Patent Publication Number: US-11640063-B2

Title: Variable pixel density display system with mechanically-actuated image projector

Description:
PRIORITY CLAIM 
     This application is a US national phase of International Patent Application No. PCT/US2019/067816 filed on Dec. 20, 2019, which claims priority from: U.S. Provisional Application No. 62/911,018 filed on Oct. 4, 2019 and titled “AUGMENTED AND VIRTUAL REALITY DISPLAY SYSTEMS WITH SHARED DISPLAY FOR LEFT AND RIGHT EYES”; U.S. Provisional Application No. 62/800,363 filed on Feb. 1, 2019 and titled “VIRTUAL AND AUGMENTED REALITY DISPLAY SYSTEMS WITH EMISSIVE MICRO-DISPLAYS”; and U.S. Provisional Application No. 62/786,199 filed on Dec. 28, 2018 and titled “LOW MOTION-TO-PHOTON LATENCY ARCHITECTURE FOR AUGMENTED AND VIRTUAL REALITY DISPLAY SYSTEMS”. The above-noted applications are hereby incorporated by reference herein in their entireties. 
     INCORPORATION BY REFERENCE 
     This application incorporates by reference the entireties of each of the following: U.S. application Ser. No. 14/555,585 filed on Nov. 27, 2014, published on Jul. 23, 2015 as U.S. Publication No. 2015/0205126; U.S. application Ser. No. 14/690,401 filed on Apr. 18, 2015, published on Oct. 22, 2015 as U.S. Publication No. 2015/0302652; U.S. application Ser. No. 14/212,961 filed on Mar. 14, 2014, now U.S. Pat. No. 9,417,452 issued on Aug. 16, 2016; U.S. application Ser. No. 14/331,218 filed on Jul. 14, 2014, published on Oct. 29, 2015 as U.S. Publication No. 2015/0309263; U.S. Patent App. Pub. No. 2018/0061121, published Mar. 1, 2018; U.S. patent application Ser. No. 16/221,065, filed Dec. 14, 2018; U.S. Patent App. Pub. No. 2018/0275410, published Sep. 27, 2018; U.S. Provisional Application No. 62/786,199, filed Dec. 28, 2018; and U.S. application Ser. No. 16/221,359, filed on Dec. 14, 2018; U.S. Provisional Application No. 62/702,707, filed on Jul. 24, 2018; U.S. application Ser. No. 15/481,255, filed Apr. 6, 2017; and U.S. application Ser. No. 15/927,808, filed Apr. 21, 2018, published on Sep. 27, 2018 as U.S. Patent App. Pub. No. 2018/0275410. 
    
    
     BACKGROUND 
     Field 
     The present disclosure relates to display systems and, more particularly, to augmented and virtual reality display 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, in which 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 the 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, an MR scenario may include AR image content that appears to be blocked by or is otherwise perceived to interact with objects in the real world. 
     Referring to  FIG.  1   , an AR scene  10  is depicted. The 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 . The user also perceives that he/she “sees” “virtual content” such as a robot statue  40  standing upon the real-world platform  30 , and a flying cartoon-like avatar character  50  which seems to be a personification of a bumble bee. These elements  50 ,  40  are “virtual” in that they do not exist in the real world. Because the human visual perception system is complex, it is challenging to produce AR technology that facilitates a comfortable, natural-feeling, rich presentation of virtual image elements amongst other virtual or real-world imagery elements. 
     SUMMARY 
     According to some embodiments, a head-mounted display system comprises a support structure configured to mount on a user&#39;s head, a light projection system supported by the support structure, an eyepiece, and one or more processors. The light projection system comprises a micro-display comprising an array of light emitters associated with a first resolution, wherein the array of light emitters is configured to output light forming frames of virtual content; projection optics; and one or more actuators. The eyepiece is supported by the support structure and configured to receive light from the light projection system and to direct the received light to the user, one or more processors. The one or more processors are configured to receive a rendered frame of virtual content, the rendered frame comprising at least a portion associated with a second resolution, wherein the second resolution is higher than the first resolution. The one or more processors are further configured to cause the emissive micro-display projector to output light forming a first subframe of the rendered frame, wherein the first subframe and the rendered frame are substantially a same size. The one or more processors are further configured to shift, via the one or more actuators, one or move parts of the light projection system to adjust positions associated with light emitter light outputted from the light projection system and cause the light projection system to output light forming a second subframe of the rendered frame. 
     According to some other embodiments, a method implemented by a head-mounted display system of one or more processors comprises providing a rendered frame of virtual content, the rendered frame comprising at least a portion associated with a second resolution. An emissive micro-display projector is caused to output light forming a first subframe of the rendered frame, the first subframe having a first resolution less than the second resolution, wherein the emissive micro-display projector comprises an array of light emitters associated with the first resolution and having a pixel pitch. The emissive micro-display projector is shifted, via one or more actuators, to adjust geometric positions associated with light output by the emissive micro-display projector, wherein the geometric positions are adjusted a distance less than the pixel pitch. The emissive micro-display projector is caused to output light forming a second subframe of the rendered frame, the second subframe having the first resolution. 
     According to yet other embodiments, a system comprises one or more processors and one or more computer storage media storing instructions that, when executed by the one or more processors, cause the one or more processors to perform operations. The operations comprise generating a rendered frame of virtual content to be displayed as augmented reality content via an emissive micro-display projector system of the system, the rendered frame being associated with a second resolution, and the emissive micro-display projector comprising one or more light emitter arrays configured to output light forming virtual content associated with a first, lower, resolution. The rendered frame of virtual content is divided into a plurality of subframes, wherein each subframe includes a subset of pixels included in the rendered frame. Light is successively output via the emissive micro-display projector system, the light forming the plurality of subframes, wherein the emissive micro-display projector system is shifted via one or more actuators for each of the subframes according to a movement pattern, wherein the emissive micro-display projector system is shifted along one or more axes on a plane parallel to a plane of an output pupil of the projector system. 
     According to some other embodiments, a method implemented by a head-mounted display system of one or more processors comprises generating a rendered frame of virtual content to be displayed as virtual content via an emissive micro-display projector system of the head-mounted display system, the rendered frame being associated with a second resolution, and the emissive micro-display projector comprising emitters configured to output light forming virtual content associated with a first, lower, resolution. The rendered frame of virtual content is divided into a plurality of subframes, wherein each subframe includes a subset of pixels included in the rendered frame. Light is successively output via the emissive micro-display projector system, the light forming the plurality of subframes, wherein the emissive micro-display projector system is shifted along one or more axes via one or more actuators for each of the subframes according to a movement pattern, wherein the emissive micro-display projector system is shifted along one or more axes on a plane parallel to a plane of an output pupil of the projector system. 
     Some additional examples are provided below. 
     Example 1. A head-mounted display system comprising: a support structure configured to mount on a user&#39;s head; a light projection system supported by the support structure and comprising: a micro-display comprising an array of light emitters associated with a first resolution, wherein the array of light emitters is configured to output light forming frames of virtual content; projection optics; and one or more actuators; an eyepiece supported by the support structure and configured to receive light from the light projection system and to direct the received light to the user; and one or more processors, the one or more processors configured to: receive a rendered frame of virtual content, the rendered frame comprising at least a portion associated with a second resolution, wherein the second resolution is higher than the first resolution; cause the emissive micro-display projector to output light forming a first subframe of the rendered frame, wherein the first subframe and the rendered frame are substantially a same size; shift, via the one or more actuators, one or move parts of the light projection system to adjust positions associated with light emitter light outputted from the light projection system; and cause the light projection system to output light forming a second subframe of the rendered frame. 
     Example 2. The head-mounted display of example 1, wherein the portion associated with the second resolution is associated with a foveal region of a user&#39;s eye. 
     Example 3. The head-mounted display of example 2, wherein the one or more processors are configured to determine that light forming the portion falls within a threshold angular distance of a fovea of the user. 
     Example 4. The head-mounted display of example 2, wherein the one or more processors are configured to cause: for the second subframe, light emitters to update emitted light forming the portion; and for the first subframe, light emitters to not update emitted light forming parts of the rendered frame outside of the portion. 
     Example 5. The head-mounted display system of example 1, wherein each emissive micro-display array has an associated emitter size, wherein the emitter size is less than the pixel pitch. 
     Example 6. The head-mounted display of example 5, wherein a total number of subframes of the rendered frame is determined based on a size associated with the pixel pitch and the emitter size. 
     Example 7. The head-mounted display of example 6, wherein the one or more processors are configured to cause the light projection system to successively output light forming the total number of subframes. 
     Example 8. The head-mounted display of example 7, wherein the one or more processors are configured to time multiplex the rendered frame by causing the one or more actuators to shift parts of the light projection system for each subframe. 
     Example 9. The head-mounted display of example 8, wherein the one or more processors are configured to cause the one or more actuators to shift the parts of the light projection system such that geometric positions associated with the array of light emitters are tiled within respective inter-emitter regions. 
     Example 10. The head-mounted display of example 1, wherein the one or more processors are configured to cause the one or more actuators to shift the parts of the light projection system according to a movement pattern, and wherein the movement pattern is a continual movement pattern. 
     Example 11. The head-mounted display of example 1, wherein the first subframe and the second subframe each comprise pixels associated with respective portions of the rendered frame. 
     Example 12. The head-mounted display of example 1, wherein the light projection system comprises a plurality of arrays of light emitters. 
     Example 13. The head-mounted display of example 12, further comprising an X-cube prism, wherein each of the arrays of light emitters face a different side of the X-cube prism. 
     Example 14. The head-mounted display of example 12, wherein each of the arrays of light emitters is configured to direct light into dedicated associated projection optics. 
     Example 15. The head-mounted display of example 12, wherein the arrays of light emitters are attached to a common back plane. 
     Example 16. The head-mounted display of example 1, wherein the one or more actuators are configured to shift the projection optics. 
     Example 17. The head-mounted display of example 1, wherein the one or more actuators are piezoelectric motors. 
     Example 18. The head-mounted display of example 1, wherein the one or more actuators shift the emissive micro-display projector along two axes. 
     Example 19. The head-mounted display of example 1, wherein the light emitters comprise light emitting diodes. 
     Example 20. The head-mounted display of example 1, wherein the array of light emitters is configured to emit light of a plurality of component colors. 
     Example 21. The head-mounted display of example 20, wherein each light emitter comprises a stack of constituent light generators, wherein each constituent light generator emits light of a different color. 
     Example 22. The head-mounted display of example 1, wherein the eyepiece comprises a waveguide assembly comprising one or more waveguides, each waveguide comprising: an in-coupling optical element configured to incouple light from the micro-display into the waveguide; and an out-coupling optical element configured to outcouple incoupled light out of the waveguide. 
     Example 23. A method implemented by a head-mounted display system of one or more processors, the method comprising: providing a rendered frame of virtual content, the rendered frame comprising at least a portion associated with a second resolution; causing an emissive micro-display projector to output light forming a first subframe of the rendered frame, the first subframe having a first resolution less than the second resolution, wherein the emissive micro-display projector comprises an array of light emitters associated with the first resolution and having a pixel pitch; shifting, via one or more actuators, the emissive micro-display projector to adjust geometric positions associated with light output by the emissive micro-display projector, wherein the geometric positions are adjusted a distance less than the pixel pitch; and causing the emissive micro-display projector to output light forming a second subframe of the rendered frame, the second subframe having the first resolution. 
     Example 24. A system comprising: one or more processors; and one or more computer storage media storing instructions that, when executed by the one or more processors, cause the one or more processors to perform operations comprising: generating a rendered frame of virtual content to be displayed as augmented reality content via an emissive micro-display projector system of the system, the rendered frame being associated with a second resolution, and the emissive micro-display projector comprising one or more light emitter arrays configured to output light forming virtual content associated with a first, lower, resolution; dividing the rendered frame of virtual content into a plurality of subframes, wherein each subframe includes a subset of pixels included in the rendered frame; and successively outputting light via the emissive micro-display projector system, the light forming the plurality of subframes, wherein the emissive micro-display projector system is shifted via one or more actuators for each of the subframes according to a movement pattern, wherein the emissive micro-display projector system is shifted along one or more axes on a plane parallel to a plane of an output pupil of the projector system. 
     Example 25. The system of example 24, wherein the one or more processors are configured to cause the one or more actuators to shift the emissive micro-display projector such that geometric positions associated with the emissive micro-display arrays are tiled within respective inter-emitter regions. 
     Example 26. The system of example 25, wherein the one or more processors are configured to cause the one or more actuators to shift the light emitter arrays along the one or axes. 
     Example 27. The system of example 25, wherein the micro-display projector system comprises projection optics, wherein the one or more processors are configured to cause the one or more actuators to shift the projection optics along the one or more axes, the projection optics being configured to output light to a user of the system. 
     Example 28. A method implemented by a head-mounted display system of one or more processors, the method comprising: generating a rendered frame of virtual content to be displayed as virtual content via an emissive micro-display projector system of the head-mounted display system, the rendered frame being associated with a second resolution, and the emissive micro-display projector comprising emitters configured to output light forming virtual content associated with a first, lower, resolution; dividing the rendered frame of virtual content into a plurality of subframes, wherein each subframe includes a subset of pixels included in the rendered frame; and successively outputting light via the emissive micro-display projector system, the light forming the plurality of subframes, wherein the emissive micro-display projector system is shifted along one or more axes via one or more actuators for each of the subframes according to a movement pattern, wherein the emissive micro-display projector system is shifted along one or more axes on a plane parallel to a plane of an output pupil of the projector system. 
     Example 28. The method of example 28, wherein the emissive micro-display projector system is shifted such that geometric positions associated with light emitter arrays are tiled within respective inter-emitter regions. 
     Example 29. The method of example 29, wherein the one or more actuators shift the light emitter arrays along the one or axes. 
     Example 30. The method of example 29, wherein the one or more actuators shift projection optics of the micro-LED projector system along the one or more axes, the projection optics being configured to output light to a user of the head-mounted display system. 
    
    
     
       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 display system for simulating three-dimensional imagery for a user. 
         FIGS.  3 A- 3 C  illustrate relationships between radius of curvature and focal radius. 
         FIG.  4 A  illustrates a representation of the accommodation-vergence response of the human visual system. 
         FIG.  4 B  illustrates examples of different accommodative states and vergence states of a pair of eyes of the user. 
         FIG.  4 C  illustrates an example of a representation of a top-down view of a user viewing content via a display system. 
         FIG.  4 D  illustrates another example of a representation of a top-down view of a user viewing content via a display system. 
         FIG.  5    illustrates aspects of an approach for simulating three-dimensional imagery by modifying wavefront divergence. 
         FIG.  6    illustrates an example of a waveguide stack for outputting image information to a user. 
         FIG.  7    illustrates an example of exit beams outputted by a waveguide. 
         FIG.  8    illustrates an example of a stacked eyepiece in which each depth plane includes images formed using multiple different component colors. 
         FIG.  9 A  illustrates a cross-sectional side view of an example of a set of stacked waveguides that each includes an in-coupling optical element. 
         FIG.  9 B  illustrates a perspective view of an example of the plurality of stacked waveguides of  FIG.  9 A . 
         FIG.  9 C  illustrates a top-down plan view of an example of the plurality of stacked waveguides of  FIGS.  9 A and  9 B . 
         FIG.  9 D  illustrates a top-down plan view of another example of a plurality of stacked waveguides. 
         FIG.  9 E  illustrates an example of wearable display system. 
         FIG.  10    illustrates an example of a wearable display system with a light projection system having a spatial light modulator and a separate light source. 
         FIG.  11 A  illustrates an example of a wearable display system with a light projection system having multiple emissive micro-displays. 
         FIG.  11 B  illustrates an example of an emissive micro-display with an array of light emitters. 
         FIG.  12    illustrates another example of a wearable display system with a light projection system having multiple emissive micro-displays and associated light redirecting structures. 
         FIG.  13 A  illustrates an example of a side-view of a wearable display system with a light projection system having multiple emissive micro-displays and an eyepiece having waveguides with overlapping and laterally-shifted light in-coupling optical elements. 
         FIG.  13 B  illustrates another example of a wearable display system with a light projection system having multiple emissive micro-displays configured to direct light to a single light in-coupling area of an eyepiece. 
         FIG.  14    illustrates an example of a wearable display system with a single emissive micro-display. 
         FIG.  15    illustrates a side view of an example of an eyepiece having a stack of waveguides with overlapping in-coupling optical elements. 
         FIG.  16    illustrates a side view of an example of a stack of waveguides with color filters for mitigating ghosting or crosstalk between waveguides. 
         FIG.  17    illustrates an example of a top-down view of the eyepieces of  FIGS.  15  and  16   . 
         FIG.  18    illustrates another example of a top-down view of the eyepieces of  FIGS.  15  and  16   . 
         FIG.  19 A  illustrates a side view of an example of an eyepiece having a stack of waveguides with overlapping and laterally-shifted in-coupling optical elements. 
         FIG.  19 B  illustrates a side view of an example of the eyepiece of  FIG.  19 A  with color filters for mitigating ghosting or crosstalk between waveguides. 
         FIG.  20 A  illustrates an example of a top-down view of the eyepieces of  FIGS.  19 A and  19 B . 
         FIG.  20 B  illustrates another example of a top-down view of the eyepieces of  FIGS.  19 A and  19 B . 
         FIG.  21    illustrates a side view of an example of re-bounce in a waveguide. 
         FIGS.  22 A- 22 C  illustrate examples of top-down views of an eyepiece having in-coupling optical elements configured to reduce re-bounce. 
         FIGS.  23 A- 23 C  illustrate additional examples of top-down views of an eyepiece having in-coupling optical elements configured to reduce re-bounce. 
         FIG.  24 A  illustrates an example of angular emission profiles of light emitted by individual light emitters of an emissive micro-display, and light captured by projection optics. 
         FIG.  24 B  illustrates an example of the narrowing of angular emission profiles using an array of light collimators. 
         FIG.  25 A  illustrates an example of a side view of an array of tapered reflective wells for directing light to projection optics. 
         FIG.  25 B  illustrates an example of a side view of an asymmetric tapered reflective well. 
         FIGS.  26 A- 26 C  illustrate examples of differences in light paths for light emitters at different positions relative to center lines of overlying lens. 
         FIG.  27    illustrates an example of a side view of individual light emitters of an emissive micro-display with an overlying nano-lens array. 
         FIG.  28    is a perspective view of an example of the emissive micro-display of  FIG.  27   . 
         FIG.  29    illustrates an example of a wearable display system with the full-color emissive micro-display of  FIG.  28   . 
         FIG.  30 A  illustrates an example of a wearable display system with an emissive micro-display and an associated array of light collimators. 
         FIG.  30 B  illustrates an example of a light projection system with multiple emissive micro-displays, each with an associated array of light collimators. 
         FIG.  30 C  illustrates an example of a wearable display system with multiple emissive micro-displays, each with an associated array of light collimators. 
         FIGS.  31 A and  31 B  illustrate examples of waveguide assemblies having variable focus elements for varying the wavefront divergence of light to a viewer. 
         FIG.  32 A  illustrates an example of an emissive micro-display having an array of light emitters that are separated by gaps. 
         FIG.  32 B  illustrates an example of how the emissive micro-display of  FIG.  32 A  may be configured to emulate a higher fill-factor micro-display via time-multiplexing and repositioning of the array or associated optics. 
         FIG.  32 C  illustrates an example of a foveated image formed by an emissive micro-display, such as the emissive micro-display of  FIG.  32 A . 
         FIG.  32 D  illustrates an example of an emissive micro-display, such as the emissive micro-display of  FIG.  32 A , configured to form foveated images with three or more levels of resolution within the image. 
         FIG.  33    illustrates another example of a foveated image provided by an emissive micro-display, such as the emissive micro-display of  FIG.  32 A . 
         FIG.  34    illustrates various example paths of movement of parts of an emissive micro-display to shift the positions of displayed pixels. 
         FIGS.  35 A and  35 B  illustrate how displacement of light emitters and projection optics may change the position of a displayed pixel. 
         FIG.  36 A  illustrates an example of a wearable display system having a light projection system with an actuator coupled to projection optics. 
         FIG.  36 B  illustrates an example of a wearable display system having a light projection system with multiple actuators, each coupled to a different micro-display. 
         FIG.  37 A  illustrates an example of a wearable display system having a light projection system with an eyepiece having a single waveguide. 
         FIG.  37 B  illustrates an example of a wearable display system having a light projection system in which a single array of light emitters outputs light of different component colors with an actuator coupled to projection optics. 
         FIG.  37 C  illustrates an example of a wearable display system similar to the wearable display system of  FIG.  37 B , except for attachment of the actuator to a micro-display rather than the projection optics. 
         FIG.  38 A  illustrates an example of a wearable display system having a light projection system that directs light of different component colors to an eyepiece without using an optical combiner to combine light of different colors with an actuator coupled to projection optics. 
         FIG.  38 B  illustrates another example of a wearable display system having a light projection system that directs light of different component colors to an eyepiece without using an optical combiner to combine the light of different colors with an actuator coupled to projection optics. 
         FIG.  38 C  illustrates an example of a wearable display system similar to the wearable display system of  FIG.  38 A , except for attachment of actuators to individual micro-displays rather than the projection optics. 
         FIG.  38 D  illustrates an example of a wearable display system similar to the wearable display system of  FIG.  38 B , except for attachment of the actuator to an integral micro-display structure rather than the projection optics. 
         FIG.  39    illustrates a flowchart of an example process for outputting subframes of a rendered frame of virtual content. 
     
    
    
     DETAILED DESCRIPTION 
     Augmented reality (AR) or virtual reality (VR) systems may display virtual content to a user, or viewer. This content may be displayed on a head-mounted display, for example, as part of eyewear, that projects image information to the user&#39;s eyes. In addition, where the system is an AR system, the display may also transmit light from a surrounding environment to the user&#39;s eyes, to allow a view of the surrounding environment. As used herein, it will be appreciated that a “head-mounted” or “head mountable” display is a display that may be mounted on the head of the user or viewer. 
     To improve the usability of AR or VR systems (also referred to simply as “display systems”), it may be beneficial to reduce the size, weight, and/or power consumption of the display systems. As an example, a user may be more likely to utilize a display system if the size, and general obtrusiveness, of the display system is decreased. As another example, a user may be more likely to utilize a display system if the weight placed on the user&#39;s head is reduced. Similarly, reduced power consumption can allow the use of smaller batteries, reduce heat generated by the display system, and so on. Various embodiments described herein facilitate such benefits, including reductions in the size of parts of display systems. 
     As described herein, light forming virtual content (also referred to herein as image light) may be generated by one or more display technologies. For example, the light may be generated by a light projection system included in a display system. This light may then be routed via optics for output as virtual content to a user of the display system. The virtual content may be represented as image pixels included in rendered frames successively presented to the user. To achieve high-quality (e.g., lifelike) virtual content, the display system may render, and then output, frames of virtual content at a sufficient resolution (e.g., greater than a threshold resolution). Accordingly, the image pixels may be sufficiently close together to achieve the sufficient resolution. 
     However, it will be appreciated that design constraints associated with a display system may limit the ability to achieve such closeness in image pixels, and thus resolution. For example, to miniaturize a display system, the display system may be required to have a reduced display size (e.g., a projector size). An example display may include a liquid crystal on silicon (LCoS) display. To output image light forming virtual content, the LCoS display may be required to utilize a separate illumination module including one or more light emitters. In this example, an LCoS panel may impose spatially varying modulation on the generated light to form virtual content. However, to decrease a size associated with an LCoS panel while preserving a high resolution, the pixel pitch associated with the LCoS panel may need to be reduced. Pixel pitch, as described herein, may represent a physical distance on a display between similar locations on similar elements of the display forming image pixels. Due to physical constraints regarding small pixel pitches, coupled with the necessity of a separate illumination module, an LCoS display may be larger than desired in some applications. 
     Some embodiments disclosed herein advantageously include an emissive micro-display, such as a micro-LED display. In some embodiments, the micro-displays are micro-OLED displays. Display systems utilizing an emissive micro-display may avoid the added bulk of an illumination module. Additionally, an emissive micro-display may facilitate the presentation of images with an apparent advantageously small pixel pitch. As described, an example display system may utilize one or more an emissive micro-displays to achieve reduced size, weight, power consumption, among other benefits. 
     Emissive micro-displays have several advantages for use in wearable display systems. As an example, the power consumption of emissive micro-displays generally varies with image content, such that dim or sparse content requires less power to display. Since AR environments may often be sparse—since it may generally be desirable for the user to be able to see their surrounding environment—emissive micro-displays may have an average power consumption below that of other display technologies that use a spatial light modulator to modulate light from a light source. In contrast, other display technologies may utilize substantial power even for dim, sparse, or “all off”, virtual content. As another example, emissive micro-displays may offer an exceptionally high frame-rate (which may enable the use of a partial-resolution array) and may provide low levels of visually apparent motion artifacts (e.g., motion blur). As another example, emissive micro-displays may not require polarization optics of the type required by LCoS displays. Thus, emissive micro-displays may avoid the optical losses present in polarization optics. 
     While arrays of light emitters, such as micro-LEDs, may provide for substantial size, weight, and/or power savings, current light emitters may not provide for sufficiently small pixel pitch to enable high resolution virtual content in small display system form factors. As a non-limiting example, some micro-LED-based micro-displays may allow for a pixel pitch of about 2 to about 3 micron. Even at such pixel pitches, to provide a desired number of pixels, the micro-LED display may still be undesirably large for use in a wearable display system, particularly since a goal for such systems may be to have a form factor and size similar to that of eyeglasses. 
     As described in more detail, a light projection system including an emissive micro-display may achieve an effective small pixel pitch via rapid physical adjustment, or displacement, to parts of the light projection system. For example, the emissive micro-display may be physically adjusted in position, or displaced, along one or more axes. As another example, optical elements (e.g., projection optics) may be physically adjusted in position, or displaced, along one or more axes. 
     As described herein, a size associated with a light emitter, such as a micro-LED, may be referred to as emitter size. The emitter size may refer to a dimension of the light emitter along a particular axis (e.g., lateral axis). Emitter size may also refer to dimensions of the light emitter along two axes (e.g., lateral and longitudinal axes). Similarly, pixel pitch may refer to a distance between similar points on directly adjacent light emitters along a particular axis (e.g., lateral axis), with different axes having their own pixel pitch. For example, in some embodiments, the light emitters may be placed more closely along a first axis than along a second axis (e.g., an orthogonal axis). An example of an array of light emitters is described in more detail herein and illustrated in  FIG.  32 A . 
     It will be appreciated that the size of a light emitter may be less than the gap that separates directly neighboring light emitters. For example, due to physical and electrical constraints, it may be challenging to form an emissive micro-display with light emitters at greater than a threshold density. Example constraints may include current crowding, substrate droop, and so on. Thus, there may be substantial gaps or spaces between adjacent light emitters. The gap between two light emitters is referred to herein as an inter-emitter region. Inter-emitter regions, an example of which is illustrated in  FIG.  32 A , may therefore delineate an area of an emissive micro-display (e.g., a maximum area) which includes a single light emitter. The size of inter-emitter regions may therefore limit the extent to which an emissive micro-display may achieve certain high densities or high resolutions. 
     Advantageously, the ability to operate light emitters, such as micro-LEDs, at high speeds may allow time-multiplexed presentation of an image using the same one or more emissive micro-displays; for example, the geometric position of light emitters relative to projection optics may be shifted to allowed the same light emitters to present different pixels of the image at different times. In some embodiments, a rendered frame of virtual content may be presented as a series of subframes in rapid succession via time-multiplexing schemes. In this example, each subframe may be associated with a particular physical position of the light emitters relative to the projection optics. Thus, it will be appreciated that the geometric position may be varied by changing the locations of the light emitters and the projection optics relative to one another (e.g., by changing the physical position of light emitters while keeping the projection optics stationary, by changing the physical position of the projection optics while keeping the light emitters stationary, or by changing the physical positions of both the light emitters and the projection optics). As described in more detail below, the geometric positions may be adjusted (e.g., via one or more actuators) to cause the light emitters to tile respective inter-emitter regions. Thus, the emissive micro-display may achieve output of advantageously high resolution virtual content. 
     Thus, a light projection system may be configured to project individual full-resolution frames of virtual content by projecting one or more partial-resolution subframes. For example, one or more partial-resolution subframes may be projected. The subframes may be projected in rapid succession and may be offset from each-other (e.g., by a less than a full pixel pitch along one or more axes on which the subframes are translated). For example, an emissive micro-display included in the projector may be physically displaced along one or more axes. As described above, an emissive micro-display may include light emitters, such as micro-LEDs, having a pixel pitch. This pixel pitch may thus inform a resolution at which the emissive micro-display may output frames of virtual content. To effectively decrease the functional pixel pitch and gap between light emitters, and thus increase a resolution for a same size display, the light emitters may be adjusted in position, for example, by less than the pixel pitch. As an example, a display may include light emitters separated by a pixel pitch of 2.5 microns, and each light emitter may have an emitter size of 0.833 microns. In some embodiments, the light emitters may be adjusted in position a number of times based on the number of times that the light emitters may be translated to different (e.g., non-overlapping) positions within an inter-emitter region. In this example, the inter-emitter region may be 6.25 microns 2 , and an example light emitter may be adjusted in position three times along a first axis and three times along a second, orthogonal axis. Thus, the example light emitter may effectively assume 9 positions within the inter-emitter region. For one or more of the 9 positions, a particular subframe of a same rendered frame of virtual content may be presented. Thus, the successively presented subframes may be perceived as a high-resolution frame of virtual content. In effect, the light emitters may form images with a higher apparent pixel density than the physical density of the light emitters. 
     In some embodiments, the visual system of users may merge together the subframes such that users perceive the full-resolution frames. For example, the pixels of the subframes may be interwoven to form a full-resolution frame. Preferably, the subframes 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 subframes as being displayed at different times. In some embodiments, the different subframes are sequentially displayed at a rate equal to or higher than the flicker fusion threshold (e.g., equal to or higher than 60 Hz). 
     As a result, an emissive micro-display can be configured to have fewer light emitters than the number of image pixels contained in each full-resolution rendered frame of virtual content. For example, a full-resolution image could include 2000×2000 pixels, while the emissive micro-display may be an array of only 1000×1000 elements. The use of lower-resolution emissive micro-displays can be particularly beneficial for wearable systems such as the display systems described herein. As an example, a lower-resolution display may be smaller, weigh less, and/or consume less power that a higher-resolution display. 
     While the above has described moving or adjusting the position of an emissive micro-display (including, for example, micro-LED arrays), it will be appreciated that the position of projection optics may alternatively, or additionally, be adjusted. For example, and as will be described in more detail below with respect to  FIGS.  35 A- 35 B , the projection optics may route light generated via the emissive micro-display to a user of a display system. As an example, the projection optics may route light to input in-coupling optical elements (e.g., in-coupling gratings) of eyepieces configured to receive and direct light encoded with image information (image light) to the user. Thus, instead of physically translating the emissive micro-display, the projection optics may be translated along one or more axes. In moving, the projection optics may change a geometric position of each array along one or more axes prior to outputting light for in-coupling via the in-coupling gratings. As described herein, a light projection system may encompass one or more emissive micro-displays, projection optics, and so on. Thus, the light output of light projection system may be adjusted by the physical translation of parts of the system (e.g., by changing the locations of pixels presented by the light projection system). 
     It will be appreciated that certain portions of a rendered frame of virtual content may be more visually apparent to a user than other portions. For example, the user may have heightened visual acuity for portions of virtual content falling on the user&#39;s fovea (herein referred to as “foveal portions”). To determine the locations of these foveal portions, a display system may determine a fixation point at which a user is fixating. Portions of virtual content falling within a threshold angular distance of this fixation point may be identified as falling on a fovea of the user. As will be described, the display system may be configured to increase a resolution associated with foveal portions. The resolution of remaining portions may be increased less or not increased. 
     As an example, an emissive micro-display may be configured to update pixels included in a foveal portion at a greater rate than for pixels included in other portions. As described above, geometric positions of light emitters may be translated, or adjusted, to tile an inter-emitter region of the array. Optionally, light emitters utilized to output light forming pixels including a foveal region may be updated for a relatively high proportion of the different geometric positions (e.g., for each different geometric position), while light emitters for forming pixels away from the foveal region may be updated for a lower proportion of the different geometric positions (e.g., these light emitters may be “off” or may simply present the same information as in a previous position). Light emitters utilized to output light forming pixels included in other regions may be updated less. For example, these light emitters may be updated twice, or only once, for a given full-resolution rendered frame of virtual content. For example, the light emitters for a foveal region may be updated for each of four different geometric positions within inter-emitter regions, while light emitters corresponding to peripheral parts of an image may be updated only for every other geometric positions. 
     As a result, a rendered frame formed from rapidly displayed or projected subframes may have an effective resolution which varies across the rendered frame. With foveated imaging, the effective resolution of an emissive micro-display can be made high in foveal regions (e.g., regions of interest, regions in which a user is focused, regions designated by a user, regions designated by a designer, etc.) and can be made lower in other regions (e.g., outside regions of interest). Configuring the emissive micro-displays to provide foveated images may further help to converse resources, for example by eliminating and/or reducing processing and power loads associated with displaying or projecting fewer interesting regions (e.g., regions unlikely to be the focus of users&#39; attentions). 
     Example Display Systems with Emissive Micro-Displays 
     Advantageously, as noted herein, display systems utilizing emissive micro-displays as described herein may allow for a low-weight and compact form factor, and may also provide a high frame rate and low motion blur. Preferably, the micro-displays are emissive micro-displays, which provide advantages for high brightness and high pixel density. In some embodiments, the emissive micro-displays are micro-LED displays. In some other embodiments, the emissive 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, 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. In some embodiments, the ratio of emitter size to pitch is 1:1 to 1:5, 1:2 to 1:4, or 1:2 to 1:3, which may have advantages for individual control of emitters and efficient utilization of emitted light by eyepieces, as discussed further herein. 
     In some embodiments, a plurality of emissive micro-displays may be utilized to form images for a head-mounted display system. The light containing the image information for forming these images may be referred to as image light. It will be appreciated that image light may vary in, e.g., wavelength, intensity, polarization, etc. The emissive micro-displays output image light to an eyepiece, which then relays the light to an eye of the user. 
     In some embodiments, the plurality of emissive micro-displays may be utilized and positioned at different sides of an optical combiner, e.g., an X-cube prism or dichroic X-cube. The X-cube prism receives light rays from different micro-displays on different faces of the cube and outputs the light rays from the same face of the cube. The outputted light may be directed towards projection optics, which is configured to converge or focus the image light onto the eyepiece. 
     In some embodiments, the plurality of emissive micro-displays comprises monochrome micro-displays, which are configured to output light of a single component color. Combining various component colors forms a full color image. In some other embodiments, one or more of the emissive micro-displays may have sub-pixels configured to emit light of two or more, but not all, component colors utilized by the display system. For example, a single emissive micro-display may have sub-pixels which emit light of the colors blue and green, while a separate emissive micro-display on a different face of the X-cube may have pixels configured to emit red light. In some embodiments, the plurality of micro-displays are each full-color displays comprising, e.g., pixels formed of multiple sub-pixels configured to emit light of different component colors. Advantageously, combining the light of multiple full-color micro-displays may increase display brightness and dynamic range. 
     It will be appreciated that the emissive micro-displays may comprise arrays of light emitters. The light emitters may emit light with a Lambertian angular emission profile. Undesirably, such an angular remission profile may “waste” light, since only a small portion of the emitted light may ultimately be incident on the eyepiece. In some embodiments, light collimators may be utilized to narrow the angular emission profile of light emitted by the light emitters. As used herein, a light collimator is an optical structure which narrows the angular emission profile of incident light; that is, the light collimator receives light from an associated light emitter 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. In some embodiments, the rays of light exiting the light collimator are more parallel than the rays of light received by the light collimator, before being transmitted through and exiting the collimator. Examples of light collimators include micro-lenses, nano-lenses, reflective wells, metasurfaces, and liquid crystal gratings. In some embodiments, the light collimators may be configured to steer light to ultimately converge on different laterally-shifted light-coupling optical elements. In some embodiments, each light emitter has a dedicated light collimator. The light collimators are preferably positioned directly adjacent or contacting the light emitters, to capture a large proportion of the light emitted by the associated light emitters. 
     In some embodiments, a single emissive micro-display may be utilized to direct light to the eyepiece. For example, the single emissive micro-display may be a full-color display comprising light emitters that emit light of different component colors. In some embodiments, the light emitters may form groups, which are localized in a common area, with each group comprising light emitters which emit light of each component color. In such embodiments, each group of light emitters may share a common micro-lens. Advantageously, light of different colors from different light emitters take a different path through the micro-lens, which may be manifested in light of different component colors being incident on different in-coupling optical elements of an eyepiece, as discussed herein. 
     In some embodiments, the full-color micro-display may comprise repeating groups of light emitters of the same component color. For instance, the micro-display may include rows of light emitters, with the light emitters of each individual row configured to emit light of the same color. Thus, different rows may emit light of different component colors. In addition, the micro-display may have an associated array of light collimators configured to direct light to a desired location on an eyepiece, e.g., to an associated in-coupling optical element. Advantageously, while the individual light emitters of such a full-color micro-display may not be positioned to form a high-quality full-color image, as viewed directly on the micro-display, the lens array appropriately steers the light from the light emitters to the eyepiece, which combines monochrome images formed by light emitters of different colors, thereby forming a high-quality full-color image. 
     In some embodiments, the eyepiece receiving image light from the micro-displays may comprise a waveguide assembly. The area of a waveguide of the waveguide assembly on which the image light is incident may include in-coupling optical elements which in-couple incident image light, such that the light propagates through the waveguide by total internal reflection (TIR). In some embodiments, the waveguide assembly may include a stack of waveguides, each of which has an associated in-coupling optical element. Different in-coupling optical elements may be configured to in-couple light of different colors, such that different waveguides may be configured to propagate light of different colors therein. The waveguides may include out-coupling optical elements, which out-couple light propagating therein, such that the out-coupled light propagates towards the eye of the user. In some other embodiments, the waveguide assembly may include a single waveguide having an associated in-coupling optical element configured to in-couple light of different component colors. 
     In some embodiments, the in-coupling optical elements are laterally shifted, as seen from the projection optics. Different in-coupling optical elements may be configured to in-couple light of different colors. Preferably, image light of different colors take different paths to the eyepiece and, thus, impinge upon different corresponding in-coupling optical elements. 
     In some other embodiments, other types of eyepieces or optics for relaying image light to the eyes of the user may be utilized. For example, as discussed herein, the eyepiece may include one or more waveguides which propagates image light therein by TIR. As another example, the eyepiece may include a birdbath combiner comprising a semitransparent mirror that both directs image light to a viewer and allows a view of the ambient environment. 
     In some embodiments, the eyepiece may be configured to selectively output light with different amounts of wavefront divergence, to provide virtual content at a plurality of virtual depth planes (also referred to simply as “depth planes” herein) perceived to be at different distances away from the user. For example, the eyepiece may comprise a plurality of waveguides each having out-coupling optical elements with different optical power to output light with different amounts of wavefront divergence. In some other embodiments, a variable focus element may be provided between the eyepiece and the user&#39;s eye. The variable focus element may be configured to dynamically change optical power to provide the desired wavefront divergence for particular virtual content. In some embodiments, as an alternative to, or in addition to waveguide optical structures for providing optical power, the display systems may also include a plurality of lenses that provide or additionally provide optical powers. 
     In addition to the compact form factor and high frame rates discussed above, emissive micro-displays according to some embodiments may provide one of more of the following advantages. For example, the micro-displays may provide exceptionally small pixel pitches and high pixel density. The micro-displays may also provide high luminance and efficiency. For example, the light emitters of the emissive micro-displays may only consume power to emit light when the light emitters are needed provide content with luminance. This is in contrast to other display technologies in which the light source may illuminate an entire panel of pixels, whether or not some of those pixels are dark. Further, it will be appreciated that the human visual system integrates received light over time and the light emitters of emissive micro-displays, such as micro-LEDs, have advantageously high duty cycles (e.g., including a short activation period for a light emitter in a micro-display to rise from an “off” to a full “on” state, and a correspondingly short time to fall from an “on” state to “off” state allow the light emitters to emit light at the on level for a large percentage of each cycle). As a result, the power used to generate an image with a given perceived brightness may be less as compared to conventional display technologies with lower duty cycles. In some embodiments, the duty cycle may be 70% or more, 80% or more, or 90% or more. In some embodiments, the duty cycle may be about 99%. In addition, as noted herein, micro-displays may facilitate exceptionally high frame rates, which may provide advantages including reducing mismatches between the position of a user&#39;s head and the displayed content. 
     Reference will now be made to the drawings, in which like reference numerals refer to like parts throughout. Unless indicated otherwise, the drawings are schematic and not necessarily drawn to scale. 
       FIG.  2    illustrates a conventional display system for simulating three-dimensional imagery for a user. It will be appreciated that a user&#39;s eyes are spaced apart and that, when looking at a real object in space, each eye will have a slightly different view of the object and may form an image of the object at different locations on the retina of each eye. This may be referred to as binocular disparity and may be utilized by the human visual system to provide a perception of depth. Conventional display systems simulate binocular disparity by presenting two distinct images  190 ,  200  with slightly different views of the same virtual object—one for each eye  210 ,  220 —corresponding to the views of the virtual object that would be seen by each eye were the virtual object a real object at a desired depth. These images provide binocular cues that the user&#39;s visual system may interpret to derive a perception of depth. 
     With continued reference to  FIG.  2   , the images  190 ,  200  are spaced from the eyes  210 ,  220  by a distance  230  on a z-axis. The z-axis is parallel to the optical axis of the viewer with their eyes fixated on an object at optical infinity directly ahead of the viewer. The images  190 ,  200  are flat and at a fixed distance from the eyes  210 ,  220 . Based on the slightly different views of a virtual object in the images presented to the eyes  210 ,  220 , respectively, the eyes may naturally rotate such that an image of the object falls on corresponding points on the retinas of each of the eyes, to maintain single binocular vision. This rotation may cause the lines of sight of each of the eyes  210 ,  220  to converge onto a point in space at which the virtual object is perceived to be present. As a result, providing three-dimensional imagery conventionally involves providing binocular cues that may manipulate the vergence of the user&#39;s eyes  210 ,  220 , and that the human visual system interprets to provide a perception of depth. 
     Generating a realistic and comfortable perception of depth is challenging, however. It will be appreciated that light from objects at different distances from the eyes have wavefronts with different amounts of divergence.  FIGS.  3 A- 3 C  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.  3 A- 3 C , the light rays become more divergent as distance to the object decreases. Conversely, 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 . While only a single eye  210  is illustrated for clarity of illustration in  FIGS.  3 A- 3 C  and other figures herein, the discussions regarding eye  210  may be applied to both eyes  210  and  220  of a viewer. 
     With continued reference to  FIGS.  3 A- 3 C , light from an object that the viewer&#39;s eyes are fixated on may have different degrees of wavefront divergence. Due to the different amounts of wavefront divergence, the light may be focused differently by the lens of the eye, which in turn may require the lens to assume different shapes to form a focused image on the retina of the eye. Where a focused image is not formed on the retina, the resulting retinal blur acts as a cue to accommodation that causes a change in the shape of the lens of the eye until a focused image is formed on the retina. For example, the cue to accommodation may trigger the ciliary muscles surrounding the lens of the eye to relax or contract, thereby modulating the force applied to the suspensory ligaments holding the lens, thus causing the shape of the lens of the eye to change until retinal blur of an object of fixation is eliminated or minimized, thereby forming a focused image of the object of fixation on the retina (e.g., fovea) of the eye. The process by which the lens of the eye changes shape may be referred to as accommodation, and the shape of the lens of the eye required to form a focused image of the object of fixation on the retina (e.g., fovea) of the eye may be referred to as an accommodative state. 
     With reference now to  FIG.  4 A , a representation of the accommodation-vergence response of the human visual system is illustrated. The movement of the eyes to fixate on an object causes the eyes to receive light from the object, with the light forming an image on each of the retinas of the eyes. The presence of retinal blur in the image formed on the retina may provide a cue to accommodation, and the relative locations of the image on the retinas may provide a cue to vergence. The cue to accommodation causes accommodation to occur, resulting in the lenses of the eyes each assuming a particular accommodative state that forms a focused image of the object on the retina (e.g., fovea) of the eye. On the other hand, the cue to vergence causes vergence movements (rotation of the eyes) to occur such that the images formed on each retina of each eye are at corresponding retinal points that maintain single binocular vision. In these positions, the eyes may be said to have assumed a particular vergence state. With continued reference to  FIG.  4 A , accommodation may be understood to be the process by which the eye achieves a particular accommodative state, and vergence may be understood to be the process by which the eye achieves a particular vergence state. As indicated in  FIG.  4 A , the accommodative and vergence states of the eyes may change if the user fixates on another object. For example, the accommodated state may change if the user fixates on a new object at a different depth on the z-axis. 
     Without being limited by theory, it is believed that viewers of an object may perceive the object as being “three-dimensional” due to a combination of vergence and accommodation. As noted above, vergence movements (e.g., 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 accommodation of the lenses of the eyes. Under normal conditions, changing the shapes of the lenses of the eyes to change focus 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, a change in vergence will trigger a matching change in lens shape under normal conditions. 
     With reference now to  FIG.  4 B , examples of different accommodative and vergence states of the eyes are illustrated. The pair of eyes  222   a  is fixated on an object at optical infinity, while the pair eyes  222   b  are fixated on an object  221  at less than optical infinity. Notably, the vergence states of each pair of eyes is different, with the pair of eyes  222   a  directed straight ahead, while the pair of eyes  222  converge on the object  221 . The accommodative states of the eyes forming each pair of eyes  222   a  and  222   b  are also different, as represented by the different shapes of the lenses  210   a ,  220   a.    
     Undesirably, many users of conventional “3-D” display systems find such conventional systems to be uncomfortable or may not perceive a sense of depth at all due to a mismatch between accommodative and vergence states in these displays. As noted above, many stereoscopic or “3-D” display systems display a scene by providing slightly different images to each eye. Such systems are uncomfortable for many viewers, since they, among other things, simply provide different presentations of a scene and cause changes in the vergence states of the eyes, but without a corresponding change in the accommodative states of those eyes. Rather, the images are shown by a display at a fixed distance from the eyes, such that the eyes view all the image information at a single accommodative state. Such an arrangement works against the “accommodation-vergence reflex” by causing changes in the vergence state without a matching change in the accommodative state. This mismatch is believed to cause viewer discomfort. Display systems that provide a better match between accommodation and vergence may form more realistic and comfortable simulations of three-dimensional imagery. 
     Without being limited by theory, it is believed that the human eye typically may 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 numbers of depth planes. In some embodiments, the different presentations may provide both cues to vergence and matching cues to accommodation, thereby providing physiologically correct accommodation-vergence matching. 
     With continued reference to  FIG.  4 B , two depth planes  240 , corresponding to different distances in space from the eyes  210 ,  220 , are illustrated. For a given depth plane  240 , vergence cues may be provided by the displaying of images of appropriately different perspectives for each eye  210 ,  220 . In addition, for a given depth plane  240 , light forming the images provided to each eye  210 ,  220  may have a wavefront divergence corresponding to a light field produced by a point at the distance of that depth plane  240 . 
     In the illustrated embodiment, the distance, along the z-axis, of the depth plane  240  containing the point  221  is 1 m. As used herein, distances or depths along the z-axis may be measured with a zero-point located at the exit pupils of the user&#39;s eyes. Thus, a depth plane  240  located at a depth of 1 m corresponds to a distance of 1 m away from the exit pupils of the user&#39;s eyes, on the optical axis of those eyes with the eyes directed towards optical infinity. As an approximation, the depth or distance along the z-axis may be measured from the display in front of the user&#39;s eyes (e.g., from the surface of a waveguide), plus a value for the distance between the device and the exit pupils of the user&#39;s eyes. That value may be called the eye relief and corresponds to the distance between the exit pupil of the user&#39;s eye and the display worn by the user in front of the eye. In practice, the value for the eye relief may be a normalized value used generally for all viewers. For example, the eye relief may be assumed to be 20 mm and a depth plane that is at a depth of 1 m may be at a distance of 980 mm in front of the display. 
     With reference now to  FIGS.  4 C and  4 D , examples of matched accommodation-vergence distances and mismatched accommodation-vergence distances are illustrated, respectively. As illustrated in  FIG.  4 C , the display system may provide images of a virtual object to each eye  210 ,  220 . The images may cause the eyes  210 ,  220  to assume a vergence state in which the eyes converge on a point  15  on a depth plane  240 . In addition, the images may be formed by a light having a wavefront curvature corresponding to real objects at that depth plane  240 . As a result, the eyes  210 ,  220  assume an accommodative state in which the images are in focus on the retinas of those eyes. Thus, the user may perceive the virtual object as being at the point  15  on the depth plane  240 . 
     It will be appreciated that each of the accommodative and vergence states of the eyes  210 ,  220  are associated with a particular distance on the z-axis. For example, an object at a particular distance from the eyes  210 ,  220  causes those eyes to assume particular accommodative states based upon the distances of the object. The distance associated with a particular accommodative state may be referred to as the accommodation distance, A d . Similarly, there are particular vergence distances, V d , associated with the eyes in particular vergence states, or positions relative to one another. Where the accommodation distance and the vergence distance match, the relationship between accommodation and vergence may be said to be physiologically correct. This is considered to be the most comfortable scenario for a viewer. 
     In stereoscopic displays, however, the accommodation distance and the vergence distance may not always match. For example, as illustrated in  FIG.  4 D , images displayed to the eyes  210 ,  220  may be displayed with wavefront divergence corresponding to depth plane  240 , and the eyes  210 ,  220  may assume a particular accommodative state in which the points  15   a ,  15   b  on that depth plane are in focus. However, the images displayed to the eyes  210 ,  220  may provide cues for vergence that cause the eyes  210 ,  220  to converge on a point  15  that is not located on the depth plane  240 . As a result, the accommodation distance corresponds to the distance from the exit pupils of the eyes  210 ,  220  to the depth plane  240 , while the vergence distance corresponds to the larger distance from the exit pupils of the eyes  210 ,  220  to the point  15 , in some embodiments. The accommodation distance is different from the vergence distance. Consequently, there is an accommodation-vergence mismatch. Such a mismatch is considered undesirable and may cause discomfort in the user. It will be appreciated that the mismatch corresponds to distance (e.g., V d −A d ) and may be characterized using diopters. 
     In some embodiments, it will be appreciated that a reference point other than exit pupils of the eyes  210 ,  220  may be utilized for determining distance for determining accommodation-vergence mismatch, so long as the same reference point is utilized for the accommodation distance and the vergence distance. For example, the distances could be measured from the cornea to the depth plane, from the retina to the depth plane, from the eyepiece (e.g., a waveguide of the display device) to the depth plane, and so on. 
     Without being limited by theory, it is believed that users may still perceive accommodation-vergence mismatches of up to about 0.25 diopter, up to about 0.33 diopter, and up to about 0.5 diopter as being physiologically correct, without the mismatch itself causing significant discomfort. In some embodiments, display systems disclosed herein (e.g., the display system  250 ,  FIG.  6   ) present images to the viewer having accommodation-vergence mismatch of about 0.5 diopter or less. In some other embodiments, the accommodation-vergence mismatch of the images provided by the display system is about 0.33 diopter or less. In yet other embodiments, the accommodation-vergence mismatch of the images provided by the display system is about 0.25 diopter or less, including about 0.1 diopter or less. 
       FIG.  5    illustrates aspects of an approach for simulating three-dimensional imagery by modifying wavefront divergence. The display system includes a waveguide  270  that is configured to receive light  770  that is encoded with image information, and to output that light to the user&#39;s eye  210 . The waveguide  270  may output the light  650  with a defined amount of wavefront divergence corresponding to the wavefront divergence of a light field produced by a point on a desired depth plane  240 . In some embodiments, the same amount of wavefront divergence is provided for all objects presented on that depth plane. In addition, it will be illustrated that the other eye of the user may be provided with image information from a similar waveguide. 
     In some embodiments, a single waveguide may be configured to output light with a set amount of wavefront divergence corresponding to a single or limited number of depth planes and/or the waveguide may be configured to output light of a limited range of wavelengths. Consequently, in some embodiments, a plurality or stack of waveguides may be utilized to provide different amounts of wavefront divergence for different depth planes and/or to output light of different ranges of wavelengths. As used herein, it will be appreciated at a depth plane may be planar or may follow the contours of a curved surface. 
       FIG.  6    illustrates an example of a waveguide stack for outputting image information to a user. A display system  250  includes a stack of waveguides, or stacked waveguide assembly,  260  that may be utilized to provide three-dimensional perception to the eye/brain using a plurality of waveguides  270 ,  280 ,  290 ,  300 ,  310 . It will be appreciated that the display system  250  may be considered a light field display in some embodiments. In addition, the waveguide assembly  260  may also be referred to as an eyepiece. 
     In some embodiments, the display system  250  may be configured to provide substantially continuous cues to vergence and multiple discrete cues to accommodation. The cues to vergence may be provided by displaying different images to each of the eyes of the user, and the cues to accommodation may be provided by outputting the light that forms the images with selectable discrete amounts of wavefront divergence. Stated another way, the display system  250  may be configured to output light with variable levels of wavefront divergence. In some embodiments, each discrete level of wavefront divergence corresponds to a particular depth plane and may be provided by a particular one of the waveguides  270 ,  280 ,  290 ,  300 ,  310 . 
     With continued reference to  FIG.  6   , the waveguide assembly  260  may also include a plurality of features  320 ,  330 ,  340 ,  350  between the waveguides. In some embodiments, the features  320 ,  330 ,  340 ,  350  may be one or more lenses. The waveguides  270 ,  280 ,  290 ,  300 ,  310  and/or 
     the plurality of lenses  320 ,  330 ,  340 ,  350  may be configured to send image information to the eye with various levels of wavefront curvature or light ray divergence. Each waveguide level may be associated with a particular depth plane and may be configured to output image information corresponding to that depth plane. Image injection devices  360 ,  370 ,  380 ,  390 ,  400  may function as a source of light for the waveguides and may be utilized to inject image information into the waveguides  270 ,  280 ,  290 ,  300 ,  310 , each of which may be configured, as described herein, to distribute incoming light across each respective waveguide, for output toward the eye  210 . Light exits an output surface  410 ,  420 ,  430 ,  440 ,  450  of the image injection devices  360 ,  370 ,  380 ,  390 ,  400  and is injected into a corresponding input surface  460 ,  470 ,  480 ,  490 ,  500  of the waveguides  270 ,  280 ,  290 ,  300 ,  310 . In some embodiments, each of the input surfaces  460 ,  470 ,  480 ,  490 ,  500  may be an edge of a corresponding waveguide, or may be part of a major surface of the corresponding waveguide (that is, one of the waveguide surfaces directly facing the world  510  or the viewer&#39;s eye  210 ). In some embodiments, a single beam of light (e.g. a collimated beam) may be injected into each waveguide to output an entire field of cloned collimated beams that are directed toward the eye  210  at particular angles (and amounts of divergence) corresponding to the depth plane associated with a particular waveguide. In some embodiments, a single one of the image injection devices  360 ,  370 ,  380 ,  390 ,  400  may be associated with and inject light into a plurality (e.g., three) of the waveguides  270 ,  280 ,  290 ,  300 ,  310 . 
     In some embodiments, the image injection devices  360 ,  370 ,  380 ,  390 ,  400  are discrete displays that each produce image information for injection into a corresponding waveguide  270 ,  280 ,  290 ,  300 ,  310 , respectively. In some other embodiments, the image injection devices  360 ,  370 ,  380 ,  390 ,  400  are the output ends of a single multiplexed display which may, e.g., pipe image information via one or more optical conduits (such as fiber optic cables) to each of the image injection devices  360 ,  370 ,  380 ,  390 ,  400 . It will be appreciated that the image information provided by the image injection devices  360 ,  370 ,  380 ,  390 ,  400  may include light of different wavelengths, or colors (e.g., different component colors, as discussed herein). 
     In some embodiments, the light injected into the waveguides  270 ,  280 ,  290 ,  300 ,  310  is provided by a light projection system  520 , which comprises a light module  530 , which may include a light emitter, such as a light emitting diode (LED). The light from the light module  530  may be directed to and modified by a light modulator  540 , e.g., a spatial light modulator, via a beam splitter  550 . The light modulator  540  may be configured to change the perceived intensity of the light injected into the waveguides  270 ,  280 ,  290 ,  300 ,  310  to encode the light with image information. Examples of spatial light modulators include liquid crystal displays (LCD) including a liquid crystal on silicon (LCOS) displays. In some other embodiments, the spatial light modulator may be a MEMS device, such as a digital light processing (DLP) device. It will be appreciated that the image injection devices  360 ,  370 ,  380 ,  390 ,  400  are illustrated schematically and, in some embodiments, these image injection devices may represent different light paths and locations in a common projection system configured to output light into associated ones of the waveguides  270 ,  280 ,  290 ,  300 ,  310 . In some embodiments, the waveguides of the waveguide assembly  260  may function as ideal lens while relaying light injected into the waveguides out to the user&#39;s eyes. In this conception, the object may be the spatial light modulator  540  and the image may be the image on the depth plane. 
     In some embodiments, the display system  250  may be a scanning fiber display comprising one or more scanning fibers configured to project light in various patterns (e.g., raster scan, spiral scan, Lissajous patterns, etc.) into one or more waveguides  270 ,  280 ,  290 ,  300 ,  310  and ultimately to the eye  210  of the viewer. In some embodiments, the illustrated image injection devices  360 ,  370 ,  380 ,  390 ,  400  may schematically represent a single scanning fiber or a bundle of scanning fibers configured to inject light into one or a plurality of the waveguides  270 ,  280 ,  290 ,  300 ,  310 . In some other embodiments, the illustrated image injection devices  360 ,  370 ,  380 ,  390 ,  400  may schematically represent a plurality of scanning fibers or a plurality of bundles of scanning fibers, each of which are configured to inject light into an associated one of the waveguides  270 ,  280 ,  290 ,  300 ,  310 . It will be appreciated that one or more optical fibers may be configured to transmit light from the light module  530  to the one or more waveguides  270 ,  280 ,  290 ,  300 ,  310 . It will be appreciated that one or more intervening optical structures may be provided between the scanning fiber, or fibers, and the one or more waveguides  270 ,  280 ,  290 ,  300 ,  310  to, e.g., redirect light exiting the scanning fiber into the one or more waveguides  270 ,  280 ,  290 ,  300 ,  310 . 
     A controller  560  controls the operation of one or more of the stacked waveguide assembly  260 , including operation of the image injection devices  360 ,  370 ,  380 ,  390 ,  400 , the light source  530 , and the light modulator  540 . In some embodiments, the controller  560  is part of the local data processing module  140 . The controller  560  includes programming (e.g., instructions in a non-transitory medium) that regulates the timing and provision of image information to the waveguides  270 ,  280 ,  290 ,  300 ,  310  according to, e.g., any of the various schemes disclosed herein. In some embodiments, the controller may be a single integral device, or a distributed system connected by wired or wireless communication channels. The controller  560  may be part of the processing modules  140  or  150  ( FIG.  9 E ) in some embodiments. 
     With continued reference to  FIG.  6   , the waveguides  270 ,  280 ,  290 ,  300 ,  310  may be configured to propagate light within each respective waveguide by total internal reflection (TIR). The waveguides  270 ,  280 ,  290 ,  300 ,  310  may each be planar or have another shape (e.g., curved), with major top and bottom surfaces and edges extending between those major top and bottom surfaces. In the illustrated configuration, the waveguides  270 ,  280 ,  290 ,  300 ,  310  may each include out-coupling optical elements  570 ,  580 ,  590 ,  600 ,  610  that are configured to extract light out of a waveguide by redirecting the light, propagating within each respective waveguide, out of the waveguide to output image information to the eye  210 . Extracted light may also be referred to as out-coupled light and the out-coupling optical elements light may also be referred to light extracting optical elements. An extracted beam of light may be outputted by the waveguide at locations at which the light propagating in the waveguide strikes a light extracting optical element. The out-coupling optical elements  570 ,  580 ,  590 ,  600 ,  610  may, for example, be gratings, including diffractive optical features, as discussed further herein. While illustrated disposed at the bottom major surfaces of the waveguides  270 ,  280 ,  290 ,  300 ,  310 , for ease of description and drawing clarity, in some embodiments, the out-coupling optical elements  570 ,  580 ,  590 ,  600 ,  610  may be disposed at the top and/or bottom major surfaces, and/or may be disposed directly in the volume of the waveguides  270 ,  280 ,  290 ,  300 ,  310 , as discussed further herein. In some embodiments, the out-coupling optical elements  570 ,  580 ,  590 ,  600 ,  610  may be formed in a layer of material that is attached to a transparent substrate to form the waveguides  270 ,  280 ,  290 ,  300 ,  310 . In some other embodiments, the waveguides  270 ,  280 ,  290 ,  300 ,  310  may be a monolithic piece of material and the out-coupling optical elements  570 ,  580 ,  590 ,  600 ,  610  may be formed on a surface and/or in the interior of that piece of material. 
     With continued reference to  FIG.  6   , as discussed herein, each waveguide  270 ,  280 ,  290 ,  300 ,  310  is configured to output light to form an image corresponding to a particular depth plane. For example, the waveguide  270  nearest the eye may be configured to deliver collimated light (which was injected into such waveguide  270 ), to the eye  210 . The collimated light may be representative of the optical infinity focal plane. The next waveguide up  280  may be configured to send out collimated light which passes through the first lens  350  (e.g., a negative lens) before it may reach the eye  210 ; such first lens  350  may be configured to create a slight convex wavefront curvature so that the eye/brain interprets light coming from that next waveguide up  280  as coming from a first focal plane closer inward toward the eye  210  from optical infinity. Similarly, the third up waveguide  290  passes its output light through both the first  350  and second  340  lenses before reaching the eye  210 ; the combined optical power of the first  350  and second  340  lenses may be configured to create another incremental amount of wavefront curvature so that the eye/brain interprets light coming from the third waveguide  290  as coming from a second focal plane that is even closer inward toward the person from optical infinity than was light from the next waveguide up  280 . 
     The other waveguide layers  300 ,  310  and lenses  330 ,  320  are similarly configured, with the highest waveguide  310  in the stack sending its output through all of the lenses between it and the eye for an aggregate focal power representative of the closest focal plane to the person. To compensate for the stack of lenses  320 ,  330 ,  340 ,  350  when viewing/interpreting light coming from the world  510  on the other side of the stacked waveguide assembly  260 , a compensating lens layer  620  may be disposed at the top of the stack to compensate for the aggregate power of the lens stack  320 ,  330 ,  340 ,  350  below. Such a configuration provides as many perceived focal planes as there are available waveguide/lens pairings. Both the out-coupling optical elements of the waveguides and the focusing aspects of the lenses may be static (i.e., not dynamic or electro-active). In some alternative embodiments, either or both may be dynamic using electro-active features. 
     In some embodiments, two or more of the waveguides  270 ,  280 ,  290 ,  300 ,  310  may have the same associated depth plane. For example, multiple waveguides  270 ,  280 ,  290 ,  300 ,  310  may be configured to output images set to the same depth plane, or multiple subsets of the waveguides  270 ,  280 ,  290 ,  300 ,  310  may be configured to output images set to the same plurality of depth planes, with one set for each depth plane. This may provide advantages for forming a tiled image to provide an expanded field of view at those depth planes. 
     With continued reference to  FIG.  6   , the out-coupling optical elements  570 ,  580 ,  590 ,  600 ,  610  may be configured to both redirect light out of their respective waveguides and to output this light with the appropriate amount of divergence or collimation for a particular depth plane associated with the waveguide. As a result, waveguides having different associated depth planes may have different configurations of out-coupling optical elements  570 ,  580 ,  590 ,  600 ,  610 , which output light with a different amount of divergence depending on the associated depth plane. In some embodiments, the light extracting optical elements  570 ,  580 ,  590 ,  600 ,  610  may be volumetric or surface features, which may be configured to output light at specific angles. For example, the light extracting optical elements  570 ,  580 ,  590 ,  600 ,  610  may be volume holograms, surface holograms, and/or diffraction gratings. In some embodiments, the features  320 ,  330 ,  340 ,  350  may not be lenses; rather, they may simply be spacers (e.g., cladding layers and/or structures for forming air gaps). 
     In some embodiments, the out-coupling optical elements  570 ,  580 ,  590 ,  600 ,  610  are diffractive features that form a diffraction pattern, or “diffractive optical element” (also referred to herein as a “DOE”). Preferably, the DOE&#39;s have a sufficiently low diffraction efficiency so that only a portion of the light of the beam is deflected away toward the eye  210  with each intersection of the DOE, while the rest continues to move through a waveguide via TIR. The light carrying the image information is thus divided into a number of related exit beams that exit the waveguide at a multiplicity of locations and the result is a fairly uniform pattern of exit emission toward the eye  210  for this particular collimated beam bouncing around within a waveguide. 
     In some embodiments, one or more DOEs may be switchable between “on” states in which they actively diffract, and “off” states in which they do not significantly diffract. For instance, a switchable DOE may comprise a layer of polymer dispersed liquid crystal, in which microdroplets comprise a diffraction pattern in a host medium, and the refractive index of the microdroplets may be switched to substantially match the refractive index of the host material (in which case the pattern does not appreciably diffract incident light) or the microdroplet may be switched to an index that does not match that of the host medium (in which case the pattern actively diffracts incident light). 
     In some embodiments, a camera assembly  630  (e.g., a digital camera, including visible light and infrared light cameras) may be provided to capture images of the eye  210  and/or tissue around the eye  210  to, e.g., detect user inputs and/or to monitor the physiological state of the user. As used herein, a camera may be any image capture device. In some embodiments, the camera assembly  630  may include an image capture device and a light source to project light (e.g., infrared light) to the eye, which may then be reflected by the eye and detected by the image capture device. In some embodiments, the camera assembly  630  may be attached to the frame or support structure  80  ( FIG.  9 E ) and may be in electrical communication with the processing modules  140  and/or  150 , which may process image information from the camera assembly  630 . In some embodiments, one camera assembly  630  may be utilized for each eye, to separately monitor each eye. 
     The camera assembly  630  may, in some embodiments, observe movements of the user, such as the user&#39;s eye movements. As an example, the camera assembly  630  may capture images of the eye  210  to determine the size, position, and/or orientation of the pupil of the eye  210  (or some other structure of the eye  210 ). The camera assembly  630  may, if desired, obtain images (processed by processing circuitry of the type described herein) used to determine the direction the user is looking (e.g., eye pose or gaze direction). In some embodiments, camera assembly  630  may include multiple cameras, at least one of which may be utilized for each eye, to separately determine the eye pose or gaze direction of each eye independently. The camera assembly  630  may, in some embodiments and in combination with processing circuitry such as the controller  560  or the local data processing module  140 , determine eye pose or gaze direction based on glints (e.g., reflections) of reflected light (e.g., infrared light) from a light source included in camera assembly  630 . 
     With reference now to  FIG.  7   , an example of exit beams outputted by a waveguide is shown. One waveguide is illustrated, but it will be appreciated that other waveguides in the waveguide assembly  260  ( FIG.  6   ) may function similarly, where the waveguide assembly  260  includes multiple waveguides. Light  640  is injected into the waveguide  270  at the input surface  460  of the waveguide  270  and propagates within the waveguide  270  by TIR. At points where the light  640  impinges on the DOE  570 , a portion of the light exits the waveguide as exit beams  650 . The exit beams  650  are illustrated as substantially parallel but, as discussed herein, they may also be redirected to propagate to the eye  210  at an angle (e.g., forming divergent exit beams), depending on the depth plane associated with the waveguide  270 . It will be appreciated that substantially parallel exit beams may be indicative of a waveguide with out-coupling optical elements that out-couple light to form images that appear to be set on a depth plane at a large distance (e.g., optical infinity) from the eye  210 . Other waveguides or other sets of out-coupling optical elements may output an exit beam pattern that is more divergent, which would require the eye  210  to accommodate to a closer distance to bring it into focus on the retina and would be interpreted by the brain as light from a distance closer to the eye  210  than optical infinity. 
     In some embodiments, a full color image may be formed at each depth plane by overlaying images in each of the component colors, e.g., three or more component colors.  FIG.  8    illustrates an example of a stacked waveguide assembly in which each depth plane includes images formed using multiple different component colors. The illustrated embodiment shows depth planes  240   a - 240   f , although more or fewer depths are also contemplated. Each depth plane may have three or more component color images associated with it, including: a first image of a first color, G; a second image of a second color, R; and a third image of a third color, B. Different depth planes are indicated in the figure by different numbers for diopters (dpt) following the letters G, R, and B. Just as examples, the numbers following each of these letters indicate diopters (1/m), or inverse distance of the depth plane from a viewer, and each box in the figures represents an individual component color image. In some embodiments, to account for differences in the eye&#39;s focusing of light of different wavelengths, the exact placement of the depth planes for different component colors may vary. For example, different component color images for a given depth plane may be placed on depth planes corresponding to different distances from the user. Such an arrangement may increase visual acuity and user comfort and/or may decrease chromatic aberrations. 
     In some embodiments, light of each component color may be outputted by a single dedicated waveguide and, consequently, each depth plane may have multiple waveguides associated with it. In such embodiments, each box in the figures including the letters G, R, or B may be understood to represent an individual waveguide, and three waveguides may be provided per depth plane where three component color images are provided per depth plane. While the waveguides associated with each depth plane are shown adjacent to one another in this drawing for ease of description, it will be appreciated that, in a physical device, the waveguides may all be arranged in a stack with one waveguide per level. In some other embodiments, multiple component colors may be outputted by the same waveguide, such that, e.g., only a single waveguide may be provided per depth plane. 
     With continued reference to  FIG.  8   , in some embodiments, G is the color green, R is the color red, and B is the color blue. In some other embodiments, other colors associated with other wavelengths of light, including magenta and cyan, may be used in addition to or may replace one or more of red, green, or blue. 
     It will be appreciated that references to a given color of light throughout this disclosure will be understood to encompass light of one or more wavelengths within a range of wavelengths of light that are perceived by a viewer as being of that given color. For example, red light may include light of one or more wavelengths in the range of about 620-780 nm, green light may include light of one or more wavelengths in the range of about 492-577 nm, and blue light may include light of one or more wavelengths in the range of about 435-493 nm. 
     In some embodiments, the light source  530  ( FIG.  6   ) may be configured to emit light of one or more wavelengths outside the visual perception range of the viewer, for example, infrared and/or ultraviolet wavelengths. In addition, the in-coupling, out-coupling, and other light redirecting structures of the waveguides of the display  250  may be configured to direct and emit this light out of the display towards the user&#39;s eye  210 , e.g., for imaging and/or user stimulation applications. 
     With reference now to  FIG.  9 A , in some embodiments, light impinging on a waveguide may need to be redirected to in-couple that light into the waveguide. An in-coupling optical element may be used to redirect and in-couple the light into its corresponding waveguide.  FIG.  9 A  illustrates a cross-sectional side view of an example of a plurality or set  660  of stacked waveguides that each includes an in-coupling optical element. The waveguides may each be configured to output light of one or more different wavelengths, or one or more different ranges of wavelengths. It will be appreciated that the stack  660  may correspond to the stack  260  ( FIG.  6   ) and the illustrated waveguides of the stack  660  may correspond to part of the plurality of waveguides  270 ,  280 ,  290 ,  300 ,  310 , except that light from one or more of the image injection devices  360 ,  370 ,  380 ,  390 ,  400  is injected into the waveguides from a position that requires light to be redirected for in-coupling. 
     The illustrated set  660  of stacked waveguides includes waveguides  670 ,  680 , and  690 . Each waveguide includes an associated in-coupling optical element (which may also be referred to as a light input area on the waveguide), 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, as seen in the illustrated head-on view in a direction of light propagating to these in-coupling optical elements. 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 . 
     Each waveguide also includes 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 total internal reflection (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. 
     Preferably, for ease of manufacturing and other considerations, the material forming the waveguides  670 ,  680 ,  690  are similar or the same, and the material forming the layers  760   a ,  760   b  are similar or the same. In some embodiments, the material forming the waveguides  670 ,  680 ,  690  may be different between one or more waveguides, and/or the material forming the layers  760   a ,  760   b  may be different, while still holding to the various refractive index relationships noted above. 
     With continued reference to  FIG.  9 A , light rays  770 ,  780 ,  790  are incident on the set  660  of waveguides. It will be appreciated that the light rays  770 ,  780 ,  790  may be injected into the waveguides  670 ,  680 ,  690  by one or more image injection devices  360 ,  370 ,  380 ,  390 ,  400  ( FIG.  6   ). 
     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. In some embodiments, the in-coupling optical elements  700 ,  710 ,  720  each selectively deflect one or more particular wavelengths of light, while transmitting other wavelengths to an underlying waveguide and associated in-coupling optical element. 
     For example, in-coupling optical element  700  may be configured to deflect ray  770 , which has a first wavelength or range of wavelengths, while transmitting rays  780  and  790 , which have different second and third wavelengths or ranges of wavelengths, respectively. 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. 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.  9 A , the deflected light rays  770 ,  780 ,  790  are deflected so that they propagate through a corresponding waveguide  670 ,  680 ,  690 ; that is, the in-coupling optical elements  700 ,  710 ,  720  of each waveguide deflects light into that corresponding waveguide  670 ,  680 ,  690  to in-couple light into that corresponding waveguide. The light rays  770 ,  780 ,  790  are deflected at angles that cause the light to propagate through the respective waveguide  670 ,  680 ,  690  by TIR. The light rays  770 ,  780 ,  790  propagate through the respective waveguide  670 ,  680 ,  690  by TIR until impinging on the waveguide&#39;s corresponding light distributing elements  730 ,  740 ,  750 . 
     With reference now to  FIG.  9 B , a perspective view of an example of the plurality of stacked waveguides of  FIG.  9 A  is illustrated. As noted above, 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 deflect or distribute light to the out-coupling optical elements  800 ,  810 ,  820  and, in some embodiments, may also increase the beam or spot size of this light as it propagates to the out-coupling optical elements. In some embodiments, 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 . For example, with reference to  FIG.  9 A , the light distributing elements  730 ,  740 ,  750  may be replaced with out-coupling optical elements  800 ,  810 ,  820 , respectively. 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  ( FIG.  7   ). 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. For example, each OPE may be configured to redirect a portion of the light striking the OPE to an EPE of the same waveguide, while allowing the remaining portion of the light to continue to propagate down the waveguide. Upon impinging on the OPE again, another portion of the remaining light is redirected to the EPE, and the remaining portion of that portion continues to propagate further down the waveguide, and so on. Similarly, upon striking the EPE, a portion of the impinging light is directed out of the waveguide towards the user, and a remaining portion of that light continues to propagate through the waveguide until it strikes the EP again, at which time another portion of the impinging light is directed out of the waveguide, and so on. Consequently, a single beam of in-coupled light may be “replicated” each time a portion of that light is redirected by an OPE or EPE, thereby forming a field of cloned beams of light, as shown in  FIG.  6   . In some embodiments, the OPE and/or EPE may be configured to modify a size of the beams of light. 
     Accordingly, with reference to  FIGS.  9 A and  9 B , in some embodiments, the set  660  of waveguides 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 finally out-couples the light ray  790  to the viewer, who also receives the out-coupled light from the other waveguides  670 ,  680 . 
       FIG.  9 C  illustrates a top-down plan view of an example of the plurality of stacked waveguides of  FIGS.  9 A and  9 B . It will be appreciated that this top-down view may also be referred to as a head-on view, as seen in the direction of propagation of light towards the in-coupling optical elements  800 ,  810 ,  820 ; that is, the top-down view is a view of the waveguides with image light incident normal to the page. As illustrated, the waveguides  670 ,  680 ,  690 , along with each waveguide&#39;s associated light distributing element  730 ,  740 ,  750  and associated out-coupling optical element  800 ,  810 ,  820 , may be vertically aligned. However, as discussed herein, the in-coupling optical elements  700 ,  710 ,  720  are not vertically aligned; rather, the in-coupling optical elements are preferably non-overlapping (e.g., laterally spaced apart as seen in the top-down view). As discussed further herein, this nonoverlapping spatial arrangement facilitates the injection of light from different sources into different waveguides on a one-to-one basis, thereby allowing a specific light source to be uniquely coupled to a specific waveguide. In some embodiments, arrangements including nonoverlapping spatially-separated in-coupling optical elements may be referred to as a shifted pupil system, and the in-coupling optical elements within these arrangements may correspond to sub-pupils. 
     It will be appreciated that the spatially overlapping areas may have lateral overlap of 70% or more, 80% or more, or 90% or more of their areas, as seen in the top-down view. On the other hand, the laterally shifted areas of less than 30% overlap, less than 20% overlap, or less than 10% overlap of their areas, as seen in top-down view. In some embodiments, laterally shifted areas have no overlap. 
       FIG.  9 D  illustrates a top-down plan view of another example of a plurality of stacked waveguides. As illustrated, the waveguides  670 ,  680 ,  690  may be vertically aligned. However, in comparison to the configuration of  FIG.  9 C , separate light distributing elements  730 ,  740 ,  750  and associated out-coupling optical elements  800 ,  810 ,  820  are omitted. Instead, light distributing elements and out-coupling optical elements are effectively superimposed and occupy the same area as seen in the top-down view. In some embodiments, light distributing elements (e.g., OPE&#39;s) may be disposed on one major surface of the waveguides  670 ,  680 ,  690  and out-coupling optical elements (e.g., EPE&#39;s) may be disposed on the other major surface of those waveguides. Thus, each waveguide  670 ,  680 ,  690  may have superimposed light distributing and out coupling optical elements, collectively referred to as combined OPE/EPE&#39;s  1281 ,  1282 ,  1283 , respectively. Further details regarding such combined OPE/EPE&#39;s may be found in U.S. application Ser. No. 16/221,359, filed on Dec. 14, 2018, the entire disclosure of which is incorporated by reference herein. The in-coupling optical elements  700 ,  710 ,  720  in-couple and direct light to the combined OPE/EPE&#39;s  1281 ,  1282 ,  1283 , respectively. In some embodiments, as illustrated, the in-coupling optical elements  700 ,  710 ,  720  may be laterally shifted (e.g., they are laterally spaced apart as seen in the illustrated top-down view) in have a shifted pupil spatial arrangement. As with the configuration of  FIG.  9 C , this laterally-shifted spatial arrangement facilitates the injection of light of different wavelengths (e.g., from different light sources) into different waveguides on a one-to-one basis. 
       FIG.  9 E  illustrates an example of wearable display system  60  into which the various waveguides and related systems disclosed herein may be integrated. In some embodiments, the display system  60  is the system  250  of  FIG.  6   , with  FIG.  6    schematically showing some parts of that system  60  in greater detail. For example, the waveguide assembly  260  of  FIG.  6    may be part of the display  70 . 
     With continued reference to  FIG.  9 E , 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. The display  70  may include one or more waveguides, such as the waveguide  270 , configured to relay in-coupled image light and to output that image light to an eye of the user  90 . 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, may optionally be positioned adjacent the other ear canal of the user to provide stereo/shapeable sound control). The display system  60  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  112  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.  9 E , 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. Optionally, the local processor and data module  140  may include one or more central processing units (CPUs), graphics processing units (GPUs), dedicated processing hardware, and so on. 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. 
     With continued reference to  FIG.  9 E , in some embodiments, the remote processing module  150  may comprise one or more processors configured to analyze and process data and/or image information, for instance including one or more central processing units (CPUs), graphics processing units (GPUs), dedicated processing hardware, and so on. 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 virtual 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. Optionally, an outside system (e.g., a system of one or more processors, one or more computers) that includes CPUs, GPUs, and so on, may perform at least a portion of processing (e.g., generating image information, processing data) and provide information to, and receive information from, modules  140 ,  150 ,  160 , for instance via wireless or wired connections. 
       FIG.  10    illustrates an example of a wearable display system with a light projection system  910  having a spatial light modulator  930  and a separate light source  940 . The light source  940  may comprise one or more light emitters and illuminates the spatial light modulator (SLM)  930 . A lens structure  960  may be used to focus the light from the light source  940  onto the SLM  930 . A beam splitter (e.g., a polarizing beam splitter (PBS))  950  reflects light from the light source  940  to the spatial light modulator  930 , which reflects and modulates the light. The reflected modulated light, also referred to as image light, then propagates through the beam splitter  950  to the eyepiece  920 . Another lens structure, projection optics  970 , may be utilized to converge or focus the image light onto the eyepiece  920 . The eyepiece  920  may include one or more waveguides or waveguides that relay the modulated to the eye  210 . 
     As noted herein, the separate light source  940  and associated lens structure  960  may undesirably add weight and size to the wearable display system. This may decrease the comfort of the display system, particularly for a user wearing the display system for an extended duration. 
     In addition, the light source  940  in conjunction with the SLM  930  may consume energy inefficiently. For example, the light source  940  may illuminate the entirety of the SLM  930 . The SLM  930  then selectively reflects light towards the eyepiece  920 . thus, not all the light produced by the light source  940  may be utilized to form an image; some of this light, e.g., light corresponding to dark regions of an image, is not reflected to the eyepiece  920 . As a result, the light source  940  utilizes energy to generate light to illuminate the entirety of the SLM  930 , but only a fraction of this light may be needed to form some images. 
     Moreover, as noted herein, in some cases, the SLM  930  may modulate light using a micro-mirror to selectively reflect incident light, or using liquid crystal molecules that modify the amount of light reflected from an underlying mirror. As a result, such devices require physical movement of optical elements (e.g., micro-mirrors or liquid crystal molecules such as in LCoS or DLP panels, respectively) in order to modulate light from the light source  940 . The physical movement required to modulate light to encode the light with image information, e.g., corresponding to a pixel, may occur at relatively slow speeds in comparison to, e.g., the ability to turn an LED or OLED “on” or “off”. This relatively slow movement may limit the frame rate of the display system and may be visible as, e.g., motion blur, color-breakup, and/or presented images that are mismatched with the pose of the user&#39;s head or changes in said pose. 
     Advantageously, wearable displays utilizing emissive micro-displays, as disclosed herein, may facilitate wearable display systems that have a relatively low weight and bulkiness, high energy efficiency, and high frame rate, with low motion blur and low motion-to-photon latency. Low blur and low motion-to-photon latency are further discussed in U.S. Provisional Application No. 62/786,199, filed Dec. 28, 2018, the entire disclosure of which is incorporated by reference herein. In addition, in comparison to scanning fiber displays, the emissive micro-displays may avoid artifacts caused by the use of coherent light sources. 
     With reference now to  FIG.  11 A , an example is illustrated of a wearable display system with a light projection system  1010  having multiple emissive micro-displays  1030   a ,  1030   b ,  1030   c . Light from the micro-displays  1030   a ,  1030   b ,  1030   c  is combined by an optical combiner  1050  and directed towards an eyepiece  1020 , which relays the light to the eye  210  of a user. Projection optics  1070  may be provided between the optical combiner  1050  and the eyepiece  1020 . In some embodiments, the eyepiece  1020  may be a waveguide assembly comprising one or more waveguides. In some embodiments, the light projection system  1010  and the eyepiece  1020  may be supported (e.g., attached to) the frame  80  ( FIG.  9 E ). 
     In some embodiments, the micro-displays  1030   a ,  1030   b ,  1030   c  may be monochrome micro-displays, with each monochrome micro-display outputting light of a different component color to provide a monochrome image. As discussed herein, the monochrome images combine to form a full-color image. 
     In some other embodiments, the micro-displays  1030   a ,  1030   b ,  1030   c  may be may each be full-color displays configured to output light of all component colors. For example, the micro-displays  1030   a ,  1030   b ,  1030   c  each include red, green, and blue light emitters. The micro-displays  1030   a ,  1030   b ,  1030   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. 
     The micro-displays may comprise an array of light emitters. 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 emissive micro-LED displays. 
     With continued reference to  FIG.  11 A , the micro-displays  1030   a ,  1030   b ,  1030   c  may each be configured to emit image light  1032   a ,  1032   b ,  1032   c . Where the micro-displays are monochrome micro-displays, the image light  1032   a ,  1032   b ,  1032   c  may each be of a different component color. The optical combiner  1050  receives the image light  1032   a ,  1032   b ,  1032   c  and effectively combines this light such that the light propagates generally in the same direction, e.g., toward the projection optics  1070 . In some embodiments, the optical combiner  1050  may be a dichroic X-cube prism having reflective internal surfaces that redirect the image light  1032   a ,  1032   b ,  1032   c  to the projection optics  1070 . It will be appreciated that the projection optics  1070  may be a lens structure comprising one or more lenses which converge or focus image light onto the eyepiece  1020 . The eyepiece  1020  then relays the image light  1032   a ,  1032   b ,  1032   c  to the eye  210 . 
     In some embodiments, the eyepiece  1020  may comprise a plurality of stacked waveguides  1020   a ,  1020   b ,  1020   c , each of which has a respective in-coupling optical element  1022   a ,  1022   b ,  1022   c . In some embodiments, the number of waveguides is proportional to the number of component colors provided by the micro-displays  1030   a ,  1030   b ,  1030   c . For example, where there are three component colors, the number of waveguides in the eyepiece  1020  may include a set of three waveguides or multiple sets of three waveguides each. In some embodiments, each set may output light with wavefront divergence corresponding to a particular depth plane, as discussed herein. It will be appreciated that the waveguides  1020   a ,  1020   b ,  1020   c  and the in-coupling optical element  1022   a ,  1022   b ,  1022   c  may correspond to the waveguides  670 ,  680 ,  690  and the in-coupling optical elements  700 ,  710 ,  720 , respectively, of  FIGS.  9 A- 9 C . As viewed from the projection optics  1070 , the in-coupling optical elements  1022   a ,  1022   b ,  1022   c  may be laterally shifted, such that they at least partly do not overlap as seen in such a view. 
     As illustrated, the various in-coupling optical elements disclosed herein (e.g., the in-coupling optical element  1022   a ,  1022   b ,  1022   c ) may be disposed on a major surface of an associated waveguide (e.g., waveguides  1020   a ,  1020   b ,  1020   c , respectively). In addition, as also illustrated, the major surface on which a given in-coupling optical element is disposed may be the rear surface of the waveguide. In such a configuration, the in-coupling optical element may be a reflective light redirecting element, which in-couples light by reflecting the light at angles which support TIR through the associated waveguide. In some other configurations, the in-coupling optical element may be disposed on the forward surface of the waveguide (closer to the projection optics  1070  than the rearward surface). In such configurations, the in-coupling optical element may be a transmissive light redirecting element, which in-couples light by changing the direction of propagation of light as the light is transmitted through the in-coupling optical element. It will be appreciated that any of the in-coupling optical elements disclosed herein may be reflective or transmissive in-coupling optical elements. 
     With continued reference to  FIG.  11 A , image light  1032   a ,  1032   b ,  1032   c  from different ones of the micro-displays  1030   a ,  1030   b ,  1030   c  may take different paths to the eyepiece  1020 , such that they impinge on different ones of the in-coupling optical element  1022   a ,  1022   b ,  1022   c . Where the image light  1032   a ,  1032   b ,  1032   c  includes light of different component colors, the associated in-coupling optical element  1022   a ,  1022   b ,  1022   c , respectively, may be configured to selectively in couple light of different wavelengths, as discussed above regarding, e.g., the in-coupling optical elements  700 ,  710 ,  720  of  FIGS.  9 A- 9 C . 
     With continued reference to  FIG.  11 A , the optical combiner  1050  may be configured to redirect the image light  1032   a ,  1032   b ,  1032   c  emitted by the micro-displays  1030   a ,  1030   b ,  1030   c  such that the image light propagates along different optical paths, in order to impinge on the appropriate associated one of the in-coupling optical element  1022   a ,  1022   b ,  1022   c . Thus, the optical combiner  1050  combines the image light  1032   a ,  1032   b ,  1032   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  1032   a ,  1032   b ,  1032   c  along different paths to the eyepiece  1020 . As a result, the image light  1032   a ,  1032   b ,  1032   c  may be incident on different associated ones of in-coupling optical elements  1022   a ,  1022   b ,  1022   c . In some embodiments, the micro-displays  1030   a ,  1030   b ,  1030   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  1022   a ,  1022   b ,  1022   c . For example, faces of one or more of the micro-displays  1030   a ,  1030   b ,  1030   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  1022   a ,  1022   b , or  1022   c . 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. 
     With continued reference to  FIG.  11 A , in some embodiments, the monochrome micro-display  1030   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. In addition, the human eye is most sensitive to the color green. Consequently, the monochrome micro-display  1030   b  opposite the output face  1051  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. 
     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 emissive micro-displays  1030   a ,  1030   b ,  1030   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, it will be appreciated that the human visual system may have a flicker fusion threshold. The flicker fusion threshold may be understood to a duration within which the human visual system is unable to differentiate images as being presented at different times. Images presented within that duration are fused or combined and, as a result, may be perceived by a user to be present simultaneously. Flickering images with temporal gaps between the images that are outside of that duration are not combined, and the flickering of the images is perceptible. In some embodiments, the duration is 1/60 seconds or less, which corresponds to a frame rate of 60 Hz or more. Preferably, image frames for any individual eye are provided to the user at a frame rate equal to or higher than the duration of the flicker fusion threshold of the user. For example, the frame rate for each of the left-eye or right-eye pieces may be 60 Hz or more, or 120 Hz or more; and, as a result, the frame rate provided by the light projection system  1010  may be 120 Hz or more, or 240 Hz or more in some embodiments. 
     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  1030   a ,  1030   b ,  1030   c.    
     As discussed herein, the micro-displays  1030   a ,  1030   b ,  1030   c  may each include arrays of light emitters.  FIG.  11 B  illustrates an example of an array  1042  of light emitters  1044 . Where the associated micro-display is a monochrome micro-display, the light emitters  1044  may all be configured to emit light of the same color. 
     Where the associated micro-display is a full-color micro-display, different ones of the light emitters  1044  may be configured to emit light of different colors. In such embodiments, the light emitters  1044  may be considered subpixels and may be arranged in groups, with each group having at least one light emitter configured to emit light of each component color. For example, where the component colors are red, green, and blue, each group may have at least one red subpixel, at least one green subpixel, in at least one blue subpixel. 
     It will be appreciated, that while the light emitters  1044  are shown arranged in a grid pattern for ease of illustration, the light emitters  1044  may have other regularly repeating spatial arrangements. For example, the number of light emitters of different component colors may vary, the sizes of the light emitters may vary, the shapes of the light emitters and/or the shapes made out by groups of light emitters may vary, etc. 
     With continued reference to  FIG.  11 B , it will be appreciated that the micro-emitters  1044  emit light. In addition, manufacturing constraints, such as lithography or other patterning and processing limitations, and/or electrical considerations, may limit how closely neighboring light-emitters  1044  are spaced. As a result, there may be an area  1045  surrounding the light emitter  1044  within which it is not practical to form other light emitters  1044 . This area  1045  forms the inter-emitter regions between light emitters  1044 . In some embodiments, taking into account the area  1045 , the light emitters have a pitch of, e.g., less than 10 μm, less than 8 μm, less than 6 μm, or less than 5 μm, and more than 1 μm, including 1-5 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. In some embodiments, the ratio of emitter size to pitch is 1:1 to 1:5, 1:2 to 1:4, or 1:2 to 1:3. 
     It will be appreciated that, given some light emitter device architectures and materials, current crowding may decrease the emitter&#39;s efficiency and pixel droop may cause unintentional activation of pixels (e.g., due to energy directed to one light emitter bleeding into a neighboring light emitter). As a result, a relatively large area  1045  may beneficially reduce current crowding and pixel droop. In some embodiments, the ratio of emitter size to pitch is preferably 1:2 to 1:4, or 1:2 to 1:3. 
     It will also be appreciated, however, that large separations between light emitters (e.g., a small light emitter to pitch ratio) may undesirably cause visible gaps, or dark regions, between the light emitters. Even when laterally translated as discussed herein, some gaps may still be visible, depending on the size of the original gap, the distance of the translation, and the number of subframes (and resulting translation increments) utilized. In some embodiments, lens structure such as light collimators may be utilized to effectively fill or partially fill in these dark regions. For example, a light collimating lens may extend on and around a light emitter  1044 , such that light from the emitter  1044  completely fills the lens. For example, the light collimating lens may have a larger width than the light emitters  1044  and, in some embodiments, the width of the collimating lens may be approximately equal to the pitch. As a result, the size of the emitter  1044  is effectively increased to extend across the area of the lens, thereby filling in some or all of the area  1045 . In some other embodiments, the width of the collimating lens may be approximately equal to the distance that the projection system is translated, as discussed herein, for each subframe. Lens structures such as light collimators are further discussed herein (e.g., in  FIG.  30 A  and the related discussion). 
     As discussed herein, the light emitters  1044  may be OLEDs or micro-LEDs. It will be appreciated that OLEDs may utilize layers of organic material, e.g., disposed between electrodes, to emit light. 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  1044  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 may emit blue light which excites a phosphor material or quantum dot that converts the blue wavelength light to green or red wavelengths. 
     With reference now to  FIG.  12   , another example is illustrated of a wearable display system with a light projection system having multiple emissive micro-displays  1030   a ,  1030   b ,  1030   c . The illustrated display system is similar to the display system of  FIG.  11 A  except that the optical combiner  1050  has a standard X-cube prism configuration and includes light redirecting structures  1080   a  and  1080   c  for modifying the angle of incidence of light on the reflective surfaces  1052 ,  1054  of the X-cube prism. It will be appreciated that a standard X-cube prism configuration will receive light which is normal to a face of the X-cube and redirect this light 45° such that it is output at a normal angle from a transverse face of the X-cube. However, this would cause the image light  1032   a ,  1032   b ,  1032   c  to be incident on the same in-coupling optical element of the eyepiece  1020 . In order to provide different paths for the image light  1032   a ,  1032   b ,  1032   c , so that the image light is incident on associated ones of the in-coupling optical elements  1022   a ,  1022   b ,  1022   c  of the waveguide assembly, the light redirecting structures  1080   a ,  1080   c  may be utilized. 
     In some embodiments, the light redirecting structures  1080   a ,  1080   c  may be lens structures. It will be appreciated that the lens structures may be configured to receive incident light and to redirect the incident light at an angle such that the light reflects off a corresponding one of the reflective surfaces  1052 ,  1054  and propagates along a light path towards a corresponding one of the in-coupling optical elements  1022   a ,  1022   c . As examples, the light redirecting structures  1080   a ,  1080   c  may comprise micro-lenses, nano-lenses, reflective wells, metasurfaces, and liquid crystal gratings. In some embodiments, the micro-lenses, nano-lenses, reflective wells, metasurfaces, and liquid crystal gratings may be organized in arrays. For example, each light emitter of the micro-displays  1030   a ,  1030   c  may be matched with one micro-lens. In some embodiments, in order to redirect light in a particular direction, the micro-lens or reflective wells may be asymmetrical and/or the light emitters may be disposed off-center relative to the micro-lens. In addition, in some embodiments, the light redirecting structures  1080   a ,  1080   c  may be collimators which narrow the angular emission profiles of associated light emitters, to increase the amount of light ultimately in-coupled into the eyepiece  1020 . Further details regarding such light redirecting structures  1080   a ,  1080   c  are discussed below regarding  FIGS.  24 A- 27 C . 
     With reference now to  FIG.  13 A , in some embodiments, two or more of the in-coupling optical elements  1022   a ,  1022   b ,  1022   c  may overlap (e.g., as seen in a head-on view in the direction of light propagation into the in-coupling optical element  1022   a ,  1022   b ,  1022   c ).  FIG.  13 A  illustrates an example of a side-view of a wearable display system with a light projection system  1010  having multiple emissive micro-displays  1032   a ,  1032   b ,  1032   c  and an eyepiece  1020  with overlapping light in-coupling optical elements  1022   a ,  1022   c  and non-overlapping light in-coupling optical element  1022   b . As illustrated, the in-coupling optical elements  1022   a ,  1022   c  overlap, while the in-coupling optical elements  1022   b  are laterally shifted. Stated another way, the in-coupling optical elements  1022   a ,  1022   c  are aligned directly in the paths of the image light  1032   a ,  1032   c , while the image light  1032   b  follows another path to the eyepiece  1020 , such that it is incident on an area of the eyepiece  1020  that is laterally shifted relative to the area in which the image light  1032   a ,  1032   c  is incident. 
     As illustrated, differences between the paths for the image light  1032   b  and image light  1032   a ,  1032   c  may be established using light redirecting structures  1080   a ,  1080   c . In some embodiments, the image light  1032   b  from the emissive micro-display  1030   b  proceeds directly through the optical combiner  1052 . The image light  1032   a  from the emissive micro-display  1032   a  is redirected by the light redirecting structure  1080   a  such that it reflects off of the reflective surface  1054  and propagates out of the optical combiner  1050  in the same direction as the image light  1032   c . It will be appreciated that the image light  1032   c  from the emissive micro-display  1032   c  is redirected by the light redirecting structure  1080   c  such that it reflects off of the reflective surface  1052  at an angle such that the image light  1032   c  propagates out of the optical combiner  1050  in the same direction as the image light  1032   b . Thus, the redirection of light by the light redirecting structures  1080   a ,  1080   c  and the angles of the reflective surfaces  1052 ,  1054  are configured to provide a common path for the image light  1032   a ,  1032   c  out of the optical combiner  1050 , with this common path being different from the path of the image light  1032   b . In some other embodiments, one or both of the light redirecting structures  1080   a ,  1080   c  may be omitted and the reflective surfaces  1052 ,  1054  in the optical combiner  1050  may be configured to reflect the image light  1032   a ,  1032   c  in the appropriate respective directions such that they exit the optical combiner  1050  propagating in the same direction, which is different from the direction of the image light  1032   b . As such, after propagating through the projection optics  1070 , the image light  1032   a ,  1032   c  exit from one exit pupil while the image light  1032   b  exits from another exit pupil. In this configuration, the light projection system  1010  may be referred to as a two-pupil projection system. 
     In some embodiments, the light projection system  1010  may have a single output pupil and may be referred to as a single-pupil projection system. In such embodiments, the light projection system  1010  may be configured to direct the image light  1032   a ,  1032   b ,  1032   c  onto a single common area of the eyepiece  1020 . Such a configuration is shown in  FIG.  13 B , which illustrates a wearable display system with a light projection system  1010  having multiple emissive micro-displays  1030   a ,  1030   b ,  1030   c  configured to direct light to a single light in-coupling area of the eyepiece  1020 . In some embodiments, as discussed further herein, the eyepiece  1020  may include a stack of waveguides having overlapping light in-coupling optical elements. In some other embodiments, a single light in-coupling optical element may be configured to in-couple light of all component colors into a single waveguide. The display system of  FIG.  13 B  is similar to the display system of  FIG.  13 A , except for the omission of the light redirecting structures  1080   a ,  1080   c  and the use of the in-coupling optical element  1122   a  and with the associated waveguide  1020   a . As illustrated, the in-coupling optical element  1122   a  in-couples each of image light  1032   a ,  1032   b ,  1032   c  into the waveguide  1020   a , which then relays the image light to the eye  210 . In some embodiments, the in-coupling optical element  1122   a  may comprise a diffractive grating. In some embodiments, the in-coupling optical element  1122   a  is a metasurface and/or liquid crystal grating. 
     As discussed herein, in some embodiments, the emissive micro-displays  1030   a ,  1030   b ,  1030   c  may be monochrome micro-displays configured to emit light of different colors. In some embodiments, one or more of the emissive micro-displays  1030   a ,  1030   b ,  1030   c  may have groups of light emitters configured to emit light of two or more, but not all, component colors. For example, a single emissive micro-display may have groups of light emitters—with at least one light emitter per group configured to emit blue light and at least one light emitter per group configured to emit green light—and a separate emissive micro-display on a different face of the X-cube  1050  may have light emitters configured to emit red light. In some other embodiments, the emissive micro-displays  1030   a ,  1030   b ,  1030   c  may each be full-color displays, each having light emitters of all component colors. As noted herein, utilizing multiple similar micro-displays may provide advantages for dynamic range and increased display brightness. 
     In some embodiments, a single full-color emissive micro-display may be utilized.  FIG.  14    illustrates an example of a wearable display system with a single emissive micro-display  1030   b . The wearable display system of  FIG.  14    is similar to the wearable display system of  FIG.  14   , except that the single emissive micro-display  1030   b  is a full color micro-display configured to emit light of all component colors. As illustrated, the micro-display  1030   b  emits image light  1032   a ,  1032   b ,  1032   c  of each component color. In such embodiments, the optical combiner  1050  ( FIG.  13 B ) may be omitted, which may advantageously reduce the weight and size of the wearable display system relative to a system with an optical combiner. 
     As discussed above, the in-coupling optical elements of the eyepiece  1020  may assume various configurations. Some examples of configurations for the eyepiece  1020  are discussed below in relation to  FIGS.  15 - 23 C . 
       FIG.  15    illustrates a side view of an example of an eyepiece  1020  having a stack of waveguides  1020   a ,  1020   b ,  1020   c  with overlapping in-coupling optical elements  1022   a ,  1022   b ,  1022   c , respectively. It will be appreciated that the illustrated waveguide stack may be utilized in place of the single illustrated waveguide  1020   a  of  FIGS.  13 B and  14   . As discussed herein, each of the in-coupling optical elements  1022   a ,  1022   b ,  1022   c  is configured to in-couple light having a specific color (e.g., light of a particular wavelength, or a range of wavelengths). In the illustrated orientation of the eyepiece  1020  in which the image light propagates vertically down the page towards the eyepiece  1020 , the in-coupling optical elements  1022   a ,  1022   b ,  1022   c  are vertically aligned with each other (e.g., along an axis parallel to the direction of propagation of the image light  1032   a ,  1032   b ,  1032   c ) such that they spatially overlap with each other as seen in a top down view (a head-on view in a direction of the image light  1032   a ,  1032   b ,  1032   c  propagating to the in-coupling optical elements). 
     With continued reference to  FIG.  15   , as discussed herein, the projection system  1010  ( FIGS.  13 ,  14   ) is configured to output a first monochrome color image, a second monochrome color image, and a third monochrome color image (e.g., red, green and blue color images) through the single-pupil of the projection system, the monochrome images being formed by the image light  1032   a ,  1032   b ,  1032   c , respectively. The in-coupling optical element  1022   c  is configured to in-couple the image light  1032   c  for the first color image into the waveguide  1020   c  such that it propagates through the waveguide  1020   c  by multiple total internal reflections at the upper and bottom major surfaces of the waveguide  1020   c , the in-coupling optical element  1022   b  is configured to in-couple the image light  1032   b  for the second color image into the waveguide  1020   b  such that it propagates through the waveguide  1020   b  by multiple total internal reflections at the upper and bottom major surfaces of the waveguide  1020   b , and the in-coupling optical element  1022   a  is configured to in-couple the image light  1032   a  for the third color image into the waveguide  1020   a  such that it propagates through the waveguide  1020   a  by multiple total internal reflections at the upper and bottom major surfaces of the waveguide  1020   a.    
     As discussed herein, the in-coupling optical element  1022   c  is preferably configured to in-couple substantially all the incident light  1032   c  corresponding to the first color image into the associated waveguide  1020   c  while allowing substantially all the incident light  1032   b ,  1032   a  corresponding to the second color image and the third color image, respectively, to be transmitted without being in-coupled. Similarly, the in-coupling optical element  1022   b  is preferably configured to in-couple substantially all the incident image light  1032   b  corresponding to the second color image into the associated waveguide  1020   b  while allowing substantially all the incident light corresponding to the third color image to be transmitted without being in-coupled. 
     It will be appreciated that, in practice, the various in-coupling optical elements may not have perfect selectivity. For example, some of the image light  1032   b ,  1032   a  may undesirably be in-coupled into the waveguide  1020   c  by the in-coupling optical element  1022   c ; and some of the incident image light  1032   a  may undesirably be in-coupled into the waveguide  1020   b  by the in-coupling optical element  1022   b . Furthermore, some of the image light  1032   c  may be transmitted through the in-coupling optical element  1022   c  and in-coupled into waveguides  1020   b  and/or  1020   a  by the in-coupling optical elements  1020   b  and/or  1020   a , respectively. Similarly, some of the image light  1032   b  may be transmitted through the in-coupling optical element  1022   b  and in-coupled into waveguide  1020   a  by the in-coupling optical element  1022   a.    
     In-coupling image light for a color image into an unintended waveguide may cause undesirable optical effects, such as, for example cross-talk and/or ghosting. For example, in-coupling of the image light  1032   c  for the first color image into unintended waveguides  1020   b  and/or  1020   a  may result in undesirable cross-talk between the first color image, the second color image and/or the third color image; and/or may result in undesirable ghosting. As another example, in-coupling of the image light  1032   b ,  1032   a  for the second or third color image, respectively, into the unintended waveguide  1020   c  may result in undesirable cross-talk between the first color image, the second color image and/or the third color image; and/or may cause undesirable ghosting. In some embodiments, these undesirable optical effects may be mitigated by providing color filters (e.g., absorptive color filters) that may reduce the amount of incident light that is in-coupled into an unintended waveguide. 
       FIG.  16    illustrates a side view of an example of a stack of waveguides with color filters for mitigating ghosting or crosstalk between waveguides. The eyepiece  1020  of  FIG.  16    is similar to that of  FIG.  15   , except for the presence of one or more of the color filters  1024   c ,  1024   b  and  1028 ,  1026 . The color filters  1024   c ,  1024   b  are configured to reduce the amount of light unintentionally in-coupled into the waveguides  1020   b  and  1020   a , respectively. The color filters  1028 ,  1026  are configured to reduce the amount of unintentionally in-coupled image light which propagates through the waveguides  1020   b ,  1020   c , respectively. 
     With continued reference to  FIG.  16   , a pair of color filters  1026  disposed on the upper and lower major surfaces of the waveguide  1020   c  may be configured to absorb image light  1032   a ,  1032   b  that may have been unintentionally been in-coupled into waveguide  1020   c . In some embodiments, the color filter  1024   c  disposed between the waveguides  1020   c  and  1020   b  is configured to absorb image light  1032   c  that is transmitted through the in-coupling optical element  1022   c  without being in-coupled. A pair of color filters  1028  disposed on the upper and lower major surfaces of the waveguide  1020   b  is configured to absorb image light  1032   a  that is in-coupled into waveguide  1020   b . A color filter  1024   b  disposed between the waveguides  1020   b  and  1020   a  is configured to absorb image light  1032   b  that is transmitted through the in-coupling optical element  710 . 
     In some embodiments, the color filters  1026  on each major surface of the waveguide  1020   c  are similar and are configured to absorb light of the wavelengths of both image light  1032   a ,  1032   b . In some other embodiments, the color filter  1026  on one major surface of the waveguide  1020   c  may be configured to absorb light of the color of image light  1032   a , and the color filter on the other major surface may be configured to absorb light of the color of image light  1032   b . In either arrangement, the color filters  1026  may be configured to selectively absorb the image light  1032   a ,  1032   b  propagating through the waveguide  1020   c  by total internal reflection. For example, at TIR bounces of the image light  1032   a ,  1032   b  off the major surfaces of the waveguide  1020   c , the image light  1032   a ,  1032   b  contacts a color filter  1026  on those major surfaces and a portion of that image light is absorbed. Preferably, due to the selective absorption of image light  1032   a ,  1032   b  by the colors filters  1026 , the propagation of the in-coupled the image light  1032   c  via TIR through the waveguide  1020   c  is not appreciably affected. 
     Similarly, the plurality of color filters  1028  may be configured as absorption filters that absorb in-coupled image light  1032   a  that propagates through the waveguide  1020   b  by total internal reflection. At TIR bounces of the image light  1032   a  off the major surfaces of the waveguide  1020   b , the image light  1032   a  contacts a color filter  1028  on those major surfaces and a portion of that image light is absorbed. Preferably, the absorption of the image light  1032   a  is selective and does not affect the propagation of the in-coupled image light  1032   b  that is also propagating via TIR through the waveguide  1020   b.    
     With continued reference to  FIG.  16   , the color filters  1024   c  and  1024   b  may also be configured as absorption filters. The color filter  1024   c  may be substantially transparent to light of the colors of the image light  1032   a ,  1032   b  such that the image light  1032   a ,  1032   b  is transmitted through the color filter  1024   c  with little to no attenuation, while light of the color of the image light  1032   c  is selectively absorbed. Similarly, the color filter  1024   b  may be substantially transparent to light of the color of the image light  1032   a  such that incident image light  1032   a  is transmitted through the color filter  1024   b  with little to no attenuation, while light of the color of the image light  1032   b  is selectively absorbed. The color filter  1024   c  may be disposed on a major surface (e.g., the upper major surface) of the waveguide  1020   b  as shown in  FIG.  16   . Alternately, the color filter  1024   c  may be disposed on a separate substrate positioned between the waveguides  1020   c  and  1020   b . Likewise, the color filter  1024   b  may be disposed on a major surface (e.g., an upper major surface) of the waveguide  1020   a . Alternately, the color filter  1024   b  may be disposed on a separate substrate positioned between the waveguides  1020   b  and  1020   a . It will be appreciated that the color filters  1024   c  and  1024   b  may be vertically aligned with the single-pupil of the projector that outputs the image light  1032   a ,  1032   b ,  1032   c  (in orientations where the image light  1032   a ,  1032   b ,  1032   c  propagates vertically to the waveguide stack  1020 , as illustrated). 
     In some embodiments, the color filters  1026  and  1028  may have single-pass attenuation factors of less than about 10%, (e.g., less than or equal to about 5%, less than or equal to about 2%, and greater than about 1%) to avoid significant undesired absorption of light propagating through the thickness the waveguides  1020   c ,  1020   b  (e.g., light of the colors of the image light  1032   a ,  1032   b  propagating through the waveguides  1020   c ,  1020   b  from the ambient environment and/or other waveguides). Various embodiments of the color filters  1024   c  and  1024   b  may be configured to have low attenuation factors for the wavelengths that are to be transmitted and high attenuation factor for the wavelengths that are to be absorbed. For example, in some embodiments, the color filter  1024   c  may be configured to transmit greater than 80%, greater than 90%, or greater than 95%, of incident light having the colors of the image light  1032   a ,  1032   b  and absorb greater than 80%, greater than 90%, or greater than 95%, of incident light having the color of the image light  1032   a . Similarly, the color filter  1024   b  may be configured to transmit greater than 80%, greater than 90%, or greater than 95%, of incident light having the color of the image light  1032   a  and absorb greater than 80%, greater than 90%, or greater than 95%, of incident light having the color of the image light  1032   b.    
     In some embodiments, the color filters  1026 ,  1028 ,  1024   c ,  1024   b  may comprise a layer of color selective absorbing material deposited on one or both surfaces of the waveguide  1020   c ,  1020   b  and/or  1020   a . The color selective absorbing material may comprise a dye, an ink, or other light absorbing material such as metals, semiconductors, and dielectrics. In some embodiments, the absorption of material such as metals, semiconductors, and dielectrics may be made color selective by utilizing these materials to form subwavelength gratings (e.g., a grating that does not diffract the light). The gratings may be made of plasmonics (e.g. gold, silver, and aluminum) or semiconductors (e.g. silicon, amorphous silicon, and germanium). 
     The color selective material may be deposited on the substrate using various deposition methods. For example, the color selective absorbing material may be deposited on the substrate using jet deposition technology (e.g., ink-jet deposition). Ink-jet deposition may facilitate depositing thin layers of the color selective absorbing material. Because ink-jet deposition allows for the deposition to be localized on selected areas of the substrate, ink-jet deposition provides a high degree of control over the thicknesses and compositions of the layers of the color selective absorbing material, including providing for nonuniform thicknesses and/or compositions across the substrate. In some embodiments, the color selective absorbing material deposited using ink-jet deposition may have a thickness between about 10 nm and about 1 micron (e.g., between about 10 nm and about 50 nm, between about 25 nm and about 75 nm, between about 40 nm and about 100 nm, between about 80 nm and about 300 nm, between about 200 nm and about 500 nm, between about 400 nm and about 800 nm, between about 500 nm and about 1 micron, or any value in a range/sub-range defined by any of these values). Controlling the thickness of the deposited layer of the color selective absorbing material may be advantageous in achieving a color filter having a desired attenuation factor. Furthermore, layers having different thickness may be deposited in different portions of the substrate. Additionally, different compositions of the color selective absorbing material may be deposited in different portions of the substrate using ink-jet deposition. Such variations in composition and/or thickness may advantageously allowing for location-specific variations in absorption. For example, in areas of a waveguide in which transmission of light from the ambient (to allow the viewer to see the ambient environment) is not necessary, the composition and/or thickness may be selected to provide high absorption or attenuation of selected wavelengths of light. Other deposition methods such as coating, spin-coating, spraying, etc. may be employed to deposit the color selective absorbing material on the substrate. 
       FIG.  17    illustrates an example of a top-down view of the waveguide assemblies of  FIGS.  15  and  16   . As illustrated, in-coupling optical elements  1022   a ,  1022   b ,  1022   c  spatially overlap. In addition, the waveguides  1020   a ,  1020   b ,  1020   c , along with each waveguide&#39;s associated light distributing element  730 ,  740 ,  750  and associated out-coupling optical element  800 ,  810 ,  820 , may be vertically aligned. The in-coupling optical elements  1022   a ,  1022   b ,  1022   c  are configured to in-couple incident image light  1032   a ,  1032   b ,  1032   c  ( FIGS.  15  and  16   ), respectively, in waveguides  1020   a ,  1020   b ,  1020   c , respectively, such that the image light propagates towards the associated light distributing element  730 ,  740 ,  750  by TIR. 
       FIG.  18    illustrates another example of a top-down view of the waveguide assemblies of  FIGS.  15  and  16   . As in  FIG.  17   , in-coupling optical elements  1022   a ,  1022   b ,  1022   c  spatially overlap and the waveguides  1020   a ,  1020   b ,  1020   c  are vertically aligned. In place of each waveguide&#39;s associated light distributing element  730 ,  740 ,  750  and associated out-coupling optical element  800 ,  810 ,  820 , however, are combined OPE/EPE&#39;s  1281 ,  1282 ,  1283 , respectively. The in-coupling optical elements  1022   a ,  1022   b ,  1022   c  are configured to in-couple incident image light  1032   a ,  1032   b ,  1032   c  ( FIGS.  15  and  16   ), respectively, in waveguides  1020   a ,  1020   b ,  1020   c , respectively, such that the image light propagates towards the associated combined OPE/EPE&#39;s  1281 ,  1282 ,  1283  by TIR. 
     While  FIGS.  15 - 18    show overlapping in-coupling optical elements for a single-pupil configuration of the display system, it will be appreciated that the display system may have a two-pupil configuration in some embodiments. In such a configuration, where three component colors are utilized, image light for two colors may have overlapping in-coupling optical elements, while image light for a third color may have a laterally-shifted in-coupling optical element. For example, the optical combiner  1050  ( FIGS.  11 A,  12 ,  13 A- 13 B ) and/or light redirecting structures  1080   a ,  1080   c  may be configured to direct image light through the projection optics  1070  such that image light of two colors are incident on directly overlapping areas of the eyepiece  1020  while another color of the image light is incident on an area that is laterally-shifted. For example, the reflective surfaces  1052 ,  1054  ( FIG.  11 A ) may be angled such that image light of one color follows a common light path with image light from the emissive micro-display  1030   b , while image light of another color follows a different light path. In some embodiments, rather than having both light redirecting structures  1080   a ,  1080   c  ( FIG.  12   ), one of these light redirecting structures may be omitted, so that only light from one of the micro-displays  1030   a ,  1030   c  is angled to provide a different light path from the light emitted by the other two micro-displays. 
       FIG.  19 A  illustrates a side view of an example of an eyepiece having a stack of waveguides with some overlapping and some laterally-shifted in-coupling optical elements. The eyepiece of  FIG.  19 A  is similar to the eyepiece of  FIG.  15   , except that one of the in-coupling optical elements is laterally shifted relative to the other in-coping optical elements. In the illustrated orientation of the eyepiece  1020  in which the image light propagates vertically down the page towards the eyepiece  1020 , the in-coupling optical elements  1022   a ,  1022   c  are vertically aligned with each other (e.g., along an axis parallel to the direction of propagation of the image light  1032   a ,  1032   c ) such that they spatially overlap with each other as seen in a head-on view in a direction of the image light  1032   a ,  1032   c  propagating to the in-coupling optical elements  1022   a ,  1022   b ,  1022   c . As seen in the same head-on view (e.g., as seen in a top-down view in the illustrated orientation), the in-coupling optical element  1022   b  is shifted laterally relative to the other in-coupling optical elements  1022   a ,  1022   c . Light for the in-coupling optical element  1022   b  is output to the eyepiece  1020  through a different exit pupil than light for the in-coupling optical elements  1022   a ,  1022   c . It will be appreciated that the illustrated waveguide stack comprising the waveguides  1020   a ,  1020   b ,  1020   c  may be utilized in place of the single illustrated waveguide  1020   a  of  FIGS.  13  and  14   . 
     With continued reference to  FIG.  19   , the in-coupling optical element  1022   c  is configured to in-couple the image light  1032   c  into the waveguide  1020   c  such that it propagates through the waveguide  1020   c  by multiple total internal reflections between the upper and bottom major surfaces of the waveguide  1020   c , the in-coupling optical element  1022   b  is configured to in-couple the image light  1032   b  into the waveguide  1020   b  such that it propagates through the waveguide  1020   b  by multiple total internal reflections between the upper and bottom major surfaces of the waveguide  1020   b , and the in-coupling optical element  1022   a  is configured to in-couple the image light  1032   a  into the waveguide  1020   a  such that it propagates through the waveguide  1020   a  by multiple total internal reflections between the upper and bottom major surfaces of the waveguide  1020   a.    
     The in-coupling optical element  1022   c  is preferably configured to in-couple all the incident light  1032   c  into the associated waveguide  1020   c  while being transmissive to all the incident light  1032   a . On the other hand, the image light  1032   b  may propagate to the in-coupling optical element  1022   b  without needing to propagate through any other in-coupling optical elements. This may be advantageous in some embodiments by allowing light, to which the eye is more sensitive, to be incident on a desired in-coupling optical element without any loss or distortion associated with propagation through other in-coupling optical elements. Without being limited by theory, in some embodiments, the image light  1032   b  is green light, to which the human eye is more sensitive. It will be appreciated that, while the waveguides  1020   a ,  1020   b ,  1020   c  are illustrated arranged a particular order, in some embodiments, the order of the waveguides  1020   a ,  1020   b ,  1020   c  may differ. 
     It will be appreciated that, as discussed herein, the in-coupling optical element  1022   c  overlying the in-coupling optical elements  1022   a  may not have perfect selectivity. Some of the image light  1032   a  may undesirably be in-coupled into the waveguide  1020   c  by the in-coupling optical element  1022   c ; and some of the image light  1032   c  may be transmitted through the in-coupling optical element  1022   c , after which the image light  1032   c  may strike the in-coupling optical element  1020   a  and be in-coupled into the waveguide  1020   a . As discussed herein, such undesired in-coupling may be visible as ghosting or crosstalk. 
       FIG.  19 B  illustrates a side view of an example of the eyepiece of  FIG.  19 A  with color filters for mitigating ghosting or crosstalk between waveguides. In particular, color filters  1024   c  and/or  1026  are added to the structures shown in  FIG.  19 A . As illustrated, the in-coupling optical element  1022   c  may unintentionally in-couple a portion of the image light  1032   a  into the waveguide  1020   c . In addition, or alternatively, a portion of the image light  1032   c  undesirably be transmitted through the in-coupling optical element  1022   c  after which it may unintentionally be in-coupled by the in-coupling optical element  1022   a.    
     To mitigate unintentionally in-couple image light  1032   a  propagating through the waveguide  1022   c , absorptive color filters  1026  may be provided on one or both major surfaces of the waveguide  1022   c . The absorptive color filters  1026  may be configured to absorb light of the color of the unintentionally in-coupled image light  1032   a . As illustrated, the absorptive color filters  1026  are disposed in the general direction of propagation of the image light through the waveguide  1020   c . Thus, the absorptive color filters  1026  are configured to absorb image light  1032   a  as that light propagates through the waveguide  1020   c  by TIR and contacts the absorptive color filters  1026  while reflecting off one or both of the major surfaces of the waveguide  1020   c.    
     With continued reference to  FIG.  19 B , to mitigate image light  1032   c  which propagates through the in-coupling optical element  1022   c  without being in-coupled, the absorptive color filter  1024   c  may be provided forward of the in-coupling optical element  1022   a . The absorptive color filter  1024   c  is configured to absorb light of the color of the image light  1032   c , to prevent that light from propagating to the in-coupling optical element  1022   a . While illustrated between the waveguides  1020   c  and  1020   b , in some other embodiments, the absorptive color filter  1024   c  may be disposed between the waveguides  1020   b  and  1020   a . It will be appreciated that further details regarding the composition, formation, and properties of the absorptive color filters  1024   c  and  1026  are provided in the discussion of  FIG.  16   . 
     It will also be appreciated that in the embodiments illustrated in  FIGS.  16  and  19 B , one or more of the color filters  1026 ,  1028 ,  1024   c , and  1024   b  may be omitted if one or more in-coupling optical elements  1022   a ,  1022   b ,  1022   c  have sufficiently high selectivity for the color of the light that is intended to be in-coupled into the associated waveguide  1020   a ,  1020   b ,  1022   c , respectively. 
       FIG.  20 A  illustrates an example of a top-down view of the eyepieces of  FIGS.  19 A and  19 B . As illustrated, in-coupling optical elements  1022   a ,  1022   c  spatially overlap, while in-coupling optical element  1022   b  is laterally-shifted. In addition, the waveguides  1020   a ,  1020   b ,  1020   c , along with each waveguide&#39;s associated light distributing element  730 ,  740 ,  750  and associated out-coupling optical element  800 ,  810 ,  820 , may be vertically aligned. The in-coupling optical elements  1022   a ,  1022   b ,  1022   c  are configured to in-couple incident image light  1032   a ,  1032   b ,  1032   c  ( FIGS.  15  and  16   ), respectively, in waveguides  1020   a ,  1020   b ,  1020   c , respectively, such that the image light propagates towards the associated light distributing element  730 ,  740 ,  750  by TIR. 
       FIG.  20 B  illustrates another example of a top-down view of the waveguide assembly of  FIGS.  19 A and  19 B . As in  FIG.  20 A , in-coupling optical elements  1022   a ,  1022   c  spatially overlap, the in-coupling optical element is laterally-shifted, and the waveguides  1020   a ,  1020   b ,  1020   c  are vertically aligned. In place of each waveguide&#39;s associated light distributing element  730 ,  740 ,  750  and associated out-coupling optical element  800 ,  810 ,  820 , however, are combined OPE/EPE&#39;s  1281 ,  1282 ,  1283 , respectively. The in-coupling optical elements  1022   a ,  1022   b ,  1022   c  are configured to in-couple incident image light  1032   a ,  1032   b ,  1032   c  ( FIGS.  15  and  16   ), respectively, in waveguides  1020   a ,  1020   b ,  1020   c , respectively, such that the image light propagates towards the associated combined OPE/EPE&#39;s  1281 ,  1282 ,  1283  by TIR. 
     With reference now to  FIG.  21   , it will be appreciated that re-bounce of in-coupled light may undesirably occur in waveguides. Re-bounce occurs when in-coupled light propagating along a waveguide strikes the in-coupling optical element a second or subsequent time after the initial in-coupling incidence. Re-bounce may result in a portion of the in-coupled light being undesirably out-coupled and/or absorbed by a material of the in-coupling optical element. The out-coupling and/or light absorption undesirably may cause a reduction in overall in-coupling efficiency and/or uniformity of the in-coupled light. 
       FIG.  21    illustrates a side view of an example of re-bounce in a waveguide  1030   a . As illustrated, image light  1032   a  is in-coupled into the waveguide  1030   a  by in-coupling optical element  1022   a . In-coupling optical element  1022   a  redirects the image light  1032   a  such that it generally propagates through the waveguide in the direction  1033 . Re-bounce may occur when in-coupled image light internally reflects or bounces off a major surface of the waveguide  1030   a  opposite the in-coupling optical element  1022   a  and is incident on or experiences a second bounce (a re-bounce) at the in-coupling optical element  1022   a . The distance between two neighboring bounces on the same surface of the waveguide  1030   a  is indicated by spacing  1034 . 
     Without being limited by theory, it will be appreciated that the in-coupling optical element  1022   a  may behave symmetrically; that is, it may redirect incident light such that the incident light propagates through the waveguide at TIR angles. However, light that is incident on the diffractive optical elements at TIR angles (such as upon re-bounce) may also be out-coupled. In addition or alternatively, in embodiments where the in-coupling optical element  1022   a  is coated with a reflective material, it will be understood that the reflection of light off of a layer of material such as metal may also involve partial absorption of the incident light, since reflection may involve the absorption and emission of light from a material. As a result, light out-coupling and/or absorption may undesirably cause loss of in-coupled light. Accordingly, re-bounced light may incur significant losses, as compared with light that interacts only once with the in-coupling optical element  1022   a.    
     In some embodiments, the in-coupling elements are configured to mitigate in-coupled image light loss due to re-bounce. Generally, re-bounce of in-coupled light occurs towards the end  1023  of the in-coupling optical element  1022   a  in the propagation direction  1033  of the in-coupled light. For example, light in-coupled at the end of the in-coupling optical element  1022   a  opposite the end  1023  may re-bounce if the spacing  1034  for that light is sufficiently short. To avoid such re-bounce, in some embodiments, the in-coupling optical element  1022   a  is truncated at the propagation direction end  1023 , to reduce the width  1022   w  of the in-coupling optical element  1022   a  along which re-bounce is likely to occur. In some embodiments, the truncation may be a complete truncation of all structures of the in-coupling optical element  1022   a  (e.g., the metallization and diffractive gratings). In some other embodiments, for example, where the in-coupling optical element  1022   a  comprises a metalized diffraction grating, a portion of the in-coupling optical element  1022   a  at the propagation direction end  1023  may not be metalized, such that the propagation direction end  1023  of the in-coupling optical element  1022   a  absorbs less re-bouncing light and/or outcouples re-bouncing light with a lower efficiency. In some embodiments, a diffractive region of an in-coupling optical element  1022   a  may have a width along a propagation direction  1033  shorter than its length perpendicular to the propagation direction  1033 , and/or may be sized and shaped such that a first portion of image light  1032   a  is incident on the in-coupling optical element  1022   a  and a second portion of the beam of light impinges on the waveguide  1030   a  without being incident on the in-coupling optical element  1022   a . While waveguide  1032   a  and light in-coupling optical element  1022   a  are illustrated alone for clarity, it will be appreciated that re-bounce and the strategies discussed for reducing re-bounce may apply to any of the in-coupling optical elements disclosed herein. It will also be appreciated that the spacing  1034  is related to the thickness of the waveguide  1030   a  (a larger thickness results in a larger spacing  1034 ). In some embodiments, the thickness of individual waveguides may be selected to set the spacing  1034  such that re-bounce does not occur. Further details regarding re-bounce mitigation may be found in U.S. Provisional Application No. 62/702,707, filed on Jul. 24, 2018, the entire disclosure of which is incorporated by reference herein. 
       FIGS.  22 A- 23 C  illustrate examples of top-down views of an eyepiece having in-coupling optical elements configured to reduce re-bounce. In-coupling optical element  1022   a ,  1022   b ,  1022   c  are configured to in-couple light such that it propagates in a propagation direction towards the associated light distributing elements  730 ,  740 ,  750  ( FIGS.  22 A- 22 C ) or combined OPE/EPE&#39;s  1281 ,  1282 ,  1283  ( FIGS.  23 A- 23 C ). As illustrated, the in-coupling optical element  1022   a ,  1022   b ,  1022   c  may have a shorter dimension along the propagation direction and a longer dimension along the transverse axis. For example, the in-coupling optical element  1022   a ,  1022   b ,  1022   c  may each be in the shape of a rectangle with a shorter side along the axis of the propagation direction and a longer side along an orthogonal axis. It will be appreciated that the in-coupling optical elements  1022   a ,  1022   b ,  1022   c  may have other shapes (e.g., orthogonal, hexagonal, etc.). In addition, different ones of the in-coupling optical elements  1022   a ,  1022   b ,  1022   c  may have different shapes in some embodiments. Also, preferably, as illustrated, non-overlapping in-coupling optical elements may be positioned such that they are not in the propagation direction of other in-coupling optical elements. For example, as shown in  FIGS.  22 A,  22 B,  23 A, and  23 B , the non-overlapping in-coupling optical elements may be arranged in a line along an axis crossing (e.g., orthogonal to) the axis of the propagation direction. 
     It will be appreciated that in the waveguide assemblies of  FIGS.  22 A- 22 C  are similar, except for the overlap of the in-coupling optical elements  1022   a ,  1022   b ,  1022   c . For example,  FIG.  22 A  illustrates in-coupling optical elements  1022   a ,  1022   b ,  1022   c  with no overlap.  FIG.  22 B  illustrates overlapping in-coupling optical elements  1022   a ,  1022   c , and non-overlapping in-coupling optical elements  1022   b .  FIG.  22 C  illustrates overlap between all the in-coupling optical elements  1022   a ,  1022   b ,  1022   c.    
     The waveguide assemblies of  FIGS.  23 A- 23 C  are also similar, except for the overlap of the in-coupling optical elements  1022   a ,  1022   b ,  1022   c .  FIG.  23 A  illustrates in-coupling optical elements  1022   a ,  1022   b ,  1022   c  with no overlap.  FIG.  23 B  illustrates overlapping in-coupling optical elements  1022   a ,  1022   c , and non-overlapping in-coupling optical elements  1022   b .  FIG.  22 C  illustrates overlap between all the in-coupling optical elements  1022   a ,  1022   b ,  1022   c.    
     With reference now to  FIG.  24 A , it will be appreciated that the emissive micro-displays have high etendue, which presents a challenge for efficient light utilization. As discussed herein, the emissive micro-displays may include a plurality of individual light emitters. Each of these light emitters may have a large angular emission profile, e.g., a Lambertian or near-Lambertian emission profile. Undesirably, not all of this light may be captured and directed to the eyepiece of the display system. 
       FIG.  24 A  illustrates an example of angular emission profiles of light emitted by individual light emitters  1044  of an emissive micro-display  1032 , and light captured by projection optics  1070 . The illustrated emissive micro-display  1032  may correspond to any of the emissive-micro-displays disclosed herein, including the emissive micro-displays  1032   a ,  1032   b ,  1032   c . As illustrated, the projection optics  1070  may be sized such that it will capture light having an angular emission profile  1046 . However, the angular emission profiles  1046  in the light emitters  1044  is significantly larger; not all of the light emitted by the light emitters  1044  will be incident on the projection optics  1070 , nor necessarily incident at angles at which the light will propagate into and through the projection optics  1070 . As a result, some of the light emitted by the light emitter  1044  may undesirably be “wasted” since it is not captured and ultimately relayed to the user&#39;s eye to form images. This may result in images that appear darker than would be expected if more of the light outputted by the light emitters  1040  ultimately reached the user&#39;s eye. 
     In some embodiments, one strategy for capturing more of the light emitted by the light emitters  1040  is to increase the size of the projection optics  1070 , to increase the size of the numerical aperture of the projection optics  1070  capturing light. In addition or alternatively, the projection optics  1070  may also be formed with high refractive index materials (e.g., having refractive indices above 1.5) which may also facilitate light collection. In some embodiments, the projection optics  1070  may utilize a lens sized to capture a desired, high proportion of the light emitted by the light emitters  1044 . In some embodiments, the projection optics  1070  may be configured to have an elongated exit pupil, e.g., to emit light beams having a cross-sectional profile similar to the shapes of the in-coupling optical elements  1022   a ,  1022   b ,  1022   c  of  FIGS.  22 A- 23 C . For example, the projection optics  1070  may be elongated in a dimension corresponding to the elongated dimension of the in-coupling optical elements  1022   a ,  1022   b ,  1022   c  of  FIGS.  22 A- 23 C . Without being limited by theory, such elongated in-coupling optical elements  1022   a ,  1022   b ,  1022   c  may improve the etendue mismatch between the emissive micro-display and the eyepiece  1020  ( FIGS.  22 A- 23 C ). In some embodiments, the thickness of the waveguides of the eyepiece  1020  (e.g.,  FIGS.  11 A, and  12 - 23 C ) may be selected to increase the percentage of light effectively captured, e.g., by reducing re-bounce by increasing the re-bounce spacing, as discussed herein. 
     In some embodiments, one or more light collimators may be utilized to reduce or narrow the angular emission profile of light from the light emitters  1044 . As a result, more of the light emitted by the light emitters  1044  may be captured by the projection optics  1070  and relayed to the eyes of a user, advantageously increasing the brightness of images and the efficiency of the display system. In some embodiments, the light collimators may allow the light collection efficiency of the projection optics (the percentage of light emitted by the light emitters  1044  that is captured by the projection optics) to reach values of 80% or more, 85% or more, or 90% or more, including about 85-95% or 85-90%. In addition, the angular emission profile of the light from the light emitters  1044  may be reduced to 60° or less, 50° or less, or 40° or less (from, e.g., 180°). In some embodiments, the reduced angular emission profiles may be in the range of about 30-60°, 30-50°, or 30-40°. It will be appreciated that light from the light emitters  1044  may make out the shape of a cone, with the light emitter  1044  at the vertex of the cone. The angular mission profile refers to the angle made out by the sides of the cone, with the associated light emitter  1044  at the vertex of the angle (as seen in a cross-section taken along a plane extending through the middle of the cone and including the cone apex). 
       FIG.  24 B  illustrates an example of the narrowing of angular emission profiles using an array of light collimators. As illustrated, the emissive micro-display  1032  includes an array of light emitters  1044 , which emit light with an angular emission profile  1046 . An array  1300  of light collimators  1302  is disposed forward of the light emitters  1044 . In some embodiments, each light emitter  1044  is matched 1-to-1 with an associated light collimator  1302  (one light collimator  1302  per light emitter  1044 ). Each light collimator  1302  redirects incident light from the associated light emitter  1044  to provide a narrowed angular emission profiles  1047 . Thus, the relatively large angular emission profiles  1046  are narrowed to the smaller angular emission profiles  1047 . 
     In some embodiments, the light collimators  1302  and array  1300  may be part of the light redirecting structures  1080   a ,  180   c  of  FIGS.  12  and  13 A . Thus, light collimators  1302  may narrow the angular emission profile of the light emitters  1044  and also redirect the light such that it propagates into the optical combiner  1050  at the appropriate angles to define multiple light paths and the related multiple exit pupils. It will be appreciated that light may be redirected in particular directions by appropriately shaping the light collimators  1302 . 
     Preferably, the light collimators  1302  are positioned in tight proximity to the light emitters  1044  to capture a large proportion of the light outputted by the light emitters  1044 . In some embodiments, there may be a gap between the light collimators  1302  and the light emitters  1044 . In some other embodiments, the light collimator  1302  may be in contact with the light emitters  1044 . It will be appreciated that the angular emission profile  1046  may make out a wide cone of light. Preferably, the entirety or majority of a cone of light from a light emitter  1044  is incident on a single associated light collimator  1302 . Thus, in some embodiments, each light emitter  1044  is smaller (occupies a smaller area) than the light receiving face of an associated light collimator  1302 . In some embodiments, each light emitter  1044  has a smaller width than the spacing between neighboring far light emitters  1044 . 
     Advantageously, the light collimators  1302  may increase the efficiency of the utilization of light and may also reduce the occurrence of crosstalk between neighboring light emitters  1044 . It will be appreciated that crosstalk between light emitters  1044  may occur when light from a neighboring light emitter is captured by a light collimator  1302  not associated with that neighboring light emitter. That captured light may be propagated to the user&#39;s eye, thereby providing erroneous image information for a given pixel. 
     With reference to  FIGS.  24 A and  24 B , the size of the beam of light captured by the projection optics  1070  may influence the size of the beam of light which exits the projection optics  1070 . As shown in  FIG.  24 A , without the use light collimators, the exit beam may have a relatively large width  1050 . As shown in  FIG.  24 B , with light collimators  1302 , the exit beam may have a smaller width  1052 . Thus, in some embodiments, the light collimators  1302  may be used to provide a desired beam size for in-coupling into an eyepiece. For example, the amount that the light collimators  1302  narrow the angular emission profile  1046  may be selected based at least partly upon the size of the intra-coupling optical elements in the eyepiece to which the light outputted by the projection optics  1070  is directed. 
     It will be appreciated that the light collimators  1302  may take various forms. For example, the light collimators  1302  may be micro-lenses or lenslets, in some embodiments. As discussed herein, each micro-lens preferably has a width greater than the width of an associated light emitter  1044 . 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  1302  may be nano-lenses, e.g., diffractive optical gratings. In some embodiments, light collimators  1302  may be metasurfaces and/or liquid crystal gratings. In some embodiments, light collimator&#39;s  1302  may take the form of reflective wells. 
     It will be appreciated that different light collimators  1302  may have different dimensions and/or shapes depending upon the wavelengths or colors of light emitted by the associated light emitter  1044 . Thus, for full-color emissive micro-displays, the array  1300  may include a plurality of light collimators  1302  with different dimensions and/or shapes depending upon the color of light emitted by the associate light emitter  1044 . In embodiments where the emissive micro-display is a monochrome micro-display, the array  1300  may be simplified, with each of the light collimators  1302  in the array being configured to redirect light of the same color. With such monochrome micro-displays, the light collimator  1302  may be similar across the array  1300  in some embodiments. 
     With continued reference to  FIG.  24 B , as discussed herein, the light collimators  1302  may have a 1-to-1 association with the light emitters  1044 . For example, each light emitter  1044  may have a discrete associated light collimator  1302 . In some other embodiments, light collimators  1302  may be elongated such that they extend across multiple light emitters  1044 . For example, in some embodiments, the light collimator  1302  may be elongated into the page and extend in front of a row of multiple light emitters  1044 . In some other embodiments, a single light collimator  1302  may extend across a column of light emitters  1044 . In yet other embodiments, the light collimator  1302  may comprise stacked columns and/or rows of lens structures (e.g., nano-lens structures, micro-lens structures, etc.). 
     As noted above, the light collimators  1302  may take the form of reflective wells.  FIG.  25 A  illustrates an example of a side view of an array of tapered reflective wells for directing light to projection optics. As illustrated, the light collimator array  1300  may include a substrate  1301  in which a plurality of light collimators  1302 , in the form of reflective wells, may be formed. Each well may include at least one light emitter  1044 , which may emit light with a Lambertian angular emission profile  1046 . The reflective walls  1303  of the wells of the light collimators  1302  are tapered and reflect the emitted light such that it is outputted from the well with a narrower angular emission profile  1047 . As illustrated, reflective walls  1303  may be tapered such that the cross-sectional size increases with distance from the light emitter  1044 . In some embodiments, the reflective walls  1303  may be curved. For example, the sides  1303  may have the shape of a compound parabolic concentrator (CPC). 
     With reference now to  FIG.  25 B , an example of a side view of an asymmetric tapered reflective well is illustrated. As discussed herein, e.g., as illustrated in  FIGS.  12 A- 13 A , it may be desirable to utilize the light collimators  1302  to steer light in a particular direction not normal to the surface of the light emitter  1044 . In some embodiments, as viewed in a side view such as illustrated in  FIG.  25 B , the light collimator  1302  may be asymmetric, with the upper side  1303   a  forming a different angle (e.g., a larger angle) with the surface of the light emitter  1044  than the lower side  1303   b ; for example, the angles of the reflective walls  1303   a ,  1303   b  relative to the light emitter  1044  may differ on different sides of the light collimators  1302  in order to direct the light in the particular non-normal direction. Thus, as illustrated, light exiting the light collimator  1302  may propagate generally in a direction  1048  which is not normal to the surface of the light emitter  1044 . In some other embodiments, in order to direct light in the direction  1048 , the taper of the upper side  1303   a  may be different than the taper of the lower side; for example, the upper side  1303   a  may flare out to a greater extent than the lower side  1303   b.    
     With continued reference to  FIG.  25   , the substrate  1301  may be formed of various materials having sufficient mechanical integrity to maintain the desired shape of the reflective walls  1303 . Examples of suitable materials include metals, plastics, and glasses. In some embodiments, the substrate  1301  may be a plate of material. In some embodiments, substrate  1301  is a continuous, unitary piece of material. In some other embodiments, the substrate  1301  may be formed by joining together two or more pieces of material. 
     The reflective walls  1303  may be formed in the substrate  1301  by various methods. For example, the walls  1303  may be formed in a desired shape by machining the substrate  1301 , or otherwise removing material to define the walls  1303 . In some other embodiments, the walls  1303  may be formed as the substrate  1301  is formed. For example, the walls  1303  may be molded into the substrate  1301  as the substrate  1301  is molded into its desired shape. In some other embodiments, the walls  1303  may be defined by rearrangement of material after formation of the body  2200 . For example, the walls  1303  may be defined by imprinting. 
     Once the contours of the walls  1303  are formed, they may undergo further processing to form surfaces having the desired degree of reflection. In some embodiments, the surface of the substrate  1301  may itself be reflective, e.g., where the body is formed of a reflective metal. In such cases, the further processing may include smoothing or polishing the interior surfaces of the walls  1303  to increase their reflectivity. In some other embodiments, the interior surfaces of the reflectors  2110  may be lined with a reflective coating, e.g., by a vapor deposition process. For example, the reflective layer may be formed by physical vapor deposition (PVD) or chemical vapor deposition (CVD). 
     It will be appreciated that the location of a light emitter relative to an associated light collimator may influence the direction of emitted light out of the light collimator. This is illustrated, for example, in  FIGS.  26 A- 26 C , which illustrate examples of differences in light paths for light emitters at different positions relative to center lines of overlying, associated light collimators. As shown in  FIG.  26 A , the emissive micro-display another 30 has a plurality of light emitters  1044   a , each having an associated light collimator  1302  which facilitates the output of light having narrowed angular emission profiles  1047 . The light passes through the projection optics  1070  (represented as a simple lens for ease of illustration), which converges the light from the various light emitters  1044   a  onto an area  1402   a.    
     With continued reference to  FIG.  26 A , in some embodiments, each of the light collimators  1302  may be symmetric and may have a center line which extends along the axis of symmetry of the light collimator. In the illustrated configuration, the light emitters  1044   a  are disposed on the center line of each of the light collimators  1302 . 
     With reference now to  FIG.  26 B , light emitters  1044   b  are offset by a distance  1400  from the center lines of their respective light collimators  1302 . This offset causes light from the light emitters  1044   b  to take a different path through the light collimators  1302 , which output light from the light emitters  1044   b  with narrowed angular emission profiles  1047   b . The projection optics  1070  then converges the light from the light emitters  1044   b  onto the area  1402   b , which is offset relative to the area  1402   a  on which light from the light emitters  1044   a  converge. 
     With reference now to  FIG.  26 C , light emitters  1044   c  offset from both the light emitters  1044   a  and  1044   b  are illustrated. This offset causes light from the light emitters  1044   c  to take a different path through the light collimators  1302  than light from the light emitters  1044   a  and  1044   b . This causes the light collimators  1302  to output light from the light emitters  1044   c  with narrowed angular emission profiles that take a different path to the projection optics  1070  than the light from the light emitters  1044   a  and  1044   b . Ultimately, the projection optics  1070  converges the light from the light emitters  1044   c  onto the area  1402   c , which is offset relative to the areas  1402   a  and  1402   b.    
     With reference to  FIGS.  26 A- 26 C , each triad of light emitters  1044   a ,  1044   b ,  1044   c  may share a common light collimator  1302 . In some embodiments, the micro-display  1030  may be a full-color micro-display and each light emitter  1044   a ,  1044   b ,  1044   c  may be configured to emit light of a different component color. Advantageously, the offset areas  1402   a ,  1402   b ,  1402   c  may correspond to the in-coupling optical elements of a waveguide in some embodiments. For example, the areas  1402   a ,  1402   b ,  1402   c  may correspond to the in-coupling optical element  1022   a ,  1022   b ,  1022   c , respectively, of  FIGS.  11 A and  12   . Thus, the light collimators  1302  and the offset orientations of the light emitters  1044   a ,  1044   b ,  1044   c  may provide an advantageously simple three-pupil projection system  1010  using a full-color emissive micro-display. 
     As noted herein, the light collimator  1302  may also take the form of a nano-lens.  FIG.  27    illustrates an example of a side view of individual light emitters  1044  of an emissive micro-display  1030  with an overlying array  1300  of light collimators  1302  which are nano-lenses. As discussed herein, individual ones of the light emitters  1044  may each have an associated light collimator  1302 . The light collimators  1302  redirect light from the light emitters  1044  to narrow the large angular emission profile  1046  of the light emitters  1044 , to output light with the narrowed angular emission profile  1047 . 
     With continued reference to  FIG.  27   , in some embodiments, the light collimators  1302  may be grating structures. In some embodiments, the light collimators  1302  may be gratings formed by alternating elongated discrete expanses (e.g., lines) of material having different refractive indices. For example, expanses of material  1306  may be elongated into and out of the page and may be formed in and separated by material of the substrate  1308 . In some embodiments, the elongated expanses of material  1306  may have sub-wavelength widths and pitch (e.g., widths and pitch that are smaller than the wavelengths of light that the light collimators  1302  are configured to receive from the associated light emitters  1044 ). In some embodiments, the pitch  1304  may be 30-300 nm, the depth of the grating may be 10-1000 nm, the refractive index of the material forming the substrate  1308  may be 1.5-3.5, and the refractive index of the material forming the grating features  1306  may be 1.5-2.5 (and different from the refractive index of the material forming the substrate  1308 ). 
     The illustrated grating structure may be formed by various methods. For example, the substrate  1308  may be etched or nano-imprinted to define trenches, and the trenches may be filled with material of a different refractive index from the substrate  1308  to form the grating features  1306 . 
     Advantageously, nano-lens arrays may provide various benefits. For example, the light collection efficiencies of the nano-lenslets may be large, e.g., 80-95%, including 85-90%, with excellent reductions in angular emission profiles, e.g., reductions to 30-40° (from 180°). In addition, low levels of cross-talk may be achieved, since each of the nano-lens light collimators  1302  may have physical dimensions and properties (e.g., pitch, depth, the refractive indices of materials forming the feature  1306  and substrate  1308 ) selected to act on light of particular colors and possibly particular angles of incidence, while preferably providing high extinction ratios (for wavelengths of light of other colors). In addition, the nano-lens arrays may have flat profiles (e.g., be formed on a flat substrate), which may facilitates integration with micro-displays that may be flat panels, and may also facilitate manufacturing and provide high reproducibility and precision in forming the nano-lens array. For example, highly reproducible trench formation and deposition processes may be used to form each nano-lens. Moreover, these processes allow, with greater ease and reproducibility, for variations between nano-lenses of an array than are typically achieved when forming curved lens with similar variations. 
     With reference now to  FIG.  28   , a perspective view of an example of an emissive micro-display  1030  is illustrated. It will be appreciated that the light collimator arrays  1300  advantageously allow light emitted from a micro-display to be routed as desired. As result, in some embodiments, the light emitters of a full-color micro-display may be organized as desired, e.g., for ease of manufacturing or implementation in the display device. In some embodiments, the light emitters  1044  may be arranged in rows or columns  1306   a ,  1306   b ,  1306   c . Each row or column may include light emitters  1044  configured to emit light of the same component color. In displays where three component colors are utilized, there may be groups of three rows or columns which repeat across the micro-display  1030 . It will be appreciated that where more component colors are utilized, each repeating group may have that number of rows or columns. For example, where four component colors are utilized, each group may have four rows or four columns, with one row or one column formed by light emitters configured to emit light of a single component color. 
     In some embodiments, some rows or columns may be repeated to increase the number of light emitters of a particular component color. For example, light emitters of some component colors may occupy multiple rows or columns. This may facilitate color balancing and/or may be utilized to address differential aging or reductions in light emission intensity over time. 
     With reference to  FIGS.  27  and  28   , in some embodiments, the light emitters  1044  may each have an associated light collimator  1302 . In some other embodiments, each line  1306   a ,  1306   b ,  1306   c  of multiple light emitters  1044  may have a single associated light collimator  1302 . That single associated light collimator  1302  may extend across substantially the entirety of the associated line  1306   a ,  1306   b , or  1306   c . In some other embodiments, the associated light collimator  1302  may be elongated and extend over a plurality of light emitters  1044  forming a portion of of an associated line  1306   a ,  1306   b , or  1306   c , and multiple similar light collimators  1302  may be provided along each of the associated lines  1306   a ,  1306   b ,  1306   c.    
     With continued reference to  FIG.  28   , each light emitter  1044  may be elongated along a particular axis (e.g., along the y-axis as illustrated); that is, each light emitter has a length along the particular axis, the length being longer than a width of the light emitter. In addition, a set of light emitters configured to emit light of the same component color may be arranged in a line  1306   a ,  1306   b , or  1306   c  (e.g. a row or column) extending along an axis (e.g., the x-axis) which crosses (e.g., is orthogonal to) the light emitter  1044 &#39;s elongate axis. Thus, in some embodiments, light emitters  1044  of the same component color form a line  1306   a ,  1306   b , or  1306   c  of light emitters, with the line extending along a first axis (e.g., the x-axis), and with individual light emitters  1044  within the line elongated along a second axis (e.g., the y-axis). 
     In contrast, it will be appreciated that full-color micro-display typically include sub-pixels of each component color, with the sub-pixels arranged in particular relatively closely-packed spatial orientations in groups, with these groups reproduced across an array. Each group of sub-pixels may form a pixel in an image. In some cases, the sub-pixels are elongated along an axis, and rows or columns of sub-pixels of the same component color extent along that same axis. It will be appreciated that such an arrangement allows the sub-pixels of each group to be located close together, which may have benefits for image quality and pixel density. In the illustrated arrangement of  FIG.  28   , however, sub-pixels of different component colors are relatively far apart, due to the elongate shape of the light emitters  1044 ; that is, the light emitters of the line  1306   a  are relatively far apart from the light emitters of the line  1306   c  since the elongated shape of the light emitters of the line  1306   b  causes the light emitters  1306   a  and  1306   c  to be spaced out more than neighboring light emitters of a given line of light emitters. While this may be expected to provide unacceptably poor image quality if the image formed on the surface of the micro-display  1030  was directly relayed to a user&#39;s eye, the use of the light collimator array  1300  advantageously allows light of different colors to be routed as desired to form a high quality image. For example, light of each component color may be used to form separate monochrome images which are then routed to and combined in an eyepiece, such as the eyepiece  1020  (e.g.,  FIGS.  11 A and  12 - 14   ). 
     With reference to  FIGS.  27  and  28   , in some embodiments, the light emitters  1044  may each have an associated light collimator  1302 . In some other embodiments, each line  1306   a ,  1306   b ,  1306   c  of light emitters  1044  may have a single associated light collimator  1302 . That single associated light collimator  1302  may extend across substantially the entirety of the associated line  1306   a ,  1306   b , or  1306   c . In some other embodiments, the associated light collimator  1302  may be elongated and extend over a plurality of light emitters  1044  forming a portion of an associated line  1306   a ,  1306   b , or  1306   c , and multiple similar light collimators  1302  may be provided along each of the associated lines  1306   a ,  1306   b ,  1306   c.    
     It will be appreciated that the light collimators  1302  may be utilized to direct light along different light paths to form multi-pupil projections systems. For example, the light collimators  1302  may direct light of different component colors to two or three areas, respectively, for light in-coupling. 
       FIG.  29    illustrates an example of a wearable display system with the full-color emissive micro-display  1030  of  FIG.  28    used to form a multi-pupil projection system  1010 . In the illustrated embodiment, the full-color emissive micro-display  1030  emits light of three component colors and forms a three-pupil projection system  1010 . The projection system  1010  has three exit pupils through which image light  1032   a ,  1032   b ,  1032   c  of different component colors propagates to three laterally-shifted light in-coupling optical elements  1022   a ,  1022   b ,  1022   c , respectively, of an eyepiece  1020 . The eyepiece  1020  then relays the image light  1032   a ,  1032   b ,  1032   c  to the eye  210  of a user. 
     The emissive-micro-display  1030  includes an array of light emitters  1044 , which may be subdivided into monochrome light emitters  1044   a ,  1044   b ,  1044   c , which emit the image light  1032   a ,  1032   b ,  1032   c , respectively. It will be appreciated that the light emitters  1044  emit image light with a broad angular emission profile  1046 . The image light propagates through the array  1300  of light collimators, which reduces the angular emission profile to the narrowed angular emission profile  1047 . 
     In addition, the array of  1300  of light collimators is configured to redirect the image light (image light  1032   a ,  1032   b ,  1032   c ) such that the image light is incident on the projection optics  1070  at angles which cause the projection optics  1070  to output the image light such that the image light propagates to the appropriate in-coupling optical element  1022   a ,  1022   b ,  1022   c . For example, the  1300  array of light collimators is preferably configured to: direct the image light  1032   a  such that it propagates through the projection optics  1070  and is incident on the in-coupling optical element  1022   a ; direct the image light  1032   b  such that it propagates through the projection optics  1070  and is incident on the in-coupling optical element  1022   b ; and direct the image light  1032   c  such that it propagates through the projection optics  1070  and is incident on the in-coupling optical element  1022   c.    
     Since different light emitters  1044  may emit light of different wavelengths and may need to be redirected into different directions to reach the appropriate in-coupling optical element, in some embodiments, the light collimators associated with different light emitters  1044  may have different physical parameters (e.g., different pitches, different widths, etc.). Advantageously, the use of flat nano-lenses as light collimators facilitates the formation of light collimators which vary in physical properties across the array  1300  of light collimators. As noted herein, the nano-lenses may be formed using patterning and deposition processes, which facilitates the formation of structures with different pitches, widths, etc. across a substrate. 
     With reference again to  FIG.  24 A , it will be appreciated that the illustrated display system shows a single emissive micro-display and omits an optical combiner  1050  ( FIGS.  11 A and  12 - 13 B ). In embodiments utilizing an optical combiner  1050 , the reflective surfaces  1052 ,  1054  ( FIGS.  11 A,  12 - 13 B, and  30 B ) in the optical combiner  1050  are preferably specular reflectors, and light from the light emitters  1044  would be expected to retain their large angular emission profiles after being reflected from the reflective surfaces  1052 ,  1054 . Thus, the problems with wasted light shown in  FIG.  24 A  are similarly present when an optical combiner  1050  is utilized. 
     With reference now to  FIG.  30 A , an example of a wearable display system with an emissive micro-display and an associated array of light collimators is illustrated.  FIG.  30 A  shows additional details regarding the interplay between the light emitters  1044 , the light collimators  1302 , and the in-coupling optical elements of the eyepiece  1020 . The display system includes a micro-display  1030   b , which may be a full-color micro-display in some embodiments. In some other embodiments, the micro-display  1030   b  may be a monochrome micro-display and additional monochrome micro-displays (not shown) may be provided at different faces of the optional optical combiner  1050  (as shown in  FIG.  30 C ). 
     With continued reference to  FIG.  30 A , the micro-display  1030   b  includes an array of light emitters  1044 , each of which emits light with a wide angular emission profile (e.g., a Lambertian angular emission profile). Each light emitter  1044  has an associated, dedicated light collimator  1302  which effectively narrows the angular emission profile to a narrowed angular emission profile  1047 . Light beams  1032   b  with the narrowed angular emission profiles pass through the projection optics  1070 , which projects or converges those light beams onto the in-coupling optical element  1022   b . It will be appreciated that the light beams  1032   b  have a certain cross-sectional shape and size  1047   a . In some embodiments, the in-coupling optical element  1022   b  has a size and shape which substantially matches or is larger than the cross-sectional shape and size of the light beam  1032   b , when that beam  1032   b  is incident on that in-coupling optical element  1022   b . Thus, in some embodiments, the size and shape of the in-coupling optical element  1022   b  may be selected based upon the cross-sectional size and shape of the light beam  1032   b  when incident on the in-coupling optical element  1022   b . In some other embodiments, other factors (re-bounce mitigation, or the angles or field of view supported by the in-coupling optical elements  1022   b ) may be utilized to determine the size and shape of the in-coupling optical element  1022   b , and the light collimator  1302  may be configured (e.g., sized and shaped) to provide the light beam  1032   b  with an appropriately sized and shaped cross-section, which is preferably fully or nearly fully encompassed by the size and shape of the in-coupling optical element  1022   b . In some embodiments, physical parameters for the light collimator  1302  and the in-coupling optical element  1022   b  may be mutually modified to provide highly efficient light utilization in conjunction with other desired functionality (e.g., re-bounce mitigation, support for the desired fields of view, etc.). Advantageously, the above-noted light collimation provided by the light collimator  1302 , and matching of the cross-sectional size and shape of the light beam  1032   b  with the size and shape of the in-coupling optical element  1022   b  allows the in-coupling optical element  1022   b  to capture a large percentage of the incident light beam  1032   b . The in-coupled light then propagates through the waveguide  1020   b  and is out-coupled to the eye  210 . 
     As illustrated, the micro-display  1030   b  may include an array  1042  of light emitters  1044 , each surrounded by non-light-emitting areas  1045  having a total width  1045   w . In addition, the light emitters  1044  have a width W and a pitch P. In arrays in which the light emitters  1044  are regularly spaced, each light emitter  1044  and surrounding area  1045  effectively forms a unit cell having the width  1045   w , which may be equal to the pitch P. 
     In some embodiments, the light collimators  1302  are micro-lenses disposed directly on and surrounding associated light emitters  1044 . In some embodiments, the width of the micro-lenses  1302  is equal to  1045   w , such that neighboring micro-lenses  1302  nearly contact or directly contact one another. It will be appreciated that light from the light emitters  1044  may fill the associated micro-lens  1302 , effectively magnifying the area encompassed by the light emitter  1044 . Advantageously, such a configuration reduces the perceptibility of the areas  1045  which do not emit light and may otherwise be visible as dark spaces to a user. However, because micro-lens  1302  effectively magnifies the associated light emitter  1044  such that it extends across the entire area of the micro-lens  1302 , the areas  1045  may be masked. 
     With continued reference to  FIG.  30 A , the relative sizes of the light emitters  1044  and light collimators  1302  may be selected such that light from the light emitters  1044  fills the associated light collimators  1302 . For example, the light emitters  1044  may be spaced sufficiently far apart such that micro-lens collimators  1302  having the desired curvature may be formed extending over individual ones of the light emitters  1044 . In addition, as noted above, the size and shape of the intra-coupling optical element  1022   b  is preferably selected such that it matches or exceeds the cross-sectional shape and size of the light beam  1032   b  when incident on that in-coupling optical element  1022   b . Consequently, in some embodiments, a width  1025  of the in-coupling optical element  1022   b  is equal to or greater than the width of the micro-lens  1302  (which may have a width equal to  1045   w  or P). Preferably, the width  1025  is greater than the width of the micro-lens  1302 , or  1045   w  or P, to account for some spread in the light beam  1032   b . As discussed herein, the width  1025  may also be selected to mitigate rebounce and may be shorter than the length (which is orthogonal to the width) of the in-coupling optical element  1022   b . In some embodiments, the width  1025  may extend along the same axis as the direction of propagation of incoupled light  1032   b  through the waveguide  1020   b  before being out-coupled for propagation to the eye  210 . 
     With reference now to  FIG.  30 B , an example of a light projection system  1010  with multiple emissive micro-displays  1030   a ,  1030   b ,  1030   c , and associated arrays  1300   a ,  1300   b ,  1300   c  of light collimators, respectively, is illustrated. The angular emission profiles of light emitted by the micro-displays  1030   a ,  1030   b ,  1030   c  are narrowed by the light collimator arrays  1300   a ,  1300   b ,  1300   c , thereby facilitating the collection of a large percentage of the emitted light by the projection optics  1070  after the light propagates through the optical combiner  1050 . The projection optics  1070  then directs the light to an eyepiece such as the eyepiece  1020  (e.g.,  FIGS.  11 A and  12 - 14   ) (not shown). 
       FIG.  30 C  illustrates an example of a wearable display system with multiple emissive micro-displays  1030   a ,  1030   b ,  1030   c , each with an associated array  1300   a ,  1300   b ,  1300   c , respectively, of light collimators. The illustrated display system includes a plurality of micro-displays  1030   a ,  1030   b ,  1030   c  for emitting light with image information. As illustrated, the micro-displays  1030   a ,  1030   b ,  1030   c  may be micro-LED panels. In some embodiments, the micro-displays may be monochrome micro-LED panels, each configured to emit a different component color. For example, the micro-display  1030   a  may be configured to emit light  1032   a  which is red, the micro-display  1030   b  may be configured to emit light  1032   b  which is green, and the micro-display  1030   c  may be configured to emit light  1032   c  which is blue. 
     Each micro-display  1030   a ,  1030   b ,  1030   c  may have an associated array  1300   a ,  1300   b ,  1300   c , respectively, of light collimators. The light collimators narrow the angular emission profile of light  1032   a ,  1032   b ,  1032   c  from light emitters of the associated micro-display. In some embodiments, individual light emitters have a dedicated associated light collimator (as shown in  FIG.  30 A ). 
     With continued reference to  FIG.  30 C , the arrays  1300   a ,  1300   b ,  1300   c  of light collimators are between the associated micro-displays  1030   a ,  1030   b ,  1030   c  and the optical combiner  1050 , which may be an X-cube. As illustrated, the optical combiner  1050  has internal reflective surfaces  1052 ,  1054  for reflecting incident light out of an output face of the optical combiner. In addition to narrowing the angular emission profile of incident light, the arrays  1300   a ,  1300   c  of light collimators may be configured to redirect light from associated micro-displays  1030   a ,  1030   c  such that the light strikes the internal reflective surfaces  1052 ,  1054  of the optical combiner  1050  at angles appropriate to propagate towards the associated light in-coupling optical elements  1022   a ,  1022   c , respectively. In some embodiments, in order to redirect light in a particular direction, the arrays  1300   a ,  1300   c  of light collimators may comprise micro-lens or reflective wells, which may be asymmetrical and/or the light emitters may be disposed off-center relative to the micro-lens or reflective wells, as disclosed herein. 
     With continued reference to  FIG.  30 C , projection optics  1070  (e.g., projection lens) is disposed at the output face of the optical combiner  1050  to receive image light exiting from that optical combiner. The projection optics  1070  may comprise lenses configured to converge or focus image light onto the eyepiece  1020 . As illustrated, the eyepiece  1020  may comprise a plurality of waveguides, each of which is configured to in-couple and out-couple light of a particular color. For example, waveguide  1020   a  may be configured to receive red light  1032   a  from the micro-display  1030   a , waveguide  1020   b  may be configured to receive green light  1032   b  from the micro-display  1030   b , and waveguide  1020   c  may be configured to receive blue light  1032   c  from the micro-display  1030   c . Each waveguide  1020   a ,  1020   b ,  1020   c  has an associated light in-coupling optical elements  1022   a ,  1022   b ,  1022   c , respectively, for in coupling light therein. In addition, as discussed herein, the waveguides  1020   a ,  1020   b ,  1020   c  may correspond to the waveguides  670 ,  680 ,  690 , respectively, of  FIG.  9 B  and may each have associated orthogonal pupil expanders (OPE&#39;s) and exit pupil expanders (EPE&#39;s), which ultimately out-couple the light  1032   a ,  1032   b ,  1032   c  to a user. 
     As discussed herein, the wearable display system incorporating micro-displays is preferably configured to output light with different amounts of wavefront divergence, to provide comfortable accommodation-vergence matching for the user. These different amounts of wavefront divergence may be achieved using out-coupling optical elements with different optical powers. As discussed herein, the out-coupling optical elements may be present on or in waveguides of an eyepiece such as the eyepiece  1020  (e.g.,  FIGS.  11 A and  12 - 14   ). In some embodiments, lenses may be utilized to augment the wavefront divergence provided by the out-couple optical elements or may be used to provide the desired wavefront divergence in configurations where the out-couple optical elements are configured to output collimated light. 
       FIGS.  31 A and  31 B  illustrate examples of eyepieces  1020  having lens for varying the wavefront divergence of light to a viewer.  FIG.  31 A  illustrates an eyepiece  1020  having a waveguide structure  1032 . In some embodiments, as discussed herein, light of all component colors may be in-coupled into a single waveguide, such that the waveguide structure  1032  includes only the single waveguide. This advantageously provides for a compact eyepiece. In some other embodiments, the waveguide structure  1032  may be understood to include a plurality of waveguides (e.g., the waveguides  1032   a ,  1032   b ,  1032   c  of  FIGS.  11 A and  12 - 13 A ), each of which may be configured to relay light of a single component color to a user&#39;s eye. 
     In some embodiments, the variable focus lens elements  1530 ,  1540  may be disposed on either side of the waveguide structure  1032 . The variable focus lens elements  1530 ,  1540  may be in the path of image light from the waveguide structure  1032  to the eye  210 , and also in the path of light from the ambient environment through the waveguide structure  1032  to the eye  210 . The variable focus optical element  1530  may modulate the wavefront divergence of image light outputted by the waveguide structure  1032  to the eye  210 . It will be appreciated that the variable focus optical element  1530  may have optical power which may distort the eye  210 &#39;s view of the world. Consequently, in some embodiments, a second variable focus optical element  1540  may be provided on the world side of the waveguide structure  1032 . The second variable focus optical element  1540  may provide optical power opposite to that of the variable focus optical element  1530  (or opposite to the net optical power of the optical element  1530  and the waveguide structure  1032 , where the waveguide structure  1032  has optical power), so that the net optical power of the variable focus lens elements  1530 ,  1540  and the waveguide structure  1032  is substantially zero. 
     Preferably, the optical power of the variable focus lens elements  1530 ,  1540  may be dynamically altered, for example, by applying an electrical signal thereto. In some embodiments, the variable focus lens elements  1530 ,  1540  may comprise a transmissive optical element such as a dynamic lens (e.g., a liquid crystal lens, an electro-active lens, a conventional refractive lens with moving elements, a mechanical-deformation-based lens, an electrowetting lens, an elastomeric lens, or a plurality of fluids with different refractive indices). By altering the variable focus lens elements&#39; shape, refractive index, or other characteristics, the wavefront of incident light may be changed. In some embodiments, the variable focus lens elements  1530 ,  1540  may comprise a layer of liquid crystal sandwiched between two substrates. The substrates may comprise an optically transmissive material such as glass, plastic, acrylic, etc. 
     In some embodiments, in addition or as alternative to providing variable amounts of wavefront divergence for placing virtual content on different depth planes, the variable focus lens elements  1530 ,  1540  and waveguide structure  1032  may advantageously provide a net optical power equal to the user&#39;s prescription optical power for corrective lenses. Thus, the eyepiece  1020  may serve as a substitute for lenses used to correct for refractive errors, including myopia, hyperopia, presbyopia, and astigmatism. Further details regarding the use of variable focus lens elements as substitutes for corrective lenses may be found in U.S. application Ser. No. 15/481,255, filed Apr. 6, 2017, the entire disclosure of which is incorporated by reference herein. 
     With reference now to  FIG.  31 B , in some embodiments, the eyepiece  1020  may include static, rather than variable, lens elements. As with  FIG.  31 B , the waveguide structure  1032  may include a single waveguide (e.g., which may relay light of different colors) or a plurality of waveguides (e.g., each of which may relay light of a single component color). Similarly, the waveguide structure  1034  may include a single waveguide (e.g., which may relay light of different colors) or a plurality of waveguides (e.g., each of which may relay light of a single component color). The one or both of the waveguide structures  1032 ,  1034  may have optical power and may output light with particular amounts of wavefront divergence, or may simply output collimated light. 
     With continued reference to  FIG.  31 B , the eyepiece  1020  may include static lens elements  1532 ,  1534 ,  1542  in some embodiments. Each of these lens elements are disposed in the path of light from the ambient environment through waveguide structures  1032 ,  1034  into the eye  210 . In addition, the lens element  1532  is between a waveguide structure  1003   2  and the eye  210 . The lens element  1532  modifies a wavefront divergence of light outputted by the waveguide structure  1032  to the eye  210 . 
     The lens element  1534  modifies a wavefront divergence of light outputted by the waveguide structure  1034  to the eye  210 . It will be appreciated that the light from the waveguide structure  1034  also passes through the lens element  1532 . Thus, the wavefront divergence of light outputted by the waveguide structure  1034  is modified by both the lens element  1534  and the lens element  1532  (and the waveguide structure  1032  in cases where the waveguide structure  1003   2  has optical power). In some embodiments, the lens elements  1532 ,  1534  and the waveguide structure  1032  provide a particular net optical power for light outputted from the waveguide structure  1034 . 
     The illustrated embodiment provides two different levels of wavefront divergence, one for light outputted from the waveguide structure  1032  and a second for light outputted by a waveguide structure  1034 . As a result, virtual objects may be placed on two different depth planes, corresponding to the different levels of wavefront divergence. In some embodiments, an additional level of wavefront divergence and, thus, an additional depth plane may be provided by adding an additional waveguide structure between lens element  1532  and the eye  210 , with an additional lens element between the additional waveguide structure and the eye  210 . Further levels of wavefront divergence may be similarly added, by adding further waveguide structures and lens elements. 
     With continued reference to  FIG.  31 B , it will be appreciated that the lens elements  1532 ,  1534  and the waveguide structures  1032 ,  1034  provide a net optical power that may distort the users view of the world. As a result, lens element  1542  may be used to counter the optical power and distortion of ambient light. In some embodiments, the optical power of the lens element  1542  is set to negate the aggregate optical power provided by the lens elements  1532 ,  1534  and the waveguide structures  1032 ,  1034 . In some other embodiments, the net optical power of the lens element  1542 ; the lens elements  1532 ,  1534 ; and the waveguide structures  1032 ,  1034  is equal to a user&#39;s prescription optical power for corrective lenses. 
     Example Light Projection Systems Having Emissive Micro-Displays Providing Enhanced Resolution 
     As described above, a display system (e.g., a wearable display system presenting AR or VR content) may utilize one or more emissive micro-displays to reduce the size, mass, and/or power consumption relative to systems utilizing various other display technologies. For example, the display system may optionally utilize a threshold number of emissive micro-displays (e.g., three displays each including an array of light emitters, such as micro-LEDs). In this example, each emissive micro-display may be configured to generate light of a particular component color. The generated light may be combined to provide the appearance of a full color image, as discussed herein. Various examples in which multiple emissive micro-displays are utilized are discussed above, and also discussed below with reference to  FIGS.  36 A- 36 B . As another example, the display system may optionally utilize a single emissive micro-display. In this example, the emissive micro-display may include light emitters (e.g., micro-LEDs) of each primary color. 
     Utilizing the one or more emissive micro-displays, the display system described herein may be configured to output AR or VR content (“virtual content”) at a greater resolution than a resolution directly corresponding to the number of light emitters included in the emissive micro-displays. For example, the display system may utilize one or more actuators to cause movement of or adjustment to one or more parts of a light projection system configured to output light forming virtual content to a user. For example, the actuators may adjust geometric positions associated with the light emitters. As an example, and as illustrated in  FIG.  36 B , the actuators may cause a change in the position of the emissive micro-display panels. In this example, the micro-LED panels may be shifted along one or two axes. As another example, and as illustrated in  FIG.  36 A , the actuator may cause a change in the position of the location of a projection optic (e.g., one or more projection lenses). As described herein, the projection optic may route light generated by one or more micro-LED panels to one or more in-coupling optical elements, such as in-coupling gratings (ICGs). The in-coupling optical elements may be configured to route the light to a user of the display system. 
     The adjustment described above may be leveraged to cause geometric positions of light emitters to assume positions located in inter-emitter regions of the arrays. As described above, an inter-emitter region (e.g., region  1045  illustrated in  FIG.  32 A ) may include a region of an emissive micro-display in which one light emitter is included. The inter-emitter region may therefore be defined based on one or more pixel pitches. For example, the inter-emitter region may be delineated by a first side having a length equal to a pixel pitch along a first axis, and second side having a length equal to a pixel pitch along a second axis. 
       FIG.  32 A  illustrates an example of an emissive micro-display  1030  having an array  1042  of light emitters (e.g., light emitter  1044 ) that are separated by an inter-emitter region  1045 . The light emitter  1044  may have an emitter size p and a pixel pitch Λ. As illustrated, the light emitter  1044  may have an emitter size p and a pixel pitch Λ that are substantially identical in the x and y directions. However, it should be appreciated that, in some embodiments, the emitter size p and pixel pitch Λ are different in the x and y directions. In addition, different ones of the light emitters of the array  1042  may have different sizes, shapes (e.g., round), compositions, etc. In the illustrated array  1042 , a second light emitter on the top row of light emitters is indicated by the reference  1044 ′ to facilitate the subsequent discussion herein. 
     With continued reference to  FIG.  32 A , in some embodiments the magnitude of the pixel pitch Λ may be larger than the emitter size p. As discussed herein, emissive micro-display  1030  may have a relatively low fill factor due to various physical and electrical constraints. For example, each light emitter  1044  may be disposed in an associated area  1049 , with only a minority of that area being occupied by the light emitter  1044 . The majority of the area  1049  is occupied by the inter-emitter region  1045 . The area  1049  may be defined as extending the pixel pitch from an extremity of an associated light emitter  1044  along the x-axis and the pixel pitch from an extremity of the associated light emitter  1044  along the y-axis. The low fill factor may undesirably limit the pixel density and ultimate resolution of images formed using the emissive micro-display  1030 . 
     In some embodiments, the positions of the light emitters of the array  1042 , as seen by a user at a first point in time, may be shifted at a second point in time to locations originally in the inter-emitter region  1045 , to thereby display pixels corresponding to those locations in an image. Thus, a high-resolution image frame may be broken up into lower resolution subframes, with a first subframe having pixels at locations corresponding to a first position of the light emitters, a second subframe having pixels at locations corresponding to a second position of the light emitters, a third subframe having pixels at locations corresponding to a third position of the light emitters, and so on. Thus, the positions of the light emitters, as seen by the user, may be adjusted in position to effectively tile (e.g., substantially tile) the subframes of the high-resolution image. It will be appreciated that the subframes and the high-resolution image frame occupy substantially the same area (e.g., are substantially the same physical size), as perceived by a user. For example, the subframes are preferably 90%, 95%, 99%, or 100% of the size of the high-resolution image frame, except that they have lower pixel density than the high-resolution image frame. 
       FIG.  32 B  illustrates an example of how the emissive micro-display  1030  of  FIG.  32 A  may be configured to emulate a higher fill-factor micro-display via time-multiplexing and repositioning of the array or associated optics. As described previously, the emissive micro-display  1030  of  FIG.  32 A  may be configured to form, in rapid succession and with offsets for the perceived positions of individual light emitters, individual partial-resolution subframes. In such embodiments, the visual system of a user may merge together the subframes such that the user perceives a full-resolution frame. 
     With continued reference to  FIG.  32 B , the illustrated pixels  1044   a - 1044   c  represent the locations of the first light emitter  1044  ( FIG.  32 A ) as seen by a user at different points in time. In addition, the illustrated pixels  1044   a ′- 1044   c ′ represent the locations of the second light emitter  1044 ′ ( FIG.  32 A ) at the same points in time as the illustrated pixels  1044   a - 1044   c , respectively. The first light emitter  1044  at a first position may emit light for pixel  1044   a  of a first subframe. This pixel may represent a first pixel  1044   a  of a rendered frame of virtual content. The perceived position of the first light emitter  1044  may then be shifted by a distance less than the pixel pitch Λ along the axis of the shift (e.g., by Δx and/or Δy) and the first light emitter  1044  may emit light for a pixel  1044   b  in a second subframe. As illustrated, the position of the first light emitter  1044  has shifted by Δx along the x-axis. The pixel  1044   b  may thus be a second pixel  1044   b  of the rendered frame of virtual content. As will be described below, the geometric position of the first light emitter  1044  may be shifted via an actuator connected to the array  1042  (FIG.  32 A). Thus, the first light emitter  1044  may be physically relocated in three-dimensional space. The geometric position may also be shifted via an actuator connected to a projection optic through which light from the first light emitter  1044  is routed. Thus, the first light emitter  1044  may remain in a same physical location, and its light may be shifted by shifting the position of the projection optics relative to the first light emitter  1044 . 
     With continued reference to  FIG.  32 B , subsequent to shifting the geometric position of the first light emitter  1044  for the pixel  1044   b , the first light emitter  1044  may again be shifted (by Δx as illustrated) and the first light emitter  1044  may emit light for a pixel  1044   c  in a third subframe of the rendered frame of virtual content. This pixel  1044   c  may represent a third pixel of the rendered frame. This process may be repeated for a total N subframes, which together form the full resolution rendered frame of virtual content. The remaining light emitters of the array  1042  may similarly emit light, over multiple offset subframes, for multiple pixels in the full frame. In the example of  FIG.  32 B , this process is repeated for 9 subframes, such that the first emitter  1044  provides all 9 pixels fitting within the area  1049 . In this way, the array  1042  ( FIG.  32 A ) may be output virtual content with a resolution which is three times greater in the x direction, and three times greater in the y direction, as compared to the resolution of the array  1042 . 
     In some embodiments, the perceived positions of the array  1042  of light emitters may be updated in a substantial continuous movement. For example, the position of emitter  1044  may be shifted continuously along an x direction until output of pixel  1044   c . The display system (e.g., one or more processors or processing elements) described herein may determine an extent to which the position of emitter  1044  has been shifted during this continual movement. The display system may be configured to determine a time at which to output light corresponding to a new pixel. For example, the display system may identify that a position of emitter  1044  has reached a distance corresponding to pixel  1044   b . The display system may then cause the emitter  1044  to output light based on an image value associated with pixel  1044   b  in the second subframe. Utilizing such continual adjustment of the geometric positions may reduce jerkiness associated with shifting the geometric positions. In some other embodiments, the geometric positions may be shifted in discrete steps. For example, emitter  1044  may output light corresponding to pixel  1044   a . The geometric position of emitter  1044  may then be shifted in a discrete step and paused to output light corresponding to pixel  1044   b . Other light emitters of the array  1042  may similarly be shifted along with the light emitter  1044 . For example, the light emitter  1044 ′ may be shifted in discrete steps to provide the pixels  1044   a ′,  1044   b ′, and  1044   c ′ at different ones of the discrete steps. 
     In some embodiments, the number of subframes N may be determined or limited by the physical properties of the array  1042 . An example property may include a maximum framerate (e.g., N is preferably not be so large that subframes fail to merge together in users&#39; visual systems; that all N subframes are preferably displayed over a time duration that is less than the flicker fusion threshold of the user, e.g., less than 1/60 of a second). Additional example properties may include the emitter pitch Λ, the emitter size p, and so on. As described above, the number of subframes N may be determined based on a number of positions in which an emitter of the array  1042  may fit within an inter-emitter region  1045 . In the example of  FIG.  32 B , the first emitter  1044  may be placed in 9 distinct positions within the inter-emitter region  1045 . Thus, there may be up to 9 subframes. If the emitter size p were larger and/or if the emitter pitch Λ were smaller, then the number of subframes N may be reduced. Similarly, if the emitter size p were smaller and/or if the emitter pitch Λ were larger, then the number of subframes N may be increased. 
     In some embodiments, N may be determined based on computing a floor of the emitter pitch Λ divided by the emitter size p. The computed floor may represent a number of times an emitter may be adjusted or moved in each direction. For example, if Λ=2.5 micron and p=0.8 micron, then N would equal 3. Thus, there may be 9 subframes (e.g., 3×3). It will be appreciated that this determination may be adjusted depending on whether emitter pitch Λ and/or emitter size p varies along the x and y directions. For example, there may be emitter pitch x  Λ x  and emitter pitch y  Λ y . In this example, N may thus vary based on direction. The number of subframes may be determined as being N x ×N y . 
     Example Emissive Micro-Display for Forming Foveated Images 
     Another potentially desirable feature in VR, AR, and MR applications is foveated imaging (also referred to simply as foveation), in which the resolution of a displayed image varies across that image. In particular, VR, AR, and MR may include eye tracking systems that determine where users are looking. Given the limitations of the human visual system, which generally detects fewer details in portions of the field of view away from a user&#39;s fixation point, presenting full resolution content (e.g., content at the full-rendered resolution) at the periphery of users&#39; vision may be undesirable. The peripheral full resolution content may consume excessive processing power in rendering and have excessive power consumption when displayed by the display system. In other words, significant benefits may be achieved in terms of reduced processing loads and display power consumption by reducing the resolution of content away from a user&#39;s fixation point and by delivering the highest resolution image content only to the part of the viewing field that the user is looking, for example, at and immediately adjacent the fixation point. It will be appreciated that the fixation point corresponds to the portion of the field of view that is focused onto the fovea of the user&#39;s eye; thus, the eye has relatively high sensitive to detail in this portion of the field of view. 
     Foveation may also be based on factors other than fixation point. As an example, content creators may specify that certain content, such as text, be displayed at full resolution even if the user is looking away from the content. As another example, content creators may specify that foveation should only be active under certain conditions. As yet other examples, foveation may be a user selectable setting or may be automatically enabled as a result of a low battery condition. In at least some embodiments, foveation may conserve display resources (data, pixels, bandwidth, computation) by delivering the highest resolution only to portions of an image in the part of the field of view that the user is fixating on (e.g., represented by foveal region A in  FIG.  33   ), while delivering a lower resolution image to the peripheral part (e.g., represented by peripheral region B in  FIG.  33   ). 
       FIGS.  32 C,  32 D, and  33    illustrate examples of configurations of the array  1042  of  FIG.  32 A  for providing foveated images  1130 ,  1140 , and  1200 , respectively. 
       FIG.  32 C  illustrates an example of a foveated image  1130  formed by an emissive micro-display, such as the emissive micro-display  1030  of  FIG.  32 A . The array  1042  of  FIG.  32 A  may be configured to provide two levels of resolution for the rendered image  1130  of virtual content. In particular, the light emitter array  1042  may provide full-resolution (or relatively high resolution) in a foveal region  1132  (the portion of the image expected to be focused onto the user&#39;s fovea) and may provide a partial-resolution (relatively low resolution) in a second region  1134 . The location of the foveal region  1132  may be determined according to the location of a user&#39;s fixation point. For example, a gaze detection scheme may be employed. The gaze detection scheme may track eyes of the user. As an example, a pupil may be identified in each eye. A vector may be extended from each identified pupil and an intersection of the vectors in three-dimensional space may be determined. This intersection may represent a fixation point of the user. The foveal region  1132  may correspond to a portion of the rendered image  1130  which is within a threshold angular distance of this fixation point. Additional details regarding foveation and the detection of a user&#39;s fixation point may be found in U.S. Patent App. Pub. No. 2018/0275410, the entire disclosure of which is incorporated by reference herein. 
     To provide a high resolution for the foveal region  1132 , while maintaining a lower resolution for the second region  1134 , the light emitters  1044  of the array  1042  ( FIG.  32 A ) may be updated differently. With reference again to the example of  FIG.  32 B , in which an emitter was updated N (e.g., 9) times for a full-resolution frame of virtual content, a portion of the light emitters  1044  that correspond to pixels in the foveal region  1132  may be updated N times. In contrast, a remaining portion of the light emitters  1044  which correspond to pixels in the second region  1134  may be updated less than N times (e.g., once, 2 times, 3, times, and so on) per rendered frame. As an example, the perceived positions of the light emitters  1044  may be shifted as described herein. For light emitters corresponding to the foveal region  1132 , the light emitters  1044  may be updated for each shift in perceived light emitter position. For example, these emitters may generate light corresponding to an updated pixel value included for each emitter position. The updated pixel values may represent pixel values included in respective subframes of the full-resolution frame of virtual content. For emitters included in the second region  1134 , the emitters may not be updated for every shift. For example, these emitters may generate light corresponding to a same pixel value for two or more geometric positions, or may simply not output light. As a result, these emitters may skip (e.g., not present) pixel values included in one or more of the subframes. 
     The second region  1134  is illustrated in  FIG.  32 C  as having the resolution of the physical array of emitters included in array  1042 , however this is merely one option. If desired, the partial-resolution in the second region  1134  may have a resolution lower than the resolution of the physical array  1042 . For example, a portion of the emitters in region  1134  may be deactivated. Alternatively, the second-region may have a resolution higher than the resolution of the physical array  1042 . For example, the perceived positions of the light emitters corresponding to region  1134  may be shifted to display pixels for multiple subframes per image frame  1130 , although the number of pixels displayed in region  1134  per rendered frame is less than in the foveal region  1132 . 
       FIG.  32 D  illustrates an example of an emissive micro-display, such as the emissive micro-display  1030  of  FIG.  32 A , configured to form foveated images with three or more levels of resolution within the image. For the illustrated displayed image  1140 , the array  1042  of  FIG.  32 A  may be configured to provide three levels of resolution in each display image  1140 , with first region  1142  having full resolution, the second region  1144  having an intermediate resolution, and the third region  1146  having a low resolution. In general, the array  1042  may be configured to implement foveation with any desired number of regions of different resolution and the resolution at any location within the display may be arbitrarily selected (e.g., by selecting how many of the pixels of the total subframes are utilized for each light emitter location of the array). For example, for pixels in the first region  1142 , the corresponding light emitters may present the pixel information for every subframe, for the second region  1144  the corresponding light emitters may present the pixel information for fewer subframes, and for the third region  1146  the corresponding light emitters may present the pixel information for yet fewer subframes. In some embodiments, it may be desirable to transition smoothly between high-resolution and low-resolution regions. Such transitioning can be accomplished by gradually reducing the number of subframes that individual light emitters present information for, as discussed above. 
     With reference to  FIGS.  32 C and  32 D , the foveal regions (e.g., regions  1132  and  1142 ) and the transition region  1144  are shown as rectangles (squares) for ease of illustration, and it will be appreciated that these regions may assume any shape. For example, these regions may have shapes that are circular, star-shaped, oval, etc.  FIG.  33    illustrates another example of a foveated image provided by an emissive micro-display. As shown in  FIG.  33   , the highest resolution may be provided only to the foveal part  1202 , which may have a circular shape and is represented by region A  1202 . Outside the foveal portion of the field of view (e.g., in region B  1204 ), the resolution of the displayed image may be reduced, thereby reducing processing loads associated with rendering and reducing display power consumption. 
     In each of  FIGS.  32 C,  32 D, and  33   , the location of the high-resolution portion of the foveated image may be determined according to the user&#39;s eye pose or gaze direction (e.g., as determined by an eye-tracking system including components such as camera assembly  630 ,  FIG.  6   ). As examples, the high-resolution portion (e.g., foveal region  1132 , first region  1142 , and region A  1202 ) may be roughly in the center of the foveated image when the user is looking straight ahead, and the high-resolution portion may be off to the left side when the user is looking left. In this manner, the user may be presented with relatively high resolution images along the direction of the user&#39;s gaze (e.g., at the fixation point), while the user is presented with lower resolution in portions of images in their peripheral vision. 
     Example Movements of Emissive Micro-Display and/or Display Optics 
     As discussed herein, the positions of displayed pixels may be shifted by shifting the physical positions of parts a light projection system, such as, for example, light emitters  1044  ( FIG.  32 A ) and/or projection optics  1070  ( FIGS.  11 A,  12 - 14 ,  24 A- 24 B,  26 A- 26 C, and  29 - 30 C ). As also discussed herein, the physical positions may be shifted using an actuator mechanically connected to the part to be shifted. It will be appreciated that the positions of light emitters may be shifted by, for example, shifting an array containing the light emitter. 
     In some embodiments, these shifts may be made in discrete steps. For example, the light emitters and/or projection optics may be stationary or substantially stationary while they emit light to form pixels of an individual subframe. The positions of the light emitters and/or projection optics may then be shifted between the presentation of different subframes. 
     In some embodiments, the light emitters and/or projection optics may be continuously moved between subframes with or without a reduction in velocity while the light emitters are displaying or projecting an individual subframe. Such continuous movement may advantageously be simpler to implement than precisely starting and stopping the movement of the light emitters and/or projection optics in small steps. In either case, the result remains that the relatively low-resolution and low-fill-factor array  1042  ( FIG.  32 A ) emulates a relatively high-resolution and high-fill factor array. 
       FIG.  34    illustrates various example paths of movement of parts of an emissive micro-display to shift the positions of displayed pixels. For example, as described herein, the movements may be made using actuators connected to one or more emissive micro-displays or connected to one or more projection optics. The actuators may cause the emissive micro-displays and/or projection optics to move on a plane along the illustrated paths. In  FIG.  34   , each numbered position may be understood to be a position at which light for a pixel of a different subframe is emitted; thus, each numbered position is associated with a different subpixel. Preferably, one loop of the various paths of movement are completed and returned to the initial position within the flicker fusion threshold. 
     In some embodiments, the movements may be made through use of two actuators. For example, a first actuator may adjust movement in a first direction (e.g., an x-direction) and a second actuator may adjust movement in a second direction (e.g., a y-direction). In some embodiments, the first actuator and second actuator may operate in quadratures (e.g., 180 degrees phase shift relative to each other). As an example, the first actuator may perform a cosine motion while the second actuator may perform a sine motion, with the two motions combining to define a circle. Consequently, in some embodiments, it will be appreciated that the various actuators (e.g. actuators  1504 ,  1504   a - c , etc.) herein may be understood to be an aggregate structure encompassing two constituent actuators, each providing movement along a particular axis. 
     With continued reference to  FIG.  34   , in movement pattern  1300 , the geometric positions may move (e.g., oscillate) back and forth between two points, thus providing a perceived pixel resolution that is double the base resolution of the array. It will be appreciated the base resolution is the resolution provided by the array without shifting the array as discussed herein. In movement pattern  1302 , the path of movement may define a triangular shape, which may increase the base resolution of the array  1042  by up to a factor of three. In movement pattern  1304 , the path of movement may define a rectangular pattern, thus increasing the base resolution of the array  1042  by up to a factor of four. In movement pattern  1306 , the geometric positions may move in a rectangular pattern, thus increasing the base resolution of the array  1042  by up to a factor of six. It will be appreciated that a different subframe is not necessarily presented at each numbered position and, as a result, as described above, the increase in resolution may be “up to” a certain factor. 
     In some embodiments, with continued reference to  FIG.  34   , it will be appreciated that other additional subframes may be presented on each leg of the various illustrated paths. For example, on the leg from position  1  to position  2  of movement path  1300 , one or more subframes may be presented at different points on that path between positions  1  and  2 . In such an arrangement, the increase in resolution may be at least the associated factors noted above for each of the movement patterns. 
     With reference now to  FIGS.  35 A and  35 B , examples are illustrated of how displacement of light emitters and projection optics may change the position of a displayed pixel. As shown in  FIG.  35 A , displacing an object point (e.g., an individual light emitter) by δ along a line on a plane  1400 , which may be representative of displacing an array along that line, changes the direction of the light ray transmitted through projection optic  1070  from a first direction α 1    1404  to a second direction α 2    1406 . This change in direction, then, will cause the pixel provided by the illustrated light emitter to shift, since the position of the light emitter has shifted. The direction of the light ray transmitted through the projection optics  1070  may have a roughly 1-1 correspondence with position of an object point. Thus, an emissive micro-display may be shifted along one or more axes to shift the location of a displayed pixel. 
     As shown in  FIG.  35 B , displacing a projection optics  1070  by δ also changes the direction of the transmitted light ray from the first position α 1    1404  to the second position α 2    1406 . It should be appreciated that a shifting of the projection optics  1070  along one or more axes may be based on physical characteristics associated with the projection optic  1070 . For example, an extent to which the projection optics  1070  is adjusted upwards may depend on physical characteristics of the projection optics  1070 , and the impact of the projection optics  1070  on the path of light. Example characteristics may include focal length, a type of the lens, refractive index, radius of curvature, and so on. 
     Thus, the techniques described herein for enhancing the resolution of an emissive micro-display may be accomplished via displacements of the projection optics or other optical component between the emissive micro-display and the user. Moreover, and as described above with respect to  FIGS.  9 A- 9 E , a full-color emissive micro-display may employ three emissive micro-displays each having a different color (e.g., a red array, a green array, and a blue array) whose light is optically combined and then projected through common projection optics. In such embodiments, it may be simpler to implement controlled displacements of the common optics, as it may require only a single displacement actuator (or set of actuators) to displace the common optics as described herein, instead of a displacement actuator (or set of actuators) for each of the emissive micro-displays. 
     Example Emissive Micro-Display Systems 
     As discussed herein, the various parts of a light projection system may be moved to provide the desired shifting of positions of displayed pixel, and this movement may be achieved using actuators mechanically connected to the parts to be moved. 
       FIG.  36 A  illustrates an example of a wearable display system having a light projection system with an actuator coupled to projection optics. The light projection system  1010  and the actuator  1504  may be referred to together as the projection system  1500 . It will be appreciated that the light projection system  1010  of the projection system  1500  may assume any of the various configurations disclosed herein (e.g., as illustrated and discussed regarding  FIGS.  11 A,  12 - 14 ,  24 A ). To the extent that micro-lenses, micro-reflectors, or gratings are utilized with the light emitters of the projection system  1500  (e.g., as illustrated in  FIGS.  24 B,  26 A- 26 C, and  29 - 30 C ), the micro-lenses, micro-reflectors, or gratings are preferably configured to provide an effective pixel size less than the pixel pitch to allow a sufficiently sparse array to facilitate the position shifting described herein. Additionally, as discussed herein, the actuator  1504 , or actuators  1504   a - 1504   c , may each include, or otherwise represent, two actuators which cause movement along different axes. 
     With continued reference to  FIG.  36 A , an example of an actuator is a piezoelectric actuator. The actuator  1504  may adjust the position of the projection optics  1070  along one or more axes on a plane (e.g. a plane parallel to the plane on which the eyepiece  1020  is disposed) as described herein. For example, the actuator  1504  may move the projection optics  1070  along two crossing axes on that plane (e.g., using a two-dimensional piezoelectric motor). As illustrated, the projection optics outputs light from the emissive micro-displays  1030   a - 1030   c  to the in-coupling optical elements  1022   a - 1022   c  of the waveguides of the eyepiece  1020 . 
     The light projection system  1010  may utilize monochrome emissive micro-displays  1030   a ,  1030   b ,  1030   c , each configured to output a different component color. An optical combiner  1050 , such as a dichroic x-cube, may redirect the light emitted from the emissive micro-displays  1030   a - 1030   c  to the projection optics  1070  as described above. 
     In some embodiments, the projection optics  1070  is configured to receive image light from the emissive micro-displays  1030   a - 1030   c , the actuator  1504  is configured to move the projection optics  1070 , which then causes the image light outputted by the light projection system  1500  to shift. Thus, the pixels presented by an array may be perceived to be adjusted in location, for example to tile subframes across an inter-emitter region as described herein; the emissive micro-displays  1030   a - 1030   c  may output light corresponding to a plurality of subframes. These subframes may be presented in rapid succession (within the flicker fusion threshold), such that a user may perceive them as being present simultaneously in a full resolution frame of virtual content. 
     In some embodiments, one or more of the emissive micro-displays  1030   a - 1030   c  are independently moveable relative to others of the emissive micro-displays, the optical combiner  1050 , and the projection optics  1070 . In such embodiments, each independently moveable micro-display may have an associated independently moveable actuator  1504 .  FIG.  36 B  illustrates an example of a wearable display system having a light projection system  1500  with multiple actuators  1504   a - 1504   c , each coupled to a different emissive micro-display  1030   a - 1030   c . The actuators  1504   a - 1504   c  may thus shift its associated component color emissive micro-displays  1030   a - 1030   c . This embodiment may allow each component color emissive micro-display  1030   a - 1030   c  to be shifted to a respective position to output a same subframe such that the subframe overlaps in the eyes of the user. In some embodiments, the component color emissive micro-displays  1030   a - 1030   c  may be shifted along different paths. 
     With reference again to  FIGS.  36 A- 36 B , the actuators  1504  or  1504   a - 1504   c  may be controlled via one or more processing elements included in the display system. Additionally, the output of image light by the emissive micro-displays  1030   a - 1030   c  may be synchronized with movement of the actuators  1504  or  1504   a - 1504   c . For example, the emissive micro-displays  1030   a - 1030   c  may output light based on a signal, or instruction, indicating that the actuators  1504  or  1504   a - 1504   c , the parts of the light rejection system  1500  moved by the actuators, have been shifted to one or more positions. This signal, or instruction, may be utilized for a continual movement pattern or a discrete movement pattern as described above. The signal or instruction may be generated by a display system, such as one or more processors or processing elements. In some embodiments, the light projection system  1500  is part of the display system  60  ( FIG.  9 E ) and the control elements for the actuator  1504  and the emissive micro-displays  1030   a - 1030   c  may be part of the processing modules  140  or  150  ( FIG.  9 E ). 
     In some embodiments, actuators  1504  or  1504   a - 1504   c  may continually move the mechanically coupled part of the light projection system  1500 , for example according to the movement patterns illustrated in  FIG.  34   , and the emissive micro-displays  1030   a - 1030   c  may periodically generate light. With respect to a continual movement pattern, the emissive micro-displays  1030   a - 1030   c  may be synchronized with a signal (e.g., a clock signal) utilized also by the actuators  1504  and/or  1504   a - 1504   c , to thereby present subframes at the desired location of those subframes. For example, the actuator  1504  may shift the projection optics  1070  according to a known movement pattern (e.g., known velocity based on the click signal). Thus, the emissive micro-displays  1030   a - 1030   c  may utilize the signal to identify an extent to which the projection optics  1070  has been moved along the movement pattern. The emissive micro-displays  1030   a - 1030   c  may then output light, for example, corresponding to a new subframe, when the projection optics  1070  is in a position associated with the new subframe. 
     In some embodiments, as discussed herein, time division multiplexing may be utilized for the micro-displays  1030   a ,  1030   b ,  1030   c . For example, different ones of the emissive micro-displays  1030   a ,  1030   b ,  1030   c , may be activated at different times to generate different component color images. 
     In some embodiments, the actuators  1504   a - 1504   c  may be moved to complete at least one movement loop (e.g., a loop of the movements paths  1300 - 1306 ,  FIG.  34   ) with only a single one of the emissive micro-displays  1030   a - 1030   c  generating subframes of a single component color during that loop. In some embodiments, after completing one loop, subframes of a second component color are generated by a second one of the micro-displays during a second loop of actuator movement; and after completing that second loop, subframes of a third component color are generated by a third one of the micro-displays during a third loop of actuator movement. Thus, each of the loops of actuator movement generate a set of tiled subframes, for a total of three sets of loops and subframes where there are three component colors (the number of sets being equal to the number of component colors). Preferably, complete sets of subframes of each component are completed within a flicker fusion threshold. 
     While the eyepiece  1020  is illustrated in  FIGS.  36 A- 36 B  as including a stack of waveguides, it will be appreciated that the eyepiece  1020  may include a single waveguide in some embodiments, as disclosed herein.  FIG.  37 A  illustrates an example of a wearable display system having a light projection system  1500  with an eyepiece  1020  having a single waveguide  1020   a . The illustrated single waveguide eyepiece  1020  may be similar to that illustrated in and discussed regarding  FIGS.  13 B,  14 ,  30 A, and  31 A- 31 B . 
     Additionally, as discussed herein, a single micro-display may emit light of two or more (e.g., all) component colors (e.g., emit red, green, and blue light). For example,  FIG.  14    illustrates an example of a wearable display system with a single full-color emissive micro-display  1030   b  which may emit light of each component color. In some embodiments, such a micro-display and/or the related projection optics may be shifted to present different pixels of an image, as discussed herein. 
       FIG.  37 B  illustrates an example of a wearable display system having a light projection system  1010  in which a single array of light emitters outputs light of one or more different component colors. As illustrated, the light projection system  1010  may include an emissive micro-display  1030   b  including an array  3742  of light emitters  3744 . In some embodiments, the array  3742  may be configured to emit light of all component colors utilized by the display system to form a full-color image. For example, the micro-display  1030   b  may include an array of light emitters which each generate red, green, and blue light. 
     Preferably, each component color for a given pixel is emitted from an overlapping area of the array  3742 , which may advantageously facilitate the shifting described herein for providing different pixels of an image. For example, the light emitters  3744  may be understood to each include a stack of constituent light generators, with each constituent light generators being configured to emit light of a different associated component color. The micro-display  1030   b  may, in some embodiments, include coaxial red, green, and blue stacked constituent light generators. 
     Advantageously, with continued reference to  FIG.  37 B , by emitting each component color, the micro-display  1030   b  may thus avoid use of an optical combiner, such as the optical combiner  1050  described herein. The light (e.g., multi-component light) from the micro-display  1030   b  may be routed through projection optics  1070  to the eyepiece  1020 . As described above, with respect to at least  FIG.  36 A , an actuator  1504  may adjust the position of the projection optics to form different pixels of an image. In some embodiments, a single waveguide eyepiece  1020  may be used to receive light (e.g., via an in-coupling optical element  1122   a  configured to in couple incident light of each of the component colors). 
     While  FIG.  37 B  illustrates the actuator  1504  adjusting the position of the projection optics  1070 , it may be understood that the actuator  1504  may in addition or alternatively be attached to the micro-display  1030   b  to adjust the position of the micro-display  1030   b , to shift the micro-display  1030   b  to provide different pixels as discussed herein.  FIG.  37 C  illustrates a wearable display system similar to that of  FIG.  37 B , except that the actuator  1504  is attached to the micro-display  1030   b  instead of the projection optics  1070 . 
     As described above, in some embodiments where different micro-displays are utilized to generate light of different component colors, the projection system may use an optical combiner to combine the separately generated light of different colors. For example, an x-cube may be employed to combine light from different micro-displays  1030   a - 1030   c  ( FIG.  36 B ). The combined light may be routed through projection optics  1070  and directed to an eyepiece  1020  including one or more waveguides. 
     In some embodiments, and as illustrated in  FIGS.  38 A- 38 D , even where different micro-displays are utilized to generate light of different component colors, an optical combiner may be omitted from the projection system  1500 . For example, the micro-displays  1030   a - 1030   c  may each route light via a dedicated associated one of the projection optics  1070   a - 1070   c  to the eyepiece  1020 . As illustrated, micro-display  1030   a  has an associated projection optics  1070   a  which focuses light onto associated in-coupling optical elements  1022   a , micro-display  1030   b  has an associated projection optics  1070   b  which focuses light onto associated in-coupling optical elements  1022   b , and micro-display  1030   c  has an associated projection optics  1070   c  which focuses light onto associated in-coupling optical elements  1022   c.    
     It will be appreciated that in embodiments in which an optical combiner  1500  is not used, several example benefits may be achieved. As an example, there may be improved light collection as the micro-displays  1030   a - 1030   c  can be placed closer to the projection optics  1070   a - 1070   c  when the intervening optical combiner  1500  is omitted. As a result, higher light utilization efficiency and image brightness may be achieved. As another example, the projection system  1500  may be simplified and tailored to light of a particular component color. For example, an optics design for each respective projection optics  1070   a - 1070 C may be calibrated separately for light of each component color generated by the micro-displays  1030   a - 1030   c . In this way, the projection system  1500  may avoid the need for achromatization of the projection optics. 
     As another example benefit, and as illustrated in  FIG.  38 A , light from each of the projection optics  1070   a - 1070   c  may advantageously be more specifically focused onto respective associated in-coupling optical elements  1022   a - 1022   c . With respect to  FIGS.  36 A- 36 B , the combined light is routed via the projection optics  1070  onto the eyepiece  1020 . As illustrated, the light may be in-coupled via different in-coupling optical elements  1022   a - 1022   c . In the examples of  FIGS.  36 A- 36 B , the eyepiece  1020  includes three example waveguides which in-couple respective component colors generated by the micro-displays  1030   a - 1030   c . However, it will be appreciated that each component color may not precisely focus on a respective in-coupling element  1022   a - 1022   c  of the eyepiece  1020 . As a non-limiting example,  FIGS.  36 A- 36 B  illustrate the combined light focusing at a depth between in-coupling elements  1022   b  and  1022   c.    
     In contrast, the examples of  FIG.  38 A- 38 B  allow for more precise focusing of each component color onto a respective in-coupling element  1022   a - 1022   c . The projection optics  1070   a - 1070   c  for each component color may be configured to precisely focus light onto a respective in-coupling element  1022   a - 1022   c . In some embodiments, this precise focusing may improve image quality by providing well-focused images of each component color. 
       FIG.  38 A  illustrates an example of a light projection system  1500  without an optical combiner (e.g., the optical combiner  1050  described above). In the illustrated example, three micro-displays  1030   a - 1030   c  provide light (e.g., component color light) to respective projection optics  1070   a - 1070   c . The projection optics  1070   a - 1070   c  may be connected to, or otherwise positioned along, a connecting element  3802 . The connecting element  3802  may be adjusted in position by an actuator  1504 . Thus, the actuator  1504  may adjust positions of the projection optics  1070   a - 1070   c , which form a unitary structure. Light from each micro-display  1030   a - 1030   c  may be routed through the projection optics  1070   a - 1070   c  and focused onto respective in-coupling elements  1022   a - 1022   c  included in the eyepiece  1020 . 
       FIG.  38 A  illustrates the actuator  1504  adjusting the positions of the projection optics  1070   a - 1070   c  via the connecting element  3802 . In some embodiments, each projection optics  1070   a - 1070   c  may include its own dedicated actuator. For example, the projection system  1500  may include three actuators for adjusting positions of a respective one of the three projection optics  1070   a - 1070   c.    
       FIG.  38 B  illustrates another example of a wearable display system having a light projection system without an optical combiner. In some embodiments, the micro-displays  1030   a - 1030   c  may form a single integral unit, e.g., the micro-displays  1030   a - 1030   c  be placed on a single back-plane  3804 . In some embodiments, the back-plane  3804  may be a silicon back-plane, which may include electrical components for the micro-displays  1030   a - 1030   c . Similar to  FIG.  38 A , the illustrated actuator  1504  may adjust position of the connecting element  3802 . 
     It will be appreciated that the actuators of  FIGS.  38 A-B  may be attached to and configured to move the micro-displays  1030   a - 1030   c  rather than the projection optics  1070   a - 1070   c . For example,  FIG.  38 C  illustrates a wearable display system which is otherwise similar to the wearable display system of  FIG.  38 A , except that the actuator  1504  attached to the projection optics  1070   a - 1070   c  is omitted, and each of the micro-displays  1030   a - 1030   c  instead has an associated actuator  1504   a - 1504   c , respectively. In some embodiments where the micro-displays  1030   a - 1030   c  are joined together (e.g., where two or more of the micro-displays or physically connected, e.g., by sharing a common back-plane), a single actuator  1504  may be utilized to change the position of the physically connected ones of the micro-displays  1030   a - 1030   c . For example,  FIG.  38 D  illustrates a wearable display system which is otherwise similar to the wearable display system of  FIG.  38 B , except that the actuator  1504  attached to the projection optics  1070   a - 1070   c  is omitted, and the actuator  1504  is instead attached to the physically conjoined micro-displays  1030   a - 1030   c  so as to move these micro-displays together. 
     In the description above, with respect to at least  FIGS.  36 A- 38 D , one or more actuators are described as causing displacement or movement of different components of the wearable display system. While in each of these figures one or more actuators are illustrated for ease of illustration and discussion as moving the same type of component (e.g., micro-display or projection optics), in some embodiments, actuators may be provided for adjusting the positions of two or more types of components (e.g., in the same display system, actuators may be attached to and configured to adjust the positions of both the micro-displays and the projection optics illustrated in these Figures). 
     For example,  FIG.  36 A  illustrates an actuator  1504  for adjusting the position of projection optics  1070 , and  FIG.  36 B  illustrates actuators  1504   a - 1504   c  for adjusting the positions of micro-displays  1030   a - 1030   c , respectively. In some embodiments, the wearable display system may include both the actuator  1504  and actuators  1504   a - 1504   c , with actuator  1504  configured to adjust the position of the projection optics  1070  while actuators  1504   a - 1504   c  are configured to adjust the positions of micro-displays  1030   a - 1030   c . For example, the positions of the projection optics  1070  may be adjusted at the same time as the positions of the micro-displays  1030   a - 1030   c . As another example, the positions of projection optics  1070  and micro-displays  1030   a - 1030   c  may be adjusted at different times (e.g., sequentially). 
     Similarly, with reference to  FIGS.  37 B and  37 C , in some embodiments, both micro-display  1030   b  and projection optics  1070  may have an associated actuator  1504  for moving the micro-display  1030   b  and projection optics  1070 , respectively (e.g., for moving these components at the same time or at different times). In some embodiments, the same display system may include and use both actuator  1504  (illustrated in  FIG.  38 A ) and actuators  1504   a - 1504   c  (illustrated in  FIG.  38 C ) for adjusting the positions of projection optics  1070   a - 1070   c  and micro-displays  1030   a - 1030   c , respectively. With reference to  FIGS.  38 B and  38 D , in some embodiments, the same display system may include the projection optics  1070   a - 1070   c  with a first associated actuator  1504  and the conjoined micro-displays  1030   a - 1030   c  may each have a second associated actuator  1504 . 
     Example Flowchart 
       FIG.  39    illustrates a flowchart of an example process for outputting subframes of a rendered frame of virtual content. For convenience, the process will be described as being performed by a display system having one or more processors (e.g., in the local processing and data module  140  or in the remote processing module  150  of  FIG.  9 E ). 
     At block  3902 , the display system obtains a rendered frame of virtual content. As described above, the display system may generate frames of virtual content for presentation to a user. For example, the local processor and data module  140  may include one or more graphics processing elements. The module  140  may then generate rendered frames of virtual content. 
     As described in  FIGS.  32 - 36   , the rendered frame may be rendered, at least in part, at a resolution (e.g., pixel density) greater than that of the light emitters (e.g., micro-LEDs) included in the micro-displays of a light projection system. Each subframe may be formed based on light generated by the light emitters. Each set of subframes for forming a full resolution frame may be successively output in rapid succession (e.g., within the flicker fusion threshold), such that they may be perceived by the user as being simultaneously present to form the full resolution rendered frame. 
     At block  3904 , the display system outputs light forming a first subframe. The display system may select pixels included in the rendered frame as forming the first subframe. For example, and as described in  FIGS.  32 A- 32 D , there may be a threshold number of subframes (e.g., 9 subframes). Thus, the display system may select the pixels corresponding to a first of these threshold number of subframes. For example, the display system may divide the rendered frames into the threshold number of subframes. The display system may then store these subframes and cause the emissive elements to generate light forming each subframe. Optionally, the module  140  may be configured to generate subframes instead of the full rendered frame. The display system then causes the light projection system to output light forming the first subframe. It will be appreciated that the pixels of the various subframes are interleaved or intermixed, such that pixels of different subframes may occupy the spaces separating pixels of other subframes. 
     At block  3906 , the display system shifts the positions of displayed pixels. As described in  FIGS.  35 A- 35 B , the pixel positions may be perceived to be adjusted in position via actuators that move parts of the light projection system, to change the path of light through that light projection system. For example, the one or more actuators may adjust the positions of one or more emissive micro-displays included in the light projection system. As another example, the one or more actuators may adjust the positions of projection optics included in the light projection system. These adjustments may cause light from the emissive elements to be adjusted in position. 
     As described above, the display system may continually move parts of the light projection system. For example, the actuators may follow a movement pattern such as the movement patterns illustrated in  FIG.  34   . The display system may also cause discrete adjustments of the light projection system via the one or more actuators. For example, the positions of the various parts of the light projection system may be discretely adjusted for each subframe. 
     At block  3908 , the display system outputs light forming a second subframe after changing the position of moveable parts of the light projection system, so that the light forming the second subframe provides pixels at a desired location for the second subframe. The display system may select pixels of the rendered frame forming the second subframe. Optionally, the module  140  may render the second subframe. The display system may then cause the light projection system to output light forming the second subframe. 
     The display system may then continue to shift the positions of displayed and output successive subframes until the full rendered frame is formed. 
     Various example embodiments of the invention are described herein. Reference is made to these examples in a non-limiting sense. They are provided to illustrate more broadly applicable aspects of the invention. Various changes may be made to the invention described and equivalents may be substituted without departing from the spirit and scope of the invention. 
     For example, while advantageously utilized with AR displays that provide images across multiple depth planes, the virtual content disclosed herein may also be displayed by systems that provide images on a single depth plane. 
     In addition, many modifications may be made to adapt a particular situation, material, composition of matter, process, process act, or step(s) to the objective(s), spirit, or scope of the present invention. Further, as will be appreciated by those with skill in the art that each of the individual variations described and illustrated herein has discrete components and features which may be readily separated from or combined with the features of any of the other several embodiments without departing from the scope or spirit of the present inventions. All such modifications are intended to be within the scope of claims associated with this disclosure. 
     The invention includes methods that may be performed using the subject devices. The methods may comprise the act of providing such a suitable device. Such provision may be performed by the user. In other words, the “providing” act merely requires the user obtain, access, approach, position, set-up, activate, power-up or otherwise act to provide the requisite device in the subject method. Methods recited herein may be carried out in any order of the recited events that is logically possible, as well as in the recited order of events. 
     In addition, 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 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. 
     Example aspects of the invention, together with details regarding material selection and manufacture have been set forth above. As for other details of the present invention, these may be appreciated in connection with the above-referenced patents and publications as well as generally known or appreciated by those with skill in the art. The same may hold true with respect to method-based aspects of the invention in terms of additional acts as commonly or logically employed. 
     In addition, though the invention has been described in reference to several examples optionally incorporating various features, the invention is not to be limited to that which is described or indicated as contemplated with respect to each variation of the invention. Various changes may be made to the invention described and equivalents (whether recited herein or not included for the sake of some brevity) may be substituted without departing from the spirit and scope of the invention. In addition, where a range of values is provided, it is understood that every intervening value, between the upper and lower limit of that range and any other stated or intervening value in that stated range, is encompassed within the invention. 
     Also, it is contemplated that any optional feature of the inventive variations described may be set forth and claimed independently, or in combination with any one or more of the features described herein. Reference to a singular item, includes the possibility that there are plural of the same items present. More specifically, as used herein and in claims associated hereto, the singular forms “a,” “an,” “said,” and “the” include plural referents unless the specifically stated otherwise. In other words, use of the articles allow for “at least one” of the subject item in the description above as well as claims associated with this disclosure. It is further noted that such claims may be drafted to exclude any optional element. As such, this statement is intended to serve as antecedent basis for use of such exclusive terminology as “solely,” “only” and the like in connection with the recitation of claim elements, or use of a “negative” limitation. Without the use of such exclusive terminology, the term “comprising” in claims associated with this disclosure shall allow for the inclusion of any additional element—irrespective of whether a given number of elements are enumerated in such claims, or the addition of a feature could be regarded as transforming the nature of an element set forth in such claims. 
     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.