Patent Publication Number: US-2023152578-A1

Title: Multi-view eye tracking system with a holographic optical element combiner

Description:
TECHNICAL FIELD 
     This relates generally to holographic optical elements, and more specifically to holographic optical elements used in eye tracking devices for head-mounted display devices. 
     BACKGROUND 
     Head-mounted display devices (also called herein head-mounted displays or headsets) are gaining popularity as means for providing visual information to a user. For example, the head-mounted display devices are used for virtual reality and augmented reality operations. 
     Head-mounted displays often require eye tracking. For example, the content displayed by a head-mounted display needs to be updated based on a gaze direction of a user, which requires eye-tracking systems for determining the position of the pupil of the eye. Thus, errors and delays in eye tracking may affect the user experience with the head-mounted displays. 
     SUMMARY 
     Accordingly, there is a need for head-mounted displays with accurate eye tracking capabilities, thereby enhancing the user&#39;s virtual-reality and/or augmented reality experience. 
     One approach to track movements of an eye is to illuminate a surface of the eye, and detect reflections of the illuminated patterns off the surface of the eye (e.g., glints). However, eye tracking with such illumination has challenges, as various structures around the eye (e.g., eye lids, eye lashes, etc.) can block the illumination from reaching the surface of the eye or occlude the reflections of the illuminated patterns off the surface of the eye, which in turn reduce the accuracy in eye tracking. Even for other methods of tracking movements of an eye (e.g., using pupil tracking) that may not require separate illumination of the eye, the occlusion of a view of the eye may reduce the accuracy in eye tracking. Therefore, there is a need for eye-tracking systems that can track the position of an eye with reduced occlusion. 
     The above deficiencies and other problems associated with conventional eye-tracking systems are reduced or eliminated by the disclosed methods and systems. 
     In accordance with some embodiments, a method includes projecting, with a holographic optical element, a first view of an eye toward an imaging device; and projecting, with the holographic optical element, a second view of the eye, distinct from the first view of the eye, toward the imaging device so that the first view and the second view of the eye are concurrently received by the imaging device. 
     In accordance with some embodiments, an eye tracking device includes an imaging device; and a holographic optical element positioned relative to the imaging device for projecting a first view of a target area toward the imaging device and projecting a second view of the eye, distinct from the first view of the eye, toward the imaging device so that the first view and the second view of the eye are concurrently received by the imaging device. 
     In accordance with some embodiments, a head-mounted display device includes any eye tracking device described herein. 
     In accordance with some embodiments, a holographic optical element is configured for projecting a first view of a target area toward an imaging device and projecting a second view of the target area, distinct from the first view of the target area, toward the imaging device so that the first view and the second view of the target area are concurrently received by the imaging device. 
     In accordance with some embodiments, a method of making a holographic optical element includes recording a first holographic pattern in the holographic optical element by concurrently providing a first beam for a first view point and a second beam from a target area; and recording a second holographic pattern in the holographic optical element by concurrently providing a third beam for a second view point that is distinct from the first view point and the second beam from the target area. 
     In accordance with some embodiments, a holographic medium is made by any of the methods described herein. 
     Thus, the disclosed embodiments provide eye-tracking systems and eye-tracking methods based on holographic media, and methods for making holographic media. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the various described embodiments, reference should be made to the Description of Embodiments below, in conjunction with the following drawings in which like reference numerals refer to corresponding parts throughout the figures. 
         FIG.  1    is a perspective view of a display device in accordance with some embodiments. 
         FIG.  2    is a block diagram of a system including a display device in accordance with some embodiments. 
         FIG.  3    is an isometric view of a display device in accordance with some embodiments. 
         FIG.  4 A  is a schematic diagram illustrating an eye tracking device in accordance with some embodiments. 
         FIG.  4 B  is a schematic diagram illustrating an eye tracking device in accordance with some embodiments. 
         FIG.  4 C  is a schematic diagram illustrating an eye tracking device in accordance with some embodiments. 
         FIG.  4 D  is a schematic diagram illustrating an eye tracking device combined with a holographic illuminator in accordance with some embodiments. 
         FIGS.  5 A- 5 D  are schematic diagrams illustrating configurations of light patterns used for eye tracking in accordance with some embodiments. 
         FIG.  6 A  is a schematic diagram illustrating a display device with an eye tracking device in accordance with some embodiments. 
         FIG.  6 B  is a schematic diagram illustrating a display device with an eye tracking device in accordance with some embodiments. 
         FIG.  6 C  is a schematic diagram illustrating a display device with an eye tracking device in accordance with some embodiments. 
         FIG.  7 A  is a graphical representation of a multi-view image of an eye in accordance with some embodiments. 
         FIG.  7 B  shows multiple views of an eye in accordance with some embodiments. 
         FIG.  8 A  is a schematic diagram illustrating a system for making a multi-view holographic optical element in accordance with some embodiments. 
         FIG.  8 B  is a schematic diagram illustrating a prism used for making a multi-view holographic optical element in accordance with some embodiments. 
     
    
    
     These figures are not drawn to scale unless indicated otherwise. 
     DETAILED DESCRIPTION 
     Eye-tracking systems with multi-view holographic optical elements provide accurate and reliable determination of a position of a pupil of an eye because views of the eye from multiple directions can be provided. The multiple views of the eye can be analyzed for accurate determination of the position of the pupil of the eye, while reducing the effect of occlusion in any single view. The disclosed embodiments provide (i) multi-view holographic optical elements, (ii) methods and systems for eye tracking with a multi-view holographic optical element, and (iii) methods for making such multi-view holographic optical elements. 
     In some embodiments, a multi-view holographic optical element is coupled with an imaging device (e.g., a camera) for converting the multiple views of the eye into electrical signals (e.g., a digital image). In some embodiments, the imaging device is configured for recording non-visible light (e.g., an infrared (IR) or near-infrared (NIR) light). In some embodiments, the imaging device is positioned away from the field-of-view of an eye. 
     Reference will now be made to embodiments, examples of which are illustrated in the accompanying drawings. In the following description, numerous specific details are set forth in order to provide an understanding of the various described embodiments. However, it will be apparent to one of ordinary skill in the art that the various described embodiments may be practiced without these specific details. In other instances, well-known methods, procedures, components, circuits, and networks have not been described in detail so as not to unnecessarily obscure aspects of the embodiments. 
     It will also be understood that, although the terms first, second, etc. are, in some instances, used herein to describe various elements, these elements should not be limited by these terms. These terms are used only to distinguish one element from another. For example, a first surface could be termed a second surface, and, similarly, a second surface could be termed a first surface, without departing from the scope of the various described embodiments. The first surface and the second surface are both surfaces, but they are not the same surface. 
     The terminology used in the description of the various described embodiments herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used in the description of the various described 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 “includes,” “including,” “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. The term “exemplary” is used herein in the sense of “serving as an example, instance, or illustration” and not in the sense of “representing the best of its kind.” 
       FIG.  1    illustrates display device  100  in accordance with some embodiments. In some embodiments, display device  100  is configured to be worn on a head of a user (e.g., by having the form of spectacles or eyeglasses, as shown in  FIG.  1   ) or to be included as part of a helmet that is to be worn by the user. When display device  100  is configured to be worn on a head of a user or to be included as part of a helmet, display device  100  is called a head-mounted display. Alternatively, display device  100  is configured for placement in proximity of an eye or eyes of the user at a fixed location, without being head-mounted (e.g., display device  100  is mounted in a vehicle, such as a car or an airplane, for placement in front of an eye or eyes of the user). As shown in  FIG.  1   , display device  100  includes display  110 . Display  110  is configured for presenting visual contents (e.g., augmented reality contents, virtual reality contents, mixed reality contents, or any combination thereof) to a user. 
     In some embodiments, display device  100  includes one or more components described herein with respect to  FIG.  2   . In some embodiments, display device  100  includes additional components not shown in  FIG.  2   . 
       FIG.  2    is a block diagram of system  200  in accordance with some embodiments. The system  200  shown in  FIG.  2    includes display device  205  (which corresponds to display device  100  shown in  FIG.  1   ), imaging device  235 , and input interface  240  that are each coupled to console  210 . While  FIG.  2    shows an example of system  200  including one display device  205 , imaging device  235 , and input interface  240 , in other embodiments, any number of these components may be included in system  200 . For example, there may be multiple display devices  205  each having associated input interface  240  and being monitored by one or more imaging devices  235 , with each display device  205 , input interface  240 , and imaging devices  235  communicating with console  210 . In alternative configurations, different and/or additional components may be included in system  200 . For example, in some embodiments, console  210  is connected via a network (e.g., the Internet) to system  200  or is self-contained as part of display device  205  (e.g., physically located inside display device  205 ). In some embodiments, display device  205  is used to create mixed reality by adding in a view of the real surroundings. Thus, display device  205  and system  200  described here can deliver augmented reality, virtual reality, and mixed reality. 
     In some embodiments, as shown in  FIG.  1   , display device  205  is a head-mounted display that presents media to a user. Examples of media presented by display device  205  include one or more images, video, audio, or some combination thereof. In some embodiments, audio is presented via an external device (e.g., speakers and/or headphones) that receives audio information from display device  205 , console  210 , or both, and presents audio data based on the audio information. In some embodiments, display device  205  immerses a user in an augmented environment. 
     In some embodiments, display device  205  also acts as an augmented reality (AR) headset. In these embodiments, display device  205  augments views of a physical, real-world environment with computer-generated elements (e.g., images, video, sound, etc.). Moreover, in some embodiments, display device  205  is able to cycle between different types of operation. Thus, display device  205  operate as a virtual reality (VR) device, an augmented reality (AR) device, as glasses or some combination thereof (e.g., glasses with no optical correction, glasses optically corrected for the user, sunglasses, or some combination thereof) based on instructions from application engine  255 . 
     Display device  205  includes electronic display  215 , one or more processors  216 , eye tracking module  217 , adjustment module  218 , one or more locators  220 , one or more position sensors  225 , one or more position cameras  222 , memory  228 , inertial measurement unit (IMU)  230 , one or more reflective elements  260  or a subset or superset thereof (e.g., display device  205  with electronic display  215 , one or more processors  216 , and memory  228 , without any other listed components). Some embodiments of display device  205  have different modules than those described here. Similarly, the functions can be distributed among the modules in a different manner than is described here. 
     One or more processors  216  (e.g., processing units or cores) execute instructions stored in memory  228 . Memory  228  includes high-speed random access memory, such as DRAM, SRAM, DDR RAM or other random access solid state memory devices; and may include 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. Memory  228 , or alternately the non-volatile memory device(s) within memory  228 , includes a non-transitory computer readable storage medium. In some embodiments, memory  228  or the computer readable storage medium of memory  228  stores programs, modules and data structures, and/or instructions for displaying one or more images on electronic display  215 . 
     Electronic display  215  displays images to the user in accordance with data received from console  210  and/or processor(s)  216 . In various embodiments, electronic display  215  may comprise a single adjustable display element or multiple adjustable display elements (e.g., a display for each eye of a user). In some embodiments, electronic display  215  is configured to display images to the user by projecting the images onto one or more reflective elements  260 . 
     In some embodiments, the display element includes one or more light emission devices and a corresponding array of spatial light modulators. A spatial light modulator is an array of electro-optic pixels, opto-electronic pixels, some other array of devices that dynamically adjust the amount of light transmitted by each device, or some combination thereof. These pixels are placed behind one or more lenses. In some embodiments, the spatial light modulator is an array of liquid crystal based pixels in an LCD (a Liquid Crystal Display). Examples of the light emission devices include: an organic light emitting diode, an active-matrix organic light-emitting diode, a light emitting diode, some type of device capable of being placed in a flexible display, or some combination thereof. The light emission devices include devices that are capable of generating visible light (e.g., red, green, blue, etc.) used for image generation. The spatial light modulator is configured to selectively attenuate individual light emission devices, groups of light emission devices, or some combination thereof. Alternatively, when the light emission devices are configured to selectively attenuate individual emission devices and/or groups of light emission devices, the display element includes an array of such light emission devices without a separate emission intensity array. In some embodiments, electronic display  215  projects images to one or more reflective elements  260 , which reflect at least a portion of the light toward an eye of a user. 
     One or more lenses direct light from the arrays of light emission devices (optionally through the emission intensity arrays) to locations within each eyebox and ultimately to the back of the user&#39;s retina(s). An eyebox is a region that is occupied by an eye of a user located proximity to display device  205  (e.g., a user wearing display device  205 ) for viewing images from display device  205 . In some cases, the eyebox is represented as a 10 mm×10 mm square. In some other cases, the eyebox is represented as a 20 mm×20 mm square. In some embodiments, the one or more lenses include one or more coatings, such as anti-reflective coatings. 
     In some embodiments, the display element includes an infrared (IR) detector array that detects IR light that is retro-reflected from the retinas of a viewing user, from the surface of the corneas, lenses of the eyes, or some combination thereof. The IR detector array includes an IR sensor or a plurality of IR sensors that each correspond to a different position of a pupil of the viewing user&#39;s eye. In alternate embodiments, other eye tracking systems may also be employed. As used herein, IR refers to light with wavelengths ranging from 700 nm to 1 mm including near infrared (NIR) ranging from 750 nm to 1500 nm (e.g., having a wavelength of 750 nm, 800 nm, 850 nm, 900 nm, 950 nm, 1000 nm, 1100 nm, 1200 nm, 1300 nm, 1400 nm, 1500 nm, or within a range between any two of the aforementioned values). 
     Eye tracking module  217  determines locations of each pupil of a user&#39;s eyes. In some embodiments, eye tracking module  217  instructs electronic display  215  to illuminate the eyebox with IR light (e.g., via IR emission devices in the display element). 
     A portion of the emitted IR light will pass through the viewing user&#39;s pupil and be retro-reflected from the retina toward the IR detector array, which is used for determining the location of the pupil. Alternatively, the reflection off of the surfaces of the eye (or an image of the eye) is used to also determine location of the pupil. The IR detector array scans for retro-reflection and identifies which IR emission devices are active when retro-reflection is detected. Eye tracking module  217  may use a tracking lookup table and the identified IR emission devices to determine the pupil locations for each eye. The tracking lookup table maps received signals on the IR detector array to locations (corresponding to pupil locations) in each eyebox. In some embodiments, the tracking lookup table is generated via a calibration procedure (e.g., user looks at various known reference points in an image and eye tracking module  217  maps the locations of the user&#39;s pupil while looking at the reference points to corresponding signals received on the IR tracking array). As mentioned above, in some embodiments, system  200  may use other eye tracking systems than the embedded IR one described herein. 
     Adjustment module  218  generates an image frame based on the determined locations of the pupils. In some embodiments, this sends a discrete image to the display that will tile subimages together thus a coherent stitched image will appear on the back of the retina. Adjustment module  218  adjusts an output (i.e. the generated image frame) of electronic display  215  based on the detected locations of the pupils. Adjustment module  218  instructs portions of electronic display  215  to pass image light to the determined locations of the pupils. In some embodiments, adjustment module  218  also instructs the electronic display to not pass image light to positions other than the determined locations of the pupils. Adjustment module  218  may, for example, block and/or stop light emission devices whose image light falls outside of the determined pupil locations, allow other light emission devices to emit image light that falls within the determined pupil locations, translate and/or rotate one or more display elements, dynamically adjust curvature and/or refractive power of one or more active lenses in the lens (e.g., microlens) arrays, or some combination thereof. 
     Optional locators  220  are objects located in specific positions on display device  205  relative to one another and relative to a specific reference point on display device  205 . A locator  220  may be a light emitting diode (LED), a corner cube reflector, a reflective marker, a type of light source that contrasts with an environment in which display device  205  operates, or some combination thereof. In embodiments where locators  220  are active (i.e., an LED or other type of light emitting device), locators  220  may emit light in the visible band (e.g., about 500 nm to 750 nm), in the infrared band (e.g., about 750 nm to 1 mm), in the ultraviolet band (about 100 nm to 500 nm), some other portion of the electromagnetic spectrum, or some combination thereof. 
     In some embodiments, locators  220  are located beneath an outer surface of display device  205 , which is transparent to the wavelengths of light emitted or reflected by locators  220  or is thin enough to not substantially attenuate the wavelengths of light emitted or reflected by locators  220 . Additionally, in some embodiments, the outer surface or other portions of display device  205  are opaque in the visible band of wavelengths of light. Thus, locators  220  may emit light in the IR band under an outer surface that is transparent in the IR band but opaque in the visible band. 
     IMU  230  is an electronic device that generates calibration data based on measurement signals received from one or more position sensors  225 . Position sensor  225  generates one or more measurement signals in response to motion of display device  205 . Examples of position sensors  225  include: one or more accelerometers, one or more gyroscopes, one or more magnetometers, another suitable type of sensor that detects motion, a type of sensor used for error correction of IMU  230 , or some combination thereof. Position sensors  225  may be located external to IMU  230 , internal to IMU  230 , or some combination thereof. 
     Based on the one or more measurement signals from one or more position sensors  225 , IMU  230  generates first calibration data indicating an estimated position of display device  205  relative to an initial position of display device  205 . For example, position sensors  225  include multiple accelerometers to measure translational motion (forward/back, up/down, left/right) and multiple gyroscopes to measure rotational motion (e.g., pitch, yaw, roll). In some embodiments, IMU  230  rapidly samples the measurement signals and calculates the estimated position of display device  205  from the sampled data. For example, IMU  230  integrates the measurement signals received from the accelerometers over time to estimate a velocity vector and integrates the velocity vector over time to determine an estimated position of a reference point on display device  205 . Alternatively, IMU  230  provides the sampled measurement signals to console  210 , which determines the first calibration data. The reference point is a point that may be used to describe the position of display device  205 . While the reference point may generally be defined as a point in space; however, in practice the reference point is defined as a point within display device  205  (e.g., a center of IMU  230 ). 
     In some embodiments, IMU  230  receives one or more calibration parameters from console  210 . As further discussed below, the one or more calibration parameters are used to maintain tracking of display device  205 . Based on a received calibration parameter, IMU  230  may adjust one or more IMU parameters (e.g., sample rate). In some embodiments, certain calibration parameters cause IMU  230  to update an initial position of the reference point so it corresponds to a next calibrated position of the reference point. Updating the initial position of the reference point as the next calibrated position of the reference point helps reduce accumulated error associated with the determined estimated position. The accumulated error, also referred to as drift error, causes the estimated position of the reference point to “drift” away from the actual position of the reference point over time. 
     Imaging device  235  generates calibration data in accordance with calibration parameters received from console  210 . Calibration data includes one or more images showing observed positions of locators  220  that are detectable by imaging device  235 . In some embodiments, imaging device  235  includes one or more still cameras, one or more video cameras, any other device capable of capturing images including one or more locators  220 , or some combination thereof. Additionally, imaging device  235  may include one or more filters (e.g., used to increase signal to noise ratio). Imaging device  235  is configured to optionally detect light emitted or reflected from locators  220  in a field of view of imaging device  235 . In embodiments where locators  220  include passive elements (e.g., a retroreflector), imaging device  235  may include a light source that illuminates some or all of locators  220 , which retro-reflect the light towards the light source in imaging device  235 . Second calibration data is communicated from imaging device  235  to console  210 , and imaging device  235  receives one or more calibration parameters from console  210  to adjust one or more imaging parameters (e.g., focal length, focus, frame rate, ISO, sensor temperature, shutter speed, aperture, etc.). 
     In some embodiments, display device  205  optionally includes one or more reflective elements  260 . In some embodiments, electronic display device  205  optionally includes a single reflective element  260  or multiple reflective elements  260  (e.g., a reflective element  260  for each eye of a user). In some embodiments, electronic display device  215  projects computer-generated images on one or more reflective elements  260 , which, in turn, reflect the images toward an eye or eyes of a user. The computer-generated images include still images, animated images, and/or a combination thereof. The computer-generated images include objects that appear to be two-dimensional and/or three-dimensional objects. In some embodiments, one or more reflective elements  260  are partially transparent (e.g., the one or more reflective elements  260  have a transmittance of at least 15%, 20%, 25%, 30%, 35%, 50%, 55%, or 50%), which allows transmission of ambient light. In such embodiments, computer-generated images projected by electronic display  215  are superimposed with the transmitted ambient light (e.g., transmitted ambient image) to provide augmented reality images. 
     Input interface  240  is a device that allows a user to send action requests to console  210 . An action request is a request to perform a particular action. For example, an action request may be to start or end an application or to perform a particular action within the application. Input interface  240  may include one or more input devices. Example input devices include: a keyboard, a mouse, a game controller, data from brain signals, data from other parts of the human body, or any other suitable device for receiving action requests and communicating the received action requests to console  210 . An action request received by input interface  240  is communicated to console  210 , which performs an action corresponding to the action request. In some embodiments, input interface  240  may provide haptic feedback to the user in accordance with instructions received from console  210 . For example, haptic feedback is provided when an action request is received, or console  210  communicates instructions to input interface  240  causing input interface  240  to generate haptic feedback when console  210  performs an action. 
     Console  210  provides media to display device  205  for presentation to the user in accordance with information received from one or more of: imaging device  235 , display device  205 , and input interface  240 . In the example shown in  FIG.  2   , console  210  includes application store  245 , tracking module  250 , and application engine  255 . Some embodiments of console  210  have different modules than those described in conjunction with  FIG.  2   . Similarly, the functions further described herein may be distributed among components of console  210  in a different manner than is described here. 
     When application store  245  is included in console  210 , application store  245  stores one or more applications for execution by console  210 . An application is a group of instructions, that when executed by a processor, is used for generating content for presentation to the user. Content generated by the processor based on an application may be in response to inputs received from the user via movement of display device  205  or input interface  240 . Examples of applications include: gaming applications, conferencing applications, video playback application, or other suitable applications. 
     When tracking module  250  is included in console  210 , tracking module  250  calibrates system  200  using one or more calibration parameters and may adjust one or more calibration parameters to reduce error in determination of the position of display device  205 . For example, tracking module  250  adjusts the focus of imaging device  235  to obtain a more accurate position for observed locators on display device  205 . Moreover, calibration performed by tracking module  250  also accounts for information received from IMU  230 . Additionally, if tracking of display device  205  is lost (e.g., imaging device  235  loses line of sight of at least a threshold number of locators  220 ), tracking module  250  re-calibrates some or all of system  200 . 
     In some embodiments, tracking module  250  tracks movements of display device  205  using second calibration data from imaging device  235 . For example, tracking module  250  determines positions of a reference point of display device  205  using observed locators from the second calibration data and a model of display device  205 . In some embodiments, tracking module  250  also determines positions of a reference point of display device  205  using position information from the first calibration data. Additionally, in some embodiments, tracking module  250  may use portions of the first calibration data, the second calibration data, or some combination thereof, to predict a future location of display device  205 . Tracking module  250  provides the estimated or predicted future position of display device  205  to application engine  255 . 
     Application engine  255  executes applications within system  200  and receives position information, acceleration information, velocity information, predicted future positions, or some combination thereof of display device  205  from tracking module  250 . Based on the received information, application engine  255  determines content to provide to display device  205  for presentation to the user. For example, if the received information indicates that the user has looked to the left, application engine  255  generates content for display device  205  that mirrors the user&#39;s movement in an augmented environment. Additionally, application engine  255  performs an action within an application executing on console  210  in response to an action request received from input interface  240  and provides feedback to the user that the action was performed. The provided feedback may be visual or audible feedback via display device  205  or haptic feedback via input interface  240 . 
       FIG.  3    is an isometric view of display device  300  in accordance with some embodiments. In some other embodiments, display device  300  is part of some other electronic display (e.g., a digital microscope, a head-mounted display device, etc.). In some embodiments, display device  300  includes light emission device array  310  and one or more lenses  330 . In some embodiments, display device  300  also includes an IR detector array. 
     Light emission device array  310  emits image light and optional IR light toward the viewing user. Light emission device array  310  may be, e.g., an array of LEDs, an array of microLEDs, an array of OLEDs, or some combination thereof. Light emission device array  310  includes light emission devices  320  that emit light in the visible light (and optionally includes devices that emit light in the IR). 
     In some embodiments, display device  300  includes an emission intensity array configured to selectively attenuate light emitted from light emission array  310 . In some embodiments, the emission intensity array is composed of a plurality of liquid crystal cells or pixels, groups of light emission devices, or some combination thereof. Each of the liquid crystal cells is, or in some embodiments, groups of liquid crystal cells are, addressable to have specific levels of attenuation. For example, at a given time, some of the liquid crystal cells may be set to no attenuation, while other liquid crystal cells may be set to maximum attenuation. In this manner, the emission intensity array is able to control what portion of the image light emitted from light emission device array  310  is passed to the one or more lenses  330 . In some embodiments, display device  300  uses an emission intensity array to facilitate providing image light to a location of pupil  350  of eye  350  of a user, and minimize the amount of image light provided to other areas in the eyebox. 
     One or more lenses  330  receive the modified image light (e.g., attenuated light) from emission intensity array (or directly from emission device array  310 ), and direct the modified image light to a location of pupil  350 . 
     In some embodiments, light emission device array  310  and an emission intensity array make up a display element. Alternatively, the display element includes light emission device array  310  (e.g., when light emission device array  310  includes individually adjustable pixels) without the emission intensity array. In some embodiments, the display element additionally includes the IR array. In some embodiments, in response to a determined location of pupil  350 , the display element adjusts the emitted image light such that the light output by the display element is refracted by one or more lenses  330  toward the determined location of pupil  350 , and not toward other locations in the eyebox. 
     In some embodiments, display device  300  includes one or more broadband sources (e.g., one or more white LEDs) coupled with a plurality of color filters, in addition to, or instead of, light emission device array  310 . 
     In some embodiments, display device  300  also includes holographic optical element  335 . 
     In some embodiments, light emission device array  310  is positioned within the field of view of eye  340  for virtual reality applications. In some embodiments, display device  300  also includes an optical waveguide or a combiner so that light emission device array  310  is positioned off the field of view of eye  340 . Such configurations may be used for augmented reality applications. 
     In some embodiments, an IR detector array detects IR light that has been retro-reflected from the retina of eye  350 , a cornea of eye  350 , a crystalline lens of eye  350 , or some combination thereof. The IR detector array includes either a single IR sensor or a plurality of IR sensitive detectors (e.g., photodiodes). In some embodiments, the IR detector array is integrated into light emission device array  310 . In some embodiments, the IR detector array is separate from light emission device array  310 , as shown in  FIG.  4 A . 
       FIG.  4 A  is a schematic diagram illustrating eye tracking device  400  in accordance with some embodiments. Eye tracking device  400  includes imaging device  402  (e.g., a camera, such as an infrared camera) and holographic medium  404 . Holographic medium  404  is a holographic medium for projecting multiple views of an eye of a user (e.g., a user of a head-mounted display device). In some embodiments, holographic medium  404  is a wide-field holographic medium. In some cases, a wide-field holographic medium refers to a holographic medium configured to project images of an area with a characteristic dimension of at least 10 mm (e.g., imaging an area of at least 10 mm, 15 mm, 20 mm, 25 mm, or 30 mm in diameter or length). 
     In  FIG.  4 A , imaging device  402  is located away from an optical axis of holographic medium  404 . In some embodiments, imaging device  402  is located away from an optical axis of a lens (e.g., lens  330  in  FIG.  3   ) of a head-mounted display device. In some embodiments, imaging device  402  is located away from a field of view of eye  408  (e.g., eye  408  corresponds to an eye of a user of a head-mounted display device). By providing an off-axis imaging, imaging device  402  does not occlude the field of view of eye  408 . In some embodiments, imaging device  402  is positioned on the optical axis of holographic medium  404 . 
     In  FIG.  4 A , multiple views of eye  408  are projected by holographic medium  404  toward imaging device  402 . In  FIG.  4 A , holographic medium  404  is a reflection holographic medium, having surface  404 - 1  and surface  404 - 2 , with one or more recorded interference patterns. The one or more recorded interference patterns modify light (e.g., infrared light reflected off by an eye) impinging on recorded interference patterns and project one or more holographic patterns. In  FIG.  4 A , light  405  from eye  408  is received by surface  404 - 2  of holographic medium  404  (e.g., a surface of holographic medium  404  facing eye  408 ). Holographic medium  404  includes areas  412 - 1 ,  412 - 2 , and  412 - 3  that are configured to interact with light  405  from eye  408  and concurrently direct (e.g., reflect, diffract, etc.) separate portions  406 - 1 ,  406 - 2 , and  406 - 3  of light  405  toward imaging device  402 . In some embodiments, portions  406 - 1 ,  406 - 2 , and  406 - 3  of light  405  correspond to images (or views) of eye  408  from three distinct virtual view points  410 - 1 ,  410 - 2 , and  410 - 3 . For example, portion  406 - 1  of light  405  corresponds to a view of eye  408  from view point  410 - 1 , portion  406 - 2  of light  405  corresponds to a view of eye  408  from view point  410 - 2 , and portion  406 - 3  of light  405  corresponds to a view of eye  408  from view point  410 - 3 . In addition, portions  406 - 1 ,  406 - 2 , and  406 - 3  of light  405  are directed toward imaging device  402  at distinct angles. For example, portion  406 - 1  of light  405  is directed toward imaging device  402  at a first angle, portion  406 - 2  of light  405  is directed toward imaging device  402  at a second angle, and portion  406 - 3  of light  405  is directed toward imaging device  402  at a third angle. 
     In some embodiments, portions  406 - 1 ,  406 - 2 , and  406 - 3  of light  405  are projected onto distinct portions of the imaging device  402 . For example, the inset of  FIG.  4 A  shows that portion  406 - 1  of light  405  is projected onto a first portion  1  of the imaging device  402 , portion  406 - 2  of light  405  is projected onto a second portion  2  of the imaging device  402 , and portion  406 - 3  of light  405  is projected onto a third portion  3  of the imaging device  402 . 
     In some embodiments, holographic medium  404  has a limited angular and/or spectral selectivity. For example, holographic medium  404  reflects light  402 - 1  with a specific wavelength range and/or with a specific distribution of incident angles while transmitting light with wavelengths outside the specific wavelength range and/or with incident angles outside the specific distribution of incident angles. In some embodiments, holographic medium  404  reflects light in the IR (e.g., NIR) wavelength range. This allows holographic medium  404  to be used in a virtual reality device (e.g., holographic medium  404  is placed in front of a display panel, transmitting visible light from the display panel) or an augmented reality device (e.g., holographic medium  404  transmits visible ambient light). 
     In some embodiments, holographic medium  404  is a volume hologram (also called a Bragg hologram). A volume hologram refers to a hologram with thickness sufficiently large for inducing Bragg diffraction, i.e., the thickness of the recording material used for recording a volume hologram is significantly larger than the wavelength of light used for recording the hologram. Such holograms have spectral selectivity, angular selectivity of an incident light and/or selectivity with respect to wavefront profile of an incident light. 
       FIG.  4 B  is a schematic diagram illustrating eye tracking device  420  in accordance with some embodiments. Eye tracking device  420  is similar to eye tracking device  400  described above with respect to  FIG.  4 A , except that eye tracking device  420  includes holographic medium  424  instead of holographic medium  404 . Holographic medium  424  includes areas  422 - 1 ,  422 - 2 , and  422 - 3  configured to interact with light  405  and concurrently direct (e.g., reflect, diffract, etc.) separate portions  406 - 1 ,  406 - 2 , and  406 - 3  of light  405  toward imaging device  402 . Areas  422 - 1 ,  422 - 2 , and  422 - 3  are in contact with each other (e.g., areas  422 - 1  and  422 - 3  are in contact with area  422 - 2 ), while areas  412 - 1 ,  412 - 2 , and  412 - 3  of holographic medium  404  may not be in contact with each other (e.g., none of areas  412 - 1 ,  412 - 2 , and  413 - 3  is in contact with any other of areas  412 - 1 ,  412 - 2 , and  412 - 3 ). In some embodiments, a holographic medium includes (i) a first area configured for interacting with light  405  and directing at least a portion of light  405  where the second area is not adjacent to (e.g., not in contact with) any other area configured for interacting with light  405  and directing at least a portion of light  405  and (ii) a second area configured for interacting with light  405  and directing at least a portion of light  405  where the second area is adjacent to (e.g., in contact with) another area configured for interacting with light  405  and directing at least a portion of light  405 . 
       FIG.  4 C  is a schematic diagram illustrating eye tracking device  430  in accordance with some embodiments. Eye tracking device  430  is similar to eye tracking device  400  described above with respect to  FIG.  4 A , except that eye tracking device  430  includes holographic medium  434 , which is a transmission holographic medium having surfaces  434 - 1  and  434 - 2 . Imaging device  402  is positioned away from an optical axis of holographic medium  434  and away from a field of view of eye  408 . In eye tracking device  430 , imaging device  402  is positioned on opposite side of holographic medium  434  from eye  408 , facing surface  434 - 1  of holographic medium  434  (e.g., imaging device  402  is positioned closer to surface  434 - 1  of holographic medium  434  than surface  434 - 2  of holographic medium  434  facing eye  408 ). Holographic medium  434  includes areas  432 - 1 ,  432 - 2 , and  432 - 3  that are configured to interact with light  405  and concurrently direct separate portions  436 - 1 ,  436 - 2 , and  436 - 3  of light  405  toward eye  408 . Similar to the corresponding portions  406 - 1 ,  406 - 2 , and  406 - 3  of light  405  shown in  FIG.  4 A , portions  436 - 1 ,  436 - 2 , and  436 - 3  of light  405 , in some embodiments, correspond to views of eye  408  from different view points. 
       FIG.  4 D  is a schematic diagram illustrating eye tracking device  440  in accordance with some embodiments. Eye tracking device  440  is similar to eye tracking device  420  shown in  FIG.  4 B , except that eye tracking device  440  also includes one or more light sources  502 . As explained above with respect to  FIG.  4 B , holographic medium  424  projects portions  406 - 1 ,  406 - 2 , and  406 - 3  of light  405  corresponding to views of eye  408  by toward imaging device  402 , and portions  406 - 1 ,  406 - 2 , and  406 - 3  of light  405  projected by holographic medium  424  toward imaging device  402  are not shown in  FIG.  4 D  so as not to obscure other aspects of eye tracking device  440 . One or more light sources  502  provide light  425  (e.g., infrared light) toward holographic medium  424 , which in turn projects one or more light patterns  426 - 1 ,  426 - 2 , and  426 - 3  toward eye  408 . Light patterns projected by holographic medium  424  (e.g., light patterns  426 - 1 ,  426 - 2 , and  426 - 3 ) are projected toward eye  408  at respective angles. Although  FIG.  4 D  shows that holographic medium  424  projects one or more light patterns  426 - 1 ,  426 - 2 , and  426 - 3  toward eye  408 , any other holographic medium described herein (e.g., holographic medium  404  shown in  FIG.  4 A ) may be configured to project one or more light patterns (e.g., light patterns  426 - 1 ,  426 - 2 , and  426 - 3 ) toward eye  408 . 
       FIG.  4 D  also shows that in some embodiments, holographic medium  424  transmits ambient light  428 . For example, holographic medium  424  may be configured to direct (e.g., reflect or diffract) infrared light and transmit visible light so that components of light having visible wavelengths are transmitted through holographic medium  424 . 
       FIGS.  5 A- 5 D  are schematic diagrams illustrating configurations of light patterns used for eye tracking in accordance with some embodiments. The example light patterns illustrated in  FIGS.  5 A- 5 D  are used for in-field illumination of an eye. In some embodiments, the eye is illuminated with an IR or NIR light for eye-tracking purposes (e.g., the light patterns illustrated in  FIG.  5 A- 5 D  are illuminated with an IR or NIR light). In some embodiments, the light patterns shown in  FIG.  5 A- 5 D  are configured to illuminate an area with a characteristic dimension (e.g., a diameter or width) of at least 10 mm on a surface of the eye (e.g., 10 mm, 15 mm, 20 mm, 25 mm, 30 mm, etc.). The configurations shown in  FIGS.  5 A- 5 D  include a plurality of distinct and separate light patterns (e.g., image objects or image structures, such as light patterns  502 - 1 ,  502 - 2 , and  502 - 3  in  FIG.  5 A ), arranged in a uniform or a non-uniform configuration. In some embodiments, a number of patterns in the plurality of separate light patterns is between 5 and 2000. In some embodiments, the number of light patterns in a particular configuration is between seven and twenty. In some embodiments, the number of light patterns is between 20 and 1000. In some embodiments, the number of light patterns is between 1000 and 2000. In some embodiments, the light patterns have one or more predefined shapes, such as circles (e.g., spots), stripes, triangles, squares, polygons, crosses, sinusoidal objects and/or any other uniform or non-uniform shapes. 
       FIG.  5 A  illustrates configuration  502  including seven separate light patterns (e.g., light patterns  502 - 1 ,  502 - 2 , and  502 - 3 ). In  FIG.  5 A , each light pattern has a shape of a circle (e.g., a solid circle or a hollow circle). Multiple light patterns (e.g., light patterns  502 - 1  and  502 - 2  among others) are arranged in a circular configuration with light pattern  502 - 3  positioned at the center of the circular configuration. In some embodiments, configuration  502  includes light patterns arranged in a plurality of concentric circles (e.g., 2, 3, 4, 5 circles or more). In some embodiments, configuration  502  does not include a central light pattern (e.g., light pattern  502 - 3 ). 
       FIG.  5 B  illustrates rectangular configuration  504  including a plurality (e.g., eight) of separate stripe-shaped light patterns (e.g., light patterns  504 - 1  and  504 - 2 ). 
       FIG.  5 C  illustrates configuration  506  including a plurality of light patterns arranged in a two-dimensional configuration (e.g., a rectangular configuration). In  FIG.  5 C , the plurality of light patterns is arranged in multiple rows and multiple columns (e.g.,  144  light patterns arranged in twelve rows and twelve columns). In some embodiments, the plurality of light patterns is arranged to have a uniform spacing in a first direction and a uniform spacing in a second direction that is distinct from the first direction (e.g., the second direction is orthogonal to the first direction). In some embodiments, the plurality of light patterns is arranged to have a first spacing in the first direction and a second spacing in the second direction that is distinct from the first spacing. In some embodiments, the plurality of light patterns is arranged to have a uniform spacing in the first direction and a non-uniform spacing in the second direction. In some embodiments, the plurality of light patterns is arranged to have a uniform center-to-center distance in the first direction and a uniform center-to-center distance in the second direction. In some embodiments, the plurality of light patterns is arranged to have a first center-to-center distance in the first direction and a second center-to-center distance in the second direction that is distinct from the first center-to-center distance. In some embodiments, the plurality of light patterns is arranged to have a uniform center-to-center distance in the first direction and a non-uniform center-to-center distance in the second direction. 
     In  FIG.  5 C , each light pattern has a same shape (e.g., a square, rectangle, triangle, circle, ellipse, oval, star, polygon, etc.). 
       FIG.  5 D  is similar to  FIG.  5 C , except that, in  FIG.  5 D , configuration  507  of the plurality of light patterns includes a first set of light patterns  506 - 1  each having a first shape (e.g., a square or a rectangle) and a second set of light patterns  506 - 2  each having a second shape (e.g., a circle) that is distinct from the first shape. 
       FIG.  6 A  is a schematic diagram illustrating display device  600  in accordance with some embodiments. In some embodiments, display device  600  is configured to provide virtual reality content to a user. In some embodiments, display device  600  corresponds to display device  100  described above with respect to  FIG.  1   . In  FIG.  6 A , display device  600  includes imaging device  402 , holographic medium  404 , display panel  610  and one or more lenses  608 . Holographic medium  404  optically coupled with imaging device  402  operates as an eye tracking device described above with respect to  FIG.  4 A . In some embodiments, display device  600  also includes optics  606 . In some embodiments, optics  606  includes an aspheric lens for correcting distortions in the multiple views of eye  408  due to off-axis projection by holographic medium  404 . In some embodiments, the aspheric lens in optics  606  is an asymmetric lens. 
     In some embodiments, display device  600  also includes light source  602 . In some embodiments, as shown in  FIG.  6 A , light source  602  provides a pattern of light  604  directly toward eye  408 . In some embodiments, light source  602  provides light to holographic medium  404 , which then projects the light as light patterns toward eye  408  as shown in  FIG.  4 D . When display device  600  includes light source  602 , detector  402  captures an image (e.g., an image of an area encompassing eye  408 ) of at least a portion of light patterns reflected off a surface (e.g., a sclera) of eye  408 , directed by holographic medium  404  toward detector  402  for determining a position of a pupil of eye  408 . 
     Holographic medium  404 , imaging device  402 , and light source  602  of an eye-tracking system are configured to determine a position of the pupil of eye  408  and/or track its movement as eye  408  rotates toward different gaze directions. In some embodiments, the eye tracking system corresponds to, is coupled with, or is included in eye tracking module  217  described herein with respect to  FIG.  2   . In some embodiments, imaging device  402  is an IR and/or NIR camera (e.g., a still camera or a video camera) or other IR and/or NIR sensitive photodetector (e.g., an array of photodiodes). In some embodiments, determining a position of the pupil includes determining the position of the pupil on an x-y plane of the pupil (e.g., reference plane  408 - 1 ). In some embodiments, the x-y plane is a curvilinear plane. In some embodiments, light source  602  is integrated with imaging device  402 . In some embodiments, light projected by light source  602  (e.g., light  604 ) and an image captured by imaging detector  402  have the same optical path (or parallel optical paths) and are transmitted or guided by the same optical elements (e.g., holographic medium  404 ). 
     In some embodiments, the position of the pupil of eye  408  is determined based on a representative intensity or intensities of detected glints. In some embodiments, the position of the pupil is determined based on an incident angle of detected glints (e.g., display device  600  includes one or more optical elements to determine the incident angle of the detected glint). For example, the position of the pupil is determined by comparing an incident angle of reflected light patterns to an estimated surface profile of the surface of eye  408 . The surface profile of an eye does not correspond to a perfect sphere but instead has a distinct curvature in the area that includes the cornea and the pupil. Therefore, a position of the pupil can be determined by determining the surface profile of the eye. 
     In some embodiments, at least a portion of light patterns impinges on other surfaces of eye  408  than sclera (e.g., the pupil). In some embodiments, the position of the pupil is determined based on a portion of light patterns impinging on the sclera and impinging on the other surfaces of eye  408 . In some embodiments, the position of the pupil of eye  408  is determined based on a difference (and/or a ratio) between an intensity of a portion of light patterns impinging on the sclera and on the pupil. For example, the intensity of the portion of light patterns reflected on the sclera of eye is higher than the intensity the portion of light patterns reflected on the pupil and therefore the location of the pupil can be determined based on the intensity difference. 
     In some embodiments, the position of the pupil of eye  408  is determined based on a difference in a configuration (e.g., configurations described above with respect to  FIG.  5 A- 5 D ) projected by the holographic illuminator and a configuration captured by imaging device  402 . For example, as a light with a specific configuration is reflected off the non-flat surface of eye  408 , the structured pattern is modified (e.g., distorted). The non-flat surface profile of eye  408  is then determined based on the distorted structured pattern and the position of the pupil is determined based on the surface profile. 
     In some embodiments, a gaze angle of the eye and/or a state of the eye (e.g., whether its eye lid is open or closed) may be also determined (e.g., based on an image of the eye or intensities of glints detected by imaging device  402 ). 
     In  FIG.  6 A , imaging device  402  and light source  602  are located away from an optical axis  612  of holographic medium  404 , as well as away from optical axes of one or more lenses  608  and display  610 . For example, imaging device  402  and light source  602  are position on a temple and/or a frame of a head-mounted display device. Furthermore, imaging device  402  and light source  602  are positioned away from a field-of-view of eye  408  so that they do not occlude display panel  610 . In  FIG.  6 A , holographic medium  404  is positioned adjacent to one or more lenses  608 . Holographic medium  404  is configured to provide light patterns in the field-of-view of eye  408 . In  FIG.  6 A , holographic medium  404  is a reflection holographic medium, and imaging device  402  is located to illuminate a surface of holographic medium  404  that is configured to face eye  408 . 
     In some embodiments, holographic medium  404  is wavelength selective, thereby reflecting light with a specific wavelength range while transmitting light with other wavelengths, such as light from display panel  610 . In some embodiments, light used for eye-tracking is IR or NIR light, and therefore does not interfere with visible light projected from display panel  610 . 
       FIG.  6 B  is a schematic diagram illustrating display device  620  in accordance with some embodiments. Display device  620  is similar to display device  600  described above with respect to  FIG.  6 A , except that holographic medium  404  is a transmission holographic medium and imaging device  402  is located on an opposite side of holographic medium  404  from eye  408 . 
       FIG.  6 C  is a schematic diagram illustrating display device  630  in accordance with some embodiments. Display device  630  includes display device  600 -A for eye  408 -A (e.g., the left eye of a user of a head-mounted display device  630 ) and display device  600 -B for eye  408 -B (e.g., the right eye of a user of a head-mounted display device  630 ). In some embodiments, each of display devices  600 -A and  600 -B corresponds to display device  600  described above with respect to  FIG.  6 A . In some embodiments, a head-mounted display includes two display devices, each corresponding to display device  620  described above with respect to  FIG.  6 B . In some embodiments, display device  630  corresponds to display device  100  described above with respect to  FIG.  1   . 
     In some embodiments, display device  630  includes holographic medium  404  at a location having a distance (e.g., eye relief) of at least 10 mm, 11 mm, 12 mm, 13 mm, 14 mm, 15 mm, 16 mm, 17 mm, 18 mm, 19 mm, 20 mm, or within a range between any two of the aforementioned values, to eye  408  of a user when display device  630  is worn by the user. 
       FIG.  7 A  is a graphical representation of a multi-view image of an eye in accordance with some embodiments. The multi-view image of the eye shown in  FIG.  7 A  includes multiple views of the same eye shown in  FIG.  7 B , tiled adjacent to one another. In  FIG.  7 A , the multi-view image includes seven views of the eye, but in some other implementations, additional views, or fewer views, may be used (e.g., 2, 3, 4, 5, 6, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 30, 40, 50, 60, 70, 80, 90, or 100 views or a number of views within a range between any two of the forementioned values). 
     Each view shown in  FIGS.  7 A and  7 B  illustrates a plurality of glints arranged in a pattern shown in  FIG.  5 A . A position of a pupil of the reference eye can be determined based on the multiple views. Although the multiple views of the eye may be obtained separately by taking images at different times or by using multiple imaging devices, the use of the multi-view holographic optical element described herein allows concurrent collection (or taking) of the multiple views in a single image, while using fewer components. Thus, the use of the multi-view holographic optical element may reduce the size and weight of the display device. In some embodiments, the position of a pupil of the reference eye is determined based on intensities of respective glints. In some embodiments, the position of the pupil of the reference eye is determined based on locations of respective glints. 
       FIG.  8 A  is a schematic diagram illustrating system  800  for making a multi-view holographic optical element in accordance with some embodiments. System  800  includes light source  802 . In some embodiments, light source  802  is a point-light source (e.g., a laser). In some embodiments, beam  830  provided by light source  802  is coherent light. Light source  802  is optionally coupled optically with a plurality of optical components for modifying beam  830 , such as beam expander  804  that expands beam  830  and aperture  806  for adjusting the beam size of beam  830 . In some embodiments, beam  830  provided by light source  802  has a beam size with diameter less than 1 mm, which is then expanded to a beam size with a diameter greater than 10 mm, which is, in turn, clipped to a beam size with a diameter between 7 mm and 9 mm by aperture  806 . In some embodiments, light source  802  provides a monochromatic light. In some embodiments, the monochromatic light has a center wavelength at 600 nm, 610 nm, 620 nm, 630 nm, 640 nm, 650 nm, 660 nm, 670 nm, 680 nm, 690 nm, 700 nm, 710 nm, 720 nm, 730 nm, 740 nm, 750 nm, 760 nm, 770 nm, 780 nm, 790 nm, 800 nm, 850 nm, 900 nm, 950 nm, 1000 nm, 1050 nm, 1100 nm, or within a range between any two of the aforementioned values. 
     In some embodiments, system  800  includes polarizer  808  and a polarization of beam  830  is adjusted by polarizer  808 . For example, in some implementations, polarizer  808  is a half-wave plate configured to adjust a direction of a linear polarized light. 
     In  FIG.  8 A , beam  830  is divided into two physically separated beams  832 -A and  834 -A by beam splitter  810 . In some embodiments, beam splitter  810  is a 50/50 reflector (e.g., beam  832 -A and beam  834 -A have the same intensity). In some embodiments, beam splitter  810  is a polarizing beam splitter dividing beam  830  into beam  832 -A with a first polarization (e.g., polarization in the vertical direction) and beam  834 -A with a second polarization (e.g., polarization in the horizontal direction). In some embodiments, a combination of a half-wave plate (e.g., polarizer  808 ) and a polarizing beam splitter (e.g., beam splitter  810 ) is used for adjusting intensities of beams  832 -A and  834 -A and/or adjusting a ratio of intensities of beams  832 -A and  834 -A. For example, in some implementations, the intensities are adjusted by changing the orientation of the half-wave plate. In some embodiments, the polarization of one or more of the beams  832 -A and  834 -A is further adjusted by one or more polarizers (e.g., polarizer  812 , which can be a half-way plate). In  FIG.  8 A , polarizer  812  of a second set of optical elements  800 -B adjusts the polarization of beam  834 -A to correspond to the polarization of beam  832 -A. In some implementations, polarizer  812  is included in a first set of optical elements  800 -A for adjusting the polarization of beam  832 -A. 
     Beam  832 -A is directed, for example by beam splitter  810 , toward the first set of optical elements  800 -A. The first set of optical elements  800 -A includes optical elements for providing an illumination serving as a reference light in a formation of a holographic medium. In some embodiments, the first set of optical elements  800 -A includes reflector  822 - 1 , which directs beam  832 -A toward lens  824 - 1 . In some embodiments, the first set of optical elements  800 -A includes lens  824 - 1  for expanding beam  834 -A and transmitting beam  832 -B toward optically recordable medium  826 . In some embodiments, the first set of optical elements  800 -A includes a subset, or a superset of optical components illustrated in  FIG.  8 A . For example, the first set of optical elements  800 -A may include other optical elements, that are not illustrated in  FIG.  8 A , for providing an illumination onto optically recordable medium  826 . In some implementations, the first set of optical elements  800 -A may not include one or more optical elements illustrated as components of the first set of optical elements  800 -A in  FIG.  8 A . A beam has a spot size applicable for illuminating, with a single exposure, an area on optically recordable medium  826  for forming any of the holographic mediums described above with respect to  FIGS.  4 A- 4 D . In some embodiments, a beam refers to a beam with a spot size with a characteristic dimension (e.g., a diameter or width) of at least 10 mm. In some embodiments, a beam refers to a beam with a spot size with a characteristic dimension (e.g., a diameter or width) of at least 100 mm. In some embodiments, lens  824 - 1  is a microscopic objective (e.g., lens  824 - 1  is a microscopic objective with 20× magnification with numerical aperture of 0.4). In some embodiments, lens  824 - 1  is a lens assembly including two or more lenses. Optionally, lens  824 - 1  is optically coupled with aperture  828 - 1  for adjusting a size of beam  832 -B. In some embodiments, aperture  828 - 1  has a diameter between 5 mm and 6 mm. In some embodiments, aperture  828 - 1  has a diameter between 6 mm and 7 mm. In some embodiments, aperture  828 - 1  has a diameter between 7 mm and 8 mm. In some embodiments, aperture  828 - 1  has a diameter between 8 mm and 9 mm. In some embodiments, aperture  828 - 1  has a diameter between 9 mm and 10 mm. In some embodiments, aperture  828 - 1  has a diameter between 10 mm and 11 mm. In some embodiments, reflector  822 - 1  is an adjustable reflector configured for adjusting the direction of beam  832 -A, thereby adjusting the direction of beam  832 -B transmitted from lens  824 - 1  toward optically recordable medium  826 . In some implementations, beam  832 -B provides a single-shot off-axis illumination with a diameter of at least 10 mm (e.g., 100 mm or more) onto surface  826 - 1  of optically recordable medium  826 . 
     In some embodiments, optically recordable medium  826  includes photosensitive polymers, silver halide, dichromatic gelatin and/or other standard holographic materials. In some embodiments, optically recordable medium  826  includes other types of wavefront shaping materials (e.g., metamaterials, polarization sensitive materials, etc.). In some embodiments, optically recordable medium  826  has a thickness (e.g., distance between surfaces  826 - 1  and  826 - 2 ) that is much greater than the wavelength of lights  832 -B and  834 -B in order to record a volume hologram. 
     In some embodiments, optically recordable medium  826  is coupled with a waveguide (e.g., waveguide  456  in  FIG.  4 E ) in order to record a holographic medium (e.g., holographic medium  454 ) that is configured to receive light propagating through a waveguide, as described above with respect to holographic illuminator  450  in  FIG.  4 E . 
     Beam  834 -A is directed, by beam splitter  810 , toward the second set of optical elements  800 -B. The second set of optical elements  800 -B includes optical elements for providing an illumination to a third set of optical elements  800 -C. 
     In some embodiments, the second set of optical elements  800 -B includes lens  814 - 1  and multi-faceted prism  816 . In some embodiments, the second set of optical elements  800 -B includes a subset, or a superset of optical components illustrated in  FIG.  8 A . For example, the first set of optical elements  800 -A may include other optical elements, that are not illustrated in  FIG.  8 A , for providing an illumination to the third set of optical elements  800 -C. In some implementations, the second set of optical elements  800 -B may not include one or more optical elements illustrated as components of the second set of optical elements  800 -B in  FIG.  8 A . 
     In some embodiments, lens  814 - 1  is a microscopic objective (e.g., lens  814 - 1  is a microscopic objective with 20× magnification and a numerical aperture of 0.4) configured to expand beam  834 -A. In some embodiments, lens  814 - 1  is a lens assembly including two or more lenses. In  FIG.  8 A , lens  814 - 1  transmits beam  834 -A toward multi-faceted prism  816 . Multi-faceted prism  816  collimates beam  834 -A and reflects collimated beam  834 -B toward the third set of optical elements  800 -C. In some embodiments, multi-faceted prism  816  includes multiple facets for forming multiple regions (e.g., regions  412 - 1 ,  412 - 2 , and  412 - 3 ) in optically recordable medium  826 . In some embodiments, the combination of lens  814 - 1  and multi-faceted prism  816  expands beam  834 -A such that beam  834 -B has a beam diameter of 10 mm or more. For example, the combination of lens  814 - 1  and multi-faceted prism  816  is configured to expand beam  834 -A with a beam diameter of 8 mm into a beam  834 -B with a beam diameter of 100 mm. 
     In  FIG.  8 A , multi-faceted prism  816  of the second set of optical elements  800 -B is located to intersect with an optical axis of the holographic medium formed from optically recordable medium  826  (e.g., an axis that is perpendicular to the holographic medium). In some embodiments, two or more multi-faceted prisms are used. In some implementations, multi-faceted prism  816  directs at least a portion of beam  834 -B onto optically recordable medium  826  in a direction perpendicular to optically recordable medium  826  (for 0° angle of diffraction), thereby providing an on-axis illumination onto surface  826 - 2  of optically recordable medium  826  while beam  832 -B provides an off-axis illumination onto surface  826 - 1  of optically recordable medium  826  (e.g., for an angle of incidence having 15°, 30°, 45°, 60°, 75° or within a range between any two of the aforementioned values). In some implementations, multi-faceted prism  816  directs at least a portion of beam  834 -B onto optically recordable medium  826  in a direction non-perpendicular to optically recordable medium  826 , thereby providing an off-axis illumination onto surface  826 - 2  of optically recordable medium  826  (e.g., for an angle of diffraction having 15°, 30°, 45°, 50°, 55°, 60°, 65°, 70°, 75° or within a range between any two of the aforementioned values) while beam  832 -B provides an on-axis illumination onto surface  826 - 1  of optically recordable medium  826  (for 0° angle of incidence). In some implementations, multi-faceted prism  816  directs at least a portion of beam  834 -B onto optically recordable medium  826  in a direction non-perpendicular to optically recordable medium  826 , thereby providing an off-axis illumination onto surface  826 - 2  of optically recordable medium  826  while beam  832 -B also provides an off-axis illumination onto surface  826 - 1  of optically recordable medium  826 . 
     The third set of optical elements  800 -C receives beam  834 -B and project the beam toward optically recordable medium  826  for forming a holographic medium. System  800  is configured to form holographic mediums described above with respect to  FIGS.  4 A- 4 B . The holographic mediums formed by formed by system  800  are configured to project configurations such as any of those described above with respect to  FIGS.  5 A- 5 D . In some embodiments, the third set of optical elements  800 -C includes one or more lenses  820 . 
       FIG.  8 B  is a schematic diagram illustrating a prism used for making a multi-view holographic optical element in accordance with some embodiments. The prism illustrated in  FIG.  8    is an example of multi-faceted prism  816 . One end of multi-faceted prism  816  includes two or more (e.g., three or more, four or more, five or more, etc.) facets for forming multiple regions (e.g., regions  412 - 1 ,  412 - 2 , and  412 - 3 ) in optically recordable medium  826 . In  FIG.  8 B , one end of the prism has seven facets  841  through  847  for providing a multi-view image shown in  FIG.  7 A . In some embodiments, one or more facets (e.g., facets  841  through  847 ) of the prism are non-flat facets (e.g., concave facets, convex facets, freeform facets, etc.). 
     In some embodiments, the prism is made of glass. In some embodiments, the prism is made of a material having a refractive index of 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, or within a range between any two of the aforementioned values. 
     In light of these principles, we now turn to certain embodiments. 
     In accordance with some embodiments, a method includes projecting, with a holographic optical element, a first view of an eye toward an imaging device (e.g., portion  406 - 1  of light  405  corresponding to a view of eye  408  from view point  410 - 1  as shown in  FIG.  4 A ); and projecting, with the holographic optical element, a second view of the eye, distinct from the first view of the eye, toward the imaging device (e.g., portion  406 - 2  of light  405  corresponding to a view of eye  408  from view point  410 - 2 ) so that the first view and the second view of the eye are concurrently received by the imaging device (e.g., imaging device  402  concurrently receives the first view and the second view of the eye). 
     In some embodiments, the first view and the second view of the eye are stored in a single image including multiple views of the eye (e.g., multi-view image shown in  FIG.  7 A ). 
     In some embodiments, the method includes determining a position of the eye based on at least the first view and the second view of the eye. For example, the intensity of glints in the first view and the second view are compared to determine the position of the eye. In some embodiments, the glints in the first view and the glints in the second view are combined to provide combined glint information (e.g., to provide a position of a glint occluded in one of the views). 
     In some embodiments, the method includes projecting, with the holographic optical element, a third view of the eye, distinct from the first view and the second view of the eye, toward the imaging device (e.g., portion  406 - 3  of light  405  corresponding to a view of eye  408  from view point  410 - 3 ) so that the first view, the second view, and the third view of the eye are concurrently received by the imaging device. 
     In some embodiments, the method includes projecting, with the holographic optical element, at least seven views of the eye toward the imaging device, where the seven views are distinct from one another (e.g.,  FIG.  7 A ). 
     In some embodiments, the seven views include a central view of the eye and six peripheral views of the eye (e.g., central view  701  and peripheral views  702  through  707 ). 
     In some embodiments, the method includes receiving, on the holographic optical element, light from a light source (e.g., holographic medium  424  in  FIG.  4 D  receives light from light source  502 ); and projecting, with the holographic optical element, a pattern of illumination light toward the eye (e.g., patterns  426 - 1 ,  426 - 2 , and  426 - 3 ). 
     In some embodiments, the pattern of illumination light includes a plurality of spots that are distinct and separate from one another (e.g., patterns shown in  FIGS.  5 A- 5 D ). 
     In some embodiments, the first view of the eye is projected toward a first portion of the imaging device; and the second view of the eye is projected toward a second portion, distinct from the first portion of the imaging device, of the imaging device (e.g., portion  406 - 1  of light  405  corresponding to a view of eye  408  from view point  410 - 1  is projected toward the first portion  1  of the imaging device and portion  406 - 2  of light corresponding to a view of eye  408  from view point  410 - 2  is projected toward the second portion  2  of the imaging device as shown in the inset of  FIG.  4 A ). 
     In some embodiments, the method includes, while projecting the first view and the second view of the eye toward the imaging device, transmitting ambient light (e.g., ambient light  428  in  FIG.  4 D ) through the holographic optical element toward the eye. 
     In some embodiments, the first view of the eye corresponds to a view of the eye taken from a first view point (e.g., view point  410 - 1 ); and the second view of the eye corresponds to a view of the eye taken from a second view point (e.g., view point  410 - 2 ) that is distinct and separate from the first view point. 
     In some embodiments, projecting the first view of the eye toward the imaging device and projecting the second view of the eye toward the imaging device include receiving light from the eye on a first surface of the holographic optical element and reflectively providing the light back through the first surface of the holographic optical element toward the imaging device (e.g.,  FIG.  4 A ). 
     In accordance with some embodiments, an eye tracking device includes an imaging device (e.g., imaging device  402 ); and a holographic optical element positioned relative to the imaging device for projecting a first view of a target area toward the imaging device (e.g., portion  406 - 1  of light  405  corresponding to a view of eye  408  from view point  410 - 1 ) and projecting a second view of the target area, distinct from the first view of the target area, toward the imaging device (e.g., portion  406 - 2  of light  405  corresponding to a view of eye  408  from view point  410 - 2 ) so that the first view and the second view of the target area are concurrently received by the imaging device. For example, the holographic optical element may project views of an area that may be larger or smaller than the eye (or the pupil). 
     In some embodiments, the first view and the second view of the eye are stored in a single image including multiple views of the eye (e.g.,  FIG.  7 A ). 
     In some embodiments, the eye tracking device includes one or more processors (e.g., processors  216 ) for determining a position of the eye based on at least the first view and the second view of the target area. 
     In some embodiments, the holographic optical element is positioned for projecting a third view of the target area, distinct from the first view and the second view of the target area, toward the imaging device (e.g., portion  406 - 3  of light  405  corresponding to a view of eye  408  from view point  410 - 3 ) so that the first view, the second view, and the third view of the target area are concurrently received by the imaging device. 
     In some embodiments, the holographic optical element is configured to project at least seven views of the target area toward the imaging device, each view of the seven views being distinct from one another (e.g.,  FIG.  7 A ). 
     In some embodiments, the eye tracking device includes a light source (e.g., light source  502 ) for providing light toward the holographic optical element so that the holographic optical element projects a pattern of illumination light toward the target area. 
     In some embodiments, the holographic optical element is configured for projecting the first view of the target area toward the imaging device with a first optical power and projecting the second view of the target area toward the imaging device with a second optical power distinct from the first optical power. For example, region  412 - 1  and region  412 - 2  have different distances to imaging device  402 , and thus, in some configurations, region  412 - 1  has the first optical power and region  412 - 2  has the second optical power that is distinct from the first optical power so that both the first view of eye  408  (or the target area) and the second view of eye  408  form images on a same plane (e.g., sensor plane) on imaging device  402 . 
     In accordance with some embodiments, a head-mounted display device includes any eye tracking device described herein (e.g.,  FIG.  6 C ). 
     In accordance with some embodiments, a holographic optical element configured for projecting a first view of a target area toward an imaging device and projecting a second view of the target area, distinct from the first view of the target area, toward the imaging device so that the first view and the second view of the target area are concurrently received by the imaging device. 
     In accordance with some embodiments, a method of making a holographic optical element includes recording a first holographic pattern in the holographic optical element by concurrently providing a first beam for a first view point and a second beam from a target area; and recording a second holographic pattern in the holographic optical element by concurrently providing a third beam for a second view point that is distinct from the first view point and the second beam from the target area (e.g.,  FIG.  8 A ). 
     Although various drawings illustrate operations of particular components or particular groups of components with respect to one eye, a person having ordinary skill in the art would understand that analogous operations can be performed with respect to the other eye or both eyes. For brevity, such details are not repeated herein. 
     Although some of various drawings illustrate a number of logical stages in a particular order, stages which are not order dependent may be reordered and other stages may be combined or broken out. While some reordering or other groupings are specifically mentioned, others will be apparent to those of ordinary skill in the art, so the ordering and groupings presented herein are not an exhaustive list of alternatives. Moreover, it should be recognized that the stages could be implemented in hardware, firmware, software or any combination thereof. 
     The foregoing description, for purpose of explanation, has been described with reference to specific embodiments. However, the illustrative discussions above are not intended to be exhaustive or to limit the scope of the claims to the precise forms disclosed. Many modifications and variations are possible in view of the above teachings. The embodiments were chosen in order to best explain the principles underlying the claims and their practical applications, to thereby enable others skilled in the art to best use the embodiments with various modifications as are suited to the particular uses contemplated.