Patent Publication Number: US-2023134442-A1

Title: In-field imaging system for eye tracking

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application No. 63/273,481 filed Oct. 29, 2021, which is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure relates generally to optics, and in particular to eye tracking technologies. 
     BACKGROUND INFORMATION 
     Eye tracking technology enables head mounted displays (HMD) to interact with users based on the users&#39; eye movement or eye orientation. Existing eye tracking systems use cameras that are positioned on a frame of the HMD. However, having a camera on a frame of an HMD makes imaging susceptible to occlusions from eyelashes and eyelids. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting and non-exhaustive embodiments of the invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified. 
         FIG.  1    illustrates a head mounted display, in accordance with aspects of the disclosure. 
         FIG.  2    illustrates an example implementation of a head mounted display in an ocular environment, in accordance with aspects of the disclosure. 
         FIG.  3    illustrates a head mounted display, in accordance with aspects of the disclosure. 
         FIG.  4    illustrates an example implementation of a head mounted display in an ocular environment, in accordance with aspects of the disclosure. 
         FIG.  5    illustrates a head mounted display, in accordance with aspects of the disclosure. 
         FIG.  6    illustrates an example implementation of a head mounted display in an ocular environment, in accordance with aspects of the disclosure. 
         FIG.  7    illustrates a flow diagram of a process for eye tracking, in accordance with aspects of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of an in-field imaging system having a cloaking optical structure is described herein. In the following description, numerous specific details are set forth to provide a thorough understanding of the embodiments. One skilled in the relevant art will recognize, however, that the techniques described herein can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring certain aspects. 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
     In aspects of this disclosure, visible light may be defined as having a wavelength range of approximately 380 nm to 700 nm. Non-visible light may be defined as light having wavelengths that are outside the visible light range, such as ultraviolet light and infrared light. In aspects of this disclosure, red light may be defined as having a wavelength range of approximately 620 to 750 nm, green light may be defined as having a wavelength range of approximately 495 to 570 nm, and blue light may be defined as having a wavelength range of approximately 450 to 495 nm. 
     Eye tracking functionality expands the services and quality of interaction that head mounted displays (HMDs) can provide to users. Eyelashes and eyelids can block and inhibit the quality of signal (e.g., image) available from eyes when cameras are placed on a frame of an HMD. A significantly better position for imaging an eye is right with a camera located in front of the eye. However, placing a camera right in front of an eye could obstruct the vision of a user and could be an annoyance that reduces the quality of a user&#39;s experience with the HMD. Disclosed herein are techniques for both placing an image sensor in the field of view of a user&#39;s eye and for concealing or cloaking the image sensor from the user&#39;s vision. In aspects of the disclosure, “in-field” can be defined as being in the field of view of a user&#39;s eye(s). 
     An HMD may include an imaging system that is at least partially disposed in a lens assembly of the HMD to receive reflections from a user&#39;s eye. The imaging system includes an image sensor and an optical structure. The optical structure is configured to direct scene light around the image sensor to enable distraction-free and in-field imaging of a user&#39;s eye. In other words, the optical structure is configured to conceal or cloak the image sensor from the user&#39;s eye by directing scene light around the image sensor to the user&#39;s eye. The optical structure also directs reflections (e.g., infrared reflections) from the user&#39;s eye to the image sensor, while the image sensor is positioned in-field of the user&#39;s eye. 
     The optical structure includes an out-coupling (eye-side) optical element, an in-coupling (scene-side) optical element, and one or more intermediate optical elements. The in-coupling optical element receives scene light and directs the scene light to the one or more intermediate optical elements. The one or more intermediate optical elements receive the scene light from the in-coupling optical element and direct the scene light to the out-coupling optical element. The out-coupling optical element receives the scene light from the one or more intermediate optical elements and directs the scene light to the user&#39;s eye. The out-coupling optical element may be configured to reflect or diffract visible light to the eye and transmit or pass non-visible light (e.g., near infrared light) from the eye to the image sensor. The out-coupling optical element and the in-coupling optical element may be dichroic mirrors, optical prisms, holographic optical elements, and/or gratings that are configured to redirect visible light while transmitting or passing non-visible light. The disclosed concealing or cloaking of the image sensor by redirecting visible light around the image sensor may result in a distraction-free and unobstructed imaging of the user&#39;s eye. 
     The image sensor is positioned between the in-coupling optical element and the out-coupling optical element to enable the optical elements to mask, cloak, or otherwise hide the image sensor from the user&#39;s eye (and from outside observers). The image sensor may have dimensions of 500 μm by 500 μm by 500 μm, for example. 
     The optical structure is a cloaking device that is at least partially integrated into a lens assembly of the HMD. The lens assembly may include one, two, three, or more optical layers. The various components of the imaging system may be incorporated into one or more optical layers and frame of the HMD, according to various embodiments. 
     The HMD may include more than one optical cloaking device for each eye of a user. For example, the HMD may include two optical cloaking devices for each eye of the user. One optical cloaking device may have an image sensor that is oriented towards the eyebox to capture eye reflection light. Another optical cloaking device may have an image sensor that is oriented outwards (away from the eyebox) to capture (or at least partially capture) scene light from a perspective of the user of the HMD. For an outward-oriented image sensor, the in-coupling (scene-side) optical element may include a beam splitter that partially directs visible light to the image sensor and that substantially directs (e.g., reflects, refracts, or diffracts) the remaining visible light towards the one or more intermediate optical elements (e.g., mirrors) and to the user&#39;s eye. Image data representing scene light can be used to identify objects in a scene, provide user interface options/menus, and reduce the size of the HMD frames by including electronics in the lens assembly (rather than in the HMD frame), to enable fabrication of lower-profile HMD frames. 
     The apparatus, system, and method for an in-field imaging system with a cloaking optical structure is described in this disclosure and enables improvements in eye tracking technologies, for example, to support operations of an HMD. These and other embodiments are described in more detail in connection with  FIGS.  1 - 7   . 
       FIG.  1    illustrates a head mounted display (HMD)  100 , in accordance with aspects of the present disclosure. As described further below, in embodiments, HMD  100  may include an imaging system  102 A/ 102 B that receives reflections from an eye in the field of vision (in-field) of a user&#39;s eye. Imaging system  102 A/ 102 B may be used to support eye tracking, user experience (UX), and other features of HMD  100 . An HMD, such as HMD  100 , is one type of head mounted display, typically worn on the head of a user to provide artificial reality content to the user. Artificial reality is a form of reality that has been adjusted in some manner before presentation to the user, which may include, e.g., virtual reality (VR), augmented reality (AR), mixed reality (MR), hybrid reality, or some combination and/or derivative thereof. 
     HMD  100  includes frame  104  coupled to arms  106 A and  106 B. Each lens assembly  108 A/ 108 B is mounted to frame  104 . Each lens assembly  108 A/ 108 B may be a prescription lens matched to a particular user of HMD  100  or may be non-prescription lens. The illustrated HMD  100  is configured to be worn on or about a head of a wearer of HMD  100 . 
     Each lens assembly  108 A/ 108 B includes a waveguide  110 A/ 110 B to direct image light generated by a display  112 A/ 112 B to an eyebox area for viewing by a user of HMD  100 . Display  112 A/ 112 B may include a liquid crystal display (LCD), an organic light emitting diode (OLED) display, micro-LED display, quantum dot display, pico-projector, or liquid crystal on silicon (LCOS) display for directing image light to a wearer of HMD  100 . 
     Lens assemblies  108  may appear transparent to a user to facilitate augmented reality or mixed reality to enable a user to view scene light from the environment around her while also receiving image light directed to her eye(s) by, for example, waveguides  110 . Consequently, lens assemblies  108  may be considered (or include) an optical combiner. Each lens assembly  108 A/ 108 B may include a lens assembly that includes two or more optical layers that carry portions of imaging system  102 A/ 102 B and/or waveguide  110 A/ 110 B, in an embodiment. In some embodiments, display light from display  112 A or  112 B is only directed into one eye of the wearer of HMD  100 . In an embodiment, both displays  112 A and  112 B are used to direct image light into waveguides  110 A and  110 B, respectively. 
     HMD  100  includes a number of light sources  116  disposed around a periphery of lens assemblies  108  in frame  104 . Light sources  116  emit light in an eyeward direction to illuminate an eyebox of HMD  100  to generate reflections from an eye (or eyes) of a wearer of HMD  100 . Light sources  116  may be light emitting diodes (LEDs), vertical-cavity surface-emitting lasers (VCSELs), micro light emitting diode (micro-LED), an edge emitting LED, a superluminescent diode (SLED), or another type of light source. Light sources  116  emit non-visible light, according to an embodiment. Light sources  116  emit near infrared light (e.g., 750 nm-1.5 μm), according to an embodiment. In one embodiment, light emitted from light sources  116  is infrared light centered around 850 nm. Infrared light from other sources may illuminate the eye as well. 
     Reflected light from light sources  116  is reflected off of a user&#39;s eye and is received by imaging system  102 A/ 102 B, according to an embodiment. Imaging systems  102  (individually, imaging system  102 A and imaging system  102 B) are positioned within lens assemblies  108  to be in-field of the user&#39;s eye. Imaging system  102 A/ 102 B includes an image sensor and a (cloaking) optical structure, according to an embodiment. The image sensor positioned in-field may provide improved eye orientation determination (e.g., eye tracking) over traditional configurations. The optical structure is configured to conceal the image sensor from the user&#39;s eye by directing scene light and/or display light around the image sensor and by directing reflected light to the image sensor. Imaging system  102 A/ 102 B is depicted as imaging system  102 A in lens assembly  108 A and as imaging system  102 B in lens assembly  108 B. However, the disclosed imaging system may be implemented in only one of lens assemblies  108 , according to one embodiment. 
     Frame  104  and arms  106  may include supporting hardware of HMD  100 . HMD  100  may include a controller  118  to receive image data from imaging system  102 A/ 102 B and to determine an orientation of a user&#39;s eye, based on the image data, according to an embodiment. Controller  118  may include processing logic  120 , wired and/or wireless data interface for sending and receiving data, graphic processors, and one or more memories  122  for storing data and computer-executable instructions. Controller  118  and/or processing logic  120  may include circuitry, logic, instructions stored in a machine-readable storage medium, ASIC circuitry, FPGA circuity, and/or one or more processors. In one embodiment, HMD  100  may be configured to receive wired power. In one embodiment, HMD  100  is configured to be powered by one or more batteries. In one embodiment, HMD  100  may be configured to receive wired data including video data via a wired communication channel. In one embodiment, HMD  100  is configured to receive wireless data including video data via a wireless communication channel. 
       FIG.  2    illustrates an ocular environment  200 , according to an embodiment of the disclosure. Ocular environment  200  includes a cross-sectional side view of an HMD  202  positioned near an eye  204  of a user. HMD  202  is an example implementation of HMD  100  (shown in  FIG.  1   ), according to an embodiment. 
     HMD  202  includes an eyeward side  206  and a scene side  210 . Eyeward side  206  is where an eyebox  208  is located and where eye  204  is located during use. Eyebox  208  is a two or three dimensional area in which eye  204  may move within. HMD  202  may be configured to direct scene light and/or display light into eyebox  208 . Scene side  210  is the side of HMD  202  from which scene light  212  is generated and enters a lens assembly  214  of HMD  202 . Lens assembly  214  is an example implementation of lens assembly  108 A (shown in  FIG.  1   ). 
     Lens assembly  214  is configured to direct scene light  212  from scene side  210  to eyeward side  206 . Lens assembly  214  may be implemented as a single optical layer (e.g., lens, plastic, glass) or may include a number of optical layers optically coupled together (e.g., stacked) and configured to support operation of HMD  202 . For example, lens assembly  214  may include one optical layer that is configured to focus scene light  212  on eyebox  208  and may have another optical layer configured to provide display light to eyebox  208 . Other layers may have other functions, as disclosed further below. 
     Lens assembly  214  includes an imaging system  216  that is positioned in-field of eye  204 . Imaging system  216  is an example implementation of imaging system  102 A (shown in  FIG.  1   ). Imaging system  216  is configured to provide scene light  212  to eyebox  208  and to receive reflections (e.g., infrared reflections) off of eye  204 . Imaging system  216  includes an image sensor  218  and an optical structure  220  that is configured to cloak or conceal image sensor  218  from eye  204 . Optical structure  220  conceals image sensor  218  from eye  204  by directing scene light  212  around image sensor  218 . By directing a portion of scene light  212  around image sensor  218 , eye  204  may see or receive portions of scene light  212  that may have been blocked or obstructed by image sensor  218 . In other words, optical structure  220  enables eye  204  to see scene side  210  of image sensor  218  without recognizing or perceiving the obstruction of image sensor  218 . In the absence of optical structure  220 , image sensor  218  may appear as a dot or obstruction in a user&#39;s field of view and may reduce the overall enjoyability of the user&#39;s experience while interacting with HMD  202 . 
     Optical structure  220  includes an in-coupling optical element  222  and an out-coupling optical element  224 , according to an embodiment. Image sensor  218  is positioned between in-coupling optical element  222  and out-coupling optical element  224 . In-coupling optical element  222  and out-coupling optical element  224  are disposed within lens assembly  214 , according to an embodiment. In-coupling optical element  222  is disposed near (e.g., almost touching) an entrance surface  226  of lens assembly  214 , and out-coupling optical element  224  is disposed near an exit surface  228  of lens assembly  214 , according to an embodiment. In-coupling optical element  222  includes a reflective surface  230 . Reflective surface  230  is configured to reflect visible light, according to an embodiment. Out-coupling optical element  224  includes a reflective surface  232 . Reflective surface  232  is configured to reflect visible light and transmit infrared light, according to an embodiment. Reflective surfaces  230  and/or  232  may be coated to selectively reflect visible light and transmit or pass non-visible light and may include a dichroic coating to enable this functionality. 
     Optical structure  220  may include one or more intermediate optical elements that optically couple in-coupling optical element  222  to out-coupling optical element  224 . Optical structure  220  includes intermediate optical element  234  and intermediate optical element  236 , according to an embodiment. Intermediate optical element  234  is positioned within lens assembly  214  to receive scene light  212  from in-coupling optical element  222  and to direct (e.g., reflect) scene light  212  to intermediate optical element  236 . Intermediate optical element  236  is positioned within lens assembly  214  to receive scene light  212  from intermediate optical element  234  and to direct (e.g., reflect) scene light  212  to out-coupling optical element  224 . As illustrated, intermediate optical elements  234  and  236  may optionally be positioned in frame  104  or near the outer portion of the field of view of eye  204  within lens assembly  214 . 
     Optical structure  220  defines an optical path that directs (a portion of) scene light  212  around image sensor  218  to conceal or cloak image sensor  218  from eye  204 . The optical path includes scene light  212 : entrance into lens assembly  214  through entrance surface  226  (with an incident angle θi) to in-coupling optical element  222 ; redirection from in-coupling optical element  222  to intermediate optical element  234 ; redirection from intermediate optical element  234  to intermediate optical element  236 ; redirection from intermediate optical element  236  to out-coupling optical element  224 ; and redirection from out-coupling optical element  224  to exit surface  228  towards eye  204 . In one embodiment, scene light  212  exits lens assembly  214  with a refraction angle θr that is similar to incident angle θi. In other embodiments, lens assembly  214  includes positive power or negative power that changes refraction angle θr to be more convergent or divergent than incident angle θi. 
     Image sensor  218  is positioned within lens assembly  214  to receive reflections from eye  204  through at least part of optical structure  220 . Image sensor  218  may be used to support eye tracking functions of HMD  202  by receiving reflections  238  of non-visible light  240  (e.g., infrared, near-infrared, etc.) off of eye  204 . Non-visible light  240  may be emitted by one or more light sources  116  that may be positioned in various locations on frame  104 . Image sensor  218  may receive reflections  238  through out-coupling optical element  224 , which may be configured to reflect visible light and transmit or pass non-visible light  240 . In other words, out-coupling optical element  224  may be configured to direct scene light  212  to eye  204  and may be configured to direct reflections  238  off of eye  204  to image sensor  218 . Image sensor  218  converts reflections  238  into image data  242 . Image data  242  includes data that may be representative of the conversion of reflections  238  into electrical signals by image sensor  218 . Image sensor  218  may be communicatively coupled to controller  118  through communications channel  244  and may use communications channel  244  to provide image data  242  to controller  118 . Image sensor  218  may be implemented as a complementary metal oxide semiconductor (“CMOS”) image sensor or a charge-coupled device (“CCD”) image sensor. 
       FIG.  3    illustrates a partially exploded perspective view of an HMD  300 , in accordance with embodiments of the disclosure. HMD  300  illustrates an example implementation of HMD  202  (shown in  FIG.  2   ), according to an embodiment. HMD  300  includes a lens assembly  302  and a lens assembly  303 . Lens assembly  302  is an illustrative example of an implementation of lens assembly  214  (shown in  FIG.  2   ). Lens assembly  303  may be similar to lens assembly  302 . 
     Lens assembly  302  may include a number of optical layers into which imaging system  216  is distributed, according to an embodiment of the disclosure. Although lens assembly  302  is illustrated with four optical layers, lens assembly  302  may be implemented with more layers or fewer layers, as described below. Lens assembly  302  includes an out-coupling layer  304 , an imaging layer  306 , an in-coupling layer  308 , and a display layer  310 , according to an embodiment. As illustrated, out-coupling layer  304  may be positioned closest to eyeward side  206 . Out-coupling layer  304  may include out-coupling optical element  224  and intermediate optical element  236 . Imaging layer  306  may be positioned between out-coupling layer  304  and in-coupling layer  308 . Imaging layer  306  may include image sensor  218 . In-coupling layer  308  may include in-coupling optical element  222  and intermediate optical element  234 . Intermediate optical element  234  and/or intermediate optical element  236  may alternatively be disposed on imaging layer  306  or in frame  104 , according to an embodiment. In-coupling optical element  222 , intermediate optical element  234 , intermediate optical element  236 , and out-coupling optical element  224  of optical structure  220  may each be disposed in their own dedicated optical layer, according to an embodiment. Image sensor  218  may alternatively be disposed in out-coupling layer  304  or in in-coupling layer  308 , according to an embodiment. Image sensor  218  may alternatively be carried on a surface of out-coupling layer  304  or on a surface of in-coupling layer  308  to be disposed between two optical layers, according to an embodiment. 
     Display layer  310  may include waveguide  110 A (shown in  FIG.  1   ) to direct display light generated by an electronic display to the eye of the user. In some implementations, at least a portion of the electronic display is included in frame  104  of HMD  300 . The electronic display may include an LCD, an organic light emitting diode (OLED) display, micro-LED display, pico-projector, or liquid crystal on silicon (LCOS) display for generating the display light. Waveguide  110 A may alternatively be incorporated into one of the other optical layers of lens assembly  302 . One or more of the optical layers of lens assembly  302  may also be fabricated to concurrently operate as an illumination layer and/or an optical combiner layer that directs virtual images to an eye of a user, in addition to scene light  212 . For example, out-coupling layer  304  may include a plurality of in-field light sources that illuminate the user&#39;s eye. 
     While  FIG.  3    illustrates HMD  300  being configured for augmented reality (AR) or mixed reality (MR) contexts, the herein disclosed embodiments of an HMD may also be used in other implementations of an HMD. For example, the optical layers of any of the lens assemblies of this disclosure may be disposed close to a display plane or focusing lens of a virtual reality (VR) HMD. 
       FIG.  4    illustrates an ocular environment  400  that includes a cross-sectional side view of an HMD  402 , in accordance with embodiments of the disclosure. HMD  402  is an example implementation of HMD  100  (shown in  FIG.  1   ), according to an embodiment. HMD  402  includes lens assembly  404 , and lens assembly  404  includes imaging system  406 . Imaging system  406  is an example implementation of imaging system  102 A (shown in  FIG.  1   ). HMD  402  and imaging system  406  include some similar components of HMD  202  (shown in  FIG.  2   ) and imaging system  216  (shown in  FIG.  2   ), respectively. Imaging system  406  includes an optical structure  408  that has a single intermediate optical element  410 . Intermediate optical element  410  receives scene light  212  from in-coupling optical element  222  and directs scene light  212  to out-coupling optical element  224 . Intermediate optical element  410  is configured to support directing scene light  212  around image sensor  218 , according to an embodiment. Intermediate optical element  410  is a reflective optical device and may be implemented as a mirror and may include one or more dichroic coatings to enable reflection of visible light or all light. As illustrated, intermediate optical element  410  may optionally be positioned in frame  104  or near the outer portion of the field of view of eye  204 . 
       FIG.  5    illustrates a partially exploded perspective view of an HMD  500 , in accordance with embodiments of the disclosure. HMD  500  illustrates an example implementation of HMD  402  (shown in  FIG.  4   ), according to an embodiment. HMD  500  includes a lens assembly  502  and a lens assembly  503 . Lens assembly  502  is an illustrative example of an implementation of lens assembly  404  (shown in  FIG.  4   ). Lens assembly  503  may be similar to lens assembly  502 . 
     Lens assembly  502  may include a number of optical layers into which imaging system  406  is distributed, according to an embodiment of the disclosure. Although lens assembly  502  is illustrated with four optical layers, lens assembly  502  may be implemented with more layers or fewer layers, in accordance with various embodiments of the disclosure. Lens assembly  502  includes an out-coupling layer  504 , an imaging layer  506 , an in-coupling layer  508 , and display layer  310  (also shown in  FIG.  3   ), according to an embodiment. As illustrated, out-coupling layer  504  may be positioned closest to eyeward side  206 . Out-coupling layer  504  may include (e.g., embedded within, surface mounted, etc.) out-coupling optical element  224 . Imaging layer  506  may be positioned between out-coupling layer  504  and in-coupling layer  508 . Imaging layer  506  may include image sensor  218  and intermediate optical element  410 . In-coupling layer  508  may include in-coupling optical element  222 . Intermediate optical element  410  may alternatively be included in out-coupling layer  504  or may be included in in-coupling layer  508 , according to an embodiment. In-coupling optical element  222 , intermediate optical element  410 , and out-coupling optical element  224  of optical structure  408  may each be disposed in their own dedicated optical layer, according to an embodiment. Image sensor  218  may alternatively be disposed in out-coupling layer  504  or in in-coupling layer  508 , according to an embodiment. Image sensor  218  may alternatively be carried on a surface of out-coupling layer  504  or on a surface of in-coupling layer  508  to be disposed between two optical layers, according to an embodiment. 
       FIG.  6    illustrates an ocular environment  600  that includes a cross-sectional side view of an HMD  602 , in accordance with embodiments of the disclosure. HMD  602  includes diffractive optical elements that direct scene light  212  around image sensor  218 , in accordance with embodiments of the disclosure. HMD  602  is an example implementation of HMD  100  (shown in  FIG.  1   ), according to an embodiment. HMD  602  includes lens assembly  604 , which includes imaging system  606 . Imaging system  606  is an example implementation of imaging system  102 A (shown in  FIG.  1   ). HMD  602  and imaging system  606  include some similar components of HMD  202  (shown in  FIG.  2   ) and imaging system  216  (shown in  FIG.  2   ), respectively, and a similar component as HMD  402  (shown in  FIG.  4   ). Imaging system  606  includes an optical structure  608  that includes intermediate optical element  410 , an in-coupling optical element  610 , and an out-coupling optical element  612 . In-coupling optical element  610  and out-coupling optical element  612  are diffractive optical elements, according to an embodiment. In-coupling optical element  610  and out-coupling optical element  612  may be implemented as diffractive gratings and/or holographic optical elements. In operation, in-coupling optical element  610  receives scene light  212  and directs (e.g., diffracts) scene light  212  to intermediate optical element  410 ; intermediate optical element  410  receives scene light  212  from in-coupling optical element  610  and directs (e.g., reflects) scene light  212  to out-coupling optical element  612 ; and out-coupling optical element  612  receives scene light  212  from intermediate optical element  410  and directs (e.g., diffracts) scene light  212  to eyebox  208 , to conceal or cloak image sensor  218  from eye  204 , according to an embodiment. Out-coupling optical element  612  is configured to transmit or pass reflections  238  to image sensor  218 . As illustrated, intermediate optical element  410  may optionally be positioned in frame  104  or near the outer portion of the field of view of eye  204 . 
       FIG.  7    illustrates a process  700  for eye tracking, according to an embodiment. Process  700  may be incorporated (e.g., in controller  120 ) into one or more HMDs disclosed herein. The order in which some or all of the process blocks appear in process  700  should not be deemed limiting. Rather, one of ordinary skill in the art having the benefit of the present disclosure will understand that some of the process blocks may be executed in a variety of orders not illustrated, or even in parallel. 
     In process block  702 , process  700  directs, with a first optical element carried by a lens assembly, scene light into a lens assembly, according to an embodiment. Process block  702  may proceed to process block  704 , according to an embodiment. 
     In process block  704 , process  700  receives, with a second optical element carried by a lens assembly, the scene light, according to an embodiment. Process block  704  may proceed to process block  706 , according to an embodiment. 
     In process block  706 , process  700  directs, with the second optical element, the scene light towards an eyebox, according to an embodiment. Process block  706  may proceed to process block  708 , according to an embodiment. 
     In process block  708 , process  700  receives, with an image sensor positioned in-field and within the lens assembly, reflected light from the eyebox, wherein the second optical element at least partially conceals the image sensor from the eyebox, according to an embodiment. 
     The first optical element and the second optical element are configured to conceal or cloak the image sensor from a user&#39;s eye within the eyebox, according to an embodiment. The first and second optical elements are configured to direct scene light around the image sensor and to the eyebox. Process  700  may also include illuminating the eyebox with, for example, infrared light to enable the image sensor to receive reflections from the user&#39;s eye. The reflections are converted to electrical signals represented by image data, which may be used to track and determine an orientation of a user&#39;s eye. The orientation of the user&#39;s eye may be used by the disclosed HMDs to provide user interface options, adjust the focus of user interface elements, or otherwise provide a custom interactive experience to the user, for example. 
     Embodiments of the invention may include or be implemented in conjunction with an artificial reality system. Artificial reality is a form of reality that has been adjusted in some manner before presentation to a user, which may include, e.g., a virtual reality (VR), an augmented reality (AR), a mixed reality (MR), a hybrid reality, or some combination and/or derivatives thereof. Artificial reality content may include completely generated content or generated content combined with captured (e.g., real-world) content. The artificial reality content may include video, audio, haptic feedback, or some combination thereof, and any of which may be presented in a single channel or in multiple channels (such as stereo video that produces a three-dimensional effect to the viewer). Additionally, in some embodiments, artificial reality may also be associated with applications, products, accessories, services, or some combination thereof, that are used to, e.g., create content in an artificial reality and/or are otherwise used in (e.g., perform activities in) an artificial reality. The artificial reality system that provides the artificial reality content may be implemented on various platforms, including a head-mounted display (HMD) connected to a host computer system, a standalone HMD, a mobile device or computing system, or any other hardware platform capable of providing artificial reality content to one or more viewers. 
     The term “processing logic” (e.g., controller  118 , processing logic  120 ) in this disclosure may include one or more processors, microprocessors, multi-core processors, Application-specific integrated circuits (ASIC), and/or Field Programmable Gate Arrays (FPGAs) to execute operations disclosed herein. In some embodiments, memories (not illustrated) are integrated into the processing logic to store instructions to execute operations and/or store data. Processing logic may also include analog or digital circuitry to perform the operations in accordance with embodiments of the disclosure. 
     A “memory” or “memories” (e.g., memories  122 ) described in this disclosure may include one or more volatile or non-volatile memory architectures. The “memory” or “memories” may be removable and non-removable media implemented in any method or technology for storage of information such as computer-readable instructions, data structures, program modules, or other data. Example memory technologies may include RAM, ROM, EEPROM, flash memory, CD-ROM, digital versatile disks (DVD), high-definition multimedia/data storage disks, or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other non-transmission medium that can be used to store information for access by a computing device. 
     A computing device may include a desktop computer, a laptop computer, a tablet, a phablet, a smartphone, a feature phone, a server computer, or otherwise. A server computer may be located remotely in a data center or be stored locally. 
     The processes explained above are described in terms of computer software and hardware. The techniques described may constitute machine-executable instructions embodied within a tangible or non-transitory machine (e.g., computer) readable storage medium, that when executed by a machine will cause the machine to perform the operations described. Additionally, the processes may be embodied within hardware, such as an application specific integrated circuit (“ASIC”) or otherwise. 
     A tangible non-transitory machine-readable storage medium includes any mechanism that provides (i.e., stores) information in a form accessible by a machine (e.g., a computer, network device, personal digital assistant, manufacturing tool, any device with a set of one or more processors, etc.). For example, a machine-readable storage medium includes recordable/non-recordable media (e.g., read only memory (ROM), random access memory (RAM), magnetic disk storage media, optical storage media, flash memory devices, etc.). 
     The above description of illustrated embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. 
     These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.