Patent Publication Number: US-11644675-B2

Title: Ghost image mitigation in see-through displays with pixel arrays

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 16/994,509, entitled GHOST IMAGE MITIGATION IN SEE-THROUGH DISPLAYS WITH PIXEL ARRAYS, filed on Aug. 14, 2020, which claims priority to Provisional Application No. 62/887,639, entitled GHOST IMAGE MITIGATION IN SEE-THROUGH DISPLAYS WITH PIXEL ARRAYS, filed on Aug. 15, 2019, the entire contents of which are incorporated by reference. 
    
    
     INCORPORATION BY REFERENCE 
     This application incorporates by reference the entirety of each of the following patent applications: U.S. patent application Ser. No. 15/479,700, filed on Apr. 5, 2017, published on Oct. 12, 2017 as U.S. Publication No. 2017/0293141; U.S. patent application Ser. No. 16/214,575, filed on Dec. 10, 2018, published on Jun. 13, 2019 as U.S. Publication No. 2019/0179057; U.S. Provisional Patent Application Ser. No. 62/725,993, entitled SPATIALLY-RESOLVED DYNAMIC DIMMING FOR AUGMENTED REALITY DEVICE, filed on Aug. 31, 2018; U.S. Provisional Patent Application Ser. No. 62/731,755, entitled SYSTEMS AND METHODS FOR EXTERNAL LIGHT MANAGEMENT, filed on Sep. 14, 2018; U.S. Provisional Patent Application Ser. No. 62/858,252, entitled SPATIALLY-RESOLVED DYNAMIC DIMMING FOR AUGMENTED REALITY DEVICE, filed on Jun. 6, 2019; and U.S. Provisional Patent Application Ser. No. 62/870,896, entitled GEOMETRIES FOR MITIGATING ARTIFACTS IN SEE-THROUGH PIXEL ARRAYS, filed on Jul. 5, 2019. For every document incorporated herein, in case of conflict, the current specification controls. 
     BACKGROUND OF THE INVENTION 
     Modern computing and display technologies have facilitated the development of systems for so called “virtual reality” or “augmented reality” experiences, wherein digitally reproduced images or portions thereof are presented to a user in a manner wherein they seem to be, or may be perceived as, real. A virtual reality, or “VR,” scenario typically involves presentation of digital or virtual image information without transparency to other actual real-world visual input; an augmented reality, or “AR,” scenario typically involves presentation of digital or virtual image information as an augmentation to visualization of the actual world around the user. 
     Despite the progress made in these display technologies, there is a need in the art for improved methods, systems, and devices related to augmented reality systems, particularly, display systems. 
     SUMMARY OF THE INVENTION 
     Segmented attenuation using a polarized TFT-LCD panel can greatly increase the visibility and solidity of content without dimming the full field of view of the world. The display light in diffractive waveguide type see through AR displays send light in two directions: one towards the user and one towards the world. The back reflection of the light (e.g., off of reflective components of the dimming assembly, such as metal traces, conductors, layer index mismatches, and TFTs) going toward the world may appear as a “ghost” image next to the primary display image. This ghost image impacts the effective contrast and viewing fidelity of the virtual content and therefore quality of immersion. Mitigating ghost images and stray light paths due to the metal traces, TFTs, layer index mismatches, elements/objects beyond the dimmer, etc. is a difficult problem. 
     The systems and techniques disclosed herein leverage polarization films of the dimmer as both a system optical isolator and an intra-dimmer optical isolator to effectively suppress such “ghost” images. The addition of index matching fluid/gel between the eyepiece cover glass and the dimmer, where the index is close to the first layer of the dimmer, can mitigate ghosting from the first dimmer surface. 
     Using quarter waveplates (QWPs) with specifically chosen achromatic properties can allow a liquid crystal dimmer, such as those using an electrically controlled birefringence (ECB) cell, to have less polarization leakage, better chromatic performance and be more color neutral across a relatively wide range of operating conditions. 
     The present disclosure relates generally to techniques for improving optical systems in varying ambient light conditions. More particularly, embodiments of the present disclosure provide systems and methods for operating an augmented reality (AR) device comprising a dimming element. Although the present invention is described in reference to an AR device, the disclosure is applicable to a variety of applications in computer vision and image display systems. 
     In general, in a first aspect, the invention features head-mounted apparatus that include an eyepiece that include a variable dimming assembly and a frame mounting the eyepiece so that, during use of the head-mounted apparatus, a user side of the eyepiece faces a towards a user of the head-mounted apparatus, and a world side of the eyepiece opposite the first side faces away from the user. The dynamic dimming assembly is configured to selectively modulate an intensity of light transmitted parallel to an optical axis from the world side of the eyepiece to the user side of the eyepiece during operation of the head-mounted apparatus. The dynamic dimming assembly includes a variable birefringence cell having multiple pixels each having an independently variable birefringence, a first linear polarizer arranged on the user side of the variable birefringence cell, the first linear polarizer being configured to transmit light propagating parallel to the optical axis linearly polarized along a pass axis of the first linear polarizer orthogonal to the optical axis, a quarter wave plate arranged between the variable birefringence cell and the first linear polarizer, a fast axis of the quarter wave plate being arranged relative to the pass axis of the first linear polarizer to transform linearly polarized light transmitted by the first linear polarizer into circularly polarized light, and a second linear polarizer on the world side of the variable birefringence cell. 
     Implementations of the head-mounted apparatus can include one or more of the following features and/or features of other aspects. For example, the dynamic dimming assembly further includes an optical retarder arranged between the variable birefringence cell and the second linear polarizer. The optical retarder can be a second quarter wave plate. The optical retarder is an A-plate with a retardation greater than a retardation of the quarter wave plate. A difference between a retardation of the optical retarder and a retardation of the quarter wave plate can correspond to a residual retardation of the variable birefringent cell in a minimum birefringence state. 
     The variable birefringence cell can include a layer of a liquid crystal. The liquid crystal can be configured in an electrically controllable birefringence mode. In some embodiments, the layer of the liquid crystal is a vertically aligned liquid crystal layer. The liquid crystal can be a nematic phase liquid crystal. 
     The pixels of the variable birefringence cell can be actively addressed pixels. 
     The eyepiece can further include a see-through display mounted in the frame on the user side of the variable dimming assembly. The see-through display can include one or more waveguide layers arranged to receive light from a light projector during operation of the head-mounted apparatus and direct the light toward the user. The head-mounted apparatus can include one or more index-matching layers arranged between the see-through display and the variable dimming assembly. 
     In some embodiments, the dynamic dimming assembly includes one or more antireflection layers. 
     In another aspect, the invention features an augmented reality system including the head-mounted apparatus. 
     In general, in another aspect, the invention features head-mounted apparatus that include an eyepiece having a variable dimming assembly and a frame mounting the eyepiece so that, during use of the head-mounted apparatus, a user side of the eyepiece faces a towards a user of the head-mounted apparatus, and a world side of the eyepiece opposite the first side faces away from the user. The dynamic dimming assembly is configured to selectively modulate an intensity of light transmitted parallel to an optical axis from the world side of the eyepiece to the user side of the eyepiece during operation of the head-mounted apparatus. The dynamic dimming assembly includes a layer of a liquid crystal, a circular polarizer arranged on the user side of the liquid crystal layer, and a linear polarizer on the world side of the liquid crystal layer. 
     Embodiments of the head-mounted apparatus can include one or more of the following features and/or features of other aspects. For example, the circular polarizer can include a linear polarizer and a quarter wave plate. The head-mounted apparatus can include an A-plate arranged between the linear polarizer on the world side of the liquid crystal layer. 
     The head-mounted apparatus can include a pixelated cell including the layer of the liquid crystal, the pixelated cell being an actively addressed pixelated cell. 
     In general, in a further aspect, the invention features a method of operating an optical system. The method may include receiving, at the optical system, light associated with a world object. The method may also include projecting a virtual image onto an eyepiece. The method may further include determining a portion of a system field of view of the optical system to be at least partially dimmed based on detected information. The method may further include adjusting a dimmer to reduce an intensity of the light associated with the world object in the portion of the system field of view. 
     In some embodiments, the optical system includes a light sensor configured to detect light information corresponding to the light associated with the world object. In some embodiments, the detected information includes the light information. In some embodiments, the light information includes multiple spatially-resolved light values. In some embodiments, the light information includes a global light value. In some embodiments, the optical system includes an eye tracker configured to detect gaze information corresponding to an eye of a user of the optical system. In some embodiments, the detected information includes the gaze information. In some embodiments, the gaze information includes a pixel location that intersects with a gaze vector of the eye of the user. In some embodiments, the gaze information includes one or more of a pupil position of the eye of the user, a center of rotation of the eye of the user, a pupil size of the eye of the user, a pupil diameter of the eye of the user, and cone and rod locations of the eye of the user. In some embodiments, the method further includes detecting image information corresponding to the virtual image. In some embodiments, the detected information includes the image information. In some embodiments, the image information includes a plurality of spatially-resolved image brightness values. In some embodiments, the image information includes a global image brightness value. 
     In some embodiments, the method further includes determining multiple spatially-resolved dimming values for the portion of the system field of view based on the detected information. In some embodiments, the dimmer is adjusted according to the plurality of dimming values. In some embodiments, the dimmer comprises a plurality of pixels. In some embodiments, the dimmer is adjusted to completely block the intensity of the light associated with the world object in all of the system field of view. In some embodiments, the method further includes adjusting a brightness associated with the virtual image. In some embodiments, the virtual image is characterized by an image field of view. In some embodiments, the image field of view is equal to the system field of view. In some embodiments, the image field of view is a subset of the system field of view. 
     In general, in another aspect, the invention features an optical system. The optical system may include a projector configured to project a virtual image onto an eyepiece. The optical system may also include a dimmer configured to dim light associated with a world object. The optical system may further include a processor communicatively coupled to the projector and the dimmer. In some embodiments, the processor is configured to perform operations including determining a portion of a system field of view of the optical system to be at least partially dimmed based on detected information. In some embodiments, the operations may also include adjusting the dimmer to reduce an intensity of the light associated with the world object in the portion of the system field of view. 
     In some embodiments, the optical system further includes a light sensor configured to detect light information corresponding to the light associated with the world object. In some embodiments, the detected information includes the light information. In some embodiments, the light information includes a plurality of spatially-resolved light values. In some embodiments, the light information includes a global light value. In some embodiments, the optical system further includes an eye tracker configured to detect gaze information corresponding to an eye of a user of the optical system. In some embodiments, the detected information includes the gaze information. In some embodiments, the gaze information includes a pixel location that intersects with a gaze vector of the eye of the user. In some embodiments, the gaze information includes one or more of a pupil position of the eye of the user, a center of rotation of the eye of the user, a pupil size of the eye of the user, a pupil diameter of the eye of the user, and cone and rod locations of the eye of the user. In some embodiments, the operations further include detecting image information corresponding to the virtual image. In some embodiments, the detected information includes the image information. In some embodiments, the image information includes a plurality of spatially-resolved image brightness values. In some embodiments, the image information includes a global image brightness value. 
     In some embodiments, the operations further include determining multiple spatially-resolved dimming values for the portion of the system field of view based on the detected information. In some embodiments, the dimmer is adjusted according to the plurality of dimming values. In some embodiments, the dimmer comprises a plurality of pixels. In some embodiments, the dimmer is adjusted to completely block the intensity of the light associated with the world object in all of the system field of view. In some embodiments, the operations further include adjusting a brightness associated with the virtual image. In some embodiments, the virtual image is characterized by an image field of view. In some embodiments, the image field of view is equal to the system field of view. In some embodiments, the image field of view is a subset of the system field of view. 
     Numerous benefits can be achieved by way of the present disclosure over conventional techniques. For example, the AR device described herein may be used in varying light levels, from dark indoors to bright outdoors, by globally dimming and/or selectively dimming the ambient light reaching the user&#39;s eyes. Embodiments of the present invention allow for AR and virtual reality (VR) capabilities in a single device by using the pixelated dimmer to attenuate the world light by greater than 99%. Embodiments of the present invention also mitigate vergence accommodation conflict using a variable focal element with discrete or continuous variable depth plane switching technologies. Embodiments of the present invention improve the battery life of the AR device by optimizing the projector brightness based on the amount of detected ambient light. Other benefits will be readily apparent to those skilled in the art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    illustrates an augmented reality (AR) scene as viewed through a wearable AR device according to some embodiments described herein. 
         FIG.  2 A  illustrates one or more general features of an AR device according to the present invention. 
         FIG.  2 B  illustrates an example of an AR device in which a dimmed area is determined based on detected light information. 
         FIG.  2 C  illustrates an example of an AR device in which a dimmed area is determined based on a virtual image. 
         FIG.  2 D  illustrates an example of an AR device in which a dimmed area is determined based on gaze information. 
         FIG.  3    illustrates a schematic view of a wearable AR device according to the present invention. 
         FIG.  4    illustrates a method for operating an optical system. 
         FIG.  5    illustrates an AR device with an eyepiece and a pixelated dimming element. 
         FIG.  6 A  is a front view of an example optically-transmissive spatial light modulator (“SLM”) or display assembly for a see-through display system, according to some embodiments of the present disclosure. 
         FIG.  6 B  is a schematic side view of the example SLM or display assembly of  FIG.  6 A . 
         FIG.  7 A  illustrates an example of a see-through display system in a first state. 
         FIG.  7 B  illustrates an example of the see-through display system of  FIG.  7 A  in a second state different from the first state. 
         FIG.  8 A  illustrates an example of another see-through display system in a first state. 
         FIG.  8 B  illustrates an example of the see-through display system of  FIG.  8 A  in a second state different from the first state. 
         FIG.  9    illustrates an example of a see-through display system according to some embodiments of the present disclosure. 
         FIG.  10    illustrates an example of a see-through display system according to other embodiments of the present disclosure. 
         FIG.  11    illustrates an example of a see-through display system according to yet other embodiments of the present disclosure. 
         FIG.  12    is a diagram of an example computer system useful with a see-through display system. 
     
    
    
     DETAILED DESCRIPTION 
     An ongoing technical challenge with optical see through (OST) augmented reality (AR) devices is the variation in the opacity and/or visibility of the virtual content under varying ambient light conditions. The problem worsens in extreme lighting conditions such as a completely dark room or outside in full bright sunlight. Embodiments disclosed herein can reduce (e.g., solve) these and other problems by dimming the world light at different spatial locations within the field of view of an AR device. In such variable dimming arrangements, the AR device can determine which a portion of the field of view to dim and the amount of dimming that is applied each portion based on various information detected by the AR device. This information may include detected ambient light, detected gaze information, and/or the detected brightness of the virtual content being projected. The functionality of the AR device can be further improved by detecting a direction associated with the ambient light by, for example, detecting spatially-resolved light intensity values. This can improve the AR device&#39;s battery life by dimming only those portions of the field of view in which dimming is needed and/or increasing the projector brightness in certain portions of the field of view. Accordingly, embodiments disclosed herein can enable usage of the AR device in a much wider variety of ambient lighting conditions than traditionally possible. 
       FIG.  1    illustrates an AR scene as viewed through a wearable AR device according to some embodiments described herein. An AR scene  100  is depicted wherein a user of an AR technology sees a real-world park-like setting  106  featuring people, trees, buildings in the background, and a concrete platform  120 . In addition to these items, the user of the AR technology also perceives that he “sees” a robot statue  110  standing upon the real-world platform  120 , and a cartoon-like avatar character  102  flying by, which seems to be a personification of a bumble bee, even though these elements (character  102  and statue  110 ) do not exist in the real world. Due to the extreme complexity of the human visual perception and nervous system, it is challenging to produce a virtual reality (VR) or AR technology that facilitates a comfortable, natural-feeling, rich presentation of virtual image elements amongst other virtual or real-world imagery elements. 
       FIG.  2 A  illustrates one or more general features of an example AR device  200 . AR device  200  includes an eyepiece  202  and a dynamic dimmer  203  configured to be transparent or semi-transparent when AR device  200  is in an inactive mode or an off mode such that a user may view one or more world objects  230  when looking through eyepiece  202  and dynamic dimmer  203 . As illustrated, eyepiece  202  and dynamic dimmer  203  are arranged in a side-by-side configuration and form a system field of view that a user sees when looking through eyepiece  202  and dynamic dimmer  203 . In some embodiments, the system field of view is defined as the entire two-dimensional region occupied by one or both of eyepiece  202  and dynamic dimmer  203 . Although  FIG.  2 A  illustrates a single eyepiece  202  and a single dynamic dimmer  203  (for illustrative reasons), in general AR device  200  includes two eyepieces and two dynamic dimmers, one for each eye of a user. 
     During operation, dynamic dimmer  203  may be adjusted to vary an intensity of a world light  232  from world objects  230  transmitted to eyepiece  202  and the user, thereby providing a dimmed area  236  within the system field of view, which transmits less world light that the other areas of dynamic dimmer  203 . Dimmed area  236  may be a portion or subset of the system field of view, and may be partially or completely dimmed. A partially dimmed area will transmit a fraction of incident world light, while a completely dimmed area will block substantially all incident world light. Dynamic dimmer  203  may be adjusted according to a plurality of spatially-resolved dimming values for dimmed area  236 . 
     Furthermore, during operation of AR device  200 , projector  214  may project a virtual image  222  onto eyepiece  202  which may be observed by the user along with world light  232 . Projecting virtual image  222  onto eyepiece  202  projects a light field  223  (i.e., an angular representation of virtual content) onto the user&#39;s retina so that the user perceives the corresponding virtual content as being positioned at some location within the user&#39;s environment. It should be noted that the virtual content (character  102  and statue  110 ) is depicted in  FIGS.  2 A- 2 D  as being displayed at eyepiece  202  for illustrative purposes only. The virtual content may actually be perceived by the user at various depths beyond eyepiece  202 . For example, the user may perceive statue  110  as being located at approximately the same distance as world objects  230  (i.e., platform  120 ) and character  102  as being located closer to the user. In some embodiments, dynamic dimmer  203  may be positioned closer to the user than eyepiece  202  and may accordingly reduce an intensity of the light associated with virtual image  222  (i.e., light field  223 ). In some embodiments, two dynamic dimmers may be utilized, one on each side of eyepiece  202 . 
     As depicted, AR device  200  includes an ambient light sensor  234  configured to detect world light  232 . Ambient light sensor  234  may be positioned such that world light  232  detected by ambient light sensor  234  is similar to and/or representative of world light  232  that impinges on dynamic dimmer  203  and/or eyepiece  202 . In some embodiments, ambient light sensor  234  may be configured to detect a plurality of spatially-resolved light values corresponding to different pixels of dynamic dimmer  203 . In some embodiments, or in the same embodiments, ambient light sensor  234  may be configured to detect a global light value corresponding to an average light intensity or a single light intensity of world light  232 . Other possibilities are contemplated. 
       FIG.  2 B  illustrates AR device  200  in a state in which dimmed area  236  is determined based on detected light information corresponding to world light  232 . Specifically, ambient light sensor  234 , in this example, detects world light  232  from the sun  233  and may further detect a direction and/or a portion of the system field of view at which world light  232  associated with the sun passes through AR device  200 . In response, dynamic dimmer  203  is adjusted to set dimmed area  236  to cover a portion of the system field of view corresponding to the detected world light, reducing an intensity of world light from sun  233  to eyepiece  202  and the user. As illustrated, dynamic dimmer  203  is adjusted to reduce the transmitted intensity of world light  232  at the center of dimmed area  236  at a greater amount than the extremities of dimmed area  236 . 
       FIG.  2 C  illustrates AR device  200  in a state in which dimmed area  236  is determined based on the location of virtual image  222  in the field of view. Specifically, dimmed area  236  is determined based on the virtual content perceived by the user resulting from the user observing virtual image  222 . In some embodiments, AR device  200  may detect image information that includes a location of virtual image  222  (e.g., a location within the system&#39;s field of view and/or the corresponding location of dynamic dimmer  203  through which the user perceives the virtual content) and/or a brightness of virtual image  222  (e.g., a brightness of the perceived virtual content), among other possibilities. As illustrated, dynamic dimmer  203  may be adjusted to set dimmed area  236  to cover a portion of the system field of view corresponding to virtual image  222  or, alternatively, in some embodiments dimmed area  236  may cover a portion of the system field of view that is not aligned with virtual image  222 . In some embodiments, the dimming values of dimmed area  236  may be determined based on world light  232  detected by ambient light sensor  234  and/or the brightness of virtual image  222 . 
       FIG.  2 D  illustrates AR device  200  in a state in which dimmed area  236  is determined based on gaze information corresponding to an eye of a user. In some embodiments, the gaze information includes a gaze vector  238  of the user and/or a pixel location of dynamic dimmer  203  at which gaze vector  238  intersects with dynamic dimmer  203 . As illustrated, dynamic dimmer  203  may be adjusted to set dimmed area  236  to cover a portion of the system field of view corresponding to an intersection point (or intersection region) between gaze vector  238  and dynamic dimmer  203  or, alternatively, in some embodiments dimmed area  236  may cover a portion of the system field of view that does not correspond to the intersection point (or intersection region) between gaze vector  238  and dynamic dimmer  203 . In some embodiments, the dimming values of dimmed area  236  may be determined based on world light  232  detected by ambient light sensor  234  and/or the brightness of virtual image  222 . In some embodiments, gaze information may be detected by an eye tracker  240  mounted to AR device  200 . 
       FIG.  3    illustrates a schematic view of a further example wearable AR device  300 . AR device  300  includes a left eyepiece  302 A and a left dynamic dimmer  303 A arranged in a side-by-side configuration and a right eyepiece  302 B and a right dynamic dimmer  303 B also arranged in a side-by-side configuration. As depicted, AR device  300  includes one or more sensors including, but not limited to: a left front-facing world camera  306 A attached directly to or near left eyepiece  302 A, a right front-facing world camera  306 B attached directly to or near right eyepiece  302 B, a left side-facing world camera  306 C attached directly to or near left eyepiece  302 A, a right side-facing world camera  306 D attached directly to or near right eyepiece  302 B, a left eye tracker  340 A positioned so as to observe a left eye of a user, a right eye tracker  340 B positioned so as to observe a right eye of a user, and an ambient light sensor  334 . AR device  300  also includes one or more image projection devices such as a left projector  314 A optically linked to left eyepiece  302 A and a right projector  314 B optically linked to right eyepiece  302 B. 
     Some or all of the components of AR device  300  may be head mounted such that projected images may be viewed by a user. In some implementations, all of the components of AR device  300  shown in  FIG.  3    are mounted onto a single device (e.g., a single headset) wearable by a user. In certain implementations, a processing module  350  is physically separate from and communicatively coupled to the other components of AR device  300  by one or more wired and/or wireless connections. For example, processing module  350  may be mounted in a variety of configurations, such as fixedly attached to a frame, fixedly attached to a helmet or hat worn by a user, embedded in headphones, or otherwise removably attached to a user (e.g., in a backpack-style configuration, in a belt-coupling style configuration, etc.). 
     Processing module  350  may include a processor  352  and an associated digital memory  356 , such as non-volatile memory (e.g., flash memory), both of which may be utilized to assist in the processing, caching, and storage of data. The data may include data captured from sensors (which may be, e.g., operatively coupled to AR device  300 ) or otherwise attached to a user, such as cameras  306 , ambient light sensor  334 , eye trackers  340 , microphones, inertial measurement units, accelerometers, compasses, GPS units, radio devices, and/or gyros. For example, processing module  350  may receive image(s)  320  from cameras  306 . Specifically, processing module  350  may receive left front image(s)  320 A from left front-facing world camera  306 A, right front image(s)  320 B from right front-facing world camera  306 B, left side image(s)  320 C from left side-facing world camera  306 C, and right side image(s)  320 D from right side-facing world camera  306 D. In some embodiments, image(s)  320  may include a single image, a pair of images, a video comprising a stream of images, a video comprising a stream of paired images, and the like. Image(s)  320  may be periodically generated and sent to processing module  350  while AR device  300  is powered on, or may be generated in response to an instruction sent by processing module  350  to one or more of the cameras. As another example, processing module  350  may receive light information from ambient light sensor  334 . As another example, processing module  350  may receive gaze information from one or both of eye trackers  340 . As another example, processing module  350  may receive image information (e.g., image brightness values) from one or both of projectors  314 . 
     Eyepieces  302 A and  302 B may include transparent or semi-transparent waveguides configured to direct light from projectors  314 A and  314 B, respectively. Specifically, processing module  350  may cause left projector  314 A to output a left virtual image  322 A onto left eyepiece  302 A (causing a corresponding light field associated with left virtual image  322 A to be projected onto the user&#39;s retina), and may cause right projector  314 B to output a right virtual image  322 B onto right eyepiece  302 B (causing a corresponding light field associated with right virtual image  322 B to be projected onto the user&#39;s retina). In some embodiments, each of eyepieces  302  includes multiple waveguides corresponding to different colors and/or different depth planes. In some embodiments, dynamic dimmers  303  may be coupled to and/or integrated with eyepieces  302 . For example, one of dynamic dimmers  303  may be incorporated into a multi-layer eyepiece and may form one or more layers that make up one of eyepieces  302 . 
     Cameras  306 A and  306 B may be positioned to capture images that substantially overlap with the field of view of a user&#39;s left and right eyes, respectively. Accordingly, placement of cameras  306  may be near a user&#39;s eyes but not so near as to obscure the user&#39;s field of view. Alternatively or additionally, cameras  306 A and  306 B may be positioned so as to align with the incoupling locations of virtual images  322 A and  322 B, respectively. Cameras  306 C and  306 D may be positioned to capture images to the side of a user, e.g., in a user&#39;s peripheral vision or outside the user&#39;s peripheral vision. Image(s)  320 C and  320 D captured using cameras  306 C and  306 D need not necessarily overlap with image(s)  320 A and  320 B captured using cameras  306 A and  306 B. 
     One or more components of AR device  300  may be similar to one or more components described in reference to  FIGS.  2 A- 2 D . For example, in the some embodiments the functionality of eyepieces  302 , dynamic dimmers  303 , projectors  314 , ambient light sensor  334 , and eye trackers  340  may be similar to eyepiece  202 , dynamic dimmer  203 , projector  214 , ambient light sensor  234 , and eye tracker  240 , respectively. In some embodiments, the functionality of processing module  350  may be implemented by two or more sets of electronic hardware components that are housed separately but communicatively coupled. For example, the functionality of processing module  350  may be carried out by electronic hardware components housed within a headset in conjunction with electronic hardware components housed within a computing device physically tethered to the headset, one or more electronic devices within the environment of the headset (e.g., smart phones, computers, peripheral devices, smart appliances, etc.), one or more remotely-located computing devices (e.g., servers, cloud computing devices, etc.), or a combination thereof. One example of such a configuration is described in further detail below with reference to  FIG.  12   . 
       FIG.  4    illustrates an example method  400  for operating an optical system (e.g., AR device  200  or  300 ). Generally, operation of optical systems can include performing the steps of method  400  as shown in  FIG.  4   , or in a different order and not all of the steps need be performed. For example, in some embodiments, one or more of steps  406 ,  408 , and  410  may be omitted during performance of method  400 . One or more steps of method  400  may be performed by a processor (e.g., processor  352 ) or by some other component within the optical system. 
     At step  402 , light (e.g., world light  232 ) associated with an object (e.g., world object  230 ) is received at the optical system. The object may be a real-world object, such as a tree, a person, a house, a building, the sun, etc., that is viewed by a user of the optical system. In some embodiments, the light associated with the object is first received by a dynamic dimmer (e.g., dynamic dimmer  203  or  303 ) or by an external cosmetic lens of the optical system. In some embodiments, the light associated with the object is considered to be received at the optical system when the light reaches one or more components of the optical system (e.g., when the light reaches the dynamic dimmer). 
     At step  404 , a virtual image (e.g., virtual image  222  or  322 ) is projected onto an eyepiece (e.g., eyepiece  202  or  302 ). The virtual image may be projected onto the eyepiece by a projector (e.g., projector  214  or  314 ) of the optical system. The virtual image may be a single image, a pair of images, a video composed of a stream of images, a video composed of a stream of paired images, and the like. In some embodiments, the virtual image is considered to be projected onto the eyepiece when any light associated with the virtual image reaches the eyepiece. In some embodiments, projecting the virtual image onto the eyepiece causes a light field (i.e., an angular representation of virtual content) to be projected onto the user&#39;s retina in a manner such that the user perceives the corresponding virtual content as being positioned at some location within the user&#39;s environment. 
     During steps  406 ,  408 , and  410 , information may be detected by the optical system using, for example, one or more sensors of the optical system. At step  406 , light information corresponding to the light associated with the object is detected. The light information may be detected using a light sensor (e.g., ambient light sensor  234  or  334 ) mounted to the optical system. In some embodiments, the light information includes a plurality of spatially-resolved light values. Each of the plurality of spatially-resolved light values may correspond to a two-dimensional position within the system field of view. For example, each of the light values may be associated with a pixel of the dynamic dimmer. In other embodiments, or in the same embodiments, the light information may include a global light value. The global light value may be associated with the entire system field of view (e.g., an average light value of light impinging on all pixels of the dynamic dimmer). 
     At step  408 , gaze information corresponding to an eye of a user of the optical system is detected. The gaze information may be detected using an eye tracker (e.g., eye tracker  240  or  340 ) mounted to the optical system. In some embodiments, the gaze information includes a gaze vector (e.g., gaze vector  238 ) of the eye of the user. In some embodiments, the gaze information includes one or more of a pupil position of the eye of the user, a center of rotation of the eye of the user, a pupil size of the eye of the user, a pupil diameter of the eye of the user, and cone and rod locations of the eye of the user. The gaze vector may be determined based on one or more components of the gaze information, such as the pupil position, the center of rotation of the eye, the pupil size, the pupil diameter, and/or the cone and rod locations. When the gaze vector is determined based on the cone and rod locations, it may further be determined based on the light information (e.g., the global light value) so as to determine an origin of the gaze vector within a retinal layer of the eye containing the cone and rod locations. In some embodiments, the gaze information includes a pixel or group of pixels of the dynamic dimmer at which the gaze vector intersects with the dynamic dimmer. 
     At step  410 , image information corresponding to a virtual image (e.g., virtual image  222  or  322 ) projected by the projector onto the eyepiece is detected. The image information may be detected by the projector, by a processor (e.g., processor  352 ), or by a separate light sensor. In some embodiments, the image information includes one or more locations within the dynamic dimmer through which the user perceives the virtual content when the user observes the virtual image. In some embodiments, the image information includes a plurality of spatially-resolved image brightness values (e.g., brightness of the perceived virtual content). For example, each of the image brightness values may be associated with a pixel of the eyepiece or of the dynamic dimmer. In one particular implementation, when the processor sends instructions to the projector to project the virtual image onto the eyepiece, the processor may determine, based on the instructions, the spatially-resolved image brightness values. In another particular implementation, when the projector receives the instructions from the processor to project the virtual image onto the eyepiece, the projector sends the spatially-resolved image brightness values to the processor. In another particular implementation, a light sensor positioned on or near the eyepiece detects and sends the spatially-resolved image brightness values to the processor. In other embodiments, or in the same embodiments, the image information includes a global image brightness value. The global image brightness value may be associated with the entire system field of view (e.g., an average image brightness value of the entire virtual image). 
     At step  412 , a portion of the system field of view to be at least partially dimmed is determined based on the detected information. The detected information may include the light information detected during step  406 , the gaze information detected during step  408 , and/or the image information detected during step  410 . In some embodiments, the portion of the system field of view is equal to the entire system field of view. In various embodiments, the portion of the system field of view may be equal to 1%, 5%, 10%, 25%, 50%, or 75%, etc., of the system field of view. In some embodiments, the different types of information may be weighted differently in determining the portion to be at least partially dimmed. For example, gaze information, when available, may be weighted more heavily in determining the portion to be at least partially dimmed than light information and image information. In one particular implementation, each type of information may independently be used to determine a different portion of the system field of view to be at least partially dimmed, and subsequently the different portions may be combined into a single portion using an AND or an OR operation. 
     At step  414 , multiple spatially-resolved dimming values for the portion of the system field of view are determined based on the detected information. In some embodiments, the dimming values are determined using a formulaic approach based on a desired opacity or visibility of the virtual content. In one particular implementation, the visibility of the virtual content may be calculated using the following equation: 
     
       
         
           
             V 
             = 
             
               
                 
                   I 
                   max 
                 
                 ⁡ 
                 
                   ( 
                   
                     1 
                     - 
                     
                       1 
                       C 
                     
                   
                   ) 
                 
               
               
                 
                   
                     I 
                     max 
                   
                   ⁡ 
                   
                     ( 
                     
                       1 
                       + 
                       
                         1 
                         C 
                       
                     
                     ) 
                   
                 
                 + 
                 
                   2 
                   ⁢ 
                   
                     I 
                     
                       b 
                       ⁢ 
                       a 
                       ⁢ 
                       c 
                       ⁢ 
                       k 
                     
                   
                 
               
             
           
         
       
     
     where V is the visibility, I max  is the brightness of the virtual image as indicated by the image information, I back  is related to a light value associated with the world object as indicated by the light information (which may be modified by the determined dimming value), and C is a desired contrast (e.g., 100:1). For example, the visibility equation may be used at each pixel location of the dimmer to calculate a dimming value for the particular pixel location using the brightness of the virtual image at the particular pixel location and the light value associated with the world object at the particular pixel location. 
     At step  416 , the dimmer is adjusted to reduce an intensity of the light associated with the object in the portion of the system field of view. For example, the dimmer may be adjusted such that the intensity of the light associated with the object impinging on each pixel location of the dimmer is reduced according to the dimming value determined for that particular pixel location. As used in the present disclosure, adjusting the dimmer may include initializing the dimmer, activating the dimmer, powering on the dimmer, modifying or changing a previously initialized, activated, and/or powered on dimmer, and the like. In some embodiments, the processor may send data to the dimmer indicating both the portion of the system field of view and the plurality of spatially-resolved dimming values. 
     At step  418 , the projector is adjusted to adjust a brightness associated with the virtual image. For example, in some embodiments it may be difficult to achieve a desired opacity or visibility of the virtual content without increasing or decreasing the brightness of the virtual object. In such embodiments, the brightness of the virtual image may be adjusted before, after, simultaneously, or concurrently with adjusting the dimmer. 
       FIG.  5    illustrates an AR device  500  with an eyepiece  502  and a pixelated dimming element  503  composed of a spatial grid of dimming areas (i.e., pixels) that can have various levels of dimming independent of each other (i.e., they are independently variable). Each of the dimming areas may have an associated size (e.g., width and height) and an associated spacing (e.g., pitch). As illustrated, the spatial grid of dimming areas may include one or more dark pixels  506  providing complete dimming of incident light and one or more clear pixels  508  providing complete transmission of incident light. Adjacent pixels within pixelated dimming element  503  may be bordering (e.g., when the pitch is equal to the width) or may be separated by gaps or channels (e.g., when the pitch is greater than the width). 
     In various embodiments, pixelated dimming element  503  uses liquid crystal technology. Such technology typically includes a layer of a liquid crystal material (e.g., having a nematic phase) aligned relative to one or more electrode layers so that the liquid crystal material can be reoriented depending on an electric field strength applied to the pixel (e.g., by applying a potential difference across the liquid crystal layer using electrodes on opposing sides of the liquid crystal layer). Examples of liquid crystal modes include twisted nematic (“TN”) or vertically aligned (“VA”) liquid crystals. Electrically controlled birefringence (“ECB”) liquid crystal modes can also be used. Liquid crystal phases other than nematic phases can be used, such as, ferroelectric liquid crystals. In some embodiments, dye doped or guest-host liquid crystal materials can be used. 
       FIG.  6 A  depicts a front view of an example optically-transmissive spatial light modulator (“SLM”) or display assembly  603  for a see-through display system. Similarly,  FIG.  6 B  depicts a schematic cross-sectional view of the example SLM or display assembly of  FIG.  6 A . In some examples, the optically-transmissive SLM or display assembly  603  may form all or part of an external cover of an augmented reality system. The assembly  603  may, for example, correspond to an optically-transmissive controllable dimming assembly that is similar or equivalent to one or more of the dimming assemblies described herein, an optically-transmissive LCD, an optically-transmissive OLED display, and the like. In some implementations, the assembly  603  of  FIGS.  6 A- 6 B  may correspond to one or more of components  203 ,  303   a ,  303 B, and  503  as described above with reference to  FIGS.  2 A- 2 C,  3 , and  5   , respectively. Additional examples of controllable dimming assembly architectures and control schemes are described in further detail in U.S. Provisional Patent Application Ser. No. 62/725,993, U.S. Provisional Patent Application Ser. No. 62/731,755, U.S. Provisional Patent Application Ser. No. 62/858,252, and U.S. Provisional Patent Application Ser. No. 62/870,896, all of which are incorporated herein by reference in their entirety. 
     In the example of  FIGS.  6 A- 6 B , the assembly  603  includes a liquid crystal layer  618  sandwiched between an outer electrode layer  616 A and an inner electrode layer  616 B, which are in turn sandwiched between an outer polarizer  612 A and an inner polarizer  612 B. The outer and inner polarizers  612 A,  612 B may be configured to linearly polarize unpolarized light that passes therethrough. The assembly  603  includes an outer substrate layer  620 A positioned between the outer polarizer  612 A and the outer electrode layer  616 A, an inner substrate layer  620 B positioned between the inner polarizer  612 B and the inner electrode layer  616 B. Substrate layers  620 A and  620 B support electrode layers  616 A and  616 B and are typically formed from an optically transparent material, such as a glass or plastic. The assembly  603  further includes an outer optical retarder  614 A (e.g., an A-plate) positioned between the outer polarizer  612 A and the outer electrode layer  616 A, an inner optical retarder  614 B (e.g., an A-plate) positioned between the inner polarizer  612 B and the inner electrode layer  616 B. 
     As mentioned above, in some implementations, the assembly  803  may include or correspond to an ECB cell. Advantageously, ECB cells can be configured to modulate circularly polarized light, e.g., to change the ellipticity of the circularly polarized light, for example. 
     In operation, the outer polarizer  612 A imparts a first polarization state (e.g., linear polarization along the vertical direction of  FIG.  6 A ) to ambient light propagating therethrough toward a user&#39;s eye. Next, depending on their orientation, liquid crystal molecules contained within the liquid crystal layer  618  further rotate/retard the polarized ambient light as it traverses the liquid crystal layer  618 . For example, liquid crystal layer  618  can rotate the linearly polarized light so that the plane of polarization is different from the plane of polarization of the first polarization state. Alternatively, or additionally, the liquid crystal layer can retard the polarization, e.g., transforming linearly polarized light into elliptically or circularly polarized light. Generally, the amount of rotation/retardation depends on the birefringence of the liquid crystal material, its orientation, and the thickness of liquid crystal layer  618 . The amount of rotation/retardation also depends on the electric field applied to liquid crystal layer  618 , e.g., by applying a potential difference across outer and inner electrode layers  616 A,  616 B. It follows that the amount of polarization rotation imparted by the pair of electrode layers  616 A,  616 B and liquid crystal layer  618  may be varied on a pixel-by-pixel basis according to the voltage applied to the electrode layers at each respective pixels. 
     Retardation of the polarized light is also influenced by the outer and inner optical retarders  614 A,  614 B. For example, use of a quarter wave plate as outer optical retarder  614 A will serve to retard the linearly polarized light transmitted by polarizer  612 A to transform the linearly polarized light to circularly polarized light where the fast axis of the quarter wave plate is appropriately oriented with respect to the pass axis of the linear polarizer (e.g., at 45°). 
     Lastly, the inner polarizer  612 B may transmit light of a second, different polarization state (e.g., horizontal polarization) compared to outer polarizer  612 A. The second polarization state may be orthogonal to the polarization state imparted on the ambient light by outer polarizer  612 A. In such circumstances, where the cumulative effect of the liquid crystal layer  618 , and the outer and/or inner optical retarders  614 A,  614 B rotates polarized light transmitted by outer polarizer  612 A, inner polarizer  612 B will transmit the light transmitted by outer polarizer  612 A, albeit rotated by 90 degrees. Alternatively, where the cumulative effect of liquid crystal layer  618  and the optical retarders  614 A and  614 B leaves the polarization state of light from polarizer  612 A unchanged, it will be blocked by inner polarizer  612 B. Accordingly, the inner polarizer  612 B may allow portions of ambient light in the second polarization state to pass therethrough unaffected, and may attenuate portions of ambient light in polarization states other than the second polarization state. The amount of polarization rotation can be controlled on a pixel-by-pixel basis by the electric field strength applied to the liquid crystal layer at each pixel, allowing light transmission of device  603  to be controlled on a pixel-by-pixel basis. 
     Generally, the pixel structure of the electrode layers can vary depending, e.g., on the nature of the liquid crystal layer, the pixel size, etc. In some embodiments, one of the outer electrode layer  616 A and the inner electrode layer  616 B may correspond to a layer of individually-addressable electrodes (i.e., pixels) arranged in a two-dimensional array. For instance, in some examples, the inner electrode layer  616 B may correspond to an array of pixel electrodes that may each be selectively controlled by the assembly  603  to generate a respective electric field/voltage in tandem with the outer electrode layer  616 A, which may correspond to a single planar electrode. In some examples, the electrodes of one or both of the outer and inner electrode layers  616 A,  616 B may be made out of an optically-transmissive conducting material, such as indium tin oxide (“ITO”). 
     As shown in  FIGS.  6 A- 6 B , the assembly  603  also includes metal line traces or conductors  617   a - n . Each pixel electrode in the array of pixel electrodes of the assembly  603  is electrically coupled to a corresponding thin film transistor (“TFT”), which in turn is electrically coupled to a corresponding pair of metal line traces or conductors in the plurality of metal line traces or conductors  617   a - n . Such metal line traces or conductors are positioned in “transmissive gap” regions between pixels (e.g., pixel electrodes of the inner electrode layer  616 B), and are further electrically coupled to one or more circuits for driving or otherwise controlling the state of each pixel. In some examples, such one or more circuits may include a chip on glass (“COG”) component  622 . The COG component  622  may be disposed upon a layer of glass in the assembly  603 , such as the inner glass layer  620 B. In some implementations, the COG component  622  may be laterally offset from the array of pixels (e.g., array of pixel electrodes of the inner electrode layer  616 B). In this way, the COG component  622  of the assembly  603  may be positioned outside of a user&#39;s field of view (“FOV”), obscured by housing or other components of the see-through display system, or a combination thereof. Pixels that use active switching elements such as TFTs are referred to as being actively addressed. 
       FIG.  7 A  illustrates an example of a see-through display system  700  in a first state (state “A”). Similarly,  FIG.  7 B  illustrates the see-through display system  700  in a second state (state “B”) that is different from the first state (state “A”). As shown in  FIGS.  7 A- 7 B , the system  700  includes an eyepiece  702  and an optically-transmissive SLM or display assembly  703 . In some examples, the eyepiece  702  of  FIGS.  7 A- 7 B  corresponds to one or more of components  202 ,  302 A,  302 B, and  502  as described above with reference to  FIGS.  2 A- 2 C,  3 , and  5   , respectively. In some implementations, the assembly  703  of  FIGS.  7 A- 7 B  may correspond to one or more of components  203 ,  303   a ,  303 B,  503 , and  603  as described above with reference to  FIGS.  2 A- 2 C,  3 ,  5 , and  6 A- 6 B  respectively. 
     The eyepiece  702  of the system  700  includes three waveguides  1210 ,  1220 , and  1230 . Each of the three waveguides  1210 ,  1220 , and  1230  may, for example, correspond to a different color of light and/or depth of virtual content. As shown in  FIGS.  7 A- 7 B , the eyepiece  702  further includes incoupling optical elements  1212 ,  1222 , and  1232  disposed upon waveguides  1210 ,  1220 , and  1230 , respectively. The incoupling optical elements  1212 ,  1222 , and  1232  may be configured to couple light into waveguides  1210 ,  1220 , and  1230 , respectively, for propagation via total internal reflection (TIR). In addition, the eyepiece  702  also includes outcoupling diffractive optical elements  1214 ,  1224 , and  1234  disposed upon waveguides  1210 ,  1220 , and  1230 , respectively. The outcoupling diffractive optical elements  1214 ,  1224 , and  1234  may be configured to couple light out of waveguides  1210 ,  1220 , and  1230 , respectively, toward one or both eyes of a viewer of the system  700 . As can be seen in  FIG.  7 A , in the first state (state “A”), no light is coupled into or out of the eyepiece  702 . 
     In the example of  FIG.  7 B , the system  700  is in a second, different state (state “B”) in which light is (i) coupled into the waveguides  1210 ,  1220 , and  1230  by way of the incoupling optical elements  1212 ,  1222 , and  1232 , (ii) propagating through the waveguides  1210 ,  1220 , and  1230  via total internal reflection (TIR), and (iii) coupled out of the waveguides  1210 ,  1220 , and  1230  by way of the outcoupling diffractive optical elements  1214 ,  1224 , and  1234 . As each light path  1240 ,  1242  and  1244  respectively incouples at locations  1212 ,  1222 , and  1232  impact a respective outcoupling diffractive optical element  1214 ,  1224 , or  1234  (outcoupled light from paths  1222  and  1232  not depicted) disposed upon waveguide  1210 ,  1220 , and  1230 , it diffracts light both towards the viewer and away from the viewer, towards the world side of the device. This light propagation is depicted as multiple beamlets each propagating in two opposite directions: one towards the viewer represented by light bundle  3010 , and one in a direction away from the viewer represented by light bundle  3020 . 
     The light bundle  3020  propagating away from the viewer may cause undesirable effects if it reflects off elements of the system in its path because these reflections can reach the viewer as stray light. For example, reflections of this light from the subsequent waveguide  1220  can interfere with light bundle  3010 , increasing blurriness and/or reducing contrast of imagery projected by system  700 . Furthermore, in some situations, the light bundle  3020  may reflect off of one or more components of the assembly  703 , such as one or more metal line traces or conductors of the assembly  703  that are equivalent or similar to the metal line traces or conductors  617   a - n  of the assembly  603  as described above with reference to  FIGS.  6 A- 6 B , which may produce “ghost” images and other undesirable artifacts. Waveguide optical systems that employ pupil expander technology can further aggravate these problems. 
       FIGS.  8 A and  8 B  respectively illustrate an example of a see-through display system  800  in a first state (state “A”) and a second state (state “B”) that is different from the first state (state “A”). As shown in  FIGS.  8 A- 8 B , the system  800  includes eyepiece  702  and an optically-transmissive SLM  803 . In some examples, the eyepiece  702  of  FIGS.  8 A- 8 B  may correspond to the eyepiece  702  of  FIGS.  7 A- 7 B , but may further include an outer glass cover  1250 A and an inner glass cover  1250 B positioned on either side of the waveguide stack. 
     In some implementations, the assembly  803  of  FIGS.  8 A- 8 B  may correspond to one or more of assemblies  603  and  703  as described above with reference to  FIGS.  6 A- 6 B and  7 A- 7 B , respectively, but where the inner and outer optical retarders  614 A and  614 B are quarter wave plates (QWP)  624 A and  624 B, respectively. The outer and inner QWPs  624 A,  624 B serve to convert linearly polarized light passing therethrough into circularly polarized light. For example, light propagating from the viewer&#39;s environment toward the viewer can be linearly polarized as it passes through the outer polarizer  612 A, and subsequently become circularly polarized as it passes through the outer QWP  624 A. Similarly, light propagating away from the viewer and toward the assembly  803 , such as one or more portions of light bundle  3020 , may become linearly polarized as it passes through the inner polarizer  612 B, and subsequently become circularly polarized as it passes through the inner QWP  624 B. As can be seen in  FIG.  8 A , in the first state (state “A”), no light is coupled into or out of the eyepiece  702 . 
     In the example of  FIG.  8 B , the system  800  is in a second, different state (state “B”) in which light is coupled out of the eyepiece  702  in two directions: one towards the viewer represented by light ray  3011 , and one in a direction away from the viewer represented by light ray  3021 . Light rays  3011  and  3021  may represent portions of light bundles similar to light bundles  3010  and  3020  as described above with reference to  FIG.  7 B . As shown in  FIG.  8 B , as the light ray  3021  propagates away from the eyepiece  702  and toward the assembly  803 , the light ray  3021  is linearly polarized as it passes through the inner polarizer  612 B, and subsequently become circularly polarized as it passes through the inner QWP  624 B. Upon reaching one or more metal line traces or conductors of the assembly  803 , the light ray  3021  may reflect back toward the eyepiece  702 , which may effectively reverse the handedness of the circular polarization of the light ray  3021  (i.e., left-handed circularly polarized light becomes right-handed circularly polarized light up reflection, and vice versa). Passing through the inner QWP  624 B on its way back toward the eyepiece  702 , the circularly polarized light is converted back to linearly polarized light, but polarized orthogonal to the pass state of polarizer  612 B. Accordingly, the reflected light is attenuated (e.g., blocked) by polarizer  612 B. In this way, the system  800  reduces the effects of “ghost” images that may be produced as a result of light from the eyepiece  702  reflecting off of components of the assembly  803  and back toward the viewer. 
     In some embodiments, the inner optical retarder  614 A is a quarter wave plate and the outer optical retarder  614 B is an A-plate (e.g., a uniaxial birefringent film with fast axis in the plane of the plate) with a retardation that is slightly different from quarter wave retardation. For example, in some implementations, the liquid crystal can retain some residual birefringence even in its state of lowest birefringence. This can occur, for instance, due to the orientation of liquid crystal molecules close to alignment layers within a liquid crystal layer. At and close to the alignment layer, the liquid crystal molecules can retain their alignment even in the presence of maximum electric field strength applied by the electrode structure. It is possible to compensate for such residual retardation of the liquid crystal material by increasing a retardation of the outer optical retarder  614 B above quarter wave retardation. For example, the outer optical retarder  614 B can be an A-plate with a retardation that is 5 nm or more greater than quarter wave retardation (e.g., 10 nm or more, 15 nm or more, 20 nm or more, 25 nm or more, 30 nm or more, 35 nm or more, 40 nm or more, 45 nm or more, such as up to 50 nm). Compensating for residual retardation of the liquid crystal layer can reduce light leakage through the dynamic dimmer. 
     The fast axes of the inner and outer optical retarders are generally arranged relative to the alignment directions of the liquid crystal layer and the pass axes of the inner and outer polarizers so that the dynamic dimmer provides a large dynamic range with good light extinction in its darkest state. In some embodiments, the inner and outer optical retarders have their fast axes oriented at 45° relative to the pass axes of their adjacent polarizer. The fast axes of the inner and outer optical retarders can be oriented at 90° with respect to each other. In certain embodiments, the fast axis of the outer optical retarder is oriented at 90° with respect to an alignment direction of the liquid crystal layer at the world-side boundary of the liquid crystal layer. 
     Generally, the performance of the dynamic dimmer is optimized for at least one light propagation direction at at least one wavelength. For example, the dynamic dimmer&#39;s performance can be optimized for “on-axis” light, i.e., light that is incident normal to the layers of the stack forming the dynamic dimmer. In certain embodiments, the performance of the dynamic dimmer is optimized for green light, e.g., light with a wavelength of 550 nm. Furthermore, while the performance of the dynamic dimmer may be optimal for one wavelength along one light propagation direction, in general, the dimmer will be configured to provide adequate performance over a range of angles and a range of wavelengths. For example, the dynamic dimmer can be configured to provide adequate performance (e.g., a dynamic range above a minimum performance threshold) over a range of light propagation angles the same size or larger than the field of view of the display. Furthermore, the performance can be adequate over a range of operative wavelengths (e.g., spanning the color gamut of the display). To achieve adequate performance over a range of wavelengths, in some implementations, achromatic optical components, such as achromatic optical retarders, can be used. Achromatic A-plates (e.g., achromatic quarter wave plates) can be provided by using two or more different birefringent materials that have different dispersions, for instance. 
       FIG.  9    illustrates a further example of a see-through display system  900 . The system  900  of  FIG.  9    may correspond to the system  800  of  FIGS.  8 A- 8 B , but may further include an outer anti-reflective layer  626 A and an inner anti-reflective layer  626 B. The outer anti-reflective layer  626 A may be positioned adjacent the outer polarizer  612 A, while the inner anti-reflective layer  626 B may be positioned adjacent the inner polarizer  612 B. The outer and inner anti-reflective layers  626 A,  626 B may serve to further reduce the presence undesirable effects, such as “ghost” images, by limiting the reflection of light from coupled out of the eyepiece  702 . In some situations, at least some such reflections may be a result of refractive index mismatches between different layers of display system  900  or between air and a layer of display system  900  (i.e., Fresnel reflections). Anti-reflective layers can reduce such reflections. A variety of different appropriate anti-reflective layers can be used, including single layer or multi-layer anti-reflective films. Such layers can be optimized for on axis light or for light at some other non-normal angle of incidence. Such layers can be optimized for one or more visible wavelengths. In some embodiments, the anti-reflective layers are optimized for light having a wavelength in the green portion of the visible spectrum, e.g., at or near the maximum photopic sensitivity of the human eye. Examples of anti-reflective layers for use in eyepieces are described in U.S. patent application Ser. No. 16/214,575, filed on Dec. 10, 2018, published on Jun. 13, 2019 as U.S. Publication No. 2019/0179057, which is expressly incorporated by reference herein in its entirety. 
       FIG.  10    illustrates yet another example of a see-through display system  1000 . The system  1000  of  FIG.  10    may correspond to the system  900  of  FIG.  9   , but may further include a second outer QWP  634 A and a second inner QWP  634 B. The second outer QWP  634 A is positioned adjacent the outer polarizer  612 A, while the second inner QWP  634 B is positioned adjacent the inner polarizer  612 B. The second QWP  634 A may serve to further reduce the presence undesirable effects, such as “ghost” images, by modulating the polarization state of light traveling away from the viewer and having passed through the outer polarizer  612 A, such that said light is attenuated by the outer polarizer  612 A in the event that it reflects off of one or more objects in the vicinity of the system  1000  (e.g., beyond the assembly  803 ). Similarly, the inner QWP  634 B may serve to further reduce the presence undesirable effects, such as “ghost” images, by modulating the polarization state of light traveling toward the viewer and having passed through the inner polarizer  612 B, such that said light is attenuated by the inner polarizer  612 A in the event that it reflects off of one or more components of the system  1000  positioned between the inner polarizer  612 B and the viewer. 
       FIG.  11    illustrates another example of a see-through display system  1100 . The system  1100  of  FIG.  11    may correspond to the system  1000  of  FIG.  10   , but may include an index-matched layer  628  in place of component  626 B. The index-matched layer  628  may be a layer of material (e.g., a gel or optical adhesive) having a refractive index equivalent or similar to the refractive index of the second inner QWP  634 B or whatever layer of assembly  803  is closest to eyepiece  702 . Including an index matching layer may serve to further reduce the presence undesirable effects, such as “ghost” images, by reducing Fresnel reflections of light from eyepiece  702  incident on assembly  803 . 
     In some implementations, a see-through display system may include some or all of the components from each of one or more of  FIGS.  8 A- 11   . Other configurations, including see-through display systems with different combinations of components from each of one or more of  FIGS.  8 A- 11   , are possible. Although described primarily within the context of optically-transmissive spatial light modulators and displays it is to be understood that one or more of the configurations and techniques described herein may be leveraged in other systems with see-through pixel arrays. 
     Some implementations described in this specification can be implemented as one or more groups or modules of digital electronic circuitry, computer software, firmware, or hardware, or in combinations of one or more of them. Although different modules can be used, each module need not be distinct, and multiple modules can be implemented on the same digital electronic circuitry, computer software, firmware, or hardware, or combination thereof. 
     Some implementations described in this specification can be implemented as one or more computer programs, i.e., one or more modules of computer program instructions, encoded on computer storage medium for execution by, or to control the operation of, data processing apparatus. A computer storage medium can be, or can be included in, a computer-readable storage device, a computer-readable storage substrate, a random or serial access memory array or device, or a combination of one or more of them. Moreover, while a computer storage medium is not a propagated signal, a computer storage medium can be a source or destination of computer program instructions encoded in an artificially generated propagated signal. The computer storage medium can also be, or be included in, one or more separate physical components or media (e.g., multiple CDs, disks, or other storage devices). 
     The term “data processing apparatus” encompasses all kinds of apparatus, devices, and machines for processing data, including by way of example a programmable processor, a computer, a system on a chip, or multiple ones, or combinations, of the foregoing. The apparatus can include special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit). The apparatus can also include, in addition to hardware, code that creates an execution environment for the computer program in question, e.g., code that constitutes processor firmware, a protocol stack, a database management system, an operating system, a cross-platform runtime environment, a virtual machine, or a combination of one or more of them. The apparatus and execution environment can realize various different computing model infrastructures, such as web services, distributed computing and grid computing infrastructures. 
     A computer program (also known as a program, software, software application, script, or code) can be written in any form of programming language, including compiled or interpreted languages, declarative or procedural languages. A computer program may, but need not, correspond to a file in a file system. A program can be stored in a portion of a file that holds other programs or data (e.g., one or more scripts stored in a markup language document), in a single file dedicated to the program in question, or in multiple coordinated files (e.g., files that store one or more modules, sub programs, or portions of code). A computer program can be deployed to be executed on one computer or on multiple computers that are located at one site or distributed across multiple sites and interconnected by a communication network. 
     Some of the processes and logic flows described in this specification can be performed by one or more programmable processors executing one or more computer programs to perform actions by operating on input data and generating output. The processes and logic flows can also be performed by, and apparatus can also be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit). 
     Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and processors of any kind of digital computer. Generally, a processor will receive instructions and data from a read only memory or a random access memory or both. A computer includes a processor for performing actions in accordance with instructions and one or more memory devices for storing instructions and data. A computer may also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto optical disks, or optical disks. However, a computer need not have such devices. Devices suitable for storing computer program instructions and data include all forms of non-volatile memory, media and memory devices, including by way of example semiconductor memory devices (e.g., EPROM, EEPROM, flash memory devices, and others), magnetic disks (e.g., internal hard disks, removable disks, and others), magneto optical disks, and CD ROM and DVD-ROM disks. The processor and the memory can be supplemented by, or incorporated in, special purpose logic circuitry. 
     To provide for interaction with a user, operations can be implemented on a computer having a display device (e.g., a monitor, or another type of display device) for displaying information to the user and a keyboard and a pointing device (e.g., a mouse, a trackball, a tablet, a touch sensitive screen, or another type of pointing device) by which the user can provide input to the computer. Other kinds of devices can be used to provide for interaction with a user as well; for example, feedback provided to the user can be any form of sensory feedback, e.g., visual feedback, auditory feedback, or tactile feedback; and input from the user can be received in any form, including acoustic, speech, or tactile input. In addition, a computer can interact with a user by sending documents to and receiving documents from a device that is used by the user; for example, by sending web pages to a web browser on a user&#39;s client device in response to requests received from the web browser. 
     A computer system may include a single computing device, or multiple computers that operate in proximity or generally remote from each other and typically interact through a communication network. Examples of communication networks include a local area network (“LAN”) and a wide area network (“WAN”), an inter-network (e.g., the Internet), a network comprising a satellite link, and peer-to-peer networks (e.g., ad hoc peer-to-peer networks). A relationship of client and server may arise by virtue of computer programs running on the respective computers and having a client-server relationship to each other. 
       FIG.  12    shows an example computer system  1200  that includes a processor  1210 , a memory  1220 , a storage device  1230  and an input/output device  1240 . Each of the components  1210 ,  1220 ,  1230  and  1240  can be interconnected, for example, by a system bus  1250 . The processor  1210  is capable of processing instructions for execution within the system  1200 . In some implementations, the processor  1210  is a single-threaded processor, a multi-threaded processor, or another type of processor. The processor  1210  is capable of processing instructions stored in the memory  1220  or on the storage device  1230 . The memory  1220  and the storage device  1230  can store information within the system  1200 . 
     The input/output device  1240  provides input/output operations for the system  1200 . In some implementations, the input/output device  1240  can include one or more of a network interface device, e.g., an Ethernet card, a serial communication device, e.g., an RS-232 port, and/or a wireless interface device, e.g., an 802.11 card, a 3G wireless modem, a 4G wireless modem, etc. In some implementations, the input/output device can include driver devices configured to receive input data and send output data to other input/output devices, e.g., wearable display system  1260 . In some implementations, mobile computing devices, mobile communication devices, and other devices can be used. 
     The methods, systems, and devices discussed above are examples. Various configurations may omit, substitute, or add various procedures or components as appropriate. For instance, in alternative configurations, the methods may be performed in an order different from that described, and/or various stages may be added, omitted, and/or combined. Also, features described with respect to certain configurations may be combined in various other configurations. Different aspects and elements of the configurations may be combined in a similar manner. Also, technology evolves and, thus, many of the elements are examples and do not limit the scope of the disclosure or claims. 
     Specific details are given in the description to provide a thorough understanding of exemplary configurations including implementations. However, configurations may be practiced without these specific details. For example, well-known circuits, processes, algorithms, structures, and techniques have been shown without unnecessary detail in order to avoid obscuring the configurations. This description provides example configurations only, and does not limit the scope, applicability, or configurations of the claims. Rather, the preceding description of the configurations will provide those skilled in the art with an enabling description for implementing described techniques. Various changes may be made in the function and arrangement of elements without departing from the spirit or scope of the disclosure. 
     Having described several example configurations, various modifications, alternative constructions, and equivalents may be used without departing from the spirit of the disclosure. For example, the above elements may be components of a larger system, wherein other rules may take precedence over or otherwise modify the application of the technology. Also, a number of steps may be undertaken before, during, or after the above elements are considered. Accordingly, the above description does not bind the scope of the claims. 
     As used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise. Thus, for example, reference to “a user” includes a plurality of such users, and reference to “the processor” includes reference to one or more processors and equivalents thereof known to those skilled in the art, and so forth. 
     Also, the words “comprise”, “comprising”, “contains”, “containing”, “include”, “including”, and “includes”, when used in this specification and in the following claims, are intended to specify the presence of stated features, integers, components, or steps, but they do not preclude the presence or addition of one or more other features, integers, components, steps, acts, or groups. 
     It is also understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims.