Patent Publication Number: US-2022214746-A1

Title: Method and Device for Image Display and Eye Tracking through a Catadioptric Lens

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Provisional Patent App. 62/564,896, filed on Sep. 28, 2017, and U.S. Non-Provisional patent application Ser. No. 16/015,769, filed on Jun. 22, 2018, which are hereby incorporated by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     The present disclosure generally relates to virtual reality and augmented reality head-mounted devices, and in particular, to systems, methods, and devices for displaying an image and tracking an eye of a user through a catadioptric lens. 
     BACKGROUND 
     Head-mounted devices (HMDs) generally include a display and an eyepiece that makes the display appear to the user to be at a virtual distance farther than the actual distance from the eye to the display. However, in many circumstances, the actual distance needed with a standard refractive lens causes the size of the HMD to be unwieldy for mounting on the head of the user. In various implementations, an eye tracking system is included in the HMD to augment the viewing experience, potentially increasing the size of the HMD further. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the present disclosure can be understood by those of ordinary skill in the art, a more detailed description may be had by reference to aspects of some illustrative implementations, some of which are shown in the accompanying drawings. 
         FIG. 1  is a block diagram of an example operating environment in accordance with some implementations. 
         FIG. 2  is a block diagram of an example controller in accordance with some implementations. 
         FIG. 3  is a block diagram of an example head-mounted device (HMD) in accordance with some implementations. 
         FIG. 4  illustrates a block diagram of a head-mounted device in accordance with some implementations. 
         FIG. 5  illustrates a block diagram of a head-mounted device having one or light sources disposed between the eyepiece and the display in accordance with some implementations. 
         FIG. 6  illustrates a block diagram of a head-mounted device having a selectively distortive eyepiece in accordance with some implementations. 
         FIG. 7  illustrates a block diagram of a head-mounted device having a catadioptric eyepiece in accordance with some implementations. 
         FIG. 8  is a ray-trace diagram of light emitted from a display towards the eye of a user through a selectively distortive split meniscus lens. 
         FIG. 9  is a functional block diagram of an optical system including a split meniscus lens in accordance with some implementations. 
         FIG. 10A  is a functional block diagram of an optical system including a selectively distortive split meniscus lens in accordance with some implementations. 
         FIG. 10B  is a plot of reflectance and transmittance versus wavelength for a partially reflective, partially transmissive surface. 
         FIG. 11A  illustrates a block diagram of a head-mounted device having a catadioptric eyepiece with an eye tracking system disposed behind the display in accordance with some implementations. 
         FIG. 11B  illustrates a block diagram of a head-mounted device having a catadioptric eyepiece with an eye tracking system disposed in front of the eyepiece in accordance with some implementations. 
         FIG. 11C  illustrates a block diagram of a head-mounted device having a catadioptric eyepiece disposed between portions of an eye tracking system in accordance with some implementations. 
         FIG. 12  illustrates a functional block diagram of an electronic device according to some implementations. 
         FIG. 13  is a flowchart representation of a method of image display and eye tracking in accordance with some implementations. 
     
    
    
     In accordance with common practice the various features illustrated in the drawings may not be drawn to scale. Accordingly, the dimensions of the various features may be arbitrarily expanded or reduced for clarity. In addition, some of the drawings may not depict all of the components of a given system, method or device. Finally, like reference numerals may be used to denote like features throughout the specification and figures. 
     SUMMARY 
     Various implementations disclosed herein include devices, systems, and methods for image display and eye tracking through a catadioptric lens. In some implementations, an apparatus includes a display to emit light in a first wavelength range and a camera to detect light in a second wavelength range. The apparatus includes an eyepiece to distort light in the first wavelength range and one or more light sources, disposed between the eyepiece and the display, to emit light in the second wavelength range. 
     In some implementations, an apparatus includes a display to emit light in a first wavelength range, one or more light sources to emit light in a second wavelength range, and a camera to detect light in a second wavelength range. The apparatus further comprises an eyepiece to reflect and refract light in the first wavelength range while passing, without substantial distortion, light in the second wavelength range. 
     In accordance with some implementations, a device includes one or more processors, a non-transitory memory, and one or more programs; the one or more programs are stored in the non-transitory memory and configured to be executed by the one or more processors and the one or more programs include instructions for performing or causing performance of any of the methods described herein. In accordance with some implementations, a non-transitory computer readable storage medium has stored therein instructions, which, when executed by one or more processors of a device, cause the device to perform or cause performance of any of the methods described herein. In accordance with some implementations, a device includes: one or more processors, a non-transitory memory, and means for performing or causing performance of any of the methods described herein. 
     DESCRIPTION 
     Numerous details are described in order to provide a thorough understanding of the example implementations shown in the drawings. However, the drawings merely show some example aspects of the present disclosure and are therefore not to be considered limiting. Those of ordinary skill in the art will appreciate that other effective aspects and/or variants do not include all of the specific details described herein. Moreover, well-known systems, methods, components, devices and circuits have not been described in exhaustive detail so as not to obscure more pertinent aspects of the example implementations described herein. 
     In order to reduce the actual distance between the eye of a user and a display in an HMD, while maintaining a virtual distance that can be easily accommodated by the eye, an eyepiece including a catadioptric lens can be used. A catadioptric lens folds the light path such that a ratio of the virtual distance to the actual distance is much greater than with a simply refractive lens. 
     When an eyepiece with a catadioptric lens is used in an HMD, and the display is moved closer to the eyepiece (and, thus, the eye), little room remains between the eyepiece and the display, making it difficult to fit an eye tracking system, such as one or more light sources and a camera. However, using a selectively distortive catadioptric lens (that distorts light from the display but does not distort light from the light sources) allows the eye tracking system to be placed very near the eyepiece without experiencing distortion from the eyepiece. 
       FIG. 1  is a block diagram of an example operating environment  100  in accordance with some implementations. While pertinent features are shown, those of ordinary skill in the art will appreciate from the present disclosure that various other features have not been illustrated for the sake of brevity and so as not to obscure more pertinent aspects of the example implementations disclosed herein. To that end, as a non-limiting example, the operating environment  100  includes a controller  110  and a head-mounted device (HMD)  120 . 
     In some embodiments, the controller  110  is configured to manage and coordinate an augmented reality/virtual reality (AR/VR) experience for the user. In some embodiments, the controller  110  includes a suitable combination of software, firmware, and/or hardware. The controller  110  is described in greater detail below with respect to  FIG. 2 . In some embodiments, the controller  110  is a computing device that is local or remote relative to the scene  105 . For example, the controller  110  is a local server located within the scene  105 . In another example, the controller  110  is a remote server located outside of the scene  105  (e.g., a cloud server, central server, etc.). In some embodiments, the controller  110  is communicatively coupled with the HMD  120  via one or more wired or wireless communication channels  144  (e.g., BLUETOOTH, IEEE 802.11x, IEEE 802.16x, IEEE 802.3x, etc.). 
     In some embodiments, the HMD  120  is configured to present the AR/VR experience to the user. In some embodiments, the HMD  120  includes a suitable combination of software, firmware, and/or hardware. The HMD  120  is described in greater detail below with respect to  FIG. 3 . In some embodiments, the functionalities of the controller  110  are provided by and/or combined with the HMD  120 . 
     According to some embodiments, the HMD  120  presents an augmented reality/virtual reality (AR/VR) experience to the user while the user is virtually and/or physically present within the scene  105 . In some embodiments, while presenting an augmented reality (AR) experience, the HMD  120  is configured to present AR content and to enable optical see-through of the scene  105 . In some embodiments, while presenting a virtual reality (VR) experience, the HMD  120  is configured to present VR content and to enable video pass-through of the scene  105 . 
     In some embodiments, the user wears the HMD 120  on his/her head. As such, the HMD  120  includes one or more AR/VR displays provided to display the AR/VR content. For example, in various implementations, the HMD  120  encloses the field-of-view of the user. In some embodiments, the HMD  120  is replaced with a handheld device (such as a smartphone or tablet) configured to present AR/VR content in which the user does not wear the HMD  120 , but holds the device with a display directed towards the field-of-view of the user and a camera directed towards the scene  105 . In some embodiments, the HMD  120  is replaced with an AR/VR chamber, enclosure, or room configured to present AR/VR content in which the user does not wear or hold the HMD  120 . 
       FIG. 2  is a block diagram of an example of the controller  110  in accordance with some implementations. While certain specific features are illustrated, those skilled in the art will appreciate from the present disclosure that various other features have not been illustrated for the sake of brevity, and so as not to obscure more pertinent aspects of the implementations disclosed herein. To that end, as a non-limiting example, in some implementations the controller  110  includes one or more processing units  202  (e.g., microprocessors, application-specific integrated-circuits (ASICs), field-programmable gate arrays (FPGAs), graphics processing units (GPUs), central processing units (CPUs), processing cores, and/or the like), one or more input/output (I/O) devices  206 , one or more communication interfaces  208  (e.g., universal serial bus (USB), FIREWIRE, THUNDERBOLT, IEEE 802.3x, IEEE 802.11x, IEEE 802.16x, global system for mobile communications (GSM), code division multiple access (CDMA), time division multiple access (TDMA), global positioning system (GPS), infrared (IR), BLUETOOTH, ZIGBEE, and/or the like type interface), one or more programming (e.g., I/O) interfaces  210 , a memory  220 , and one or more communication buses  204  for interconnecting these and various other components. 
     In some implementations, the one or more communication buses  204  include circuitry that interconnects and controls communications between system components. In some implementations, the one or more I/O devices  206  include at least one of a keyboard, a mouse, a touchpad, a joystick, one or more microphones, one or more speakers, one or more image sensors, one or more displays, and/or the like. 
     The memory  220  includes high-speed random-access memory, such as dynamic random-access memory (DRAM), static random-access memory (SRAM), double-data-rate random-access memory (DDR RAM), or other random-access solid-state memory devices. In some implementations, the memory  220  includes non-volatile memory, such as one or more magnetic disk storage devices, optical disk storage devices, flash memory devices, or other non-volatile solid-state storage devices. The memory  220  optionally includes one or more storage devices remotely located from the one or more processing units  202 . The memory  220  comprises a non-transitory computer readable storage medium. In some implementations, the memory  220  or the non-transitory computer readable storage medium of the memory  220  stores the following programs, modules and data structures, or a subset thereof including an optional operating system  230  and an augmented reality/virtual reality (AR/VR) experience module  240 . 
     The operating system  230  includes procedures for handling various basic system services and for performing hardware dependent tasks. In some implementations, the AR/VR experience module  240  is configured to manage and coordinate one or more AR/VR experiences for one or more users (e.g., a single AR/VR experience for one or more users, or multiple AR/VR experiences for respective groups of one or more users). To that end, in various implementations, the AR/VR experience module  240  includes a data obtaining unit  242 , a tracking unit  244 , a coordination unit  246 , and a data transmitting unit  248 . 
     In some implementations, the data obtaining unit  242  is configured to obtain data (e.g., presentation data, interaction data, sensor data, location data, etc.) from at least the HMD  120 . To that end, in various implementations, the data obtaining unit  242  includes instructions and/or logic therefor, and heuristics and metadata therefor. 
     In some implementations, the tracking unit  244  is configured to map the scene  105  and to track the position/location of at least the HMD  120  with respect to the scene  105 . To that end, in various implementations, the tracking unit  244  includes instructions and/or logic therefor, and heuristics and metadata therefor. 
     In some implementations, the coordination unit  246  is configured to manage and coordinate the AR/VR experience presented to the user by the HMD  120 . To that end, in various implementations, the coordination unit  246  includes instructions and/or logic therefor, and heuristics and metadata therefor. 
     In some implementations, the data transmitting unit  248  is configured to transmit data (e.g., presentation data, location data, etc.) to at least the HMD  120 . To that end, in various implementations, the data transmitting unit  248  includes instructions and/or logic therefor, and heuristics and metadata therefor. 
     Although the data obtaining unit  242 , the tracking unit  244 , the coordination unit  246 , and the data transmitting unit  248  are shown as residing on a single device (e.g., the controller  110 ), it should be understood that in other implementations, any combination of the data obtaining unit  242 , the tracking unit  244 , the coordination unit  246 , and the data transmitting unit  248  may be located in separate computing devices. 
     Moreover,  FIG. 2  is intended more as functional description of the various features which are present in a particular embodiment as opposed to a structural schematic of the implementations described herein. As recognized by those of ordinary skill in the art, items shown separately could be combined and some items could be separated. For example, some functional modules shown separately in  FIG. 2  could be implemented in a single module and the various functions of single functional blocks could be implemented by one or more functional blocks in various implementations. The actual number of modules and the division of particular functions and how features are allocated among them will vary from one embodiment to another and, in some implementations, depends in part on the particular combination of hardware, software, and/or firmware chosen for a particular embodiment. 
       FIG. 3  is a block diagram of an example of the head-mounted device (HMD)  120  in accordance with some implementations. While certain specific features are illustrated, those skilled in the art will appreciate from the present disclosure that various other features have not been illustrated for the sake of brevity, and so as not to obscure more pertinent aspects of the implementations disclosed herein. To that end, as a non-limiting example, in some implementations the HMD  120  includes one or more processing units  302  (e.g., microprocessors, ASICs, FPGAs, GPUs, CPUs, processing cores, and/or the like), one or more input/output (I/O) devices and sensors  306 , one or more communication interfaces  308  (e.g., USB, FIREWIRE, THUNDERBLOT, IEEE 802.3x, IEEE 802.11x, IEEE 802.16x, GSM, CDMA, TDMA, GPS, IR, BLUETOOTH, ZIGBEE, and/or the like type interface), one or more programming (e.g., I/O) interfaces  310 , one or more AR/VR displays  312 , one or more optional interior and/or exterior facing image sensors  314 , a memory  320 , and one or more communication buses  304  for interconnecting these and various other components. 
     In some implementations, the one or more communication buses  304  include circuitry that interconnects and controls communications between system components. In some implementations, the one or more I/O devices and sensors  306  include at least one of an inertial measurement unit (IMU), an accelerometer, a gyroscope, a thermometer, one or more physiological sensors (e.g., blood pressure monitor, heart rate monitor, blood oxygen sensor, blood glucose sensor, etc.), one or more microphones, one or more speakers, a haptics engine, one or more depth sensors (e.g., a structured light, a time-of-flight, or the like), and/or the like. 
     In some implementations, the one or more AR/VR displays  312  are configured to present the AR/VR experience to the user. In some embodiments, the one or more AR/VR displays  312  correspond to holographic, digital light processing (DLP), liquid-crystal display (LCD), liquid-crystal on silicon (LCoS), organic light-emitting field-effect transitory (OLET), organic light-emitting diode (OLED), surface-conduction electron-emitter display (SED), field-emission display (FED), quantum-dot light-emitting diode (QD-LED), micro-electro-mechanical system (MEMS), and/or the like display types. In some embodiments, the one or more AR/VR displays  312  correspond to diffractive, reflective, polarized, holographic, etc. waveguide displays. For example, the HMD  120  includes a single AR/VR display. In another example, the HMD  120  includes an AR/VR display for each eye of the user. In some embodiments, the one or more AR/VR displays  312  are capable of presenting AR and VR content. In some embodiments, the one or more AR/VR displays  312  are capable of presenting AR or VR content. 
     In some implementations, the one or more optional image sensors  314  are configured to obtain image data that corresponds to at least a portion of the face of the user that includes the eyes of the user. For example, the one or more optional image sensors  314  correspond to one or more RGB camera (e.g., with a complimentary metal-oxide-semiconductor (CMOS) image sensor or a charge-coupled device (CCD) image sensor), IR camera, event-based camera, and/or the like. 
     The memory  320  includes high-speed random-access memory, such as DRAM, SRAM, DDR RAM, or other random-access solid-state memory devices. In some implementations, the memory  320  includes non-volatile memory, such as one or more magnetic disk storage devices, optical disk storage devices, flash memory devices, or other non-volatile solid-state storage devices. The memory  320  optionally includes one or more storage devices remotely located from the one or more processing units  302 . The memory  320  comprises a non-transitory computer readable storage medium. In some implementations, the memory  320  or the non-transitory computer readable storage medium of the memory  320  stores the following programs, modules and data structures, or a subset thereof including an optional operating system  330  and an AR/VR presentation module  340 . 
     The operating system  330  includes procedures for handling various basic system services and for performing hardware dependent tasks. In some implementations, the AR/VR presentation module  340  is configured to present AR/VR content to the user via the one or more AR/VR displays  312 . To that end, in various implementations, the AR/VR presentation module  340  includes a data obtaining unit  342 , an AR/VR presenting unit  344 , an eye tracking unit  346 , and a data transmitting unit  348 . 
     In some implementations, the data obtaining unit  342  is configured to obtain data (e.g., presentation data, interaction data, sensor data, location data, etc.) from at least o the controller  110 . To that end, in various implementations, the data obtaining unit  342  includes instructions and/or logic therefor, and heuristics and metadata therefor. 
     In some implementations, the AR/VR presenting unit  344  is configured to present AR/VR content via the one or more AR/VR displays  312 . To that end, in various implementations, the AR/VR presenting unit  344  includes instructions and/or logic therefor, and heuristics and metadata therefor. In some implementations, the AR/VR presenting unit  344  is configured to projecting an image comprising emitted light in a first wavelength range through an eyepiece that distorts light in the first wavelength range. In some embodiments, the AR/VR presenting unit  344  is configured to project an image comprising emitted light in a first wavelength through an eyepiece that reflects and refracts light in the first wavelength range while passing, without substantial distortion, light in the second wavelength range. 
     In some implementations, the eye tracking unit  346  is configured to emit, using one or more light sources disposed between the eyepiece and the display, light in a second wavelength range and detect, using a camera, the light in the second wavelength range. In various implementations, the one or more light sources illuminate the eye of a user and the camera detect light reflected from the eye of the user. To that end, in various implementations, the eye tracking unit  346  includes instructions and/or logic therefor, and heuristics and metadata therefor. In some implementations, the eye tracking unit  346  is configured to emitting light in a second wavelength range through the eyepiece and detecting the light in the second wavelength range reflected by the eye of a user. 
     In some implementations, the data transmitting unit  348  is configured to transmit data (e.g., presentation data, location data, etc.) to at least the controller  110 . To that end, in various implementations, the data transmitting unit  348  includes instructions and/or logic therefor, and heuristics and metadata therefor. 
     Although the data obtaining unit  342 , the AR/VR presenting unit  344 , the eye tracking unit  346 , and the data transmitting unit  348  are shown as residing on a single device (e.g., the HMD  120 ), it should be understood that in other implementations, any combination of the data obtaining unit  342 , the AR/VR presenting unit  344 , the eye tracking unit  346 , and the data transmitting unit  348  may be located in separate computing devices. 
     Moreover,  FIG. 3  is intended more as functional description of the various features which are present in a particular embodiment as opposed to a structural schematic of the implementations described herein. As recognized by those of ordinary skill in the art, items shown separately could be combined and some items could be separated. For example, some functional modules shown separately in  FIG. 3  could be implemented in a single module and the various functions of single functional blocks could be implemented by one or more functional blocks in various implementations. The actual number of modules and the division of particular functions and how features are allocated among them will vary from one embodiment to another and, in some implementations, depends in part on the particular combination of hardware, software, and/or firmware chosen for a particular embodiment. 
       FIG. 4  illustrates a block diagram of a head-mounted device  400  in accordance with some implementations. The head-mounted device  400  includes a housing  401  (or enclosure) that houses various components of the head-mounted device  400 . The housing  401  includes (or is coupled to) an eye pad  405  disposed at a proximal (to the user  10 ) end of the housing  401 . In various implementations, the eye pad  405  is a plastic or rubber piece that comfortably and snugly keeps the head-mounted device  400  in the proper position on the face of the user  10  (e.g., surrounding the eye of the user  10 ). 
     The housing  401  houses a display  410  that projects an image, through an eyepiece  430 , onto the eye of a user  10 . The eyepiece  430  refracts light emitted by the display  410 , making the display appear to the user  10  to be at a virtual distance further than the actual distance from the eye to the display  410 . For the user to be able to focus on the display  410 , in various implementations, the virtual distance is at least greater than a minimum focal distance of the eye (e.g., 7 cm). Further, in order to provide a better user experience, in various implementations, the virtual distance is greater than 1 m. 
     For example, in one embodiment, the distance from the eye to the eyepiece  430  is approximately 15 mm, the eyepiece  430  is approximately 8 mm thick, and the distance between the eyepiece  430  and the display  410  is approximately 38 mm. Thus, the distance from the eye to the display  410  is approximately 61 mm. However, the eyepiece  430  refracts the light emitted by the display  410  to provide a virtual distance of approximately 2 m. 
     The housing  401  also houses an eye tracking system including one or more light sources  422  and a camera  424 . The one or more light sources  422  project, onto the eye of the user  10 , a light pattern (e.g., a circle of glints) that can be detected by the camera  424 . Based on the light pattern, reflected by the eye of the user  10  and detected by the camera  424 , the relative position of the eye of the user  10  can be tracked. For example, a gaze direction of the user  10  can be determined. 
     In various implementations, eye tracking (or, in particular, the determined gaze direction) is used to enable user interaction (e.g., the user  10  selects an option on the display  410  by looking at it), provide foveated rendering (e.g., present a higher resolution in an area of the display  410  the user  10  is looking at and a lower resolution elsewhere on the display  410 ), or reduce geometric distortion (e.g., in 3D rendering of objects on the display  410 ). 
     The display  410  emits light in a first wavelength range and the one or more light sources  422  emit light in a second wavelength range. Similarly, the camera  424  detects light in the second wavelength range. In various implementations, the first wavelength range is a visible wavelength range (e.g., a wavelength range within the visible spectrum of approximately 400-700 nm) and the second wavelength range is a near-infrared wavelength range (e.g., a wavelength range within the near-infrared spectrum of approximately 700-1400 nm). In the head-mounted device  400  of  FIG. 4 , the eyepiece  430  refracts light in both the first wavelength range and the second wavelength range. 
     In various implementations, the camera  424  is disposed to have a frontal view of the eye of the user  10 , thereby minimizing distortion and ensuring the pupil of the eye is visible for accurate eye tracking. Thus, in various implementations, the camera  424  is disposed between the eyepiece  430  and the display  410 . In various implementations, the one or more light sources  422  are disposed between the eye and the eyepiece  430 . 
       FIG. 5  illustrates a block diagram of a head-mounted device  500  having one or more light sources  422  disposed between the eyepiece  430  and the display  410  in accordance with some implementations. The head-mounted device  500  of  FIG. 5  is substantially similar to the head-mounted device  400  of  FIG. 4 , except that, in the head-mounted device  500  of  FIG. 5 , the one or more light sources  422  are disposed between the eyepiece  430  and the display  410 . In particular, in various implementations, the one or more light sources  422  are disposed closer to the display than to the eyepiece  430 . 
     Because the one or more light sources  422  are disposed between the eyepiece  430  and the display  410  (and further from the eyepiece  430  than the focal length in at least the first wavelength range), light emitted by the one or more light sources  422  is collimated by the eyepiece  430  and appears out-of-focus in the image of the eye detected by the camera  424 , thereby reducing eye tracking accuracy. Further, because the eyepiece  430  increases the virtual distance between the eye and the one or more light sources  422 , the user  10  is able focus on the one or more light sources  122 , potentially causing retinal damage. 
       FIG. 6  illustrates a block diagram of a head-mounted device  600  having a selectively distortive eyepiece  630  in accordance with some implementations. The head-mounted device  600  of  FIG. 6  is substantially similar to the head-mounted device  500  of  FIG. 5 , except that, in the head-mounted device  600  of  FIG. 6 , the eyepiece  630  refracts light in the first wavelength range while passing, without substantially distortion, light in the second wavelength range. Accordingly, the eyepiece  630  increases the virtual distance from the eye to the display  410  without affecting the virtual distance from the eye to the one or more light sources  422  or the camera  424 . 
     Accordingly, the one or more light sources  422  are disposed in the head-mounted display  600  closer to the optical axis than their position in the head-mounted display of  FIG. 4 , thereby improving detectability by the camera  424  and localization by the eye tracking system. Further, light emitted by the one or more light sources  422  is not collimated by the eyepiece  630  as is the case in the head-mounted device  500  of  FIG. 5 . Further, because the eyepiece  630  does not increase the virtual distance between the eye and the one or more light sources  422 , the user  10  cannot focus on the one or more light sources  422  as is the case in the head-mounted device  500  of  FIG. 5 . 
     In order to reduce the form factor of the head-mounted device  600  (e.g., the size of the housing  401 ), in various implementations, the display  410  is brought closer to the eyepiece  630  and, thus, the eye of the user  10 . In various implementations, an eyepiece that simply refracts light is unable to provide a sufficient virtual distance when the actual distance between the eyepiece  630  and the display  410  is reduced. Accordingly, in various implementations, a catadioptric eyepiece that both refracts and reflects light is used. 
       FIG. 7  illustrates a block diagram of a head-mounted device  700  having a catadioptric eyepiece  730  in accordance with some implementations. The head-mounted device  700  of  FIG. 7  is substantially similar to the head-mounted device  600  of  FIG. 6 , except that, in the head-mounted device  700  of  FIG. 7 , the eyepiece  730  includes a selectively distortive catadioptric lens that reflects and refracts light in the first wavelength range while passing, without substantial distortion, light in the second wavelength range and, in the head-mounted device  700  of  FIG. 7 , the size of the housing  701  is reduced, with the display  410  (and the eye tracking system) closer to the eyepiece  730 . For example, in various implementations, the distance between the eyepiece  730  and the display  410  is between 0 and 3 mm. However, because, in various implementations, the selectively distortive catadioptric lens folds the optical path of light rays in the first wavelength range, the virtual distance between the eye and the display  410  is still approximately 2 m. 
     In various implementations, the selectively distortive catadioptric lens is a split meniscus lens including two lens halves separated by a quarter-wave retarder. In various implementations, the quarter-wave retarder includes a quarter-wave plate or another birefringent material that retards by a quarter-wave. Thus, the eyepiece  730  passes, without substantial distortion, light in the second wavelength range, and the image of the eye detected by the camera  424  has less distortion than if the eyepiece  730  reflected and refracted light in the second wavelength range. Further, in various implementations, the eye tracking system is hidden from the view of the user  10  with the one or more light sources  422  and/or camera  424  disposed out of the field of view in the first wavelength range of the eyepiece  730  (while being in the field of view in the second wavelength range of the eyepiece  730 ). 
       FIG. 8  is a ray-trace diagram of light emitted from a display  410  towards the eye of a user  10  through a selectively distortive split meniscus lens  830 . The selectively distortive split meniscus lens  830  includes two lens halves separated by a quarter-wave retarder. In various implementations, The lens halves and quarter-wave retarder can be adhesively bonded together as shown in  FIG. 8  or separated with air gaps between one or more of the lens elements. Visible light rays  810  are reflected and refracted by the selectively distortive split meniscus lens  830 , whereas near-infrared rays  820  pass through the selectively distortive split meniscus lens  830  without substantial distortion. Accordingly, the camera  424  cannot be seen in the visible spectrum by the user. With a gaze direction towards the camera  424 , the user sees the bottom of the display  410  (as indicated by the lowermost visible light rays  810 ). Accordingly, the camera  424  is hidden from the view of the user  10 . 
       FIG. 9  is a functional block diagram of an optical system  900  including a split meniscus lens  930  in accordance with some implementations. The optical system  900  includes a display  910  that projects an image through the split meniscus lens  930  to the eye of a user  10 . 
     The display  910  includes a light emitter  911 , such as an array of LEDs, that emits light  920  in the first wavelength range. The display  910  includes a linear polarizer  912  that linear polarizes the light  920 . The display  910  includes a quarter-wave retarder  913  that changes linearly polarized light into circularly polarized light. Accordingly, the light  920  emitted by the display  610  is circularly polarized with a first circular orientation (also referred to as a handedness or polarization). In various implementations, elements described herein that circularly polarize light can also elliptically polarize light. 
     The circularly polarized light  920  strikes a partially reflective, partially transmissive (PR/PT) surface  931  of the split meniscus lens  930 . In various implementations, the PR/PT surface  931  is a 50/50 mirror, which is 50% reflective and 50% transmissive within the first wavelength range. Accordingly, a portion of the circularly polarized light  920  is reflected off the PR/PT surface  931  back towards the display  931  and a portion of the circularly polarized light  920  refracts (based on the geometry of the PR/PT surface  931  and relative refractive indexes) into the split meniscus lens  930 . The refracted portion of the circularly polarized light  920  passes through a quarter-wave retarder  932  that changes the circularly polarized light to linearly polarized light. Accordingly, after passing through the quarter-wave retarder  932 , the light  920  is linearly polarized in a first linear orientation (in either the same linear orientation as previously after passing through the linear polarizer  912  or an orthogonal linear orientation). 
     The light  920 , having passed through the quarter-wave retarder  932 , encounters a reflective polarizer  933  that reflects linearly polarized light of the first linear orientation and transmits linearly polarized light of a second linear orientation orthogonal to the first linear orientation. Because the light  920  is linearly polarized in the first linear orientation, the light  920  reflects off the reflective polarizer  933  back towards the quarter-wave retarder  932 . The light  920  then passes through the quarter-wave retarder  932  a second time becoming circularly polarized in the first circular orientation (e.g., the same circular orientation that the light  920  had when it entered the split meniscus  930 ). The light  920  encounters the PR/PT surface  931  and one portion of the light  920  is emitted from the split meniscus lens  930  back towards the display  910  while another portion of the light  920  is reflected back towards the quarter-wave retarder  932 . In reflecting off the PR/PT surface  931 , the circular orientation of the light  920  is changed to a second circular orientation opposite the first orientation. The portion of the light  920  that is reflected back towards the quarter-wave retarder  932  passes through the quarter-wave retarder  932  a third time and, having the second circular orientation entering the quarter-wave plate  932 , becomes linearly polarized in the second linear orientation. Thus, being linearly polarized in the second linear orientation, the portion of light  920  passes through the reflective polarizer  933  out of the split meniscus lens  930  towards the eye of the user  10 , being refracted according the geometry of the reflective polarizer  933  and the relative refractive indexes. 
       FIG. 10A  is a functional block diagram of an optical system  1000  including a selectively distortive split meniscus lens  1030  in accordance with some implementations. The optical system  1000  includes one or more light sources  1022  that emit light  1020  in a second wavelength range and a camera  1024  that detects light  1020  in the second wavelength range. 
     In various implementations, the selectively distortive split meniscus lens  1030  includes a PR/PT surface  1031 , a quarter-wave retarder  1032 , and a reflective polarizer  1033  which operate, in the first wavelength range, as described above with respect to the split meniscus lens  930 , the PR/PT surface  931 , the quarter-wave retarder  932 , and the reflective polarizer  933  of  FIG. 9 . However, in the second wavelength range, the PR/PT surface  1031 , the quarter-wave retarder  1032 , and the reflective polarizer  1033  operate differently. 
     In one embodiment, the light  1020  is unpolarized or randomly polarized. Thus, in various implementations, the PR/PT surface  1031  is partially reflective in the first wavelength range, but the PR/PT surface  1031  transmits, without substantial reflection, light  1020  in the second wavelength range.  FIG. 10B  illustrates a plot of the reflectance and transmittance versus wavelength for a PR/PT circusurface in accordance with some implementations. The plot  1090  illustrates that reflectance  1091  and the transmittance  1092  of the PR/PT surface is approximately 50% over a first wavelength range (e.g., between 450-650 nm), while the reflectance  1091  is between 0% and 10% and the transmittance  1092  is between 90% and 100% over a second wavelength range (e.g., between 850-950 nm). 
     Because the light  1020  is unpolarized, the quarter-wave retarder  1032  transmits, without substantial distortion, light  1020  in the second wavelength range without changing the polarization and the light  1020  remains unpolarized. The reflective polarizer  1033  reflects linearly polarized light of a first linear polarization and transmits linearly polarized light of a second linear orientation orthogonal to the first linear orientation in the first wavelength range, but the reflective polarizer  1033  transmits, without substantial distortion, light  1020  in the second wavelength range of any polarization. Accordingly, the light  1020  passes through the selectively distortive split meniscus lens  1030  without substantial distortion. 
     In one embodiment, the light  1020  provided by the one or more light sources  1022  is circularly polarized in the second circular orientation. Thus, in various implementations, the PR/PT surface  1031  is partially reflective in the first wavelength range, but the PR/PT surface  1031  transmits, without substantial reflection, light  1020  in the second wavelength range. As noted above,  FIG. 10B  illustrates a plot of the reflectance and transmittance versus wavelength for a PR/PT surface in accordance with some implementations. 
     The quarter-wave retarder  1032  changes linearly polarized light to circularly polarized light (and changes circularly polarized light into linearly polarized light) in the first wavelength range and the second wavelength range. Thus, the light  1020  passing through the quarter-wave retarder  1032  is linearly polarized in the second linear orientation. The reflective polarizer  1033  reflects linearly polarized light of a first linear polarization and transmits linearly polarized light of a second linear orientation orthogonal to the first linear orientation in the first wavelength wave and in the second wavelength. Thus, the light  1020 , being linearly polarized in the second linear orientation after passing through the quarter wave retarder  1032 , passes through the reflective polarizer  1033  the first time it encounters the reflective polarizer  1033 . 
     The light  1020  then reflects off the eye of the user  10  and, still being linearly polarized in the second linear orientation, passes back through the reflective polarizer  1033 , passes back through the quarter-wave retarder  1022  (thereby becoming circularly polarized), and passes through the PR/PT surface  1031  (which is fully transmissive in the second wavelength range). Accordingly, the light  1020  passes through the selectively distortive split meniscus lens  1030  without substantial distortion and without being reflected by the reflective polarizer  1033 . The camera  1024  then detects the light  1020  that is circularly polarized in the second wavelength range that has been reflected by the eye of the user  10 . 
     It is to be appreciated that the light path shown for the light  920  in  FIG. 9  and the light path shown for the light  1020  can occur simultaneously. In various implementations, the difference in polarization state between the display light  920  and the eye tracking light  1020  and the resulting difference in interaction with the quarter wave retarder cause the light  920  from the display to be first reflected and then transmitted by the reflective polarizer  933 , while the light  1020  from the one or more light sources is simply transmitted by the reflective polarizer  1033 . 
       FIG. 11A  illustrates a block diagram of a head-mounted device  1100 A having a catadioptric eyepiece  1130  with an eye tracking system disposed behind the display  410  in accordance with some implementations. The head-mounted device  1100 A of  FIG. 11A  is substantially similar to the head-mounted device  700  of  FIG. 7 , except that, in the head-mounted device  1100 A of  FIG. 11A , the one or more light sources  422  and the camera  424  are disposed at a distal (from the user  10 ) end of the housing, further from the eyepiece  730  than the display  410 . 
       FIG. 11B  illustrates a block diagram of a head-mounted device  1100 B having a catadioptric eyepiece  730  with an eye tracking system disposed in front of the eyepiece  730  in accordance with some implementations. The head-mounted device  1100 B of  FIG. 11B  is substantially similar to the head-mounted device  700  of  FIG. 7 , except that, in the head-mounted device  1100 B of  FIG. 11B , the one or more light sources  422  and the camera  424  are disposed between the user  10  and the eyepiece  430 . The camera  424  detects the light in the second wavelength range emitted by the light source after reflecting off a mirror  1132  that is reflective in the second wavelength range and transmissive in the first wavelength range. Alternatively, the mirror  1132  can be removed and the eyepiece  730  can include a PR/PT surface as described above that is fully reflective in the second wavelength range rather than fully transmissive. 
       FIG. 11C  illustrates a block diagram of a head-mounted device  1100 C having a catadioptric eyepiece  730  disposed between portions of an eye tracking system in accordance with some implementations. The head-mounted device  1100 C of  FIG. 11C  is substantially similar to the head-mounted device  700  of  FIG. 7 , except that, in the head-mounted device  1100 C of  FIG. 11C , the one or more light sources  422  are disposed between the user  10  and the eyepiece and the camera  424  is disposed between the eyepiece  730  and the display  410 . 
       FIG. 12  illustrates a functional block diagram of an electronic device according to some implementations. The electronic device  1200  includes a display  1210  to emit light in a first wavelength range. The electronic device  1200  includes one or more light sources  1222  to emit light in a second wavelength range and a camera  1224  to detect light in the second wavelength range. In various implementations, the first wavelength range and the second wavelength range are non-overlapping. For example, in various implementations, the first wavelength range is a visible wavelength range and the second wavelength range is a near-infrared wavelength range. In various implementations, the first wavelength range and the second wavelength range are at least partially overlapping. In various implementations, the first wavelength range and the second wavelength range are substantially the same. 
     The electronic device  1200  includes an eyepiece  1230  to distort light in the first wavelength range. In various implementations, the eyepiece  1230  reflects and refracts light in the first wavelength range. In various implementations, the eyepiece  1230  passes, without substantially distortion, light in the second wavelength range. In various implementations, the eyepiece  1230  includes a catadioptric lens. In various implementations, the eyepiece  1230  includes a split meniscus lens. In various implementations, the eyepiece  1230  includes a 50/50 mirror, a quarter-wave retarder, and a reflective polarizer. In various implementations, the 50/50 mirror is substantially transparent in the second wavelength. In various implementations, the quarter-wave retarder is to change linearly polarized light into circularly polarized light in the first wavelength range and pass, without substantial distortion, unpolarized light in the second wavelength range. In various implementations, the one or more light sources  1222  are to emit circularly polarized light in the second wavelength range with a first circular orientation, the display  1210  is to emit circularly polarized light in the first wavelength range with a second circular orientation opposite to the first circular polarization, and the quarter-wave retarder is to change circularly polarized light to linearly polarized light in the first wavelength range and change circularly polarized light into linearly polarized light in the second wavelength range. 
     The electronic device  1200  includes an augmented reality engine  1280  (which, in various implementations, is a processor executing an augmented reality module) that provides data to the display indicating an image to be presented on the display  1210 . The augmented reality engine  1280  includes an eye tracking module  1281  to track a gaze direction of a user of the electronic device  1200  based on the light in the second wavelength range detected by the camera  1224 . In various implementations, the one or more light sources project a series of bright spots or glints onto the eye of a user so that the relative position or gaze direction of the user&#39;s eye can be tracked. 
     In various implementations, similar to the head-mounted device  700  of  FIG. 7 , the camera  1224  is disposed between the eyepiece  1230  and the display  1210 . In various implementations, similar to the head-mounted device  1100 A of  FIG. 11A , the display  1210  is disposed between the eyepiece  1230  and the camera  1224 . In various implementations, similar to the head-mounted device  1100 B of  FIG. 11B , the eyepiece  1230  is disposed between the camera  1224  and the display  1210 . 
     In various implementations, similar to the head-mounted device  700  of  FIG. 7 , the one or more light sources  1222  are disposed between the eyepiece  1230  and the display  1210 . In various implementations, similar to the head-mounted device  1100 A of  FIG. 11A , the display  1210  is disposed between the eyepiece  1230  and the one or more light sources  1222 . In various implementations, similar to the head-mounted device  1100 B of  FIG. 11B , the eyepiece  1230  is disposed between the one or more light sources  1222  and the display  1210 . 
     In various implementations, the display  1210  is disposed less than five millimeters from the eyepiece  1230 , but the eyepiece  1230  provides a virtual distance of at least half a meter. In various implementations, the display  1210  is disposed less than five millimeters from the eyepiece  1230 , but the eyepiece  1230  provides a virtual distance of at one meter. In various implementations, the one or more light sources  1222  are disposed less than thirty millimeters from the eye of the user. However, by placing the one or more light sources  1222  between the eyepiece  1230  and the display  1210 , the one or more light sources  422  are disposed closer to the optical axis, thereby improving detectability by the camera  424  and localization by the eye tracking system. 
       FIG. 13  is a flowchart representation of a method  1300  of image display and eye tracking in accordance with some implementations. In various implementations (and as described below as an example), the method  1300  is performed by a head-mounted device, such as the HMD  1200  of  FIG. 12 . In various implementations, the method  1200  is performed by a device with one or more processors, non-transitory memory, and one or more AR/VR displays (e.g., the HMD  120  of  FIG. 3 ). In some implementations, the method  1300  is performed by processing logic, including hardware, firmware, software, or a combination thereof. In some implementations, the method  1300  is performed by a processor executing code stored in a non-transitory computer-readable medium (e.g., a memory). 
     The method  1300  begins, at block  1310 , with the HMD projecting, using a display, an image comprising emitted light in a first wavelength through a selectively distortive catadioptric lens. Thus, in some implementations, the HMD projects, using a display, an image comprising emitted light in a first wavelength range through an eyepiece that distorts light in the first wavelength range. In some implementations, the HMD projects, using a display, an image comprising emitted light in a first wavelength through an eyepiece that reflects and refracts light in the first wavelength range while passing, without substantial distortion, light in a second wavelength range. 
     The method  1300  continues, at block  1320 , with the HMD emitting, using one or more light sources, light in the second wavelength range through the selectively distortive catadioptric lens. Thus, in some implementations, the HMD emits, using one or more light sources disposed between the eyepiece and the display, light in the second wavelength range. In some implementations, the HMD emitting, using one or more light sources, light in the second wavelength range through the eyepiece. 
     The method continues  1300 , at block  1330 , with the HMD detecting, using a camera, the light in the light in the second wavelength range reflected by the eye of a user. 
     The method continues  1300 , at block  1340 , with the HMD performing eye tracking on the eye of the user based on the detected light in the second wavelength range reflected by the eye of the user. For example, in various implementations, the HMD performs eye tracking by determining a gaze direction of the user based on the detected light in the second wavelength range reflected by the eye of the user and the relative position of the user&#39;s eye. 
     While various aspects of implementations within the scope of the appended claims are described above, it should be apparent that the various features of implementations described above may be embodied in a wide variety of forms and that any specific structure and/or function described above is merely illustrative. Based on the present disclosure one skilled in the art should appreciate that an aspect described herein may be implemented independently of any other aspects and that two or more of these aspects may be combined in various ways. For example, an apparatus may be implemented and/or a method may be practiced using any number of the aspects set forth herein. In addition, such an apparatus may be implemented and/or such a method may be practiced using other structure and/or functionality in addition to or other than one or more of the aspects set forth herein. 
     It will also be understood that, although the terms “first,” “second,” etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first node could be termed a second node, and, similarly, a second node could be termed a first node, which changing the meaning of the description, so long as all occurrences of the “first node” are renamed consistently and all occurrences of the “second node” are renamed consistently. The first node and the second node are both nodes, but they are not the same node. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the claims. As used in the description of the embodiments and the appended claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that the term “and/or” as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     As used herein, the term “if” may be construed to mean “when” or “upon” or “in response to determining” or “in accordance with a determination” or “in response to detecting,” that a stated condition precedent is true, depending on the context. Similarly, the phrase “if it is determined [that a stated condition precedent is true]” or “if [a stated condition precedent is true]” or “when [a stated condition precedent is true]” may be construed to mean “upon determining” or “in response to determining” or “in accordance with a determination” or “upon detecting” or “in response to detecting” that the stated condition precedent is true, depending on the context.