Patent Publication Number: US-2022221724-A1

Title: Display system and method for providing variable accommodation cues using multiple intra-pupil parallax views formed by light emitter arrays

Description:
PRIORITY CLAIM 
     This application is a continuation application of U.S. application Ser. No. 16/803,563, filed Feb. 27, 2020, which claims priority to U.S. Provisional Application No. 62/812,142, filed on Feb. 28, 2019, and U.S. Provisional Application No. 62/815,225, filed on Mar. 7, 2019, which are incorporated herein by reference. 
    
    
     INCORPORATION BY REFERENCE 
     This application incorporates by reference the entirety of each of the following patent applications and publications: U.S. Application Publ. No. 2018/0113311, published Apr. 26, 2018; U.S. application Ser. No. 14/555,585, filed on Nov. 27, 2014; U.S. application Ser. No. 14/690,401, filed on Apr. 18, 2015; U.S. application Ser. No. 14/212,961, filed on Mar. 14, 2014; U.S. application Ser. No. 14/331,218, filed on Jul. 14, 2014; U.S. application Ser. No. 15/072,290, filed on Mar. 16, 2016; and WO 2016/179246, published Nov. 10, 2016; and U.S. Prov. Application No. 62/800363, filed Feb. 1, 2019. 
     BACKGROUND 
     Field 
     The present disclosure relates to optical devices, including augmented reality and virtual reality imaging and visualization systems. 
     Description of the Related Art 
     Modern computing and display technologies have facilitated the development of systems for so called “virtual reality” or “augmented reality” experiences, wherein digitally reproduced images or portions thereof are presented to a user in a manner wherein they seem to be, or may be perceived as, real. A virtual reality, or “VR”, scenario typically involves presentation of digital or virtual image information without transparency to other actual real-world visual input; an augmented reality, or “AR”, scenario typically involves presentation of digital or virtual image information as an augmentation to visualization of the actual world around the user. A mixed reality, or “MR”, scenario is a type of AR scenario and typically involves virtual objects that are integrated into, and responsive to, the natural world. For example, in an MR scenario, AR image content may be blocked by or otherwise be perceived as interacting with objects in the real world. 
     Referring to  FIG. 1 , an augmented reality scene  10  is depicted wherein a user of an AR technology sees a real-world park-like setting  20  featuring people, trees, buildings in the background, and a concrete platform  30 . In addition to these items, the user of the AR technology also perceives that he “sees” “virtual content” such as a robot statue  40  standing upon the real-world platform  30 , and a cartoon-like avatar character  50  flying by, which seems to be a personification of a bumble bee, even though these elements  40 ,  50  do not exist in the real world. Because the human visual perception system is complex, it is challenging to produce an AR technology that facilitates a comfortable, natural-feeling, rich presentation of virtual image elements amongst other virtual or real-world imagery elements. 
     Systems and methods disclosed herein address various challenges related to AR and VR technology. 
     SUMMARY 
     In some embodiments, a head-mounted display system is provided. The display system includes an image projection system comprising: a micro-display configured to output image light defining images; and projection optics configured to direct the image light from the micro-display for propagation to an eye of a viewer. The display system also comprises an array of selectively-activated shutters for selectively transmitting the image light to the eye from different locations. The array of selectively-activated shutters is disposed within an eye-box volume of the projection optics. 
     In some other embodiments, a method for displaying image content is provided. The method comprises injecting, from a head-mounted display system, a set of parallactically-disparate intra-pupil images of a virtual object into an eye of a viewer. Each image of the intra-pupil images is provided by: forming the image on a micro-display of the head-mounted display system; outputting image light from the micro-display through projection optics; and opening a shutter of an array of shutters to propagate image light through the opened shutter to the eye. The array of shutters is disposed within an eye box volume of the projection optics. Different images of the set of parallactically-disparate intra-pupil images propagate through different opened shutters. 
     In yet other embodiments, a head-mounted display system is provided. The display system comprises a micro-display comprising an array of groups of light emitters; an array of light collimators overlying the light emitters; and projection optics. Each light collimator is associated with one of the groups of light emitters and extends across all light emitters of the associated group of light emitters. The array of light collimators is between the light emitters and the projection optics. The display system is configured to display a virtual object on a depth plane by injecting a set of parallactically-disparate intra-pupil images of the object into an eye of a viewer. 
     In some other embodiments, a method for displaying image content is provided. The method comprises injecting, from a head-mounted display system, a set of parallactically-disparate intra-pupil images into an eye of a viewer. Injecting the set of parallactically-disparate intra-pupil images comprises: providing an array of groups of light emitters; providing an array of light collimators overlying the light emitters, wherein each light collimator is associated with a group of the light emitters; providing projection optics, wherein the array of light collimators is between the array of groups of light emitters and the projection optics, injecting a first parallactically-disparate intra-pupil image into the eye by emitting light from a first light emitter of the groups of light emitters; and injecting a second parallactically-disparate intra-pupil image into the eye by emitting light from a second light emitter of the groups of light emitters. 
     In addition, various innovative aspects of the subject matter described in this disclosure may be implemented in the following examples: 
     EXAMPLE 1 
     A head-mounted display system comprising:
         an image projection system comprising:
           a micro-display configured to output image light defining images; and   projection optics configured to direct the image light from the micro-display for propagation to an eye of a viewer; and   
           an array of selectively-activated shutters for selectively transmitting the image light to the eye from different locations, wherein the array of selectively-activated shutters is disposed within an eye-box volume of the projection optics.       

     EXAMPLE 2 
     The display system of Example 1, further comprising a control system comprising one or more processors and memory storing instructions that, when executed by the one or more processors, cause the display system to perform operations comprising:
         determining a desired depth plane for a virtual object;   determining shutters of the array of selectively-activated shutters to be opened based upon the desired depth plane;   synchronizing presentation of different images, by the image projection system, with opening of different ones of the shutters, wherein the different images provide different views of the virtual object.       

     EXAMPLE 3 
     The display system of any of Examples 1 or 2, wherein the shutters are moveable physical structures. 
     EXAMPLE 4 
     The display system of Example 3, wherein the physical structures are mems-based micro-mechanical structures. 
     EXAMPLE 5 
     The display system of Example 3, wherein the shutters are ferro-electric shutters. 
     EXAMPLE 6 
     The display system of any of Examples 1 or 2, wherein the shutters comprise chemical species having reversibly changeable states, the states providing different amounts of light transmission. 
     EXAMPLE 7 
     The display system of Example 6, wherein the chemical species comprise liquid crystals, wherein the shutters are formed by pixels of a pixelated liquid crystal display. 
     EXAMPLE 8 
     The display system of any of Examples 1-7, wherein the micro-display is an emissive micro-display comprising an array of light emitters. 
     EXAMPLE 9 
     The display system of Example 8, wherein the light emitters are micro-LEDs. 
     EXAMPLE 10 
     The display system of any of Examples 1-9, further comprising an array of light collimators between the light emitters and the projection optics. 
     EXAMPLE 11 
     The display system of Example 10, wherein each of the array of light collimators extends across a plurality of the light emitters, wherein each light collimator corresponds to a pixel in images outputted by the image projection system. 
     EXAMPLE 12 
     The display system of any of Examples 1-11, wherein the micro-display is one of a plurality of monochrome micro-displays forming the projection system, wherein each of the monochrome micro-displays is configured to emit light of a different component color. 
     EXAMPLE 13 
     The display system of Example 12, further comprising an X-cube prism, wherein each of the monochrome micro-displays is arranged to output image light into a different face of the X-cube prism. 
     EXAMPLE 14 
     The display system of any of Examples 1-13, further comprising a pupil relay combiner eyepiece configured to relay the image light to the eye of the viewer, wherein the array of selectively-activated shutters are configured to regulate propagation of the image light to the pupil relay combiner eyepiece. 
     EXAMPLE 15 
     The display system of Example 14, wherein the pupil relay combiner eyepiece comprises a waveguide comprising:
         in-coupling optical elements for in-coupling the image light into the waveguide; and   out-coupling optical elements for out-coupling in-coupled image light out of the waveguide.       

     EXAMPLE 16 
     The display system of Example 15, wherein the waveguide is one of a plurality of waveguides comprising in-coupling optical elements and out-coupling optical elements. 
     EXAMPLE 17 
     The display system of any of Examples 1-16, wherein the projection system has a pupil diameter of 0.2-0.5 mm. 
     EXAMPLE 18 
     A method for displaying image content, the method comprising:
         injecting, from a head-mounted display system, a set of parallactically-disparate intra-pupil images of a virtual object into an eye of a viewer,   wherein each image of the intra-pupil images is provided by:
           forming the image on a micro-display of the head-mounted display system;   outputting image light from the micro-display through projection optics; and   opening a shutter of an array of shutters to propagate image light through the opened shutter to the eye, wherein the array of shutters is disposed within an eye box volume of the projection optics,   wherein different images of the set of parallactically-disparate intra-pupil images propagate through different opened shutters.   
               

     EXAMPLE 19 
     The method of Example 18, wherein all images of the set of parallactically-disparate intra-pupil images are injected into the eye within a flicker fusion threshold. 
     EXAMPLE 20 
     The method of Example 18, wherein the flicker fusion threshold is 1/60 of a second. 
     EXAMPLE 21 
     The method of any of Examples 18-20, further comprising:
         determining a desired depth plane for the virtual object to be displayed to the viewer;   determining shutters of the array of selectively-activated shutters to be opened based upon the desired depth plane; and   synchronizing presentation of different ones of the set of parallactically-disparate intra-pupil images with opening of different ones of the shutters.       

     EXAMPLE 22 
     The method of any of Examples 18-21, further comprising:
         determining a gaze of the eye using an eye tracking sensor; and   selecting content for the intra-pupil images based upon the determined gaze of the eye.       

     EXAMPLE 23 
     The method of any of Examples 18-22, wherein the micro-display is an emissive micro-display. 
     EXAMPLE 24 
     The method of any of Examples 18-23, wherein the array of shutters comprises selectively-movable physical structures. 
     EXAMPLE 25 
     The method of any of Examples 18-23, wherein the array of shutters comprises chemical species having reversibly changeable states, the states providing different amounts of light transmission. 
     EXAMPLE 26 
     The method of any of Examples 18-25, wherein the different images provide different views of the virtual object. 
     EXAMPLE 27 
     A head-mounted display system comprising:
         a micro-display comprising an array of groups of light emitters;   an array of light collimators overlying the light emitters, wherein each light collimator is associated with one of the groups of light emitters and extends across all light emitters of the associated group of light emitters;   projection optics, wherein the array of light collimators is between the light emitters and the projection optics,   wherein the display system is configured to display a virtual object on a depth plane by injecting a set of parallactically-disparate intra-pupil images of the object into an eye of a viewer.       

     EXAMPLE 28 
     The display system of Example 27, further comprising one or more processors and memory storing instructions that, when executed by the one or more processors, cause the display system to perform operations comprising:
         determining light emitters of each of the groups of light emitters to activate based upon a desired level of parallax disparity for images formed by light emitters;   activating a first light emitter of the groups of light emitters to form a first parallactically-disparate intra-pupil image; and   activating a second light emitter of the groups of light emitters to form a second parallactically-disparate intra-pupil image,   wherein the first and second parallactically-disparate intra-pupil images provide different views of the virtual object.       

     EXAMPLE 29 
     The display system of Example 27, wherein activating the first light emitter of the groups of light emitters overlaps in time with activating the second light emitter of the groups of light emitters, to inject the first and second parallactically-disparate intra-pupil images into the eye simultaneously. 
     EXAMPLE 30 
     The display system of any of Examples 27-29, wherein the light collimators are lenslets. 
     EXAMPLE 31 
     The display system of any of Examples 27-30, further comprising an array of selectively-activated shutters for selectively transmitting the image light to the eye from different locations, wherein the array of selectively-activated shutters is disposed within an eye-box volume of the projection optics. 
     EXAMPLE 32 
     The display system of Example 31, wherein the array of shutters comprises selectively-movable physical structures. 
     EXAMPLE 33 
     The display system of Example 31, wherein the array of shutters comprises chemical species having reversibly changeable states, the states providing different amounts of light transmission. 
     EXAMPLE 34 
     The display system of any of Examples 31-33, further comprising one or more processors and memory storing instructions that, when executed by the one or more processors, cause the display system to perform operations comprising:
         determining a desired depth plane for a virtual object;   determining shutters of the array of selectively-activated shutters to be opened based upon the desired depth plane;   synchronizing presentation of different images, by the image projection system, with opening of different ones of the shutters, wherein the different images provide different views of the virtual object.       

     EXAMPLE 35 
     The display system of any of Examples 27-34, wherein the light collimators are lenticular lenslets configured to provide different beams of light, from light emitters of an associated group of light emitters, to different locations along a first axis, wherein the array of shutters are arranged to form subpupils along a second axis orthogonal to the first axis. 
     EXAMPLE 36 
     The display system of any of Examples 27-35, wherein the micro-display is an emissive micro-display, wherein the light emitters are micro-LEDs. 
     EXAMPLE 37 
     The display system of any of Examples 27-36, wherein the micro-display is one of a plurality of monochrome micro-displays, wherein each of the monochrome micro-displays is configured to emit light of a different component color. 
     EXAMPLE 38 
     The display system of Example 37, further comprising an X-cube prism, wherein each of the monochrome micro-displays is arranged to output image light into a different face of the X-cube prism. 
     EXAMPLE 39 
     The display system of any of Examples 27-38, further comprising a pupil relay combiner eyepiece configured to relay the image light to the eye of the viewer, wherein the array of selectively-activated shutters are configured to regulate propagation of the image light to the pupil relay combiner eyepiece. 
     EXAMPLE 40 
     The display system of Example 39, wherein the pupil relay combiner eyepiece comprises a waveguide comprising:
         in-coupling optical elements for in-coupling the image light into the waveguide; and   out-coupling optical elements for out-coupling in-coupled image light out of the waveguide.       

     EXAMPLE 41 
     The display system of Example 40, wherein the waveguide is one of a plurality of waveguides comprising in-coupling optical elements and out-coupling optical elements. 
     EXAMPLE 42 
     A method for displaying image content, the method comprising:
         injecting, from a head-mounted display system, a set of parallactically-disparate intra-pupil images into an eye of a viewer,   wherein injecting the set of parallactically-disparate intra-pupil images comprises:
           providing an array of groups of light emitters;   providing an array of light collimators overlying the light emitters, wherein each light collimator is associated with a group of the light emitters;   providing projection optics, wherein the array of light collimators is between the array of groups of light emitters and the projection optics,   injecting a first parallactically-disparate intra-pupil image into the eye by emitting light from a first light emitter of the groups of light emitters; and   injecting a second parallactically-disparate intra-pupil image into the eye by emitting light from a second light emitter of the groups of light emitters.   
               

     EXAMPLE 43 
     The method of Example 42, wherein each of the images of the set of parallactically-disparate intra-pupil images are injected into the eye at different angles and all images of the set of parallactically-disparate intra-pupil images are injected into the eye within a flicker fusion threshold. 
     EXAMPLE 44 
     The method of Example 43, wherein the flicker fusion threshold is 1/60 of a second. 
     EXAMPLE 45 
     The method of any of Examples 42-43, wherein the different images provide different views of the virtual object. 
     EXAMPLE 46 
     The method of any of Examples 42-45, wherein injecting the first parallactically-disparate intra-pupil image and injecting the second parallactically-disparate intra-pupil image are performed simultaneously. 
     EXAMPLE 47 
     The method of any of Examples 42-46, further comprising providing an array of selectively-activated shutters for selectively transmitting the image light to the eye from different locations, wherein the array of selectively-activated shutters is disposed within an eye-box volume of the projection optics. 
     EXAMPLE 48 
     The method of any of Examples 42-47, wherein the light collimators are lenticular lenslets configured to provide different beams of light, from light emitters of an associated group of light emitters, to different locations along a first axis, wherein the array of shutters are arranged to form subpupils along a second axis orthogonal to the first axis. 
     EXAMPLE 49 
     The method of any of Examples 47-48, further comprising spatially-multiplexing multiple images formed by different light emitters of the groups of light emitters to localize a display subpupil along the first axis, and temporally-multiplexing multiple images by synchronizing opening of the shutters with activation of corresponding light emitters. 
     EXAMPLE 50 
     The method of any of Examples 47-49, wherein the array of shutters comprises selectively-movable physical structures. 
     EXAMPLE 51 
     The method of any of Examples 47-49, wherein the array of shutters comprises chemical species having reversibly changeable states, the states providing different amounts of light transmission. 
     EXAMPLE 52 
     The method of any of Examples 42-51, wherein injecting the first parallactically-disparate intra-pupil image and injecting the second parallactically-disparate intra-pupil image comprise routing light from the light emitters to the eye through a pupil relay combiner eyepiece. 
     EXAMPLE 53 
     The method of Example 52, wherein the pupil relay combiner eyepiece comprises a waveguide comprising:
         in-coupling optical elements for in-coupling the image light into the waveguide; and   out-coupling optical elements for out-coupling in-coupled image light out of the waveguide.       

     EXAMPLE 54 
     The method of any of Examples 42-53, further comprising injecting, from the head-mounted display system, a second set of parallactically-disparate intra-pupil images into a second eye of a viewer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a user&#39;s view of augmented reality (AR) through an AR device. 
         FIG. 2  illustrates a conventional scheme for simulating three-dimensional imagery for a user. 
         FIG. 3  illustrates aspects of an approach for simulating three-dimensional imagery using multiple depth planes. 
         FIGS. 4A-4C  illustrate relationships between wavefront curvature and focal distance. 
         FIG. 5  illustrates an example of a waveguide stack for outputting image information to a user. 
         FIG. 6A  illustrates pre-accommodation and post-accommodation conditions of an eye for a continuous incoming wavefront. 
         FIG. 6B  illustrates pre-accommodation and post-accommodation conditions of an eye to a piecewise approximation for a continuous incoming wavefront. 
         FIG. 7A  illustrates an eye accommodating to a divergent wavefront emanating from a finite focal-distance virtual image provided by a projection system. 
         FIG. 7B  illustrates a system for forming an approximation of the divergent wavefront of  FIG. 7A  utilizing wavefront segments formed by infinity-focused virtual images. 
         FIG. 8  illustrates examples of parallax views forming the divergent wavefront approximation of  FIG. 7B . 
         FIG. 9  illustrates an example of a display system comprising a projection system for forming the divergent wavefront approximation of  FIGS. 7B and 8 . 
         FIGS. 10A-10C  illustrate examples of arrays of shutters for providing different parallax views. 
         FIG. 11  illustrates an example of the projection system of  FIG. 9  providing a different parallactically-disparate intra-pupil image at a different time than that shown in  FIG. 9 . 
         FIG. 12  illustrates an example of the projection systems of  FIGS. 9 and 11  with an array of light collimators between an emissive micro-display and projection optics. 
         FIG. 13  illustrates another example of a display system comprising a projection system for forming the divergent wavefront approximation of  FIGS. 7B and 8 . 
         FIG. 14  illustrates an example of a projection system having an array of light collimators and an array of shutters for forming different parallactically-disparate intra-pupil images. 
         FIG. 15  illustrates an example of a range of depth planes provided by projection systems according to various embodiments. 
         FIG. 16A  illustrates an example of a display system comprising a projection system, comprising an array of shutters, and a pupil relay combiner eyepiece for superimposing image content on a user&#39;s view of the world. 
         FIG. 16B  illustrates an example of a display system comprising a projection system, comprising an array of light collimators for providing different intra-pupil images, and a pupil relay combiner eyepiece for superimposing image content on a user&#39;s view of the world. 
         FIG. 17A  illustrates another example of a display system comprising a projection system, comprising an array of shutters, and a pupil relay combiner eyepiece for superimposing image content on a user&#39;s view of the world. 
         FIG. 17B  illustrates another example of a display system comprising a projection system, comprising an array of light collimators for providing different intra-pupil images, and a pupil relay combiner eyepiece for superimposing image content on a user&#39;s view of the world. 
         FIG. 18  illustrates an example of a display system comprising an eye tracking system and a combiner eyepiece with a pupil expander. 
         FIG. 19  illustrates an example of a display system comprising an eye tracking system and a pupil rely combiner eyepiece with a pupil expander configured to produce a non-infinity depth plane. 
         FIGS. 20A-20B  illustrate examples of projection systems having multiple micro-displays. 
         FIG. 21  illustrates an example of an eyepiece comprising a stacked waveguide assembly for outputting light of different wavelengths corresponding to different component colors. 
         FIG. 22  illustrates an example of a wearable display system. 
     
    
    
     DETAILED DESCRIPTION 
     The human visual system may be made to perceive images presented by a display as being “3-dimensional” by providing slightly different presentations of the image to each of a viewer&#39;s left and right eyes. Depending on the images presented to each eye, the viewer perceives a “virtual” object in the images as being at a selected distance (e.g., at a certain “depth plane”) from the viewer (also referred to as “user” herein). Simply providing different presentations of the image to the left and right eyes, however, may cause viewer discomfort. As discussed further herein, viewing comfort may be increased by causing the eyes to accommodate to the images as the eyes would accommodate to a real object at the depth plane on which the virtual object is placed. 
     The proper accommodation for a virtual object on a given depth plane may be elicited by presenting images to the eyes with light having a wavefront divergence that matches the wavefront divergence of light coming from a real object on that depth plane. Some display systems use distinct structures having distinct optical powers to provide the appropriate wavefront divergence. For example, one structure may provide a specific amount of wavefront divergence (to place virtual objects on one depth plane) and another structure may provide a different amount of wavefront divergence (to place virtual objects on a different depth plane). Thus, there may be a one-to-one correspondence between physical structures and the depth planes in these display systems. Due to the need for a separate structure for each depth plane, such display systems may be bulky and/or heavy, which may be undesirable for some applications, such as portable head-mounted displays. In addition, such display systems may be limited in the numbers of different accommodative responses they may elicit from the eyes, due to practical limits on the number of structures of different optical powers that may be utilized. 
     It has been found that a continuous wavefront, e.g. a continuous divergent wavefront, may be approximated by injecting parallactically-disparate intra-pupil images directed into an eye. For example, a display system may provide a range of accommodative responses without requiring a one-to-one correspondence between the accommodative response and the optical structures in the display. The display system may output light with a selected amount of perceived wavefront divergence, corresponding to a desired depth plane, by injecting a set of parallactically-disparate intra-pupil images into the eye. These images may be referred to as “parallactically-disparate” intra-pupil images since each image may be considered to be a different parallax view of the same virtual object or scene, on a given depth plane. These are “intra-pupil” images since a set of images possessing parallax disparity is projected into the pupil of a single eye, e.g., the right or left eye of a viewer. Although the images may have some overlap, the light beams forming these images will have at least some areas without overlap and will impinge on the pupil from slightly different angles. In some embodiments, the other eye of the viewer, e.g., the left eye, may be provided with its own set of parallactically-disparate intra-pupil images. The sets of parallactically-disparate intra-pupil images projected into each eye may be slightly different, e.g., the images may show slightly different views of the same scene due to the slightly different perspectives provided by each eye. 
     The wavefronts of light forming each of the intra-pupil images of the different views, when projected into a pupil of an eye, may, in the aggregate, approximate a continuous divergent wavefront. The amount of perceived divergence of this approximated wavefront may be varied by varying the amount of parallax disparity between the intra-pupil images; changes in the parallax disparity change the angular range spanned by the wavefronts of light forming the intra-pupil images. Preferably, this angular range mimics the angular range spanned by the continuous wavefront being approximated. In some embodiments, the wavefronts of light forming the individual intra-pupil images are collimated or quasi-collimated, as discussed herein. Examples of systems for providing intra-pupil images are disclosed in U.S. Application Publ. No. 2018/0113311 published Apr. 26, 2018. Some embodiments disclosed in that application utilize light emitters to illuminate a spatial light modulator which encodes light from the light emitters with image information; multiple light emitters may be provided and the different intra-pupil images may be formed using light emitters at different locations to provide desired amounts of parallax disparity between images. 
     Preferably, the set of intra-pupil images for approximating a particular continuous wavefront is injected into the eye sufficiently rapidly for the human visual system to not detect that the images were provided to the eye at different times. Without being limited by theory, the term flicker fusion threshold may be used to denote the duration within which images presented to the human eye are perceived as being present simultaneously; that is, the visual system may perceive images formed on the retina within a flicker fusion threshold as being present simultaneously. In some embodiments, approximating a continuous wavefront may include sequentially injecting beams of light for each of a set of intra-pupil images into the eye, with the total duration for injecting all of the beams of light being less than the flicker fusion threshold. It will be appreciated that presenting the set of images over a duration greater than the flicker fusion threshold may result in the human visual system perceiving at least some of the images as being separately injected into the eye. As an example, the flicker fusion threshold may be about 1/60 of a second. Consequently, each set of intra-pupil images may consist of a particular number of parallax views, e.g., two or more views, three or more views, four or more views, etc. and preferably all of these views are provided to the eye within the flicker fusion threshold. 
     Providing all of the desired views within the flicker fusion threshold presents a challenge for some display technologies, such as those using spatial light modulators in which physical elements are moved to modulate the intensity of outputted light. The need to physically move these elements may limit the speed at which individual pixels may change states and also constrains the frame rate of displays using these optical elements. In addition, spatial light modulators may require a separate light source, which can undesirably add to the complexity, size, and weight of the display system, and may potentially limit the brightness of displayed images. 
     In some embodiments, a display system includes an emissive micro-display, which advantageously may provide different intra-pupil images at exceptionally high rates. In addition, the display system may include an array of shutters. The shutters of the array of shutters may be individually selectively opened, or activated to allow light transmission, to allow light to propagate into the retina from different locations. The emissive micro-display emits image light for forming an intra-pupil images and the image light propagates to the array of shutters. Different ones of the shutters at different locations may be selectively opened (made transmissive to the image light) to allow the image light to further propagate into a viewer&#39;s eye from those different locations. The amount of parallax disparity between the intra-pupil images may be varied by changing the locations at which an shutter is opened. Consequently, spatial differences in the opened shutter locations may translate into differences in the paths that the light takes into the eye. The different paths may correspond to different amounts of parallax disparity. In some embodiments, an array of light collimators may be disposed proximate the emissive micro-display. For example, each pixel of the emissive micro-display may have an associated light collimator. The light collimators narrow the angular emission profile of light emitted by the emissive micro-display, and thereby nay increase the amount of the emitted light that ultimately reaches the eyes of the viewer. 
     It will be appreciated that the images formed by the emissive micro-display may be temporally synchronized with the shutter that is opened in the array of shutters. For example, the opening of one shutter (or multiple adjacent or contiguous shutters) corresponding to one intra-pupil image may be synchronized, or simultaneous, with the activation of pixels in the micro-display. Once another shutter, at a location desired for a second intra-pupil image, is opened, the micro-display may emit light for forming that second intra-pupil image. Additional intra-pupil images may be formed by synchronizing with the opening of shutters at different locations. This time-based sequential injection of intra-pupil images to the eye may be referred to as temporal multiplexing or temporally multiplexed display of the intra-pupil images. As a result, in some embodiments, the presentation of the intra-pupil images by the micro-display may be temporally multiplexed, such that different parallax views may be provided by the emissive micro-display at different times and synchronized with the opening of different shutters providing the desired parallax disparity. 
     Preferably, the emissive micro-displays are micro-LED displays, which provide advantages for high brightness and high pixel density. In some other embodiments, the micro-displays are micro-OLED displays. 
     In some embodiments, the emissive micro-displays comprise arrays of light emitters having a pitch of, e.g., less than 10 μm, less than 8 μm, less than 6 μm, less than 5 μm, or less than 2 μm, including 1-5 μm, 1-4 μm or 1-2 μm; and an emitter size of 2 μm or less, 1.7 μm or less, or 1.3 μm or less. In some embodiments, the emitter size is within a range having an upper limit of the above-noted sizes and a lower limit of 1 μm. Examples of the ratio of emitter size to pitch include 1:1 to 1:5, 1:2 to 1:4, or 1:2 to 1:3, 
     In some embodiments, the display system may utilize an emissive micro-display in conjunction with a light collimator array to provide different amounts of parallax disparity. The collimator array may be configured to direct light, emitted by the micro-display, along different paths which correspond to the different amounts of parallax disparity. For example, the collimator array may be positioned proximate to or directly on the emissive micro-display. In some embodiments, the light collimators are lenslets. Each collimator of the collimator array may include a group of associated subpixels, each disposed at a different location relative to the collimator. As a result, light from different subpixels of the group of subpixels interfaces differently with the collimator and is directed along slightly different paths by the collimator. These different paths may correspond to different amounts of parallax disparity. Thus, each collimator may correspond to a different pixel of an intra-pupil image, and each subpixel may provide a different light path for that pixel, such that the parallax disparity between two or more pixels may be selected by appropriate activation of the sub-pixels forming those pixels. In some embodiments, advantageously, different intra-pupil images may be formed and provided to the eye simultaneously, with the parallax disparity determined by the locations of the subpixels forming the images and with the collimator array directing the propagation of light from those subpixels. 
     As noted above, light from different subpixels will take different paths to the projection optic and thus to the viewer&#39;s eyes. Consequently, lateral and/or vertical displacement of the active subpixels translates into angular displacement in the light leaving the light collimator array and ultimately propagating towards the viewer&#39;s pupil through the projection optic. In some embodiments, increases in lateral displacement between the activated subpixels used to form different images may be understood to translate to increases in angular displacement as measured with respect to the micro-display. In some embodiments, each of the intra-pupil images, used to approximate a particular wavefront, may be formed by outputting light from a different subpixel, thereby providing the angular displacement between the beams of light forming each of the images. 
     In some embodiments, the display systems, whether utilizing a collimator array or an array of shutters, may include projection optics for injecting light into the eye. The emissive micro-display may be configured to output the light, encoded with image information, to form an intra-pupil image. Light subsequently impinges on and propagates through the projection optics and, ultimately, to the eye of a viewer. 
     In some embodiments, the display system may include a combiner eyepiece, which allows virtual image content to be overlaid with the viewer&#39;s view of the world, or ambient environment. For example, the combiner eyepiece may be an optically transmissive waveguide that allows the viewer to see the world. In addition, the waveguide may be utilized to receive, guide, and ultimately output light, forming the intra-pupil images, to the viewer&#39;s eyes. Because the waveguide may be positioned between the viewer and the world, the light outputted by the waveguide may be perceived to form virtual images that are placed on various depth planes in the world. In essence, the combiner eyepiece allows the viewer to receive a combination of light from the display system and light from the world. 
     In some embodiments, the display system may also include an eye tracking system to detect the viewer&#39;s gaze direction. Such an eye tracking system allows appropriate content to be selected and displayed based upon where the viewer is looking. 
     Preferably, the display system has a sufficiently small exit pupil that the depth of field provided by light forming individual intra-pupil images is substantially infinite and the visual system operates in an “open-loop” mode in which the eye is unable to accommodate to an individual intra-pupil image. In some embodiments, the light beams forming individual images occupy an area having a width or diameter less than about 0.5 mm when incident on the eye. It will be appreciated, however, that light beams forming a set of intra-pupil images are at least partially non-overlapping and the set of light beams preferably define an area larger than 0.5 mm, to provide sufficient information to the lens of the eye to elicit a desired accommodative response based on the wavefront approximation formed by the wavefronts of the light forming the intra-pupil images. 
     Without being limited by theory, the area defined by a set of beams of light may be considered to mimic a synthetic aperture through which an eye views a scene. It will be appreciated that viewing a scene through a sufficiently small pinhole in front of the pupil provides a nearly infinite depth of field. Given the small aperture of the pinhole, the lens of the eye is not provided with adequate scene sampling to discern distinct depth of focus. As the pinhole enlarges, additional information is provided to the eye&#39;s lens, and natural optical phenomena allow a limited depth of focus to be perceived. Advantageously, the area defined by the set of beams of light and the corresponding sets of parallactically-disparate intra-pupil images may be made larger than the pinhole producing the infinite depth of field and the multiple intra-pupil images may produce an approximation of the effect provided by the enlarged pinhole noted above. 
     Some embodiments disclosed herein may provide various advantages. For example, because the micro-displays are emissive, no external illumination is required, thereby facilitating reductions in the size and weight of the projection system. The small size of these micro-displays allows the use of a single projector with separate component color (e.g., red, green, blue) micro-display panels, without requiring an unduly large or complicated projector. In some embodiments, because of the advantageously small size and weight of various micro-display disclosed herein, different projectors may be used for different component colors. In addition, in contrast to typical displays, such as LCOS displays, polarization is not needed to provide light with image information. As a result, light loss associated with polarization may be avoided. Also, the individual light emitters of the micro-displays have high etendue and, as a result, light from each pixel naturally fills a large super-pupil area, which can provide a desirably large eye-box volume. In some embodiments, the emissive micro-display is a micro-LED display, which may have exceptionally high frame rates (e.g., frame rate of 1 kHz or more, including 1-2 kHz). In addition, the emissive micro-displays may have exceptionally small pixel pitches (e.g., 1-4 μm, including 2-4 μm or 1-2 μm) and high pixel density, which may provide desirably high image resolutions. 
     Reference will now be made to the figures, in which like reference numerals refer to like parts throughout. 
     As discussed herein, the perception of an image as being “three-dimensional” or “3-D” may be achieved by providing slightly different presentations of the image to each eye of the viewer.  FIG. 2  illustrates a conventional scheme for simulating three-dimensional imagery for a user. Two distinct images  190 ,  200 —one for each eye  210 ,  220 —are outputted to the user. The images  190 ,  200  are spaced from the eyes  210 ,  220  by a distance  230  along an optical or z-axis that is parallel to the line of sight of the viewer. The images  190 ,  200  are flat and the eyes  210 ,  220  may focus on the images by assuming a single accommodated state. Such 3-D display schemes rely on the human visual system to combine the images  190 ,  200  to provide a perception of depth and/or scale for the combined image. 
     It will be appreciated, however, that the human visual system is more complicated and providing a realistic perception of depth is more challenging. For example, many viewers of conventional “3-D” display systems find such systems to be uncomfortable or may not perceive a sense of depth at all. Without being limited by theory, it is believed that viewers of an object may perceive objects as being “three-dimensional” due to a combination of vergence and accommodation. Vergence movements (i.e., rotation of the eyes so that the pupils move toward or away from each other to converge the lines of sight of the eyes to fixate upon an object) of the two eyes relative to each other are closely associated with focusing (or “accommodation”) of the lenses and pupils of the eyes. Under normal conditions, changing the focus of the lenses of the eyes (or accommodating the eyes) from one object to another object at a different distance will automatically cause a matching change in vergence to the same distance, under a relationship known as the “accommodation-vergence reflex”. Likewise, under normal conditions, a change in vergence will trigger a matching change in accommodation, with changes in lens shape and pupil size. As noted herein, many stereoscopic or “3-D” display systems display a scene using slightly different presentations (and, so, slightly different images) to each eye such that a three-dimensional perspective is perceived by the human visual system. Such systems are uncomfortable for many viewers, however, since they, among other things, simply provide a different presentation of a scene; the eyes view all the image information at a single accommodated state, even image information for objects at different depths. This, however, which works against the “accommodation-vergence reflex.” Display systems that provide a better match between accommodation and vergence may form more realistic and comfortable simulations of three-dimensional imagery, which may facilitate users wearing the displays for longer durations. 
       FIG. 3  illustrates aspects of an approach for simulating three-dimensional imagery using multiple depth planes. Objects at various distances from eyes  210 ,  220  on the z-axis are accommodated by the eyes  210 ,  220  so that those objects are in focus; that is, the eyes  210 ,  220  assume particular accommodated states to bring into focus objects at different distances along the z-axis. Consequently, a particular accommodated state may be said to be associated with a particular one of depth planes  240 , which has an associated focal distance, such that objects or parts of objects in a particular depth plane are in focus when the eye is in the accommodated state for that depth plane. In some embodiments, three-dimensional imagery may be simulated by providing different presentations of an image for each of the eyes  210 ,  220 , with the presentations of the image also being different for different depth planes. While shown as being separate for clarity of illustration, it will be appreciated that the fields of view of the eyes  210 ,  220  may overlap, for example, as distance along the z-axis increases. In addition, while shown as flat for ease of illustration, it will be appreciated that the contours of a depth plane may be curved in physical space, such that all features in a depth plane are in focus with the eye in a particular accommodated state. 
     The distance between an object and the eye  210  or  220  may also change the amount of divergence of light from that object, as viewed by that eye.  FIGS. 4A-4C  illustrate relationships between distance and the divergence of light rays. The distance between the object and the eye  210  is represented by, in order of decreasing distance, R 1 , R 2 , and R 3 . As shown in  FIGS. 4A-4C , the light rays become more divergent as distance to the object decreases. As distance increases, the light rays become more collimated. Stated another way, it may be said that the light field produced by a point (the object or a part of the object) has a spherical wavefront curvature, which is a function of how far away the point is from the eye of the user. The curvature increases with decreasing distance between the object and the eye  210 . Consequently, at different depth planes, the degree of divergence of light rays is also different, with the degree of divergence increasing with decreasing distance between depth planes and the viewer&#39;s eye  210 . While only a single eye  210  is illustrated for clarity of illustration in  FIGS. 4A-4C  and other figures herein, it will be appreciated that the discussions regarding eye  210  may be applied to both eyes  210  and  220  of a viewer. 
     Without being limited by theory, it is believed that the human eye typically can interpret a finite number of depth planes to provide depth perception. Consequently, a highly believable simulation of perceived depth may be achieved by providing, to the eye, different presentations of an image corresponding to each of these limited number of depth planes. The different presentations may be separately focused by the viewer&#39;s eyes, thereby helping to provide the user with depth cues based on the accommodation of the eye required to bring into focus different image features in the scene, where the different features are located on different depth planes. This may also cause other image features on other depth planes to appear out of focus, which provides an additional sense of depth for the viewer. 
     Because each depth plane has an associated wavefront divergence, to display image content appearing to be at a particular depth plane, some displays may utilize waveguides that have optical power to output light with a wavefront divergence corresponding to that depth plane. A plurality of similar waveguides, but having different optical powers, may be utilized to display image content on a plurality of depth planes. For example, such systems may utilize a plurality of such waveguides formed in a stack.  FIG. 5  illustrates an example of a waveguide stack for outputting image information to a user. A display system  250  includes a stack of waveguides  260  that may be utilized to provide three-dimensional perception to the eye/brain using a plurality of waveguides  270 ,  280 ,  290 ,  300 ,  310  to output image information. Image injection devices  360 ,  370 ,  380 ,  390 ,  400  may be utilized to inject light containing image information into the waveguides  270 ,  280 ,  290 ,  300 ,  310 . In some embodiments, the image injection devices  360 ,  370 ,  380 ,  390 ,  400  may be understood to be different light projection systems and/or different pupils of one or more projection systems. The waveguides  270 ,  280 ,  290 ,  300 ,  310  may be separated from other waveguides by, e.g., air or other low refractive index material which facilitates total internal reflection of light through individual ones of the waveguides. Each waveguide  270 ,  280 ,  290 ,  300 ,  310  may include a structure (e.g., an optical grating and/or lens  570 ,  580 ,  590 ,  600 ,  610 , respectively) that provides optical power, such that each waveguide outputs light with a preset amount of wavefront divergence, which corresponds to a particular depth plane. Thus, each waveguide  270 ,  280 ,  290 ,  300 ,  310  places image content on an associated depth plane determined by the amount of wavefront divergence provided by that waveguide. 
     It will be appreciated, however, that the one-to-one correspondence between a waveguide and a depth plane may lead to a bulky and heavy device in systems in which multiple depth planes are desired. In such embodiments, multiple depth planes would require multiple waveguides. In addition, where color images are desired, even larger numbers of waveguides may be required, since each depth plane may have multiple corresponding waveguides, one waveguide for each component color (e.g., red, green, or blue) may be required to form the color images. 
     Advantageously, various embodiments herein may provide a simpler display system that approximates a desired continuous wavefront by using discrete light beams that form intra-pupil images that present different parallax views of an object or scene. Moreover, the light projection system utilizes emissive micro-displays, which may reduce the size and weight of the display system relative to projection systems utilizing separate spatial light modulators and light sources. In addition, some embodiments provide exceptionally high frame rates, which may provide advantages for flexibility in providing desired numbers of intra-pupil images within a given duration. 
     With reference now to  FIG. 6A , the pre-accommodation and post-accommodation conditions of an eye  210  upon receiving a continuous input wavefront  1000  are illustrated. Illustration a) shows the pre-accommodation condition, before the visual system brings the wavefront  1000  into focus on the retina  211 . Notably, the focal point  212  is not on the retina  211 . For example, the focal point  212  may be forward of the retina  211  as illustrated. Illustration b) shows the post-accommodation condition, after the human visual system flexes pupillary musculature of the eye  210  of the viewer to bring the wavefront  1000  into focus on the retina  211 . As illustrated, the focal point  212  may be on the retina  211 . 
     It has been found that a continuous wavefront such as the wavefront  1000  of  FIG. 6A  may be approximated using a plurality of wavefronts.  FIG. 6B  illustrates the pre-accommodation and post-accommodation conditions of the eye  210  upon receiving a piecewise approximation of the continuous wavefront  1000  of  FIG. 6A . Illustration a) of  FIG. 6B  shows the pre-accommodation condition and illustration b) shows the post-accommodation condition of the eye  210 . The approximation may be formed using a plurality of constituent wavefronts  1010   a,    1010   b,  and  1010   c,  each of which is associated with separate beams of light. As used herein, references numerals  1010   a,    1010   b,  and  1010   c  may indicate both a light beam and that light beam&#39;s associated wavefront. In some embodiments, the constituent wavefronts  1010   a  and  1010   b  may be planar wavefronts, such as formed by a collimated beam of light. As shown in illustration b), the wavefront approximation  1010  formed by the constituent wavefronts  1010   a  and  1010   b  are focused by the eye  210  onto the retina  211 , with the focal point  212  on the retina  211 . Advantageously, the pre- and post-accommodation conditions are similar to that caused by the continuous wavefront  1000  shown in  FIG. 6A . 
     It will be appreciated that continuous divergent wavefronts may be formed using optical projection systems. As noted above, U.S. Application Publ. No. 2018/0113311, published Apr. 26, 2018, discloses examples of systems using a light source to illuminate a spatial light modulator  1018 .  FIG. 7A  illustrates an eye accommodating to a divergent wavefront emanating from a finite focal-distance virtual image provided by a projection system. The system includes spatial light modulator  1018  and projection optics  1020  with focal length “F” and an external stop. A light source (not illustrated) provides light to the spatial light modulator  1018  and an image may be formed by the spatial light modulator  1018  by modulating the light. The modulated light containing image information may be directed through projection optics  1020  to the eye  210 . As indicated in  FIG. 7A , the spacing (less than F) between the micro-display  2000  and the projection optics  1020  may be chosen such that a divergent wavefront  1000  is outputted towards the eye  210 . As noted above regarding  FIG. 6A , the eye  210  may then focus the wavefront  1000  on the retina  211 . 
       FIG. 7B  illustrates a system for forming an approximation of the divergent wavefront of  FIG. 7A  utilizing wavefront segments formed by infinity-focused virtual images. As above, the system includes spatial light modulator  1018  and projection optics  1020 . As above, a light source (not illustrated) provides light to the spatial light modulator  1018 , which modulates the light to form images. The spatial light modulator  1018  may form two images that are shifted relative to one another. The spatial light modulator  1018  is placed at distance F from the back focal plane of projection optics  1020 , which have a back focal length of F. Light beam  1010   a,  containing image information for a first image, propagates through the projection optics  1020  into the eye  210 . Light beam  1010   b  containing image information for a second image takes a different path through the projection optics  1020  into the eye  210 . The light beams  1010   a  and  1010   b  propagate away from the spatial light modulator along paths through the projection optics  1020  and into the eye  210  such that those light beams define an angular range, from one light beam to the other, that matches the angular range of the divergent wavefront  1000  ( FIG. 7A ). It will be appreciated that the angular separation between light beams  1010   a  and  1010   b  increases with increases in the amount of wavefront divergence that is approximated. 
     With reference now to  FIG. 8 , examples of parallax views forming the divergent wavefront approximation of  FIG. 7B  are illustrated. It will be appreciated that each of light beams  1010   a,    1010   b,  and  1010   c  form a distinct image of one view of the same objects or scene from slightly different perspectives corresponding to the different placements of the images in space. As illustrated, the images may be injected into the eye  210  sequentially at different times. Alternatively, the images may be injected simultaneously if the optical system permits, or a group of images may be simultaneously injected, as discussed herein. In some embodiments, the total duration over which light forming all of the images is injected into the eye  210  is less than the flicker fusion threshold of the viewer. For example, the flicker fusion threshold may be 1/60 of a second, and all of the light beams  1010   a,    1010   b,  and  1010   c  are injected into the eye  210  over a duration less than that flicker fusion threshold. As such, the human visual system integrates all of these images and they appear to the eye  210  as if the light beams  1010   a,    1010   b,  and  1010   c  were simultaneously injected into that eye  210 . The light beams  1010   a,    1010   b,  and  1010   c  thus form the wavefront approximation  1010 . 
     With reference now to  FIG. 9 , an example is illustrated of a display system  1001  comprising a projection system  1003  for forming the divergent wavefront approximation  1010  of  FIGS. 7B and 8 . The projection system  1003  comprises a micro-display  2000  comprising a plurality of light emitters  2002 , which function as pixels for forming images. The light emitters  2002  emit light  2002   a,    2002   b,    2002   c,  which propagate through the relay/projection optics  1020 , and is outputted by the relay/projection optics  1020 . The light then continues through an open shutter  2006   1  of an array  2004  of shutters. The open shutter  2006   1  may be understood to form a shutter-aperture sub-pupil. In addition, the light propagating through the open shutter  2006   1  may be understood to be a light beam  1010   a,  which propagates into the eye  210  to form an intra-pupil image (see, e.g., the image by formed by the beam of light  1010   a  in  FIG. 8 ). Preferably, the array  2004  of shutters is located on or proximate the plane of the pupil of the eye  210 . 
     It will be appreciated that the numerical aperture of light accepted by the projection optics  1020  is determined by the focal length and diameter of the projection optics. Light emerging from the projection optics  1020  forms a pupil, which may be at the exit aperture of the projection optics  1020  in some embodiments. In addition, the light  2002   a,    2002   b,    2002   c  propagating through and exiting the projection optics  1020  continue to propagate in different directions, but also overlap to define a volume  2003 . The volume  2003  is an eye-box and may be pyramid shaped in some embodiments. As noted above, the eye-box includes contributions from all of the light emitters  2002  of the micro-display  2000 . Preferably, the size (e.g., the lateral dimension) of the projection optics pupil and the size of the eye-box (e.g., the lateral dimension) in which the eye  210  is placed is as large or larger than the size (e.g., the lateral dimension) of the eye  210 &#39;s pupil when viewing an image from the micro-display  2000 . As a result, preferably, the entirety of an image formed on the micro-display  2000  may be viewed by the eye  210 . In addition, as noted herein, the array  2004  of shutters is preferably located on or proximate the plane of the pupil of the eye  210 , so that the array  2004  of shutters is also within the eye-box volume  2003 . 
     With continued reference to  FIG. 9 , as noted herein, the micro-display  2000  may comprise an array of light emitters  2002 . Examples of light emitters include organic light-emitting diodes (OLEDs) and micro-light-emitting diodes (micro-LEDs). It will be appreciated that OLEDs utilize organic material to emit light and micro-LEDs utilize inorganic material to emit light. 
     Advantageously, some micro-LEDs provide higher luminance and higher efficiency (in terms of lux/W) than OLEDs. In some embodiments, the micro-displays are preferably micro-LED displays. The micro-LEDs may utilize inorganic materials, e.g., Group III-V materials such as GaAs, GaN, and/or GaIn for light emission. Examples of GaN materials include InGaN, which may be used to form blue or green light emitters in some embodiments. Examples of GaIn materials include AlGaInP, which may be used to form red light emitters in some embodiments. In some embodiments, the light emitters  2002  may emit light of an initial color, which may be converted to other desired colors using phosphor materials or quantum dots. For example, the light emitter  20002  may emit blue light which excites a phosphor material or quantum dot that converts the blue wavelength light to green or red wavelengths. 
     In some embodiments, the display system  1001  may include an eye tracking device  1022 , e.g., a camera, configured to monitor the gaze of the eye. Such monitoring may be used to determine the direction in which the viewer is looking, which may be used to select image content appropriate for that direction. The eye tracking device  1022  may track both eyes of the viewer, or each eye may include its own associated eye tracking device. In some embodiments, vergence of both eyes of the viewer may be tracked and the convergence point of the gaze of the eyes may be determined to determine in what direction and at what distance the eyes are directed. 
     With continued reference to  FIG. 9 , the display system  1001  may also include control systems  1024  for determining the timing and the type of image content provided by the display system. In some embodiments, the control system  1024  comprises one or more hardware processors with memory storing programs for controlling the display system  1001 . For example, the system  1024  may be configured to control opening of individual shutters of the shutter array  2004 , activation of the emitters  2002  of the micro-display  2000 , and/or the interpretation and reaction of the display system  1001  to data received from the eye tracking device  1022 . Preferably, the system  1024  includes a computation module  1024   a  configured to receive an input regarding a desired depth plane or wavefront divergence and to calculate the appropriate shutters to open, in order to form parallax views with the proper amount of disparity for the desired depth plane or wavefront divergence. In addition, computation module  1024   a  may be configured to determine the appropriate actuation of the light emitters  2002  of the micro-display  2000  to form images of the desired parallax views. The system  1024  may also include a synchronization module  1024   b  that is configured to synchronize the activation of particular shutters of the shutter array  2004  with emission of light by light emitters  2002  of the micro-display  2000  to form images providing the desired parallax views. In addition, the system  1024  may include an eye tracking module  1024   c  that receives inputs from the eye tracking device  1022 . For example, the eye tracking device  1022  may be a camera configured to image the eye  210 . Based on images captured by the eye tracking device  1022 , the eye tracking module  1024   c  may be configured to determine the orientation of the pupil and to extrapolate the line of sight of the eye  210 . This information may be electronically conveyed to the computation module  1024   a.  The computation module  1024   a  may be configured to select image content based upon the line of sight or the gaze of the eye  210  (and preferably also based upon the line of sight or gaze of the other eye of the viewer, e.g., based upon a fixation point of the eyes of the viewer). 
     With reference now to  FIG. 10A , an example of the array  2004  of shutters  2006  is illustrated. The illustrated view is a head-on view, as seen from the direction of light from the micro-display  2000  impinging on the array  2004 . As illustrated, the array  2004  may extend in two dimensions and may comprise rows and columns of shutters  2006 . Preferably, each of the shutters  2006  may be individually activated, or opened. The location of an opened shutter  2006  may correspond to the desired location of an intra-pupil image. For example, the shutter array  2004  shown in  FIG. 9  may have an opened shutter  2006   a  that corresponds to the location of the light beam  1010   a  shown in  FIG. 8 . 
     It will be appreciated that the array  2004  may have other overall configurations. For example, for horizontal parallax-only driven accommodation, the array  2004  may be a horizontal line of shutters  2006 , as shown in  FIG. 10A . In some other embodiments, for vertical parallax-only driven accommodation, the array  2004  may be a vertical line of shutters  2006 , as shown in  FIG. 10B . 
     While shown as a grid of squares, the shutters  2006  may be arrayed in any configuration which provides parallax disparity when opening different ones of the shutters  2006 . For example, the shutter  2006  may be arranged in a hexagonal grid. In addition, the shutters  2006  may have shapes other than squares. For example, the shutters  2006  may have a hexagonal shape, a circular shape, a triangular-shape, etc. In some embodiments, multiple shutters may be opened, which may allow the sizes and/or shapes of the opening to be selected as desired. 
     It will be appreciated that the shutters discussed herein are structures that may be independently made transmissive to light. Consequently, an opened shutter is one which has been controlled to be transmissive to light and a closed shutter is one which has been controlled to block light or to be opaque. In some embodiments, the shutters may be physical structures which move between first and second positions to transmit or block light, respectively, by moving out of or into the path of light. In some other embodiments, the shutters may include chemical species which reversibly change states, or orientations, to change the light transmission properties of the shutters. Examples of structures suitable for use as shutters include pixels forming transmissive pixelated liquid crystal displays, mems-based micro-mechanical structures (which may, e.g., move vertically and/or horizontally into and out of the path of light), or other array-segmented structures capable of high switching rates between at least two states. Additional examples of shutters include ferro-electric shutters. Preferably, the shutters are able to change states (switch between transmissive (open) and non-transmissive (closed) states) at a rate greater than the frame update rate of the micro-display  2000  by a factor determined by the number of shutter-aperture sub-pupils (or intra-pupil images) desired for the display system. For example, if the micro-display  2000  is producing images at 120 Hz, and 3 sub-pupils are desired, the shutters  2006  of the shutter array  2004  are preferably capable of a switching rate of at least 3 times the micro-display frame rate, or 3×120 Hz (e.g., 360 Hz or higher). 
     Preferably, the projection system  1003  ( FIG. 9 ) produces a relatively long depth of field, which may be controlled by limiting the aperture in the system. Without being limited by theory and, as discussed herein, the projection systems may be configured to provide images to the eye with an effective pupil diameter sufficiently small to force the human visual system to operate in “open-loop” mode, as the eye is unable to accommodate to such images individually. Multiple intra-pupil images, however, are believed to provide sufficient information to the eye to elicit a desired accommodative response. As discussed herein, the wavefronts of the light forming the intra-pupil images, in the aggregate, provide a wavefront approximation to which the eye accommodates. In some embodiments, the projection system  1003  may be configured to provide images with an effective pupil diameter of about 0.5 mm or less (e.g., 0.2-0.5 mm), which is believed to be sufficiently small to force the human visual system to operate in “open-loop” mode. Consequently, in some embodiments, the shutters  2006 , when opened, provide an opening having a width of 0.5 mm or less (e.g., 0.2-0.5 mm). 
     With reference again to  FIG. 9 , it will be appreciated that the illustrated opened shutter  2006   1  is opened at a first time (t=t 1 ) to provide an initial intra-pupil image, showing a first parallax view. Subsequently, at a later time (t=t 2 ) another shutter may be opened to provide a second intra-pupil image, showing a second parallax view. 
     With reference now to  FIG. 11 , an example is illustrated of the projection system of  FIG. 9  providing a different parallactically-disparate intra-pupil image at the later time (e.g., at time t=t 2 ). As illustrated, shutter  2006   2  is opened, to provide a beam of light  1010   b  into the eye  210 . The light beam  1010   b  may form the intra-pupil image shown in the right side of  FIG. 8 . It will be appreciated that additional shutters may be opened at other times to provide other intra-pupil images, with a set of intra-pupil images for approximating a wavefront curvature preferably being provided to the eye within a flicker fusion threshold. 
     With reference again to  FIG. 9 , as noted herein, the flicker fusion threshold of the human visual system places a time constraint on the number of images that may be injected into the eye  210  while still being perceived as being injected simultaneously. For example, the processing bandwidth of the control system  1024  and the ability to switch light-emitting regions of the light source  1026  and light modulators of the micro-display  2000  may limit the number of images that may be injected into the eye  210  within the duration allowed by the flicker fusion threshold. Given this finite number of images, the control system  1024  may be configured to make choices regarding the images that are displayed. For example, within the flicker fusion threshold, the display system may be required to inject a set of parallactically-disparate intra-pupil images into the eye, and in turn each parallax view may require images of various component colors in order to form a full color image. In some embodiments, the formation of full color images using component color images is bifurcated from the elucidation of a desired accommodation response. For example, without being limited by theory, it may be possible to elicit the desired accommodation response with a single color of light. In such a case, the parallactically-disparate intra-pupil images used to elicit the accommodation response would only be in the single color. As a result, it would not be necessary to form parallactically-disparate intra-pupil images using other colors of light, thereby freeing up time within the flicker fusion threshold for other types of images to be displayed. For example, to better approximate the wavefront, a larger set of parallactically-disparate intra-pupil images may be generated. 
     In some other embodiments, the control system  1024  may be configured to devote less time within the flicker fusion threshold for displaying images of colors of light for which the human visual system is less sensitive. For example, the human visual system is less sensitive to blue light then green light. As a result, the display system may be configured to generate a higher number of images formed with green light than images formed with blue light. 
     It will be appreciated that the light emitters of the micro-display  2000  may emit light with a large angular emission profile (e.g., a Lambertian angular emission profile). Undesirably, such an angular emission profile may “waste” light, since only a small portion of the emitted light may ultimately propagate through the projection optics  1020  and reach the eyes of the viewer. 
     With reference now to  FIG. 12 , in some embodiments, light collimators may be utilized to narrow the angular emission profile of light emitted by the light emitters.  FIG. 12  illustrates an example of the projection systems of  FIGS. 9 and 11  with an array of light collimators  2010  between emissive micro-display  2000  and projection optics  1020 . The array of light collimators  2010  is disposed forward of the light emitters  2002 . In some embodiments, each light emitter  2002  has a dedicated light collimator  2010 ; each light emitter  2002  is matched 1-to-1 with an associated light collimator  2010  (one light collimator  2010  per light emitter  2002 ). The light collimator may be configured to take light from an associated light emitter  2002  and image that light at optical infinity (or some other not infinite plane, in some other embodiments). 
     The light collimator  2010  may be configured to narrow the angular emission profile of incident light; for example, as illustrated, each light collimator  2010  receives light from an associated light emitter  2002  with a relatively wide initial angular emission profile and outputs that light with a narrower angular emission profile than the wide initial angular emission profile of the light emitter  2002 . In some embodiments, the rays of light exiting a light collimator  2010  are more parallel than the rays of light received by the light collimator  2010 , before being transmitted through and exiting the collimator  2010 . Advantageously, the light collimators  2010  may increase the efficiency of the utilization of light to form images by allowing more of that light to be directed into the eye of the viewer than if the collimators  2010  are not present. 
     Preferably, the light collimators  2010  are positioned in tight proximity to the light emitters  2002  to capture a large proportion of the light outputted by the light emitters  2002 . In some embodiments, there may be a gap between the light collimators  2010  and the light emitters  2002 . In some other embodiments, the light collimator  2010  may be in contact with the light emitters  2002 . Preferably, the entirety or majority of a cone of light from a light emitter  2002  is incident on a single associated light collimator  2010 . Thus, in some embodiments, each light emitter  2002  is smaller (occupies a smaller area) than the light receiving face of an associated light collimator  2010 . In some embodiments, each light emitter  2002  has a smaller width than the spacing between neighboring light emitters  2002 . 
     The light collimators  2010  may take various forms. For example, in some embodiments, the light collimators  2010  may be micro-lenses or lenslets, including including spherical or lenticular lenslets. As discussed herein, each micro-lens preferably has a width greater than the width of an associated light emitter  2002 . The micro-lenses may be formed of curved transparent material, such as glass or polymers, including photoresist and resins such as epoxy. In some embodiments, light collimators  2010  may be nano-lenses, e.g., diffractive optical gratings. In some embodiments, light collimators  2010  may be metasurfaces and/or liquid crystal gratings. In some embodiments, light collimators  2010  may take the form of reflective wells. 
     With reference now to  FIG. 13 , another example of a projection system  1003  for forming the divergent wavefront approximation of  FIGS. 7B and 8  is illustrated. In some embodiments, the projection system  1003  may comprise the micro-display  2000  and an array of light collimators  2010  disposed between the micro-display  2000  and the projection optics  1020 . The projection system is part of the display system  1001 , having the control system  1024  for controlling the projection system  1003  and the eye tracking device  1022 , as discussed above regarding  FIG. 9 . 
     With continued reference to  FIG. 13 , the micro-display  2000  may comprise a plurality of light emitters  2002 . The light emitters  2002  may be arranged in groups comprising two or more light emitters per group. Each group of light emitters  2002  includes an associated light collimator  2010 . As illustrated, each group  2002   g  of light collimators may include two light emitters  2002   1 ,  2002   2 , and the associated light collimator  2010   1  is configured to capture light from both light emitters. Preferably, the associated light collimator  2010   1  has a width that extends across both light emitters  2002   1 ,  2002   2 , so that the associated light collimator  2010   1  encompasses an area greater than the area of both the light emitters  2002   1 ,  2002   2 . In some embodiments, the light collimators  2010  are spaced from the light emitters  2002   1 ,  2002   2 . In some other embodiments, the light collimators  2010  may contact the light emitters  2002   1 ,  2002   2 . For example, the light collimators  2002  may be lenslets, and the light emitters  2002   1 ,  2002   2  may be disposed within the associated lenslet  2010   1 . 
     It will be appreciated that the location of a light emitter relative to an associated light collimator may influence the direction of light propagating out of the light collimator. The light emitters  2002   1 ,  2002   2  interface differently (e.g., are located at different locations) with the associated light collimator  2010   1 , which causes the light collimator  2010   1  to direct light from each of the light emitters  2002   1 ,  2002   2  along different paths to the projections optics  1020  and then to the eye  210 . Preferably, the locations of the light emitters  2002   1 ,  2002   2  and the physical parameters (e.g., the size and shape) of the associated light collimator  2010   1  are configured to provide different light paths corresponding to different intra-pupil images. While two light emitters  2002   1 ,  2002   2  are provided for clarity of illustration, it will be appreciated that each group of light emitters associated with a light collimator may have more than two light emitters. For example, each of the groups  2002   g  may have three or more light emitters, four or more light emitters, etc. Larger numbers of spatially distinct light emitters facilitate larger numbers of potential levels of parallax disparity. 
     In some embodiments, the image light from a light emitter of each group of light emitters may be directed by the associated light collimators  2010  and the projection optics  1020  to form a first intra-pupil image as the beam of light  1010   a,  and the image light from another light emitter of each group of light emitters may be directed by the associated light collimators  2010  and the projection optics  1020  to form a second intra-pupil image as the beam of light  1010   b.  As discussed herein, the light beams  1010   a,    1010   b  preferably have a sufficiently small diameter that the eye is unable to accommodate to the individual images formed by each beam of light. In some embodiments, the diameter of each beam of light  1010   a,    1010   b,  at the pupil of the eye  210 , is about 0.5 mm or less (e.g., 0.2-0.5 mm). 
     With continued reference to  FIG. 13 , the projection system  1003 , including the groups  2002   g  of light emitters  2002   1 ,  2002   2  may form different intra-pupil images using spatial multiplexing. For such spatial multiplexing, light emitters at different locations may be used to form different intra-pupil images; that is, different light emitters of each group of light emitters may provide image information for different intra-pupil images. The light from corresponding light emitters of each group of light emitters is directed by optical structures (e.g., the light collimator of the array  2010  of light collimators and the projection optics  1020 ) to form one of the light beams  1010   a,    1010   b  for forming a corresponding intra-pupil image. Advantageously, the intra-pupil images may be provided to the eye simultaneously, obviating the need to rapidly sequentially provided images under the flicker fusion threshold. This may relax requirements for the frame rate of the micro-display  2000  and the supporting control system  1024 . 
     With reference now to  FIG. 14 , it will be appreciated that the projection system  1003  of  FIG. 13  may include both light collimators with associated groups of light emitters, and also an array  2004  of shutters, to provide a combination of spatial and temporal multiplexing. The array  2004  of shutters may further refine the locations and/or sizes of beams of light entering the eye and forming intra-pupil images. For example, the beams of light outputted by the array  2010  of light collimators may be larger than desired for forming intra-pupil images (e.g., larger than that desired for open loop operation of the eye, including larger than 0.5 mm in diameter). The shutters of the shutter array  2004  may be utilized to limit the size of the light beam that ultimately enters the eye  210 . In addition, each beam of light provided by the light collimators of the collimator array  2010  may impinge on multiple shutters. As a result, the shutters may provide additional levels of parallax disparity for each incident beam of light, which may facilitate the presentation of virtual content on a larger number of depth planes than would be provided by only the beams of light outputted by the light collimator array  2010 . 
     In some embodiments, the display system may temporally multiplex in one dimension (e.g., along the x-axis) and spatially multiplex in another dimension (e.g., along an orthogonal dimension, such as the y-axis). For example, the light collimators may be lenticular lenslets, rather than spherical lenslets, and may be configured to provide different beams of light from spatially distinct light emitters to different locations along a first axis, which is orthogonal to a second axis which may be the elongate axis of the lenticular lenslet. An array of shutters may be utilized to form subpupils at different points and different times along that second axis. For example, localization of a subpupil along the first axis may be achieved by spatial multiplexing, by activating different light emitters of each group of light emitters associated with a lenticular lenslet (which is elongated along the second axis), while the localization of the subpupil along the second axis may be provided by temporal-multiplexing, by opening shutters on the second axis at desired times (e.g., times overlapping the activation of the corresponding light emitters used for spatial multiplexing). 
     With reference now to  FIG. 15 , an example is illustrated of a range of depth planes provided by the projection systems disclosed herein. The range spans from a far plane at optical infinity to a near plane closer to the eye  210 . The far plane at optical infinity may be provided by intra-pupil images formed by light beams (e.g., light beams  1010   a  and  1010   b ) that are substantially collimated or parallel relative to one another. For such a far plane, the light beams may be understood to have no or low parallax disparity. 
     With continued reference to  FIG. 15 , closer depth planes may be provided using higher levels of parallax disparity. In some embodiments, the perceived nearness of the near plane to the eye  210  may be determined by the maximum parallax disparity between the light beams forming intra-pupil images. For example, the maximum parallax disparity between the light beams  1010   a  and  1010   b  may be determined by the maximum distance between shutters of the array  2004  of shutters ( FIGS. 9-12 and 14 ) and/or the maximum difference in direction provided by each light collimator  1010   a  for light from light emitters  2002  of its associated group of light emitters ( FIGS. 13 and 14 ). 
     With reference now to  FIG. 16A , an example is illustrated of a display system comprising a projection system, comprising an array of shutters, and a pupil relay combiner eyepiece for superimposing image content on a user&#39;s view of the world. Preferably, the eyepiece  1030  is optically transmissive, allowing light  2012  from the world to propagate through the eyepiece into the eye  210  of the viewer, so that the viewer can see the world. In some embodiments, the eyepiece  1030  comprises one or more waveguides  1032  having in-coupling optical elements and out-coupling optical elements, such as in-coupling  770  and out-coupling optical elements  800 . The in-coupling optical element  770  receives image light  2014  from the projection system  1003  and redirects that image light such that it propagates through the eyepiece  1030  by total internal reflection to the out-coupling optical element  800 . The out-coupling optical element  800  outputs the image light  2014  to the viewer&#39;s eye  210 . Advantageously, eyepiece  1030  preserves all of the image attributes of provided by the projection system  1003 , and thus rapidly switching parallax views are accurately portrayed through the eyepiece  1030 . It will be appreciated that the image light  2014  is the light emitted by the micro-display  2000  and may correspond to, e.g., the light  200   a,    2002   b,    2002   c  ( FIG. 9 ) or the light provided through the array  2010  of light collimators ( FIGS. 13 and 14 ). 
     As illustrated, the projection system  1003  may comprise the array  2004  of shutters on the pupil plane  2005 , through which light exits the projection system  1003  towards the in-coupling optical element  770 . A lens structure  2016  may be provided between the shutter array  2004  and in-coupling optical element  770  to relay the image light  2014 . In some embodiments, the lens structure  2016  may also collimate the image light  2014  for propagation within a waveguide  1032  forming the eyepiece  1030 . 
     The in-coupling optical element  770  and the out-coupling optical element  800  may be refractive or reflective structures. Preferably, the in-coupling optical element  770  and the out-coupling optical element  800  are diffractive optical elements. Examples of diffractive optical elements include surface relief features, volume-phase features, meta-materials, or liquid-crystal polarization gratings. While illustrated being disposed on the same side of the waveguide  1032 , it will be appreciated that the in-coupling optical element  770  and the out-coupling optical element  800  may be disposed on different sides of the waveguide  1032 . Also, while shown on the side of the waveguide  1032  opposite the projection optics  1020 , one or both of the in-coupling optical element  770  and the out-coupling optical element  800  may be deposed on the same side of the waveguide  1032  as the projection optics  1020 . 
       FIG. 16B  illustrates an example of a display system comprising a projection system, comprising an array of light collimators for providing different intra-pupil images, and a pupil relay combiner eyepiece for superimposing image content on a user&#39;s view of the world. The display system of  FIG. 16B  is similar to that of  FIG. 16A , except that the projection system  1003  is similar to that of  FIG. 13 , rather than the projection system of  FIG. 9 . As illustrated, the projection system  1003  may comprise the micro-display  2000  and an array of light collimators  2010  between the micro-display  2000  and the projection optics  1020 . The micro-display  2000  may comprise a plurality of light emitters, formed in groups  2002   g,  with one group per light collimator  2010 . The location of a light emitter relative to the associated light collimator determines the direction of light propagating out of the light collimator and, thus, the parallax disparity between images formed using different light emitters  2002   1 ,  2002   2  of a group  2002   g  of light emitters. As a result, as noted herein, activation of different light emitters within a group of light emitters sets the parallax disparity between intra-pupil images formed using different light emitters. In some embodiments, light from a first light emitter  2002   1  of each group  2002   g  of light emitters is directed by the associated light collimators  2010  and the projection optics  1020  to form a first intra-pupil image with the beam of light  1010   a,  and light from a second light emitter  2002   2  of each group  2002   g  of light emitters may be directed by the associated light collimators  2010  and the projection optics  1020  to form a second intra-pupil image with the beam of light  1010   b.    
     It will be appreciated that any of the embodiments of projection systems disclosed herein may be utilized in the display systems of  FIGS. 16A and 16B , e.g., as the projection system  1003  of these figures. For example, the projection system  1003  may utilize both light collimators and an array of shutters as shown in, e.g.,  FIGS. 12 and 14 . 
       FIG. 17A  illustrates another example of a projection system, comprising an array of shutters, and a pupil relay combiner eyepiece for superimposing image content on a user&#39;s view of the world. The illustrated display system is similar to that of  FIG. 16A , except for the optical structures associated with the eyepiece  1030 . The eyepiece  1030  may comprise a partially transparent relay mirror  2020  for reflecting image light  2014  into the eye  210 . As illustrated, the image light  2014  is in-coupled by the in-coupling optical element  770 , which redirects the light  2014  such that it propagates through the waveguide  1032 . The eyepiece  1030  includes an outcoupling optical element  800  which directs the image light  2014  forward, through a quarter waveplate  2018  on the forward face of the waveguide  1032 . As illustrated, the partially transparent relay mirror  2020  may be forward of and spaced apart from the quarter wave plate  2018  and waveguide  1032 . 
       FIG. 17B  illustrates another example of a display system comprising a projection system, comprising an array of light collimators for providing different intra-pupil images, and a pupil relay combiner eyepiece for superimposing image content on a user&#39;s view of the world. The illustrated display system is similar to that of  FIG. 16B , except for the optical structures associated with the eyepiece  1030 . The eyepiece  1030  and associated structures are similar to that illustrated in  FIG. 17A . As noted above, image light  2014  is in-coupled by the in-coupling optical element  770  and propagates through the waveguide  1032  by total internal reflection. The outcoupling optical element  800  that outputs the image light  2014  forward away from the eye  210  through a quarter waveplate  2018  to the partially transparent relay mirror  2020 . As illustrated, the partially transparent relay mirror  2020  may be forward of and spaced apart from the quarter wave plate  2018  and waveguide  1032  and reflects the image light  2014  into the eye  210 . In some embodiments, the partially transparent relay mirror  2020  may be configured to selectively reflect light of the polarization transmitted through the quarter wave plate  2018 . 
     With reference now to  FIG. 18 , an example is illustrated of a display system  1001  having a projection system  1003  comprising the eye tracking system  1022  and a combiner eyepiece  1030  with a pupil expander  1034 . The pupil expander  1034  may comprise, e.g., diffractive optical elements configured to replicate the projection system pupil across the eyepiece  1030 . Since the pupil expander  1034  replicates the projection system pupil across a large area that may be traversed by the viewer&#39;s pupil through eye motion, the images formed by the micro-display  2000  and locations of the light-emitting regions of the light source  1026  can be updated based on input from the eye tracking system  1022  in real time. Advantageously, this configuration enables a larger eye-box for more comfortable viewing, relaxing restrictions on eye-to-combiner relative positioning and variations in inter-pupillary distance. 
     With reference now to  FIG. 19 , an example is illustrated of a display system  1001  having a projection system  1003  comprising eye tracking system  1022  and a combiner eyepiece  1030  with a pupil expander  1035  configured to produce a non-infinity depth plane. In some embodiments, the non-infinity depth plane may be at 3 meters, which offers an in-budget accommodation of ˜2.5 meters to infinity. For example, given the tolerance of the human visual system for accommodation-vergence mismatches, virtual content at distances of ˜2.5 meters to infinity from the viewer may be placed on the  3  meter depth plane with little discomfort. In such a system, the parallactically-disparate intra-pupil images may be used to drive accommodation for a narrower range of depth planes, possibly all closer to the viewer than the fixed “default” focal plane. In some embodiments, this system may also incorporate the eye tracking system  1022  to determine the distance of the viewer&#39;s fixation based, e.g., on vergence angles of both eyes of the viewer. 
     It will be appreciated that the micro-display  2000  may be a monochrome display, and the display system  1001  may be configured to provide monochrome images to the eye  210 . More preferably, the display system  1001  is configured to provide full color images to the eye  210 . In such embodiments, the micro-display  2000  may be a full color micro-display. For example, the full color images may be formed by providing different images formed by different component colors (e.g., three or more component colors, such as red, green, and blue), which in combination are perceived to be full color images by the viewer. Micro-display  2000  may be configured to emit light of all component colors. For example, different colors may be emitted by different light emitters. 
     In some other embodiments, full color images may be formed using component color images provided by a plurality of micro-displays, at least some of which are monochrome micro-displays. For example, different monochrome micro-displays may be configured to provide different component-color images. The component color images may be provided simultaneously to the eye  210 , or may be temporally multiplexed (e.g., all of the different component color images for forming a single full color image may be provided to the eye  210  within a flicker fusion threshold). 
       FIGS. 20A-20B  illustrate examples of projection systems having multiple emissive micro-displays  2000   a,    2000   b,    2000   c.  The micro-displays  2000   a,    2000   b,    2000   c  may each be similar to the micro-display  2000  disclosed herein (see, e.g.,  FIGS. 9, 11-14, and 16A-17B ). Light from the micro-displays  2000   a,    2000   b,    2000   c  are combined by an optical combiner  1050  and directed towards projection optics  1020 , and ultimately to the eye  210  of a viewer. As discussed herein, in some embodiments, the light from the projection optics  1020  may be directed to the eyepiece  1030 , which may be a waveguide assembly comprising one or more waveguides. 
     In some embodiments, the micro-displays  2000   a,    2000   b,    2000   c  may be monochrome micro-displays. Each monochrome micro-display may output light of a different component color to provide different monochrome images, which may be combined by the viewer to form a full-color image. 
     With continued reference to  FIG. 20A , the optical combiner  1050  receives image light  2001   a,    2001   b,    2001   c,  respectively, from each of the micro-displays  2000   a,    2000   b,    2000   c  and combines this light such that the light propagates generally in the same direction, e.g., toward the projection optics  1020 . In some embodiments, the optical combiner  1050  may be a dichroic X-cube prism having reflective internal surfaces  1052 ,  1054  that redirect the image light  2001   c,    2001   a,  respectively, to the projection optics  1020 . The projection optics  1020  converges or focuses image light, which impinges on the array  2004  of shutters. The image light  2001   a,    2001   b,    2001   c  then propagates through an open shutter (e.g., shutter  2006   i ) and into the eye  210 . 
     With reference now to  FIG. 20B , an example of a light projection system  1010  with multiple emissive micro-displays  2000   a,    2000   b,    2000   c,  and associated arrays  2010   a,    2010   b,    2010   c  of light collimators, respectively, is illustrated. The arrays  2010   a,    2010   b,    2010   c  may each be similar to the array  2010  disclosed herein (see, e.g.,  FIGS. 12-14, 16B, and 17B ). 
     In some embodiments, the arrays  2010   a,    2010   b,    2010   c  may each comprise light collimators configured to narrow the angular emission profiles of image light  2001   a,    2001   b,    2001   c  emitted by the micro-displays  1030   a,    1030   b,    1030   c.  The image light  2001   a,    2001   b,    2001   c  subsequently propagates through the optical combiner  1050  to the projection optics  1020 , and then into the eye  210 . In addition, each light collimator may have an associated group of light emitters (e.g., as illustrated in  FIGS. 13, 14, 16B, and 17B ) and may direct light from different light emitters of each group of emitters along different paths, corresponding to different amount of parallax disparity, for forming different intra-pupil images, as discussed herein. 
     With reference to  FIGS. 20A and 20B , in some other embodiments, the micro-displays  2000   a,    2000   b,    2000   c  may each be full-color displays configured to output light of all component colors. For example, the micro-displays  2000   a,    2000   b,    2000   c  may each include light emitters configured to emit red, green, and blue light. The micro-displays  2000   a,    2000   b,    2000   c  may be identical and may display the same image. However, utilizing multiple micro-displays may provide advantages for increasing the brightness and brightness dynamic range of the brightness of the image, by combining the light from the multiple micro-displays to form a single image. In some embodiments, two or more (e.g., three) micro-displays may be utilized, with the optical combiner  1050  is configured to combine light from all of these micro-displays. 
     With continued reference to  FIGS. 20A and 20B , in some embodiments, the monochrome micro-display  2000   b  directly opposite the output face  1051  may advantageously output green light. It will be appreciated that the reflective surfaces  1052 ,  1054  may have optical losses when reflecting light from the micro-displays  2000   c,    2000   a,  respectively. In addition, of the component colors red, green and blue, the human eye is most sensitive to the color green. Consequently, the monochrome micro-display  2000   b  opposite the output face of the optical combiner  1050  preferably outputs green light, so that the green light may proceed directly through the optical combiner  1050  without needing to be reflected to be outputted from the optical combiner  1050 . It will be appreciated, however, that the green monochrome micro-display may face other surfaces of the optical combiner  1050  in some other embodiments. 
     In some embodiments, as discussed herein, the display system may comprise an eyepiece (e.g., eyepiece  1030 ,  FIGS. 16A-19 ) to relay light outputted by the projection optics  1020  to the eye  210 . In some embodiments, the eyepiece may comprise a single waveguide configured to in-couple and out-couple light of all component colors. 
     In some other embodiments, the eyepiece  1030  may comprise a plurality of waveguides that form a stack of waveguides. Each waveguide has a respective in-coupling optical element for in-coupling image light. For example, each waveguide may have an associated in-coupling optical element configure to in-couple light of a different component color or a different range of wavelengths. In some embodiments, the number of waveguides is proportional to the number of component colors provided by the micro-displays  2000   a,    2000   b,    2000   c.  For example, where there are three component colors, the number of waveguides in the eyepiece  1030  may include a set of three waveguides or multiple sets of three waveguides. Examples of arrangements of waveguides and associated structures are discussed in U.S. Prov. Application No. 62/800363, filed Feb. 1, 2019, the entire disclosure of which is incorporated by reference herein. 
     With reference now to  FIG. 21 , an example is illustrated of an eyepiece  660  (which may correspond to the eyepiece  1030 ,  FIGS. 16A-19 ) comprising a stacked waveguide assembly for outputting light of different wavelengths corresponding to different component colors. In some embodiments, the waveguide assembly includes waveguides  670 ,  680 , and  690 , which individually may correspond to the waveguide  1032  ( FIGS. 16A-17B ). Each waveguide includes an associated in-coupling optical element, with, e.g., in-coupling optical element  700  disposed on a major surface (e.g., an upper major surface) of waveguide  670 , in-coupling optical element  710  disposed on a major surface (e.g., an upper major surface) of waveguide  680 , and in-coupling optical element  720  disposed on a major surface (e.g., an upper major surface) of waveguide  690 . In some embodiments, one or more of the in-coupling optical elements  700 ,  710 ,  720  may be disposed on the bottom major surface of the respective waveguide  670 ,  680 ,  690  (particularly where the one or more in-coupling optical elements are reflective, deflecting optical elements). As illustrated, the in-coupling optical elements  700 ,  710 ,  720  may be disposed on the upper major surface of their respective waveguide  670 ,  680 ,  690  (or the top of the next lower waveguide), particularly where those in-coupling optical elements are transmissive, deflecting optical elements. In some embodiments, the in-coupling optical elements  700 ,  710 ,  720  may be disposed in the body of the respective waveguide  670 ,  680 ,  690 . In some embodiments, as discussed herein, the in-coupling optical elements  700 ,  710 ,  720  are wavelength selective, such that they selectively redirect one or more wavelengths of light, while transmitting other wavelengths of light. While illustrated on one side or corner of their respective waveguide  670 ,  680 ,  690 , it will be appreciated that the in-coupling optical elements  700 ,  710 ,  720  may be disposed in other areas of their respective waveguide  670 ,  680 ,  690  in some embodiments. 
     As illustrated, the in-coupling optical elements  700 ,  710 ,  720  may be laterally offset from one another. In some embodiments, each in-coupling optical element may be offset such that it receives light without that light passing through another in-coupling optical element. For example, each in-coupling optical element  700 ,  710 ,  720  may be configured to receive light from a different image injection device  360 ,  370 ,  380 ,  390 , and  400  as shown in  FIG. 6 , and may be separated (e.g., laterally spaced apart) from other in-coupling optical elements  700 ,  710 ,  720  such that it substantially does not receive light from the other ones of the in-coupling optical elements  700 ,  710 ,  720 . In some embodiments, the in-coupling optical elements  700 ,  710 ,  720  are vertically aligned and are not laterally offset. 
     Each waveguide may also include associated light distributing elements, with, e.g., light distributing elements  730  disposed on a major surface (e.g., a top major surface) of waveguide  670 , light distributing elements  740  disposed on a major surface (e.g., a top major surface) of waveguide  680 , and light distributing elements  750  disposed on a major surface (e.g., a top major surface) of waveguide  690 . In some other embodiments, the light distributing elements  730 ,  740 ,  750 , may be disposed on a bottom major surface of associated waveguides  670 ,  680 ,  690 , respectively. In some other embodiments, the light distributing elements  730 ,  740 ,  750 , may be disposed on both top and bottom major surface of associated waveguides  670 ,  680 ,  690 , respectively; or the light distributing elements  730 ,  740 ,  750 , may be disposed on different ones of the top and bottom major surfaces in different associated waveguides  670 ,  680 ,  690 , respectively. 
     The waveguides  670 ,  680 ,  690  may be spaced apart and separated by, e.g., gas, liquid, and/or solid layers of material. For example, as illustrated, layer  760   a  may separate waveguides  670  and  680 ; and layer  760   b  may separate waveguides  680  and  690 . In some embodiments, the layers  760   a  and  760   b  are formed of low refractive index materials (that is, materials having a lower refractive index than the material forming the immediately adjacent one of waveguides  670 ,  680 ,  690 ). Preferably, the refractive index of the material forming the layers  760   a,    760   b  is 0.05 or more, or 0.10 or less than the refractive index of the material forming the waveguides  670 ,  680 ,  690 . Advantageously, the lower refractive index layers  760   a,    760   b  may function as cladding layers that facilitate TIR of light through the waveguides  670 ,  680 ,  690  (e.g., TIR between the top and bottom major surfaces of each waveguide). In some embodiments, the layers  760   a,    760   b  are formed of air. While not illustrated, it will be appreciated that the top and bottom of the illustrated set  660  of waveguides may include immediately neighboring cladding layers. 
     With continued reference to  FIG. 21 , light rays  770 ,  780 ,  790  are incident on and injected into the waveguides  670 ,  680 ,  690  by projection system  1003  ( FIGS. 9, 11-14, and 16A-17B ). 
     In some embodiments, the light rays  770 ,  780 ,  790  have different properties, e.g., different wavelengths or different ranges of wavelengths, which may correspond to different colors. The in-coupling optical elements  700 ,  710 ,  720  each deflect the incident light such that the light propagates through a respective one of the waveguides  670 ,  680 ,  690  by TIR. 
     For example, in-coupling optical element  700  may be configured to deflect ray  770 , which has a first wavelength or range of wavelengths. Similarly, the transmitted ray  780  impinges on and is deflected by the in-coupling optical element  710 , which is configured to deflect light of a second wavelength or range of wavelengths. Likewise, the ray  790  is deflected by the in-coupling optical element  720 , which is configured to selectively deflect light of third wavelength or range of wavelengths. 
     With continued reference to  FIG. 21 , the in-coupled light rays  770 ,  780 ,  790  are deflected by the in-coupling optical elements  700 ,  710 ,  720 , respectively, and then propagate by TIR within the waveguides  670 ,  680 ,  690 , respectively. The light rays  770 ,  780 ,  790  then impinge on the light distributing elements  730 ,  740 ,  750 , respectively. The light distributing elements  730 ,  740 ,  750  deflect the light rays  770 ,  780 ,  790  so that they propagate towards the out-coupling optical elements  800 ,  810 ,  820 , respectively. 
     In some embodiments, the light distributing elements  730 ,  740 ,  750  are orthogonal pupil expanders (OPE&#39;s). In some embodiments, the OPE&#39;s both deflect or distribute light to the out-coupling optical elements  800 ,  810 ,  820  and also increase the beam or spot size of this light as it propagates to the out-coupling optical elements. In some embodiments, e.g., where the beam size is already of a desired size, the light distributing elements  730 ,  740 ,  750  may be omitted and the in-coupling optical elements  700 ,  710 ,  720  may be configured to deflect light directly to the out-coupling optical elements  800 ,  810 ,  820 . In some embodiments, the out-coupling optical elements  800 ,  810 ,  820  are exit pupils (EP&#39;s) or exit pupil expanders (EPE&#39;s) that direct light in a viewer&#39;s eye  210  (see, e.g.,  FIGS. 18 ). It will be appreciated that the OPE&#39;s may be configured to increase the dimensions of the eye-box in at least one axis and the EPE&#39;s may be to increase the eye-box in an axis crossing, e.g., orthogonal to, the axis of the OPEs. 
     Accordingly, in some embodiments, the eyepiece  660  includes waveguides  670 ,  680 ,  690 ; in-coupling optical elements  700 ,  710 ,  720 ; light distributing elements (e.g., OPE&#39;s)  730 ,  740 ,  750 ; and out-coupling optical elements (e.g., EP&#39;s)  800 ,  810 ,  820  for each component color. The waveguides  670 ,  680 ,  690  may be stacked with an air gap/cladding layer between each one. The in-coupling optical elements  700 ,  710 ,  720  redirect or deflect incident light (with different in-coupling optical elements receiving light of different wavelengths) into its waveguide. The light then propagates at an angle which will result in TIR within the respective waveguide  670 ,  680 ,  690 . In the example shown, light ray  770  (e.g., blue light) is deflected by the first in-coupling optical element  700 , and then continues to bounce down the waveguide, interacting with the light distributing element (e.g., OPE&#39;s)  730  and then the out-coupling optical element (e.g., EPs)  800 , in a manner described earlier. The light rays  780  and  790  (e.g., green and red light, respectively) will pass through the waveguide  670 , with light ray  780  impinging on and being deflected by in-coupling optical element  710 . The light ray  780  then bounces down the waveguide  680  via TIR, proceeding on to its light distributing element (e.g., OPEs)  740  and then the out-coupling optical element (e.g., EP&#39;s)  810 . Finally, light ray  790  (e.g., red light) passes through the waveguide  690  to impinge on the light in-coupling optical elements  720  of the waveguide  690 . The light in-coupling optical elements  720  deflect the light ray  790  such that the light ray propagates to light distributing element (e.g., OPEs)  750  by TIR, and then to the out-coupling optical element (e.g., EPs)  820  by TIR. The out-coupling optical element  820  then ultimately out-couples the light ray  790  to the viewer, who also receives the out-coupled light from the other waveguides  670 ,  680 . 
     As noted above, the in-coupling optical elements  700 ,  710 ,  720  may be laterally offset from one another. In such embodiments, image light  2001   a,    2001   b,    2001   c  from different ones of the micro-displays  2000   a,    2000   b,    2000   c  may take different paths to the eyepiece  1030 , such that they impinge on different ones of the in-coupling optical element  700 ,  710 ,  720 . Where the image light  2001   a,    2001   b,    2001   c  includes light of different component colors, the associated in-coupling optical element  700 ,  710 ,  720 , respectively, may be configured to selectively in-couple light of different wavelengths, as discussed herein. 
     The different light paths for the image light  2001   a,    2001   b,    2001   c  may be provided by the combiner  1050  ( FIGS. 20A-20B ); by one or more of the arrays  2010   a,    2010   b,    2010   c;  and/or by angling one or more of the micro-displays  2000   a,    2000   b,    2000   c  at appropriate angles relative to the reflective surfaces  1052 ,  1054  of the optical combiner  1050 . For example, with reference to  FIGS. 20A-20B , the optical combiner  1050  may be configured to redirect the image light  2001   a,    2001   b,    2001   c  emitted by the micro-displays  2000   a,    2000   b,    2000   c  such that the image light propagates along different optical paths, in order to impinge on the associated one of the in-coupling optical elements  700 ,  710 ,  720 . Thus, the optical combiner  1050  combines the image light  2001   a,    2001   b,    2001   c  in the sense that the image light is outputted from a common face of the optical combiner  1050 , although light may exit the optical combiner in slightly different directions. For example, the reflective internal surfaces  1052 ,  1054  of the X-cube prism may each be angled to direct the image light  2001   a,    2001   b,    2001   c  along different paths to the eyepiece  1030 . As a result, the image light  2001   a,    2001   b,    2001   c  may be incident on different associated ones of in-coupling optical elements  700 ,  710 ,  720 . In some embodiments, the micro-displays  2000   a,    2000   b,    2000   c  may be appropriately angled relative to the reflective internal surfaces  1052 ,  1054  of the X-cube prism to provide the desired light paths to the in-coupling optical elements  700 ,  710 ,  720 . For example, faces of one or more of the micro-displays  2000   a,    2000   b,    2000   c  may be angled to matching faces of the optical combiner  1050 , such that image light emitted by the micro-displays is incident on the reflective internal surfaces  1052 ,  1054  at an appropriate angle to propagate towards the associated in-coupling optical element  700 ,  710 ,  720 . It will be appreciated that, in addition to a cube, the optical combiner  1050  may take the form of various other polyhedra. For example, the optical combiner  1050  may be in the shape of a rectangular prism having at least two faces that are not squares. 
     As discussed herein, the perception of a full color image by a user may be achieved with time division multiplexing in some embodiments. For example, different ones of the micro-LED arrays  2000   a,    2000   b,    2000   c  may be activated at different times to generate different component color images. In such embodiments, the different component color images that form a single full color image may be sequentially displayed sufficiently quickly that the human visual system does not perceive the component color images as being displayed at different times; that is, the different component color images that form a single full color image may all be displayed within a duration that is sufficiently short that the user perceives the component color images as being simultaneously presented, rather than being temporally separated. For example, the component color images may be sequentially displayed at a frame rate higher than the flicker fusion threshold of the human visual system. As an example, the flicker fusion threshold may be 60 Hz, which is considered to be sufficiently fast that most users do not perceive the component color images as being displayed at different times. In some embodiments, the different component color images are sequentially displayed at a rate higher than 60 Hz. It will be appreciated that time division multiplexing may advantageously reduce the computational load on processors (e.g., graphics processors) utilized to form displayed images. In some other embodiments, such as where sufficient computational resources are available, all component color images that form a full color image may be displayed simultaneously by the micro-displays  2000   a,    2000   b,    2000   c.    
     With reference now to  FIG. 22 , an example is illustrated of a wearable display system  60 . The display system  60  may correspond to the display system  1001  of  FIGS. 9 and 13 , with a projection system  1003  for each eye of the viewer or user  90 . 
     The display system  60  includes a display  70 , and various mechanical and electronic modules and systems to support the functioning of that display  70 . The display  70  may be coupled to a frame  80 , which is wearable by a display system user or viewer  90  and which is configured to position the display  70  in front of the eyes of the user  90 . The display  70  may be considered eyewear in some embodiments. In some embodiments, a speaker  100  is coupled to the frame  80  and configured to be positioned adjacent the ear canal of the user  90  (in some embodiments, another speaker, not shown, is positioned adjacent the other ear canal of the user to provide stereo/shapeable sound control). In some embodiments, the display system may also include one or more microphones  110  or other devices to detect sound. In some embodiments, the microphone is configured to allow the user to provide inputs or commands to the system  60  (e.g., the selection of voice menu commands, natural language questions, etc.), and/or may allow audio communication with other persons (e.g., with other users of similar display systems. The microphone may further be configured as a peripheral sensor to collect audio data (e.g., sounds from the user and/or environment). In some embodiments, the display system  60  may further include one or more outwardly-directed environmental sensors  112  configured to detect objects, stimuli, people, animals, locations, or other aspects of the world around the user. For example, environmental sensors  121  may include one or more cameras, which may be located, for example, facing outward so as to capture images similar to at least a portion of an ordinary field of view of the user  90 . In some embodiments, the display system may also include a peripheral sensor  120   a,  which may be separate from the frame  80  and attached to the body of the user  90  (e.g., on the head, torso, an extremity, etc. of the user  90 ). The peripheral sensor  120   a  may be configured to acquire data characterizing a physiological state of the user  90  in some embodiments. For example, the sensor  120   a  may be an electrode. 
     With continued reference to  FIG. 22 , the display  70  is operatively coupled by communications link  130 , such as by a wired lead or wireless connectivity, to a local data processing module  140  which may be mounted in a variety of configurations, such as fixedly attached to the frame  80 , fixedly attached to a helmet or hat worn by the user, embedded in headphones, or otherwise removably attached to the user  90  (e.g., in a backpack-style configuration, in a belt-coupling style configuration). Similarly, the sensor  120   a  may be operatively coupled by communications link  120   b,  e.g., a wired lead or wireless connectivity, to the local processor and data module  140 . The local processing and data module  140  may comprise a hardware processor, as well as digital memory, such as non-volatile memory (e.g., flash memory or hard disk drives), both of which may be utilized to assist in the processing, caching, and storage of data. The data may include data a) captured from sensors (which may be, e.g., operatively coupled to the frame  80  or otherwise attached to the user  90 ), such as image capture devices (such as cameras), microphones, inertial measurement units, accelerometers, compasses, GPS units, radio devices, gyros, and/or other sensors disclosed herein; and/or b) acquired and/or processed using remote processing module  150  and/or remote data repository  160  (including data relating to virtual content), possibly for passage to the display  70  after such processing or retrieval. The local processing and data module  140  may be operatively coupled by communication links  170 ,  180 , such as via a wired or wireless communication links, to the remote processing module  150  and remote data repository  160  such that these remote modules  150 ,  160  are operatively coupled to each other and available as resources to the local processing and data module  140 . In some embodiments, the local processing and data module  140  may include one or more of the image capture devices, microphones, inertial measurement units, accelerometers, compasses, GPS units, radio devices, and/or gyros. In some other embodiments, one or more of these sensors may be attached to the frame  80 , or may be standalone structures that communicate with the local processing and data module  140  by wired or wireless communication pathways. In some embodiments, the local processing and data module  140  may include one or more graphics processors, and may correspond to the control system  1024  ( FIGS. 9 and 13 ). 
     With continued reference to  FIG. 22 , in some embodiments, the remote processing module  150  may comprise one or more processors configured to analyze and process data and/or image information. In some embodiments, the remote data repository  160  may comprise a digital data storage facility, which may be available through the internet or other networking configuration in a “cloud” resource configuration. In some embodiments, the remote data repository  160  may include one or more remote servers, which provide information, e.g., information for generating augmented reality content, to the local processing and data module  140  and/or the remote processing module  150 . In some embodiments, all data is stored and all computations are performed in the local processing and data module, allowing fully autonomous use from a remote module. 
     It will be appreciated that each of the processes, methods, and algorithms described herein and/or depicted in the figures may be embodied in, and fully or partially automated by, code modules executed by one or more physical computing systems, hardware computer processors, application-specific circuitry, and/or electronic hardware configured to execute specific and particular computer instructions. For example, computing systems may include general purpose computers (e.g., servers) programmed with specific computer instructions or special purpose computers, special purpose circuitry, and so forth. A code module may be compiled and linked into an executable program, installed in a dynamic link library, or may be written in an interpreted programming language. In some embodiments, particular operations and methods may be performed by circuitry that is specific to a given function. 
     Further, certain embodiments of the functionality of the present disclosure are sufficiently mathematically, computationally, or technically complex that application-specific hardware or one or more physical computing devices (utilizing appropriate specialized executable instructions) may be necessary to perform the functionality, for example, due to the volume or complexity of the calculations involved or to provide results substantially in real-time. For example, a video may include many frames, with each frame having millions of pixels, and specifically programmed computer hardware is necessary to process the video data to provide a desired image processing task or application in a commercially reasonable amount of time. 
     Code modules or any type of data may be stored on any type of non-transitory computer-readable medium, such as physical computer storage including hard drives, solid state memory, random access memory (RAM), read only memory (ROM), optical disc, volatile or non-volatile storage, combinations of the same and/or the like. In some embodiments, the non-transitory computer-readable medium may be part of one or more of the local processing and data module ( 140 ), the remote processing module ( 150 ), and remote data repository ( 160 ). The methods and modules (or data) may also be transmitted as generated data signals (e.g., as part of a carrier wave or other analog or digital propagated signal) on a variety of computer-readable transmission mediums, including wireless-based and wired/cable-based mediums, and may take a variety of forms (e.g., as part of a single or multiplexed analog signal, or as multiple discrete digital packets or frames). The results of the disclosed processes or process steps may be stored, persistently or otherwise, in any type of non-transitory, tangible computer storage or may be communicated via a computer-readable transmission medium. 
     Any processes, blocks, states, steps, or functionalities in flow diagrams described herein and/or depicted in the attached figures should be understood as potentially representing code modules, segments, or portions of code which include one or more executable instructions for implementing specific functions (e.g., logical or arithmetical) or steps in the process. The various processes, blocks, states, steps, or functionalities may be combined, rearranged, added to, deleted from, modified, or otherwise changed from the illustrative examples provided herein. In some embodiments, additional or different computing systems or code modules may perform some or all of the functionalities described herein. The methods and processes described herein are also not limited to any particular sequence, and the blocks, steps, or states relating thereto may be performed in other sequences that are appropriate, for example, in serial, in parallel, or in some other manner. Tasks or events may be added to or removed from the disclosed example embodiments. Moreover, the separation of various system components in the embodiments described herein is for illustrative purposes and should not be understood as requiring such separation in all embodiments. It should be understood that the described program components, methods, and systems may generally be integrated together in a single computer product or packaged into multiple computer products. 
     In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than restrictive sense. 
     Indeed, it will be appreciated that the systems and methods of the disclosure each have several innovative aspects, no single one of which is solely responsible or required for the desirable attributes disclosed herein. The various features and processes described above may be used independently of one another, or may be combined in various ways. All possible combinations and subcombinations are intended to fall within the scope of this disclosure. 
     Certain features that are described in this specification in the context of separate embodiments also may be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment also may be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination may in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. No single feature or group of features is necessary or indispensable to each and every embodiment. 
     It will be appreciated that conditional language used herein, such as, among others, “can,” “could,” “might,” “may,” “e.g.,” and the like, unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements and/or steps. Thus, such conditional language is not generally intended to imply that features, elements and/or steps are in any way required for one or more embodiments or that one or more embodiments necessarily include logic for deciding, with or without author input or prompting, whether these features, elements and/or steps are included or are to be performed in any particular embodiment. The terms “comprising,” “including,” “having,” and the like are synonymous and are used inclusively, in an open-ended fashion, and do not exclude additional elements, features, acts, operations, and so forth. Also, the term “or” is used in its inclusive sense (and not in its exclusive sense) so that when used, for example, to connect a list of elements, the term “or” means one, some, or all of the elements in the list. In addition, the articles “a,” “an,” and “the” as used in this application and the appended claims are to be construed to mean “one or more” or “at least one” unless specified otherwise. Similarly, while operations may be depicted in the drawings in a particular order, it is to be recognized that such operations need not be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. Further, the drawings may schematically depict one more example processes in the form of a flowchart. However, other operations that are not depicted may be incorporated in the example methods and processes that are schematically illustrated. For example, one or more additional operations may be performed before, after, simultaneously, or between any of the illustrated operations. Additionally, the operations may be rearranged or reordered in other embodiments. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments, and it should be understood that the described program components and systems may generally be integrated together in a single software product or packaged into multiple software products. Additionally, other embodiments are within the scope of the following claims. In some cases, the actions recited in the claims may be performed in a different order and still achieve desirable results. 
     Accordingly, the claims are not intended to be limited to the embodiments shown herein, but are to be accorded the widest scope consistent with this disclosure, the principles and the novel features disclosed herein.