 - 804  and an overmolding  13 . 11 - 806 . The support structure  13 . 11 - 804  can be rigid (e.g., a metal material). By contrast, the overmolding  13 . 11 - 806  can include a plastic material, elastomer material, etc. The support structure  13 . 11 - 804  can be positioned at predetermined areas of the display  13 . 11 - 800 , for instance, at areas corresponding to the connector(s)  13 . 11 - 106  (e.g., for increased rigidity and/or structural integrity per finite element analysis). To illustrate, the support structure  13 . 11 - 804  is positioned directly underneath the connector(s)  13 . 11 - 106  and extends a distance past at least one relief cutout. In certain implementations, the support structure  13 . 11 - 804  is positioned all along the display  13 . 11 - 800 . 
     In particular implementations, the support structure  13 . 11 - 804  is positioned at an offset distance  13 . 11 - 808  from the overmolding  13 . 11 - 806 . The offset distance  13 . 11 - 808  can range from about 0.2 mm to about 4 mm. In certain examples, the offset distance  13 . 11 - 808  is about 0.75 mm. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 11 - 8 A- 13 . 11 - 8 B  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 11 - 8 A- 13 . 11 - 8 B . 
     The head-mountable devices, and in particular the frames of the displays of the present disclosure, can be formed of various materials and with different geometric variations. These example display frames can provide various advantages, including decreased weight. In accordance with one or more such examples,  FIGS.  13 . 11 - 9 A- 13 . 11 - 9 B  respectively show example head-mountable devices  13 . 11 - 900   a ,  13 . 11 - 900   b  with example displays  13 . 11 - 902 ,  13 . 11 - 904 . 
     In  FIGS.  13 . 11 - 9 A , the head-mountable device  13 . 11 - 900   a  includes the display  13 . 11 - 902  connected to the facial interface  13 . 11 - 104  via the connector(s)  13 . 11 - 106 . The display  13 . 11 - 902  in this example includes a frame including a single, lightweight material. For instance, the display  13 . 11 - 902  includes an all-metal frame. In certain implementations, the display  13 . 11 - 902  includes stainless steel (e.g., SUS 17-4PH). In other implementations, the display  13 . 11 - 902  includes aluminum (e.g., AL 6061). In alternative embodiments, the display  13 . 11 - 902  includes at least one of a composite material (e.g., carbon fiber), polycarbonate or thermoplastic material (e.g., Lexan), or other suitable material with a higher strength-to-weight ratio (e.g., titanium, magnesium, graphene, ceramics, etc.). 
     In  FIGS.  13 . 11 - 9 B , the head-mountable device  13 . 11 - 900   b  similarly includes the display  13 . 11 - 904  connected to the facial interface  13 . 11 - 104  via the connector(s)  13 . 11 - 106 . The display  13 . 11 - 904  can include one or more materials as just described for the display  13 . 11 - 902 . In particular, however, the display  13 . 11 - 904  includes a frame including cored-out regions  13 . 11 - 906  in the forehead and temple areas of a face. The cored-out regions  13 . 11 - 906  can reduce the amount of material in the display  13 . 11 - 904 , thereby reducing the weight of the display  13 . 11 - 904 . In certain implementations, the cored-out regions  13 . 11 - 906  correspond to areas of the display  13 . 11 - 904  between the connector(s)  13 . 11 - 106 . However, it will be appreciated that the cored-out regions  13 . 11 - 906  can be positioned in additional or alternative locations along the frame of the display  13 . 11 - 904 , as may be desired. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 11 - 9 A- 13 . 11 - 9 B  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 11 - 9 A- 13 . 11 - 9 B . 
     The present disclosure also contemplates stiffener members disposed between the facial interface  13 . 11 - 104  and the display  13 . 11 - 102 . In accordance with one or more such examples,  FIGS.  13 . 11 - 10 A- 13 . 11 - 10 B  illustrate head-mountable devices  13 . 11 - 1000   a ,  13 . 11 - 1000   b . In particular,  FIGS.  13 . 11 - 10 A  shows the head-mountable device  13 . 11 - 1000   a  includes a display  13 . 11 - 1002   a  (i.e., a display frame) connected to the facial interface  13 . 11 - 104  via the connector(s)  13 . 11 - 106 , as similarly described above. In addition, the head-mountable device  13 . 11 - 1000   a  includes a stiffener member  13 . 11 - 1004   a  disposed between the display  13 . 11 - 1002   a  and the facial interface  13 . 11 - 104 . The stiffener member  13 . 11 - 1004   a  can include an additional frame, skeleton, hoop, or support structure. The stiffener member  13 . 11 - 1004   a  can extend partially along the display  13 . 11 - 1002   a  (e.g., until the zygoma connector), or in certain cases all along the display  13 . 11 - 1002   a  (e.g., until the maxilla connector). 
     In particular implementations, a gap exists between the stiffener member  13 . 11 - 1004   a  and the display  13 . 11 - 1002   a . In these cases, the connectors  13 . 11 - 106  can bridge the gap between the stiffener member  13 . 11 - 1004   a  and the display  13 . 11 - 1002   a.    
     The display  13 . 11 - 1002   a  further includes cored-out regions  13 . 11 - 1006 . The cored-out regions  13 . 11 - 1006  can provide weight savings, as may be desired. 
     Likewise,  FIGS.  13 . 11 - 10 B  shows the head-mountable device  13 . 11 - 1000   b  includes a display  13 . 11 - 1002   b  connected to the facial interface  13 . 11 - 104  via the connector(s)  13 . 11 - 106 , as similarly described above. In addition, the head-mountable device  13 . 11 - 1000   b  includes a stiffener member  13 . 11 - 1004   b  disposed between the display  13 . 11 - 1002   b  and the facial interface  13 . 11 - 104 . The stiffener member  13 . 11 - 1004   b  can include an additional frame, skeleton, hoop, or support structure. The stiffener member  13 . 11 - 1004   b  can extend partially along the display  13 . 11 - 1002   b  (e.g., until the zygoma connector), or in certain cases all along the display  13 . 11 - 1002   a  (e.g., until the maxilla connector). The display  13 . 11 - 1002   b  further includes cored-out regions  13 . 11 - 1010 , similar to the cored-out regions  13 . 11 - 1006  described above. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 11 - 10 A- 13 . 11 - 10 B  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 11 - 10 A- 13 . 11 - 10 B . 
     As mentioned above, the head-mountable devices of the present disclosure can include myriad different configurations of cored-out portions. In accordance with one or more such examples,  FIGS.  13 . 11 - 11 A- 13 . 11 - 11 C  illustrate display frames  13 . 11 - 1100   a - 13 . 11 - 1100   c  with respective cored-out portions (e.g., thru-holes, partial holes, recesses, etc.). In particular,  FIGS.  13 . 11 - 11 A  shows the display frame  13 . 11 - 1100   a  includes cored-out portions  13 . 11 - 1102   a - 13 . 11 - 1102   d  at the forehead and temple regions of a face.  FIGS.  13 . 11 - 11 B  shows the display frame  13 . 11 - 1100   b  including cored-out portions  13 . 11 - 1102   a - 13 . 11 - 1102   b  (thus only including cored-out regions at the forehead). In addition,  FIGS.  13 . 11 - 11 C  shows the display frame  13 . 11 - 1100   c  including cored-out portions  13 . 11 - 1104 . The cored-out portions  13 . 11 - 1104  are defined by one or more backside surfaces oriented towards a facial interface (not shown). Thus, the cored-out portions  13 . 11 - 1104  are perpendicular to the cored-out portions shown in  FIGS.  13 . 11 - 11 A- 13 . 11 - 11 B . 
     It will be appreciated that the cored-out portions of the display frames  13 . 11 - 1100   a - 13 . 11 - 1100   c  can be located in various positions and in various configurations or patterns (e.g., to decrease the overall weight of the display frame). The cored-out portions can also be configured to maintain a structural stability of the display frame. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 11 - 11 A- 13 . 11 - 11 C  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 11 - 11 A- 13 . 11 - 11 C . 
       FIGS.  13 . 11 - 12    illustrates a display frame  13 . 11 - 1200  including an array of holes  13 . 11 - 1202  in accordance with one or more examples of the present disclosure. The array of holes  13 . 11 - 1202  can reduce an amount of material (and therefore, weight) of the display frame  13 . 11 - 1200 . The array of holes  13 . 11 - 1202  can include a hole density such that at least (or at most) a certain number of holes is found within a predetermined area of the display frame  13 . 11 - 1200 . As an alternative to the array of holes  13 . 11 - 1202 , dimples or non-thru-holes can similarly be implemented in the display frame  13 . 11 - 1200 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 11 - 12    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 11 - 12   . 
       FIGS.  13 . 11 - 13    shows a display frame  13 . 11 - 1300  in accordance with one or more examples of the present disclosure. As illustrated, the display frame  13 . 11 - 1300  includes a single-shot plastic portion  13 . 11 - 1302 . A variety of different plastics can be implemented. Moreover, the single-shot plastic portion  13 . 11 - 1302  can assume any number of different shapes and forms (albeit schematically shown as a block with various straight and curved edges). 
     In addition, the display frame  13 . 11 - 1300  can include metal stiffeners  13 . 11 - 1304 . Examples of the metal stiffeners  13 . 11 - 1304  include sheets or pieces of metal (whether molded, shaped, cast, etc.). Further, the metal stiffeners  13 . 11 - 1304  can be arranged in different ways. For example, the metal stiffeners  13 . 11 - 1304  can be arranged in an array that structurally couples or connects them to each other (e.g., via attachment mechanisms  13 . 11 - 1308  and fasteners  13 . 11 - 1310 ). For instance, the metal stiffeners  13 . 11 - 1304  can be arranged partially around a perimeter of the single-shot plastic portion  13 . 11 - 1302 . In other examples, the metal stiffeners can be arranged entirely around a perimeter of the single-shot plastic portion  13 . 11 - 1302 . 
     The attachment mechanisms  13 . 11 - 1308  can include various pins, clasps, hinges, cams, swivels, balls, knuckles, springs, etc. Likewise, the fasteners  13 . 11 - 1310  can include screws, bolts, pins, press-fits, welds, bonds, etc. 
     In some examples, the display frame  13 . 11 - 1300  includes adhesives (not shown) to structurally couple the metal stiffeners  13 . 11 - 1304  (and associated attachment mechanisms  13 . 11 - 1308 ) and the single-shot plastic portion  13 . 11 - 1302 . Examples of adhesives include glue, epoxy, thermal bonds, tape strips, etc. In alternative examples, the metal stiffeners  13 . 11 - 1304  and the single-shot plastic portion  13 . 11 - 1302  include no adhesives in between these components. For example, the metal stiffeners  13 . 11 - 1304  and the single-shot plastic portion  13 . 11 - 1302  can be sandwiched together via applied pressure from one or more additional frame layers, fabric, etc. 
     In some examples, the display frame  13 . 11 - 1300  can include additional layers (not shown). For instance, the display frame  13 . 11 - 1300  can include a final plastic “cover” to provide cosmetic and/or additional stiffness properties to the display frame  13 . 11 - 1300 . The additional cover can be attached to the metal stiffeners  13 . 11 - 1304  in a same or similar fashion as just described between the metal stiffeners  13 . 11 - 1304  and the single-shot plastic portion  13 . 11 - 1302 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 11 - 13    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 11 - 13   . 
       FIGS.  13 . 11 - 14 A  illustrates a top view of a frame  13 . 11 - 1410  of a device seal according to one embodiment. The frame  13 . 11 - 1410  can be substantially similar to, including some or all of the features of, the frames described herein. 
     In some examples, the frame  13 . 11 - 1410  can include one or more stiffeners. For example, the frame  13 . 11 - 1410  can include a top stiffener  13 . 11 - 1421  that is positioned on a top forehead region of the frame  13 . 11 - 1410 . The top stiffener  13 . 11 - 1421  can be curved or angled to match a curve of the frame  13 . 11 - 1410 . In some examples, the curve of the top of the frame  13 . 11 - 1410  and the top stiffener  13 . 11 - 1421  are designed to match the curvature of a user&#39;s forehead. In some examples, the curvature of the top of the frame  13 . 11 - 1410  and the top stiffener  13 . 11 - 1421  are designed to allow for a gap between the user&#39;s forehead and the frame  13 . 11 - 1410 . The top stiffener  13 . 11 - 1421  can aid in maintaining the gap between the frame  13 . 11 - 1410  and the user&#39;s forehead. Thus, the top stiffener  13 . 11 - 1421  can provide structural support to the frame  13 . 11 - 1410  and can be used to modify the stiffness and/or structural integrity of the frame. In some examples, there can be multiple top stiffeners in the forehead region of the frame  13 . 11 - 1410 , and each of the multiple top stiffeners can have different profiles, geometries, and/or material properties that effect the properties imparted to the frame  13 . 11 - 1410 . 
     The top stiffener  13 . 11 - 1421  can include or can be made from any suitable stiff or rigid material capable of maintaining its shape under the amounts of stress likely to be encountered from typical use of the HMD, thereby increasing comfort and predictability to the user. In some examples, the top stiffener  13 . 11 - 1421  can undergo elastic deformation. The top stiffener  13 . 11 - 1421  can include sheet metal, aluminum, steel, carbon fiber, plastic, combinations thereof, or any other suitable material capable of stiffening the frame  13 . 11 - 1410 . 
     The top stiffener  13 . 11 - 1421  can be secured to the frame  13 . 11 - 1410  using any suitable attachment methods. In some examples, the top stiffener  13 . 11 - 1421  is fastened to the frame  13 . 11 - 1410  using fastening screws  13 . 11 - 1423 . In some examples, the fastening screws  13 . 11 - 1423  can attached ends of the top stiffener  13 . 11 - 1421  to a metallic anchor  13 . 11 - 1425  of the frame  13 . 11 - 1410 , which in turn is connected to posts  13 . 11 - 1427  that engage the facial interface of the device seal. In some examples, the top stiffener  13 . 11 - 1421  can be laser welded to the frame  13 . 11 - 1410 , such as via the metallic anchor  13 . 11 - 1425  of the frame  13 . 11 - 1410 . In some examples, the top stiffener  13 . 11 - 1421  can be adhered to the frame  13 . 11 - 1421  using any suitable adhesive. 
     In some examples, the frame  13 . 11 - 1410  can include one or more side stiffeners  13 . 11 - 1429  that are positioned on along a side region of the frame  13 . 11 - 1410 . The side stiffeners  13 . 11 - 1429  can be curved or angled to match a curvature of the frame  13 . 11 - 1410 . In some examples, the curve of the side of the frame  13 . 11 - 1410  and the side stiffeners  13 . 11 - 1429  are designed to match the curvature of a user&#39;s brow, temple, and/or cheeks. In some examples, the curvature of the side of the frame  13 . 11 - 1410  and the side stiffener  13 . 11 - 1429  are designed to allow for a gap between the user&#39;s face and the frame  13 . 11 - 1410 . The side stiffeners  13 . 11 - 1429  can aid in maintaining the gap between the frame  13 . 11 - 1410  and the user&#39;s face. Thus, the side stiffeners  13 . 11 - 1429  can provide structural support and added comfort to the frame  13 . 11 - 1410 . 
     The side stiffeners  13 . 11 - 1429  can include or be made from any suitable stiff or rigid material. In some examples, the side stiffeners  13 . 11 - 1429  can experience elastic deformation during use. For example, the side stiffeners  13 . 11 - 1429  can include sheet metal, aluminum, steel, carbon fiber, polymer, or any other suitable material capable of stiffening the frame  13 . 11 - 1410 . The side stiffener  13 . 11 - 1429  can be made from the same material or a different material than the top stiffener  13 . 11 - 1421 . 
     The side stiffeners  13 . 11 - 1429  can be secured to the frame  13 . 11 - 1410  using any suitable attachment methods or systems. In some examples, the side stiffeners  13 . 11 - 1429  are fastened to the frame  13 . 11 - 1410  using fastening screws  13 . 11 - 1423 . In some examples, the fastening screws  13 . 11 - 1423  can attached ends of the side stiffeners  13 . 11 - 1429  to a metallic anchor  13 . 11 - 1425  of the frame  13 . 11 - 1410 , which in turn is connected to posts  13 . 11 - 1427  that engage the facial interface of the device seal. In some examples, the side stiffeners  13 . 11 - 1429  can be laser welded to the frame  13 . 11 - 1410 , such as the metallic anchor  13 . 11 - 1425  of the frame  13 . 11 - 1410 . In some examples, the side stiffeners  13 . 11 - 1429  can be adhered to the frame  13 . 11 - 1421  using any suitable adhesive. 
       FIGS.  13 . 11 - 14 B  illustrates a cross-sectional view of the frame  13 . 11 - 1410  of a device seal according to one embodiment. As illustrated, the stiffener  13 . 11 - 1421  can be positioned within a housing of the frame  13 . 11 - 1410 . For instance, the frame  13 . 11 - 1410  can include an upper portion  13 . 11 - 1411  and a lower portion  13 . 11 - 1412  that engage to form an internal volume. The upper portion  13 . 11 - 1411  and the lower portion  13 . 11 - 1412  can be plastic, metal, or any other suitable material for the frame  13 . 11 - 1410 . The stiffener  13 . 11 - 1421  can be positioned directly on the lower portion  13 . 11 - 1412  of the frame  13 . 11 - 1410  to directly strengthen and stabilize the frame  13 . 11 - 1410 . In some examples, the stiffener  13 . 11 - 1421  is secured in place by adhesive  13 . 11 - 1433 . The adhesive  13 . 11 - 1433  can be located at a number of locations between the stiffener  13 . 11 - 1421  and the lower portion  13 . 11 - 1412 . Adhesive can also be placed between the stiffener  13 . 11 - 1421  and the upper cover  13 . 11 - 1411 . In this way, forces experienced by the lower portion  13 . 11 - 1412  and the upper cover  13 . 11 - 1411  can be passed through the adhesive  13 . 11 - 1433  to be carried by the stiffener  13 . 11 - 1421 . In some examples, the stiffener  13 . 11 - 1421  is secured in place by being sandwiched or pressed between the upper cover  13 . 11 - 1411  and the lower portion  13 . 11 - 1412  of the frame  13 . 11 - 1410 . 
     In some examples, the stiffener  13 . 11 - 1421  can include an angled edge. For example, the edge of the stiffener  13 . 11 - 1421  closest to the user&#39;s face can be angle so as to run substantially perpendicular or flat to the user&#39;s face. The angled edge can prevent a hard or sharp edge of the stiffener from being directed toward the user to enhance prolonged user comfort. The angled edge of the stiffener  13 . 11 - 1421  can also assist in securing the stiffener  13 . 11 - 1421  in place and prevent movement within the internal volume of the frame  13 . 11 - 1410 . 
       FIGS.  13 . 11 - 14 C  illustrates a bottom view of the frame  13 . 11 - 1410  of a device seal according to one exemplary embodiment. The frame  13 . 11 - 1410  can include one or more stiffeners along the sides of the frame  13 . 11 - 1410 . For example, the frame  13 . 11 - 1410  can include two stiffeners  13 . 11 - 1429  along each side of the frame  13 . 11 - 1410 . In some examples, the frame  13 . 11 - 1410  can include a nose stiffener  13 . 11 - 1431  positioned along the nose region  13 . 11 - 1415 . 
     The nose stiffener  13 . 11 - 1431  can be substantially similar to, including some or all of the features of, the stiffeners described herein, such as top stiffener  13 . 11 - 1421  and side stiffeners  13 . 11 - 1429 . In some examples, there can be multiple nose stiffeners in the nose region  13 . 11 - 1415  of the frame  13 . 11 - 1410   
     The nose stiffener  13 . 11 - 1431  can be curved or angled to match a curve of the nose region  13 . 11 - 1415 . In some examples, the curve of the nose region  13 . 11 - 1415  and the nose stiffener  13 . 11 - 1431  is designed to match the curvature of a user&#39;s nose. In some examples, the curvature of the nose region  13 . 11 - 1415  and the nose stiffener  13 . 11 - 1431  is designed to allow for a gap between the user&#39;s nose and the frame  13 . 11 - 1410 . The nose stiffener  13 . 11 - 1431  can aid in maintaining the gap between the frame  13 . 11 - 1410  and the user&#39;s nose. Thus, the nose stiffener  13 . 11 - 1431  can provide structural support to the frame  13 . 11 - 1410 . The nose stiffener  13 . 11 - 1431  can be made from the same or different material than the top stiffener  13 . 11 - 1421  or the side stiffeners  13 . 11 - 1429 . The nose stiffener  13 . 11 - 1431  can be attached to the nose portion  13 . 11 - 1415  of the frame  13 . 11 - 1410  by at least one of the following: laser welding, fastening screws, adhesive, or any other suitable coupling method. 
       FIGS.  13 . 11 - 14 D  illustrates a top view of the frame  13 . 11 - 1410  including stiffeners  13 . 11 - 1421 ,  13 . 11 - 1429  to improve the structural stability and rigidity of the frame  13 . 11 - 1410 . In some examples, adhesive  13 . 11 - 1433  can be placed on a top surface of the stiffeners  13 . 11 - 1421 ,  13 . 11 - 1429  to secure the stiffeners to the top cover of the frame (shown in  FIG.  13 . 11 - 14 B ). 
     In some examples, the frame  13 . 11 - 1410  can include a hard stop component  13 . 11 - 1435 . The hard stop component  13 . 11 - 1435  can be an adhesive or any other suitable coupler to fixedly secure the ends of the stiffeners, and the anchor  13 . 11 - 1425  in place on the frame  13 . 11 - 1410 . Thus, when a force is applied to the post  13 . 11 - 1427 , the hard stop component  13 . 11 - 1435  can prevent the anchor  13 . 11 - 1425  from moving relative to the frame  13 . 11 - 1410  and stiffeners  13 . 11 - 1421 ,  13 . 11 - 1429 . Thus, the hard stop component  13 . 11 - 1435  acts as a deflection limiter. In the case that the hard stop component  13 . 11 - 1435  is an adhesive, it can be the same or a different adhesive than the adhesives  13 . 11 - 1433  applied on the stiffeners. 
     As mentioned above, many different types of connectors between a facial interface and a display frame are herein contemplated. The following description for  FIGS.  13 . 11 - 14 - 13 . 11 - 19    corresponds to an example connector that includes a floating connection between a facial interface and a display frame. As a floating connection, the connector can be attached to one of the display frame or the facial interface, but not both, as will be described in more detail below. 
       FIGS.  13 . 11 - 14    illustrates a perspective view of an example connector  13 . 11 - 1400  in accordance with one or more examples of the present disclosure. The connector  13 . 11 - 1400  can include a floating connection. The connector  13 . 11 - 1400  can therefore include a bumper, a compliant stop, or a contact mechanism positioned between a display frame  13 . 11 - 1402  and a facial interface  13 . 11 - 1404 . The display frame  13 . 11 - 1402  and the facial interface  13 . 11 - 1404  are, respectively, the same as or similar to the display frames and facial interfaces discussed above. However, as shown specifically in  FIGS.  13 . 11 - 14   , the connector  13 . 11 - 1400  is only attached to the display frame  13 . 11 - 1402  (i.e., not also attached to the facial interface  13 . 11 - 1404 ). This constitutes a floating connection where the connector  13 . 11 - 1400  can contact or abut the facial interface  13 . 11 - 1404 , but can also move relative to the facial interface  13 . 11 - 1404 . For example, the connector  13 . 11 - 1400  can translate or rotate along a surface of the facial interface  13 . 11 - 1404 . In certain examples, rounded edges or smooth surfaces of the connector  13 . 11 - 1400  can facilitate such movement or range of motion. As another example, the connector  13 . 11 - 1400  can compress against (or have more surface area contact with) the facial interface  13 . 11 - 1404 , such as when donning the head-mountable device. In yet another example, the connector  13 . 11 - 1400  can retract or bias away from the facial interface  13 . 11 - 1404 . Accordingly, the connector  13 . 11 - 1400  can provide desired flexibility and compliance (e.g., for improved comfort when wearing or adjusting the head-mountable device), yet also provide desired rigidity, support, and load-bearing functionality. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 11 - 14    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 11 - 14   . 
       FIGS.  13 . 11 - 15 A  illustrates a side view of an example connector positioned between a display frame and a facial interface in accordance with one or more examples of the present disclosure. As shown, the connector  13 . 11 - 1400  can include one or more posts. In particular implementations, the connector  13 . 11 - 1400  includes a post  13 . 11 - 1502   a . In other implementations, the connector  13 . 11 - 1400  includes a post  13 . 11 - 1502   b.    
     In these or other examples, a post can include an element with rigid and/or pliant portions. In some examples, a post can include a soft outer portion and a rigid inner portion (e.g., a sheath or sheet metal overmolded by a softer, cushioning material). A post can include a contact mechanism for contacting the facial interface  13 . 11 - 1404 . Additionally or alternatively, a post can include an arm, a bumper, a mechanical stop, a damper, a shock absorber, a spring, an actuator, a crumple element, a mechanical fuse, a break-away element, etc. In at least some examples, a post can include surfaces, joints, etc. that include a radius (or curvature) allowing movement, deflection, and/or energy absorption. 
     As shown in  FIGS.  13 . 11 - 15 A , the post  13 . 11 - 1502   a  is longer than the post  13 . 11 - 1502   b . It will be appreciated that the length of the posts  13 . 11 - 1502   a ,  13 . 11 - 1502   b  can determine the gap or distance between the connector  13 . 11 - 1400  and the facial interface  13 . 11 - 1404 . For example, the post  13 . 11 - 1502   a  is positioned a distance  13 . 11 - 1506  from a contact surface  13 . 11 - 1508  of the facial interface  13 . 11 - 1404 . Similarly, the post  13 . 11 - 1502   b  is positioned a distance  13 . 11 - 1504  from the contact surface  13 . 11 - 1508  of the facial interface  13 . 11 - 1404 . 
     In addition to length of the posts  13 . 11 - 1502   a ,  13 . 11 - 1502   b , an angle of the posts  13 . 11 - 1502   a ,  13 . 11 - 1502   b  relative to the display frame  13 . 11 - 1402  can also affect the distances  13 . 11 - 1504 ,  13 . 11 - 1506  to the facial interface  13 . 11 - 1404 . The post  13 . 11 - 1502   a  is positioned at an angle  13 . 11 - 1510  relative to the display frame  13 . 11 - 1402 . The post  13 . 11 - 1502   b  is positioned at an angle  13 . 11 - 1512  relative to the display frame  13 . 11 - 1402 . These angles may be preset, adjusted, or actuated. In particular examples, the angles  13 . 11 - 1510 ,  13 . 11 - 1512  can be dynamically modified (e.g., decreased) in response to the post contacting the contact surface  13 . 11 - 1508  and flexing upward. The angles  13 . 11 - 1510 ,  13 . 11 - 1512  can also correspond to the angle of incidence or angle of contact with the contact surface  13 . 11 - 1508 . Likewise, the angles  13 . 11 - 1510 ,  13 . 11 - 1512  can affect the point of contact with the contact surface  13 . 11 - 1508 . The point of contact and angle of incidence (as well as the distances  13 . 11 - 1504 ,  13 . 11 - 1506 ) can include tunable factors for tuning comfort, fit, adjustability, load responsiveness, etc. as may be desired. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 11 - 15 A  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 11 - 15 A . 
     With respect to the facial interface  13 . 11 - 1404 , it will be appreciated that the facial interface  13 . 11 - 1404  can include many different variations. In some examples, the facial interface  13 . 11 - 1404  can include thicker regions and/or thinner regions at certain locations along the facial interface  13 . 11 - 1404 , can be fixed or removable, and can be removably attached by any number of systems such as hook and loop, magnets, fasteners, and the like. In accordance with one or more such examples,  FIGS.  13 . 11 - 15 B  illustrates a cross-sectional view of a facial interface  15 - 1502 .  FIG.  15 C  illustrates a cross-sectional view of a forehead region  13 . 11 - 15 - 1503  of the facial interface  13 . 11 - 15 - 1502 , while  FIG.  15 D  illustrates a cross-sectional view of a cheek region  13 . 11 - 15 - 1505  of the facial interface  13 . 11 - 15 - 1502 . The facial interface  13 . 11 - 15 - 1502  can be substantially similar to, including some or all of the features of, the facial interfaces described herein. 
     In some examples, the facial interface  13 . 11 - 15 - 1502  includes a compressible portion that can include a central region  13 . 11 - 15 - 1509  and an extended region  13 . 11 - 15 - 1507 . In some examples, the central region  13 . 15 - 1509  and the extended region  13 . 11 - 15 - 1507  can be a single unitary component (e.g., a foam piece). In some examples, the central region  13 . 11 - 15 - 1509  can be a separate and distinct material from the extended region  13 . 11 - 15 - 1507  (e.g., two different types of foam pieces). The central region  13 . 11 - 15 - 1509  can be shaped, sized, and positioned to correspond to the base of the frame. In other words, the central region  13 . 11 - 15 - 1509  can reside directly adjacent the base plastic. In contrast, the extended region  13 . 11 - 15 - 1507  can extend beyond a footprint of the base, such that the extended region  13 . 11 - 15 - 1507  overhangs the base. 
     In some examples, the extended region  13 . 11 - 15 - 1507  can run along an entirety of the facial interface  13 . 11 - 15 - 1502 . The compressible portion (or combination of the extended region  13 . 11 - 15 - 1507  and the central region  13 . 11 - 15 - 1509 ) can have a varying cross-section or be asymmetric relative to the base. For example, the extended region  13 . 11 - 15 - 1507  can taper as it travels from the forehead region  13 . 11 - 15 - 1503  to the cheek region  13 . 11 - 15 - 1505 . In other words, the extended region  13 . 11 - 15 - 1507  can decrease in size as it approaches the cheek region  13 . 11 - 15 - 1505 , so that it is wider at the forehead, and less wide at the cheeks. For example, at the forehead region  13 . 11 - 15 - 1503  shown in  FIGS.  13 . 11 - 15 C , the extended region  13 . 11 - 15 - 1507  can overhang a base by approximately equal to or greater than 3.4 mm. In contrast, at the cheek region  13 . 11 - 15 - 1505  shown in  FIGS.  13 . 11 - 15 C , the extended region  13 . 11 - 15 - 1507  can overhang a base by approximately equal to or less than 3.4 mm. Accordingly, the extended region  13 . 11 - 15 - 1507  can extend beyond or overhang the central region  13 . 11 - 15 - 1509  and the base plastic both above and below, as oriented in the figures. The varying asymmetric cross-section of the extended region  13 . 11 - 15 - 1507  can be intentionally varied to correspond to user facial features, thereby increasing comfort and fit of the head-mountable device. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 11 - 15 B- 13 . 11 - 15 D  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 11 - 15 B- 13 . 11 - 15 D . 
     As mentioned above, a connector of the present disclosure can include multiple portions with differing material properties. In accordance with one or more such examples,  FIGS.  13 . 11 - 16    illustrates a cross-sectional view of the connector  13 . 11 - 1400  including a connector frame  13 . 11 - 1600  for a post  13 . 11 - 1602 . The cross-sectional view of  FIGS.  13 . 11 - 16    is taken along the cross-sectional slice indicated in  FIGS.  13 . 11 - 15 A . 
     As shown in  FIGS.  13 . 11 - 16   , the connector frame  13 . 11 - 1600  can include a rigid or semi-rigid core extending at least partially through the post  13 . 11 - 1602 . The connector frame  13 . 11 - 1600  can provide a desired stiffness or shape to the post  13 . 11 - 1602 . For example, as shown in  FIGS.  13 . 11 - 16   , the connector frame  13 . 11 - 1600  includes a bend adjoining a portion attached to the display frame  1402  and the post  13 . 11 - 1602  extending away from the display frame  13 . 11 - 1402 . In certain examples, the post  13 . 11 - 1602  is overmolded onto, adhered to, or fastened to the connector frame  13 . 11 - 1600 . In other examples, the post  13 . 11 - 1602  is formed integral with the connector frame  13 . 11 - 1600 . In at least one example, the connector frame  13 . 11 - 1600  includes sheet metal and the surrounding portion of the post  13 . 11 - 1602  can include an elastomer material, a rubber, a silicone, a composite, a plastic, or any combination thereof. 
     As will be shown more below in relation to  FIGS.  13 . 11 - 17   , the connector frame  13 . 11 - 1600  can include one or more elements for attaching the connector  13 . 11 - 1400  to the display frame  13 . 11 - 1402 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 11 - 16    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 11 - 16   . 
       FIGS.  13 . 11 - 17    illustrates a top view of an example connector in accordance with one or more examples of the present disclosure. As shown, the connector  13 . 11 - 1400  can include the connector frame  13 . 11 - 1600  discussed above. In particular, the connector frame  1600  can include one or more through holes  13 . 11 - 1700 . Via the through-holes  13 . 11 - 1700 , the connector frame  13 . 11 - 1600  can be fastened to the display frame  13 . 11 - 1402  (having corresponding holes that are align-able with the connector frame  13 . 11 - 1600 ). In this way, the connector frame  13 . 11 - 1600  can be secured to the display frame  13 . 11 - 1402 . 
     In some examples, the connector  13 . 11 - 1400  can be attached to a stiffener  13 . 11 - 1604 . The stiffener  13 . 11 - 1604  can provide certain material property advantages to the connector  13 . 11 - 1400 . For example, the stiffener  13 . 11 - 1604  can be used to tune the spring force, biasing strength, flexibility, etc. of the connector frame  13 . 11 - 1600 . Additionally or alternatively, the spring force, biasing strength, flexibility, etc. for the connector  13 . 11 - 1400  as a whole can be tuned via pre-biasing the connector frame  13 . 11 - 1600  according to a predetermined force-displacement curve. In other examples, the spring force, biasing strength, flexibility, etc. for the connector  13 . 11 - 1400  can be tuned via an overmolded material for the post  13 . 11 - 1602  or by adding a spring member, a torsion bar, or an actuator to the connector  13 . 11 - 1400 . 
     Additionally or alternatively, the stiffener  13 . 11 - 1604  can provide a datum surface (e.g., a flat mating surface) for the connector frame  1600 . In certain examples, the fixture biasing can be implemented to properly position, orient, and/or attach the connector frame  13 . 11 - 1600  to the stiffener  13 . 11 - 1604 . Additionally or alternatively to the stiffener being used as a datum, an adhesive can be used to flow into a desired position and solidify into a datum-like element relative to the connector frame  13 . 11 - 1600 . 
     Additionally, in particular examples, the connector frame  13 . 11 - 1600  (and the connector  13 . 11 - 1400  as a whole) can be removed from the display frame  13 . 11 - 1402 . For example, fasteners (not shown) can be removed from the through-holes  13 . 11 - 1700  to allow removal of the connector  13 . 11 - 1400  from the display frame  13 . 11 - 1402 . In some examples, the fasteners and the through-holes  13 . 11 - 1700  can be user-accessible and/or manufacturer-accessible for swapping out the connector  13 . 11 - 1400  for a connector with a different size post and/or a post with a different angle relative to the display frame (e.g., a shorter or longer post as shown in  FIGS.  13 . 11 - 15 A ). Accordingly, in some examples, the connector  13 . 11 - 1400  is modular. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 11 - 17    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 11 - 17   . For example, a connector  13 . 11 - 1606  shown in the background of  FIGS.  13 . 11 - 16    can include one or more other connectors described above in the present disclosure. 
     It will be appreciated that one or more connectors of the present disclosure can be modified to exhibit certain material properties (e.g., increased strength, resilience, flexibility, etc.). In accordance with one or more such examples,  FIGS.  13 . 11 - 18    illustrates a side perspective view of a base of a connector where it is attached to a display frame. In particular,  FIGS.  13 . 11 - 18    illustrates a gusset  13 . 11 - 1800  formed on the surface of the connector frame  13 . 11 - 1600 . The gusset  13 . 11 - 1800  can include a feature on the connector  13 . 11 - 1400  that helps to reinforce the connector attachment to the display frame  13 . 11 - 1402  (e.g., via the stiffener  13 . 11 - 1604 ). In some examples, the gusset  13 . 11 - 1800  includes a plate adhered, bonded, welded, or molded onto the connector frame  13 . 11 - 1600 . In certain examples, the gusset  1800  includes a reinforced joint. In at least one example, the gusset  13 . 11 - 1800  is shaped to distribute loads from concentrating around fasteners  13 . 11 - 1802  on the connector frame  13 . 11 - 1600 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 11 - 18    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 11 - 18   . 
       FIGS.  13 . 11 - 19    illustrates another cross-sectional view of the connector  13 . 11 - 1400  in accordance with one or more examples of the present disclosure. As shown in  FIGS.  13 . 11 - 19   , the connector frame  13 . 11 - 1600  can be discontinuous at certain areas of the connector frame  1600 . For example, the connector frame  13 . 11 - 1600  can include one or more reliefs  13 . 11 - 1900  (e.g., cutouts for added flexibility, decreased weight, or less material consumption). In these or other examples, the post  13 . 11 - 1602  can include corresponding portions positioned inside the relief  13 . 11 - 1900  that are solid plastic, elastomer, or other material. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 11 - 19    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 11 - 19   . The following description discusses example adhesives in a head-mountable device. Indeed, a head-mountable device of the present disclosure can include a variety of different adhesives that perform various functions and/or impart certain properties. 
     In particular,  FIGS.  13 . 11 - 20 - 13 . 11 - 21    respectively show perspective and top views of example adhesives in an example head-mountable device. The term adhesives can refer to one or more types of glue, epoxy adhesives, polyurethane adhesives, polyimide adhesives, paste adhesives, liquid adhesives, film adhesives, pellet adhesives, strip adhesives, hot melt, thermosetting adhesives, pressure sensitive adhesives, contact adhesives, structural adhesives, non-structural adhesives, semi-structure adhesives, tape, bonds, welds, etc. 
     Specifically, as shown, the display frame  13 . 11 - 1402  is depicted with a cover or housing portion removed for illustration purposes (e.g., to expose various adhesives that can be included within the display frame  13 . 11 - 1402 ). In some examples, the display frame  13 . 11 - 1402  includes a connector  13 . 11 - 2000 , as described above. In at least certain implementations, the connector  13 . 11 - 2000  can include a different material than the stiffener  13 . 11 - 1604 . For instance, in one example, the connector  13 . 11 - 2000  can include a  13 . 11 - 316  stainless steel, and the stiffener  13 . 11 - 1604  can include a  17 - 4  steel. In such an example, laser welding the dissimilar materials can cause oxidation, such as oxidation of the chromium in the  13 . 11 - 316  stainless steel. In specific examples, adhesives  13 . 11 - 2002  can help prevent (or at least reduce an amount of) oxidation on one or both of the connector  2000  or the stiffener  13 . 11 - 1604 . Thus, the adhesives  13 . 11 - 2002  can be positioned on the surface of the stiffener  13 . 11 - 1604  at an end portion adjacent to the connector  13 . 11 - 2000 . In at least one example, the adhesives  13 . 11 - 2002  are positioned at least partially around fasteners attaching the stiffener  13 . 11 - 1604  to the display frame  13 . 11 - 1402 . In these or other examples, the adhesives  13 . 11 - 2002  can interact with the surface of the stiffener  13 . 11 - 1604  and/or the connector  13 . 11 - 2000  (e.g., when under thermal loads from laser welding applications) to inhibit oxidation. Other cosmetic advantages can also be imparted by the adhesives  13 . 11 - 2002 , as desired. 
     The display frame  13 . 11 - 1402  can also include adhesives  13 . 11 - 2004 . The adhesives  13 . 11 - 2004  are strips of adhesive, which can be positioned longitudinally along a top surface of the stiffener  13 . 11 - 1604 . In particular, the adhesives  13 . 11 - 2004  can couple the outer portion or shell (removed for interior illustration purposes) of the display frame to the stiffener  13 . 11 - 1604 . This coupling can impart increased stiffness to the display frame  13 . 11 - 1402  as a whole (and in particular, impart more stiffness to the less rigid or stiff outer shell)—by leveraging stiffness of the stiffener  13 . 11 - 1604 . 
     In addition, the display frame  13 . 11 - 1402  can include adhesives  13 . 11 - 2006 . The adhesives  13 . 11 - 2006  can include adhesives positioned underneath the stiffener  13 . 11 - 1604 . The adhesives  13 . 11 - 2006  can specifically adhere the stiffener  13 . 11 - 1604  to the bottom housing or shell of the display frame  13 . 11 - 1402 . 
     In some examples, the display frame  13 . 11 - 1402  can include adhesives  13 . 11 - 2008 . The adhesives  13 . 11 - 2008  can include a discontinuous glue strip between top and bottom shells of the display frame  13 . 11 - 1402 . In at least one example, the gaps between the adhesives  13 . 11 - 2008  can impart thermal advantages. For instance, the adhesives  13 . 11 - 2008  can reduce or eliminate deformation in the display frame  13 . 11 - 1402  caused by the different coefficients of thermal expansion in response to higher heat and/or humid environments. 
     In one or more examples, the display frame  13 . 11 - 1402  can include an adhesive  13 . 11 - 2010 . The adhesive  13 . 11 - 2010  can include a gap filler or hardstop glue to limit travel of the connector  13 . 11 - 2000 . For example, the adhesive  13 . 11 - 2010  can bound the travel of the connector  2000  towards the optical system such that the adhesive  13 . 11 - 2010  serves as a mechanical stop. In these or other examples, the adhesive  13 . 11 - 2010  can include an adhesive with higher compressive stiffness compared to other adhesives. For example, the adhesive  13 . 11 - 2010  can be load bearing (e.g., to at least partially transfer an applied load to the connector  13 . 11 - 2000 ). 
     Additionally or alternatively to the foregoing adhesives, it will be appreciated that other examples are also contemplated. For instance, in some implementations, the display frame  13 . 11 - 1402  can include near net molding (which can obviate one or more adhesives discussed above). 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 11 - 20 - 13 . 11 - 21    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 11 - 20 - 13 . 11 - 21   . The following description discusses example adhesives in a head-mountable device. Indeed, a head-mountable device of the present disclosure can include a variety of different adhesives that perform various functions and/or impart certain properties. 
     It will be noted that in addition to forming the frame in the manner described herein, it is also contemplated to form the frame and its structural components using machined configurations, carbon fiber configurations, 3D modeled configurations, fixed molded configurations, etc. 
     13.12: Face Engaging Structure 
     The following disclosure relates to connectors of a head-mountable device used for AR/VR experiences. More particularly, the present embodiments relate to connectors positioned at a maxilla region (and nearby zygoma regions) of a face when the head-mountable device is donned. These connectors can dynamically adjust to different heads, as well as dynamically adjust in real time in response to force events. The connectors can also include a variety of different types of connections, as will be described below. 
     In one example, a head-mountable device includes a connector between a display and a facial interface. In certain implementations, the connector between the display and the facial interface has a stop (e.g., a hard stop). The connector allows for free movement (or multiple degrees of freedom) of the facial interface while still grossly constraining the facial interface. Because the facial interface can freely react to the user&#39;s face, pressure can be more evenly distributed to the user&#39;s face. The improved load distribution across a user&#39;s face can create a pleasant and enjoyable AR/VR experience for the user. 
     Conventional head-mountable devices do not include connections with a stop. By contrast, the head-mountable device of the present disclosure includes connections with a stop. A stop can stabilize (and in some cases protect from undue force distribution to) the HMD and internal components. A stop can also improve HMD robustness by preventing damage to sensitive components and efficiently transferring a force load through the stop (e.g., to a force-dissipation member, force-spreader member, or predetermined facial feature), thereby improving user comfort. 
     In one example, a head-mountable device of the present disclosure includes a display and a facial interface with a connector positioned at the maxilla facial region. The connector can include two degrees of freedom and can be a pin-and-bowl connection. 
     In another example, the connector can include a foam stop adhered to the facial interface. In certain implementations, the foam stop is also coupled to the display. For instance, the foam stop can be preloaded to remain positioned in a “home” (e.g., non-deflected or deflected) state until the head-mountable device is donned. 
     In yet another example, the connector can include a slidable post. The slidable post can slide relative to the facial interface or the display, as desired. 
     Accordingly, the apparatuses and systems described herein can provide facial feature adjustments and connections that can increase user customization, reduce facial pressure, and improve distribution of force loads for a head-mountable device, thereby increasing comfort. 
     These and other embodiments are discussed below with reference to  FIGS.  13 . 12 - 1 - 13 . 12 - 8 B . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these Figures is for explanatory purposes only and should not be construed as limiting. Furthermore, as used herein, a system, a method, an article, a component, a feature, or a sub-feature including at least one of a first option, a second option, or a third option should be understood as referring to a system, a method, an article, a component, a feature, or a sub-feature that can include one of each listed option (e.g., only one of the first option, only one of the second option, or only one of the third option), multiple of a single listed option (e.g., two or more of the first option), two options simultaneously (e.g., one of the first option and one of the second option), or combination thereof (e.g., two of the first option and one of the second option). 
       FIGS.  13 . 12 - 1    illustrates a top view profile of a head-mountable device  13 . 12 - 100  worn on a user head  13 . 12 - 101 . The head-mountable device  13 . 12 - 100  can include a display  13 . 12 - 102  (e.g., one or more optical lenses or display screens in front of the eyes of the user). The display  13 . 12 - 102  can include a display for presenting augmented reality visualizations, a virtual reality visualization, or other suitable visualizations. 
     The head-mountable device  13 . 12 - 100  also includes a facial interface  13 . 12 - 104 . As used herein, the term “facial interface” refers to a portion of the head-mountable device  13 . 12 - 100  that engages with a user face via direct contact. In particular, a facial engagement portion includes portions of the head-mountable device  13 . 12 - 100  that conform to (e.g., press against) regions of a user face. To illustrate a facial interface can include a pliant (or semi-plaint) face track that spans the forehead, wraps around the eyes, contacts the zygoma and maxilla regions of the face, and bridges the nose. In addition, a facial interface can include various components forming a structure, webbing, cover, fabric, or frame of a head-mountable device disposed between the display  13 . 12 - 102  and the user skin. In particular implementations, a facial interface can include a seal (e.g., a light seal, environment seal, dust seal, air seal, etc.). It will be appreciated that the term “seal” can include partial seals or inhibitors, in addition to complete seals (e.g., a partial light seal where some ambient light is blocked and a complete light seal where all ambient light is blocked when the head-mountable device is donned). 
     The head-mountable device  13 . 12 - 100  further includes connector(s)  13 . 12 - 103 . As used herein, the term “connector” refers to a connection or touch point between the display  13 . 12 - 102  and the facial interface  13 . 12 - 104 . For instance, a connector can movably constrain the facial interface  13 . 12 - 104  to the display  13 . 12 - 102 . In certain implementations, a connector includes a joint that physically connects the display  13 . 12 - 102  and the facial interface  13 . 12 - 104 . In these or other implementations, a connector includes a floating joint or mechanical stop where the facial interface  13 . 12 - 104  and the display  13 . 12 - 102  can contact each other (e.g., at rest, after certain amounts of deflection, etc.). 
     To illustrate, the connector(s)  13 . 12 - 103  can allow the facial interface  13 . 12 - 104  to translate (e.g., left-right, up-down,) in one plane. Additionally or alternatively, the connector(s)  13 . 12 - 103  can allow the facial interface  13 . 12 - 104  to translate (e.g., depth-wise) in another plane relative to the display  13 . 12 - 102 . For example, the connector(s)  13 . 12 - 103  allow the facial interface  13 . 12 - 104  to move between a first state (e.g., a home state when the head-mountable device  13 . 12 - 100  is not donned by a user) to a second state (e.g., when the head-mountable device  13 . 12 - 100  is donned by a user). The connector(s)  13 . 12 - 103  help enable the facial interface  13 . 12 - 104  to flexibly accommodate the facial profile of the user by deflecting according to the user&#39;s facial features. When the head-mountable device  13 . 12 - 100  is removed from the user&#39;s head  13 . 12 - 101 , the connector(s)  13 . 12 - 103  can return to the first state-thereby causing the facial interface  13 . 12 - 104  to correspondingly move. 
     The connector(s)  13 . 12 - 103  can include one or more components (e.g., pins, bowls, slidable posts, foam, mechanical stops, dampeners, spring connections, etc.) that can allow (or actively provide) translation in one or more planes (e.g., at a maxilla region of a face). The connector(s)  13 . 12 - 103  can include proud engagement portions (e.g., pin, slidable post) and recessed engagement portions (e.g., receptacle, cup, slidable post-track). The proud engagement portions and recessed engagement portions can mate, forming a floating connection (e.g., a receptacle, touch point, mechanical stop, etc. constraining a post from moving in certain directions or beyond certain distances or depths). 
     Additionally, as shown in  FIGS.  13 . 12 - 1   , the head-mountable device  13 . 12 - 100  can include one or more arms  13 . 12 - 108 ,  13 . 12 - 110 . The arm  13 . 12 - 108 ,  13 . 12 - 110  are connected to the display  13 . 12 - 102  and extend distally toward the rear of the head. The arms  13 . 12 - 108 ,  13 . 12 - 110  are configured to secure the display  13 . 12 - 102  in a position relative to the user head  13 . 12 - 101  (e.g., such that the display is maintained in front of a user&#39;s eyes). For example, the arms  13 . 12 - 108 ,  13 . 12 - 110  extend over the user&#39;s ears  13 . 12 - 106 . In certain examples, the arms  13 . 12 - 108 ,  13 . 12 - 110  rest on the user&#39;s ears  13 . 12 - 106  to secure the head-mountable device  13 . 12 - 100  via friction between the arms  13 . 12 - 108 ,  13 . 12 - 110  and the user head  13 . 12 - 101 . Optionally, the arms  13 . 12 - 108 ,  13 . 12 - 110  can be connected to each other via a strap  13 . 12 - 112  (shown in the dashed lines) that can compress the head-mountable device  13 . 12 - 100  against the user head  13 . 12 - 101 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 12 - 1    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 12 - 1   . 
       FIGS.  13 . 12 - 2 A- 13 . 12 - 2 B  illustrate side and front view profiles, respectively, of an example of the head-mountable device  13 . 12 - 100 . As discussed above, the head-mountable device  13 . 12 - 100  includes the display  13 . 12 - 102 , the facial interface  13 . 12 - 104 , and the connector(s)  13 . 12 - 103 . In particular, as shown in  FIGS.  13 . 12 - 2 A- 13 . 12 - 2 B , the facial interface  13 . 12 - 104  can indeed wrap around the eyes  13 . 12 - 206 , bridge the nose  13 . 12 - 208 , span a forehead region  13 . 12 - 203 , contact a zygoma facial region  13 . 12 - 202 , and contact a maxilla facial region  13 . 12 - 204 . 
     In addition, the connector(s)  13 . 12 - 103  can be positioned along the maxilla facial region  13 . 12 - 204  and connect directly (e.g., having a direct connection) or indirectly (e.g., having a floating connection) to the facial interface  13 . 12 - 104  and the display  13 . 12 - 102 . The connector(s)  13 . 12 - 103  can similarly be positioned along the zygoma facial region  13 . 12 - 202  and/or the forehead region  13 . 12 - 203 . In at least some examples, the connector(s)  13 . 12 - 103  are spatially configured to provide a particular force profile when donned and/or during a force event applying a force load to the display  13 . 12 - 102 . 
     Further below, this disclosure describes with more particularity how the connector(s)  13 . 12 - 103  can be constructed. For example, the connector(s)  13 . 12 - 103  can include pivot connectors, floating connectors, foam connectors, stop connectors, or other connection types (e.g., sliding connectors, rigid connectors), including combinations thereof (such as a hybrid elastomer and mechanical slider). 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 12 - 2 A- 13 . 12 - 2 B  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 12 - 2 A- 13 . 12 - 2 B . 
     The connector(s)  13 . 12 - 103  can provide a stable and flexible interface between the facial interface  13 . 12 - 104  and the display  13 . 12 - 102 . Additionally or alternatively, the connector(s)  13 . 12 - 103  include an amount of travel (e.g., an amount of possible displacement). The connector(s)  13 . 12 - 103  can also provide a datum or a hard stop limiting the travel to a particular separation distance (e.g., a maximum displacement) of the display  13 . 12 - 102  relative to the facial interface  13 . 12 - 104 . These connector(s)  13 . 12 - 103  can also increase movement of the facial interface  13 . 12 - 104  in the maxilla facial region  13 . 12 - 204  in such a way that the facial interface  13 . 12 - 104  freely interacts (e.g., conforms) with the user&#39;s face without applying excessive pressure back on to the user&#39;s face. 
     In accordance with one or more such examples of the present disclosure,  FIGS.  13 . 12 - 3    illustrates an example of the head-mountable device  13 . 12 - 100  including the display  13 . 12 - 102 , the facial interface  13 . 12 - 104 , and the connector(s)  13 . 12 - 103  (namely, connectors  13 . 12 - 303   a - 13 . 12 - 303   f ). The connector(s)  13 . 12 - 103  (albeit positionable in different places) can specifically be positioned at the maxilla facial region  13 . 12 - 204  to distribute pressure exerted on a user&#39;s face. For example, elastic forces exerted by the strap  13 . 12 - 112  (not shown) when donning the head-mountable device  13 . 12 - 100  can be distributed from the display  13 . 12 - 102  to the facial interface  13 . 12 - 104  in a comfortable manner via the connectors  13 . 12 - 303   a - 13 . 12 - 303   f . In particular, the connectors  13 . 12 - 103   a - 13 . 12 - 103   b  positioned at the maxilla facial region  13 . 12 - 204  can conform (e.g., flex, pivot, rotate, etc.) to a user&#39;s face evenly distributing the weight of the head-mountable device  13 . 12 - 100  across the maxilla facial region  13 . 12 - 204 , thereby helping to provide increased comfort to a user. 
     As shown in  FIGS.  13 . 12 - 3   , the connectors  13 . 12 - 303   a - 13 . 12 - 303   b  at the maxilla facial region  13 . 12 - 204  include a pivot connection. A pivot connection can include one or more different connections that allow pivot action relative to an anchored portion. Thus, a pivot connection can include a ball-and-socket joint, hinged joint, condyloid joint, atlantoaxial joint, radioulnar joint, serial manipulators, etc. 
     In certain implementations, the anchored portion of the pivot connection for the connectors  13 . 12 - 303   a - 13 . 12 - 303   b  is positioned on the display  13 . 12 - 102 . Thus, the facial interface  13 . 12 - 104  can move or pivot relative to the display  13 . 12 - 102  (e.g., for a comfortable, conforming fit to the user face). In other implementations, however, the anchored portion of the pivot connection for the connectors  13 . 12 - 303   a - 13 . 12 - 303   b  is positioned on the facial interface  13 . 12 - 104 . 
     The pivot connection for the connectors  13 . 12 - 303   a - 13 . 12 - 303   b  can include travel defined by the amount of possible deflection or linear distance movement. In some cases, the pivot connection includes a range of travel between about 2 mm to about 20 mm. In particular implementations, the pivot connection includes about 6 mm of travel. It will be appreciated that the pivot connection also includes a stop (e.g., a portion of the socket joint) bounding the travel. 
     In at least some examples, the head-mountable device  13 . 12 - 100  implements additional connectors to change a pressure profile to reduce or avoid “hot-spot loading” lending to acute pressure against a user&#39;s face. For example, the head-mountable device  13 . 12 - 100  includes the connectors  13 . 12 - 303   e - 13 . 12 - 303   f  positioned at the zygoma facial region  13 . 12 - 202  to at least partially offload an applied force distributed through the connectors  13 . 12 - 303   a - 13 . 12 - 303   b  at the maxilla facial region  13 . 12 - 204 . In certain implementations, the connectors  13 . 12 - 303   e - 13 . 12 - 303   f  include the same type of connection as the connectors  13 . 12 - 303   a - 13 . 12 - 303   b . In other implementations, the connectors  13 . 12 - 303   e - 13 . 12 - 303   f  include a different type of connection than the connectors  13 . 12 - 303   a - 13 . 12 - 303   b . For instance, the connectors  13 . 12 - 303   e - 13 . 12 - 303   f  can include a floating foam joint or a leaf spring joint, and the connectors  13 . 12 - 303   a - 13 . 12 - 303   b  can include a pivot connection. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 12 - 3    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 12 - 3   . 
     As mentioned above, the head-mountable device of the present disclosure can implement a pin-and-bowl connection between the display  13 . 12 - 102  and the facial interface  13 . 12 - 104 . In accordance with one or more such examples,  FIGS.  13 . 12 - 4 A  illustrates an example of the head-mountable device  13 . 12 - 100  including the display  13 . 12 - 102 , the facial interface  13 . 12 - 104 , and the connector(s)  13 . 12 - 103  (namely connectors  13 . 12 - 402 ).  FIGS.  13 . 12 - 4 B- 13 . 12 - 4 C  show a front profile view and side profile view, respectively, of the connectors  13 . 12 - 402  located at the maxilla facial region  13 . 12 - 204 . 
     As shown, the connectors  13 . 12 - 402  include a pin-and-bowl connection. In particular, the connectors  13 . 12 - 402  include a bowl  13 . 12 - 404  and a pin  13 . 12 - 405 . The bowl  13 . 12 - 404  includes a receptacle defined within the display  13 . 12 - 102  that is configured to receive the pin  13 . 12 - 405  affixed to the facial interface  13 . 12 - 104 . In particular implementations, the bowl  13 . 12 - 404  is sized and shaped to receive a particular portion of the pin  13 . 12 - 405  (e.g., an entirety of the pin  13 . 12 - 405 , an end portion of the pin  13 . 12 - 405 , or a predetermined depth of the pin  13 . 12 - 405 ). 
     In these or other examples, the bowl  13 . 12 - 404  includes a stop  13 . 12 - 410  (e.g., an axial dead stop) coupled to the display  13 . 12 - 102 . The stop  13 . 12 - 410  can control the penetration depth of the pin  13 . 12 - 405  into the bowl  13 . 12 - 404 . Specifically, the stop  13 . 12 - 410  allows a maximum displacement or penetration of the pin  13 . 12 - 405  in an axial direction  13 . 12 - 412 . When the pin  13 . 12 - 405  abuts the stop  13 . 12 - 410 , the pin  13 . 12 - 405  is stopped out and cannot proceed further in the axial direction  13 . 12 - 412 —at which point the facial interface  13 . 12 - 104  and the display  13 . 12 - 102  are positioned at a predetermined distance from each other (e.g., a minimum separation distance). 
     The stop  13 . 12 - 410  can also control other movement of the pin  13 . 12 - 405  within the bowl  13 . 12 - 404 . In particular, the stop  13 . 12 - 410  can limit or define translational movement of the pin  13 . 12 - 405  in directions perpendicular to the axial direction  13 . 12 - 412 . 
     In particular implementations, the connectors  13 . 12 - 402  include two degrees of freedom. As illustrated in  FIGS.  13 . 12 - 4 B , a first degree of freedom includes a translational degree of freedom in a first plane (e.g., defined by a first axis  13 . 12 - 406  and a second axis  13 . 12 - 408 ). A second degree of freedom includes depth-based motion in the axial direction  13 . 12 - 412 , as illustrated in  FIGS.  13 . 12 - 4 C . The combination of the first degree of freedom and the second degree of freedom allow the facial interface  13 . 12 - 104  to conform to a user&#39;s face while also providing rigidity and stability where appropriate. 
     According to some examples, the connectors  13 . 12 - 402  can move along the first degree of freedom (i.e., along the first axis  13 . 12 - 406 ) when the pin  13 . 12 - 405  is positioned at a variety of different depths along the axial direction  13 . 12 - 412 . For example, the connectors  13 . 12 - 402  can move along the first axis  13 . 12 - 406  even when the pin  13 . 12 - 405  is fully depressed (i.e., when the pin  13 . 12 - 405  is stopped out). In this manner, the connectors  13 . 12 - 402  can move in response to various forces, such as at least one of an applied load or a shear force. Nonetheless, such movement of the connectors  13 . 12 - 402  can be bounded by virtue of the pin  13 . 12 - 405  being retained within the bowl  13 . 12 - 404 . 
     The pin  13 . 12 - 405  can include a variety of components and/or materials. In one example, the pin  13 . 12 - 405  includes a contact portion  13 . 12 - 418 . The contact portion  13 . 12 - 418  can be firm and rigid. In other examples, the contact portion  13 . 12 - 418  is soft or compressive. In some examples, the contact portion  13 . 12 - 418  includes a material such as a foam, an elastomer, a gel, a silicon, a metal, or a thermoplastic. 
     The pin  13 . 12 - 405  can also include a variety of different shapes and sizes. In some examples, the pin  13 . 12 - 405  is cone-shaped. In some instances, a cone shape can allow more freedom of motion at greater distances of separation between the display  13 . 12 - 102  and the facial interface  13 . 12 - 104 . That is, as the pin  13 . 12 - 405  compresses further into the bowl  13 . 12 - 404 , the cone-shape of the pin  13 . 12 - 405  fills more and more volume of the bowl  13 . 12 - 404 , leaving little or no motion for the pin  13 . 12 - 405 . Other shapes, however, are herein contemplated. For instance, the pin  13 . 12 - 405  can be shaped to resemble a square, rectangle, cylinder, oval, triangle, or other suitable shape. 
     Further, the connectors  13 . 12 - 402  can include one or more mechanisms for retaining the pin  13 . 12 - 405  within the bowl  13 . 12 - 404  (not shown). For example, a tip of the pin  13 . 12 - 405  can be sized greater than the opening of the bowl  13 . 12 - 404 . According to another example, the pin  13 . 12 - 405  and/or the bowl  13 . 12 - 404  can be sized and shaped such that the pin  13 . 12 - 405  cannot escape the bowl  13 . 12 - 404 . For example, the pin  13 . 12 - 405  and/or the bowl  13 . 12 - 404  can be sized greater than the amount of pin displacement between positional states. In yet another example, the bowl  13 . 12 - 404  can include a socket-like structure that retains the pin  13 . 12 - 405 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 12 - 4 A- 13 . 12 - 4 C  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 12 - 4 A- 13 . 12 - 4 C . 
     In one example, the connectors of the present disclosure are preloaded (e.g., via springs, elastic materials, etc.) to return to a certain position or state after being moved. In accordance with one or more such examples,  FIGS.  13 . 12 - 5 A- 13 . 12 - 5 B  respectively illustrate states (or positions) of the connectors  13 . 12 - 402  described above. In particular,  FIGS.  13 . 12 - 5 A  illustrates a first state  13 . 12 - 500  of the connectors  13 . 12 - 402  where the pin  13 . 12 - 405  is stopped out (i.e., bottomed out) within the bowl  13 . 12 - 404 . When in the first state  13 . 12 - 500 , the display  13 . 12 - 102  and the facial interface  13 . 12 - 104  are positioned at a predetermined separation distance (e.g., a minimum separation distance). In the first state  13 . 12 - 500 , further travel of the pin  13 . 12 - 405  into the bowl  13 . 12 - 404  is disallowed. 
     In some examples, the first state  13 . 12 - 500  includes a home state. The home state refers to a position that the connectors  13 . 12 - 402  rebound to after being deflected. In particular, the home state is the unperturbed position of the connectors  13 . 12 - 402 . 
     In other examples, however, the first state  13 . 12 - 500  includes a deflected state. The deflected state refers to a position that the connectors  13 . 12 - 402  can achieve in response to deflection (e.g., when donning the head-mountable device or during a force event. 
       FIGS.  13 . 12 - 5 B  illustrates a second state  13 . 12 - 502  of the connectors  13 . 12 - 402  where the pin  13 . 12 - 405  is only partially disposed within the bowl  13 . 12 - 404 . In certain cases, the pin  13 . 12 - 405  is complete withdrawn from the bowl  13 . 12 - 404 . When in the second state  13 . 12 - 502 , the display  13 . 12 - 102  and the facial interface  13 . 12 - 104  are movable relative to each other (either closer to or farther away). In the second state  13 . 12 - 502 , further travel of the pin  13 . 12 - 405  into the bowl  13 . 12 - 404  is allowed. 
     Similar to the first state  13 . 12 - 500 , the second state  13 . 12 - 502  can include a home state. In other implementations, the second state  13 . 12 - 502  includes a deflected state. 
     In at least some examples, fabric  13 . 12 - 504  shown in  FIGS.  13 . 12 - 5 A- 13 . 12 - 5 B  can be stretched or tensioned to accommodate the first state  13 . 12 - 500  and the second state  13 . 12 - 502  just described. The fabric  13 . 12 - 504  can include an adjustable external shell that flexes with movement of the display  13 . 12 - 102  and the facial interface  13 . 12 - 104 . Thus, as the connectors  13 . 12 - 402  move to and from the first state  13 . 12 - 500  and the second state  13 . 12 - 502 , the fabric  13 . 12 - 504  can correspondingly adjust (e.g., stretch, tighten, loosen, wrinkle, fold, etc.). 
     According to some examples, the fabric  13 . 12 - 504  shown in  FIGS.  13 . 12 - 5 A- 13 . 12 - 5 B  can be implemented to bias the connectors of the present disclosure. For example, the fabric  13 . 12 - 504  can be stretched or tensioned to bias the connectors  13 . 12 - 402  in a particular home state or a deflected state. In at least some examples, the induced bias of the connectors  13 . 12 - 402  (provided by the tension from the fabric  13 . 12 - 504 ) can facilitate a broad range of connector architectures, ranges of motion, etc. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 12 - 5 A- 13 . 12 - 5 B  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 12 - 5 A- 13 . 12 - 5 B . 
     As mentioned above, the connectors of the present disclosure can include a variety of different connectors, including foam connectors positioned at a maxilla facial region. In accordance with one or more such examples,  FIGS.  13 . 12 - 6 A  illustrates another example of the head-mountable device  13 . 12 - 100  including the display  13 . 12 - 102  and the facial interface  13 . 12 - 104 , and the connector(s)  13 . 12 - 103  (namely connectors  13 . 12 - 602 ). 
     In these or other examples, the connectors  13 . 12 - 602  include foam connectors. Specifically, the connectors  13 . 12 - 602  include a foam portion  13 . 12 - 604  affixed to the facial interface  13 . 12 - 104  and a base  13 . 12 - 606  affixed to the display  13 . 12 - 102 . The base  13 . 12 - 606  is sized and shaped to receive the foam portion  13 . 12 - 604 . In particular implementations, the foam portion  13 . 12 - 604  is permanently adhered to the base  13 . 12 - 606  via an attachment  13 . 12 - 608 . The attachment  13 . 12 - 608  can include glue, bond, or other type of adhesion. Additionally or alternatively, the attachment  13 . 12 - 608  includes a fastener, such as a bolt, screw, etc. 
     In at least one example, the connectors  13 . 12 - 602  include a compressible portion  13 . 12 - 610 , as illustrated in  13 . 12 - 6 B. The compressible portion  13 . 12 - 610  can also include a foam or other compressible material. In certain implementations, the compressible portion  13 . 12 - 610  may include a predefined thickness (e.g., about 2 mm, about 3 mm, about 10 mm, etc.) that dictates a corresponding amount of displacement or compression. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 12 - 6 A- 13 . 12 - 6 B  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 12 - 6 A- 13 . 12 - 6 B . 
     As similarly described above, the connectors of the present disclosure can move between states or positions. Foam connections can likewise move between states or positions. In accordance with one or more such examples,  FIGS.  13 . 12 - 7 A- 13 . 12 - 7 B  illustrate positional states of the connectors  13 . 12 - 602 . 
     A first state  13 . 12 - 700  of the connector  13 . 12 - 602  is shown in  FIGS.  13 . 12 - 7 A . In the first state  13 . 12 - 700 , the compressible portion  13 . 12 - 610  is uncompressed and has a first thickness. The second state  13 . 12 - 702  of the connector  13 . 12 - 602  is shown in  FIGS.  13 . 12 - 7 B . Specifically, in the second state  13 . 12 - 702 , the compressible portion  13 . 12 - 610  is compressed (e.g., reduced in thickness from the first thickness) and has a second thickness less than the first thickness. The compressible portion  13 . 12 - 610  in the second state  13 . 12 - 702  need not fully compress. Indeed, the compressible portion  13 . 12 - 610  can compress partially or fully, as may be the case (e.g., when donning the head-mountable device or during a force event). 
     It will be appreciated that the first state  13 . 12 - 700  and the second state  13 . 12 - 702  can be, respectively, home states and deflected states. However, in certain implementations, the first state  13 . 12 - 700  and the second state  13 . 12 - 702  can oppositely configured (i.e., deflected states and home states, respectively). 
     In at least some examples, the fabric  13 . 12 - 504  described above can correspondingly move and flex to accommodate the first state  13 . 12 - 700  and the second state  13 . 12 - 702  for the connectors  13 . 12 - 602 . Alternatively, the fabric  13 . 12 - 504  can be attached to non-deflecting portions (e.g., exclusive of the compressible portion  13 . 12 - 610 ) such that the fabric  13 . 12 - 504  need not flex or move. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 12 - 7 A- 13 . 12 - 7 B  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 12 - 7 A- 13 . 12 - 7 B . 
     Also discussed above, the connectors of the present disclosure can include sliding posts that slide relative to the display  13 . 12 - 102  or the facial interface  13 . 12 - 104 . These sliding connectors can provide improved ranges of motion (e.g., lateral motion of the maxilla region) while still maintaining comfortable stability and rigidity. In accordance with one or more such examples,  FIGS.  13 . 12 - 8 A- 13 . 12 - 8 B  illustrate connectors  13 . 12 - 802   a - 13 . 12 - 802   b  positioned at the maxilla region of the facial interface  13 . 12 - 104 . 
     As shown in  FIGS.  13 . 12 - 8 A , the connector  13 . 12 - 802   a  is affixed to the facial interface  13 . 12 - 104  and slidably engaged with the display  13 . 12 - 102 . In particular, the connector  13 . 12 - 802   a  includes a slidable post with a proud engagement portion  13 . 12 - 801 . The proud engagement portion  13 . 12 - 801  interfaces with a recessed engagement portion  13 . 12 - 804  positioned within the display  13 . 12 - 102 . The recessed engagement portion  13 . 12 - 804  can include a slidable-post track (e.g., slide mechanism, track, guide, groove, recess, or raceway) defined by one or more surfaces of the display  13 . 12 - 102 . The recessed engagement portion  13 . 12 - 804  can guide a range of motion for the connector  13 . 12 - 802   a  along a direction  13 . 12 - 808 . In this manner, the connector  13 . 12 - 802   a  allows the facial interface  13 . 12 - 104  to comfortably conform to or move with the user&#39;s face in a stabilized fashion. 
     Alternatively, as shown in  FIGS.  13 . 12 - 8 B , the connector  13 . 12 - 802   b  is affixed to the display  13 . 12 - 102  and slidably engaged with the facial interface  13 . 12 - 104 . The connector  13 . 12 - 802   b  is the same as the connector  13 . 12 - 802   a , except configured in a different positional relationship. In particular, the connector  13 . 12 - 802   b  includes a slidable post with a proud engagement portion  13 . 12 - 801 . The proud engagement portion  13 . 12 - 801  interfaces with a recessed engagement portion  13 . 12 - 804  positioned within the facial interface  13 . 12 - 104 . The proud engagement portion  13 . 12 - 801  can move along the direction  13 . 12 - 808  within the facial interface  13 . 12 - 104 . 
     According to some examples, the mechanical slider examples illustrated in  FIGS.  13 . 12 - 8 A- 13 . 12 - 8 B  can be implemented utilizing a variety of different materials. For example, at least one of the proud engagement portion  13 . 12 - 801  or the recessed engagement portion  13 . 12 - 804  can be an elastomer material, metal material, plastic material, composite materials, and the like. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 12 - 8 A- 13 . 12 - 8 B  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 12 - 8 A- 13 . 12 - 8 B . 
     Myriad other types of connectors are herein contemplated. For instance, the connector(s)  13 . 12 - 106  of the present disclosure can include a piston-based mechanism. In accordance with one or more such examples,  FIGS.  13 . 12 - 9 A- 13 . 12 - 9 F  illustrate a head-mountable device  13 . 12 - 900  with a maxilla connector  13 . 12 - 902  and a zygoma connector  13 . 12 - 904 . The maxilla connector  13 . 12 - 902  and the zygoma connector  13 . 12 - 904  movably join the facial interface  13 . 12 - 104  and the display  13 . 12 - 102  as similarly described above. 
     As shown, however, the maxilla connector  13 . 12 - 902  includes a piston connection, and the zygoma connector  13 . 12 - 904  includes a pivot connection. The maxilla connector  13 . 12 - 902  can move in and out in response to application and release of a compressive force at the maxilla region of a face. As the maxilla connector  13 . 12 - 902  moves, the zygoma connector  13 . 12 - 904  correspondingly moves (e.g., pivots). 
       FIGS.  13 . 12 - 9 B  shows that the maxilla connector  13 . 12 - 902  includes a cap  13 . 12 - 906 , a plunger  13 . 12 - 908 , a compression spring  13 . 12 - 910 , and a housing  13 . 12 - 912 . The cap  13 . 12 - 906  maintains the components of the maxilla connector  13 . 12 - 902  in place. The plunger  13 . 12 - 908  moves in and out of the housing  13 . 12 - 912 , with resistance provided by the compression spring  13 . 12 - 910 . The plunger  13 . 12 - 908  is also a stop, where compression of the plunger  13 . 12 - 908  is mechanically stopped due to the body of the plunger  13 . 12 - 908  abutting a bottom of the housing  13 . 12 - 912 . 
       FIGS.  13 . 12 - 9 C  illustrates the maxilla connector  13 . 12 - 902  in a nominal (e.g., uncompressed) position.  FIGS.  13 . 12 - 9 D  shows the maxilla connector  13 . 12 - 902  in a compressed position. 
       FIGS.  13 . 12 - 9 E- 13 . 12 - 9 F  show a positional relationship of the maxilla connector  13 . 12 - 902  relative to the zygoma connector  13 . 12 - 904  at depth positions  13 . 12 - 914 - 918  of the maxilla connector  13 . 12 - 902 . The maxilla connector  13 . 12 - 902  is positioned in a perpendicular fashion relative to the zygoma connector  13 . 12 - 904 . In particular, the maxilla connector  13 . 12 - 902  is positioned perpendicular to the zygoma connector  13 . 12 - 904  at a median (i.e., middle) position, namely the depth position  13 . 12 - 916 . By aligning the piston direction of the maxilla connector  13 . 12 - 902  to be perpendicular to the depth position  13 . 12 - 916 , an amount of horizontal displacement can be minimized or reduced. In turn, the risk of binding the maxilla connector  13 . 12 - 902  due to horizontal displacement is also minimized or reduced. 
     Specifically, a displacement distance  13 . 12 - 920  in  FIGS.  13 . 12 - 9 F  corresponds to the piston alignment with the depth position  13 . 12 - 916 . In addition, a displacement distance  13 . 12 - 922  corresponds to an alternative piston alignment (i.e., with the depth position  13 . 12 - 914 ). Comparatively, the displacement distance  13 . 12 - 922  is far greater than the displacement distance  13 . 12 - 920 . In certain implementations, the displacement distance  13 . 12 - 920  is (for example) about 0.2 mm, and the displacement distance  13 . 12 - 922  is (for example) about 0.8 mm (e.g., for a 0 to 15 degree rotation of the zygoma connector  13 . 12 - 904  and a radial distance of about 24 mm). Myriad other positional relationships of the maxilla connector  13 . 12 - 902  relative to the zygoma connector  13 . 12 - 904  are herein contemplated. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 12 - 9 A- 13 . 12 - 9 F  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 12 - 9 A- 13 . 12 - 9 F . 
       FIGS.  13 . 12 - 10    illustrates a cutaway view of an example connector  13 . 12 - 1000  in accordance with one or more examples of the present disclosure. As shown, the connector  13 . 12 - 1000  includes a spring element  13 . 12 - 1002 . The spring element  13 . 12 - 1002  can bias internal portions of the connector  13 . 12 - 1000  (e.g., to facilitate more or less travel or compression of the connector  13 . 12 - 1000 ). 
     Additionally or alternatively to the spring element  13 . 12 - 1002  (or other suitable internal biasing components), the connector  13 . 12 - 1000  can include external biasing component(s). For example, a textile  13 . 12 - 1004  forming an exterior surface of the connector  13 . 12 - 1000  can also bias the connector  13 . 12 - 1000 . Indeed, the textile  13 . 12 - 1004  can be stretched or otherwise positioned over the connector  13 . 12 - 1000  to bias internal components of the connector  13 . 12 - 1000 . In these or other examples, the textile  13 . 12 - 1004  can include silicone, stretchy fabric (e.g., spandex, polyester, etc.), or other suitable materials. 
     It will be appreciated that the connector  13 . 12 - 1000  can be implemented in a variety of ways. In some examples, the connector  13 . 12 - 1000  includes a floating connection that does not integrally secure the display  13 . 12 - 102  and the facial interface  13 . 12 - 104 . Rather, the connector  13 . 12 - 1000  can make contact with (and can compress against) the display  13 . 12 - 102  or the facial interface  13 . 12 - 104 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 12 - 10    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 12 - 10   . 
     As discussed above, the connectors of the present disclosure can include a variety of different materials. In accordance with one or more examples of the present disclosure,  FIGS.  13 . 12 - 11    illustrates an example connector  13 . 12 - 1100  with an elastomer portion  13 . 12 - 1102  adjoining the display  13 . 12 - 102  and the facial interface  13 . 12 - 104 . In some examples, the elastomer portion  13 . 12 - 1102  can include a thickness of a predetermined amount (e.g., based on a force-displacement profile). Further, in some examples, the elastomer portion  13 . 12 - 1102  can vary in thickness (e.g., in a tapered fashion). In these or other examples, the elastomer portion  13 . 12 - 1102  can include various different types of elastomers, such as cis-polyisoprene (natural rubber), cis-polybutadiene (butadiene rubber), styrene-butadiene rubber, and ethylene-propylene monomer. Additionally or alternatively, the elastomer portion  13 . 12 - 1102  can include a hybrid elastomer material (e.g., part elastomer and part non-elastomer). 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 12 - 11    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 12 - 11   . 
       FIGS.  13 . 12 - 12    illustrates another example connector  13 . 12 - 1200  in accordance with one or more examples of the present disclosure. As shown, the connector  13 . 12 - 1200  includes a ball joint  13 . 12 - 1202  and a leaf spring  13 . 12 - 1204 . In this manner, the connector  13 . 12 - 1200  can allow at least some pivoting or twisting about the ball joint  13 . 12 - 1202 . In addition, the connector  13 . 12 - 1200  can allow tensioned (and tunable) depth adjustment via the leaf spring  13 . 12 - 1204 . Indeed, the leaf spring  13 . 12 - 1204  can include a variety of different lengths, thicknesses, etc. to provide the desired stiffness (or flexibility) of connection between the display  13 . 12 - 102  and the facial interface  13 . 12 - 104 . As with other connectors, the leaf spring  13 . 12 - 1204  can also be formed of various materials, such as metal, plastic, or composite material. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 12 - 12   , can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 12 - 12   . 
     13.13: Adjustment Mechanism 
     The following disclosure relates to adjustment mechanisms of a head-mountable device (e.g., for AR/VR experiences). More particularly, the present embodiments relate to adjustment mechanisms of a head-mountable device for altering angle and/or distance of a head-mountable device relative to the user head (or eyes). These adjustment mechanisms include depth (e.g., translation) adjustments and/or angular (e.g., rotation) adjustments. 
     In one example, a head-mountable device includes a connection between a display and a facial interface. The connection allows the display to be translatable or rotatable relative to the facial interface. The translatable or rotatable movements allow the head-mountable device to achieve positions and combinations of positions that otherwise could not be achieved. 
     Conventional head-mountable devices do not include connections between the display and facial interfaces that allow translatable or rotatable movements. Therefore, conventional head-mountable devices do not allow the movement required to adjust depth for eye relief or angular adjustments to optimize viewing. Thus, conventional head-mountable devices cannot accommodate for different facial profiles, viewing preferences, eyesight capacities, etc. 
     By contrast, a head-mountable device of the present disclosure provides connections allowing translatable or rotatable motion relative to a facial interface. Such a head-mountable device has many advantages over a conventional head-mountable device. A head-mountable device with connections allowing movement of a display relative to the user&#39;s face allows a user adjust the depth and angle of the display screen as needed to accommodate their personal preferences or requirements. Additionally, a head mountable display with connections allows motion of the display relative to different users&#39; faces, thereby improving a shareability of head-mountable device over conventional HMDs. 
     As disclosed herein, controlling the movement (e.g., linear translation or angular tilt) of a head-mounted display entails user friendly and simple actuation controls (e.g., lever, button, dial, rocker, slider, toggle, etc.) that are intuitive and easy to manipulate. Several examples of these devices are discussed below. 
     In one example, a head-mountable device includes a display, a strap connected to the display (e.g., the display frame), a facial interface, which is customizable to a user&#39;s facial profile, and a connection. The connection can be between the display and the facial interface such that the display is translatable and/or rotatable relative to the facial interface via the connection. 
     In another example, a head-mountable device includes a display, a strap connected to the display (e.g., the display frame), a facial interface, which is customizable to a user&#39;s facial profile, and an actuator. The actuator can moveably constrain the display relative to the facial interfaces. 
     In yet another example, a head-mountable device includes a display, a facial interface, which is customizable to a user&#39;s facial profile, and an angular and linear adjustment connection between the display and facial interface. 
     Accordingly, the apparatus and systems described herein provide angular and linear adjustments for a display of a head-mountable device in a user-friendly way, creating an enhanced more customizable user experience. 
     These and other embodiments are discussed below with reference to  FIGS.  13 . 13 - 1 - 13 . 13 - 26 B . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these Figures is for explanatory purposes only and should not be construed as limiting. Furthermore, as used herein, a system, a method, an article, a component, a feature, or a sub-feature including at least one of a first option, a second option, or a third option should be understood as referring to a system, a method, an article, a component, a feature, or a sub-feature that can include one of each listed option (e.g., only one of the first option, only one of the second option, or only one of the third option), multiple of a single listed option (e.g., two or more of the first option), two options simultaneously (e.g., one of the first option and one of the second option), or combination thereof (e.g., two of the first option and one of the second option). 
       FIGS.  13 . 13 - 1    illustrates a top view profile of a head-mountable device  13 . 13 - 100  worn on a user head  13 . 13 - 101 . While the present systems and methods are described in the context of a head-mountable device  13 . 13 - 100 , the systems and methods can be used with any wearable apparatus, or any apparatus or system that can be physically attached to a user&#39;s body, but are particularly relevant to an electronic device worn on a user&#39;s head. The head-mountable device  13 . 13 - 100  can include a display  13 . 13 - 102  (e.g., one or more optical lenses or display screens in front of the eyes of the user). The display  13 . 13 - 102  can include a display for presenting augmented reality visualizations, a virtual reality visualization, or other suitable visualization. The display  13 . 13 - 102  can also include a display frame  13 . 13 - 105  that houses optical components of the display  13 . 13 - 102 . It will therefore be appreciated that when referencing the display  13 . 3 - 102  herein, the display frame  13 . 3 - 105  can be implemented as part of specific aspects or functionality for the display  13 . 3 - 102 . For example, connection(s)  13 . 3 - 106  (described further below) can connect the facial interface  13 . 3 - 104  to at least one of the display  13 . 3 - 102  (e.g., optical elements of the display  13 . 3 - 102 ) or structural elements of the display  13 . 3 - 102  (e.g., the display frame  13 . 3 - 105 ). 
     The head-mountable device  13 . 13 - 100  also includes a facial interface  13 . 13 - 104  and a connection(s)  13 . 13 - 106  between the display  13 . 13 - 102  (e.g., the display frame  13 . 13 - 105 ) and the facial interface  13 . 13 - 104 . As used herein, the term “facial interface” refers to a portion of the head-mountable device  13 . 13 - 100  that engages a user face via direct contact. In particular, a facial interface includes portions of the head-mountable device  13 . 13 - 100  that conform to (e.g., compress against) regions of a user face. To illustrate, a facial interface can include a pliant (or semi-pliant) face track that spans the forehead, wraps around the eyes, contacts the zygoma and maxilla regions of the face, and bridges the nose. In addition, a facial interface can include various components forming a structure, webbing, cover, fabric, or frame of a head-mountable device disposed between the display  13 . 13 - 102  and the user skin. In particular implementations, a facial interface can include a seal (e.g., a light seal, environment seal, dust seal, air seal, etc.). It will be appreciated that the term “seal” can include partial seals or inhibitors, in addition to complete seals (e.g., a partial light seal where some ambient light is blocked and a complete light seal where all ambient light is blocked when the head-mountable device is donned). 
     In addition, the term “connection” refers to a joining between the display  13 . 13 - 102  (e.g., the display frame  13 . 13 - 105 ) and the facial interface  13 . 13 - 104 . In some examples, a connection allows the display  13 . 13 - 102  to translate or rotate relative to the facial interface  13 . 13 - 104 . In other examples, a connection allows the display  13 . 13 - 102  to both translate and rotate relative to the facial interface  13 . 13 - 104 . For instance, the connection(s)  13 . 13 - 106 , when adjusted, can slide (e.g., translate) the display  13 . 13 - 102  toward or away from the facial interface  13 . 13 - 104  (e.g., in a linear fashion). In another example, the connection(s)  13 . 13 - 106 , when adjusted, can rotate the display  13 . 13 - 102  up or down relative to the facial interface  13 . 13 - 104  (e.g., in an angular fashion). Thus a connection can slidably join or rotatably join the display  13 . 13 - 102  and the facial interface  13 . 13 - 104 . 
     In this manner, the connection(s)  13 . 13 - 106  can movably constrain the display  13 . 13 - 102  relative to the facial interface  13 . 13 - 104 . As used herein, the term “movably constrain” or “movably constraining” refers to the type of connection that can dynamically move (e.g., translate or rotate), yet retain control over a particular element&#39;s movement or position. For example, to “movably constrain” means the connection(s)  13 . 13 - 106  can bound movement of the display  13 . 13 - 102  within two degrees of freedom (e.g., along a horizontal plane and along an additional plane non-planar with the horizontal plane) relative to the facial interface  13 . 13 - 104 . 
     The connection(s)  13 . 13 - 106  can include one or more components (e.g., actuators, sliders, gears, levers, mechanical advantage devices, posts, mechanical stops, dampeners, etc.) that can allow (or actively provide) translation or rotation of the display  13 . 13 - 102  relative to the facial interface  13 . 13 - 104 . The connection(s)  13 . 13 - 106  can have an adjustable resolution that is a continuous resolution or a non-continuous resolution, and capable of transitioning between states (e.g., between a first display position and a second display position). The connection(s)  13 . 13 - 106  can include a manual actuator control or an automated actuator control that is operable from a single interface location (or multiple interface locations). 
     In certain implementations, the connection(s)  13 . 13 - 106  are positioned between inner and outer surfaces of the head-mountable device  13 . 13 - 100 . As used herein, an “outer surface” refers to an exterior surface of the head-mountable device  13 . 13 - 100  that outwardly faces the ambient environment. By contrast, as used herein, an “inner surface” refers to an exterior surface of the head-mountable device  13 . 13 - 100  that is oriented to face towards (or contact) a human face or skin. 
     Furthermore, the connection  13 . 13 - 106  can include a variety of different adjustment mechanisms, members, or movement mechanisms or movable connectors (e.g., bearings/bushings, dovetail slides, scissor mechanisms, sarrus linkages, gears, movable arms, etc.), cables (e.g., tensioned cables), actuators (e.g., mechanical, electromechanical, hydraulic, pneumatic, piezoelectric, etc.), and locking mechanisms (toggles, latches, ratchets, clamps, friction brakes, toothed brakes, hydraulic brakes, pneumatic bladders, non-back-drivable mechanisms, etc.), and other components. Locations of the connection  13 . 13 - 106  can be modified or tuned, as can the number of connections. For instance, the location and/or the number of the connection(s)  13 . 13 - 106  can correlate to an amount of force or pressure exerted on the user at any one datum (e.g., forehead region, maxilla region, zygoma region, etc.). In other instances, the location and/or the number of the connection(s)  13 . 13 - 106  can correspond to rigidity (or rigidity variances) between the connection(s)  13 . 13 - 106 . 
     As used herein, the term “forehead region” refers to an area of a human face between the eyes and the scalp of a human. Additionally, the term “maxilla region” refers to an area of a human face corresponding to the zygomatic bone structure of a human. Similarly, the term “maxilla region” refers to an area of a human face corresponding to the maxilla bone structure of a human. 
     Additionally shown in  FIGS.  13 . 13 - 1   , the head-mountable device  13 . 13 - 100  includes one or more arms  13 . 13 - 108 ,  13 . 13 - 110 . The arms  13 . 13 - 108 ,  13 . 13 - 110  are connected to the display  13 . 13 - 102  and extend distally toward the rear of the head. The arms  13 . 13 - 108 ,  13 . 13 - 110  are configured to secure the display in a position relative to the user head  13 . 13 - 101  (e.g., such that the display  13 . 13 - 102  is maintained in front of a user&#39;s eyes). For example, the arms  13 . 13 - 108 ,  13 . 13 - 110  extend over the user&#39;s ears  13 . 13 - 112 . In certain examples, the arms  13 . 13 - 108 ,  13 . 13 - 110  rest on the user&#39;s ears  13 . 13 - 112  to secure the head-mountable device  13 . 13 - 100  via friction between the arms  13 . 13 - 108 ,  13 . 13 - 110  and the user head  13 . 13 - 101 . For example, the arms  13 . 13 - 108 ,  13 . 13 - 110  can apply opposing pressures to the sides of the user head  13 . 13 - 101  to secure the head-mountable device  13 . 13 - 100  to the user head  13 . 13 - 101 . Optionally, the arms  13 . 13 - 108 ,  13 . 13 - 110  can be connected to each other via a strap  13 . 13 - 103  (shown in the dashed lines) that can compress the head-mountable device  13 . 13 - 100  against the user head  13 . 13 - 101 . In particular examples, the strap  13 . 13 - 103  is connected to at least the frame of the display  13 . 13 - 102  or the facial interface  13 . 13 - 104 . 
     In specific examples, the arms  13 . 13 - 108 ,  13 . 13 - 110  are connected to the display  13 . 13 - 102  (by way of the display frame  13 . 13 - 105 ) via a pivot joint  13 . 13 - 114 . The pivot joint  13 . 13 - 114  can be external or internal with respect to the arms  13 . 13 - 108 ,  13 . 13 - 110  and/or the display frame  13 . 13 - 105 . In these or other examples, the pivot joint  13 . 13 - 114  can allow the display frame  13 . 13 - 105  to rotate about the pivot joint  13 . 13 - 114  independent of the arms  13 . 13 - 108 ,  13 . 13 - 110 . To do so, the pivot joint  13 . 13 - 114  can include a detent hinge, a friction hinge, a passive hinge, a full lock-out hinge, etc. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 13 - 1    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 13 - 1   . 
       FIGS.  13 . 13 - 2 A- 13 . 13 - 2 B  respectively illustrate side and front view profiles of an example of the head-mountable device  13 . 13 - 100 . As discussed above, the head-mountable device  13 . 13 - 100  includes the display  13 . 13 - 102 , the facial interface  13 . 13 - 104 , and the connection(s)  13 . 13 - 106 . In particular, as shown in  FIGS.  13 . 13 - 2 A- 13 . 13 - 2 B , the facial interface  13 . 13 - 104  can indeed wrap around the eyes  13 . 13 - 201 , bridge the nose  13 . 13 - 202 , span the forehead  13 . 13 - 203 , and contact the zygoma and maxilla regions  13 . 13 - 205  of the face. 
     Additionally shown in  FIGS.  13 . 13 - 2 A- 13 . 13 - 2 B , the head-mountable device  13 . 13 - 100  includes sensors  13 . 13 - 204  which can be attached to (or embedded within) the facial interface  13 . 13 - 104 . As used herein, the term “sensor” refers to one or more different sensing devices, such as a camera or imaging device, temperature device, oxygen device, movement device, brain activity device, sweat gland activity device, breathing activity device, muscle contraction device, etc. some particular examples of sensors include an electrooculography sensor, electrocardiogramansor, EKG sensor, heart rate variability sensor, blood volume pulse sensor, SpO2 sensor, compact pressure sensor, electromyography sensor, core-body temperature sensor, galvanic skin sensor, accelerometer, gyroscope, magnetometer, inclinometer, barometer, infrared sensor, global positioning system sensor, etc. 
     Further below, this disclosure describes with more particularity how the connection(s)  13 . 13 - 106  as adjustment mechanisms can be positioned within the head-mountable device  13 . 13 - 100 . In addition, subsequent portions of this disclosure provide additional detail with respect to user interactions for controlling or actuating adjustment mechanisms. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 13 - 2 A- 13 . 13 - 2 B  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 13 - 2 A- 13 . 13 - 2 B . 
     The location and positioning of the connection(s)  13 . 13 - 106  can provide a stable yet flexible and adjustable interface between the facial interface  13 . 13 - 104  and the display  13 . 13 - 102 . In accordance with one or more examples of the present disclosure,  FIGS.  13 . 13 - 3 A- 13 . 13 - 3 D  illustrate a plurality of positions where the connection(s)  13 . 13 - 106  can be located between the display  13 . 13 - 102  and the facial interface  13 . 13 - 104 . Although  FIGS.  13 . 13 - 3 A- 13 . 13 - 3 D  illustrate particular locations, it will be appreciated that the locations of the connection(s)  13 . 13 - 106  can be modified within the scope of the present disclosure. 
     In one example,  FIGS.  13 . 13 - 3 A , shows the connection(s)  13 . 13 - 106  includes a single adjustment mechanism located at the forehead  13 . 13 - 203  of a user. The connection(s)  13 . 13 - 106  can be configured to provide a linear or angular connection between the display  13 . 13 - 102  and the facial interface  13 . 13 - 104 . In particular implementations, the connection(s)  13 . 13 - 106  provides only angular (tilt) adjustment by pivoting the display  13 . 13 - 102  relative to the facial interface  13 . 13 - 104 . 
     In at least some instances, a single-point system as shown in  FIGS.  13 . 13 - 3 A  can provide the simplest and most direct means of adjustment. For example, the connection(s)  13 . 13 - 106  in  FIGS.  13 . 13 - 3 A  can avoid potential angular misalignment or racking issues of multi-point systems. As another example, the connection(s)  13 . 13 - 106  in  FIGS.  13 . 13 - 3 A  can facilitate single-handed user adjustment of the head-mountable device  13 . 13 - 100 . 
       FIGS.  13 . 13 - 3 B  shows another example where the connection(s)  13 . 13 - 106  include two adjustment mechanisms oppositely positioned between the facial interface  13 . 13 - 104  and the display  13 . 13 - 102 . Specifically, the connection(s)  13 . 13 - 106  in  FIGS.  13 . 13 - 3 B  include a first adjustment mechanism on a left side and a second adjustment mechanism on a right side (e.g., near regions of the user eye). Similarly, in another example, the connection(s)  13 . 13 - 106  may be located near the zygoma or maxilla regions  13 . 13 - 205  of the face. The two connections can provide at least two points of angular or linear adjustment of the display  13 . 13 - 102  relative to the facial interface  13 . 13 - 104 . For example, both of the connection(s)  13 . 13 - 106  in  FIGS.  13 . 13 - 3 B  can be adjusted linearly together to provide a depth adjustment, or adjusted angularly together to provide a vertical (up/down) tilt adjustment. In another example, one of the connections  13 . 13 - 106  can be extended outward and the other of the connection(s)  13 . 13 - 106  can be pushed inward (so as to provide a side-to-side or horizontal tilt adjustment). 
       FIGS.  13 . 13 - 3 C  shows the connection(s)  13 . 13 - 106  include three adjustment mechanisms-one near the forehead  13 . 13 - 203  and two at bottom corner areas at the zygoma or maxilla regions. Additionally,  FIGS.  13 . 13 - 3 D  shows the connection(s)  13 . 13 - 106  include four connection(s)  13 . 13 - 106 —two at the top corners and two at the bottom corners. In at least some examples, multi-point systems like those shown in  FIGS.  13 . 13 - 3 B- 13 . 13 - 3 D  can provide improved load distribution and/or rigidity. Multi-point systems like those shown in  FIGS.  13 . 13 - 3 B- 13 . 13 - 3 D  can also satisfy larger load requirements, such as increased loads during a sudden impact event to the head-mountable device  13 . 13 - 100 . In the case of multi-point systems, synchronization mechanisms can be added to ensure that each of the connection(s)  13 . 13 - 106  move in lockstep. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 13 - 3 A- 13 . 13 - 3 D  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 13 - 3 A- 13 . 13 - 3 D . Moreover, although not shown, it will be appreciated that more than four of the connection(s)  13 . 13 - 106  are also contemplated, and the locations of any one the connection(s)  13 . 13 - 106  described can vary in position and location relative to the figures in which they were described. 
     As mentioned above, the connection(s)  13 . 13 - 106  can include adjustment mechanisms that allow (or provide) movement of the display  13 . 13 - 102  relative to the facial interface  13 . 13 - 104 . This movement can include motion in a single degree of freedom or multiple degrees of freedom. In accordance with one or more such examples,  FIGS.  13 . 13 - 4 A- 13 . 13 - 4 C  illustrate an example of depth adjustment via depth adjustment mechanisms  13 . 13 - 402  (e.g., linear adjustment connections). For example,  FIGS.  13 . 13 - 4 A  shows the display  13 . 13 - 102  positioned relative to the facial interface  13 . 13 - 104  in a home state. In the home state, depth adjustment mechanisms  13 . 13 - 402  can be positioned at a default position, a customized position, etc. in which the display  13 . 13 - 102  is a predetermined distance (i.e., eye relief distance) away from the facial interface  13 . 13 - 104 . In other instances, the home state can include a current positioning of the display  13 . 13 - 102  relative to the display  13 . 13 - 102 . 
       FIGS.  13 . 13 - 4 B  shows the depth adjustment mechanisms  13 . 13 - 402  in an extended state. In the extended state, the depth adjustment mechanisms  13 . 13 - 402  are extended or otherwise altered from the home state (or another state) to increase a relative distance between the display  13 . 13 - 102  and the facial interface  13 . 13 - 104 . 
       FIGS.  13 . 13 - 4 C  shows the depth adjustment mechanisms  13 . 13 - 402  in a retracted state. In the retracted state, the depth adjustment mechanisms  13 . 13 - 402  are retracted or otherwise altered from the home state (or another state) to decrease a relative distance between the display  13 . 13 - 102  and the facial interface  13 . 13 - 104 . 
     In some examples, the various states shown in  FIGS.  13 . 13 - 4 A- 13 . 13 - 4 C  can include discrete or predefined positions of the display  13 . 13 - 102  relative to the facial interface  13 . 13 - 104 —where the predefined positions are differentiated by set (or customizable) step-sizes. For instance, the depth adjustment mechanisms  13 . 13 - 402  can be utilized to vary the position of the display  13 . 13 - 102  relative to the facial interface  13 . 13 - 104  in positional steps from 0 mm eye relief distance in a fully retracted state, to 6 mm eye relief distance in a fully extended state. In other examples, however, the various states shown in  FIGS.  13 . 13 - 4 A- 13 . 13 - 4 C  can include non-discrete positions of the display  13 . 13 - 102  relative to the facial interface  13 . 13 - 104 —where the positions are configurable via continuous adjustment of the depth adjustment mechanisms  13 . 13 - 402 , due to a continuous interface that is gradually adjusting over a substantially smooth surface, providing a continuous adjustment resolution. 
     Adjustment of the depth adjustment mechanisms  13 . 13 - 402  can be performed in different ways. In some examples, user interactions to the depth adjustment mechanisms  13 . 13 - 402  (e.g., via one or more user controls) can actuate or move the depth adjustment mechanisms  13 . 13 - 402 . Additionally or alternatively, the depth adjustment mechanisms  13 . 13 - 402  can actuate in response to one or more sensors of the head-mountable device  13 . 13 - 100  detecting a voice command or gesture (e.g., eye gesture, hand gesture, touch, etc.). Still further, in some examples, the head-mountable device  13 . 13 - 100  can automatically adjust the depth adjustment mechanisms  13 . 13 - 402  (e.g., based on one or more sensors of the head-mountable device  13 . 13 - 100  detecting a user field of view indicating adjustment is needed). Thus, in certain implementations, the head-mountable device  13 . 13 - 100  includes a motor to auto-drive certain actuation. In this example, the motor can mechanically interface with the connections or adjustment mechanisms to automate or controllably drive the adjustment mechanisms. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 13 - 4 A- 13 . 13 - 4 C  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 13 - 4 A- 13 . 13 - 4 C . Moreover, although not shown, it will be appreciated that more than four of the connection(s)  13 . 13 - 106  are also contemplated, and the locations of any one the connection(s)  13 . 13 - 106  described can vary in position and location relative to the figures in which they were described. 
     The connection(s)  13 . 13 - 106  can also allow (or provide) angular or rotational adjustment of the display  13 . 13 - 102  relative to the facial interface  13 . 13 - 104 . In accordance with one or more such examples,  FIGS.  13 . 13 - 5 A- 13 . 13 - 5 C  illustrate an example of tilt adjustment via angular adjustment mechanisms  13 . 13 - 502  (e.g., angular adjustment connections). Via adjustment of the angular adjustment mechanisms  13 . 13 - 502 , the display  13 . 13 - 102  can pivot (e.g., rotate) the display  13 . 13 - 102  up or down by an angular displacement relative to the facial interface  13 . 13 - 104 , as illustrated in  FIGS.  13 . 13 - 5 A -C. 
     In particular,  FIGS.  13 . 13 - 5 A  shows the angular adjustment mechanisms  13 . 13 - 502  in a home state (e.g., with no angular tilt). It will be appreciated, however, that alternative home states with angular tilt are herein contemplated. 
       FIGS.  13 . 13 - 5 B  shows the angular adjustment mechanisms  13 . 13 - 502  in a negative angular state (e.g., downward angular tilt) relative to the home state. By contrast,  FIGS.  13 . 13 - 5 C  shows the angular adjustment mechanisms  13 . 13 - 502  in a positive angular state (e.g., upward angular tilt) relative to the home state. 
     As with the depth adjustment mechanisms  13 . 13 - 402  described above, the angular adjustment mechanisms  13 . 13 - 502  can also be adjusted in myriad different ways. For example, the angular adjustment mechanisms  13 . 13 - 502  can be adjusted manually (e.g., via user controls, voice commands, or user gestures). As another example, the head-mountable device  13 . 13 - 100  can automatically adjust the angular adjustment mechanisms  13 . 13 - 502  based on a detected user field of view. 
     In certain implementations, the angular state can have angular range of negative 7.5 degrees in a fully extended negative angular tilt state, to positive 7.5 degrees in a fully extended positive angular tilt state. The angular state can be discrete (e.g., with limited positional states) or continuous (e.g., unlimited positional states). 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 13 - 5 A- 13 . 13 - 5 C  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 13 - 5 A- 13 . 13 - 5 C . 
     In some cases, the connection(s)  13 . 13 - 106  can be statically adjusted, customized, or set. For instance, the connection(s)  13 . 13 - 106  may be adjusted upon initially donning the head-mountable device  13 . 13 - 100 , and thereafter the connection(s)  13 . 13 - 106  is set without the ability to externally modify the connection(s)  13 . 13 - 106 . That is, the connection(s)  13 . 13 - 106  nor a corresponding actuator control may be externally accessible when the apparatus is donned (e.g., without some disassembly of the head-mountable device  13 . 13 - 100 ). In accordance with one or more such examples,  FIGS.  13 . 13 - 6 A- 13 . 13 - 6 B  illustrate an example head-mountable device  13 . 13 - 600  with adjustment connections  13 . 13 - 602  between the display  13 . 13 - 102  and the facial interface  13 . 13 - 104 . In particular,  FIGS.  13 . 13 - 6 A  shows the head-mountable device assembly, while  FIGS.  13 . 13 - 6 B  provides a zoomed-in view of one of the adjustment connections  13 . 13 - 602 . 
     The adjustment connections  13 . 13 - 602  can connectively join the display  13 . 13 - 102  at a distance from the facial interface  13 . 13 - 104  (as similarly described above for the connection(s)  13 . 13 - 106 ). The adjustment connections  13 . 13 - 602  can be positioned in myriad different locations. As shown, however, the adjustment connections  13 . 13 - 602  are positioned in upper corners of the forehead region and at the zygoma regions. 
     Additionally, the adjustment connections  13 . 13 - 602  as shown include an adjustable pillar  13 . 13 - 604 . The adjustable pillar  13 . 13 - 604  is configured to slide into and out of an adjustment connection. Specifically, the adjustable pillar  13 . 13 - 604  moves relative to a base  13 . 13 - 603  of the adjustment connection. The adjustable pillar  13 . 13 - 604  is connected to the facial interface  13 . 13 - 104 , and the base  13 . 13 - 603  is connected to the display  13 . 13 - 102 . Alternatively, opposite configurations are herein contemplated (e.g., the adjustable pillar  13 . 13 - 604  connected to the display  13 . 13 - 102  and the base  13 . 13 - 603  connected to the facial interface  13 . 13 - 104 ). Thus, adjusting the adjustable pillar  13 . 13 - 604  (in or out of the base  13 . 13 - 603 ) causes a corresponding change in distance between the display  13 . 13 - 102  and the facial interface  13 . 13 - 104 . The adjustable pillar  13 . 13 - 604  further includes holes  13 . 13 - 606  configured to receive a set screw (or other fastener) by which the adjustable pillar  13 . 13 - 604  can be positionally affixed upon setting to the desired position. 
     The adjustable pillar  13 . 13 - 604  can include a variety of different specifications (e.g., according to desired load distribution). For example, in some instances, the adjustable pillars  13 . 13 - 604  are constructed to withstand the compressive force of the strap  13 . 13 - 103  (not shown) pulling the display  13 . 13 - 102  towards the facial interface  13 . 13 - 104 . In certain implementations, the adjustment connections  13 . 13 - 602  can distribute up to a 6 kN applied load (e.g., from the display  13 . 13 - 102  along the facial interface  13 . 13 - 104 ). 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 13 - 6 A- 13 . 13 - 6 B  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 13 - 6 A- 13 . 13 - 6 B . 
     In contrast to static adjustment, the following description for  FIGS.  13 . 13 - 7 A- 27    discusses various examples of dynamic adjustment (e.g., for different users, head sizes, head shapes, etc.)—where “dynamic” refers to on-the-fly or real time adjustment as may be desired for a given user at a given time. Moreover, as discussed above, the head-mountable device of the present disclosure can include single-point systems with a singular adjustment mechanism. In accordance with one or more examples of the present disclosure,  FIGS.  13 . 13 - 7 A- 13 . 13 - 7 B  show an example head-mountable device  13 . 13 - 700  including the display  13 . 13 - 102  and the facial interface  13 . 13 - 104 . In particular, the head-mountable device  13 . 13 - 700  includes an angular adjustment connection  13 . 13 - 702  that movably constrains the display  13 . 13 - 102  relative to the facial interface  13 . 13 - 104 . 
     The angular adjustment connection  13 . 13 - 702  specifically includes a linkage  13 . 13 - 704  attached to the display  13 . 13 - 102 . The linkage  13 . 13 - 704  includes notches  13 . 13 - 706  configured to receive a locking element  13 . 13 - 708 , where the locking element  13 . 13 - 708  is connected to the facial interface  13 . 13 - 104 . The notches  13 . 13 - 706  correspond to positions at which the locking element  13 . 13 - 708  can engage the linkage  13 . 13 - 704  for maintaining a position of the linkage  13 . 13 - 704 . 
     The display  13 . 13 - 102  can be moved (e.g., rotated) by sliding the linkage  13 . 13 - 704  relative to the locking element  13 . 13 - 708 . For example, the linkage  13 . 13 - 704  allows the display  13 . 13 - 102  to pivot relative to the facial interface  13 . 13 - 104  when the linkage  13 . 13 - 704  extends or the linkage  13 . 13 - 704  retracts relative to the locking element  13 . 13 - 708 . 
     In certain implementations, the linkage  13 . 13 - 704  automatically moves in response to direct manipulation of the display  13 . 13 - 102 . For example, the linkage  13 . 13 - 704  can extend or retract relative to the locking element  13 . 13 - 708  in response to user manipulation (e.g., a user hand tilting) of the display  13 . 13 - 102 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 13 - 7 A- 13 . 13 - 7 B  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 13 - 7 A- 13 . 13 - 7 B . 
     The head-mountable device of the present disclosure can also include a multi-point system where multiple adjustment mechanisms in combination can allow (or provide) multiple degrees of freedom (e.g., multiple directions of display mobility). In accordance with one or more such examples,  FIGS.  13 . 13 - 8    illustrates the display  13 . 13 - 102  and facial interface  13 . 13 - 104  connected by adjustable connections  13 . 13 - 802   a - 13 . 13 - 802   d . The adjustable connections  13 . 13 - 802   a - 13 . 13 - 802   d  are disposed between the display  13 . 13 - 102  and the facial interface  13 . 13 - 104 . Each of the adjustable connections  13 . 13 - 802  can be individually adjusted such that the display and be displaced linearly and/or angularly. 
     For example, the adjustable connections  13 . 13 - 802   a ,  13 . 13 - 802   b  can be adjusted so the top of the display is a first distance d 1  from the facial interface. The adjustable connections  13 . 13 - 802   c ,  13 . 13 - 802   d  can be adjusted differently than the adjustable connections  13 . 13 - 802   a ,  13 . 13 - 802   b  so that the bottom of the display is a second distance d 2  from the facial interface  13 . 13 - 104 . In downward tilting cases, the second distance d 2  can be less that the first distance d 1 . By contrast, for upward tilting cases, the second distance d 2  can be more than the first distance d 1 . In this way, the adjustable connections  13 . 13 - 802   a - 13 . 13 - 802   d  make the angular adjustment connection between the facial interface  13 . 13 - 104  and the display  13 . 13 - 102 , and the adjustable connections  13 . 13 - 802  make the linear adjustment connection between the facial interface  13 . 13 - 104  and the display  13 . 13 - 102 . 
     As another example, the adjustable connections  13 . 13 - 802   a - 13 . 13 - 802   d  can be adjusted in other ways. For instance, the adjustable connections  13 . 13 - 802   a - 13 . 13 - 802   d  can be adjusted in a same or similar fashion (e.g., all lengthened or retracted by a same distance) to linearly translate the display  13 . 13 - 102  towards or away from the facial interface  13 . 13 - 104 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 13 - 8    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 13 - 8   . 
     Where  FIGS.  13 . 13 - 6 - 8    depict example adjustment mechanisms without actuator controls (moving instead via direct user manipulation or setting of the head-mountable device), the following figures depict adjustment mechanisms with dynamic actuator controls. Indeed, as mentioned above, the connections (i.e., adjustment mechanisms or actuators) of the present disclosure can include adjustment controls configured for myriad different modes of user input and in various locations. In accordance with one or more such embodiments,  FIGS.  13 . 13 - 9 A- 13 . 13 - 24 B  show example adjustment mechanism variations of the head-mountable device  13 . 13 - 100 . In particular, the head-mountable device  13 . 13 - 100  includes the display  13 . 13 - 102 , the facial interface  13 . 13 - 104 , and the strap  13 . 13 - 103  (each described above). 
     In addition,  FIGS.  13 . 13 - 9 A- 13 . 13 - 24 B  respectively depict front and side-view profiles of head-mountable devices  13 . 13 - 900 - 13 . 13 - 2400  with corresponding connections  13 . 13 - 902 - 13 . 13 - 2402  (which are the same as or similar to the connection(s)  13 . 13 - 106  also described above). The connections  13 . 13 - 902 - 13 . 13 - 2402  movably constrain the display  13 . 13 - 102  relative to the facial interface  13 . 13 - 104 . The connections  13 . 13 - 902 - 13 . 13 - 2402  can therefore include myriad different forms of connections (e.g., mechanical connections) as indicated previously. Further, the connections  13 . 13 - 902 - 13 . 13 - 2402  can include myriad different forms of control mechanisms (e.g., actuator controls) that actuate the connections  13 . 13 - 902 - 13 . 13 - 2402  in response to user input. 
     In many examples, the head-mountable devices  13 . 13 - 900 - 13 . 13 - 2400  include actuator controls such as levers (e.g., side peel lever, vertical slide lever, etc.), buttons (push-button, side push button, top push button, automatic release button, etc.), dials (vertical scroll wheel dial, horizontal scroll wheel dial, top scroll wheel dial, etc.), rockers, sliders, or toggles. The actuator controls can be combined with multiple control mechanisms. For example, a sliding actuator controller can include a lever which can be user to lock a head-mountable device in position at the desired viewing state. In another example, a combined actuator control may include a rocker combined with a scroll wheel, such that the scroll wheel, when engaged by the rocker rotates, causing a change in distance between the display  13 . 13 - 102  and the facial interface  13 . 13 - 104 . It can be appreciated that a number of variations and examples are contemplated herein and the examples provided below serve to illustrate a few of the possible configurations. 
       FIGS.  13 . 13 - 9 A- 13 . 13 - 9 B  illustrate a head-mountable device  13 . 13 - 900  with a connection  13 . 13 - 902 . The connection  13 . 13 - 902  includes an actuator control  13 . 13 - 904 . The actuator control  13 . 13 - 904  is positioned on a side portion of the display  13 . 13 - 102  or the facial interface  13 . 13 - 104 . The actuator control  13 . 13 - 904  includes a side push button  13 . 13 - 906 . In response to actuation of the side push button  13 . 13 - 906  (e.g., an inward push, press, tap), the connection  13 . 13 - 902  can translate the display  13 . 13 - 102  to or from a first state  13 . 13 - 908 , a second state  13 . 13 - 910 , or a third state  13 . 13 - 912 . Although  FIGS.  13 . 13 - 9 A- 13 . 13 - 9 B  illustrate discrete states or positions (i.e., the first state  13 . 13 - 908 , the second state  13 . 13 - 910 , and the third state  13 . 13 - 912 ), it will be appreciated that head-mountable device  13 . 13 - 900  can include more or fewer positional states. In certain implementations, the head-mountable device  13 . 13 - 900  includes a continuous resolution of positional states (as opposed to discrete, predetermined positional states). 
     In one or more examples, the side push button  13 . 13 - 906  can be actuated in various ways. For instance, the side push button  13 . 13 - 906  can include a push-to-release button, a push-and-hold button, slide-push buttons, top-push buttons, etc.). The buttons can be located in various locations (e.g., top portion, side portion, bottom portion, etc.) of the head-mountable device  13 . 13 - 900 , either on the display  13 . 13 - 102  or on the facial interface  13 . 13 - 104 . 
       FIGS.  13 . 13 - 10 A- 13 . 13 - 10 B  illustrate a head-mountable device  13 . 13 - 1000  with a connection  13 . 13 - 1002 . The connection  13 . 13 - 1002  includes an actuator control  13 . 13 - 1004 . The actuator control  13 . 13 - 1004  is similar to the actuator control  13 . 13 - 904 . In particular, the actuator control  13 . 13 - 1004  includes a top push button positioned at a top portion of the display  13 . 13 - 102  or the facial interface  13 . 13 - 104  (e.g., at one or both top corners of the head-mountable device  13 . 13 - 1000 ). When the actuator control  13 . 13 - 1004  is pressed, the connection  13 . 13 - 1002  can translate the display  13 . 13 - 102  to or from the first state  13 . 13 - 908 , the second state  13 . 13 - 910 , or the third state  13 . 13 - 912 , as described above. 
     In another example, the head-mountable device  13 . 13 - 100  includes actuation components that have actuator controls that slide or twist (e.g., by levers, buttons, rotatable components, etc.) causing the display  13 . 13 - 102  to move relative to the facial interface  13 . 13 - 104 .  FIGS.  13 . 13 - 11 A- 13 . 13 - 14 B  illustrate such examples. 
     In particular,  FIGS.  13 . 13 - 11 A and  13 . 13 - 11 B  illustrate a head-mountable device  13 . 13 - 1100  with a connection  13 . 13 - 1102 . The connection  13 . 13 - 1102  includes an actuator control  13 . 13 - 1104 . The actuator control  13 . 13 - 1104  is positioned on a side portion of the display  13 . 13 - 102  or the facial interface  13 . 13 - 104 . The actuator control  13 . 13 - 1104  includes a side peel lever. The side peel lever of the actuator control  13 . 13 - 1104 , when pulled downward, releases a lock (e.g., a cam lock) allowing the connection  13 . 13 - 1102  to move the display  13 . 13 - 102  relative to the facial interface  13 . 13 - 104 . Additionally, the actuator control  13 . 13 - 1104 , when pushed upward into a locked position, causes the connection  13 . 13 - 1102  to lock the display  13 . 13 - 102  into position relative to the facial interface  13 . 13 - 104 . 
       FIGS.  13 . 13 - 12 A and  13 . 13 - 12 B  illustrate a head-mountable device  13 . 13 - 1200  with a connection  13 . 13 - 1202 . The connection  13 . 13 - 1202  includes an actuator control  13 . 13 - 1204 . The actuator control  13 . 13 - 1204  is also positioned on a side portion of the display  13 . 13 - 102  or the facial interface  13 . 13 - 104 . The actuator control  13 . 13 - 1204  includes a twist sleeve lever. Thus, the twist sleeve lever of the actuator control  13 . 13 - 1204  can rotate (e.g., outward in a clockwise direction), allowing the connection  13 . 13 - 1202  to move the display  13 . 13 - 102  relative to the facial interface  13 . 13 - 104 . Likewise, the actuator control  13 . 13 - 1204  can be rotated in an opposite direction (e.g., inward in a counterclockwise direction), allowing the connection  13 . 13 - 1202  to positionally fix the display  13 . 13 - 102  relative to the facial interface  13 . 13 - 104 . 
       FIGS.  13 . 13 - 13 A and  13 . 13 - 13 B  illustrate a head-mountable device  13 . 13 - 1300  with a connection  13 . 13 - 1302 . The connection  13 . 13 - 1302  includes an actuator control  13 . 13 - 1304 . The actuator control  13 . 13 - 1304  is similarly positioned on a side portion of the display  13 . 13 - 102  or the facial interface  13 . 13 - 104 . The actuator control  13 . 13 - 1304  includes a finger pinch slider. The finger pinch slider of the actuator control  13 . 13 - 1304  can slide frontwards and backwards allowing the connection  13 . 13 - 1302  to positionally move (or lock) the display  13 . 13 - 102  relative to the facial interface  13 . 13 - 104 . 
       FIGS.  13 . 13 - 14 A and  14 B  illustrate a head-mountable device  13 . 13 - 1400  with a connection  13 . 13 - 1402 . The connection  13 . 13 - 1402  includes an actuator control  13 . 13 - 1404 . The actuator control  13 . 13 - 1404  is positioned on a side portion of the display  13 . 13 - 102  or the facial interface  13 . 13 - 104 . The actuator control  13 . 13 - 1404  includes a sleeve slide. The sleeve slide of the actuator control  13 . 13 - 1404  can slide along the actuator control  13 . 13 - 904  allowing the connection  13 . 13 - 1402  to move the display  13 . 13 - 102  relative to the facial interface  13 . 13 - 104 . 
       FIGS.  13 . 13 - 15 A and  13 . 13 - 15 B  illustrate a head-mountable device  13 . 13 - 1500  with a connection  13 . 13 - 1502 . The connection  13 . 13 - 1502  includes an actuator control  13 . 13 - 1504 . The actuator control  13 . 13 - 1504  is positioned along an outer periphery of the head-mountable device  13 . 13 - 1500  (e.g., from top to bottom corners at a side portion of the display  13 . 13 - 102  or the facial interface  13 . 13 - 104 ). The actuator control  13 . 13 - 1504  includes a vertical slide that moves approximately up and down along the outer periphery of the head-mountable device  13 . 13 - 1500 . The vertical slide of the actuator control  13 . 13 - 1504  can be toggled between positions (e.g., the first state  13 . 13 - 908 , the second state  13 . 13 - 910 , and the third state  13 . 13 - 912 ). Additional detail of this embodiment is discussed further below in relation to  FIGS.  13 . 13 - 25 A- 13 . 13 - 25 D . 
       FIGS.  13 . 13 - 16 A and  13 . 13 - 16 B  illustrate a head-mountable device  13 . 13 - 1500  with a connection  13 . 13 - 1602 . The connection  13 . 13 - 1602  includes an actuator control  13 . 13 - 1604 . The actuator control  13 . 13 - 1604  is positioned on a side portion of the display  13 . 13 - 102  or the facial interface  13 . 13 - 104 . The actuator control  13 . 13 - 1604  includes a side temple squeeze button (e.g., spring button) that, when squeezed, allows the connection  13 . 13 - 1602  to contemporaneously move the display  13 . 13 - 102  relative to the facial interface  13 . 13 - 104 . The side temple squeeze button of the actuator control  13 . 13 - 1604  can automatically release when one of the first state  13 . 13 - 908 , the second state  13 . 13 - 910 , or the third state  13 . 13 - 912  is reached. The side temple squeeze button of the actuator control  13 . 13 - 1604  can have the button oriented upward (or in alternative cases, downward) relative to the head-mountable device  13 . 13 - 1600 . 
       FIGS.  13 . 13 - 17 A and  13 . 13 - 17 B  illustrate a head-mountable device  13 . 13 - 1700  with a connection  13 . 13 - 1702 . The connection  13 . 13 - 1702  includes an actuator control  13 . 13 - 1704 . The actuator control  13 . 13 - 1704  is positioned on a side portion of the display  13 . 13 - 102  or the facial interface  13 . 13 - 104 . The actuator control  13 . 13 - 1704  includes a scroll wheel rotatable about an axis  13 . 13 - 1706  proceeding inward and outward of the head-mountable device  13 . 13 - 1700 . In some examples, the scroll wheel of the actuator control  13 . 13 - 1704  rotates in response to tangential forces (e.g., a finger swipe from front to back or up and down). In other examples, the scroll wheel of the actuator control  13 . 13 - 1704  rotates in response to twisting fingers. The scroll wheel of the actuator control  13 . 13 - 1704 , when rotated about the axis  13 . 13 - 1706 , can cause the connection  13 . 13 - 1702  to correspondingly adjust the distance between the display  13 . 13 - 102  and the facial interface  13 . 13 - 104 . 
       FIGS.  13 . 13 - 18 A and  13 . 13 - 18 B  illustrate a head-mountable device  13 . 13 - 1800  with a connection  13 . 13 - 1802 . The connection  13 . 13 - 1802  includes an actuator control  13 . 13 - 1804 . The actuator control  13 . 13 - 1804  is positioned on a side portion of the display  13 . 13 - 102  or the facial interface  13 . 13 - 104 . The actuator control  13 . 13 - 1804  includes a scroll wheel rotatable about an axis  13 . 13 - 1806 . The actuator control  13 . 13 - 1804 , like the actuator control  13 . 13 - 1704 , can respond to tangential forces (such as a finger swipe up or down). The scroll wheel of the actuator control  13 . 13 - 1804 , when rotated about the axis  13 . 13 - 1806 , can cause the connection  13 . 13 - 1802  to correspondingly adjust the distance between the display  13 . 13 - 102  relative to the facial interface  13 . 13 - 104 . 
       FIGS.  13 . 13 - 19 A and  13 . 13 - 19 B  illustrate a head-mountable device  13 . 13 - 1900  with a connection  13 . 13 - 1902 . The connection  13 . 13 - 1902  includes an actuator control  13 . 13 - 1904 . The actuator control  13 . 13 - 1904  is positioned on a side portion of the display  13 . 13 - 102  or the facial interface  13 . 13 - 104 . The actuator control  13 . 13 - 1904  is similar to the actuator control  13 . 13 - 1804 . In particular, the actuator control  13 . 13 - 1904  includes a scroll wheel rotatable about an axis  13 . 13 - 1906 . The scroll wheel of the actuator control  13 . 13 - 1904  can therefore respond to tangential forces (e.g., a finger swipe front to back, and vice-versa). In response to this user interaction, the connection  13 . 13 - 1902  can move the display  13 . 13 - 102  relative to the facial interface  13 . 13 - 104 . 
       FIGS.  13 . 13 - 20 A and  13 . 13 - 13 . 13 - 20 B  illustrate a head-mountable device  13 . 13 - 2000  with a connection  13 . 13 - 2002 . The connection  13 . 13 - 2002  includes an actuator control  13 . 13 - 2004 . The actuator control  13 . 13 - 2004  includes a slider (e.g., toggle) positioned on the top region of the display  13 . 13 - 102  or the facial interface  13 . 13 - 104  (e.g., along a forehead region). The actuator control  13 . 13 - 904  moves laterally (e.g., inward and outward) to correspondingly cause the connection  13 . 13 - 2002  to adjust the distance between the display  13 . 13 - 102  relative to the facial interface  13 . 13 - 104  (e.g., frontwards or backwards). 
       FIGS.  13 . 13 - 21 A and  13 . 13 - 13 . 13 - 21 B  illustrate a head-mountable device  13 . 13 - 2100  with a connection  13 . 13 - 2102 . The connection  13 . 13 - 2102  includes an actuator control  13 . 13 - 2104 . The actuator control  13 . 13 - 2104  includes a rocker positioned on the top region of the display  13 . 13 - 102  or the facial interface  13 . 13 - 104 . The actuator control  13 . 13 - 2104  moves up or down about a pivot  13 . 13 - 2106 . In response to up/down actuation of the actuator control  13 . 13 - 2104 , the connection  13 . 13 - 2102  adjusts the distance between the display  13 . 13 - 102  relative to the facial interface  13 . 13 - 104 . 
       FIGS.  13 . 13 - 22 A- 13 . 13 - 24 B  illustrate an actuator control that includes a scroll wheel disposed on top of a head-mountable device. The scroll wheel can be rotated around different axes to adjust the distance between the display  13 . 13 - 102  relative to the facial interface  13 . 13 - 104 . Further detail is provided below. 
     In particular,  FIGS.  13 . 13 - 22 A and  13 . 13 - 13 . 13 - 22 B  illustrate a head-mountable device  13 . 13 - 2200  with a connection  13 . 13 - 2202 . The connection  13 . 13 - 2202  includes the actuator control  13 . 13 - 1804  described above. Here, however, the actuator control  13 . 13 - 1804  is positioned on a top portion of the display  13 . 13 - 102  or the facial interface  13 . 13 - 104 . Indeed, the actuator control  13 . 13 - 1804  includes a scroll wheel rotatable about an axis  13 . 13 - 2204 . The scroll wheel of the actuator control  13 . 13 - 1804 , when rotated up or down about the axis  13 . 13 - 2204 , causes the connection  13 . 13 - 2202  to adjust the distance between the display  13 . 13 - 102  relative to the facial interface  13 . 13 - 104 . 
       FIGS.  13 . 13 - 23 A and  13 . 13 - 13 . 13 - 23 B  illustrate a head-mountable device  13 . 13 - 2300  with a connection  13 . 13 - 2302 . The connection  13 . 13 - 2302  also includes the actuator control  13 . 13 - 1804  described above. Differently though, the actuator control  13 . 13 - 1804  is positioned on a top portion of the display  13 . 13 - 102  or the facial interface  13 . 13 - 104 . The actuator control  13 . 13 - 1804  also includes a scroll wheel rotatable about an axis  13 . 13 - 2304 . The scroll wheel of the actuator control  13 . 13 - 1804 , when rotated up or down about the axis  13 . 13 - 2304 , causes the connection  13 . 13 - 2302  to adjust the distance between the display  13 . 13 - 102  relative to the facial interface  13 . 13 - 104 . 
       FIGS.  13 . 13 - 24 A and  13 . 13 - 13 . 13 - 24 B  illustrate a head-mountable device  13 . 13 - 2400  with a connection  13 . 13 - 2402 . The connection  13 . 13 - 2402  includes the actuator control  13 . 13 - 1704  described above, albeit differently located in  FIGS.  13 . 13 - 24 A- 13 . 13 - 24 B . In particular, the actuator control  13 . 13 - 1704  is positioned on a top portion of the display  13 . 13 - 102  or the facial interface  13 . 13 - 104 . The actuator control  13 . 13 - 1704  includes a scroll wheel rotatable about an axis  13 . 13 - 2404 . The scroll wheel of the actuator control  13 . 13 - 1704 , when rotated about the axis  13 . 13 - 2404 , causes the connection  13 . 13 - 2402  to adjust the distance between the display  13 . 13 - 102  relative to the facial interface  13 . 13 - 104 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 13 - 9 A- 13 . 13 - 24 B  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 13 - 9 A- 13 . 13 - 24 B . 
     As discussed above, a head-mountable device of the present disclosure can include connections (e.g., adjustment mechanisms) that can toggle from position to position via a sliding actuation. In accordance with one or more such examples,  FIGS.  13 . 13 - 25 A  shows a perspective view of the head-mountable device  13 . 13 - 1500  shown in  FIGS.  13 . 13 - 15 A and  13 . 13 - 15 B . The head-mountable device  13 . 13 - 1500  includes connections  13 . 13 - 2502   a ,  13 . 13 - 2502   b  having an actuator control  13 . 13 - 1504 . The actuator control  13 . 13 - 1504  includes a handle or lever that juts out from the head-mountable device  13 . 13 - 1500  for convenient user manipulation of the connections  13 . 13 - 2502   a ,  13 . 13 - 2502   b  (e.g., a sliding motion along the periphery of the head-mountable device  13 . 13 - 1500 ). 
     As shown, user interaction with the actuator control  13 . 13 - 1504  can move the connections  13 . 13 - 2502   a ,  13 . 13 - 2502   b  in a manner that toggles the connections  13 . 13 - 2502   a ,  13 . 13 - 2502   b  between three discrete states (e.g., positions) including the first state  13 . 13 - 908 , the second state  13 . 13 - 910 , and the third state  13 . 13 - 912 . The connections  13 . 13 - 2502   a ,  13 . 13 - 2502   b  are independently movable. In other implementations, however, the connections  13 . 13 - 2502   a ,  13 . 13 - 2502   b  are movably connected to each other. It will be appreciated that the connections  13 . 13 - 2502   a ,  13 . 13 - 2502   b  can move within the head-mountable device  13 . 13 - 1500  in myriad different ways. In particular examples, the connections  13 . 13 - 2502   a ,  13 . 13 - 2502   b  follow a track having a constant radius. 
     As the connections  13 . 13 - 2502   a ,  13 . 13 - 2502   b  move, the display  13 . 13 - 102  correspondingly moves relative to the facial interface  13 . 13 - 104 . The particular movement of the connections  13 . 13 - 2502   a ,  13 . 13 - 2502   b  causing the head-mountable device adjustment is described below in relation to  FIGS.  13 . 13 - 25 B- 13 . 13 - 25 D  (depicting the connection  13 . 13 - 2502   a  in greater detail). In particular,  FIGS.  13 . 13 - 25 B  illustrates the connection  13 . 13 - 2502   a  including a stepped adjustment portion  13 . 13 - 2503  in a first state. In the first state, the facial interface  13 . 13 - 104  and the display  13 . 13 - 102  are positioned a distance  13 . 13 - 2504   a  apart (e.g., about zero mm). 
     By contrast,  FIGS.  13 . 13 - 25 C  illustrates the connection  13 . 13 - 2502   a  moving left as indicated in the provided arrow. In doing so, the stepped adjustment portion  13 . 13 - 2503  correspondingly moves in a stepwise fashion, thereby creating increased separation between the display  13 . 13 - 102  and the facial interface  13 . 13 - 104  of a distance  13 . 13 - 2504   b  (e.g., about 3 mm) to a second positional state. Still further,  FIGS.  13 . 13 - 1 - 13 . 13 - 25 D  illustrates the connection  13 . 13 - 2502   a  moving further left as indicated in the provided arrow. With this additional movement, the stepped adjustment portion  13 . 13 - 2503  moves another step to create more separation between the display  13 . 13 - 102  and the facial interface  13 . 13 - 104  of a distance  13 . 13 - 2504   c  (e.g., about 6 mm) to a third positional state. Thus, the geometry or structure of the stepped adjustment portion  13 . 13 - 2503  can provide a desired stepwise resolution between positional states by causing the display  13 . 13 - 102  to move away from (or towards) the facial interface  13 . 13 - 104 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 13 - 25 A- 13 . 13 - 25 D  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 13 - 25 A- 13 . 13 - 25 D . 
     As briefly mentioned above, multiple connections of a head-mountable device can be linked together as a single connection. The links between connections of a head-mountable device form a timing mechanism that causes the connections to move together uniformly. In accordance with one or more such examples,  FIGS.  13 . 13 - 26 - 27    illustrate respective perspective views of assemblies  13 . 13 - 2600 - 2700  for head-mountable devices. 
     In particular,  FIGS.  13 . 13 - 26    depicts the assembly  13 . 13 - 2600  for a head-mountable device with a connection  13 . 13 - 2602  that is movably adjustable via an actuator control  13 . 13 - 2604 . The actuator control  13 . 13 - 2604  is positioned along a forehead region, but oriented downward (albeit myriad other orientations are herein contemplated). In response to actuation of the actuator control  13 . 13 - 2604 , stepped adjustment portions  13 . 13 - 2606  each move in unison in a stepwise fashion (as similarly described above for the stepped adjustment portion  13 . 13 - 2503  in relation to  FIGS.  13 . 13 - 25 B- 13 . 13 - 25 D ). Linkage arms  13 . 13 - 2608  movably connect portions of the connection  13 . 13 - 2602  to provide such a timing mechanism that transfers motion in a coordinated manner. While  FIGS.  13 . 13 - 26    illustrates stepped adjustment portions  13 . 13 - 2606 , the adjustment portions can have additional steps or even a gradual incline surface that provides a finer adjustment capability. 
     In contrast to stepped adjustment portions,  FIGS.  13 . 13 - 27    depicts the assembly  13 . 13 - 2700  for a head-mountable device with a connection  13 . 13 - 2702  with toggle links. In particular, the connection  13 . 13 - 2702  is movably adjustable via the actuator control  13 . 13 - 2604 . In response to actuation of the actuator control  13 . 13 - 2604 , toggle links  13 . 13 - 2704  toggle between stowed and locked positions. When the toggle links  13 . 13 - 2704  move in unison, the display  13 . 13 - 102  correspondingly moves relative to the facial interface  13 . 13 - 104 . Moreover, in this configuration, the connection  13 . 13 - 2702  moves in binary fashion between a first positional state and a second positional state. The linkage arms  13 . 13 - 2608  in  FIGS.  13 . 13 - 27    also movably connect portions of the connection  13 . 13 - 2702  to provide such a timing mechanism that transfers motion in a coordinated manner. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 13 - 26 - 27    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 13 - 26 - 27   . The following figure descriptions for  FIGS.  13 . 13 - 28 - 13 . 13 - 33    relate to example head-mountable devices, wearable apparatuses, or wearable electronic devices that can implement two degrees of freedom-namely angular tilt and linear translation toward and away from the user&#39;s face. In these examples, a linear slide can be implemented with certain structure to control actuation of the linear slide (whether in response to user adjustment or an applied force). 
       FIGS.  13 . 13 - 28 - 13 . 13 - 30    illustrate top, front, and side views, respectively, of a head-mountable device  13 . 13 - 2800  in accordance with one or more examples of the present disclosure. As shown, the head-mountable device  13 . 13 - 2800  can include a display frame  13 . 13 - 2802 , a facial interface  13 . 13 - 2804 , and connections  13 . 13 - 2806   a - 13 . 13 - 2806   d . The display frame  13 . 13 - 2802  and associated display can be the same as, or similar to, the display frame  13 . 13 - 105  (and associated display  13 . 13 - 102 ) discussed above. The facial interface  13 . 13 - 2804  can be the same as, or similar to, the facial interface  13 . 13 - 104  discussed above. Likewise, the connections  13 . 13 - 2806   a - 13 . 13 - 2806   d  can be the same as or similar to the connection(s)  13 . 13 - 106  discussed above. 
     In particular, however, the head-mountable device  13 . 13 - 2800  can include actuators  13 . 13 - 2808 ,  13 . 13 - 2810 . The actuators  13 . 13 - 2808 ,  13 . 13 - 2810  can include a variety of different actuators. In particular examples, the actuators  13 . 13 - 2808 ,  13 . 13 - 2810  include linear actuators of a linear adjustment system (e.g., that facilitate translational movement of the facial interface  13 . 13 - 2804  toward or away from the display frame  13 . 13 - 2802 ). For instance, the actuators  13 . 13 - 2808 ,  13 . 13 - 2810  can include linear slides and/or linear springs that can translate in and out to vary a distance (e.g., a depth distance, eye relief, etc.) between the facial interface  13 . 13 - 2804  and the display frame  13 . 13 - 2802 . In particular, the actuators  13 . 13 - 2808  correspond to forehead actuators or top actuators, and the actuators  13 . 13 - 2810  correspond to zygoma actuators or side actuators. It will be appreciated that additional or alternative configurations of the actuators  13 . 13 - 2808 ,  13 . 13 - 2810  are herein contemplated. For instance, actuators can be positioned in the cheek (maxilla region) along a bottom portion of the head-mountable device  13 . 13 - 2800 . Additional details regarding the actuators  13 . 13 - 2808 ,  13 . 13 - 2810  are described below in relation to  FIGS.  13 . 13 - 31 - 13 . 13 - 33   . 
     In some examples, the actuators  13 . 13 - 2808 ,  13 . 13 - 2810  are controlled or manipulated via at least one of actuator controls  13 . 13 - 2812   a ,  13 . 13 - 2812   b . In these or other examples, the actuator controls  13 . 13 - 2812   a ,  13 . 13 - 2812   b  can include a variety of different types of controls, such as a lever, button, dial, rocker, slider, or toggle (as described above). The actuator controls  13 . 13 - 2812   a ,  13 . 13 - 2812   b  can be engaged via manual manipulation and/or via hands-free or automated methods (also mentioned above). The actuator controls  13 . 13 - 2812   a ,  13 . 13 - 2812   b  can be positioned in various locations on or within the head-mountable device  13 . 13 - 2800 . In particular examples, the actuator controls  13 . 13 - 2812   a ,  13 . 13 - 2812   b  can be positioned adjacent to a maxilla region of a human face when the head-mountable device  13 . 13 - 2800  is donned. Other example positions for the actuator controls  13 . 13 - 2812   a ,  13 . 13 - 2812   b  can include the zygoma region, forehead region, on the arms (not shown), or within an interior portion of the optical space adjacent the display. 
     In these or other examples, the actuator controls  13 . 13 - 2812   a ,  13 . 13 - 2812   b  can control or manipulate the actuators in a variety of ways. In some examples, the actuator controls  13 . 13 - 2812   a ,  13 . 13 - 2812   b  are connected to flexible drive shafts  13 . 13 - 2814 ,  13 . 13 - 2816 . In particular examples, the flexible drive shafts  13 . 13 - 2814 ,  13 . 13 - 2816  can transfer energy or movement from engagement (e.g., depression) of at least one of the actuator controls  13 . 13 - 2812   a ,  13 . 13 - 2812   b  to the actuators  13 . 13 - 2808 ,  13 . 13 - 2810  (or associated locks, as described below). Examples of the flexible drive shafts  13 . 13 - 2814 ,  13 . 13 - 2816  can include cables, tensioned wires, rods, dowels, etc. In some examples, the flexible drive shafts  13 . 13 - 2814 ,  13 . 13 - 2816  are positioned inside the display frame  13 . 13 - 2802  (hidden from exterior viewing). In other examples, the flexible drive shafts  13 . 13 - 2814 ,  13 . 13 - 2816  are positioned externally (e.g., on an outside surface of the display frame  13 . 13 - 2802 ). 
     In at least some examples, the flexible drive shafts  13 . 13 - 2814 ,  13 . 13 - 2816  can provide synchronized movement or parallel motion for each of the actuators  13 . 13 - 2808 ,  13 . 13 - 2810 . For example, the flexible drive shafts  13 . 13 - 2814 ,  13 . 13 - 2816  can be connected to each other via a linkage  13 . 13 - 2818 . Examples of the linkage  13 . 13 - 2818  include a gear system, a pulley system, a pivot connection, etc. 
     The flexible drive shafts  13 . 13 - 2814 ,  13 . 13 - 2816  can be motion-synchronized via the linkage  13 . 13 - 2818  to mitigate (or prevent) uneven translation between the actuators  13 . 13 - 2808 ,  13 . 13 - 2810 . Further, in some examples, the linkage  13 . 13 - 2818  can allow a single input to one of the actuators  13 . 13 - 2808 ,  13 . 13 - 2810  to control both of the flexible drive shafts  13 . 13 - 2814 ,  13 . 13 - 2816 . That is, in certain implementations, engagement of both of the actuator controls  13 . 13 - 2812   a ,  13 . 13 - 2812   b  is not required to actuate all of the actuators  13 . 13 - 2808 ,  13 . 13 - 2810  (and/or their corresponding locks, described below). 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 13 - 28 - 13 . 13 - 30    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 13 - 28 - 13 . 13 - 30   . The following figure descriptions in relation to  FIGS.  13 . 13 - 31 - 13 . 13 - 33    relate to specific examples of the actuators  13 . 13 - 2808 ,  13 . 13 - 2810  discussed above. Indeed, the actuators  13 . 13 - 2808 ,  13 . 13 - 2810  can include a linear adjustment connection between the display and the facial interface, where the linear adjustment connection includes a linear slide actuator with a locking mechanism. 
       FIGS.  13 . 13 - 31 - 13 . 13 - 33    respectively illustrate a perspective view of lock-slider disengagement, a front view of lock-slider disengagement, and a front view of lock-slider engagement of a portion of a linear adjustment connection  13 . 13 - 3100  in accordance with one or more examples of the present disclosure. In these or other examples, the linear adjustment connection  13 . 13 - 3100  includes an actuator  13 . 13 - 3102  (i.e., a linear slide), an actuator lock  13 . 13 - 3104  (otherwise referred to as a lock), and an actuator control (e.g., the actuator controls  13 . 13 - 2808 ,  13 . 13 - 2810  not shown). As will be discussed below, in response to an actuator control, the actuator lock  13 . 13 - 3104  can engage the actuator  13 . 13 - 3102  to hold the actuator  13 . 13 - 3102  in position, or else to release the actuator  13 . 13 - 3102  for desired adjustability. 
     As shown, the actuator  13 . 13 - 3102  can be designed to translate in a slide direction  13 . 13 - 3116  (e.g., to move the facial interface  13 . 13 - 2804  away from the display frame  13 . 13 - 2802 ). The actuator  13 . 13 - 3102  can also freely translate in a direction opposite the slide direction  13 . 13 - 3116  (e.g., to move the facial interface  13 . 13 - 2804  toward the display frame  13 . 13 - 2802 ) when the actuator lock  13 . 13 - 3104  is disengaged from the actuator  13 . 13 - 3102 . To perform this function, the actuator  13 . 13 - 3102  can include certain structures. For example, the actuator  13 . 13 - 3102  can include teeth  13 . 13 - 3106  positioned on a top surface  13 . 13 - 3107 . The teeth  13 . 13 - 3106  can extend upward from the top surface  13 . 13 - 3107  for a tooth height  13 . 13 - 3110 . In some examples, the tooth height  13 . 13 - 3110  can be tuned based on various design factors. For example, the tooth height  13 . 13 - 3110  can affect a stack thickness of one or more portions of the display frame  13 . 13 - 2802 . As another example, the tooth height  13 . 13 - 3110  can affect an ability of the actuator lock  13 . 13 - 3104  to deflect upwards in a direction  13 . 13 - 3122  (e.g., without damage to the actuator lock  13 . 13 - 3104 ). Thus, in some examples, the tooth height  13 . 13 - 3110  can be minimized or reduced. 
     As an additional example, the teeth  13 . 13 - 3106  can include a tooth pitch (e.g., an amount of teeth per inch defining a distance  13 . 13 - 3112  between teeth). Like the tooth height  13 . 13 - 3110 , the tooth pitch can also be tuned based on various design factors. For example, a finer tooth pitch (e.g., a smaller distance  13 . 13 - 3112  or more teeth per inch) can be utilized as the tooth height  13 . 13 - 3110  is decreased. Additionally or alternatively, tooth pitch and/or the tooth height  13 . 13 - 3110  can be tuned for a desired amount of force at which back-drivability can occur (or to entirely prevent back-drivability according to some examples). In these or other examples, the term “back-drive” or “back-drag” refers to translation of the actuator  13 . 13 - 3102  in a direction opposite the slide direction  13 . 13 - 3116  (e.g., when the facial interface  13 . 13 - 2804  and the display frame  13 . 13 - 2802  move toward each other in response to an applied force from one of the display frame  13 . 13 - 2802  or the facial interface  13 . 13 - 2804 ). 
     In more detail, the teeth  13 . 13 - 3106  can include leading tips  13 . 13 - 3114 . The leading tips  13 . 13 - 3114  can include a lead-in structure where teeth  13 . 13 - 3108  (and specifically leading tips  13 . 13 - 3118 ) of the actuator lock  13 . 13 - 3104  can begin to engage. The leading tips  13 . 13 - 3114 ,  13 . 13 - 3118  can include tapered portions or angled portions of the teeth  13 . 13 - 3106 ,  13 . 13 - 3108  that serve as ramps (e.g., to avoid dead-band or teeth misalignment/blunt collisions where the actuator lock  13 . 13 - 3104  is prevented from engaging the actuator  13 . 13 - 3102 ). That is, when transitioning from slider-lock disengagement (shown in  FIGS.  13 . 13 - 32   ) to slider-lock engagement (shown in  FIGS.  13 . 13 - 33   ), the leading tips  13 . 13 - 3114 ,  13 . 13 - 3118  allow the teeth  13 . 13 - 3108  of the actuator lock to smoothly slide into and seat between the teeth  13 . 13 - 3106  of the actuator  13 . 13 - 3102 . 
     With respect to  FIGS.  13 . 13 - 32    specifically, the actuator lock  13 . 13 - 3104  can translate in a lock translation direction  13 . 13 - 3120  (e.g., perpendicular to the slide direction  13 . 13 - 3116 ). In particular, the actuator lock  13 . 13 - 3104  can translate in the lock translation direction  13 . 13 - 3120  in response to at least one of the actuator controls  13 . 13 - 2812   a ,  13 . 13 - 2812   b  being engaged (e.g., depressed) and the flexible drive shafts  13 . 13 - 2814 ,  13 . 13 - 2816  correspondingly engaging the actuator lock  13 . 13 - 3104 . Thereafter, the actuator  13 . 13 - 3102  can be adjusted to translate the facial interface  13 . 13 - 2804  toward or away from the display frame  13 . 13 - 2802  (or associated display). Additionally, upon release of the actuator controls  13 . 13 - 2812   a ,  13 . 13 - 2812   b , the actuator lock  13 . 13 - 3104  can move to an engaged position (shown in  FIGS.  13 . 13 - 33   ) in which the teeth  13 . 13 - 3106  and the teeth  13 . 13 - 3108  are engaged or otherwise positioned adjacent each other. In this position, the actuator  13 . 13 - 3102  is positionally locked in place to inhibit translation of the facial interface  13 . 13 - 2804  toward or away from the display frame  13 . 13 - 2802  (or associated display). As further shown in  FIGS.  13 . 13 - 33   , the leading tips  13 . 13 - 3114  can extend beyond the teeth  13 . 13 - 3108 , and the leading tips  13 . 13 - 3118  can extend beyond the teeth  13 . 13 - 3106 . 
     When the teeth  13 . 13 - 3106 ,  13 . 13 - 3108  are engaged (as shown in  FIGS.  13 . 13 - 33   ), the actuator lock  13 . 13 - 3104  can be deflected upward in a lock deflection direction  13 . 13 - 3122 . The lock deflection direction  13 . 13 - 3122  can be perpendicular to both the slide direction  13 . 13 - 3116  and the lock translation direction  13 . 13 - 3120 . In these or other examples, the actuator lock  13 . 13 - 3104  can deflect in the lock deflection direction  13 . 13 - 3122  during a back-driving or back-dragging event in which the actuator  13 . 13 - 3102  moves opposite the slide direction  13 . 13 - 3116 . To facilitate this back-driving or back-dragging, the actuator lock  13 . 13 - 3104  can act as a cantilever spring to deflect in the lock deflection direction  13 . 13 - 3122  in response to an applied force at the facial interface  13 . 13 - 2804  or the display frame  13 . 13 - 2802 . In some examples, actuator lock  13 . 13 - 3104  can be tuned to deflect in response to a predetermined amount of force (e.g., about 5 Newtons to about 500 Newtons per actuator, about 10 Newtons to about 100 Newtons per actuator, or about 20 Newtons to about 40 Newtons per actuator). In these or other examples, various design factors for the actuator lock  13 . 13 - 3104 , such as tooth angles or bending stiffness, can be adjusted to provide the desired force tuning. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 13 - 31 - 13 . 13 - 33    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 13 - 31 - 13 . 13 - 33   . The following description provides more detail regarding an example implementation of multiple linear adjustment connections connected to a flexible driveshaft (or multiple flexible drive shafts). 
       FIGS.  13 . 13 - 34    illustrates a perspective view of a portion of a head-mountable device  13 . 13 - 3400  in accordance with one or more examples of the present disclosure. As shown, the head-mountable device  13 . 13 - 3400  includes multiple actuators (e.g., multiple linear adjustment connections  13 . 13 - 3100   a - 13 . 13 - 3100   d ) discussed above with respect to the linear adjustment connection  13 . 13 - 3100 . As also mentioned, each linear adjustment connection  13 . 13 - 3100   a - 13 . 13 - 3100   d  can be actuated via the flexible drive shafts  13 . 13 - 2814 ,  13 . 13 - 2816 . The flexible drive shafts  13 . 13 - 2814 ,  13 . 13 - 2816  can be motion-synchronized via the linkage  13 . 13 - 2818  to mitigate (or prevent) uneven translation between each linear adjustment connection  13 . 13 - 3100   a - 13 . 13 - 3100   d . Further, in some examples, the linkage  13 . 13 - 2818  can allow a single input (e.g., a single button press) to actuate each of the linear adjustment connections  13 . 13 - 3100   a - 13 . 13 - 3100   d . The following description of  FIGS.  13 . 13 - 34    gives an example implementation in which this can be performed. 
     As shown in  FIGS.  13 . 13 - 34   , the head-mountable device  13 . 13 - 3400  can include actuator controls  13 . 13 - 3401   a ,  13 . 13 - 3401   b  (which can be the same as or similar to the actuator controls  13 . 13 - 2812   a ,  13 . 13 - 2812   b  discussed above). In response to a force  13 . 13 - 3402   a  applied to the actuator control  13 . 13 - 3401   a , the lock  13 . 13 - 3104   a  can correspondingly deflect, translate, or otherwise move to release the slide  13 . 13 - 3102   a  of the linear adjustment connection  13 . 13 - 3100   a . In addition, the flexible drive shaft  13 . 13 - 2816  can be attached to a joint  13 . 13 - 3404  (e.g., a ball joint) connected to the lock  13 . 13 - 3104   a . As the lock  13 . 13 - 3104   a  deflects, translates, or otherwise moves in response to the applied force  13 . 13 - 3402   a , the flexible drive shaft  13 . 13 - 2816  can be tensioned or pulled in a direction  13 . 13 - 3406 . 
     The flexible drive shaft  13 . 13 - 2816  can be attached to a gear anchor  13 . 13 - 3410  such that, upon pulling of the flexible drive shaft  13 . 13 - 2816  in the direction  13 . 13 - 3406 , a first gear  13 . 13 - 3409   a  connected to the gear anchor  13 . 13 - 3410  rotates in a direction  13 . 13 - 3412 . In turn, the lock  13 . 13 - 3104   b  (which is movably attached to the first gear  13 . 13 - 3409 ) is pulled back away from the slide  13 . 13 - 3102   b  of the linear adjustment connection  13 . 13 - 3100   b  as the first gear  13 . 13 - 3409  rotates in the direction  13 . 13 - 3412 . 
     Furthermore, rotation of the first gear  13 . 13 - 3409   a  in the direction  13 . 13 - 3412  can simultaneously induce rotation of the second gear  13 . 13 - 3409   b  in a direction  13 . 13 - 3414 . The rotation of the second gear  13 . 13 - 3409   b  in the direction  13 . 13 - 3414  can cause the lock  13 . 13 - 3104   c  (which is movably attached to the second gear  13 . 13 - 3409   b ) to pull back away from the slide  13 . 13 - 3102   c  of the linear adjustment connection  13 . 13 - 3100   c.    
     In addition, rotation of the second gear  13 . 13 - 3409   b  in the direction  13 . 13 - 3414  can cause a gear anchor  13 . 13 - 3416  connected to the second gear  13 . 13 - 3409   b  to push the flexible drive shaft  13 . 13 - 2814 . Specifically, the flexible drive shaft  13 . 13 - 2814  can be connected to the gear anchor  13 . 13 - 3416  such that, upon rotation of the second gear  13 . 13 - 3409   b  in the direction  13 . 13 - 3414 , the flexible drive shaft  13 . 13 - 2814  is pushed in a direction  13 . 13 - 3408 . Pushing of the flexible drive shaft  13 . 13 - 2814  can cause the lock  13 . 13 - 3104   d  to release from the slide  13 . 13 - 3102   d  of the linear adjustment connection  13 . 13 - 3100   d . For instance, the flexible drive shaft  13 . 13 - 2814  can be connected to the lock  13 . 13 - 3104   d  in a same or similar manner as the flexible drive shaft  13 . 13 - 2816  is attached to the lock  13 . 13 - 3104   a  to impart slide-lock disengagement. 
     Additionally or alternatively, it will be appreciated that a force  13 . 13 - 3402   b  can be applied to the actuator control  13 . 13 - 3401   b , which is in mechanical communication with the lock  13 . 13 - 3104   d  (in a same or similar manner as the actuator control  13 . 13 - 3401   a  is mechanically coupled to the lock  13 . 13 - 3104   a ). Thus, dual inputs via both of the actuator controls  13 . 13 - 3401   a ,  13 . 13 - 3401   b  can be applied, but are not required, for synchronized actuation of each of the linear adjustment connections  13 . 13 - 3100   a - 13 . 13 - 3100   d . Alternatively, the applied force  13 . 13 - 3402   b  alone (i.e., without the applied force  13 . 13 - 3402   a ) can be used to actuate each of the linear adjustment connections  13 . 13 - 3100   a - 13 . 13 - 3100   d.    
     Further, in some examples, additional or alternative modes of actuating the linear adjustment connections  13 . 13 - 3100   a - 13 . 13 - 3100   d  can be implemented. For example, solenoids or small motors can be utilized to actuate linear adjustment connections  13 . 13 - 3100   a - 13 . 13 - 3100   d  (with or without the flexible drive shafts  13 . 13 - 2814 ,  13 . 13 - 2816 ). Similarly, in some examples, the actuator controls  13 . 13 - 3401   a ,  13 . 13 - 3401   b  can include alternative types of actuator controls. For example, the actuator controls  13 . 13 - 3401   a ,  13 . 13 - 3401   b  can be arch sliders that move in an arc path (e.g., along a display frame not shown of the head-mountable display  13 . 13 - 3400 ). The arch sliders can be connected to the flexible drive shafts  13 . 13 - 2814 ,  13 . 13 - 2816  as discussed above to actuate the linear adjustment connections  13 . 13 - 3100   a - 13 . 13 - 3100   d.    
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 13 - 34    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 13 - 34   . 
     13.14: Nosepiece 
     Head-mounted devices include head-mounted support structures that allow the devices to be worn on the heads of users. The head-mounted support structures may include device housings for housing components such as displays that are used for presenting a user with visual content. Head-mounted devices may also include a light-shielding nosepiece that rests on the nose of the user. The light-shielding nosepiece may include an elastomeric layer with perforations and fabric that covers the elastomeric layer. The elastomeric layer with the perforations may allow the light-shielding nosepiece to conform to the user&#39;s nose while the device is worn, while maintaining enough rigidity to be wrapped by a low force, high stretch textile or other low force, high stretch material. Additional structures, such as rigid structures or semi-rigid structures, may be included in the light-shielding nosepiece to provide additional support for the device while it is worn. 
     A schematic diagram of an illustrative system having an electronic device with a light-shielding nosepiece is shown in  FIGS.  13 . 14 - 1   . As shown in  FIGS.  13 . 14 - 1   , system  13 . 14 - 8  may include one or more electronic devices such as electronic device  13 . 14 - 10 . The electronic devices of system  13 . 14 - 8  may include computers, cellular telephones, head-mounted devices, wristwatch devices, and other electronic devices. Configurations in which electronic device  13 . 14 - 10  is a head-mounted device are sometimes described herein as an example. 
     As shown in  FIGS.  13 . 14 - 1   , electronic devices such as electronic device  13 . 14 - 10  may have control circuitry  13 . 14 - 12 . Control circuitry  13 . 14 - 12  may include storage and processing circuitry for controlling the operation of device  13 . 14 - 10 . Circuitry  13 . 14 - 12  may include storage such as hard disk drive storage, nonvolatile memory (e.g., electrically-programmable-read-only memory configured to form a solid-state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. 
     Processing circuitry in control circuitry  13 . 14 - 12  may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors, power management units, audio chips, graphics processing units, application specific integrated circuits, and other integrated circuits. Software code may be stored on storage in circuitry  13 . 14 - 12  and run on processing circuitry in circuitry  13 . 14 - 12  to implement control operations for device  13 . 14 - 10  (e.g., data gathering operations, operations involved in processing three-dimensional facial image data, operations involving the adjustment of components using control signals, etc.). Control circuitry  13 . 14 - 12  may include wired and wireless communications circuitry. For example, control circuitry  13 . 14 - 12  may include radio-frequency transceiver circuitry such as cellular telephone transceiver circuitry, wireless local area network (WiFi®) transceiver circuitry, millimeter wave transceiver circuitry, and/or other wireless communications circuitry. 
     During operation, the communications circuitry of the devices in system  13 . 14 - 8  (e.g., the communications circuitry of control circuitry  13 . 14 - 12  of device  13 . 14 - 10 ), may be used to support communication between the electronic devices. For example, one electronic device may transmit video and/or audio data to another electronic device in system  13 . 14 - 8 . Electronic devices in system  13 . 14 - 8  may use wired and/or wireless communications circuitry to communicate through one or more communications networks (e.g., the internet, local area networks, etc.). The communications circuitry may be used to allow data to be received by device  13 . 14 - 10  from external equipment (e.g., a tethered computer, a portable device such as a handheld device or laptop computer, online computing equipment such as a remote server or other remote computing equipment, or other electrical equipment) and/or to provide data to external equipment. 
     Device  13 . 14 - 10  may include input-output devices  13 . 14 - 22 . Input-output devices  13 . 14 - 22  may be used to allow a user to provide device  13 . 14 - 10  with user input. Input-output devices  13 . 14 - 22  may also be used to gather information on the environment in which device IO is operating. Output components in devices  13 . 14 - 22  may allow device  13 . 14 - 10  to provide a user with output and may be used to communicate with external electrical equipment. 
     As shown in  FIG.  1   , input-output devices  13 . 14 - 22  may include one or more displays such as display  13 . 14 - 14 . In some configurations, display  13 . 14 - 14  of device  13 . 14 - 10  includes left and right display panels (sometimes referred to as left and right portions of display  13 . 14 - 14  and/or left and right displays) that are in alignment with the user&#39;s left and right eyes and are viewable through left and right lens assemblies, respectively. In other configurations, display  13 . 14 - 14  includes a single display panel that extends across both eyes. 
     Display  13 . 14 - 14  may be used to display images. The visual content that is displayed on display  13 . 14 - 14  may be viewed by a user of device  13 . 14 - 10 . Displays in device  13 . 14 - 10  such as display  13 . 14 - 14  may be organic light-emitting diode displays or other displays based on arrays of light-emitting diodes, liquid crystal displays, liquid-crystal-on-silicon displays, projectors or displays based on projecting light beams on a surface directly or indirectly through specialized optics (e.g., digital micromirror devices), electrophoretic displays, plasma displays, electrowetting displays, microLED displays, or any other suitable displays. 
     Display  13 . 14 - 14  may present computer-generated content such as virtual reality content and mixed reality content to a user. Virtual reality content may be displayed in the absence of real-world content. Mixed reality content, which may sometimes be referred to as augmented reality content, may include computer-generated images that are overlaid on real-world images. The real-world images may be captured by a camera (e.g., a forward-facing camera) and merged with overlaid computer-generated content or an optical coupling system may be used to allow computer-generated content to be overlaid on top of real-world images. As an example, a pair of mixed reality glasses or other augmented reality head-mounted display may include a display device that provides images to a user through a beam splitter, prism, holographic coupler, or other optical coupler. Configurations in which display  13 . 14 - 14  is used to display virtual reality content to a user through lenses are described herein as an example. 
     Input-output devices  13 . 14 - 22  may include sensors  13 . 14 - 16 . Sensors  13 . 14 - 16  may include, for example, three-dimensional sensors (e.g., three-dimensional image sensors such as structured light sensors that emit beams of light and that use two-dimensional digital image sensors to gather image data for three-dimensional images from light spots that are produced when a target is illuminated by the beams of light, binocular three-dimensional image sensors that gather three-dimensional images using two or more cameras in a binocular imaging arrangement, three-dimensional LIDAR (light detection and ranging) sensors, three-dimensional radio-frequency sensors, or other sensors that gather three-dimensional image data), cameras (e.g., infrared and/or visible digital image sensors), gaze tracking sensors (e.g., a gaze tracking system based on an image sensor and, if desired, a light source that emits one or more beams of light that are tracked using the image sensor after reflecting from a user&#39;s eyes), touch sensors, buttons, force sensors, sensors such as contact sensors based on switches, gas sensors, pressure sensors, moisture sensors, magnetic sensors, audio sensors (microphones), ambient light sensors, microphones for gathering voice commands and other audio input, sensors that are configured to gather information on motion, position, and/or orientation (e.g., accelerometers, gyroscopes, compasses, and/or inertial measurement units that include all of these sensors or a subset of one or two of these sensors), fingerprint sensors and other biometric sensors, optical position sensors (optical encoders), and/or other position sensors such as linear position sensors, and/or other sensors. 
     User input and other information may be gathered using sensors and other input devices in input-output devices  13 . 14 - 22 . If desired, input-output devices  13 . 14 - 22  may include other devices  13 . 14 - 24  such as haptic output devices (e.g., vibrating components), light-emitting diodes and other light sources, speakers such as ear speakers for producing audio output, and other electrical components. Device  13 . 14 - 10  may include circuits for receiving wireless power, circuits for transmitting power wirelessly to other devices, batteries and other energy storage devices (e.g., capacitors), joysticks, buttons, and/or other components. 
     Electronic device  13 . 14 - 10  may have housing structures (e.g., housing walls, straps, etc.), as shown by illustrative support structures  13 . 14 - 26  of  FIGS.  13 . 14 - 1   . In configurations in which electronic device IO is a head-mounted device (e.g., a pair of glasses, goggles, a helmet, a hat, a headband, etc.), support structures  13 . 14 - 26  may include head-mounted support structures (e.g., a helmet housing, head straps, temples in a pair of eyeglasses, goggle housing structures, and/or other head-mounted structures). The head-mounted support structures may be configured to be worn on a head of a user during operation of device IO and may support display(s)  13 . 14 - 14 , sensors  13 . 14 - 16 , other components  13 . 14 - 24 , other input-output devices  13 . 14 - 22 , and control circuitry  13 . 14 - 12 . 
     In some embodiments, support structures  13 . 14 - 26  may include a light-shielding nosepiece. The light-shielding nosepiece may be attached to support structures  13 . 14 - 26 , such as a main housing portion of electronic device  13 . 14 - 10 , and may rest on the user&#39;s nose while device  13 . 14 - 10  is worn. The light-shielding nosepiece may be flexible, to allow the nosepiece to conform to the user&#39;s nose, while retaining enough rigidity to support device  13 . 14 - 10  on the user&#39;s face while it is being worn (i.e., to maintain its shape on the user&#39;s nose while the device is worn) and to be wrapped by a fabric or other material. If desired, the light-shielding nosepiece may also include stiffeners or other components that help maintain the nosepiece on the user&#39;s nose to prevent light from reaching the user&#39;s eyes. An example of an illustrative electronic device having a nosepiece is shown in  FIGS.  13 . 14 - 2   . 
     As shown in  FIGS.  13 . 14 - 2   , head-mounted device  13 . 14 - 10  may include support structures  13 . 14 - 26 , which may include a head-mounted housing (sometimes referred to as a main housing, main housing unit, head-mounted support structure, etc.). The housing may have walls or other structures that separate an interior housing region from an exterior region surrounding the housing. For example, the housing may have walls formed from polymer, glass, metal, and/or other materials. Electrical and optical components may be mounted in the housing. These components may include components such as integrated circuits, sensors, control circuitry, input-output devices, etc. 
     To present a user with images for viewing from eye boxes (e.g., eye boxes in which the user&#39;s eyes are located when device  13 . 14 - 10  is being worn on the user&#39;s head), device  13 . 14 - 10  may include displays and lenses. These components may be mounted in optical modules or other supporting structure in the housing to form respective left and right optical systems. There may be, for example, a left display for presenting an image through a left lens to a user&#39;s left eye in a left eye box and a right display for presenting an image through a right lens to a user&#39;s right eye in a right eye box. 
     If desired, the housing may have forward-facing components such as cameras and other sensors on a front side for gathering sensor measurements and other input and may have a soft cushion on an opposing rear side of the housing. The rear side of the housing may have openings that allow the user to view images (image light  13 . 14 - 32 ) from the left and right optical systems (e.g., when the rear side of the housing is resting on the user&#39;s head). 
     If desired, device  13 . 14 - 10  may have an adjustable strap or headband, and if desired, may have other structures (e.g., an over-the-head strap) to help hold the housing on the user&#39;s head. 
     As shown in  FIGS.  13 . 14 - 2   , when worn by the user, device  13 . 14 - 10  may include a nosepiece, such as nosepiece  13 . 14 - 28  that rests on the nasal region the user&#39;s head (e.g., on the user&#39;s nose). In particular, nosepiece  13 . 14 - 28  (sometimes referred to as a light-shielding structure or a light-shielding nosepiece herein) may serve as an extension of the housing that rests on the user&#39;s nose and bridges between the opposing cheeks of the user. If desired, nosepiece  13 . 14 - 28  may be attached to the housing and/or may include one or more members formed from a portion of the housing. In some embodiments, nosepiece  13 . 14 - 28  may be a portion of a light seal or attached to a light seal that extends around some or all of a periphery of the housing (e.g., the light seal is attached to the housing around the periphery). When device  13 . 14 - 10  is in use, the light seal may compress against the user&#39;s face and prevent interference from ambient light. In general, however, nosepiece  13 . 14 - 28  may be attached to the housing in any desired manner. 
     Nosepiece  13 . 14 - 28  may be configured as a light-shielding structure and may therefore be sometimes referred to as light-shielding structure  13 . 14 - 28  or light-shielding nosepiece  13 . 14 - 28 . As an example, it may be desirable to enhance the viewing experience of the user by blocking external environmental light from entering the interior of device  13 . 14 - 10  (e.g., from entering the eye boxes) when device  13 . 14 - 10  is worn by the user. Nosepiece  13 . 14 - 28  may conform to the facial topology of the user around the user&#39;s nose and block light from entering the eye boxes. In some illustrative configurations, nosepiece  13 . 14 - 28  may be adjustable to conform to varying facial topologies of different users (e.g., portions of nosepiece  13 . 14 - 28  may deform differently based on the nose shapes of the users). 
     Nosepiece  13 . 14 - 28  may be mounted to a housing portion of electronic device  13 . 14 - 10 , such as head-mounted support structures  13 . 14 - 26 , at mounting points  13 . 14 - 30 . The housing may include a housing frame that runs along the periphery of device  13 . 14 - 10 . If desired, the housing frame may be overlapped by a cushion member on the rear side of the housing facing the user. As an example, the cushion member may include foam structures or other soft compressible structures affixed to the housing frame. A fabric may overlap and extend over the housing frame and/or the cushion member on the rear side of the housing. If desired, the fabric may enclose only the cushion member, and the fabric-enclosed cushion member may be removably coupled to the housing frame. 
     Mounting points  13 . 14 - 30  may be located at a bottom portion of the housing frame (e.g., a bottom portion of support structures  13 . 14 - 26 ). As examples, mounting points  13 . 14 - 30  may include coupling mechanisms such as magnets, adhesive, hinges, or any other suitable coupling mechanisms. 
     In the example of  FIGS.  13 . 14 - 2   , nosepiece  13 . 14 - 28  is attached to support structures  13 . 14 - 26  such that nosepiece  13 . 14 - 28  extends into a portion of support structures  13 . 14 - 26 . This is merely illustrative. If desired, the housing frame, a cushion member, and/or fabric may split into multiple portions to surround nosepiece  13 . 14 - 28 . As an example, a fabric-enclosed cushion member may run in front of nosepiece  13 . 14 - 28 , and a housing frame may run behind nosepiece  13 . 14 - 28 . In general, the housing portions, cushion member, and/or fabric may define an opening in which nosepiece  13 . 14 - 28  is disposed. 
     In some illustrative examples, nosepiece  13 . 14 - 28  may be removably coupled (via magnetics) to the housing frame or other portions of the housing. In some illustrative examples, a portion of nosepiece  13 . 14 - 28  may form an integral portion of the housing frame and/or may not be removable from the housing. 
     Support structures  13 . 14 - 26  (e.g., housing frames) and nosepiece  13 . 14 - 28  (along with other desired structures) may define the periphery of the eye boxes of device  13 . 14 - 10  at which the user&#39;s eyes are located. Components, such as displays, lenses, sensors, etc., may overlap and/or be located within the eye boxes of device  13 . 14 - 10 , and may be enclosed by and/or mounted to support structures  13 . 14 - 26  and/or nosepiece  13 . 14 - 28 . As illustratively shown in  FIGS.  13 . 14 - 2   , display  13 . 14 - 14  emit image light  13 . 14 - 32  through a lens to an eye box. Nosepiece  13 . 14 - 28  may be configured to block environmental light from an exterior of device  13 . 14 - 10  from entering the eye box and interfering with image light  13 . 14 - 24 . An example of a nosepiece that may be used in device  13 . 14 - 10  is shown in  FIGS.  13 . 14 - 3   . 
     As shown in  FIGS.  13 . 14 - 3   , a nosepiece, such as nosepiece  13 . 14 - 28 , may include elastomer  13 . 14 - 34  (also referred to as elastomeric layer  13 . 14 - 34  herein). Elastomer  13 . 14 - 34  may be any desired elastomeric material, such as thermoplastic polyurethane (TPU), nitrile butadiene rubber (NBR), or silicone (such as a low-durometer silicone). Elastomer  13 . 14 - 34  may have a thickness of greater than 0.1 mm, greater than 0.25 mm, greater than 0.5 mm, about 1 mm, less than 2 mm, less than 5 mm, etc. 
     In some embodiments, elastomer  13 . 14 - 34  may include perforations  13 . 14 - 36 . Perforations  13 . 14 - 36  may allow elastomer  13 . 14 - 34  to bend to accommodate a user&#39;s nose (i.e., in the horizontal left and right directions of  FIGS.  13 . 14 - 3   ), while maintaining sufficient rigidity (i.e., in the vertical up and down directions of  FIGS.  13 . 14 - 3   ) to support device IO on the user&#39;s nose. The rigidity of elastomer  13 . 14 - 34  may also allow elastomer  13 . 14 - 34  to be wrapped by fabric  13 . 14 - 38  or other low force, high stretch material. 
     Perforations  13 . 14 - 36  may be formed in any desired pattern. In the example of  FIGS.  13 . 14 - 3   , perforations  13 . 14 - 36  are formed as an array of perforations across the entire surface of elastomer  13 . 14 - 34 . Perforations  13 . 14 - 36  may be formed in a brick pattern, as in  FIG.  13 . 14 - 3   , or may be formed in any other desired pattern. Additionally, any desired number of perforations  13 . 14 - 36  may be formed in elastomer  13 . 14 - 34 , such as at least one perforation, at least  13 . 14 - 5  perforations, or any other desired number of perforations. Perforations  13 . 14 - 36  may extend entirely through elastomer  13 . 14 - 34 , or may extend partially through elastomer  13 . 14 - 34 . 
     Elastomer  13 . 14 - 34  may have a shape that generally conforms to a user&#39;s nose, such as a chevron shape, as shown in  FIG.  13 . 14 - 3   , or any other desired shape, such as a rounded shape, or a rectangular shape. Elastomer  13 . 14 - 34  may be covered by fabric  13 . 14 - 38 . In particular, fabric  13 . 14 - 38  may overlap and/or encapsulate elastomer  13 . 14 - 34  in region  13 . 14 - 40 . Fabric region  13 . 14 - 40  may have the same shape as elastomer  13 . 14 - 34 , as shown in  FIG.  13 . 14 - 3   , or may have a different shape than elastomer  13 . 14 - 34 . Fabric members  13 . 14 - 40  and/or  13 . 14 - 38  may serve as a light-shielding member or layer for light-shielding structure  13 . 14 - 28 . In particular, illustrative configurations in which fabric  13 . 14 - 40 / 38  is a fabric cover (sometimes referred to as a fabric cover layer or a cover layer) are described herein as an illustrative example. To serve light-shielding functions, the fabric cover may be formed from an opaque or light-shielding material (e.g., black yam) or may formed from an underlying material coated with an opaque or light-shielding material (e.g., black dye or ink). As examples, the fabric cover may be formed any suitable type of fabric such as knit fabric, woven fabric, braided fabric, etc. 
     Fabric  13 . 14 - 40  may be tented over elastomer  13 . 14 - 34 . Tenting of the fabric cover over an underlying structure may be achieved by the underlying structure contacting or otherwise supporting the fabric cover at one or more points or areas of support as the fabric cover extends over one or more sides of the underlying structure. The tenting of the fabric cover over the underlying structure may cause the fabric cover to follow the general outline of the underlying structure, especially around the areas of support. If desired, differences in the outlines of the fabric cover and of the underlying structure may exist, especially in some regions away from the areas of support, thereby causing some portions of the fabric cover to be suspended in air and therefore readily deflectable. As an example, the fabric cover may be deflectable to the boundary of the underlying structure (or even beyond the boundary of the underlying structure if the boundary is defined by a flexible or deformable member). 
     The rigidity of elastomer  13 . 14 - 34 , which is preserved in the vertical direction by perforations  13 . 14 - 36 , may allow elastomer  13 . 14 - 34  to be wrapped by fabric  13 . 14 - 40  (or other low force, high stretch textile, or other low force, high stretch material) without deforming. In other words, elastomer  13 . 14 - 34  may be rigid enough to maintain its shape while fabric  13 . 14 - 40  is applied to/wrapped around elastomer  13 . 14 - 34  (as well as to maintain its shape when the device is worn by a user), but flexible enough that it can conform to a user&#39;s nose. 
     In such a way, the fabric cover may have a three-dimensional shape (based on an outline of the underlying structure) that includes portions (e.g., directly supported by the underlying structure) that are more defined and portions (e.g., not directly supported by the underlying structure, suspended in air, etc.) that are less defined, and more flexible or yielding. These less-defined portions (e.g., a yielding fabric surface) may help form flexible boundaries such as those for an opening configured to receive a user&#39;s nose. 
     As a particular illustrative example, the underlying structure may have surfaces that define an opening for accommodating a user&#39;s nose. The surfaces may be surrounded by peripheral edges. The fabric cover may be tented over the underlying structure such that the fabric cover is directly supported by the underlying structure along one or more of the peripheral edges of the underlying structure and may be suspended in air around the opening, thereby providing a fabric surface that is deflectable by the user&#39;s nose. This may help with improving user comfort as well as providing a more conformal fit when the light shielding structure rests on the user&#39;s nose. 
     Regardless of the shape of fabric region  13 . 14 - 40 , fabric region  13 . 14 - 40  (and/or elastomer  13 . 14 - 34 ) may be bonded to support structures  13 . 14 - 26  (such as a housing frame) using adhesive  13 . 14 - 42 . However, the use of adhesive  13 . 14 - 42  is merely illustrative. Fabric  13 . 14 - 40  and/or elastomer  13 . 14 - 34  may be formed integrally with support structures  13 . 14 - 26 , or may be attached to support structures  13 . 14 - 26  using any desired attachment mechanism. 
     Although nosepiece  13 . 14 - 28  is shown as including both elastomer  13 . 14 - 34  and fabric  13 . 14 - 40 , this is merely illustrative. If desired, nosepiece  13 . 14 - 28  may include elastomer  13 . 14 - 34  without an overlapping fabric layer. In this case, another layer, such as a polymer or rubber, may overlap elastomer  13 . 14 - 34 , or elastomer  13 . 14 - 34  may directly contact a user&#39;s nose as they wear device  13 . 14 - 10 . Alternatively, nosepiece  13 . 14 - 28  may include fabric  13 . 14 - 40  without elastomer  13 . 14 - 34 , if desired. In this case, fabric  13 . 14 - 40  may be a flat knit fabric to provide sufficient stretching over a user&#39;s nose, while providing enough support for device  13 . 14 - 10 . 
     Although fabric  13 . 14 - 38 / 40  is described as fabric, this is merely illustrative. In general, fabric  13 . 14 - 38 / 40  may be any low force, high stretch material, such as a low force, high stretch textile. 
     In some embodiments, although elastomer  13 . 14 - 34  may have rigidity to support device  13 . 14 - 10  on a user&#39;s nose (i.e., in the vertical direction of  FIGS.  13 . 14 - 3   ) and to be wrapped by fabric, it may be desirable to add additional rigid or semi-rigid structures to a nosepiece. An illustrative example of a nosepiece having a rigid structure is shown in  FIGS.  13 . 14 - 4   . 
     As shown in  FIGS.  13 . 14 - 4   , nosepiece  13 . 14 - 28  may include fabric  13 . 14 - 40  (which may cover an elastomeric layer, such as elastomer  13 . 14 - 34 ) and adhesive  13 . 14 - 42 , which in turn may attach fabric  13 . 14 - 40  to structural frame  13 . 14 - 43 . Structural frame  13 . 14 - 43  may be a plastic frame, as an example. Structural frame  13 . 14 - 43  may provide additional rigidity to ensure that nosepiece  13 . 14 - 28  retains its shape and support when worn by a user. 
     Although  FIGS.  13 . 14 - 4    shows the use of adhesive  13 . 14 - 42 , this is merely illustrative. Adhesive  13 . 14 - 42  may be omitted, if desired. In some examples, elastomer  13 . 14 - 34  may be co-molded to structural frame  13 . 14 - 43 . Alternatively, elastomer  13 . 14 - 34  may be fit into a recessed portion of structural frame  13 . 14 - 43  or otherwise attached to structural frame  13 . 14 - 43 . 
     Additionally, although  FIGS.  13 . 14 - 4    shows structural frame  13 . 14 - 43  on three sides of fabric  13 . 14 - 40 /elastomer  13 . 14 - 34 , this is merely illustrative. Structural frame  13 . 14 - 43  may be attached to one side (e.g., the bottom side) of fabric  13 . 14 - 40 /elastomer  13 . 14 - 34  or any other desired number of sides. Regardless of the attachment of structural frame  13 . 14 - 43  to elastomer  13 . 14 - 34  and/or fabric  13 . 14 - 40 , fabric  13 . 14 - 40  may extend over all or some of both frame  13 . 14 - 43  and/or elastomer  13 . 14 - 34 . An example of a stack up of nosepiece  13 . 14 - 28  is shown in  FIGS.  13 . 14 - 5   . 
     As shown in  FIGS.  13 . 14 - 5   , nosepiece  13 . 14 - 28  may include fabric  13 . 14 - 40 , which is shown as having fabric portions  13 . 14 - 40 - 1  and  13 . 14 - 40 - 2 , on either side of elastomer  13 . 14 - 34 . If desired, fabric  13 . 14 - 40  may be bonded to elastomer  13 . 14 - 34  at a periphery of elastomer  13 . 14 - 34 , which is shown in  FIGS.  13 . 14 - 5    as points  13 . 14 - 44 . For example, fabric  13 . 14 - 40  may be bonded to elastomer  13 . 14 - 34  around a periphery (such as an entire periphery or a portion of the periphery) of the elastomer layer. Fabric  13 . 14 - 40  may be bonded to elastomer  13 . 14 - 34  using an adhesive or other desired mounting mechanism. In some cases, it may be desirable to allow elastomer  13 . 14 - 34  to move to a greater extent, and therefore leave elastomer  13 . 14 - 34  un-bonded from fabric  13 . 14 - 40 . For example, elastomer  13 . 14 - 34  may be fully surrounded fabric  13 . 14 - 40  and float within the fabric. In some embodiments portions of fabric  13 . 14 - 40  may be bonded directly to each other, rather than to elastomer  13 . 14 - 34 . In this way, elastomer  13 . 14 - 34  may float within fabric  13 . 14 - 40 , which may allow  13 . 14 - 34  to move more freely. 
     Because nosepiece  13 . 14 - 28  is designed to block light from reaching the eye boxes of a user wearing device  13 . 14 - 10 , it may be desirable to ensure a tight fit between nosepiece  13 . 14 - 28  and the user&#39;s nose and a similarly tight fit between nosepiece  13 . 14 - 28  and device  13 . 14 - 10 . An example of an extension piece that may allow for nosepiece  13 . 14 - 28  to fit tightly with device  13 . 14 - 10  is shown in  FIGS.  13 . 14 - 6   . 
     As shown in  FIGS.  13 . 14 - 6   , extension piece  13 . 14 - 46  may be added below nosepiece  13 . 14 - 28 . In particular, if a user&#39;s nose bridge protrudes only a small amount, nosepiece  13 . 14 - 28  may not reach the user&#39;s nose. Therefore, extension piece  13 . 14 - 46  may be attached between support structures of device  13 . 14 - 10 , such as support structures  13 . 14 - 26 , and nosepiece  13 . 14 - 28 . Extension piece  13 . 14 - 28  may be formed from fabric, foam, plastic, and/or any other desired material. In some cases, extension piece  13 . 14 - 28  may allow nosepiece  13 . 14 - 28  to align with other light-blocking structures in device  13 . 14 - 10 , such as a foam member that fits against the user&#39;s face. In this way, light may be prevented from reaching the user&#39;s eye boxes and interfering with the user&#39;s visibility of a display in device  13 . 14 - 10 . 
     In addition or as an alternative to extension piece  13 . 14 - 46 , it may be desirable to ensure a close fit to a user&#39;s nose to prevent light from entering the user&#39;s eye boxes. Examples of nosepieces that have additional structures to improve the fit to a user&#39;s nose are shown in  FIGS.  13 . 14 - 7  and  13 . 14 - 8   . 
     As shown in  FIGS.  13 . 14 - 7   , service loop  13 . 14 - 49  may be included within nosepiece  13 . 14 - 28 . In particular, service loop  13 . 14 - 49  (and other portions of nosepiece  13 . 14 - 28 , if desired) may be attached to supports  13 . 14 - 50 - 1  and  13 . 14 - 50 - 2 . Service loop  13 . 14 - 49  may be tightened and loosened as desired to conform nosepiece  13 . 14 - 28  to the nose of a user. For example, as shown in  FIGS.  13 . 14 - 7   , service loop  13 . 14 - 49  may be tightened to move nosepiece  13 . 14 - 28  to position  13 . 14 - 28 ′ by moving nosepiece  13 . 14 - 28  downward in direction  13 . 14 - 56 , right in direction  13 . 14 - 52 , and left in direction  13 . 14 - 54  toward to the nose of the user. In this way, service loop  13 . 14 - 49  may allow nosepiece  13 . 14 - 28  to fit more securely to the user&#39;s nose, preventing light from entering the user&#39;s eye boxes through gaps between nosepiece  13 . 14 - 28  and the nose. 
     Alternatively or additionally, nosepiece  13 . 14 - 28  may include foam  13 . 14 - 48 . Foam  13 . 14 - 48  may fill a gap between nosepiece  13 . 14 - 28  and the user&#39;s nose, and may be compressible to allow for a secure fit between nosepiece  13 . 14 - 28  and the nose. Foam  13 . 14 - 48  may directly contact the nose of the user (e.g., may be on a surface of a fabric layer, such as fabric  13 . 14 - 40 ), as shown in  FIGS.  13 . 14 - 7   , may be embedded within nosepiece  13 . 14 - 28  (e.g., covered by fabric, such as fabric  13 . 14 - 40 ), or may otherwise be attached to nosepiece  13 . 14 - 28 . 
     Instead of service loop  13 . 14 - 49 , a deformable stiffener, such as deformable stiffener  13 . 14 - 58 , may be incorporated into nosepiece  13 . 14 - 28 . As shown in  FIGS.  13 . 14 - 8   , deformable stiffener  13 . 14 - 58  may allow for nosepiece  13 . 14 - 28  to be adjusted in directions  13 . 14 - 52 ,  13 . 14 - 54 , and  13 . 14 - 56  toward the nose of the user to conform nosepiece  13 . 14 - 28  to the user&#39;s nose at position  13 . 14 - 28 ′. Deformable stiffener  13 . 14 - 58  may be formed within nosepiece  13 . 14 - 28  (i.e., may be covered by fabric), as shown in  FIG.  13 . 14 - 8   , or may be formed on a surface of the fabric (e.g., an inner or outer surface of fabric  13 . 14 - 40 ). Although not shown in  FIGS.  13 . 14 - 8   , a gap-filling foam, such as foam  13 . 14 - 48  of  FIGS.  13 . 14 - 7   , may be incorporated in or on nosepiece  13 . 14 - 28  in addition to deformable stiffener  13 . 14 - 58 . 
     In some embodiments, to ensure that nosepiece  13 . 14 - 28  is tightly sealed to the nose of a user, internal components of device  13 . 14 - 10 , such as fans, may be used to move air toward nosepiece  13 . 14 - 28 , thereby sealing nosepiece  13 . 14 - 28  around the user&#39;s nose. 
     In addition to improving the fit of nosepiece  13 . 14 - 28  to prevent light from entering the eye boxes of the user, it may also be desirable to incorporate layers into nosepiece  13 . 14 - 28  that improve the comfort for the user. Examples of various modifications that may be made to nosepiece  13 . 14 - 28  to improve user comfort are shown in  FIGS.  13 . 14 - 9 A- 13 . 14 - 9 G . 
     As shown in  FIGS.  13 . 14 - 9 A , a nosepiece, which includes elastomer  13 . 14 - 34  and fabric  13 . 14 - 40  (such as nosepiece  13 . 14 - 28 ) may have a rolled edge and may approach the user&#39;s nose at a shallow angle of contact to conform to a curved portion of nose  13 . 14 - 60 . The rolled edge may prevent elastomer  13 . 14 - 34  from digging into the user&#39;s skin on nose  13 . 14 - 60 , while the shallower angle of contact may allow the nosepiece to move up the user&#39;s nose rather than hitting the user&#39;s nose directly. Therefore, by having a rolled edge and/or a shallower angle of contact, nosepiece  13 . 14 - 28  may improve comfort for a user of device  13 . 14 - 10 . 
     Alternatively or additionally, a nosepiece may include foam, such as foam  13 . 14 - 62 , between some or all of elastomer  13 . 14 - 34  and fabric  13 . 14 - 40 , as shown in  FIGS.  13 . 14 - 9 B . In particular, foam  13 . 14 - 62  may be at an edge portion of the nosepiece that contacts the user&#39;s nose, and therefore prevent elastomer  13 . 14 - 34  from hurting the user&#39;s nose or reducing discomfort as the nosepiece pushes against the nose. 
     In some examples, it may be desirable to have a series of openings in elastomer  13 . 14 - 34  that are either unfilled or filled with different material, such as foam. For example, in  FIGS.  13 . 14 - 9 C , openings  13 . 14 - 64  may be provided in a portion of the nosepiece that will contact the user&#39;s nose. Openings  13 . 14 - 64  may be through openings, partial openings, may be filled with other material, such as foam, or may otherwise be more flexible than the surrounding regions of elastomer  13 . 14 - 34 . As the nosepiece pushes against the user&#39;s nose, openings  13 . 14 - 64  may crumple, thereby preventing or reducing discomfort to the user. 
     If desired, a portion of fabric may be extended from the portion that contacts the user&#39;s nose to provide an additional buffer between the nose and elastomer  13 . 14 - 34 . As shown in  FIGS.  13 . 14 - 9 D , fabric portion  13 . 14 - 66  may extend from fabric  13 . 14 - 40 . When worn by a user, fabric portion  13 . 14 - 66  may contact the nose of the user first, and therefore increase the amount of fabric between the nose and elastomer  13 . 14 - 34 , which may improve user comfort. In some examples, fabric portion  13 . 14 - 66  may be a hemmed edge of fabric  13 . 14 - 40 . 
     If desired, at least some portions of elastomer  13 . 14 - 34  that would otherwise contact the user&#39;s nose may be cut and replaced by more flexible material, such as foam. As shown in  FIGS.  13 . 14 - 9 E , foam portions  13 . 14 - 68  may replace portions of elastomer  13 . 14 - 34  that would otherwise contact the user&#39;s nose. Although fabric  13 . 14 - 40  does not cover foam portions  13 . 14 - 68  in  FIGS.  13 . 14 - 9 E , fabric  13 . 14 - 40  may overlap foam portions  13 . 14 - 68 , if desired. 
     In addition to, or instead of, the modifications of  FIGS.  13 . 14 - 9 A- 13 . 14 - 9 E , in which an edge portion of a nosepiece is modified to prevent the elastomer from providing discomfort to a user&#39;s nose, a bottom portion of the nosepiece may be modified. As shown in  FIGS.  13 . 14 - 9 F , segmented material  13 . 14 - 69  may be added to a bottom surface of nosepiece  13 . 14 - 28 . For example, segmented material  13 . 14 - 69  may contact the top of the user&#39;s nose (i.e., the nose bridge), and prevent nosepiece  13 . 14 - 28  from directly contacting the nose and causing discomfort. Material  13 . 14 - 69  may be foam or may be elastomer flaps. 
     Although  FIGS.  13 . 14 - 9 F  shows material  13 . 14 - 69  as being segmented and non-overlapping, material  13 . 14 - 69  may be segmented and overlapping, or may be one connected piece of material, if desired. For example, in  FIGS.  13 . 14 - 9 G , foam  13 . 14 - 73  may be provided between nosepiece  13 . 14 - 28  and the user&#39;s nose while device  13 . 14 - 10  is worn. If desired, optional deformable stiffener  13 . 14 - 71  may be provided on the bottom surface of nosepiece  13 . 14 - 28  to improve the fit of nosepiece  13 . 14 - 28  on the user&#39;s nose. If stiffener  13 . 14 - 71  is included, foam  13 . 14 - 73  may also prevent stiffener  13 . 14 - 71  from directly contacting the user&#39;s nose and providing discomfort. 
     All of the examples in  FIGS.  13 . 14 - 9 A- 13 . 14 - 9 F  are merely illustrative examples of improving user comfort while ensuring that a nosepiece fits tightly to the user&#39;s nose. The examples are not limiting and may be implemented individually or together in any combination. Regardless of whether any structures in  FIGS.  13 . 14 - 9 A- 13 . 14 - 9 F  are used, it may be desirable to strengthen a portion of nosepiece  13 . 14 - 28 . Although the use of a structural frame was described in connection with  FIGS.  13 . 14 - 4   , such a frame may add unnecessary bulk or add too much rigidity to nosepiece  13 . 14 - 28  in some cases. Therefore, a semi-rigid member may be used. An example of using a semi-rigid member on a nosepiece is shown in  FIGS.  13 . 14 - 10   . 
     As shown in  FIGS.  13 . 14 - 10   , a nosepiece, such as nosepiece  13 . 14 - 28 , may include portions  13 . 14 - 70  and  13 . 14 - 72 . Portion  13 . 14 - 70  may include an elastomer, such as elastomer  13 . 14 - 34 , and/or a fabric, such as fabric portion  13 . 14 - 72  may be coupled to portion  13 . 14 - 70  and may include a semi-rigid member. In particular, the semi-rigid member may be formed on a bottom portion of nosepiece  13 . 14 - 28  (i.e., toward the bottom of the user&#39;s nose when device  13 . 14 - 10  is worn), and may provide additional rigidity to nosepiece  13 . 14 - 28 . The semi-rigid members may be formed from any desired material, such as rubber or plastic. In this way, nosepiece  13 . 14 - 28  may remain sufficiently flexible to conform to the shape of a user&#39;s nose, while retaining its shape when after it has been conformed to the shape of the nose and the device is being worn by the user. 
     The foregoing is merely illustrative and various modifications can be made to the described embodiments. The foregoing embodiments may be implemented individually or in any combination. 
     13.15: Removable Facial Interface 
     According to some aspects of the present disclosure, a head-mountable device (HMD) can include a display; a facial interface frame at least partially surrounding the display; a removable facial interface attached to the facial interface frame; a first attachment mechanism attached to one of the facial interface frame or the removable facial interface; and a second attachment mechanism attached to the other of the facial interface frame or the removable facial interface. In some examples, the removable facial interface can be attached to the facial interface frame by the first attachment mechanism and the second attachment mechanism. 
     In some examples, the first attachment mechanism and the second attachment mechanism can include magnets. In some examples, the first attachment mechanism can include a spherical magnetic receptacle. The second attachment mechanism can include a spherical magnet. In some examples, the first attachment mechanism can include a capsule-dished magnetic receptacle. The second attachment mechanism can include a spherical magnet. In some examples, the first attachment mechanism can include a magnetic post. The second attachment mechanism can include a magnetic protrusion. The first attachment mechanism can be configured to slidably engage and disengage the second attachment mechanism. 
     In some examples, the first attachment mechanism and the second attachment mechanism can include hook-and-loop fasteners. In some examples, the first attachment mechanism can include a post and a flange on the post. The second attachment mechanism can include a spring-snap feature. The spring-snap feature can include an opening; a detent adjacent the opening; and a spring attached to the detent. 
     In some examples, the first attachment mechanism can include a receptacle and a first magnet in the receptacle. The second attachment mechanism can include a protrusion and a second magnet in the protrusion. The protrusion can have a first shape complementary to a second shape of the receptacle. 
     In some examples, the first attachment mechanism and the second attachment mechanism can include interlocking elastic fasteners. In some examples, the first attachment mechanism can include a suction cup. The second attachment mechanism can include a curved surface shaped to interface with the suction cup. 
     In some examples, the removable facial interface can be C-shaped. The facial interface frame can include a C-shaped base complementary to the removable facial interface. 
     In some examples, the removable facial interface can include a fabric material encircling one of the first attachment mechanism or the second attachment mechanism. The facial interface frame can include a compressible material encircling the other of the first attachment mechanism or the second attachment mechanism. The first attachment mechanism can be directly attached to the second attachment mechanism. 
     According to some aspects, a wearable electronic device can include a display; a facial interface frame physically coupled to the display; and a removable facial interface removably attached to the connector of the facial interface frame by a first attachment mechanism. The facial interface frame may include a frame partially surrounding the display; a connector attached to the frame; and a base attached to the connector. The connector may extend through the base. 
     In some examples, the first attachment mechanism can include a magnetic attachment mechanism. The magnetic attachment mechanism can include a post on one of the connector or the removable facial interface, and a receptacle on the other of connector or the removable facial interface. 
     In some examples, the first attachment mechanism can include a hook-and-loop attachment mechanism. The hook-and-loop attachment mechanism can include a post on one of the connector or the removable facial interface and a receptacle on the other of connector or the removable facial interface. 
     In some examples, the wearable electronic device can further include a compressible member attached to the base. The compressible member can encircle the connector and the connector can be exposed through the compressible member. 
     According to some aspects, a light seal for a head-mountable device can include a facial interface frame. The light seal can include a base; a compressible portion; and a magnet attached to the base. 
     In some examples, the light seal can further include a hook-and-loop fastener attached to the base. In some examples, the light seal can further include an interlocking fastener attached to the base. In some examples, the light seal can further include a receptacle attached to the base. 
     The following disclosure relates to wearable electronic devices (e.g., head-mountable devices (HMDs)), including those that include a removable facial interface. Prolonged use of a head-mountable device can cause components of the head-mountable device to wear and to become dirty. In particular, components of the head-mountable device that contact a user&#39;s face are subject to wear and becoming dirty due to the contact with the user&#39;s face. It can be desirable to clean the portions of the head-mountable device that contact the user&#39;s face in order to remove oils, sweat, and the like. However, the head-mountable device can include sensitive components, small components, and oddly shaped components that are difficult to clean in-situ. Further, it can be desirable to replace portions of the head-mountable device that contact the user&#39;s face when those portions become worn out, in order to obtain a more comfortable fit for specific users, based on other user preferences, or the like. 
     A head-mountable device of the present disclosure includes a removable facial interface. As will be discussed in detail below, the removable facial interface can be attached to a facial interface frame through various attachment mechanisms, such as magnets, interlocking features, sliding features, hook-and-loop features (e.g., Velcro), spring snaps, suction features, bi-stable features, stretch features, re-usable adhesives, mating posts, combinations thereof, or the like. The removable facial interface can be easily removed from the facial interface frame, and can be re-installed on the facial interface frame in order to allow for the removable facial interface to be cleaned, replaced, or the like. Allowing the removable facial interface to be removed and cleaned increases the longevity of the removable facial interface. Replacing the removable facial interface improves the longevity of the head-mountable device, reduces user costs, provides users with choices of removable facial interfaces based on fit and personal preferences, and provides additional benefits. 
     These and other examples are discussed below with reference to  FIGS.  13 . 15 - 1 A through  13 . 15 - 15 C . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these Figures is for explanatory purposes only and should not be construed as limiting. Furthermore, as used herein, a system, a method, an article, a component, a feature, or a sub-feature including at least one of a first option, a second option, or a third option should be understood as referring to a system, a method, an article, a component, a feature, or a sub-feature that can include one of each listed option (e.g., only one of the first option, only one of the second option, or only one of the third option), multiple of a single listed option (e.g., two or more of the first option), two options simultaneously (e.g., one of the first option and one of the second option), or combination thereof (e.g., two of the first option and one of the second option). 
       FIGS.  13 . 15 - 1 A  illustrates a block diagram of a head-mountable device (HMD)  13 . 15 - 100  including a display  13 . 15 - 106 , a frame  13 . 15 - 104 , a device seal  13 . 15 - 116 , and a retention band  13 . 15 - 118 . The display  13 . 15 - 106  can include one or more optical lenses or display screens that are configured to be positioned in front of the eyes of a user. The display  13 . 15 - 106  can be configured to present an augmented reality visualization, a virtual reality visualization, or another suitable visualization to the user. The display  13 . 15 - 106  can be positioned at least partially in or on the frame  13 . 15 - 104 . The frame  13 . 15 - 104  can be a housing of the display  13 . 15 - 106 . The device seal  13 . 15 - 116  can be physically coupled to the frame  13 . 15 - 104 . The retention band  13 . 15 - 118  can be physically coupled to the device seal  13 . 15 - 116  and/or the frame  13 . 15 - 104  and can be used to secure the HMD  13 . 15 - 100  on the head of the user. In some examples, the device seal  13 . 15 - 116  includes the frame  13 . 15 - 104  (e.g., the frame  13 . 15 - 104  is part of the device seal  13 . 15 - 116 ). 
     The device seal  13 . 15 - 116  includes a facial interface frame  13 . 15 - 112 , a removable facial interface  13 . 15 - 110 , a cover  13 . 15 - 114 , and electrical components (e.g., sensors  13 . 15 - 120 ). The device seal  13 . 15 - 116  can also be referred to as a light seal. In some examples, the device seal  13 . 15 - 116  can refer to a portion of the HMD  13 . 15 - 100  that engages or shields a user&#39;s face. The device seal  13 . 15 - 116  can include portions of the HMD  13 . 15 - 100  that conform to, contact, or press against regions of the user&#39;s face (e.g., the facial interface frame  13 . 15 - 112  and the removable facial interface  13 . 15 - 110 ). 
     The removable facial interface  13 . 15 - 110  refers to a portion of the HMD  13 . 15 - 100  that directly contacts a user&#39;s face. The facial interface frame  13 . 15 - 112  refers to a portion of the HMD  13 . 15 - 100  to which the removable facial interface  13 . 15 - 110  is attached, and which is physically coupled between the removable facial interface  13 . 15 - 110  and the frame  13 . 15 - 104 . The removable facial interface  13 . 15 - 110  and/or the facial interface frame  13 . 15 - 112  can also be referred to as a face track. The removable facial interface  13 . 15 - 110  and the facial interface frame  13 . 15 - 112  can conform to (e.g., compress against and assume the shape of) regions of the user&#39;s face. In some examples, the removable facial interface  13 . 15 - 110  and the facial interface frame  13 . 15 - 112  include pliant (or semi-pliant) materials that span the forehead, wrap partially around the eyes, and contact the zygoma and maxilla regions of the user&#39;s face. 
     In some examples, the facial interface frame  13 . 15 - 112  can be formed of a relatively stiff or rigid material, while the removable facial interface  13 . 15 - 110  can be formed of a relatively soft, deformable, elastic material. In some examples, the facial interface frame  13 . 15 - 112  can be formed from plastics, metals, polymers, combinations thereof, or the like. The removable facial interface  13 . 15 - 110  can be formed from plastics, metals, polymers, fabrics, foams, rubbers, silicone, elastomers, hydrogels, combinations thereof, or the like. In some examples, the removable facial interface  13 . 15 - 110  can include a stiffener, which can be formed from metals, plastics, polymers, combinations thereof, or the like. The removable facial interface  13 . 15 - 110  can include a pliable material surrounding the stiffener, which can be formed from rubbers, foams, polymers, silicone, elastomers, hydrogen gels, combinations thereof, or the like. The pliable material can be formed around the stiffener through overmolding or the like. The removable facial interface  13 . 15 - 110  can further include a fabric material surrounding the pliable material. The fabric material can be wrapped around the pliable material. The stiffener can provide the removable facial interface  13 . 15 - 110  with a desired degree of stiffness. The pliable material can be pliant or semi-pliant and can be provided to increase user comfort. The fabric material can be configured to contact a user&#39;s skin and can also be provided to increase user comfort. 
     The removable facial interface  13 . 15 - 110  can be attached to the facial interface frame  13 . 15 - 112  through various attachment mechanisms. For example, the attachment mechanisms can include magnets, interlocking features, sliding features, hook-and-loop features (e.g., Velcro), spring snaps, suction features, bi-stable features, stretch features, re-usable adhesives, mating posts, combinations thereof, or the like. The attachment mechanisms can be provided on both the removable facial interface  13 . 15 - 110  and the facial interface frame  13 . 15 - 112 . For example, a male attachment mechanism can be provided on the removable facial interface  13 . 15 - 110  and a female attachment mechanism can be provided on the facial interface frame  13 . 15 - 112 , or vice versa. The attachment mechanisms can be self-aligning or can entail alignment by a user. The attachment mechanisms can be configured such that the removable facial interface  13 . 15 - 110  is removed from, and attached to, the facial interface frame  13 . 15 - 112  in one-handed operations. Each of the respective attachment mechanisms can datum the removable facial interface  13 . 15 - 110  to the facial interface frame  13 . 15 - 112 , or allow the removable facial interface  13 . 15 - 110  to float or move relative to the facial interface frame  13 . 15 - 112 . The respective attachment mechanisms can have variable shapes and/or sizes around the perimeters of the removable facial interface  13 . 15 - 110  and the facial interface frame  13 . 15 - 112 , which can improve the ability to interchange various removable facial interfaces  13 . 15 - 110  and facial interface frames  13 . 15 - 112 . More specifically, the variable shapes and/or sizes of the attachment mechanisms around the perimeters of the removable facial interface  13 . 15 - 110  and the facial interface frame  13 . 15 - 112  can allow for different shaped removable facial interfaces  13 . 15 - 110  and facial interface frames  13 . 15 - 112  to be connected together. 
     Providing the removable facial interface  13 . 15 - 110  and the facial interface frame  13 . 15 - 112  allows for the removable facial interface  13 . 15 - 110  to be easily removed and cleaned. The removable facial interface  13 . 15 - 110  can be replaced based on wear, user preferences, or the like. For example, specific removable facial interfaces  13 . 15 - 110  can be provided based on different facial structures of various users. Removable facial interfaces  13 . 15 - 110  can be provided based on different intended user uses, such as a specific removable facial interface  13 . 15 - 110  designed for sport use. Providing the removable facial interface  13 . 15 - 110  allows for a portion of the HMD  13 . 15 - 100  that is subject to the greatest amount of wear to be cheaply and easily replaced. 
     The cover  13 . 15 - 114  can include a seal, such as an environment seal, a dust seal, an air seal, a light seal, or the like. The cover  13 . 15 - 114  can be positioned in a gap between the display  13 . 15 - 106  and the user&#39;s face. The cover  13 . 15 - 114  can form an eye-box through which the user can view the display  13 . 15 - 106 . It will be appreciated that the term “seal” can include partial seals or inhibitors, in addition to complete seals. For example, a seal can be a partial light seal, where some ambient light is blocked. In some examples, a seal can be a complete light seal where all ambient light is blocked when the HMD  13 . 15 - 100 . 
     The cover  13 . 15 - 114  can be a woven fabric that is non-rigid or deformable. The cover  13 . 15 - 114  can be elastically deformable. In some examples, the cover  13 . 15 - 114  can be formed from a plastic, a rubber, a polymer material, or the like. The cover  13 . 15 - 114  can be a cosmetic textile material with stretch and can feature an open mesh pattern. In some examples, the cover  13 . 15 - 114  can be rigid. Providing the removable facial interface  13 . 15 - 110  separate from the cover  13 . 15 - 114  allows for the cover  13 . 15 - 114  and a material covering the removable facial interface  13 . 15 - 110  to be formed from different materials, such as the cover  13 . 15 - 114  being formed from a cosmetic material and the removable facial interface  13 . 15 - 110  being formed from a comfortable material. In some examples, the cover  13 . 15 - 114  and the covering of the removable facial interface  13 . 15 - 110  can be formed from the same materials. 
     The device seal  13 . 15 - 116  can be removably attached to the frame  13 . 15 - 104 . The device seal  13 . 15 - 116  can be electrically coupled to the display  13 . 15 - 106 . The device seal  13 . 15 - 116  can include various electrical components, such as the sensors  13 . 15 - 120 . The sensors  13 . 15 - 120  can include various sensors, such as sensors that collect user data, or environmental data. In some examples, the sensors  13 . 15 - 120  can collect biometric information. The sensors  13 . 15 - 120  can transmit signals to other components of the HMD  13 . 15 - 100 , such as the display  13 . 15 - 106 . The sensors  13 . 15 - 120  can transmit signals to various outputs, such as an output configured to perform an action in response to information collected by the sensors  13 . 15 - 120 . 
       FIGS.  13 . 15 - 1 B  illustrates a top view of an HMD  13 . 15 - 100 . The HMD  13 . 15 - 100  of  FIGS.  13 . 15 - 1 B  can be substantially similar to, including some or all of the features of, the HMD  13 . 15 - 100  described with respect to  FIGS.  13 . 15 - 1 A . The HMD  13 . 15 - 100  includes a display (also referred to as a display unit)  13 . 15 - 106  and a retention band  13 . 15 - 118 . In some examples, the display  13 . 15 - 106  includes an opaque, translucent, transparent, or semi-transparent screen, including any number lenses, for presenting visual data to a user. The display  13 . 15 - 106  can include any number of internal electronic components  13 . 15 - 108 . The HMD  13 . 15 - 100  can be mounted on a user&#39;s head  13 . 15 - 102  using the retention band  13 . 15 - 118 . 
     The HMD  13 . 15 - 100  further includes a frame  13 . 15 - 104  (also referred to as a housing), a facial interface frame  13 . 15 - 112 , a removable facial interface  13 . 15 - 110 , and a cover  13 . 15 - 114 . The frame  13 . 15 - 104  can be physically coupled to the display  13 . 15 - 106 . The frame  13 . 15 - 104  can at least partially border one or more edges of the display  13 . 15 - 106 . One end of the cover  13 . 15 - 114  can be attached to the frame  13 . 15 - 104  and an opposite end of the cover  13 . 15 - 114  can be attached to the facial interface frame  13 . 15 - 112 . The facial interface frame  13 . 15 - 112  and the removable facial interface  13 . 15 - 110  provide an interface between a user&#39;s head  13 . 15 - 102  and the frame  13 . 15 - 104 . The combination of the frame  13 . 15 - 104 , the facial interface frame  13 . 15 - 112 , the removable facial interface  13 . 15 - 110 , and the cover  13 . 15 - 114  can form a device seal  13 . 15 - 116 . It will be understood, however, that the device seal  13 . 15 - 116  can include fewer or additional components from those listed or shown. 
     The HMD  13 . 15 - 100  can be worn on the user&#39;s head  13 . 15 - 102  such that the display  13 . 15 - 106  is positioned on the user&#39;s face and disposed in front of one or both of the user&#39;s eyes. The display  13 . 15 - 106  can be physically coupled to the retention band  13 . 15 - 118  and/or the device seal  13 . 15 - 116 . In some examples, the retention band  13 . 15 - 118  can be positioned against sides of the user&#39;s head  13 . 15 - 102  and in contact therewith. In some examples, the retention band  13 . 15 - 118  can be at least partially positioned above the user&#39;s ear or ears. In some examples, the retention band  13 . 15 - 118  can be positioned adjacent to the user&#39;s ear or ears. The retention band  13 . 15 - 118  can extend around the user&#39;s head  13 . 15 - 102 . In this way, the display  13 . 15 - 106  and the retention band  13 . 15 - 118  can form a loop configured to retain the HMD  13 . 15 - 100  on the user&#39;s head  13 . 15 - 102 . It should be understood, however, that this configuration is just one example of how the components of the HMD  13 . 15 - 100  can be arranged. In some examples, a different number of connector straps and/or retention bands can be included. Although the HMD  13 . 15 - 100  is referred to as an HMD, it should be understood that the terms wearable device, wearable electronic device, HMD device, and/or HMD system can be used to refer to any wearable device, including smart glasses. 
     In some examples, the frame  13 . 15 - 104  is physically coupled to the facial interface frame  13 . 15 - 112 . The removable facial interface  13 . 15 - 110  is removably attached to the facial interface frame  13 . 15 - 112 . The removable facial interface  13 . 15 - 110  can contact the user&#39;s head  13 . 15 - 102 , such as a user&#39;s face. In some examples, the cover  13 . 15 - 114  can be a light blocking component that extends between the frame  13 . 15 - 104  and the removable facial interface  13 . 15 - 110 , such as along portions of the facial interface frame  13 . 15 - 112 . The cover  13 . 15 - 114  can cover or surround a perimeter of the frame  13 . 15 - 104  and/or the facial interface frame  13 . 15 - 112 . 
     The cover  13 . 15 - 114  can be formed from a cloth, fabric, woven material, plastic, rubber, or any other suitable opaque or semi-opaque material. In some examples, the cover  13 . 15 - 114  is flexible, having the ability to repeatedly stretch, compress, and deform. The cover  13 . 15 - 114  can be elastically or in-elastically deformable. The facial interface frame  13 . 15 - 112 , the cover  13 . 15 - 114 , and the removable facial interface  13 . 15 - 110  can be configured to block outside light and limit the peripheral view of the user. In some examples, the cover  13 . 15 - 114  and the facial interface frame  13 . 15 - 112  are part of the same or a unitary component. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 15 - 1 A and  13 . 15 - 1 B  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures, including in any of  FIGS.  13 . 15 - 1 A and  13 . 15 - 1 B , described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 15 - 1 A and  13 . 15 - 1 B . 
       FIGS.  13 . 15 - 2 A  illustrates a perspective view of a device seal  13 . 15 - 200 . The device seal  13 . 15 - 200  can be substantially similar to, including some or all of the features of, the device seals described herein, such as the device seal  13 . 15 - 116 . The device seal  13 . 15 - 200  can be implemented on an HMD, such as the HMD  13 . 15 - 100  described in reference to  FIGS.  13 . 15 - 1 A  and  13 . 15 - 1 B. The device seal  13 . 15 - 200  can include a removable facial interface  13 . 15 - 202 , a facial interface frame  13 . 15 - 204 , and a cover  13 . 15 - 206 . The removable facial interface  13 . 15 - 202 , the facial interface frame  13 . 15 - 204 , and the cover  13 . 15 - 206  can be substantially similar to, including some or all of the features of, the removable facial interface  13 . 15 - 110 , the facial interface frame  13 . 15 - 112 , and the cover  13 . 15 - 114 , respectively. The cover  13 . 15 - 206  can be included in the device seal  13 . 15 - 200  to provide a seal between the device seal  13 . 15 - 200  and the outer environment. 
     The facial interface frame  13 . 15 - 204  can include a frame  13 . 15 - 210 , connectors  13 . 15 - 212 , a base  13 . 15 - 214 , and a compressible portion  13 . 15 - 216 . The frame  13 . 15 - 210  can be physically coupled to a display of the HMD, and the frame  13 . 15 - 210  can be configured provide an interface between the display and the facial interface frame  13 . 15 - 204 . The removable facial interface  13 . 15 - 202  can be removably attached to the base  13 . 15 - 214 , and the base  13 . 15 - 214  can be configured provide an interface between the removable facial interface  13 . 15 - 202  and the facial interface frame  13 . 15 - 204 . The connectors  13 . 15 - 212  can be configured to provide a desired distance between the display and a user&#39;s eyes when the HMD is donned by the user. The connectors  13 . 15 - 212  and/or the base  13 . 15 - 214  can include attachment mechanisms that are used to attach the removable facial interface  13 . 15 - 202  to the facial interface frame  13 . 15 - 204 . 
     The compressible portion  13 . 15 - 216  can be included in the facial interface frame  13 . 15 - 204  to fill a gap between the base  13 . 15 - 214  and the removable facial interface  13 . 15 - 202 , and to provide improved user comfort. The compressible portion  13 . 15 - 216  can also be referred to as a compressible member or a compressible material. In some examples, the connectors  13 . 15 - 212  (e.g., an attachment mechanism) can extend through the compressible portion  13 . 15 - 216 . The compressible portion  13 . 15 - 216  can encircle portions of the connectors  13 . 15 - 212  that extend through the compressible portion  13 . 15 - 216 . The connectors  13 . 15 - 212  can also extend through the base  13 . 15 - 214 , and attachment mechanisms provided on the connectors  13 . 15 - 212  can be exposed through both the compressible portion  13 . 15 - 216  and the base  13 . 15 - 214  for access and connection. In some examples, the compressible portion  13 . 15 - 216  can include a foam material. In some examples, the compressible portion  13 . 15 - 216  can include materials with properties that impart flexibility, softness, compressibility, deformability, and the like. Examples of materials that can be used for the compressible portion  13 . 15 - 216  include silicone, polymers, elastomers, hydrogels, combinations thereof, or the like. The compressible portion  13 . 15 - 216  can be formed by molding, printing, casting, or the like. In some examples, attachment mechanisms, such as perimeter attachments, can be included on the compressible portion  13 . 15 - 216 . The compressible portion  13 . 15 - 216  provides a soft, flexible, deformable interface between the removable facial interface  13 . 15 - 202  and rigid components of the removable facial interface  13 . 15 - 202 , such as the frame  13 . 15 - 210 , the connectors  13 . 15 - 212 , and the base  13 . 15 - 214 . This interface provides for improved user comfort when the HMD is donned. 
     The removable facial interface  13 . 15 - 202  is a removable component that is configured to contact a user&#39;s face. The removable facial interface  13 . 15 - 202  can be formed of conformable and comfortable materials. The removable facial interface  13 . 15 - 202  is configured to be easily removed from and attached to the facial interface frame  13 . 15 - 204 . This allows for the removable facial interface  13 . 15 - 202  to be removed, cleaned, replaced, and the like. In some examples, the attachment mechanisms used to attach the removable facial interface  13 . 15 - 202  to the facial interface frame  13 . 15 - 204  can be simple mechanisms that allow the removable facial interface  13 . 15 - 202  to be removed from and attached to the facial interface frame  13 . 15 - 204  in a one-handed manner. Alignment features can be provided on the removable facial interface  13 . 15 - 202  and/or the facial interface frame  13 . 15 - 204  to aid in the alignment of the removable facial interface  13 . 15 - 202  with respect to the facial interface frame  13 . 15 - 204 . Providing the removable facial interface  13 . 15 - 202  as a removable component allows the removable facial interface  13 . 15 - 202  to be easily cleaned, replaced as a result of wear, replaced due to user preferences, and the like. This reduces maintenance costs for HMDs including the removable facial interface  13 . 15 - 202 . 
     The removable facial interface  13 . 15 - 202  can be formed of a relatively pliant material, while the components of the facial interface frame  13 . 15 - 204  are formed of relatively rigid materials. This allows the removable facial interface  13 . 15 - 202  to conform to the shape of the facial interface frame  13 . 15 - 204  and the shape of a user&#39;s face or head when the user dons the HMD. As illustrated in  FIGS.  13 . 15 - 2 A , the base  13 . 15 - 214 , the compressible portion  13 . 15 - 216 , and the removable facial interface  13 . 15 - 202  can be C-shaped and can have shapes corresponding to one another. In other words, the removable facial interface  13 . 15 - 202  is C-shaped, and the base  13 . 15 - 214  and the compressible portion  13 . 15 - 216  are C-shaped and complementary to the C-shaped removable facial interface  13 . 15 - 202 . This provides a visual cue to users when the removable facial interface  13 . 15 - 202  is removed from the base  13 . 15 - 214  that the removable facial interface  13 . 15 - 202  is intended to be attached to the base  13 . 15 - 214 . In some examples, attachment mechanisms on the removable facial interface  13 . 15 - 202  and the facial interface frame  13 . 15 - 204  can be visible when the removable facial interface  13 . 15 - 202  is removed from the facial interface frame  13 . 15 - 204 , which provides further visual cues as to how the removable facial interface  13 . 15 - 202  is intended to be attached to the facial interface frame  13 . 15 - 204 . Moreover, upon seeing the attachment mechanisms on the facial interface frame  13 . 15 - 204 , a user can realize that something is missing from the facial interface frame  13 . 15 - 204  when the removable facial interface  13 . 15 - 202  is removed from the facial interface frame  13 . 15 - 204 . 
       FIGS.  13 . 15 - 2 B  illustrates a perspective view of a facial interface frame  13 . 15 - 204 . The facial interface frame  13 . 15 - 204  can include a frame  13 . 15 - 210 , connectors  13 . 15 - 212 , and a base  13 . 15 - 214 . The frame  13 . 15 - 210 , the connectors  13 . 15 - 212 , and the base  13 . 15 - 214  of the facial interface frame  13 . 15 - 204  can be formed of relatively rigid materials, such as plastics, metals, polymers, combinations thereof, or the like. In some examples, the base  13 . 15 - 214  can be formed of a relatively flexible material, such that the base  13 . 15 - 214  is allowed to flex when an HMD including the facial interface frame  13 . 15 - 204  is donned by a user. In some examples, the frame  13 . 15 - 210 , the connectors  13 . 15 - 212 , and/or the base  13 . 15 - 214  can include rigid materials that can be coated in relatively soft materials, such as polymers, fabrics, foams, rubbers, silicone, elastomers, hydrogels, combinations thereof, or the like to increase user comfort. 
     The frame  13 . 15 - 210  can provide an interface between a display of the HMD and the facial interface frame  13 . 15 - 204 . For example, the frame  13 . 15 - 210  can be physically coupled to the display of the HMD and can hold the display in a desired position. The connectors  13 . 15 - 212  can control the spacing between the display and a user&#39;s face. For example, depending on a length of the connectors  13 . 15 - 212 , the display can be disposed closer to or further from a user&#39;s face. The base  13 . 15 - 214  and/or the connectors  13 . 15 - 212  can provide an interface between the facial interface frame  13 . 15 - 204  and a removable facial interface. For example, attachment mechanisms can be included on the base  13 . 15 - 214  and/or the connectors  13 . 15 - 212  and can be used to removably attach the removable facial interface to the base  13 . 15 - 214  and/or the connectors  13 . 15 - 212 . 
     In some examples, post attachments can be provided on the connectors  13 . 15 - 212  and perimeter attachments can be provided on the base  13 . 15 - 214 . As will be discussed in detail below, the post attachments and corresponding attachment mechanisms on the removable facial interface can be configured to remain fixed relative to one another or have a limited amount of travel when the removable facial interface is mounted to the facial interface frame  13 . 15 - 204 . The perimeter attachments and corresponding attachment mechanisms on the removable facial interface can be configured to slide relative to one another when the removable facial interface is mounted to the facial interface frame  13 . 15 - 204 . Including both the post attachments and the perimeter attachments allows the removable facial interface to be securely mounted on the facial interface frame  13 . 15 - 204 , while allowing the removable facial interface to conform comfortably to a user&#39;s face. 
       FIGS.  13 . 15 - 2 C  illustrates a perspective view of portions of a removable facial interface  13 . 15 - 202  and a facial interface frame  13 . 15 - 204 . The removable facial interface  13 . 15 - 202  can include a body portion  13 . 15 - 220 , a side mount  13 . 15 - 222 , a top mount  13 . 15 - 224 , and a mounting area  13 . 15 - 226 . The body portion  13 . 15 - 220  can be formed of materials such as plastics, metals, polymers, fabrics, foams, rubbers, silicone, elastomers, hydrogels, combinations thereof, or the like. The facial interface frame  13 . 15 - 204  can include a frame  13 . 15 - 210 , connectors  13 . 15 - 212 , and a base  13 . 15 - 214 . 
     The side mount  13 . 15 - 222 , the top mount  13 . 15 - 224 , and the mounting area  13 . 15 - 226  are examples of attachment mechanisms that can be included in the removable facial interface  13 . 15 - 202 . The side mount  13 . 15 - 222 , the top mount  13 . 15 - 224 , and the mounting area  13 . 15 - 226  can be used to removably attach the removable facial interface  13 . 15 - 202  to the facial interface frame  13 . 15 - 204 . In some examples, post attachments can be provided on the connectors  13 . 15 - 212  corresponding to the side mount  13 . 15 - 222  and the top mount  13 . 15 - 224 . Perimeter attachments can be provided on the base  13 . 15 - 214  corresponding to the mounting area  13 . 15 - 226 . Specifically, when the removable facial interface  13 . 15 - 202  is aligned with and mounted to the facial interface frame  13 . 15 - 204 , the side mount  13 . 15 - 222  and the top mount  13 . 15 - 224  are aligned with corresponding post attachments on the connectors  13 . 15 - 212  and the mounting area  13 . 15 - 226  is aligned with corresponding perimeter attachments on the base  13 . 15 - 214 . 
     The side mount  13 . 15 - 222 , the top mount  13 . 15 - 224 , and corresponding post attachments of the connectors  13 . 15 - 212  can include attachment mechanisms that are configured to remain fixed relative to one another, or to have a relatively small travel relative to one another (e.g., the attachment mechanisms can float relative to one another). In some examples, the side mount  13 . 15 - 222  and a corresponding post attachment can be configured to remain fixed relative to one another and the top mount  13 . 15 - 224  and a corresponding post attachment can be configured to have some travel relative to one another. In some examples, the side mount  13 . 15 - 222 , the top mount  13 . 15 - 224 , and corresponding post attachments can include magnets, interlocking features, sliding features, hook-and-loop features (e.g., Velcro), spring snaps, suction features, bi-stable features, stretch features, re-usable adhesives, mating posts, combinations thereof, or the like. Posts and corresponding holes or other self-aligning features can be included in the side mount  13 . 15 - 222 , the top mount  13 . 15 - 224 , and the post attachments in order to align the removable facial interface  13 . 15 - 202  with the facial interface frame  13 . 15 - 204 . Male features can be provided in the side mount  13 . 15 - 222  and the top mount  13 . 15 - 224  and corresponding female features can be provided in the post attachments of the facial interface frame  13 . 15 - 204 , or vice versa. By using the prescribed attachment mechanisms and fixing the positions of the side mount  13 . 15 - 222  and the top mount  13 . 15 - 224  relative to the post attachments, the removable facial interface  13 . 15 - 202  can be aligned with the facial interface frame  13 . 15 - 204 . Moreover, the relative positions of the removable facial interface  13 . 15 - 202  and the facial interface frame  13 . 15 - 204  can be fixed to one another during use of an HMD by a user. This prevents misalignment of the removable facial interface  13 . 15 - 202  and the facial interface frame  13 . 15 - 204  from negatively impacting a user&#39;s experience when using the HMD. 
     The mounting area  13 . 15 - 226  and the corresponding perimeter attachments of the facial interface frame  13 . 15 - 204  can include attachment mechanisms that are configured to slide relative to one another. For example, the mounting area  13 . 15 - 226  and the corresponding perimeter attachments can include longitudinal interlocking features, rows of interlocking features, rows of magnets, lengths of flexible magnets, lengths of hook-and-loop fasteners, combinations thereof, or the like. Male features can be provided in the mounting area  13 . 15 - 226  and corresponding female features can be provided in the perimeter attachments of the facial interface frame  13 . 15 - 204 , or vice versa. In some examples, the mounting area  13 . 15 - 226  and the corresponding perimeter attachments are disposed in curved or corner regions of the removable facial interface  13 . 15 - 202  and the facial interface frame  13 . 15 - 204 , respectively. The mounting area  13 . 15 - 226  and the corresponding perimeter attachments can be disposed in areas of the removable facial interface  13 . 15 - 202  and the facial interface frame  13 . 15 - 204  that move or bend when the removable facial interface  13 . 15 - 202  is mounted on the facial interface frame  13 . 15 - 204  and when an HMD including the removable facial interface  13 . 15 - 202  and the facial interface frame  13 . 15 - 204  is donned by a user. This causes an arc length of the mounting area  13 . 15 - 226  and the corresponding perimeter attachments to change. By using the prescribed attachment mechanisms, the mounting area  13 . 15 - 226  and the corresponding perimeter attachments are allowed to slide relative to one another, and the removable facial interface  13 . 15 - 202  is securely fastened to the facial interface frame  13 . 15 - 204  even when the removable facial interface  13 . 15 - 202  moves relative to the facial interface frame  13 . 15 - 204 . 
     In the example of  FIGS.  13 . 15 - 2 C , a single top mount  13 . 15 - 224  and a single side mount  13 . 15 - 222  are illustrated. The top mount  13 . 15 - 224  can be configured to be disposed adjacent a user&#39;s forehead when the HMD is donned and the side mount  13 . 15 - 222  can be configured to be disposed adjacent the user&#39;s cheek bones when the HMD is donned. However, any number of top mounts  13 . 15 - 224  and side mounts  13 . 15 - 222  can be included in the removable facial interface  13 . 15 - 202 , and the top mounts  13 . 15 - 224  and the side mounts  13 . 15 - 222  can be configured to be disposed adjacent any portions of the user&#39;s face or head when the HMD is donned. Moreover, although the mounting area  13 . 15 - 226  is illustrated as being in a corner area of the removable facial interface  13 . 15 - 202  between the top mount  13 . 15 - 224  and the side mount  13 . 15 - 222 , the mounting area  13 . 15 - 226  can be disposed in any area along the perimeter of the removable facial interface  13 . 15 - 202  and any number or length of mounting areas  13 . 15 - 226  can be included. 
       FIGS.  13 . 15 - 2 D  illustrates a cross-sectional view of a facial interface shim  13 . 15 - 219 . The facial interface shim  13 . 15 - 219  can be used in conjunction with connectors of an HMD in order to control the spacing between a display of the HMD and a user&#39;s face. In some examples, the facial interface shim  13 . 15 - 219  can be disposed between a removable facial interface and a connector of a facial interface frame. For example, the facial interface shim  13 . 15 - 219  can be disposed in a mount opening  13 . 15 - 218  of the removable facial interface to which the connector of the facial interface frame is attached. In some examples, the facial interface shim  13 . 15 - 219  can be disposed between a base of a facial interface frame and a connector of the facial interface frame. For example, the connector can pass through the mount opening  13 . 15 - 218 , and the facial interface shim  13 . 15 - 219  can be disposed between the connector and the base. The facial interface shim  13 . 15 - 219  can have a thickness in a range from about 2 mm to about 4 mm. The facial interface shim  13 . 15 - 219  allows for distances between a display of an HMD and a user&#39;s face to be altered while standard length connectors are used, which decreases manufacturing costs. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 15 - 2 A through  13 . 15 - 2 D  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures, including in any of  FIGS.  13 . 15 - 2 A through  13 . 15 - 2 D , described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 15 - 2 A through  13 . 15 - 2 D . 
       FIGS.  13 . 15 - 3 A and  13 . 15 - 3 B  illustrate a perspective view of a device seal  13 . 15 - 200  and a plan view of a removable facial interface  13 . 15 - 202 , respectively. The device seal  13 . 15 - 200  includes the removable facial interface  13 . 15 - 202 , a facial interface frame  13 . 15 - 204 , and a cover  13 . 15 - 206 . The device seal  13 . 15 - 200  can be substantially similar to, including some or all of the features of, the device seals described herein, such as the device seals  13 . 15 - 116  and  13 . 15 - 200 . The device seal  13 . 15 - 200  can be implemented on an HMD, such as the HMD  13 . 15 - 100  described in reference to  FIGS.  13 . 15 - 1 A and  13 . 15 - 1 B . 
       FIGS.  13 . 15 - 3 A and  13 . 15 - 3 B  illustrate an example in which the attachment mechanisms used to attach the removable facial interface  13 . 15 - 202  to the facial interface frame  13 . 15 - 204  include magnets. In some examples, six connectors  13 . 15 - 212  can be provided between a base  13 . 15 - 214  and a frame  13 . 15 - 210  of the facial interface frame  13 . 15 - 204 . Four circular receptacles  13 . 15 - 250  can be provided corresponding to four of the connectors  13 . 15 - 212 . The connectors  13 . 15 - 212  can extend through the base  13 . 15 - 214 , such that the attachment mechanisms provided on the connectors  13 . 15 - 212  are exposed through the base  13 . 15 - 214 . The circular receptacles  13 . 15 - 250  can be disposed along portions of the facial interface frame  13 . 15 - 204  configured to be disposed adjacent to a user&#39;s cheek bones when an HMD including the device seal  13 . 15 - 200  is donned. A spherically-dished magnet  13 . 15 - 252  can be disposed in each of the circular receptacles  13 . 15 - 250 . Each of the circular receptacles  13 . 15 - 250  in combination with a corresponding spherically-dished magnet  13 . 15 - 252  may be referred to as a spherical magnetic receptacle. Two stadium or lozenge-shaped receptacles  13 . 15 - 254  can be provided corresponding to two of the connectors  13 . 15 - 212 . The stadium or lozenge-shaped receptacles  13 . 15 - 254  can be disposed along portions of the facial interface frame  13 . 15 - 204  configured to be disposed adjacent to a user&#39;s forehead when the HMD is donned. A capsule-dished magnet  13 . 15 - 256  can be disposed in each of the stadium or lozenge-shaped receptacles  13 . 15 - 254 . Each of the stadium or lozenge-shaped receptacles  13 . 15 - 254  in combination with a corresponding capsule-dished magnet  13 . 15 - 256  may be referred to as a capsule-dished magnetic receptacle. 
     In the example of  FIGS.  13 . 15 - 3 B , four circular receptacles  13 . 15 - 260  can be provided in the replaceable facial interface  13 . 15 - 202 . The circular receptacles  13 . 15 - 260  can be disposed along portions of the replaceable facial interface  13 . 15 - 202  configured to be disposed adjacent to a user&#39;s cheek bones when the HMD is donned. A spherical magnet  13 . 15 - 262  can be disposed in each of the circular receptacles  13 . 15 - 260 . Two stadium or lozenge-shaped receptacles  13 . 15 - 264  can be provided in the replaceable facial interface  13 . 15 - 202 . The stadium or lozenge-shaped receptacles  13 . 15 - 264  can be disposed along portions of the replaceable facial interface  13 . 15 - 202  configured to be disposed adjacent to a user&#39;s forehead when the HMD is donned. A spherical magnet  13 . 15 - 266  can be disposed in each of the stadium or lozenge-shaped receptacles  13 . 15 - 264 . Although the receptacles  13 . 15 - 250 ,  13 . 15 - 254 ,  13 . 15 - 260 , and  13 . 15 - 264  and the magnets  13 . 15 - 252 ,  13 . 15 - 256 ,  13 . 15 - 262 , and  13 . 15 - 266  are described as being various rounded shapes, any suitable shapes can be used. 
     The spherical magnets  13 . 15 - 262  can be configured to interface with the spherically-dished magnets  13 . 15 - 252 , and the spherical magnets  13 . 15 - 266  can be configured to interface with the capsule-dished magnets  13 . 15 - 256 . The spherical magnets  13 . 15 - 262  and the spherically-dished magnets  13 . 15 - 252  can have corresponding shapes, which provides for a small amount of relative travel between the spherical magnets  13 . 15 - 262  and the spherically-dished magnets  13 . 15 - 252  in the x direction and the y direction illustrated in  FIGS.  13 . 15 - 3 A and  13 . 15 - 3 B . The spherical magnets  13 . 15 - 266  and the capsule-dished magnets  13 . 15 - 256  can have corresponding shapes, which provides for a small amount of relative travel between the spherical magnets  13 . 15 - 266  and the capsule-dished magnets  13 . 15 - 256  in the y direction illustrated in  FIGS.  13 . 15 - 3 A and  13 . 15 - 3 B . The capsule-dished magnets  13 . 15 - 256  have a longitudinal axis extending in the x direction of  FIGS.  13 . 15 - 3 A . This provides for a greater amount of relative travel between the spherical magnets  13 . 15 - 266  and the capsule-dished magnets  13 . 15 - 256  in the x direction illustrated in  FIGS.  13 . 15 - 3 A and  13 . 15 - 3 B . The distances and directions that each of the magnets  13 . 15 - 252 ,  13 . 15 - 256 ,  13 . 15 - 262 , and  13 . 15 - 266  are allowed to travel relative to one another can be controlled by appropriately selecting the sizes and shapes of each of the magnets  13 . 15 - 252 ,  13 . 15 - 256 ,  13 . 15 - 262 , and  13 . 15 - 266 . 
     In the example illustrated in  FIGS.  13 . 15 - 3 A and  13 . 15 - 3 B , the receptacles  13 . 15 - 250  and  13 . 15 - 254  and the magnets  13 . 15 - 252  and  13 . 15 - 256  in the facial interface frame  13 . 15 - 204  and the receptacles  13 . 15 - 260  and  13 . 15 - 264  and the magnets  13 . 15 - 262  and  13 . 15 - 266  in the removable facial interface  13 . 15 - 202  are visible to a user. In addition, the base  13 . 15 - 214  of the facial interface frame  13 . 15 - 204  and the removable facial interface  13 . 15 - 202  are C-shaped and have shapes corresponding to one another. This provides visual cues to users that the removable facial interface  13 . 15 - 202  is meant to mount to the facial interface frame  13 . 15 - 204 . Moreover, if the removable facial interface  13 . 15 - 202  is not attached to the facial interface frame  13 . 15 - 204 , the facial interface frame  13 . 15 - 204  looks as though a piece is missing. This encourages a user to install the removable facial interface  13 . 15 - 202  onto the facial interface frame  13 . 15 - 204  so that the HMD can be comfortably donned. In some examples, any of the receptacles  13 . 15 - 250 ,  13 . 15 - 254 ,  13 . 15 - 260 , and  13 . 15 - 264  and/or the magnets  13 . 15 - 252 ,  13 . 15 - 256 ,  13 . 15 - 262 , and  13 . 15 - 266  can be covered or otherwise hidden from view. 
     The magnets  13 . 15 - 252 ,  13 . 15 - 262  can datum relative to one another (e.g., maintain relatively fixed positions relative to one another), and the magnets  13 . 15 - 256 ,  13 . 15 - 266  can float relative to one another (e.g., move relative to one another). Although the magnets  13 . 15 - 252 ,  13 . 15 - 256 ,  13 . 15 - 262 , and  13 . 15 - 266  are illustrated in particular positions in the example of  FIGS.  13 . 15 - 3 A and  13 . 15 - 3 B , any of the magnets  13 . 15 - 252 ,  13 . 15 - 256 ,  13 . 15 - 262 , and  13 . 15 - 266  can be positioned in any desired positions relative to the removable facial interface  13 . 15 - 202  and the facial interface frame  13 . 15 - 204 , depending on areas of the removable facial interface  13 . 15 - 202  and the facial interface frame  13 . 15 - 204  that are desired to remain fixed or float. Moreover, the magnets  13 . 15 - 252 ,  13 . 15 - 256 ,  13 . 15 - 262 , and  13 . 15 - 266  and the receptacles  13 . 15 - 250 ,  13 . 15 - 254 ,  13 . 15 - 260 , and  13 . 15 - 264  can have varying shapes and sizes, which can improve the ability to interchange various removable facial interfaces  13 . 15 - 202  and facial interface frames  13 . 15 - 204 , and can aid in the alignment of the removable facial interface  13 . 15 - 202  relative to the facial interface frame  13 . 15 - 204 . In some examples, the variable shapes and/or sizes of the magnets  13 . 15 - 252 ,  13 . 15 - 256 ,  13 . 15 - 262 , and  13 . 15 - 266  and the receptacles  13 . 15 - 250 ,  13 . 15 - 254 ,  13 . 15 - 260 , and  13 . 15 - 264  around the perimeters of the removable facial interface  13 . 15 - 202  and the facial interface frame  13 . 15 - 204  can allow for different shaped removable facial interfaces  13 . 15 - 202  and facial interface frames  13 . 15 - 204  to be connected together. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 15 - 3 A and  13 . 15 - 3 B  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures, including in any of  FIGS.  13 . 15 - 3 A and  13 . 15 - 3 B , described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 15 - 3 A and  13 . 15 - 3 B . 
       FIGS.  13 . 15 - 4    illustrates a cross-sectional view of a magnetic attachment mechanism  13 . 15 - 300 , which is an example of a post mount. The magnetic attachment mechanism  13 . 15 - 300  includes a connector  13 . 15 - 301 , which can be magnetically attached to a base  13 . 15 - 310 . The connector  13 . 15 - 301  can be substantially similar to, including some or all of the features of, the connectors described herein, such as the connectors  13 . 15 - 212 . The base  13 . 15 - 310  can be substantially similar to, including some or all of the features of, the stiffeners described herein, such as the stiffeners of the removable facial interfaces  13 . 15 - 202 . The connector  13 . 15 - 301  can be physically coupled to a receptacle  13 . 15 - 302 , which can be physically coupled to a magnet  13 . 15 - 304 . The base  13 . 15 - 310  can be physically coupled to a protrusion  13 . 15 - 312 , which can surround or otherwise house a magnet  13 . 15 - 316 . 
     As illustrated in  FIGS.  13 . 15 - 4   , the protrusion  13 . 15 - 312  and the receptacle  13 . 15 - 302  can have shapes that are configured to correspond to one another. For example, the receptacle  13 . 15 - 302  can include a squared opening, and the protrusion  13 . 15 - 312  can have a squared shape configured to fit into the squared opening of the receptacle  13 . 15 - 302 . The protrusion  13 . 15 - 312  can be an alignment post. Providing the protrusion  13 . 15 - 312  and the receptacle  13 . 15 - 302  with corresponding shapes aids in self-aligning a removable facial interface with a facial interface frame when the removable facial interface is attached to the facial interface frame. The magnet  13 . 15 - 304  and the magnet  13 . 15 - 316  are then attracted to one another and retain the protrusion  13 . 15 - 312  in the receptacle  13 . 15 - 302 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 15 - 4    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures, including in any of  FIGS.  13 . 15 - 4   , described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 15 - 4   . 
       FIGS.  13 . 15 - 5 A  illustrates a cross-sectional view of an interlocking attachment mechanism  13 . 15 - 400 , which is an example of a post mount. The interlocking attachment mechanism  13 . 15 - 400  can be referred to as an interlocking fastener. The interlocking attachment mechanism  13 . 15 - 400  includes a connector  13 . 15 - 401 , to which a base  13 . 15 - 410  can be attached. The connector  13 . 15 - 401  can be substantially similar to, including some or all of the features of, the connectors described herein, such as the connectors  13 . 15 - 212 . The base  13 . 15 - 410  can be substantially similar to, including some or all of the features of, the stiffeners described herein, such as the stiffeners of the removable facial interfaces  13 . 15 - 202 . The connector  13 . 15 - 401  can be physically coupled to a receptacle  13 . 15 - 402 , which can include one or more openings  13 . 15 - 404 . In the cross-sectional view illustrated in  FIGS.  13 . 15 - 5 A , the receptacle  13 . 15 - 402  includes three openings  13 . 15 - 404 ; however, the receptacle  13 . 15 - 402  of  FIGS.  13 . 15 - 5 A  can include two openings  13 . 15 - 404  that are circular- or annular-shaped in a bottom-up view. Any number of openings  13 . 15 - 404  can be included in the receptacle  13 . 15 - 402 . The base  13 . 15 - 410  can be physically coupled to protrusions  13 . 15 - 412 . The protrusions  13 . 15 - 412  can have shapes corresponding to the openings  13 . 15 - 404 , such as having circular- or annular-shapes in a top-down view. In some examples, the openings  13 . 15 - 404  and the protrusions  13 . 15 - 412  can have triangular shapes, rectangular shapes, or any other suitable shapes. Providing the openings  13 . 15 - 404  and the protrusions  13 . 15 - 412  with corresponding shapes aids in self-aligning the base  13 . 15 - 410  with the connector  13 . 15 - 401 . 
     The protrusions  13 . 15 - 412  can be configured to interface with the openings  13 . 15 - 404  in the receptacle  13 . 15 - 402  to attach the base  13 . 15 - 410  to the connector  13 . 15 - 401  when the base  13 . 15 - 410  and the connector  13 . 15 - 401  are pressed together. The protrusions  13 . 15 - 412  and the receptacle  13 . 15 - 402  can be formed of elastic, flexible materials with high coefficients of friction. In examples in which the protrusions  13 . 15 - 412  and the receptacle  13 . 15 - 402  are formed from elastic materials, the interlocking attachment mechanism  13 . 15 - 400  may be referred to as an interlocking elastic fastener. Forming the protrusions  13 . 15 - 412  and the receptacle  13 . 15 - 402  from flexible materials allows the protrusions  13 . 15 - 412  to be inserted into the openings  13 . 15 - 404 . Forming the protrusions  13 . 15 - 412  and the receptacle  13 . 15 - 402  from elastic materials having high coefficients of friction helps to retain the protrusions  13 . 15 - 412  in the openings  13 . 15 - 404  once the protrusions  13 . 15 - 412  are inserted into the openings  13 . 15 - 404 . In some examples, the protrusions  13 . 15 - 412  and the receptacle  13 . 15 - 402  can be formed of polymers, elastomers, plastics, combinations thereof, or the like. Providing the protrusions  13 . 15 - 412  and the openings  13 . 15 - 404  in the receptacle  13 . 15 - 402  with corresponding shapes aids in self-aligning a removable facial interface with a facial interface frame when the removable facial interface is attached to the facial interface frame. 
       FIGS.  13 . 15 - 5 B  illustrates a cross-sectional view of an interlocking attachment mechanism  13 . 15 - 420 , which is an example of a perimeter mount. The interlocking attachment mechanism  13 . 15 - 420  can be referred to as an interlocking fastener. The interlocking attachment mechanism  13 . 15 - 420  includes a base  13 . 15 - 421 , which can be physically attached to a base  13 . 15 - 430 . The base  13 . 15 - 421  can be substantially similar to, including some or all of the features of, the bases of the facial interface frames described herein, such as the base  13 . 15 - 214 . The base  13 . 15 - 430  can be substantially similar to, including some or all of the features of, the stiffeners described herein, such as the stiffeners of the removable facial interfaces  13 . 15 - 202 . The base  13 . 15 - 421  can be physically coupled to an outer layer  13 . 15 - 424 . The base  13 . 15 - 421  can include one or more protrusions  13 . 15 - 422  and the outer layer  13 . 15 - 424  can include one or more protrusions  13 . 15 - 426 . The base  13 . 15 - 430  can be physically coupled to an outer layer  13 . 15 - 434 . The base  13 . 15 - 430  can include one or more protrusions  13 . 15 - 436  and the outer layer  13 . 15 - 434  can include one or more protrusions  13 . 15 - 436 . Any number of protrusions can be included in the base  13 . 15 - 421 , the outer layer  13 . 15 - 424 , the base  13 . 15 - 430 , and the outer layer  13 . 15 - 434 . 
     The protrusions  13 . 15 - 432  and  13 . 15 - 436  of the base  13 . 15 - 430  can have shapes corresponding to the protrusions  13 . 15 - 422  and  13 . 15 - 426  of the base  13 . 15 - 421 . For example, the protrusion  13 . 15 - 422  can have a shape corresponding to an opening between the protrusions  13 . 15 - 432 ; the protrusions  13 . 15 - 432  can have shapes corresponding to openings between the protrusion  13 . 15 - 422  and the protrusions  13 . 15 - 426 ; and the protrusions  13 . 15 - 426  can have shapes corresponding to openings between the protrusions  13 . 15 - 432  and the protrusions  13 . 15 - 436 . The protrusions  13 . 15 - 422 ,  13 . 15 - 426 ,  13 . 15 - 432 , and  13 . 15 - 436  can have longitudinal shapes with open ends and longitudinal axes extending in a direction into and out of the page in the view of  FIGS.  13 . 15 - 5 B . This allows for the base  13 . 15 - 430  to slide relative to the base  13 . 15 - 421 . This can aid a user in attaching a removable facial interface of an HMD to a facial interface frame. Further, this allows for the removable facial interface to conform to the user&#39;s face when the user dons the HMD. 
     The protrusions  13 . 15 - 432  and  13 . 15 - 436  of the base  13 . 15 - 430  can be configured to interface with the protrusions  13 . 15 - 422  and  13 . 15 - 426  of the base  13 . 15 - 421  to attach the base  13 . 15 - 430  to the base  13 . 15 - 421  when the base  13 . 15 - 430  and the base  13 . 15 - 421  are pressed together. The protrusions  13 . 15 - 422 ,  13 . 15 - 426 ,  13 . 15 - 432 , and  13 . 15 - 436 , the outer layers  13 . 15 - 424  and  13 . 15 - 434 , and the bases  13 . 15 - 421  and  13 . 15 - 430  can be formed of elastic, flexible materials with high coefficients of friction. In examples in which the protrusions  13 . 15 - 422 ,  13 . 15 - 426 ,  13 . 15 - 432 , and  13 . 15 - 436  are formed from elastic materials, the interlocking attachment mechanism  13 . 15 - 420  may be referred to as an interlocking elastic fastener. The outer layers  13 . 15 - 424  and  13 . 15 - 434  and the bases  13 . 15 - 421  and  13 . 15 - 430  can be formed of any combinations of materials, such as the base  13 . 15 - 421  and the outer layer  13 . 15 - 424  being formed of the same or different materials and the base  13 . 15 - 430  and the outer layer  13 . 15 - 434  being formed of the same or different materials. Forming the protrusions  13 . 15 - 422 ,  13 . 15 - 426 ,  13 . 15 - 432 , and  13 . 15 - 436  from flexible materials allows the protrusions  13 . 15 - 422 ,  13 . 15 - 426 ,  13 . 15 - 432 , and  13 . 15 - 436  to be inserted into corresponding openings. Forming the protrusions  13 . 15 - 422 ,  13 . 15 - 426 ,  13 . 15 - 432 , and  13 . 15 - 436  from elastic materials having high coefficients of friction helps to retain the protrusions  13 . 15 - 422 ,  13 . 15 - 426 ,  13 . 15 - 432 , and  13 . 15 - 436  in the corresponding openings once the protrusions  13 . 15 - 422 ,  13 . 15 - 426 ,  13 . 15 - 432 , and  13 . 15 - 436  are inserted into the corresponding openings. In some examples, the protrusions  13 . 15 - 422 ,  13 . 15 - 426 ,  13 . 15 - 432 , and  13 . 15 - 436 , the outer layers  13 . 15 - 424  and  13 . 15 - 434 , and the bases  13 . 15 - 421  and  13 . 15 - 430  can be formed of polymers, elastomers, plastics, combinations thereof, or the like. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 15 - 5 A and  13 . 15 - 5 B  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures, including in any of  FIGS.  13 . 15 - 5 A and  13 . 15 - 5 B , described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 15 - 5 A and  13 . 15 - 5 B . 
       FIGS.  13 . 15 - 6    illustrates a cross-sectional view of a magnetic slide attachment mechanism  13 . 15 - 500 , which is an example of a post mount. The magnetic slide attachment mechanism  13 . 15 - 500  includes a connector  13 . 15 - 501 , to which a base  13 . 15 - 510  can be attached. The connector  13 . 15 - 501  can be substantially similar to, including some or all of the features of, the connectors described herein, such as the connectors  13 . 15 - 212 . The base  13 . 15 - 510  can be substantially similar to, including some or all of the features of, the stiffeners described herein, such as the stiffeners of the removable facial interfaces  13 . 15 - 202 . 
     The connector  13 . 15 - 501  can be physically coupled to a magnetic post  13 . 15 - 504  through a fastener  13 . 15 - 502 . The magnetic post  13 . 15 - 504  can include a flange  13 . 15 - 506 . The base  13 . 15 - 510  can be physically coupled to a magnetic protrusion  13 . 15 - 512 . The magnetic post  13 . 15 - 504  is magnetically attracted to the magnetic protrusion  13 . 15 - 512 , which helps to align the connector  13 . 15 - 501  with the base  13 . 15 - 510  and helps to secure the flange  13 . 15 - 506  of the magnetic post  13 . 15 - 504  under a lip  13 . 15 - 514  of the magnetic protrusion  13 . 15 - 512 . A magnet  13 . 15 - 516  of the magnetic protrusion  13 . 15 - 512  can be disposed under the lip  13 . 15 - 514  of the magnetic protrusion  13 . 15 - 512  to pull the magnetic post  13 . 15 - 504  into an opening  13 . 15 - 518  of the magnetic protrusion  13 . 15 - 512  under the lip  13 . 15 - 514  of the magnetic protrusion  13 . 15 - 512 . In the view of  FIGS.  13 . 15 - 6   , the magnetic post  13 . 15 - 504  can move from left to right as the connector  13 . 15 - 501  is attached to the base  13 . 15 - 510 . In other words, the magnetic post  13 . 15 - 504  can be slidably engaged with the magnetic protrusion  13 . 15 - 512  by sliding the flange  13 . 15 - 506  under the lip  13 . 15 - 514 . The connector  13 . 15 - 501  can be removed from the base  13 . 15 - 510  by moving the magnetic post  13 . 15 - 504  from right to left, then moving the magnetic post  13 . 15 - 504  in an upwards direction once the flange  13 . 15 - 506  of the magnetic post  13 . 15 - 504  clears the lip  13 . 15 - 514  of the protrusion  13 . 15 - 512 . In other words, the magnetic post  13 . 15 - 504  can be slidably disengaged with the magnetic protrusion  13 . 15 - 512  by sliding the flange  13 . 15 - 506  out from under the lip  13 . 15 - 514 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 15 - 6    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures, including in any of  FIGS.  13 . 15 - 6   , described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 15 - 6   . 
       FIGS.  13 . 15 - 7    illustrates a cross-sectional view of a hook-and-loop attachment mechanism  13 . 15 - 600 , which is an example of a post mount. The hook-and-loop attachment mechanism  13 . 15 - 600  includes a connector  13 . 15 - 601 , to which a base  13 . 15 - 610  can be attached. The connector  13 . 15 - 601  can be substantially similar to, including some or all of the features of, the connectors described herein, such as the connectors  13 . 15 - 212 . The base  13 . 15 - 610  can be substantially similar to, including some or all of the features of, the stiffeners described herein, such as the stiffeners of the removable facial interfaces  13 . 15 - 202 . 
     The connector  13 . 15 - 601  can be physically coupled to a hook-and-loop feature  13 . 15 - 604  through a fastener  13 . 15 - 602 . The hook-and-loop feature  13 . 15 - 604  can include a post  13 . 15 - 606 . The base  13 . 15 - 610  can be physically coupled to a hook-and-loop feature  13 . 15 - 612 . The hook-and-loop feature  13 . 15 - 604  can include an opening  13 . 15 - 614  configured to receive the post  13 . 15 - 606 . In the example of  FIGS.  13 . 15 - 7   , the post  13 . 15 - 606  and the opening  13 . 15 - 614  can have corresponding frustum shapes, however any suitable shapes can be used. The opening  13 . 15 - 614  and the post  13 . 15 - 606  can be included to aid in self-aligning the connector  13 . 15 - 601  and the base  13 . 15 - 610 ; however, in some examples, the opening  13 . 15 - 614  and the post  13 . 15 - 606  can be omitted. 
     One of the hook-and-loop feature  13 . 15 - 604  or the hook-and-loop feature  13 . 15 - 612  can include a plurality of hooks, and the other of the hook-and-loop feature  13 . 15 - 604  or the hook-and-loop feature  13 . 15 - 612  can include a plurality of loops. The hooks of the hook-and-loop feature  13 . 15 - 604  or the hook-and-loop feature  13 . 15 - 612  are retained in the corresponding loops of the hook-and-loop feature  13 . 15 - 604  or the hook-and-loop feature  13 . 15 - 612  in order to attach the base  13 . 15 - 610  to the connector  13 . 15 - 601 . Velcro is an example of a hook-and-loop attachment mechanism that can be used for the hook-and-loop feature  13 . 15 - 604  and the hook-and-loop feature  13 . 15 - 612 . Fine hook and loop features can be provided in the hook-and-loop feature  13 . 15 - 604  and/or the hook-and-loop feature  13 . 15 - 612  in order to lessen a tearing sound when the connector  13 . 15 - 601  and the base  13 . 15 - 610  are separated from one another. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 15 - 7    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures, including in any of  FIGS.  13 . 15 - 7   , described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 15 - 7   . 
       FIGS.  13 . 15 - 8    illustrates a cross-sectional view of a magnetic attachment mechanism  13 . 15 - 700 , which is an example of a post mount. The magnetic attachment mechanism  13 . 15 - 700  includes a connector  13 . 15 - 701 , which can be magnetically attached to a base  13 . 15 - 710 . The connector  13 . 15 - 701  can be substantially similar to, including some or all of the features of, the connectors described herein, such as the connectors  13 . 15 - 212 . The base  13 . 15 - 710  can be substantially similar to, including some or all of the features of, the stiffeners described herein, such as the stiffeners of the removable facial interfaces  13 . 15 - 202 . 
     The connector  13 . 15 - 701  can be physically coupled to a magnet  13 . 15 - 706  through a fastener  13 . 15 - 702 . The connector  13 . 15 - 701  can be further physically coupled to a post  13 . 15 - 704 , which houses the magnet  13 . 15 - 706 . In some examples, the post  13 . 15 - 704  can be referred to as a protrusion, and the magnet  13 . 15 - 706  can be disposed in the protrusion. The base  13 . 15 - 710  can be physically coupled to a magnet  13 . 15 - 714 , and a receptacle  13 . 15 - 712  can be physically coupled to the base  13 . 15 - 710  and/or the magnet  13 . 15 - 714 . In some examples, the magnet  13 . 15 - 714  can be disposed in the receptacle  13 . 15 - 712 . As illustrated in  FIGS.  13 . 15 - 8   , the receptacle  13 . 15 - 712  can include an opening configured to receive the post  13 . 15 - 704 . In the example of  FIGS.  13 . 15 - 8   , the post  13 . 15 - 704  and the opening of the receptacle  13 . 15 - 712  can have corresponding frustum shapes, however any suitable shapes can be used. In some examples, the post  13 . 15 - 704  (e.g., the protrusion) and the receptacle  13 . 15 - 712  have shapes that are complementary to one another, such that they can nest, stack, or be received in one-another. The opening of the receptacle  13 . 15 - 712  and the post  13 . 15 - 704  can be included to aid in self-aligning the connector  13 . 15 - 701  and the base  13 . 15 - 710 ; however, in some examples, the opening of the receptacle  13 . 15 - 712  and the post  13 . 15 - 704  can be omitted. The magnet  13 . 15 - 706  and  13 . 15 - 714  are magnetically attracted to one another and retain the post  13 . 15 - 704  of the connector  13 . 15 - 701  within the opening of the receptacle  13 . 15 - 712  of the base  13 . 15 - 710 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 15 - 8    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures, including in any of  FIGS.  13 . 15 - 8   , described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 15 - 8   . 
       FIGS.  13 . 15 - 9    illustrates a cross-sectional view of a spring-snap attachment mechanism  13 . 15 - 800 , which is an example of a post mount. The spring-snap attachment mechanism  13 . 15 - 800  includes a connector  13 . 15 - 801 , which can be physically attached to a base  13 . 15 - 801 . The connector  13 . 15 - 801  can be substantially similar to, including some or all of the features of, the connectors described herein, such as the connectors  13 . 15 - 212 . The base  13 . 15 - 810  can be substantially similar to, including some or all of the features of, the stiffeners described herein, such as the stiffeners of the removable facial interfaces  13 . 15 - 202 . 
     The connector  13 . 15 - 801  is physically coupled to a spring-snap feature  13 . 15 - 804  through a fastener  13 . 15 - 802 . The spring-snap feature  13 . 15 - 804  includes detents  13 . 15 - 806  that are attached to the spring-snap feature  13 . 15 - 804  through springs in the spring-snap feature  13 . 15 - 804 . The base  13 . 15 - 810  is physically coupled to a post  13 . 15 - 812 , which includes flange  13 . 15 - 814 . When the base  13 . 15 - 810  is attached to the connector  13 . 15 - 801 , the flange  13 . 15 - 814  of the post  13 . 15 - 812  push the detents  13 . 15 - 806  outwards relative to the post  13 . 15 - 812  and into openings in the spring-snap feature  13 . 15 - 804 . Once the flange  13 . 15 - 814  pass the detents  13 . 15 - 806 , the springs of the spring-snap feature  13 . 15 - 804  push the detents  13 . 15 - 806  against the post  13 . 15 - 812  to retain the flange  13 . 15 - 814  of the post  13 . 15 - 812  within an opening of the spring-snap feature  13 . 15 - 804 . In other words, the spring force of the spring-snap feature  13 . 15 - 804  pressing the detents  13 . 15 - 806  against the post  13 . 15 - 812  adjacent the flange  13 . 15 - 814  retains the post  13 . 15 - 812  within the spring-snap feature  13 . 15 - 804 , attaching the base  13 . 15 - 810  to the connector  13 . 15 - 801 . When the base  13 . 15 - 810  is removed from the connector  13 . 15 - 801 , this same process occurs in reverse. In the example of  FIGS.  13 . 15 - 9   , the post  13 . 15 - 812  and the opening of the spring-snap feature  13 . 15 - 804  that receives the post can have corresponding cylindrical shapes, however any suitable shapes can be used. The opening of the spring-snap feature  13 . 15 - 804  and the post  13 . 15 - 812  aid in self-aligning the connector  13 . 15 - 801  and the base  13 . 15 - 810 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 15 - 9    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures, including in any of  FIGS.  13 . 15 - 9   , described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 15 - 9   . 
       FIGS.  13 . 15 - 10    illustrates a cross-sectional view of an interlocking attachment mechanism  13 . 15 - 900 , which is an example of a post mount or a perimeter mount. The interlocking attachment mechanism  13 . 15 - 900  can be referred to as an interlocking fastener. The interlocking attachment mechanism  13 . 15 - 900  includes a base  13 . 15 - 910 , which can be interlocked with a base  13 . 15 - 901 . The base  13 . 15 - 901  or  13 . 15 - 910  can be substantially similar to, including some or all of the features of, the bases of the facial interface frames described herein, such as the base  13 . 15 - 214 . The other of the base  13 . 15 - 901  or  13 . 15 - 910  can be substantially similar to, including some or all of the features of, the stiffeners described herein, such as the stiffeners of the removable facial interfaces  13 . 15 - 202 . The base  13 . 15 - 901  can include a plurality of protrusions  13 . 15 - 902 , such as two protrusions  13 . 15 - 902  in the example of  FIGS.  13 . 15 - 10   . The base  13 . 15 - 910  can include a protrusion  13 . 15 - 912 . In some examples, any number of protrusions  13 . 15 - 912  and  13 . 15 - 902  can be included in the base  13 . 15 - 910  and the base  13 . 15 - 901 . 
     As illustrated in  FIGS.  13 . 15 - 10   , the protrusion  13 . 15 - 912  can be configured to interlock with the protrusions  13 . 15 - 902 . The protrusion  13 . 15 - 912  can be mushroom-shaped and can have a flange at an end of a post. The protrusions  13 . 15 - 902  can define an opening and include lips adjacent to the protrusion  13 . 15 - 912  when the protrusion  13 . 15 - 912  is inserted into the opening defined by the protrusions  13 . 15 - 902 . The lips of the protrusions  13 . 15 - 902  and the flange of the protrusion  13 . 15 - 912  can secure the protrusion  13 . 15 - 912  within the opening defined between the protrusions  13 . 15 - 902 . 
     In some examples, the protrusions  13 . 15 - 902  and  13 . 15 - 912  can have longitudinal shapes with open ends and longitudinal axes extending in a direction into and out of the page in the view of  FIGS.  13 . 15 - 10   . This allows for the base  13 . 15 - 901  to slide relative to the base  13 . 15 - 910 . This can aid a user in attaching a removable facial interface of an HMD to a facial interface frame. Further, this allows for the removable facial interface to conform to the user&#39;s face when the user dons the HMD. In some examples, the protrusion  13 . 15 - 912  can have a circular shape in a bottom-up view and the protrusions  13 . 15 - 902  can have an annular shape in a top-down view. In some examples, the protrusions  13 . 15 - 902  and  13 . 15 - 912  can have triangular, rectangular, or other shapes in bottom-up and top-down views, respectively. Providing the protrusions  13 . 15 - 902  and  13 . 15 - 912  with corresponding shapes aids in self-aligning the base  13 . 15 - 901  with the base  13 . 15 - 910 . 
     The protrusions  13 . 15 - 902  and  13 . 15 - 912  can be formed of elastic, flexible materials with high coefficients of friction. In examples in which the protrusions  13 . 15 - 902  and  13 . 15 - 912  are formed from elastic materials, the interlocking attachment mechanism  13 . 15 - 900  may be referred to as an interlocking elastic fastener. The protrusions  13 . 15 - 902  and  13 . 15 - 912  can be formed from the same or different materials. Forming the protrusions  13 . 15 - 902  and  13 . 15 - 912  from flexible materials allows the protrusion  13 . 15 - 912  to be inserted into the corresponding opening between the protrusions  13 . 15 - 902 . Forming the protrusions  13 . 15 - 902  and  13 . 15 - 912  from elastic materials having high coefficients of friction helps to retain the protrusion  13 . 15 - 912  in the corresponding opening between the protrusions  13 . 15 - 902 . In some examples, the protrusions  13 . 15 - 902  and  13 . 15 - 912  can be formed of polymers, elastomers, plastics, combinations thereof, or the like. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 15 - 10    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures, including in any of  FIGS.  13 . 15 - 10   , described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 15 - 10   . 
       FIGS.  13 . 15 - 11    illustrates a cross-sectional view of a suction attachment mechanism  13 . 15 - 1000 , which is an example of a post mount. The suction attachment mechanism  13 . 15 - 1000  includes a connector  13 . 15 - 1010 , which can be attached to a base  13 . 15 - 1001  through a suction force. The connector  13 . 15 - 1010  can be substantially similar to, including some or all of the features of, the connectors described herein, such as the connectors  13 . 15 - 212 . The base  13 . 15 - 1001  can be substantially similar to, including some or all of the features of, the stiffeners described herein, such as the stiffeners of the removable facial interfaces  13 . 15 - 202 . 
     The base  13 . 15 - 1001  can include a curved opening  13 . 15 - 1002  formed at a top surface thereof. In some examples, the curved opening  13 . 15 - 1002  may be referred to as a curved surface. A suction cup  13 . 15 - 1012  can be physically coupled to the connector  13 . 15 - 1010 . The base  13 . 15 - 1001  can be attached to the connector  13 . 15 - 1010  by pressing the suction cup  13 . 15 - 1012  into the curved opening with sufficient force. The connector  13 . 15 - 1010  can be removed from the base  13 . 15 - 1001  by pulling the connector  13 . 15 - 1010  away from the base  13 . 15 - 1001  with sufficient force. Removing the connector  13 . 15 - 1010  from the base  13 . 15 - 1001  can be aided by tilting the connector  13 . 15 - 1010  relative to the base  13 . 15 - 1001 . The curved opening  13 . 15 - 1002  of the base  13 . 15 - 1001  can have a smooth hard surface, which can be surrounded by softer surfaces, such as fabrics or the like. This aids in self-aligning the base  13 . 15 - 1001  and the connector  13 . 15 - 1010 , as the suction cup  13 . 15 - 1012  cannot easily be mounted to portions of the base  13 . 15 - 1001  surrounding the curved opening  13 . 15 - 1002 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 15 - 11    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures, including in any of  FIGS.  13 . 15 - 11   , described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 15 - 11   . 
       FIGS.  13 . 15 - 12    illustrates a cross-sectional view of a bi-stable attachment mechanism  13 . 15 - 1100 , which is an example of a post mount. The bi-stable attachment mechanism  13 . 15 - 1100  includes a connector  13 . 15 - 1101 , which can be attached to a base  13 . 15 - 1110  through a suction force. The connector  13 . 15 - 1101  can be substantially similar to, including some or all of the features of, the connectors described herein, such as the connectors  13 . 15 - 212 . The base  13 . 15 - 1110  can be substantially similar to, including some or all of the features of, the stiffeners described herein, such as the stiffeners of the removable facial interfaces  13 . 15 - 202 . 
     The connector  13 . 15 - 1101  can include a curved opening  13 . 15 - 1102  formed at a bottom surface thereof. A bi-stable element  13 . 15 - 1114  can be physically coupled to the base  13 . 15 - 1110  through a joint  13 . 15 - 1112 . The bi-stable element  13 . 15 - 1114  can be stable in two positions, such as a first position  13 . 15 - 1114 A and a second position  13 . 15 - 1114 C. The third position  13 . 15 - 1114 B is an intermediate position between the first position  13 . 15 - 1114 A and the second position  13 . 15 - 1114 C. The bi-stable element  13 . 15 - 1114  can flip between the first position  13 . 15 - 1114 A and the second position  13 . 15 - 1114 C. For example, when the connector  13 . 15 - 1101  is pressed into the base  13 . 15 - 1110 , the bi-stable element  13 . 15 - 1114  can move from the first position  13 . 15 - 1114 A to the second position  13 . 15 - 1114 C. When the connector  13 . 15 - 1101  is pulled from the base  13 . 15 - 1110 , the bi-stable element  13 . 15 - 1114  can move from the second position  13 . 15 - 1114 C to the first position  13 . 15 - 1114 A. The connector  13 . 15 - 1101  can be attached to the base  13 . 15 - 1110  through a suction force similar to the suction attachment mechanism  13 . 15 - 1100 . Removing the connector  13 . 15 - 1101  from the base  13 . 15 - 1110  can be aided by tilting the connector  13 . 15 - 1101  relative to the base  13 . 15 - 1110 . The curved opening  13 . 15 - 1102  of the connector  13 . 15 - 1101  can have a smooth hard surface, which can be surrounded by softer surfaces, such as fabrics or the like. This aids in self-aligning the base  13 . 15 - 1110  and the connector  13 . 15 - 1101 , as the bi-stable element  13 . 15 - 1114  cannot easily be mounted to portions of the connector  13 . 15 - 1101  surrounding the curved opening  13 . 15 - 1102 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 15 - 12    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures, including in any of  FIGS.  13 . 15 - 12   , described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 15 - 12   . 
       FIGS.  13 . 15 - 13 A and  13 . 15 - 13 B  are plan views of removable facial interfaces  13 . 15 - 1200 A and  13 . 15 - 1200 B, respectively. The removable facial interfaces  13 . 15 - 1200 A and  13 . 15 - 1200 B include a base  13 . 15 - 1204 A and a base  13 . 15 - 1204 B, respectively. A pliant material  13 . 15 - 1202  can be formed surrounding the respective bases  13 . 15 - 1204 A and  13 . 15 - 1204 B. Top mounts  13 . 15 - 1214 , side mounts  13 . 15 - 1210 , and bottom mounts  13 . 15 - 1208  can be physically coupled to the respective bases  13 . 15 - 1204 A and  13 . 15 - 1204 B through the pliant material  13 . 15 - 1202 . Any of the above-described attachment mechanisms, such as the post attachments, can be provided on the top mounts  13 . 15 - 1214 , the side mounts  13 . 15 - 1210 , and the bottom mounts  13 . 15 - 1208 . Additional attachment mechanisms, such as the perimeter attachments, can be provided on the removable facial interfaces  13 . 15 - 1200 A and  13 . 15 - 1200 B, such as along the bases  13 . 15 - 1204 A and  13 . 15 - 1204 B between the top mounts  13 . 15 - 1214 , the side mounts  13 . 15 - 1210 , and the bottom mounts  13 . 15 - 1208 .  FIGS.  13 . 15 - 13 A and  13 . 15 - 13 B  illustrate magnets  13 . 15 - 1212  and  13 . 15 - 1206  physically coupled to the top mounts  13 . 15 - 1214  and the bottom mounts  13 . 15 - 1208 , respectively. Although  13 . 15 - 6  mounts are illustrated as being provided for the removable facial interfaces  13 . 15 - 1200 A and  13 . 15 - 1200 B; any number of mounts can be provided. 
     In the example of  FIGS.  13 . 15 - 13 A , the base  13 . 15 - 1204 A includes thick portions and thin portions. The thick portions can be included to provide stiffness to the removable facial interface  13 . 15 - 1200 A, while the thin portions can be included to provide flexibility to the removable facial interface  13 . 15 - 1200 A. In the example of  FIGS.  13 . 15 - 13 B , the base  13 . 15 - 1204 B includes slits formed along the perimeter of the base  13 . 15 - 1204 B. The slits portions can be included to provide flexibility to the removable facial interface  13 . 15 - 1200 B. Any suitable bases with any suitable configurations can be included to provide desired flexibility and stiffness to the removable facial interfaces. 
     The bases  13 . 15 - 1204 A and  13 . 15 - 1204 B can be formed from metals, plastics, polymers, combinations thereof, or the like. The pliable material  13 . 15 - 1202  can be formed from rubbers, foams, polymers, silicone, elastomers, hydrogen gels, combinations thereof, or the like. The removable facial interfaces  13 . 15 - 1200 A and  13 . 15 - 1200 B further include a fabric material surrounding the pliable material  13 . 15 - 1202 , the top mounts  13 . 15 - 1214 , the side mounts  13 . 15 - 1210 , and the bottom mounts  13 . 15 - 1208 . The fabric material may encircle the top mounts  13 . 15 - 1214 , the side mounts  13 . 15 - 1210 , and the bottom mounts  13 . 15 - 1208 , and the top mounts  13 . 15 - 1214 , the side mounts  13 . 15 - 1210 , and the bottom mounts  13 . 15 - 1208  may be exposed through the fabric material. The bases  13 . 15 - 1204 A and  13 . 15 - 1204 B can provide the removable facial interfaces  13 . 15 - 1200 A and  13 . 15 - 1200 B with a desired degree of stiffness. The pliable material  13 . 15 - 1202  can be pliant or semi-pliant and can be provided to increase user comfort. The fabric material can be configured to contact a user&#39;s skin and can also be provided to increase user comfort. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  13 . 15 - 13 A and  13 . 15 - 13 B  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures, including in any of  FIGS.  13 . 15 - 13 A and  13 . 15 - 13 B , described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  13 . 15 - 13 A and  13 . 15 - 13 B . 
     In some examples, the device seal can include an asymmetric compressible portion that contacts a user&#39;s face. The asymmetry in the design of the compressible portion improves comfort and reduces contact between the user&#39;s face and hard edges of the device seal. While the asymmetric compressible portion is described herein as part of a removable device seal, the asymmetric compressible portion can similarly be a part of a fixed or non-removable facial interface to be used with an HMD. 
       FIGS.  13 . 15 - 14    illustrates a cross-sectional view of a device seal  13 . 15 - 1400 . The device seal  13 . 15 - 1400  can be substantially similar to, including some or all of the features of, the device seals described herein. The device seal  13 . 15 - 1400  can include a facial interface  13 . 15 - 1402 , and a cover  13 . 15 - 1406 . The cover  13 . 15 - 1406  can be included in the device seal  13 . 15 - 1400  to provide a seal between the device seal  13 . 15 - 1400  and the outer environment. 
     The device seal  13 . 15 - 1400  can include a frame  13 . 15 - 1410 , a base  13 . 15 - 1414 , and a compressible portion  13 . 15 - 1416 . The frame  13 . 15 - 1410  can be physically coupled to a display portion of the HMD, and the frame  13 . 15 - 1410  can be configured provide an interface between the display portion and the facial interface  13 . 15 - 1402 . The facial interface  13 . 15 - 1402  can be removably attached to the base  13 . 15 - 1414 , or non-removably fixed to the base. 
     The compressible portion  13 . 15 - 1416  can fill a gap between the base  13 . 15 - 1414  and the facial interface  13 . 15 - 1402 , and can provide improved user comfort relative to traditional interfaces. The compressible portion  13 . 15 - 1416  can also be referred to as a compressible member or a compressible material. In some examples, the compressible portion  13 . 15 - 1416  can include a foam material. In some examples, the compressible portion  13 . 15 - 1416  can include materials with properties that impart flexibility, softness, compressibility, deformability, and the like. Examples of materials that can be used for the compressible portion  13 . 15 - 1416  include, but are in no way limited to, silicone, polymers, elastomers, hydrogels, combinations thereof, or the like. The compressible portion  13 . 15 - 1416  can be formed by molding, printing, casting, or the like. In some examples, attachment mechanisms, such as perimeter attachments, can be included on the compressible portion  13 . 15 - 1416 . The compressible portion  13 . 15 - 1416  provides a soft, flexible, deformable interface between the removable facial interface  202  and rigid components of the removable facial interface  13 . 15 - 1402 , such as the frame  13 . 15 - 1410 , the connectors, and the base  13 . 15 - 1414 . This interface can provide for improved user comfort when the HMD is donned. 
     In some examples, the compressible portion  13 . 15 - 1416  extends beyond the edges of the base  13 . 15 - 1414 . In other words, the compressible portion  13 . 15 - 1416  can be wider than the base  13 . 15 - 1414 , such that the compressible portion  13 . 15 - 1416  overhangs the base  13 . 15 - 1414 . The compressible portion  13 . 15 - 1416  can overhang, extend from, or be proud of the cover  13 . 15 - 1406 . For example, the compressible portion  13 . 15 - 1416  can extend a distance d above the edge of the base  13 . 15 - 1414 . In some examples, the distance “d” can be approximately 3.4 mm. However, as discussed in greater detail below, the width of the compressible portion  13 . 15 - 1416  can vary based on the position of the compressible portion  13 . 15 - 1416  relative to the user&#39;s face when donned. In some examples, the lower part of the compressible portion  13 . 15 - 1416  is substantially even or flush with the lower edge of the base  13 . 15 - 1416 , while the upper part of the compressible portion  13 . 15 - 1416  is proud of or extended beyond the upper edge of the base  13 . 15 - 1414 . 
     The increased width of the compressible portion  13 . 15 - 1416  advantageously increases comfort and wearability of the HMD. As the HMD is worn, the compressible portion  13 . 15 - 1416  can bend or fold around the upper edge of the base  13 . 15 - 1414 . The base  13 . 15 - 1414  can, in some examples, be made from a rigid material, such as plastic. Thus, the overhanging compressible portion  13 . 15 - 1416  can shield the user&#39;s face from the edges of the base  13 . 15 - 1414  during use. The overhanging compressible portion  13 . 15 - 1416  can also, in some examples, become more compliant over time, improving comfort. 
       FIGS.  13 . 15 - 15 A  illustrates a cross-sectional view of a facial interface  13 . 15 - 1502 .  FIGS.  13 . 15 - 15 B  illustrates a cross-sectional view of a forehead region  13 . 15 - 1503  of the facial interface  13 . 15 - 1502 .  FIGS.  13 . 15 - 15 C  illustrates a cross-sectional view of a cheek region  13 . 15 - 1505  of the facial interface  13 . 15 - 1502 . The facial interface  13 . 15 - 15  can be substantially similar to, including some or all of the features of, the facial interfaces described herein. 
     In some examples, the compressible portion  13 . 15 - 1516  can include a central region  13 . 15 - 1509  and an extended region  13 . 15 - 1507 . In some examples, the central region  13 . 15 - 1509  and the extended region  13 . 15 - 1507  can be a single unitary component (e.g., a foam piece). In some examples, the central region  13 . 15 - 1509  can be a separate and distinct material from the extended region  13 . 15 - 1507  (e.g., two different types of foam pieces). The central region  13 . 15 - 1509  can be shaped, sized, and positioned to correspond to the base of the frame. In other words, the central region  13 . 15 - 1509  can reside directly adjacent the base plastic. In contrast, the extended region  13 . 15 - 1507  can extend beyond a footprint of the base, such that the extended region  13 . 15 - 1507  overhangs the base. 
     In some examples, the extended region  13 . 15 - 1507  can run along an entirety of the facial interface  13 . 15 - 1502 . The compressible portion  13 . 15 - 1516  can have a varying cross-section or be asymmetric relative to the base. The extended region  13 . 15 - 1507  can taper as it travels from the forehead region  13 . 15 - 1503  to the cheek region  13 . 15 - 1505 . In other words, the extended region  13 . 15 - 1507  can decrease in size as it approaches the cheek region  13 . 15 - 1505 , so that it is wider at the forehead, and less wide at the cheeks. For example, at the forehead region  13 . 15 - 1503 , the extended region  13 . 15 - 1507  can overhang a base by approximately equal to or greater than 3.4 mm, while at the forehead region  13 . 15 - 1505 , the extended region  13 . 15 - 1507  can overhang a base by approximately equal to or less than 3.4 mm. As shown in  FIGS.  13 . 15 - 15 B . As shown, the compressible portion  13 . 15 - 1516  can extend beyond or overhang the central region  13 . 15 - 1509  and the base plastic both above and below, as oriented in the figures. The varying asymmetric cross-section of the extended region  13 . 15 - 1507  can be intentionally varied to correspond to user facial features, thereby increasing comfort and fit of the device seal  13 . 15 - 1400  during use. 
     13.16: Electronic Devices with Light Blocking Structures 
     Head-mounted devices include head-mounted support structures that allow the devices to be worn on the heads of users. The head-mounted support structures may include device housings for housing components such as displays that are used for presenting a user with visual content. Head-mounted devices may also include a light-shielding nosepiece that rests on the nose of the user. The light-shielding nosepiece may include an elastomeric layer with perforations and fabric that covers the elastomeric layer. The elastomeric layer with the perforations may allow the light-shielding nosepiece to conform to the user&#39;s nose while the device is worn, while maintaining enough rigidity to be wrapped by a low force, high stretch textile or other low force, high stretch material. Additional structures, such as rigid structures or semi-rigid structures, may be included in the light-shielding nosepiece to provide additional support for the device while it is worn. 
     A schematic diagram of an illustrative system having an electronic device with a light-shielding nosepiece is shown in  FIGS.  13 . 16 - 1   . As shown in  FIGS.  13 . 16 - 1   , system  13 . 16 - 8  may include one or more electronic devices such as electronic device  13 . 16 - 10 . The electronic devices of system  13 . 16 - 8  may include computers, cellular telephones, head-mounted devices, wristwatch devices, and other electronic devices. Configurations in which electronic device  13 . 16 - 10  is a head-mounted device are sometimes described herein as an example. 
     As shown in  FIGS.  13 . 16 - 1   , electronic devices such as electronic device  13 . 16 - 10  may have control circuitry  13 . 16 - 12 . Control circuitry  13 . 16 - 12  may include storage and processing circuitry for controlling the operation of device  13 . 16 - 10 . Circuitry  13 . 16 - 12  may include storage such as hard disk drive storage, nonvolatile memory (e.g., electrically-programmable-read-only memory configured to form a solid-state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Processing circuitry in control circuitry  13 . 16 - 12  may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors, power management units, audio chips, graphics processing units, application specific integrated circuits, and other integrated circuits. Software code may be stored on storage in circuitry  13 . 16 - 12  and run on processing circuitry in circuitry  13 . 16 - 12  to implement control operations for device  13 . 16 - 10  (e.g., data gathering operations, operations involved in processing three-dimensional facial image data, operations involving the adjustment of components using control signals, etc.). Control circuitry  13 . 16 - 12  may include wired and wireless communications circuitry. For example, control circuitry  13 . 16 - 12  may include radio-frequency transceiver circuitry such as cellular telephone transceiver circuitry, wireless local area network (WiFi®) transceiver circuitry, millimeter wave transceiver circuitry, and/or other wireless communications circuitry. 
     During operation, the communications circuitry of the devices in system  13 . 16 - 8  (e.g., the communications circuitry of control circuitry  13 . 16 - 12  of device  13 . 16 - 10 ), may be used to support communication between the electronic devices. For example, one electronic device may transmit video and/or audio data to another electronic device in system  13 . 16 - 8 . Electronic devices in system  13 . 16 - 8  may use wired and/or wireless communications circuitry to communicate through one or more communications networks (e.g., the internet, local area networks, etc.). The communications circuitry may be used to allow data to be received by device  13 . 16 - 10  from external equipment (e.g., a tethered computer, a portable device such as a handheld device or laptop computer, online computing equipment such as a remote server or other remote computing equipment, or other electrical equipment) and/or to provide data to external equipment. 
     Device  13 . 16 - 10  may include input-output devices  13 . 16 - 22 . Input-output devices  13 . 16 - 22  may be used to allow a user to provide device  13 . 16 - 10  with user input. Input-output devices  13 . 16 - 22  may also be used to gather information on the environment in which device  13 . 16 - 10  is operating. Output components in devices  13 . 16 - 22  may allow device  13 . 16 - 10  to provide a user with output and may be used to communicate with external electrical equipment. 
     As shown in  FIGS.  13 . 16 - 1   , input-output devices  13 . 16 - 22  may include one or more displays such as display  13 . 16 - 14 . In some configurations, display  13 . 16 - 14  of device  13 . 16 - 10  includes left and right display panels (sometimes referred to as left and right portions of display  13 . 16 - 14  and/or left and right displays) that are in alignment with the user&#39;s left and right eyes and are viewable through left and right lens assemblies, respectively. In other configurations, display  13 . 16 - 14  includes a single display panel that extends across both eyes. 
     Display  13 . 16 - 14  may be used to display images. The visual content that is displayed on display  13 . 16 - 14  may be viewed by a user of device  13 . 16 - 10 . Displays in device  13 . 16 - 10  such as display  13 . 16 - 14  may be organic light-emitting diode displays or other displays based on arrays of light-emitting diodes, liquid crystal displays, liquid-crystal-on-silicon displays, projectors or displays based on projecting light beams on a surface directly or indirectly through specialized optics (e.g., digital micromirror devices), electrophoretic displays, plasma displays, electrowetting displays, microLED displays, or any other suitable displays. 
     Display  13 . 16 - 14  may present computer-generated content such as virtual reality content and mixed reality content to a user. Virtual reality content may be displayed in the absence of real-world content. Mixed reality content, which may sometimes be referred to as augmented reality content, may include computer-generated images that are overlaid on real-world images. The real-world images may be captured by a camera (e.g., a forward-facing camera) and merged with overlaid computer-generated content or an optical coupling system may be used to allow computer-generated content to be overlaid on top of real-world images. As an example, a pair of mixed reality glasses or other augmented reality head-mounted display may include a display device that provides images to a user through a beam splitter, prism, holographic coupler, or other optical coupler. Configurations in which display  13 . 16 - 14  is used to display virtual reality content to a user through lenses are described herein as an example. 
     Input-output devices  13 . 16 - 22  may include sensors  13 . 16 - 16 . Sensors  13 . 16 - 16  may include, for example, three-dimensional sensors (e.g., three-dimensional image sensors such as structured light sensors that emit beams of light and that use two-dimensional digital image sensors to gather image data for three-dimensional images from light spots that are produced when a target is illuminated by the beams of light, binocular three-dimensional image sensors that gather three-dimensional images using two or more cameras in a binocular imaging arrangement, three-dimensional lidar (light detection and ranging) sensors, three-dimensional radio-frequency sensors, or other sensors that gather three-dimensional image data), cameras (e.g., infrared and/or visible digital image sensors), gaze tracking sensors (e.g., a gaze tracking system based on an image sensor and, if desired, a light source that emits one or more beams of light that are tracked using the image sensor after reflecting from a user&#39;s eyes), touch sensors, buttons, force sensors, sensors such as contact sensors based on switches, gas sensors, pressure sensors, moisture sensors, magnetic sensors, audio sensors (microphones), ambient light sensors, microphones for gathering voice commands and other audio input, sensors that are configured to gather information on motion, position, and/or orientation (e.g., accelerometers, gyroscopes, compasses, and/or inertial measurement units that include all of these sensors or a subset of one or two of these sensors), fingerprint sensors and other biometric sensors, optical position sensors (optical encoders), and/or other position sensors such as linear position sensors, and/or other sensors. 
     User input and other information may be gathered using sensors and other input devices in input-output devices  13 . 16 - 22 . If desired, input-output devices  13 . 16 - 22  may include other devices  13 . 16 - 24  such as haptic output devices (e.g., vibrating components), light-emitting diodes and other light sources, speakers such as ear speakers for producing audio output, and other electrical components. Device  13 . 16 - 10  may include circuits for receiving wireless power, circuits for transmitting power wirelessly to other devices, batteries and other energy storage devices (e.g., capacitors), joysticks, buttons, and/or other components. 
     Electronic device  13 . 16 - 10  may have housing structures (e.g., housing walls, straps, etc.), as shown by illustrative support structures  13 . 16 - 26  of  FIGS.  13 . 16 - 1   . In configurations in which electronic device  13 . 16 - 10  is a head-mounted device (e.g., a pair of glasses, goggles, a helmet, a hat, a headband, etc.), support structures  13 . 16 - 26  may include head-mounted support structures (e.g., a helmet housing, head straps, temples in a pair of eyeglasses, goggle housing structures, and/or other head-mounted structures). The head-mounted support structures may be configured to be worn on a head of a user during operation of device  13 . 16 - 10  and may support display(s)  13 . 16 - 14 , sensors  13 . 16 - 16 , other components  13 . 16 - 24 , other input-output devices  13 . 16 - 22 , and control circuitry  13 . 16 - 12 . 
     In some embodiments, support structures  13 . 16 - 26  may include a light-shielding nosepiece. The light-shielding nosepiece may be attached to support structures  13 . 16 - 26 , such as a main housing portion of electronic device  13 . 16 - 10 , and may rest on the user&#39;s nose while device  13 . 16 - 10  is worn. The light-shielding nosepiece may be flexible, to allow the nosepiece to conform to the user&#39;s nose, while retaining enough rigidity to support device  13 . 16 - 10  on the user&#39;s face while it is being worn (i.e., to maintain its shape on the user&#39;s nose while the device is worn) and to be wrapped by a fabric or other material. If desired, the light-shielding nosepiece may also include stiffeners or other components that help maintain the nosepiece on the user&#39;s nose to prevent light from reaching the user&#39;s eyes. An example of an illustrative electronic device having a nosepiece is shown in  FIGS.  13 . 16 - 2   . 
     As shown in  FIGS.  13 . 16 - 2   , head-mounted device  13 . 16 - 10  may include support structures  13 . 16 - 26 , which may include a head-mounted housing (sometimes referred to as a main housing, main housing unit, head-mounted support structure, etc.). The housing may have walls or other structures that separate an interior housing region from an exterior region surrounding the housing. For example, the housing may have walls formed from polymer, glass, metal, and/or other materials. Electrical and optical components may be mounted in the housing. These components may include components such as integrated circuits, sensors, control circuitry, input-output devices, etc. 
     To present a user with images for viewing from eye boxes (e.g., eye boxes in which the user&#39;s eyes are located when device  13 . 16 - 10  is being worn on the user&#39;s head), device  13 . 16 - 10  may include displays and lenses. These components may be mounted in optical modules or other supporting structure in the housing to form respective left and right optical systems. There may be, for example, a left display for presenting an image through a left lens to a user&#39;s left eye in a left eye box and a right display for presenting an image through a right lens to a user&#39;s right eye in a right eye box. 
     If desired, the housing may have forward-facing components such as cameras and other sensors on a front side for gathering sensor measurements and other input and may have a soft cushion on an opposing rear side of the housing. The rear side of the housing may have openings that allow the user to view images (image light  13 . 16 - 32 ) from the left and right optical systems (e.g., when the rear side of the housing is resting on the user&#39;s head). 
     If desired, device  13 . 16 - 10  may have an adjustable strap or headband, and if desired, may have other structures (e.g., an over-the-head strap) to help hold the housing on the user&#39;s head. 
     As shown in  FIGS.  13 . 16 - 2   , when worn by the user, device  13 . 16 - 10  may include a nosepiece, such as nosepiece  13 . 16 - 28  that rests on the nasal region the user&#39;s head (e.g., on the user&#39;s nose). In particular, nosepiece  13 . 16 - 28  (sometimes referred to as a light-shielding structure or a light-shielding nosepiece herein) may serve as an extension of the housing that rests on the user&#39;s nose and bridges between the opposing cheeks of the user. If desired, nosepiece  13 . 16 - 28  may be attached to the housing and/or may include one or more members formed from a portion of the housing. In some embodiments, nosepiece  13 . 16 - 28  may be a portion of a light seal or attached to a light seal that extends around some or all of a periphery of the housing (e.g., the light seal is attached to the housing around the periphery). When device  13 . 16 - 10  is in use, the light seal may compress against the user&#39;s face and prevent interference from ambient light. In general, however, nosepiece  13 . 16 - 28  may be attached to the housing in any desired manner. 
     Nosepiece  13 . 16 - 28  may be configured as a light-shielding structure and may therefore be sometimes referred to as light-shielding structure  13 . 16 - 28  or light-shielding nosepiece  13 . 16 - 28 . As an example, it may be desirable to enhance the viewing experience of the user by blocking external environmental light from entering the interior of device  13 . 16 - 10  (e.g., from entering the eye boxes) when device  13 . 16 - 10  is worn by the user. Nosepiece  13 . 16 - 28  may conform to the facial topology of the user around the user&#39;s nose and block light from entering the eye boxes. In some illustrative configurations, nosepiece  13 . 16 - 28  may be adjustable to conform to varying facial topologies of different users (e.g., portions of nosepiece  13 . 16 - 28  may deform differently based on the nose shapes of the users). 
     Nosepiece  13 . 16 - 28  may be mounted to a housing portion of electronic device  13 . 16 - 10 , such as head-mounted support structures  13 . 16 - 26 , at mounting points  13 . 16 - 30 . The housing may include a housing frame that runs along the periphery of device  13 . 16 - 10 . If desired, the housing frame may be overlapped by a cushion member on the rear side of the housing facing the user. As an example, the cushion member may include foam structures or other soft compressible structures affixed to the housing frame. A fabric may overlap and extend over the housing frame and/or the cushion member on the rear side of the housing. If desired, the fabric may enclose only the cushion member, and the fabric-enclosed cushion member may be removably coupled to the housing frame. 
     Mounting points  13 . 16 - 30  may be located at a bottom portion of the housing frame (e.g., a bottom portion of support structures  13 . 16 - 26 ). As examples, mounting points  13 . 16 - 30  may include coupling mechanisms such as magnets, adhesive, hinges, or any other suitable coupling mechanisms. 
     In the example of  FIGS.  13 . 16 - 2   , nosepiece  13 . 16 - 28  is attached to support structures  13 . 16 - 26  such that nosepiece  13 . 16 - 28  extends into a portion of support structures  13 . 16 - 26 . This is merely illustrative. If desired, the housing frame, a cushion member, and/or fabric may split into multiple portions to surround nosepiece  13 . 16 - 28 . As an example, a fabric-enclosed cushion member may run in front of nosepiece  13 . 16 - 28 , and a housing frame may run behind nosepiece  13 . 16 - 28 . In general, the housing portions, cushion member, and/or fabric may define an opening in which nosepiece  13 . 16 - 28  is disposed. 
     In some illustrative examples, nosepiece  13 . 16 - 28  may be removably coupled (via magnetics) to the housing frame or other portions of the housing. In some illustrative examples, a portion of nosepiece  13 . 16 - 28  may form an integral portion of the housing frame and/or may not be removable from the housing. 
     Support structures  13 . 16 - 26  (e.g., housing frames) and nosepiece  13 . 16 - 28  (along with other desired structures) may define the periphery of the eye boxes of device  13 . 16 - 10  at which the user&#39;s eyes are located. Components, such as displays, lenses, sensors, etc., may overlap and/or be located within the eye boxes of device  13 . 16 - 10 , and may be enclosed by and/or mounted to support structures  13 . 16 - 26  and/or nosepiece  13 . 16 - 28 . As illustratively shown in  FIGS.  13 . 16 - 2   , display  13 . 16 - 14  emit image light  13 . 16 - 32  through a lens to an eye box. Nosepiece  13 . 16 - 28  may be configured to block environmental light from an exterior of device  13 . 16 - 10  from entering the eye box and interfering with image light  13 . 16 - 24 . An example of a nosepiece that may be used in device  13 . 16 - 10  is shown in  FIGS.  13 . 16 - 3   . 
     As shown in  FIGS.  13 . 16 - 3   , a nosepiece, such as nosepiece  13 . 16 - 28 , may include elastomer  13 . 16 - 34  (also referred to as elastomeric layer  13 . 16 - 34  herein). Elastomer  13 . 16 - 34  may be any desired elastomeric material, such as thermoplastic polyurethane (TPU), nitrile butadiene rubber (NBR), or silicone (such as a low-durometer silicone). Elastomer  13 . 16 - 34  may have a thickness of greater than 0.1 mm, greater than 0.25 mm, greater than 0.5 mm, about 1 mm, less than 2 mm, less than 5 mm, etc. 
     In some embodiments, elastomer  13 . 16 - 34  may include perforations  13 . 16 - 36 . Perforations  13 . 16 - 36  may allow elastomer  13 . 16 - 34  to bend to accommodate a user&#39;s nose (i.e., in the horizontal left and right directions of  FIGS.  13 . 16 - 3   ), while maintaining sufficient rigidity (i.e., in the vertical up and down directions of  FIGS.  13 . 16 - 3   ) to support device  13 . 16 - 10  on the user&#39;s nose. The rigidity of elastomer  13 . 16 - 34  may also allow elastomer  13 . 16 - 34  to be wrapped by fabric  13 . 16 - 38  or other low force, high stretch material. 
     Perforations  13 . 16 - 36  may be formed in any desired pattern. In the example of  FIGS.  13 . 16 - 3   , perforations  13 . 16 - 36  are slits formed as an array of perforations across the entire surface of elastomer  13 . 16 - 34 . Perforations  13 . 16 - 36  may be formed in a brick pattern, as in  FIG.  13 . 16 - 3   , or may be formed in any other desired pattern. Additionally, any desired number of perforations  13 . 16 - 36  may be formed in elastomer  13 . 16 - 34 , such as at least 1 perforation, at least 5 perforations, or any other desired number of perforations. Perforations  13 . 16 - 36  may extend entirely through elastomer  13 . 16 - 34 , or may extend partially through elastomer  13 . 16 - 34 . 
     The use of slits formed in a brick pattern, as shown in  FIGS.  13 . 16 - 3   , are merely illustrative. Other illustrative examples of perforations in elastomer  13 . 16 - 34  are shown in  FIGS.  13 . 16 - 4 A and  13 . 16 - 4 B . 
     As shown in  FIGS.  13 . 16 - 4 A , elastomer  13 . 16 - 34  may include three-part openings  13 . 16 - 37 . Three-part openings  13 . 16 - 37  may be arranged in a brick pattern, similar to the slits of  FIGS.  13 . 16 - 3   . Alternatively, three-part openings  13 . 16 - 37  may be formed in any other suitable pattern. 
     Three-part openings  13 . 16 - 37  may allow elastomer  13 . 16 - 34  to stretch/deform in two or more axes. As a result, elastomer  13 . 16 - 34  (and therefore nosepiece  13 . 16 - 28 ) may contour to a user&#39;s nose more accurately (e.g., elastomer  13 . 16 - 34  may be able to deform to a user&#39;s nose and therefore adapt to the shape of the user&#39;s nose). In this way, elastomer  13 . 16 - 34  may seal to the user&#39;s nose and prevent light from interfering with displays in the head-mounted device. 
     Different types of perforations may be used in a single elastomeric member. For example, as shown in  FIGS.  13 . 16 - 4 B , slits  13 . 16 - 36  may be formed on a top half of elastomer  13 . 16 - 34 , while three-part openings  13 . 16 - 37  may be formed on a bottom half of elastomer  13 . 16 - 34 . In some embodiments, using three-part openings  13 . 16 - 37  at the bottom portion of elastomer  13 . 16 - 34  may allow elastomer  13 . 16 - 34  to stretch more (e.g., along two or more axes) at the user&#39;s nose. However, the example of an upper half of elastomer  13 . 16 - 34  having slits  13 . 16 - 36  and the bottom half having three-part openings  13 . 16 - 37  is merely illustrative. In general, slits  13 . 16 - 36  and three-part openings (or other suitable perforations) may be formed anywhere on elastomer  13 . 16 - 34 , and may fill any suitable portion of elastomer  13 . 16 - 34  to allow elastomer  13 . 16 - 34  to conform to a user&#39;s nose. 
     Elastomer  13 . 16 - 34  may have a shape that generally conforms to a user&#39;s nose, such as a chevron shape, as shown in  FIG.  13 . 16 - 3   , or any other desired shape, such as a rounded shape, or a rectangular shape. 
     As shown in  FIGS.  13 . 16 - 3   , elastomer  13 . 16 - 34  may be covered by fabric  13 . 16 - 38 . In particular, fabric  13 . 16 - 38  may overlap and/or encapsulate elastomer  13 . 16 - 34  in region  13 . 16 - 40 . Fabric region  13 . 16 - 40  may have the same shape as elastomer  13 . 16 - 34 , as shown in  FIG.  13 . 16 - 3   , or may have a different shape than elastomer  13 . 16 - 34 . Fabric members  13 . 16 - 40  and/or  13 . 16 - 38  may serve as a light-shielding member or layer for light-shielding structure  13 . 16 - 28 . In particular, illustrative configurations in which fabric  13 . 16 - 40 / 13 . 16 - 38  is a fabric cover (sometimes referred to as a fabric cover layer or a cover layer) are described herein as an illustrative example. To serve light-shielding functions, the fabric cover may be formed from an opaque or light-shielding material (e.g., black yarn) or may formed from an underlying material coated with an opaque or light-shielding material (e.g., black dye or ink). As examples, the fabric cover may be formed any suitable type of fabric such as knit fabric, woven fabric, braided fabric, etc. 
     Fabric  13 . 16 - 40  may be tented over elastomer  13 . 16 - 34 . Tenting of the fabric cover over an underlying structure may be achieved by the underlying structure contacting or otherwise supporting the fabric cover at one or more points or areas of support as the fabric cover extends over one or more sides of the underlying structure. The tenting of the fabric cover over the underlying structure may cause the fabric cover to follow the general outline of the underlying structure, especially around the areas of support. If desired, differences in the outlines of the fabric cover and of the underlying structure may exist, especially in some regions away from the areas of support, thereby causing some portions of the fabric cover to be suspended in air and therefore readily deflectable. As an example, the fabric cover may be deflectable to the boundary of the underlying structure (or even beyond the boundary of the underlying structure if the boundary is defined by a flexible or deformable member). 
     The rigidity of elastomer  13 . 16 - 34 , which is preserved in the vertical direction by perforations  13 . 16 - 36 , may allow elastomer  13 . 16 - 34  to be wrapped by fabric  13 . 16 - 40  (or other low force, high stretch textile, or other low force, high stretch material) without deforming. In other words, elastomer  13 . 16 - 34  may be rigid enough to maintain its shape while fabric  13 . 16 - 40  is applied to/wrapped around elastomer  13 . 16 - 34  (as well as to maintain its shape when the device is worn by a user), but flexible enough that it can conform to a user&#39;s nose. 
     In such a way, the fabric cover may have a three-dimensional shape (based on an outline of the underlying structure) that includes portions (e.g., directly supported by the underlying structure) that are more defined and portions (e.g., not directly supported by the underlying structure, suspended in air, etc.) that are less defined, and more flexible or yielding. These less-defined portions (e.g., a yielding fabric surface) may help form flexible boundaries such as those for an opening configured to receive a user&#39;s nose. 
     As a particular illustrative example, the underlying structure may have surfaces that define an opening for accommodating a user&#39;s nose. The surfaces may be surrounded by peripheral edges. The fabric cover may be tented over the underlying structure such that the fabric cover is directly supported by the underlying structure along one or more of the peripheral edges of the underlying structure and may be suspended in air around the opening, thereby providing a fabric surface that is deflectable by the user&#39;s nose. This may help with improving user comfort as well as providing a more conformal fit when the light shielding structure rests on the user&#39;s nose. 
     Regardless of the shape of fabric region  13 . 16 - 40 , fabric region  13 . 16 - 40  (and/or elastomer  13 . 16 - 34 ) may be bonded to support structures  13 . 16 - 26  (such as a housing frame) using adhesive  13 . 16 - 42 . However, the use of adhesive  13 . 16 - 42  is merely illustrative. Fabric  13 . 16 - 40  and/or elastomer  13 . 16 - 34  may be formed integrally with support structures  13 . 16 - 26 , or may be attached to support structures  13 . 16 - 26  using any desired attachment mechanism. 
     Although nosepiece  13 . 16 - 28  is shown as including both elastomer  13 . 16 - 34  and fabric  13 . 16 - 40 , this is merely illustrative. If desired, nosepiece  13 . 16 - 28  may include elastomer  13 . 16 - 34  without an overlapping fabric layer. In this case, another layer, such as a polymer or rubber, may overlap elastomer  13 . 16 - 34 , or elastomer  13 . 16 - 34  may directly contact a user&#39;s nose as they wear device  13 . 16 - 10 . Alternatively, nosepiece  13 . 16 - 28  may include fabric  13 . 16 - 40  without elastomer  13 . 16 - 34 , if desired. In this case, fabric  13 . 16 - 40  may be a flat knit fabric to provide sufficient stretching over a user&#39;s nose, while providing enough support for device  13 . 16 - 10 . 
     Although fabric  13 . 16 - 38 / 13 . 16 - 40  is described as fabric, this is merely illustrative. In general, fabric  13 . 16 - 38 / 13 . 16 - 40  may be any low force, high stretch material, such as a low force, high stretch textile. 
     In some embodiments, although elastomer  13 . 16 - 34  may have rigidity to support device  13 . 16 - 10  on a user&#39;s nose (i.e., in the vertical direction of  FIGS.  13 . 16 - 3   ) and to be wrapped by fabric, it may be desirable to add additional rigid or semi-rigid structures to a nosepiece. An illustrative example of a nosepiece having a rigid structure is shown in  FIGS.  13 . 16 - 5   . 
     As shown in  FIGS.  13 . 16 - 5   , nosepiece  13 . 16 - 28  may include fabric  13 . 16 - 40  (which may cover an elastomeric layer, such as elastomer  13 . 16 - 34 ) and adhesive  13 . 16 - 42 , which in turn may attach fabric  13 . 16 - 40  to structural frame  13 . 16 - 43 . Structural frame  13 . 16 - 43  may be a plastic frame, as an example. Structural frame  13 . 16 - 43  may provide additional rigidity to ensure that nosepiece  13 . 16 - 28  retains its shape and support when worn by a user. 
     Although  FIGS.  13 . 16 - 5    shows the use of adhesive  13 . 16 - 42 , this is merely illustrative. Adhesive  13 . 16 - 42  may be omitted, if desired. In some examples, elastomer  13 . 16 - 34  may be co-molded to structural frame  13 . 16 - 43 . Alternatively, elastomer  13 . 16 - 34  may be fit into a recessed portion of structural frame  13 . 16 - 43  or otherwise attached to structural frame  13 . 16 - 43 . 
     Additionally, although  FIGS.  13 . 16 - 5    shows structural frame  13 . 16 - 43  on three sides of fabric  13 . 16 - 40 /elastomer  13 . 16 - 34 , this is merely illustrative. Structural frame  13 . 16 - 43  may be attached to one side (e.g., the bottom side) of fabric  13 . 16 - 40 /elastomer  13 . 16 - 34  or any other desired number of sides. 
     Regardless of the attachment of structural frame  13 . 16 - 43  to elastomer  13 . 16 - 34  and/or fabric  13 . 16 - 40 , fabric  13 . 16 - 40  may extend over all or some of both frame  13 . 16 - 43  and/or elastomer  13 . 16 - 34 . An example of a stack up of nosepiece  13 . 16 - 28  is shown in  FIGS.  13 . 16 - 6   . 
     As shown in  FIGS.  13 . 16 - 6   , nosepiece  13 . 16 - 28  may include fabric  13 . 16 - 40 , which is shown as having fabric portions  13 . 16 - 40 - 1  and  13 . 16 - 40 - 2 , on either side of elastomer  13 . 16 - 34 . If desired, fabric  13 . 16 - 40  may be bonded to elastomer  13 . 16 - 34  at a periphery of elastomer  13 . 16 - 34 , which is shown in  FIGS.  13 . 16 - 6    as points  13 . 16 - 44 . For example, fabric  13 . 16 - 40  may be bonded to elastomer  13 . 16 - 34  around a periphery (such as an entire periphery or a portion of the periphery) of the elastomer layer. Fabric  13 . 16 - 40  may be bonded to elastomer  13 . 16 - 34  using an adhesive or other desired mounting mechanism. 
     In some cases, it may be desirable to allow elastomer  13 . 16 - 34  to move to a greater extent, and therefore leave elastomer  13 . 16 - 34  un-bonded from fabric  13 . 16 - 40 . For example, elastomer  13 . 16 - 34  may be fully surrounded fabric  13 . 16 - 40  and float within the fabric. In some embodiments portions of fabric  13 . 16 - 40  may be bonded directly to each other, rather than to elastomer  13 . 16 - 34 . In this way, elastomer  13 . 16 - 34  may float within fabric  13 . 16 - 40 , which may allow  13 . 16 - 34  to move more freely. 
     Because nosepiece  13 . 16 - 28  is designed to block light from reaching the eye boxes of a user wearing device  13 . 16 - 10 , it may be desirable to ensure a tight fit between nosepiece  13 . 16 - 28  and the user&#39;s nose and a similarly tight fit between nosepiece  13 . 16 - 28  and device  13 . 16 - 10 . An example of an extension piece that may allow for nosepiece  13 . 16 - 28  to fit tightly with device  13 . 16 - 10  is shown in  FIGS.  13 . 16 - 7   . 
     As shown in  FIGS.  13 . 16 - 7   , extension piece  13 . 16 - 46  may be added below nosepiece  13 . 16 - 28 . In particular, if a user&#39;s nose bridge protrudes only a small amount, nosepiece  13 . 16 - 28  may not reach the user&#39;s nose. Therefore, extension piece  13 . 16 - 46  may be attached between support structures of device  13 . 16 - 10 , such as support structures  13 . 16 - 26 , and nosepiece  13 . 16 - 28 . Extension piece  13 . 16 - 28  may be formed from fabric, foam, plastic, and/or any other desired material. In some cases, extension piece  13 . 16 - 28  may allow nosepiece  13 . 16 - 28  to align with other light-blocking structures in device  13 . 16 - 10 , such as a foam member that fits against the user&#39;s face. In this way, light may be prevented from reaching the user&#39;s eye boxes and interfering with the user&#39;s visibility of a display in device  13 . 16 - 10 . 
     In addition or as an alternative to extension piece  13 . 16 - 46 , it may be desirable to ensure a close fit to a user&#39;s nose to prevent light from entering the user&#39;s eye boxes. Examples of nosepieces that have additional structures to improve the fit to a user&#39;s nose are shown in  FIGS.  13 . 16 - 8  and  13 . 16 - 9   . 
     As shown in  FIGS.  13 . 16 - 8   , service loop  13 . 16 - 49  may be included within nosepiece  13 . 16 - 28 . In particular, service loop  13 . 16 - 49  (and other portions of nosepiece  13 . 16 - 28 , if desired) may be attached to supports  13 . 16 - 50 - 1  and  13 . 16 - 50 - 2 . Service loop  13 . 16 - 49  may be tightened and loosened as desired to conform nosepiece  13 . 16 - 28  to the nose of a user. For example, as shown in  FIGS.  13 . 16 - 7   , service loop  13 . 16 - 49  may be tightened to move nosepiece  13 . 16 - 28  to position  13 . 16 - 28 ′ by moving nosepiece  13 . 16 - 28  downward in direction  13 . 16 - 56 , right in direction  13 . 16 - 52 , and left in direction  13 . 16 - 54  toward to the nose of the user. In this way, service loop  13 . 16 - 49  may allow nosepiece  13 . 16 - 28  to fit more securely to the user&#39;s nose, preventing light from entering the user&#39;s eye boxes through gaps between nosepiece  13 . 16 - 28  and the nose. 
     Alternatively or additionally, nosepiece  13 . 16 - 28  may include foam  13 . 16 - 48 . Foam  13 . 16 - 48  may fill a gap between nosepiece  13 . 16 - 28  and the user&#39;s nose, and may be compressible to allow for a secure fit between nosepiece  13 . 16 - 28  and the nose. Foam  13 . 16 - 48  may directly contact the nose of the user (e.g., may be on a surface of a fabric layer, such as fabric  13 . 16 - 40 ), as shown in  FIGS.  13 . 16 - 8   , may be embedded within nosepiece  13 . 16 - 28  (e.g., covered by fabric, such as fabric  13 . 16 - 40 ), or may otherwise be attached to nosepiece  13 . 16 - 28 . 
     Instead of service loop  13 . 16 - 49 , a deformable stiffener, such as deformable stiffener  13 . 16 - 58 , may be incorporated into nosepiece  13 . 16 - 28 . As shown in  FIGS.  13 . 16 - 9   , deformable stiffener  13 . 16 - 58  may allow for nosepiece  13 . 16 - 28  to be adjusted in directions  13 . 16 - 52 ,  13 . 16 - 54 , and  13 . 16 - 56  toward the nose of the user to conform nosepiece  13 . 16 - 28  to the user&#39;s nose at position  13 . 16 - 28 ′. Deformable stiffener  13 . 16 - 58  may be formed within nosepiece  13 . 16 - 28  (i.e., may be covered by fabric), as shown in  FIG.  13 . 16 - 9   , or may be formed on a surface of the fabric (e.g., an inner or outer surface of fabric  13 . 16 - 40 ). Although not shown in  FIGS.  13 . 16 - 9   , a gap-filling foam, such as foam  13 . 16 - 48  of  FIGS.  13 . 16 - 8   , may be incorporated in or on nosepiece  13 . 16 - 28  in addition to deformable stiffener  13 . 16 - 58 . 
     In some embodiments, to ensure that nosepiece  13 . 16 - 28  is tightly sealed to the nose of a user, internal components of device  13 . 16 - 10 , such as fans, may be used to move air toward nosepiece  13 . 16 - 28 , thereby sealing nosepiece  13 . 16 - 28  around the user&#39;s nose. 
     In addition to improving the fit of nosepiece  13 . 16 - 28  to prevent light from entering the eye boxes of the user, it may also be desirable to incorporate layers into nosepiece  13 . 16 - 28  that improve the comfort for the user. Examples of various modifications that may be made to nosepiece  13 . 16 - 28  to improve user comfort are shown in  FIGS.  13 . 16 - 10 A- 13 . 16 - 10 G . 
     As shown in  FIGS.  13 . 16 - 10 A , a nosepiece, which includes elastomer  13 . 16 - 34  and fabric  13 . 16 - 40  (such as nosepiece  13 . 16 - 28 ) may have a rolled edge and may approach the user&#39;s nose at a shallow angle of contact to conform to a curved portion of nose  13 . 16 - 60 . The rolled edge may prevent elastomer  13 . 16 - 34  from digging into the user&#39;s skin on nose  13 . 16 - 60 , while the shallower angle of contact may allow the nosepiece to move up the user&#39;s nose rather than hitting the user&#39;s nose directly. Therefore, by having a rolled edge and/or a shallower angle of contact, nosepiece  13 . 16 - 28  may improve comfort for a user of device  13 . 16 - 10 . 
     Alternatively or additionally, a nosepiece may include foam, such as foam  13 . 16 - 62 , between some or all of elastomer  13 . 16 - 34  and fabric  13 . 16 - 40 , as shown in  FIGS.  13 . 16 - 10 B . In particular, foam  13 . 16 - 62  may be at an edge portion of the nosepiece that contacts the user&#39;s nose, and therefore prevent elastomer  13 . 16 - 34  from hurting the user&#39;s nose or reducing discomfort as the nosepiece pushes against the nose. 
     In some examples, it may be desirable to have a series of openings in elastomer  13 . 16 - 34  that are either unfilled or filled with different material, such as foam. For example, in  FIGS.  13 . 16 - 10 C , openings  13 . 16 - 64  may be provided in a portion of the nosepiece that will contact the user&#39;s nose. Openings  13 . 16 - 64  may be through openings, partial openings, may be filled with other material, such as foam, or may otherwise be more flexible than the surrounding regions of elastomer  13 . 16 - 34 . As the nosepiece pushes against the user&#39;s nose, openings  13 . 16 - 64  may crumple, thereby preventing or reducing discomfort to the user. 
     If desired, a portion of fabric may be extended from the portion that contacts the user&#39;s nose to provide an additional buffer between the nose and elastomer  13 . 16 - 34 . As shown in  FIGS.  13 . 16 - 10 D , fabric portion  13 . 16 - 66  may extend from fabric  13 . 16 - 40 . When worn by a user, fabric portion  13 . 16 - 66  may contact the nose of the user first, and therefore increase the amount of fabric between the nose and elastomer  13 . 16 - 34 , which may improve user comfort. In some examples, fabric portion  13 . 16 - 66  may be a hemmed edge of fabric  13 . 16 - 40 . 
     If desired, at least some portions of elastomer  13 . 16 - 34  that would otherwise contact the user&#39;s nose may be cut and replaced by more flexible material, such as foam. As shown in FIGS.  13 . 16 - 10 E, foam portions  13 . 16 - 68  may replace portions of elastomer  13 . 16 - 34  that would otherwise contact the user&#39;s nose. Although fabric  13 . 16 - 40  does not cover foam portions  13 . 16 - 68  in  FIGS.  13 . 16 - 10 E , fabric  13 . 16 - 40  may overlap foam portions  13 . 16 - 68 , if desired. 
     In addition to, or instead of, the modifications of  FIGS.  13 . 16 - 10 A- 13 . 16 - 10 E , in which an edge portion of a nosepiece is modified to prevent the elastomer from providing discomfort to a user&#39;s nose, a bottom portion of the nosepiece may be modified. As shown in  FIGS.  13 . 16 - 10 F , segmented material  13 . 16 - 69  may be added to a bottom surface of nosepiece  13 . 16 - 28 . For example, segmented material  13 . 16 - 69  may contact the top of the user&#39;s nose (i.e., the nose bridge), and prevent nosepiece  13 . 16 - 28  from directly contacting the nose and causing discomfort. Material  13 . 16 - 69  may be foam or may be elastomer flaps. 
     Although  FIGS.  13 . 16 - 10 F  shows material  13 . 16 - 69  as being segmented and non-overlapping, material  13 . 16 - 69  may be segmented and overlapping, or may be one connected piece of material, if desired. For example, in  FIGS.  13 . 16 - 10 G , foam  13 . 16 - 73  may be provided between nosepiece  13 . 16 - 28  and the user&#39;s nose while device  13 . 16 - 10  is worn. If desired, optional deformable stiffener  13 . 16 - 71  may be provided on the bottom surface of nosepiece  13 . 16 - 28  to improve the fit of nosepiece  13 . 16 - 28  on the user&#39;s nose. If stiffener  13 . 16 - 71  is included, foam  13 . 16 - 73  may also prevent stiffener  13 . 16 - 71  from directly contacting the user&#39;s nose and providing discomfort. 
     All of the examples in  FIGS.  13 . 16 - 10 A- 13 . 16 - 10 G  are merely illustrative examples of improving user comfort while ensuring that a nosepiece fits tightly to the user&#39;s nose. The examples are not limiting and may be implemented individually or together in any combination. Regardless of whether any structures in  FIGS.  13 . 16 - 10 A- 13 . 16 - 10 G  are used, it may be desirable to strengthen a portion of nosepiece  13 . 16 - 28 . Although the use of a structural frame was described in connection with  FIGS.  13 . 16 - 5   , such a frame may add unnecessary bulk or add too much rigidity to nosepiece  13 . 16 - 28  in some cases. Therefore, a semi-rigid member may be used. An example of using a semi-rigid member on a nosepiece is shown in  FIGS.  13 . 16 - 11   . 
     As shown in  FIGS.  13 . 16 - 11   , a nosepiece, such as nosepiece  13 . 16 - 28 , may include portions  13 . 16 - 70  and  13 . 16 - 72 . Portion  13 . 16 - 70  may include an elastomer, such as elastomer  13 . 16 - 34 , and/or a fabric, such as fabric  13 . 16 - 40 . Portion  13 . 16 - 72  may be coupled to portion  13 . 16 - 70  and may include a semi-rigid member. In particular, the semi-rigid member may be formed on a bottom portion of nosepiece  13 . 16 - 28  (i.e., toward the bottom of the user&#39;s nose when device  13 . 16 - 10  is worn), and may provide additional rigidity to nosepiece  13 . 16 - 28 . The semi-rigid members may be formed from any desired material, such as rubber or plastic. In this way, nosepiece  13 . 16 - 28  may remain sufficiently flexible to conform to the shape of a user&#39;s nose, while retaining its shape when after it has been conformed to the shape of the nose and the device is being worn by the user. 
     Although nosepieces, such as nosepiece  13 . 16 - 28 , have been described as including an elastomeric member, such as elastomer  13 . 16 - 34 , this is merely illustrative. In some embodiments, elastomer  13 . 16 - 34  may be omitted. For example, in the example of  FIG.  13 . 16 - 3   , elastomer  13 . 16 - 34  may be omitted or may be replaced with thick fabric, which may be surrounded by thinner, more stretchy fabric. In the illustrative example of  FIGS.  13 . 16 - 12   , nosepiece  13 . 16 - 28  may include fabric  13 . 16 - 73  that may be formed from fabric layers  13 . 16 - 74  and  13 . 16 - 76 . Fabric layers  13 . 16 - 74  and  13 . 16 - 76  may be bonded at points  13 . 16 - 78 , such as with laser welding, ultrasonic welding, or adhesive. The use of two (or more) fabric layers  13 . 16 - 74  and  13 . 16 - 76  may make fabric  13 . 16 - 73  thicker, thereby providing more support in the region of a user&#39;s nose. In general, however, fabric  13 . 16 - 73  may include any suitable number of fabric layers. 
     The foregoing is merely illustrative and various modifications can be made to the described embodiments. The foregoing embodiments may be implemented individually or in any combination. 
     XIV: Powerstraps and Securement Band 
       FIGS.  14 . 0 - 1    illustrates a view of an HMD  14 . 0 - 100  including power straps  14 . 0 - 102  and a securement band  14 . 0 - 104 . The power straps  14 . 0 - 102  and a securement band  14 . 0 - 104  are described in more detail here in section XIV. 
     14.1: Electrical Connectors 
     Portable electronic devices, such as smart phones, laptops, tablet computing devices, smart watches, head-mountable devices (also referred to as head-mountable displays (HMDs)), and headphones, have become commonplace for persons undertaking daily activities (travel, communication, education, entertainment, employment, and the like). Indeed, portable electronic devices can provide assistance in completing daily tasks and errands, such as, watching an instructional video or monitoring progress during and after an exercise routine. However, some electronic devices incorporate a temporary or permanent cabled connection to operate (e.g., charging the device, providing electrical power to an electronic component, interconnecting a peripheral input or output device, and the like). 
     Regarding HMDs, an electronic device can include one or more cabled connections, each of which can have unique and specific requirements. As an example, a temporary or permanent cabled connection can be coupled to a display portion of the HMD. The HMD can include one or more battery packs or electrical power sources that demand regular charging to operate the HMD for an extended period of time. However, a cabled connection to the display portion can inhibit (e.g., snag on a user or objects around the user while in use) or otherwise limit a user from operating the HMD while charging. 
     In some examples of the present disclosure, a display portion of the HMD can receive electrical power and/or control signals through a support electrically coupled to the display portion. The support can include a plug connector which is connected (structurally and electrically) within a receptacle connector of the display portion. While connected, the support can define an electrical path that enables electrical power and/or control signals to pass between the display portion and another electronic device (e.g., an electrical power source, such as, a battery pack) electrically connected to the support. The support can include a receptacle connector configured to electrically connect an electronic device to the support. 
     One aspect of the present disclosure relates to an electronic device, (e.g., an HMD) including an enclosure and a receptacle connector coupled or joined to the enclosure. The receptacle connector can include a housing defining an aperture and an internal volume. In some examples, the housing is configured to receive a plug connector of a support within the internal volume. The receptacle connector can include one or more electrical contacts at least partially disposed within the internal volume and contacting one or more correlating electrical contacts on the plug connector. In some examples, the plug connector and the receptacle connector can define respective convex surfaces and respective concave surfaces such that a cross-sectional shape of the plug connector and/or the receptacle connector has a curved longitudinal axis. The electrical contacts of the receptacle connector can be coupled or connected to the housing at angles normal to the concave and convex surfaces of the plug connector, which provides for improved electrical contact between the electrical contacts of the receptacle connector and the plug connector. In some examples, the receptacle connector can include a spring-driven fastener configured to retain the plug connector within the receptacle connector and configured to release the plug connector in response to a tool actuation. Bumpers can be provided in the housing to supply an ejection force to the plug connector, and seals can be provided adjacent an opening of the cavity to provide sealing and/or an ejection force to the plug connector. 
     In some examples, abruptly or accidentally removing electrical power and/or control signals from an electronic device can damage the components of the electronic device and/or data stored on the electronic device. As such, cabled connections that do not unwantedly or accidentally disconnect can be desirable. In some examples, a cable assembly that reliably interlocks with the electronic device to limit unwanted or accidental extraction of the cable assembly is provided. In some examples, a connection that requires more force to remove the connection than to establish the connection (e.g., disconnecting a cable assembly from a port versus inserting the cable assembly into the port) is provided. In some examples, a cable assembly and a receptacle connector of an electronic device can be configured to prevent damage to the electronic device and loss of data, even in cases in which the cable assembly is accidentally or undesirably extracted from the receptacle connector. 
     One aspect of the present disclosure relates to an electronic device including an enclosure and a receptacle connector attached to the enclosure. The receptacle connector can form a recess that interlocks or otherwise engages with a plug connector of a cable assembly. In some examples, the plug connector can include a boot, a central member coupled to the boot, and one or more protrusions extending laterally from the central member. The one or more protrusions can extend laterally into a channel, or respective channels, formed within the recess of the receptacle connector. 
     In some examples, the receptacle connector can include one more detents disposed within the recess. Each of the one or more detents can interlock with a respective protrusion of the central member. The detents can be biased to extend into the recess by one or more leaf springs, canted coil springs, elastic foam, a combination thereof, or another biasing element. Each of the detents can form an angled surface which interfaces with a respective protrusion to retain the plug connector within the receptacle connector. At least one of the spring force of the biasing element and/or the angle of the angled surface can correlate to a force required to extract the plug connector from the receptacle connector. In some examples, each respective angle of the angled surfaces can vary so the plug connector can be more easily extracted by lifting one side of the plug connector (e.g., a non-cable-side) than another side of the plug connector (e.g., a cable-side of the plug connector). 
     In some examples, the central member can be rotated to interlock (or release) the plug connector and the receptacle connector. In other words, the plug connector can be rotatably received within the receptacle connector. Alternatively, a push-button, a latch, a slide-button, or another actuating mechanism can be used to interlock (or release) the plug connector and the receptacle connector. For example, an actuating mechanism can be incorporated into the boot of the plug connector. Additionally, or alternatively, one or more magnets can be disposed within the plug connector and/or the receptacle connector to orient the plug connector relative to the receptacle connector and/or retain the plug connector to the receptacle connector. 
     One aspect of the present disclosure relates to an electronic device including a housing that defines an exterior surface of the electronic device. The housing can form an aperture within the exterior surface. An electronic component can be disposed within the housing and can include a receptacle connector coupled to the electronic component. The receptacle connector can be fastened to a trim ring, which can be fastened to the housing. The housing, the trim ring, and the receptacle connector can form an electrical port. The electronic device can further include a first engagement feature and a cable having a plug connector. The plug connector can include a boot, a plug, and a second engagement feature. The boot is receivable within the aperture formed within the exterior surface of the housing and can be receivable within the trim ring. The plug is receivable within the receptacle connector to electrically couple the electronic device with the cable. The second engagement feature can interconnect or otherwise engage the first engagement feature to retain the cable to the electronic device. 
     In some examples, the first and second engagement features can be one or more: biasing elements, latches, arms, protrusions, recesses, channels, magnets, canted coiled springs, rotating members, sliding members, another type of engagement feature, or a combination thereof. In some examples, the first engagement feature can be included in the trim ring, the second engagement feature can be included in the boot, and the first and second engagement features can latch, interlock, or otherwise secure the boot in the trim ring such that the cable is interconnected with the electronic device. In some examples, the first engagement feature and the second engagement feature can involve a lesser force to connect the cable to the electronic device (e.g., insert the plug connector into the electrical port) and a greater force to disconnect the cable from the electronic device (e.g., remove the plug connector from the electrical port). In other words, a force required to withdraw the plug connector from the electrical port can be sufficiently large to prevent or limit unwanted or accidental removal of the cable from the electronic device. In some examples, the first engagement feature and the second engagement feature can be spring-driven features that are actuated by a tool in order to release the plug connector from the electrical port. In other words, a tool actuation to withdraw the plug connector from the electrical port can prevent or limit unwanted or accidental removal of the cable from the electronic device. 
     While the electronic devices are described herein as HMDs, the electronic devices can include a smart phone, a laptop, a tablet computing device, a smart watch, headphones, or any other electronic device. 
     These and other examples are discussed below with reference to  FIGS.  14 . 1 - 1 A through  14 . 1 - 35 B . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these figures is for explanatory purposes only and should not be construed as limiting. Furthermore, as used herein, a system, a method, an article, a component, a feature, or a sub-feature comprising at least one of a first option, a second option, or a third option should be understood as referring to a system, a method, an article, a component, a feature, or a sub-feature that can include one of each listed option (e.g., only one of the first option, only one of the second option, or only one of the third option), multiple of a single listed option (e.g., two or more of the first option), two options simultaneously (e.g., one of the first option and one of the second option), or combination thereof (e.g., two of the first option and one of the second option). 
       FIGS.  14 . 1 - 1 A and  14 . 1 - 1 B  show a first electronic device  14 . 1 - 100 , a cable assembly  14 . 1 - 140 , and a second electronic device  14 . 1 - 130 . In some examples, the first electronic device  14 . 1 - 100  can be a head-mountable display (HMD) including a display portion  14 . 1 - 102  and one or more supports  14 . 1 - 120 . While the first electronic device  14 . 1 - 100  is illustrated as an HMD, the first electronic device  14 . 1 - 100  can be a tablet computing device, smart phone, smart watch, or any other electronic device in other examples. The display portion  14 . 1 - 102  can output visual content viewable by a user of the electronic device  14 . 1 - 100 . For example, the display portion  14 . 1 - 102  can include a light-emitting diode (LED) display, an organic light-emitting diode (OLED) display, a liquid-crystal display (LCD) display, a micro-LED display, or the like. In some examples, the display portion  14 . 1 - 102  can be any form of display now known in the art, or as can be developed in the future. 
     The supports  14 . 1 - 120  can retain the electronic device  14 . 1 - 100  relative to a user&#39;s head  14 . 1 - 110 . In some examples, the first electronic device  14 . 1 - 100  can include a first support  14 . 1 - 120  disposed on a first side of the user&#39;s head  14 . 1 - 110  and illustrated in  FIGS.  14 . 1 - 1 A and  14 . 1 - 1 B , and a second support  14 . 1 - 120  (not separately illustrated) coupled to the display portion  14 . 1 - 102  and disposed on a second side of the user&#39;s head  14 . 1 - 110  opposite the side of the user&#39;s head  14 . 1 - 110  illustrated in  FIGS.  14 . 1 - 1 A and  14 . 1 - 1 B . In some examples, a single support  14 . 1 - 120  can be included, and can be a band  14 . 1 - 123  or can include a band portion coupled to the display portion  14 . 1 - 102 . The support  14 . 1 - 120  can be configured to wrap around or otherwise encircle a portion of the user&#39;s head  14 . 1 - 110  and can be coupled to the display portion  14 . 1 - 102  at two or more locations. In some examples in which the support  14 . 1 - 120  includes a band  14 . 1 - 123  or band portion, the support  14 . 1 - 120  can be made of an elastomer material which can flex or stretch and thereafter return to an initial state. In other examples, the supports  14 . 1 - 120  can be separate structural straps including electronics, processors, speakers, plugs, and/or other functional elements. These structural straps can then be connected to a band  14 . 1 - 123  that flexibly engages a user&#39;s head to impart a retention force on the system, in connection with facial engagement points. As used herein, the supports will be described as separate structural straps including wires, electronics, speakers, and/or other functional elements. 
     The supports  14 . 1 - 120  can include plug connectors  14 . 1 - 122 , which can be coupled to receptacle connectors  14 . 1 - 104  of the display portion  14 . 1 - 120 . The receptacle connectors  14 . 1 - 104  can include recesses that are configured to receive portions of the supports  14 . 1 - 120 , such as the plug connectors  14 . 1 - 122 . As illustrated in  FIGS.  14 . 1 - 1 A , the plug connectors  14 . 1 - 122  can be disposed at proximal ends of the supports  14 . 1 - 120 . The plug connectors  14 . 1 - 122  and the receptacle connectors  14 . 1 - 104  can each include one or more electrical contacts (not separately illustrated) that electrically couple the plug connectors  14 . 1 - 122  and the supports  14 . 1 - 120  to the display portion  14 . 1 - 102 . In other words, as shown in  FIGS.  14 . 1 - 1 B , the supports  14 . 1 - 120  can be electrically coupled to the display portion  14 . 1 - 102  through the plug connectors  14 . 1 - 122  and the receptacle connectors  14 . 1 - 104  when the plug connectors  14 . 1 - 122  are coupled to the receptacle connectors  14 . 1 - 104  (e.g., the plug connectors  14 . 1 - 122  are at least partially disposed within recesses of the receptacle connectors  14 . 1 - 104 ). Examples of the plug connectors  14 . 1 - 122  and the receptacle connectors  14 . 1 - 104  will be discussed in greater detail below with reference to  FIGS.  14 . 1 - 2  through  14 . 1 - 15   . In some examples, the supports  14 . 1 - 120  can be welded, adhered, fastened, crimped, clipped, mechanically engaged with, or otherwise retained by the display portion  14 . 1 - 102 . 
     The supports  14 . 1 - 120  can each include an enclosure  14 . 1 - 121  (also referred to as a housing) formed from a polymer, metal, ceramic, or combinations thereof. In some examples, the enclosure  14 . 1 - 121  can form a channel or cavity extending between a receptacle connector  14 . 1 - 124  of each support  14 . 1 - 120  and the plug connector  14 . 1 - 122 . The supports  14 . 1 - 120  can be electrically coupled to the plug connector  14 . 1 - 122  (and the display portion  14 . 1 - 102  through the plug connector  14 . 1 - 122  and the receptacle connector  14 . 1 - 104 ) such that electrical signals and/or electrical power received at the receptacle connector  14 . 1 - 124  can be provided to the plug connector  14 . 1 - 122  or other electronic components of the first electronic device  14 . 1 - 100 . For example, one or more electronic components (e.g., printed circuit boards, processors, electrical wires, digital logic circuitry, digital processing circuitry, or the like) can be positioned within the cavity formed within the enclosure  14 . 1 - 121  and extend between the receptacle connector  14 . 1 - 124  and the plug connector  14 . 1 - 122  to form an electrical path between the receptacle connector  14 . 1 - 124  and the plug connector  14 . 1 - 122 . 
     Each of the supports  14 . 1 - 120  can include a receptacle connector  14 . 1 - 124  disposed on or within an enclosure  14 . 1 - 121 . The receptacle connector  14 . 1 - 124  can form a recess  14 . 1 - 126  that can receive a portion of the cable assembly  14 . 1 - 140 . The cable assembly  14 . 1 - 140  can include a plug connector  14 . 1 - 142  at a distal end of the cable assembly  14 . 1 - 140 . The plug connector  14 . 1 - 142  can include a boot  14 . 1 - 143  and a central member  14 . 1 - 145  at least partially disposed within the boot  14 . 1 - 143 . The central member  14 . 1 - 145  can include one or more electrical contacts (not separately illustrated) that electrically couple the plug connector  14 . 1 - 142  to the first electronic device  14 . 1 - 100 . In other words, as shown in  FIGS.  14 . 1 - 1 B , the first electronic device  14 . 1 - 100  can be electrically coupled to the second electronic device  14 . 1 - 130  through the cable assembly  14 . 1 - 140  when the plug connector  14 . 1 - 142  is at least coupled to the receptacle connector  14 . 1 - 124  (e.g., the central member  14 . 1 - 145  is at least partially disposed within the recess  14 . 1 - 126 ). Examples of the receptacle connector  14 . 1 - 124  and the plug connector  14 . 1 - 142  will be discussed in greater detail below with reference to  FIGS.  14 . 1 - 16 A through  14 . 1 - 25 B . 
     As illustrated in  FIGS.  14 . 1 - 1 A and  14 . 1 - 1 B , each receptacle connector  14 . 1 - 124  can be positioned on or within the enclosure  14 . 1 - 121  of a respective support  14 . 1 - 120 . In some examples, the receptacle connector  14 . 1 - 124  can be positioned on the enclosure  14 . 1 - 121  and disposed a distance from the display portion  14 . 1 - 102 . The plug connector  14 . 1 - 142  can be operably coupled to the enclosure  14 . 1 - 121  (e.g., through the receptacle connector  14 . 1 - 124  of the enclosure  14 . 1 - 121 ) rather than directly coupling the plug connector  14 . 1 - 142  to the display portion  14 . 1 - 102 . The receptacle connector  14 . 1 - 124  can be disposed on or within the enclosure  14 . 1 - 121  such that the display portion  14 . 1 - 102  and the receptacle connector  14 . 1 - 124  are disposed on opposing sides of a user&#39;s ear  14 . 1 - 112 . In some examples, the display portion  14 . 1 - 102  can be coupled to a proximal end of the support  14 . 1 - 120  and the receptacle connector  14 . 1 - 124  can be positioned on or within a distal end of the support  14 . 1 - 120  opposite the display portion  14 . 1 - 102 . In some examples, the receptacle connector  14 . 1 - 124  can be disposed on or within the enclosure  14 . 1 - 121  between the proximal end and the distal end of the support  14 . 1 - 120  such that the display portion  14 . 1 - 102  and the user&#39;s ear  14 . 1 - 112  are disposed on opposing sides of the receptacle connector  14 . 1 - 124  (as shown in  FIGS.  14 . 1 - 1 A and  14 . 1 - 1 B ). In some examples, the receptacle connector  14 . 1 - 124  can be disposed within the enclosure  14 . 1 - 121  at a location that is closer to the distal end than the proximal end. In some examples, the receptacle connector  14 . 1 - 124  can be disposed within the enclosure  14 . 1 - 121  at a location that is closer to the proximal end than the distal end. In some examples, the receptacle connector  14 . 1 - 124  can be disposed behind the user&#39;s ear  14 . 1 - 112  such that the user&#39;s ear  14 . 1 - 112  is between the display portion  14 . 1 - 102  and the receptacle connector  14 . 1 - 124 . 
     Some HMDs utilize a re-chargeable power source (e.g., a battery) affixed to the HMD to provide electrical power to the electronic components (e.g., processors, displays, speakers, and the like). The size or capacity of the re-chargeable power source can be limited by the desired size, shape, and weight of the HMD. After the re-chargeable power source has been substantially depleted of electrical power, the user can be required to discontinue use of the HMD to allow re-charging of the electrical power source. 
     In some aspects of the present disclosure, rather than solely relying on a power source disposed within the HMD (e.g., the first electronic device  14 . 1 - 100 ), at least one power source (e.g., the second electronic device  14 . 1 - 130 ) can be additionally, or alternatively, electrically coupled to the HMD by a cabled connection (e.g., the cable assembly  14 . 1 - 140 ), which is electrically coupled to the enclosure  14 . 1 - 121  of the support  14 . 1 - 120  via the receptacle connector  14 . 1 - 124 . Electrically coupling the power source (e.g., the second electronic device  14 . 1 - 130 ) to the receptacle connector  14 . 1 - 124  of the support  14 . 1 - 120  can be advantageous. For example, electrical power and/or electrical signals can be provided to the first electronic device  14 . 1 - 100 , while the second electronic device  14 . 1 - 130  is disposed within a case, pocket, pouch, or otherwise retained by the user. Relocating the electrical power source away from the first electronic device  14 . 1 - 100  can accommodate a larger electrical power source than can be directly disposed on the first electronic device  14 . 1 - 100 , which can provide for extended use of the first electronic device  14 . 1 - 100 . Positioning the receptacle connector  14 . 1 - 124  between the proximal end and distal end of the support  14 . 1 - 120  can be beneficial in at least partially limiting user contact with the cable assembly  14 . 1 - 140  by positioning the cable assembly  14 . 1 - 140  at the side of the user rather than dangling or hanging the cable assembly  14 . 1 - 140  in front of the user near the first electronic device  14 . 1 - 100 . 
     Electrically coupling a power source (e.g., the second electronic device  14 . 1 - 130 ) to the receptacle connector  14 . 1 - 124  of the support  14 . 1 - 120 , as opposed to disposing the power source directly on the display portion  14 . 1 - 102  can also enable a reduction in the weight and/or a size of the display portion  14 . 1 - 102 . A reduction in the weight and/or size of the display portion  14 . 1 - 102  can render the first electronic device  14 . 1 - 100  more comfortable during use, more convenient to transport, and more convenient to store. 
     While the receptacle connectors  14 . 1 - 124  and the plug connectors  14 . 1 - 142  are shown in  FIGS.  14 . 1 - 1 A and  14 . 1 - 1 B  as having cubic shape or square cross-sections, the receptacle connectors  14 . 1 - 124  and the plug connectors  14 . 1 - 142  can define other shapes and cross-sections in some examples. For example, the cross-sections of the receptacle connectors  14 . 1 - 124  and the plug connectors  14 . 1 - 142  can be circular or rounded to enable rotation of the plug connectors  14 . 1 - 142  relative to the receptacle connectors  14 . 1 - 124 . In other words, the plug connectors  14 . 1 - 142  can be rotatably received within the receptacle connectors  14 . 1 - 124 . 
     In some examples, the second electronic device  14 . 1 - 130  can provide electrical power and/or electrical signals to the first electronic device  14 . 1 - 100  through the cable assembly  14 . 1 - 140 . For example, the second electronic device  14 . 1 - 130  can be an external electrical power source, such as, a lithium battery pack or other device capable of supplying electrical power to the first electronic device  14 . 1 - 100 . The cable assembly  14 . 1 - 140  can include a plug connector  14 . 1 - 144 , which can be coupled to a receptacle connector  14 . 1 - 132  of the second electronic device  14 . 1 - 130 . 
     The plug connector  14 . 1 - 144  can include a boot  14 . 1 - 146  and a plug  14 . 1 - 147  at least partially disposed within the boot  14 . 1 - 146 . The receptacle connector  14 . 1 - 132  of the second electronic device  14 . 1 - 130  can include a recess configured to receive portions of the plug connector  14 . 1 - 144 . The plug  14 . 1 - 147  can include one or more electrical contacts (not separately illustrated) that electrically couple the plug connector  14 . 1 - 144  to the receptacle connector  14 . 1 - 132  of the second electronic device  14 . 1 - 130 . In other words, as shown in  FIGS.  14 . 1 - 1 B , the second first electronic device  14 . 1 - 130  can be electrically coupled to the cable assembly  14 . 1 - 140  through the plug connector  14 . 1 - 144  and the receptacle connector  14 . 1 - 132 . The second electronic device  14 . 1 - 130  can be electrically coupled to the first electronic device  14 . 1 - 100  through the cable assembly  14 . 1 - 140  and the supports  14 . 1 - 120 . Examples of the receptacle connector  14 . 1 - 132  and the plug connector  14 . 1 - 144  will be discussed in greater detail below with reference to  FIGS.  14 . 1 - 26 A through  14 . 1 - 35 B . 
     As will be discussed in detail below, the various plug connectors and receptacle connectors included in an HMD (e.g., coupling the first electronic device  14 . 1 - 100  and the second electronic device  14 . 1 - 130 ) can be fastened to one another through various fastening mechanisms. In some examples, the fastening mechanisms can require a relatively small force to mate a plug connector and a receptacle connector and can require a relatively large force to un-mate the plug connector and the receptacle connector. In some examples, a tool actuation can be required to un-mate a plug connector and a receptacle connector once the plug connector and the receptacle connector are mated. In some examples, various seals can be provided on plug connectors and/or receptacle connectors to provide sealing between the plug connectors and receptacle connectors. The seals can provide forces between the plug connectors and receptacle connectors, such as ejection forces to aid in the removal of plug connectors from receptacle connectors. 
       FIGS.  14 . 1 - 2  through  14 . 1 - 15    illustrate various examples of receptacle connectors and plug connectors that can be used as the receptacle connectors  14 . 1 - 104  of the display portion  14 . 1 - 102  and the plug connectors  14 . 1 - 122  of the supports  14 . 1 - 120 . 
       FIGS.  14 . 1 - 2    shows a first electronic device  14 . 1 - 200  including a display portion  14 . 1 - 202 , supports  14 . 1 - 210 , and a cable assembly  14 . 1 - 220 . In some examples, the first electronic device  14 . 1 - 200  can be a head-mountable device (HMD). While the first electronic device  14 . 1 - 200  is illustrated as an HMD, the first electronic device  14 . 1 - 200  can be a tablet computing device, a smart phone, a smart watch, headphones, or any other electronic device. The display portion  14 . 1 - 202  can output visual content viewable by a user of the first electronic device  14 . 1 - 200 . The first electronic device  14 . 1 - 200 , the display portion  14 . 1 - 202 , the supports  14 . 1 - 210 , and the cable assembly  14 . 1 - 220  can be similar to or the same as the first electronic device  14 . 1 - 100 , the display portion  14 . 1 - 102 , the supports  14 . 1 - 120 , and the cable assembly  14 . 1 - 140 , respectively, discussed above with respect to  FIGS.  14 . 1 - 1 A and  14 . 1 - 1 B . 
     The supports  14 . 1 - 210  can retain the first electronic device  14 . 1 - 200  relative to a user&#39;s head. As illustrated in  FIGS.  14 . 1 - 2   , the first electronic device  14 . 1 - 200  can include two supports  14 . 1 - 210  coupled to the display portion  14 . 1 - 202  and configured to be positioned on either side of the user&#39;s head. The supports  14 . 1 - 210  can be connected to the display portion  14 . 1 - 202  at two or more locations. 
     The supports  14 . 1 - 210  can each include a housing or an enclosure  14 . 1 - 212  formed from a polymer, metal, ceramic, combination thereof, or the like. In some examples, the enclosure  14 . 1 - 212  can form a channel or cavity extending between a receptacle connector  14 . 1 - 222  of the support  14 . 1 - 210  and a plug connector  14 . 1 - 214  of the support  14 . 1 - 210 . The support  14 . 1 - 210  can be electrically coupled to the display portion  14 . 1 - 202  such that electrical signals and/or electrical power received at the receptacle connector  14 . 1 - 222  can be provided to the display portion  14 . 1 - 202  through the plug connector  14 . 1 - 214 . For example, one or more electronic components (e.g., printed circuit boards, processors, electrical wires, digital logic circuitry, digital processing circuitry, or the like) can be positioned within the cavity formed within the enclosure  14 . 1 - 212  and extend between the receptacle connector  14 . 1 - 222  and the plug connector  14 . 1 - 214  to form an electrical path between the receptacle connector  14 . 1 - 222  and the plug connector  14 . 1 - 214 . 
     In some examples, the support  14 . 1 - 210  can be coupled to the display portion  14 . 1 - 202 . For example, the support  14 . 1 - 210  can be welded, adhered, fastened, crimped, clipped, or otherwise retained within a receptacle connector  14 . 1 - 206  of the display portion  14 . 1 - 202 . In some examples, a proximal end of the support  14 . 1 - 210  can be electrically conductive or include the plug connector  14 . 1 - 214  that enables electrical power and/or electrical signals received by the receptacle connector  14 . 1 - 222  of the support  14 . 1 - 210  to be transferred to the display portion  14 . 1 - 202  through the plug connector  14 . 1 - 214  and the receptacle connector  14 . 1 - 206 . 
     As illustrated in  FIGS.  14 . 1 - 2   , each of the supports  14 . 1 - 210  can include a plug connector  14 . 1 - 214  disposed at a proximal end of the support  14 . 1 - 210 . The plug connector  14 . 1 - 214  can be coupled (e.g., electrically and structurally) to the display portion  14 . 1 - 202  by disposing the proximal end of the support  14 . 1 - 210  into a receptacle connector  14 . 1 - 206  coupled to a housing  14 . 1 - 204  of the display portion  14 . 1 - 202 . The receptacle connector  14 . 1 - 206  can be positioned on or within the housing  14 . 1 - 204  of the display portion  14 . 1 - 202 . In some examples, the receptacle connector  14 . 1 - 206  of the display portion  14 . 1 - 202  can be in electrical communication with the receptacle connector  14 . 1 - 222  of the support  14 . 1 - 210 . That is, the cable assembly  14 . 1 - 220  can operably couple to the receptacle connector  14 . 1 - 222  of the support  14 . 1 - 210  rather than directly coupling to the display portion  14 . 1 - 202  to electrical power and/or electrical signals can be transferred between the receptacle connector  14 . 1 - 222  of the support  14 . 1 - 210  and the receptacle connector  14 . 1 - 206  of the display portion  14 . 1 - 202 . 
     The plug connector  14 . 1 - 214  can include one or more electrical contacts  14 . 1 - 216 A,  14 . 1 - 216 B,  14 . 1 - 216 C which electrically couple to respective electrical contacts  14 . 1 - 208 A,  14 . 1 - 208 B, and  14 . 1 - 208 C disposed within the receptacle connector  14 . 1 - 206  while the plug connector  14 . 1 - 214  is disposed within the receptacle connector  14 . 1 - 206 . The one or more electrical contacts  14 . 1 - 216 A,  14 . 1 - 216 B,  14 . 1 - 216 C and the correlating one or more electrical contacts  14 . 1 - 208 A,  14 . 1 - 208 B,  14 . 1 - 208 C can provide electrical signals, electrical power, a grounding path, another electrical communication, or a combination thereof between the support  14 . 1 - 210  and the display portion  14 . 1 - 202 . 
     Electrically coupling a power source (e.g., the second electronic device  104 ) to the receptacle connector  14 . 1 - 222  within the support  14 . 1 - 210 , as opposed to disposing the power source directly on the display portion  14 . 1 - 202  can also enable a reduction in the weight and/or a size of the display portion  14 . 1 - 202 . A reduction in the weight and/or size of the display portion  14 . 1 - 202  can render the first electronic device  14 . 1 - 200  more comfortable during use, more convenient to transport, and more convenient to store. While the receptacle connector  14 . 1 - 206  and the plug connector  14 . 1 - 214  are shown in  FIGS.  14 . 1 - 2    as having a cubic shape or square cross-section, the receptacle connector  14 . 1 - 206  and the plug connector  14 . 1 - 214  can define other shapes and cross-sections in other examples. For example, the cross-section of the receptacle connector  14 . 1 - 206  and the plug connector  14 . 1 - 214  can be elongated and/or curved such that the support  14 . 1 - 210  can only be inserted into the receptacle connector  14 . 1 - 206  of the display portion  14 . 1 - 202  in a single orientation. 
       FIGS.  14 . 1 - 3 A and  14 . 1 - 3 B  show perspective views of a receptacle connector  14 . 1 - 300  and a plug connector  14 . 1 - 320 . The receptacle connector  14 . 1 - 300  can be used as the receptacle connector  14 . 1 - 104  or the receptacle connector  14 . 1 - 206  and the plug connector  14 . 1 - 320  can be used as the plug connector  14 . 1 - 122  or the plug connector  14 . 1 - 214  of the first electronic devices  14 . 1 - 100 / 14 . 1 - 200  of  FIGS.  14 . 1 - 1 A through  14 . 1 - 2   . The receptacle connector  14 . 1 - 300  can include a receptacle housing  14 . 1 - 304 , one or more electrical contacts (e.g., electrical contacts  14 . 1 - 308 A,  14 . 1 - 308 B, and  14 . 1 - 308 C), a cover member  14 . 1 - 302 , and a printed circuit board (PCB)  14 . 1 - 316 . 
     In some examples, the receptacle housing  14 . 1 - 304  can be manufactured from an electrically insulating material, such as, a polymer or ceramic and can be molded, machined, cast, stamped, or a combination thereof. The receptacle housing  14 . 1 - 304  can one include or more sidewalls defining a recess  14 . 1 - 310 . The recess  14 . 1 - 310  can be configured to accept and retain the plug connector  14 . 1 - 320 . For example, at least a portion of the plug connector  14 . 1 - 320  can be inserted into and received within the recess  14 . 1 - 310 . The recess  14 . 1 - 310  can define a cross-sectional shape having a longitudinal axis that curves or bends. For example, the recess  14 . 1 - 310  can have a cross-sectional shape that substantially conforms to a cross-sectional shape of a plug connector (e.g., the plug connector  14 . 1 - 320 ). In other words, the sidewalls of the receptacle housing  14 . 1 - 304  can define a convex surface and a concave surface that correlate in size, shape, and contour with convex and concave surfaces of the plug connector  14 . 1 - 320 . 
     The one or more electrical contacts (e.g., the electrical contacts  14 . 1 - 308 A,  14 . 1 - 308 B, and  14 . 1 - 308 C) can extend from a sidewall of the receptacle connector  14 . 1 - 300  into the recess  14 . 1 - 310 . The electrical contacts can provide electrical signals, electrical power, a grounding path, another electrical communication, or a combination thereof to the PCB  14 . 1 - 316 . In some examples, a first set of electrical contacts, including the electrical contacts  14 . 1 - 308 A,  14 . 1 - 308 B, and  14 . 1 - 308 C, can be disposed on a first side of the recess  14 . 1 - 310  and a second set of electrical contacts (not separately illustrated) can be disposed on a second, opposite, side of the recess  14 . 1 - 310 . As will be discussed in detail below, the electrical contacts can be attached to the receptacle housing  14 . 1 - 304  at angles normal to a surface of the plug connector  14 . 1 - 320 , which the electrical contacts are configured to contact. This provides improved contact between the electrical contacts of the receptacle connector  14 . 1 - 300  and electrical contacts of the plug connector  14 . 1 - 320 . 
     In  FIGS.  14 . 1 - 3 B , the plug connector  14 . 1 - 320  can define a top surface and a bottom surface. The top surface can be a convex external surface or otherwise form a radius of curvature. The bottom surface can be a concave external surface extending parallel or substantially parallel to the top surface. One or more electrical contacts (e.g., electrical contacts  14 . 1 - 324 A,  14 . 1 - 324 B, and  14 . 1 - 324 C) can be disposed on the top surface and/or the bottom surface of the plug connector  14 . 1 - 320 . In some examples, the electrical contacts can be provided on the bottom surface of the plug connector  14 . 1 - 320 , and the top surface of the plug connector  14 . 1 - 320  can be free from any electrical contacts; or the electrical contacts can be provided on the top surface of the plug connector  14 . 1 - 320 , and the bottom surface of the plug connector  14 . 1 - 320  can be free from any electrical contacts. 
     In some examples, the plug connector  14 . 1 - 320  can include one or more guide channels  14 . 1 - 328  and one or more retention channels  14 . 1 - 326 . The guide channels  14 . 1 - 328  and/or the retention channels  14 . 1 - 326  can be formed within the top surface, the bottom surface, or other surfaces (e.g., side surfaces) of the plug connector  14 . 1 - 320 . The guide channels  14 . 1 - 328  can receive guide rails (such as guide rails  14 . 1 - 313 , illustrated in  FIGS.  14 . 1 - 5 A ) of a receptacle connector of a display portion to limit or inhibit movement between a support including the plug connector  14 . 1 - 320  and the display portion. The guide channels  14 . 1 - 328  can also be beneficial to enable insertion of the plug connector  14 . 1 - 320  into a receptacle connector in a limited combination of configurations. For example, a guide rail will prevent insertion of the plug connector  14 . 1 - 320  within a receptacle connector unless the plug connector  14 . 1 - 320  has a correlating guide channel  14 . 1 - 328 . 
     The retention channels  14 . 1 - 326  can receive latches  14 . 1 - 312  of the receptacle connector  14 . 1 - 300  in order to retain the plug connector  14 . 1 - 320  within the recess  14 . 1 - 310  of the receptacle connector  14 . 1 - 300 . The retention channels  14 . 1 - 326  and the latches  14 . 1 - 312  can prevent undesired removal of the plug connector  14 . 1 - 320  from the recess  14 . 1 - 310 . In some examples, the retention channels  14 . 1 - 326  and the latches  14 . 1 - 312  can retain the plug connector  14 . 1 - 320  in the recess  14 . 1 - 310  of the receptacle connector  14 . 1 - 300  in a position in which electrical contacts of the plug connector  14 . 1 - 320  are aligned with electrical contacts of the receptacle connector  14 . 1 - 300 . 
     As will be discussed in detail below, the receptacle connector  14 . 1 - 314  can include a push block  14 . 1 - 314  operably coupled to the latches  14 . 1 - 312 . The push block  14 . 1 - 314  can be actuated by a tool and actuating the push block  14 . 1 - 314  can rotate the latches  14 . 1 - 312  out of the recess  14 . 1 - 310  such that the plug connector  14 . 1 - 320  can be removed from the recess  14 . 1 - 310 . The latches  14 . 1 - 312  can be configured such that a large force is required to pull the plug connector  14 . 1 - 320  from the recess  14 . 1 - 310 , but a small force is required to pull the plug connector  14 . 1 - 320  from the recess  14 . 1 - 310  when the push block  14 . 1 - 314  is actuated and the latches  14 . 1 - 312  release the plug connector  14 . 1 - 320 . This allows for the plug connector  14 . 1 - 320  to be removed when needed but prevents the plug connector  14 . 1 - 320  from being accidentally or unintentionally removed from the receptacle connector  14 . 1 - 300 . 
     The guide channels  14 . 1 - 328  and the retention channels  14 . 1 - 326  are illustrated with partial cylindrical shapes. However, any suitable shapes can be used for the guide channels  14 . 1 - 328  and the retention channels  14 . 1 - 326 , and the guide channels  14 . 1 - 328  and the retention channels  14 . 1 - 326  can have square, rectangular, semi-circular, triangular, U-shaped, V-shaped, other geometric-shaped, combinations thereof, or the like cross-sectional shapes. In some examples, the cross-sectional shape of the guide channels  14 . 1 - 328  and the retention channels  14 . 1 - 326  can vary along a length of the channels. For example, the guide channels  14 . 1 - 328  and the retention channels  14 . 1 - 326  can form rectangular cross-sectional shapes that taper to square cross-section shapes along the length of the respective channels. 
     The receptacle connector  14 . 1 - 300  can include a face seal  14 . 1 - 306  adjacent the recess  14 . 1 - 310 . The face seal  14 . 1 - 306  can encircle at least a portion of the recess  14 . 1 - 310 , such as a distal portion of the recess  14 . 1 - 310 . The face seal  14 . 1 - 306  can act as a seal which prevents or limits ingress of contaminants (e.g., liquid, dust, lint, debris, or the like) into the receptacle connector  14 . 1 - 300  through the recess  14 . 1 - 310 , such as when the plug connector  14 . 1 - 320  is inserted into the recess  14 . 1 - 310  of the receptacle connector  14 . 1 - 300 . In some examples, the face seal  14 . 1 - 306  can be configured to provide an ejection force to the plug connector  14 . 1 - 320 . This can aid a user in removing the plug connector  14 . 1 - 320  from the receptacle connector  14 . 1 - 300 , such as when the push block  14 . 1 - 314  is actuated. 
     The cover member  14 . 1 - 302  can include an upper portion and a lower portion. The upper portion and the lower portion can be machined, molded, stamped, extruded, or otherwise manufactured from one or more materials, such as a metal, a ceramic, or a polymer. In some examples, the lower portion can at least partially form the holes, which can be used to mount the receptacle connector to a housing or enclosure of a display portion. Each of the upper portion and the lower portion can provide a support structure for the receptacle connector  14 . 1 - 300 . The upper portion and the lower portion can shield the receptacle connector  14 . 1 - 300  from propagating errant electromagnetic waves out of the receptacle connector  14 . 1 - 300 . 
     The PCB  14 . 1 - 316  can be electrically coupled to the electrical contacts of the receptacle connector  14 . 1 - 300  (e.g., the electrical contacts  14 . 1 - 308 A,  14 . 1 - 308 B, and  14 . 1 - 308 C). For example, the PCB  14 . 1 - 316  can include one or more electrical traces which carry electrical signals and/or electrical power from the electrical contacts to an electronic component (e.g., a processor, electrical wires, digital logic circuitry, digital processing circuitry, or the like) disposed on the PCB  14 . 1 - 316 . The PCB  14 . 1 - 316  and/or the one or more electronic components electrically coupled to the PCB  14 . 1 - 316  can be electrically coupled to one or more electrical wires (not shown) to form at least a portion of an electrical path for electrical signals, electrical power, an electrical ground, another electrical communication, or a combination thereof to transfer between the receptacle connector  14 . 1 - 300  and a display or other electronic components within the display portion (e.g., the display portion  14 . 1 - 102 / 14 . 1 - 202 ). Similarly, the PCB  14 . 1 - 316  can form at least a portion of an electrical path for electrical signals, electrical power, an electrical ground, another electrical communication, or a combination thereof to transfer between the receptacle connector  14 . 1 - 300  and the plug connector  14 . 1 - 320 . 
       FIGS.  14 . 1 - 4    shows an exploded view of the receptacle connector  14 . 1 - 300  including the receptacle housing  14 . 1 - 304 . The receptacle housing  14 . 1 - 304  can form slits or slots within the sidewalls of the receptacle housing  14 . 1 - 304 , which enable a first set of electrical contacts (e.g., the electrical contacts  14 . 1 - 308 A,  14 . 1 - 308 B, and  14 . 1 - 308 C) to extend through the sidewalls and into the recess  14 . 1 - 310 . In some examples, the receptacle housing  14 . 1 - 304  can further include slits or slots within the sidewalls, which enable a second set of electrical contacts (e.g., electrical contacts  14 . 1 - 309 A,  14 . 1 - 309 B, and  14 . 1 - 309 C) to extend through the sidewalls of the receptacle housing  14 . 1 - 304  and into the recess  14 . 1 - 310 . The second set of electrical contacts can extend into the recess  14 . 1 - 310  through a different sidewall of the receptacle housing  14 . 1 - 304  from the first set of electrical contacts. In some examples, the plug connector  14 . 1 - 320  can be devoid of electrical contacts that make contact with both the first set of electrical contacts and the second set of electrical contacts. In other words, one example of a support can include electrical contacts that only utilize one of the sets of electrical contacts shown in  FIGS.  14 . 1 - 4   . Another example of a support can include electrical contacts that utilize both sets of electrical contacts shown in  FIGS.  14 . 1 - 4   . Thus, a single configuration of the receptacle connector  14 . 1 - 300  having two sets of electrical contacts can be operable with multiple configurations of supports (e.g., supports having varied electrical contact configurations). 
     As shown in  FIGS.  14 . 1 - 4   , in some examples, each of the electrical contacts of the first set of electrical contacts can be provided with a corresponding electrical contact of the second set of electrical contacts. Each set of corresponding electrical contacts can be disposed in and coupled to the receptacle housing  14 . 1 - 304  at an oblique angle to a longitudinal axis of the recess  14 . 1 - 310  (e.g., a lateral axis of the receptacle housing  14 . 1 - 304 ). Each set of electrical contacts can be coupled to the receptacle housing  14 . 1 - 304  at an angle normal to a surface of the plug connector  14 . 1 - 310  that the electrical contacts are configured to contact. This improves the electrical contact between the electrical contacts of the receptacle connector  14 . 1 - 300  and the plug connector  14 . 1 - 320 . Each set of electrical contacts can also be disposed in and coupled to the receptacle housing  14 . 1 - 304  at an angle oblique to other sets of the electrical contacts, such as adjacent sets of electrical contacts. 
     The receptacle connector  14 . 1 - 300  can include a fastening mechanism for retaining the plug connector  14 . 1 - 320  in the recess  14 . 1 - 310 . In the example illustrated in  FIGS.  14 . 1 - 4   , the fastening mechanism includes a push block  14 . 1 - 314 , a spring  14 . 1 - 318 , a guide rail  14 . 1 - 319 , and latches  14 . 1 - 312 . The latches  14 . 1 - 312  extend through openings in the receptacle housing  14 . 1 - 304  into the recess  14 . 1 - 310 . The latches  14 . 1 - 312  are pivotally coupled to the receptacle housing  14 . 1 - 304 . The latches  14 . 1 - 312  can be in an engaged configuration when the latches  14 . 1 - 312  are pivoted into the recess  14 . 1 - 310 , and in an un-engaged configuration when the latches are pivoted out of the recess  14 . 1 - 310 . 
     The push block  14 . 1 - 314  is coupled to the latches  14 . 1 - 312 , the spring  14 . 1 - 319 , and the guide rail  14 . 1 - 319 . The push block  14 . 1 - 314  is configured to slide along the guide rail  14 . 1 - 319  and rotate the latches  14 . 1 - 312  between the engaged configuration and the un-engaged configuration. The spring  14 . 1 - 318  is configured to retain the push block  14 . 1 - 314  in an initial position in which the latches  14 . 1 - 312  are in the engaged configuration. A user can use a tool or the like to apply force to the push block  14 . 1 - 314  and push the push block  14 . 1 - 314  along the guide rail  14 . 1 - 319 . This rotates the latches  14 . 1 - 312  to the un-engaged configuration. Once the latches are in the un-engaged configuration, a plug connector  14 . 1 - 320  can be released from the recess  14 . 1 - 310  and removed from the receptacle connector  14 . 1 - 300 . The spring  14 . 1 - 318  resists force applied to the push block  14 . 1 - 314  and movement of the push block  14 . 1 - 314  and returns the push block  14 . 1 - 314  to the initial position with the latches  14 . 1 - 312  in the engaged configuration once the force is no longer applied to the push block  14 . 1 - 314 . As such, the latches  14 . 1 - 312 , the push block  14 . 1 - 314 , the spring  14 . 1 - 318 , and the guide rail  14 . 1 - 319  are used to retain the plug connector  14 . 1 - 320  in the recess  14 . 1 - 310  of the receptacle connector  14 . 1 - 300  and release the plug connector  14 . 1 - 320  from the receptacle connector  14 . 1 - 300 . 
     The face seal  14 . 1 - 306  can be coupled to a front surface of the receptacle housing  14 . 1 - 304  and can encircle the recess  14 . 1 - 310 . The face seal  14 . 1 - 306  can be molded, adhered, welded, fastened, or otherwise secured to a surface of the receptacle housing  14 . 1 - 304 . In some examples, the face seal  14 . 1 - 306  can include an elastic material that can be stretched around a lip of the receptacle housing  14 . 1 - 304  and can be secured to the receptacle housing  14 . 1 - 304  by interference with the receptacle housing  14 . 1 - 304 . Bumpers  14 . 1 - 307  can be coupled to the receptacle housing  14 . 1 - 304  at a back surface of the recess  14 . 1 - 310 . The bumpers  14 . 1 - 307  can be molded, adhered, welded, fastened, or otherwise secured to a surface of the receptacle housing  14 . 1 - 304 . The bumpers  14 . 1 - 307  and the face seal  14 . 1 - 306  can provide an ejection force to a plug connector  14 . 1 - 320  inserted into the recess  14 . 1 - 310  of the receptacle connector  14 . 1 - 300 . For example, the bumpers  14 . 1 - 307  and the face seal  14 . 1 - 306  can provide resistance to the plug connector  14 . 1 - 320  as the plug connector  14 . 1 - 320  is inserted into the recess  14 . 1 - 310  and can help to eject the plug connector  14 . 1 - 320  from the recess  14 . 1 - 310  when the latches  14 . 1 - 312  are rotated to the un-engaged configuration. This can allow the plug connector  14 . 1 - 320  to be ejected from the recess by actuating the push block  14 . 1 - 314  with a tool, and without requiring a user to otherwise pull on the plug connector  14 . 1 - 320  or the like. The bumpers  14 . 1 - 307  and the face seal  14 . 1 - 306  can also retain the electrical contacts of the plug connector  14 . 1 - 320  in a desired position relative to the electrical contacts of the receptacle connector  14 . 1 - 300 . 
     The cover member  14 . 1 - 302  can include an upper portion  14 . 1 - 302 A, a lower portion  14 . 1 - 302 B, front portions  14 . 1 - 302 C, and a rear portion  14 . 1 - 302 D. Each of the portions of the cover member  14 . 1 - 302  can be coupled to the receptacle housing  14 . 1 - 304 . For example, the portions of the cover member  14 . 1 - 302  can adhered (such as by a polymer-based adhesive tape), fastened, welded, or otherwise affixed to the receptacle housing  14 . 1 - 304 . The cover member  14 . 1 - 302  can provide shielding and a support structure for the receptacle housing  14 . 1 - 304 . 
       FIGS.  14 . 1 - 5 A and  14 . 1 - 5 B  illustrate front views of the receptacle connector  14 . 1 - 300 . As illustrated in  FIGS.  14 . 1 - 5 A , the face seal  14 . 1 - 306  can encircle the recess  14 . 1 - 310  of the receptacle housing  14 . 1 - 304 . The bumpers  14 . 1 - 307  can be disposed at a rear surface of the receptacle housing  14 . 1 - 304  adjacent the recess  14 . 1 - 310 . When the plug connector  14 . 1 - 320  is inserted into the recess  14 . 1 - 310 , the bumpers  14 . 1 - 307  can be disposed between the plug connector  14 . 1 - 320  and a rear inner surface of the receptacle housing  14 . 1 - 304 . The bumpers  14 . 1 - 307  and the face seal  14 . 1 - 306  can each provide an ejection force to the plug connector  14 . 1 - 320 , as discussed previously. 
     As illustrated in  FIGS.  14 . 1 - 5 A , the receptacle housing  14 . 1 - 304  can include one or more guide rails  14 . 1 - 313 . Each guide rail  14 . 1 - 313  can be disposed on a respective sidewall of the receptacle housing  14 . 1 - 304  and can extend into the recess  14 . 1 - 310 . The guide rails  14 . 1 - 313  can be molded, co-molded, or otherwise formed with the receptacle housing  14 . 1 - 304  as a singular and unitary structure. The guide rails  14 . 1 - 313  can be received within guide channels  14 . 1 - 328  of the plug connector  14 . 1 - 320  to provide a rigid connection between a support including the plug connector  14 . 1 - 320  and a display portion including the receptacle connector  14 . 1 - 300 . In other words, the guide rails  14 . 1 - 313  and the guide channels  14 . 1 - 328  can limit motion or free play between the support and the display portion. Additionally, or alternatively, the guide rails  14 . 1 - 313  can limit or prevent the plug connector  14 . 1 - 320  from being inserted into the receptacle connector  14 . 1 - 300  in an undesirable orientation and/or configuration. 
       FIGS.  14 . 1 - 5 B  illustrates orientations of the electrical contacts (e.g., the electrical contacts  14 . 1 - 308 A,  14 . 1 - 308 B,  14 . 1 - 308 C,  14 . 1 - 309 A,  14 . 1 - 309 B, and  14 . 1 - 309 C) of the receptacle connector  14 . 1 - 300  relative to the receptacle housing  14 . 1 - 304 . As illustrated in  FIGS.  14 . 1 - 5 B , each of the electrical contacts can be disposed at an angle oblique to a lateral axis and a vertical axis of the receptacle housing  14 . 1 - 304 . The electrical contacts can be disposed such that surfaces of the electrical contacts configured to contact a plug connector  14 . 1 - 320  are normal to surfaces of the plug connector  14 . 1 - 320  those electrical contacts are configured to contact. This provides for improved contact between the electrical contacts of the receptacle connector  14 . 1 - 300  and electrical contacts of the plug connector  14 . 1 - 320 , as surfaces of the electrical contacts of the receptacle connector  14 . 1 - 300  rather than just edges or corners are configured to contact the plug connector  14 . 1 - 320 . Each of the electrical contacts of the receptacle connector  14 . 1 - 300  can also be disposed at an angle oblique to adjacent electrical contacts. 
       FIGS.  14 . 1 - 6 A and  14 . 1 - 6 B  illustrate side sectional views of receptacle connectors. In  FIGS.  14 . 1 - 6 A , a receptacle connector  14 . 1 - 400 A includes an electrical contact  14 . 1 - 402 A vertically stitched to a receptacle housing  14 . 1 - 404 A. The electrical contact  14 . 1 - 402 A can have a longitudinal axis extending in a horizontal direction, and the electrical contact  14 . 1 - 402 A can be stitched to the receptacle housing  14 . 1 - 404 A in a vertical direction. The electrical contact  14 . 1 - 402 A can be stitched to the receptacle housing  14 . 1 - 404 A in a direction perpendicular to an insertion direction of a plug connector. The electrical contact  14 . 1 - 402 A is configured to contact the plug connector when the plug connector is inserted into a recess  14 . 1 - 410  of the receptacle housing  14 . 1 - 404 A. 
     A cover member  14 . 1 - 406  can be provided around the receptacle housing  14 . 1 - 404 A to provide support for and shielding of the receptacle housing  14 . 1 - 404 A. A face seal  14 . 1 - 408  can be provided along an inner surface of the receptacle housing  14 . 1 - 404 A adjacent the recess  14 . 1 - 410  into which a plug connector is to be inserted. A bumper  14 . 1 - 412  can be provided along an inner surface of the receptacle housing  14 . 1 - 404 A adjacent the recess  14 . 1 - 410 , at a back surface of the recess  14 . 1 - 410  relative to an opening of the recess  14 . 1 - 410  through which the plug connector is to be inserted. The bumper  14 . 1 - 412  provides a stop when the plug connector is inserted into the recess  14 . 1 - 410 , and the bumper  14 . 1 - 412  and the face seal  14 . 1 - 408  can provide ejection forces to the plug connector when the plug connector is un-latched from the receptacle connector  14 . 1 - 400 A. The bumper  14 . 1 - 412  and the face seal  14 . 1 - 408  are configured to be disposed between the plug connector and the receptacle housing  14 . 1 - 404 A when the plug connector is inserted into the recess  14 . 1 - 410 . 
     In  FIGS.  14 . 1 - 6 B , a receptacle connector  14 . 1 - 400 B includes an electrical contact  14 . 1 - 402 B horizontally stitched to a receptacle housing  14 . 1 - 404 B. The electrical contact  14 . 1 - 402 B can have a longitudinal axis extending in a horizontal direction, and the electrical contact  14 . 1 - 402 B can be stitched to the receptacle housing  14 . 1 - 404 B in the horizontal direction. The electrical contact  14 . 1 - 402 B can be stitched to the receptacle housing  14 . 1 - 404 B in a direction parallel to an insertion direction of a plug connector. The electrical contact  14 . 1 - 402 B is configured to contact the plug connector when the plug connector is inserted into a recess  14 . 1 - 410  of the receptacle housing  14 . 1 - 404 B. 
     A cover member  14 . 1 - 406  can be provided around the receptacle housing  14 . 1 - 404 B to provide support for and shielding of the receptacle housing  14 . 1 - 404 B. A face seal  14 . 1 - 408  can be provided along an inner surface of the receptacle housing  14 . 1 - 404 B adjacent the recess  14 . 1 - 410  into which a plug connector is to be inserted. A bumper  14 . 1 - 412  can be provided along an inner surface of the receptacle housing  14 . 1 - 404 B adjacent the recess  14 . 1 - 410 , at a back surface of the recess  14 . 1 - 410  relative to an opening of the recess  14 . 1 - 410  through which the plug connector is to be inserted. The bumper  14 . 1 - 412  provides a stop when the plug connector is inserted into the recess  14 . 1 - 410 , and the bumper  14 . 1 - 412  and the face seal  14 . 1 - 408  can provide ejection forces to the plug connector when the plug connector is un-latched from the receptacle connector  14 . 1 - 400 B. The bumper  14 . 1 - 412  and the face seal  14 . 1 - 408  are configured to be disposed between the plug connector and the receptacle housing  14 . 1 - 404 B when the plug connector is inserted into the recess  14 . 1 - 410 . 
       FIGS.  14 . 1 - 7 A and  14 . 1 - 7 B  illustrate perspective views of a receptacle connector  14 . 1 - 500  and a plug connector  14 . 1 - 520 , respectively. The receptacle connector  14 . 1 - 500  includes a receptacle housing  14 . 1 - 504  defining a recess  14 . 1 - 505  and a cover member  14 . 1 - 502  coupled to the receptacle housing  14 . 1 - 504 . The recess  14 . 1 - 505  can be configured to receive the plug connector  14 . 1 - 520 . The cover member  14 . 1 - 502  can be configured to provide support and shielding for the receptacle housing  14 . 1 - 504 . 
     A face seal  14 . 1 - 510  can be coupled to the receptacle housing  14 . 1 - 504 , such as adjacent to the recess  14 . 1 - 505 . The face seal  14 . 1 - 510  can surround or encircle the recess  14 . 1 - 505  in the receptacle housing  14 . 1 - 504 . The face seal  14 . 1 - 510  can provide sealing between the receptacle housing  14 . 1 - 504  and the plug connector  14 . 1 - 520  when the plug connector is received in the recess  14 . 1 - 505 . A bumper  14 . 1 - 512  can be coupled to the receptacle housing  14 . 1 - 504 . The bumper  14 . 1 - 512  can be coupled to a surface of the receptacle housing  14 . 1 - 504  at a back of the recess  14 . 1 - 505 . The bumper  14 . 1 - 512  can be configured to be disposed between the receptacle housing  14 . 1 - 504  and the plug connector  14 . 1 - 520  when the plug connector  14 . 1 - 520  is inserted into the recess  14 . 1 - 505 . The bumper  14 . 1 - 512  and/or the face seal  14 . 1 - 510  can be configured to provide an ejection force to the plug connector  14 . 1 - 520 . For example, the bumper  14 . 1 - 512  and/or the face seal  14 . 1 - 510  can resist movement of the plug connector  14 . 1 - 520  as the plug connector  14 . 1 - 520  is inserted into the recess  14 . 1 - 505 . After the plug connector  14 . 1 - 520  is latched in the recess  14 . 1 - 505 , the bumper  14 . 1 - 512  and/or the face seal  14 . 1 - 510  can provide the ejection force to the plug connector  14 . 1 - 520  such that the plug connector  14 . 1 - 520  is ejected from the recess  14 . 1 - 505  when the plug connector is un-latched from the receptacle housing  14 . 1 - 504 . 
     The receptacle housing  14 . 1 - 504  can further include a latch  14 . 1 - 506  rotatably or pivotally coupled to the receptacle housing  14 . 1 - 504 . The latch  14 . 1 - 506  can correspond to a retention channel  14 . 1 - 526  formed in a body portion  14 . 1 - 522  of the plug connector  14 . 1 - 520 . The latch  14 . 1 - 506  can be pivotable between an engaged configuration and an un-engaged configuration. When the latch  14 . 1 - 506  is in the engaged configuration, the latch  14 . 1 - 506  extends into the recess  14 . 1 - 505  of the receptacle housing  14 . 1 - 504 , as illustrated in  FIG.  14 . 1 - 7 A . Although the latch  14 . 1 - 506  is illustrated as extending into a side portion of the recess  14 . 1 - 505 , the latch  14 . 1 - 506  can be provided in a top portion, a bottom portion, or the like of the recess  14 . 1 - 505 . When the latch  14 . 1 - 506  is in the un-engaged configuration, the latch  14 . 1 - 506  can be disposed outside of the recess  14 . 1 - 505 , or less of the latch  14 . 1 - 506  can be disposed in the recess  14 . 1 - 505 . The latch  14 . 1 - 506  can be biased in the engaged configuration by a spring or the like, and can be un-latched by a user, such as with a tool actuation. 
     The retention channel  14 . 1 - 526  can be disposed in a sidewall of the plug connector  14 . 1 - 520 . The retention channel  14 . 1 - 526  can be etched, drilled, molded, co-molded, or otherwise formed with the plug connector  14 . 1 - 520  as a singular and unitary structure. As illustrated in  FIGS.  14 . 1 - 7 B , the retention channel  14 . 1 - 526  can be disposed in the body portion  14 . 1 - 522  at an angle perpendicular to an insertion direction  14 . 1 - 528  in which the plug connector  14 . 1 - 520  is to be inserted into the recess  14 . 1 - 505  of the receptacle connector  14 . 1 - 500 , which can aid in the retention of the plug connector  14 . 1 - 520  by the latch  14 . 1 - 506 . The latch  14 . 1 - 506  can be received within the retention channel  14 . 1 - 526  of the plug connector  14 . 1 - 520  to retain the plug connector  14 . 1 - 520  within the recess  14 . 1 - 505  of the receptacle connector  14 . 1 - 500 , when the latch  14 . 1 - 506  is in the engaged configuration. The latch  14 . 1 - 506  can then be transitioned into the un-engaged configuration to release the plug connector  14 . 1 - 520  from the recess  14 . 1 - 505  of the receptacle connector  14 . 1 - 500 . 
     The receptacle housing  14 . 1 - 504  can further include a guide rail  14 . 1 - 508 , which can correspond to a guide channel  14 . 1 - 524  formed in a body portion  14 . 1 - 522  of the plug connector  14 . 1 - 520 . The guide rail  14 . 1 - 508  can be disposed on a sidewall of the receptacle housing  14 . 1 - 504  and can extend into the recess  14 . 1 - 505 . The guide rail  14 . 1 - 508  can be molded, co-molded, or otherwise formed with the receptacle housing  14 . 1 - 504  as a singular and unitary structure. The guide channel  14 . 1 - 524  can be disposed in a sidewall of the plug connector  14 . 1 - 520 . The guide channel  14 . 1 - 524  can be etched, drilled, molded, co-molded, or otherwise formed with the plug connector  14 . 1 - 520  as a singular and unitary structure. As illustrated in  FIGS.  14 . 1 - 7 B , the guide channel  14 . 1 - 524  can be disposed in the body portion  14 . 1 - 522  at an angle parallel to an insertion direction  14 . 1 - 528  in which the plug connector  14 . 1 - 520  is to be inserted into the recess  14 . 1 - 505  of the receptacle connector  14 . 1 - 500 . The guide rail  14 . 1 - 508  can be received within the guide channel  14 . 1 - 524  of the plug connector  14 . 1 - 520  to provide a rigid connection between a support including the plug connector  14 . 1 - 520  and a display portion including the receptacle connector  14 . 1 - 500 . In other words, the guide rail  14 . 1 - 508  and the guide channel  14 . 1 - 524  can limit motion or free play between the support and the display portion. Additionally, or alternatively, the guide rail  14 . 1 - 508  can limit or prevent the plug connector  14 . 1 - 520  from being inserted into the receptacle connector  14 . 1 - 500  in an undesirable orientation and/or configuration. 
     As illustrated in  FIGS.  14 . 1 - 7 A , a fastener  14 . 1 - 514  can extend through holes formed in the receptacle housing  14 . 1 - 504  and the cover member  14 . 1 - 502 . The fastener  14 . 1 - 514  can be used to attach the receptacle connector to an enclosure or housing of the display portion. 
       FIGS.  14 . 1 - 8 A  illustrates a sectional view of a support  14 . 1 - 620  including a plug connector  14 . 1 - 624  retained in a receptacle connector  14 . 1 - 600 .  FIGS.  14 . 1 - 8 B  illustrates a detailed view of a region  14 . 1 - 610  of  FIGS.  14 . 1 - 8 A . The receptacle connector  14 . 1 - 600  can include a receptacle housing  14 . 1 - 602 , latches  14 . 1 - 604  extending through the receptacle housing  14 . 1 - 602  and configured to retain the plug connector  14 . 1 - 622 , a face seal  14 . 1 - 606  coupled to the receptacle housing  14 . 1 - 602 , and bumper seals  14 . 1 - 608  coupled to the receptacle housing  14 . 1 - 602 . The support  14 . 1 - 620  can include the plug connector  14 . 1 - 624 , an enclosure  14 . 1 - 622 , and retention channels  14 . 1 - 626  formed in the plug connector  14 . 1 - 624 . 
     The latches  14 . 1 - 604  can be configured to retain the plug connector  14 . 1 - 624  in a recess of the receptacle connector  14 . 1 - 600 . As illustrated in  FIGS.  14 . 1 - 8 B , the latches  14 . 1 - 604  can extend through a cover member  14 . 1 - 612  or the like, which can be coupled to the receptacle housing  14 . 1 - 602 . The latches  14 . 1 - 604  can have trapezoidal shapes, and the retention channels  14 . 1 - 626  in the plug connector  14 . 1 - 624  can have corresponding trapezoidal shapes. In some examples, the latches  14 . 1 - 604  and the retention channels can have rounded shapes, U-shapes, V-shapes, triangular shapes, or any other suitable shapes. The latches  14 . 1 - 604  can have widths less than widths of the retention channels  14 . 1 - 626 , which can provide for deeper engagement of the latches  14 . 1 - 604  in the retention channels  14 . 1 - 626 . In some examples, the latches  14 . 1 - 604  can have widths equal to or greater than widths of the retention channels  14 . 1 - 626 , such that only portions of the latches  14 . 1 - 604  extend into the retention channels  14 . 1 - 626 . This can result in shallower engagement between the latches  14 . 1 - 604  and the retention channels  14 . 1 - 626  but can also provide a tighter connection between the latches  14 . 1 - 604  and the retention channels  14 . 1 - 626  and can reduce wiggle or other movement between the plug connector  14 . 1 - 624  and the receptacle connector  14 . 1 - 600 . 
     The face seal  14 . 1 - 606  can be coupled to the receptacle housing  14 . 1 - 602  adjacent the recess configured to accept the plug connector  14 . 1 - 624 . The face seal  14 . 1 - 606  can provide sealing between the plug connector  14 . 1 - 624  and the receptacle housing  14 . 1 - 602 . The bumpers  14 . 1 - 608  can be provided on a surface of the receptacle housing  14 . 1 - 602  at a back of the recess, such that the bumpers  14 . 1 - 608  are between the plug connector  14 . 1 - 624  and the receptacle housing  14 . 1 - 602  when the plug connector  14 . 1 - 624  is disposed in the recess. The face seal  14 . 1 - 606  and the bumpers  14 . 1 - 608  can each be configured to provide an ejection force to the plug connector  14 . 1 - 624 . For example, when the latches  14 . 1 - 604  are in an un-engaged configuration, the face seal  14 . 1 - 606  and/or the bumpers  14 . 1 - 608  can eject the plug connector  14 . 1 - 624  from the recess in the receptacle housing  14 . 1 - 602 , or aid in the ejection of the plug connector  14 . 1 - 624  from the recess in the receptacle housing  14 . 1 - 602 . 
       FIGS.  14 . 1 - 9 A through  14 . 1 - 9 F  illustrate various configurations of latches and retention channels that can be included in receptacle connectors and plug connectors.  FIGS.  14 . 1 - 9 A,  14 . 1 - 9 C, and  14 . 1 - 9 E  illustrate sectional views, and  FIGS.  14 . 1 - 9 B,  14 . 1 - 9 D, and  14 . 1 - 9 F  illustrate detail views of regions  14 . 1 - 710 A,  14 . 1 - 710 B, and  14 . 1 - 710 C of  FIGS.  14 . 1 - 9 A,  14 . 1 - 9 C, and  14 . 1 - 9 E , respectively. 
     In  FIGS.  14 . 1 - 9 A and  14 . 1 - 9 B , a latch  14 . 1 - 706 A of a receptacle connector  14 . 1 - 700 A extends through a receptacle housing  14 . 1 - 702  and a cover member  14 . 1 - 704  adjacent a face seal  14 . 1 - 702 . The latch  14 . 1 - 706 A extends into a retention channel  14 . 1 - 722 A formed in a plug connector  14 . 1 - 720 . The retention channel  14 . 1 - 722 A and the latch  14 . 1 - 706 A can have corresponding trapezoidal cross-sectional shapes; however, any suitable corresponding shapes can be used. The latch  14 . 1 - 706 A extends into the retention channel  14 . 1 - 722 A a distance  14 . 1 -L 1 , which can be in a range from about 0 mm to about 10 mm, from about 1 mm to about 2 mm, from about 2 mm to about 5 mm, from about 0 mm to about 2 mm, or the like. In some examples, side surfaces of the latch  14 . 1 - 706 A and the retention channel  14 . 1 - 722 A can taper at an angle in a range from about 0° to about 10°. In the example of  FIGS.  14 . 1 - 9 A and  14 . 1 - 9 B , tolerances between the latch  14 . 1 - 706 A and the retention channel  14 . 1 - 722 A can be relatively large, which results in the latch  14 . 1 - 706 A extending further into the retention channel  14 . 1 - 722 A. For example, a ratio of a width of the retention channel  14 . 1 - 722 A to a width of the latch  14 . 1 - 706 A can be in a range from about 0.8 to about 1.2, from about 1.0 to about 1.2, from about 0.5 to about 2.0, from about 1.0 to about 1.5, or the like. 
     In  FIGS.  14 . 1 - 9 C and  14 . 1 - 9 D , a latch  14 . 1 - 706 B of a receptacle connector  14 . 1 - 700 B extends through a receptacle housing  14 . 1 - 702  and a cover member  14 . 1 - 704  adjacent a face seal  14 . 1 - 702 . The latch  14 . 1 - 706 B extends into a retention channel  14 . 1 - 722 B formed in a plug connector  14 . 1 - 720 . The retention channel  14 . 1 - 722 B and the latch  14 . 1 - 706 B can have corresponding trapezoidal cross-sectional shapes; however, any suitable corresponding shapes can be used. The latch  14 . 1 - 706 B extends into the retention channel  14 . 1 - 722 B a distance  14 . 1 -L 2 , which can be in a range from about 0 mm to about 10 mm, from about 1 mm to about 2 mm, from about 2 mm to about 5 mm, from about 0 mm to about 2 mm, or the like. In some examples, side surfaces of the latch  14 . 1 - 706 B and the retention channel  14 . 1 - 722 B can taper at an angle in a range from about 0° to about 10°. In the example of  FIGS.  14 . 1 - 9 C and  14 . 1 - 9 D , tolerances between the latch  14 . 1 - 706 B and the retention channel  14 . 1 - 722 B can be intermediate, which results in the latch  14 . 1 - 706 B extending less far into the retention channel  14 . 1 - 722 B than in the example of  FIGS.  14 . 1 - 9 A and  14 . 1 - 9 B . For example, a ratio of a width of the retention channel  14 . 1 - 722 B to a width of the latch  14 . 1 - 706 B can be in a range from about 0.8 to about 1.2, from about 1.0 to about 1.2, from about 0.5 to about 2.0, from about 1.0 to about 1.5, or the like. 
     In  FIGS.  14 . 1 - 9 E and  14 . 1 - 9 F , a latch  14 . 1 - 706 C of a receptacle connector  14 . 1 - 700 C extends through a receptacle housing  14 . 1 - 702  and a cover member  14 . 1 - 704  adjacent a face seal  14 . 1 - 702 . The latch  14 . 1 - 706 C extends into a retention channel  14 . 1 - 722 C formed in a plug connector  14 . 1 - 720 . The retention channel  14 . 1 - 722 C and the latch  14 . 1 - 706 C can have corresponding trapezoidal cross-sectional shapes; however, any suitable corresponding shapes can be used. The latch  14 . 1 - 706 C extends into the retention channel  14 . 1 - 722 C a distance  14 . 1 -L 3 , which can be in a range from about 0 mm to about 10 mm, from about 1 mm to about 2 mm, from about 2 mm to about 5 mm, from about 0 mm to about 2 mm, or the like. In some examples, side surfaces of the latch  14 . 1 - 706 C and the retention channel  14 . 1 - 722 C can taper at an angle in a range from about 0° to about 10°. In the example of  FIGS.  14 . 1 - 9 E and  14 . 1 - 9 F , tolerances between the latch  14 . 1 - 706 C and the retention channel  14 . 1 - 722 C can be tight, which results in the latch  14 . 1 - 706 C extending less far into the retention channel  14 . 1 - 722 C than in the examples of  FIGS.  14 . 1 - 9 A through  14 . 1 - 9 D . For example, a ratio of a width of the retention channel  14 . 1 - 722 C to a width of the latch  14 . 1 - 706 C can be in a range from about 0.8 to about 1.2, from about 1.0 to about 1.2, from about 0.5 to about 2.0, from about 1.0 to about 1.5, or the like. 
     Examples with larger tolerances between the latch and the retention channel can result in the latch extending deeper into the retention channel, but can result in wiggle or other movement between the latch and the retention channel, and between the plug connector and the receptacle connector. On the other hand, examples with tighter tolerances can result in the latch extending a shallow distance into the retention channel, but can result in less wiggle or other movement between the latch and the retention channel, and between the plug connector and the receptacle connector. Forming the latch and the retention channel with too tight of tolerances, or with the latch having too great a width relative to a width of the retention channel can result in the latch not being capable of engaging with the retention channel. 
       FIGS.  14 . 1 - 10    illustrates actuation of a push block  14 . 1 - 806  of a receptacle connector  14 . 1 - 800  with a tool  14 . 1 - 840 , which can be used to release a plug connector  14 . 1 - 820  from the receptacle connector  14 . 1 - 800 . The receptacle connector  14 . 1 - 800  can include a receptacle housing  14 . 1 - 802  and a cover member  14 . 1 - 804  coupled to the receptacle housing  14 . 1 - 802 . A fastening mechanism of the receptacle connector  14 . 1 - 800  can include the push block  14 . 1 - 806 , a spring  14 . 1 - 808 , and a guide rail  14 . 1 - 810 . Although not separately illustrated in  FIGS.  14 . 1 - 10   , the push block  14 . 1 - 806  can be coupled to latches that interact with retention channels of the plug connector  14 . 1 - 820  to retain the plug connector  14 . 1 - 820  within the receptacle connector  14 . 1 - 800 . The spring  14 . 1 - 808  retains the push block  14 . 1 - 806  in an initial position in which the latches are in an engaged configuration. The spring  14 . 1 - 808  also resists a force supplied to the push block  14 . 1 - 806  by the tool  14 . 1 - 840 . When the push block  14 . 1 - 806  is pushed by the force of the tool  14 . 1 - 840 , the push block  14 . 1 - 806  slides or translates along the guide rail  14 . 1 - 810 . As the push block  14 . 1 - 806  moves along the guide rail  14 . 1 - 810 , the push block  14 . 1 - 806  pivots the latches from the engaged configuration to an un-engaged configuration in which the plug connector  14 . 1 - 820  can be removed from the receptacle connector  14 . 1 - 800 . Once the force is no longer supplied by the tool  14 . 1 - 840 , the spring  14 . 1 - 808  returns the push block  14 . 1 - 806  to the initial position and the latches pivot from the un-engaged configuration back to the engaged configuration. 
     The receptacle connector  14 . 1 - 800  further includes a face seal  14 . 1 - 814  and bumpers  14 . 1 - 812 . The face seal  14 . 1 - 814  and the bumpers  14 . 1 - 812  can be disposed between the plug connector  14 . 1 - 820  and the receptacle housing  14 . 1 - 802  in a direction parallel to an insertion/extraction direction of the plug connector  14 . 1 - 820 . This helps the face seal  14 . 1 - 814  and/or the bumpers  14 . 1 - 812  to provide an ejection force to the plug connector  14 . 1 - 820 . For example, when the latches are in the un-engaged configuration, the face seal  14 . 1 - 814  and/or the bumpers  14 . 1 - 812  can provide the ejection force to the plug connector  14 . 1 - 820  to push the plug connector  14 . 1 - 820  out of the receptacle connector  14 . 1 - 800 . This allows a user to remove the plug connector  14 . 1 - 820  by actuating the push block  14 . 1 - 806  with the tool  14 . 1 - 840 , without having to apply a separate force to the plug connector  14 . 1 - 820 . In some examples, the ejection force of the face seal  14 . 1 - 814  and the bumpers  14 . 1 - 812  can be relatively small, and a pull force on the plug connector  14 . 1 - 820  can be used in addition to the ejection force to eject the plug connector  14 . 1 - 820  from the receptacle connector  14 . 1 - 800 . In some examples, the bumpers  14 . 1 - 812  and the face seal  14 . 1 - 814  do not provide the ejection force. For example, the bumpers  14 . 1 - 812  can be provided to ensure proper alignment of electrical contacts of the plug connector  14 . 1 - 820  with electrical contacts of the receptacle connector  14 . 1 - 800 . The face seal  14 . 1 - 814  can provide sealing between the plug connector  14 . 1 - 820  and the receptacle connector  14 . 1 - 800 . 
     The receptacle connector  14 . 1 - 800  can be mounted to an enclosure  14 . 1 - 830  (also referred to as a housing) of a display. In some examples, the receptacle connector  14 . 1 - 800  can be mounted to the enclosure  14 . 1 - 830  using fasteners; however, any suitable mounting means can be used. The enclosure  14 . 1 - 830  can include an opening through which the plug connector  14 . 1 - 820  can be inserted into a recess of the receptacle connector  14 . 1 - 800 . The enclosure  14 . 1 - 830  can further include an opening through which the tool  14 . 1 - 840  is inserted to interact with the push block  14 . 1 - 806 . 
       FIGS.  14 . 1 - 11 A through  14 . 1 - 11 D  illustrate various configurations of tools that can be used to release a plug connector from a receptacle connector. In  FIGS.  14 . 1 - 11 A through  14 . 1 - 11 D , a display  14 . 1 - 904  is provided in an enclosure  14 . 1 - 902  (also referred to as a housing), and a support  14 . 1 - 906  is coupled to the enclosure  14 . 1 - 902 . The support  14 . 1 - 906  can include a plug connector, which can be retained in a receptacle connector provided in the enclosure  14 . 1 - 902 . In  FIGS.  14 . 1 - 11 A , a tool  14 . 1 - 900 A can include a stadium-shaped handle with a stadium-shaped cutout therein, and a cylindrical-shaped protrusion. The cylinder-shaped protrusion of the tool  14 . 1 - 900 A can be inserted into a hole in the enclosure  14 . 1 - 902  and can actuate a push block or the like to release the plug connector of the support  14 . 1 - 906  from the receptacle connector of the enclosure  14 . 1 - 902 . 
     In  FIGS.  14 . 1 - 11 B , a tool  14 . 1 - 900 B can include a stadium-shaped handle and a cylindrical-shaped protrusion. The cylinder-shaped protrusion of the tool  14 . 1 - 900 B can be inserted into a hole in the enclosure  14 . 1 - 902  and can actuate a push block or the like to release the plug connector of the support  14 . 1 - 906  from the receptacle connector of the enclosure  14 . 1 - 902 . 
     In  FIGS.  14 . 1 - 11 C , a tool  14 . 1 - 900 C can include a cylinder-shaped handle and a cylindrical-shaped protrusion. The cylinder-shaped protrusion of the tool  14 . 1 - 900 C can be inserted into a hole in the enclosure  14 . 1 - 902  and can actuate a push block or the like to release the plug connector of the support  14 . 1 - 906  from the receptacle connector of the enclosure  14 . 1 - 902 . 
     In  FIGS.  14 . 1 - 11 D , a tool  14 . 1 - 900 D can include a handle that interfaces with the support  14 . 1 - 906  and a cylindrical-shaped protrusion. The handle of the tool  14 . 1 - 900 D can be slidably attached to the support  14 . 1 - 906  and can be used to align the cylinder-shaped protrusion with a hole in the enclosure. The cylinder-shaped protrusion of the tool  14 . 1 - 900 D can be inserted into the hole in the enclosure  14 . 1 - 902  and can actuate a push block or the like to release the plug connector of the support  14 . 1 - 906  from the receptacle connector of the enclosure  14 . 1 - 902 . 
       FIGS.  14 . 1 - 12 A and  14 . 1 - 12 B  illustrate a configuration of a guide rail  14 . 1 - 1006  of a receptacle connector  14 . 1 - 1000  and a guide channel  14 . 1 - 1014  of a plug connector  14 . 1 - 1010 . The receptacle connector  14 . 1 - 1000  can include a receptacle housing  14 . 1 - 1002  having a recess  14 . 1 - 1004  formed therein. The recess  14 . 1 - 1004  can be configured to receive and retain the plug connector  14 . 1 - 1010 . The guide rail  14 . 1 - 1006  can be formed in the receptacle housing  14 . 1 - 1002  and can extend into a portion of the recess  14 . 1 - 1004 . The plug connector  14 . 1 - 1012  can include a main body portion  14 . 1 - 1012  having the guide channel  14 . 1 - 1014  formed therein. As illustrated in  FIGS.  14 . 1 - 12 A and  14 . 1 - 12 B , the guide rail  14 . 1 - 1006  and the guide channel  14 . 1 - 1014  can be formed in longitudinally central portions of the receptacle housing  14 . 1 - 1002  and the main body  14 . 1 - 1012  of the plug connector  14 . 1 - 1010 , respectively. The guide rail  14 . 1 - 1006  and the guide channel  14 . 1 - 1014  can be disposed on corresponding portions of the receptacle housing  14 . 1 - 1002  and the main body  14 . 1 - 1012  and can have shapes corresponding to one another. The guide rail  14 . 1 - 1006  can be received within the guide channel  14 . 1 - 1014  of the plug connector  14 . 1 - 1010  to provide a rigid connection between a support including the plug connector  14 . 1 - 1010  and a display portion including the receptacle connector  14 . 1 - 1000 . In other words, the guide rail  14 . 1 - 1006  and the guide channel  14 . 1 - 1014  can limit motion or free play between the support and the display portion. Additionally, or alternatively, the guide rail  14 . 1 - 1006  can limit or prevent the plug connector  14 . 1 - 1010  from being inserted into the receptacle connector  14 . 1 - 1000  in an undesirable orientation and/or configuration. 
       FIGS.  14 . 1 - 13 A through  14 . 1 - 13 D  illustrate various configurations of face seals that can be included in a receptacle connector  14 . 1 - 1100 .  FIGS.  14 . 1 - 13 B through  14 . 1 - 13 D  illustrate detail views of region  14 . 1 - 1108  illustrated in  FIGS.  14 . 1 - 13 A . As illustrated in  FIGS.  14 . 1 - 13 A , the receptacle connector  14 . 1 - 1100  can include a receptacle housing  14 . 1 - 1102 , which defines a recess  14 . 1 - 1104 . A plug connector can be received in the recess  14 . 1 - 1104 . A face seal  14 . 1 - 1106  can be coupled to the receptacle housing  14 . 1 - 1102  adjacent the recess  14 . 1 - 1104  and can be used to provide sealing between the receptacle housing  14 . 1 - 1102  and the plug connector. The face seal  14 . 1 - 1106  can further provide an ejection force to the plug connector to assist in the removal of the plug connector from the recess  14 . 1 - 1104  of the receptacle connector  14 . 1 - 1100 . 
     In  FIGS.  14 . 1 - 13 B , a face seal  14 . 1 - 1106 A coupled to the receptacle housing  14 . 1 - 1102  in a region  14 . 1 - 1108 A extends from the receptacle housing  14 . 1 - 1102  at an angle perpendicular to an insertion/extraction direction of the plug connector with respect to the recess  14 . 1 - 1104 . The face seal  14 . 1 - 1106 A is then ramped down and extends distally past the receptacle housing  14 . 1 - 1102 . 
     In  FIGS.  14 . 1 - 13 C , a face seal  14 . 1 - 1106 B coupled to the receptacle housing  14 . 1 - 1102  in a region  14 . 1 - 1108 B extends from the receptacle housing  14 . 1 - 1102  at an angle perpendicular to an insertion/extraction direction of the plug connector with respect to the recess  14 . 1 - 1104 . The face seal  14 . 1 - 1106 B is then ramped down and extends distally past the receptacle housing  14 . 1 - 1102 . The face seal  14 . 1 - 1106 B can extend a smaller distance from the receptacle housing  14 . 1 - 1102  relative to the face seal  14 . 1 - 1106 A and can be ramped from a proximal edge to a distal edge to a lesser degree relative to the face seal  14 . 1 - 1106 A. This provides less resistance to insertion and extraction of the plug connector, while still providing good sealing between the plug connector and the receptacle housing  14 . 1 - 1102 . 
     In  FIGS.  14 . 1 - 13 D , a face seal  14 . 1 - 1106 C coupled to the receptacle housing  14 . 1 - 1102  in a region  14 . 1 - 1108 C extends from flush with the receptacle housing  14 . 1 - 1102 , ramps out from the receptacle housing  14 . 1 - 1102 , and then ramps back towards the receptacle housing  14 . 1 - 1102 . The face seal  14 . 1 - 1106 C extends distally past the receptacle housing  14 . 1 - 1102 . The face seal  14 . 1 - 1106 C can extend a smaller distance from the receptacle housing  14 . 1 - 1102  relative to the face seals  14 . 1 - 1106 A and  14 . 1 - 1106 B and can be ramped to lesser degrees relative to the face seals  14 . 1 - 1106 A and  14 . 1 - 1106 B. This provides less resistance to insertion and extraction of the plug connector, while still providing good sealing between the plug connector and the receptacle housing  14 . 1 - 1102 . 
       FIGS.  14 . 1 - 14 A and  14 . 1 - 14 B  illustrate a perspective view and a sectional view of a receptacle connector  14 . 1 - 1200 .  FIGS.  14 . 1 - 14 A and  14 . 1 - 14 B  illustrate gaskets  14 . 1 - 1208  that can be provided with fasteners  14 . 1 - 1206  to fasten a receptacle housing  14 . 1 - 1202  of the receptacle connector  14 . 1 - 1200  to an enclosure or housing of a display portion or the like. The receptacle housing  14 . 1 - 1202  can include a cover member  14 . 1 - 1204  coupled thereto, which provides structural support and shielding for the receptacle housing  14 . 1 - 1202 . Holes can be provided in the receptacle housing  14 . 1 - 1202  and the cover member  14 . 1 - 1204 , and the gaskets  14 . 1 - 1208  and the fasteners  14 . 1 - 1206  can be provided in the holes extending through the receptacle housing  14 . 1 - 1202  and the cover member  14 . 1 - 1204 . The fasteners  14 . 1 - 1206  can be used to fasten the receptacle housing  14 . 1 - 1202  and the cover member  14 . 1 - 1204  to the enclosure of the display portion. The gaskets  14 . 1 - 1208  can be provided between the fasteners  14 . 1 - 1206  and the cover member  14 . 1 - 1204 /receptacle housing  14 . 1 - 1202  to provide give or flexibility between the receptacle connector  14 . 1 - 1200  and the enclosure. The gaskets  14 . 1 - 1208  can be formed of a relatively soft material, such as a rubber, a polymer, an elastomer, or the like. Providing the gaskets  14 . 1 - 1208  allows the receptacle connector  14 . 1 - 1200  and a plug connector inserted into the receptacle connector  14 . 1 - 1200  to move together, and to move relative to the enclosure. Large forces can be transferred between the plug connector and the receptacle connector  14 . 1 - 1200  and providing the gaskets  14 . 1 - 1208  limits any negative impacts of these forces from being transferred between the receptacle connector  14 . 1 - 1200  and the enclosure. 
     A plurality of electrical contacts  14 . 1 - 1210 A,  14 . 1 - 1210 B,  14 . 1 - 1210 C,  14 . 1 - 1211 A,  14 . 1 - 1211 B, and  14 . 1 - 1211 B can be coupled to the receptacle housing  14 . 1 - 1202 . As illustrated in  FIGS.  14 . 1 - 14 A , the electrical contacts can be coupled to the receptacle housing  14 . 1 - 1202  along a radius of curvature, with each electrical contact being provided at an oblique angle relative to other electrical contacts, such as adjacent electrical contacts. This allows for surfaces of the electrical contacts that are to be in contact with a plug connector inserted into the receptacle connector to be disposed at angles normal to surfaces of the plug connector that the electrical contacts are to contact. This provides for improved electrical contact between the electrical contacts of the receptacle connector  14 . 1 - 1200  and electrical contacts of the plug connector. 
     A fastening mechanism can be provided on the receptacle connector  14 . 1 - 1200 . The fastening mechanism can include a primary push block  14 . 1 - 1212 , a spring  14 . 1 - 1214 , latches  14 . 1 - 1220 , a secondary push block  14 . 1 - 1222 , and a guide rail  14 . 1 - 1224 . The fastening mechanism can be provided to transition the latches  14 . 1 - 1220  between an engaged configuration and an un-engaged configuration, which can be used to retain and release the plug connector in the receptacle connector  14 . 1 - 1200 . The spring  14 . 1 - 1214  can be coupled to the cover member  14 . 1 - 1204  and/or the receptacle housing  14 . 1 - 1202 . The primary push block  14 . 1 - 1212  can be pivotally coupled to the spring  14 . 1 - 1214  and the cover member  14 . 1 - 1204  and/or the receptacle housing  14 . 1 - 1202 . The primary push block  14 . 1 - 1212  can be configured to interact with a tool in order to transition the latches  14 . 1 - 1220  between the engaged configuration and the un-engaged configuration. The spring  14 . 1 - 1214  retains the primary push block  14 . 1 - 1212  in an initial position in which the latches  14 . 1 - 1220  are in the engaged configuration. The spring  14 . 1 - 1214  also resists a force supplied to the primary push block  14 . 1 - 1212  by the tool. 
     The secondary push block  14 . 1 - 1222  transfers movement between the primary push block  14 . 1 - 1212  and the latches  14 . 1 - 1220 . The secondary push block  14 . 1 - 1222  can be configured to translate along the guide rail  14 . 1 - 1224 , such that the secondary push block  14 . 1 - 1222  moves back and forth in a single direction. Using the tool to actuate the primary push block  14 . 1 - 1212  moves the secondary push block  14 . 1 - 1222  along the guide rail  14 . 1 - 1224 , and the secondary push block  14 . 1 - 1222  pivots the latches  14 . 1 - 1220  from the engaged configuration to the un-engaged configuration. After force from the tool is removed from the primary push block  14 . 1 - 1212 , the secondary push block  14 . 1 - 1222  returns to its initial position and the latches  14 . 1 - 1220  return to the engaged configuration. Thus, the fastening mechanism is used to retain and release a plug connector with respect to the receptacle connector  14 . 1 - 1200 . 
       FIGS.  14 . 1 - 15    illustrates an example in which a support  14 . 1 - 1320  is fastened to a receptacle connector  14 . 1 - 1300  and/or an enclosure  14 . 1 - 1330  using fasteners  14 . 1 - 1338 . The fastening mechanism of  FIGS.  14 . 1 - 15    can be used in addition to or alternative to the fastening mechanisms of  FIGS.  14 . 1 - 3 A through  14 . 1 - 14 B . The receptacle connector  14 . 1 - 1300  can include a receptacle housing  14 . 1 - 1302  having a recess  14 . 1 - 1304  formed therein. The recess  14 . 1 - 1304  can be configured to receive and retain the support  14 . 1 - 1320 . Fasteners  14 . 1 - 1306  can extend through holes in the receptacle housing  14 . 1 - 1302  and can be used to fasten the receptacle connector  14 . 1 - 1300  to the enclosure  14 . 1 - 1330 . 
     The enclosure  14 . 1 - 1330  (also referred to as a housing) can be an enclosure of a display portion or the like. The enclosure  14 . 1 - 1330  can include a housing body  14 . 1 - 1332 , openings  14 . 1 - 1334 , and openings  14 . 1 - 1336 . The openings  14 . 1 - 1336  can be configured to receive the fasteners  14 . 1 - 1306  and can be used to mount the receptacle connector  14 . 1 - 1300  to the enclosure  14 . 1 - 1330 . As will be discussed in greater detail below, the support  14 . 1 - 1320  can be fastened to the enclosure  14 . 1 - 1330  through the openings  14 . 1 - 1334 . 
     The support  14 . 1 - 1320  includes a plug connector  14 . 1 - 1322  and an enclosure  14 . 1 - 1324 . Openings  14 . 1 - 1326  can be included in the plug connector  14 . 1 - 1322  and/or the enclosure  14 . 1 - 1324 . The plug connector  14 . 1 - 1322  can include electrical contacts  14 . 1 - 1328 A,  14 . 1 - 1328 B, and  14 . 1 - 1328 C. Fasteners  14 . 1 - 1338  can be provided and can extend through the openings  14 . 1 - 1326  in the support  14 . 1 - 1320  and into the openings  14 . 1 - 1334  in the enclosure  14 . 1 - 1330 . In some examples, the openings  14 . 1 - 1334  can be provided on the receptacle connector  14 . 1 - 1300  rather than the enclosure  14 . 1 - 1330 . Fastening the support  14 . 1 - 1320  to the enclosure  14 . 1 - 1330  and/or the receptacle connector  14 . 1 - 1300  prevents unwanted disconnects of the support  14 . 1 - 1320  from the receptacle connector  14 . 1 - 1300  and/or the enclosure  14 . 1 - 1330 . The fasteners  14 . 1 - 1338  can be contained within the enclosure  14 . 1 - 1330 , unseen to a user, and thus accidental removal by a user can be avoided. Fastening the support  14 . 1 - 1320  directly to the enclosure  14 . 1 - 1330  with the fasteners  14 . 1 - 1338  further ensures the support  14 . 1 - 1320  does not move relative to the chassis  14 . 1 - 1330 . 
       FIGS.  14 . 1 - 16 A through  14 . 1 - 25 B  illustrate various examples of receptacle connectors and plug connectors that can be used as the receptacle connectors  14 . 1 - 124  of the supports  14 . 1 - 120  and the plug connectors  14 . 1 - 142  of the cable assembly  14 . 1 - 140  illustrated in  FIGS.  14 . 1 - 1 A and  14 . 1 - 1 B . 
       FIGS.  14 . 1 - 16 A  illustrates a side section view of a receptacle connector  14 . 1 - 1400  and a cable assembly  14 . 1 - 1420  including a plug connector  14 . 1 - 1422  inserted into and retained by the receptacle connector  14 . 1 - 1400 .  FIGS.  14 . 1 - 16 B  illustrates an electrical contact of the plug connector  14 . 1 - 1422  and a raised portion of the receptacle connector  14 . 1 - 1400 . The cable assembly  14 . 1 - 1420  includes a shielded cable  14 . 1 - 1426  coupled to the plug connector  14 . 1 - 1422 . The plug connector  14 . 1 - 1422  can include a boot  14 . 1 - 1424  and a central member  14 . 1 - 1430 . In some examples, the boot  14 . 1 - 1424  can have a circular or semi-circular cross-sectional shape. The plug connector  14 . 1 - 1422  can be centered or substantially centered about a central axis of the circular cross-sectional shape of the boot  14 . 1 - 1424 . The boot  14 . 1 - 1424  can form a cavity or volume and at least a portion of the central member  14 . 1 - 1430  and/or a portion of the shielded cable  14 . 1 - 1426  can be disposed within the volume. 
     The central member  14 . 1 - 1430  can extend from the boot  14 . 1 - 1424 . In some examples, the central member  14 . 1 - 1430  can have a cylindrical shape that enables rotation of the central member  14 . 1 - 1430  within the receptacle connector  14 . 1 - 1400 . The central member  14 . 1 - 1430  can form an external surface and an internal surface. In some examples, the central member  14 . 1 - 1430  can include one or more protrusions  14 . 1 - 1428  extending laterally from, or substantially perpendicular to, the external surface. In some examples, the central member  14 . 1 - 1430  can form a cavity or volume. For example, the internal surface can form a cavity or volume. One or more electrical contacts can be disposed on the internal surface, such as an electrical contact  14 . 1 - 1425  illustrated in  FIGS.  14 . 1 - 16 B . The one or more electrical contacts can extend into the volume of the central member  14 . 1 - 1430  from the internal surface. The one or more electrical contacts can define an electrical grounding path, provide electrical power, and/or provide one or more control signals to the receptacle connector  14 . 1 - 1400 . 
     The shielded cable  14 . 1 - 1426  can be one or more wires (e.g., metallic wires capable of transferring electrical signals and/or electrical power). The one or more wires can be individually shielded or collectively shielded. The shielding can prevent each of the one or more wires from contacting one another. Additionally, or alternatively, the shielding can prevent or limit the influence of electromagnetic waves on each of the one or more wires. 
     The receptacle connector  14 . 1 - 1400  can be disposed on an enclosure of an electronic device (e.g., the supports  14 . 1 - 120  of the first electronic device  14 . 1 - 100 ). In some examples, the receptacle connector  14 . 1 - 1400  can be disposed on or within the enclosure, such that a trim ring  14 . 1 - 1402  of the receptacle connector  14 . 1 - 1400  is at least partially disposed within the enclosure. In some examples, the trim ring  14 . 1 - 1402  can be disposed on or within the enclosure such that the trim ring  14 . 1 - 1402  is flush with an exterior surface of the enclosure. 
     In some examples, the receptacle connector  14 . 1 - 1400  can include the trim ring  14 . 1 - 1402 , a base, one or more detents  14 . 1 - 1406 , and a raised portion  14 . 1 - 1404 . The trim ring  14 . 1 - 1402  can be coupled to the base to form a recess and one or more undercut regions at a periphery of the recess. Each of the undercut regions can house or retain a respective detent  14 . 1 - 1406 . When the central member  14 . 1 - 1430  of the cable assembly  14 . 1 - 1420  is disposed within the recess, the protrusion  14 . 1 - 1428  can extend into a channel defined by a respective detent  14 . 1 - 1406 . In some examples, each of the detents  14 . 1 - 1406  can be biased toward the raised portion  14 . 1 - 1404  by a biasing element. In other words, each detent  14 . 1 - 1406  can radially translate within their respective undercut regions toward and away from the raised portion  14 . 1 - 1404 . 
     In some examples, the raised portion  14 . 1 - 1404  can be disposed in the center of the recess, such that the recess forms a ring within the receptacle connector  14 . 1 - 1400 . The raised portion  14 . 1 - 1404  can be a distinct component of the receptacle connector  14 . 1 - 1400 . For example, the raised portion  14 . 1 - 1404  can be adhered, fastened, interlocked, welded, or otherwise coupled to the base. In some examples, one or more electrical contacts  14 . 1 - 1407 A and  14 . 1 - 1407 B can be disposed on the raised portion  14 . 1 - 1404  and can be used to provide connections between the raised portion  14 . 1 - 1404  and the plug connector  14 . 1 - 1422 , such as the electrical contact  14 . 1 - 1425  of the plug connector  14 . 1 - 1422 . Each of the one or more electrical contacts  14 . 1 - 1407 A and  14 . 1 - 1407 B can physically contact one or more electrical contacts of the plug connector  14 . 1 - 1422  (e.g., the electrical contact  14 . 1 - 1425 ) when the plug connector  14 . 1 - 1422  is coupled to the receptacle connector  14 . 1 - 1400 . The raised portion  14 . 1 - 1404  can further include one or more electrical contacts  14 . 1 - 1409 A and  14 . 1 - 1409 B that provide connections between the raised portion  14 . 1 - 1404  and electronic components contained within the enclosure (e.g., processors, electrical wires, digital logic circuitry, digital processing circuitry, or the like). 
     In some examples, the raised portion  14 . 1 - 1404  can include a body  14 . 1 - 1414  and a cap  14 . 1 - 1412 . In some examples, the body  14 . 1 - 1414  can be manufactured from a material that electrically insulates the one or more electrical contacts  14 . 1 - 1407 A,  14 . 1 - 1407 B,  14 . 1 - 1409 A, and  14 . 1 - 1409 B, such that each of the one or more electrical contacts are electrically isolated from one another. For example, each of the one or more electrical contacts can be at least partially disposed within a respective through-hole defined within the body  14 . 1 - 1414 . The cap  14 . 1 - 1412  can be coupled to the body  14 . 1 - 1414 . 
     The raised portion  14 . 1 - 1404  can include a seal  14 . 1 - 1408  extending around a periphery of the body  14 . 1 - 1414 . As illustrate in  FIGS.  14 . 1 - 16 B , the seal  14 . 1 - 1408  can encircle the body  14 . 1 - 1414  of the raised portion  14 . 1 - 1404 . The seal  14 . 1 - 1408  can act as a seal which prevents or limits ingress of contaminants (e.g., liquid, dust, lint, debris, or the like) into the receptacle connector  14 . 1 - 1400  through the recess, such as when the plug connector  14 . 1 - 1422  is inserted into the recess of the receptacle connector  14 . 1 - 1400 . 
       FIGS.  14 . 1 - 17 A and  14 . 1 - 17 B  show respective side and bottom views of a cable assembly  14 . 1 - 1500  including a plug connector  14 . 1 - 1502  and a shielded cable  14 . 1 - 1504 . The plug connector  14 . 1 - 1502  can include a boot  14 . 1 - 1506  and a central member  14 . 1 - 1508 . In some examples, the boot  14 . 1 - 1506  can have a circular or semi-circular cross-sectional shape. The plug connector  14 . 1 - 1502  can be centered or substantially centered about a central axis of the circular cross-sectional shape of the boot  14 . 1 - 1506 . For example, as shown in  FIG.  14 . 1 - 17 A , the central member  14 . 1 - 1508  can be centered about an axis A that extends through a center of the boot  14 . 1 - 1506 . The boot  14 . 1 - 1506  can form a cavity or volume and at least a portion of the central member  14 . 1 - 1508  and/or a portion of the shielded cable  14 . 1 - 1504  can be disposed within the volume. 
     The central member  14 . 1 - 1508  can extend from the boot  14 . 1 - 1506 . In some examples, the central member  14 . 1 - 1508  can have a cylindrical shape that enables rotation of the central member  14 . 1 - 1508  within a receptacle connector (e.g., the receptacle connector  14 . 1 - 1400  of  FIGS.  14 . 1 - 16 A ). The central member  14 . 1 - 1508  can form an external surface  14 . 1 - 1510  and an internal surface  14 . 1 - 1512 . In some examples, the central member  14 . 1 - 1508  can include one or more protrusions  14 . 1 - 1514  extending laterally from, or substantially perpendicular to, the external surface  14 . 1 - 1510 . In some examples, the central member  14 . 1 - 1508  can form a cavity or volume, for example, the internal surface  14 . 1 - 1512  can form a cavity or volume  14 . 1 - 1516 . One or more electrical contacts  14 . 1 - 1518  can be disposed on the internal surface  14 . 1 - 1512 , such that the one or more electrical contacts  14 . 1 - 1518  extend into the volume  14 . 1 - 1516  from the internal surface  14 . 1 - 1512 . The one or more electrical contacts  14 . 1 - 1518  can define an electrical grounding path, provide electrical power, and/or provide one or more control signals to a receptacle connector. 
     The shielded cable  14 . 1 - 1504  can be one or more wires (e.g., metallic wires capable of transferring electrical signals and/or electrical power). The one or more wires can be individually shielded or collectively shielded. The shielding can prevent each of the one or more wires from contacting one another. Additionally, or alternatively, the shielding can prevent or limit the influence of electromagnetic waves on each of the one or more wires. 
       FIGS.  14 . 1 - 18 A and  14 . 1 - 18 B  show a perspective view and a top view of a receptacle connector  14 . 1 - 1600  disposed on an enclosure  14 . 1 - 1602  of an electronic device (e.g., the supports  14 . 1 - 120  of the first electronic device  14 . 1 - 100  of  FIGS.  14 . 1 - 1 A and  14 . 1 - 1 B ). In some examples, the receptacle connector  14 . 1 - 1600  can be disposed on or within the enclosure  14 . 1 - 1602 , such that a trim ring  14 . 1 - 1604  of the receptacle connector  14 . 1 - 1600  is at least partially disposed within the enclosure  14 . 1 - 1602 . Alternatively, the trim ring  14 . 1 - 1604  can be disposed on or within the enclosure  14 . 1 - 1602  such that the trim ring  14 . 1 - 1604  is flush with an exterior surface  14 . 1 - 1606  of the enclosure  14 . 1 - 1602 . 
     In some examples, the receptacle connector  14 . 1 - 1600  can include the trim ring  14 . 1 - 1604 , a base  14 . 1 - 1608 , one or more detents  14 . 1 - 1610 , and a raised portion  14 . 1 - 1612 . The trim ring  14 . 1 - 1604  can couple to the base  14 . 1 - 1608  to form a recess  14 . 1 - 1614  and one or more undercut regions at a periphery of the recess  14 . 1 - 1614 . Each of the one or more undercut regions can house or retain a respective detent  14 . 1 - 1610 . While a central member (e.g., the central member  14 . 1 - 1508  of  FIGS.  14 . 1 - 17 A and  14 . 1 - 17 B ) is disposed within the recess  14 . 1 - 1614 , a protrusion (e.g., the protrusions  14 . 1 - 1514  of  FIGS.  14 . 1 - 17 A and  14 . 1 - 17 B ) can extend into a channel defined by each of the detents  14 . 1 - 1610 . In some examples, each of the detents  14 . 1 - 1610  can be biased toward the raised portion  14 . 1 - 1612  by a biasing element. In other words, each detent  14 . 1 - 1610  can radially translate within their respective undercut regions toward and away from the raised portion  14 . 1 - 1612 . 
     In some examples, the raised portion  14 . 1 - 1612  can be disposed in the center of the recess  14 . 1 - 1614 , such that the recess  14 . 1 - 1614  forms a ring within the receptacle connector  14 . 1 - 1600 . The raised portion  14 . 1 - 1612  can be a distinct component of the receptacle connector  14 . 1 - 1600 . For example, the raised portion  14 . 1 - 1612  can be adhered, fastened, interlocked, welded, or otherwise coupled to the base  14 . 1 - 1608 . In some examples, one or more electrical contacts  14 . 1 - 1618  can be disposed on the raised portion  14 . 1 - 1612 . Each of the one or more electrical contacts  14 . 1 - 1618  can physically contact one or more electrical contacts of the plug connector (e.g., the electrical contacts  14 . 1 - 1518  of  FIGS.  14 . 1 - 17 A and  14 . 1 - 17 B ) when the plug connector is coupled to the receptacle connector  14 . 1 - 1600 . 
       FIGS.  14 . 1 - 18 C  illustrates a sectional view of the receptacle connector of  FIGS.  14 . 1 - 18 A and  14 . 1 - 18 B . As illustrated in  FIGS.  14 . 1 - 18 C , biasing elements  14 . 1 - 1611  can be housed within the trim ring  14 . 1 - 1604  between the detents  14 . 1 - 1610  and the trim ring  14 . 1 - 1604 . The biasing elements  14 . 1 - 1611  can be configured to bias the detents  14 . 1 - 1610  radially towards protrusions of the plug connector when the plug connector is inserted into and retained in the recess  14 . 1 - 1614  of the receptacle connector  14 . 1 - 1600 . In other words, each of the detents  14 . 1 - 1610  can radially translate within their respective undercut regions toward and away from the raised portion  14 . 1 - 1612  to engage and disengage with respective protrusions of the plug connector. The biasing elements  14 . 1 - 1611  can provide forces to the detents  14 . 1 - 1610  to retain the detents  14 . 1 - 1610  in the radially extended position to engage the protrusions of the plug connector, and to resist the detents being disengages from the protrusions of the plug connector. The biasing elements  14 . 1 - 1611  are illustrated as a canted coil; however various biasing elements  14 . 1 - 1611  can be included, such as a coiled spring, an elastic foam, leaf springs, or the like. 
     The raised portion  14 . 1 - 1612  can include a body  14 . 1 - 1626  and a cap  14 . 1 - 1628 . In some examples, the body  14 . 1 - 1626  can be manufactured from a material that electrically insulates one or more electrical contacts of the raised portion  14 . 1 - 1612 , such that each of the one or more electrical contacts are electrically isolated from one another. For example, each of the one or more electrical contacts can be at least partially disposed within a respective through-hole defined within the body  14 . 1 - 1626 . The cap  14 . 1 - 1628  can be coupled to the body  14 . 1 - 1626 . 
     The raised portion  14 . 1 - 1612  can include a seal  14 . 1 - 1632  extending around a periphery of the body  14 . 1 - 1626 . As illustrate in  FIGS.  14 . 1 - 18 C , the seal  14 . 1 - 1632  can encircle the body  14 . 1 - 1626  of the raised portion  14 . 1 - 1612 . The seal  14 . 1 - 1632  can act as a seal which prevents or limits ingress of contaminants (e.g., liquid, dust, lint, debris, or the like) into the receptacle connector  14 . 1 - 1600  through the recess  14 . 1 - 1614 , such as when the plug connector is inserted into the recess  14 . 1 - 1614  of the receptacle connector  14 . 1 - 1600 . In the example of  FIGS.  14 . 1 - 18 C , the seal  14 . 1 - 1632  is disposed between the body  14 . 1 - 1626  of the raised portion  14 . 1 - 1612  and the base  14 . 1 - 1608 . The seal  14 . 1 - 1632  can further prevent the ingress of contaminants between the raised portion  14 . 1 - 1612  and the base  14 . 1 - 1608 . 
       FIGS.  14 . 1 - 18 D  shows the cable assembly  14 . 1 - 1500 , including the plug connector  14 . 1 - 1502  and the shielded cable  14 . 1 - 1504  of  FIGS.  14 . 1 - 17 A and  14 . 1 - 17 B , being pulled away from the receptacle connector  14 . 1 - 1600  of  FIGS.  14 . 1 - 18 A through  14 . 1 - 18 C . The plug connector  14 . 1 - 1502  can be unintentionally and abruptly extracted from the receptacle connector  14 . 1 - 1600 , for example, when the shielded cable  14 . 1 - 1504  snags on an object and pulls the plug connector  14 . 1 - 1502  in a direction away from the receptacle connector  14 . 1 - 1600  (as represented by the arrow shown in  FIGS.  14 . 1 - 18 D ). When the plug connector  14 . 1 - 1502  is abruptly and/or undesirably extracted from the receptacle connector  14 . 1 - 1600 , the electrical contacts  14 . 1 - 1518 A and  14 . 1 - 1518 B of the plug connector  14 . 1 - 1502  can be pulled out of contact with the correlating electrical contacts  14 . 1 - 1618 A and  14 . 1 - 1618 B of the receptacle connector  14 . 1 - 1600  causing a loss of: electrical power, control signals, an electrical ground, or a combination thereof. An abrupt loss of electrical power, electrical ground, or one or more control signals can damage the electronic device and/or compromise or corrupt information stored on the electronic device. 
     In some examples, at least one of the electrical contacts (e.g., electrical contact  14 . 1 - 1618 A) within the receptacle connector  14 . 1 - 1600  can detect when the plug connector  14 . 1 - 1502  is being removed from the receptacle connector  14 . 1 - 1600 . For example, the electronic device can detect an undesirable extraction of the plug connector  14 . 1 - 1502  from the receptacle connector  14 . 1 - 1600  when the electrical contact  14 . 1 - 1518 A of the plug connector  14 . 1 - 1502  is pulled out of contact with the electrical contact  14 . 1 - 1618 A of the receptacle connector  14 . 1 - 1600 . An undesirable extraction of the plug connector  14 . 1 - 1502  from the receptacle connector  14 . 1 - 1600  can occur when the plug connector  14 . 1 - 1502  is pulled away from the receptacle connector  14 . 1 - 1600  from the side of the plug connector  14 . 1 - 1502  where the shielded cable  14 . 1 - 1504  enters the boot  14 . 1 - 1506 . This undesirable extraction scenario can be most probable as the shielded cable  14 . 1 - 1504  can get snagged on an object and can pull the plug connector  14 . 1 - 1502  away from the receptacle connector  14 . 1 - 1600  (as illustrated by the arrow). 
     In some examples, the electrical contacts  14 . 1 - 1618  can be disposed about the raised portion  14 . 1 - 1612  such that the electrical contact  14 . 1 - 1518 A moves out of contact from the electrical contact  14 . 1 - 1518 A before the electrical contacts  14 . 1 - 1518 B of the plug connector  14 . 1 - 1502  move out of contact with the electrical contact  14 . 1 - 1618 B of the receptacle connector  14 . 1 - 1600 . While the electrical contact  14 . 1 - 1518 A is out of contact from the electrical contact  14 . 1 - 1618 A but before the electrical contacts  14 . 1 - 1518 B of the plug connector  14 . 1 - 1502  move out of contact with the electrical contact  14 . 1 - 1618 B of the receptacle connector  14 . 1 - 1600 , a duration of time can pass. The duration of time can be sufficient to enable the electronic device to mitigate any destructive effects of unintentionally extracting the plug connector  14 . 1 - 1502  from the receptacle connector  14 . 1 - 1600 . The size, shape, and position of each of the electrical contacts  14 . 1 - 1518 ,  14 . 1 - 1618  can impact the duration of time when the electrical contact  14 . 1 - 1518 A is out of contact from the electrical contact  14 . 1 - 1618 A but before the electrical contacts  14 . 1 - 1518 B move out of contact with the electrical contact  14 . 1 - 1618 B. 
     In some examples, a contact area defined by the electrical contact  14 . 1 - 1618 A (e.g., a surface area of the electrical contact  14 . 1 - 1618  that can be engaged by one or more of the electrical contacts  14 . 1 - 1518 ) can be different from a contact area defined by the electrical contact  14 . 1 - 1618 B. In other words, each electrical contact  14 . 1 - 1618 A,  14 . 1 - 1618 B can be sized or shaped such that one or more correlating electrical contacts  14 . 1 - 1518  can physically contact the contact area defined by each respective electrical contact  14 . 1 - 1618 . As shown in  FIGS.  14 . 1 - 18 D , the contact area of the electrical contact  14 . 1 - 1618 B can be larger than the contact area of the electrical contact  14 . 1 - 1618 A. As such, the electrical contact  14 . 1 - 1618 B can interface with more than one corresponding electrical contacts  14 . 1 - 1518 B while the plug connector  14 . 1 - 1502  is inserted into the receptacle connector  14 . 1 - 1600 . The size, shape, and position of the contact area of each electrical contact  14 . 1 - 1518 ,  14 . 1 - 1618  can impact the duration of time when the electrical contact  14 . 1 - 1518 A is out of contact from the electrical contact  14 . 1 - 1618 A but before the electrical contacts  14 . 1 - 1518 B move out of contact with the electrical contact  14 . 1 - 1618 B. 
     A loss of the electrical signal passed between the electrical contacts  14 . 1 - 1518 A,  14 . 1 - 1618 A can alert the electronic device (e.g., a processor of the electronic device) that an unexpected or undesirable extraction of the plug connector  14 . 1 - 1502  is occurring. Accordingly, the electronic device can react to the loss of signal by implementing one or more actions to safeguard the electrical components and information of the electronic device. For example, a processor within the electronic device can: withdraw electrical power from one or more electronic components, cause data to be saved, or perform another preventative task or action. While  FIGS.  14 . 1 - 18 D  shows the plug connector  14 . 1 - 1502  being extracted from the receptacle connector  14 . 1 - 1600  from the side of the plug connector  14 . 1 - 1502  adjacent the shielded cable  14 . 1 - 1504  (e.g., rocking out of the receptacle connector  14 . 1 - 1600  starting from the side of the plug connector  14 . 1 - 1502  adjacent the shielded cable  14 . 1 - 1504 ), the plug connector  14 . 1 - 1502  can be undesirably detached from another side of the plug connector  14 . 1 - 1502  in some examples. 
       FIGS.  14 . 1 - 18 E  illustrates various configurations of the raised portion  14 . 1 - 1612  that can be used to alter the force required to insert and extract the plug connector  14 . 1 - 1502  into and out of the receptacle connector  14 . 1 - 1600 . As illustrated in  FIGS.  14 . 1 - 18 E , side surfaces of the raised portion  14 . 1 - 1612  can be disposed at different angles with respect to the base  14 . 1 - 1608 . In some examples, an angle  14 . 1 - 01  between side surfaces of the raised portion  14 . 1 - 1612  and the base  14 . 1 - 1608  can be in a range from about 95° to about 105°, from about 90° to about 180°, from about 90° to about 120°, from about 95° to about 135°, from about 92° to about 110°, or the like. In some examples, an angle  14 . 1 - 02  between side surfaces of the raised portion  14 . 1 - 1612  and the base  14 . 1 - 1608  can be relatively larger than the angle  14 . 1 - 01  and can be in a range from about 95° to about 105°, from about 90° to about 180°, from about 90° to about 120°, from about 95° to about 135°, from about 92° to about 110°, or the like. Providing the angle  14 . 1 - 02  between the raised portion  14 . 1 - 1612  and the base  14 . 1 - 1608  allows the plug connector  14 . 1 - 1502  to be inserted into and extracted from the receptacle connector  14 . 1 - 1600  with a relatively small force. Providing the angle  14 . 1 - 01  between the raised portion  14 . 1 - 1612  and the base  14 . 1 - 1608  allows the plug connector  14 . 1 - 1502  to be inserted into and extracted from the receptacle connector  14 . 1 - 1600  with a relatively large force. Thus, the angle between the raised portion  14 . 1 - 1612  and the base  14 . 1 - 1608  can be selected based on the forces desired to insert the plug connector  14 . 1 - 1502  into the receptacle connector  14 . 1 - 1600  and to extract the plug connector  14 . 1 - 1502  from the receptacle connector  14 . 1 - 1600 . 
       FIGS.  14 . 1 - 19 A through  14 . 1 - 19 M  illustrate various configurations of detents  14 . 1 - 1710  that can be included in a receptacle connector  14 . 1 - 1700 . The receptacle connector  14 . 1 - 1700  can include a trim ring  14 . 1 - 1704 , a base  14 . 1 - 1708 , a raised portion  14 . 1 - 1712 , and a recess between the trim ring  14 . 1 - 1704  and the raised portion  14 . 1 - 1712 . The detents  14 . 1 - 1710  can be disposed within a cavity formed in the trim ring  14 . 1 - 1704 . 
       FIGS.  14 . 1 - 19 A through  14 . 1 - 19 C  illustrate a detent  14 . 1 - 1710 A that includes a central region  14 . 1 - 1711  and peripheral regions  14 . 1 - 1713  on either side of the central region  14 . 1 - 1711 . The central region  14 . 1 - 1711  can be notched to form a channel  14 . 1 - 1733  that can receive a protrusion  14 . 1 - 1724  of a plug connector  14 . 1 - 1720 . For example, a lower portion of the central region  14 . 1 - 1711  can be notched to form an undercut portion of the detent  14 . 1 - 1710 A in the central region  14 . 1 - 1711 . The notches can be omitted from the peripheral regions  14 . 1 - 1713 , which can prevent, limit, or reduce rotation of the plug connector  14 . 1 - 1720 . Including the un-notched peripheral regions  14 . 1 - 1713  can also increase mating forces and un-mating forces between the plug connector  14 . 1 - 1720  and the receptacle connector  14 . 1 - 1700 . 
       FIGS.  14 . 1 - 19 C  shows a cross-sectional view of the protrusion  14 . 1 - 1724  of the plug connector  14 . 1 - 1720  disposed within the channel  14 . 1 - 1733  and interfacing the detent  14 . 1 - 1710 A taken along reference line A-A′ of  FIGS.  14 . 1 - 19 A . In some examples, the detent  14 . 1 - 1710 A can define the channel  14 . 1 - 1733  such that a first surface  14 . 1 - 1731  of the channel  14 . 1 - 1733  is angled relative to a second surface  14 . 1 - 1732  of the channel  14 . 1 - 1733  to form an angle  14 . 1 - 03 . The protrusion  14 . 1 - 1714  can engage or contact the first surface  14 . 1 - 1730  while disposed within the channel  14 . 1 - 1733 . The angle  14 . 1 - 03  can directly correlate to a force required to overcome a biasing element coupled to the detent  14 . 1 - 1710 A to cause the detent  14 . 1 - 1710 A to move away from the central member  14 . 1 - 1712  and enable extraction of the protrusion  14 . 1 - 1724  from the channel  14 . 1 - 1733  (e.g., enabling extraction of the plug connector  14 . 1 - 1720  from the receptacle connector  14 . 1 - 1700 ). For example, a relatively smaller angle  14 . 1 - 03  can require a relatively greater force to extract the plug connector  14 . 1 - 1720  from the receptacle connector  14 . 1 - 1700 . Alternatively, a relatively larger angle  14 . 1 - 03  can require relatively smaller force to extract the plug connector  14 . 1 - 1720  from the receptacle connector  14 . 1 - 1700 . The angle  14 . 1 - 03  can be about 30 degrees, between about 30 degrees and about 60 degrees, between about 60 degrees and about 90 degrees, between about 90 degrees and about 120 degrees, between about 120 degrees and about 150 degrees, between about 150 degrees and about 180 degrees, or less than 180 degrees. A third surface  14 . 1 - 1730  adjacent the first surface  14 . 1 - 1731  can extend in a direction parallel to the second surface  14 . 1 - 1732 . 
       FIGS.  14 . 1 - 19 D through  14 . 1 - 19 F  illustrate a detent  14 . 1 - 1710 B that includes a central region  14 . 1 - 1711  and peripheral regions  14 . 1 - 1713  on either side of the central region  14 . 1 - 1711 . The central region  14 . 1 - 1711  can be notched to form a channel  14 . 1 - 1733  that can receive a protrusion  14 . 1 - 1724  of a plug connector  14 . 1 - 1720 . For example, a lower portion of the central region  14 . 1 - 1711  can be notched to form an undercut portion of the detent  14 . 1 - 1710 B in the central region  14 . 1 - 1711 . Further, an upper portion of the central region  14 . 1 - 1711  can be notched to form a sloped upper surface of the detent  14 . 1 - 1710 B in the central region  14 . 1 - 1711 . The notches can be omitted from the peripheral regions  14 . 1 - 1713 , which can prevent, limit, or reduce rotation of the plug connector  14 . 1 - 1720 . Including the un-notched peripheral regions  14 . 1 - 1713  can also increase mating forces and un-mating forces between the plug connector  14 . 1 - 1720  and the receptacle connector  14 . 1 - 1700 . In the example of  FIGS.  14 . 1 - 19 D through  14 . 1 - 19 F , one of the peripheral regions  14 . 1 - 1713  can be un-notched, and the other peripheral region  14 . 1 - 1713  can be notched, which reduces breakaway forces (e.g., reduces un-mating forces). The notched upper surface of the detent  14 . 1 - 1710 B can reduce a mating force for mating the plug connector  14 . 1 - 1720  in the receptacle connector  14 . 1 - 1700 . In other words the notched upper surface of the detent  14 . 1 - 1710 B can reduce the force require to move the protrusions  14 . 1 - 1724  past the detent  14 . 1 - 1710 B into the channel  14 . 1 - 1733 . Further, the peripheral regions  14 . 1 - 1713  can extend radially inward relative to the central region  14 . 1 - 1711  and can include protuberances or the like. In the example of  FIGS.  14 . 1 - 19 D through  14 . 1 - 19 F , protuberances can be included in both of the peripheral regions  14 . 1 - 1713 . This can increase mating and/or un-mating forces of the plug connector  14 . 1 - 1720  relative to the receptacle connector  14 . 1 - 1700 . 
       FIGS.  14 . 1 - 19 F  shows a cross-sectional view of the protrusion  14 . 1 - 1724  of the plug connector  14 . 1 - 1720  disposed within the channel  14 . 1 - 1733  and interfacing the detent  14 . 1 - 1710 B taken along reference line A-A′ of  FIGS.  14 . 1 - 19 D . In some examples, the detent  14 . 1 - 1710 B can define the channel  14 . 1 - 1733  such that a first surface  14 . 1 - 1737  of the channel  14 . 1 - 1733  is angled relative to a second surface  14 . 1 - 1738  of the channel  14 . 1 - 1733  to form an angle  14 . 1 - 04 . The protrusion  14 . 1 - 1714  can engage or contact the first surface  14 . 1 - 1737  while disposed within the channel  14 . 1 - 1733 . The angle  14 . 1 - 04  can directly correlate to a force required to overcome a biasing element coupled to the detent  14 . 1 - 1710 B to cause the detent  14 . 1 - 1710 B to move away from the central member  14 . 1 - 1712  and enable extraction of the protrusion  14 . 1 - 1724  from the channel  14 . 1 - 1733  (e.g., enabling extraction of the plug connector  14 . 1 - 1720  from the receptacle connector  14 . 1 - 1700 ). For example, a relatively smaller angle  14 . 1 - 04  can require a relatively greater force to extract the plug connector  14 . 1 - 1720  from the receptacle connector  14 . 1 - 1700 . Alternatively, a relatively larger angle  14 . 1 - 04  can require relatively smaller force to extract the plug connector  14 . 1 - 1720  from the receptacle connector  14 . 1 - 1700 . The angle  14 . 1 - 04  can be about 30 degrees, between about 30 degrees and about 60 degrees, between about 60 degrees and about 90 degrees, between about 90 degrees and about 120 degrees, between about 120 degrees and about 150 degrees, between about 150 degrees and about 180 degrees, or less than 180 degrees. A third surface  14 . 1 - 1736  adjacent the first surface  14 . 1 - 1737  can extend in a direction parallel to the second surface  14 . 1 - 1738 . 
     In some examples, a top surface of the detent  14 . 1 - 1710 B is also notched. As such, a fourth surface  14 . 1 - 1734  of the detent  14 . 1 - 1710 B is angled relative to the third surface  14 . 1 - 1736  of the detent  14 . 1 - 1710 B to form an angle. The protrusion  14 . 1 - 1714  can engage or contact the fourth surface  14 . 1 - 1734  as the protrusion  14 . 1 - 1714  is inserted into the channel  14 . 1 - 1733 . The angle can directly correlate to a force required to overcome a biasing element coupled to the detent  14 . 1 - 1710 B to cause the detent  14 . 1 - 1710 B to move away from the central member  14 . 1 - 1712  and enable insertion of the protrusion  14 . 1 - 1724  into the channel  14 . 1 - 1733  (e.g., enabling insertion of the plug connector  14 . 1 - 1720  from the receptacle connector  14 . 1 - 1700 ). For example, a relatively smaller angle can require a relatively greater force to extract the plug connector  14 . 1 - 1720  from the receptacle connector  14 . 1 - 1700 . Alternatively, a relatively larger angle can require relatively smaller force to extract the plug connector  14 . 1 - 1720  from the receptacle connector  14 . 1 - 1700 . The angle can be about 30 degrees, between about 30 degrees and about 60 degrees, between about 60 degrees and about 90 degrees, between about 90 degrees and about 120 degrees, between about 120 degrees and about 150 degrees, between about 150 degrees and about 180 degrees, or less than 180 degrees. 
       FIGS.  14 . 1 - 19 G and  14 . 1 - 19 H  illustrate a detent  14 . 1 - 1710 C that includes a central region  14 . 1 - 1711  and a peripheral region  14 . 1 - 1713  on one side of the central region  14 . 1 - 1711 . The central region  14 . 1 - 1711  can be notched to form a channel  14 . 1 - 1733  that can receive a protrusion  14 . 1 - 1724  of a plug connector  14 . 1 - 1720 . The central region  14 . 1 - 1711  can also be notched to form a notched top surface. For example, a lower portion of the central region  14 . 1 - 1711  can be notched to form an undercut portion of the detent  14 . 1 - 1710 C in the central region  14 . 1 - 1711 . Further, an upper portion of the central region  14 . 1 - 1711  can be notched to form a sloped upper surface of the detent  14 . 1 - 1710 C in the central region  14 . 1 - 1711 . The notches can be omitted from the peripheral region  14 . 1 - 1713 , which can prevent, limit, or reduce rotation of the plug connector  14 . 1 - 1720 . Including the un-notched peripheral region  14 . 1 - 1713  can also increase mating forces and un-mating forces between the plug connector  14 . 1 - 1720  and the receptacle connector  14 . 1 - 1700 . In the example of  FIGS.  14 . 1 - 19 G and  14 . 1 - 19 H , the peripheral region  14 . 1 - 1713  can be un-notched, which can increase mating and/or un-mating forces between the plug connector  14 . 1 - 1720  and the receptacle connector  14 . 1 - 1700 . The notched upper surface of the detent  14 . 1 - 1710 C can reduce a mating force for mating the plug connector  14 . 1 - 1720  in the receptacle connector  14 . 1 - 1700 . In other words, the notched upper surface of the detent  14 . 1 - 1710 C can reduce the force require to move the protrusions  14 . 1 - 1724  past the detent  14 . 1 - 1710 C into the channel  14 . 1 - 1733 . Further, the peripheral region  14 . 1 - 1713  can extend radially inward relative to the central region  14 . 1 - 1711  and can include a protuberance or the like. In the example of  FIGS.  14 . 1 - 19 G and  14 . 1 - 19 H , a protuberance can be included in the un-notched peripheral region  14 . 1 - 1713  of the detent  14 . 1 - 1710 C. This can increase mating and/or un-mating forces of the plug connector  14 . 1 - 1720  relative to the receptacle connector  14 . 1 - 1700 . A cross-sectional view of the detent  14 . 1 - 1710 C taken along cross-section A-A′ of  FIGS.  14 . 1 - 19 G  can be the same as or similar to the cross-sectional view of  FIGS.  14 . 1 - 19 F , discussed above. 
       FIGS.  14 . 1 - 19 I through  14 . 1 - 19 K  illustrate a detent  14 . 1 - 1710 D that includes a central region  14 . 1 - 1711  and a peripheral region  14 . 1 - 1713  on one side of the central region  14 . 1 - 1711 . The central region  14 . 1 - 1711  can be notched to form a channel  14 . 1 - 1733  that can receive a protrusion  14 . 1 - 1724  of a plug connector  14 . 1 - 1720 . For example, a lower portion of the central region  14 . 1 - 1711  can be notched to form an undercut portion of the detent  14 . 1 - 1710 D in the central region  14 . 1 - 1711 . Further, an upper portion of the central region  14 . 1 - 1711  and the peripheral region  14 . 1 - 1713  can be notched to form a sloped upper surface of the detent  14 . 1 - 1710 D in the central region  14 . 1 - 1711  and the peripheral region  14 . 1 - 1713 . In the example of  FIGS.  14 . 1 - 19 I through  14 . 1 - 19 K , the notched portions of the detent  14 . 1 - 1710 D can reduce mating and/or un-mating forces of the plug connector  14 . 1 - 1720  relative to the receptacle connector  14 . 1 - 1700 . The peripheral region  14 . 1 - 1713  can extend radially inward relative to the central region  14 . 1 - 1711  and can include a protuberance or the like. In the example of  FIGS.  14 . 1 - 19 I through  14 . 1 - 19 K , a protuberance can be included in the peripheral region  14 . 1 - 1713  with the un-notched lower portion and the notched upper portion. This can increase mating and/or un-mating forces of the plug connector  14 . 1 - 1720  relative to the receptacle connector  14 . 1 - 1700 . 
       FIGS.  14 . 1 - 19 K  shows a cross-sectional view of the protrusion  14 . 1 - 1724  of the plug connector  14 . 1 - 1720  disposed within the channel  14 . 1 - 1733  and interfacing the detent  14 . 1 - 1710 D taken along reference line A-A′ of  FIGS.  14 . 1 - 19 I . In some examples, the detent  14 . 1 - 1710 D can define the channel  14 . 1 - 1733  such that a first surface  14 . 1 - 1744  of the channel  14 . 1 - 1733  is angled relative to a second surface  14 . 1 - 1746  of the channel  14 . 1 - 1733  to form an angle  14 . 1 - 05 . The protrusion  14 . 1 - 1714  can engage or contact the first surface  14 . 1 - 1744  while disposed within the channel  14 . 1 - 1733 . The angle  14 . 1 - 05  can directly correlate to a force required to overcome a biasing element coupled to the detent  14 . 1 - 1710 D to cause the detent  14 . 1 - 1710 D to move away from the central member  14 . 1 - 1712  and enable extraction of the protrusion  14 . 1 - 1724  from the channel  14 . 1 - 1733  (e.g., enabling extraction of the plug connector  14 . 1 - 1720  from the receptacle connector  14 . 1 - 1700 ). For example, a relatively smaller angle  14 . 1 - 05  can require a relatively greater force to extract the plug connector  14 . 1 - 1720  from the receptacle connector  14 . 1 - 1700 . Alternatively, a relatively larger angle  14 . 1 - 05  can require relatively smaller force to extract the plug connector  14 . 1 - 1720  from the receptacle connector  14 . 1 - 1700 . The angle  14 . 1 - 05  can be about 30 degrees, between about 30 degrees and about 60 degrees, between about 60 degrees and about 90 degrees, between about 90 degrees and about 120 degrees, between about 120 degrees and about 150 degrees, between about 150 degrees and about 180 degrees, or less than 180 degrees. A third surface  14 . 1 - 1742  adjacent the first surface  14 . 1 - 1744  can extend in a direction parallel to the second surface  14 . 1 - 1746 . 
     In some examples, a top surface of the detent  14 . 1 - 1710 D is also notched. As such, a fourth surface  14 . 1 - 1740  of the detent  14 . 1 - 1710 D is angled relative to the third surface  14 . 1 - 1742  of the detent  14 . 1 - 1710 D to form an angle  14 . 1 - 06 . The protrusion  14 . 1 - 1714  can engage or contact the fourth surface  14 . 1 - 1740  as the protrusion  14 . 1 - 1714  is inserted into the channel  14 . 1 - 1733 . The angle  14 . 1 - 06  can directly correlate to a force required to overcome a biasing element coupled to the detent  14 . 1 - 1710 D to cause the detent  14 . 1 - 1710 D to move away from the central member  14 . 1 - 1712  and enable insertion of the protrusion  14 . 1 - 1724  into the channel  14 . 1 - 1733  (e.g., enabling insertion of the plug connector  14 . 1 - 1720  from the receptacle connector  14 . 1 - 1700 ). For example, a relatively smaller angle  14 . 1 - 06  can require a relatively greater force to extract the plug connector  14 . 1 - 1720  from the receptacle connector  14 . 1 - 1700 . Alternatively, a relatively larger angle  14 . 1 - 06  can require relatively smaller force to extract the plug connector  14 . 1 - 1720  from the receptacle connector  14 . 1 - 1700 . The angle  14 . 1 - 06  can be about 30 degrees, between about 30 degrees and about 60 degrees, between about 60 degrees and about 90 degrees, between about 90 degrees and about 120 degrees, between about 120 degrees and about 150 degrees, between about 150 degrees and about 180 degrees, or less than 180 degrees. 
       FIGS.  14 . 1 - 19 L and  14 . 1 - 19 M  illustrate a detent  14 . 1 - 1710 E that includes a central region  14 . 1 - 1711  and a peripheral region  14 . 1 - 1713  on one side of the central region  14 . 1 - 1711 . The central region  14 . 1 - 1711  can be notched to form a channel  14 . 1 - 1733  that can receive a protrusion  14 . 1 - 1724  of a plug connector  14 . 1 - 1720 . For example, a lower portion of the central region  14 . 1 - 1711  can be notched to form an undercut portion of the detent  14 . 1 - 1710 E in the central region  14 . 1 - 1711 . The notches can be omitted from lower portion of the peripheral regions  14 . 1 - 1713 , which can prevent, limit, or reduce rotation of the plug connector  14 . 1 - 1720 . Including the un-notched peripheral regions  14 . 1 - 1713  can also increase mating forces and un-mating forces between the plug connector  14 . 1 - 1720  and the receptacle connector  14 . 1 - 1700 . The central region  14 . 1 - 1711  and the peripheral region  14 . 1 - 1713  can be notched to form a notched top surface. The notched upper surface of the detent  14 . 1 - 1710 E can reduce a mating force for mating the plug connector  14 . 1 - 1720  in the receptacle connector  14 . 1 - 1700 . In other words, the notched upper surface of the detent  14 . 1 - 1710 E can reduce the force require to move the protrusions  14 . 1 - 1724  past the detent  14 . 1 - 1710 E into the channel  14 . 1 - 1733 . A cross-sectional view of the detent  14 . 1 - 1710 E taken along cross-section A-A′ of  FIGS.  14 . 1 - 19 L  can be the same as or similar to the cross-sectional view of  FIGS.  14 . 1 - 19 F , discussed above. 
       FIGS.  14 . 1 - 20 A and  14 . 1 - 20 B  illustrate different configurations of a cable assembly  14 . 1 - 1800  relative to a receptacle connector  14 . 1 - 1802 .  FIGS.  14 . 1 - 20 A and  14 . 1 - 20 B  show a portion of the cable assembly  14 . 1 - 1800  inserted into the receptacle connector  14 . 1 - 1802 . The cable assembly  14 . 1 - 1800  can be substantially similar to, and can include some or all of, the features of any of the previously described cable assemblies. For example, the cable assembly  14 . 1 - 1800  can include a shielded cable  14 . 1 - 1803  and a plug connector  14 . 1 - 1804  having a boot  14 . 1 - 1806  and a central member  14 . 1 - 1808 A/ 14 . 1 - 1808 B. The central member  14 . 1 - 1808  can form an external surface  14 . 1 - 1810  and an internal surface  14 . 1 - 1812 . In some examples, the central member  14 . 1 - 1808  can include one or more protrusions  14 . 1 - 1814  extending laterally from, or substantially perpendicular to, the external surface  14 . 1 - 1810 . In some examples, the central member  14 . 1 - 1808  can form a cavity or volume, for example, the internal surface  14 . 1 - 1812  can form a cavity or volume. 
     The receptacle connector  14 . 1 - 1802  can be substantially similar to, and can include some or all of, the features of the previously described receptacle connectors. For example, the receptacle connector  14 . 1 - 1802  can include a trim ring  14 . 1 - 1816 , one or more detents  14 . 1 - 1818 A/ 14 . 1 - 1818 B/ 14 . 1 - 1818 C (collectively referred to as detents  14 . 1 - 1818 ), and a raised portion  14 . 1 - 1820 . The trim ring  14 . 1 - 1816  can form one or more undercut regions which house or retain the detents  14 . 1 - 1818 . While a central member  14 . 1 - 1808  is disposed within the receptacle connector  14 . 1 - 1802 , the protrusions  14 . 1 - 1814  can extend into respective channels defined by the detents  14 . 1 - 1818 . In some examples, each detent  14 . 1 - 1818  can be biased toward the raised portion  14 . 1 - 1820  by a biasing element. In other words, each detent  14 . 1 - 1818  can radially translate within their respective undercut regions toward and away from the raised portion  14 . 1 - 1820  to engage and disengage respective protrusions  14 . 1 - 1814 . 
     Each of the detents  14 . 1 - 1818  can include different configurations and different biasing elements to customize mating and un-mating forces for each of the detents  14 . 1 - 1818 . Specific configurations of the detents  14 . 1 - 1818  and the shielded cable  14 . 1 - 1803  relative to the detents  14 . 1 - 1818  can be used to provide different mating an un-mating forces, and to provide different breakaway forces. In the example of  FIGS.  14 . 1 - 20 A , the detents  14 . 1 - 1818 A and  14 . 1 - 1818 C are offset from the shielded cable  14 . 1 - 1803  (e.g., the shielded cable  14 . 1 - 1803  splits the detents  14 . 1 - 1818 A and  14 . 1 - 1818 B) distal the shielded cable  14 . 1 - 1803 , and the detent  14 . 1 - 1818 B is aligned with the shielded cable  14 . 1 - 1803  proximal the shielded cable  14 . 1 - 1803 . Protrusions  14 . 1 - 1814  distal the shielded cable  14 . 1 - 1803  are offset from the shielded cable  14 . 1 - 1803  (e.g., the shielded cable  14 . 1 - 1803  splits the protrusions  14 . 1 - 1814 ), and the protrusion  14 . 1 - 1814  proximal the shielded cable  14 . 1 - 1803  is aligned with the shielded cable  14 . 1 - 1803 . In the example of  FIGS.  14 . 1 - 20 B , the detents  14 . 1 - 1818 A and  14 . 1 - 1818 C are offset from the shielded cable  14 . 1 - 1803  (e.g., the shielded cable  14 . 1 - 1803  splits the detents  14 . 1 - 1818 A and  14 . 1 - 1818 B) proximal the shielded cable  14 . 1 - 1803 , and the detent  14 . 1 - 1818 B is aligned with the shielded cable  14 . 1 - 1803  distal the shielded cable  14 . 1 - 1803 . Protrusions  14 . 1 - 1814  proximal the shielded cable  14 . 1 - 1803  are offset from the shielded cable  14 . 1 - 1803  (e.g., the shielded cable  14 . 1 - 1803  splits the protrusions  14 . 1 - 1814 ), and the protrusion  14 . 1 - 1814  distal the shielded cable  14 . 1 - 1803  is aligned with the shielded cable  14 . 1 - 1803 . The detents  14 . 1 - 1818  can include any of the previously described detents, such as the detents described with respect to  FIGS.  14 . 1 - 19 A through  14 . 1 - 19 M , and the biasing elements can include any of the previously described biasing elements. Any combination of detents  14 . 1 - 1818  and biasing elements can be provided. 
       FIGS.  14 . 1 - 21 A and  14 . 1 - 21 B  illustrate examples in which the receptacle connector  14 . 1 - 1802  includes protrusions  14 . 1 - 1830  and the plug connector  14 . 1 - 1804  includes openings  14 . 1 - 1831  configured to receive the protrusions. The protrusions  14 . 1 - 1830  and the openings  14 . 1 - 1831  can be configured to ensure that the plug connector  14 . 1 - 1804  is properly aligned with and installed in a proper orientation relative to the receptacle connector  14 . 1 - 1802 .  FIGS.  14 . 1 - 21 A and  14 . 1 - 21 B  further illustrate protrusions  14 . 1 - 1832  in dashed lines. In some examples, the receptacle connector  14 . 1 - 1802  includes a single protrusion  14 . 1 - 1830 ; however, the receptacle connector  14 . 1 - 1802  can include the protrusions  14 . 1 - 1832  in addition to or in place of the protrusion  14 . 1 - 1830 , and the plug connector  14 . 1 - 1804  can include any number of the openings  14 . 1 - 1831  configured to receive the protrusion  14 . 1 - 1830  and/or the protrusions  14 . 1 - 1832 . In some examples, the receptacle connector  14 . 1 - 1802  can include one protrusion  14 . 1 - 1830 ; one protrusion  14 . 1 - 1830  and one protrusion  14 . 1 - 1832 ; one protrusion  14 . 1 - 1830  and two protrusions  14 . 1 - 1832 ; or a greater number of protrusions. 
       FIGS.  14 . 1 - 21 A and  14 . 1 - 21 B  illustrate alternative angular spacings of the protrusions  14 . 1 - 1830 / 14 . 1 - 1832  of the receptacle connector  14 . 1 - 1802 . In  FIGS.  14 . 1 - 21 A , the protrusions  14 . 1 - 1830 / 14 . 1 - 1832  are evenly spaced relative to one another, with angles  14 . 1 - 07 ,  14 . 1 - 08 , and  14 . 1 - 09  being about 60°. A protrusion  14 . 1 - 1830  and a protrusion  14 . 1 - 1832  distal the shielded cable  14 . 1 - 1803  can be offset from the shielded cable  14 . 1 - 1803  (e.g., the shielded cable  14 . 1 - 1803  splits the protrusion  14 . 1 - 1830  and the protrusion  14 . 1 - 1832 ), and a protrusion  14 . 1 - 1832  proximal the shielded cable  14 . 1 - 1803  can be aligned with the shielded cable  14 . 1 - 1803 . Any rotational configuration of the protrusions  14 . 1 - 1830 / 14 . 1 - 1832  relative to the shielded cable  14 . 1 - 1803  can be provided. In  FIGS.  14 . 1 - 21 B , the protrusions  14 . 1 - 1830 / 14 . 1 - 1832  are spaced at different intervals relative to one another. Angles  14 . 1 - 010 ,  14 . 1 - 011 , and  14 . 1 - 012  between adjacent protrusions  14 . 1 - 1830 / 14 . 1 - 1832  can be about 30 degrees, between about 30 degrees and about 60 degrees, between about 60 degrees and about 90 degrees, between about 90 degrees and about 120 degrees, between about 120 degrees and about 150 degrees, between about 150 degrees and about 180 degrees, or less than 180 degrees. A protrusion  14 . 1 - 1830  and a protrusion  14 . 1 - 1832  distal the shielded cable  14 . 1 - 1803  can be offset from the shielded cable  14 . 1 - 1803  (e.g., the shielded cable  14 . 1 - 1803  splits the protrusion  14 . 1 - 1830  and the protrusion  14 . 1 - 1832 ), and a protrusion  14 . 1 - 1832  proximal the shielded cable  14 . 1 - 1803  can also be offset from the shielded cable  14 . 1 - 1803 . Any rotational configuration of the protrusions  14 . 1 - 1830 / 14 . 1 - 1832  relative to the shielded cable  14 . 1 - 1803  can be provided. Various configurations of the protrusions  14 . 1 - 1830 / 14 . 1 - 1832  can be used to ensure proper alignment of the plug connector  14 . 1 - 1804  with the receptacle connector  14 . 1 - 1802  and can also be used to ensure that incompatible plug connectors are not inadvertently installed in the receptacle connector  14 . 1 - 1802 . 
       FIGS.  14 . 1 - 22 A through  14 . 1 - 22 E  show cross-sectional side views of a plug connector  14 . 1 - 1904  of a cable assembly  14 . 1 - 1900  disposed within a receptacle connector  14 . 1 - 1902 . The plug connector  14 . 1 - 1904  includes a central member  14 . 1 - 1908  with a protrusion  14 . 1 - 1914 . The protrusion  14 . 1 - 1914  engages a channel of a detent  14 . 1 - 1918 . The detent  14 . 1 - 1918  is biased towards a raised portion  14 . 1 - 1920  of the receptacle connector  14 . 1 - 1902  by a biasing element  14 . 1 - 1924 . The biasing element  14 . 1 - 1924  and the detent  14 . 1 - 1918  are disposed within a trim ring  14 . 1 - 1916 , which is coupled to a base  14 . 1 - 1930  of the receptacle connector  14 . 1 - 1902 . An external surface  14 . 1 - 1910  of the central member  14 . 1 - 1908  includes the protrusion  14 . 1 - 1914  and is configured to engage the detent  14 . 1 - 1918 . An internal surface  14 . 1 - 1912  of the central member  14 . 1 - 1908  includes an electrical contact  14 . 1 - 1919  and is electrically coupled to the raised portion  14 . 1 - 1920  of the receptacle connector  14 . 1 - 1902 . 
       FIGS.  14 . 1 - 22 A through  14 . 1 - 22 E  show various seals  14 . 1 - 1932  disposed between the central member  14 . 1 - 1908  and the receptacle connector  14 . 1 - 1902  at various locations to prevent ingress of contaminants into the receptacle connector  14 . 1 - 1902 , such as, fluids, dust, liquids, and other materials that can degrade or impede the functionality of the plug connector  14 . 1 - 1904  and the receptacle connector  14 . 1 - 1902 . The seal  14 . 1 - 1932  can be affixed to any surface of the plug connector  14 . 1 - 1904  and/or the receptacle connector  14 . 1 - 1902 . For example, the seal  14 . 1 - 1932  can be molded, adhered, welded, fastened, or otherwise secured to a surface of the plug connector  14 . 1 - 1904  and/or the receptacle connector  14 . 1 - 1902 . 
       FIGS.  14 . 1 - 22 A  shows a seal  14 . 1 - 1932 A disposed between the base  14 . 1 - 1930  and the central member  14 . 1 - 1908 . As illustrated in  FIGS.  14 . 1 - 22 A , an opening  14 . 1 - 1934 A can be provided in a bottom portion of the central member  14 . 1 - 1908 , and the seal  14 . 1 - 1932 A can be disposed in the opening  14 . 1 - 1934 A in the central member  14 . 1 - 1908 . 
       FIGS.  14 . 1 - 22 B  shows a seal  14 . 1 - 1932 B disposed between the base  14 . 1 - 1930  and the central member  14 . 1 - 1908 . As illustrated in  FIGS.  14 . 1 - 22 B , an opening  14 . 1 - 1934 B can be provided in a portion of the base  14 . 1 - 1930  adjacent a bottom portion of the central member  14 . 1 - 1908 , and the seal  14 . 1 - 1932 B can be disposed in the opening  14 . 1 - 1934 B in the base  14 . 1 - 1930 . 
       FIGS.  14 . 1 - 22 C  shows a seal  14 . 1 - 1932 C disposed between the raised portion  14 . 1 - 1920  and the central member  14 . 1 - 1908 . As illustrated in  FIGS.  14 . 1 - 22 C , an opening  14 . 1 - 1934 C can be provided in a portion of the raised portion  14 . 1 - 1920  adjacent a bottom portion of the central member  14 . 1 - 1908 , and the seal  14 . 1 - 1932 C can be disposed in the opening  14 . 1 - 1934 C in the raised portion  14 . 1 - 1920 . 
       FIGS.  14 . 1 - 22 D  shows a seal  14 . 1 - 1932 D disposed between the trim ring  14 . 1 - 1916  and the central member  14 . 1 - 1908 . As illustrated in  FIGS.  14 . 1 - 22 D , the seal  14 . 1 - 1932 D can be coupled to the central member  14 . 1 - 1908  between the central member  14 . 1 - 1908  and the trim ring  14 . 1 - 1916 . The seal  14 . 1 - 1932 D can include a protrusion that interacts with the trim ring  14 . 1 - 1916  to provide sealing between the central member  14 . 1 - 1908  and the trim ring  14 . 1 - 1916 . 
       FIGS.  14 . 1 - 22 E  shows a seal  14 . 1 - 1932 E disposed between the trim ring  14 . 1 - 1916  and the central member  14 . 1 - 1908 . As illustrated in  FIGS.  14 . 1 - 22 E , the seal  14 . 1 - 1932 E can be coupled to the central member  14 . 1 - 1908  between the central member  14 . 1 - 1908  and the trim ring  14 . 1 - 1916 . The seal  14 . 1 - 1932 E can include an opening, which gives flexibility to the seal  14 . 1 - 1932 E as the seal  14 . 1 - 1932 E interacts with the trim ring  14 . 1 - 1916  to provide sealing between the central member  14 . 1 - 1908  and the trim ring  14 . 1 - 1916 . While  FIGS.  14 . 1 - 22 A through  14 . 1 - 22 E  only depict a single seal  14 . 1 - 1932  in each example disposed between the central member  14 . 1 - 1908  and the receptacle connector  14 . 1 - 1902 , other examples can include multiple seals  14 . 1 - 1932 , each disposed at one of a plurality of positions within the receptacle connector  14 . 1 - 1902  and/or the central member  14 . 1 - 1908 . 
       FIGS.  14 . 1 - 23 A through  14 . 1 - 23 G  illustrate various configurations of seals  14 . 1 - 2002  that can be included between a raised portion  14 . 1 - 2000  and a base  14 . 1 - 2006  of a receptacle connector and a central member  14 . 1 - 2008  of a plug connector. The seals  14 . 1 - 2002  can be disposed at various locations and can include different sizes and shapes to prevent ingress of contaminants into the receptacle connector, such as, fluids, dust, liquids, and other materials that can degrade or impede the functionality of the plug connector and the receptacle connector. The seals  14 . 1 - 2002  can be affixed to any surface of the plug connector and/or the receptacle connector. For example, the seals  14 . 1 - 2002  can be molded, adhered, welded, fastened, or otherwise secured to a surface of the plug connector and/or the receptacle connector. 
       FIGS.  14 . 1 - 23 A  shows a seal  14 . 1 - 2002 A disposed vertically between the base  14 . 1 - 2006  and the raised portion  14 . 1 - 2000  and horizontally between the raised portion  14 . 1 - 2000  and the central member  14 . 1 - 2008 . The seal  14 . 1 - 2002 A can include a triangular portion that interfaces with the central member  14 . 1 - 2008 . An opening  14 . 1 - 2004 A can be provided in the raised portion  14 . 1 - 2000  and the seal  14 . 1 - 2002 A can be disposed in the opening  14 . 1 - 2004 A. The seal  14 . 1 - 2002 A can include a protrusion  14 . 1 - 2012 A that interfaces with an opening  14 . 1 - 2010 A of the base  14 . 1 - 2006 . The protrusion  14 . 1 - 2012 A can be generally rectangular. The seal  14 . 1 - 2002 A can be attached to the raised portion  14 . 1 - 2000 . The seal  14 . 1 - 2002 A can further be attached to the base  14 . 1 - 2006  through the protrusion  14 . 1 - 2012 A. 
       FIGS.  14 . 1 - 23 B  shows a seal  14 . 1 - 2002 B disposed vertically between the base  14 . 1 - 2006  and the raised portion  14 . 1 - 2000  and horizontally between the raised portion  14 . 1 - 2000  and the central member  14 . 1 - 2008 . The seal  14 . 1 - 2002 B can be generally L-shaped, with a rectangular portion interfacing with the central member  14 . 1 - 2008 . An opening  14 . 1 - 2004 B can be provided in the raised portion  14 . 1 - 2000  and the seal  14 . 1 - 2002 B can be disposed in the opening  14 . 1 - 2004 B. The seal  14 . 1 - 2002 B can be attached to the raised portion  14 . 1 - 2000 . 
       FIGS.  14 . 1 - 23 C  shows a seal  14 . 1 - 2002 C disposed vertically between the base  14 . 1 - 2006  and the raised portion  14 . 1 - 2000  and horizontally between the raised portion  14 . 1 - 2000  and the central member  14 . 1 - 2008 . The seal  14 . 1 - 2002 C can include a rounded portion that interfaces with the central member  14 . 1 - 2008 . An opening  14 . 1 - 2004 C can be provided in the raised portion  14 . 1 - 2000  and the seal  14 . 1 - 2002 C can be disposed in the opening  14 . 1 - 2004 C. The seal  14 . 1 - 2002 C can include a protrusion  14 . 1 - 2012 C that interfaces with an opening  14 . 1 - 2010 C of the base  14 . 1 - 2006 . The protrusion  14 . 1 - 2012 C can be generally rounded. The seal  14 . 1 - 2002 C can be generally D-shaped. The seal  14 . 1 - 2002 C can be attached to the raised portion  14 . 1 - 2000 . The seal  14 . 1 - 2002 C can further be attached to the base  14 . 1 - 2006  through the protrusion  14 . 1 - 2012 C. 
       FIGS.  14 . 1 - 23 D  shows a seal  14 . 1 - 2002 D disposed vertically between the base  14 . 1 - 2006  and the raised portion  14 . 1 - 2000  and horizontally between the raised portion  14 . 1 - 2000  and the central member  14 . 1 - 2008 . The seal  14 . 1 - 2002 D can further extend vertically below a top surface of the base  14 . 1 - 2006  and laterally adjacent the base  14 . 1 - 2006  in a direction opposite the central member  14 . 1 - 2008 . The seal  14 . 1 - 2002 D can include a rounded/generally rectangular portion that interfaces with the central member  14 . 1 - 2008 . An opening  14 . 1 - 2004 D can be provided in the raised portion  14 . 1 - 2000  and the seal  14 . 1 - 2002 D can be disposed in the opening  14 . 1 - 2004 A. The seal  14 . 1 - 2002 A can include a protrusion  14 . 1 - 2012 D that interfaces with an opening  14 . 1 - 2010 D of the base  14 . 1 - 2006 . The opening  14 . 1 - 2010 D of the base can be laterally opposite the central member  14 . 1 - 2008 . The seal  14 . 1 - 2002 D can further include an opening  14 . 1 - 2014 D disposed therein, which can be generally U-shaped. The opening  14 . 1 - 2014 D can provide additional flexibility to the seal  14 . 1 - 2002 D and can improve sealing between the central member  14 . 1 - 2008  and the receptacle connector. The seal  14 . 1 - 2002 D can be attached to the base  14 . 1 - 2006  through the protrusion  14 . 1 - 2012 D in the opening  14 . 1 - 2010 D. The seal  14 . 1 - 2002 D can further be attached to the raised portion  14 . 1 - 2000 . 
       FIGS.  14 . 1 - 23 E  shows a seal  14 . 1 - 2002 E disposed vertically between the base  14 . 1 - 2006  and the raised portion  14 . 1 - 2000  and horizontally between the raised portion  14 . 1 - 2000  and the central member  14 . 1 - 2008 . The seal  14 . 1 - 2002 E can include a rounded portion that interfaces with the central member  14 . 1 - 2008 . An opening  14 . 1 - 2004 E can be provided in the raised portion  14 . 1 - 2000  and the seal  14 . 1 - 2002 E can be disposed in the opening  14 . 1 - 2004 E. The seal  14 . 1 - 2002 E can be generally stadium-shaped, or rectangular with rounded corners. The seal  14 . 1 - 2002 E can be attached to the raised portion  14 . 1 - 2000 . The seal  14 . 1 - 2002 E can further be attached to the base  14 . 1 - 2006 . 
       FIGS.  14 . 1 - 23 F  shows a seal  14 . 1 - 2002 F disposed vertically between the base  14 . 1 - 2006  and the raised portion  14 . 1 - 2000  and horizontally between the raised portion  14 . 1 - 2000  and the central member  14 . 1 - 2008 . The seal  14 . 1 - 2002 F can include a rounded portion that interfaces with the central member  14 . 1 - 2008 . An opening  14 . 1 - 2004 F can be provided in the raised portion  14 . 1 - 2000  and the seal  14 . 1 - 2002 F can be disposed in the opening  14 . 1 - 2004 F. The seal  14 . 1 - 2002 F can be generally stadium-shaped, or rectangular with rounded corners. In the example of  FIGS.  14 . 1 - 23 F , corners of the seal  14 . 1 - 2002 F proximal the central member  14 . 1 - 2008  are more rounded relative to corners of the seal  14 . 1 - 2002 F distal the central member  14 . 1 - 2008 . The seal  14 . 1 - 2002 F can be attached to the raised portion  14 . 1 - 2000 . The seal  14 . 1 - 2002 F can further be attached to the base  14 . 1 - 2006 . 
       FIGS.  14 . 1 - 23 G  shows a seal  14 . 1 - 2002 G disposed vertically between the base  14 . 1 - 2006  and the raised portion  14 . 1 - 2000  and horizontally between the raised portion  14 . 1 - 2000  and the central member  14 . 1 - 2008 . The seal  14 . 1 - 2002 G can include a rounded portion that interfaces with the central member  14 . 1 - 2008 . An opening  14 . 1 - 2004 G can be provided in the raised portion  14 . 1 - 2000  and the seal  14 . 1 - 2002 G can be disposed in the opening  14 . 1 - 2004 G. The seal  14 . 1 - 2002 G can be generally stadium-shaped, or rectangular with rounded corners. In the example of  FIGS.  14 . 1 - 23 G , the seal  14 . 1 - 2002 E further includes an opening  14 . 1 - 2014 G in a bottom surface thereof, adjacent to the base  14 . 1 - 2006 . The opening  14 . 1 - 2014 G can provide improved flexibility to the seal  14 . 1 - 2002 G. The seal  14 . 1 - 2002 G can be attached to the raised portion  14 . 1 - 2000 . The seal  14 . 1 - 2002 G can further be attached to the base  14 . 1 - 2006 . 
     While  FIGS.  14 . 1 - 23 A through  14 . 1 - 23 G  only depict a single seal  14 . 1 - 2002  in each example disposed between the central member  14 . 1 - 2008  and the raised portion  14 . 1 - 2000  and the base  14 . 1 - 2006  of a receptacle connector, other examples can include multiple seals, each disposed at one of a plurality of positions within the receptacle connector and/or the central member. The seals  14 . 1 - 2002  of  FIGS.  14 . 1 - 23 A through  14 . 1 - 23 G  can be used in addition to or in place of any of the seals discussed above, such as the seals discussed with respect to  FIGS.  14 . 1 - 22 A through  14 . 1 - 22 E . 
       FIGS.  14 . 1 - 24 A and  14 . 1 - 24 B  show exploded views of a receptacle connector  14 . 1 - 2100 . The receptacle connector  14 . 1 - 2100  includes a base  14 . 1 - 2102 , a raised portion  14 . 1 - 2110 , mounting clips  14 . 1 - 2114 , a lower mounting ring  14 . 1 - 2116 , and an upper mounting ring  14 . 1 - 2122 . The mounting clips  14 . 1 - 2114 , the lower mounting ring  14 . 1 - 2116 , and the upper mounting ring  14 . 1 - 2122  can affix the raised portion  14 . 1 - 2110  to the base  14 . 1 - 2102 . For example, the upper mounting ring  14 . 1 - 2122  and the lower mounting ring  14 . 1 - 2116  can be coupled to the base  14 . 1 - 2102 . The base  14 . 1 - 2102  can include protrusions  14 . 1 - 2106  and a raised electronic component  14 . 1 - 2104 . The upper mounting ring  14 . 1 - 2122  can include openings  14 . 1 - 2124  configured to receive the protrusions  14 . 1 - 2106  of the base  14 . 1 - 2102 . The lower mounting ring  14 . 1 - 2116  can include openings  14 . 1 - 2120  configured to receive the protrusions  14 . 1 - 2106  of the base  14 . 1 - 2102 . The openings  14 . 1 - 2124  and the openings  14 . 1 - 2120  can have shapes corresponding to shapes of the protrusions  14 . 1 - 2106  (e.g., capsule shapes, rounded shapes, rectangular shapes, or the like). The lower mounting ring can further include an opening  14 . 1 - 2118  configured to receive the raised electronic component  14 . 1 - 2104  of the base  14 . 1 - 2102 . The opening  14 . 1 - 2118  can have a shape corresponding to a shape of the raised electronic component  14 . 1 - 2104  (e.g., a capsule shape, a rounded shape, a rectangular shape, or the like). 
     The mounting clips  14 . 1 - 2114  can be configured to receive a raised component  14 . 1 - 2112  of the raised portion  14 . 1 - 2110 . Each of the mounting clips  14 . 1 - 2114  can be generally L-shaped, with an outer surface of the mounting clips  14 . 1 - 2114  being rounded. Central portions of the mounting clips  14 . 1 - 2114  can receive the raised component  14 . 1 - 2112 . The lower mounting ring  14 . 1 - 2116  and the upper mounting ring  14 . 1 - 2122  can be affixed to the base  14 . 1 - 2102 , and the mounting clips  14 . 1 - 2114  can affix the raised component  14 . 1 - 2112  of the raised portion  14 . 1 - 2110  to the lower mounting ring  14 . 1 - 2116 , the upper mounting ring  14 . 1 - 2122 , and the base  14 . 1 - 2102 . The upper mounting ring  14 . 1 - 2122  and the lower mounting ring  14 . 1 - 2116  can be affixed to the base  14 . 1 - 2102 , the mounting clips  14 . 1 - 2114  can be affixed to the raised component  14 . 1 - 2112 , and the raised portion  14 . 1 - 2110  can be affixed to the base  14 . 1 - 2102  by adhesives, adhesive tapes, fasteners, welding, a combination thereof, or any other mechanism from permanently or temporarily securing the various components to one another. 
     In the example of  FIGS.  14 . 1 - 24 B , the mounting clips  14 . 1 - 2114  are replaced by mounting clips  14 . 1 - 2126 . The mounting clips  14 . 1 - 2126  interface with openings  14 . 1 - 2128  in the lower mounting ring  14 . 1 - 2116  to attach the raised portion  14 . 1 - 2110  to the base  14 . 1 - 2102  through the lower mounting ring  14 . 1 - 2116 . 
       FIGS.  14 . 1 - 25 A and  14 . 1 - 25 B  show exploded views of a receptacle connector  14 . 1 - 2200 . The receptacle connector  14 . 1 - 2200  includes a base  14 . 1 - 2210  and a raised portion that includes a body  14 . 1 - 2204  and a cap  14 . 1 - 2202 . The base includes a contact tab  14 . 1 - 2212  with contacts  14 . 1 - 2214  disposed in an opening  14 . 1 - 2218  in a raised portion  14 . 1 - 2216  of the base  14 . 1 - 2210 . In the example of  FIGS.  14 . 1 - 25 A , the contact tab  14 . 1 - 2212  is relatively wider and includes a greater number of contacts, such as six contacts. In the example of  FIGS.  14 . 1 - 25 B , the contact tab  14 . 1 - 2212  is relatively narrower and includes a lower number of contacts, such as three contacts. Any number of contacts and appropriate width of the contact tab  14 . 1 - 2212  can be provided. Providing the contact tab  14 . 1 - 2212  with a greater width and a greater number of contacts improves the strength of the connection between the body  14 . 1 - 2204  and the base  14 . 1 - 2210  and provides for more electrical connections between the receptacle connector  14 . 1 - 2200  and other electronic components, such as a plug connector. 
       FIGS.  14 . 1 - 25 A and  14 . 1 - 25 B  further illustrate different configurations of contacts  14 . 1 - 2206  included in an opening  14 . 1 - 2208  in the body  14 . 1 - 2204 . In the example of  FIGS.  14 . 1 - 25 A , four contacts  14 . 1 - 2206  are included in the opening  14 . 1 - 2208  in the body  14 . 1 - 2204 . In the example of  FIGS.  14 . 1 - 25 B , two contacts  14 . 1 - 2206  are included in the opening  14 . 1 - 2208  in the body  14 . 1 - 2204 . Any number of contacts  14 . 1 - 2206  can be provided. Providing a greater number of the contacts  14 . 1 - 2206  in the body  14 . 1 - 2204  provides for more electrical connections between the receptacle connector  14 . 1 - 2200  and other electronic components, such as a plug connector. 
       FIGS.  14 . 1 - 26 A through  14 . 1 - 35 B  illustrate various examples of receptacle connectors and plug connectors that can be used as the plug connectors  14 . 1 - 144  of the cable assembly  14 . 1 - 140  and the receptacle connectors  14 . 1 - 132  of the second electronic device  14 . 1 - 130  illustrated in  FIGS.  14 . 1 - 1 A and  14 . 1 - 1 B . 
       FIGS.  14 . 1 - 26 A  shows an electronic device  14 . 1 - 2300  having a housing  14 . 1 - 2302  forming an external surface  14 . 1 - 2304  of the electronic device  14 . 1 - 2300 . The housing  14 . 1 - 2302  can form or define one or more apertures  14 . 1 - 2306 ,  14 . 1 - 2308 . Each of the one or more apertures  14 . 1 - 2306 ,  14 . 1 - 2308  can receive a connector of an electrical cord or cable (see cable  14 . 1 - 2310  of  FIGS.  14 . 1 - 26 B ) to electrically couple the electronic device  14 . 1 - 2300  to another electronic device (not shown) and/or a power source (e.g., a wall-mounted electrical outlet). 
     The electronic device  14 . 1 - 2300  can be a smart phone, a laptop, a tablet computing device, a smart watch, a head-mounted display (HMD), a pair of headphones, a combination thereof, or any other electronic device. Alternatively, or additionally, the electronic device  14 . 1 - 2300  can be a sub-component or an accessory to one or more of the electronic devices previously listed above, such as, an external power supply or electrical power source, a display, a user interface, an audio component or speaker, or a combination thereof. As such, the electronic device  14 . 1 - 2300  can be electrically and/or communicatively coupled to one or more other electronic devices (not shown) via electrical receptacle connectors or ports disposed within the one or more apertures  14 . 1 - 2306 ,  14 . 1 - 2308 . 
     The housing  14 . 1 - 2302  can define a cavity or volume wherein one or more electronic components can be disposed. For example, the one or more electrical components disposed within the housing  14 . 1 - 2302  can be a battery (e.g., a lithium battery pack), a processor, a wireless communication module, one or more antennas, or any other component or assembly housed within portable electronic devices. In some examples, the one or more electronic components can be electrically coupled to a receptacle connector (see receptacle connector  2424  in  FIGS.  14 . 1 - 27 A ) accessible by a cable or cord inserted into one of the apertures  14 . 1 - 2306 ,  14 . 1 - 2308 . For example, the one or more electronic components can provide electrical power to the receptacle connector and the cable or cord can transfer the electrical power to another electronic device (i.e., the electronic device  14 . 1 - 2300  can be a power source or external battery electrically coupled to a portable electronic device). The housing  14 . 1 - 2302  can be molded, machined, cast, stamped, or otherwise assembled from a metal, polymer, ceramic, or combination thereof. 
     The aperture  14 . 1 - 2306  can be formed within or defined by the external surface  14 . 1 - 2304  of the housing  14 . 1 - 2302 . The aperture  14 . 1 - 2306  can be sized and shaped to receive and retain a plug connector of a cable (see cable  14 . 1 - 2310  in  FIGS.  14 . 1 - 26 B ). In some examples, a cable operably coupled to a receptacle connector within the aperture  14 . 1 - 2306  can transfer electrical power to the electronic device  14 . 1 - 2300  while the plug connector is disposed within the aperture  14 . 1 - 2306 . In other words, a receptacle connector within the aperture  14 . 1 - 2306  can be configured to receive electrical power (e.g., from a wall-mounted electrical outlet) and deliver the electrical power to the one or more components within the housing  14 . 1 - 2302  (e.g., charging a battery disposed within the housing  14 . 1 - 2302 ). 
     The aperture  14 . 1 - 2308  can be formed within or defined by the external surface  14 . 1 - 2304  of the housing  14 . 1 - 2302 . The aperture  14 . 1 - 2308  can be sized and shaped to receive a plug connector of a cable. In some examples, a cable operably coupled to a receptacle connector within the aperture  14 . 1 - 2308  can transfer electrical power from the electronic device  14 . 1 - 2300  while the plug connector is disposed within the aperture  14 . 1 - 2308 . In other words, the receptacle connector within the aperture  14 . 1 - 2308  can be configured to deliver or provide electrical power from the one or more components within the housing  14 . 1 - 2302  to a portable electronic device electrically coupled to the cable. 
     While the apertures  14 . 1 - 2306 ,  14 . 1 - 2308  are shown as being positioned on a particular side of the housing  14 . 1 - 2302 , the apertures  14 . 1 - 2306 ,  14 . 1 - 2308  can be formed or otherwise positioned at any location on the external surface  14 . 1 - 2304  of the housing  14 . 1 - 2302 . Similarly, while the apertures  14 . 1 - 2306 ,  14 . 1 - 2308  are shown as being disposed adjacent or substantially adjacent to one another, in some examples, the apertures  14 . 1 - 2306 ,  14 . 1 - 2308  can be formed or otherwise positioned at nonadjacent locations on the external surface  14 . 1 - 2304  of the housing  14 . 1 - 2302  (e.g., on opposite sides of the housing  14 . 1 - 2302 ). While the apertures  14 . 1 - 2306 ,  14 . 1 - 2308  are depicted as having different dimensions (i.e., the aperture  14 . 1 - 2306  is illustrated as being relatively smaller than the aperture  14 . 1 - 2308 ), the apertures  14 . 1 - 2306 ,  14 . 1 - 2308  can have the same dimensions or substantially similar dimensions to one another in some examples. While two apertures (e.g., apertures  14 . 1 - 2306 ,  14 . 1 - 2308 ) are shown in  FIGS.  14 . 1 - 26 A and  14 . 1 - 26 B , some examples can have more or fewer apertures. For example, the housing  14 . 1 - 2302  can form a single aperture which is operable to provide electrical power and/or signals to a portable electronic device and also operable to receive electrical power and/or signals (e.g., from a portable electronic device and/or a wall-mounted electrical outlet). In some examples, the housing  14 . 1 - 2302  can form two apertures, such as a first aperture operable to provide electrical power and/or signals to a portable electronic device and also operable to receive electrical power and/or signals (e.g., from a portable electronic device and/or a wall-mounted electrical outlet), and a second aperture operable to receive a tool that can be used to remove a cable from the first aperture. 
       FIGS.  14 . 1 - 26 B  shows the electronic device  14 . 1 - 2300  and a cable  14 . 1 - 2310  receivable within one or more of the apertures  14 . 1 - 2306 ,  14 . 1 - 2308 . The cable  14 . 1 - 2310  can have a plug connector  14 . 1 - 2312  having a plug  14 . 1 - 2314  and a boot  14 . 1 - 2316 . The plug connector  14 . 1 - 2312  can be electrically coupled to a length of shielded wire  14 . 1 - 2318 . The shielded wire  14 . 1 - 2318  can be one or more wires (e.g., metallic wires capable of transferring electrical signals and/or electrical power). The one or more wires can be individually shielded or collectively shielded. The shielding can prevent each of the one or more wires from contacting one another. Additionally, or alternatively, the shielding can prevent or limit the influence of electromagnetic waves on each of the one or more wires. The plug connector  14 . 1 - 2312  can be received within one of the apertures  14 . 1 - 2306 ,  14 . 1 - 2308  to electrically couple the cable  14 . 1 - 2310  with the electronic device  14 . 1 - 2300 . For example, the plug  14 . 1 - 2314  can be inserted within a receptacle connector (e.g., receptacle connector  14 . 1 - 2424  of  FIGS.  14 . 1 - 27 A ) disposed within the housing  14 . 1 - 2302  of the electronic device  14 . 1 - 2300  to enable transfer of electrical power and/or signals relative to the electronic device  14 . 1 - 2300 . The boot  14 . 1 - 2316  can be inserted within a trim ring (e.g., trim ring  14 . 1 - 2422  of  FIGS.  14 . 1 - 27 A ) disposed within the housing  14 . 1 - 2302  of the electronic device  14 . 1 - 2300 , between the receptacle connector and the housing  14 . 1 - 2302 . One or more of the plug  14 . 1 - 2314 , the boot  14 . 1 - 2316 , the apertures  14 . 1 - 2306 ,  14 . 1 - 2308 , the receptacle connector, and the trim ring can include one or more engagement features that retain the plug connector  14 . 1 - 2312  to the electronic device  14 . 1 - 2300 . The mechanisms and components capable of inter coupling the cable  14 . 1 - 2310  and the electronic device  14 . 1 - 2300  will be discussed in greater detail below with reference to  FIGS.  14 . 1 - 27 A through  14 . 1 - 35 B . 
       FIGS.  14 . 1 - 27 A  shows an electronic device  14 . 1 - 2400  and a cable  14 . 1 - 2410 . The electronic device  14 . 1 - 2400  can be substantially similar to, and can include some or all of, the features of the electronic device  14 . 1 - 2300 . For example, the electronic device  14 . 1 - 2400  can include a housing  14 . 1 - 2402  forming a cavity or volume  14 . 1 - 2420 . One or more electronic components can be disposed within the volume  14 . 1 - 2420 . For example, the one or more electronic components disposed within the volume  14 . 1 - 2420  can include a trim ring  14 . 1 - 2422  and a receptacle connector  14 . 1 - 2424  operably coupled to a battery  14 . 1 - 2426 , a processor  14 . 1 - 2428 , a combination thereof, or any other electronic component. The trim ring  14 . 1 - 2422  and the receptacle connector  14 . 1 - 2424  can form an electrical port disposed within the volume  14 . 1 - 2420 . In some examples, an external surface  14 . 1 - 2404  of the housing  14 . 1 - 2402  can form an aperture  14 . 1 - 2408  capable of receiving a plug connector  14 . 1 - 2412  of the cable  14 . 1 - 2410 . The trim ring  14 . 1 - 2422  can be positioned adjacent the aperture  14 . 1 - 2408  such that a boot  14 . 1 - 2416  of the plug connector  14 . 1 - 2412  is disposed within the trim ring  14 . 1 - 2422 . The receptacle connector  14 . 1 - 2424  can be positioned adjacent the trim ring  14 . 1 - 2422  such that a plug of the plug connector  14 . 1 - 2412  is disposed within the receptacle connector  14 . 1 - 2424 . The trim ring  14 . 1 - 2422  and the receptacle connector  14 . 1 - 2424  will be discussed in detail below with reference to  FIGS.  14 . 1 - 27 B . 
       FIGS.  14 . 1 - 27 B  shows a partially exploded view of the cable  14 . 1 - 2410 , the trim ring  14 . 1 - 2422 , and the receptacle connector  14 . 1 - 2424 . The cable  14 . 1 - 2410  can be substantially similar to, and can include some or all of, the features of the cable  14 . 1 - 2310 . For example, the cable  14 . 1 - 2410  can include the plug connector  14 . 1 - 2412  having a plug  14 . 1 - 2414  and a boot  14 . 1 - 2416 . The plug connector  14 . 1 - 2412  can be electrically coupled to a length of shielded wire  14 . 1 - 2418 . 
     In some examples, the boot  14 . 1 - 2416  can be sized or otherwise shaped to be at least partially receivable within a cavity  14 . 1 - 2430  formed within the trim ring  14 . 1 - 2422 . In some examples, the boot  14 . 1 - 2416  can be sized or otherwise shaped to be at least partially receivable within the cavity  14 . 1 - 2430  of the trim ring  14 . 1 - 2422  and partially receivable within the aperture  14 . 1 - 2408  of the housing  14 . 1 - 2402 . The cavity  14 . 1 - 2430  can be shaped as cubic, cylindrical, triangular, or any other geometric shape. The boot  14 . 1 - 2416  can form a corresponding geometric shape, which can be inserted or otherwise positioned within the cavity  14 . 1 - 2430 . In some examples, the boot  14 . 1 - 2416  can be partially disposed within the cavity  14 . 1 - 2430  such that a portion of the boot  14 . 1 - 2416  protrudes from the cavity  14 . 1 - 2430 . In some examples, the boot  14 . 1 - 2416  can be entirely surrounded or substantially surrounded by the trim ring  14 . 1 - 2422  and within the cavity  14 . 1 - 2430 , such that a wire-side surface  14 . 1 - 2434  of the boot  14 . 1 - 2416  adjacent the shielded wire  14 . 1 - 2418  is flush or substantially flush with a surface  14 . 1 - 2436  of the trim ring  14 . 1 - 2422  adjacent the cavity  14 . 1 - 2430 . In some examples, the surface  14 . 1 - 2432  of the boot  14 . 1 - 2416  can be disposed within the cavity  14 . 1 - 2430  of the trim ring  14 . 1 - 2422  such that the wire-side surface  14 . 1 - 2434  is flush or substantially flush with the external surface  14 . 1 - 2404  of the housing  14 . 1 - 2402  and the boot  14 . 1 - 2416  is substantially concealed or hidden within the cavity  14 . 1 - 2430  of the trim ring  14 . 1 - 2422 . In some examples, while the boot  14 . 1 - 2416  is disposed within the cavity  14 . 1 - 2430 , the boot  14 . 1 - 2416  can be substantially covered by the trim ring  14 . 1 - 2422 . This prevents the boot  14 . 1 - 2416  from being gripped (e.g., by a user) and pulled from the trim ring  14 . 1 - 2416 . In other words, a substantial portion of the boot  14 . 1 - 2416  can be disposed within the cavity  14 . 1 - 2430  such that the boot  14 . 1 - 2416  is inaccessible to grasp to pull the plug connector  14 . 1 - 2412  from the trim ring  14 . 1 - 2422  and the receptacle connector  14 . 1 - 2424 . 
     The plug  14 . 1 - 2414  can include one or more electrical contacts  14 . 1 - 2438 A,  14 . 1 - 2438 B, and  14 . 1 - 2438 C, which can provide electrical signals, electrical power, a grounding path, another electrical communication, or a combination thereof between the shielded wire  14 . 1 - 2418  and the receptacle connector  14 . 1 - 2424 . 
     The trim ring  14 . 1 - 2422  can be molded, machined, cast, stamped, or otherwise manufactured from a metal, polymer, ceramic, or combination thereof. For example, the trim ring  14 . 1 - 2422  can be molded or co-molded from a thermoplastic or other polymer. The cavity  14 . 1 - 2430  formed within the trim ring  14 . 1 - 2422  can be at least partially defined by internal walls  14 . 1 - 2440 . In some examples, the boot  14 . 1 - 2416  can contact one or more of the internal walls  14 . 1 - 2440  while the boot  14 . 1 - 2416  is positioned within the cavity  14 . 1 - 2430  such that the boot  14 . 1 - 2416  is at least partially retained within the cavity  14 . 1 - 2430  by a frictional engagement between the boot  14 . 1 - 2416  and the internal walls  14 . 1 - 2440 . 
     In some examples, the receptacle connector  14 . 1 - 2424  can be adhered, molded, welded, fastened, or otherwise affixed to the trim ring  14 . 1 - 2422 . For example, the receptacle connector  14 . 1 - 2424  can define one or more receptacle apertures  14 . 1 - 2444 A,  14 . 1 - 2444 B, which correspond to and align with respective trim ring apertures  14 . 1 - 2442 A,  14 . 1 - 2442 B of the trim ring  14 . 1 - 2422 . A respective fastener (not shown) can be disposed within the receptacle aperture  14 . 1 - 2444 A and the trim ring aperture  14 . 1 - 2442 A to couple the receptacle connector  14 . 1 - 2424  to the trim ring  14 . 1 - 2422 . Similarly, a respective fastener (not shown) can be disposed within the receptacle aperture  14 . 1 - 2444 B and the trim ring aperture  14 . 1 - 2442 B to couple the receptacle connector  14 . 1 - 2424  to the trim ring  14 . 1 - 2422 . The fastener can be a pin, rivet, screw, bolt, a combination thereof, or any other fastener. 
     The boot  14 . 1 - 2416  can be molded, machined, cast, stamped, or otherwise manufactured from a metal, polymer, ceramic, or combination thereof. As illustrated in  FIGS.  14 . 1 - 27 B , the boot  14 . 1 - 2416  can include recesses  14 . 1 - 2462 A and  14 . 1 - 2462 B formed in opposite sides of the boot  14 . 1 - 2416 . The recesses  14 . 1 - 2462 A and  14 . 1 - 2462 B can be machined, molded, etched, or otherwise recessed into an exterior surface of the boot  14 . 1 - 2416 . In the example of  FIGS.  14 . 1 - 27 B , the recesses  14 . 1 - 2462 A and  14 . 1 - 2462 B extend along only a portion of the periphery of the boot  14 . 1 - 2416 . For example, the recesses  14 . 1 - 2462 A and  14 . 1 - 2462 B can extend along one or more sides of the boot  14 . 1 - 2416  (e.g., two sides in the example of  FIGS.  14 . 1 - 27 B ). In some examples, recesses can extend along an entire periphery of the boot  14 . 1 - 2416 . In other words, the recesses can form a channel encircling the boot  14 . 1 - 2416 . The recesses  14 . 1 - 2462 A and  14 . 1 - 2462 B can be formed between a plug-side surface and a wire-side surface of the boot  14 . 1 - 2416 . In some examples, the recesses  14 . 1 - 2462 A and  14 . 1 - 2462 B can extend parallel to and between the plug-side surface and the wire-side surface of the boot  14 . 1 - 2416 . The recesses  14 . 1 - 2462 A and  14 . 1 - 2462 B can be included to interface with structures of the trim ring  14 . 1 - 2422  in order to retain the boot  14 . 1 - 2416  and the plug connector  14 . 1 - 2412  within the channel  14 . 1 - 2430 . The trim ring  14 . 1 - 2422  and the boot  14 . 1 - 2416  will be discussed in greater detail below with reference to  FIGS.  14 . 1 - 28 A through  14 . 1 - 28 F . 
     In some examples, the receptacle connector  14 . 1 - 2424  can include an exterior portion  14 . 1 - 2446  and an interior portion  14 . 1 - 2448 . For example, the exterior portion  14 . 1 - 2446  can at least partially encompass the interior portion  14 . 1 - 2448 . In other words, the exterior portion  14 . 1 - 2446  can act as a sleeve or shell that at least partially surrounds the interior portion  14 . 1 - 2448 . The exterior portion  14 . 1 - 2446  and interior portion  14 . 1 - 2448  can be interlocked or affixed to one another. For example, the exterior portion  14 . 1 - 2446  can include a through-hole  14 . 1 - 2450  that receives a protrusion  14 . 1 - 2452  of the interior portion  14 . 1 - 2448  when the interior portion  14 . 1 - 2448  is affixed within the exterior portion  14 . 1 - 2446 . In some examples, the receptacle connector  14 . 1 - 2424  can be formed from a singular or monolithic structure instead of the external portion  14 . 1 - 2446  and the internal portion  14 . 1 - 2448 . The receptacle connector  14 . 1 - 2424  can form a slot  14 . 1 - 2454  capable of receiving and retaining the plug  14 . 1 - 2414 . 
       FIGS.  14 . 1 - 28 A  shows sectional views of a cable assembly  14 . 1 - 2500  including a plug connector  14 . 1 - 2502  received within an opening  14 . 1 - 2522  of a trim ring  14 . 1 - 2520  and a slot  14 . 1 - 2554  of a receptacle connector  14 . 1 - 2550 . In some examples, one or more internal contacts  14 . 1 - 2512 A,  14 . 1 - 2512 B,  14 . 1 - 2512 C are disposed within the slot  14 . 1 - 2554  of the receptacle connector  14 . 1 - 2550  such that the contacts  14 . 1 - 2512 A,  14 . 1 - 2512 B,  14 . 1 - 2512 C of the plug connector  14 . 1 - 2502  touch or otherwise engage internal contacts  14 . 1 - 2552 A,  14 . 1 - 2552 B,  14 . 1 - 2552 C positioned within the slot  14 . 1 - 2554 . In some examples, the internal contacts  14 . 1 - 2552 A,  14 . 1 - 2552 B,  14 . 1 - 2552 C disposed within the slot  14 . 1 - 2554  can be one or more metal prongs biased to engage the contacts  14 . 1 - 2512 A,  14 . 1 - 2512 B,  14 . 1 - 2512 C while the plug connector  14 . 1 - 2502  is disposed within the slot  14 . 1 - 2554 . The internal contacts  14 . 1 - 2552 A,  14 . 1 - 2552 B,  14 . 1 - 2552 C can be electrically connected to a printed circuit board (PCB), a processor, one or more wires, a combination thereof, or another component positioned within the housing (e.g., the housing  14 . 1 - 2302 ,  14 . 1 - 2402 , discussed above with respect to  FIGS.  14 . 1 - 26 A through  14 . 1 - 27 B ). 
     The cable assembly  14 . 1 - 2500  includes the plug connector  14 . 1 - 2502  and a shielded cable  14 . 1 - 2508 . The plug connector  14 . 1 - 2502  includes a boot  14 . 1 - 2506  having openings  14 . 1 - 2510  formed in the side surfaces thereof and a plug  14 . 1 - 2504  including the contacts  14 . 1 - 2512 A,  14 . 1 - 2512 B, and  14 . 1 - 2512 C. 
     In the example of  FIGS.  14 . 1 - 28 A through  14 . 1 - 28 F , the trim ring  14 . 1 - 2520  includes a detent element  14 . 1 - 2530  that can be configured to interface the recesses  14 . 1 - 2510  of the boot  14 . 1 - 2506  in order to retain the plug connector  14 . 1 - 2502  within the cavity  14 . 1 - 2522 . Although a single detent element  14 . 1 - 2530  is illustrated in  FIGS.  14 . 1 - 28 A through  14 . 1 - 28 F , some examples can include multiple detent elements  14 . 1 - 2530 . For example, in some examples, the trim ring  14 . 1 - 2520  can include two detent elements  14 . 1 - 2530  disposed on opposite sides of the plug connector  14 . 1 - 2502  when the plug connector  14 . 1 - 2502  is inserted into the cavity  14 . 1 - 2522 . While the plug connector  14 . 1 - 2502  is disposed within the cavity  14 . 1 - 2522 , the detent element  14 . 1 - 2530  can be inserted into the recesses  14 . 1 - 2510  defined in the boot  14 . 1 - 2506 . 
       FIGS.  14 . 1 - 28 A  illustrates the plug connector  14 . 1 - 2502  and the detent element  14 . 1 - 2530  as the plug connector  14 . 1 - 2502  is inserted into the cavity  14 . 1 - 2522 . The detent element  14 . 1 - 2530  includes a protrusion  14 . 1 - 2532 , which is used couple the detent element  14 . 1 - 2530  to a lever arm  14 . 1 - 2540 . The lever arm  14 . 1 - 2540  can be included in the trim ring  14 . 1 - 2520  to apply a force, such as a spring force, to the detent element  14 . 1 - 2530 . The lever arm  14 . 1 - 2540  can be rotatable around a pivot  14 . 1 - 2542 . A spring (not illustrated in  FIGS.  14 . 1 - 28 A ) can be wrapped around the pivot  14 . 1 - 2542  to apply a force to the lever arm  14 . 1 - 2540  in a direction  14 . 1 -D 1 . The lever arm  14 . 1 - 2540  includes protrusions  14 . 1 - 2544 , which interface with the protrusion  14 . 1 - 2532  of the detent element  14 . 1 - 2530 . The detent element  14 . 1 - 2530  is physically coupled to the lever arm  14 . 1 - 2540  through the protrusion  14 . 1 - 2532  and the protrusions  14 . 1 - 2544 . The lever arm  14 . 1 - 2540  can apply the force to the detent element  14 . 1 - 2530  through the protrusions  14 . 1 - 2544  and the protrusion  14 . 1 - 2532  in a direction  14 . 1 -D 2  in order to retain the detent element  14 . 1 - 2530  in an initial position (illustrated in  FIGS.  14 . 1 - 28 A ) prior to the plug connector  14 . 1 - 2502  being inserted into the cavity  14 . 1 - 2522 . When the plug connector  14 . 1 - 2502  is further inserted into the cavity  14 . 1 - 2522  (illustrated in  FIGS.  14 . 1 - 28 C ), the force applied by the lever arm  14 . 1 - 2540  can cause friction between the detent element  14 . 1 - 2530  and the boot  14 . 1 - 2506 . Once the plug connector  14 . 1 - 2502  is fully inserted into the cavity  14 . 1 - 2522 , the force applied by the lever arm  14 . 1 - 2540  causes the detent element  14 . 1 - 2530  to slide into one of the recesses  14 . 1 - 2510  (illustrated in  FIGS.  14 . 1 - 28 D ). 
     The lever arm  14 . 1 - 2540  can be rotatable around the pivot  14 . 1 - 2542  in the direction  14 . 1 -D 1  and a direction  14 . 1 -D 3  (illustrated in  FIGS.  14 . 1 - 28 C and  14 . 1 - 28 E ) opposite the direction  14 . 1 -D 1 . The lever arm  14 . 1 - 2540  can include the spring, which exerts a force on the lever arm  14 . 1 - 2540  in the direction  14 . 1 -D 1 . The lever arm  14 . 1 - 2540  can include a protrusion  14 . 1 - 2546 , which can be used to rotate the lever arm  14 . 1 - 2540  in the direction  14 . 1 -D 3 . The detent element  14 . 1 - 2530  can translate or slide in the direction  14 . 1 -D 2  and a direction  14 . 1 -D 4  (illustrated in  FIGS.  14 . 1 - 28 C and  14 . 1 - 28 E ) opposite the direction  14 . 1 -D 2 . 
     The detent element  14 . 1 - 2530  and the boot  14 . 1 - 2506  of the plug connector  14 . 1 - 2502  can include sloped portions  14 . 1 - 2534  and  14 . 1 - 2511 , respectively. The sloped portions  14 . 1 - 2534  and  14 . 1 - 2511  are provided in the detent element  14 . 1 - 2530  and the boot  14 . 1 - 2506  to allow the boot  14 . 1 - 2506  and the detent element  14 . 1 - 2530  to slide over one another as the plug connector  14 . 1 - 2502  is inserted into the cavity  14 . 1 - 2522 . This reduces the amount of force required to translate the detent element  14 . 1 - 2530  in the direction  14 . 1 -D 4  as the plug connector  14 . 1 - 2502  is inserted into the cavity  14 . 1 - 2522  and reduces the force required to insert the plug connector  14 . 1 - 2502  into the cavity  14 . 1 - 2522 . The sloped portions  14 . 1 - 2534  and  14 . 1 - 2511  of the detent element  14 . 1 - 2530  and the boot  14 . 1 - 2506  will be described in detail below with reference to  FIGS.  14 . 1 - 28 B . 
       FIGS.  14 . 1 - 28 B  shows a detail view of the detent element  14 . 1 - 2530  and the boot  14 . 1 - 2506 . Specifically,  FIGS.  14 . 1 - 28 B  illustrates details of the sloped portions  14 . 1 - 2534  and  14 . 1 - 2511  of the detent element  14 . 1 - 2530  and the boot  14 . 1 - 2506 .  FIGS.  14 . 1 - 28 B  also illustrates details of a sloped portion  14 . 1 - 2514  of the boot  14 . 1 - 2506  adjacent the recess  14 . 1 - 2510 . 
     In the example of  FIGS.  14 . 1 - 28 B , the detent element  14 . 1 - 2530  includes the sloped portion  14 . 1 - 2534  adjacent a linear portion  14 . 1 - 2536 . In some examples, the sloped portion  14 . 1 - 2534  can be curved, rather than sloped. The linear portion  14 . 1 - 2536  and the sloped portion  14 . 1 - 2534  can form a first angle  14 . 1 - 013 . The first angle  14 . 1 - 013  can be less than 90 degrees, such as, between about 89 degrees and about 60 degrees, between about 60 degrees and about 30 degrees, between about 30 degrees and about 5 degrees, or greater than about 5 degrees. 
     The boot  14 . 1 - 2506  includes the sloped portion  14 . 1 - 2511  adjacent a linear portion  14 . 1 - 2513 . In some examples, the sloped portion  14 . 1 - 2511  can be curved, rather than sloped. The linear portion  14 . 1 - 2513  and the sloped portion  14 . 1 - 2511  can form a second angle  14 . 1 - 014 . The second angle  14 . 1 - 014  can be less than 90 degrees, such as, between about 89 degrees and about 60 degrees, between about 60 degrees and about 30 degrees, between about 30 degrees and about 5 degrees, or greater than about 5 degrees. In some examples, the second angle  14 . 1 - 014  can be the same as the first angle  14 . 1 - 013 , or the second angle  14 . 1 - 014  can be different from the first angle  14 . 1 - 013 . 
     In some examples, a force required to insert the plug connector  14 . 1 - 2502  into the cavity  14 . 1 - 2522  can correlate to the first angle  14 . 1 - 013  and/or the second angle  14 . 1 - 014 . For example, a relatively smaller first angle  14 . 1 - 013  (e.g., between about 5 degrees and 45 degrees) can cause the force required to insert the plug connector  14 . 1 - 2502  into the cavity  14 . 1 - 2522  to be relatively greater. Similarly, a relatively smaller second angle  14 . 1 - 014  (e.g., between about 5 degrees and 45 degrees) can cause the force required to insert the plug connector  14 . 1 - 2502  into the cavity  14 . 1 - 2522  to be relatively greater. Alternatively, a relatively larger first angle  14 . 1 - 013  (e.g., between about 45 degrees and 89 degrees) can cause the force required to insert the plug connector  14 . 1 - 2502  into the cavity  14 . 1 - 2522  to be relatively smaller. A relatively larger second angle  14 . 1 - 014  (e.g., between about 45 degrees and 89 degrees) can cause the force required to insert the plug connector  14 . 1 - 2502  into the cavity  14 . 1 - 2522  to be relatively smaller. In other words, sizes of the first angle  14 . 1 - 013  and the second angle  14 . 1 - 014  can be chosen to generate a desired force required to insert the plug connector  14 . 1 - 2502  into the cavity  14 . 1 - 2522  (i.e., move the detent element  14 . 1 - 2530  out of the path of the boot  14 . 1 - 2506  as the plug connector  14 . 1 - 2502  is inserted into the cavity  14 . 1 - 2522 ). The force required to insert the plug connector  14 . 1 - 2502  into the cavity  14 . 1 - 2522  can be in a range from about 1 kgf to about 1.2 kgf, in a range from about 1 kgf to about 2 kgf, in a range from about 0.5 kgf to about 5 kgf, in a range from about 0 kgf to about 1 kgf, in a range from about 1 kgf to about 10 kgf, or the like. 
     In the example of  FIGS.  14 . 1 - 28 B , the boot  14 . 1 - 2506  includes the sloped portion  14 . 1 - 2514  adjacent a linear portion  14 . 1 - 2512 . The sloped portion  14 . 1 - 2514  defines a side surface of the recess  14 . 1 - 2510 . In some examples, the sloped portion  14 . 1 - 2514  can be curved, rather than sloped. The linear portion  14 . 1 - 2512  and the sloped portion  14 . 1 - 2514  can form a third angle  14 . 1 - 015 . The third angle  14 . 1 - 015  can be less than 90 degrees, such as, between about 89 degrees and about 60 degrees, between about 60 degrees and about 30 degrees, between about 30 degrees and about 5 degrees, or greater than about 5 degrees. 
     In some examples, a force required to remove the plug connector  14 . 1 - 2502  from the cavity  14 . 1 - 2522  can correlate to the third angle  14 . 1 - 015 . For example, a relatively smaller third angle  14 . 1 - 015  (e.g., between about 5 degrees and 45 degrees) can cause the force required to remove the plug connector  14 . 1 - 2502  from the cavity  14 . 1 - 2522  to be relatively smaller. Alternatively, a relatively larger third angle  14 . 1 - 015  (e.g., between about 45 degrees and 89 degrees) can cause the force required to remove the plug connector  14 . 1 - 2502  from the cavity  14 . 1 - 2522  to be relatively greater. In other words, a size of the third angle  14 . 1 - 015  can be chosen to generate a desired force required to remove the plug connector  14 . 1 - 2502  from the cavity  14 . 1 - 2522  (i.e., move the detent element  14 . 1 - 2530  out of the path of the boot  14 . 1 - 2506  as the plug connector  14 . 1 - 2502  is removed from the cavity  14 . 1 - 2522 ). The force required to remove the plug connector  14 . 1 - 2502  from the cavity  14 . 1 - 2522  can be in a range from about 5 kgf to about 7 kgf, in a range from about 1 kgf to about 12 kgf, in a range from about 10 kgf to about 20 kgf, in a range from about 5 kgf to about 50 kgf, in a range from about 1 kgf to about 10 kgf, in a range from about 10 kgf to about 100 kgf, or the like. 
       FIGS.  14 . 1 - 28 C  illustrates the plug connector  14 . 1 - 2502  and the detent element  14 . 1 - 2530  as the plug connector  14 . 1 - 2502  is being inserted into the cavity  14 . 1 - 2522 . As illustrated in  FIGS.  14 . 1 - 28 C , inserting the boot  14 . 1 - 2506  into the cavity  14 . 1 - 2522  causes the detent element  14 . 1 - 2470  to slide/translate in the direction  14 . 1 -D 4 . The movement of the detent element  14 . 1 - 2530  causes the lever arm  14 . 1 - 2540  to rotate around the pivot  14 . 1 - 2542  in the direction  14 . 1 -D 3  through the protrusions  14 . 1 - 2544  and the protrusion  14 . 1 - 2532 . The spring of the lever arm  14 . 1 - 2540  causes a force to be applied to the detent element  14 . 1 - 2530  in the direction  14 . 1 -D 2 , which results in friction between the detent element  14 . 1 - 2530  and the boot  14 . 1 - 2506  as the plug connector  14 . 1 - 2502  is inserted into the cavity  14 . 1 - 2522 . The force required to insert the plug connector  14 . 1 - 2502  into the cavity  14 . 1 - 2522  can be in a range from about 1 kgf to about 1.2 kgf, in a range from about 1 kgf to about 2 kgf, in a range from about 0.5 kgf to about 5 kgf, in a range from about 0 kgf to about 1 kgf, in a range from about 1 kgf to about 10 kgf, or the like. 
       FIGS.  14 . 1 - 28 D  illustrates the plug connector  14 . 1 - 2502  and the detent element  14 . 1 - 2530  after the plug connector  14 . 1 - 2502  has been fully inserted into the cavity  14 . 1 - 2522 . As illustrated in  FIGS.  14 . 1 - 28 D , the spring of the lever arm  14 . 1 - 2540  causes a force to be applied to the detent element  14 . 1 - 2530 . This rotates the lever arm  14 . 1 - 2540  in the direction  14 . 1 -D 1  and translates the detent element  14 . 1 - 2530  in the direction  14 . 1 -D 2  such that the detent element  14 . 1 - 2530  slides into the recess  14 . 1 - 2510 . The detent element  14 . 1 - 2530  can contact a side surface of the boot  14 . 1 - 2506  when the plug connector  14 . 1 - 2502  is fully inserted into the cavity  14 . 1 - 2522  (e.g., the detent element  14 . 1 - 2530  is fully inserted into the recess  14 . 1 - 2510 ). For example, the detent element  14 . 1 - 2530  can contact a side surface of the boot  14 . 1 - 2506  adjacent the recess  14 . 1 - 2510  and the sloped portion  14 . 1 - 2514 . 
     As described previously, the sloped portion  14 . 1 - 2514  of the boot  14 . 1 - 2506  can be configured such that a force required to remove the plug connector  14 . 1 - 2502  from the cavity  14 . 1 - 2522  can be in a range from about 5 kgf to about 7 kgf, in a range from about 1 kgf to about 12 kgf, in a range from about 10 kgf to about 20 kgf, in a range from about 5 kgf to about 50 kgf, in a range from about 1 kgf to about 10 kgf, in a range from about 10 kgf to about 100 kgf, or the like. A ratio of the force required to remove the plug connector  14 . 1 - 2502  from the cavity  14 . 1 - 2522  to the force required to insert the plug connector  14 . 1 - 2502  into the cavity  14 . 1 - 2522  can be in a range from about 1 to about 10, from about 1.5 to about 3, from about 2 to about 4, from about 3 to about 5, from about 1.5 to about 5, or the like. In some examples, the detent element  14 . 1 - 2530  can also include a sloped portion, which can be configured to interface with the sloped portion  14 . 1 - 2514  and can reduce the force required to remove the plug connector  14 . 1 - 2502  from the cavity  14 . 1 - 2522 . As illustrated in  FIGS.  14 . 1 - 28 D , in some examples, the boot  14 . 1 - 2506  can be flush with the trim ring  14 . 1 - 2520  (i.e., an external-facing surface of the boot  14 . 1 - 2506  can be flush with an external-facing surface of the trim ring  14 . 1 - 2520 ) when the plug connector  14 . 1 - 2502  is disposed within the trim ring  14 . 1 - 2520 . 
       FIGS.  14 . 1 - 28 E  illustrates the detent element  14 . 1 - 2530  and the lever arm  14 . 1 - 2540  after a tool  14 . 1 - 2560  is used to release the plug connector  14 . 1 - 2502 . In some examples, the trim ring  14 . 1 - 2520  can include an aperture  14 . 1 - 2524  that can receive the tool  14 . 1 - 2560 . The tool  14 . 1 - 2560  can be shaped as cubic, cylindrical, triangular, or any other geometric shape. The aperture  14 . 1 - 2524  can form a corresponding geometric shape, into which the tool  14 . 1 - 2560  can be inserted or otherwise positioned. A user can use the tool  14 . 1 - 2560  to rotate the lever arm  14 . 1 - 2540  in the direction  14 . 1 -D 3 , which moves the detent element  14 . 1 - 2530  in the direction  14 . 1 -D 4 . This slides the detent element  14 . 1 - 2530  out of the recess  14 . 1 - 2510  and allows the plug connector  14 . 1 - 2502  to be removed from the cavity  14 . 1 - 2522 . Once the detent element  14 . 1 - 2530  is moved out of the recess  14 . 1 - 2510 , a force required to remove the plug connector  14 . 1 - 2502  from the cavity  14 . 1 - 2522  can be in a range from about 1 kgf to about 1.2 kgf, in a range from about 1 kgf to about 2 kgf, in a range from about 0.5 kgf to about 5 kgf, in a range from about 0 kgf to about 1 kgf, in a range from about 1 kgf to about 10 kgf, or the like. A force required to use the tool  14 . 1 - 2560  to remove the detent element  14 . 1 - 2530  from the recess  14 . 1 - 2519  can be in a range from about 1 kgf to about 1.2 kgf, in a range from about 1 kgf to about 2 kgf, in a range from about 0.5 kgf to about 5 kgf, in a range from about 0 kgf to about 1 kgf, in a range from about 1 kgf to about 10 kgf, or the like. 
       FIGS.  14 . 1 - 28 F  illustrates a perspective view of the lever arm  14 . 1 - 2540 . As illustrated in  FIGS.  14 . 1 - 28 F , the lever arm  14 . 1 - 2540  includes a body portion that is rotatable around a pivot  14 . 1 - 2542 . The pivot  14 . 1 - 2542  can be a pin, rivet, screw, bolt, a combination thereof, or any other fastener. A spring  14 . 1 - 2541  is disposed in the body portion and wrapped around the pivot  14 . 1 - 2542 . The spring  14 . 1 - 2541  is configured to apply a spring force to the body portion of the lever arm  14 . 1 - 2540  in a direction  14 . 1 -D 1 . The body portion of the lever arm  14 . 1 - 2540  includes protrusions  14 . 1 - 2544  and a protrusion  14 . 1 - 2546 . The protrusions  14 . 1 - 2544  can be used to physically interact with a detent element, as discussed previously. The protrusion  14 . 1 - 2546  can be used to interact with a tool, as discussed previously, and can be used to aid in removing a plug connector from a trim ring. 
     Providing the detent element  14 . 1 - 2530  and the lever arm  14 . 1 - 2540  to retain the plug connector  14 . 1 - 2502  in the cavity  14 . 1 - 2522  allows the plug connector  14 . 1 - 2502  to be inserted into the cavity  14 . 1 - 2522  with a relatively small force and requires a relatively large force to remove the plug connector  14 . 1 - 2502  from the cavity  14 . 1 - 2522 . This prevents the plug connector  14 . 1 - 2502  from unwantedly or accidentally being disconnected from an electronic device. Moreover, by providing the aperture  14 . 1 - 2524  and the tool  14 . 1 - 2560 , the plug connector  14 . 1 - 2502  can be easily removed when desired. 
       FIGS.  14 . 1 - 29 A through  14 . 1 - 29 E  illustrate seals  14 . 1 - 2626  that can be included in a trim ring  14 . 1 - 2620  for providing sealing between the trim ring  14 . 1 - 2620  and a boot  14 . 1 - 2606  of a plug connector  14 . 1 - 2602  of a cable assembly  14 . 1 - 2600 . The cable assembly  14 . 1 - 2600  includes the plug connector  14 . 1 - 2602  and a shielded cable  14 . 1 - 2608  coupled to the plug connector  14 . 1 - 2602 . The plug connector  14 . 1 - 2602  includes a plug  14 . 1 - 2604  and the boot  14 . 1 - 2606  coupled to the plug  14 . 1 - 2604  and the shielded cable  14 . 1 - 2608 . 
     The plug connector  14 . 1 - 2602  can be received in a cavity  14 . 1 - 2622  of an enclosure  14 . 1 - 2610  and the trim ring  14 . 1 - 2620 . The trim ring  14 . 1 - 2620  can be coupled to the enclosure  14 . 1 - 2610 , and a receptacle connector  14 . 1 - 2650  can be coupled to the trim ring  14 . 1 - 2620 . The trim ring  14 . 1 - 2620  can include a detent element  14 . 1 - 2630  and a lever arm  14 . 1 - 2640 . The detent element  14 . 1 - 2630  can include a protrusion  14 . 1 - 2632  configured to interface with the lever arm  14 . 1 - 2640 . The detent element  14 . 1 - 2630  can be configured to interface with openings  14 . 1 - 2609  in the boot  14 . 1 - 2606  to fasten the plug connector  14 . 1 - 2602  in the cavity  14 . 1 - 2622 . The lever arm  14 . 1 - 2640  can be rotatable around a pivot  14 . 1 - 2642  and can include protrusions  14 . 1 - 2644  configured to interface with the protrusion  14 . 1 - 2632  of the detent element  14 . 1 - 2632 . The lever arm  14 . 1 - 2640  can further include a protrusion  14 . 1 - 2646  configured to interface with a tool  14 . 1 - 2660  in order to un-fasten the plug connector  14 . 1 - 2602  from the cavity  14 . 1 - 2622 . An aperture  14 . 1 - 2624  can be provided in the trim ring  14 . 1 - 2620  and the enclosure  14 . 1 - 2610  in order to provide access to the tool  14 . 1 - 2660 . As such, the tool  14 . 1 - 2660  can extend through the enclosure  14 . 1 - 2610  and the trim ring  14 . 1 - 2620  to interface with the protrusion  14 . 1 - 2646 . 
       FIGS.  14 . 1 - 29 A  shows a sectional view of the cable assembly  14 . 1 - 2600 , the enclosure  14 . 1 - 2610 , the trim ring  14 . 1 - 2620 , and the receptacle connector  14 . 1 - 2650 . As shown in  FIGS.  14 . 1 - 29 A , the seal  14 . 1 - 2626  can be L-shaped in a cross-sectional view. The seal  14 . 1 - 2626  can be provided on the trim ring  14 . 1 - 2620 , between the trim ring  14 . 1 - 2620  and the enclosure  14 . 1 - 2610 . This provides sealing between the trim ring  14 . 1 - 2620  and the enclosure  14 . 1 - 2610 . The seal  14 . 1 - 2626  is also provided between a body of the trim ring  14 . 1 - 2620  and the cavity  14 . 1 - 2622  of the trim ring  14 . 1 - 2620 . This provides sealing between the trim ring  14 . 1 - 2620  and the boot  14 . 1 - 2606  of the plug connector  14 . 1 - 2602 . Placing the seal  14 . 1 - 2626  in this configuration provides sealing between the trim ring  14 . 1 - 2620  and both the boot  14 . 1 - 2606  and the enclosure  14 . 1 - 2610 . The seal  14 . 1 - 2626  can include an elastic material that can be stretched around a lip of the trim ring  14 . 1 - 2620  and can be secured to the trim ring  14 . 1 - 2620  by interference with the lip of the trim ring  14 . 1 - 2620 . 
       FIGS.  14 . 1 - 29 B  shows a sectional view of the cable assembly  14 . 1 - 2600 , the enclosure  14 . 1 - 2610 , the trim ring  14 . 1 - 2620 , and the receptacle connector  14 . 1 - 2650 .  FIGS.  14 . 1 - 29 E  shows a perspective view of the lever arm  14 . 1 - 2640 . The lever arm  14 . 1 - 2640  includes a spring  14 . 1 - 2641  wrapped around the pivot  14 . 1 - 2642  and configured to apply a spring force to a body portion of the lever arm  14 . 1 - 2640 . A seal  14 . 1 - 2647  is coupled to the protrusion  14 . 1 - 2646  of the lever arm  14 . 1 - 2640 . The spring  14 . 1 - 2641  can apply a spring force to the lever arm  14 . 1 - 2640  that presses the seal  14 . 1 - 2647  against the enclosure  14 . 1 - 2610 . This provides sealing between the trim ring  14 . 1 - 2620  and an exterior of an electronic device. The tool  14 . 1 - 2660  can be configured to pass through an opening  14 . 1 - 2643  in the seal  14 . 1 - 2647  when the tool  14 . 1 - 2660  is used to actuate the protrusion  14 . 1 - 2646 . The seal  14 . 1 - 2647  can be a soft gasket or the like. 
       FIGS.  14 . 1 - 29 C  shows a perspective view of the plug connector  14 . 1 - 2602  and the trim ring  14 . 1 - 2620  and  FIGS.  14 . 1 - 29 D  illustrates a partial sectional view of the trim ring  14 . 1 - 2620 . In some examples, the boot  14 . 1 - 2606 , the plug  14 . 1 - 2604 , and the cavity  14 . 1 - 2622  can be stadium-shaped in a cross-sectional view. In some examples, the boot  14 . 1 - 2606 , the plug  14 . 1 - 2604 , and the cavity  14 . 1 - 2622  can be rectangular, triangular, rounded, or can have any other shape in a cross-sectional view. The trim ring  14 . 1 - 2620  can include a first cavity portion  14 . 1 - 2622 A configured to receive the boot  14 . 1 - 2606  and a second cavity portion  14 . 1 - 2622 B configured to receive the plug  14 . 1 - 2604 . The trim ring  14 . 1 - 2620  can include a third cavity portion  14 . 1 - 2622 C configured to receive the detent element  14 . 1 - 2630 . The third cavity portion  14 . 1 - 2622 C can be adjacent the first cavity portion  14 . 1 - 2622 A, and can align the  14 . 1 - 2630  with openings  14 . 1 - 2609  in the side surfaces of the boot  14 . 1 - 2606 . 
       FIGS.  14 . 1 - 29 C and  14 . 1 - 29 D  further illustrate details of the seal  14 . 1 - 2626 . As illustrated in  FIGS.  14 . 1 - 29 D , the seal  14 . 1 - 2626  can have an L-shape in a cross-sectional view. The seal  14 . 1 - 2626  can be wrapped around at least a portion of a body of the trim ring  14 . 1 - 2620 , such as a lip of the trim ring  14 . 1 - 2620 . The seal  14 . 1 - 2626  can be formed of an elastic material, and an elastic force of the elastic material can retain the seal  14 . 1 - 2626  on the lip of the trim ring  14 . 1 - 2620 . In some examples, an adhesive or the like can be provided between the seal  14 . 1 - 2626  and the body of the trim ring  14 . 1 - 2620  to secure the seal  14 . 1 - 2626  to the trim ring  14 . 1 - 2620 . 
     The seal  14 . 1 - 2626  can act as a seal which prevents or limits ingress of contaminants (e.g., liquid, dust, lint, debris, or the like) into the trim ring  14 . 1 - 2620  and the receptacle connector  14 . 1 - 2650  through the cavity  14 . 1 - 2622 , such as when the plug connector  14 . 1 - 2602  is inserted into the cavity  14 . 1 - 2622 . The seal  14 . 1 - 2626  can provided sealing between the trim ring  14 . 1 - 2620  and the plug connector  14 . 1 - 2602  and between the trim ring  14 . 1 - 2620  and the enclosure  14 . 1 - 2610 . The seal  14 . 1 - 2647  can act as a seal which prevents or limits ingress of contaminants (e.g., liquid, dust, lint, debris, or the like) into the trim ring  14 . 1 - 2620  and the through the aperture  14 . 1 - 2624 . 
       FIGS.  14 . 1 - 30 A through  14 . 1 - 30 C  illustrate a seal  14 . 1 - 2726  included on a trim ring  14 . 1 - 2720  and a seal  14 . 1 - 2705  included on a plug connector  14 . 1 - 2702  of a cable assembly  14 . 1 - 2700 . The seal  14 . 1 - 2726  can provide sealing between an enclosure  14 . 1 - 2710  and the trim ring  14 . 1 - 2720  and between the trim ring  14 . 1 - 2720  and the plug connector  14 . 1 - 2702 . The seal  14 . 1 - 2705  can provide sealing between the plug connector  14 . 1 - 2702  and the enclosure  14 . 1 - 2710 . The plug assembly  14 . 1 - 2700  includes the plug connector  14 . 1 - 2702  coupled to a shielded cable  14 . 1 - 2708 . The plug connector  14 . 1 - 2702  includes a plug  14 . 1 - 2704  and the boot  14 . 1 - 2706  coupled to the plug  14 . 1 - 2704  and the shielded cable  14 . 1 - 2708 . 
     The plug connector  14 . 1 - 2702  can be received in a cavity  14 . 1 - 2722  of an enclosure  14 . 1 - 2710  and the trim ring  14 . 1 - 2720 . The trim ring  14 . 1 - 2720  can be coupled to the enclosure  14 . 1 - 2710 , and a receptacle connector  14 . 1 - 2750  can be coupled to the trim ring  14 . 1 - 2720 . The trim ring  2620  can include a detent element  14 . 1 - 2730  and a lever arm  14 . 1 - 2740 . The detent element  14 . 1 - 2730  can be configured to interface with openings  14 . 1 - 2709  in the boot  14 . 1 - 2706  to fasten the plug connector  14 . 1 - 2702  in the cavity  14 . 1 - 2722 . The lever arm  14 . 1 - 2740  can be rotatable around a pivot. The lever arm  14 . 1 - 2740  can include a protrusion  14 . 1 - 2746  configured to interface with a tool in order to un-fasten the plug connector  14 . 1 - 2702  from the cavity  14 . 1 - 2722 . 
     The seal  14 . 1 - 2705  can be coupled to the boot  14 . 1 - 2706  between the shielded cable  14 . 1 - 2708  and the openings  14 . 1 - 2709 . In the example illustrated in  FIGS.  14 . 1 - 30 B , the seal  14 . 1 - 2705  can be provided adjacent the enclosure  14 . 1 - 2710 . Providing the seal  14 . 1 - 2705  in this position provides sealing between the boot  14 . 1 - 2706  and the enclosure  14 . 1 - 2710 , which also seals the trim ring  14 . 1 - 2720  from an external environment. In the example illustrated in  FIGS.  14 . 1 - 30 C , the seal  14 . 1 - 2705  can be provided adjacent the enclosure  14 . 1 - 2710  and the trim ring  14 . 1 - 2720 . Providing the seal  14 . 1 - 2705  in this position can provide sealing between the boot  14 . 1 - 2706  and each of the enclosure  14 . 1 - 2710  and the trim ring  14 . 1 - 2720  and can also provide sealing between the enclosure  14 . 1 - 2710  and the trim ring  14 . 1 - 2720 . Although the seal  14 . 1 - 2705  is illustrated as being between the seal  14 . 1 - 2726  and the shielded cable  14 . 1 - 2708 , the seal  14 . 1 - 2705  can be positioned at any point along the boot  14 . 1 - 2706 , such as between the seal  14 . 1 - 2726  and the plug  14 . 1 - 2704 , between the seal  14 . 1 - 2726  and the openings  14 . 1 - 2709 , between the openings  14 . 1 - 2704  and the plug  14 . 1 - 2704 , or the like. Moreover, the seal  14 . 1 - 2705  can be provided in place of, or in addition to the seal  14 . 1 - 2726 . 
       FIGS.  14 . 1 - 30 B  shows a sectional view of the cable assembly  14 . 1 - 2700 , the enclosure  14 . 1 - 2710 , the trim ring  14 . 1 - 2720 , and the receptacle connector  14 . 1 - 2750 . As shown in  FIGS.  14 . 1 - 30 B , the seal  14 . 1 - 2726  can be L-shaped in a cross-sectional view. The seal  14 . 1 - 2726  can be provided on the trim ring  14 . 1 - 2720 , between the trim ring  14 . 1 - 2720  and the enclosure  14 . 1 - 2710 . This provides sealing between the trim ring  14 . 1 - 2720  and the enclosure  14 . 1 - 2710 . The seal  14 . 1 - 2726  is also provided between a body of the trim ring  14 . 1 - 2720  and the cavity  14 . 1 - 2722  of the trim ring  14 . 1 - 2720 . This provides sealing between the trim ring  14 . 1 - 2720  and the boot  14 . 1 - 2706  of the plug connector  14 . 1 - 2702 . Placing the seal  14 . 1 - 2726  in this configuration provides sealing between the trim ring  14 . 1 - 2720  and both the boot  14 . 1 - 2706  and the enclosure  14 . 1 - 2710 . The seal  14 . 1 - 2726  can include an elastic material that can be stretched around a lip of the trim ring  14 . 1 - 2720  and can be secured to the trim ring  14 . 1 - 2720  by interference with the lip of the trim ring  14 . 1 - 2720 . 
       FIGS.  14 . 1 - 30 C  shows a sectional view of the cable assembly  14 . 1 - 2700 , the enclosure  14 . 1 - 2710 , the trim ring  14 . 1 - 2720 , and the receptacle connector  14 . 1 - 2750 . As shown in  FIGS.  14 . 1 - 30 C , the seal  14 . 1 - 2726  can be rectangular in a cross-sectional view. Other shapes can be used for the seal  14 . 1 - 2726 , such as rounded, triangular, or the like. The seal  14 . 1 - 2726  can be provided between the body of the trim ring  14 . 1 - 2720  and the boot  14 . 1 - 2706 , between an outer surface of the trim ring  14 . 1 - 2720  facing the enclosure  14 . 1 - 2710  and an inner surface of the trim ring  14 . 1 - 2720  facing the receptacle connector  14 . 1 - 2750 . The seal  14 . 1 - 2726  can provide sealing between the trim ring  14 . 1 - 2720  and the boot  14 . 1 - 2706 . 
     The seal  14 . 1 - 2726  can act as a seal which prevents or limits ingress of contaminants (e.g., liquid, dust, lint, debris, or the like) into the trim ring  14 . 1 - 2720  and the receptacle connector  14 . 1 - 2750  through the cavity  14 . 1 - 2722 , such as when the plug connector  14 . 1 - 2702  is inserted into the cavity  14 . 1 - 2722 . The seal  14 . 1 - 2726  can provided sealing between the trim ring  14 . 1 - 2720  and the plug connector  14 . 1 - 2702  and between the trim ring  14 . 1 - 2720  and the enclosure  14 . 1 - 2710 . The seal  14 . 1 - 2705  can act as a seal which prevents or limits ingress of contaminants (e.g., liquid, dust, lint, debris, or the like) into the trim ring  14 . 1 - 2720  and the receptacle connector  14 . 1 - 2750  through the cavity  14 . 1 - 2722 , such as when the plug connector  14 . 1 - 2702  is inserted into the cavity  14 . 1 - 2722 . The seal  14 . 1 - 2705  can provided sealing between the plug connector  14 . 1 - 2702  and the enclosure  14 . 1 - 2710 , between the plug connector  14 . 1 - 2702  and both of the trim ring  14 . 1 - 2720  and the enclosure  14 . 1 - 2710 , or the like. 
       FIGS.  14 . 1 - 31 A and  14 . 1 - 31 B  show sectional views of a plug connector  14 . 1 - 2800  inserted into a receptacle connector  14 . 1 - 2820  and fastened by a detent element  14 . 1 - 2830 . The plug connector  14 . 1 - 2800  includes a boot  14 . 1 - 2804  and a plug  14 . 1 - 2802  coupled to the boot  14 . 1 - 2804 . The boot includes recesses  14 . 1 - 2806  formed in side surfaces thereof. The detent element  14 . 1 - 2830  can interface with the recesses  14 . 1 - 2806  to retain the boot  14 . 1 - 2806  and the plug connector  14 . 1 - 2800  in a cavity of the receptacle connector  14 . 1 - 2820  and a trim ring. 
     The detent element  14 . 1 - 2830  includes a protrusion  14 . 1 - 2832  configured to interface with a lever arm  14 . 1 - 2810 . The detent element  14 . 1 - 2830  and the lever arm  14 . 1 - 2810  can be included in a trim ring, as discussed in examples previously. The lever arm  14 . 1 - 2810  includes protrusions  14 . 1 - 2814  configured to interface with the protrusion  14 . 1 - 2832  of the detent element  14 . 1 - 2830 . The lever arm  14 . 1 - 2810  can include a spring and can be configured to supply a spring force to the detent element  14 . 1 - 2830  through the protrusions  14 . 1 - 2814  and the protrusion  14 . 1 - 2832  to retain the detent element  14 . 1 - 2830  in an engaged configuration (illustrated in  FIGS.  14 . 1 - 31 A ). The lever arm  14 . 1 - 2810  can include a protrusion  14 . 1 - 2816  configured to interact with a tool  14 . 1 - 2840  in order to transition the lever arm  14 . 1 - 2810  and the detent element  14 . 1 - 2830  from the engaged configuration to an un-engaged configuration (illustrated in  FIGS.  14 . 1 - 31 B ). As such, the lever arm  14 . 1 - 2810  can be used to retain the detent element  14 . 1 - 2830  in an engaged configuration and transition the detent element  14 . 1 - 2830  to an un-engaged configuration. The detent element  14 . 1 - 2830  can also include fasteners  14 . 1 - 2834 , which can couple the detent element  14 . 1 - 2830  to a body of a trim ring. The detent element  14 . 1 - 2830  can translate relative to the fasteners  14 . 1 - 2834  as the detent element  14 . 1 - 2830  moves from the engaged configuration to the un-engaged configuration. 
     The lever arm  14 . 1 - 2810  can include a protrusion  14 . 1 - 2811  configured to interact with a spring  14 . 1 - 2819 . The spring  14 . 1 - 2819  can interact with the protrusion  14 . 1 - 2811  to resist movement of the lever arm  14 . 1 - 2810 . In some examples, the spring  14 . 1 - 2819  can provide a desired feel to a user using the tool  14 . 1 - 2840  to un-latch the detent element  14 . 1 - 2830  and the lever arm  14 . 1 - 2810 . For example, as illustrated in  FIGS.  14 . 1 - 31 A and  14 . 1 - 31 B , the spring  14 . 1 - 2819  can move over the protrusion  14 . 1 - 2811  as the lever arm  14 . 1 - 2810  rotates from the engaged configuration (shown in  FIGS.  14 . 1 - 31 A ) to the un-engaged configuration (shown in  FIGS.  14 . 1 - 31 B ). This can provide a physical click through the tool  14 . 1 - 2840 , which informs the user that the lever arm  14 . 1 - 2810  and the detent element  14 . 1 - 2830  are un-latched, and the plug connector  14 . 1 - 2800  can be removed from the trim ring and the receptacle connector  14 . 1 - 2820 . Positions of the spring  14 . 1 - 2819  and the protrusion  14 . 1 - 2811 , as well as lengths of the spring  14 . 1 - 2819 , angles of the spring  14 . 1 - 2819 , shapes of the protrusion  14 . 1 - 2811 , and the like can be altered to alter the force required to rotate the lever arm  14 . 1 - 2810 , as well as the feel to a user of moving the lever arm  14 . 1 - 2810 . 
     In some examples, the receptacle connector  14 . 1 - 2820  can include an exterior portion  14 . 1 - 2824  and an interior portion  14 . 1 - 2822 . For example, the exterior portion  14 . 1 - 2824  can at least partially encompass the interior portion  14 . 1 - 2822 . In other words, the exterior portion  14 . 1 - 2824  can act as a sleeve or shell that at least partially surrounds the interior portion  14 . 1 - 2822 . The exterior portion  14 . 1 - 2824  and interior portion  14 . 1 - 2822  can be interlocked or affixed to one another. In some examples, the receptacle connector  14 . 1 - 2820  can be formed from a singular or monolithic structure instead of the external portion  14 . 1 - 2824  and the internal portion  14 . 1 - 2822 . The receptacle connector  14 . 1 - 2820  can form a slot capable of receiving and retaining the plug connector  14 . 1 - 2800 . 
       FIGS.  14 . 1 - 32 A through  14 . 1 - 32 C  illustrate a method of assembling a trim ring  14 . 1 - 2930  and a plug connector  14 . 1 - 2940  in an enclosure  14 . 1 - 2900 . In  FIGS.  14 . 1 - 32 A , a frame  14 . 1 - 2910  is attached to the enclosure  14 . 1 - 2900 . The enclosure can include a side surface  14 . 1 - 2902 , an opening  14 . 1 - 2904  for receiving a plug connector, a bottom surface  14 . 1 - 2906 , and a top surface  14 . 1 - 2908 . The frame  14 . 1 - 2910  can be attached to structures of the bottom surface  14 . 1 - 2906  and the side surface  14 . 1 - 2902  through fasteners  14 . 1 - 2920 . The fasteners  14 . 1 - 2920  can extend longitudinally in a direction from the top surface  14 . 1 - 2908  towards the bottom surface  14 . 1 - 2906  and can pull the frame  14 . 1 - 2910  down towards the bottom surface  14 . 1 - 2906 . Although three fasteners  14 . 1 - 2920  are illustrated for attaching the frame  14 . 1 - 2910  to the enclosure  14 . 1 - 2900 , any number of the fasteners  14 . 1 - 2920  can be included. 
     In  FIGS.  14 . 1 - 32 B , the trim ring  14 . 1 - 2930  is attached to the frame  14 . 1 - 2910 . The trim ring  14 . 1 - 2930  can be attached to the frame  14 . 1 - 2910  through fasteners  14 . 1 - 2922 . The fasteners  14 . 1 - 2922  can extend into openings  14 . 1 - 2912  of the frame  14 . 1 - 2910 , illustrated in  FIGS.  14 . 1 - 32 A . The fasteners  14 . 1 - 2922  can extend longitudinally in a direction parallel to the bottom surface  14 . 1 - 2906 , and can pull the trim ring  14 . 1 - 2930  towards the frame  14 . 1 - 2910  and the side surface  14 . 1 - 2902  of the enclosure  14 . 1 - 2900 . A dummy trim ring can be inserted through the opening  14 . 1 - 2904  in the enclosure  14 . 1 - 2900  and into an opening  14 . 1 - 2932  in the trim ring  14 . 1 - 2930  in order to align the trim ring  14 . 1 - 2930  with the enclosure  14 . 1 - 2900 . Although three fasteners  14 . 1 - 2922  are illustrated for attaching the trim ring  14 . 1 - 2930  to the frame  14 . 1 - 2910 , any number of the fasteners  14 . 1 - 2922  can be included. 
     In  FIGS.  14 . 1 - 32 C , the receptacle connector  14 . 1 - 2940  is attached to the trim ring  14 . 1 - 2930 . The receptacle connector  14 . 1 - 2940  can be attached to the trim ring  14 . 1 - 2930  through fasteners  14 . 1 - 2924 . The fasteners  14 . 1 - 2924  can extend into openings  14 . 1 - 2934  of the trim ring  14 . 1 - 2930 , illustrated in  FIGS.  14 . 1 - 32 A . The fasteners  14 . 1 - 2924  can extend longitudinally in a direction parallel to the bottom surface  14 . 1 - 2906  and can pull the receptacle connector  14 . 1 - 2940  towards the trim ring  14 . 1 - 2930 , the frame  14 . 1 - 2910 , and the side surface  14 . 1 - 2902  of the enclosure  14 . 1 - 2900 . The dummy trim ring can be inserted through the opening  14 . 1 - 2904  in the enclosure  14 . 1 - 2900 , the opening  14 . 1 - 2932  in the trim ring  14 . 1 - 2930 , and into an opening in the receptacle connector  14 . 1 - 2940  in order to align the receptacle connector  14 . 1 - 2940  with the trim ring  14 . 1 - 2930  and the enclosure  14 . 1 - 2900 . Although two fasteners  14 . 1 - 2924  are illustrated for attaching the receptacle connector  14 . 1 - 2940  to the trim ring  14 . 1 - 2930 , any number of the fasteners  14 . 1 - 2924  can be included. 
       FIGS.  14 . 1 - 33 A through  14 . 1 - 33 F  illustrate an example of a plug connector  14 . 1 - 3000  inserted into a receptacle connector  14 . 1 - 3030  and fastened in the receptacle connector  14 . 1 - 3030  by a lever arm  14 . 1 - 3010 . The plug connector  14 . 1 - 3000  includes a boot  14 . 1 - 3002  and a plug  14 . 1 - 3004  coupled to the boot  14 . 1 - 3002 . The boot  14 . 1 - 3002  includes recesses  14 . 1 - 3003 . 
     The lever arm  14 . 1 - 3010  can be included in a trim ring coupled to the receptacle connector  14 . 1 - 3030 . The lever arm  14 . 1 - 3010  is rotatable around a pivot  14 . 1 - 3012 . The lever arm  14 . 1 - 3010  includes a protrusion  14 . 1 - 3014  configured to interact with the recesses  14 . 1 - 3003  of the boot  14 . 1 - 3002  to retain the plug connector  14 . 1 - 3000  in the trim ring and the receptacle connector  14 . 1 - 3030 . The lever arm  14 . 1 - 3010  can be rotatable between an engaged configuration, with the protrusion  14 . 1 - 3014  disposed in a respective recess  14 . 1 - 3003 , and an un-engaged configuration, with the protrusion  14 . 1 - 3014  retracted from the recess  14 . 1 - 3003 . 
     The lever arm  14 . 1 - 3010  further includes a protrusion  14 . 1 - 3016 . The protrusion  14 . 1 - 3016  can be configured to interact with a tool  14 . 1 - 3060  to transition the lever arm  14 . 1 - 3010  from the engaged configuration to the un-engaged configuration. The lever arm  14 . 1 - 3010  includes a protrusion  14 . 1 - 3018  configured to interact with a spring mechanism  14 . 1 - 3020 . The spring mechanism  14 . 1 - 3020  can interact with the protrusion  14 . 1 - 3018  to retain the lever arm  14 . 1 - 3010  in the engaged configuration and can resist movement or forces of the lever arm  14 . 1 - 3010  transitioning to the un-engaged configuration. 
     The spring mechanism  14 . 1 - 3020  can include a central member  14 . 1 - 3022  extending into a spring  14 . 1 - 3024  and a stop  14 . 1 - 3026 . The protrusion  14 . 1 - 3018  interacts with the central member  14 . 1 - 3022  to compress the spring  14 . 1 - 3024  into the stop  14 . 1 - 3026 . This compression of the spring  14 . 1 - 3024  exerts a spring force on the protrusion  14 . 1 - 3018  through the central member  14 . 1 - 3022  to retain the lever arm  14 . 1 - 3010  in the engaged configuration, and to resist the lever arm  14 . 1 - 3010  from transitioning to the un-engaged configuration. 
     In some examples, the receptacle connector  14 . 1 - 3030  can include an exterior portion  14 . 1 - 3034  and an interior portion  14 . 1 - 3032 . For example, the exterior portion  14 . 1 - 14 . 1 - 3024  can at least partially encompass the interior portion  14 . 1 - 3032 . In other words, the exterior portion  14 . 1 - 3034  can act as a sleeve or shell that at least partially surrounds the interior portion  14 . 1 - 3032 . The exterior portion  14 . 1 - 3034  and interior portion  14 . 1 - 3032  can be interlocked or affixed to one another. In some examples, the receptacle connector  14 . 1 - 3030  can be formed from a singular or monolithic structure instead of the external portion  14 . 1 - 3034  and the internal portion  14 . 1 - 3032 . The receptacle connector  14 . 1 - 3030  can form a slot capable of receiving and retaining the plug connector  14 . 1 - 3000 . 
       FIGS.  14 . 1 - 33 B  is a detailed view of a region  14 . 1 - 3050  of  FIGS.  14 . 1 - 33 A . As illustrated in  FIGS.  14 . 1 - 33 A and  14 . 1 - 33 B , as the plug connector  14 . 1 - 3000  is inserted into a trim ring and the receptacle connector  14 . 1 - 3030 , the protrusion  14 . 1 - 3014  extends along a side surface of the boot  14 . 1 - 3002 . Once the plug connector  14 . 1 - 3000  is fully inserted into the trim ring and the receptacle connector  14 . 1 - 3030 , the protrusion  14 . 1 - 3014  can extend into the recess  14 . 1 - 3003  and can retain the plug connector  14 . 1 - 3000  in the trim ring and the receptacle connector  14 . 1 - 3030 . The protrusion  14 . 1 - 3014  can extend through a body portion  14 . 1 - 3046  of the trim ring. The spring  14 . 1 - 3024  of the spring mechanism  14 . 1 - 3020  is in a compressed state and pushes the protrusion  14 . 1 - 3014  of the lever arm  14 . 1 - 3010  against the side surface of the boot  14 . 1 - 3002  through the interaction between the central member  14 . 1 - 3022  and the protrusion  14 . 1 - 3018 . The lever arm  14 . 1 - 3010  is in the engaged configuration, such that the lever arm  14 . 1 - 3010  applies a force against the boot  14 . 1 - 3002  in a clockwise direction. 
       FIGS.  14 . 1 - 33 C and  14 . 1 - 33 D  illustrate the lever arm  14 . 1 - 3010  after the plug connector  14 . 1 - 3000  is fully inserted into the trim ring and the receptacle connector  14 . 1 - 3030 , such that the protrusion  14 . 1 - 3014  extend into the recess  14 . 1 - 3003 . Spring force from the spring  14 . 1 - 3024  of the spring mechanism  14 . 1 - 3020  through the central member  14 . 1 - 3022  and the protrusion  14 . 1 - 3018  causes the lever arm  14 . 1 - 3010  to rotate in a counterclockwise direction. The protrusion  14 . 1 - 3014  extends through the body portion  14 . 1 - 3046  of the trim ring into the recess  14 . 1 - 3003  of the boot  14 . 1 - 3002 . This is the engaged configuration in which the protrusion  14 . 1 - 3014  secures and fastens the boot  14 . 1 - 3002  into the trim ring and the receptacle connector  14 . 1 - 3030 . The spring  14 . 1 - 3024  of the spring mechanism  14 . 1 - 3020  is in a partially extended state and pushes the protrusion  14 . 1 - 3014  of the lever arm  14 . 1 - 3010  against a side surface of the boot  14 . 1 - 3002  adjacent the recess  14 . 1 - 3003  through the interaction between the central member  14 . 1 - 3022  and the protrusion  14 . 1 - 3018 . The lever arm  14 . 1 - 3010  is in the engaged configuration, such that the lever arm  14 . 1 - 3010  applies a force against the boot  14 . 1 - 3002  in a clockwise direction. 
       FIGS.  14 . 1 - 33 E and  14 . 1 - 33 F  illustrate the lever arm  14 . 1 - 3010  after the tool  14 . 1 - 3060  is used to actuate the lever arm  14 . 1 - 3010 , transitioning the lever arm  14 . 1 - 3010  from the engaged configuration to the un-engaged configuration. Force from the tool  14 . 1 - 3060  overcomes a spring force of the spring  14 . 1 - 3024  of the spring mechanism  14 . 1 - 3020  and causes the lever arm  14 . 1 - 3010  to rotate in a clockwise direction. The protrusion  14 . 1 - 3014  retracts from the recess  14 . 1 - 3003  through the body portion  14 . 1 - 3046  of the trim ring such that the plug connector  14 . 1 - 3000  can be removed from the trim ring and the receptacle connector  14 . 1 - 3030 . This is the un-engaged configuration in which the protrusion  14 . 1 - 3014  no longer secures and fastens the boot  14 . 1 - 3002  into the trim ring and the receptacle connector  14 . 1 - 3030 . The spring  14 . 1 - 3024  of the spring mechanism  14 . 1 - 3020  is in a compressed state and resists the force supplied by the tool  14 . 1 - 3060 . The lever arm  14 . 1 - 3010  is in the un-engaged configuration, such that the lever arm does not apply a force against the boot  14 . 1 - 3002 . 
     The spring mechanism  14 . 1 - 3020  can be part of the trim ring, or part of the receptacle connector  14 . 1 - 3030  and fastened to the receptacle connector  14 . 1 - 3030 . The lever arm  14 . 1 - 3010  and the spring mechanism  14 . 1 - 3020  can overlap portions of the plug connector  14 . 1 - 3000  and the receptacle  14 . 1 - 3030 , which can enable the fastener of  FIGS.  14 . 1 - 33 A through  14 . 1 - 33 F  to be formed in a smaller package, providing more space within an enclosure for other components, or providing for a smaller enclosure. 
       FIGS.  14 . 1 - 34 A and  14 . 1 - 34 B  illustrate perspective views of a cable assembly  14 . 1 - 3006  inserted into a trim ring  14 . 1 - 3040  and a receptacle connector  14 . 1 - 3030 . The cable assembly  14 . 1 - 3006  includes a shielded cable  14 . 1 - 3007  and a plug connector  14 . 1 - 3000 . The plug connector  14 . 1 - 3002  includes a boot  14 . 1 - 3002 . The boot includes recesses  14 . 1 - 3003  formed in the side surfaces thereof. 
     The trim ring  14 . 1 - 3040  includes a lever arm  14 . 1 - 3010  and a spring mechanism  14 . 1 - 3020 . The lever arm  14 . 1 - 3010  is provided for fastening the plug connector  14 . 1 - 3030  in the trim ring  14 . 1 - 3040  and the receptacle connector  14 . 1 - 3030 . The lever arm  14 . 1 - 3010  includes a protrusion  14 . 1 - 3014  configured to be inserted into a respective recess  14 . 1 - 3003  of the boot  14 . 1 - 3002  (such as by rotation around a pivot  14 . 1 - 3012 ) and to retain the plug connector  14 . 1 - 3002  within the trim ring  14 . 1 - 3040  and the receptacle connector  14 . 1 - 3030 . The lever arm  14 . 1 - 3010  includes a protrusion  14 . 1 - 3016  configured to interact with a tool in order to transition the lever arm  14 . 1 - 3010  to an un-engaged configuration and to release the plug connector  14 . 1 - 3000  from the trim ring  14 . 1 - 3040  and the receptacle connector  14 . 1 - 3030 . The lever arm  14 . 1 - 3010  includes a protrusion  14 . 1 - 3018  configured to interact with the spring mechanism  14 . 1 - 3020 , to retain the lever arm in the engaged configuration, and to resist force supplied by the tool through the protrusion  14 . 1 - 3016 . 
     The spring mechanism  14 . 1 - 3020  is provided for retaining the lever arm  14 . 1 - 3010  in an engaged configuration. The spring mechanism  14 . 1 - 3020  includes a central member  14 . 1 - 3022 , a spring  14 . 1 - 3024 , and a stop  14 . 1 - 3026 . The central member  14 . 1 - 3022  is provided for interacting with the protrusion  14 . 1 - 3018  of the lever arm  14 . 1 - 3010 . The spring  14 . 1 - 3024  is provided for providing a spring force to the central member  14 . 1 - 3022  to retain the lever arm  14 . 1 - 3010  in the engaged configuration through the protrusion  14 . 1 - 3018 . The stop  14 . 1 - 3026  is provided for compressing the spring  14 . 1 - 3024 , thereby supplying the spring force to the central member  14 . 1 - 3022 . 
     As illustrated in  FIGS.  14 . 1 - 34 A and  14 . 1 - 34 B , the lever arm  14 . 1 - 3010  can be disposed in a side surface of the trim ring  14 . 1 - 3040 , and the spring mechanism  14 . 1 - 3020  can be disposed on a top surface of the trim ring  14 . 1 - 3040  and/or the receptacle connector  14 . 1 - 3030 . The lever arm  14 . 1 - 3010  can include components extending through a portion of a sidewall of the trim ring  14 . 1 - 3040 , which allows for the trim ring  14 . 1 - 3040  and the lever arm  14 . 1 - 3010  to take up less space in an enclosure. The spring mechanism  14 . 1 - 3020  can be disposed in different position to maximize space use within the enclosure. 
     In some examples, the receptacle connector  14 . 1 - 3030  can include an exterior portion  14 . 1 - 3034  and an interior portion  14 . 1 - 3032 . For example, the exterior portion  2824  can at least partially encompass the interior portion  14 . 1 - 3032 . In other words, the exterior portion  14 . 1 - 3034  can act as a sleeve or shell that at least partially surrounds the interior portion  14 . 1 - 3032 . The exterior portion  14 . 1 - 3034  and interior portion  14 . 1 - 3032  can be interlocked or affixed to one another. In some examples, the receptacle connector  14 . 1 - 3030  can be formed from a singular or monolithic structure instead of the external portion  14 . 1 - 3034  and the internal portion  14 . 1 - 3032 . The receptacle connector  14 . 1 - 3030  can form a slot capable of receiving and retaining the plug connector  14 . 1 - 3000 . The receptacle connector  14 . 1 - 3030  can further include a printed circuit board (PCB)  14 . 1 - 3036 . The PCB  14 . 1 - 3036  can be coupled to the plug of the plug connector  14 . 1 - 3000  through the interior portion  14 . 1 - 3032 . The receptacle connector  14 . 1 - 3030  can be coupled to the trim ring  14 . 1 - 3040  through a portion of the interior portion  14 . 1 - 3032 . 
       FIGS.  14 . 1 - 35 A and  14 . 1 - 35 B  illustrate an exploded view and a side sectional view of a receptacle connector  14 . 1 - 3100 . The receptacle connector  14 . 1 - 3100  includes an interior portion  14 . 1 - 3102 , electrical contacts  14 . 1 - 3110 , a contact block  14 . 1 - 3112 , an anchor  14 . 1 - 3114 , and an exterior portion  14 . 1 - 3120 . The interior portion  14 . 1 - 3102  includes an opening  14 . 1 - 3104  for receiving a plug of a plug connector, openings  14 . 1 - 3106 A,  14 . 1 - 3106 B, and  14 . 1 - 3106 C for receiving the electrical contacts  14 . 1 - 3110  (e.g., the electrical contacts  14 . 1 - 3110 A,  14 . 1 - 3110 B, and  14 . 1 - 3110 C, respectively), and openings  14 . 1 - 3108  for fastening the interior portion  14 . 1 - 3102  to a trim ring or the like. 
     The electrical contacts  14 . 1 - 3110  are provided in the openings  14 . 1 - 3106  of the interior portion  14 . 1 - 3102 . The contact block  14 . 1 - 3112  can be used to isolate the electrical contacts  14 . 1 - 3110  from one another, and to aid in positioning the electrical contacts  14 . 1 - 3110  relative to the openings  14 . 1 - 3106  in the interior portion  14 . 1 - 3102 . The electrical contacts  14 . 1 - 3110  can be stitched to the anchor  14 . 1 - 3114 , which can be stitched to the interior portion  14 . 1 - 3102  to couple the electrical contacts  14 . 1 - 3110  to the interior portion  14 . 1 - 3102  of the receptacle connector  14 . 1 - 3100 . In some examples, the electrical contacts  14 . 1 - 3110  can be stitched to the contact block  14 . 1 - 3112  and/or the anchor  14 . 1 - 3114 . 
     The exterior portion  14 . 1 - 3120  can surround at least a portion of the interior portion  14 . 1 - 3102 . The exterior portion can include openings  14 . 1 - 3122  for fastening the exterior portion  14 . 1 - 3120  to the trim ring or the like. The openings  14 . 1 - 3122  can also fasten the exterior portion  14 . 1 - 3120  to the interior portion  14 . 1 - 3102 . 
     The following description is directed to certain example connectors (including example method steps of manufacturing these connectors) shown in  FIGS.  14 . 1 - 36 A through  14 . 1 - 41 B . In particular, the below description will include examples of electrical connectors with a protective overmold onto a flexible circuit board at a wire termination region. In some examples, the structural features of these electrical connectors can help mitigate potential issues (e.g., solder crack, electrical discontinuity, loading stresses, heat stresses, etc.) involved with over-molding elements with lower mechanical stiffness (e.g., a flex circuit board and wires). In particular implementations, and as discussed below, a stiffener element and a wrap element can allow such sensitive components to be overmolded in an effective manner. For instance, at least one of the stiffener element or the wrap element can help decrease high-temperature exposure to the flexible circuit board and wires during the overmolding process. 
       FIGS.  14 . 1 - 36 A and  14 . 1 - 36 B  respectively illustrate a semi-transparent view and a solid view of an electrical connector portion  14 . 1 - 3600  in accordance with one or more examples of the present disclosure. In  FIGS.  14 . 1 - 36 A , a connector assembly  14 . 1 - 3612  is hidden (or made partially transparent) for illustration purposes; whereas in  FIGS.  14 . 1 - 36 B , the connector assembly  14 . 1 - 3612  is depicted for viewing. The connector assembly  14 . 1 - 3612  can include an assembled (or partially assembled) harness with prongs, sleeves, frame members, etc. The connector assembly  14 . 1 - 3612  can thus be sized and shaped to receive electrical components inside a receptacle  14 . 1 - 3616  defined by the internal walls of the connector assembly  14 . 1 - 3612 . In particular implementations, the connector assembly  14 . 1 - 3612  is sized and shaped such that one or more electrical components can be withdrawn from inside the receptacle  14 . 1 - 3616  to expose at least a portion of one or more such components for manufacturing (as will be discussed below, and as shown in  FIGS.  14 . 1 - 36 A ). Once these manufacturing steps have been performed, the electrical components can be inserted (e.g., pushed) back into the receptacle  14 . 1 - 3616  (as will also be discussed below, and as shown in  FIGS.  14 . 1 - 36 B ). 
       FIGS.  14 . 1 - 36 A and  14 . 1 - 36 B  illustrate the electrical connector portion  14 . 1 - 3600  including a wrap  14 . 1 - 3602 . The wrap  14 . 1 - 3602  can include one or more elements that can help support or protect wires  14 . 1 - 3606  leading into a termination region. In certain examples, the wrap  14 . 1 - 3602  can help stiffen the wires  14 . 1 - 3606  (e.g., so as to decrease bending or flex at the wire termination region). Additionally or alternatively, the wrap  14 . 1 - 3602  help keep the wires  14 . 1 - 3606  from fraying under pressure from the overmold tooling. In these or other examples, the wrap  14 . 1 - 3602  includes a jacket, coating, or tape to at least partially wrap around the wires  14 . 1 - 3606  and provide the foregoing functionality. To do so, the wrap  14 . 1 - 3602  can be made of a variety of materials. In particular examples, however, the wrap  14 . 1 - 3602  includes at least one of a silicone jacket or an acetate tape. 
     The electrical connector portion  14 . 1 - 3600  further includes an overmold  14 . 1 - 3608  positioned adjacent to (e.g., over, around, at least partially encompassing, etc.) a wire termination region. The overmold  14 . 1 - 3608  can be injection molded, form molded, etc. The overmold can include a variety of materials. In some examples, the overmold  14 . 1 - 3608  includes a polymer material. In particular examples, the overmold  14 . 1 - 3608  includes a liquid crystal polymer material. 
     In addition, the electrical connector portion  14 . 1 - 3600  can include a stiffener  14 . 1 - 3610  and  14 . 1 - 3614 . The stiffener  14 . 1 - 3610  can include stiffener surfaces that can be used to glue and weld to portions of the connector assembly  14 . 1 - 3612 . Additionally or alternatively, the stiffener  14 . 1 - 3610  can include stiffener surfaces that can be used to glue or welded to portions of a flex circuit  14 . 1 - 3604 . In these or other examples, the attachment regions at the stiffener surfaces of the stiffener  14 . 1 - 3610  can help prevent assembly loading stresses from transferring to electrical components (e.g., the flex circuit  14 . 1 - 3604 ). Accordingly, the stiffener  14 . 1 - 2610  can help increase the likelihood of electrical continuity after the overmold and product assembly process. 
     The stiffener  14 . 1 - 3610  also includes surfaces that function as contact points for overmold tooling components (e.g., heated tooling surfaces). In some examples, surfaces of the stiffener  14 . 1 - 3610  can also include datum surfaces (e.g., to guide or center overmolded parts with respect to the tool structure, thereby reducing scalding risk). 
     In these or other examples, the stiffener  14 . 1 - 3610  includes one or more surfaces for hotbar termination of the flex circuit  14 . 1 - 3604  to an interfacing connector (shown in  FIG.  14 . 1 - 40   ). In these or other examples, one or more surfaces of the stiffener  14 . 1 - 3610  can be exposed (e.g., withdrawn from inside the receptacle  14 . 1 - 3616 ) to perform the hotbar termination, which reduces manufacturing complexity of needing to make the hotbar termination inside the connector assembly  14 . 1 - 3612 . 
     The stiffener  14 . 1 - 3610  can include a variety of materials. In some examples, the stiffener  14 . 1 - 3610  comprises a rigid material. In some examples, the stiffener  14 . 1 - 3610  comprises a conductive material. In some examples, the stiffener  14 . 1 - 3610  comprises a metal material. In particular examples, the stiffener  14 . 1 - 3610  comprises a steel stiffener. 
     The following description provides more detail at least with respect to one or more stiffener surfaces (e.g., datum surfaces).  FIGS.  14 . 1 - 37  through  14 . 1 - 38    illustrate respective top and bottom views of the electrical connector portion  14 . 1 - 3600  discussed above. As mentioned, the stiffener  14 . 1 - 3610  can include datum surfaces for contacting the overmold tooling. Examples of these datum surfaces are depicted as datum surfaces  14 . 1 - 3702 . The datum surfaces  14 . 1 - 3702  can include extensions, wings, jut-outs, arms, flaps, flat spots, faces, curved surfaces, etc. that are designed to contact the overmold tooling at specific locations. In these or other examples, the datum surfaces  14 . 1 - 3702  can be specially designed for custom tooling. Alternatively, the datum surfaces  14 . 1 - 3702  can be designed for many different overmold tooling. 
     The following description provides example method steps for manufacturing the electrical connector portion  14 . 1 - 3600  discussed above. Indeed,  FIGS.  14 . 1 - 39    illustrates example method steps  1 - 6  for manufacturing the electrical connector portion  14 . 1 - 3600  in accordance with one or more examples of the present disclosure. It will be appreciated that the following steps can be modified, omitted, or added thereto (as may be desired). 
     At step  1 , the flex circuit  14 . 1 - 3604  is provided. At step  2 , at least a portion of the stiffener  14 . 1 - 3610  is attached to the flex circuit  14 . 1 - 3604 . For example, the stiffener  14 . 1 - 3610  is adhered (e.g., via pressure sensitive adhesive) or fastened via screws or the like. 
     At step  3 , the wires  14 . 1 - 3606  (wrapped by the wrap  14 . 1 - 3602 ) are terminated onto the flex circuit  14 . 1 - 3604 . At this step, the stiffener  14 . 1 - 3610  acts as support during the termination process. At step  4 , the overmold  14 . 1 - 3608  is applied to the termination region. At step  5 , the wire harness portion assembled thus far can be inserted into the receptacle  14 . 1 - 3616  of the connector assembly  14 . 1 - 3612 . At step  6 , an end region of the electrical connector portion  14 . 1 - 3600  is left exposed for hot-bar termination (now discussed below in relation to  FIGS.  14 . 1 - 40   ). 
       FIGS.  14 . 1 - 40    depicts example method steps of providing an interface connector to the electrical connector portion  14 . 1 - 3600  discussed above in accordance with one or more examples of the present disclosure. At step  1 , the electrical connector portion  14 . 1 - 3600  is provided (e.g., as discussed above in connection with  FIGS.  14 . 1 - 39   ). At step  2 , the end region  14 . 1 - 3900  of the electrical connector portion  14 . 1 - 3600  is exposed. 
     At step  3 , a connection  14 . 1 - 4002  is formed between the stiffener  14 . 1 - 3610  and an interface connector  14 . 1 - 4000  (e.g., a plug-in connection, prong connection, magnetic connection, etc.). In these or other examples, the connection  14 . 1 - 4002  includes a hot-bar connection, welding connection, solder connection, metal bond connection, etc. In some examples, the stiffener  14 - 3610  is used to transfer loading stresses from the interface connector  14 . 1 - 4000 , thereby bypassing stresses to the sensitive electrical components (e.g., the flex circuit board). At step  4 , the interface connector  14 . 1 - 4000  is pushed back into (e.g., flush with) the electrical connector portion  3600 . 
       FIGS.  14 . 1 - 41 A and  14 . 1 - 41 B  show side schematic views of the assembling the interface connector  14 . 1 - 4000  to the electrical connector portion  14 . 1 - 3600  in accordance with one or more examples of the present disclosure. In particular,  FIGS.  14 . 1 - 41 A  illustrates the end region  14 . 1 - 3900  of the electrical connector portion  3600  exposed for attaching the interface connector  14 . 1 - 4000 . Then,  FIGS.  14 . 1 - 41 B  illustrates the end region  14 . 1 - 3900  of the electrical connector portion  3600  pushed inside the receptacle of the connector assembly (with the interface connector  14 . 1 - 4000  flush against the electrical connector portion  14 . 1 - 3600 ). 
     As mentioned above in relation to  FIGS.  14 . 1 - 40   , the stiffener  14 . 1 - 3610  can prevent the transfer of stress between the interface connector. To do so, the stiffener  14 . 1 - 3610  can include one or more attachment regions to the various components. For example, the stiffener  14 . 1 - 3610  can attach to the flex circuit  14 . 1 - 3604  at the attachment region  14 . 1 - 4104 , which is located adjacent to the interface connector  14 . 1 - 4000  at an underside of the stiffener  14 . 1 - 3610 . Additionally or alternatively, a top side of a stiffener  14 . 1 - 4110  can attach to the flex circuit  14 . 1 - 3604  at an attachment region  14 . 1 - 4108  positioned adjacent to the overmold  14 . 1 - 3608 . The underside of the stiffener  14 . 1 - 4110  can be attached to the connector assembly  14 . 1 - 3612  (e.g., a frame member of the connector assembly). 
     Additionally shown, an underside of the stiffener  14 . 1 - 3610  can attach to the topside of the interface connector  14 . 1 - 4000  at an attachment region  14 . 1 - 4102 . Further, a top side of the flex circuit  14 . 1 - 3604  can attach to the bottom side of the interface connector  14 . 1 - 4000 . In addition, the wires  14 . 1 - 3606  can attach to the flex circuit  14 . 1 - 3604  at an attachment region  14 . 1 - 4100 . 
     With the foregoing attachments at the attachment regions discussed, the stiffener  14 . 1 - 3610  can avoid the transfer of loading stress from the interface connector  14 . 1 - 4000  to either of the flex circuit  14 . 1 - 3604  (at associated attachment regions) or the wires (and associated attachment regions). The foregoing configuration can also enable the flex circuit  14 . 1 - 3604  to be rolled up or folded (e.g., into a serpentine pattern) for positioning/storage of the extended portions of the flex circuit  14 . 1 - 3604 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  14 . 1 - 36  through  14 . 1 - 41 B  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  14 . 1 - 36  through  14 . 1 - 41 B . 
     14.2: Modular Components for Wearable Electronic Devices 
     As virtual reality (VR) and mixed reality (MR) become more ubiquitous, the demand for user friendly head-mounted displays with quality components increases. Traditionally, these VR/MR systems have been devices that include a wearable display component, often referred to as a head-mounted display (HMD) and a supplemental unit. The supplemental unit can be part of the HMD, or can be a separate component that is permanently connected to the HMD through a cable, wires, or other conductors. The supplemental unit can provide power and/or added processing functionality to the system. The components of these systems are not interchangeable and thus only have a single configuration. VR/MR systems are increasingly being used in a variety of different environments and scenarios. Thus, it can be desirable for a VR/MR system to have different properties depending on the desired use or uses. 
     For example, certain components can maximize comfort levels and audio quality, while other components can reduce the weight of the wearable and maximize securement of the device. The modular nature of a wearable VR/MR system and its associated components can allow particular components to be selected for incorporation into the device. For example, the components that maximize comfort and audio quality can be selected for scenarios where a user may wear a device for long periods of time, while the components that maximize weight reduction and securement may be selected for scenarios where a user desired to be highly mobile. 
     Further, traditional VR/MR systems, including HMD systems and devices, have packaged most or all of the components of the device in a housing that is worn on the user&#39;s face. Even in systems that contain a separate processing unit, almost all of the electronic components of the HMD portion are packaged in a single housing that is mounted to the user&#39;s head. In addition to reducing the customizability of the HMD device, this type of design can concentrate the devices weight in a single location, potentially reducing comfort for the user. In contrast, the HMD devices and systems described herein can include multiple modular and connectable parts that can distribute the electronic and functional components of the device in more than one location, increasing both modularity and comfort for a user. 
     These and other embodiments are discussed below with reference to  FIGS.  14 . 2 - 1  through  14 . 2 - 13   . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these figures is for explanatory purposes only and should not be construed as limiting. Furthermore, as used herein, a system, a method, an article, a component, a feature, or a sub-feature including at least one of a first option, a second option, or a third option should be understood as referring to a system, a method, an article, a component, a feature, or a sub-feature that can include one of each listed option (e.g., only one of the first option, only one of the second option, or only one of the third option), multiple of a single listed option (e.g., two or more of the first option), two options simultaneously (e.g., one of the first option and one of the second option), or combination thereof (e.g., two of the first option and one of the second option). 
       FIGS.  14 . 2 - 1 A  shows a wearable electronic device  14 . 2 - 100  being worn on the head  14 . 2 - 101  of a user. The wearable electronic device  14 . 2 - 100  can include a number of modular components. For example, the wearable device  14 . 2 - 100  can include a head-mounted display (HMD)  14 . 2 - 110  including a housing  14 . 2 - 105  and a display  14 . 2 - 107  attached to the housing for displaying images to a user (shown in  FIGS.  14 . 2 - 1 B ), one or more intermediate members, flexible straps, or connector straps  14 . 2 - 130 , a retention band  14 . 2 - 120  that can be a flexible band or a band that is moldable to a user&#39;s head, and an optional supplemental unit  14 . 2 - 140  such as an external power supply, memory component, and/or processor. Although referred to as a wearable electronic device  14 . 2 - 100 , it should be understood that the device  14 . 2 - 100  can include multiple modular components or devices and can be interchangeably referred to as a wearable electronic device, wearable electronic device system, and/or wearable electronic system. Additionally, although the particular component  14 . 2 - 110  can be referred to as an HMD, it should be understood that the terms HMD, HMD device, and/or HMD system can be used to refer to the wearable device  14 . 2 - 100  as a whole. 
     In some examples, and as shown, the device  14 . 2 - 100  can be worn on the user&#39;s head  14 . 2 - 101  such that the HMD  14 . 2 - 110  is worn on the user&#39;s face and disposed over one or both of their eyes. The HMD can be removably and/or releasably connected to the connector strap or straps  14 . 2 - 130  as described further herein. In some examples, the connector straps  14 . 2 - 130  can be positioned against the side of a user&#39;s head  14 . 2 - 101  and in contact therewith. In some examples, the connector straps  14 . 2 - 130  can be positioned above the user&#39;s ear  14 . 2 - 150  or ears, as shown. In some examples, the connector straps  14 . 2 - 130  can be positioned adjacent to the user&#39;s ear or ears. The connector straps  14 . 2 - 130  can be removably connected to the retention band  14 . 2 - 120 , which can extend around the user&#39;s head  14 . 2 - 101  and removably connect to the other of the connector straps  14 . 2 - 130  (not shown). In this way, the HMD, connector straps  14 . 2 - 130 , and retention band  14 . 2 - 120  can form a loop that can retain the wearable electronic device  14 . 2 - 100  on the user&#39;s head  14 . 2 - 101 . As shown in  FIGS.  14 . 2 - 1 A , the connector straps  14 . 2 - 130  can connect to the HMD  14 . 2 - 110 , both mechanically and electrically, at an HMD connection location  14 . 2 - 154  that can include an electrical input or electrical connector that is attached to the housing  14 . 2 - 105  and electrically connected to the display  14 . 2 - 107  (as illustrated in  FIG.  14 . 2 - 2   ). This location can be identified as a temple area “Y” that can be defined as an area near a user&#39;s temple adjacent to the user&#39;s eye, and can span from in front of the user&#39;s eye to approximately 1-1.5 inches past the outer corner of a user&#39;s eye, along the side of the user&#39;s head. Similarly, the connector straps  14 . 2 - 130  can connect to the retention band  14 . 2 - 120  at a retention band connection location  14 . 2 - 152  identified as an ear area “X” that can span to include the area above the user&#39;s ear  14 . 2 - 150  or within 0.5 inches of the outer edge of the ear on either side. In this manner, the connector straps  14 . 2 - 130  are able to provide structural support between the HMD  14 . 2 - 110  and the user&#39;s ear  14 . 2 - 150 , while securely connecting the retention band  14 . 2 - 120  and translating the retention forces of the retention band through the wearable electronic device  14 . 2 - 100 . It should be understood, however, that this configuration is just one example of how the components of a modular wearable electronic device  14 . 2 - 100  can be arranged, and that in some, a different number of connector straps and/or retention bands can be included. 
       FIGS.  14 . 2 - 1 B  shows a top view of the wearable electronic device  14 . 2 - 100 , demonstrating how the components of the device  14 . 2 - 100  can be mechanically and/or electrically connected to one another in some examples. In the example shown in  FIGS.  14 . 2 - 1 B , a first connector strap  14 . 2 - 130  and a second connector strap  14 . 2 - 131  can each be removably connected or coupled to either side of the HMD  14 . 2 - 110 . In some examples, the connector straps  14 . 2 - 130 ,  14 . 2 - 131  can both be electrically and mechanically coupled or connected to the HMD and can operate as an intermediate member located between the HMD and the retention band  14 . 2 - 120 . In some examples, one of the connector straps  14 . 2 - 130 ,  14 . 2 - 131  can be mechanically connected, but may not be electrically connected to the HMD. The connector straps  14 . 2 - 130 ,  14 . 2 - 131  can each be connected to the retention band  14 . 2 - 120  at a different location on the connector strap  14 . 2 - 130 ,  14 . 2 - 131  than the connection to the HMD. For example, the HMD can be connected to a first end of each connector strap  14 . 2 - 130 ,  14 . 2 - 131 , and the retention band  14 . 2 - 120  can be connected to a second, opposite end of each connector strap  14 . 2 - 130 ,  14 . 2 - 131 . 
     In some examples, and as shown, at least one of the connector straps  14 . 2 - 130  can also be connected to a supplemental unit  14 . 2 - 140 . The supplemental unit  14 . 2 - 140  can contain one or more processors, batteries or power supplies, antennas, and any other electrical and/or processing components, as desired. In some examples, power and/or data from the supplemental unit can be provided to the HMD through the connector strap  14 . 2 - 130  via a connection between the supplemental unit  14 . 2 - 140  and the connector strap  14 . 2 - 130 , as described herein. In the present example, a conductor or cable  14 . 2 - 142  can electrically connect the supplemental unit  14 . 2 - 140  to the connector strap  14 . 2 - 130 , through which the supplemental unit can provide power and/or communicate with the HMD  14 . 2 - 110 , second connector strap  14 . 2 - 131 , and/or retention band  14 . 2 - 120 . 
     Although the supplemental unit  14 . 2 - 140  is shown as being connected to the connector strap  14 . 2 - 130  through a wired connection  14 . 2 - 142 , it should be understood that in some examples the supplemental unit  14 . 2 - 140  may wirelessly connect or communicate data and/or power with the connector strap  14 . 2 - 130  and/or HMD  14 . 2 - 110  by any desired method or technology. Additionally, in some examples, the supplemental unit  14 . 2 - 140  can be incorporated into, or can be an integral part of, one or more of the other components of the device  14 . 2 - 100 , including the retention band  14 . 2 - 120 , the connector straps  14 . 2 - 130 ,  14 . 2 - 131 , and/or the HMD  14 . 2 - 110 . Further, although the components of the wearable electronic device  14 . 2 - 100  are shown as being connected to one another at certain locations, it should be understood that any of the components of the device  14 . 2 - 100  can be electrically and/or mechanically connected to one or more of any of the other components of the device  14 . 2 - 100 , in any manner and location, as desired. 
       FIGS.  14 . 2 - 1 C  shows an exploded view of the wearable electronic device  14 . 2 - 100 , including the locations of connectors on the various components, as described herein. As can be seen, the connector strap  14 . 2 - 130  can include a first connector  14 . 2 - 132 , also referred to as an HMD connector, to provide electrical communication between the connector strap  14 . 2 - 130  and the HMD  14 . 2 - 110  by removably or releasably attaching to a corresponding connector  14 . 2 - 112  on the HMD  14 . 2 - 110 . As used herein, providing electrical communication between two elements can include providing any type of electrical signal including power and/or data. Additionally, in some examples, an intermediate component can include conductive elements that are in electrical communication with two or more components and can transmit an electrical signal from one component to another. In other words, multiple components can be in electrical communication with one another, including components that conduct electrical signals, and not just components that are transmitting or receiving the electrical signals. The first connector  14 . 2 - 132  can be positioned at a first end of the connector strap  14 . 2 - 130 , and/or the flexible housing thereof. The connector strap  14 . 2 - 130  can also include a second connector  14 . 2 - 134  positioned at a second end, such as an opposite end, of the connector strap  14 . 2 - 130 . The second connector  14 . 2 - 134  can be referred to as a retention band connector or a flexible band connector, and can be removably connectable to the retention band  14 . 2 - 120  by attaching to a corresponding connector  14 . 2 - 124  on the retention band  14 . 2 - 120 . The second connector  14 . 2 - 134  can at least partially define an external surface of the connector strap  14 . 2 - 130 . 
     The connector strap  14 . 2 - 130  can further include a third connector  14 . 2 - 136 , also referred to as a supplemental unit connector, that can provide electrical communication between the connector strap  14 . 2 - 130  and the supplemental unit  14 . 2 - 140 , such as through a corresponding connector  14 . 2 - 141  that can be coupled to the conductor  14 . 2 - 142 . In some examples, the supplemental unit connector  14 . 2 - 136  can be positioned at or near an end of the flexible housing of the connector strap  14 . 2 - 130 , opposite the first end. In some examples, the connector  14 . 2 - 136  can include a mechanical connector surrounding an electrical connector, and the connector  14 . 2 - 141  can be configured to rotate relative to the supplemental unit connector  14 . 2 - 136  when connected thereto to facilitate placement and relative freedom for the positioning of the supplemental unit  14 . 2 - 140 . 
     Similar to the connector strap  14 . 2 - 130 , the second connector strap  14 . 2 - 131  can include a first connector  14 . 2 - 133 , also referred to as an HMD connector or a display connector, and a second connector  14 . 2 - 135 , also referred to as a retention band connector. These connectors  14 . 2 - 133 ,  14 . 2 - 135  can be positioned at or near opposite ends of the housing of the connector strap  14 . 2 - 131 . The connectors  14 . 2 - 133 ,  14 . 2 - 135 , can have substantially the same or similar functionalities as any of the connectors  14 . 2 - 132 ,  14 . 2 - 134 ,  14 . 2 - 136 . In some examples, the connector  14 . 2 - 133  can be releasably attached to a corresponding connector of the HMD (not shown), and the connector  14 . 2 - 135  can be releasably attached to a corresponding connector  14 . 2 - 125  of the retention band  14 . 2 - 120 . 
     Any number or variety of components in any of the configurations described herein can be included in a wearable electronic device, such as the HMD devices and/or HMD systems described herein. The components can include any combination of the features described herein, and can be arranged in any of the various configurations described herein. The structure and arrangement of components of a device, as well as the concepts regarding their use can apply not only to the specific examples discussed herein, but to any number of embodiments in any combination. Various examples of electronic devices and electronic device components including some having various features in various arrangements are described below, with reference to  FIGS.  14 . 2 - 2 A through  14 . 2 - 4   . 
       FIGS.  14 . 2 - 2 A  shows an exploded view of a wearable electronic device  14 . 2 - 200  that can include modular components which can be selectively and removably or releasably attached or coupled to one another as described herein. In some examples, the device  14 . 2 - 200  can be substantially similar to, or can include some or all of the features of the other wearable electronic devices described herein. As with the electronic device  14 . 2 - 100 , the device  14 . 2 - 200  can include an HMD  14 . 2 - 210 , a retention band  14 . 2 - 220 , and one or more connector straps  14 . 2 - 230 ,  14 . 2 - 231 . The device  14 . 2 - 200  can also include a supplemental unit (not shown) that can be connected to one or more of the components of the device  14 . 2 - 200  to provide additional functionality, power, processing speed, and the like. 
     Similar to the connector strap  14 . 2 - 130 , the connector strap  14 . 2 - 230  can include an HMD connector  14 . 2 - 232  that can releasably couple to a corresponding connector  14 . 2 - 212 , a retention band connector  14 . 2 - 234  that can releasably couple to a corresponding connector  14 . 2 - 224 , and a supplemental unit connector  14 . 2 - 236  that can releasably couple to a supplemental unit. In some examples, the supplemental unit connector  14 . 2 - 236  includes a battery input and/or a data input, and can alternatively be referred to as a power connector and/or a data connector. In some examples, the HMD connected  14 . 2 - 232  can be positioned at an edge of the connector strap  14 . 2 - 230 , while the retention band connector  14 . 2 - 234  and/or the supplemental unit connector  14 . 2 - 236  can be positioned at a major surface(s) of the connector strap  14 . 2 - 230 , such as a side surface(s). 
     In some examples, and as shown, the retention band connector  14 . 2 - 234  and its associated connector  14 . 2 - 224  on the retention band  14 . 2 - 220 , can include multiple connection points. In some examples, these multiple connection points can be aligned in a single line and can thus rotationally fix the connector strap  14 . 2 - 230  and the retention band  14 . 2 - 220  to one another. In some examples, any other type of connection can be implemented that can provide rotational fixation between the connector strap  14 . 2 - 230  and the retention band  14 . 2 - 220 . In some examples, where the device  14 . 2 - 200  includes a second connector strap  14 . 2 - 231 , that connector strap  14 . 2 - 231  can also include an HMD connector  14 . 2 - 233  and a retention band connector  14 . 2 - 235  that can be releasably coupled to a corresponding connector  14 . 2 - 225  so as to be rotationally fixed thereto. 
     In some examples, and as described further herein, one or more of the connector straps  14 . 2 - 230 ,  14 . 2 - 231  can include one or more operational elements or components  14 . 2 - 237 ,  14 . 2 - 238  disposed at least partially in an internal volume defined by the housing of the connector strap  14 . 2 - 230 ,  14 . 2 - 231 . The operational element, component, or components  14 . 2 - 237 ,  14 . 2 - 238  can include any number of functional and/or electrical components, including output components, such as an audio module or component, display module, or data output component, input components, such as a touch sensor or touch sensitive element or component, button, toggle, switch, fingerprint sensor, camera, or other form of sensor, or any other functional and/or operational component desired. Further details of the connector straps  14 . 2 - 230 ,  14 . 2 - 231  are described below with respect to  FIGS.  14 . 2 - 2 B through  14 . 2 - 2 D . 
       FIGS.  14 . 2 - 2 B  shows a side view of a connector strap  14 . 2 - 231  of the electronic device  14 . 2 - 200  shown in  FIGS.  14 . 2 - 2 A , including an HMD connector  14 . 2 - 233  disposed at an end of the strap  14 . 2 - 231 , and a retention band connector  14 . 2 - 235  including multiple connection locations disposed at an opposite end of the strap  14 . 2 - 231 . As can be seen, an operational component  14 . 2 - 237  can be disposed at least partially in the internal volume of the strap  14 . 2 - 231  and between the HMD connector  14 . 2 - 233  and the retention band connector  14 . 2 - 235 . In some examples, and as described further herein, the housing of the strap  14 . 2 - 231  can be raised or offset at the location of the operational component  14 . 2 - 237 , as desired. In some examples, the operational component  14 . 2 - 237  itself can at least partially define an exterior surface of the strap  14 . 2 - 231 . Additionally, and as described with respect to  FIGS.  14 . 2 - 2 D , the operational component  14 . 2 - 237  can be electrically connected to at least one of the first connector  14 . 2 - 233  or the second connector  14 . 2 - 235 . 
       FIGS.  14 . 2 - 2 C  shows a side view of the other connector strap  14 . 2 - 230  of the wearable electronic device  14 . 2 - 200 . Whereas the connector strap  14 . 2 - 231  of  FIGS.  14 . 2 - 2 B  included an HMD connector  14 . 2 - 233  and a retention strap connector  14 . 2 - 235 , but may not have included a supplemental unit connector, the connector strap  14 . 2 - 230  of  FIGS.  14 . 2 - 2 C  includes an HMD connector  14 . 2 - 232 , a retention band connector (not shown), and a supplemental unit connector  14 . 2 - 236 . As with the connector strap  14 . 2 - 231 , the strap  14 . 2 - 230  can include an operational component  14 . 2 - 238  that can be disposed between the HMD connector  14 . 2 - 232  and the supplemental unit connector  14 . 2 - 236 , and at least partially within an internal volume defined by the housing  14 . 2 - 239  of the strap  14 . 2 - 230 . 
       FIG.  14 . 2 - 2 D  shows a cross-sectional or partial cut-away view of the connector strap  14 . 2 - 230  shown in  FIGS.  14 . 2 - 2 A and  14 . 2 - 2 C . In this view, a portion of the housing  14 . 2 - 239 , as well as the operational component  14 . 2 - 238 , are not shown in order to illustrate the internal volume of the strap  14 . 2 - 230 . As can be seen, in some examples, the HMD connector  14 . 2 - 232  can be electrically connected to the supplemental unit connector  14 . 2 - 236 . In this particular example, a transmission array or transmission component(s)  14 . 2 - 251 ,  14 . 2 - 253 , such as wires or other conductive elements, can be disposed in the internal volume to electrically connect the HMD connector  14 . 2 - 232  and the supplemental unit connector  14 . 2 - 236 , such as to provide data and/or power therebetween. In some examples, the transmission array can include multiple conductors, cables, wires, or other transmission lines  14 . 2 - 251 ,  14 . 2 - 253  for transmitting data and/or power between the HMD connector  14 . 2 - 232  and the supplemental unit connector  14 . 2 - 236 . In some examples, the lines  14 . 2 - 251 ,  14 . 2 - 253  can be in communication to the connectors through connection points  14 . 2 - 250 , as shown. Additionally, in some examples, the supplemental unit connector  14 . 2 - 236  can include a printed circuit board  14 . 2 - 256  that can be at least partially disposed in the internal volume. In some examples, the connector strap  14 . 2 - 230  can include a processor or a processing unit  14 . 2 - 257 . The conductors  14 . 2 - 251 ,  14 . 2 - 253  of the transmission array can be coupled to one or more locations on this printed circuit board  14 . 2 - 256 , and/or can be directly connected to the supplemental unit connector  14 . 2 - 236 . 
     In some examples, the housing  14 . 2 - 239  of the strap  14 . 2 - 230  can include a relatively flexible material, such as polymeric material, fabric, and/or any desired flexible material or materials. In some examples, the strap  14 . 2 - 230  can further include a stiffening component or a stiffener  14 . 2 - 254  disposed at least partially in the internal volume defined by the housing  14 . 2 - 239 . In some examples, the stiffener  14 . 2 - 254  can be positioned at any desired location or locations in the internal volume, and can extend along any portion of the length of the strap  14 . 2 - 230 . In some examples, the stiffener  14 . 2 - 254  can be disposed adjacent to the internal components associated with one or both of the HMD connector  14 . 2 - 232 , the supplemental unit connector  14 . 2 - 236 , and/or the retention band connector (not shown). 
     In some examples, such as where the housing  14 . 2 - 239  of the strap  14 . 2 - 230  is relatively flexible, the stiffener  14 . 2 - 254  can serve to provide structural support to the strap  14 . 2 - 230  in one or more directions. For example, the stiffener  14 . 2 - 254  can be flexible along a first axis, and rigid along at least one axis that is perpendicular to the first axis. In some examples, the stiffener  14 . 2 - 254  can be flexible along a first axis and can be rigid along two axes that are perpendicular to the first axis and to each other. That is, the stiffener  14 . 2 - 254  can be flexible along a first bend direction, such as a direction extending into and out of the page of  FIG.  14 . 2 - 2 D , while being rigid in one or more other bend directions, such as one or more directions parallel to the plane of the page of  FIGS.  14 . 2 - 2 D . In some examples, the stiffener  14 . 2 - 254  can include a polymer material, metallic material, and/or combinations thereof. In some examples, the stiffener  14 . 2 - 254  can have a higher stiffness than the material including the housing  14 . 2 - 239  in at least one bend direction or axis. In some examples, the stiffener  14 . 2 - 254  can include metal in the form of a metallic sheet. Further details regarding the connector straps of a modular wearable electronic device, including details regarding the operational components thereof are provided with respect to  FIGS.  14 . 2 - 3  and  14 . 2 - 4   . 
       FIGS.  14 . 2 - 3    shows a side view of a component  14 . 2 - 330  that can be used as part of a module wearable electronic device as described herein. In some examples, the component  14 . 2 - 330  can be a connector strap  14 . 2 - 330  and can be substantially similar to, or can include some or all of the features of the other connector straps and/or electronic device described herein. The example shown in  FIGS.  14 . 2 - 3    can be substantially similar to the connector strap  14 . 2 - 230  shown in  FIGS.  14 . 2 - 2 C , and can include an HMD connector  14 . 2 - 332 , a supplemental unit connector  14 . 2 - 336 , and a retention band connector (not shown) disposed on an opposite side of the supplemental unit connector  14 . 2 - 336 . 
     In the example shown in  FIGS.  14 . 2 - 3   , the operational component  14 . 2 - 338  of the strap  14 . 2 - 330  can include a data output component. That is, the operational component  14 . 2 - 338  can be configured to transmit data from the strap  14 . 2 - 330  and/or any of the components of a wearable device in communication with the strap, such as an HMD and/or supplemental unit, to another electronic device. In some examples, the output device  14 . 2 - 338  can output data at a relatively high bitrate. In some examples, the output component  14 . 2 - 338  can have a direct electrical connection to a secondary device through a conductor or cable  14 . 2 - 339  as shown. In some examples, however, the output component  14 . 2 - 338  can include one or more antennas to wireless transmit data to a secondary device as desired. 
     In some examples, a user can include the strap  14 . 2 - 330  including the output component  14 . 2 - 338  in a module electronic device in place of, for example the strap  14 . 2 - 230  shown in  FIGS.  14 . 2 - 2 A,  14 . 2 - 2 C, and  14 . 2 - 2 D  in situations where high throughput transfer of data from the wearable device to a secondary device is desired. In other scenarios where this functionality may not be needed or desired, however, the strap  14 . 2 - 230  can be included in the wearable device instead. 
       FIGS.  14 . 2 - 4    shows a side view of a component  14 . 2 - 430  that can be used as part of a module wearable electronic device, as described herein. In some examples, the component  14 . 2 - 430  can be a connector strap  14 . 2 - 430  and can be substantially similar to, or can include some or all of the features of the other connector straps and/or electronic device described herein. The example shown in  FIGS.  14 . 2 - 4    can be substantially similar to the connector strap  230  shown in  FIGS.  14 . 2 - 2 C , and can include an HMD connector  14 . 2 - 432 , a supplemental unit connector  14 . 2 - 436 , and a retention band connector (not shown) disposed on an opposite side of the supplemental unit connector  14 . 2 - 436 . 
     The strap  14 . 2 - 430  can also include an operational component  14 . 2 - 439  including a visual or display component. In the example shown in  FIGS.  14 . 2 - 4   , the operational component  14 . 2 - 439  can take the form of one or more LEDs that can be selectively illuminated in order to visually display information. In some examples, the operational component  14 . 2 - 439  can include any other type of display or display technology, or combinations thereof. In some examples, the display  14 . 2 - 439  can be used to present visual information to persons other than the user when the wearable electronic device of which the strap  14 . 2 - 430  is part. In some examples, the display  14 . 2 - 439  can be configured to present visual information to a user when the wearable device is not being worn. For example, the display  14 . 2 - 439  can communicate a battery level of the device before a user puts the device on. Further, as shown, the strap  14 . 2 - 430  can include a second additional operational component  14 . 2 - 438  in addition to the display  14 . 2 - 439 . 
     Any number or variety of components in any of the configurations described herein can be included in a wearable electronic device, such as the HMD devices and/or HMD systems described herein. The components can include any combination of the features described herein, and can be arranged in any of the various configurations described herein. The structure and arrangement of components of a device, as well as the concepts regarding their use can apply not only to the specific examples discussed herein, but to any number of embodiments in any combination. Various examples of electronic devices and electronic device components including some having various features in various arrangements are described below, with reference to  FIGS.  14 . 2 - 5 A through  14 . 2 - 10 C . 
       FIGS.  14 . 2 - 5 A  shows a top view of a component  14 . 2 - 530  that can be part of, or can be used with, a wearable electronic device, as described herein. In some examples, the component  14 . 2 - 530  can be a connector strap  14 . 2 - 530  and can be substantially similar to, or can include some or all of the features of the other connector straps and/or electronic device described herein. As with other connector straps described herein, the connector strap  14 . 2 - 530  can include a first connector  14 . 2 - 532  that can be an HMD connector  14 . 2 - 532 , a second connector  14 . 2 - 536  that can be a supplemental unit connector, and a third connector  14 . 2 - 534  that can be a retention band connector. As described herein, in some examples, the retention band connector  14 . 2 - 534  can include multiple connection portions. In the example shown in  FIGS.  14 . 2 - 5 A , the retention band connector  14 . 2 - 534  can include a first connection portion and a second connection portion  14 . 2 - 534 ′ spaced apart from the first connection portion on the same side of the connector strap  14 . 2 - 530 . 
     The connector strap  14 . 2 - 530  can have a housing  14 . 2 - 539  that can at least partially define an internal volume and an offset surface that is proud of a major surface of the housing  14 . 2 - 539 . That is, the housing can define a protrusion or protruding portion  14 . 2 - 538  and one or more operational components can be positioned in or at this protrusion  14 . 2 - 538 . Additionally, in some examples, one or more connectors of the connector strap  14 . 2 - 530  can also be positioned at or in this protrusion  14 . 2 - 538 . For example, the supplemental unit connector  14 . 2 - 536  can be positioned at least partially at an external surface of the housing  14 . 2 - 539  defined by the protrusion  14 . 2 - 538 . In some examples, the external surface of the housing can include the outer surface of the housing. In some additional examples, the external surface can further include an outer surface of the housing that is oriented away from a user&#39;s head when worn by the user. 
       FIGS.  14 . 2 - 5 B  shows a side view of the connector strap  14 . 2 - 530 . In addition to defining an internal volume and external surface of the connector strap  14 . 2 - 530 , the housing  14 . 2 - 539  of the connector strap  14 . 2 - 530  can define one or more openings or ports  14 . 2 - 533 . In some examples, the housing  14 . 2 - 539  can define one or more audio ports  14 . 2 - 533  that are acoustically connected to an audio module or speaker and can allow an audio module or a speaker positioned in the internal volume defined by the housing  14 . 2 - 539  to communicate with the ambient environment. In some examples, the housing  14 . 2 - 539  can define two opposing major surfaces and the audio port  14 . 2 - 533  can be position at an edge or a minor surface joining the two opposing surfaces. Additionally, while one of the major surfaces, such as the major surface shown in  FIGS.  14 . 2 - 5 B  can be interrupted by a protrusion  14 . 2 - 538 , at least one of the major surfaces (not shown) can extend an entire length of the connector strap  14 . 2 - 530  and/or an entire length of the internal volume of the strap  14 . 2 - 530 . 
       FIGS.  14 . 2 - 5 C  shows a cross-sectional view of the connector strap  14 . 2 - 530  in the position shown in  FIGS.  14 . 2 - 5 B , but with a major surface of the housing  14 . 2 - 539  not shown in order to illustrate the positioning of components in the internal volume. In the present example, the strap  14 . 2 - 530  can include two or more operational components  14 . 2 - 557 ,  14 . 2 - 558 , at least one or which can be positioned at the protrusion  14 . 2 - 538 . In some examples, a first operational component  14 . 2 - 557  can include a first audio module disposed in the internal volume and can be configured to output a first range of audio frequencies, while a second operational component  14 . 2 - 558  can include a second audio module disposed in the internal volume and configured to output a second range of audio frequencies different than the first range. Additionally drivers of the first and/or second audio modules  14 . 2 - 557 ,  14 . 2 - 558  can be in communication with the audio port  14 . 2 - 533  in order to output sound therefrom. In some examples, one or more of the audio modules  14 . 2 - 557 ,  14 . 2 - 558  can have open-box or ported speaker construction or closed-box or sealed speaker construction. In some examples, one audio module  14 . 2 - 557  can function as a tweeter, while the second audio module  14 . 2 - 558  can function as a woofer. 
     As with other straps  14 . 2 - 530  described herein, the strap  14 . 2 - 530  can include a stiffener  14 . 2 - 552  that is flexible in a first direction, and rigid in at least one direction that is perpendicular to the first direction. One or more of the operational components  14 . 2 - 557 ,  14 . 2 - 558  and/or connectors  14 . 2 - 536  can be positioned on, or can be connected to a printed circuit board  14 . 2 - 556  positioned in the internal volume. The printed circuit board  14 . 2 - 556  can also include components or electronic elements, including memory and processors positioned thereon. In some examples, the strap  14 . 2 - 530  can also include a rigid support member  14 . 2 - 553  positioned between the printed circuit board  14 . 2 - 556  and the stiffener  14 . 2 - 552 , the rigid support member  14 . 2 - 553  being more rigid than the stiffener in the first direction. 
     In this particular example, a transmission array or transmission component(s)  14 . 2 - 551  can be disposed in the internal volume to electrically connect the HMD connector  14 . 2 - 532  to the printed circuit board  14 . 2 - 556 , the supplemental unit connector  14 . 2 - 536  and/or one or more operational components  14 . 2 - 557 ,  14 . 2 - 558 . In some examples, the transmission component  14 . 2 - 551  can extend along a center-line of the housing  14 . 2 - 539 . In some examples including multiple conductors or transmission components, a first conductor can be positioned or disposed adjacent to one internal surface of the housing of the strap  14 . 2 - 530 , and a second conductor can be positioned or disposed adjacent to another different internal surface of the housing of the strap. In some examples, different conductors may be positioned on opposite sides of internal components, such as the printed circuit board  14 . 2 - 556 , operational components  14 . 2 - 557 ,  14 . 2 - 558 , stiffeners, and the like. In some examples, the components of the strap  14 . 2 - 530  can be maintained in their desired locations by internal fixtures and/or attachment features defined by, or attached to the housing  14 . 2 - 539 . In some examples, such as where the housing  14 . 2 - 539  includes a relatively flexible material, such as a polymeric material, the housing  14 . 2 - 539  be at least partially overmolded around at least some of the components of the strap  14 . 2 - 530 , including the printed circuit board  14 . 2 - 556 , operational components  14 . 2 - 557 ,  14 . 2 - 558 , and/or connectors  14 . 2 - 532 ,  14 . 2 - 536 . Further details of connector straps for wearable electronic devices are described with respect to  FIGS.  14 . 2 - 6 A through  14 . 2 - 6 C . 
       FIGS.  14 . 2 - 6 A  shows a top view of a component  14 . 2 - 630  that can be part of, or can be used with a wearable electronic device, as described herein. In some examples, the component  14 . 2 - 630  can be a connector strap  14 . 2 - 630  and can be substantially similar to, or can include some or all of the features of the other connector straps and/or electronic device described herein. As with other connector straps described herein, the connector strap  14 . 2 - 630  can include a first connector  14 . 2 - 632  that can be an HMD connector  14 . 2 - 632 , a second connector  14 . 2 - 636  that can be a supplemental unit connector, and a third connector  14 . 2 - 634  that can be a retention band connector. As described herein, in some examples, the retention band connector  14 . 2 - 634  can include multiple connection portions. The connector strap  14 . 2 - 630  can have a housing  14 . 2 - 639  that can at least partially define an internal volume and an offset surface that is proud of a major surface of the housing  14 . 2 - 639 . That is, the housing can define a protrusion or protruding portion  14 . 2 - 638  and one or more operational components can be positioned in or at this protrusion  14 . 2 - 638 . The protruding portion  14 . 2 - 638  can be a portion of the housing that is wider that another portion, in any direction. 
       FIGS.  14 . 2 - 6 B  shows a side view of the connector strap  14 . 2 - 630 . In addition to defining an internal volume and external surface of the connector strap  14 . 2 - 630 , the housing  14 . 2 - 639  of the connector strap  14 . 2 - 630  can define one or more openings or ports  14 . 2 - 633 . In some examples, the housing  14 . 2 - 639  can define one or more audio ports  14 . 2 - 633  that can allow an audio module or speaker positioned in the internal volume defined by the housing  14 . 2 - 639  to communicate with the ambient environment. Additionally, as shown, the connector strap  14 . 2 - 630  may not have a constant height or thickness along its entire length. In some examples, a portion of the housing can be enlarged relative to other portions of the strap  14 . 2 - 630 , such as to provide room for operational components, and/or to position operational components nearer to their desired location relative to a user. 
       FIGS.  14 . 2 - 6 C  shows a cross-sectional view of the connector strap  14 . 2 - 630  in the position shown in  FIGS.  14 . 2 - 6 B , but with a major surface of the housing  14 . 2 - 639  not shown in order to illustrate the positioning of components in the internal volume. In the present example, the strap  14 . 2 - 630  can include two or more operational components  14 . 2 - 657 ,  14 . 2 - 658 , at least one or which can be positioned at the protrusion  14 . 2 - 638 . In some examples, a first operational component  14 . 2 - 657  can include a first audio module disposed in the internal volume and configured to output a first range of audio frequencies, while a second operational component  14 . 2 - 658  can include a second audio module disposed in the internal volume and configured to output a second range of audio frequencies different than the first range. Additionally drivers of the first and/or second audio modules  14 . 2 - 657 ,  14 . 2 - 658  can be in communication with the audio port  14 . 2 - 633  in order to output sound therefrom. In some examples, one or more of the audio modules  14 . 2 - 657 ,  14 . 2 - 658  can have open-box speaker construction or closed-box speaker construction. In some examples, one audio module  14 . 2 - 657  may function as a tweeter, while the second audio module  14 . 2 - 658  can function as a woofer. In some examples, the strap  14 . 2 - 630  can include one or more sensors, for example a sensor that can detect a location of a user&#39;s ear relative to the strap and can aid in directing sound from the audio port  14 . 2 - 633  to the user&#39;s ear. 
     As with other straps  14 . 2 - 630  described herein, the strap  14 . 2 - 630  can include a stiffener  14 . 2 - 652  that is flexible in a first direction and rigid in at least one direction that is perpendicular to the first direction. One or more of the operational components  14 . 2 - 657 ,  14 . 2 - 658  and/or connectors  14 . 2 - 636  can be positioned on, or connected to, a printed circuit board  14 . 2 - 656  positioned in the internal volume. The printed circuit board  14 . 2 - 656  can also include components, including memory and processors positioned thereon. In some examples, the strap  14 . 2 - 630  can also include a support member  14 . 2 - 653  positioned between the printed circuit board  14 . 2 - 656  and the stiffener  14 . 2 - 652 , the support member  14 . 2 - 653  being more rigid than the stiffener in the first direction. 
     In this particular example, a transmission array or transmission component(s)  14 . 2 - 650 ,  14 . 2 - 651  can be disposed in the internal volume to electrically connect the HMD connector  14 . 2 - 632  to the printed circuit board  14 . 2 - 656 , the supplemental unit connector  14 . 2 - 636  and/or one or more operational components  14 . 2 - 657 ,  14 . 2 - 658 . In some examples, the transmission components  14 . 2 - 650 ,  14 . 2 - 651  can extend along a center-line “C” of the housing  14 . 2 - 639 . In some examples, the components of the strap  14 . 2 - 630  can be maintained in their desired locations by internal fixtures and/or attachment features defined by, or attached to the housing  14 . 2 - 639 . In some examples, such as where the housing  14 . 2 - 639  includes a relatively flexible material, such as a polymeric material, the housing  14 . 2 - 639  be at least partially overmolded around at least some of the components of the strap  14 . 2 - 630 , including the printed circuit board  14 . 2 - 656 , operational components  14 . 2 - 657 ,  14 . 2 - 658 , and/or connectors  14 . 2 - 632 ,  14 . 2 - 636 . 
     In some examples, the housing  14 . 2 - 639  can include a first portion, such as the portion containing the stiffener  14 . 2 - 652 , and a second portion, such as the portion containing the first audio module  14 . 2 - 657  and the second audio module  14 . 2 - 658 . In some examples, this second portion can have a greater height than the first portion, as shown. Additionally, because this second portion can also be where the protrusion  14 . 2 - 638  is located, the second portion can also have a thickness or width that is greater than a thickness or width of the first portion. Further details of connector straps for wearable electronic devices are described with respect to  FIGS.  14 . 2 - 7 A through  14 . 2 - 7 C . 
       FIGS.  14 . 2 - 7 A  shows a top view of a component  14 . 2 - 730  that can be part of, or used with a wearable electronic device, as described herein. In some examples, the component  14 . 2 - 730  can be a connector strap  14 . 2 - 730  and can be substantially similar to, or can include some or all of the features of the other connector straps and/or electronic device described herein. As with other connector straps described herein, the connector strap  14 . 2 - 730  can include a first connector  14 . 2 - 732  that can be an HMD connector  14 . 2 - 732 , a second connector  14 . 2 - 736  that can be a supplemental unit connector, and a third connector  14 . 2 - 734  that can be a retention band connector. As described herein, in some examples, the retention band connector  14 . 2 - 734  can include multiple connection portions. The connector strap  14 . 2 - 730  can have a housing  14 . 2 - 739  that can at least partially define an internal volume and an offset surface that is proud of a major surface of the housing  14 . 2 - 739 . That is, the housing can define a protrusion or protruding portion  14 . 2 - 738  and one or more operational components can be positioned in or at this protrusion  14 . 2 - 738 . 
       FIGS.  14 . 2 - 7 B  shows a side view of the strap  14 . 2 - 730 . As can be seen in  FIGS.  14 . 2 - 7 A and  14 . 2 - 7 B , in some examples, the protrusion  14 . 2 - 738  defined by the housing  14 . 2 - 739  of the strap  14 . 2 - 730  can be positioned at any desired location on the strap  14 . 2 - 730 . Additionally, the protrusion  14 . 2 - 738  can be spaced apart from an audio port  14 . 2 - 733  defined by the housing  14 . 2 - 739 . Further, as can be seen in  FIGS.  14 . 2 - 7 C , this means that an operational component, such as an audio module  14 . 2 - 758  can be positioned in the internal volume of the strap  14 . 2 - 730  away from the audio port  14 . 2 - 733 . It can still be desirable, however, for the audio module  14 . 2 - 758  and/or a driver thereof to be in communication with the audio port  14 . 2 - 733  (shown with dashed lines). In order to facilitate this communication, the strap  14 . 2 - 730  can include an acoustic guide  14 . 2 - 759  that can include one or more channels in communication with the audio module  14 . 2 - 758  and the audio port  14 . 2 - 733 . In this way, the audio module  14 . 2 - 758  can be positioned away from the audio port  14 . 2 - 733 , but can still direct sound to a user from the audio port  14 . 2 - 733 . In some examples, the acoustic guide  14 . 2 - 759  can include any material desired, including metals or polymers. In some examples, the acoustic guide  14 . 2 - 759  can be flexible and can bend or flex with the strap  14 . 2 - 730  while still providing acoustic communication. In some examples, the acoustic guide  14 . 2 - 759  may not be a separate or discrete component and may be at least partially defined by the housing  14 . 2 - 739  itself. 
     The strap  14 . 2 - 730  can also include some or all of the other components and features of the other straps described herein. For example, while the audio module  14 . 2 - 758  can be positioned away from the audio port  14 . 2 - 733  and communicate with the audio port  14 . 2 - 733  through an acoustic guide  14 . 2 - 759 , the strap  14 . 2 - 730  can include another audio module  14 . 2 - 757  that can be positioned near to the audio port  14 . 2 - 733  to communicate therewith. The strap can also include a printed circuit board that can be connected through various transmission components  14 . 2 - 751  to other connectors  14 . 2 - 732  and/or components  14 . 2 - 757 ,  14 . 2 - 758  of the strap  14 . 2 - 730 . Further details of connector straps for wearable electronic devices are described with respect to  FIGS.  14 . 2 - 8 A through  14 . 2 - 8 C . 
       FIGS.  14 . 2 - 8 A  shows a top view of a component  14 . 2 - 830  that can be part of, or can be used with a wearable electronic device, as described herein. In some examples, the component  14 . 2 - 830  can be a connector strap  14 . 2 - 830  and can be substantially similar to, or can include some or all of the features of the other connector straps and/or electronic device described herein. As with other connector straps described herein, the connector strap  14 . 2 - 830  can include a first connector  14 . 2 - 832  that can be an HMD connector  14 . 2 - 832 , a second connector  14 . 2 - 836  that can be a supplemental unit connector, and a third connector  14 . 2 - 834  that can be a retention band connector. As described herein, in some examples, the retention band connector  14 . 2 - 834  can include multiple connection portions. The connector strap  14 . 2 - 830  can have a housing  14 . 2 - 839  that can at least partially define an internal volume and an offset surface that is proud of a major surface of the housing  14 . 2 - 839 . That is, the housing can define a protrusion or protruding portion  14 . 2 - 838  and one or more operational components can be positioned in or at this protrusion  14 . 2 - 838 . 
     As seen in the side view of  FIGS.  14 . 2 - 8 B , the strap  14 . 2 - 830  can have a similar configuration as other straps described herein, such as straps  14 . 2 - 530 ,  14 . 2 - 630 , where the protrusion  14 . 2 - 838  is positioned at an end of the strap that includes the supplemental unit connector  14 . 2 - 836 . In addition to the audio port  14 . 2 - 833 , the housing  14 . 2 - 839  can further define a vent, an exit, or a back port  14 . 2 - 835  that can be positioned at an opposite side of the strap from the audio port  14 . 2 - 833 . In some examples, the back port  14 . 2 - 835  can also be positioned at an opposite end of the strap  14 . 2 - 830  from the audio port  14 . 2 - 833 . In some examples, it can be desirable to position the back port  14 . 2 - 835  at a location on the strap  14 . 2 - 830  that is as far away from the audio port as possible or feasible. 
       FIGS.  14 . 2 - 8 C  shows a cross-sectional view of the strap  14 . 2 - 830 , including first and second audio modules  14 . 2 - 857 ,  14 . 2 - 858 , printed circuit board  14 . 2 - 856 , and transmission component  14 . 2 - 851  including a conductor such as cables or wires. In the present example, the strap  14 . 2 - 830  also includes an acoustic guide  14 . 2 - 859  that can be in communication with the audio module  14 . 2 - 858  or a driver thereof. Whereas the acoustic guide  14 . 2 - 759  shown in  FIGS.  14 . 2 - 7 C  is configured to direct sound generated by the audio module  14 . 2 - 758  to the audio port  14 . 2 - 733 , the acoustic guide  14 . 2 - 859  can be configured to direct the back wave generated by the driver of the audio module  14 . 2 - 858  to the back port, and thus, away from the user&#39;s ear. Accordingly, the audio module  14 . 2 - 858  can include an open-box or open-back speaker and the acoustic guide  14 . 2 - 859  can be in communication with the back or rear surface of the speaker&#39;s diaphragm so as to release backpressure and/or direct negative sound waves generated by the speaker away from the user&#39;s ear. This construction can thus allow for the audio modules  14 . 2 - 857 ,  14 . 2 - 858  to generate high quality audio in a relatively small package. Further details of connector straps for wearable electronic devices are described with respect to  FIGS.  14 . 2 - 9 A through  14 . 2 - 9 C . 
       FIGS.  14 . 2 - 9 A  shows a top view of a component  14 . 2 - 930  that can be part of, or used with a wearable electronic device, as described herein. In some examples, the component  14 . 2 - 930  can be a connector strap  14 . 2 - 930  and can be substantially similar to, or can include some or all of the features of the other connector straps and/or electronic device described herein. As with other connector straps described herein, the connector strap  14 . 2 - 930  can include a first connector  14 . 2 - 932  that can be an HMD connector  14 . 2 - 932 , a second connector  14 . 2 - 936  that can be a supplemental unit connector, and a third connector  14 . 2 - 934  that can be a retention band connector. As described herein, in some examples, the retention band connector  14 . 2 - 934  can include multiple connection portions. The connector strap  14 . 2 - 930  can have a housing  14 . 2 - 939  that can at least partially define an internal volume and an offset surface that is proud of a major surface of the housing  14 . 2 - 939 . That is, the housing can define a protrusion or protruding portion  14 . 2 - 938  and one or more operational components can be positioned in or at this protrusion  14 . 2 - 938 . 
       FIGS.  14 . 2 - 9 B  shows a side view of the strap  14 . 2 - 930 . As can be seen in  FIGS.  14 . 2 - 9 A and  14 . 2 - 9 B , in some examples, the protrusion  14 . 2 - 938  defined by the housing  14 . 2 - 939  of the strap  14 . 2 - 930  can be positioned at any desired location on the strap  14 . 2 - 930 . Additionally, the protrusion  14 . 2 - 938  can be spaced apart from an audio port  14 . 2 - 933  defined by the housing  14 . 2 - 939 . Further, as can be seen in  FIGS.  14 . 2 - 9 C , this means that an operational component, such as an audio module  14 . 2 - 958  can be positioned in the internal volume of the strap  14 . 2 - 930  away from the audio port  14 . 2 - 933 . In some examples, the strap  14 . 2 - 930  can include an acoustic guide  14 . 2 - 959  that can include one or more channels in communication with the audio module  14 . 2 - 958  and the audio port  14 . 2 - 933 . In some examples, the acoustic guide  14 . 2 - 959  can define two or more, three or more, four or more, or even more channels. In some examples, the acoustic guide  14 . 2 - 959  can define three channels. In some examples, the acoustic guide  14 . 2 - 959  can also function as a stiffener for the strap  14 . 2 - 930 . That is, the acoustic guide  14 . 2 - 959  can be flexible along a first bend direction, and rigid along a second, different bend direction. In some examples, the acoustic guide  14 . 2 - 959  can thus be considered a flexible acoustic guide, or a semi-rigid acoustic guide. 
     The strap  14 . 2 - 930  can also include some or all of the other components and features of the other straps described herein. For example, while the audio module  14 . 2 - 958  can be positioned away from the audio port  14 . 2 - 933  and communicate with the audio port  14 . 2 - 933  through an acoustic guide  14 . 2 - 959 , the strap  14 . 2 - 930  can include another audio module  14 . 2 - 957  that can be positioned near to the audio port  14 . 2 - 933  to communicate therewith. The strap can also include a printed circuit board that can be connected through various transmission components  14 . 2 - 951  to other connectors  14 . 2 - 932  and/or components  14 . 2 - 957 ,  14 . 2 - 958  of the strap  14 . 2 - 930 . Further details of connector straps for wearable electronic devices are described with respect to  FIGS.  14 . 2 - 10 A through  14 . 2 - 10 C . 
       FIGS.  14 . 2 - 10 A  shows a top view of a component  14 . 2 - 1030  that can be part of, or can be used with, a wearable electronic device, as described herein. In some examples, the component  14 . 2 - 1030  can be a connector strap  14 . 2 - 1030  and can be substantially similar to, or can include some or all of the features of the other connector straps and/or electronic device described herein. As with other connector straps described herein, the connector strap  14 . 2 - 1030  can include a first connector  14 . 2 - 1032  that can be an HMD connector  14 . 2 - 1032 , a second connector  14 . 2 - 1036  that can be a supplemental unit connector, and a third connector  14 . 2 - 1034  that can be a retention band connector. As described herein, in some examples, the retention band connector  14 . 2 - 1034  can include multiple connection portions. The connector strap  14 . 2 - 1030  can have a housing  14 . 2 - 1039  that can at least partially define an internal volume and an offset surface that is proud of a major surface of the housing  14 . 2 - 1039 . That is, the housing can define a protrusion or protruding portion  14 . 2 - 1038  and one or more operational components can be positioned in or at this protrusion  14 . 2 - 1038 . 
       FIGS.  14 . 2 - 10 B  shows a side view of the strap  14 . 2 - 1030 . As can be seen in  FIGS.  14 . 2 - 10 A and  14 . 2 - 10 B , in some examples, the protrusion  14 . 2 - 1038  defined by the housing  14 . 2 - 1039  of the strap  14 . 2 - 1030  can be positioned at any desired location on the strap  14 . 2 - 1030 . Additionally, the protrusion  14 . 2 - 1038  can be spaced apart from an audio port  14 . 2 - 1033  defined by the housing  14 . 2 - 1039 . Further, as can be seen in  FIGS.  14 . 2 - 10 C , this means that an operational component, such as an audio module  14 . 2 - 1058  can be positioned in the internal volume of the strap  14 . 2 - 1030  away from the audio port  14 . 2 - 1033 . In some examples, the strap  14 . 2 - 1030  can include an acoustic guide  14 . 2 - 1059  that can include one or more channels in communication with the audio module  14 . 2 - 1058  and the audio port  14 . 2 - 1033 . In some examples, the acoustic guide  14 . 2 - 1059  can include multiple channels. In some examples, the multiple channels of the acoustic guide  14 . 2 - 1059  can have a same size and cross-sectional area, or different sizes and/or cross-sectional areas. In some examples, one or more of the channels defined by the acoustic guide  14 . 2 - 1059  can have a rectangular cross-sectional shape, a round or circular cross-sectional shape, or any cross-sectional shape or shapes as desired. In some examples, the number of channels and their cross-sectional area can allow for the acoustic guide  14 . 2 - 1059  to be relatively flexible in at least one direction, while being relatively resistance to compression or collapse along other directions. 
     The strap  14 . 2 - 1030  can also include some or all of the other components and features of the other straps described herein. For example, while the audio module  14 . 2 - 1058  can be positioned away from the audio port  14 . 2 - 1033  and communicate with the audio port  14 . 2 - 1033  through an acoustic guide  14 . 2 - 1059 , the strap  14 . 2 - 1030  can include another audio module  14 . 2 - 1057  that can be positioned near to the audio port  14 . 2 - 1033  to communicate therewith. The strap can also include a printed circuit board that can be connected through various transmission components  14 . 2 - 1051  to other connectors  14 . 2 - 1032  and/or components  14 . 2 - 1057 ,  14 . 2 - 1058  of the strap  14 . 2 - 1030 . 
     Any number or variety of components in any of the configurations described herein can be included in a wearable electronic device, such as the HMD devices and/or HMD systems described herein. The components can include any combination of the features described herein, and can be arranged in any of the various configurations described herein. The structure and arrangement of components of a device, as well as the concepts regarding their use can apply not only to the specific examples discussed herein, but to any number of embodiments in any combination. Various examples of electronic devices and electronic device components including some having various features in various arrangements are described below, with reference to  FIGS.  14 . 2 - 11  through  14 . 2 - 13   . 
       FIGS.  14 . 2 - 11    shows an exploded view of a wearable electronic device  14 . 2 - 1100  that can include modular components which can be selectively and removably or releasably attached or coupled to one another as described herein. In some examples, the device  14 . 2 - 1100  can be substantially similar to, or can include some or all of the features of the other wearable electronic devices described herein. As with the electronic device  14 . 2 - 200 , the device  14 . 2 - 1100  can include an HMD  14 . 2 - 1110 , a retention band  14 . 2 - 1120 , and one or more connector straps  14 . 2 - 1130 ,  14 . 2 - 1131 . 
     The connector straps  14 . 2 - 1130 ,  14 . 2 - 1131  can include a number of connectors, such as HMD connectors  14 . 2 - 1132 ,  14 . 2 - 1133  that can releasably couple to corresponding connectors on the HMD, such as connector  14 . 2 - 1112 . While the straps  14 . 2 - 230 ,  14 . 2 - 231  of  FIGS.  14 . 2 - 2 A  included separate retention band connectors and supplemental unit connectors, the device  14 . 2 - 1100  can include a retention band  14 . 2 - 1120  that includes an integrated supplemental unit  14 . 2 - 1122 . Accordingly, the straps  14 . 2 - 1130 ,  14 . 2 - 1131  can include a combined retention band and supplemental unit connector  14 . 2 - 1136 ,  14 . 2 - 1137  that can both mechanically couple the straps  14 . 2 - 1130 ,  14 . 2 - 1131  to the retention band  14 . 2 - 1120  and electrically communicate data and/or power with the supplemental unit  14 . 2 - 1122  of the retention band  14 . 2 - 1120 . The retention band  14 . 2 - 1120  itself can include corresponding connectors  14 . 2 - 1126 ,  14 . 2 - 1127  that can be releasably coupled to the retention band connectors  14 . 2 - 1136 ,  14 . 2 - 1137 . 
     Additionally, as noted above, the connector straps  14 . 2 - 1130 ,  14 . 2 - 1131  can include a stiffener  14 . 2 - 1152  disposed in the flexible housing, the stiffener  14 . 2 - 1152  can be flexible along a first bend direction, such as a direction extending into and out of the page of  FIGS.  14 . 2 - 11   , represented as the “Z” direction. Additionally, the stiffener  14 . 2 - 1152  can be rigid in one or more other bend directions, such as one or more directions parallel to the plane of the page of  FIGS.  14 . 2 - 11   , represented as the “X” and “Y” directions, which are each substantially perpendicular to the “Z” direction. Stiffness in the “X” and “Y” directions allow the connector straps  14 . 2 - 1130 ,  14 . 2 - 1131  to support both the HMD  14 . 2 - 1110  and the retention band  14 . 2 - 1120  during operation, counteracting the moment or rotational force caused by the HMD  14 . 2 - 1110  and the retention band  14 . 2 - 1120 . In this manner, the connector straps  14 . 2 - 1130 ,  14 . 2 - 1131  provide sufficient structural rigidity to maintain the relative position of each component in the system during use. Simultaneously, the flexibility provided in the “Z” direction provides a snug and comfortable fit to the side of a user&#39;s head during use, and can continue and enhance the secured fit provided by the retention band  14 . 2 - 1120 . In some examples, the stiffener  14 . 2 - 1152  can include a polymer material, a metallic material, and/or combinations thereof. In some examples, the stiffener  14 . 2 - 1152  can have a higher stiffness than the material forming a majority of the connector straps  14 . 2 - 1130 ,  14 . 2 - 1131  in at least one bend direction or axis. In some examples, the stiffener  14 . 2 - 1152  can include metal in the form of a metallic sheet. 
     According to one example, the shape and material of the connector straps  14 . 2 - 1130 ,  14 . 2 - 1131  provide the desired combination of flexibility in the “Z” direction and rigidity in the “X” and “Y” directions even without the stiffener  14 . 2 - 1152 . According to this example, the approximately rectangular cross-sectional profile of the connector straps  14 . 2 - 1130 ,  14 . 2 - 1131  having a height that is approximately between 5 and 10, between 10 and 20, between 20 and 50, or between 50 and 100 times that of the width facilitates bending or low-force buckling of the connector strap in the “Z” direction, while resisting buckling in the “X” and “Y” directions. In other words, buckling the connector straps  14 . 2 - 1130 ,  14 . 2 - 1131  in the “X” and “Y” directions can require 10, 20, 30, 50, or more times the force that is needed to buckle the connector straps in the “Z” direction. Additional modifications, such as profile curves or directional stiffeners, can be added to the connector straps  14 . 2 - 1130 ,  14 . 2 - 1131  to further strengthen the connector straps in desired directions. 
     In some examples, the retention band  14 . 2 - 1120  can include other operational or functional components in addition, or in alternative, to the supplemental unit  14 . 2 - 1122 . For example, the component  14 . 2 - 1122  can include a battery module that can be in electrical communication with the straps  14 . 2 - 1130 ,  14 . 2 - 1131 . Additionally, the device  14 . 2 - 1100  may include a separate processing unit (not shown), such as the supplemental unit  14 . 2 - 140  shown in  FIGS.  14 . 2 - 1 C  that can be connected to the strap  14 . 2 - 1130 , while the retention band  14 . 2 - 1120  includes a second additional processing unit  14 . 2 - 1122 . Further details regarding various configurations of wearable electronic devices are discussed with respect to  FIGS.  14 . 2 - 12   . 
       FIGS.  14 . 2 - 12    shows an exploded view of a wearable electronic device  14 . 2 - 1200  that can include modular components which can be selectively and removably or releasably attached or coupled to one another, as described herein. In some examples, the device  14 . 2 - 1200  can be substantially similar to, or can include some or all of the features of the other wearable electronic devices described herein. As with the electronic device  14 . 2 - 100 , the device  14 . 2 - 1200  can include an HMD  14 . 2 - 1210 , a retention band  14 . 2 - 1220 , and one or more connector straps  14 . 2 - 1230 ,  14 . 2 - 1231 , and a supplemental unit  14 . 2 - 1240 . 
     The connector straps  14 . 2 - 1230 ,  14 . 2 - 1231  can include a number of connectors, such as HMD connectors  14 . 2 - 1232 ,  14 . 2 - 1233 , that can releasably couple to corresponding connectors on the HMD, such as connector  14 . 2 - 1212 . The connector straps  14 . 2 - 1230 ,  14 . 2 - 1231  can also include retention band connectors  14 . 2 - 1234 ,  14 . 2 - 1235  that can releasably couple to corresponding connectors  14 . 2 - 1224 ,  14 . 2 - 1225  on the retention band  14 . 2 - 1220 . The connector strap  14 . 2 - 1230  can include a supplemental unit connector  14 . 2 - 1236  that can be releasably coupled to the supplemental unit  14 . 2 - 1240  through a conductor or cable  14 . 2 - 1242 . In some examples, the retention band  14 . 2 - 1220  can include a band  14 . 2 - 1222  designed to secure the device  14 . 2 - 1200  to a user, and a pocket or compartment  14 . 2 - 1226  that can be attached to the band  14 . 2 - 1222  and which can be configured to hold or retain the supplemental unit  14 . 2 - 1240 . In this way, the supplemental unit  14 . 2 - 1240  can be carried entirely on the device  14 . 2 - 1200  without the need to attach the supplemental unit  14 . 2 - 1240  to the user at another location. 
       FIGS.  14 . 2 - 13    shows an exploded view of a wearable electronic device  14 . 2 - 1300  that can include modular components which can be selectively and removably or releasably attached or coupled to one another as described herein. In some examples, the device  14 . 2 - 1300  can be substantially similar to, or can include some or all of the features of the other wearable electronic devices described herein. As with the electronic device  14 . 2 - 100 , the device  14 . 2 - 1300  can include an HMD  14 . 2 - 1310 , a retention band  14 . 2 - 1320 , and one or more connector straps  14 . 2 - 1330 ,  14 . 2 - 1331 . 
     The connector straps  14 . 2 - 1330 ,  14 . 2 - 1331  can include a number of connectors, such as HMD connectors  14 . 2 - 1332 ,  14 . 2 - 1333  that can releasably couple to corresponding connectors on the HMD, such as connector  14 . 2 - 1312 . The connector straps  14 . 2 - 1330 ,  14 . 2 - 1331  can also include retention band connectors  14 . 2 - 1334 ,  14 . 2 - 1335  that can releasably couple to corresponding connectors  14 . 2 - 1324 ,  14 . 2 - 1325  on the retention band  14 . 2 - 1320 . The connector strap  14 . 2 - 1330  can include a supplemental unit connector  14 . 2 - 1336  that can be releasably coupled to the supplemental unit  14 . 2 - 1340  through a conductor or cable  14 . 2 - 1342 . Whereas some examples described herein can include two or more modular or separate connector straps, in some examples, the connector straps  14 . 2 - 1330 ,  14 . 2 - 1331  can be releasably or permanently joined to one another, such as by a third strap or a head strap  14 . 2 - 1339 . In some examples, this head strap  14 . 2 - 1339  can extend over a user&#39;s head when the wearable electronic device  14 . 2 - 1300  is worn by a user. The head strap  14 . 2 - 1339  can thus provide a secured and comfortable fit to the user. Further, in examples where the strap  14 . 2 - 1339  is removably or releasably attached to the straps  14 . 2 - 1330 ,  14 . 2 - 1331 , the user can opt whether or not to use the straps  14 . 2 - 1339  depending on the particular use scenario of the device  14 . 2 - 1300 . In some examples, the strap  14 . 2 - 1339  can include any material as desired, including polymeric materials, fabric materials, and any of the materials described with respect to connector straps and/or retention bands. 
     14.3: Modular Strap for an Electronic Device 
     As virtual reality (VR) and mixed reality (MR) become more ubiquitous, the demand for user friendly head-mounted displays with quality components increases. Traditionally, these VR/MR systems have been devices that include a wearable display component, often referred to as a head-mounted display (HMD) and a supplemental unit. The supplemental unit can be part of the HMD, or can be a separate component that is permanently or removably connected to the HMD through a cable, wires, or other conductors. The supplemental unit can provide power and/or added processing functionality to the system. 
     Devices of the present disclosure include one or more additional dongles for improved connectivity to such supplemental components. For example, an HMD system of the present disclosure includes a first dongle for attaching a power supply to a first removable strap (e.g., a modular band of the HMD system). In addition, the HMD system includes a second dongle for attaching an external computing device or display to a second removable strap (e.g., another modular band of the HMD system). With separate, designated dongles (e.g., one for power and the other for data transmission), the HMD system of the present disclosure can significantly improve throughput. Advantageously, this can facilitate developers or programmers to rapidly upload and download data from the HMD system, while separately powering the device. 
     In addition, the present disclosure relates to various thermal ergonomic improvements for a removable strap. Removable straps can include a variety of electrical components, including a battery, integrated circuit, and a speaker (to name a few). Such electrical components can generate large amounts of thermal energy, particularly for longer use durations and heavy computing applications. Thus, given the location of a removable strap (and its associated electrical components) being next to a user&#39;s head, managing thermal output is important. 
     To reduce or mitigate hotspots along the removable straps, the present disclosure includes electrical pods positioned along the removable straps. These electrical pods can include a thermal shell and other thermal elements that provide a combination of heat shielding and heat dissipation. In so doing, the removable straps can provide increased comfort to a user. In addition, the removable straps can reduce (or more efficiently spread) a thermal load. 
     These and other embodiments are discussed below with reference to  FIGS.  14 . 3 - 1  through  14 . 3 - 8   . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these figures is for explanatory purposes only and should not be construed as limiting. Furthermore, as used herein, a system, a method, an article, a component, a feature, or a sub-feature comprising at least one of a first option, a second option, or a third option should be understood as referring to a system, a method, an article, a component, a feature, or a sub-feature that can include one of each listed option (e.g., only one of the first option, only one of the second option, or only one of the third option), multiple of a single listed option (e.g., two or more of the first option), two options simultaneously (e.g., one of the first option and one of the second option), or combination thereof (e.g., two of the first option and one of the second option). 
       FIGS.  14 . 3 - 1    shows a top view of a wearable electronic device  14 . 3 - 100  being worn on the head  14 . 3 - 101  of a user. The wearable electronic device  14 . 3 - 100 , as well as other wearable electronic devices disclosed herein, can also be referred to as HMD systems, electronic devices, or simply as devices. The device  14 . 3 - 100  can include a number of modular components. For example, the device  14 . 3 - 100  can include a head-mounted display (HMD)  14 . 3 - 102  including a housing  14 . 3 - 104  and a display  14 . 3 - 106  attached to the housing  14 . 3 - 104  for displaying images to a user. 
     The HMD can also be referred to as a display portion or display module having the display  14 . 3 - 106 . The display portion can include the housing  14 . 3 - 104  and display  14 . 3 - 106  that at least partially constitutes the HMD. In one or more examples, including the example shown in  FIGS.  14 . 3 - 1    and other examples shown in other figures, the HMD  14 . 3 - 102  can also be referred to as an output component, output module. Such output components, modules, or portions can include one or more outputs other than visual outputs from a display. For example, an output module similar to the HMD  14 . 3 - 102  can include a speaker that outputs sound instead of or in addition to the display  14 . 3 - 106  shown in  FIGS.  14 . 3 - 1   . 
     In addition, one example of the device  14 . 3 - 100  can include a modular securement assembly  14 . 3 - 108  that secures the HMD  14 . 3 - 102  to the user&#39;s head  14 . 3 - 101 . The modular securement assembly  14 . 3 - 108  includes removable straps  14 . 3 - 110 ,  14 . 3 - 112 . The modular securement assembly  14 . 3 - 108  further includes a retention band  14 . 3 - 114 . The modular securement assembly  14 . 3 - 108  is therefore configured to removably secure the wearable electronic device  14 . 3 - 100 , including the HMD  14 . 3 - 102 , to the head  14 . 3 - 101  of the user when the removable straps  14 . 3 - 110 ,  14 . 3 - 112  and retention band  14 . 3 - 114  are connected as shown in  FIGS.  14 . 3 - 1   . 
     In particular examples, the removable straps  14 . 3 - 110 ,  14 . 3 - 112  removably (e.g., detachably) connect to both of the HMD  14 . 3 - 102  and the retention band  14 . 3 - 114 . For example, each of the removable straps  14 . 3 - 110 ,  14 . 3 - 112  can be removably connected to the HMD  14 . 3 - 102  (or the housing or the display portion thereof) and the retention band  14 . 3 - 114  at opposing ends of each removable strap  14 . 3 - 110 ,  14 . 3 - 112  as shown in  FIGS.  14 . 3 - 1   . In such an example, the modular securement assembly  14 . 3 - 108  is modular in that each of the removable straps  14 . 3 - 110 ,  14 . 3 - 112 , and the retention band  14 . 3 - 114  can be connected and detached, as may be desired. For instance, each of the removable straps  14 . 3 - 110 ,  14 . 3 - 112  can be removed from the modular securement assembly  14 . 3 - 108  and swapped out for one or more other modules, straps, or electronic components. 
     One or more other examples of the device  14 . 3 - 100  can include alternative configurations of the removable straps  14 . 3 - 110 ,  14 . 3 - 112  shown in  FIGS.  14 . 3 - 1   . For example, the device  14 . 3 - 100  can be positioned elsewhere along the modular securement assembly  14 . 3 - 108  (differently than what is shown in  FIGS.  14 . 3 - 1   ). In addition, one or more examples of wearable electronic devices described herein can include one or more intermediate members, flexible straps, or other optional supplemental components and electronic modules such as external power supplies, memory components, and/or processors. 
     In the example shown in  FIGS.  14 . 3 - 1   , when the device  14 . 3 - 100  is worn on the head of the user, the removable strap  14 . 3 - 110  is positioned on the left side of the user&#39;s head and the removable strap  14 . 3 - 112  is positioned on the right side of the user&#39;s head. The retention band  14 . 3 - 114  can span between the removable straps  14 . 3 - 110 ,  14 . 3 - 112  to wrap around the back of the user&#39;s head  14 . 3 - 101 , as shown. 
     In some examples, and as shown, the device  14 . 3 - 100  can be worn on the user&#39;s head  14 . 3 - 101  such that the HMD  14 . 3 - 102  is worn on the user&#39;s face and disposed over one or both of their eyes. The HMD  14 . 3 - 102  can be removably and/or releasably connected to one or more of the removable straps  14 . 3 - 110 ,  14 . 3 - 112  as mentioned above. In some examples, the removable straps  14 . 3 - 110 ,  14 . 3 - 112  can be positioned against the side of a user&#39;s head  14 . 3 - 101  and in contact therewith. In some examples, the removable straps  14 . 3 - 110 ,  14 . 3 - 112  can be positioned above the user&#39;s ear or ears. In some examples, the removable straps  14 . 3 - 110 ,  14 . 3 - 112  can be positioned adjacent to the user&#39;s ear or ears. The removable straps  14 . 3 - 110 ,  14 . 3 - 112  can be removably connected to the retention band  14 . 3 - 114 , which can extend around the user&#39;s head  14 . 3 - 101  and removably connect to the other of the removable straps  14 . 3 - 110 ,  14 . 3 - 112 . In this way, the HMD  14 . 3 - 102 , removable straps  14 . 3 - 110 ,  14 . 3 - 112 , and retention band  14 . 3 - 114  can form a loop that can retain the wearable electronic device  14 . 3 - 100  on the user&#39;s head  14 . 3 - 101 . 
     As shown in  FIGS.  14 . 3 - 1   , the removable straps  14 . 3 - 110 ,  14 . 3 - 112  can connect to the HMD  14 . 3 - 102 , both mechanically and electrically. In particular examples, the removable straps  14 . 3 - 110 ,  14 . 3 - 112  can receive and/or relay at least one of data or power via such connections. In these or other examples, the removable straps  14 . 3 - 110 ,  14 . 3 - 112  can connect to the HMD  14 . 3 - 102  at an HMD connection location that can include an electrical input or electrical connector that is attached to the housing  14 . 3 - 104  and electrically connected to the display  14 . 3 - 106 . This location can be identified as a temple area that can be defined as an area near a user&#39;s temple adjacent to the user&#39;s eye, and can span from in front of the user&#39;s eye to approximately 1-1.5 inches past the outer corner of a user&#39;s eye, along the side of the user&#39;s head  14 . 3 - 101 . 
     Similarly, the removable straps  14 . 3 - 110 ,  14 . 3 - 112  can connect to the retention band  14 . 3 - 114  at a retention band connection location identified as an area that can span to include the area above the user&#39;s ear or within 0.5 inches of the outer edge of the ear on either side. In this manner, the removable straps  14 . 3 - 110 ,  14 . 3 - 112  are able to provide structural support between the HMD  14 . 3 - 102  and the user&#39;s ear, while securely connecting the retention band  14 . 3 - 114  and transferring the retention forces of the retention band  14 . 3 - 114  through the device  14 . 3 - 100 . It should be understood, however, that this configuration is just one example of how the components of a modular wearable electronic device  14 . 3 - 100  can be arranged, and that in some, a different number of removable straps and/or retention bands can be included. 
     While a user wearing an HMD  14 . 3 - 102  on his or her head  14 . 3 - 101  is shown as one example of a wearable electronic device, the modular components, features, and advantages of various examples of electronic devices disclosed herein can also apply to other wearable electronic devices having securement mechanisms, including but not limited to wearable smart watches, fitness trackers, smart glasses, medical monitor devices, and so forth. For example, the housing  14 . 3 - 104  and display  14 . 3 - 106  of HMD  14 . 3 - 102  shown in  FIGS.  14 . 3 - 1    can also be configured as a housing and display for a smart watch module secured to the user&#39;s arm or wrist via a modular securement mechanism or assembly similar to the securement assembly  14 . 3 - 108  shown in  FIGS.  14 . 3 - 1   . Although referred to as a wearable electronic device  14 . 3 - 100 , it should be understood that the device  14 . 3 - 100  can include multiple modular components or devices and can be interchangeably referred to as a wearable electronic device, wearable electronic device system, and/or wearable electronic system. Additionally, although the particular component  14 . 3 - 102  can be referred to as an HMD, it should be understood that the terms HMD, HMD device, and/or HMD system can be used to refer to the wearable device  14 . 3 - 100  as a whole. 
     In particular examples, the device  14 . 3 - 100  (or HMD system) includes one or more dongles  14 . 3 - 116 ,  14 . 3 - 118  that are connectable to external components or devices, as will now be discussed. As used herein, the term “dongle” can refer to a piece of electrical hardware such as a connection, adapter, or coupling between an external device/component and one or more components of the device  14 . 3 - 100  to provide additional functionality, or enable a pass-through to another device that adds functionality. A dongle can include electrical connections for data and/or power transmission through the dongle. In particular examples, a dongle can be unidirectional or bidirectional in terms of data and/or a power transmission. 
     As shown in  FIGS.  14 . 3 - 1   , the dongle  14 . 3 - 116  connects the removable strap  14 . 3 - 110  to a power supply  14 . 3 - 120 . As used herein, the term “power supply” refers to any power source that supplies power to one or more components of the device  14 . 3 - 100  (e.g., to charge a battery or power a processor such as a microcontroller within the removable strap  14 . 3 - 110 ). For example, a power supply can include fuel cells, battery cells, generators, alternators, solar power converters, motion-based converters (e.g., that convert vibrations or oscillations into power), etc. In particular implementations, a power supply can convert alternating current to direct current (or vice-versa) for charging or recharging components of the device  14 . 3 - 100 . Some particular examples of a power supply can include a switched mode power supply, an uninterruptible power supply, an alternating current power supply, a direct current power supply, a regulated power supply, a programmable power supply, a computer power supply, and a linear power supply. 
     Additionally shown in  FIGS.  14 . 3 - 1   , the dongle  14 . 3 - 118  connects the removable strap  14 . 3 - 112  to an external device or display (DoD)  14 . 3 - 122 . An external device can include a smartphone, a tablet, a smart television, a desktop computer, a laptop computer, a server/network device, a virtual reality device, an augmented reality device, a smart watch, a sound/speaker device, a camera device, or other computing device. Further, an external display can include a monitor, device screen (e.g., a display screen for a laptop, television, phone, tablet, etc.), projector, or other suitable visual medium. 
     In at least some examples, the external DoD  14 . 3 - 122  includes a developer computer (e.g., for receiving and/or transmitting data from one or more components of the device  14 . 3 - 100 ). In particular examples, the external DoD  14 . 3 - 122  uploads computer-executable instructions to one or more components of the device  14 . 3 - 100  (e.g., the removable strap  14 . 3 - 112 ) through the dongle  14 . 3 - 118 . Additionally or alternatively, the external DoD  14 . 3 - 122  can also provide data to and/or receive data from certain component(s) of the device  14 . 3 - 100  (e.g., the removable strap  14 . 3 - 110 ) through other dongle configurations, such as through the dongle  14 . 3 - 116  or a split-dongle configuration (described below in relation to  FIGS.  14 . 3 - 6   ). 
     In some examples, the external DoD  14 . 3 - 122  receives performance data (e.g., thermal data, power data, data throughput data, etc.). In certain implementations, the external DoD  14 . 3 - 122  provides power to one or more components of the device  14 . 3 - 100 . For example, the external DoD  14 . 3 - 122  can power or charge components of the removable strap  14 . 3 - 112  by providing power through the dongle  14 . 3 - 118 . In other examples, the external DoD  14 . 3 - 122  provides no power, in which case the power supply  14 . 3 - 120  can provide power to components of both the removable strap  14 . 3 - 110  and the removable strap  14 . 3 - 112 . For instance, the power supply  14 . 3 - 120  can relay power to the removable strap  14 . 3 - 112  via electrical connections between the power supply  14 . 3 - 120 , the removable strap  14 . 3 - 110 , the HMD  14 . 3 - 102 , and the removable strap  14 . 3 - 112 . Alternatively, the power supply  14 . 3 - 120  can relay power to the removable strap  14 . 3 - 112  via alternative dongle configurations (as described below in relation to  FIGS.  14 . 3 - 6   ). 
     Additionally or alternatively, the external DoD  14 . 3 - 122  includes a display that outputs visual information. For example, the external DoD  14 . 3 - 122  includes a monitor that outputs the same visual content that is shown at the display  14 . 3 - 106 . Thus, advantageously, the external DoD  14 . 3 - 122  can provide visual feedback (e.g., to developers or programmers) without needing to wear the device  14 . 3 - 100 . Similarly, the external DoD  14 . 3 - 122  can provide visual feedback (e.g., on a developer computer screen) while a user is simultaneously wearing the device  14 . 3 - 100 . 
     In one or more examples of the present disclosure, the dongles  14 . 3 - 116 ,  14 . 3 - 118  can include a variety of bandwidth capacities. In certain examples, the dongle  14 . 3 - 116  includes a first bandwidth, and the dongle  14 . 3 - 118  includes a second bandwidth greater than the first bandwidth. For example, the dongle  14 . 3 - 118  can provide (relative to the dongle  14 . 3 - 116 ) increased throughput of at least one of data or power. In this manner, the dongle  14 . 3 - 118  can facilitate larger and/or faster data uploads and downloads (for developer work or testing on the device  14 . 3 - 100 ). 
     In some examples, the dongles  14 . 3 - 116 ,  14 . 3 - 118  can also include variable bandwidths that can be throttled (e.g., reduced) or increased, as may be desired. For example, the dongles  14 . 3 - 116 ,  14 . 3 - 118  can include adjusted bandwidths based on one or more performance metrics (e.g., throughput speeds, visual content quality/resolution, thermal output, etc.). Indeed, as will be discussed more below in relation to  FIGS.  14 . 3 - 4  and  14 . 3 - 5   , an electronics pod on one or both of the removable straps  14 . 3 - 110 ,  14 . 3 - 112  can generate a thermal output that is spread out or dissipated into the ambient environment (e.g., to reduce heat transfer to a user). To help lessen the thermal output of the electronics pod (and/or an amount of thermal output to dissipate), one or more components of the device  14 . 3 - 100  can vary (e.g., decrease) the bandwidth through the dongles  14 . 3 - 116 ,  14 . 3 - 118 . As used herein, the term “bandwidth” refers to a rate of power transfer (e.g., power bandwidth) or a rate of data transfer (e.g., data bandwidth) through electrical wiring and connections, or else wirelessly. In some cases, the term bandwidth can include a maximum rate of transfer across a given path. 
     In these or other examples, at least one of data bandwidth or power bandwidth through the dongles  14 . 3 - 116 ,  14 . 3 - 118  can be reduced when the thermal output achieves a threshold thermal output (e.g., a predetermined temperature reading). For instance, a micro controller (discussed below) can throttle bandwidth through at least one of the dongles  14 . 3 - 116 ,  14 . 3 - 118  in response to a temperature sensor in the removable straps  14 . 3 - 110 ,  14 . 3 - 112  indicating a temperature in excess of a certain temperature, such as one hundred degrees Fahrenheit. 
     Any of the features, components, parts, including the arrangements and configurations thereof shown in  FIGS.  14 . 3 - 1    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures. Likewise, any of the features, components, parts, including the arrangements and configurations thereof shown in the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  14 . 3 - 1   . 
       FIGS.  14 . 3 - 2    shows a perspective view of an example of a wearable electronic device  14 . 3 - 200  that includes a display portion  14 . 3 - 202  in the form of an HMD with a housing  14 . 3 - 204 . The display portion  14 . 3 - 202  can be secured to the head of a user via a modular securement assembly  14 . 3 - 208  that can include a first removable strap  14 . 3 - 210 , a second removable strap  14 . 3 - 212 , and a retention band  14 . 3 - 214 . The first removable strap  14 . 3 - 210  can be removably connected to the display portion  14 . 3 - 202 , or the housing  14 . 3 - 204  thereof, at a first end  14 . 3 - 216  of the first removable strap  14 . 3 - 210 . A second end  14 . 3 - 218  of the first removable strap  14 . 3 - 210  can be removably connected to a first end  14 . 3 - 220  of the retention band  14 . 3 - 214 . Likewise, the second removable strap  14 . 3 - 212  can be removably connected to the display portion  14 . 3 - 202 , or the housing  14 . 3 - 204  thereof, at a first end  14 . 3 - 222  of the second removable strap  14 . 3 - 212 . A second end  14 . 3 - 224  of the second removable strap  14 . 3 - 212  can be removably connected to a second end  14 . 3 - 226  of the retention band  14 . 3 - 214 . 
       FIGS.  14 . 3 - 3    shows an exploded perspective view of an example of a wearable electronic device  14 . 3 - 300 , similar to the device  14 . 3 - 200  shown in  FIGS.  14 . 3 - 2   . The device  14 . 3 - 300  of  FIGS.  14 . 3 - 3    includes a display portion  14 . 3 - 302  in the form of an HMD with a housing  14 . 3 - 304 . The display portion  14 . 3 - 302  can be secured to the head of a user via a modular securement assembly  14 . 3 - 308  that can include a first removable strap  14 . 3 - 310 , a second removable strap  14 . 3 - 312 , and a retention band  14 . 3 - 314 . The first removable strap  14 . 3 - 310  can be removably connected to the display portion  14 . 3 - 302 , or the housing  14 . 3 - 304  thereof, at a first end  14 . 3 - 316  of the first removable strap  14 . 3 - 310 . A second end  14 . 3 - 318  of the first removable strap  14 . 3 - 310  can be removably connected to a first end  14 . 3 - 320  of the retention band  14 . 3 - 314 . Likewise, the second removable strap  14 . 3 - 312  can be removably connected to the display portion  14 . 3 - 302 , or the housing  14 . 3 - 304  thereof, at a first end  14 . 3 - 322  of the second removable strap  14 . 3 - 312 . A second end  14 . 3 - 324  of the second removable strap  14 . 3 - 312  can be removably connected to a second end  14 . 3 - 326  of the retention band  14 . 3 - 314 . 
     The example of the device  14 . 3 - 300  in  FIGS.  14 . 3 - 3    is shown in an exploded view to illustrate the modularity of the securement assembly  14 . 3 - 308 , where each of the first and second removable straps  14 . 3 - 310 ,  14 . 3 - 312  can be individually removed or replaced and removably connected with the retention band  14 . 3 - 314  and display portion  14 . 3 - 302  or other display component or other output component such as a speaker component or module. 
     Any of the features, components, parts, including the arrangements and configurations thereof shown in  FIGS.  14 . 3 - 2  and  14 . 3 - 3    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown in the other figures. Likewise, any of the features, components, parts, including the arrangements and configurations thereof shown in the other figures can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  14 . 3 - 2  and  14 . 3 - 3   . For example, at least one of the dongles  14 . 3 - 116 ,  14 . 3 - 118  shown in  FIGS.  14 . 3 - 1    above can be correspondingly implemented with the removable straps  14 . 3 - 210 ,  14 . 3 - 212  and/or the removable straps  14 . 3 - 310 ,  14 . 3 - 312  of  FIGS.  14 . 3 - 2  and  14 . 3 - 3   . 
       FIGS.  14 . 3 - 4    illustrates a side profile view of an example removable strap of an HMD system in accordance with one or more examples of the present disclosure. In particular,  FIGS.  14 . 3 - 4    illustrates a removable strap  14 . 3 - 412  that includes an electronics pod  14 . 3 - 400 , a thermal spreader  14 . 3 - 402 , and a thermal spreader  14 . 3 - 404 . Although a single removable strap is shown, it will be appreciated that both removable straps of an HMD system of the present disclosure can include the same or similar structure shown and described in relation to  FIGS.  14 . 3 - 4   . 
     As used herein, the term “electronics pod” refers to a subassembly, an enclosure, or a shell dedicated for housing certain electronics. In certain implementations, electrical components of the electronics pod  14 . 3 - 400  are communicatively coupled to the HMD  14 . 3 - 102  (e.g., via wire(s)  14 . 3 - 534  shown in  FIG.  14 . 3 - 5   ). Additionally or alternatively, the electronics pod  14 . 3 - 400  can be coupled to one or more dongles (e.g., the dongles  14 . 3 - 116 ,  14 . 3 - 118  discussed above). 
     Furthermore, the electronics pod  14 . 3 - 400  can generate thermal energy or heat as a result of operation. In some examples, it is desirable to dissipate this thermal energy or heat away from a user (e.g., towards an ambient environment and away from a user&#39;s head). In some examples, it is also desirable to spread this thermal energy or heat away from the electronics pod  14 . 3 - 400  (e.g., by utilizing the thermal spreaders  14 . 3 - 402 ,  14 . 3 - 404 ). In particular examples, the thermal spreader  14 . 3 - 404  dissipates thermal energy from inside the electronics pod  14 . 3 - 400 . Optionally, the thermal spreader  14 . 3 - 402  also dissipates thermal energy from inside electronics pod  14 . 3 - 400 . 
     As will be shown below, a thermal spreaders  14 . 3 - 402 ,  14 . 3 - 404  are in thermal communication with the electronics pod  14 . 3 - 400 . For example, the thermal spreaders  14 . 3 - 402 ,  14 . 3 - 404  contact conductive portions of the electronics pod  14 . 3 - 400 . In particular examples, the thermal spreaders  14 . 3 - 402 ,  14 . 3 - 404  are positioned beyond the electronics pod  14 . 3 - 400 . In these or other examples, the term “beyond” refers to an exterior or outside positioning relative to a boundary/perimeter. For instance, the thermal spreaders  14 . 3 - 402 ,  14 . 3 - 404  are at least partially positioned beyond the electronics pod  14 . 3 - 400  (e.g., outside of the exterior shell of the electronics pod  14 . 3 - 400 ). 
     As used herein, the term “thermal spreader” refers to a material that transfers or distributes heat from a hotter source to a colder source. In some examples, a thermal spreader is a thermally conductive element that disperses or spreads out thermal energy from the electronics pod  14 . 3 - 400 . To illustrate, a thermal spreader can include conductive wires, foil, strips, sheets, layers, contacts, etc. Thermal spreaders can also include a variety of materials. For example, thermal spreaders can include copper, graphite, diamond, silver, gold, silken carbide, aluminum, tungsten, zinc, carbon fiber (e.g., pitched carbon fiber), etc. In the example of pitched carbon fiber, it will be appreciated that the fibers can be oriented in one or more predetermined directions for directional heat transfer. 
       FIGS.  14 . 3 - 5    illustrates a top cross-sectional profile view of an example electronics pod in accordance with one or more examples of the present disclosure. As shown, the electronics pod  14 . 3 - 400  includes a thermal shell  14 . 3 - 500  and a cover  14 . 3 - 502 . Together, the thermal shell  14 . 3 - 500  and the cover  14 . 3 - 502  define an internal volume  14 . 3 - 504 . Multiple electrical components can be housed within the internal volume  14 . 3 - 504 . These electrical components can also generate thermal energy during operation. 
     In addition, the thermal shell  14 . 3 - 500  and the cover  14 . 3 - 502  can form an at least partially enclosed bowl or U-shaped cross-section. In particular, the example thermal shell  14 . 3 - 500  includes a plateaued surface. In some examples the plateaued surface can be oriented towards a user&#39;s head. In this manner, audio can be transmitted from the electronics pod  14 . 3 - 400  in a directional fashion towards a user&#39;s ear. In alternative examples, the plateaued surface of the electronics pod  14 . 3 - 400  can contact a user skin (e.g., for vibrational sound via bone conduction). 
     In certain examples, the cover  14 . 3 - 502  includes one or more of a variety of materials. In some examples, the cover  14 . 3 - 502  includes at least one of a foam portion or an aluminum portion. In particular examples, the cover  14 . 3 - 502  includes a stainless steel portion. Further in some examples, the cover  14 . 3 - 502  forms a complete enclosure over the internal volume  14 . 3 - 504 . In other examples, the cover  14 . 3 - 502  forms a partial enclosure over the internal volume  14 . 3 - 504 . 
     In these or other examples, a bottom surface of the cover  14 . 3 - 502  can attach to a removable strap. Additionally or alternatively, the cover  14 . 3 - 502  can be integrated into a removable strap itself. Further, in some examples, the thermal shell  14 . 3 - 500  includes a variety of layers of material. In some examples, the thermal shell  14 . 3 - 500  includes a clad (e.g., an overmolded clad). The clad can include a first metal layer  14 . 3 - 508 , a thermal interface material  14 . 3 - 510 , and a second metal layer  14 . 3 - 512 . 
     The metal layers  14 . 3 - 508 ,  14 . 3 - 512  can include one or more of a variety of metal materials. In some examples, the metal layers  14 . 3 - 508 ,  14 . 3 - 512  include stainless steel. In other examples, the metal layers  14 . 3 - 508 ,  14 . 3 - 512  include chrome, titanium, etc. It will be appreciated, however, that the metal layers  14 . 3 - 508 ,  14 . 3 - 512  need not be metal. For instance, the layers  14 . 3 - 508 ,  14 . 3 - 512  can be a variety of insulative materials, such as foam, plastic, rubber, ceramic, etc. It will also be appreciated that the layers  14 . 3 - 508 ,  14 . 3 - 512  can be different materials, as may be desired. 
     In some examples, the thermal interface material  14 . 3 - 510  can include a variety of thermally conductive materials. As used herein, the term “thermal interface material” refers to a material inserted between components or surfaces to enhance a thermal coupling therebetween and/or provide a thermal pathway. Examples of a thermal interface material include a thermal epoxy, thermal paste, thermal adhesive, thermal gap filler, thermally conductive pad, thermal tape, phase-change materials, or thermally conductive coatings (e.g., metallic coatings, diamond coatings, etc.). A thermal interface material can thus have high thermal conductivity. In specific examples, the thermal interface material  14 . 3 - 510  includes one or more copper layers. In one or more examples, the thermal interface material  14 . 3 - 510  can move, mold, or conform to the shape of the electronics pod  14 . 3 - 400 . Moreover, as will be discussed below, the thermal interface material  14 . 3 - 510  can transfer or dissipate thermal energy out of the internal volume  14 . 3 - 504 . 
     An overmold  14 . 3 - 506  can be positioned over the thermal shell  14 . 3 - 500 . The overmold  14 . 3 - 506  can include a variety of materials. To illustrate, the overmold  14 . 3 - 506  can include softer materials, textured materials, fabric materials, etc. In particular examples, the overmold  14 . 3 - 506  includes an elastomer material. For instance, the overmold  14 . 3 - 506  includes a fluorocarbon rubber (e.g., Viton® rubber). In these or other examples, the overmold  14 . 3 - 506  can conform to a shape of the thermal shell  14 . 3 - 500 . Additionally or alternatively, the overmold  14 . 3 - 506  can provide rigidity or support to the thermal shell  14 . 3 - 500 . 
     As mentioned, the electronics pod  400  can house a variety of electrical components. Shown in  FIGS.  14 . 3 - 5   , the electrical components can include a printed circuit board (PCB)  14 . 3 - 514 , a battery  14 . 3 - 516 , a processor such as a microcontroller  14 . 3 - 518 , a system-on-chip (SoC)  14 . 3 - 520 , and a speaker  14 . 3 - 522 . 
     The terms “PCB” or “printed circuit board” refer to a logic assembly that includes electronic components. The PCB  14 . 3 - 514  includes electrical connections and circuitry for mounting various components, including the SoC  14 . 3 - 520 . The PCB  14 . 3 - 514  can also relay power to mounted electrical components from a power supply (e.g., a battery, not separately illustrated). In certain examples, the PCB  14 . 3 - 514  is a main logic board. The PCB  14 . 3 - 514  can be a rigid board (e.g., composed of glass-epoxy compounds). In some examples, the PCB  14 . 3 - 514  is a multi-layer PCB (e.g., a laminated sandwich structure of conductive and insulating layers). In some examples, the PCB  14 . 3 - 514  is flexible (e.g., with flexible circuitry made with polyimide). In certain examples, the PCB  14 . 3 - 514  includes stiffeners added via lamination or pressure sensitive adhesive. 
     The battery  14 . 3 - 516  can include a myriad of different types of batteries or power sources. Indeed, the battery  14 . 3 - 516  can include one or more electrochemical cells with external connections for powering electrical devices or electrical components. For example, in some implementations, the battery  14 . 3 - 516  includes a lithium ion battery, alkaline battery, etc. In a specific example, the battery  14 . 3 - 516  includes a rechargeable battery. In other words, the battery  14 . 3 - 516  can be dispensable or rechargeable, as may be desired. 
     The microcontroller  14 . 3 - 518  can direct operation of one or more other components (e.g., within the electronics pod  14 . 3 - 400  the display  14 . 3 - 106 , etc.). As used herein, the term “microcontroller” refers to a device capable of generating an electrical (or digital) signal responsive to sensor data from a sensor. In one or more embodiments, a microcontroller includes any of a variety of processors (e.g., a system-on-chip, integrated circuit, driver, application processor, crossover processor, etc.). In some embodiments, a microcontroller further includes one or more memory devices (e.g., individual nonvolatile memory, processor-embedded nonvolatile memory, random access memory, memory integrated circuits, DRAM chips, stacked memory modules, storage devices, memory partitions, etc.). In particular embodiments, a microcontroller further includes one or more of input/output ports, counters, timers, etc. It will be appreciated that such a microcontroller can be mounted on a printed circuit board (e.g., a rigid circuit board or a flexible printed circuit). 
     The SoC  14 . 3 - 520  can include an integrated circuit that integrates a combination of electrical components for operation of the HMD system. The terms “SoC,” and “system-on-chip” refer to an electronic chip that generates thermal energy during operation. One or more of the SoC  14 . 3 - 520 , the microcontroller  14 . 3 - 518 , or the battery  14 . 3 - 516  can be a “heat source,” which term generally refers to component(s) that can generate thermal energy. In some examples, the SoC  14 . 3 - 520  can include a microchip to generate (e.g., drive) visualizations presented at the display  14 . 3 - 106  (shown in  FIGS.  14 . 3 - 1   ). It will be appreciated that the thermal energy or heat generated by the SoC  14 . 3 - 520  can become greater over a duration of use. Similarly, the thermal energy or heat generated by the SoC  14 . 3 - 520  can become greater when executing more data-intensive operations (e.g., for more complex or high fidelity visualizations). 
     The speaker  14 . 3 - 522  can include one or more audio-generating components. Example components of the speaker  14 . 3 - 522  include a transducer, magnet, coil, diaphragm/driver, etc. 
     Myriad other components in the electronics pod  14 . 3 - 400  can be included (albeit not expressly shown in  FIGS.  14 . 3 - 5   ). For example, the electronics pod  14 . 3 - 400  can include a heat sink, microphone, antenna, one or more sensors (e.g., temperature sensors, pressure sensor, accelerometer, magnetometer, etc.), or other suitable electronic elements. 
     In one or more examples, components of the electronics pod  14 . 3 - 400  can be electrically coupled to a power supply positioned outside of the electronics pod  14 . 3 - 400 . For example, wire(s)  14 . 3 - 534  can extend through a wire opening  14 . 3 - 536  defined by the cover  14 . 3 - 502 . The wire(s)  14 . 3 - 534  can connect to the PCB  14 . 3 - 514  on one end, and the wire(s)  14 . 3 - 534  can connect to a power supply (not shown) on the other end. In this manner, the wire(s)  14 . 3 - 534  can provide power to each component electrically coupled to the PCB  14 . 3 - 514 . 
     Components of the electronics pod  14 . 3 - 400  can also be thermally coupled to at least one of the ambient environment outside the electronics pod  14 . 3 - 400  or various thermal elements. To illustrate, the electronics pod  14 . 3 - 400  comprises a thermal conduit. As used herein, the term “thermal conduit” refers to a thermal path for heat flow. A thermal conduit may be conceptualized as linear or direct (e.g., along wire paths). However, a thermal conduit is not limited to such a flow path. Indeed, thermal paths can be linear or curved (e.g., non-linear), as a function of thermal properties for a given medium (whether solid, liquid, or gaseous). Further, thermal conduits are not limited to two-dimensional space. It will be appreciated that thermal conduits of the present disclosure include three-dimensional thermal paths through a given volume. In at least some examples, however, a thermal conduit of the present disclosure includes at least one of a thermal connector  14 . 3 - 524  or a thermal connector  14 . 3 - 526 , both of which can be composed of one or more wires (e.g., copper wires) for conductive heat transfer. 
     In some examples, the thermal conduit includes a thermal connector  14 . 3 - 524 . The thermal connector  14 . 3 - 524  is positioned inside the internal volume  14 . 3 - 504  and connects the PCB  14 . 3 - 514  and each associated component to the thermal interface material  14 . 3 - 510  at a connection point  14 . 3 - 528 . Alternatively, the thermal connector  14 . 3 - 524  connects a specific component of the electronics pod  14 . 3 - 400  (e.g., the SoC  14 . 3 - 520 ) to the thermal interface material  14 . 3 - 510 . 
     Thermal energy can be conductively transferred from components of the electronics pod  14 . 3 - 400  through the thermal connector  14 . 3 - 524 . In turn, the thermal energy can be spread or dissipated along the thermal interface material  14 . 3 - 510 . In at least some examples, a thermal conduit includes a thermal connector  14 . 3 - 526 . The thermal connector  14 . 3 - 526  is attached to the thermal interface material  14 . 3 - 510  at the connection point  14 . 3 - 528 . The thermal connector  14 . 3 - 526  then extends between clad layers of the thermal shell  14 . 3 - 500  (or over the thermal shell  14 . 3 - 500  underneath the overmold  14 . 3 - 506  as shown). The thermal connector  14 . 3 - 526  further extends to a connection  14 . 3 - 538  beyond the electronics pod  14 . 3 - 400 , where the thermal connector  14 . 3 - 526  and the thermal interface material  14 . 3 - 510  thermally couple to the thermal spreader  14 . 3 - 404 . By coupling the thermal connector  14 . 3 - 526  and the thermal interface material  14 . 3 - 510  to the thermal spreader  14 . 3 - 404 , thermal energy from the electronics pod  14 . 3 - 400  can be even further dissipated or spread along the removable strap. In specific examples, the thermal spreader  14 . 3 - 404  directs this thermal energy towards a second end  14 . 3 - 424  that opposes a first end  14 . 3 - 422  of the removable strap  14 . 3 - 412 . In doing so, the thermal energy is directed away from a user&#39;s temple. 
     However, as indicated via dashed lines, the removable strap  14 . 3 - 412  can include the thermal spreader  14 . 3 - 402  and associated heat-dissipation elements. The thermal spreader  14 . 3 - 402 , like the thermal spreader  14 . 3 - 404 , can connect to the thermal interface material  14 . 3 - 510  and a thermal connector, in a similar manner as just discussed above. In this way, thermal energy from the electronics pod  14 . 3 - 400  can be directed along the thermal spreader  14 . 3 - 402  towards the first end  14 . 3 - 422  of the removable strap  14 . 3 - 412 , as may be desired (e.g., to decrease a thermal load through the thermal connector  14 . 3 - 524 , the thermal connector  14 . 3 - 526 , and/or the thermal spreader  14 . 3 - 404 ). 
     To accommodate these and/or other thermal couplings of the present disclosure, the thermal shell  14 . 3 - 500  includes certain etched portions that expose segments of the thermal interface material  14 . 3 - 510 . Indeed, the thermal shell  14 . 3 - 500  includes an etched opening  14 . 3 - 530  and an etched opening  14 . 3 - 532 . The etched openings  14 . 3 - 530 ,  14 . 3 - 532  can be formed in a variety of ways. In some examples, chemical etching may be used to form the etched openings  14 . 3 - 530 ,  14 . 3 - 532 . In other examples, laser etching may be used to form the etched openings  14 . 3 - 530 ,  14 . 3 - 532 . In such examples, the etched opening  14 . 3 - 530  is defined by a removed portion of the metal layer  14 . 3 - 512 . Similarly, the etched opening  14 . 3 - 532  is defined by a removed portion of the metal layer  14 . 3 - 508 . Through the etched openings  14 . 3 - 530 ,  14 . 3 - 532 , the thermal interface material  14 . 3 - 510  is exposed for one or more conductive connections (e.g., for conductive heat transfer). 
     Additionally or alternatively to the thermal connector  14 . 3 - 524 , heat can be transferred from electrical components of the electronics pod  14 . 3 - 400  to the thermal interface material  14 . 3 - 510  via forced convection or non-force convection. For example, thermal energy can convectively move through the etched opening  14 . 3 - 530  and impinge upon the thermal interface material  14 . 3 - 510 . As used herein, the terms “conduction” or “natural conduction” refer to the heat transfer via conductively connected components. By contrast, the term “non-force convection” refers to free or natural convection, where air motion is caused by natural, buoyancy forces that result from the density variations due to variations of thermal temperature in the air. Non-forced convection differs from forced convection, where a fluid (e.g., air) is forced to flow by an internal source such as fans, by stirring, or pumps to create an artificially induced convection current. 
       FIGS.  14 . 3 - 6    shows a top view of a wearable electronic device  14 . 3 - 600  being worn on the head  14 . 3 - 101  of a user. The wearable electronic device  14 . 3 - 600  is the same as or similar to the wearable electronic device  100  discussed above in relation to  FIGS.  14 . 3 - 1   . Differently however, the wearable electronic device  14 . 3 - 600  includes a dongle  14 . 3 - 618 . As shown, the dongle  14 . 3 - 618  is a split dongle. In particular, the dongle  14 . 3 - 618  includes a first portion  14 . 3 - 619  connecting the removable strap  14 . 3 - 112  and the external DoD  14 . 3 - 122 . In addition, the dongle  14 . 3 - 618  includes a second portion  14 . 3 - 621  connecting the removable strap  14 . 3 - 112  to at least one of the power supply  14 . 3 - 120  or the removable strap  14 . 3 - 110 . 
     Advantageously, the dongle  14 . 3 - 618  can provide a variety of use-case applications. For instance, the power supply  14 . 3 - 120  can additionally provide power to the removable strap  14 . 3 - 112  via the second portion  14 . 3 - 621 . In another example, the external DoD  14 . 3 - 122  can provide data to both of the removable straps  14 . 3 - 110 ,  14 . 3 - 112  via the dongle  14 . 3 - 618 . In at least some examples, the dongle  14 . 3 - 618  can improve bandwidth through the wearable electronic device  14 . 3 - 600  (e.g., by circumventing the HMD  14 . 3 - 102  for transmission of data around the wearable electronic device  14 . 3 - 600 ). By transmitting data to the removable strap  14 . 3 - 110  through the dongle  14 . 3 - 618 , and in particular through the second portion  14 . 3 - 621 , the wearable electronic device  14 . 3 - 600  can also include reduced thermal loads in the bypassed components (e.g., through the removable strap  14 . 3 - 112  and the HMD  14 . 3 - 102 ). 
       FIGS.  14 . 3 - 7  and  14 . 3 - 8    illustrate schematic views of example cable management mechanisms for HMD systems in accordance with one or more examples of the present disclosure. In particular,  FIGS.  14 . 3 - 7    shows a dongle  14 . 3 - 718  connected to the removable strap  14 . 3 - 112 . The dongle  14 . 3 - 718  can be the same as or similar to the dongles  14 . 3 - 118 ,  14 . 3 - 618  discussed above. The dongle  14 . 3 - 718  also includes slack  14 . 3 - 702  (e.g., a cable portion for routing to the external DoD  14 . 3 - 122 ). To help prevent unwanted positioning or tangling, the retention band  14 . 3 - 114  can include a cable management mechanism  14 . 3 - 700 . 
     The cable management mechanism  14 . 3 - 700  can route, position, tension, or otherwise manage the slack  14 . 3 - 702  for the dongle  14 . 3 - 718 . In some examples, the cable management mechanism  14 . 3 - 700  includes an external ring, clip, button snap, Velcro® strip, tie, etc. attached to the retention band  14 . 3 - 114 . In other examples, the cable management mechanism  14 . 3 - 700  includes an aperture, slot, or recess within the retention band  14 . 3 - 114 . 
     Similarly,  FIGS.  14 . 3 - 8    shows a dongle  14 . 3 - 818  connected to the removable strap  14 . 3 - 112 . The dongle  14 . 3 - 818  can be the same as or similar to the dongles  14 . 3 - 118 ,  14 . 3 - 618  discussed above. Like the dongle  14 . 3 - 718 , the dongle  14 . 3 - 818  includes slack  14 . 3 - 802  (e.g., for routing to the external DoD  14 . 3 - 122 ). A cable management mechanism  14 . 3 - 800  is configured to efficiently handle the slack  14 . 3 - 802 , including one or more of a length or tension of the slack  14 . 3 - 802 . 
     In some examples, the cable management mechanism  14 . 3 - 800  includes a retractable spool. Via the retractable spool, the slack  14 . 3 - 802  can be pulled out or lengthened with manual interaction. In some examples, the slack  14 . 3 - 802  can be pulled out to a desired length and locked in place such that the retractable spool maintains an amount or tension of the slack  14 . 3 - 802 . In certain examples, the cable management mechanism  14 . 3 - 800  can automatically wind the slack  14 . 3 - 802 . In other examples, the cable management mechanism  14 . 3 - 800  winds the slack  14 . 3 - 802  in response to user interaction with at least one of the slack  14 . 3 - 802  or the cable management mechanism  14 . 3 - 800 . For instance, the cable management mechanism  14 . 3 - 800  includes a button release to initiate winding of the slack  14 . 3 - 802 . In other instances, the cable management mechanism  14 . 3 - 800  can wind the slack  14 . 3 - 802  in response to a pull (e.g., a directional tug) of the slack  14 . 3 - 802 . 
     14.4: Devices with Detachable Headbands 
     Head-mounted devices include head-mounted support structures that allow the devices to be worn on the heads of users. The head-mounted support structures may include device housings that enclose components such as displays. The displays may be used for presenting a user with visual content. The head-mounted support structures for a head-mounted device may also include headbands and other structures that help hold a device housing on the face of a user. The headband of a head-mounted device may be removable. This allows users to swap different headbands into use to accommodate different head sizes and/or to update the style of headband being used. 
       FIGS.  14 . 4 - 1    is a side view of a head-mounted electronic device with a detachable headband. As shown in  FIGS.  14 . 4 - 1   , head-mounted device  14 . 4 - 10  may include head-mounted housing  14 . 4 - 12  (sometimes referred to as a main housing, main housing unit, head-mounted support structure, main housing portion, etc.). Housing  14 . 4 - 12  may have walls or other structures that separate an interior housing region from an exterior region surrounding housing  14 . 4 - 12 . For example, housing  14 . 4 - 12  may have walls formed from polymer, glass, metal, and/or other materials. Electrical and optical components may be mounted in housing  14 . 4 - 12 . These components may include components such as integrated circuits, sensors, control circuitry, input-output devices, etc. 
     To present a user with images for viewing from eye boxes (e.g., eye boxes in which the user&#39;s eyes are located when device  14 . 4 - 10  is being worn on the users&#39; head), device  14 . 4 - 10  may include displays and lenses. These components may be mounted in optical modules  14 . 4 - 20  that face towards rear R of device  14 . 4 - 10  or may be mounted in other supporting structures in housing  14 . 4 - 12  to form respective left and right optical systems. There may be, for example, a left display for presenting an image through a left lens to a user&#39;s left eye in a left eye box and a right display for presenting an image to a user&#39;s right eye in a right eye box. 
     If desired, housing  14 . 4 - 12  may have forward-facing components such as cameras and other sensors on front F for gathering sensor measurements and other input and may have a soft cushion on opposing rear R. Rear R may have openings that allow the user to view images from left and right optical modules  14 . 4 - 20  (e.g., when rear R is resting on the front of the user&#39;s head). 
     Device  14 . 4 - 10  may have a strap such as headband  14 . 4 - 26  and, if desired, may have other structures (e.g., an optional over-the-head strap) to help hold housing  14 . 4 - 12  on the user&#39;s head. Headband  14 . 4 - 26  may have a fixed length or may be adjustable. Headband  14 . 4 - 26  may have first and second ends coupled, respectively, to the left and right sides of housing  14 . 4 - 12 . In the example of  FIGS.  14 . 4 - 1   , coupling members  14 . 4 - 24 , which serve as extensions of housing  14 . 4 - 12 , are provided on the left and right sides of housing  14 . 4 - 12 . Members  14 . 4 - 24  may be formed from rigid materials such as rigid polymer and/or other materials and may contain sensors, buttons, speakers, and other electrical components. Hinges and/or other mechanisms may be used to couple members  14 . 4 - 24  to housing  14 . 4 - 12  or members  14 . 4 - 24  may be formed as integral portions of a main housing unit. The ends of headband  14 . 4 - 26  may have coupling mechanisms such as openings configured to receive posts  14 . 4 - 30  (pins) or other protrusions on members  14 . 4 - 24  or other housing structures. In some examples, these posts face inwardly towards the user&#39;s head and are not visible to people in the vicinity of device  14 . 4 - 10  when device  14 . 4 - 10  is being worn by the user. Releasable latch mechanisms can be used to help secure the ends of headband  14 . 4 - 26  to member  14 . 4 - 24 . For example, a first detachable latch may be used to removably couple the left end of headband  14 . 4 - 26  to a left post in a left member  14 . 4 - 24  on a left side of housing  14 . 4 - 12  and a second detachable latch may be used to removably couple the right end of headband  14 . 4 - 26  to a right post in a right member  14 . 4 - 24  on a right side of housing  14 . 4 - 12 . If desired, a user may flip the headband  14 . 4 - 26  over so that the first detachable latch removable couples the end of headband  14 . 4 - 26  that was previously coupled to the left post to the right post and so that the second detachable latch removably couples the end of headband  14 . 4 - 26  that was previously coupled to the right post to the left post (e.g., the user may flip the left and right sides of the band without flipping the band inside out). A user may open and close the latches when housing  14 . 4 - 12  is being worn or when housing  14 . 4 - 12  is not being worn. 
     The use of latch-based coupling mechanisms in device  14 . 4 - 10  may help allow a user to removably attach headband  14 . 4 - 26  to members  14 . 4 - 24  and thereby removably attach headband  14 . 4 - 26  to housing  14 . 4 - 12 . Members  14 . 4 - 24  may have elongated shapes of the type shown in  FIGS.  14 . 4 - 1    and/or other suitable shapes and may sometimes be referred to as rigid straps, rigid coupling members, power straps head-mounted device housing structures, elongated head-mounted device housing members, elongated housing structures, elongated housing members, or head-mounted device housing members (as examples). 
     Headband  14 . 4 - 26  may have soft flexible portions and/or rigid portions. As an example, a central portion of headband  14 . 4 - 26  may be formed from stretchable fabric. Left and right end portions of headband  14 . 4 - 26  may be coupled to opposing ends of this central portion. The left and right end portions may, as an example, have stiffening structures (e.g., the left and right end portions may be stiffer than the central stretchable portion). Other types of configuration may be used for headband  14 . 4 - 26 , if desired (e.g., arrangements with adjustable tensioning cables, etc.). 
       FIGS.  14 . 4 - 2    is a diagram of an end portion of a headband  14 . 4 - 26  and a corresponding end portion of a member  14 . 4 - 24 . Headband  14 . 4 - 26  and housing structures such as members  14 . 4 - 24  may sometime be collectively referred to as a forming a head-mounted device headband system (headband system  14 . 4 - 22 ). As shown in the example of  FIGS.  14 . 4 - 2   , member  14 . 4 - 24  may have a protruding post such as post  14 . 4 - 30  (e.g., a post that protrudes out of member  14 . 4 - 24  towards a user head while device  14 . 4 - 10  is being worn by a user). Headband  14 . 4 - 26  may have a corresponding opening  14 . 4 - 32  that is configured to receive post  14 . 4 - 30 . 
     Opening  14 . 4 - 32  may be a through-hole opening with a shape that matches the outline of post  14 . 4 - 30 . In the present example, post  14 . 4 - 30  and opening  14 . 4 - 32  have elongated shapes when viewed end-on (e.g., rectangular shapes with rounded corners). These elongated shapes may help resist rotational motion between longitudinal axis  14 . 4 - 34  of headband  14 . 4 - 26  and longitudinal axis  14 . 4 - 36  of member  14 . 4 - 24 . This helps prevent headband  14 . 4 - 26  from slipping up or down along the rear surface of a user&#39;s head during use. In general, post  14 . 4 - 30  and/or mating opening  14 . 4 - 32  may have any suitable shapes (e.g., the shape of post  14 . 4 - 30  and/or opening  14 . 4 - 32  may be circular, oval, rectangular, triangular, may be a shape with curved edges and/or straight edges, may be a shape with drafted edges to help with alignment and/or insertion, etc.). The use of rectangular shapes with rounded corners and/or other shapes that are elongated (e.g., along respective longitudinal axes  14 . 4 - 40  and  14 . 4 - 38  of  FIGS.  14 . 4 - 2   , respectively), is merely for illustrative purposes. 
       FIGS.  14 . 4 - 3    is a cross-sectional side view of system  14 . 4 - 22 . As shown in  FIGS.  14 . 4 - 3   , member  14 . 4 - 24  may have main portion  14 . 4 - 24 M. Main portion  14 . 4 - 24 M may serve as a support for post  14 . 4 - 30  and may be formed from rigid polymer, metal, fabric, carbon-fiber composite materials, and/or other materials. Post  14 . 4 - 30  may be attached to main portion  14 . 4 - 24 M and may protrude inwardly (or, in some examples, outwardly) from portion  14 . 4 - 24 M. Post  14 . 4 - 30  may be formed from metal, rigid polymer, other materials, and/or combinations of these materials. Post  14 . 4 - 30  may have main portion  14 . 4 - 30 C, which protrudes away from the surface of portion  14 . 4 - 24 M. Post  14 . 4 - 30  may also have a peripheral protruding portion such as peripheral protruding portion  14 . 4 - 30 P. Portion  14 . 4 - 30 P may, as an example, run along the periphery of post  14 . 4 - 30  and may protrude radially outward from main portion  14 . 4 - 30 C. In the example of  FIGS.  14 . 4 - 3   , protruding portion  14 . 4 - 30 P has bevels to facilitate entry of post  14 . 4 - 30  into opening  14 . 4 - 32  of headband  14 . 4 - 26  in direction  14 . 4 - 54  and to facilitate removal of post  14 . 4 - 30  from opening  14 . 4 - 32  in the opposite direction when detaching headband  14 . 4 - 26  from member  14 . 4 - 24 . 
     Headband  14 . 4 - 26  has a main strap portion  14 . 4 - 44 . Strap portion  14 . 4 - 44 , which may sometimes be referred to as a strap or headband member, may have internal stiffening members, external fabric coverings and or other covering layers, strips of strengthening fabric, stretchable fabric portions (e.g., stretchable knit fabric), cosmetic coverings, and/or other headband structures. Through-hole opening  14 . 4 - 32  may be formed by cutting or otherwise forming an opening in portion  14 . 4 - 44 . The periphery of opening  14 . 4 - 32  may be strengthened using a mating pair of ring members  14 . 4 - 40 . Members  14 . 4 - 40 , which may sometimes be referred to as a cap and socket, may have ring shapes and may capture portions of strap  14 . 4 - 44 . Members  14 . 4 - 40  may be attached to each other using laser welding and/or other attachment mechanisms. Adhesive may optionally be used to help secure members  14 . 4 - 40  to strap  14 . 4 - 44 . 
     When members  14 . 4 - 50  are attached to each other, a ring shaped recess such as recess  14 . 4 - 52  is formed. Spring  14 . 4 - 42  (e.g., a spring formed from metal, foam, stretchy rubber gasket material or other elastomeric material, and/or other spring structures) may have a ring shape surrounding opening  14 . 4 - 32  and may be received within recess  14 . 4 - 52 . Gap  14 . 4 - 46  may be formed on one side of spring  14 . 4 - 42 . As shown in the top view of spring  14 . 4 - 42  of  FIGS.  14 . 4 - 4   , the presence of gap  14 . 4 - 46  allows spring  14 . 4 - 42  to expand outwardly in direction  14 . 4 - 56  when post  14 . 4 - 30  is inserted within opening  14 . 4 - 32  and the bevels on protruding portion  14 . 4 - 30 P press spring  14 . 4 - 42  radially outward. Once post  14 . 4 - 30  has been inserted within opening  14 . 4 - 32  sufficiently for protruding portion  14 . 4 - 30 P to have passed spring  14 . 4 - 42 , spring  14 . 4 - 42  may contract inwardly against main portion  14 . 4 - 30 C, thereby retaining post  14 . 4 - 30  within opening  14 . 4 - 32  and securing headband  14 . 4 - 26  to member  14 . 4 - 24 . When it is desired to remove headband  14 . 4 - 26 , headband  14 . 4 - 26  may be pulled away from member  14 . 4 - 24 . During headband removal, the bevels on portion  14 . 4 - 30 P press against spring  14 . 4 - 42  and expand spring  14 . 4 - 42  radially, thereby allowing post  14 . 4 - 30  to be removed from opening  14 . 4 - 32 . 
     The illustrative spring-based headband retention arrangement of  FIGS.  14 . 4 - 3  and  14 . 4 - 4    allows headband  14 . 4 - 26  to be attached and detached from member  14 . 4 - 24 . If desired a latch-based mechanism maybe used to secure and release headband  14 . 4 - 26 . This type of arrangement is shown in  FIGS.  14 . 4 - 5   . As shown in the example of  FIGS.  14 . 4 - 5   , system  14 . 4 - 22  may include a latching mechanism such as latch  14 . 4 - 62 . Latch  14 . 4 - 62 , which may sometimes be referred to as a latch mechanism or latch structures, may be opened and closed using magnets, springs, sliding members, toggling members, rotating members such as knobs, buttons, and/or other latch structures that may be manipulated by a user (e.g., a user&#39;s fingers). In the example of  FIGS.  14 . 4 - 5   , the latch of headband  14 . 4 - 26  has a movable latch member that engages post  14 . 4 - 30  when post  14 . 4 - 30  is within opening  14 . 4 - 32  and has an associated release mechanism such as release tab  14 . 4 - 62 T. Tab  14 . 4 - 62 T, which may be formed from a flexible strip of material (e.g., fabric, polymer, and/or other material) may be pulled in direction  14 . 4 - 64  by a user (e.g., when a user grasps tab  14 . 4 - 62 T between the user&#39;s fingers), thereby moving the movable latch member out of engagement with post  14 . 4 - 30 . This releases post  14 . 4 - 30  and allows post  14 . 4 - 30  to be removed from opening  14 . 4 - 32 . Magnets, spring structures, and/or other biasing structures may be used in closing the latch. In some examples, member  14 . 4 - 24  and headband  14 . 4 - 26  may have magnets that facilitate attachment of headband  14 . 4 - 26  and member  14 . 4 - 24 . As shown in  FIGS.  14 . 4 - 5   , for example, post  14 . 4 - 30  may have one or more magnets such as magnet  14 . 4 - 60 . Magnet  14 . 4 - 60  may be used to attract and/or repel corresponding magnets in strap  14 . 4 - 44 , which can assist in attaching headband  14 . 4 - 26  to member  14 . 4 - 24  and/or can assist in closing latch  14 . 4 - 62 . 
       FIGS.  14 . 4 - 6    is a side view of system  14 . 4 - 22  in a configuration in which latch  14 . 4 - 62  has a release tab. Tab  14 . 4 - 62 T may be attached to slidable (movable) latch member  14 . 4 - 62 M. Latch member  14 . 4 - 62 M may be moved laterally towards post  14 . 4 - 30  in response to magnetic force, spring force, or other biasing force from a latch member biasing mechanism so that the tip of the member  14 . 4 - 62 M protrudes under a lip portion (portion  14 . 4 - 30 P) of post  14 . 4 - 30 . In this way, post  14 . 4 - 30  is retained within opening  14 . 4 - 32  until latch  14 . 4 - 62  is released (e.g., by pulling on tab  14 . 4 - 62 T in direction  14 . 4 - 64 ). 
       FIGS.  14 . 4 - 7    is a top view of system  14 . 4 - 22  in an example in which post  14 . 4 - 30  of member  14 . 4 - 24  and headband  14 . 4 - 26  have magnets. Post  14 . 4 - 30  has magnet  14 . 4 - 60 . Strap  14 . 4 - 44  of headband  14 . 4 - 26  has magnets  14 . 4 - 66 ,  14 . 4 - 68 , and  14 . 4 - 70 . When post  14 . 4 - 30  is inserted within opening  14 . 4 - 32  of headband  14 . 4 - 26 , magnet  14 . 4 - 60  lies between magnets  14 . 4 - 66  and  14 . 4 - 68 . In this configuration, the south pole of magnet  14 . 4 - 60  attracts the north pole of magnet  14 . 4 - 68 . Magnet  14 . 4 - 68  is attached to slidable latch member  14 . 4 - 62 M. The magnetic attraction between magnet  14 . 4 - 60  and magnet  14 . 4 - 68  pulls latch member  14 . 4 - 62 M towards post  14 . 4 - 30  until the tip of latch member  14 . 4 - 62 M is received under protruding portion (lip)  14 . 4 - 30 P of post  14 . 4 - 30  (as shown in  FIGS.  14 . 4 - 6   ). In this position, latch  14 . 4 - 62  is closed and post  14 . 4 - 30  is retained within opening  14 . 4 - 32  (e.g., headband  14 . 4 - 62  is attached to member  14 . 4 - 24 ). 
     When post  14 . 4 - 30  is not inserted in opening  14 . 4 - 32 , the south poles of magnets  14 . 4 - 70  attract the north poles of magnet  14 . 4 - 68  and vice versa, so that magnet  14 . 4 - 68  aligns with magnet  14 . 4 - 70  when post  14 . 4 - 30  is not present. Magnets  14 . 4 - 70  are attached to strap  14 . 4 - 44 , whereas magnet  14 . 4 - 68  is attached to slidable latch member  14 . 4 - 62 M. Due to the magnetic attraction between magnets  14 . 4 - 70  and magnet  14 . 4 - 68 ,  14 . 4 - 70 , member  14 . 4 - 62 M is moved to open latch position  14 . 4 - 62 M′. This ensures that the tip of member  14 . 4 - 62 M that faces opening  14 . 4 - 32  will not protrude into opening  14 . 4 - 32  in the absence of post  14 . 4 - 30  and will therefore not visible within opening  14 . 4 - 32 . The magnetic retraction of latch member  14 . 4 - 62 M thereby helps enhance the visual appearance of headband  14 . 4 - 26  when headband  14 . 4 - 26  is not attached to member  14 . 4 - 24  and latch  14 . 4 - 62  is open. 
     When a user desires to attach headband  14 . 4 - 26  to member  14 . 4 - 24 , the user places opening  14 . 4 - 32  of headband  14 . 4 - 26  adjacent to post  14 . 4 - 30 . As opening  14 . 4 - 32  moves towards post  14 . 4 - 30 , the south pole of magnet  14 . 4 - 66  attracts the north pole of magnet  14 . 4 - 60  while the north pole of magnet  14 . 4 - 68  attracts the south pole of magnet  14 . 4 - 60  in an almost perfectly symmetrical fashion, so that the force exhibited on post  14 . 4 - 30  feels balanced. This tends to align post  14 . 4 - 30  with opening  14 . 4 - 32  and pull post  14 . 4 - 30  into opening  14 . 4 - 32 , thereby reducing the need for the user to accurately align post  14 . 4 - 30  with opening  14 . 4 - 32 . 
     The illustrative example of  FIGS.  14 . 4 - 7    uses magnetic force for three different functions. Firstly, while headband  14 . 4 - 26  and member  14 . 4 - 24  are detached, magnetic force is used to automatically retract latch member  14 . 4 - 62 M into position  14 . 4 - 62 M′. This automatic magnetic latch opening mechanism helps maintain latch member  14 . 4 - 62 M out of opening  14 . 4 - 32  when not in use. Secondly, when a user is first attaching headband  14 . 4 - 26  to member  14 . 4 - 24 , magnetic attraction is used to guide post  14 . 4 - 30  into opening  14 . 4 - 32  without excessive user attention. Thirdly, after post  14 . 4 - 30  has been received within opening  14 . 4 - 32 , magnetic attraction is used to pull latch member  14 . 4 - 62 M towards post  14 . 4 - 30  under protruding (lip) portion  14 . 4 - 30 P, thereby magnetically closing latch  14 . 4 - 62 . 
     Other biasing mechanisms may be used in addition to or in the alternative to magnetic biasing arrangements. For example,  FIGS.  14 . 4 - 8 ,  14 . 4 - 9 ,  14 . 4 - 10 , and  14 . 4 - 11    and their related description disclose spring-based biasing mechanisms that can be used in addition to or in the alternative to magnetic biasing arrangements. 
     As shown in  FIGS.  14 . 4 - 8   , latch member  14 . 4 - 26 M may be biased using biasing member  14 . 4 - 80 . Biasing member  14 . 4 - 80  may be a spring (e.g., a compression spring formed from metal or other springy material, etc.), may be a spring formed from elastomeric material (e.g., polymer foam), and/or may be formed from other spring structures that exhibit spring force. Member  14 . 4 - 80  of  FIGS.  14 . 4 - 8    may be used, for example, to push away from strap  14 . 4 - 44  and thereby push latch member  14 . 4 - 62 M into engagement with post  14 . 4 - 30  when post  14 . 4 - 30  has been received within opening  14 . 4 - 32 . Compression springs or other expanding biasing members may be used to move any suitable movable structures associated with latch  14 . 4 - 62  (e.g., member  14 . 4 - 62 M and/or other movable members). 
     As shown in  FIGS.  14 . 4 - 9   , biasing members  14 . 4 - 80  may be used in tension (e.g., members  14 . 4 - 80  may be tension springs). In the illustrative configuration of  FIGS.  14 . 4 - 9   , members  14 . 4 - 80  are pulling member  14 . 4 - 62 M towards opening  14 . 4 - 32 . Tension springs or other tensioned biasing members may, in general, pull on any suitable latch structures in latch  14 . 4 - 62 . 
       FIGS.  14 . 4 - 10  and  14 . 4 - 11    illustrate the use of a spring formed from an elastomeric band. As shown in the top view of  FIGS.  14 . 4 - 10   , for example, spring member  14 . 4 - 80  (e.g., an elastomeric band having a ring shape) may extend between a portion of headband  14 . 4 - 26  (e.g., strap  14 . 4 - 44 ) and latch member  14 . 4 - 62 M. When member  14 . 4 - 62 M is moved away from opening  14 . 4 - 32 , member  14 . 4 - 80  is stretched. This creates an opposing restoring spring force that tends to pull member  14 . 4 - 62 M towards opening  14 . 4 - 32 , allowing member  14 . 4 - 62 M to engage post  14 . 4 - 30  in opening  14 . 4 - 32 . This type of elastomeric band spring biasing mechanism is further illustrated in the cross-sectional side view of system  14 . 4 - 22  of  FIGS.  14 . 4 - 11   . As shown in  FIGS.  14 . 4 - 11   , biasing member  14 . 4 - 80  (e.g., a spring formed from an elastomeric band) may have a ring shape that extends (on the left) through a portion of strap  14 . 4 - 44  and (on the right) through a grooved portion of latch member  14 . 4 - 62 M. Post  14 . 4 - 30  may have a protruding portion such as portion  14 . 4 - 30 P that creates a post recess (recess  14 . 4 - 82 ) into which latch member  14 . 4 - 62 M can be received when latch  14 . 4 - 62  is closed. When it is desired to open latch  14 . 4 - 62 , a release tab or other release mechanism that is coupled to member  14 . 4 - 62 M may pull the tip of latch member  14 . 4 - 62 M out of recess  14 . 4 - 82 . This releases protruding portion  14 . 4 - 30 P from the tip of latch member  14 . 4 - 62 M and thereby unlocks latch  14 . 4 - 62  so that post  14 . 4 - 30  can be removed from opening  14 . 4 - 32 . At the same time, moving latch member  14 . 4 - 62 M away from opening  14 . 4 - 32  tensions member  14 . 4 - 80 . When it is desired to attach headband  14 . 4 - 26  to member  14 . 4 - 24 , this tension can be used to move member  14 . 4 - 62 M into recess  14 . 4 - 82 , so that member  14 . 4 - 62 M engages post  14 . 4 - 30 . 
     In the latch arrangement of  FIGS.  14 . 4 - 11   , latch member  14 . 4 - 62 M has a cam surface such as angled surface  14 . 4 - 84 . This surface may interact with protruding portion  14 . 4 - 30 P. For example, as post  14 . 4 - 30  is being inserted into opening  14 . 4 - 32 , portion  14 . 4 - 30 P may bear against surface  14 . 4 - 84  and thereby force latch member  14 . 4 - 62 M away from post  14 . 4 - 30  to open latch  14 . 4 - 62 . Once portion  14 . 4 - 30 P has moved across all of surface  14 . 4 - 84 , latch member  14 . 4 - 62 M may move into recess  14 . 4 - 82  under the latch-closing biasing force of biasing member  14 . 4 - 80 . 
       FIGS.  14 . 4 - 12    shows how latch member  14 . 4 - 62 M may have both outwardly and inwardly facing cam surfaces  14 . 4 - 88 . Protruding portion  14 . 4 - 30 P may have corresponding outwardly and inwardly cam surfaces  14 . 4 - 86 . With this type of arrangement, the outwardly facing cam surface of protrusion  14 . 4 - 30 P may bear against the inwardly facing cam surface of latch member  14 . 4 - 62 M when post  14 . 4 - 30  is being inserted into opening  14 . 4 - 32 , thereby moving latch member  14 . 4 - 62 M away from post  14 . 4 - 30  to place latch  14 . 4 - 62  in its open state. After inserting post  14 . 4 - 30  in opening  14 . 4 - 32 , the biasing mechanism that is being used to close latch  14 . 4 - 62  (e.g., magnetic biasing, spring biasing, etc.) pushes latch member  14 . 4 - 62 M toward post  14 . 4 - 30  and engages protrusion  14 . 4 - 30 P to hold post  14 . 4 - 30  in place in opening  14 . 4 - 32 . When a user desires to remove headband  14 . 4 - 26  from member  14 . 4 - 24 , the user may pull headband  14 . 4 - 26  and member  14 . 4 - 24  apart, causing the inwardly facing cam surface  14 . 4 - 86  of protruding portion  14 . 4 - 30 P to bear against the outwardly facing cam surface  14 . 4 - 88  of latch member  14 . 4 - 62 M and thereby push latch member  14 . 4 - 62 M into its open position. Because a pulling force from a user is used to disengage member  14 . 4 - 24  and headband  14 . 4 - 26  in systems such as system  14 . 4 - 22  of  FIGS.  14 . 4 - 12   , these systems may sometimes be referred to as pull-to-release systems (e.g., latch  14 . 4 - 62  of  FIGS.  14 . 4 - 12    may be referred to as a pull-to-release latch). Any suitable latch closing mechanism may be used with this type of latch (e.g., member  14 . 4 - 62 M may be biased towards post  14 . 4 - 30  using magnetic biasing, metal spring biasing, elastomeric band spring biasing, etc. 
     In addition to or instead of using a pull-to-release mechanism to release latch  14 . 4 - 62 , latch  14 . 4 - 62  may be provided with one or more of the latch release mechanisms of  FIG.  14 . 4 - 13   . As shown in  FIGS.  14 . 4 - 13   , latch  14 . 4 - 62  may have a movable latch member such as latch member  14 . 4 - 62 M that has a cam surface such as cam surface  14 . 4 - 94 . A biasing mechanism (magnetic, spring-based, etc.) may be used to push latch member  14 . 4 - 62 M into its closed position in which latch member  14 . 4 - 62 M engages protruding portion  14 . 4 - 30 P of post  14 . 4 - 30  and holds member  14 . 4 - 24  and headband  14 . 4 - 26  together. 
     In some examples, latch  14 . 4 - 62  uses a push-button release mechanism. With this arrangement, latch  14 . 4 - 62  is provided with a button. The button may have a movable button member such as button member  14 . 4 - 90  that is coupled to post  14 . 4 - 30 . When a use desires to release latch  14 . 4 - 62 , the user may press inwardly on button member  14 . 4 - 90 , to move button  14 . 4 - 90  inwardly and cause button member cam surface  14 . 4 - 92  to bear against cam surface  14 . 4 - 94  of latch member  14 . 4 - 62 M. This pushes latch member  14 . 4 - 62 M away from post  14 . 4 - 30  and thereby opens the latch so that post  14 . 4 - 30  can be removed from opening  14 . 4 - 32 . 
     In some examples, latch  14 . 4 - 62  is provided with a toggle release mechanism. As shown in  FIGS.  14 . 4 - 13   , for example, toggle lever  14 . 4 - 96  may be coupled to latch member  14 . 4 - 62 M by pivoting connection  14 . 4 - 100  and may be coupled to structures in strap  14 . 4 - 44  of headband  14 . 4 - 26  by pivoting connection  14 . 4 - 98 . This allows a user to move latch member  14 . 4 - 62 M to its open position (disengaged from post  14 . 4 - 30 ) by flipping lever  14 . 4 - 96 . 
     In some examples, a slider mechanism may be used for latch release operations. As shown in  FIGS.  14 . 4 - 13   , for example, slider member  14 . 4 - 102  may be attached to latch member  14 . 4 - 62 M. With this arrangement, a user may open latch  14 . 4 - 62  by sliding member  14 . 4 - 102  (and thereby sliding latch member  14 . 4 - 62 M) away from post  14 . 4 - 30 . 
     Additional latch release arrangements may be used, if desired (e.g., release mechanisms based on pull tabs, rotating knobs, push buttons, pull buttons, sliders, toggle switches, other release structures and/or combinations of these structures). These latch release arrangements may be used with any suitable latch-biasing scheme (e.g., arrangements in which latch member  14 . 4 - 62 M is biased towards its closed position magnetically, using a metal spring, using a tension spring, using a compression spring, using a coil spring, using a leaf spring, using a spring based on a stretchable band such as an elastomeric ring, and/or other spring, using a biasing device based on compressible foam, etc.). There may be a single latch member  14 . 4 - 62 M in latch  14 . 4 - 62  or multiple latch members  14 . 4 - 62 M may be used in latch  14 . 4 - 62 . 
       FIGS.  14 . 4 - 14    is a perspective view of headband  14 . 4 - 26  in an example in which strap  14 . 4 - 44  has latch member recesses  14 . 4 - 104 . Recesses  14 . 4 - 104  may be formed on opposing sides of opening  14 . 4 - 32  and may be configured to receive respective latch members that are located on opposing sides of post  14 . 4 - 30 . With this type of arrangement, the latch members are retracted into post  14 . 4 - 30  to open latch  14 . 4 - 62  and are extended out of post  14 . 4 - 30  into openings  14 . 4 - 104  of headband strap  14 . 4 - 44  to close latch  14 . 4 - 62 . 
       FIGS.  14 . 4 - 15    is a top view of a post for member  14 . 4 - 24  that has movable latch members  14 . 4 - 62 M for forming latch  14 . 4 - 62 . In this example, post  14 . 4 - 30  has a main portion  14 . 4 - 30 MP to which latch members  14 . 4 - 26 M are slidably coupled. When it is desired to release headband  14 . 4 - 26  from member  14 . 4 - 24 , movable post latch members  14 . 4 - 26 M are placed in the retracted position shown in  FIGS.  14 . 4 - 15   . After inserting post  14 . 4 - 30  into opening  14 . 4 - 32  of strap  14 . 4 - 44  of headband  14 . 4 - 26 , latch members  14 . 4 - 26 M may be moved outwardly in directions  14 . 4 - 106 . This causes latch members  14 . 4 - 62 M to move into openings  14 . 4 - 104  of strap  14 . 4 - 44  of  FIGS.  14 . 4 - 14   , thereby closing the latch and securing headband  14 . 4 - 26  to member  14 . 4 - 24 . 
     A cross-sectional side view of a system based on a strap of the type shown in  FIGS.  14 . 4 - 14    and a post with latch members of the type shown in  FIGS.  14 . 4 - 15    is shown in  FIG.  14 . 4 - 16   . As shown in  FIGS.  14 . 4 - 16   , latch  14 . 4 - 62  may have movable latch members  14 . 4 - 26 M that are slidably coupled to post  14 . 4 - 30  (e.g., in slots in main post portion  14 . 4 - 30 MP). Each latch member  14 . 4 - 26 M may have a biasing mechanism (magnetic, spring-based, etc.) that pushes that member outwardly into a corresponding strap recess  14 . 4 - 104  in strap  14 . 4 - 44  of headband  14 . 4 - 26  when post  14 . 4 - 30  is inserted into opening  14 . 4 - 32 . This closes latch  14 . 4 - 62  and secures headband  14 . 4 - 26  to member  14 . 4 - 24 . 
     Latch  14 . 4 - 62  may have a button release or other release mechanism. As shown in  FIGS.  14 . 4 - 16   , for example, button member  14 . 4 - 110  may be coupled to post  14 . 4 - 30  and may move in and out with respect to main post portion  14 . 4 - 30 MP. When the biasing mechanisms coupled to latch members  14 . 4 - 26 M are pushing members  14 . 4 - 26 M to their greatest outward extent (away from each other), cam surfaces  14 . 4 - 116  of members  14 . 4 - 62 M bear against structures  14 . 4 - 108  (e.g., a hollow tube or other structure coupled to member  14 . 4 - 24 ). This forces latch members  14 . 4 - 62 M in upward direction  14 . 4 - 120 . When it is desired to release latch  14 . 4 - 62 , a user may press on button member  14 . 4 - 130 . This causes button member  14 . 4 - 130  to move inwardly in direction  14 . 4 - 122 , so that button member surface  14 . 4 - 112  bears against surfaces  14 . 4 - 114  of latch members  14 . 4 - 62 M, causing cam surfaces  14 . 4 - 116  of latch members  14 . 4 - 62 M to bear against structures  14 . 4 - 108 . As cam surfaces  14 . 4 - 116  interact with structures  14 . 4 - 108  under pressure in direction  14 . 4 - 122  from button member  14 . 4 - 110 , latch members  14 . 4 - 62 M are forced inwardly towards each other (overcoming their outward biases). Once latch members  14 . 4 - 62 M have been fully retracted into post  14 . 4 - 30 , latch members  14 . 4 - 62 M will become disengaged with recesses  14 . 4 - 104  of strap  14 . 4 - 44 , thereby allowing strap  14 . 4 - 44  to be removed from post  14 . 4 - 30 . 
     In the example of  FIGS.  14 . 4 - 16   , recesses  14 . 4 - 104  are located in strap  14 . 4 - 44 . If desired, recesses for receiving latch members  14 . 4 - 62 M may be located in post  14 . 4 - 30 . Consider, as an example, the arrangement shown in the cross-sectional side view of system  14 . 4 - 22  of  FIGS.  14 . 4 - 17   . As shown in  FIGS.  14 . 4 - 17   , latch members  14 . 4 - 26 M may be slidably coupled to strap  14 . 4 - 44  of headband  14 . 4 - 26  (e.g., latch members  14 . 4 - 26 M may slide back and forth laterally within recesses in strap  14 . 4 - 44 ). Biasing mechanisms (magnetic biasing structures, spring-based biasing structures, etc.) may be used to bias latch members  14 . 4 - 26 M towards post  14 . 4 - 30 . Post  14 . 4 - 30  may have one or more recesses  14 . 4 - 124  that are configured to receive latch members  14 . 4 - 62 M. When latch members  14 . 4 - 26 M are biased into recesses  14 . 4 - 124 , post  14 . 4 - 30  will be retained within opening  14 . 4 - 32  and headband  14 . 4 - 26  will therefore be secured to member  14 . 4 - 24 . When it is desired to release latch  14 . 4 - 62 , members  14 . 4 - 26 M may be moved out of post recesses  14 . 4 - 120  (e.g., using a release mechanism based on a press or pull button, a sliding button, a toggling button, a pull tab, etc.). Recess  14 . 4 - 124  may extend around the entire periphery of post  14 . 4 - 30  or there may be one or more recesses  14 . 4 - 124  located at different locations about the periphery of post  14 . 4 - 30 , each of which is aligned with a respective member  14 . 4 - 26 M. 
     Although sometimes described herein in the context of examples where posts  14 . 4 - 30  are formed on members  14 . 4 - 24  and openings  14 . 4 - 32  are formed in headband  14 . 4 - 26 , posts  14 . 4 - 30  may, if desired, be formed as part of headband  14 . 4 - 26  and openings  14 . 4 - 32  may be formed in members  14 . 4 - 24 . 
     14.5: Cable Tensioning System and Dial 
     Dial-based cable tensioning systems are used in an increasing number of applications, including clothing, such as lacing systems for footwear, tensioning systems for headwear, and the like. Dial-based tensioning systems can feature various improvements over conventional tensioning systems, such as low friction and equilibrated tensioning pressure across contact surfaces (e.g., in the case of headwear, equal pressure is applied around a user&#39;s head). However, current dial-based tensioning systems are often noisy when users manipulate the dial, which can be problematic or irritating. In addition, current dial-based tensioning systems can fail to maintain the desired pressure across the contact surface over time. Improvements and advances to dial-based tensioning systems can be desirable to provide additional functionality in a variety of situations and environments. 
     Portable electronic devices, such as smart phones, laptops, tablet computing devices, smart watches, head-mountable displays (HMDs), and headphones, have become commonplace for persons undertaking daily activities (travel, communication, education, entertainment, employment, etc.). Indeed, portable electronic devices can provide assistance in completing daily tasks and errands, such as, watching an instructional video or monitoring progress during and after an exercise routine. Some electronic devices, such as HMDs, require mounting systems for mounting the electronic devices on a user&#39;s body, such as on a user&#39;s head. As an example, an HMD can be mounted on a user&#39;s head using an adjustable headband, which can include a dial-based tensioning system. Tensioning systems described herein can be silent and non-back-drivable, for example tensioning systems for HMDs, so as to not cause unwanted sounds near a user&#39;s ear and to maintain the HMD on a user&#39;s head. The tensioning systems described herein can be applicable to other applications, such as lacing systems for footwear, tensioning systems for other headwear, tensioning systems for other clothing, tensioning systems for bindings, and the like. 
     Various examples disclosed herein relate to improved tensioning systems, such as improved dial-based tensioning systems, which can be used for adjustable headbands in head-mountable display devices along with any other applications. The improved tensioning systems include a clutch that interacts with a spool on which a cable can be wound and a wheel that can be used by a user to wind the spool. The clutch is a roller-and-wedge system, which includes rollers in roller openings of a cam wheel. When the rollers are wedged between the cam wheel and a housing of the tensioning system, the cam wheel is locked. When the rollers are not wedged between the cam wheel and the housing, the cam wheel is unlocked. The roller-and-wedge system is non-back-drivable (cable can be pushed into the tensioning system, but not pulled out of the tensioning system without rotating the wheel) and silent. The tensioning system can also include various angular restraint systems, which prevent the tensioning system from over-spooling (winding too much cable into the spool and damaging the tensioning system) and operating in a backwards direction. 
     HMDs described herein can include head-mountable support structures that allow the devices to be worn on the heads of users. The head-mountable support structures can include device housings for housing components, such as displays that are used for presenting a user with visual content. The head-mountable support structures for an HMD can also include headbands and other structures that help hold a device housing on the face of a user. The headband of an HMD device can be adjustable, and can include a tensioning system or the like for adjusting the headband. 
     These and other examples are discussed below with reference to  FIGS.  14 . 5 - 1  through  14 . 5 - 9 C . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these figures is for explanatory purposes only and should not be construed as limiting. Furthermore, as used herein, a system, a method, an article, a component, a feature, or a sub-feature comprising at least one of a first option, a second option, or a third option should be understood as referring to a system, a method, an article, a component, a feature, or a sub-feature that can include one of each listed option (e.g., only one of the first option, only one of the second option, or only one of the third option), multiple of a single listed option (e.g., two or more of the first option), two options simultaneously (e.g., one of the first option and one of the second option), or combination thereof (e.g., two of the first option and one of the second option). 
       FIGS.  14 . 5 - 1    is a side view of a head-mountable display (HMD)  14 . 5 - 110  with an adjustable headband  14 . 5 - 126  (sometimes referred to herein as a “band”), in accordance with some examples. As illustrated in  FIGS.  14 . 5 - 1   , the HMD  14 . 5 - 110  can include a head-mountable housing  14 . 5 - 112 . The head-mountable housing  14 . 5 - 112  can include a main housing, a main housing unit, a head-mountable support structure, or the like. The head-mountable housing  14 . 5 - 112  can have walls or other structures that separate an interior housing region from an exterior housing region of the head-mountable housing  14 . 5 - 112 . In some examples, the walls of the head-mountable housing  14 . 5 - 112  can be formed from a material such as polymer, glass, metal, combinations thereof, or the like. Electrical and optical components can be mounted in the head-mountable housing  14 . 5 - 112 . These components can include components such as integrated circuits, sensors, control circuitry, input-output devices, and the like. 
     The HMD  14 . 5 - 110  can be used to present images to a user for viewing from eye boxes (e.g., eye boxes in which the user&#39;s eyes are located when the HMD  14 . 5 - 110  is being worn on the users&#39; head, as representatively illustrated by a user&#39;s head  14 . 5 - 122  in  FIGS.  14 . 5 - 1   ). Specifically, the HMD  14 . 5 - 110  can include displays, lenses, and the like in the eye boxes. These components can be mounted in optical modules or other supporting structure in the head-mountable housing  14 . 5 - 112  to form respective left and right optical systems. A left display can be included for presenting an image through a left lens to a user&#39;s left eye in a left eye box and a right display can be included for presenting an image to a user&#39;s right eye in a right eye box. 
     In some examples, the head-mountable housing  14 . 5 - 112  can include forward-facing components, such as cameras, other sensors, and the like on a front-side F of the head-mountable housing  14 . 5 - 112 . The forward-facing components can be used for gathering sensor measurements and other inputs. A soft cushion can be disposed on an opposing rear side of the head-mountable housing  14 . 5 - 112 . The rear side of the head-mountable housing  14 . 5 - 112  can include openings that allow the user to view images from the left and right optical systems (e.g., when the rear of the head-mountable housing  14 . 5 - 112  is resting on a front surface  14 . 5 - 120 F of the user&#39;s head  14 . 5 - 122 ). 
     The HMD  14 . 5 - 110  can include an adjustable band, such as an adjustable headband  14 . 5 - 126  or band. In some examples, the HMD  14 . 5 - 110  can include other structures (e.g., an optional over-the-head band or the like) to help hold the head-mountable housing  14 . 5 - 112  on the user&#39;s head  14 . 5 - 122 . The adjustable headband  14 . 5 - 126  can have first and second ends that are physically coupled to left and right sides of the head-mountable housing  14 . 5 - 112 , respectively. In the example illustrated in  FIGS.  14 . 5 - 1   , coupling members  14 . 5 - 124  or straps, which serve as extensions of the head-mountable housing  14 . 5 - 112 , are provided on the left and right sides of the head-mountable housing  14 . 5 - 112  to physically couple the adjustable headband  14 . 5 - 126  to the head-mountable housing  14 . 5 - 112 . The coupling members  14 . 5 - 124  or straps can be formed from rigid materials, such as rigid polymers and/or other materials. The coupling members  14 . 5 - 124  can contain sensors, buttons, speakers, and/or other electrical components. Hinges and/or other mechanisms can be used to couple the coupling members  14 . 5 - 124  to the head-mountable housing  14 . 5 - 112 . In some examples, the coupling members  14 . 5 - 124  or straps can be formed integrally as portions of the head-mountable housing  14 . 5 - 112 . 
     The first and second ends of the adjustable headband  14 . 5 - 126  can have coupling mechanisms, such as openings, that are configured to receive posts or other protrusions  14 . 5 - 124 P of the coupling members  14 . 5 - 124  or straps (or other coupling structures of the head-mountable housing  14 . 5 - 112 ). The adjustable headband  14 . 5 - 126  can be coupled to the coupling members  14 . 5 - 124  or straps such that a user removably attaches the adjustable headband  14 . 5 - 126  to the coupling members  14 . 5 - 124  or straps. As such, the user can remove the adjustable headband  14 . 5 - 126  from the head-mountable housing  14 . 5 - 112 . The coupling members  14 . 5 - 124  or straps can have elongated shapes, as illustrated in  FIGS.  14 . 5 - 1   , and/or other suitable shapes. Examples of the coupling members  14 . 5 - 124  or straps can include rigid straps, rigid coupling members, power straps, or the like. 
     The adjustable headband  14 . 5 - 126  can include a soft flexible portion, such as a central portion  14 . 5 - 130 . The central portion  14 . 5 - 130  can be formed between two stiffer portions, such as end portions  14 . 5 - 128  on the left and right ends of the adjustable headband  14 . 5 - 126  (e.g., the first and second ends of the adjustable headband  14 . 5 - 126 ). The end portions  14 . 5 - 128  can be stiffened using embedded polymer stiffeners (e.g., single-layer or multi-layer polymer stiffening strips) and/or other stiffening members. The central portion  14 . 5 - 130  can be formed from a stretchable material such as stretchy fabric. In some examples, the central portion  14 . 5 - 130  can be formed from a band of flat knit fabric that includes stretchable strands of material (e.g., elastomeric strands) and/or which uses a stretchable fabric construction (e.g., a stretchable knit construction). The central portion  14 . 5 - 130  can include narrowed end portions, which can extend over stiffening members in the end portions  14 . 5 - 128 . This can ensure that the adjustable headband  14 . 5 - 126  has a uniform external appearance. Forming the central portion  14 . 5 - 130  of a stretchable material and including the central portion in the adjustable headband  14 . 5 - 126  allows the adjustable headband  14 . 5 - 126  be stretched along its length. This allows the length of the adjustable headband  14 . 5 - 126  to be temporarily increased to help a user place the adjustable headband  14 . 5 - 126  over the user&#39;s head  14 . 5 - 122  when a user is donning the HMD  14 . 5 - 110 . When the adjustable headband  14 . 5 - 126  is released, the stretchiness and the elastic nature of the central portion  14 . 5 - 130  of the adjustable headband  14 . 5 - 126  help to shorten the adjustable headband  14 . 5 - 126  and pull the adjustable headband  14 . 5 - 126  against the user&#39;s head. This allows the adjustable headband  14 . 5 - 126  to rest against a rear surface  14 . 5 - 120 R the user&#39;s head. 
     A tensioning mechanism (such as the tensioning mechanism  14 . 5 - 100 , discussed below with respect to  FIGS.  14 . 5 - 2  through  14 . 5 - 5   ) can be included to provide further adjustment of the effective length and tension of the adjustable headband  14 . 5 - 126  to secure the adjustable headband  14 . 5 - 126  and the HMD  14 . 5 - 110  on the user&#39;s head  14 . 5 - 122 . The tensioning mechanism can be provided to adjust the effective length and tension in a headband tensioning cable  14 . 5 - 132 . The tensioning mechanism can be a rotatable knob, lever, slider, or other mechanism for adjusting the headband tensioning cable  14 . 5 - 132 . When the effective length of the cable  14 . 5 - 132  is greater, or the tension lower, the adjustable headband  14 . 5 - 126  will be looser. The increased effective length and low cable tension of the headband tensioning cable  14 . 5 - 132  helps to create slack in the adjustable headband  14 . 5 - 126 , which can aid in donning and doffing of the HMD  14 . 5 - 110 . When cable tension is increased, the adjustable headband  14 . 5 - 126  is secured against the user&#39;s head. 
     As illustrated in  FIGS.  14 . 5 - 1   , the headband tensioning cable  14 . 5 - 132  can have a loop shape and can include an upper cable segment  14 . 5 - 132 T that runs along an upper edge of the adjustable headband  14 . 5 - 126  and an opposing lower cable segment  14 . 5 - 132 B that runs along the opposing lower edge of the adjustable headband  14 . 5 - 126 . In the middle of the central portion  14 . 5 - 130  of the adjustable headband  14 . 5 - 126 , the upper and lower cable segments  14 . 5 - 132 T and  14 . 5 - 132 B can be separated by a distance BW (sometimes referred to as a cable bifurcation distance). The separation between the upper and lower cable segments  14 . 5 - 132 T and  14 . 5 - 132 B helps secure the adjustable headband  14 . 5 - 126  on the curved rear surface  14 . 5 - 120 R of the user&#39;s head  14 . 5 - 122 . When the adjustable headband  14 . 5 - 126  is worn on the rear surface  14 . 5 - 120 R, as illustrated in  FIGS.  14 . 5 - 1   , the upper cable segment  14 . 5 - 132 T can apply an upward force F 1  on the adjustable headband  14 . 5 - 126  that can prevent the headband  14 . 5 - 126  from slipping downwards off of the user&#39;s head  14 . 5 - 122 . Similarly, the lower cable segment  14 . 5 - 132 B can apply a downward force F 2  that can prevent the adjustable headband  14 . 5 - 126  from slipping upwards off of the user&#39;s head  14 . 5 - 122 . 
       FIGS.  14 . 5 - 2    illustrates a plan view of an adjustable headband  14 . 5 - 126 . The adjustable headband  14 . 5 - 126  can be substantially similar to, and can include some or all of, the features of the adjustable headband  14 . 5 - 126  discussed above with respect to  FIGS.  14 . 5 - 1   . As illustrated in  FIGS.  14 . 5 - 2   , the adjustable headband  14 . 5 - 126  can include a central portion  14 . 5 - 130  and end portions  14 . 5 - 128 . As discussed above, the central portion  14 . 5 - 130  can be stretchable and can be formed from a stretchy knit fabric, and the end portions can be stiffened using stiffeners embedded in fabric. One or more openings  14 . 5 - 140  can be formed in the end portions  14 . 5 - 128  of the adjustable headband  14 . 5 - 126 . The openings  14 . 5 - 140  can receive posts or other protrusions, such as the protrusions  14 . 5 - 124 P discussed above with respect to  FIGS.  14 . 5 - 1   . The openings  14 . 5 - 140  can be used in conjunction with the protrusions  14 . 5 - 124 P to secure the end portions  14 . 5 - 128  of the adjustable headband  14 . 5 - 126  to coupling members  14 . 5 - 124  or straps of a head-mountable housing  14 . 5 - 112 . In some examples, a single opening  14 . 5 - 140  or other attachment mechanism can be disposed on each of the end portions  14 . 5 - 128  of the adjustable headband  14 . 5 - 126 . In some examples, each of the end portions  14 . 5 - 128  of the adjustable headband  14 . 5 - 126  can include two or more of the openings  14 . 5 - 140 . In some examples, other attachment mechanisms (e.g., magnets, snaps, hook-and-loop fasteners, screws or other fasteners, combinations thereof, or the like) can be used to attach the adjustable headband  14 . 5 - 126  to the coupling members  14 . 5 - 124  or straps, or other portions of the head-mountable housing  14 . 5 - 112  in an HMD  14 . 5 - 110 . 
     A headband tensioning cable  14 . 5 - 132  can extend in a loop around the perimeter of the central portion  14 . 5 - 130  of the adjustable headband  14 . 5 - 126 . In the example illustrated in  FIGS.  14 . 5 - 2   , the adjustable headband  14 . 5 - 126  includes a pulley  14 . 5 - 142  coupled to a left end portion  14 . 5 - 128  of the adjustable headband  14 . 5 - 126 . The adjustable headband  14 . 5 - 126  includes a tensioning mechanism  14 . 5 - 100  coupled to a right end portion  14 . 5 - 128  of the adjustable headband  14 . 5 - 126 . In some examples, the tensioning mechanism  14 . 5 - 100  can be an adjustment knob or dial. The tensioning mechanism  14 . 5 - 100  can be a two-way dial that can be rotated in a first direction  14 . 5 - 148  about a rotational axis  14 . 5 - 146  (e.g., a clockwise direction) to shorten the length of the loop of the headband tensioning cable  14 . 5 - 132  and thereby increase tension on the headband tensioning cable  14 . 5 - 132 . The length of the loop of the headband tensioning cable  14 . 5 - 132  outside the tensioning mechanism  14 . 5 - 100  and disposed in the central portion  14 . 5 - 130  can be referred to as the “effective length” of the tensioning cable  14 . 5 - 132 . A dial of the tensioning mechanism  14 . 5 - 100  can be rotated in an opposing second direction  14 . 5 - 150  about the rotational axis  14 . 5 - 146  (e.g., a counterclockwise direction) to increase the length of the loop of the headband tensioning cable  14 . 5 - 132  and thereby decrease tension on the headband tensioning cable  14 . 5 - 132 . By tightening the headband tensioning cable  14 . 5 - 132 , tension in the adjustable headband  14 . 5 - 126  can be increased and the HMD  14 . 5 - 110  can be held more snugly on a user&#39;s head  14 . 5 - 122 . The pulley  14 . 5 - 142  can be included to allow tension in upper and lower portions of the headband tensioning cable  14 . 5 - 132  to be balanced, ensuring a consistent cupping of the adjustable headband  14 . 5 - 126  across various head shapes. By loosening the headband tensioning cable  14 . 5 - 132 , effective length can be increased and the tension in the adjustable headband  14 . 5 - 126  can be decreased. 
     The tensioning mechanism  14 . 5 - 100  can include a variety of improvements. In some examples, the tensioning mechanism  14 . 5 - 100  can include a cam and wedge system that allows the tensioning mechanism  14 . 5 - 100  to silently spool the headband tensioning cable  14 . 5 - 132  into the tensioning mechanism  14 . 5 - 100  and silently unspool the headband tensioning cable  14 . 5 - 132  from the tensioning mechanism  14 . 5 - 100 . The tensioning mechanism  14 . 5 - 100  can be non-back-drivable. In other words, the tensioning mechanism  14 . 5 - 100  can allow the headband tensioning cable  14 . 5 - 132  to be spooled into the tensioning mechanism  14 . 5 - 100  without rotating the tensioning mechanism  14 . 5 - 100 , but cannot allow the headband tensioning cable  14 . 5 - 132  to be unspooled from the tensioning mechanism  14 . 5 - 100  without rotating the tensioning mechanism  14 . 5 - 100 . The tensioning mechanism  14 . 5 - 100  can include an angular stop mechanism (also referred to as a hard stop, an angular stop, or the like). The angular stop mechanism prevents too much of the headband tensioning cable  14 . 5 - 132  from being spooled into the tensioning mechanism  14 . 5 - 100 , preventing damage to the tensioning mechanism  14 . 5 - 100 . The angular stop mechanism also prevents the tensioning mechanism  14 . 5 - 100  from operating in a backwards direction, which can impact the user experience. The tensioning mechanism  14 . 5 - 100  can further include a spring detent or the like. The spring detent can provide a desired sound, feel, and other user experience features to the tensioning mechanism  14 . 5 - 100 . These and other features of the tensioning mechanism  14 . 5 - 100  will be discussed below with respect to  FIGS.  14 . 5 - 3  through  14 . 5 - 9 C . 
       FIGS.  14 . 5 - 3    illustrates a perspective view of a tensioning mechanism or system  14 . 5 - 300  that can be included in an HMD  14 . 5 - 110  to adjust the tension in a headband tensioning cable  14 . 5 - 132  of an adjustable headband  14 . 5 - 126 , in accordance with some examples. The tensioning system  14 . 5 - 300  includes a dial cap  14 . 5 - 302  and a housing  14 . 5 - 304 . The housing  14 . 5 - 304  can be directly fixed to an adjustable headband  14 . 5 - 126  of the head-mounted device  14 . 5 - 110 , or can be fixed to the adjustable headband  14 . 5 - 126  through an interface. The housing  14 . 5 - 104  can be fixed to the adjustable headband  14 . 5 - 126  such that the housing  14 . 5 - 304  cannot rotate relative to the adjustable headband  14 . 5 - 126 . 
     In one example, the dial cap  14 . 5 - 302  includes a center cap  14 . 5 - 306  and a wheel  14 . 5 - 308  (also referred to as a tire or the like). The wheel  14 . 5 - 308  can be configured to be rotated by a user in order to spool the headband tensioning cable  14 . 5 - 132  into the tensioning system  14 . 5 - 300  or unspool the headband tensioning cable  14 . 5 - 132  from the tensioning system  14 . 5 - 300 . The wheel  14 . 5 - 308  rotates around a central axis of the center cap  14 . 5 - 306 . The center cap  14 . 5 - 306  and the housing  14 . 5 - 304  can be fixed to one another such that the center cap  14 . 5 - 306  does not rotate relative to the housing  14 . 5 - 304 . As such, the wheel  14 . 5 - 308  can be rotatable relative to the center cap  14 . 5 - 306  and the housing  14 . 5 - 304 . 
     As will be discussed in more detail below, the housing  14 . 5 - 304  can be configured to contain a spool within the housing  14 . 5 - 304 . The housing  14 . 5 - 304  includes a cable opening  14 . 5 - 310 . The headband tensioning cable  14 . 5 - 132  extends through the cable opening  14 . 5 - 310  into the housing  14 . 5 - 304  and is fixed to the spool. The headband tensioning cable  14 . 5 - 132  moves through the cable opening  14 . 5 - 310  as the headband tensioning cable  14 . 5 - 132  is spooled into or unspooled from the tensioning system  14 . 5 - 300 , such as by rotating the wheel  14 . 5 - 308  of the dial cap  14 . 5 - 302 . 
     Any number or variety of components in any of the configurations described herein can be included in the tensioning system and the electronic device. The components can include any combination of the features described herein and can be arranged in any of the various configurations described herein. The structure and arrangement of components of a tensioning system having a roller-and-wedge locking system described herein, can apply not only to the specific examples discussed herein, but to any number of examples in any combination. Examples of a configuration of the various components of the tensioning system relative to one another are described below, with reference to  FIGS.  14 . 5 - 4   . 
       FIGS.  14 . 5 - 4    illustrates a partially exploded view of a tensioning system  14 . 5 - 300 . The tensioning system  14 . 5 - 300  includes a headband tensioning cable  14 . 5 - 132 , a dial cap  14 . 5 - 302 , a housing  14 . 5 - 304 , a cam wheel  14 . 5 - 312 , rollers  14 . 5 - 314 , a spool  14 . 5 - 316 , and a cap post  14 . 5 - 318 . The dial cap  14 . 5 - 302  and the housing  14 . 5 - 304  can be substantially similar to, and can include some or all of, the features of the dial cap  14 . 5 - 302  and the housing  14 . 5 - 304  discussed above with respect to  FIGS.  14 . 5 - 3   . 
     The housing  14 . 5 - 304  can include a housing post  14 . 5 - 322 . The housing post  14 . 5 - 322  can be disposed centrally in the housing  14 . 5 - 304 . The housing  14 . 5 - 304  can further include an outer wall  14 . 5 - 320  in which a cable opening  14 . 5 - 310  is disposed. 
     In at least one example, the spool  14 . 5 - 316  is configured to sit in the housing  14 . 5 - 304  between the outer wall  14 . 5 - 320  and the housing post  14 . 5 - 322 . The spool  14 . 5 - 316  can include a hollow central portion  14 . 5 - 330 , which surrounds the housing post  14 . 5 - 322 . The spool  14 . 5 - 316  can be configured to rotate around the housing post  14 . 5 - 322  while the headband tensioning cable  14 . 5 - 132  is spooled into or unspooled from the tensioning system  14 . 5 - 300 . The spool  14 . 5 - 316  includes a cable opening  14 . 5 - 334 , into which the headband tensioning cable  14 . 5 - 132  extends. The headband tensioning cable  14 . 5 - 132  can be retained or fixed within the cable opening  14 . 5 - 334  of the spool  14 . 5 - 316 . The spool  14 . 5 - 316  includes a barrel  14 . 5 - 338  around which the headband tensioning cable  14 . 5 - 132  is wound, and flanges  14 . 5 - 336 , which prevent the headband tensioning cable  14 . 5 - 132  from being wound around or into other portions of the tensioning system  14 . 5 - 300  or otherwise escaping the spool  14 . 5 - 316 . The spool  14 . 5 - 316  further includes protrusions  14 . 5 - 332 , which allow the spool  14 . 5 - 316  to interface with and be rotatably coupled to the cam wheel  14 . 5 - 312 . 
     The cam wheel  14 . 5 - 312  and the rollers  14 . 5 - 314  are used to transfer rotation between the dial cap  14 . 5 - 302  and the spool  14 . 5 - 316 . The cam wheel  14 . 5 - 312  can be configured to rotate the spool  14 . 5 - 316  in response to a user rotating the wheel  14 . 5 - 308  in either direction. The cam wheel  14 . 5 - 312  and the rollers  14 . 5 - 314  can be configured to allow the headband tensioning cable  14 . 5 - 132  to be spooled into the tensioning system  14 . 5 - 300  by pushing the headband tensioning cable  14 . 5 - 132  into the housing  14 . 5 - 304  (e.g., without a user rotating the wheel  14 . 5 - 308 ). The cam wheel  14 . 5 - 312  and the rollers  14 . 5 - 314  can be configured not to allow the cable to be unspooled from of the tensioning system  14 . 5 - 300  when the headband tensioning cable  14 . 5 - 132  is pulled. The cam wheel  14 . 5 - 312  and the rollers  14 . 5 - 314  are configured to resist torque in a first direction (e.g., a direction in which the headband tensioning cable  14 . 5 - 132  is unspooled from the tensioning system  14 . 5 - 300 ) and allow torque in a second direction (e.g., a direction in which the headband tensioning cable  14 . 5 - 132  is spooled into the tensioning system  14 . 5 - 300 ). Details of the cam wheel  14 . 5 - 312  and the rollers  14 . 5 - 314  are discussed below with respect to  FIGS.  14 . 5 - 5   . The cam wheel  14 . 5 - 312  can include protrusions  14 . 5 - 340 , which interact with the protrusions  14 . 5 - 332  of the spool  14 . 5 - 316  to transfer rotation between the spool  14 . 5 - 316  and the cam wheel  14 . 5 - 312 . The cam wheel  14 . 5 - 312  includes an opening  14 . 5 - 342  and is configured to sit on top of the spool  14 . 5 - 316  between the spool  14 . 5 - 316  and the dial cap  14 . 5 - 302 . 
     The cap post  14 . 5 - 318  can be disposed in a post opening  14 . 5 - 324  in the housing  14 . 5 - 304 . The cap post  14 . 5 - 318  can be fixed to the housing  14 . 5 - 304 , such that the cap post  14 . 5 - 318  is in a fixed position. In some examples, the cap post  14 . 5 - 318  can be fixed to the housing  14 . 5 - 304  through an adhesive, a fastener, or the like. In some examples, the cap post  14 . 5 - 318  can have a semi-circular cross-sectional shape. The cap post  14 . 5 - 318  can be configured to interface with a center cap post  14 . 5 - 307  of the center cap  14 . 5 - 306  to hold the center cap  14 . 5 - 306  in place in the housing  14 . 5 - 304 . The cap post  14 . 5 - 318  can prevent the center cap  14 . 5 - 306  from rotating relative to the housing  14 . 5 - 304 . The center cap post  14 . 5 - 307  can have a complementary cross-sectional shape to the cap post  14 . 5 - 318  (e.g., the center cap post  14 . 5 - 307  and the cap post  14 . 5 - 318  can each have semi-circular cross-sectional shapes). In some examples, the center cap post  14 . 5 - 307  can have a key-shape, and the cap post  14 . 5 - 318  can have a complementary key-hole shape. 
     Any number or variety of components in any of the configurations described herein can be included in the tensioning system and the electronic device. The components can include any combination of the features described herein and can be arranged in any of the various configurations described herein. The structure and arrangement of components of a tensioning system having a roller-and-wedge locking system described herein, can apply not only to the specific examples discussed herein, but to any number of examples in any combination. Examples of an operation of the tensioning system are described below, with reference to  FIGS.  14 . 5 - 5   . 
       FIGS.  14 . 5 - 5    illustrates a partial cross-sectional view of a portion of a tensioning system  14 . 5 - 300 .  FIGS.  14 . 5 - 5    illustrates the operation of a cam wheel  14 . 5 - 312  and rollers  14 . 5 - 314  included in the tensioning system  14 . 5 - 300 , in accordance with some examples. The tensioning system  14 . 5 - 300  includes a housing  14 . 5 - 304 , a spool  14 . 5 - 316  in the housing  14 . 5 - 304 , and a wheel  14 . 5 - 308 . The wheel  14 . 5 - 308  can include an external portion  14 . 5 - 308 A and an internal portion  14 . 5 - 308 B. The external portion  14 . 5 - 308 A is configured to be interacted with by a user. The external portion  14 . 5 - 308 A can include ridges, bumps, grooves, or other features, which can be used by a user to grip the external portion. The external portion  14 . 5 - 308 A can be formed of a relatively soft material, such as a rubber, a polymer, or the like. The internal portion  14 . 5 - 308 B can include protrusions  14 . 5 - 308 B.i, which are configured to interact with the rollers  14 . 5 - 314  and the cam wheel  14 . 5 - 312 . The internal portion  14 . 5 - 308 B can also be used to connect the external portion  14 . 5 - 308 A to other structures of the tensioning system  14 . 5 - 300 . In some examples, the internal portion  14 . 5 - 308 B can include posts, and the external portion  14 . 5 - 308 A can include corresponding holes in which the posts are inserted to fix the internal portion  14 . 5 - 308 B and the external portion  14 . 5 - 308 A to one another. In some examples, an adhesive can be disposed between the internal portion  14 . 5 - 308 B and the external portion  14 . 5 - 308 A to fix the internal portion  14 . 5 - 308 B and the external portion  14 . 5 - 308 A to one another. 
     The wheel  14 . 5 - 312  can include a protrusion  14 . 5 - 342 , which is configured to interface with the spool  14 . 5 - 316 . For example, as illustrated in  FIGS.  14 . 5 - 5   , the spool  14 . 5 - 316  can include an opening  14 . 5 - 343 , which has a complementary shape to the protrusion  14 . 5 - 342 . This allows for the spool  14 . 5 - 316  and the wheel  14 . 5 - 312  to be rotationally coupled to one another. A cap post  14 . 5 - 318  is disposed in an opening in the housing  14 . 5 - 304 . The cap post  14 . 5 - 318  can be adhered to the housing  14 . 5 - 304 . The housing  14 . 5 - 304  can be physically coupled to a center cap post  14 . 5 - 307  of a center cap by a protrusion  14 . 5 - 319  of the center cap post  14 . 5 - 307 . The center cap post  14 . 5 - 307 , including the protrusion  14 . 5 - 319 , and the cap post  14 . 5 - 318  can have complementary shapes, such that the center cap post  14 . 5 - 307  is physically coupled to (e.g., rotationally coupled to) the cap post  14 . 5 - 318  by the protrusion  14 . 5 - 319 . For example, the protrusion  14 . 5 - 319  and the center cap post  14 . 5 - 307  can have a key shape, and the cap post  14 . 5 - 318  can have a complementary key-hole shape, as illustrated in  FIGS.  14 . 5 - 5   . 
     The housing  14 . 5 - 304  can include a main body portion  14 . 5 - 304 A and an inner ring  14 . 5 - 304 B. The inner ring  14 . 5 - 304 B can be configured to interact with the rollers  14 . 5 - 314 , and can be formed of a relatively hard material, such as metals (e.g., stainless steel, such SAE 304 stainless steel, 420 stainless steel, or the like; aluminum, brass, or any other suitable metals), high-hardness polymers, other polymers, plastics, or the like. The main body portion  14 . 5 - 304 A can be formed of a relatively soft material, such as polymers, plastics, or the like. The spool  14 . 5 - 316  can be formed of a relatively soft material, such as polymers, plastics, or the like. The inner ring  14 . 5 - 304 B can be fixed to the main body portion  14 . 5 - 304 A through an adhesive, fasteners, or the like. 
     The cam wheel  14 . 5 - 312  can include roller openings  14 . 5 - 313 , in which the rollers  14 . 5 - 314  and the protrusions  14 . 5 - 308 B.i are disposed. The roller openings  14 . 5 - 313  are configured to interact with the rollers  14 . 5 - 314  and the protrusions  14 . 5 - 308 B.i to lock and unlock the cam wheel  14 . 5 - 312  and the housing  14 . 5 - 304  together. Specifically, the cam wheel  14 . 5 - 312  and the housing  14 . 5 - 304  can be locked together when the rollers  14 . 5 - 314  are wedged between the cam wheel  14 . 5 - 312  and the housing  14 . 5 - 304 . This system can be referred to as a roller-and-wedge system or a roller-and-wedge clutch. The roller openings  14 . 5 - 313  include ramped surfaces  14 . 5 - 340  that are inclined relative to the inner ring  14 . 5 - 304 B of the housing  14 . 5 - 304  at a cam angle  14 . 5 - 0 . Specifically, the cam angle  14 . 5 - 0  for each ramped surface  14 . 5 - 340  defining a respective roller opening  14 . 5 - 313  can be measured between the respective ramped surface  14 . 5 - 340  and a line tangential to a point on the inner ring  14 . 5 - 304 B of the housing  14 . 5 - 304  in contact with a roller  14 . 5 - 314  in the respective roller opening  14 . 5 - 313 . The cam angle  14 . 5 - 0  can be in a range from about 5° to about 30°. 
     In at least one example, tension from the cable can supply a force to the cam wheel  14 . 5 - 312  through the spool  14 . 5 - 316 , which causes the cam wheel to rotate in a third direction  14 . 5 - 354  (e.g., a counterclockwise direction). When the cam wheel  14 . 5 - 312  rotates in third direction  14 . 5 - 354  as a result of the tension in the cable  14 . 5 - 132 , the rollers  14 . 5 - 314  move in a fourth direction  14 . 5 - 356  (e.g., a clockwise direction) up the ramped surfaces  14 . 5 - 340 , and are wedged between the protrusions  14 . 5 - 308 B.i of the internal portion  14 . 5 - 308 B of the wheel  14 . 5 - 308  and the ramped surfaces  14 . 5 - 344  of the cam wheel  14 . 5 - 312  defining the roller openings  14 . 5 - 313 . The wedged rollers  14 . 5 - 314  lock the cam wheel  14 . 5 - 312  and the housing  14 . 5 - 304  together due to friction being present between the cam wheel  14 . 5 - 312 , the inner ring  14 . 5 - 304 B of the housing  14 . 5 - 304 , and the wedged rollers  14 . 5 - 314 , and prevents the cable  14 . 5 - 132  from rotating the spool  14 . 5 - 316  in the third direction  14 . 5 - 354 . When the cable is pushed into the tensioning system  14 . 5 - 300 , the cam wheel  14 . 5 - 312  and the housing  14 . 5 - 304  are unlocked from one another. Specifically, when the cable  14 . 5 - 132  is pushed into the tensioning system  14 . 5 - 300 , the cam wheel  14 . 5 - 312  rotates in a direction opposite the third direction  14 . 5 - 354 , the rollers  14 . 5 - 314  are un-wedged and roll down the ramped surfaces  14 . 5 - 344  in a direction opposite the fourth direction  14 . 5 - 356 , and the spool  14 . 5 - 316  is free to rotate such that the cable  14 . 5 - 132  wraps around the spool  14 . 5 - 316 . 
     In some examples, the cam wheel  14 . 5 - 312 , the rollers  14 . 5 - 314 , and the inner ring  14 . 5 - 304 B of the housing  14 . 5 - 304  can be formed of materials such as metals (e.g., stainless steel, such SAE 304 stainless steel, 420 stainless steel, or the like; aluminum, brass, or any other suitable metals), high-hardness polymers, other polymers, plastics, or the like. In some examples, one or more of the cam wheel  14 . 5 - 312 , the rollers  14 . 5 - 314 , and the inner ring  14 . 5 - 304 B can be coated or plated with a material to increase the hardness of the cam wheel  14 . 5 - 312 , the rollers  14 . 5 - 314 , and/or the inner ring  14 . 5 - 304 B. In some examples, the rollers  14 . 5 - 314  can be formed of materials having a higher hardness than materials of the cam wheel  14 . 5 - 312  and the inner ring  14 . 5 - 304 B. For example, the cam wheel  14 . 5 - 312 , the rollers  14 . 5 - 314 , and the inner ring  14 . 5 - 304 B can be formed of 14.5-304 stainless steel, with a coating being applied to the rollers  14 . 5 - 314  to increase the hardness of the rollers  14 . 5 - 314  relative to the cam wheel  14 . 5 - 312  and the inner ring  14 . 5 - 304 B. 
     The cam angle  14 . 5 - 0  can be selected based on friction resulting from the materials selected for the cam wheel  14 . 5 - 312 , the inner ring  14 . 5 - 304 B of the housing  14 . 5 - 304 , and the rollers  14 . 5 - 314 ; the scale of the tensioning system  14 . 5 - 300 ; and clearances and tolerances required for the tensioning system; among other factors. Providing a smaller cam angle  14 . 5 - 0  allows the cam wheel  14 . 5 - 312  and the rollers  14 . 5 - 314  to lock with less friction being present between the cam wheel  14 . 5 - 312 , the inner ring  14 . 5 - 304 B of the housing  14 . 5 - 304 , and the rollers  14 . 5 - 314 . Providing too large of a cam angle  14 . 5 - 0  can allow for slipping between the cam wheel  14 . 5 - 312  and the housing  14 . 5 - 304 , such that the cam wheel  14 . 5 - 312  and the housing  14 . 5 - 304  are not sufficiently locked. Providing a smaller cam angle  14 . 5 - 0  requires more space for the rollers  14 . 5 - 314  to move in order to lock the cam wheel  14 . 5 - 312  and the housing  14 . 5 - 304 , which requires larger tolerances and clearances. This results in a larger dead zone between movement of the cable  14 . 5 - 132  before the cam wheel  14 . 5 - 312  and the housing  14 . 5 - 304  lock, and in movement of the wheel  14 . 5 - 308  before the spool  14 . 5 - 316  spools or unspools the cable. The dead zone (also referred to as a roller travel) can be a distance or an angular distance  14 . 5 -D 1  between a center of the rollers  14 . 5 - 314  in the locked position and the unlocked position, and can be in a range from about 0.5 mm to about 8 mm, or in a range from about 5° and about 30°, respectively. A smaller cam angle  14 . 5 - 0  results in higher normal forces, which requires more force to be used to remove the rollers  14 . 5 - 314  from between the cam wheel  14 . 5 - 312  and the inner ring  14 . 5 - 304 B when the cam wheel  14 . 5 - 312  and the housing  14 . 5 - 304  are locked. Providing the cam angle  14 . 5 - 0  in the prescribed range allows for the cam wheel  14 . 5 - 312  and the housing  14 . 5 - 304  to be locked without slipping, while a minimal dead zone is present and minimal force is required to rotate the wheel  14 . 5 - 308 . 
     Although six protrusions  14 . 5 - 308 B.i, six rollers  14 . 5 - 314 , and six roller openings  14 . 5 - 313  are illustrated in  FIGS.  14 . 5 - 5   , any number of the protrusions  14 . 5 - 308 B.i, the rollers  14 . 5 - 314 , and the roller openings  14 . 5 - 313  can be provided. Providing larger numbers of the protrusions  14 . 5 - 308 B.i, the rollers  14 . 5 - 314 , and the roller openings  14 . 5 - 313  can increase strength, while also increasing complexity, size, and cost. Providing smaller numbers of the protrusions  14 . 5 - 308 B.i, the rollers  14 . 5 - 314 , and the roller openings  14 . 5 - 313  can decrease strength, complexity, size, and cost. 
       FIGS.  14 . 5 - 5    illustrates all of the protrusions  14 . 5 - 308 B.i, the rollers  14 . 5 - 314 , and the roller openings  14 . 5 - 313  facing in the same direction with respect to an axis of rotation. In some examples, half or another percentage of the protrusions  14 . 5 - 308 B.i, the rollers  14 . 5 - 314 , and the roller openings  14 . 5 - 313  can face in opposite directions with respect to the axis of rotation. This allows for the tensioning system  14 . 5 - 300  to be locked in both directions, rather than a single direction. In examples in which the protrusions  14 . 5 - 308 B.i, the rollers  14 . 5 - 314 , and the roller openings  14 . 5 - 313  face opposite directions with respect to the axis of rotation, the wheel  14 . 5 - 308  can be used to spool the cable into the spool  14 . 5 - 316  and out of the spool  14 . 5 - 316 . 
     The wheel  14 . 5 - 308  can be rotated in a first direction  14 . 5 - 350  (e.g., a clockwise direction) around a center cap post  14 . 5 - 307  of a center cap  14 . 5 - 306  (illustrated in  FIGS.  14 . 5 - 4   ) and the housing  14 . 5 - 304  and a second direction  14 . 5 - 352  (e.g., a counterclockwise direction) opposite the first direction  14 . 5 - 350  around the center cap post  14 . 5 - 307  and the housing  14 . 5 - 304 . Rotating the wheel  14 . 5 - 308  in the first direction  14 . 5 - 350  can spool a cable onto the spool  14 . 5 - 316 , adding tension to the cable. In other words, rotating the wheel  14 . 5 - 308  in the first direction  14 . 5 - 350  wraps the cable onto the spool  14 . 5 - 316  in order to tighten the cable. Rotating the wheel  14 . 5 - 308  in the second direction  14 . 5 - 352  can unspool the cable from the spool  14 . 5 - 316 , reducing tension in the cable. In other words, rotating the wheel  14 . 5 - 308  in the second direction  14 . 5 - 352  unwraps the cable from the spool  14 . 5 - 316  in order to loosen the cable. 
     Rotating the wheel  14 . 5 - 308  in the first direction  14 . 5 - 350  or the second direction  14 . 5 - 352  unlocks the cam wheel  14 . 5 - 312  and the housing  14 . 5 - 304 , and allows the cable to be spooled into the tensioning system  14 . 5 - 300  or out of the tensioning system  14 . 5 - 300 . For example, when the wheel  14 . 5 - 308  is rotated in the first direction  14 . 5 - 350 , the rollers  14 . 5 - 314  roll down the ramped surfaces  14 . 5 - 344  and the tensioning system  14 . 5 - 300  is unlocked. When the wheel  14 . 5 - 308  is rotated in the second direction  14 . 5 - 352 , the protrusions  14 . 5 - 308 B.i push the rollers  14 . 5 - 314  down the ramped surfaces and the tensioning system  14 . 5 - 300  is unlocked. This allows the wheel  14 . 5 - 308  to rotate in either direction in order to increase or decrease tension on the cable. 
     The tensioning system  14 . 5 - 300 , which includes the rollers  14 . 5 - 314  and the cam wheel  14 . 5 - 312 , can be locked in either direction or both directions. In contrast to conventional systems, which can use spring-driven pawls, ratcheting systems, or the like, the tensioning system  14 . 5 - 300  uses the rollers  14 . 5 - 314 , which roll within the roller openings  14 . 5 - 313  to lock and unlock the tensioning system  14 . 5 - 300 . This is a silent or relatively silent system. The tensioning system  14 . 5 - 300  can be locked in one or both directions, and is thus non-back-drivable. In other words, the cable can be pushed into the tensioning system  14 . 5 - 300 , but cannot be pulled from the tensioning system  14 . 5 - 300  without using the wheel  14 . 5 - 308 . This allows for the cable to be spooled into the tensioning system  14 . 5 - 300  and for the tensioning system  14 . 5 - 300  to hold tension in the cable. 
     Any number or variety of components in any of the configurations described herein can be included in the tensioning system and the electronic device. The components can include any combination of the features described herein and can be arranged in any of the various configurations described herein. The structure and arrangement of components of a tensioning system having a roller-and-wedge locking system described herein, can apply not only to the specific examples discussed herein, but to any number of examples in any combination. Examples of a configuration of the various components of a dial cap included in the tensioning system relative to one another, and a configuration of an angular restraint are described below, with reference to  FIGS.  14 . 5 - 6   . 
       FIGS.  14 . 5 - 6    illustrates a partially exploded view of dial cap  14 . 5 - 302  and a perspective view of a cam wheel  14 . 5 - 312 . The dial cap  14 . 5 - 302  includes a center cap  14 . 5 - 306 , a center cap post  14 . 5 - 307 , a wheel  14 . 5 - 308  including an external portion  14 . 5 - 308 A and an internal portion  14 . 5 - 308 B, a wheel adhesive  14 . 5 - 360 , and a clamp ring  14 . 5 - 364 . The center cap  14 . 5 - 306 , the center cap post  14 . 5 - 307 , the wheel  14 . 5 - 308 , and the cam wheel  14 . 5 - 312  can be substantially similar to, and can include some or all of, the features of the center cap  14 . 5 - 306 , the center cap post  14 . 5 - 307 , the wheel  14 . 5 - 308 , and the cam wheel  14 . 5 - 312 , respectively, discussed above with respect to  FIGS.  14 . 5 - 3  through  14 . 5 - 5   . 
     As discussed previously, a user can interact with the external portion  14 . 5 - 308 A of the wheel  14 . 5 - 308  to turn the wheel  14 . 5 - 308  and wind or unwind a cable in a tensioning system to tighten or loosen the cable. The internal portion  14 . 5 - 308 B of the wheel  14 . 5 - 308  can include protrusions  14 . 5 - 308 B.i. The protrusions  14 . 5 - 308 B.i can be used to interact with rollers and the cam wheel  14 . 5 - 312  to lock and unlock the cam wheel  14 . 5 - 312  and a housing of the tensioning system. The wheel adhesive  14 . 5 - 360  and the clamp ring  14 . 5 - 364  can be used to fasten the inner portion  14 . 5 - 308 B to the external portion  14 . 5 - 308 A of the wheel  14 . 5 - 308 . 
     The example of  FIGS.  14 . 5 - 6    includes a disc-type angular stop mechanism. Specifically, in the example of  FIGS.  14 . 5 - 6   , a first disc  14 . 5 - 351  and a second disc  14 . 5 - 355  are included, which provide angular stops for the dial cap  14 . 5 - 302 . The first disc  14 . 5 - 351  and the second disc  14 . 5 - 355  can be mounted on the center cap post  14 . 5 - 307  by a clip  14 . 5 - 358 . The first disc  14 . 5 - 351  and the second disc  14 . 5 - 355  can be fastened on the center cap post  14 . 5 - 307  by any method, which allows the first disc  14 . 5 - 351  and the second disc  14 . 5 - 355  to rotate around the center cap post  14 . 5 - 307 . For example, the first disc  14 . 5 - 351  and the second disc  14 . 5 - 355  can be fastened on the center cap post  14 . 5 - 307  by a pin, a clip, another fastener, or the like. The center cap  14 . 5 - 306  can include a protrusion  14 . 5 - 309 , which provides an angular stop for the first disc  14 . 5 - 351 . 
     The first disc  14 . 5 - 351  can include a first protrusion  14 . 5 - 352 A, which interacts with a first side of the protrusion  14 . 5 - 309  to stop angular rotation of the first disc  14 . 5 - 351  in a first direction, or interacts with a second side of the protrusion  14 . 5 - 309  opposite the first side to stop angular rotation of the first disc  14 . 5 - 351  in a second direction opposite the first direction. The first disc  14 . 5 - 351  includes a second protrusion (such as the second protrusion  14 . 5 - 350 B, discussed below with respect to  FIGS.  14 . 5 - 7 A and  14 . 5 - 7 B ) opposite the first protrusion, which interacts with the second disc  14 . 5 - 355 . The second disc  14 . 5 - 355  interacts with a protrusion  14 . 5 - 359  on the cam wheel  14 . 5 - 312 . The cam wheel  14 . 5 - 312  can rotate freely until the protrusion  14 . 5 - 359  interacts with a first protrusion  14 . 5 - 356 A on the second disc  14 . 5 - 355 . The cam wheel  14 . 5 - 312  and the second disc  14 . 5 - 355  can then rotate freely until a second protrusion (such as the second protrusion  14 . 5 - 356 B, discussed below with respect to  FIGS.  14 . 5 - 7 B ) of the second disc  14 . 5 - 355  interacts with the second protrusion of the first disc  14 . 5 - 351 . The cam wheel  14 . 5 - 312 , the second disc  14 . 5 - 355 , and the first disc  14 . 5 - 351  can then rotate freely until the first protrusion  14 . 5 - 352 A of the first disc  14 . 5 - 351  interacts with the protrusion  14 . 5 - 309  of the center cap  14 . 5 - 306 . When the first protrusion  14 . 5 - 352 A of the first disc  14 . 5 - 351  interacts with the protrusion  14 . 5 - 309  of the center cap  14 . 5 - 306 , the protrusion  14 . 5 - 309  provides an angular stop, which prevents the first disc  14 . 5 - 351 , the second disc  14 . 5 - 355 , and the cam wheel  14 . 5 - 312  from turning in one direction, but allows the first disc  14 . 5 - 351 , the second disc  14 . 5 - 355 , and the cam wheel  14 . 5 - 312  to turn in the opposite direction. This prevents a user from over-spooling a tensioning system (e.g., spooling more cable into a housing of the tensioning system than the housing is configured to hold), or from loosening the tensioning system too much such that the tensioning system winds the cable into the housing in the wrong direction. 
     Each of the first disc  14 . 5 - 351  and the second disc  14 . 5 - 355  can allow for roughly 360 degrees of rotation and the cam wheel  14 . 5 - 312  can allow for an additional roughly 360 degrees of rotation. Specifically, each of the first disc  14 . 5 - 351  and the second disc  14 . 5 - 355  can allow for 360 degrees of rotation minus the angular width of the protrusions included in the first disc  14 . 5 - 351 , the second disc  14 . 5 - 355 , the center cap  14 . 5 - 306 , and the cam wheel  14 . 5 - 312 . Including a larger number of discs in the angular stop increases the angular rotation allowed by the angular stop, while decreasing the number of discs in the angular stop decreases the angular rotation allowed by the angular stop. The first disc  14 . 5 - 351  and the second disc  14 . 5 - 355  can be formed of a relatively hard material, such as metals (e.g., stainless steel, such SAE 304 stainless steel, 420 stainless steel, or the like; aluminum, brass, or any other suitable metals), high-hardness polymers, other polymers, plastics, or the like. Including the first disc  14 . 5 - 351  and the second disc  14 . 5 - 355  in the angular stop and forming the first disc  14 . 5 - 351  and the second disc  14 . 5 - 355  of relatively hard materials provides a strong, robust angular stop, which is difficult for a user to damage. 
     Any number or variety of components in any of the configurations described herein can be included in the tensioning system and the electronic device. The components can include any combination of the features described herein and can be arranged in any of the various configurations described herein. The structure and arrangement of components of a tensioning system having a roller-and-wedge locking system and an angular restraint described herein, can apply not only to the specific examples discussed herein, but to any number of examples in any combination. Examples of a configuration and operation of a disc-type angular restraint included in the tensioning system are described below, with reference to  FIGS.  14 . 5 - 7 A and  14 . 5 - 7 B . 
       FIGS.  14 . 5 - 7 A and  14 . 5 - 7 B  illustrate plan views of a center cap  14 . 5 - 306 , a first disc  14 . 5 - 351 , a second disc  14 . 5 - 355 , and a cam wheel  14 . 5 - 312 .  FIGS.  14 . 5 - 7 A and  14 . 5 - 7 B  illustrate additional details of the disc-type angular stop mechanism of the example illustrated in  FIGS.  14 . 5 - 6   . The center cap  14 . 5 - 306 , the first disc  14 . 5 - 351 , the second disc  14 . 5 - 355 , and the cam wheel  14 . 5 - 312  can be substantially similar to, and can include some or all of, the features of the center cap  14 . 5 - 306 , the first disc  14 . 5 - 351 , the second disc  14 . 5 - 355 , and the cam wheel  14 . 5 - 312 , respectively, discussed above with respect to  FIGS.  14 . 5 - 6   . 
     As illustrated in  FIGS.  14 . 5 - 7 A , the first disc  14 . 5 - 351  can sit in a channel  14 . 5 - 311  of the center cap  14 . 5 - 306 . The first disc  14 . 5 - 351  is free to rotate in the channel  14 . 5 - 311 , but is stopped by the first protrusion  14 . 5 - 352 A of the first disc  14 . 5 - 351  contacting either side of the protrusion  14 . 5 - 309  of the center cap  14 . 5 - 306 . The first disc  14 . 5 - 351  can rotate 360 degrees relative to the center cap  14 . 5 - 306 , minus the angular widths of the protrusion  14 . 5 - 309  and the first protrusion  14 . 5 - 352 A. The first disc  14 . 5 - 351  further includes a second protrusion  14 . 5 - 352 B opposite the first protrusion  14 . 5 - 352 A, which interacts with the second disc  14 . 5 - 355 , illustrated in  FIGS.  14 . 5 - 7 B . 
     As illustrated in  FIGS.  14 . 5 - 7 B , the second disc  14 . 5 - 355  sits on the first disc  14 . 5 - 351 . The second protrusion  14 . 5 - 352 B of the first disc  14 . 5 - 351  can sit in a channel  14 . 5 - 353  of the second disc  14 . 5 - 355 , which includes a second protrusion  14 . 5 - 356 B opposite the first protrusion  14 . 5 - 356 A. The second disc  14 . 5 - 355  is free to rotate, and begins to rotate the first disc  14 . 5 - 351  when the second protrusion  14 . 5 - 356 B of the second disc  14 . 5 - 355  contacts either side of the second protrusion  14 . 5 - 352 B of the first disc  14 . 5 - 351 . The second disc  14 . 5 - 355  can rotate 360 degrees relative to the first disc  14 . 5 - 351 , minus the angular widths of the second protrusion  14 . 5 - 352 B and the second protrusion  14 . 5 - 356 B. 
     In at least one example, the cam wheel  14 . 5 - 312  sits on the second disc  14 . 5 - 355 . The cam wheel  14 . 5 - 312  can include a channel in which the second disc  14 . 5 - 355  is disposed, and the protrusion  14 . 5 - 359  of the cam wheel  14 . 5 - 312  and the first protrusion  14 . 5 - 356 A of the second disc  14 . 5 - 355  can be disposed in the channel of the cam wheel  14 . 5 - 312 . The cam wheel  14 . 5 - 312  is free to rotate, and begins to rotate the second disc  14 . 5 - 355  when the protrusion  14 . 5 - 359  of the cam wheel  14 . 5 - 312  contacts either side of the first protrusion  14 . 5 - 356 A of the second disc  14 . 5 - 355 . The cam wheel  14 . 5 - 312  can rotate 360 degrees relative to the second disc  14 . 5 - 355 , minus the angular widths of the protrusion  14 . 5 - 359  and the first protrusion  14 . 5 - 356 A. Although the protrusion  14 . 5 - 359  has been discussed as being disposed on the cam wheel  14 . 5 - 312 , the protrusion  14 . 5 - 359  can be located on any of the parts that rotate relative to the housing and the center cap  14 . 5 - 306 , such as on the spool, the wheel, or the like. 
     Any number or variety of components in any of the configurations described herein can be included in the tensioning system and the electronic device. The components can include any combination of the features described herein and can be arranged in any of the various configurations described herein. The structure and arrangement of components of a tensioning system having a roller-and-wedge locking system and an angular restraint described herein, can apply not only to the specific examples discussed herein, but to any number of examples in any combination. Examples of a configuration and operation of a dial cap including a ball detent mechanism that can be included in the tensioning system are described below, with reference to  FIGS.  14 . 5 - 8   . 
       FIGS.  14 . 5 - 8    illustrates a partial cross-sectional view of a dial cap  14 . 5 - 302 . The dial cap  14 . 5 - 302  includes a center cap  14 . 5 - 306 , a wheel  14 . 5 - 308 , and a ball detent mechanism. The center cap  14 . 5 - 306  and the wheel  14 . 5 - 308  can be substantially similar to, and can include some or all of, the features of the center cap  14 . 5 - 306  and the wheel  14 . 5 - 308 , respectively, discussed above with respect to  FIGS.  14 . 5 - 3  through  14 . 5 - 6   . The ball detent mechanism includes a ball  14 . 5 - 370 , a detent spring  14 . 5 - 372 , springs  14 . 5 - 374 , and a grooved surface  14 . 5 - 376  in the wheel  14 . 5 - 308 . The ball  14 . 5 - 370 , the detent spring  14 . 5 - 372 , and the springs  14 . 5 - 374  can be contained within protrusions  14 . 5 - 315  of the center cap  14 . 5 - 306 . As described previously, the wheel  14 . 5 - 308  can rotate relative to the center cap  14 . 5 - 306 . As the wheel  14 . 5 - 308  rotates, the springs  14 . 5 - 374  push the detent spring  14 . 5 - 372 , which pushes the ball  14 . 5 - 370  into the grooves of the grooved surface  14 . 5 - 376  in the wheel  14 . 5 - 308 . The ball detent mechanism generates both a clicking sound and a clicking feel as wheel  14 . 5 - 308  turns and the ball  14 . 5 - 370  moves between adjacent grooves of the grooved surface  14 . 5 - 376 . Because the locking mechanism provided by the rollers and the cam wheel is silent, the ball detent mechanism can be included in a tensioning system in order to provide a desired user experience that includes a clicking feel and sound when the wheel  14 . 5 - 308  is rotated. 
     Any number or variety of components in any of the configurations described herein can be included in the tensioning system and the electronic device. The components can include any combination of the features described herein and can be arranged in any of the various configurations described herein. The structure and arrangement of components of a tensioning system having a roller-and-wedge locking system and an angular restraint described herein, can apply not only to the specific examples discussed herein, but to any number of examples in any combination. Examples of configurations and operations of various angular restraints that can be included in the tensioning system are described below, with reference to  FIGS.  14 . 5 - 9 A through  14 . 5 - 9 C . 
       FIGS.  14 . 5 - 9 A through  14 . 5 - 9 C  illustrate various angular stop mechanisms, which can be used in place of or in addition to the disc-type angular stop mechanism of  FIGS.  14 . 5 - 6    through  14 . 5 - 7 B. In the example of  FIGS.  14 . 5 - 9 A , a cable-type angular stop mechanism is illustrated. A cable  14 . 5 - 380  can be provided and attached to a center cap  14 . 5 - 306  and a cam wheel  14 . 5 - 312 . The cable  14 . 5 - 380  can be attached to the center cap  14 . 5 - 306  and the cam wheel  14 . 5 - 312  using an adhesive or the like. The cable  14 . 5 - 380  can be provided such that the cable  14 . 5 - 380  wraps around a center cap post  14 . 5 - 307  of the center cap  14 . 5 - 306  as the cam wheel  14 . 5 - 312  is rotated relative to the center cap  14 . 5 - 306 . Once the cable  14 . 5 - 380  is wrapped tightly around the center cap post  14 . 5 - 307 , the cable  14 . 5 - 380  prevents the cam wheel  14 . 5 - 312  from rotating relative to the center cap  14 . 5 - 306 , while allowing the cam wheel  14 . 5 - 312  to rotate in the opposite direction. The cable  14 . 5 - 380  can be a cheap method for providing an angular stop mechanism for the tensioning system. This prevents a user from over-spooling a tensioning system (e.g., spooling more cable into a housing of the tensioning system than the housing is configured to hold), or from loosening the tensioning system too much such that the tensioning system winds cable into the housing in the wrong direction. Although the cable  14 . 5 - 380  has been discussed as being attached to the center cap  14 . 5 - 306  and the cam wheel  14 . 5 - 312 , the cable  14 . 5 - 380  can be attached to any components of the tensioning system that are desired to include an angular stop. For example, the cable  14 . 5 - 380  can be attached to the center cap  14 . 5 - 306  and a spool of the tensioning system, a wheel of the tensioning system, or the like. In some examples, the cable  14 . 5 - 380  can be attached to a housing of the tensioning system and the cam wheel  14 . 5 - 312 , the housing and the spool, the housing and the wheel, or the like. 
     In the example of  FIGS.  14 . 5 - 9 B , a screw-type angular stop mechanism is illustrated. A helical ridge  14 . 5 - 382  is formed on a center cap post  14 . 5 - 307  of a center cap  14 . 5 - 306  and a corresponding helical groove  14 . 5 - 384  is formed on a cam wheel  14 . 5 - 312 . As the cam wheel  14 . 5 - 312  rotates relative to the center cap  14 . 5 - 306 , the cam wheel  14 . 5 - 312  moves up or down the center post  14 . 5 - 307  of the center cap  14 . 5 - 306 . Vertical stops can be included on the center cap post  14 . 5 - 307 , which prevent the cam wheel  14 . 5 - 312  from rotating too far in either direction. This prevents a user from over-spooling a tensioning system (e.g., spooling more cable into a housing of the tensioning system than the housing is configured to hold), or from loosening the tensioning system too much such that the tensioning system winds cable into the housing in the wrong direction. Although the helical ridge  14 . 5 - 382  has been discussed as being disposed on the center cap post  14 . 5 - 307  of the center cap  14 . 5 - 306 , the helical ridge  14 . 5 - 382  can be disposed on a portion of a housing of a tensioning system or the like. The helical groove  14 . 5 - 384  can be disposed on a spool of the tensioning system or the like. 
     In the example of  FIGS.  14 . 5 - 9 C , a spiral-type angular stop mechanism is illustrated. A spiral  14 . 5 - 386  is formed in a center cap  14 . 5 - 306 . A center cap post  14 . 5 - 307  of the center cap  14 . 5 - 306  can be formed adjacent the spiral  14 . 5 - 386 . The spiral  14 . 5 - 386  can be a spiral-shaped groove formed in the center cap  14 . 5 - 306 . The spiral  14 . 5 - 386  can interact with a protrusion  14 . 5 - 388  of a cam wheel  14 . 5 - 312 . The protrusion  14 . 5 - 388  can be attached to the cam wheel  14 . 5 - 312  such that the protrusion can slide relative to the cam wheel  14 . 5 - 312 . For example, the protrusion  14 . 5 - 388  can slide in a direction along a radius of the cam wheel  14 . 5 - 312 . As the cam wheel  14 . 5 - 312  rotates relative to the center cap  14 . 5 - 306 , the protrusion  14 . 5 - 388  slides relative to the cam wheel  14 . 5 - 312  along the radius of the cam wheel  14 . 5 - 312  closer to or further from a center of the cam wheel  14 . 5 - 312  depending on which direction the cam wheel  14 . 5 - 312  turns. When the protrusion  14 . 5 - 388  reaches either end of the spiral  14 . 5 - 386 , the protrusion  14 . 5 - 388  and the spiral  14 . 5 - 386  prevent the cam wheel  14 . 5 - 312  from rotating relative to the center cap  14 . 5 - 306 , while allowing the cam wheel  14 . 5 - 312  to rotate in the opposite direction. This prevents a user from over-spooling a tensioning system (e.g., spooling more cable into a housing of the tensioning system than the housing is configured to hold), or from loosening the tensioning system too much such that the tensioning system winds cable into the housing in the wrong direction. Although the spiral  14 . 5 - 386  has been discussed as being disposed on the center cap  14 . 5 - 306 , the spiral  14 . 5 - 386  can be disposed on a portion of a housing of a tensioning system or the like. The protrusion  14 . 5 - 388  can be disposed on a spool of the tensioning system, a wheel of the tensioning system, or the like. 
     14.6: Two-Part Speaker System 
     The following disclosure relates to speaker assemblies for electronic devices. More specifically, the following disclosure describes a two-part speaker assembly having a single outlet port. Electronic devices typically include a housing that surrounds internal system components, such as audio speaker assemblies, circuitry, processing units, display elements, and other electronic components. However, as electronic devices become smaller and smaller the available space for such components is reduced. Further, it may be desirable to reduce the number of openings or ports visible on the electronic device. The speaker assembly described herein uniquely addresses the challenges that arise from reductions in available space, as well as aesthetic and performance requirements. 
     In a particular embodiment, an electronic device, such as a head-mounted device (“HMD”), can include a housing defining an internal volume. The housing can also define an opening that is in fluid communication with the internal volume of the housing and with the ambient environment. A speaker assembly or unit can be positioned within the internal volume of the electronic device. In some examples, the speaker assembly is positioned within a support of an HMD, such as within a support arm or support band positioned along a user&#39;s head. The speaker assembly can be positioned proximate a user&#39;s ear and can be oriented to direct sound in the direction of the user&#39;s ear. 
     In some examples, the speaker assembly can include a speaker pod or housing that is positioned within the electronic device. The speaker pod can house various components of the speaker assembly. The speaker pod can further define a port or outlet that is aligned with the opening of the electronic device. Thus, the port can be in fluid communication with an interior volume of the speaker pod and with the outside environment. The speaker assembly can include multiple audio transducers or speakers. 
     The speaker can include a diaphragm coupled to a magnetic component. The magnetic component can include gaps or grooves within which is disposed a wound coil, such as a copper coil or any conductive coil that is capable of being influenced by an electromagnetic field. In some examples, the coil is capable of generating or being influenced by a desired electromagnetic field. The diaphragm can be affixed to the coil. As pulses of electricity pass through the coil, the direction of its magnetic field is rapidly changed, resulting in alternating attraction and repulsion to the magnetic component, causing vibrations back and forth. The coil can be attached to the diaphragm which amplifies these vibrations, pumping sound waves into the surrounding air. Notably, despite having multiple audio transducers, the speaker assembly may include only one outlet or port. 
     In one example, the speaker assembly includes a first speaker, such as a woofer, that is positioned within the pod and is in fluid communication with a first channel. The woofer channel can include a winding, tortuous path to the outlet port. A second speaker, such as a tweeter, can also be positioned within the pod and can be in fluid communication with a second channel or tweeter channel. As described in greater detail below, the woofer channel and the tweeter channel can merge, converge, or combine into a shared channel that leads to the port. 
     In some examples, the tweeter can at least partially overlap the woofer, thereby reducing the footprint of the assembly along at least one major dimension. This can enable the speaker pod to better fit within an electronic device with space constraints. In some examples, the speaker assembly can include a port barrier positioned at the port. The port barrier can cover the port. The port barrier can include a woven mesh, a metallic screen defining an exterior of the electronic device, and a support frame to reinforce the woven mesh. Likewise, the speaker assembly can include a woofer mesh positioned in a channel leading to the woofer. The woofer mesh can have a higher density than the mesh of the port barrier. In some examples, the woofer mesh can have a lower density than the port barrier mesh. Thus, the speaker assembly can include multiple meshes. 
     A challenge that arises from a shared port system as described herein is potential interference of the tweeter from the acoustic waves or sound generated by the woofer. The woofer diaphragm is larger than the tweeter diaphragm, and can therefore negatively impact operation of the tweeter. Accordingly, in some examples, the speaker assembly can include a wall positioned between the tweeter and the woofer channel. The wall can shield the tweeter from the acoustic waves generated by the woofer. In some examples, the shielding wall can at least partially defining the one or more of the channels of the speaker assembly and can be shaped to direct the flow of the sound. 
     These and other embodiments are discussed below with reference to  FIGS.  14 . 6 - 1  through  14 . 6 - 4   . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these figures is for explanatory purposes only and should not be construed as limiting. Furthermore, as used herein, a system, a method, an article, a component, a feature, or a sub-feature comprising at least one of a first option, a second option, or a third option should be understood as referring to a system, a method, an article, a component, a feature, or a sub-feature that can include one of each listed option (e.g., only one of the first option, only one of the second option, or only one of the third option), multiple of a single listed option (e.g., two or more of the first option), two options simultaneously (e.g., one of the first option and one of the second option), or combination thereof (e.g., two of the first option and one of the second option). 
     It will further be understood that various directional indicators used herein (e.g., up, down, sideways, vertical, horizontal, top, bottom, above, below, etc.) are made in relation to the orientation of the figure at issue. For example, a vertical axis is to be understood as an axis extending from a top of the page to a bottom of the page. 
       FIGS.  14 . 6 - 1 A through  14 . 6 - 1 D  illustrate various electronic devices that are capable of being used in conjunction with the speaker assembly described herein. The electronic devices can correspond to any form of a wearable electronic device, a portable media player, a media storage device, a portable digital assistant (“PDA”), a tablet computer, a computer, a mobile communication device, a GPS unit, a remote-control device, or any other electronic device. Specific examples of such devices are described below. 
       FIGS.  14 . 6 - 1 A  illustrates an electronic device, such as a head-mounted device  14 . 6 - 100   a  positioned on a user&#39;s head  14 . 6 - 120 . The head-mounted device can include a display device used for virtual or augmented reality simulations. The head-mounted device  14 . 6 - 100   a  can further relate to any form of computer glasses or smart glasses. The head-mounted device  14 . 6 - 100   a  can include a display  14 . 6 - 104  and a support  14 . 6 - 108 , such as a headband or support arms. In some examples, the display  14 . 6 - 104  includes an opaque, translucent, transparent, or semi-transparent screen, including any number lenses, for presenting visual data. 
     The head-mounted device  14 . 6 - 100   a  can be worn on the user&#39;s head  14 . 6 - 120  such that the display  14 . 6 - 104  is positioned over the user&#39;s face and disposed over one or both of the user&#39;s eyes. The display  14 . 6 - 104  can be connected to the support  14 . 6 - 108 . In some examples, the support  14 . 6 - 108  can be positioned against the side of a user&#39;s head  14 . 6 - 120  and in contact therewith. In some examples, the support  14 . 6 - 108  can be at least partially positioned above the user&#39;s ear or ears  14 . 6 - 124 . In some examples, the support  14 . 6 - 108  can be positioned adjacent to the user&#39;s ear or ears  14 . 6 - 124 . The support  14 . 6 - 108  can extend around the user&#39;s head  14 . 6 - 120 . In this way, the display  14 . 6 - 104  and the support  14 . 6 - 108  can retain the head-mounted device  14 . 6 - 100   a  on the user&#39;s head  14 . 6 - 120 . It should be understood, however, that this configuration is just one example of how the components of a modular head-mounted device  14 . 6 - 100   a  can be arranged, and that in some examples, a different number of connector straps and/or retention bands can be included. 
     The head-mounted device  14 . 6 - 100   a  can include a frame or housing that houses various electrical components. Various components of the display  14 . 6 - 104  can be housed within the housing. For example, the hardware and electronics which allow functionality of the HMD can be housed within the housing. The housing can be defined by any portion of the support  14 . 6 - 108  and/or the display  14 . 6 - 104 . 
     In some examples, the head-mounted device  14 . 6 - 100   a  can include a speaker assembly  14 . 6 - 112 . The speaker assembly  14 . 6 - 112  can be positioned in a housing of the head-mounted device  14 . 6 - 100   a . For example, the speaker assembly  14 . 6 - 112  can be positioned in or on the support  14 . 6 - 108 . The support  14 . 6 - 108  can define an opening  14 . 6 - 116  that is positioned proximate the speaker assembly  14 . 6 - 112  to permit passage of acoustic waves generated by the speaker assembly  14 . 6 - 112  to emit into the outside environment. In some examples, when the head-mounted device  14 . 6 - 100   a  is worn on the user&#39;s head  14 . 6 - 120 , the speaker assembly  14 . 6 - 112  and the opening  14 . 6 - 116  are positioned proximate the user&#39;s ear  14 . 6 - 124 . The support  14 . 6 - 108  and/or the opening  14 . 6 - 116  can be oriented such that sound can be directed toward the user&#39;s ear  14 . 6 - 124  when the user  14 . 6 - 120  is wearing the head-mounted device  14 . 6 - 100   a . Further details regarding electronic devices capable of implementing a single port speaker system are described below with reference to  FIGS.  14 . 6 - 1 B . 
       FIGS.  14 . 6 - 1 B  illustrates an example in which the electronic device is a wearable device  14 . 6 - 100   b . The wearable device  14 . 6 - 100   b  can be a watch, such as a smartwatch. The wearable device  14 . 6 - 100   b  of  FIGS.  14 . 6 - 1 B  is merely one representative example of a device that can be used in conjunction with the components and methods disclosed herein. In other words, the wearable device  14 . 6 - 100   b  is one non-limiting example of a device that can include a single port speaker assembly as described herein. 
     The wearable device  14 . 6 - 100   b  can include an enclosure or housing  14 . 6 - 103 . The housing  14 . 6 - 103  can be connected to a front display  14 . 6 - 105  and can have a strap  14 . 6 - 109  designed to attach the wearable device  14 . 6 - 100   b  to a user, or to provide wearable functionality. The housing  14 . 6 - 103  can define a port  14 . 6 - 116  that is in fluid communication with a speaker assembly disposed within the housing  14 . 6 - 103 . A number of input elements, such as a rotatable crown and/or a button  14 . 6 - 107  can be attached to and can protrude from the housing  14 . 6 - 103 . The housing  14 . 6 - 103  can define a single speaker port  14 . 6 - 116  that can be integrally formed in and defined by the housing  14 . 6 - 103 . 
     In some examples, the wearable device  14 . 6 - 100   b  can include a speaker assembly (not shown in  FIGS.  14 . 6 - 1 B ). The speaker assembly can be positioned in the housing  14 . 6 - 103  of the wearable device  14 . 6 - 100   b . The housing  14 . 6 - 103  can define the port  14 . 6 - 116  that is positioned in relation to the speaker assembly  14 . 6 - 112  to permit passage of acoustic waves generated by the speaker assembly to emit into the outside environment. In some examples, when the wearable device  14 . 6 - 100   b  is worn by a user, the port  14 . 6 - 116  is positioned or oriented to be directed toward the user&#39;s ear  14 . 6 - 124 . The port  14 . 6 - 116  can be positioned such that sound can be directed toward the user&#39;s ear when the user is wearing the wearable device  14 . 6 - 100   b . Further details regarding electronic devices capable of implementing a single port speaker system are described below with reference to  FIGS.  14 . 6 - 1 C . 
       FIGS.  14 . 6 - 1 C  illustrates an example in which the electronic device is a computer  14 . 6 - 100   c , such as a laptop or desktop. The computer  14 . 6 - 100   c  of  FIGS.  14 . 6 - 1 C  is merely one representative example of a device that can be used in conjunction with the components and methods disclosed herein. Specifically, the computer  14 . 6 - 100   c  is one representative example of a device that can include a single port speaker assembly as described herein. 
     The computer  14 . 6 - 100   c  can include an enclosure or housing  14 . 6 - 111 . The housing  14 . 6 - 111  can be connected to a display  14 . 6 - 113 . The housing  14 . 6 - 111  can define a port  14 . 6 - 116  that is in fluid communication with a speaker assembly housed within the housing  14 . 6 - 111 . A number of input members  14 . 6 - 115 , such as a keyboard, can be attached to and can protrude from the housing  14 . 6 - 111 . 
     The housing  14 . 6 - 111  can substantially define an internal volume and at least a portion of an exterior surface of the computer  14 . 6 - 100   c . The display  14 . 6 - 113  can include a touch sensitive surface, such as a touchscreen. The display  14 . 6 - 113  can define an exterior surface of the computer  14 . 6 - 100   c.    
     The housing  14 . 6 - 111  can also include features, such as such as button apertures or charging port apertures. In some examples, the housing  14 . 6 - 111  defines only a single speaker port  14 . 6 - 116  by the process described herein. The speaker port  14 . 6 - 116  can be integrally formed in and defined by the housing  14 . 6 - 111 . 
     In some examples, the computer  14 . 6 - 100   c  can include a speaker assembly (not shown in  FIGS.  14 . 6 - 1 C ). The speaker assembly can be positioned in the internal volume of housing  14 . 6 - 111  of the computer  14 . 6 - 100   c . The housing  14 . 6 - 111  can define the port  14 . 6 - 116  that is positioned in relation to the speaker assembly to permit passage of acoustic waves generated by the speaker assembly to emit into the outside environment. The port  14 . 6 - 116  can be positioned such that sound can be directed toward the user&#39;s ear when the user is using the computer  14 . 6 - 100   c . Further details regarding electronic devices capable of implementing a single port speaker system are described below with reference to  FIGS.  14 . 6 - 1 D . 
       FIGS.  14 . 6 - 1 D  illustrates an example in which the electronic device is a mobile device  14 . 6 - 100   d , such as a smartphone or tablet computer. The mobile device  14 . 6 - 100   d  of  FIG.  14 . 6 - 1 D  is merely one representative example of a device that can be used in conjunction with the components and methods disclosed herein. Specifically, the mobile device  14 . 6 - 100   d  is merely one representative example of a device that can include a single port speaker assembly as described herein. 
     The mobile device  14 . 6 - 100   d  can include an enclosure or housing  14 . 6 - 117 . The housing  14 . 6 - 117  can be connected to a display  14 . 6 - 119 . The housing  14 . 6 - 117  can define a port  14 . 6 - 116  that is in fluid communication with a speaker assembly disposed within the housing  14 . 6 - 117 . 
     The housing  14 . 6 - 117  can substantially define at least a portion of an exterior surface of the mobile device  14 . 6 - 100   d . The display  14 . 6 - 119  can include a touch sensitive surface, such as a touchscreen. The display  14 . 6 - 119  can define an exterior surface of the mobile device  14 . 6 - 100   d.    
     The housing  14 . 6 - 117  can also include features, such as such as button apertures or charging port apertures. In some examples, the housing  14 . 6 - 117  defines only a single speaker port  14 . 6 - 116  by the process described herein. The speaker port  14 . 6 - 116  can be integrally formed in the housing  14 . 6 - 117 . 
     In some examples, the mobile device  14 . 6 - 100   d  can include a speaker assembly (not shown in  FIGS.  14 . 6 - 1 C ). The speaker assembly can be positioned in the housing  14 . 6 - 117  of the mobile device  14 . 6 - 100   d . The housing  14 . 6 - 117  can define the port  14 . 6 - 116  that is positioned in relation to the speaker assembly  14 . 6 - 112  to permit passage of acoustic waves generated by the speaker assembly to emit into the outside environment. The port  14 . 6 - 116  can be positioned such that sound can be directed toward the user&#39;s ear when the user is using the mobile device  14 . 6 - 100   d . Further details regarding speaker assemblies are described below with reference to  FIGS.  14 . 6 - 2   . 
       FIGS.  14 . 6 - 2    shows a schematic view of an electronic device  14 . 6 - 200  that includes a speaker assembly  14 . 6 - 212 . The electronic device  14 . 6 - 200  can be substantially similar to, and can include some or all of the features of the electronic devices described herein, such as the head-mounted device  14 . 6 - 100   a , the wearable device  14 . 6 - 100   b , the computer  14 . 6 - 100   c , and the mobile device  14 . 6 - 100   d.    
     The electronic device  14 . 6 - 200  can define an interior volume  14 . 6 - 201  to house electronic components. The speaker assembly  14 . 6 - 212  can be positioned within the internal volume  14 . 6 - 201 . In some examples, an entirety of the speaker assembly  14 . 6 - 212  is positioned within the internal volume  14 . 6 - 201 . The electronic device  14 . 6 - 200  can define an aperture or opening  14 . 6 - 216   a  that is in fluid communication with the ambient environment and the speaker assembly  14 . 6 - 212 . 
     The speaker assembly  14 . 6 - 212  can include a pod  14 . 6 - 213 , which can otherwise be referred to as a speaker assembly housing or enclosure, which defines an internal volume  14 . 6 - 215  of the speaker assembly. The pod  14 . 6 - 213  can house various electrical and acoustic components. The speaker assembly  14 . 6 - 212  can include various acoustic components, such as audio transducers, woofers, tweeters, midrange drivers and any other types of drivers or speaker components. 
     In some examples, the speaker pod  14 . 6 - 213  includes a woofer  14 . 6 - 230  and a tweeter  14 . 6 - 234 . The woofer  14 . 6 - 230  (also known as a bass speaker or a loudspeaker driver) can be designed to produce low frequency sounds. The woofer  14 . 6 - 230  can be an electrodynamic driver. In some examples, the woofer  14 . 6 - 230  can produce sound from about 50 Hz to about 2000 Hz. 
     The speaker assembly  14 . 6 - 212  can include a tweeter  14 . 6 - 234 . The tweeter  14 . 6 - 234  can also be referred to as a treble speaker or loudspeaker. The tweeter  14 . 6 - 234  can be a speaker driver that produces acoustic output at a high frequency range or a range of frequencies higher than the range of frequencies output by the woofer. In some examples, the tweeter  14 . 6 - 234  produces sound from about 2 kHz to about 20 kHz. In some examples, the tweeter  14 . 6 - 234  can produce sound up to about 100 kHz. The tweeter  14 . 6 - 234  can be positioned to be proximate an opening, outlet, or port  14 . 6 - 216   b  in the speaker pod  14 . 6 - 213  that leads to the ambient environment. The port  14 . 6 - 216   b  can be aligned with or in fluid communication with the opening  14 . 6 - 216   a  defined by the housing of the electronic device  14 . 6 - 200 . 
     Notably, in some examples, the speaker assembly  14 . 6 - 212  includes only one port  14 . 6 - 216   b  that vents sounds into the ambient environment. The tweeter  14 . 6 - 234  and the woofer  14 . 6 - 230  can be stacked vertically. For example, the tweeter  14 . 6 - 234  can be positioned over or on top (in relation to the view of  FIGS.  14 . 6 - 2   ) of the woofer  14 . 6 - 230 . In some examples, the tweeter  14 . 6 - 234  overlaps the woofer  14 . 6 - 230  along a horizontal axis such that the woofer  14 . 6 - 230  and tweeter are stacked along a vertical axis. The stacking of the woofer  14 . 6 - 230  and the tweeter  14 . 6 - 234  can allow the speaker pod  14 . 6 - 213  to have a reduced dimension (e.g., along the horizontal axis). As shown, the tweeter  14 . 6 - 234  can be positioned closer to the port  14 . 6 - 216   b  than the woofer  14 . 6 - 230 . In other words, a distance between the port  14 . 6 - 216   b  and the woofer  14 . 6 - 230  is greater than a distance between the port  14 . 6 - 216   b  and the tweeter  14 . 6 - 234 . 
     The speaker assembly  14 . 6 - 212  can include internal walls or barrier components that define one or more channels, passages, or tunnels that are designed to guide sound within the speaker assembly  14 . 6 - 212 . In some examples, a woofer channel  14 . 6 - 250  can be in fluid communication with an interior of the woofer  14 . 6 - 230  to receive sound produced by the woofer  14 . 6 - 230  and guide the sound, at least partially, to the port  14 . 6 - 216   b . Likewise, a tweeter channel  14 . 6 - 251  can be in fluid communication with the tweeter  14 . 6 - 234  to receive sound produced by the tweeter  14 . 6 - 234  and guide the sound to the port  14 . 6 - 216   b.    
     As described herein, the pod  14 . 6 - 213  can define a single outlet port  14 . 6 - 216   b . Thus, in some configurations the woofer channel  14 . 6 - 250  and the tweeter channel  14 . 6 - 251  converge into a shared channel  14 . 6 - 252  leading to the port  14 . 6 - 216   b . In at least one example, the woofer channel  14 . 6 - 250 , tweeter channel  14 . 6 - 251 , and the shared channel  14 . 6 - 252  can be formed as a single channel leading to a single port  14 . 6 - 216   b . In other words, the woofer channel  14 . 6 - 250  can direct primarily low frequency sound generated by the woofer  14 . 6 - 230  and the tweeter channel  14 . 6 - 251  can direct primarily high frequency sound generated by the tweeter  14 . 6 - 234 . Accordingly, the shared channel  14 . 6 - 252  can include both high and low frequency acoustic waves produced by both the woofer  14 . 6 - 230  and the tweeter  14 . 6 - 234 . In some examples, the woofer channel  14 . 6 - 250  and the tweeter channel  14 . 6 - 251  terminate at the port  14 . 6 - 216   b.    
     In at least one example, the shared channel  14 . 6 - 252  can be removed or reduced in size with the woofer channel  14 . 6 - 250  and the tweeter channel  14 . 6 - 252  extending to the port  14 . 6 - 216   b . In such an example, the woofer channel  14 . 6 - 250  can terminate at the tweeter channel  14 . 6 - 251  and the tweeter channel can terminate at the port  14 . 6 - 216 . In this way, in at least one example, the woofer channel  14 . 6 - 250  and the tweeter channel  14 . 6 - 251  can join to form a single channel that terminates at a single port  14 . 6 - 216   b . As shown in  FIGS.  14 . 6 - 2   , the port  14 . 6 - 216   b  and opening  14 . 6 - 216  can align to form a single opening or port defined at least in part by an external housing of the device  14 . 6 - 200  and visible from the outside environment. 
     In some examples, a mesh  14 . 6 - 258  can be positioned within or across the woofer channel  14 . 6 - 250 . The mesh  14 . 6 - 258  can include cloth or fabric (also referred to as grille cloth, acoustic cloth, or speaker mesh) that is designed to allow for sound transmission through the material. The mesh  14 . 6 - 258  can aid in acoustic dampening, distortion control, and flow noise control. 
     The speaker mesh  14 . 6 - 258  can include a predetermined sound transmissibility suitable for sound generated by the woofer  14 . 6 - 230 . The woofer  14 . 6 - 230  can have a frequency range of about 20 Hz to about 2 kHz. The tweeter  14 . 6 - 234  can have a frequency range of about 200 Hz to about 20 kHz. In some examples, the woofer  14 . 6 - 230  can have a frequency range as low as about 20 Hz to about 50 Hz, and as high as about 200 Hz to about 2 kHz. The tweeter  14 . 6 - 234  can have a frequency range as low as about 200 Hz to about 2 kHz, and as high as about 20 kHz. In some examples, the mesh  14 . 6 - 258  can allow for passage of frequencies lower than frequencies produced by the tweeter  14 . 6 - 234 . For example, the mesh  14 . 6 - 258  can substantially allow sound generated by the woofer  14 . 6 - 230  below 2000 Hz to pass there through but substantially prevent or reduce the transmission of frequencies produced by the tweeter  14 . 6 - 234  from transferring therethrough. For example, the mesh  14 . 6 - 258  can substantially block or prevent sound frequencies above about 2000 Hz from transferring from the tweeter channel  14 . 6 - 252  backward into the woofer channel  14 . 6 - 250 . The mesh  14 . 6 - 258  can at least partially block or prevent sound from the tweeter  14 . 6 - 234  from traveling into the woofer channel  14 . 6 - 250 . The mesh  14 . 6 - 258  can be made from synthetic materials or threads in a woven pattern, or can include a foam. The mesh  14 . 6 - 258  can be an air-permeable component. 
     The mesh  14 . 6 - 258  can be positioned at or near a terminus or outlet  14 . 6 - 249  of the woofer channel  14 . 6 - 250 . The mesh  14 . 6 - 258  can occupy substantially an entire volume defined by the woofer channel  14 . 6 - 250 . In some other examples, however, the mesh  14 . 6 - 258  may only occupy a portion of the woofer channel  14 . 6 - 250 . In use, the mesh  14 . 6 - 258  can serve to reduce the velocity of air flowing through the woofer channel  14 . 6 - 250 , thereby reducing undesirable flow noise and providing for a clearer acoustic signal to be heard by the user. In some examples, the mesh  14 . 6 - 258  can block or prevent particle ingress through the woofer channel  14 . 6 - 250 . 
     The speaker assembly  14 . 6 - 212  can include a port mesh  14 . 6 - 260  that is positioned within or across the opening  14 . 6 - 216   a , the port  14 . 6 - 216   b , and/or the shared channel  14 . 6 - 252 . In some examples, the port mesh  14 . 6 - 260  is positioned across the tweeter channel  14 . 6 - 251 . The port mesh can be substantially similar to the mesh  14 . 6 - 258 . However, in at least one example, the port mesh  14 . 6 - 260  can have a lower density than the mesh  14 . 6 - 258 . The port mesh  14 . 6 - 260  can include cloth or fabric that is designed to allow for easy sound transmission through the material. The port mesh  14 . 6 - 260  can include a predetermined sound transmissibility suitable for sound generated by the tweeter  14 . 6 - 234 . The port mesh  14 . 6 - 260  can supplement the mesh  14 . 6 - 258  in further dampening the sound. In some examples, the port mesh  14 . 6 - 260  can allow for passage of frequencies as high as 20 kHz. The port mesh  14 . 6 - 260  can be made from synthetic materials or threads in a woven pattern. As described in greater detail with regard to  FIGS.  14 . 6 - 4   , a cosmetic steel screen can be combined with the port mesh  14 . 6 - 260  to define an exterior of the electronic device  14 . 6 - 200 . Further, a frame defined by a series of support ribs can support the port mesh  14 . 6 - 260  to prevent damage to the port mesh  14 . 6 - 260 . 
     The port mesh  14 . 6 - 260  can be an air-permeable component. In some examples, the port mesh  14 . 6 - 260  can occupy substantially an entire volume defined by at least one of the shared channel  14 . 6 - 252 , the port  14 . 6 - 216   b , and the opening  14 . 6 - 216   a . In some other examples, however, the port mesh  14 . 6 - 260  may only occupy a portion of at least one of the shared channel  14 . 6 - 252 , the port  14 . 6 - 216   b , and the opening  14 . 6 - 216   a . In use, the port mesh  14 . 6 - 260  can serve to reduce the velocity of air flowing through the shared channel  14 . 6 - 252 , thereby reducing undesirable flow noise and providing for a clearer acoustic signal to be heard by a user. In some examples, the port mesh  14 . 6 - 260  can block or prevent particle ingress through the shared channel  14 . 6 - 252 . 
     As noted above and shown in  FIGS.  14 . 6 - 2   , the mesh  14 . 6 - 258  can be disposed between the woofer channel  14 . 6 - 250  and the tweeter channel  14 . 6 - 252  such that the mesh  14 . 6 - 258  aid in acoustic dampening, distortion control, and flow noise control for specific frequencies output by the woofer  14 . 6 - 230  without attenuating or otherwise interfering with the passage of sound frequencies passing through the tweeter channel  14 . 6 - 252  and out the port mesh  14 . 6 - 260 . In this way, the denser mesh  14 . 6 - 258  provides the advantages tuned for the sound output by the woofer  14 . 6 - 230  and the port mesh is configured to optimally provide dampening, distortion control, and flow noise control of the higher frequencies output by the tweeter  14 . 6 - 234  without those same high frequencies having to first passing through the mesh  14 . 6 - 258 . As noted above, the port mesh  14 . 6 - 260  is also configured to allow the passage of lower frequencies from the woofer  14 . 6 - 230  to pass through the port  14 . 6 - 216   b  and the opening  14 . 6 - 216   a  without substantially or noticeably attenuating or otherwise interfering with the woofer sound. In this way, with both the mesh  14 . 6 - 258  and port mesh  14 . 6 - 260  positioned as shown in  FIGS.  14 . 6 - 2   , both the lower woofer frequencies and the higher tweeter frequencies produced by the speaker assembly  14 . 6 - 212  can be optimally controlled for quality as the sound passes through a single shared outlet or aperture, such as the opening  14 . 6 - 216   a . Further details regarding speaker assemblies are described below with reference to  FIGS.  14 . 6 - 3 A through  14 . 6 - 3 E . 
       FIGS.  14 . 6 - 3 A  illustrates a top perspective view of an example of a speaker assembly  14 . 6 - 312 . The speaker assembly  14 . 6 - 312  can be substantially similar to, and can include some or all of the features of the speaker assemblies described herein, such a speaker assembly  14 . 6 - 112  and  14 . 6 - 212 .  FIGS.  14 . 6 - 3 A  is merely one illustrative example of how the speaker assembly  14 . 6 - 312  can be designed and is not limited to the depicted design or componentry. As illustrated, the speaker assembly  14 . 6 - 312  can include a housing or pod  14 . 6 - 313  that houses various components of the speaker assembly  14 . 6 - 312 , such as audio transducers. The pod  14 . 6 - 313  can include stainless steel. In some examples, the pod  14 . 6 - 313  can include a bottom portion (shown in  FIGS.  14 . 6 - 3 A ) and a top portion (not shown). The top and bottom portions of the pod  14 . 6 - 313  can substantially surround or encapsulate the components of the speaker assembly  14 . 6 - 312 . The bottom portion of the pod  14 . 6 - 313  can include a curved or non-planar surface that defines an internal volume, such as internal volume  14 . 6 - 215  of  FIGS.  14 . 6 - 2   . The speaker assembly  14 . 6 - 312  can include an elastomeric gasket (not shown in  FIGS.  14 . 6 - 3 A ) positioned around a perimeter of the pod  14 . 6 - 313 . The elastomeric gasket can provide a seal between the top portion and bottom portion of the pod  14 . 6 - 313 . In some examples, the pod  14 . 6 - 313  does not include a top portion, but is instead sealed directly to an interior surface of the housing of the electronic device. 
     As illustrated in  FIGS.  14 . 6 - 3 A , the pod  14 . 6 - 313  can house a woofer  14 . 6 - 330  and a tweeter  14 . 6 - 334 . The tweeter  14 . 6 - 334  can be positioned at least partially adjacent to the woofer  14 . 6 - 330 . As described herein, the tweeter  14 . 6 - 334  can at least partially stack or overlap the woofer  14 . 6 - 330  in order to accommodate for a reduced dimension of the pod  14 . 6 - 313 . 
     The pod  14 . 6 - 313  can define the port  14 . 6 - 316 , out of which sound from both the woofer  14 . 6 - 330  and tweeter  14 . 6 - 334  are emitted into the ambient environment. As discussed herein, the pod  14 . 6 - 313  can include or define a single outlet port  14 . 6 - 316 . The single outlet port  14 . 6 - 316  can improve an aesthetic appearance of the electronic device. Further, the single outlet port  14 . 6 - 316  can provide the advantage of having fewer entry points for dust, particles, or debris, thereby reducing the likelihood of foreign objects entering the electronic device. The pod  14 . 6 - 313  can define grooves  14 . 6 - 338  that are shaped to allow for electrical wires to pass into the internal volume of the pod  14 . 6 - 313  even when the top portion is secured to the bottom portion. The electrical wires can be in electrical communication with the woofer  14 . 6 - 330  and the tweeter  14 . 6 - 334 . In some examples, a printed circuit board or PCB (not shown in  FIGS.  14 . 6 - 3 A ) can be secured to a top of the woofer  14 . 6 - 330 . The PCB can be secured, at least partially, by protrusions  14 . 6 - 342  that anchor the PCB in place above the woofer  14 . 6 - 330 . The protrusions  14 . 6 - 342  can be helical coils. A heat sink can be positioned on top of or above the PCB. In some examples, the speaker assembly  14 . 6 - 312  can include a vent  14 . 6 - 346 . The vent  14 . 6 - 346  can be a barometric vent that allows for air equalization within the speaker assembly  14 . 6 - 312 . 
       FIGS.  14 . 6 - 3 B  shows a cross-sectional side view of the speaker assembly  14 . 6 - 312  according to one embodiment. Similarly,  FIGS.  14 . 6 - 3 C  illustrates a perspective cross-sectional view of the speaker assembly  14 . 6 - 312  according to one embodiment. The woofer  14 . 6 - 330  can include a diaphragm  14 . 6 - 332  that oscillates along the vertical axis when an electrical current is provided to the woofer  14 . 6 - 330 . Sound produced by the woofer diaphragm  14 . 6 - 332  can travel downward in a woofer volume  14 . 6 - 349  and can then be guided laterally or horizontally through the woofer channel  14 . 6 - 350 . In some examples, at least a portion of the woofer volume  14 . 6 - 349  and the woofer channel  14 . 6 - 350  can be defined by an interior surface of a bottom wall of the pod  14 . 6 - 313 . 
     In some examples, the sound produced by the woofer  14 . 6 - 330  can take a winding or tortuous path as it travels to the port  14 . 6 - 316 . For example, the sound from the woofer  14 . 6 - 330  may need to change directions several times in order to reach the port  14 . 6 - 316 . A long tortuous path can impact acoustic modes of that path, consequently impacting the output. As used herein, a tortuous path can refer to a winding or snaking path (e.g., non-linear). The tortuous path described herein can be a result of a horizontal interface  14 . 6 - 357  between the woofer  14 . 6 - 330  and tweeter  14 . 6 - 334  with the tweeter stacked or overlapping the woofer  14 . 6 - 330  as shown. 
     As illustrated in  FIGS.  14 . 6 - 3 B , the pod  14 . 6 - 313  can include an overhang or lip  14 . 6 - 315 . A perimeter or footprint of the lip  14 . 6 - 315  can be smaller than a perimeter or footprint of the combined audio components (e.g., woofer  14 . 6 - 330 , tweeter  14 . 6 - 334 , etc.) housed within the pod  14 . 6 - 313 . Accordingly, the system may need to be assembled in a certain order and fashion due to the lip  14 . 6 - 315 . For example, the dimensions of the pod  14 . 6 - 313  may prevent the woofer  14 . 6 - 330  and tweeter  14 . 6 - 334  from being simply attached or assembled before being placed into the pod  14 . 6 - 313 . Instead, it may be necessary to first position the tweeter  14 . 6 - 334  or woofer  14 . 6 - 330  within the pod  14 . 6 - 313  and then place the remaining components into position. The components of the speaker assembly  14 . 6 - 312  may need to be angled or slid horizontally to be set into the correct position within the pod  14 . 6 - 313 . The lip  14 . 6 - 315  can overlap or overhang a portion of the tweeter  14 . 6 - 334 . 
     Further, as described herein, modern electronic devices often demand reduced dimensions that require novel solutions to fit the electronics within the housings or enclosures. For example, the electronic devices described herein can include integrated speaker assemblies that maintain a broad frequency range and desirable acoustic performance levels, despite residing in smaller, more compact housings. In some examples, this is achieved by overlapping or stacking the tweeter  14 . 6 - 334  and the woofer  14 . 6 - 330 . 
     The overlapping tweeter  14 . 6 - 334  and woofer  14 . 6 - 330  configuration increases the compactness of the speaker assembly  14 . 6 - 312  and allows for assembly tolerances in the horizontal direction when assembling the speaker assembly  14 . 6 - 312 . In addition, in at least one example, the overlapping configuration of the tweeter  14 . 6 - 334  and woofer  14 . 6 - 330  can result in the woofer channel  14 . 6 - 350  and/or tweeter channel  14 . 6 - 351  to change directions in a tortuous manner, for example from horizontal to vertical and/or other channel angles. The stacked nature of the speaker assembly  14 . 6 - 312  in combination with a single port  14 . 6 - 316  may result in a tortuous and winding path for the sound waves. For example, the horizontal interface  14 . 6 - 357  allows for horizontal assembly tolerances while causing the woofer channel  14 . 6 - 350  to direct upward in order to accommodate for the horizontal interface  14 . 6 - 357 . In some examples, the tortuous path can be defined by smooth or curved walls to reduce or eliminate turbulence. 
     As shown in  FIGS.  14 . 6 - 3 B , the horizontal interface  14 . 6 - 357  can include contacting or at least overlapping portions of the tweeter  14 . 6 - 334  and woofer  14 . 6 - 330 , or the contacting or overlapping of certain walls or other components of the tweeter  14 . 6 - 334  and woofer  14 . 6 - 330 , due to the stacked nature of the tweeter  14 . 6 - 334  and the woofer  14 . 6 - 330 . Also, as used herein and as viewed in the orientation of  FIGS.  14 . 6 - 3 B , the term “horizontal” can refer to a plane generally parallel to the planes in which the tweeter diaphragm  14 . 6 - 335  and the woofer diaphragm  14 . 6 - 332  are disposed. In one or more other examples, the stacked configuration/positioning of the tweeter  14 . 6 - 334  and woofer  14 . 6 - 330  can form an interface at an angle from the referenced “horizontal” orientation of the diaphragms  14 . 6 - 335 ,  14 . 6 - 332 , such as any angle between horizontal and vertical (with “vertical” referencing an angle, plane, or orientation perpendicular to the diaphragms  14 . 6 - 335 ,  14 . 6 - 332  shown in  FIGS.  14 . 6 - 3 B ). Accordingly, in at least on example, the interface  14 . 6 - 357  included surfaces or components of the tweeter  14 . 6 - 334  and the woofer  14 . 6 - 330  coming together at an angle other than vertical, for example at a 45-degree angle from horizontal or any other angle from horizontal or vertical directions. As noted above, this angled or horizontal interface  14 . 6 - 357  can cause the path of sound generated by the woofer  14 . 6 - 330  and/or tweeter  14 . 6 - 334  to be tortuous or winding as shown in  FIGS.  14 . 6 - 3 B . 
     A sloped or curved ramp  14 . 6 - 354  can be positioned and shaped to direct sound from the woofer volume  14 . 6 - 349  to the shared volume  14 . 6 - 352 . For example, as the sound travels laterally, the curved ramp  14 . 6 - 354  guides the sound upward. The curved ramp  14 . 6 - 354  can include a smooth transition surface to reduce turbulence. The curved ramp  14 . 6 - 354  can include a radius of curvature of about 0.2 cm to 0.3 cm. It will be understood that the ramp  14 . 6 - 354  is optional and that in some examples, the sound produced by the woofer  14 . 6 - 330  travels in a primarily lateral or horizontal direction to the port  14 . 6 - 316 . 
     In some examples, a mesh  14 . 6 - 358  (similar to the mesh  14 . 6 - 258 ) can be positioned at a terminus of the woofer channel  14 . 6 - 350 . The mesh  14 . 6 - 358  can be positioned at an interface between the woofer  14 . 6 - 330  and the tweeter  14 . 6 - 334 . 
     The speaker assembly  14 . 6 - 312  can include a wall  14 . 6 - 353 . The wall  14 . 6 - 353  can be made from plastic and can be positioned below the tweeter  14 . 6 - 334  (e.g., between the tweeter  14 . 6 - 334  and the woofer channel  14 . 6 - 350 ). The wall  14 . 6 - 353  can be positioned to shield the tweeter  14 . 6 - 334  from the sound produced by the woofer  14 . 6 - 330 . In some examples, the wall  14 . 6 - 353  can be curved to provide a smooth transition surface to direct upward directed sound laterally toward the port  14 . 6 - 316 . In some examples, the wall  14 . 6 - 353  is positioned and/or shaped to direct sound from the tweeter  14 . 6 - 334  toward the port  14 . 6 - 316 . The wall  14 . 6 - 353  can at least partially prevent sound from the tweeter  14 . 6 - 334  from passing into the woofer channel  14 . 6 - 350 . The wall  14 . 6 - 353  can at least partially define the tweeter channel  14 . 6 - 351  and/or the woofer channel  14 . 6 - 350 . In some examples, the wall  14 . 6 - 353  can extend substantially across the tweeter  14 . 6 - 334 . The wall  14 . 6 - 353  can extend to the port mesh  14 . 6 - 360 . Thus, the shared channel  14 . 6 - 352  can be omitted due to the wall  14 . 6 - 353  maintaining a separation of the woofer channel  14 . 6 - 350  and the tweeter channel  14 . 6 - 351 . 
     The tweeter  14 . 6 - 334  can include a diaphragm  14 . 6 - 335 . The diaphragm  14 . 6 - 335  can oscillate or vibrate along a vertical axis. The diaphragm  14 . 6 - 335  can oscillate in the same direction(s) as the diaphragm  14 . 6 - 332 . The woofer diaphragm  14 . 6 - 332  can include a surface area that is approximately 10 times larger than that of the tweeter diaphragm  14 . 6 - 335 . 
     In some examples, the tweeter  14 . 6 - 334  can be positioned closer to the port  14 . 6 - 316  than the woofer  14 . 6 - 330 . The path taken by sound produced by the tweeter  14 . 6 - 334  can be substantially linear, or at least substantially less tortuous than the path taken by sound from the woofer  14 . 6 - 330 . In some examples, sound generated by the tweeter  14 . 6 - 334  is initially directed in a downward direction but is guided substantially laterally, toward the port  14 . 6 - 316 . 
     The speaker assembly  14 . 6 - 312  can include several interfaces (e.g., horizontal interface  14 . 6 - 357 ) between the woofer  14 . 6 - 330  and the tweeter  14 . 6 - 334  or between the speaker components and the pod  14 . 6 - 313 . Foams can be positioned between the interfaces of the components and the speaker housing  14 . 6 - 313 . Specifically,  FIGS.  14 . 6 - 3 B  shows a foam  14 . 6 - 363   a  positioned between a sidewall  14 . 6 - 364  of the pod  14 . 6 - 313  and the tweeter  14 . 6 - 334 . Further, a foam  14 . 6 - 363   b  can be positioned between the interior surface of the pod  14 . 6 - 313  and a channel wall  14 . 6 - 366 . 
     The foams  14 . 6 - 363   a ,  14 . 6 - 363   b  can include re-workable properties such as flexibility and malleability to allow free play within the speaker assembly  14 . 6 - 312 . The foams  14 . 6 - 363   a ,  14 . 6 - 363   b  can be used in addition to or as a replacement for pressure sensitive adhesive (PSA). The foams  14 . 6 - 363   a ,  14 . 6 - 363   b  beneficially may not be permanently set, allowing for adjustment and repositioning of the components of the assembly. In some examples, the foams  14 . 6 - 363   a ,  14 . 6 - 363   b  can be used in conjunction with an adhesive. Advantageously, the foams between various interfaces of the components and the pod  14 . 6 - 313  can take up tolerances within the system. For example, a minimum sealing tolerance can be about 300 microns. The position of the foams  14 . 6 - 363   a ,  14 . 6 - 363   b  are merely example locations that foam can reside. It will be understood that foam can be located at various positions or interfaces not shown in  FIGS.  14 . 6 - 3 B and  14 . 6 - 3 C . 
     In some examples, the port  14 . 6 - 316  is about 30 mm 2  across its cross-sectional face. The size of the port  14 . 6 - 316  can depend on the capability of the woofer  14 . 6 - 330 . The size of the port  14 . 6 - 316  can be driven by a desired particle velocity. For example, particle velocity can be a volume velocity (e.g., the volume of sound waves or air passing an area per unit time) divided by a cross sectional area of the port, where the volume velocity is dictated by the maximum output of the system at low frequencies. The particle velocity can be up to 50 meters per second. In some examples, the particle velocity is 10 meters per second. The port  14 . 6 - 316  can include a sleeve  14 . 6 - 319  positioned within the port  14 . 6 - 316  and extending around a perimeter of the port  14 . 6 - 316 . The sleeve  14 . 6 - 319  can be made from plastic. The sleeve  14 . 6 - 319  can extend through the port opening  14 . 6 - 316  in the pod  14 . 6 - 313  and can also extend through the opening in the housing of the electronic device (e.g., opening  14 . 6 - 216   a  in  FIGS.  14 . 6 - 2   ). 
       FIGS.  14 . 6 - 3 D  shows a top perspective view of a speaker module  14 . 6 - 317 . The speaker module  14 . 6 - 317  can represent the certain electronics removed from the pod  14 . 6 - 313  described above. The speaker module  14 . 6 - 317  can include the woofer  14 . 6 - 330  and the tweeter  14 . 6 - 334 . In some examples, the speaker module  14 . 6 - 317  can include a frame  14 . 6 - 347 . The frame  14 . 6 - 347  can be secured to an interior surface of the pod  14 . 6 - 313 .  FIGS.  14 . 6 - 3 E  shows a bottom perspective view of a speaker module  14 . 6 - 317 . An underside surface of the frame  14 . 6 - 347  can be sealed to a bottom of the pod  14 . 6 - 313 . In some examples, the speaker module  14 . 6 - 317  is sealed to the pod using an adhesive and/or foam, such as the foam  14 . 6 - 363  discussed above with regard to  FIGS.  14 . 6 - 3 B . The woofer diaphragm  14 . 6 - 332 , woofer channel  14 . 6 - 350 , and mesh  14 . 6 - 358  are also visible from the underside of the speaker module  14 . 6 - 317  shown in  FIGS.  14 . 6 - 3 E . In some examples, a barometric vent  14 . 6 - 346  can be positioned as illustrated between the mesh  14 . 6 - 358  and the woofer diaphragm  14 . 6 - 332 . 
       FIGS.  14 . 6 - 4    shows a perspective view of a port barrier  14 . 6 - 460 . The port barrier  14 . 6 - 460  can be substantially similar to, including some or all of the features of, the meshes described herein, such as mesh  14 . 6 - 260  and  14 . 6 - 360 . The port barrier  14 . 6 - 460  can be positioned within or across the opening of the electronic device, the port of the speaker assembly, and/or a shared channel (e.g., shared channel  14 . 6 - 252 ). The port barrier  14 . 6 - 460  can include an external screen  14 . 6 - 460   a , a woven mesh  14 . 6 - 460   b , and a frame  14 . 6 - 460   c.    
     The external screen  14 . 6 - 460   a  can be a metallic structure that defines at least a portion of an exterior surface of the electronic device. In other words, the external screen  14 . 6 - 460   a  can be a cosmetic feature, visible to a user. In some examples, the external screen  14 . 6 - 460   a  can provide structural support to the woven mesh  14 . 6 - 460   b . The external screen  14 . 6 - 460   a  can act as a particle or debris barrier to prevent particle ingress into the speaker assembly. 
     As discussed in greater detail above, with regard to  FIGS.  14 . 6 - 3 B , the woven mesh  14 . 6 - 460   b  can include a cloth or fabric that is designed to allow for easy sound transmission through the material. The speaker port barrier  14 . 6 - 460  can include a predetermined sound transmissibility suitable for sound generated by the tweeter  14 . 6 - 234 . The port barrier  14 . 6 - 460  can partially dampen the sound. In some examples, the woven mesh  14 . 6 - 460   b  can allow passage of frequencies from 20 Hz to 20 kHz. In some examples, the woven mesh  14 . 6 - 460   b  can allow passage of frequencies from 2 kHz to 20 kHz. The woven mesh  14 . 6 - 460   b  can be made from synthetic materials or threads in a woven pattern. 
     The port barrier  14 . 6 - 460  can be air-permeable. The port barrier  14 . 6 - 460  can occupy substantially an entire volume defined by at least one of the shared channel, the port (e.g., port  14 . 6 - 216   b ,  14 . 6 - 316 ), and the opening (e.g., opening  14 . 6 - 116 ,  14 . 6 - 216   a ). In some other examples, however, the port barrier  14 . 6 - 460  may only occupy a portion of at least one of the shared channel  14 . 6 - 252 , the port  14 . 6 - 216   b , and the opening  14 . 6 - 216   a . In use, the port barrier  14 . 6 - 460  can serve to reduce the velocity of air flowing through the shared channel  14 . 6 - 252 , thereby reducing undesirable flow noise and providing for a clearer acoustic signal to be heard by a user. In some examples, the port barrier  14 . 6 - 460  can block or prevent particle ingress through the shared channel  14 . 6 - 252 . The port barrier  14 . 6 - 460  can have a lower density than the mesh  14 . 6 - 258 . 
     The frame  14 . 6 - 460   c  can be positioned interior to the external screen  14 . 6 - 460   a  and woven mesh  14 . 6 - 460   b . In some examples, the woven mesh  14 . 6 - 460   b  is positioned between or sandwiched between the external screen  14 . 6 - 460   a  and the frame  14 . 6 - 460   c . The frame  14 . 6 - 460   c  can include a series of support ribs  14 . 6 - 461  to strengthen, shield, or provide support the woven mesh  14 . 6 - 460   b . In some examples, the frame  14 . 6 - 460   c  is positioned external to the woven mesh  14 . 6 - 460   b  (e.g., between the external screen  14 . 6 - 460   a  and the woven mesh  14 . 6 - 460   b . The frame  14 . 6 - 460   c  can be made from any suitable material, such as plastic or metal. 
     14.7: Bifurcated Band 
     Head-mounted devices include head-mounted support structures that allow the devices to be worn on the heads of users. The head-mounted support structures may include device housings for housing components such as displays that are used for presenting a user with visual content. The head-mounted support structures for a head-mounted device may also include headbands and other structures that help hold a device housing on the face of a user. The headband of a head-mounted device may be adjustable. 
     In some embodiments, it may be desirable to incorporate a bifurcated headband that contacts the user&#39;s head in multiple locations while the head-mounted device is worn. The headband may include different woven textures on opposite sides of the headband (e.g., on the side in contact with the user&#39;s head vs. facing away from the user&#39;s head), and may include hair-safe hook and loop fasteners. The headband may have ends with seamless curved webbings. To prevent the headband from slipping with respect to the user&#39;s head, stiffeners and/or cords may be included on and/or in the headband. If cords are included in the headband, the headband may have adaptive curvature based on the location of the cords within the headband. As a result, the headband may conform to the user&#39;s head. 
       FIGS.  14 . 7 - 1    is a side view of an illustrative head-mounted electronic device with an adjustable headband. As shown in  FIGS.  14 . 7 - 1   , head-mounted device  14 . 7 - 10  may include head-mounted housing  14 . 7 - 12  (sometimes referred to as a main housing, main housing unit, head-mounted support structure, etc.). Housing  14 . 7 - 12  may have walls or other structures that separate an interior housing region from an exterior region surrounding housing  14 . 7 - 12 . For example, housing  14 . 7 - 12  may have walls formed from polymer, glass, metal, and/or other materials. Electrical and optical components may be mounted in housing  14 . 7 - 12 . These components may include components such as integrated circuits, sensors, control circuitry, input-output devices, etc. 
     To present a user with images for viewing from eye boxes (e.g., eye boxes in which the user&#39;s eyes are located when device  14 . 7 - 10  is being worn on the users&#39; head such as head  14 . 7 - 22  of  FIGS.  14 . 7 - 1   ), device  14 . 7 - 10  may include displays and lenses. These components may be mounted in optical modules or other supporting structure in housing  14 . 7 - 12  to form respective left and right optical systems. There may be, for example, a left display for presenting an image through a left lens to a user&#39;s left eye in a left eye box and a right display for presenting an image to a user&#39;s right eye in a right eye box. 
     If desired, housing  14 . 7 - 12  may have forward-facing components such as cameras, other sensors, and/or a display on front F for gathering sensor measurements/other input and/or display information on front F. Housing  14 . 7 - 12  and may have a soft cushion on an opposing rear side of housing  14 . 7 - 12 . The rear of housing  14 . 7 - 12  may have openings that allow the user to view images from the left and right optical systems (e.g., when the rear of housing  14 . 7 - 12  is resting on front surface  14 . 7 - 20 F of the user&#39;s head  14 . 7 - 22 . 
     Device  14 . 7 - 10  may have an adjustable strap such as adjustable headband  14 . 7 - 26  and, if desired, may have other structures (e.g., an optional over-the-head strap) to help hold housing  14 . 7 - 12  on head  14 . 7 - 22 . Headband  14 . 7 - 26  may have first and second ends coupled, respectively, to the left and right sides of housing  14 . 7 - 12 . In the example of  FIGS.  14 . 7 - 1   , coupling members  14 . 7 - 24 , which serve as extensions of housing  14 . 7 - 12  (e.g., extend from housing  14 . 7 - 12  directly or are attached directly to housing  14 . 7 - 12 ), are provided on the left and right sides of housing  14 . 7 - 12 . Members  14 . 7 - 24  may be formed from rigid materials such as rigid polymer and/or other materials and may contain sensors, buttons, speakers, and other electrical components. Hinges and/or other mechanisms may be used to couple members  14 . 7 - 24  to housing  14 . 7 - 12  or members  14 . 7 - 24  may be formed as integral portions of a main housing unit. The ends of headband  14 . 7 - 26  may have coupling mechanisms such as openings configured to receive posts or other protrusions  14 . 7 - 24 P on members  14 . 7 - 24  or other housing structures. These coupling mechanisms allow a user to removably attach headband  14 . 7 - 26  to members  14 . 7 - 24  and thereby removable attach headband  14 . 7 - 26  to housing  14 . 7 - 12 . Members  14 . 7 - 24  may have elongated shapes of the type shown in  FIGS.  14 . 7 - 1    and/or other suitable shapes and may sometimes be referred to as rigid straps, rigid coupling members, or power straps. 
     Headband  14 . 7 - 26  may have a soft flexible portion such as central portion  14 . 7 - 30 . Portion  14 . 7 - 30  may be formed between two stiffer portions such as end portions  14 . 7 - 28  on the left and right ends of headband  14 . 7 - 26 . Portions  14 . 7 - 28  may be stiffened using embedded polymer stiffeners (e.g., single-layer or multilayer polymer stiffening strips) and/or other stiffening members. 
     Portion  14 . 7 - 30  may be formed from a stretchable material such as stretchy fabric. Portion  14 . 7 - 30  may, as an example, be formed from a band of flat knit fabric that includes stretchable strands of material (e.g., elastomeric strands) and/or which uses a stretchable fabric construction (e.g., a stretchable knit construction). Alternatively, portion  14 . 7 - 30  may be formed from a band of woven fabric, which may include stretchable strands of material and/or may use a stretchable fabric construction. Narrowed end portions of the band of knit fabric may, if desired, extend over stiffening members in end portions  14 . 7 - 28  (e.g., to ensure that headband  14 . 7 - 26  has a uniform external appearance). 
     Portion  14 . 7 - 30  may include two bifurcated headband portions  14 . 7 - 32  and  14 . 7 - 34 . In particular, bifurcated headband portions  14 . 7 - 32  and  14 . 7 - 34  may form two portions of the same headband  14 . 7 - 26  (e.g., headband portions  14 . 7 - 32  and  14 . 7 - 34  may be connected to the same points in portions  14 . 7 - 28 ). In the illustrative embodiment of  FIGS.  14 . 7 - 1   , portion  14 . 7 - 32  may be attached to protrusions  14 . 7 - 24 P, and portion  14 . 7 - 34  may extend from portion  14 . 7 - 32  at a fixed angle. In other embodiments, headband portions  14 . 7 - 32  and  14 . 7 - 34  may both be attached to protrusions  14 . 7 - 24 P, thereby being able to pivot about protrusions  14 . 7 - 24 P individually. 
     Headband portions  14 . 7 - 32  and  14 . 7 - 34  may both contact rear portions of head  14 . 7 - 22 . In particular, headband portion  14 . 7 - 32  may contact an upper rear portion of head  14 . 7 - 22  (e.g., in the upper half of the rear of head  14 . 7 - 22 ), while headband portion  14 . 7 - 34  may contact a lower rear portion of head  14 . 7 - 22  (e.g., in the lower half of the rear of head  14 . 7 - 22 ). By contacting the rear of head  14 . 7 - 22  in two different locations, headband portions  14 . 7 - 32  and  14 . 7 - 34  may secure device  14 . 7 - 10  to head  14 . 7 - 22  while mitigating the amount of stress applied to any portion of head  14 . 7 - 22 . 
     The stretchability of headband portion  14 . 7 - 30  (and therefore headband portions  14 . 7 - 32  and  14 . 7 - 34  of headband  14 . 7 - 26 ) allows headband  14 . 7 - 26  be stretched along its length. This allows the length of headband  14 . 7 - 26  to be temporarily increased to help a user place headband  14 . 7 - 26  over the user&#39;s head when a user is donning device  14 . 7 - 10 . When headband  14 . 7 - 26  is released, the stretchiness and elastic nature of portion  14 . 7 - 30  of headband  14 . 7 - 26  will help shorten headband  14 . 7 - 26  and pull headband  14 . 7 - 26  against the user&#39;s head so that headband  14 . 7 - 26  rests against rear surface  14 . 7 - 20 R the user&#39;s head. 
     Further adjustment of the tension of headband  14 . 7 - 26  to secure headband  14 . 7 - 26  and device  14 . 7 - 10  on the user&#39;s head may be provided by tightening doubled-back portions  14 . 7 - 33  and  14 . 7 - 35  of headband  14 . 7 - 26 . In particular, doubled-back portions  14 . 7 - 33  and  14 . 7 - 35  may pass through adjustment loops  14 . 7 - 36  and  14 . 7 - 38 , respectively. Doubled-back portions  14 . 7 - 33  and  14 . 7 - 35  may have hook-and-loop fasteners on an inner surface, allowing doubled-back portions  14 . 7 - 33  and  14 . 7 - 35  to be secured to headband portions  14 . 7 - 32  and  14 . 7 - 34 . In this way, headband portions  14 . 7 - 32  and  14 . 7 - 34  may be tightened or loosened as desired by a user of device  14 . 7 - 10 . 
       FIGS.  14 . 7 - 2 A  is an illustrative side view of an outer surface (e.g., the surface that faces away from head  14 . 7 - 22  when worn) of headband  14 . 7 - 26  in a configuration in which headband  14 . 7 - 26  is not attached to housing  14 . 7 - 12 . As shown in  FIGS.  14 . 7 - 2 A , headband  14 . 7 - 26  may have a stretchable central portion  14 . 7 - 30  that includes portions  14 . 7 - 32  and  14 . 7 - 34  formed from stretchy woven or knit fabric and may have ends  14 . 7 - 28  (e.g., end portions that are stiffened using stiffeners embedded in the fabric)—one of ends  14 . 7 - 28  is shown in the side view of  FIG.  14 . 7 - 2 A . One or more openings  14 . 7 - 40  may be formed in the ends of headband  14 . 7 - 26  in portion  14 . 7 - 31 . Openings  14 . 7 - 40  may receive posts or other protrusions such as protrusion  14 . 7 - 24 P of  FIGS.  14 . 7 - 1    to secure the left and right ends of headband  14 . 7 - 26  to the left and right members  14 . 7 - 24  of device  14 . 7 - 10 . There may be a single opening  14 . 7 - 40  or other attachment mechanism located on each end of headband  14 . 7 - 26  or each end of headband  14 . 7 - 26  may have two or more openings  14 . 7 - 40 . If desired, other attachment mechanism (e.g., magnets, snaps, latches, hook-and-loop fasteners, screws or other fasteners, etc.) may be used in attaching headband  14 . 7 - 26  to members  14 . 7 - 24  or other portions of the housing of device  14 . 7 - 10 . 
     As shown in  FIGS.  14 . 7 - 2 A , portions  14 . 7 - 32  and  14 . 7 - 34  of headband  14 . 7 - 26  may both extend from portion  14 . 7 - 31  of headband  14 . 7 - 26 . In other words, portions  14 . 7 - 32  and  14 . 7 - 34  may extend from portion  14 . 7 - 31  at a fixed angle to one another. Portion  14 . 7 - 31  may be more rigid (e.g., less stretchy) than portions  14 . 7 - 32  and  14 . 7 - 34 , if desired. For example, portion  14 . 7 - 31  may include one or more rigid or semi-rigid layers, such as polymer layers or metal layers. 
     By coupling (e.g., weaving, knitting, or otherwise intertwining) portions  14 . 7 - 32  and  14 . 7 - 34  with portion  14 . 7 - 31 , portions  14 . 7 - 32  and  14 . 7 - 34  may be kept at a constant angle with respect to one another. In other words, portions  14 . 7 - 32  and  14 . 7 - 34  may remain separated by a fixed angle when worn on head  14 . 7 - 22 . 
     Headband portions  14 . 7 - 32  and  14 . 7 - 34  may respectively include hook-and-loop fasteners  14 . 7 - 42  and  14 . 7 - 44 , which may secure the ends of portions  14 . 7 - 32  and  14 . 7 - 34  when the end portions are doubled back to tighten headband  14 . 7 - 26  (as shown in  FIGS.  14 . 7 - 1   ). Hook-and-loop fasteners  14 . 7 - 42  may be, for example, hair-safe hook-and-loop fasteners that prevent hair from being caught in fasteners  14 . 7 - 42 . 
     The outer surface of headband  14 . 7 - 26  may be woven. As shown in  FIGS.  14 . 7 - 2 A , on the outer side of headband  14 . 7 - 26 , headband  14 . 7 - 26  may have solid loop weave  14 . 7 - 46 . Solid loop weave  14 . 7 - 46  may be tightly woven and have a uniform appearance across headband  14 . 7 - 26 . The loops in solid loop weave  14 . 7 - 46  may provide the loops for hook-and-loop fasteners  14 . 7 - 42 . 
       FIGS.  14 . 7 - 2 B  is an illustrative side view of an inner surface (e.g., the surface in contact with head  14 . 7 - 22  when worn) of headband  14 . 7 - 22  in a configuration in which headband  14 . 7 - 26  is not attached to housing  14 . 7 - 12 . 
     Headband  14 . 7 - 26  may have a tab, such as tab  14 . 7 - 46 . Tab  14 . 7 - 46  may be attached to a latch (or other suitable attachment mechanism) in opening  14 . 7 - 40  that attaches headband  14 . 7 - 26  to post  14 . 7 - 24 P. By pulling on tab  14 . 7 - 46 , a user may release the latch from post  14 . 7 - 24 P, and headband  14 . 7 - 26  may be removed from member  14 . 7 - 24 . However, this attachment mechanism of headband  14 . 7 - 26  to member  14 . 7 - 24  is merely illustrative. In general, headband  14 . 7 - 26  may attach to member  14 . 7 - 24  in any suitable manner. 
     The inner surface of headband  14 . 7 - 26  may be woven. As shown in  FIGS.  14 . 7 - 2 B , on the inner side of headband  14 . 7 - 26 , headband  14 . 7 - 26  may have ribbed loop weave  14 . 7 - 47 . Ribbed loop weave  14 . 7 - 47  may provide sufficient friction for headband  14 . 7 - 26  to remain in place on head  14 . 7 - 22 , while being comfortable on head  14 . 7 - 22  (e.g., on a user&#39;s hair). Ribbed loop weave  14 . 7 - 47  may include loops  14 . 7 - 49  and rib portions  14 . 7 - 48 . By having ribbed loop weave  14 . 7 - 47 , the inner surface of headband  14 . 7 - 26  may have less tightly-packed loops than solid loop weave  14 . 7 - 46  on the outer surface of headband  14 . 7 - 26 , thereby improving the user&#39;s comfort. 
     Although  FIGS.  14 . 7 - 2 A and  14 . 7 - 2 B  show headband  14 . 7 - 26  as being woven, this is merely illustrative. In general, headband  14 . 7 - 26  may be formed from fabric including strands that are intertwined using knitting, weaving, braiding, and/or other strand intertwining techniques. 
     In some embodiments, it may be desirable to create a curved edge that appears seamless at the end of headband  14 . 7 - 26 . An illustrative headband strap with a curved edge with a seam that is invisible to the naked eye is shown in  FIGS.  14 . 7 - 3   . 
     As shown in  FIGS.  14 . 7 - 3   , strap  14 . 7 - 50  may be formed from inner woven portion  14 . 7 - 52  and webbing  14 . 7 - 54 . In particular, webbing  14 . 7 - 54  may be sewn, woven, or otherwise coupled to woven portion  14 . 7 - 52 . Woven portion  14 . 7 - 52  may have a solid loop weave (e.g., solid loop weave  14 . 7 - 45  of  FIG.  14 . 7 - 2 A ) or a ribbed loop weave (e.g., ribbed loop weave  14 . 7 - 47  of  FIGS.  14 . 7 - 2 B ), as examples. 
     Although portion  14 . 7 - 52  has been described as being woven, this is merely illustrative. In general, portion  14 . 7 - 52  may be knitted, woven, braided, and/or formed using other strand intertwining techniques. 
     Webbing  14 . 7 - 54  may have portion  14 . 7 - 56  that wraps from one side of inner woven portion  14 . 7 - 52  (e.g., the left side of inner woven portion  14 . 7 - 52 ), across edge  14 . 7 - 55  of strap  14 . 7 - 50 , to the opposite side of inner woven portion  14 . 7 - 52  (e.g., the right side of inner woven portion  14 . 7 - 52 ). 
     A stiffener may be inserted into webbing  14 . 7 - 54 , if desired. In the illustrative example of  FIGS.  14 . 7 - 3   , stiffener  14 . 7 - 58  may be inserted into webbing  14 . 7 - 54 , such as one side of webbing  14 . 7 - 54 . In particular, webbing  14 . 7 - 54  may have multiple layers, and stiffener  14 . 7 - 58  may be inserted between the multiple layers. Alternatively, webbing  14 . 7 - 54  may be formed as a single piece using a flat knitting technique and includes a built-in channel (sometimes referred to as a pocket or cavity), and stiffener  14 . 7 - 58  may be inserted into the built-in channel. In general, however, stiffener  14 . 7 - 58  may be inserted into webbing  14 . 7 - 54  in any desired manner. 
     Stiffener  14 . 7 - 58  may be formed from a cord, such as a braided cord, or a flexible strip of polymer (e.g., an elastomer such as thermoplastic polyurethane). Stiffener  14 . 7 - 58  may be sufficiently flexible to permit the headband to bend and twist, but may not stretch substantially along its length and may therefore sometimes be referred to as a non-stretchable stiffener, non-stretchable member, non-stretchable stiffening structure, etc. Stiffener  14 . 7 - 58  may significantly less stretchy and soft than the fabric of strap  14 . 7 - 50  and may serve to increase the stiffness and decrease (or eliminate) stretchiness at desired portions along strap  14 . 7 - 50 . At the same time, the flexibility of stiffener  14 . 7 - 58  may allow strap  14 . 7 - 50  to bend around the curvature of a user&#39;s head. Stiffener  14 . 7 - 58  may be inserted into selected portions of strap  14 . 7 - 50  to selectively stiffen strap  14 . 7 - 50  at desired portions along its length, if desired. 
     Headband  14 . 7 - 26  may have rounded corners  14 . 7 - 62  of webbing  14 . 7 - 54  and may not have visible seams. An illustrative side view of headband  14 . 7 - 26  is shown in  FIGS.  14 . 7 - 4   . 
     As shown in  FIGS.  14 . 7 - 4   , headband  14 . 7 - 26  may include layers  14 . 7 - 64  and loops  14 . 7 - 66  (e.g., in webbing  14 . 7 - 54  of  FIGS.  14 . 7 - 3   ). Webbing from one side of headband  14 . 7 - 26  (e.g., portion  14 . 7 - 56  of  FIGS.  14 . 7 - 3   ) may be bonded to the other side of the webbing at point  14 . 7 - 71  (e.g., headband  14 . 7 - 26  may have single seam at point  14 . 7 - 71  one only one side of headband  14 . 7 - 26 ). As shown in  FIGS.  14 . 7 - 5   , there may be no visible seam at point  14 . 7 - 71 . In this way, a one-sided seam that is not visible to the naked eye may be formed in headband  14 . 7 - 26 . 
     Although  FIGS.  14 . 7 - 2 A and  14 . 7 - 2 B  show portions  14 . 7 - 32  and  14 . 7 - 34  of headband  14 . 7 - 26  extending from common portion  14 . 7 - 31 , this is merely illustrative. If general, portions  14 . 7 - 32  and  14 . 7 - 34  may be coupled and attached to a head-mounted device in any suitable manner, such as attaching portions  14 . 7 - 32  and  14 . 7 - 34  separately to a common post or wrapping a single band through a ring to double back on itself. An illustrative embodiment of a single band wrapped through a ring is shown in  FIGS.  14 . 7 - 5   . 
     As shown in  FIGS.  14 . 7 - 5   , headband  14 . 7 - 26  may be wrapped through ring  14 . 7 - 70  to form portions  14 . 7 - 32  and  14 . 7 - 34 . Ring  14 . 7 - 70  may be a metal ring, a rigid polymer ring, or a ring of other suitable material. To ensure that portions  14 . 7 - 32  and  14 . 7 - 34  remain at a desired angle with respect to one another when worn by a user, stiffeners  14 . 7 - 72  may be added to headband  14 . 7 - 26 . Stiffeners  14 . 7 - 72  may be formed from rigid polymer, metal, or other suitable material. 
     Stiffeners  14 . 7 - 72  may locally increase the stiffness of band  14 . 7 - 26  in the area around ring  14 . 7 - 70 . As a result, portions  14 . 7 - 32  and  14 . 7 - 34  may maintain the angle between each other about ring  14 . 7 - 70  when band  14 . 7 - 26  is worn on a user&#39;s head. Moreover, stiffeners  14 . 7 - 72  may create detents to provide a user of headband  14 . 7 - 26  with feedback regarding how tight headband  14 . 7 - 26  is on the user&#39;s head. In other words, because stiffeners  14 . 7 - 72  have to pass through ring  14 . 7 - 70  when a user tightens or loosens headband  14 . 7 - 26 , stiffeners  14 . 7 - 72  may indicate to the user how tight the headband is (e.g., by how may stiffeners the user feels pass through ring  14 . 7 - 70 ). 
     The stiffeners shown in  FIGS.  14 . 7 - 5    are merely illustrative. In general, stiffeners may be added to headband  14 . 7 - 26  in any suitable manner. For example, as shown in illustrative  FIGS.  14 . 7 - 6 A and  14 . 7 - 6 B , stiffeners  14 . 7 - 72  may be added with different sizes, pitches, and/or geometries to adjust the stiffness of headband  14 . 7 - 26  about ring  14 . 7 - 70 . Stiffeners  14 . 7 - 72  may be proud of the surface of headband  14 . 7 - 26 , or may be formed flush or sub-flush with the surface of headband  14 . 7 - 26 . As another illustrative example, as shown in  FIGS.  14 . 7 - 6 C , cuts  14 . 7 - 74  may be made in headband  14 . 7 - 26 . Cuts  14 . 7 - 74  may selectively modify the stiffness of headband  14 . 7 - 26  in different regions, and cuts  14 . 7 - 74  may be made so that headband  14 . 7 - 26  has more stiffness in the region adjacent to ring  14 . 7 - 70  than in other regions. In general, stiffeners in headband  14 . 7 - 26  may have any suitable geometries and may help maintain the angle between portions  14 . 7 - 32  and  14 . 7 - 34  about ring  14 . 7 - 70 . 
     Although stiffeners  14 . 7 - 72  are shown in  FIGS.  14 . 7 - 5    as being used when headband  14 . 7 - 26  extends through ring  14 . 7 - 70 , this is merely illustrative. Stiffeners  14 . 7 - 72  may be included on a surface of headband  14 . 7 - 26  if the portions of headband  14 . 7 - 26  are in a fixed position (as shown in  FIGS.  14 . 7 - 1   , as an example). 
     In addition to, or instead of, stiffeners  14 . 7 - 72 , headband  14 . 7 - 26  may be have stiffeners that run along a length of headband  14 . 7 - 26 . An illustrative example of a headband with stiffeners of this type is shown in  FIGS.  14 . 7 - 7   . 
     As shown in  FIGS.  14 . 7 - 7   , portions  14 . 7 - 32  and  14 . 7 - 34  of headband  14 . 7 - 26  may include stiffeners  14 . 7 - 76 . Stiffeners  14 . 7 - 76  may be the same as stiffener  14 . 7 - 58  of  FIGS.  14 . 7 - 3   , as an example. Stiffeners  14 . 7 - 76  may be formed from a cord, such as a braided cord, or a flexible strip of polymer (e.g., an elastomer such as thermoplastic polyurethane). Stiffeners  14 . 7 - 76  may be more rigid (e.g., less flexible/stretchy) than the other portions of headband  14 . 7 - 26 . 
     Stiffeners  14 . 7 - 76  may be inserted along one side of portions  14 . 7 - 32  and  14 . 7 - 34 , as shown in  FIGS.  14 . 7 - 7   . In particular, because stiffeners  14 . 7 - 76  stretch less than the rest of portions  14 . 7 - 32  and  14 . 7 - 34 , portions  14 . 7 - 32  and  14 . 7 - 34  may conform to a user&#39;s head (e.g., head  14 . 7 - 22  of  FIGS.  14 . 7 - 1   ) and support the head-mounted device against the user&#39;s face more effectively. 
     Stiffeners  14 . 7 - 76  may extend entirely along the lengths of portions  14 . 7 - 32  and  14 . 7 - 34 , or may extend across parts of the lengths of portions  14 . 7 - 32  and  14 . 7 - 34 . In general, stiffeners  14 . 7 - 76  may be coupled to suitable parts of portions  14 . 7 - 32  and  14 . 7 - 34  to adjust the stiffness of portions  14 . 7 - 32  and  14 . 7 - 34  across their length as needed. 
     Stiffeners  14 . 7 - 76  may be inserted into channels within headband  14 . 7 - 26 . As shown in illustrative  FIGS.  14 . 7 - 8   , headband  14 . 7 - 26  may include strap  14 . 7 - 78  (which may correspond to strap  14 . 7 - 50  of  FIGS.  14 . 7 - 3   ) having surfaces  14 . 7 - 80  and  14 . 7 - 82 . Between surfaces  14 . 7 - 80  and  14 . 7 - 82 , strap  14 . 7 - 78  may have channel  14 . 7 - 85  with opening  14 . 7 - 84  at an edge of strap  14 . 7 - 78 . Stiffener  14 . 7 - 76  may be mounted within channel  14 . 7 - 85 . In other words, stiffener  14 . 7 - 76  may be embedded within strap  14 . 7 - 78  (e.g., within headband  14 . 7 - 26 ). 
     Stiffeners  14 . 7 - 76  may adjust the stiffness and curvature of headband  14 . 7 - 26 . In particular, by adding stiffeners  14 . 7 - 76  to different regions of headband  14 . 7 - 26 , headband  14 . 7 - 26  may curve differently in different regions. An illustrative example of a headband having stiffeners in different regions to create regions with different curvatures is shown in  FIGS.  14 . 7 - 9 A and  14 . 7 - 9 B . 
     As shown in  FIGS.  14 . 7 - 9 A , headband  14 . 7 - 26  may be straight when not under tension. In other words, when headband  14 . 7 - 26  is not on a user&#39;s head or otherwise under tension, headband  14 . 7 - 26  may return to a straight configuration. However, when headband  14 . 7 - 26  is pulled in opposite directions, headband may curve differently based on the locations of stiffeners  14 . 7 - 76 . 
     As shown in  FIGS.  14 . 7 - 9 B , portions  14 . 7 - 90  of headband  14 . 7 - 26  may curve more than portions  14 . 7 - 92  in response to headband  14 . 7 - 26  being pulled in directions  14 . 7 - 88  and  14 . 7 - 86 . In particular, because stiffeners  14 . 7 - 76  are formed in portions  14 . 7 - 92 , portions  14 . 7 - 92  may be stiffer than regions  14 . 7 - 90 , which do not have stiffeners  14 . 7 - 76  (in other words, stiffeners  14 . 7 - 76  are stiffer than fabric  14 . 7 - 78 ). As a result, in response to pulling both ends of headband  14 . 7 - 26  in directions  14 . 7 - 88  and  14 . 7 - 86 , headband  14 . 7 - 26  may extend in a serpentine pattern. 
     In some embodiments, headband  14 . 7 - 26  may be straight when not under tension (e.g.,  FIGS.  14 . 7 - 9 A ) an may bend to conform to compound curvature (e.g., a user&#39;s head) when under tension (e.g.,  FIGS.  14 . 7 - 9 B ). However, this is merely illustrative. In general, stiffeners  14 . 7 - 76  may be included in any suitable portion of headband  14 . 7 - 26  so that headband  14 . 7 - 26  bends as desired when under tension. 
     Returning to  FIGS.  14 . 7 - 7   , by forming stiffener  14 . 7 - 76  on the bottom edge of portion  14 . 7 - 32  and stiffener  14 . 7 - 76  on the upper edge of portion  14 . 7 - 34 , headband  14 . 7 - 26  may conform to a user&#39;s head when worn by the user. In other words, headband  14 . 7 - 26  may sit flat against the user&#39;s head across different curvatures of the head (e.g., headband  14 . 7 - 26  may distribute pressure evenly across a non-uniform surface). However, this is merely illustrative. If desired, stiffeners  14 . 7 - 76  may be formed on the upper edge of portion  14 . 7 - 32  or on the lower edge of portion  14 . 7 - 34 , depending on the shape to which headband  14 . 7 - 26  needs to conform. 
     The foregoing is merely illustrative and various modifications can be made to the described embodiments. The foregoing embodiments may be implemented individually or in any combination. 
     14.8: Over the Head Strap 
     Head-mounted devices include head-mounted support structures that allow the devices to be worn on the heads of users. The head-mounted support structures may include device housings that enclose components such as displays. The displays may be used for presenting a user with visual content. The head-mounted support structures for a head-mounted device may also include headbands and other structures that help hold a device housing on the face of a user. The headbands of a head-mounted device may be removable. This allows users to swap different headbands into use to accommodate different head sizes and/or to update the style of headband being used. 
     In some embodiments, it may be desirable to attach multiple headbands to a head-mounted device. For example, one detachable headband may extend from the head-mounted support structures around the back of a user&#39;s head when worn, and another detachable headband may extend over the user&#39;s head. The user of two (or more) headbands may hold the head-mounted device against the user&#39;s face and prevent excessive pressure on the face when the device is worn. Both headbands may attach and detach from the same portion of the head-mounted support structures. 
       FIGS.  14 . 8 - 1    is a side view of an illustrative head-mounted electronic device with multiple detectable headbands. As shown in  FIGS.  14 . 8 - 1   , head-mounted device  14 . 8 - 10  may include head-mounted housing  14 . 8 - 12  (sometimes referred to as a main housing, main housing unit, head-mounted support structure, main housing portion, etc.). Housing  14 . 8 - 12  may have walls or other structures that separate an interior housing region from an exterior region surrounding housing  14 . 8 - 12 . For example, housing  14 . 8 - 12  may have walls formed from polymer, glass, metal, and/or other materials. Electrical and optical components may be mounted in housing  14 . 8 - 12 . These components may include components such as integrated circuits, sensors, control circuitry, input-output devices, etc. 
     To present a user with images for viewing from eye boxes (e.g., eye boxes in which the user&#39;s eyes are located when device  14 . 8 - 10  is being worn on the users&#39; head), device  14 . 8 - 10  may include displays and lenses. These components may be mounted in optical modules  14 . 8 - 20  that face towards rear R of device  14 . 8 - 10  or may be mounted in other supporting structures in housing  14 . 8 - 12  to form respective left and right optical systems. There may be, for example, a left display for presenting an image through a left lens to a user&#39;s left eye in a left eye box and a right display for presenting an image to a user&#39;s right eye in a right eye box. 
     If desired, housing  14 . 8 - 12  may have forward-facing components such as cameras and other sensors on front F for gathering sensor measurements and other input and may have a soft cushion on opposing rear R. Rear R may have openings that allow the user to view images from left and right optical modules  14 . 8 - 20  (e.g., when rear R is resting on the front of the user&#39;s head). 
     Device  14 . 8 - 10  may have strap such as headband  14 . 8 - 26  and over-the-head headband  14 . 8 - 27  to help hold housing  14 . 8 - 12  on the user&#39;s head. Headbands  14 . 8 - 26  and  14 . 8 - 27  may have a fixed length or may be adjustable. Headbands  14 . 8 - 26  and  14 . 8 - 27  may have first and second ends coupled, respectively, to the left and right sides of housing  14 . 8 - 12 . In the example of  FIGS.  14 . 8 - 1   , coupling members  14 . 8 - 24 , which serve as extensions of housing  14 . 8 - 12 , are provided on the left and right sides of housing  14 . 8 - 12 . Members  14 . 8 - 24  may be formed from rigid materials such as rigid polymer and/or other materials and may contain sensors, buttons, speakers, and other electrical components. Hinges and/or other mechanisms may be used to couple members  14 . 8 - 24  to housing  14 . 8 - 12  or members  14 . 8 - 24  may be formed as integral portions of a main housing unit. 
     The ends of headbands  14 . 8 - 26  and  14 . 8 - 27  may have coupling mechanisms such as openings, magnets, second posts, or other structures configured to attach to posts  14 . 8 - 30  (pins) or other protrusions on members  14 . 8 - 24  or other housing structures. In an illustrative configuration, these posts face inwardly towards the user&#39;s head and are not visible to people in the vicinity of device  14 . 8 - 10  when device  14 . 8 - 10  is being worn by the user. Releasable latch mechanisms can be used to help secure the ends of headband  14 . 8 - 26  and/or headband  14 . 8 - 27  to member  14 . 8 - 24 . For example, a first detachable latch may be used to removably couple the left end of headband  14 . 8 - 26  to a left post in a left member  14 . 8 - 24  on a left side of housing  14 . 8 - 12  and a second detachable latch may be used to removably couple the right end of headband  14 . 8 - 26  to a right post in a right member  14 . 8 - 24  on a right side of housing  14 . 8 - 12 . Third and fourth detachable latches may couple respective left and right ends of headband  14 . 8 - 27  to the left and right posts. Alternatively or additionally, headband  14 . 8 - 27  may be attached to the posts between the head-mounted support structures  14 . 8 - 24  and headband  14 . 8 - 26  to keep headband  14 . 8 - 27  attached to structures  14 . 8 - 24  when headband  14 . 8 - 26  is attached, and/or may use another structures, such as a magnetic structure or additional post, to attach to posts  14 . 8 - 30 . 
     If desired, a user may flip the headband over so that the first detachable latch removable couples the end of headband  14 . 8 - 26  that was previously coupled to the left post to the right post and so that the second detachable latch removably couples the end of headband  14 . 8 - 26  that was previously coupled to the right post to the left post (e.g., the user may flip the left and right sides of the band without flipping the band inside out). Headband  14 . 8 - 27  also may have reversible ends, if desired. A user may open and close the latches when housing  14 . 8 - 12  is being worn or, in an illustrative configuration that is sometimes described herein as an example, a user may open and close the latches when housing  14 . 8 - 12  is not being worn. 
     The use of latch-based coupling mechanisms, magnetic structures, and/or or other mechanisms in device  14 . 8 - 10  may help allow a user to removably attach headbands  14 . 8 - 26  and  14 . 8 - 27  to members  14 . 8 - 24  and thereby removably attach headbands  14 . 8 - 26  and  14 . 8 - 27  to housing  14 . 8 - 12 . Members  14 . 8 - 24  may have elongated shapes of the type shown in  FIGS.  14 . 8 - 1    and/or other suitable shapes and may sometimes be referred to as rigid straps, rigid coupling members, power straps, head-mounted device housing structures, elongated head-mounted device housing members, elongated housing structures, elongated housing members, or head-mounted device housing members (as examples). 
     Headbands  14 . 8 - 26  and  14 . 8 - 27  may be straps with soft flexible portions and/or rigid portions. As an example, a central portion of headband  14 . 8 - 26  and/or headband  14 . 8 - 27  may be formed from stretchable fabric. The central portions of headband  14 . 8 - 26  and/or headband  14 . 8 - 27  may have internal stiffening members, external fabric coverings and or other covering layers, strips of strengthening fabric, stretchable fabric portions (e.g., stretchable knit fabric), cosmetic coverings, and/or other headband structures. 
     Left and right end portions of headband  14 . 8 - 26  and/or headband  14 . 8 - 27  may be coupled to opposing ends of this central portion. The left and right end portions may, as an example, have stiffening structures (e.g., the left and right end portions may be stiffer than the central stretchable portion). Other types of configuration may be used for headband  14 . 8 - 26 , if desired (e.g., arrangements with adjustable tensioning cables, etc.). 
       FIGS.  14 . 8 - 2    is an illustrative diagram of an end portion of an illustrative headband  14 . 8 - 26 , an end portion of an illustrative headband  14 . 8 - 27 , and a corresponding end portion of an illustrative member  14 . 8 - 24 . Headband  14 . 8 - 26 , headband  14 . 8 - 27 , and housing structures such as members  14 . 8 - 24  may sometimes be collectively referred to as a forming a head-mounted device headband system (headband system)  14 . 8 - 22 . As shown in the example of  FIGS.  14 . 8 - 2   , member  14 . 8 - 24  may have a protruding post such as post  14 . 8 - 30  (e.g., a post that protrudes out of member  14 . 8 - 24  towards a user&#39;s head while device  14 . 8 - 10  is being worn by the user). Headband  14 . 8 - 26  may have a corresponding opening  14 . 8 - 32  that is configured to receive post  14 . 8 - 30 . 
     Post  14 . 8 - 30  may protrude inwardly (or, in some embodiments, outwardly) from member  14 . 8 - 24 . Post  14 . 8 - 30  may be formed from metal, rigid polymer, other materials, and/or combinations of these materials. Member  14 . 8 - 24  may have a rigid portion to which post  14 . 8 - 30  is attached. This rigid portion may be formed from rigid polymer, metal, fabric, carbon-fiber composite materials, and/or other materials. 
     In the illustrative embodiment of  FIGS.  14 . 8 - 2   , to attach headband  14 . 8 - 27  to post  14 . 8 - 30 , headband  14 . 8 - 27  may include post  14 . 8 - 31 . Post  14 . 8 - 31  may be formed from metal, rigid polymer, other materials, and/or combinations of these materials. In the present example, post  14 . 8 - 31  and post  14 . 8 - 30  have elongated shapes when viewed end-on (e.g., rectangular shapes with rounded corners). These elongated shapes may help resist rotational motion between longitudinal axis  14 . 8 - 34  of headband  14 . 8 - 26  and longitudinal axis  14 . 8 - 36  of member  14 . 8 - 24 . This helps prevent headband  14 . 8 - 26  from slipping up or down along the rear surface of a user&#39;s head during use. In general, post  14 . 8 - 30  and/or mating opening  14 . 8 - 32  may have any suitable shapes (e.g., the shape of post  14 . 8 - 30  and/or opening  14 . 8 - 32  may be circular, oval, rectangular, triangular, may be a shape with curved edges and/or straight edges, may be a shape with drafted edges to help with alignment and/or insertion, etc.). The use of rectangular shapes with rounded corners and/or other shapes that are elongated is illustrative. 
     Post  14 . 8 - 31  may be slightly larger than post  14 . 8 - 30  to fit on top of post  14 . 8 - 30 . If desired, post  14 . 8 - 31  may snugly fit onto post  14 . 8 - 30  to attach headband  14 . 8 - 27  to member  14 . 8 - 24 . Additionally or alternatively, post  14 . 8 - 31  may include a latch, such as a structure biased with a spring or other suitable latch that engages an opening in the side of post  14 . 8 - 30 , a magnet, or another suitable attachment to attach headband  14 . 8 - 27  to member  14 . 8 - 24 . If a latch is used to attach headband  14 . 8 - 27  to post  14 . 8 - 30 , tab  14 . 8 - 61 T may be attached to the latch, and may be pulled to disengage the latch so that headband  14 . 8 - 27  may be released from member  14 . 8 - 24 . 
     Opening  14 . 8 - 32  in headband  14 . 8 - 26  may be a through-hole opening with a shape that matches the outline of post  14 . 8 - 31 . Through-hole opening  14 . 8 - 32  may be formed by cutting or otherwise forming an opening in headband  14 . 8 - 26 . The periphery of opening  14 . 8 - 32  may be strengthened using a mating pair of ring members, which may sometimes be referred to as a cap and socket and may capture portions of headband  14 . 8 - 26 . 
     In the present example, post  14 . 8 - 31  and opening  14 . 8 - 32  have elongated shapes when viewed end-on (e.g., rectangular shapes with rounded corners). Headband  14 . 8 - 26  may have a latch such as a structure biased with a spring or other suitable latch, in opening  14 . 8 - 32  that engages an opening in the side of post  14 . 8 - 31  to attach headband  14 . 8 - 26  to member headband  14 . 8 - 27 . Tab  14 . 8 - 62 T may be attached to the latch, and may be pulled to disengage the latch so that headband  14 . 8 - 26  may be released from member  14 . 8 - 27 . 
     By including post  14 . 8 - 31  on headband  14 . 8 - 27  to overlap and attach to post  14 . 8 - 30 , both headband  14 . 8 - 27  and band  14 . 8 - 26  may be attached to member  14 . 8 - 24 . An illustrative cross-sectional side view of member  14 . 8 - 24 , headband  14 . 8 - 26 , and headband  14 . 8 - 27  attached together is shown in  FIGS.  14 . 8 - 3   . 
     As shown in  FIGS.  14 . 8 - 3   , post  14 . 8 - 30  on member  14 . 8 - 24  may extend into post  14 . 8 - 31  of headband  14 . 8 - 27 . For example, post  14 . 8 - 31  may be a small amount larger than post  14 . 8 - 30  (e.g., less than 1 mm larger, less than 2 mm larger, or other suitable size difference) to allow post  14 . 8 - 31  to press onto post  14 . 8 - 30  and remain attached to post  14 . 8 - 30 . Post  14 . 8 - 31  may then be inserted into opening  14 . 8 - 32  of headband  14 . 8 - 26 , thereby attaching headband  14 . 8 - 26  to headband  14 . 8 - 27  and member  14 . 8 - 24 . 
     In some embodiments, post  14 . 8 - 31  may be attached to post  14 . 8 - 30  with a latch, and/or headband  14 . 8 - 26  may be attached to post  14 . 8 - 31  with a latch. A cross-sectional side view of an illustrative latch that may be used to attach posts  14 . 8 - 30  and  14 . 8 - 31 , and/or headband  14 . 8 - 26  and post  14 . 8 - 31  is shown in  FIGS.  14 . 8 - 4   . 
     As shown in  FIGS.  14 . 8 - 4    system  14 . 8 - 22  may include a latching mechanism such as latch  14 . 8 - 62 . Latch  14 . 8 - 62 , which may sometimes be referred to as a latch mechanism or latch structures, may be opened and closed using magnets, springs, sliding members, toggling members, rotating members such as knobs, buttons, and/or other latch structures that may be manipulated by a user (e.g., a user&#39;s fingers). In the example of  FIGS.  14 . 8 - 4   , the latch of headband  14 . 8 - 26  has a movable latch member that engages post  14 . 8 - 31  when post  14 . 8 - 31  is within opening  14 . 8 - 32  and has an associated release mechanism such as release tab  14 . 8 - 62 T. Tab  14 . 8 - 62 T, which may be formed from a flexible strip of material (e.g., fabric, polymer, and/or other material) may be pulled in direction  14 . 8 - 64  by a user (e.g., when a user grasps tab  14 . 8 - 62 T between the user&#39;s fingers), thereby moving the movable latch member out of engagement with post  14 . 8 - 31 . This releases post  14 . 8 - 31  and allows post  14 . 8 - 31  to be removed from opening  14 . 8 - 32 . Magnets, spring structures, and/or other biasing structures may be used in closing the latch. In some embodiments, headband  14 . 8 - 27  and headband  14 . 8 - 26  may have magnets that facilitate attachment of headband  14 . 8 - 27  and headband  14 . 8 - 26 . As shown in  FIGS.  14 . 8 - 4   , for example, post  14 . 8 - 31  may have one or more magnets such as magnet  14 . 8 - 60 . Magnet  14 . 8 - 60  may be used to attract and/or repel corresponding magnets in headband  14 . 8 - 26 , which can assist in attaching headband  14 . 8 - 26  to headband  14 . 8 - 27  and/or can assist in closing latch  14 . 8 - 62 . 
     Although  FIGS.  14 . 8 - 4    shows latch  14 . 8 - 62  attaching headband  14 . 8 - 26  to headband  14 . 8 - 27 , latch  14 . 8 - 62  may additionally or alternatively be used to attach headband  14 . 8 - 27  to member  14 . 8 - 24 . In particular, latch  14 . 8 - 62  may be formed in post  14 . 8 - 31  of headband  14 . 8 - 27 , and may be moved in and out of a recess within post  14 . 8 - 30  of member  14 . 8 - 24  using tab  14 . 8 - 61 T. In this way, headband  14 . 8 - 27  may be removably coupled to member  14 . 8 - 24  with a latch, such as latch  14 . 8 - 62 . 
     In addition to, or instead of, using a post on headband  14 . 8 - 27  to attach headband  14 . 8 - 27  to member  14 . 8 - 24 , magnets may be used for this attachment. An illustrative embodiment in which headband  14 . 8 - 27  is attached to member  14 . 8 - 24  using magnets is shown in  FIGS.  14 . 8 - 5   . 
     As shown in  FIGS.  14 . 8 - 5   , headband  14 . 8 - 26  may attach to post  14 . 8 - 30  at opening  14 . 8 - 32 . For example, headband  14 . 8 - 26  may attach to post  14 . 8 - 30  in the same manner as headband  14 . 8 - 26  attaches to post  14 . 8 - 31  in  FIGS.  14 . 8 - 3   . In particular, a latch in opening  14 . 8 - 32  (e.g., latch  14 . 8 - 62  of  FIGS.  14 . 8 - 4   ) may move into an opening within post  14 . 8 - 30 , removably attaching headband  14 . 8 - 26  to member  14 . 8 - 24 . 
     Headband  14 . 8 - 27  may be magnetically attached to post  14 . 8 - 30 . In particular, magnet  14 . 8 - 33  may be inserted within post  14 . 8 - 30 , and magnet  14 . 8 - 35  of headband  14 . 8 - 27  may be magnetically attached to magnet  14 . 8 - 33  of post  14 . 8 - 30 . Because post  14 . 8 - 30  passes through opening  14 . 8 - 32  of headband  14 . 8 - 26 , headband  14 . 8 - 27  may contact the upper surface of post  14 . 8 - 30 , as magnets  14 . 8 - 33  and  14 . 8 - 35  attract post  14 . 8 - 30  and headband  14 . 8 - 27  together. An illustrative cross-sectional view of a magnetic attachment of headband  14 . 8 - 27  and member  14 . 8 - 24  is shown in  FIGS.  14 . 8 - 6   . 
     As shown in  FIGS.  14 . 8 - 6   , member  14 . 8 - 24  may have recess  14 . 8 - 37  formed in post  14 . 8 - 30 . Magnet  14 . 8 - 33  may be coupled to post  14 . 8 - 30  within recess  14 . 8 - 37 . Similarly, headband  14 . 8 - 27  may include recess  14 . 8 - 39 , and magnet  14 . 8 - 35  may be coupled to headband  14 . 8 - 27  within recess  14 . 8 - 39 . Magnets  14 . 8 - 33  and  14 . 8 - 35  may attach headband  14 . 8 - 27  to member  14 . 8 - 24 . 
     Headband  14 . 8 - 26  may be between member  14 . 8 - 24  and headband  14 . 8 - 27  when headband  14 . 8 - 27  is magnetically attached to member  14 . 8 - 24 . In particular, opening  14 . 8 - 32  of headband  14 . 8 - 26  may surround post  14 . 8 - 30 . Headband  14 . 8 - 26  may be removably attached to member  14 . 8 - 24  with a latch in opening  14 . 8 - 32  (as shown in  FIGS.  14 . 8 - 5   ), and/or headband  14 . 8 - 26  may remain attached to member  14 . 8 - 24  due to headband  14 . 8 - 27  being magnetically attached to member  14 . 8 - 24 . 
     The presence of recesses  14 . 8 - 37  and  14 . 8 - 39  to accommodate magnets  14 . 8 - 33  and  14 . 8 - 35  are merely illustrative. In general, one or more magnets may be attached to each of member  14 . 8 - 24  and headband  14 . 8 - 27  in any suitable manner. 
     Instead of, or in addition to, having recesses to accommodate magnets in member  14 . 8 - 24  and/or headband  14 . 8 - 27 , recesses and protrusions may help attach and align headband  14 . 8 - 27  to member  14 . 8 - 24 . An illustrative embodiment in which a recess and a protrusion are used for aligning and attaching member  14 . 8 - 24  and headband  14 . 8 - 27  is shown in  FIGS.  14 . 8 - 7   . 
     As shown in  FIGS.  14 . 8 - 7   , in addition to having magnets  14 . 8 - 35  and  14 . 8 - 37  coupling headband  14 . 8 - 27  to member  14 . 8 - 24 , member  14 . 8 - 24  may have recess  14 . 8 - 40  in post  14 . 8 - 30 , and headband  14 . 8 - 27  may have protrusion  14 . 8 - 42 . Protrusion  14 . 8 - 42  may be inserted within recess  14 . 8 - 40  when headband  14 . 8 - 27  and member  14 . 8 - 24  are attached, which may improve the alignment and/or the attachment of headband  14 . 8 - 27  and member  14 . 8 - 24 . 
     As in the embodiment of  FIGS.  14 . 8 - 6   , headband  14 . 8 - 26  may be between member  14 . 8 - 24  and headband  14 . 8 - 27  when headband  14 . 8 - 27  is magnetically attached to member  14 . 8 - 24 . In particular, opening  14 . 8 - 32  of headband  14 . 8 - 26  may surround post  14 . 8 - 30 . Headband  14 . 8 - 26  may be removably attached to member  14 . 8 - 24  with a latch in opening  14 . 8 - 32  (as shown in  FIGS.  14 . 8 - 5   ), and/or headband  14 . 8 - 26  may remain attached to member  14 . 8 - 24  due to headband  14 . 8 - 27  being magnetically attached to member  14 . 8 - 24 . 
     Although member  14 . 8 - 24  is shown as having recess  14 . 8 - 40 , and headband  14 . 8 - 27  is shown as having protrusion  14 . 8 - 42 , this is merely illustrative. Headband  14 . 8 - 27  may have a recess in which a protrusion of member  14 . 8 - 24  is inserted, if desired. 
     In other embodiments, headband  14 . 8 - 27  may be attached to member  14 . 8 - 24  directly, rather than at post  14 . 8 - 30 , such as by wrapping around member  14 . 8 - 24  or clipping onto member  14 . 8 - 24 . An illustrative embodiment in which headband  14 . 8 - 27  is attached to member  14 . 8 - 24  directly is shown in  FIGS.  14 . 8 - 8   . 
     As shown in  FIGS.  14 . 8 - 8   , headband  14 . 8 - 27  may be attached to member  14 . 8 - 24  with portions  14 . 8 - 44  and  14 . 8 - 46  that wrap around member  14 . 8 - 24 . In particular, portions  14 . 8 - 44  and  14 . 8 - 46  may extend from the rest of headband  14 . 8 - 27 , and may be on either side of post  14 . 8 - 30  of member  14 . 8 - 24 . In some embodiments, headband  14 . 8 - 27  may be slid onto member  14 . 8 - 24  at end  14 . 8 - 25 , and portions  14 . 8 - 44  and  14 . 8 - 46  may attach headband  14 . 8 - 27  to member  14 . 8 - 24 . 
     Headband  14 . 8 - 26  may attach to post  14 . 8 - 30  directly, as opening  14 . 8 - 32  of headband  14 . 8 - 26  may surround post  14 . 8 - 30 . Headband  14 . 8 - 26  may be removably attached to member  14 . 8 - 24  with a latch in opening  14 . 8 - 32  (as shown in  FIG.  14 . 8 - 5   ) or another mechanism. 
     Although  FIGS.  14 . 8 - 8    shows portions  14 . 8 - 44  and  14 . 8 - 46  wrapping entirely around member  14 . 8 - 24 , this is merely illustrative. If desired, headband  14 . 8 - 27  may have a single or multiple portions that clip onto (e.g., extend partially around) edges  14 . 8 - 21  and  14 . 8 - 23  of member  14 . 8 - 24 . In this arrangement, headband  14 . 8 - 27  may be detached from member  14 . 8 - 24  (e.g., by unclipping the clip-on portions) without removing headband  14 . 8 - 26  from post  14 . 8 - 31 . Alternatively or additionally, portion  14 . 8 - 46  may extend around and cover end  14 . 8 - 25  of member  14 . 8 - 24 . 
     Instead of having portions of headband  14 . 8 - 27  that surround member  14 . 8 - 24  or clip onto member  14 . 8 - 24 , headband  14 . 8 - 27  may include lugs that fit into openings in member  14 . 8 - 24  (e.g., headband  14 . 8 - 27  and member  14 . 8 - 24  may have a lug and socket system to attach headband  14 . 8 - 27  to member  14 . 8 - 24 ). An illustrative example of headband  14 . 8 - 27  attached to member  14 . 8 - 24  with a lug and socket system is shown in  FIGS.  14 . 8 - 9   . 
     As shown in  FIGS.  14 . 8 - 9   , headband  14 . 8 - 27  may have lugs  14 . 8 - 48  that are received within sockets  14 . 8 - 50  of member  14 . 8 - 24 . Lugs  14 . 8 - 48  may be coupled by one or more springs or otherwise biased outward (e.g., into sockets  14 . 8 - 50 ). To release headband  14 . 8 - 27  from member  14 . 8 - 24 , sliding member  14 . 8 - 52  may be slid within opening  14 . 8 - 54  of headband  14 . 8 - 27  to retract one of lugs  14 . 8 - 50 . Headband  14 . 8 - 27  may then be removed from member  14 . 8 - 24 . Headband  14 . 8 - 27  may be reattached to member  14 . 8 - 24  by inserting lugs  14 . 8 - 48  into sockets  14 . 8 - 50 , which may include retracting one of the lugs by sliding member  14 . 8 - 52  within opening  14 . 8 - 54 . In this way, headband  14 . 8 - 27  may be attached to member  14 . 8 - 24  using a lug and socket system. 
     Headband  14 . 8 - 26  may attach to post  14 . 8 - 30  directly, as opening  14 . 8 - 32  of headband  14 . 8 - 26  may surround post  14 . 8 - 30 . Headband  14 . 8 - 26  may be removably attached to member  14 . 8 - 24  with a latch in opening  14 . 8 - 32  (as shown in  FIG.  14 . 8 - 5   ) or another mechanism. 
     In some embodiments, post  14 . 8 - 30  of member  14 . 8 - 24  may be modified to accommodate both headband  14 . 8 - 26  and headband  14 . 8 - 27 . For example, headband  14 . 8 - 27  may have an additional latch that connects headband  14 . 8 - 27  to member  14 . 8 - 24 . An illustrative example of attaching headband  14 . 8 - 27  to member  14 . 8 - 24  with an additional latch is shown in  FIGS.  14 . 8 - 10   . 
     As shown in  FIGS.  14 . 8 - 10   , headband  14 . 8 - 27  may include latch  14 . 8 - 56 , and member  14 . 8 - 24  may have post  14 . 8 - 30  with recess  14 . 8 - 66 . Latch  14 . 8 - 56  may be formed from metal, rigid polymer, or other material. Latch  14 . 8 - 56  may include protruding portion  14 . 8 - 68  that attaches to post  14 . 8 - 31  within recess  14 . 8 - 66 . For example, latch  14 . 8 - 56  may be biased toward recess  14 . 8 - 66  by a spring, magnet, or other component. If it is desired to remove headband  14 . 8 - 27  from member  14 . 8 - 24 , a user may push on portion  14 . 8 - 60  of latch  14 . 8 - 56  so that latch  14 . 8 - 56  rotates about hinge  14 . 8 - 58 . Protruding portion  14 . 8 - 68  may slide out of recess  14 . 8 - 66 , and headband  14 . 8 - 27  may be removed from member  14 . 8 - 24 . To reattach headband  14 . 8 - 27 , the user may use portion  14 . 8 - 60  to move protruding portion, which may move into position when released by the user. In some embodiments, post  14 . 8 - 30  and/or protruding portion  14 . 8 - 68  may be beveled so that user may insert headband  14 . 8 - 27  into post  14 . 8 - 30  without engaging portion  14 . 8 - 60  of latch  14 . 8 - 56 . 
     Headband  14 . 8 - 26  may attach to post  14 . 8 - 30  directly, as opening  14 . 8 - 32  of headband  14 . 8 - 26  may surround post  14 . 8 - 30 . Headband  14 . 8 - 26  may be removably attached to member  14 . 8 - 24  with a slidable latch member  14 . 8 - 62 M that may be moved with tab  14 . 8 - 62 T. As shown in  FIGS.  14 . 8 - 10   , headband  14 . 8 - 26  may be attached to member  14 . 8 - 24  between member  14 . 8 - 24  and headband  14 . 8 - 27 . 
     Instead of using latch  14 . 8 - 56  to attach headband  14 . 8 - 27  to member  14 . 8 - 24 , an internal structure may be used. An illustrative example in which headband  14 . 8 - 27  is attached to member  14 . 8 - 24  using an internal structure is shown in  FIGS.  14 . 8 - 11   . 
     As shown in  FIGS.  14 . 8 - 11   , headband  14 . 8 - 27  may have protruding portion  14 . 8 - 70  that is inserted into opening  14 . 8 - 72  of post  14 . 8 - 30 . Protruding portion  14 . 8 - 70  be formed from metal, rigid plastic, or other material. In some embodiments, protruding portion  14 . 8 - 70  may be slightly smaller (e.g., less than 1 mm smaller, less than 2 mm smaller, or other suitable difference) than opening  14 . 8 - 72 . By inserting protruding portion  14 . 8 - 70  into opening  14 . 8 - 72 , headband  14 . 8 - 27  may remain attached to member  14 . 8 - 24 . 
     To remove headband  14 . 8 - 27  from, or attach headband  14 . 8 - 27  to, member  14 . 8 - 24 , a user would merely have to apply enough force to overcome the friction between protruding portion  14 . 8 - 70  and opening  14 . 8 - 72 . 
     If desired, additional material, such as material  14 . 8 - 74 , may be added to protruding portion  14 . 8 - 70  to further secure headband  14 . 8 - 27  to member  14 . 8 - 24 . Material  14 . 8 - 74  may be, for example, a polymer material, a gasket, or another suitable material. Post  14 . 8 - 30  may optionally have recessed portion  14 . 8 - 76  that matches the shape of material  14 . 8 - 74  to hold protruding portion  14 . 8 - 70  in place within opening  14 . 8 - 72 . 
     To remove headband  14 . 8 - 27  from, or attach headband  14 . 8 - 27  to, member  14 . 8 - 24 , a user would merely have to apply enough force to overcome the friction between material  14 . 8 - 74  and the sides of opening  14 . 8 - 72  (or the recess  14 . 8 - 76  once inserted). 
     Another attachment mechanism for coupling headband  14 . 8 - 27  to member  14 . 8 - 24  is a twist-to-lock system. An illustrative example of a twist-to-lock system is shown in  FIGS.  14 . 8 - 12     
     As shown in  FIGS.  14 . 8 - 12   , headband  14 . 8 - 27  may have protrusion  14 . 8 - 78 . Member  14 . 8 - 24  may have post  14 . 8 - 30  with opening  14 . 8 - 80 . Protrusion  14 . 8 - 78  may fit through opening  14 . 8 - 80  perpendicularly, and may be twisted into proper position  14 . 8 - 82  once protrusion  14 . 8 - 78  passes through opening  14 . 8 - 80 . Post  14 . 8 - 30  may have an at least partially hollow interior to allow protrusion  14 . 8 - 78  to swivel within post  14 . 8 - 30  into position  14 . 8 - 82 . Once protrusion  14 . 8 - 78  is in position  14 . 8 - 82  within post  14 . 8 - 30 , it may be locked into place. A user may detach headband  14 . 8 - 27  from member  14 . 8 - 24  by twisting headband  14 . 8 - 27  so that protrusion  14 . 8 - 78  is aligned with opening  14 . 8 - 82 . In this way, a twist-to-lock system may attach headband  14 . 8 - 27  to member  14 . 8 - 24 . 
     Although not shown in  FIGS.  14 . 8 - 12   , headband  14 . 8 - 27  and post  14 . 8 - 30  may include magnets to further attach headband  14 . 8 - 27  to member  14 . 8 - 24 . These magnets may be similar to those shown in  FIGS.  14 . 8 - 5 - 14 . 8 - 7   . 
     Moreover, although not shown in  FIGS.  14 . 8 - 12   , headband  14 . 8 - 26  may be between member  14 . 8 - 24  and headband  14 . 8 - 27  prior to attaching headband  14 . 8 - 27  to member  14 . 8 - 24  with the twist-to-lock system. In particular, opening  14 . 8 - 32  of headband  14 . 8 - 26  may surround post  14 . 8 - 30 . Headband  14 . 8 - 26  may be attached with a latch, such as the latch of  FIGS.  14 . 8 - 5   , and/or may be held in place by headband  14 . 8 - 27  once it is attached to post  14 . 8 - 30  with the twist-to-lock system. 
     In some embodiments, headband  14 . 8 - 27  may be attached to member  14 . 8 - 24  and held in place by headband  14 . 8 - 26  attaching to member  14 . 8 - 24  with a latch. An illustrative example of such an embodiment is shown in  FIGS.  14 . 8 - 13   . 
     As shown in  FIGS.  14 . 8 - 13   , member  14 . 8 - 24  may have post  14 . 8 - 30  with shoulder  14 . 8 - 84 . Member  14 . 8 - 24  may also have pins  14 . 8 - 86 , although pins  14 . 8 - 86  may be omitted if desired. Headband  14 . 8 - 27  may have opening  14 . 8 - 88  that is slightly larger (e.g., less than 1 mm larger, less than 2 mm larger, or other suitable difference) than shoulder  14 . 8 - 84  of post  14 . 8 - 30 . Headband  14 . 8 - 27  may be attached to member  14 . 8 - 24  by passing post  14 . 8 - 30  through opening  14 . 8 - 88 , so that headband  14 . 8 - 27  surrounds shoulder  14 . 8 - 84 . 
     To prevent headband  14 . 8 - 27  from detaching from member  14 . 8 - 24 , headband  14 . 8 - 26  may be attached to post  14 . 8 - 30 . In particular, opening  14 . 8 - 32  of headband  14 . 8 - 26  may surround post  14 . 8 - 30 , and headband  14 . 8 - 26  may be removably attached to member  14 . 8 - 24  with a latch in opening  14 . 8 - 32  (as shown in  FIG.  14 . 8 - 5   ) or another mechanism. When the latch is engaged and headband  14 . 8 - 26  is attached to member  14 . 8 - 24 , headband  14 . 8 - 26  will be unable to detach from member  14 . 8 - 24 . 
     Optional pins  14 . 8 - 86  may pass through optional holes  14 . 8 - 90  in headband  14 . 8 - 27 . Pins  14 . 8 - 26  may prevent headband  14 . 8 - 27  from moving/rotating with respect to member  14 . 8 - 24 . In this way, pins  14 . 8 - 26  may maintain headband  14 . 8 - 27  in a desired position on the user&#39;s head when worn. 
     Elastomer  14 . 8 - 92  may surround opening  14 . 8 - 88 . Elastomer  14 . 8 - 92  may be formed from any desired elastomeric material, and may help prevent headband  14 . 8 - 27  from moving with respect to shoulder  14 . 8 - 84  or detaching from post  14 . 8 - 30 . However, elastomer  14 . 8 - 92  may be omitted, if desired. 
     Regardless of whether elastomer  14 . 8 - 92  or pins  14 . 8 - 86  are included in the embodiment of  FIGS.  14 . 8 - 13   , a cross-sectional side view is shown in  FIGS.  14 . 8 - 14   . As shown in  FIG.  14 . 8 - 14   , headband  14 . 8 - 27  may surround shoulder portions  14 . 8 - 84  of post  14 . 8 - 30 . Headband  14 . 8 - 26  may then rest on top of shoulders  14 . 8 - 84  of post  14 . 8 - 30 . In this way, headband  14 . 8 - 27  may be held in place against member  14 . 8 - 24  by headband  14 . 8 - 26 . 
     Although not shown in  FIGS.  14 . 8 - 13  and  14 . 8 - 14   , because headband  14 . 8 - 27  will be directly on member  14 . 8 - 24 , material may be included on member  14 . 8 - 24  to keep headband  14 . 8 - 27  from moving (e.g., swiveling) when the head-mounted device is worn. For example, a gasket, such as a compressible gasket, elastomeric material, or other material may be formed on the surface of member  14 . 8 - 24  that is contacted by headband  14 . 8 - 27  when headband  14 . 8 - 27  is attached to member  14 . 8 - 24 . 
     In some embodiments, headband  14 . 8 - 27  may be attached to member  14 . 8 - 24  with extendable magnets. Illustrative embodiments in which headband  14 . 8 - 27  is attached to member  14 . 8 - 24  with extendable magnets are shown in  FIGS.  14 . 8 - 15 A- 14 . 8 - 17 B . 
     As shown in  FIGS.  14 . 8 - 15 A , post  14 . 8 - 30  may have magnet  14 . 8 - 94  in a stowed (e.g., non-extended) position. Post  14 . 8 - 30  may also include opening  14 . 8 - 96 , which may accommodate a latch mechanism to attach headband  14 . 8 - 26  (e.g., the latch mechanism of  FIGS.  14 . 8 - 5   ). As shown in  FIGS.  14 . 8 - 15 B , magnet  14 . 8 - 94  may extend into an extended position. Magnet  14 . 8 - 94  may include optional recess  14 . 8 - 98 , which may be used to attach headband  14 . 8 - 27  to member  14 . 8 - 24 . 
     As shown in  FIGS.  14 . 8 - 16   , headband  14 . 8 - 27  may have opening  14 . 8 - 100 . Opening  14 . 8 - 100  may receive magnet  14 . 8 - 94  when it is in the expanded position. Illustrative examples of magnet  14 . 8 - 94  expanding and attaching headband  14 . 8 - 27  is shown in  FIGS.  14 . 8 - 17 A and  14 . 8 - 17 B . 
     As shown in  FIGS.  14 . 8 - 17 A , headband  14 . 8 - 27  may be brought into contact with post  14 . 8 - 30  (e.g., with member  14 . 8 - 24 ). Headband  14 . 8 - 27  may have stationary magnet  14 . 8 - 102  within opening  14 . 8 - 100 . As headband  14 . 8 - 27  is moved in direction  14 . 8 - 101 , magnet  14 . 8 - 102  may attract magnet  14 . 8 - 94  in post  14 . 8 - 30 . Eventually, as shown in  FIGS.  14 . 8 - 17 B , magnet  14 . 8 - 94  may be moved into opening  14 . 8 - 100  of headband  14 . 8 - 27  due to the attractive magnetic force of magnet  14 . 8 - 102 . Recess  14 . 8 - 98  of magnet  14 . 8 - 94  may be coupled to portion  14 . 8 - 104  of opening  14 . 8 - 100 , thereby attaching headband  14 . 8 - 27  to post  14 . 8 - 30  with magnet  14 . 8 - 94 . In this way, an extendable magnet may attach headband  14 . 8 - 27  to post  14 . 8 - 30  (and to member  14 . 8 - 24 ). 
     Although the embodiments of  FIGS.  14 . 8 - 2 - 17 B  have described attaching headband  14 . 8 - 27  to member  14 . 8 - 24  in a variety of ways, this is merely illustrative. In general, headband  14 . 8 - 26  or any other headband may be attached to member  14 . 8 - 24  using any of the attachments of  FIGS.  14 . 8 - 2 - 17 B . Alternatively or additionally, although sometimes described herein in the context of examples where posts  14 . 8 - 30  are formed on members  14 . 8 - 24  and openings  14 . 8 - 32  are formed in headband  14 . 8 - 26 , posts  14 . 8 - 30  may, if desired, be formed as part of headband  14 . 8 - 26  and openings  14 . 8 - 32  may be formed in members  14 . 8 - 24 . 
     The foregoing is merely illustrative and various modifications can be made to the described embodiments. The foregoing embodiments may be implemented individually or in any combination. 
     XV: User Interface 
       FIGS.  1 - 1 A to  14 . 6 - 4    illustrate various examples of computer systems that can be used to perform the methods and provide audio, visual and/or haptic feedback as part of user interfaces described herein. In some embodiments, the exemplary computer system includes one or more display generation components (e.g., first and second display assemblies  1 - 120   a ,  1 - 120   b  and/or first and second optical modules  11 . 1 . 1 - 104   a  and  11 . 1 . 1 - 104   b ) for displaying virtual elements and/or a representation of a physical environment to a user of the computer system, optionally generated based on detected events and/or user inputs detected by the computer system. User interfaces generated by the computer system are optionally corrected by one or more corrective lenses  11 . 3 . 2 - 216  that are optionally removably attached to one or more of the optical modules to enable the user interfaces to be more easily viewed by users who would otherwise use glasses or contacts to correct their vision. While many user interfaces illustrated herein show a single view of a user interface, user interfaces in an HMD are optionally displayed using two optical modules (e.g., first and second display assemblies  1 - 120   a ,  1 - 120   b  and/or first and second optical modules  11 . 1 . 1 - 104   a  and  11 . 1 . 1 - 104   b ), one for a user&#39;s right eye and a different one for a user&#39;s left eye, and slightly different images are presented to the two different eyes to generate the illusion of stereoscopic depth, the single view of the user interface would typically be either a right-eye or left-eye view and the depth effect is explained in the text or using other schematic charts or views. In some embodiments, the computer system includes one or more external displays (e.g., display assembly  1 - 108 ) for displaying status information for the computer system to the user of the computer system (when the computer system is not being worn) and/or to other people who are near the computer system, optionally generated based on detected events and/or user inputs detected by the computer system. In some embodiments, the computer system includes one or more audio output components (e.g., electronic component  1 - 112 ) for generating audio feedback, optionally generated based on detected events and/or user inputs detected by the computer system. In some embodiments, the computer system includes one or more input devices for detecting input such as one or more sensors (e.g., one or more sensors in sensor assembly  1 - 356 , and/or  FIG.  1 I ) for detecting information about a physical environment of the device which can be used (optionally in conjunction with one or more illuminators such as the illuminators describe in  FIG.  61   ) to generate a digital passthrough image, capture visual media corresponding to the physical environment (e.g., photos and/or video), or determine a pose (e.g., position and/or orientation) of physical objects and/or surfaces in the physical environment so that virtual objects ban be placed based on a detected pose of physical objects and/or surfaces. In at least one example, the virtual objects can represent structural objects, and can be referred to as virtual structural objects, with which the user can interact via detected hand gestures and other body movement. In one example, the virtual structural objects can include virtual buttons, dials, crowns, keyboards, or other physical input mechanisms and devices represented virtually and manipulatable by the user. In some embodiments, the computer system includes one or more input devices for detecting input such as one or more sensors for detecting hand position and/or movement (e.g., one or more sensors in sensor assembly  1 - 356 , and/or  FIG.  1 I ) that can be used (optionally in conjunction with one or more illuminators such as the illuminators  6 - 124  describe in  FIG.  61   ) to determine when one or more air gestures have been performed. In some embodiments, the computer system includes one or more input devices for detecting an input such as one or more sensors for detecting eye movement (e.g., eye tracking and gaze tracking sensors in  FIG.  1 I ) which can be used (optionally in conjunction with one or more lights such as lights  11 . 3 . 2 - 110  in  FIG.  10   ) to determine attention or gaze position and/or gaze movement, which can optionally be used to detect gaze-only inputs based on gaze movement and/or dwell. A combination of the various sensors described above can be used to determine user facial expressions and/or hand movements for use in generating an avatar or representation of the user such as an anthropomorphic avatar or representation for use in a real-time communication session where the avatar has facial expressions, hand movements, and/or body movements that are based on or similar to detected facial expressions, hand movements, and/or body movements of a user of the device. Gaze and/or attention information is, optionally, combined with hand tracking information to determine interactions between the user and one or more user interfaces based on direct and/or indirect inputs such as air gestures or inputs that use one or more hardware input devices such as one or more buttons (e.g., first button  1 - 128 , button  11 . 1 . 1 - 114 , second button  1 - 132 , and or dial or button  1 - 328 ), knobs (e.g., first button  1 - 128 , button  11 . 1 . 1 - 114 , and/or dial or button  1 - 328 ), digital crowns (e.g., first button  1 - 128  which is depressible and twistable or rotatable, button  11 . 1 . 1 - 114 , and/or dial or button  1 - 328 ), trackpads, touch screens, keyboards, mice and/or other input devices. One or more buttons (e.g., first button  1 - 128 , button  11 . 1 . 1 - 114 , second button  1 - 132 , and or dial or button  1 - 328 ) are optionally used to perform system operations such as recentering content in a three-dimensional environment that is visible to a user of the device, displaying a home user interface for launching applications, starting real-time communication sessions, or initiating display of virtual three-dimensional backgrounds. Knobs or digital crowns (e.g., first button  1 - 128  which is depressible and twistable or rotatable, button  11 . 1 . 1 - 114 , and/or dial or button  1 - 328 ) are optionally rotatable to adjust parameters of the visual content such as a level of immersion of a virtual three-dimensional environment (e.g., a degree to which virtual-content occupies the viewport of the user into the three-dimensional environment) or other parameters associated with the three-dimensional environment and the virtual content that is displayed via the optical modules (e.g., first and second display assemblies  1 - 120   a ,  1 - 120   b  and/or first and second optical modules  11 . 1 . 1 - 104   a  and  11 . 1 . 1 - 104   b ). 
     Viewpoint/View 
     In an augmented reality, mixed reality, or virtual reality environment, a view of a three-dimensional environment is visible to a user. The view of the three-dimensional environment is typically visible to the user via one or more display generation components (e.g., a display or a pair of display modules that provide stereoscopic content to different eyes of the same user) through a virtual viewport that has a viewport boundary that defines an extent of the three-dimensional environment that is visible to the user via the one or more display generation components. In some examples, the region defined by the viewport boundary is smaller than a range of vision of the user in one or more dimensions (e.g., based on the range of vision of the user, size, optical properties or other physical characteristics of the one or more display generation components, and/or the location and/or orientation of the one or more display generation components relative to the eyes of the user). In some embodiments, the region defined by the viewport boundary is larger than a range of vision of the user in one or more dimensions (e.g., based on the range of vision of the user, size, optical properties or other physical characteristics of the one or more display generation components, and/or the location and/or orientation of the one or more display generation components relative to the eyes of the user). The viewport and viewport boundary typically move as the one or more display generation components move (e.g., moving with a head of the user for a head mounted device or moving with a hand of a user for a handheld device such as a tablet or smartphone). A viewpoint of a user determines what content is visible in the viewport, a viewpoint generally specifies a location and a direction relative to the three-dimensional environment, and as the viewpoint shifts, the view of the three-dimensional environment will also shift in the viewport. For a head mounted device, a viewpoint is typically based on a location and direction of the head, face, and/or eyes of a user to provide a view of the three-dimensional environment that is perceptually accurate and provides an immersive experience when the user is using the head-mounted device. For a handheld or stationed device, the viewpoint shifts as the handheld or stationed device is moved, and/or as a position of a user relative to the handheld or stationed device changes (e.g., a user moving toward, away from, up, down, to the right, and/or to the left of the device). For devices that include display generation components with virtual passthrough, portions of the physical environment that are visible (e.g., displayed, and/or projected) via the one or more display generation components are based on a field of view of one or more cameras in communication with the display generation components which typically move with the display generation components (e.g., moving with a head of the user for a head mounted device or moving with a hand of a user for a handheld device such as a tablet or smartphone) because the viewpoint of the user moves as the field of view of the one or more cameras moves (and the appearance of one or more virtual objects displayed via the one or more display generation components is updated based on the viewpoint of the user (e.g., displayed positions and poses of the virtual objects are updated based on the movement of the viewpoint of the user)). For display generation components with optical passthrough, portions of the physical environment that are visible (e.g., optically visible through one or more partially or fully transparent portions of the display generation component) via the one or more display generation components are based on a field of view of a user through the partially or fully transparent portion(s) of the display generation component (e.g., moving with a head of the user for a head mounted device or moving with a hand of a user for a handheld device such as a tablet or smartphone) because the viewpoint of the user moves as the field of view of the user through the partially or fully transparent portions of the display generation components moves (and the appearance of one or more virtual objects is updated based on the viewpoint of the user). 
     Air Gestures 
     In some embodiments, a gesture includes an air gesture. An air gesture is a gesture that is detected without the user touching (or independently of) an input element that is part of a device (e.g., computer system  101 , one or more input device  125 , and/or hand tracking device  140 ) and is based on detected motion of a portion (e.g., the head, one or more arms, one or more hands, one or more fingers, and/or one or more legs) of the user&#39;s body through the air including motion of the user&#39;s body relative to an absolute reference (e.g., an angle of the user&#39;s arm relative to the ground or a distance of the user&#39;s hand relative to the ground), relative to another portion of the user&#39;s body (e.g., movement of a hand of the user relative to a shoulder of the user, movement of one hand of the user relative to another hand of the user, and/or movement of a finger of the user relative to another finger or portion of a hand of the user), and/or absolute motion of a portion of the user&#39;s body (e.g., a tap gesture that includes movement of a hand in a predetermined pose by a predetermined amount and/or speed, or a shake gesture that includes a predetermined speed or amount of rotation of a portion of the user&#39;s body). 
     In some embodiments, input gestures used in the various examples and embodiments described herein include air gestures performed by movement of the user&#39;s finger(s) relative to other finger(s) or part(s) of the user&#39;s hand) for interacting with an XR environment (e.g., a virtual or mixed-reality environment), in accordance with some embodiments. In some embodiments, an air gesture is a gesture that is detected without the user touching an input element that is part of the device (or independently of an input element that is a part of the device) and is based on detected motion of a portion of the user&#39;s body through the air including motion of the user&#39;s body relative to an absolute reference (e.g., an angle of the user&#39;s arm relative to the ground or a distance of the user&#39;s hand relative to the ground), relative to another portion of the user&#39;s body (e.g., movement of a hand of the user relative to a shoulder of the user, movement of one hand of the user relative to another hand of the user, and/or movement of a finger of the user relative to another finger or portion of a hand of the user), and/or absolute motion of a portion of the user&#39;s body (e.g., a tap gesture that includes movement of a hand in a predetermined pose by a predetermined amount and/or speed, or a shake gesture that includes a predetermined speed or amount of rotation of a portion of the user&#39;s body). 
     In some embodiments in which the input gesture is an air gesture (e.g., in the absence of physical contact with an input device that provides the computer system with information about which user interface element is the target of the user input, such as contact with a user interface element displayed on a touchscreen, or contact with a mouse or trackpad to move a cursor to the user interface element), the gesture takes into account the user&#39;s attention (e.g., gaze) to determine the target of the user input (e.g., for direct inputs, as described below). Thus, in implementations involving air gestures, the input gesture is, for example, detected attention (e.g., gaze) toward the user interface element in combination (e.g., concurrent) with movement of a user&#39;s finger(s) and/or hands to perform a pinch and/or tap input, as described in more detail below. 
     Air Tap and Air Pinch 
     In some embodiments, input gestures that are directed to a user interface object are performed directly or indirectly with reference to a user interface object. For example, a user input is performed directly on the user interface object in accordance with performing the input gesture with the user&#39;s hand at a position that corresponds to the position of the user interface object in the three-dimensional environment (e.g., as determined based on a current viewpoint of the user). In some embodiments, the input gesture is performed indirectly on the user interface object in accordance with the user performing the input gesture while a position of the user&#39;s hand is not at the position that corresponds to the position of the user interface object in the three-dimensional environment while detecting the user&#39;s attention (e.g., gaze) on the user interface object. For example, for direct input gesture, the user is enabled to direct the user&#39;s input to the user interface object by initiating the gesture at, or near, a position corresponding to the displayed position of the user interface object (e.g., within 0.5 cm, 1 cm, 5 cm, or a distance between 0-5 cm, as measured from an outer edge of the option or a center portion of the option). For an indirect input gesture, the user is enabled to direct the user&#39;s input to the user interface object by paying attention to the user interface object (e.g., by gazing at the user interface object) and, while paying attention to the option, the user initiates the input gesture (e.g., at any position that is detectable by the computer system) (e.g., at a position that does not correspond to the displayed position of the user interface object). 
     In some embodiments, input gestures (e.g., air gestures) used in the various examples and embodiments described herein include pinch inputs and tap inputs, for interacting with a virtual or mixed-reality environment, in accordance with some embodiments. For example, the pinch inputs and tap inputs described below are performed as air gestures. 
     In some embodiments, a pinch input is part of an air gesture that includes one or more of: a pinch gesture, a long pinch gesture, a pinch and drag gesture, or a double pinch gesture. For example, a pinch gesture that is an air gesture includes movement of two or more fingers of a hand to make contact with one another, that is, optionally, followed by an immediate (e.g., within 0-1 seconds) break in contact from each other. A long pinch gesture that is an air gesture includes movement of two or more fingers of a hand to make contact with one another for at least a threshold amount of time (e.g., at least 1 second), before detecting a break in contact with one another. For example, a long pinch gesture includes the user holding a pinch gesture (e.g., with the two or more fingers making contact), and the long pinch gesture continues until a break in contact between the two or more fingers is detected. In some embodiments, a double pinch gesture that is an air gesture comprises two (e.g., or more) pinch inputs (e.g., performed by the same hand) detected in immediate (e.g., within a predefined time period) succession of each other. For example, the user performs a first pinch input (e.g., a pinch input or a long pinch input), releases the first pinch input (e.g., breaks contact between the two or more fingers), and performs a second pinch input within a predefined time period (e.g., within 1 second or within 2 seconds) after releasing the first pinch input. 
     In some embodiments, a pinch and drag gesture that is an air gesture includes a pinch gesture (e.g., a pinch gesture or a long pinch gesture) performed in conjunction with (e.g., followed by) a drag input that changes a position of the user&#39;s hand from a first position (e.g., a start position of the drag) to a second position (e.g., an end position of the drag). In some embodiments, the user maintains the pinch gesture while performing the drag input, and releases the pinch gesture (e.g., opens their two or more fingers) to end the drag gesture (e.g., at the second position). In some embodiments, the pinch input and the drag input are performed by the same hand (e.g., the user pinches two or more fingers to make contact with one another and moves the same hand to the second position in the air with the drag gesture). In some embodiments, the pinch input is performed by a first hand of the user and the drag input is performed by the second hand of the user (e.g., the user&#39;s second hand moves from the first position to the second position in the air while the user continues the pinch input with the user&#39;s first hand. In some embodiments, an input gesture that is an air gesture includes inputs (e.g., pinch and/or tap inputs) performed using both of the user&#39;s two hands. For example, the input gesture includes two (e.g., or more) pinch inputs performed in conjunction with (e.g., concurrently with, or within a predefined time period of) each other. For example, a first pinch gesture performed using a first hand of the user (e.g., a pinch input, a long pinch input, or a pinch and drag input), and, in conjunction with performing the pinch input using the first hand, performing a second pinch input using the other hand (e.g., the second hand of the user&#39;s two hands). 
     In some embodiments, a tap input (e.g., directed to a user interface element) performed as an air gesture includes movement of a user&#39;s finger(s) toward the user interface element, movement of the user&#39;s hand toward the user interface element optionally with the user&#39;s finger(s) extended toward the user interface element, a downward motion of a user&#39;s finger (e.g., mimicking a mouse click motion or a tap on a touchscreen), or other predefined movement of the user&#39;s hand. In some embodiments a tap input that is performed as an air gesture is detected based on movement characteristics of the finger or hand performing the tap gesture movement of a finger or hand away from the viewpoint of the user and/or toward an object that is the target of the tap input followed by an end of the movement. In some embodiments the end of the movement is detected based on a change in movement characteristics of the finger or hand performing the tap gesture (e.g., an end of movement away from the viewpoint of the user and/or toward the object that is the target of the tap input, a reversal of direction of movement of the finger or hand, and/or a reversal of a direction of acceleration of movement of the finger or hand). 
     Attention 
     In some embodiments, attention of a user is determined to be directed to a portion of the three-dimensional environment based on detection of gaze directed to the portion of the three-dimensional environment (optionally, without requiring other conditions). In some embodiments, attention of a user is determined to be directed to a portion of the three-dimensional environment based on detection of gaze directed to the portion of the three-dimensional environment with one or more additional conditions such as requiring that gaze is directed to the portion of the three-dimensional environment for at least a threshold duration (e.g., a dwell duration) and/or requiring that the gaze is directed to the portion of the three-dimensional environment while the viewpoint of the user is within a distance threshold from the portion of the three-dimensional environment in order for the device to determine that attention of the user is directed to the portion of the three-dimensional environment, where if one of the additional conditions is not met, the device determines that attention is not directed to the portion of the three-dimensional environment toward which gaze is directed (e.g., until the one or more additional conditions are met). Further details of the user interface and user experience are provided below with reference to  FIG.  15 - 1   . 
     In  FIG.  15 - 1   , electronic device  15 . 1 - 700  detects that an interpupillary distance (IPD) setting of electronic device  15 . 1 - 700  does not match an interpupillary distance of a user of electronic device  15 . 1 - 700  (e.g., as detected, estimated, and/or determined by electronic device  15 . 1 - 700 ). For example, in some embodiments, electronic device  15 . 1 - 700  is a head-mounted system that includes two or more optical components (e.g., two or more optical lenses and/or two or more display generation components such as, for example, two or more transparent display generation components), with a first optical component positioned in front of a first eye of the user and a second optical component positioned in front of a second eye of the user. In some embodiments, the IPD setting of electronic device  15 . 1 - 700  corresponds to the distance between the two optical components, and the IPD setting is adjustable to move the two optical components further apart or closer together so that they are positioned correctly relative to the two eyes of the user (e.g., the IPD setting is adjustable to move the two optical components further apart or closer together until the IPD setting of electronic device  15 . 1 - 700  matches the interpupillary distance of the user). At  FIG.  15 - 1   , electronic device  15 . 1 - 700  detects the interpupillary distance of the user (e.g., via sensors  15 . 1 - 707 ), and determines that the IPD setting of electronic device  15 . 1 - 700  does not match the interpupillary distance of the user (e.g., the two optical components should be moved closer together or further apart). In  FIG.  15 - 1   , in response to detecting that the IPD setting of electronic device  15 . 1 - 700  does not match the interpupillary distance of the user (and, optionally, in response to determining that electronic device  15 . 1 - 700  does not satisfy the one or more error conditions), electronic device  15 . 1 - 700  displays user interface  15 . 1 - 712 . In some embodiments, when electronic device  15 . 1 - 700  detects that the IPD setting of electronic device  15 . 1 - 700  matches the interpupillary distance of the user (e.g., the two optical components of electronic device  15 . 1 - 700  are positioned in a predetermined position, or within a predetermined range of positions, relative to the two eyes of the user), electronic device  15 . 1 - 700  forgoes display of user interface  15 . 1 - 712  (and, optionally, displays a different user interface). 
     In  FIG.  15 - 1   , user interface  15 . 1 - 712  includes object  15 . 1 - 714 , which is representative of electronic device  15 . 1 - 700 , and objects  15 . 1 - 716   a - 15 . 1 - 716   b , which are representative of buttons  15 . 1 - 706   a  and  15 . 1 - 706   b , respectively. User interface  15 . 1 - 712  also includes objects  15 . 1 - 718   a - 15 . 1 - 718   c . In some embodiments, one or more of objects  15 . 1 - 718   a - 15 . 1 - 718   c  are representative of physical components of electronic device  15 . 1 - 700 . For example, in some embodiments, object  15 . 1 - 718   a  is representative of a first optical component (e.g., a first optical lens and/or a first display generation component), and object  15 . 1 - 718   b  is representative of a second optical component (e.g., a second optical lens and/or a second display generation component). As will be described and shown in greater detail below, in some embodiments, object  15 . 1 - 718   a  and object  15 . 1 - 718   b  move relative to one another on display  15 . 1 - 702  to indicate movement of the corresponding physical components of electronic device  15 . 1 - 700 . User interface  15 . 1 - 712  also includes arrow  15 . 1 - 720  and prompt  15 . 1 - 722 , which instruct the user to hold down button  15 . 1 - 706   b  (represented in user interface  15 . 1 - 712  by object  15 . 1 - 716   b ) in order to align one or more physical components of electronic device  15 . 1 - 700  (e.g., in order to adjust the IPD setting of electronic device  15 . 1 - 700  and/or to move one or more optical components of electronic device  15 . 1 - 700 ). In  FIG.  15 - 1   , while the IPD setting of electronic device  15 . 1 - 700  is not being adjusted (e.g., while a user does not provide user input to button  15 . 1 - 706   a  and/or button  15 . 1 - 706   b ), electronic device  15 . 1 - 700  outputs audio output  15 . 1 - 711   a . In some embodiments, audio output  15 . 1 - 711   a  is an ambient audio output that indicates that the IPD setting of the electronic device  15 . 1 - 700  is not currently being modified and/or changed. At  FIG.  15 - 1   , electronic device  15 . 1 - 700  detects user input  15 . 1 - 724 , which is a button press of button  15 . 1 - 706   b . In  FIG.  15 - 1   , user input  15 . 1 - 724  is a button press of button  15 . 1 - 706   b . However, in some embodiments, user input  15 . 1 - 724  is a different type of user input (e.g., a gesture, an air gesture, a touch input, a rotation input, a gaze-based input, and/or any combination of the foregoing). In addition, the device  15 . 1 - 700  can include other buttons, including button  15 . 1 - 704 , which can also be pressed, turned, or otherwise actuated to perform similar or different functions to those described above with reference to buttons  15 . 1 - 706   a - b . Buttons  15 . 1 - 706   a - b  and  15 . 1 - 704  can include mechanically depressible buttons, touch sensitive buttons such as capacitive sensing buttons, twistable/rotatable dials such as the crowns or buttons described elsewhere herein, or any other type of button. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  15 - 1    can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown elsewhere and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to figures shown elsewhere can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  15 - 1   . 
     As illustrated in  FIG.  15 - 2 A , while displaying mode control affordance  15 . 2 - 746  having photo mode affordance  15 . 2 - 746 A and video mode affordance  15 . 2 - 746 B, with video mode affordance  15 . 2 - 746 B highlighted and shutter affordance  15 . 2 - 718  with the color fill, computer system  15 . 2 - 700  detects a sixth potential media capture input, such as button press input  15 . 2 - 748 A of hardware button  15 . 2 - 706 , air gesture input  15 . 2 - 748 B, and/or tap input  15 . 2 - 748 C while gaze  15 . 2 - 732  is directed at shutter affordance  15 . 2 - 718  and accordingly initiates media capture. As the video media capture mode is currently selected, computer system  15 . 2 - 700  begins capturing video media. The computer system  15 . 2 - 700  can include an electronic device  15 . 2 - 702 , which in one example can be a part of a head-mountable device or other wearable electronic device including a display screen displaying images. The device  15 . 2 - 700  can include a border  15 . 2 - 714  of a housing or other structural frame, a user interface  15 . 2 - 710  displayed on a display  15 . 2 - 708 . The device  15 . 2 - 702  can be configured to detect eye gaze and hand  15 . 2 - 726  gestures for controlling the user interface  15 . 2 - 710 . The user interface  15 . 2 - 710  of the display  15 . 2 - 708  can include indicators  15 . 2 - 722 A and  15 . 2 - 722 B as well as darkened area  15 . 2 - 716 . The display  15 . 2 - 708  can be a user facing display of the HMDs described herein. Finger  15 . 2 - 724  can be used by the user to depress any number of buttons of the HMD to input commands to alter the user interface  15 . 2 - 710  as described herein. 
     As illustrated in  FIG.  15 - 2 B , while capturing video media, computer system  15 . 2 - 700  ceases display of mode control affordance  15 . 2 - 746  and instead displays video status affordance  15 . 2 - 750  in the lower region of camera viewfinder  15 . 2 - 712 . Video status affordance  15 . 2 - 750  indicates that video media is currently being captured and includes the currently elapsed time of the video capture. Upon beginning the video capture, computer system  15 . 2 - 700  initially displays video status affordance  15 . 2 - 750  opaquely. After a threshold amount of time (e.g., a few seconds) without detecting gaze  15 . 2 - 732  directed at video status affordance  15 . 2 - 750 , computer system  15 . 2 - 700  increases the translucency (e.g., reduces the opacity) of video status affordance  15 . 2 - 750  (e.g., as described above with respect to changing the visual prominence of captured media icon  15 . 2 - 738  and/or mode control affordance  15 . 2 - 746 ). In response to detecting gaze  15 . 2 - 732  directed at video status affordance  15 . 2 - 750 , computer system  15 . 2 - 700  decreases the translucency (e.g., increases the opacity) of video status affordance  15 . 2 - 750 . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  15 - 2 A  and  FIG.  15 - 2 B  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown elsewhere and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to figures shown elsewhere can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  15 - 2 A  and  FIG.  15 - 2 B . 
       FIG.  15 - 3 A  and  FIG.  15 - 3 B  show an example of a computer system  15 . 3 - 101 , which can include a device such as an HMD. The HMD can include rearward facing displays or display screens, similar to the display showing the three-dimensional environment  15 . 3 - 702  illustrated in  FIGS.  15 - 3 A and  15 - 3 B . The device can include one or more image sensors  314 , similar to other sensors and cameras of HMDs shown and described elsewhere herein. The display can show various virtual objects  15 . 3 - a - 15 . 3 - h ,  15 . 3 - 706 ,  15 . 3 - 708 ,  15 . 3 - 732   a  and  15 . 3 - 732   c , as well indicators for thresholds  15 . 3 - 734   a - d  and  15 . 3 - 738   a - b . The display can be configured to project light toward a user&#39;s eyes when donning the HMD and forward facing cameras and sensors can capture the environment external to the device and the display can display those images, as well as various virtual objects, to the user through the display projecting the various virtual objects noted herein and shown in the figures. 
     In some embodiments, if the attention input indicates that the attention of the user has changed away from the gaze virtual object (or corresponding virtual object) and subsequently moved back to the gaze virtual object (or corresponding virtual object), the computer system  15 . 3 - 101  updates the visual indication of progress of the attention of the user directed towards satisfying the one or more criteria for performing an operation associated with a corresponding virtual object based on whether the attention input moved back to the object within a threshold time (e.g., 0.03, 0.05, 0.07, 0.09, 0.1, 0.15, 0.2, 0.25, 0.3, 0.5, 1, 3, 5, 10, 15, 20, or 30 seconds). For example, in  FIG.  15 - 3 A , the visual indication of progress in the gaze target for object  15 . 3 - 704   a  is maintained (e.g., continued from the amount of fill prior to the attention of the user changed away) because the attention of the user (e.g., attention input  15 . 3 - 716 ) changed from being directed away from the gaze virtual object  15 . 3 - 704   a ′ of virtual object  15 . 3 - 704   a  to being directed to the gaze virtual object  15 . 3 - 704   a ′ of virtual object  15 . 3 - 704   a  within the time threshold. In some embodiments, the computer system  15 . 3 - 101  updates the visual indication of progress, different from maintaining the indication of progress as will be described in the following figures. 
     In some embodiments, if the attention input includes an activation input, the computer system  15 . 3 - 101  performs the operation associated with the virtual object. For example, in  FIG.  15 - 3 A , the computer system  15 . 3 - 101  detects an activation input (e.g., finger of hand  15 . 3 - 710  touching trackpad  15 . 3 - 746  and/or an air pinch gesture from hand  15 . 3 - 710 ) before attention  15 . 3 - 716  satisfies the one or more criteria with respect to the gaze virtual object  15 . 3 - 704   a ′ for object  15 . 3 - 704   a  for performing the operation associated with the virtual object; for example, before the duration of the attention input  15 . 3 - 716  directed towards the gaze virtual object  15 . 3 - 704   a ′ associated with virtual object  15 . 3 - 704   a  reaches threshold  15 . 3 - 734   d  as shown by timer  15 . 3 - 734  in  FIG.  15 - 3 A . In response to detecting the activation input before satisfying the one or more criteria (e.g., reaching threshold  15 . 3 - 734   d ), the computer system  15 . 3 - 101  optionally performs the operation associated with the virtual object  15 . 3 - 704   a  such as updating the right side of virtual object  15 . 3 - 704  to include content associated with virtual object  15 . 3 - 704   a  as shown in  FIG.  15 - 3 B . 
     In some embodiments, in response to detecting that recent interactions with computer system include attention-only inputs or non-attention inputs, the computer system  15 . 3 - 101  optionally changes (e.g., shortens or lengthens) the threshold requirement to display gaze virtual objects. For example, after detecting that the attention input includes attention-only inputs (e.g., does not include non-attention inputs, such as inputs from hand  15 . 3 - 710 ), the computer system  15 . 3 - 101  shortens the time threshold for displaying a gaze virtual object for a virtual object to  15 . 3 - 732   b ′, as shown in  FIG.  15 - 3 A . In another example, after detecting that recent interaction with computer system  15 . 3 - 101  includes non-attention inputs, the computer system  15 . 3 - 101  optionally lengthens the time threshold for displaying a gaze virtual object for a virtual object to  738   b , as shown in  FIG.  15 - 3 B . 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  15 - 3 A and  15 - 3 B  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown elsewhere and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to figures shown elsewhere can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  15 - 3 A and  15 - 3 B . 
       FIG.  15 - 4 A  and  FIG.  15 - 4 B  show first and second display generation components  15 . 4 - 7100  and  15 . 4 - 7102 , which can include cameras or sensors  15 . 4 - 7104  and  15 . 4 - 7106 , respectively. The display generation components  15 . 4 - 7100  and  15 . 4 - 7102  be or include head mountable display devices operated in different locations  15 . 4 - 7000 - a  and  15 . 4 - 7000   b , as shown in  FIGS.  15 - 4 A and  15 - 4 B . In at least one example, the HMDs can also include one or more buttons  15 . 4 - 7302  to control the displayed content or other functionalities of the device, as described elsewhere herein. The button  15 . 4 - 7302  can include a dial. In at least one example, the display generation components  15 . 4 - 7100  and  15 . 4 - 7102  can display mixed/virtual reality (MR, VR, or XR) content  15 . 4 - 7002 . Various virtual objects, including a progress bar  15 . 4 - 7004  and overlay  15 . 4 - 7008  can be displayed as shown. 
     In some embodiments, the computer system detects whether criteria for changing the current level of immersion are met, while displaying the XR content via the first display generation component  15 . 4 - 7100  with a respective level of immersion. In response to detecting that the criteria for changing the current level of immersion are met, the computer system changes the level of immersion with which the XR content is displayed via the first display generation component  15 . 4 - 7100 , and changes the appearance of the graphical elements that represent the status associated with the user shown via the second display generation component  15 . 4 - 7102 . For example, as shown in  FIGS.  15 - 4 A- 15 - 4 B , in some embodiments, the computer system determines that the criteria for changing the level of immersion are met in accordance with detection of actions of the second user  15 . 4 - 7204  that is present in the physical environment (e.g., in the location B  15 . 4 - 7000 - b  in  FIG.  15 - 4 A ). For example, in some embodiments, detecting the actions of the second user  15 . 4 - 7204  that cause the computer system to change the current level of immersion (e.g., when the current level of immersion is a fully immersive level, or more than a threshold level of immersion) includes detecting that the second user  15 . 4 - 7204  has moved into a region in front of the second display generation component  15 . 4 - 7102  (e.g., also the region in front of the first user  15 . 4 - 7202  if the first user is wearing the HMD including the first display generation component  15 . 4 - 7100  and the second display generation component  15 . 4 - 7102 ). In some embodiments, detecting the actions of the second user  15 . 4 - 7204  that cause the computer system to change the current level of immersion (e.g., when the current level of immersion is a fully immersive level, or more than a threshold level of immersion) includes detecting that the second user  15 . 4 - 7204  is making a gesture (e.g., waving to the first user  15 . 4 - 7202 , tapping on the shoulder or arm of the first user  15 . 4 - 7202 , or other gestures that calls for the attention of the first user  15 . 4 - 7202 ) to the first user  15 . 4 - 7202 . As shown in  FIG.  15 - 4 A , the second user  15 . 4 - 7204  has moved to a region in front of the second display generation component  15 . 4 - 7102 , is waving to the first user  15 . 4 - 7202 , and/or is calling out the name of the first user  15 . 4 - 7202 ; and upon detecting one or more of the above, the computer system determines that the criteria for changing the current level of immersion are met, in accordance with some embodiments. In Figure M, in response to detecting that the criteria for changing the current level of immersion are met, the computer system changes the way that XR content is displayed via the first display generation component in accordance with the change of the current level of immersion, in accordance with some embodiments. In this example scenario shown in  FIGS.  15 - 4 A- 15 - 4 B , the computer system decreases the level of immersion from a fully immersive level to an intermediate level of immersion, and displays a representation of the physical environment (e.g., including a representation  15 . 4 - 7312  of the physical object  15 . 4 - 7314  in the physical environment (e.g., in location B  15 . 4 - 7000 - b ), and a representation  15 . 4 - 7010  of the second user  15 . 4 - 7204 ) in front of the first user (e.g., in location B  15 . 4 - 7000 - b ) among the XR content displayed via the first display generation component  15 . 4 - 7100 . In addition, when the computer system decreases the level of immersion from a fully immersive level to an intermediate level of immersion, the computer system also displays the representation  15 . 4 - 7006  of the portion of the body of the first user  15 . 4 - 7202  via the second display generation component  15 . 4 - 7102 , in accordance with some embodiments. The representation  15 . 4 - 7006  of the portion of the body of the first user  15 . 4 - 7202  are updated in accordance with the changes in appearance of the first user  15 . 4 - 7202 , and the graphical elements that indicate the status of the XR content and the current level of immersion are updated in accordance with the changes in the status of the XR content and the level of immersion. 
     In at least one example, the computer system can vary a level of engagement between the user and the user interface, including changing the level at which a user can engage with (e.g., manipulate, interact with, move . . . etc.) a virtual object displayed to the user. In at least one example, the computer system can vary a level of engagement between the user and the user interface, including changing the level at which a user can engage with (e.g., manipulate, interact with, move . . . etc.) a 3-D space or 3-D environment, including virtual 3-D spaces displayed to the user. In one example, the levels of engagement between the user and the virtual objects and/or 3-D spaces can be automatically changed based on detected facial features, hand gestures, gaze direction, and so forth. In one example, the levels of engagement between the user and the virtual objects and/or 3-D spaces can be manually adjusted using the buttons, dials, crowns, and other physical interactions between the user and the device having the computer system described herein. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  15 - 4 A and  15 - 4 B  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown elsewhere and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to figures shown elsewhere can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  15 - 4 A and  15 - 4 B . 
       FIGS.  15 - 5 A- 15 - 5 C  illustrate examples of setting levels of immersion for operating system user interfaces as compared with application user interfaces.  FIG.  15 - 5 A  illustrates an electronic device  15 . 5 - 101  displaying, via a display generation component (e.g., a display screen or generation component of a head mountable device), a three-dimensional environment  15 . 5 - 901  in a user interface. The electronic device  15 . 5 - 101  optionally includes a display generation component (e.g., a touch screen) and a plurality of image sensors (e.g., image sensors including cameras of an HMD described herein). The image sensors optionally include one or more of a visible light camera, an infrared camera, a depth sensor, or any other sensor the electronic device  15 . 5 - 101  would be able to use to capture one or more images of a user or a part of the user while the user interacts with the electronic device  15 . 5 - 101 . In some embodiments, the user interfaces shown below could also be implemented on a head-mounted display that includes a display generation component that displays the user interface to the user and sensors to detect the physical environment and/or movements of the user&#39;s hands (e.g., external sensors facing outwards from the user), and/or gaze of the user (e.g., internal sensors facing inwards towards the face of the user). 
     As shown in  FIG.  15 - 5 A , device  15 . 5 - 101  captures one or more images of the physical environment  15 . 5 - 900  around device  15 . 5 - 101 , including one or more objects in the physical environment  15 . 5 - 900  around device  15 . 5 - 101 . In some embodiments, device  15 . 5 - 101  displays representations of the physical environment in three-dimensional environment  15 . 5 - 901 . For example, three-dimensional environment  15 . 5 - 901  includes a representation  15 . 5 - 902   b  of a tree (e.g., corresponding to tree  15 . 5 - 902   a  in physical environment  15 . 5 - 900 ) and a representation  15 . 5 - 904   b  of a person (e.g., corresponding to person  15 . 5 - 904   a  in physical environment  15 . 5 - 900 ). In some embodiments, device  15 . 5 - 101  also displays one or more virtual objects (e.g., objects that are not in the physical environment  15 . 5 - 900 ) in the three-dimensional environment  15 . 5 - 901 . For example, in  FIG.  15 - 5 A , device  15 . 5 - 101  is displaying virtual object  15 . 5 - 910  (e.g., an ornament) on representation  15 . 5 - 902   b  of a tree, virtual object  15 . 5 - 906  (e.g., a hat) on representation  15 . 5 - 904   b  of a person, and representation  15 . 5 - 908  of the sun. In some embodiments, representation  15 . 5 - 908  of the sun is displayed and treated as a light source in the three-dimensional environment  15 . 5 - 901 , and the lighting effects resulting from representation  15 . 5 - 908  of the sun are applied, by device  15 . 5 - 101 , to representations of physical objects and/or virtual objects displayed in three-dimensional environment  15 . 5 - 901 . 
     In  FIG.  15 - 5 A , device  15 . 5 - 101  is also displaying a user interface  15 . 5 - 912  of the operating system of the device  15 . 5 - 101 . User interface  15 . 5 - 912  is optionally an application browsing user interface of the operating system of the device  15 . 5 - 101  from which one or more applications that are accessible on device  15 . 5 - 101  can be displayed/launched. In some embodiments, user interface  15 . 5 - 912  is any other user interface of the operating system of the device  15 . 5 - 101  (e.g., as compared with being a user interface of an application on device  15 . 5 - 101 ). In  FIG.  15 - 5 A , user interface  15 . 5 - 912  includes icons corresponding to different applications that are accessible on device  15 . 5 - 101  (e.g., an icon for App A, an icon for App B, an icon for App C, and an icon for App D), which are selectable to display the respective selected application via the display generation component  15 . 5 - 120  of device  15 . 5 - 101 . In at least one example, the display generation component  15 . 5 - 120  can be a display module or screen of a head mountable device configured to project light toward a user&#39;s eyes when the user dons the head mountable device. The button  15 . 5 - 920  can be similar to or the same as the crowns, dials, and other buttons shown and described on other head mountable devices described herein and shown in other figures. 
     In some embodiments, user interface  15 . 5 - 912  is displayed at a location within three-dimensional environment  15 . 5 - 901  such that it overlays one or more representations of physical objects and/or virtual objects in three-dimensional environment  15 . 5 - 901  (e.g., overlaying and in front of part of representation  15 . 5 - 904   b  of the person, corresponding to person  15 . 5 - 904   a  in physical environment  15 . 5 - 900 ). In some embodiments, user interface  15 . 5 - 912  is located within three-dimensional environment  15 . 5 - 901  such that one or more virtual objects are in front of and overlay part of user interface  15 . 5 - 912  and/or one or more representations of physical objects are in front of and overlay part of user interface  15 . 5 - 912 . 
     In some embodiments, device  15 . 5 - 101  displays three-dimensional environment  15 . 5 - 901  and/or user interfaces displayed via display generation component  15 . 5 - 120  at a particular level of immersion. In  FIG.  15 - 5 A , device  15 . 5 - 101  is displaying three-dimensional environment  15 . 5 - 901  at a particular level of immersion  15 . 5 - 918  (e.g., indicated on immersion scale  15 . 5 - 916 , the left-most side of which corresponds to no immersion, and the right-mode side of which corresponds to maximum immersion) at which representations  15 . 5 - 902   b  and  15 . 5 - 904   b  of physical objects are visible via display generation component  15 . 5 - 120 . In some embodiments, a level of immersion includes an associated degree to which the content displayed by the electronic device obscures background content (e.g., content other than the user interface  15 . 5 - 912 ) around/behind the user interface  15 . 5 - 912 , optionally including the number of items of background content displayed and the visual characteristics (e.g., colors, contrast, opacity) with which the background content is displayed. In some embodiments, the background content is included in a background over which the user interface  15 . 5 - 912  is displayed. In some embodiments, the background content includes additional user interfaces (e.g., user interfaces generated by the device  15 . 5 - 101  corresponding to applications, system user interfaces, etc.), virtual objects (e.g., files, representations of other users, etc. generated by the device  15 . 5 - 101 ) not associated with or included in the user interface  15 . 5 - 912 , and/or real objects (e.g., pass-through objects representing real objects in the physical environment of the electronic device  15 . 5 - 101  that are displayed by the device such that they are visible via the display generation component). In some embodiments, at a first (e.g., low) level of immersion, the background, virtual and/or real objects are displayed in an unobscured manner. For example, a respective user interface with a low level of immersion is optionally displayed concurrently with the background content, which is optionally displayed with full brightness, color, and/or translucency. In some embodiments, at a second (e.g., higher) level of immersion, the background, virtual and/or real objects are displayed in an obscured manner (e.g., dimmed, blurred, removed from display, etc.). For example, a respective user interface with a high level of immersion is displayed without concurrently displaying the background content (e.g., in a full screen or fully immersive mode). As another example, a user interface displayed with a medium level of immersion is displayed concurrently with darkened, blurred, or otherwise de-emphasized background content. In some embodiments, the visual characteristics of the background objects vary among the background objects. For example, at a particular immersion level, one or more first background objects are visually de-emphasized (e.g., dimmed, blurred, displayed with increased transparency) more than one or more second background objects, and one or more third background objects cease to be displayed. 
     In  FIG.  15 - 5 A , an input is received on input element  15 . 5 - 920  of device  15 . 5 - 101 . Input element  15 . 5 - 920  is optionally a mechanical input element that can be manipulated to change the level of immersion of the user interface(s) currently-displayed by device  15 . 5 - 101 , such as a slider element or a rotational input element, where the amount and direction (e.g., increase or decrease) of the change in immersion is based on the magnitude and direction of the manipulation of the input element  15 . 5 - 920  (e.g., the direction and/or magnitude of the movement of input element  15 . 5 - 920 , or the direction and/or magnitude of the rotation of a rotatable input element). In some embodiments, device  15 . 5 - 101  and/or input element  15 . 5 - 920  generate tactile feedback as the level of immersion is changes (e.g., minor tactile outputs for each increases or decreases stage of immersion, and different major tactile outputs upon reaching minimum or maximum immersion). 
     In  FIG.  15 - 5 A , the input received on input element  15 . 5 - 920  is an input to slide the input element  15 . 5 - 920  upward to increase the level of immersion of the currently displayed user interface(s) by an amount based on the amount of movement of element  15 . 5 - 920  upward. In response to the input in  FIG.  15 - 5 A , device  15 . 5 - 101  increases the level of immersion at which operating system user interface  15 . 5 - 912  is displayed to an intermediate level of immersion, as shown in  FIG.  15 - 5 B  and immersion scale  15 . 5 - 916 . In particular, in some embodiments, increasing the level of immersion at which user interface  15 . 5 - 912  is displayed causes device  15 . 5 - 101  to display user interface at a larger size via display generation component  15 . 5 - 120 , as shown in  FIG.  15 - 5 B . Further, as described previously, additionally or alternatively, the content displayed surrounding and/or behind user interface  15 . 5 - 912  (e.g., representations of physical objects  15 . 5 - 902   b  and  15 . 5 - 904   b , and virtual objects  15 . 5 - 910  and  15 . 5 - 906 ) is darkened and/or blurred in  FIG.  15 - 5 B , while user interface  15 . 5 - 912  is not darkened or blurred. In some embodiments, increasing the level of immersion at which the currently-displayed user interface(s) is displayed additionally or alternatively increases atmospheric visual and/or audio effects associated with virtual or physical objects displayed via display generation component  15 . 5 - 120 . For example, in  FIG.  15 - 5 B , the rays of light being emitted from representation  15 . 5 - 908  of the sun have been lengthened (e.g., corresponding to more virtual light being emitted from representation  15 . 5 - 908 ), and ornament  15 . 5 - 910  on representation  15 . 5 - 902   b  of the tree has begun to shine as a result. Additional or alternative changes in atmospheric lighting, visual, audio, etc. effects in conjunction with changes in levels of immersion are similarly contemplated. 
     As previously mentioned, in some embodiments, the levels of immersion at which different user interfaces are displayed are optionally set independently from one another. For example, changes in the level of immersion at which operating system user interfaces are displayed optionally do not change the level of immersion at which application user interfaces are displayed, and in some embodiments, changes in the level of immersion at which the user interface of a first application is displayed do not change the level of immersion at which the user interface of a second application is displayed. Thus, in some embodiments, in response to an input to switch from displaying one user interface to displaying another user interface, device  15 . 5 - 101  optionally displays the switched-to user interface at the level of immersion at which that user interface was last displayed, independent of any changes of immersion that were applied to the currently-displayed user interface. 
     For example, in  FIG.  15 - 5 B , device  15 . 5 - 101  detects an input selecting the icon corresponding to App C in user interface  15 . 5 - 912 . In response, device  15 . 5 - 101  displays user interface  15 . 5 - 934  of App C, as shown in  FIG.  15 - 5 C . App C is optionally an application that is accessible via (e.g., installed on) device  15 . 5 - 101 , and is optionally any type of application (e.g., a content viewing application, a word processing application, etc.). App C is optionally different from the operating system of device  15 . 5 - 101 , and thus user interface  15 . 5 - 934  is optionally not a user interface of the operating system of device  15 . 5 - 101 , but rather a user interface of App C. 
     As shown in  FIG.  15 - 5 C , in response to the input to display App C, device  15 . 5 - 101  is displaying user interface  15 . 5 - 934  at a level of immersion  15 . 5 - 932  that is higher than the level of immersion  15 . 5 - 918  at which device  15 . 5 - 101  was displaying user interface  15 . 5 - 912  in  FIG.  15 - 5 B  (e.g., as indicated by immersion scale  15 . 5 - 930  for App C). This is optionally because, as previously described, the levels of immersion set for applications are optionally independent of the level of immersion set for the operating system of device  15 . 5 - 101 . Therefore, it is optionally the case that the higher level of immersion at which device  15 . 5 - 101  is displaying user interface  15 . 5 - 934  of App C in  FIG.  15 - 5 C  is the level of immersion at which user interface  15 . 5 - 934  was last displayed by device  15 . 5 - 101 . If user interface  15 . 5 - 934  had last been displayed at a lower level of immersion than the level of immersion  15 . 5 - 918  for user interface  15 . 5 - 912  shown in  FIG.  15 - 5 B , device  15 . 5 - 101  would have optionally displayed user interface  15 . 5 - 934  in  FIG.  15 - 5 C  at that lower level of immersion rather than at the higher level of immersion shown in  FIG.  15 - 5 C . Also shown in  FIG.  15 - 5 C  is the increased size of user interface  15 . 5 - 934  on display generation component  15 . 5 - 120  (as compared with the size of user interface  15 . 5 - 912  in  FIG.  15 - 5 B ) and the increased darkening and/or blurring of the content outside of/surrounding user interface  15 . 5 - 934  (as compared with the darkening and/or blurring of the content outside of/surrounding user interface  15 . 5 - 912  in  FIG.  15 - 5 B )—these changes in display characteristics are optionally some of the changes that occur in response to increased immersion, as previously described. Additional or alternative changes in display characteristics, such as those previously described, are also contemplated. 
     As previously described, changes in immersion at which a user interface in one application is displayed optionally do not change the level of immersion at which user interfaces of other applications and/or user interfaces of the operating system are displayed. For example, if device  15 . 5 - 101  detects an input to change the level of immersion of user interface  15 . 5 - 934  of App C in  FIG.  15 - 5 C  (e.g., via input element  15 . 5 - 920 ), device  15 . 5 - 101  would change the level of immersion at which user interface  15 . 5 - 934  is displayed in accordance with that input, as previously described. After changing the level of immersion at which user interface  15 . 5 - 934  is displayed, in response to an input to redisplay user interface  15 . 5 - 912  (e.g., an input to cease display of user interface  15 . 5 - 934 ), device would optionally return to the display of  FIG.  15 - 5 B  in which user interface  15 . 5 - 912  is displayed at level of immersion  15 . 5 - 918 , which is the level of immersion at which user interface  15 . 5 - 912  was last displayed, and is independent from the changed level of immersion at which user interface  15 . 5 - 934  was displayed when the input to redisplay user interface  15 . 5 - 912  was detected. 
     In some embodiments, applications (e.g., App C) include controls in their user interfaces for changing the level of immersion at which those applications are displayed. For example, in some embodiments, user interface  15 . 5 - 934  of App C includes controls for increasing or decreasing the level of immersion at which user interface  15 . 5 - 934  is displayed, and the immersion at which App C is displayed is changed by device  15 . 5 - 101  in response to interaction with those controls additionally or alternatively to interaction with input element  15 . 5 - 920 . Further, in some embodiments, immersion above a threshold (e.g. maximum immersion) for an operating system user interface is reachable in response to inputs detected at input element  15 . 5 - 920 . However, in some embodiments, immersion above that threshold for an application user interface is not reachable in response to inputs detected at input element  15 . 5 - 920 —in some embodiments, the immersion at which an application user interface is displayed can only reach the threshold immersion using the input element, and once at that threshold, a different type of input is required to increase the immersion at which the application user interface is displayed beyond that threshold. For example, in  FIG.  15 - 5 C , the level of immersion  15 . 5 - 932  at which user interface  15 . 5 - 934  is displayed is optionally the highest immersion that can be reached for user interface  15 . 5 - 934  via inputs at input element  15 . 5 - 920 . To increase immersion past the level of immersion  15 . 5 - 932 , device  15 . 5 - 101  optionally requires an input selecting a user interface element shown in user interface  15 . 5 - 934 . For example, in  FIG.  15 - 5 C , user interface  15 . 5 - 934  includes user interface element  15 . 5 - 950  that is selectable to increase the immersion at which user interface  15 . 5 - 934  is displayed past level of immersion  15 . 5 - 932  (e.g., a user input selecting user interface element  15 . 5 - 950  optionally results in device  15 . 5 - 101  displaying user interface  15 . 5 - 934  at maximum immersion, or full screen, where no content surrounding user interface  15 . 5 - 934  is displayed by device  15 . 5 - 101 ). In some embodiments, user interface  15 . 5 - 934  only includes element  15 . 5 - 950  once the level of immersion at which user interface  15 . 5 - 934  is displayed has reached the threshold level of immersion previously described. In some embodiments, user interface  15 . 5 - 934  includes element  15 . 5 - 950  irrespective of whether the level of immersion at which user interface  15 . 5 - 934  is displayed has reached the threshold level of immersion previously described. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIG.  15 - 5 A- 15 - 5 C  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown elsewhere and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to figures shown elsewhere can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIG.  15 - 5 A- 15 - 5 C . 
       FIG.  15 - 6 A  shows application user interface  15 . 6 - 11002  displayed in three-dimensional environment  15 . 6 - 11000 ,  15 . 6 - 7128  that includes representation  15 . 6 - 7014 ′ of physical object  15 . 6 - 7014  (e.g., a physical table), representations  15 . 6 - 7004 ′ and  15 . 6 - 7006 ′ of physical walls  15 . 6 - 7004  and  15 . 6 - 7006 , respectively, and representation  15 . 6 - 7008 ′ of physical floor  15 . 6 - 7008 . In some embodiments, both computer-generated virtual objects that do not correspond to any specific objects in the physical environment (e.g., box  15 . 6 - 7016 ) and representations of objects in the physical environment are displayed to a user. 
     In some embodiments, as shown in  FIG.  15 - 6 A , application user interface  15 . 6 - 11002  provides media content to a user. For example, application user interface  15 . 6 - 11002  is a video player that includes a display of playback controls. In some embodiments, physical environment is a spatially constrained space (e.g., indoor space) surrounded by walls, physical objects, or other obstructions. Features of physical environment can be visible in the three-dimensional environment  15 . 6 - 11000  (e.g., an XR environment) displayed to a user, as shown in  FIG.  15 - 6 A , by the presence of representations  15 . 6 - 7004 ′ and  15 . 6 - 7006 ′ of physical walls  15 . 6 - 7004  and  15 . 6 - 7006 , representation  15 . 6 - 7008 ′ of physical floor  15 . 6 - 7008 , and representation  15 . 6 - 7014 ′ of physical object  15 . 6 - 7014  (e.g., a table). In some embodiments, computer generated virtual content, such as box  15 . 6 - 7016 , which has no correspondence in the physical environment is also displayed to a user by computer system. 
     As shown in  FIG.  15 - 6 A , a first portion of the three-dimensional environment  15 . 6 - 11000  includes computer-generated virtual objects that are not present in the physical environment, and a second portion of the virtual three-dimensional environment includes representations of objects in physical environment displayed as a part of three-dimensional environment  15 . 6 - 11000 .  FIG.  15 - 6 A  shows a first level of immersion generated by the display generation component of computer system in which a display of the XR environment concurrently includes a passthrough portion of a physical environment of computer system, virtual content from the application user interface  15 . 6 - 11002 , and computer-generated virtual content, such as box  15 . 6 - 7016 , that is distinct from the content provided by application user interface  15 . 6 - 11002 . 
     In some embodiments, when a level of immersion is increased, an amount of input signals from the physical environment that user receives is reduced. For example, reducing inputs from the physical environment by not displaying representations  15 . 6 - 7004 ′,  15 . 6 - 7006 ′, and  15 . 6 - 7008 ′ to a user, and/or displaying computer generated virtual content that provide visual inputs to a user to simulate a more spacious environment (e.g., open fields, outdoor spaces, or outer space) than that offered by physical environment, increases a level of immersion that is provided to a user in the three-dimensional environment  15 . 6 - 11000 . In some embodiments, increasing a level of immersion that is provided to a user decreases an influence the physical environment has on user. 
     User may wish to be more fully immersed in the content of the media that is provided through application user interface  15 . 6 - 11002 . Without exiting the application user interface  15 . 6 - 11002 , user provides rotational inputs to rotational input element  15 . 6 - 7108  (e.g., by turning rotational input element  15 . 6 - 7108  or  15 . 6 - 7508 ). The input elements  15 . 6 - 7108  and  15 . 6 - 7508  can be similar to the buttons, crowns, dials, and other input mechanisms of head mountable display devices described herein. In some embodiments, user provides a first rotational input to rotational input element  15 . 6 - 7108  by turning in a first rotational direction (e.g., a clockwise rotation, or a counter-clockwise rotation). The first rotational direction changes a level of immersion presented by the display generation component (e.g., increases a level of immersion, or decreases a level of immersion). For example, by turning in a first rotational direction that increases the level of immersion, passthrough portions of physical environment are displayed with lower fidelity and/or fewer passthrough portions of physical environment are displayed (e.g., some passthrough portions of the experience cease to be displayed) than before the first rotational input. For example, representation  15 . 6 - 7014  ceases to be displayed, as shown in  FIG.  15 - 6 B . Instead, computer generated content, shown by dash lines, is presented as virtual content  15 . 6 - 11004  to a user. 
     In addition to increasing a level of immersion during a media consumption experience (e.g., watching a video, or listening to music), the level of immersion can also be increased to help user&#39;s concentration and focus while working. For example, increasing the level of immersion reduces noises from the ambient physical environment, and/or content from a smaller number of applications (e.g., a single application, a word processing application) is presented to a user (e.g., notifications from other applications are blocked while user is using a particular application at a high level of immersion). 
     Increasing a proportion of virtual (e.g., computer generated) content that is not a representation of the physical environment, increases a level of immersion presented to a user. Virtual content refers generally to content that is distinct from representations of the physical world (e.g., physical environment). For example, presenting a larger number of computer-generated virtual content, such as box  15 . 6 - 7016 , which has no correspondence in the physical environment, increases a level of immersion represented to a user. In some embodiments, as the immersion level increases, computer generated content previously presented at a lower immersion level continues to be displayed (e.g., maintaining display of computer-generated content as level of immersion increases). 
     In some embodiments, a level of immersion presented to a user is related to an associated size (e.g., a magnitude) of a spatial extent (e.g., an angular range) of a field of view within which computer-generated content is displayed. At a lower level of immersion, computer generated virtual content is displayed within a smaller field of view. At a higher level of immersion, computer generated virtual content is displayed within and covers a larger field of view. 
     For example, in  FIG.  15 - 6 A , although box  15 . 6 - 7016  is computer-generated virtual content, it appears in a peripheral portion of a field of view of user. In contrast, as shown in  FIG.  15 - 6 B , in response to detecting a rotational input to the rotatable input element  15 . 6 - 7108  along the first direction, the passthrough portion showing representation  15 . 6 - 7014  ceases to be displayed, and a first portion (e.g., central portion) of the user&#39;s field of view is replaced with computer-generated virtual content  15 . 6 - 11004 . In some embodiments, the computer-generated virtual content occupies an extended portion of user&#39;s field of view, starting in the central portion of user&#39;s field of view. 
     In some embodiments, the central portion of the user&#39;s field of view coincides with a middle region of the application user interface  15 . 6 - 110002 . In some embodiments, a level of immersion has an associated angle of view, which is an angular size of a view cone of the field of view within which computer-generated virtual content is displayed. A higher level of immersion has a larger associated angle of view, and a lower level of immersion has a smaller associated angle of view. 
     In some embodiments, for the level of immersion shown in  FIG.  15 - 6 B , the angle of view is about #10°. For example, for an angle of view of about +10°, computer generated virtual content is displayed, from a perspective of user, starting at an angle of about 10° to the left, to about 10° to the right, and from about 10° above, to about 10° below an axis of user (e.g., an axis lying in a sagittal plane of user, for example, the axis of user projects forward, perpendicularly from a front surface plane of user&#39;s body). In some embodiments, computer generated virtual content occupies (e.g., completely occupies) the field of view of user within the angle of view. In some embodiments, computer generated virtual content partially occupies a portion of the field of view of user within the angle of view, but no input from the physical environment of computer system is provided, within the angle of view, in the field of view of user. 
     In some embodiments, decreasing a level of immersion involves changing a level of immersion from an initial virtual reality (VR) environment in which no passthrough portion of a physical environment of the computer system is displayed to a first immersion level that includes a display of the XR environment. In some embodiments, a highest level of immersion for a three-dimensional environment is a virtual reality environment in which no passthrough portion of the physical environment is provided. 
     Level of immersion influences a user&#39;s perception experience by changing properties of a mixed reality three-dimensional environment. Changing a level of immersion changes a relative prominence of virtual content to content from the physical world (visual and/or audio). For example, for audio components, increasing an immersion level includes, for example, increasing noise cancellation, increasing a spatiality of spatial audio associated with the XR environment (e.g., by moving audio sources to more points around the user or increasing a number and/or volume of point sources of audio), and/or by increasing a volume of audio associated with the virtual environment. In some embodiments, increasing a level of immersion changes a degree to which the mixed-reality environment reduces (or eliminates) signals from the physical world that are presented to the user (e.g., audio and/or visual passthrough of a portion of the physical environment of the computer system). For example, increasing an immersion level includes increasing a proportion of the visual field of view that displays the virtual content, or decreasing a prominence of a representation of the real world (e.g., physical environment) by dimming, fading, or reducing an amount of the representation of the real world that is displayed to the user. 
     Changing a level of immersion can also include changing a visual presentation of the mixed-reality environment, including an extent of a field of view and a degree to which visibility of the external physical environment is reduced. Changing a level of immersion can include varying a number or extent of sensory modalities that a user can use to interact with the mixed-reality three-dimensional environment (e.g., interacting through user&#39;s voice, gaze, and body motion). Changing a level of immersion can also include changing an extent of a fidelity and a resolution with which the mixed-reality environment simulates a desired environment. Changing a level of immersion can also include modifying an extent to which a viewpoint of the mixed-reality environment is modified to match a user&#39;s viewpoint or perspective, e.g., through capture of the user&#39;s motion and timely adjustment of portions of the three-dimensional environment that lie within a field of view. 
     At the level of immersion shown in  FIG.  15 - 6 B , some visual inputs from the physical environment are still provided to a user. In some embodiments, the representations  15 . 6 - 7004 ′,  15 . 6 - 7006 ′, and  15 . 6 - 7008 ′ are provided at a lower fidelity (e.g., less focused, with less contrast, or more monochromatic) in the level of immersion shown in  FIG.  15 - 6 B  compared to the level of immersion shown in  FIG.  15 - 6 A . Thus, while inputs from the physical environment are still provided to a user, the visibility of the external physical environment is reduced at a higher level of immersion compared to a lower level of immersion. 
     Rotatable input mechanism  15 . 6 - 7108 , in addition to changing a level of immersion presented to a user, is also able to receive one or more press inputs that cause the computer system (e.g., a wearable device) to perform various operations, as described in Table 3 below. 
     The use of a single input device (e.g., rotatable input mechanism  15 . 6 - 7108 ) that accepts two (or more) different types of input (e.g., rotational inputs as a first type of input, and/or press inputs as a second type of input) reduces the number of distinct input devices that have to be provided to request or instruct performance of different functionalities. Reducing the number of input devices that have to be provided reduces manufacturing costs of the computer system, and reduces the number of components in the computer system that can fails. Reducing the number of components reduces the cost and complexity of manufacturing the computer system, and increases the reliability of the computer system. Reducing the number of input devices also reduces physical clutter on the computer system, freeing up more physical space on the computer system and helps to prevent accidental inputs from inadvertent contacts. 
     Table 15-1 below describes the behavior of the computer system in response to different operations on button  15 . 6 - 7108  (e.g., a hardware button, a solid-state button), in accordance with some embodiments. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 15-1 
               
               
                   
               
               
                 Number of 
                   
                 Device On  
                 Device  
               
               
                 presses 
                 Device On (Worn) 
                 (Not Worn) 
                 Off 
               
               
                   
               
             
            
               
                 1 press 
                 Show home UI or 
                 Standby (temporary 
                 N/A 
               
               
                   
                 passthrough or  
                 tracking state) 
                   
               
               
                   
                 exit full screen 
                   
                   
               
               
                 2 presses 
                 Force quit menu 
                 N/A 
                 N/A 
               
               
                 3 presses 
                 Accessibility mode 
                 N/A 
                 N/A 
               
               
                 Turn 
                 Change immersion 
                 N/A 
                 N/A 
               
               
                 Press and 
                 Re-center (fade out  
                 N/A 
                 N/A 
               
               
                 hold 
                 and fade in) 
               
               
                   
               
            
           
         
       
     
     For a computer system that is a wearable device (e.g., a head-mounted device, a strapped-on device, or a watch), while the wearable device is turned on and worn on the body of user, in response to detecting a rotational input to rotatable input element  15 . 6 - 7108  (e.g., a bidirectional rotatable input element), a level of immersion presented by the wearable device changes in the manner described above in reference to  FIGS.  15 - 6 A and  15 - 6 B . For example, a level of immersion increases when a rotational input in the first rotational direction is provided to rotatable input element  15 . 6 - 7108 . In another example, when the wearable device is turned on but not worn on the body of a user (e.g., user), in response to detecting a rotational input to rotatable input element  15 . 6 - 7108 , no operation is triggered (e.g., the wearable device remains in a sleep state). 
     For a computer system that is a wearable device (e.g., a head-mounted device, a strapped-on device, or a watch), while the wearable device is turned on and worn on the body of user, in response to detecting a single press input to rotatable input element  15 . 6 - 7108 , a home menu user interface is presented to a user, or an application user interface that was in an immersive or full-screen display mode when the single press input was detected exits the full-screen or immersive display mode. Thus, the wearable device is configured to perform different operations depending on a type of user input provided to the rotatable input element  15 . 6 - 7108  (e.g., a press input, or a rotational input). 
     For a computer system that is a wearable device (e.g., a head-mounted device, a strapped-on device, or a watch), while the wearable device is turned on but not worn on the body of user, in response to detecting a single press input to rotatable input element  15 . 6 - 7108 , the wearable device transitions from a sleep state into a standby state. 
     For a computer system that is a wearable device (e.g., a head-mounted device, a strapped-on device, or a watch), while the wearable device is turned on and worn on the body of user, in response to detecting two press inputs within a preset time interval (e.g., less than 3 seconds, less than 2 seconds, less than 1 second) to rotatable input element  15 . 6 - 7108 , an operating system menu (e.g., a force quit menu) is presented to a user. 
     For a computer system that is a wearable device (e.g., a head-mounted device, a strapped-on device, or a watch), while the wearable device is turned on and worn on the body of user, in response to detecting three press inputs within a preset time interval (e.g., less than 5 seconds, less than 3 seconds, less than 2 second) to rotatable input element  15 . 6 - 7108 , an option to enter an accessibility mode is presented to a user. 
     In some embodiments, the rotatable input mechanism  15 . 6 - 7108  is also used to re-center a field of view of user. For example, instead of having a central portion of the view of field of view of user be aligned with a middle portion of the application user interface  15 . 6 - 11002 , user recenters a central portion of his field of view to a different location in the three-dimensional environment  15 . 6 - 11000 . For example, the new center of user&#39;s field of view corresponds to a point along an intersection of representations  15 . 6 - 7004 ′ and  15 . 6 - 7006 ′. In some embodiments, recentering a field of view includes a display of computer generated virtual content fading out of the user&#39;s field of view at the previous center location of the user&#39;s field of view and then fading in, at the newly defined center location of the field of view of user, computer generated virtual content in that region. Optionally, the virtual content is presented with a higher fidelity or displayed with a higher contrast than prior to the recentering of the field of view of the user. While the wearable device is turned on and worn on the body of user, a press input on the rotatable input mechanism  15 . 6 - 7108  that is held (e.g., persists) for a preset time duration (e.g., about 2 second, or about 5 second) causes the wearable device to begin the re-centering operation described above. While the press input is held, user can rotate or move her head so a central portion of her field of view is re-positioned to a new location. Upon selecting the new location, releasing the press input recenters the central portion of user&#39;s field of view to the new location. 
     As shown in Table 3, different numbers of inputs (e.g., individual or sequential inputs) of a type distinct from rotational inputs (e.g., press inputs) cause different operations to be performed by computer system. For example, in some embodiments, for a single press input: (1) a home menu user interface is displayed, (2) a passthrough portion of the physical environment is provided, or (3) the application exits a full-screen mode. For two press inputs provided in close succession (e.g., within 3 seconds, within 2 seconds, within 1 second), a force quit menu is displayed. For three press inputs provided in close succession (e.g., within 5 seconds, within 3 seconds, within 2 seconds), an accessibility mode is activated, or an option to activate the accessibility mode is provided. 
     Using the number of press inputs to determine which operation(s) to perform reduces the number of distinct input devices that have to be provided to accomplish different. Reducing the number of input devices that have to be provided reduces physical clutter on the device, freeing up more physical space on the device and helps to prevent accidental inputs from inadvertent contacts. Reducing the number of input devices also reduces the need to provide additional hardware wiring within the device, and instead, the processor can be programmed to interpret more types of inputs (e.g., based on a number of press inputs) from a particular input device. 
     The use of a rotational input mechanism allows the user to easily provide a range of inputs, which may be a continuous range or a range that encompasses a sequence of discrete steps or values, and bidirectionality of the rotational input mechanism allows the input to be easily and intuitively varied, in either direction, without having to display additional controls to the user. The same rotational input mechanism  15 . 6 - 7108  is able to receive a second type of input (e.g., a press input) that requests or instructs performance of discrete functions (e.g., dismiss or display a user interface object). Reducing the number of input devices that have to be provided reduces physical clutter on the device, freeing up more physical space on the device and helps to prevent accidental inputs from inadvertent contacts. The use of the rotational input mechanism provides direct access to changes in immersion levels and the performance of different operations, reducing the amount of time needed to effect particular outcomes (e.g., the user does not have to navigate through menus or visually displayed control elements to make a selection for performing the operation and/or changing an immersion level), thereby improving operational efficiency of the computer system. 
     With respect to immersion level, increasing an immersion level can helps to lessen constraints imposed by the physical environment of the computer system. For example, a more spacious virtual or XR environment can be realistically simulated by blocking out sensory out inputs from the physical environment (e.g., blocking visual input of a small/confined room, removing (audio) echoes from a small physical space) to provide a virtual environment that is more conducive for the user to interact with an application in the three-dimensional environment  15 . 6 - 11000 . 
     User further increases a level of immersion from the level of immersion shown in  FIG.  15 - 6 B  by applying a further rotational input in the same direction (e.g., the first rotational direction) as that applied to increase the level of immersion from that shown in  FIG.  15 - 6 A  to  FIG.  15 - 6 B . In response to a second rotational input, the immersion level associated with display of the XR environment generated by the display generation component shown in  FIG.  15 - 6 B , increases to a second immersion level by displaying additional virtual content to a user. 
     Any of the features, components, and/or parts, including the arrangements and configurations thereof shown in  FIGS.  15 - 6 A and  15 - 6 B  can be included, either alone or in any combination, in any of the other examples of devices, features, components, and parts shown elsewhere and described herein. Likewise, any of the features, components, and/or parts, including the arrangements and configurations thereof shown and described with reference to figures shown elsewhere can be included, either alone or in any combination, in the example of the devices, features, components, and parts shown in  FIGS.  15 - 6 A and  15 - 6 B . 
     While a number of user interface features and experiences that can be achieved with the system described in  FIGS.  1 - 1 A to  14 . 6 - 4    are provided above, additional user input systems and methods can operate in conjunction with the exemplary systems and methods in a variety of ways. According to one exemplary embodiment, the user can dynamically change the level of immersion by actuating an input. For example, a user can manipulate a input to gradually increases the level of immersion of the AR experience as an input progresses. This gradual change in the immersion can include, but is in no way limited to, a switch between a plurality of levels of immersion, a smooth animation between levels of immersion, the display of a window from AR space to VR space, where light from VR space is cast onto objects in AR space. Additionally, any number of motions can be detected and cause the initiation of a designated action, including, but in no way limited to, a user swipe to change a view, a swipe to change lighting, a horizontal swipe to change location or time of day while a vertical swipe changes level of immersion (and vice-versa), or a detection of a progression of swipes, such as a first swipe to expand windows, a second swipe that removes a wall, and a third swipe removes multiple walls and a roof in the displayed environment, as an example. Other detected input variations can also be used to modify the user&#39;s experience with the HMD, including, but in no way limited to, detecting repeated inputs to change levels of immersion, detecting a single continuous input to change through multiple levels of immersion, detecting a relatively long press to get navigation menu (geographic location, time of day, level of immersion). Similarly, as the AR/VR environment is being adjusted or presented, different combinations of removal and immersion can be performed, such as the floor remains even when the walls disappear, some furniture remains in place even when walls disappear, and/or a detected air swipe over by the user can change content, while a detected air swipe up or down changes a level of immersion. Additional input motions and resulting system outputs can be incorporated by the exemplary system. 
     Additionally, as noted above, some examples of the HMD include a set of power straps that include a speaker system, or can be in communication with earphones or another separate audio device. According to some examples, the audio output style can change or otherwise be modified to correspond to a level of immersion. According to one example, the power strap can be designed to produce spatial audio. As the dial or button  1 - 328  is actuated (either by pushing, pulling, rotating, etc.), the actuation can cause a dedicated audio change or setting. By way of example, if the user is watching a movie on the HMD, the system can provide stereo audio when movie is displayed in frame, and can provide surround sound audio when movie is displayed in an immersive environment. Furthermore, in some examples the audio level corresponds to a change in a level of immersion (e.g. window slide down, walls slide away, etc.). 
     In other examples, the effect of the level of immersion can be related to the operating system only. For example, the system can use dedicated control to adjust a level of immersion for the operating system, and according to this example, applications have their own immersion controls (while the operating system is in a first level of immersion, a user can enter an application, adjust the immersion in the application, and then return to the operating system that is at the first level of immersion). According to one example of this configuration, the Digital Crown can be a dedicated control wherein the immersion varies based on a direction and/or magnitude of rotation of the Digital Crown. In some examples, major haptics can be disposed at either end of the crown, and minor haptics can be located throughout the dial of the crown to provide desired haptic effects. In some examples, the system can gradually change the level of immersion in the first experience in accordance with a user input (e.g., based on magnitude and/or direction of the user input). Additionally, application controls can be different for different applications, and can be displayed in the user interface. Furthermore, in some examples, the level of immersion can include directing amount of passthrough content that is replaced. The level of immersion include an amount of de-emphasis of passthrough content or an amount of display area occupied with the corresponding experience. In some examples the level of immersion also controls or corresponds to atmospheric effects, and/or audio effects. In some examples, the controls enable a 360 degree experience. The video content provided to the user can be in either a VR or an AR environment. 
     In some examples, the user will leave a first application and enter a second application, each having a different level of immersion. According to one example, when the user toggles between the applications, an adjustment to a level of immersion in a first application, will not affect the second application or the system as a whole. If the user returns to the first application (or the second application) after exiting to the operating system, the system will return to the level of immersion set in the application previously (e.g., without regard to any changes in level of immersion for the operating system). Similarly, the system can remember the user&#39;s level of immersion when toggling between AR/VR modes. For example, while displaying a first experience with a first level of immersion, the system can detect an input to adjust the level of immersion, increase to a second level of immersion selected based on user input, and detect a request to reduce to a low level of immersion. Upon resuming the experience, the user can resume at the second level of immersion. 
     Additionally, the present system includes various levels of immersion and passthrough. According to one example, a low level of immersion is essentially a passthrough. In one example, a level of immersion can be adjusted by rotating a digital crown (e.g., based on direction and/or magnitude of rotation). In some examples, a request to reduce to a low level of immersion can be a press of the digital crown (e.g., in a direction perpendicular to the direction of rotation). Similarly, in some examples, rotating the crown can resume the experience from the second level of immersion, or can resume the experience gradually increasing the level of immersion from the low level of immersion (e.g., not from the second level of immersion). In addition to the crown rotation, any number of gestures can be used to trigger the system to resume an experience. Similarly, a long press that exceeds a threshold amount of time can cause the system to initiate a passthrough setting, and a release of the long press can cause the system to go back out of passthrough mode. 
     In some examples, when displaying a simulated environment, the system can use a first transition if the physical environment meets a first criteria, and can use a second transition if the physical environment meets a second criteria. According to this example, the first criteria can be that a portion of the physical environment in front of the user is more than a threshold distance away from a viewpoint of the user. The second criteria can be that a portion of the physical environment in front of the user is less than the threshold distance away from the viewpoint of the user. According to some embodiments, the environment can replace at least a portion of a view of the physical environment, and the transition can be gradual based on user input, such as a rotation of the crown. In some examples, the beginning and end states can be the same, but the intermediate states can be different. In some examples, the first transition can take into account depth information about a respective portion of the environment and the second transition does not take into account depth information about the respective portion of the environment. The first transition can be a cross dissolve starting with a portion of the simulated environment that is furthest away from a viewpoint of the user, and the second transition can be a transition that gradually encompasses portions of the physical environment based on a distance from the viewpoint of the user to the portions of the physical environment. According to these examples, portions of the room (e.g., architecture or furniture) that are “in front of” the portion of the physical environment that has been taken over by the simulated environment can be shown in front of the simulated environment (e.g., occlude portions of the simulated environment) and portions of the room (e.g., architecture or furniture) that are “behind” the portion of the physical environment that has been taken over by the simulated environment can be hidden by the simulated environment (e.g., are occluded by portions of the simulated environment). In some examples, the environment includes light sources that shine onto portions of the physical environment, the environment can take into account the geometry of the room, the system can take into account the architecture of the room (location of walls/windows/doors, etc.), the environment can include volumetric effects that fill space within the physical environment, can include lighting effects, and/or particle effects. 
     In some examples, the present system includes a spatial effect (environments/atmosphere) immersion adjustment. According to this example, when initiating a spatial effect, the spatial effect can be in an intermediate immersive state, detect an input, and if the input is a first input, increase immersion in spatial effect if the input is a second input, decrease immersion in spatial effect. In some examples, the spatial effect can be an environment that replaces at least a portion of the view of the physical space. In some examples, the system can detect if the user looks to the side and can, in response to the detected motion, fade the audio component of the environment. The user can also, in some examples, choose extent of visuals for maximum level of immersion (e.g., between 180 degree immersion and 360 degree immersion as maximum level of immersion). Alternatively, the application can designate an extent of visuals for maximum level of immersion (e.g., between 180 degree immersion and 360 degree immersion as maximum level of immersion). 
     In some examples, the spatial effect is an atmosphere that modifies at least a portion of the view of the physical space (e.g., with lighting or particle effects). In some examples, the system can detect the user look to the side to fade the atmosphere effect. In some examples, there are more levels of immersion for environments than atmospheres. Additionally, displaying the atmosphere can include replacing something outside of the room (e.g., ceiling, imagery outside of windows), can include increasing an amount of light in the room, and/or can include reducing the amount of light in the room. 
     Any number of visual indicators can be used to communicate the level of immersion, including, but in no way limited to a graphical indicator. According to this example, major indicators can indicate major levels of immersion, and minor indicators can indicate minor levels of immersion. In some examples, the first major level of immersion can be audio only, the second major level of immersion can be audio and some visuals, and the third major level can include audio and expanded visuals. 
     In some examples, a threshold distance is established by the system such that if a user moves more than a threshold distance while displaying an environment, the system can cease to display the environment. According to this example, the environment gradually fades out as the user approaches the threshold distance. In some examples, if the user moves more than a threshold distance while displaying an environment, the system can cease to display the environment/if displaying an atmosphere, maintain display of the atmosphere. 
     In one example, the system can detect the motion of the user and adjust the user interface accordingly. According to one example, when the user looks to the side or back, the system can add duck audio for the environment. In response to a detected movement, the system can reduce spatial audio, reduce noise cancellation, and or reduce complexity of audio (e.g., number of tracks). Similarly, when the user looks to the side or back, the system can fade out some of the visuals of the environment. For example, the system can feather cutout around user&#39;s feet. 
     In some examples, the system can include an environment breakthrough where the system will reveal a portion of the physical environment during use. According to one example, the environment appears to approach a user as the user increases immersion. Further, in some examples, the environment rotates based on body movement, but not based on head movement. For example, a virtual environment is viewable from a particular region, and when a user shifts position toward a boundary of the region, the system can reveal a portion of the physical environment to the user. Alternatively, the system can reduce a level of immersion in the environment when revealing the portion of the physical environment; can reveal the portion of the physical environment without regard to the presence or absence of obstacles; and/or if user moves away from the boundary, cease to display the portion of the physical environment. In some examples, the amount of the physical environment that is revealed can be based on the distance from the boundary, the speed of movement toward the boundary, and/or the acceleration of the user. In some examples, if the user continues moving toward a boundary, the system can cease to display the virtual environment. In some examples, the physical environment is not displayed, such as if the user is just looking around, or if the user moves toward the boundary but is not within a threshold distance of the boundary. In some examples, the system uses spatial transition to reveal physical space. In some examples, the breakthrough is not applied to atmospheres. Similarly, if a user leaves a virtual environment when an application is in a virtual environment, the system can cease to display the application. If, however, the user leaves a virtual environment when the application is outside the virtual environment, the system can continue to display the application on the system. Alternatively, if the user leaves a virtual environment when the application is in the virtual environment, the system can reduce the size of the application and move into physical space. 
     Additionally, the present system can, in some examples, set time and day for the system, to be used with different applications. In one example, when a user requests to display an environment, the system can pick a time of day for the environment based on a system setting (e.g., if time of day has a first setting, environment is displayed with a first time of day, if time of day has a second setting, environment is displayed with second time of day). In some examples the user can select the time of day manually or automatically by the device. In some examples, the selection can be based on a current time of the day, or alternatively, can be selected based on a lighting level of the room. In some examples, the system can provide the user with access to a control center that includes setting inputs for setting the time of day, using any number of system controls. According to this example, the control center can fade from display after the time of day is set. Additionally, according to one example, when displaying an environment picker user interface, the system can update displayed icons to represent current time of day (e.g., daytime or nighttime); can display different simulated times of day for different nighttime environments (e.g., dusk, midnight, or predawn); and can display different simulated times of day for different daytime environments (e.g., morning, afternoon, or early evening). According to one embodiment, the system can include a night mode that darkens passthrough for portions of the field of view of the user that are showing passthrough content. According to one example, if an atmosphere is currently selected, the system can disable time of day picker. Similarly, the time of day setting can be ignored for some applications/experiences (e.g., mindfulness and other contemplative experiences). In some examples, changing the time of day setting can affect UI outside of the app/experience. Further variations can also include, but are in no way limited to, setting different degrees of passthrough dimming for different times of day (e.g., more dimming for nighttime, less dimming for daytime); changing immersion in environment with a physical control or a control center setting; allowing access to settings for environment even if environment is hidden (e.g., at 0% immersion); and/or displaying the environment volume control. 
     In at least one example, when joining a communication session while in a simulated environment with a user in a different environment, the devices described herein can maintain a simulated environment. The first user can see a representation of the second user in the first user&#39;s environment and the second user can see a representation of the first user in the second user&#39;s environment. In one example, when predetermined criteria are met, the environment(s) can be shared between users. In such an example, the environment can automatically switch when one user switches, the switch can occur when a user accepts a prompt to switch environments, the switch can occur after the environment is downloaded after a user accepts a prompt to switch environments, and/or the switch can occur after the new environment has been fully downloaded. 
     In one example, the current environment can be shared from an information panel. The system/user-interface of the HMD can prompt the user to share the environment with other users if the user switches while in a communication session. An environmental picker can indicate which environment is currently being shared with the user. The environment picker can indicate which environments have been downloaded. The user-interface can present an option to switch for all users or just switch for themselves. Switching environments can be based on a user request that switches the system environment even when a communication session ends, switching the environment based on a prompt to share other user&#39;s environment may not switch the system environment. In one example, if a user switches the environment while not in a communication session, the user may not be prompted to share with others. 
     In one example, the user-interface can prompt a user to join an environment of another user and the other user has the option to decline or accept. In one example, an even can be based on a user switching the environment. In one example, an even can be based on a user starting to share content. In one example, if a user declines to switch environments, that user can leave the sharing session. In one example, when content sharing sessions end, the user-interface can revert back to the original environment. In one example, when users are in a shared environment, the users can have shared spatial truth. In one example, the users can be seeing different portions of the environment when there is a shared environment. In one example, there is no spatial truth for even if both users have selected the same environment if the users are only in a shared environment and if they are not in a shared environment. 
     In one example, when switching to a shared environment, the current immersion level can be maintained. In one example, when switching to a shared environment, if the level of immersion is below a threshold, the current immersion level may not be maintained. In one example, different users can set different immersion levels. In one example, different users can set different volume levels. In one example, different users can set different immersion levels while not in full screen viewing mode, but if the content is changed to full screen viewing mode, all the user can have immersion set to at least a threshold level (e.g., the same level or a level that is a “high” level). In one example, and if users are sharing an environment, the time of day is synchronized, if users aren&#39;t sharing an environment, the time of day is not synchronized. In one example, any user can change the time of day. In one example, other users can receive a notice of the change of time of day. 
     In one example, while participating in a communication session and the environment changes, and if a first set of conditions is met, the user-interface can change the environment for multiple users in the communication session. If the first set of conditions are not met, the environment for different users remain different from each other. The first set of conditions can include a user confirmation that the environment should be shared. In one example, when a user displays an environment, the user is displayed at a predefined orientation. In one example, if the user joins a communication session with a shared environment, the user can be placed at a different orientation. In one example, the different orientation can be based on the positions of other participants in the environment. The different orientation can be based on the number of other participants in the environment. In one example, when an activity of the communication session changes, the participants can be shifted based on the selected activity. In one example, the participants can be shifted to face the same direction for shared visual experience (e.g., video). In one example, the participants can be shifted to face different directions (e.g., toward each other) for a participatory experience (e.g., a game). 
     In at least one example, the immersion state can be maintained for different participants and a notification can be displayed when the shared environment is changed. The notification can indicate who changed the environment. In one example, other users may need download the environment before displaying the environment. In one example, multiple participants (e.g., all participants) can be placed in a center of the environment, and in some examples facing different directions. In one example, one user can see other users at locations that are different from where those users see themselves. In one example, the time of day for the environment can be synchronized between different users. In one example, when one user changes the time of day, the time of day can change for multiple or all users. 
     In one example, the user can pick a new environment from session UI for communication session (e.g., with audio/video start/stop option, call end option, and/or user status information). In one example, the user can pick a new environment from home UI (e.g., with multiple environment options, option to display available apps, and/or option to display available people for a communication session). The UI can highlight around a currently selected environment. Different color highlights can be used for shared environments vs. non-shared environments. In one example, the home UI can be displayed over representations of one or more other participants. In one example, new participants can be added to the conversation. 
     In one example, while the user is in a first viewing angle viewing an environment from a first angle, the system can detect a predetermined event in accordance with a determination that the perspective of the user meets respective criteria, change a viewing angle of the environment in accordance with a determination that the perspective of the user doesn&#39;t meet the respective criteria, and maintain the viewing angle of the environment. In one example, the angle of the environment can determine a location of the portal into the environment. In one example, the predetermined even can be dragging a window. In one example, the predetermined even can be detecting a change in an orientation of the user. In one example, the change in orientation can be a change in orientation of the user&#39;s body. In one example, the change in orientation can be a change in orientation of the user&#39;s head or a change in orientation of a portion of the user relative to the horizon of the environment or relative to the horizon of the physical world. In one example, the viewing angle for a different type of change in perspective of the user is not shifted (e.g., turning left/right as opposed to tilting with respect to the horizon). In one example, the predetermined event includes detecting a request to change a position of content. In one example, the predetermined event includes detecting a request to re-center content. These can be carried out be pressing a hardware button or by activating a software button. The software button can be displayed in response to a determination that the perspective of the user has changed. 
     In one example, a horizon of the environment remains consistent as the viewing angle of the environment changes. In one example, a degree of immersion of the environment remains consistent as the viewing angle of the environment changes. In one example, the environment can be viewed through a portal in the real world and the location of the portal can shift. In one example, when the environment is shifted, content that was at a first distance into the environment is shifted in distance relative to the users, for example shifted closer or content was docked at a predefined location and is turned into a window that can be repositioned at a user defined position. In one example, if a first kind of content was displayed with the environment, the first kind of content can shift to maintain the same or similar angle relative to the perspective of the user and if a second kind of content was displayed with the environment (e.g., user-positionable windows), that content can remain at a world locked position. 
     In one example, another predetermined event can be detected after the perspective of the user shifts again and in response, the viewing angle of the environment can be shifted. In one example, if the perspective of the user shifts back, the viewing angle of the environment can shift back. If the perspective of the user shifted to a different perspective, the viewing angle of the environment can shift to a different viewing angle. If a docked window was undocked when the viewing angle shifted the first time, the window can move back to the docked location. 
     In one example, a mechanical or software button of the HMD can be pressed once to go to home UI and again to hide home UI. In one example, while displaying an application, a button can be pressed to replace at least a portion of the application with a home UI. When pressed again, the home UI can be hidden. In one example, home UI can include representations of people, environments, and apps. Hiding home UI can include replacing home UI with passthrough display. In one example, hiding home UI can include ceasing to display a virtual environment in which the home UI is display. The home UI can be used to switch which virtual environment is displayed. IN one example, if an application is a media application with media playing, the UI can display a min-player, including video PiP, audio player, including representation of video or audio content, playback controls. In one example, the mini-player can persist after home UI is hidden. 
     In one example, when the home UI is displayed, the user can scroll through home UI and the user can navigate through sections of the UI (e.g., environments, people, and/or apps). In one example, a current section can be maintained when the user leaves and returns to home UI. In one example, the current selection can be maintained if the user returns to home UI within a time threshold and resets to a predetermined section (e.g., first page of applications) if the user returns to home UI after the time threshold (e.g., the next day, next session, or one hour). In one example, a quick look object (e.g., an object dragged out of an application) can remain when the application is dismissed. In one example, the quick look object is an object pulled out of the application before it was replaced with the home UI. In one example, the quick look objects hide when home UI is hidden (e.g., on a second button press). In one example, a second application can be launched from home UI while the quick look object is displayed. In one example, the quick look object can be dragged by the user in the UI into a second application to perform an operation in the second application based on the quick look object (e.g., added to a message or document). 
     In one example, the user can press the home button again to re-display the home UI. In one example, the button is a hardware button. In one example, the button is a solid state button. In one example, the system UI can be treated as the same as was the application (e.g., a button press while they are displayed can cause the system UI to be replaced with the home UI). In one example, a quick double press of the button can be a command to get an application management UI (e.g., force quit menu or multitasking UI). 
     In one example, the system can detect a press input on a button. If the device is displaying an immersive experience of an application, the UI can display the application in a non-immersive mode. If the device is displaying a non-immersive experience, the UI can replace the non-immersive experience with the home UI. In one example, the button can be pressed again during a non-immersive experience to cease to display the home UI. In one example, the non-immersive experience can be displayed in a virtual environment that continues to be displayed after the home button is pressed. In one example, virtual environment continues to be displayed while the home UI is displayed. In one example, the home UI can be used to switch which virtual environment is displayed. 
     In one example, the rotatable input mechanism of the HMD can be used to perform a UI immersion level change operation when a first input type is detected and a different operation when a second input type is detected. For example, the change in immersion can be based on a rotation of the mechanism, a direction of rotation, an amount of rotation, and so forth. In one example, the first input type is a rotation and the second input type is a depression of the mechanism. In one example, the number of presses can change what operation is performed. In one example, the duration of the press can change what operation is performed. In one example, the second operation can be a home display operation. In one or more other examples, the second operation can include an application dismiss operation, a virtual object dismiss operation, an application manager UI display operation, an accessibility mode operation, reentering virtual UI elements in the AR/VR environment, and/or an operation that can be used in accordance with another input device to perform a third operation. In one or more other examples, the third operation can depend on the duration and/or pattern of inputs and can include taking a screenshot, powering off the device, restarting the device, and/or entering a hardware reset mode. 
     As described above, one aspect of the present technology is the gathering and use of information such as information from input-output devices. The present disclosure contemplates that in some instances, data may be gathered that includes personal information data that uniquely identifies or can be used to contact or locate a specific person. Such personal information data can include demographic data, location-based data, telephone numbers, email addresses, twitter ID&#39;s, home addresses, data or records relating to a user&#39;s health or level of fitness (e.g., vital signs measurements, medication information, exercise information), date of birth, username, password, biometric information, or any other identifying or personal information. 
     The present disclosure recognizes that the use of such personal information, in the present technology, can be used to the benefit of users. For example, the personal information data can be used to deliver targeted content that is of greater interest to the user. Accordingly, use of such personal information data enables users to calculated control of the delivered content. Further, other uses for personal information data that benefit the user are also contemplated by the present disclosure. For instance, health and fitness data may be used to provide insights into a user&#39;s general wellness, or may be used as positive feedback to individuals using technology to pursue wellness goals. 
     The present disclosure contemplates that the entities responsible for the collection, analysis, disclosure, transfer, storage, or other use of such personal information data will comply with well-established privacy policies and/or privacy practices. In particular, such entities should implement and consistently use privacy policies and practices that are generally recognized as meeting or exceeding industry or governmental requirements for maintaining personal information data private and secure. Such policies should be easily accessible by users, and should be updated as the collection and/or use of data changes. Personal information from users should be collected for legitimate and reasonable uses of the entity and not shared or sold outside of those legitimate uses. Further, such collection/sharing should occur after receiving the informed consent of the users. Additionally, such entities should consider taking any needed steps for safeguarding and securing access to such personal information data and ensuring that others with access to the personal information data adhere to their privacy policies and procedures. Further, such entities can subject themselves to evaluation by third parties to certify their adherence to widely accepted privacy policies and practices. In addition, policies and practices should be adapted for the particular types of personal information data being collected and/or accessed and adapted to applicable laws and standards, including jurisdiction-specific considerations. For instance, in the United States, collection of or access to certain health data may be governed by federal and/or state laws, such as the Health Insurance Portability and Accountability Act (HIPAA), whereas health data in other countries may be subject to other regulations and policies and should be handled accordingly. Hence different privacy practices should be maintained for different personal data types in each country. 
     Despite the foregoing, the present disclosure also contemplates embodiments in which users selectively block the use of, or access to, personal information data. That is, the present disclosure contemplates that hardware and/or software elements can be provided to prevent or block access to such personal information data. For example, the present technology can be configured to allow users to select to “opt in” or “opt out” of participation in the collection of personal information data during registration for services or anytime thereafter. In another example, users can select not to provide certain types of user data. In yet another example, users can select to limit the length of time user-specific data is maintained. In addition to providing “opt in” and “opt out” options, the present disclosure contemplates providing notifications relating to the access or use of personal information. For instance, a user may be notified upon downloading an application (“app”) that their personal information data will be accessed and then reminded again just before personal information data is accessed by the app. 
     Moreover, it is the intent of the present disclosure that personal information data should be managed and handled in a way to minimize risks of unintentional or unauthorized access or use. Risk can be minimized by limiting the collection of data and deleting data once it is no longer needed. In addition, and when applicable, including in certain health related applications, data de-identification can be used to protect a user&#39;s privacy. De-identification may be facilitated, when appropriate, by removing specific identifiers (e.g., date of birth, etc.), controlling the amount or specificity of data stored (e.g., collecting location data at a city level rather than at an address level), controlling how data is stored (e.g., aggregating data across users), and/or other methods. 
     Therefore, although the present disclosure broadly covers use of information that may include personal information data to implement one or more various disclosed embodiments, the present disclosure also contemplates that the various embodiments can also be implemented without the need for accessing personal information data. That is, the various embodiments of the present technology are not rendered inoperable due to the lack of all or a portion of such personal information data. 
     Physical environment: A physical environment refers to a physical world that people can sense and/or interact with without aid of electronic systems. Physical environments, such as a physical park, include physical articles, such as physical trees, physical buildings, and physical people. People can directly sense and/or interact with the physical environment, such as through sight, touch, hearing, taste, and smell. 
     Computer-generated reality: in contrast, a computer-generated reality (CGR) environment refers to a wholly or partially simulated environment that people sense and/or interact with via an electronic system. In CGR, a subset of a person&#39;s physical motions, or representations thereof, are tracked, and, in response, one or more characteristics of one or more virtual objects simulated in the CGR environment are adjusted in a manner that comports with at least one law of physics. For example, a CGR system may detect a person&#39;s head turning and, in response, adjust graphical content and an acoustic field presented to the person in a manner similar to how such views and sounds would change in a physical environment. In some situations (e.g., for accessibility reasons), adjustments to characteristic(s) of virtual object(s) in a CGR environment may be made in response to representations of physical motions (e.g., vocal commands). A person may sense and/or interact with a CGR object using any one of their senses, including sight, sound, touch, taste, and smell. For example, a person may sense and/or interact with audio objects that create 3D or spatial audio environment that provides the perception of point audio sources in 3D space. In another example, audio objects may enable audio transparency, which selectively incorporates ambient sounds from the physical environment with or without computer-generated audio. In some CGR environments, a person may sense and/or interact only with audio objects. Examples of CGR include virtual reality and mixed reality. 
     Virtual reality: A virtual reality (VR) environment refers to a simulated environment that is designed to be based entirely on computer-generated sensory inputs for one or more senses. A VR environment comprises a plurality of virtual objects with which a person may sense and/or interact. For example, computer-generated imagery of trees, buildings, and avatars representing people are examples of virtual objects. A person may sense and/or interact with virtual objects in the VR environment through a simulation of the person&#39;s presence within the computer-generated environment, and/or through a simulation of a subset of the person&#39;s physical movements within the computer-generated environment. 
     Mixed reality: In contrast to a VR environment, which is designed to be based entirely on computer-generated sensory inputs, a mixed reality (MR) environment refers to a simulated environment that is designed to incorporate sensory inputs from the physical environment, or a representation thereof, in addition to including computer-generated sensory inputs (e.g., virtual objects). On a virtuality continuum, a mixed reality environment is anywhere between, but not including, a wholly physical environment at one end and virtual reality environment at the other end. In some MR environments, computer-generated sensory inputs may respond to changes in sensory inputs from the physical environment. Also, some electronic systems for presenting an MR environment may track location and/or orientation with respect to the physical environment to enable virtual objects to interact with real objects (that is, physical articles from the physical environment or representations thereof). For example, a system may account for movements so that a virtual tree appears stationery with respect to the physical ground. Examples of mixed realities include augmented reality and augmented virtuality. 
     Augmented reality: an augmented reality (AR) environment refers to a simulated environment in which one or more virtual objects are superimposed over a physical environment, or a representation thereof. For example, an electronic system for presenting an AR environment may have a transparent or translucent display through which a person may directly view the physical environment. The system may be configured to present virtual objects on the transparent or translucent display, so that a person, using the system, perceives the virtual objects superimposed over the physical environment. Alternatively, a system may have an opaque display and one or more imaging sensors that capture images or video of the physical environment, which are representations of the physical environment. The system composites the images or video with virtual objects, and presents the composition on the opaque display. A person, using the system, indirectly views the physical environment by way of the images or video of the physical environment, and perceives the virtual objects superimposed over the physical environment. As used herein, a video of the physical environment shown on an opaque display is called “pass-through video,” meaning a system uses one or more image sensor(s) to capture images of the physical environment, and uses those images in presenting the AR environment on the opaque display. Further alternatively, a system may have a projection system that projects virtual objects into the physical environment, for example, as a hologram or on a physical surface, so that a person, using the system, perceives the virtual objects superimposed over the physical environment. An augmented reality environment also refers to a simulated environment in which a representation of a physical environment is transformed by computer-generated sensory information. For example, in providing pass-through video, a system may transform one or more sensor images to impose a select perspective (e.g., viewpoint) different than the perspective captured by the imaging sensors. As another example, a representation of a physical environment may be transformed by graphically modifying (e.g., enlarging) portions thereof, such that the modified portion may be representative but not photorealistic versions of the originally captured images. As a further example, a representation of a physical environment may be transformed by graphically eliminating or obfuscating portions thereof. 
     Augmented virtuality: an augmented virtuality (AV) environment refers to a simulated environment in which a virtual or computer generated environment incorporates one or more sensory inputs from the physical environment. The sensory inputs may be representations of one or more characteristics of the physical environment. For example, an AV park may have virtual trees and virtual buildings, but people with faces photorealistically reproduced from images taken of physical people. As another example, a virtual object may adopt a shape or color of a physical article imaged by one or more imaging sensors. As a further example, a virtual object may adopt shadows consistent with the position of the sun in the physical environment. 
     Hardware: there are many different types of electronic systems that enable a person to sense and/or interact with various CGR environments. Examples include head mounted systems, projection-based systems, heads-up displays (HUDs), vehicle windshields having integrated display capability, windows having integrated display capability, displays formed as lenses designed to be placed on a person&#39;s eyes (e.g., similar to contact lenses), headphones/earphones, speaker arrays, input systems (e.g., wearable or handheld controllers with or without haptic feedback), smartphones, tablets, and desktop/laptop computers. A head mounted system may have one or more speaker(s) and an integrated opaque display. Alternatively, a head mounted system may be configured to accept an external opaque display (e.g., a smartphone). The head mounted system may incorporate one or more imaging sensors to capture images or video of the physical environment, and/or one or more microphones to capture audio of the physical environment. Rather than an opaque display, a head mounted system may have a transparent or translucent display. The transparent or translucent display may have a medium through which light representative of images is directed to a person&#39;s eyes. The display may utilize digital light projection, OLEDs, LEDs, uLEDs, liquid crystal on silicon, laser scanning light sources, or any combination of these technologies. The medium may be an optical waveguide, a hologram medium, an optical combiner, an optical reflector, or any combination thereof. In one embodiment, the transparent or translucent display may be configured to become opaque selectively. Projection-based systems may employ retinal projection technology that projects graphical images onto a person&#39;s retina. Projection systems also may be configured to project virtual objects into the physical environment, for example, as a hologram or on a physical surface. 
     The foregoing is merely illustrative and various modifications can be made to the described embodiments. The foregoing embodiments may be implemented individually or in any combination.

Metadata:
Filing Date: 20230929
Publication Date: 20241119
Grant Date: 20241119
Priority Date: 20230515
Inventors: HUO, EDWARD S.
Assignee: APPLE INC
CPC Classifications: [{"code": "G06F3/011", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N23/56", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/013", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N23/90", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G2354/00", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G3/002", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04N23/56", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N23/90", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G2354/00", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F3/013", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G3/002", "inventive": true, "first": true, "tree": "[]"}, {"code": "G09G2354/00", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04N23/90", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N23/56", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/013", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G3/002", "inventive": true, "first": true, "tree": "[]"}]
Family ID: 93464464