Patent Publication Number: US-11391951-B2

Title: Dual depth exit pupil expander

Description:
INCORPORATION BY REFERENCE 
     This application is a continuation of U.S. application Ser. No. 15/855,906, filed Dec. 27, 2017, entitled “DUAL DEPTH EXIT PUPIL EXPANDER,” which claims priority under 35 U.S.C. § 119(e) from U.S. Provisional Application No. 62/439,848, filed Dec. 28, 2016, entitled “DUAL DEPTH EXIT PUPIL EXPANDER,” which is hereby incorporated by reference herein in its entirety and for all purposes. This application additionally incorporates by reference the entirety of each of the following patent applications: U.S. application Ser. No. 14/555,585 filed on Nov. 27, 2014; U.S. application Ser. No. 14/690,401 filed on Apr. 18, 2015; U.S. application Ser. No. 14/212,961 filed on Mar. 14, 2014; U.S. application Ser. No. 14/331,218 filed on Jul. 14, 2014; and U.S. application Ser. No. 15/072,290 filed on Mar. 16, 2016. 
    
    
     BACKGROUND 
     Field 
     The present disclosure relates to optical devices, including virtual reality and augmented reality imaging and visualization systems. 
     Description of the Related Art 
     Modern computing and display technologies have facilitated the development of systems for so called “virtual reality” or “augmented reality” experiences, wherein digitally reproduced images or portions thereof are presented to a user in a manner wherein they seem to be, or may be perceived as, real. A virtual reality, or “VR”, scenario typically involves presentation of digital or virtual image information without transparency to other actual real-world visual input; an augmented reality, or “AR”, scenario typically involves presentation of digital or virtual image information as an augmentation to visualization of the actual world around the user. A mixed reality, or “MR”, scenario is a type of AR scenario and typically involves virtual objects that are integrated into, and responsive to, the natural world. For example, in an MR scenario, AR image content may be blocked by or otherwise be perceived as interacting with objects in the real world. 
     Referring to  FIG. 1 , an augmented reality scene  10  is depicted wherein a user of an AR technology sees a real-world park-like setting  20  featuring people, trees, buildings in the background, and a concrete platform  30 . In addition to these items, the user of the AR technology also perceives that he “sees” “virtual content” such as a robot statue  40  standing upon the real-world platform  30 , and a cartoon-like avatar character  50  flying by which seems to be a personification of a bumble bee, even though these elements  40 ,  50  do not exist in the real world. Because the human visual perception system is complex, it is challenging to produce an AR technology that facilitates a comfortable, natural-feeling, rich presentation of virtual image elements amongst other virtual or real-world imagery elements. 
     Systems and methods disclosed herein address various challenges related to AR and VR technology. 
     SUMMARY 
     Details of one or more implementations of the subject matter described in this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages will become apparent from the description, the drawings, and the claims. Neither this summary nor the following detailed description purports to define or limit the scope of the inventive subject matter. 
     Examples 
     1. An eyepiece for a head-mounted display comprising:
         a waveguide having a front face, a rear face and a plurality of edges, said front face and said rear face having lateral dimensions, said edges having a thickness less than the lateral dimensions of said front face and said rear face such that said waveguide can guide light therein from a location closer to one edge toward a location closer to another edge by total internal reflection from said front and rear faces;   at least one out-coupling optical element configured to receive light guided within said waveguide by total internal reflection from said front face and said rear face and to out-couple light out of said front face of said waveguide; and   at least one optical element having optical power such that said eyepiece outputs a first portion of said light guided within said waveguide from a first region of said eyepiece as if said light originated from a first depth with respect to said waveguide and a second portion of light guided within said waveguide from a second region of said eyepiece as if said light originated from a second depth with respect to said waveguide, said second region being laterally displaced with respect to said first region.       

     2. The eyepiece of Example 1, wherein said at least one optical element having optical power is included in said at least one out-coupling element. 
     3. The eyepiece of Example 2, wherein said at least one out-coupling optical element includes a first region configured to out-couple light guided within said waveguide as if said light originated from a first depth with respect to said waveguide and a second region configured to out-couple light guided within said waveguide as if said light originated from a second depth with respect to said waveguide, said first region of said out-coupling optical element corresponding to said first region of said eyepiece and said second region of said out-coupling optical element corresponding to said second region of said eyepiece. 
     4. The eyepiece of any of the examples above, wherein the at least one out-coupling optical element comprises a diffractive or holographic optical element. 
     5. The eyepiece of any of the examples above, wherein the at least one optical element having optical power comprises a diffractive or holographic optical element. 
     6. The eyepiece of Example 4 or 5, wherein the diffractive or holographic optical element comprises an analog surface relief grating (ASR), a binary surface relief structure (BSR), or a switchable diffractive optical element. 
     7. The eyepiece of Example 1, wherein said at least one optical element having optical power is disposed on said at least one out-coupling element. 
     8. The eyepiece of any of the examples above, wherein the at least one optical element having optical power comprises a refracting surface that has optical power. 
     9. The eyepiece of any of the examples above, wherein the at least one optical element having optical power comprises a diffractive or holographic optical element. 
     10. The eyepiece of any of the examples above, further comprising an in-coupling optical element configured to in-couple light into the waveguide. 
     11. The eyepiece of Example 10, wherein the in-coupling optical element comprises a wavelength selective optical element configured to couple more light of a first visible wavelength into said waveguide to be guided therein than a second visible wavelength. 
     12. The eyepiece of Examples 10 or 11, wherein the in-coupling optical element comprises a diffractive optical element. 
     13. The eyepiece of Example 10, wherein the in-coupling optical element comprises a refractive lens. 
     14. The eyepiece of any of the example above, further comprising a light distributing element configured to receive light from the in-coupling optical element and redirect said light toward said at least one out-coupling optical element. 
     15. The eyepiece of any of the examples above, further comprising a display configured to provide light for injection into said waveguide. 
     16. The eyepiece of Example 15, wherein the display comprises a fiber scanning display. 
     17. The eyepiece of any of the examples above, wherein at least one of the first region and the second region of the eyepiece has a fixed optical power. 
     18. The eyepiece of any of the above examples, further comprising a multiplexing system configured to selectively distribute a first portion of image content through the first region of the eyepiece and a second portion of the image content through the second region of the eyepiece. 
     19. The eyepiece of Example 18, wherein the eyepiece is included in a head mounted display to be worn on the head of a wearer having a field of view that includes said eyepiece and objects forward said eyepiece, the first portion of the image content comprising image content to be presented in the portion of the field of view of the wearer coinciding with the first region of the eyepiece while the second portion of the image content comprises image content to be presented in the portion of the field of view of the wearer coinciding with the second region of the eyepiece. 
     20. The eyepiece of Example 19, wherein the image content in the first portion of the image content is not visible to the wearer in the portion of the field of view of the wearer coinciding with the second region of the eyepiece while the image content in the second portion of the image content is not visible to the wearer in the portion of the field of view of the wearer coinciding with the first region of the eyepiece. 
     21. The eyepiece of any of Examples 18-20, wherein the first region of the eyepiece is forward and central of a wearer&#39;s eye while the second region of the eyepiece is below the first region, and the image content presented through the first region of the eyepiece corresponds to far image content while the image content presented through the second region of the eyepiece corresponds to near image content. 
     22. The eyepiece of any of Examples 18-21, wherein the multiplexing system comprises a polarization modulator and respective first and second polarization filters associated with respective first and second regions of the eyepiece. 
     23. The eyepiece of any of Examples 18-22, wherein the multiplexing system is configured to present a first portion of the image content corresponding to first far image content through the first region of the eyepiece while blocking the first portion of the image content from exiting the second region of the eyepiece. 
     24. The eyepiece of any of Examples 18-23, where the multiplexing system is configured to present a second portion of the image content corresponding to second near image content through the second region of the eyepiece while blocking the second portion of the image content from exiting the first region of the eyepiece. 
     25. The eyepiece of any of Examples 22-24, wherein the polarization modulator comprises a liquid crystal modulator comprising a liquid crystal layer disposed between electrodes configured to apply a voltage across the liquid crystal layer so as to cause linearly polarized light to rotate polarization angle. 
     26. The eyepiece of any of Examples 18-21, 23, and 24 wherein, the multiplexer system comprises first and second shutters and the eyepiece is configured to present far image content through the first region of the eyepiece while blocking the second region of the eyepiece with the second shutter and present near image content through the second region of the eyepiece at a different time while blocking the first region of the eyepiece with the first shutter. 
     27. The eyepiece of Example 26, wherein the shutters each comprise liquid crystal layers disposed between electrodes arranged to apply a voltage across the liquid crystal layers. 
     28. The eyepiece of Example 27, wherein the liquid crystal shutters further comprise a polarizer and an analyzer. 
     29. The eyepiece of Example 28, wherein the polarizer is configured to provide linearly polarized light that is received by the liquid crystal layer and the liquid crystal layer is configured to rotate the polarization angle of the linear polarized light depending on the voltage applied to the liquid crystal layer via the electrodes such that said linear polarization of the light may be made to be parallel or perpendicular to the linear polarization state that is transmitted by the analyzer. 
     30. The eyepiece of any of Examples 26-29, wherein when distant image content is to be displayed, the two shutters may be set such that the distant image content is passed through the first region of the eyepiece while the distant image content from the second region of the eyepiece is blocked while when near image content is being displayed, the two shutters may be set such that the near image content is passed through the second region of the eyepiece while the near image content from the first region of the eyepiece is blocked. 
     31. The eyepiece of Example 26, 27, or 30, wherein the eyepiece is included in a head mounted display to be worn on the head of a wearer having a field of view that includes said eyepiece and objects forward said eyepiece, and the shutters are configured such that light from objects forward of the wearer and the head mounted display may pass through the eyepiece to the wearer&#39;s eye regardless of whether the shutters are open or closed. 
     32. The eyepiece of Example 27, 30, or 31, wherein the eyepiece is included in a head mounted display to be worn on the head of a wearer having a field of view that includes said eyepiece and objects forward said eyepiece, and the shutters are configured such that light from objects forward of the wearer and the head mounted display may pass through the analyzer regardless of whether the liquid crystal layer rotates linearly polarized light or not. 
     33. The eyepiece of Example 32, wherein the shutters further comprise an analyzer but not a polarizer in the optical path between the liquid crystal layer and objects forward the head mounted display. 
     34. The eyepiece of any of Examples 31-33, wherein the shutter is configured to be selectively opened and closed to allow light from a display to pass or be blocked, said display providing image content. 
     35. The eyepiece of Example 34, further comprising a polarizer disposed to receive light from the display and yield linearly polarized light that may be rotated by a liquid crystal layer depending on the voltage applied to the liquid crystal layer such that the liquid crystal layer and an analyzer that receives light from the liquid crystal layer can operate as a shutter for the light from the display that may be selectively open and closed to allow light from the display to pass or be blocked. 
     36. A head mounted display comprising the eyepiece of any of the claims above. 
     37. The head mounted display of Example 36, further comprising at least one eye tracking camera configured to track a gaze of a wearer&#39;s eye. 
     38. The head mounted display of Example 36 or 37, further comprising at least one head pose sensor configured to assist in determining head movement, head orientation, head position or any combination of thereof. 
     39. The head mounted display of Example 37 or 38, wherein the eye tracking sensor or the head pose sensor or both are configured to such that when a wearer&#39;s gaze is directed to the first region of the eyepiece, a display may couple into the eye piece near image content and not distant image content, and when the wearer&#39;s gaze is directed toward the second region of the eyepiece, the display may couple light into the eyepiece distant image content and not near image content. 
     40. The head mounted display of any of Examples 36-39, wherein said head mounted display comprises an augmented reality head mounted display. 
     41. The head mounted display of any of Examples 36-39, wherein said head mounted display comprises a virtual reality head mounted display. 
     42. An eyepiece for a head-mounted display comprising:
         a first waveguide having a front face, a rear face and a plurality of edges, said front face and said rear face of said first waveguide having lateral dimensions, said edges of said first waveguide having a thickness less than the lateral dimensions of said front face and said rear face of said first waveguide such that said first waveguide can guide light therein from a location closer to one edge toward a location closer to another edge by total internal reflection from said front and rear faces of said first waveguide;   at least one first in-coupling optical element configured to in-couple light into said first waveguide, said at least one first in-coupling optical element comprising a wavelength selective optical element configured to couple more light of a first visible wavelength into said first waveguide to be guided therein than a second visible wavelength;   at least one first out-coupling optical element configured to receive light guided within said first waveguide by total internal reflection from said front face of said first waveguide and said rear face of said first waveguide and to out-couple light out of said front face of said first waveguide;   a second waveguide having a front face, a rear face and a plurality of edges, said front face and said rear face of said second waveguide having lateral dimensions, said edges of said second waveguide having a thickness less than the lateral dimensions of said front face and said rear face of said second waveguide such that said second waveguide can guide light therein from a location closer to one edge toward a location closer to another edge by total internal reflection from said front and rear faces of said second waveguide;   at least one second in-coupling optical element configured to in-couple light into said second waveguide, said at least one second in-coupling optical element comprising a wavelength selective optical element configured to couple more light of said second visible wavelength into said second waveguide to be guided therein than the first visible wavelength;   at least one second out-coupling optical element configured to receive light guided with said second waveguide by total internal reflection from said front face of said second waveguide and said rear face of said second waveguide and to out-couple light out of said front face of said second waveguide; and   at least one optical element having optical power such that said eyepiece outputs a first portion of said light guided within said first waveguide and a first portion of said light guided within said second waveguide from a first region of said eyepiece as if said light originated from a first depth with respect to said first waveguide and said second waveguide, and outputs a second portion of said light guided within said first waveguide and a second portion of said light guided within said second waveguide from a second region of said eyepiece as if said light originated from a second depth with respect to said first waveguide and said second waveguide, said second region being laterally displaced with respect to said first region.       

     43. The eyepiece of Example 42, wherein said light of a first visible wavelength comprises one of red, green, and blue light, and wherein said light of a second visible wavelength comprises a different one of red, green, and blue light. 
     44. The eyepiece of Example 42, wherein said at least one first out-coupling optical element includes a first region configured to out-couple light guided within said first waveguide as if said light originated from a first depth with respect to said first waveguide and a second region configured to out-couple light guided within said first waveguide as if said light originated from a second depth with respect to said first waveguide, and said at least one second out-coupling optical element includes a first region configured to out-couple light guided within said second waveguide as if said light originated from a first depth with respect to said waveguide and a second region configured to out-couple light guided within said second waveguide as if said light originated from a second depth with respect to said second waveguide, said first region of said at least one first out-coupling optical element and said first region of said at least one second out-coupling optical element corresponding to said first region of said eyepiece, and said second region of said at least one first out-coupling optical element and said second region of said at least one second out-coupling optical element corresponding to said second region of said eyepiece. 
     45. A head-mounted display comprising:
         a display configured to output light; and   an eyepiece comprising:
           at least one waveguide configured to guide light therein by total internal reflection;   at least one in-coupling optical element configured to receive light output by said display and in-couple said light into said waveguide;   at least one out-coupling optical element configured to receive light guided within said waveguide and to out-couple light out of said waveguide; and   at least one optical element having optical power such that said eyepiece outputs a first portion of said light guided within said at least one waveguide as if said light originated from a first depth with respect to said at least one waveguide, and a second portion of said light guided within said at least one waveguide as if said light originated from a second depth with respect to said at least one waveguide, said second region being laterally displaced with respect to said first region.   
               

     46. The head-mounted display of Example 45, wherein said display comprises a fiber scanning display. 
     47. The head-mounted display of Example 45, wherein said at least one out-coupling optical element includes a first region configured to out-couple light guided within said at least one waveguide as if said light originated from a first depth with respect to said at least one waveguide and a second region configured to out-couple light guided within said at least one waveguide as if said light originated from a second depth with respect to said at least one waveguide, said first region of said at least one out-coupling optical element corresponding to said first region of said eyepiece and said second region of said at least one out-coupling optical element corresponding to said second region of said eyepiece. 
     48. A head-mounted display comprising an eyepiece partitioned into a first section configured to project a first image to an eye of a wearer and a second section configured to project a second image to the eye of the wearer, wherein the first section has a first optical power, and the second section has a second optical power different from the first optical power, such that the second section image is projected as if from a different depth plane than the first image. 
     49. The head-mounted display of Example 48, wherein the first region and the section region are laterally spaced within the field of view of the eye of the wearer. 
     50. The head-mounted display of Example 48, wherein the optical power of at least one of the first section and the second section is reconfigurable. 
     51. The head-mounted display of Example 48, wherein the head-mounted display comprises a virtual reality display. 
     52. The head-mounted display of Example 48, wherein the head-mounted display comprises an augmented reality display. 
     53. The eyepiece of any of Examples 18-22, wherein the multiplexing system is configured to present a first portion of the image content through the first region of the eyepiece while blocking the first portion of the image content from exiting the second region of the eyepiece. 
     54. The eyepiece of any of Examples 18-22, and 53, where the multiplexing system is configured to present a second portion of the image content through the second region of the eyepiece while blocking the second portion of the image content from exiting the first region of the eyepiece. 
     55. An eyepiece for a head mounted display comprising:
         a waveguide having a front face, a rear face and a plurality of edges, said front face and said rear face having lateral dimensions, said edges having a thickness less than the lateral dimensions of said front face and said rear face such that said waveguide can guide light therein from a location closer to one edge toward a location closer to another edge by total internal reflection from said front and rear faces;   at least one out-coupling optical element configured to receive light guided within said waveguide by total internal reflection from said front face and said rear face and to out-couple light out of said front face of said waveguide; and   at least one optical element having optical power such that said eyepiece outputs a first portion of said light guided within said waveguide from a first region of said eyepiece as if said light originated from a first depth with respect to said waveguide and a second portion of light guided within said waveguide from a second region of said eyepiece as if said light originated from a second depth with respect to said waveguide, said second region being laterally displaced with respect to said first region.       

     56. The eyepiece of Example 55, wherein said at least one optical element having optical power is included in or disposed on said at least one out-coupling element. 
     57. The eyepiece of Example 56, wherein said at least one out-coupling optical element includes a first region configured to out-couple light guided within said waveguide as if said light originated from a first depth with respect to said waveguide and a second region configured to out-couple light guided within said waveguide as if said light originated from a second depth with respect to said waveguide, said first region of said at least one out-coupling optical element corresponding to said first region of said eyepiece and said second region of said at least one out-coupling optical element corresponding to said second region of said eyepiece. 
     58. The eyepiece of Example 55, wherein at least one of the at least one out-coupling optical element or the at least one element having optical power comprises a diffractive or holographic optical element. 
     59. The eyepiece of Example 58, wherein the diffractive or holographic optical element comprises an analog surface relief grating (ASR), a binary surface relief structure (BSR), or a switchable diffractive optical element. 
     60. The eyepiece of Example 55, wherein the at least one optical element having optical power comprises a refracting surface that has optical power. 
     61. The eyepiece of Example 55, further comprising an in-coupling optical element configured to in-couple light into the waveguide. 
     62. The eyepiece of Example 55, further comprising a light distributing element configured to receive light from the in-coupling optical element and redirect said light toward said at least one out-coupling optical element. 
     63. The eyepiece of Example 55, wherein at least one of the first region and the second region of the eyepiece has a fixed optical power. 
     64. The eyepiece of Example 55, further comprising a multiplexing system configured to selectively distribute a first portion of image content through the first region of the eyepiece and a second portion of the image content through the second region of the eyepiece. 
     65. The eyepiece of Example 64, wherein the eyepiece is included in a head mounted display to be worn on the head of a wearer having a field of view that includes said eyepiece and objects forward said eyepiece, the first portion of the image content comprising image content to be presented in the portion of the field of view of the wearer coinciding with the first region of the eyepiece while the second portion of the image content comprises image content to be presented in the portion of the field of view of the wearer coinciding with the second region of the eyepiece. 
     66. The eyepiece of Example 65, wherein the image content in the first portion of the image content is not visible to the wearer in the portion of the field of view of the wearer coinciding with the second region of the eyepiece while the image content in the second portion of the image content is not visible to the wearer in the portion of the field of view of the wearer coinciding with the first region of the eyepiece. 
     67. The eyepiece of Example 64, wherein the first region of the eyepiece is forward and central of a wearer&#39;s eye while the second region of the eyepiece is below the first region, and the image content presented through the first region of the eyepiece corresponds to far image content while the image content presented through the second region of the eyepiece corresponds to near image content. 
     68. The eyepiece of Example 64, wherein the multiplexing system comprises a polarization modulator and respective first and second polarization filters associated with respective first and second regions of the eyepiece. 
     69. The eyepiece of Example 68, wherein the polarization modulator comprises a liquid crystal modulator comprising a liquid crystal layer disposed between electrodes configured to apply a voltage across the liquid crystal layer so as to cause linearly polarized light to rotate polarization angle. 
     70. The eyepiece of Example 64, wherein the multiplexing system is configured to present a first portion of the image content corresponding to first far image content through the first region of the eyepiece while blocking the first portion of the image content from exiting the second region of the eyepiece. 
     71. The eyepiece of Example 64, where the multiplexing system is configured to present a second portion of the image content corresponding to second near image content through the second region of the eyepiece while blocking the second portion of the image content from exiting the first region of the eyepiece. 
     72. The eyepiece of Example 64, wherein the multiplexer system comprises first and second shutters and the eyepiece is configured to present far image content through the first region of the eyepiece while blocking the second region of the eyepiece with the second shutter and present near image content through the second region of the eyepiece at a different time while blocking the first region of the eyepiece with the first shutter. 
     73. The eyepiece of Example 72, wherein the shutters each comprise liquid crystal layers disposed between electrodes arranged to apply a voltage across the liquid crystal layers. 
     74. The eyepiece of Example 73, wherein the liquid crystal shutters further comprise a polarizer and an analyzer. 
     75. The eyepiece of Example 74, wherein the polarizer is configured to provide linearly polarized light that is received by the liquid crystal layer and the liquid crystal layer is configured to rotate the polarization angle of the linear polarized light depending on the voltage applied to the liquid crystal layer via the electrodes such that said linear polarization of the light may be made to be parallel or perpendicular to the linear polarization state that is transmitted by the analyzer. 
     76. The eyepiece of Example 72, wherein when distant image content is to be displayed, the two shutters may be set such that the distant image content is passed through the first region of the eyepiece while the distant image content from the second region of the eyepiece is blocked while when near image content is being displayed, the two shutters may be set such that the near image content is passed through the second region of the eyepiece while the near image content from the first region of the eyepiece is blocked. 
     77. A head mounted display comprising:
         a frame configured to mount on a wearer;   an eyepiece comprising:
           a waveguide having a front face, a rear face and a plurality of edges, said front face and said rear face having lateral dimensions, said edges having a thickness less than the lateral dimensions of said front face and said rear face such that said waveguide can guide light therein from a location closer to one edge toward a location closer to another edge by total internal reflection from said front and rear faces;   at least one out-coupling optical element configured to receive light guided within said waveguide by total internal reflection from said front face and said rear face and to out-couple light out of said front face of said waveguide; and   at least one optical element having optical power such that said eyepiece outputs a first portion of said light guided within said waveguide from a first region of said eyepiece as if said light originated from a first depth with respect to said waveguide and a second portion of light guided within said waveguide from a second region of said eyepiece as if said light originated from a second depth with respect to said waveguide, said second region being laterally displaced with respect to said first region; and   
           a display configured to selectively couple into the eyepiece light comprising near image content and light comprising distant image content.       

     78. The head mounted display of Example 77, further comprising at least one eye tracking camera configured to track a gaze of the wearer&#39;s eye. 
     79. The head mounted display of Example 78, wherein the eye tracking camera is configured such that when the wearer&#39;s gaze is directed to the first region of the eyepiece, the display may couple into the eye piece light comprising near image content and not distant image content, and when the wearer&#39;s gaze is directed toward the second region of the eyepiece, the display may couple into the eyepiece light comprising distant image content and not near image content. 
     80. The head mounted display of Example 77, further comprising at least one head pose sensor configured to assist in determining at least one of head movement, head orientation, and head position. 
     81. The head mounted display of Example 80, wherein the head pose sensor is configured such that the display selectively couples into the eyepiece light comprising near image content and not distant image content, or light comprising distant image content and not near image content, based on at least one of the head movement, head orientation, and head position of the wearer. 
     82. The head mounted display of Example 77, wherein said head mounted display comprises an augmented reality head mounted display. 
     83. The head mounted display of Example 77, wherein said head mounted display comprises a virtual reality head mounted display. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a user&#39;s view of augmented reality (AR) through an AR device. 
         FIG. 2  illustrates an example of a wearable display system. 
         FIG. 3  illustrates a conventional display system for simulating three-dimensional imagery for a user. 
         FIG. 4  illustrates aspects of an approach for simulating three-dimensional imagery using multiple depth planes. 
         FIGS. 5A-5C  illustrate relationships between radius of curvature and focal radius. 
         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 waveguide assembly in which each depth plane includes images formed using multiple different component colors. 
         FIG. 9A  illustrates a cross-sectional side view of an example of a set of stacked waveguides that each includes an incoupling optical element. 
         FIG. 9B  illustrates a perspective view of an example of the plurality of stacked waveguides of  FIG. 9A . 
         FIG. 9C  illustrates a top-down plan view of an example of the plurality of stacked waveguides of  FIGS. 9A and 9B . 
         FIG. 10A  schematically illustrates a perspective view of an example of a plurality of stacked waveguides including a dual depth exit pupil expander. 
         FIGS. 10B-10E  schematically illustrate top-down plan views of example of a plurality of stacked waveguides including a dual depth exit pupil expander. 
         FIG. 11A  schematically illustrates a multiplexing system comprising a polarization modulator and respective first and second polarization filters associated with respective first and second regions of the exit pupil expander that are configured to present far image content through the first region while blocking the second region and present near image content through the second region at a different time while blocking the first region by modulating the polarization depending on whether far or near image content is to be presented. 
         FIG. 11B  schematically illustrates another time division multiplexing system comprising first and second shutters associated with first and second regions of the exit pupil expander that are configured to present far image content through the first region while blocking the second region with the second shutter and present near image content through the second region at a different time while blocking the first region with the first shutter. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made to the figures, in which like reference numerals refer to like parts throughout. It will be appreciated that embodiments disclosed herein include optical systems, including display systems, generally. In some embodiments, the display systems are wearable, which may advantageously provide a more immersive VR or AR experience. For example, displays containing one or more waveguides (e.g., a stack of waveguides) may be configured to be worn positioned in front of the eyes of a user, or viewer. In some embodiments, two stacks of waveguides, one for each eye of a viewer, may be utilized to provide different images to each eye. 
     Example Display Systems 
       FIG. 2  illustrates an example of wearable display system  60 . The display system  60  includes a display  70 , and various mechanical and electronic modules and systems to support the functioning of that display  70 . The display  70  may be coupled to a frame  80 , which is wearable by a display system user or viewer  90  and which is configured to position the display  70  in front of the eyes of the user  90 . The display  70  may be considered eyewear in some embodiments. In some embodiments, a speaker  100  is coupled to the frame  80  and configured to be positioned adjacent the ear canal of the user  90  (in some embodiments, another speaker, not shown, is positioned adjacent the other ear canal of the user to provide stereo/shapeable sound control). In some embodiments, the display system may also include one or more microphones  110  or other devices to detect sound. In some embodiments, the microphone is configured to allow the user to provide inputs or commands to the system  60  (e.g., the selection of voice menu commands, natural language questions, etc.), and/or may allow audio communication with other persons (e.g., with other users of similar display systems. The microphone may further be configured as a peripheral sensor to collect audio data (e.g., sounds from the user and/or environment). In some embodiments, the display system 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 the physiological state of the user  90  in some embodiments. For example, the sensor  120   a  may be an electrode. 
     With continued reference to  FIG. 2 , the display  70  is operatively coupled by communications link  130 , such as by a wired lead or wireless connectivity, to a local data processing module  140  which may be mounted in a variety of configurations, such as fixedly attached to the frame  80 , fixedly attached to a helmet or hat worn by the user, embedded in headphones, or otherwise removably attached to the user  90  (e.g., in a backpack-style configuration, in a belt-coupling style configuration). Similarly, the sensor  120   a  may be operatively coupled by communications link  120   b , e.g., a wired lead or wireless connectivity, to the local processor and data module  140 . The local processing and data module  140  may comprise a hardware processor, as well as digital memory, such as non-volatile memory (e.g., flash memory or hard disk drives), both of which may be utilized to assist in the processing, caching, and storage of data. The data 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. 2 , in some embodiments, the remote processing module  150  may comprise one or more processors configured to analyze and process data and/or image information. In some embodiments, the remote data repository  160  may comprise a digital data storage facility, which may be available through the internet or other networking configuration in a “cloud” resource configuration. In some embodiments, the remote data repository  160  may include one or more remote servers, which provide information, e.g., information for generating augmented reality content, to the local processing and data module  140  and/or the remote processing module  150 . In some embodiments, all data is stored and all computations are performed in the local processing and data module, allowing fully autonomous use from a remote module. 
     The perception of an image as being “three-dimensional” or “3-D” may be achieved by providing slightly different presentations of the image to each eye of the viewer.  FIG. 3  illustrates a conventional display system for simulating three-dimensional imagery for a user. Two distinct images  190 ,  200 —one for each eye  210 ,  220 —are outputted to the user. The images  190 ,  200  are spaced from the eyes  210 ,  220  by a distance  230  along an optical or z-axis that is parallel to the line of sight of the viewer. The images  190 ,  200  are flat and the eyes  210 ,  220  may focus on the images by assuming a single accommodated state. Such 3-D display systems rely on the human visual system to combine the images  190 ,  200  to provide a perception of depth and/or scale for the combined image. 
     It will be appreciated, however, that the human visual system is more complicated and providing a realistic perception of depth is more challenging. For example, many viewers of conventional “3-D” display systems find such systems to be uncomfortable or may not perceive a sense of depth at all. Without being limited by theory, it is believed that viewers of an object may perceive the object as being “three-dimensional” due to a combination of vergence and accommodation. Vergence movements (i.e., rotation of the eyes so that the pupils move toward or away from each other to converge the lines of sight of the eyes to fixate upon an object) of the two eyes relative to each other are closely associated with focusing (or “accommodation”) of the lenses and pupils of the eyes. Under normal conditions, changing the focus of the lenses of the eyes, or accommodating the eyes, 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,” as well as pupil dilation or constriction. Likewise, a change in vergence will trigger a matching change in accommodation of lens shape and pupil size, under normal conditions. As noted herein, many stereoscopic or “3-D” display systems display a scene using slightly different presentations (and, so, slightly different images) to each eye such that a three-dimensional perspective is perceived by the human visual system. Such systems are uncomfortable for many viewers, however, since they, among other things, simply provide a different presentation of a scene, but with the eyes viewing all the image information at a single accommodated state, and work against the “accommodation-vergence reflex.” Display systems that provide a better match between accommodation and vergence may form more realistic and comfortable simulations of three-dimensional imagery contributing to increased duration of wear and in turn compliance to diagnostic and therapy protocols. 
       FIG. 4  illustrates aspects of an approach for simulating three-dimensional imagery using multiple depth planes. With reference to  FIG. 4 , objects at various distances from eyes  210 ,  220  on the z-axis are accommodated by the eyes  210 ,  220  so that those objects are in focus. The eyes  210 ,  220  assume particular accommodated states to bring into focus objects at different distances along the z-axis. Consequently, a particular accommodated state may be said to be associated with a particular one of depth planes  240 , with has an associated focal distance, such that objects or parts of objects in a particular depth plane are in focus when the eye is in the accommodated state for that depth plane. In some embodiments, three-dimensional imagery may be simulated by providing different presentations of an image for each of the eyes  210 ,  220 , and also by providing different presentations of the image corresponding to each of the depth planes. While shown as being separate for clarity of illustration, it will be appreciated that the fields of view of the eyes  210 ,  220  may overlap, for example, as distance along the z-axis increases. In addition, while shown as flat for ease of illustration, it will be appreciated that the contours of a depth plane may be curved in physical space, such that all features in a depth plane are in focus with the eye in a particular accommodated state. 
     The distance between an object and the eye  210  or  220  may also change the amount of divergence of light from that object, as viewed by that eye.  FIGS. 5A-5C  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. 5A-5C , the light rays become more divergent as distance to the object decreases. As distance increases, the light rays become more collimated. Stated another way, it may be said that the light field produced by a point (the object or a part of the object) has a spherical wavefront curvature, which is a function of how far away the point is from the eye of the user. The curvature increases with decreasing distance between the object and the eye  210 . Consequently, at different depth planes, the degree of divergence of light rays is also different, with the degree of divergence increasing with decreasing distance between depth planes and the viewer&#39;s eye  210 . While only a single eye  210  is illustrated for clarity of illustration in  FIGS. 5A-5C  and other figures herein, it will be appreciated that the discussions regarding eye  210  may be applied to both eyes  210  and  220  of a viewer. 
     Without being limited by theory, it is believed that the human eye typically can interpret a finite number of depth planes to provide depth perception. Consequently, a highly believable simulation of perceived depth may be achieved by providing, to the eye, different presentations of an image corresponding to each of these limited number of depth planes. The different presentations may be separately focused by the viewer&#39;s eyes, thereby helping to provide the user with depth cues based on the accommodation of the eye required to bring into focus different image features for the scene located on different depth plane and/or based on observing different image features on different depth planes being out of focus. 
       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 . In some embodiments, the display system  250  is the system  60  of  FIG. 2 , with  FIG. 6  schematically showing some parts of that system  60  in greater detail. For example, the waveguide assembly  260  may be part of the display  70  of  FIG. 2 . It will be appreciated that the display system  250  may be considered a light field display in some embodiments. 
     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, the 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 projector 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 . Examples of spatial light modulators include liquid crystal displays (LCD) including a liquid crystal on silicon (LCOS) displays. 
     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. 2 ) 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 can 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 can 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  80  ( FIG. 2 ) and may be in electrical communication with the processing modules  140  and/or  150 , which may process image information from the camera assembly  630  to make various determinations regarding, e.g., the physiological state of the user, as discussed herein. It will be appreciated that information regarding the physiological state of user may be used to determine the behavioral or emotional state of the user. Examples of such information include movements of the user and/or facial expressions of the user. The behavioral or emotional state of the user may then be triangulated with collected environmental and/or virtual content data so as to determine relationships between the behavioral or emotional state, physiological state, and environmental or virtual content data. In some embodiments, one camera assembly  630  may be utilized for each eye, to separately monitor each eye. 
     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 without-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. In some embodiments, features  320 ,  330 ,  340 , and  350  may be active or passive optical filters configured to block or selectively light from the ambient environment to the viewer&#39;s eyes. 
     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. 9A , 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. 9A  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. 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 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. 9A , 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. Light rays  770 ,  780 ,  790  may also be laterally displaced to different locations corresponding to the lateral locations of in-coupling optical elements  700 ,  710 ,  720 . The in-coupling optical elements  700 ,  710 ,  720  each deflect the incident light such that the light propagates through a respective one of the waveguides  670 ,  680 ,  690  by TIR. 
     For example, in-coupling optical element  700  may be configured to deflect ray  770 , which has a first wavelength or range of wavelengths. Similarly, the transmitted ray  780  impinges on and is deflected by the in-coupling optical element  710 , which is configured to deflect light of a second wavelength or range of wavelengths. Likewise, the ray  790  is deflected by the in-coupling optical element  720 , which is configured to selectively deflect light of third wavelength or range of wavelengths. 
     With continued reference to  FIG. 9A , 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, and thus be guided therein. For example, deflection of light rays  770 ,  780 ,  790  may be caused by one or more reflective, diffractive, and/or holographic optical elements, such as a holographic, diffractive, and/or reflective turning feature, reflector, or mirror. Deflection may in some cases be caused by microstructure such as diffractive features in one or more gratings, and/or holographic and/or diffractive optical elements configured to turn or redirect light, for example, so as to be guided with the light guide. The light rays  770 ,  780 ,  790  propagate through the respective waveguide  670 ,  680 ,  690  by TIR, being guided therein until impinging on the waveguide&#39;s corresponding light distributing elements  730 ,  740 ,  750 . 
     With reference now to  FIG. 9B , a perspective view of an example of the plurality of stacked waveguides of  FIG. 9A  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 and are guided within the waveguides  670 ,  680 ,  690 , respectively. The guided light rays  770 ,  780 ,  790  then impinge on the light distributing elements  730 ,  740 ,  750 , respectively. The light distributing elements  730 ,  740 ,  750  may comprise one or more reflective, diffractive, and/or holographic optical elements, such as a holographic, diffractive, and/or reflective turning feature, reflector, or mirror. Deflection may in some cases be caused by microstructure such as diffractive features in one or more gratings, and/or holographic and/or diffractive optical elements configured to turn or redirect light, for example, so as to be guided with the light guide. The light rays  770 ,  780 ,  790  propagate through the respective waveguide  670 ,  680 ,  690  by TIR being guided therein until impinging on the waveguide&#39;s corresponding light distributing elements  730 ,  740 ,  750 , where they are deflected, however, in a manner so that the light rays  770 ,  780 ,  790  are still guided within the waveguide. 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. 
     The out-coupling optical elements  800 ,  810 ,  820  are configured to direct light guided within the waveguide, e.g., the light rays  770 ,  780 ,  790 , out of the waveguide and toward the viewer&#39;s eye. The out-coupling optical elements  800 ,  810 ,  820  may be configured therefore to deflect and redirect the light guided within the waveguide, e.g., light rays  770 ,  780 ,  790 , at a more normal angle with respect to the surfaces of the waveguide so as to reduce the effects of total internal reflection (TIR) such that light is not guided within the waveguide but instead exits therefrom. Moreover, these out-coupling optical elements  800 ,  810 ,  820  may be configured to deflect and redirect this this light, e.g., light rays  770 ,  780 ,  790 , toward the viewer&#39;s eye. Accordingly, the out-coupling optical elements  800 ,  810 ,  820  may comprise one or more reflective, diffractive, and/or holographic optical elements, such as a holographic, diffractive, and/or reflective turning feature, reflector, or mirror. Deflection may in some cases be caused by microstructure such as diffractive features in one or more gratings, and/or holographic and/or diffractive optical elements configured to turn or redirect light, for example, so as to be guided with the light guide. The optical elements  800 ,  810 ,  820  may be configured to reflect, deflect, and/or diffract the light rays  770 ,  780 ,  790  so that they propagate out of the waveguide toward the users eye. 
     In some embodiments, the light distributing elements  730 ,  740 ,  750  are orthogonal pupil expanders (OPE&#39;s). In some embodiments, the OPE&#39;s both deflect or distribute light to the out-coupling optical elements  800 ,  810 ,  820  and also replicate the beam or beams to form a larger number of beams which propagate to the out-coupling optical elements. As a beam travels along the OPE&#39;s, a portion of the beam may be split from the beam and travel in a direction orthogonal to the beam, in the direction of out-coupling optical elements  800 ,  810 ,  820 . Orthogonal splitting of the beam in the OPE&#39;s may occur repeatedly along the path of the beam through the OPE&#39;s. For example, OPE&#39;s may include a grating having an increasing reflectance along the beam path such that a series of substantially uniform beamlets are produced from a single beam. 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 ). The OPE&#39;s may be configured to increase the dimensions of the eye box, for example, along the x direction, and the EPE&#39;s may be to increase the eye box in an axis crossing, for example, orthogonal to, the axis of the OPE&#39;s, e.g., along the y direction. 
     Accordingly, with reference to  FIGS. 9A and 9B , 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., EPE&#39;s)  800 ,  810 ,  820  for each component color. The waveguides  670 ,  680 ,  690  may be stacked with an air gap and/or 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 respective waveguide. The light then propagates at an angle which will result in TIR within the respective waveguide  670 ,  680 ,  690 , and the light is guided therein. 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 propagate within the waveguide being guided therein, interacting with the light distributing element (e.g., OPE&#39;s)  730  where it is replicated into a plurality of rays propagating to the out-coupling optical element (e.g., EPE&#39;s)  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., OPE&#39;s)  740  where it is replicated into a plurality of rays propagating to the out-coupling optical element (e.g., EPE&#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., OPE&#39;s)  750  by TIR, where it is replicated into a plurality of rays propagating to to the out-coupling optical element (e.g., EPE&#39;s)  820  by TIR. The out-coupling optical element  820  then finally further replicates and out-couples the light rays  790  to the viewer, who also receives the out-coupled light from the other waveguides  670 ,  680 . 
       FIG. 9C  illustrates a top-down plan view (or front view) of an example of the plurality of stacked waveguides of  FIGS. 9A and 9B . 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 (e.g., along the x and y directions). 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 along the x direction as seen in the top-down view of front view in this example). Shifting in other directions, such as the y direction, can also be employed. This non-overlapping spatial arrangement facilitates the injection of light from different resources such as different light sources and/or displays 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 non-overlapping laterally-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. 
     In addition to coupling light out of the waveguides, the out-coupling optical elements  800 ,  810 ,  820  may cause the light to be collimated or to diverge as if the light originated from an object at a far distance or a closer distance, depth, or depth plane. Collimated light, for example, is consistent with light from an object that is far from the view. Increasing diverging light is consistent with light from an object that is closer, for example, 5-10 feet or 1-3 feet, in front of the viewer. The natural lens of the eye will accommodate when viewing an object closer to the eye and the brain may sense this accommodation, which also then serves as a depth cue. Likewise, by causing the light to be diverging by a certain amount, the eye will accommodate and perceive the object to be at closer distance. Accordingly, the out-coupling optical elements  800 ,  810 ,  820  can be configured to cause the light to be collimated or to diverge as if the light emanated from a far or close distance, depth, or depth plane. To do so, the out-coupling optical elements  800 ,  810 ,  820  may include optical power. For example, the out-coupling optical elements  800 ,  810 ,  820 , may include holographic, diffractive, and/or reflective optical elements that in addition to deflecting or re-directing the light out of the waveguides, these holographic, diffractive, and/or reflective optical elements may further include optical power to cause the light to be collimated or diverging. The out-coupling optical elements  800 ,  810 ,  820  may in the alternative or in addition include refracting surfaces that include optical power that cause the light to be collimated or diverging. The out-coupling optical elements  800 ,  810 ,  820  may therefore comprise, for example, in addition to diffractive or holographic turning features, a refractive surface the provides optical power. Such refractive surface may also be included in addition to the out-coupling optical elements  800 ,  810 ,  820 , for example, on top of the out-coupling optical elements  800 ,  810 ,  820 . In certain embodiments, for example, optical elements, for example, diffractive optical element, holographic optical elements, refractive lens surfaces, or other structures may be disposed with respect to the out-coupling optical elements  800 ,  810 ,  820  to provide the optical power cause the collimation or divergence of the light. A layer with optical power such as a layer with a refractive surface or a layer with diffractive and/or holographic features may for example be disposed with respect to the out-coupling optical elements  800 ,  810 ,  820  to additionally provide optical power. A combination of contributions from both the out-coupling optical elements  800 ,  810 ,  820  having optical power and an additional layer with optical power such as a layer with a refractive surface or a layer with diffractive and/or holographic features is also possible. 
     Referring now to  FIGS. 10A-10E , the out-coupling optical elements  800 ,  810 ,  820  depicted in  FIGS. 9B and 9C  may include a plurality of sub-regions configured to project light as if the light is emanating from different depths or depth planes. For example, the stacked waveguide assembly  660  may include EPE&#39;s  800 ′,  810 ′,  820 ′ having sub-regions  805 ,  815 ,  825 , each configured to out-couple the in-coupled light rays  770 ,  780 ,  790  in a sub-region of an EPE  800 ′,  810 ′,  820 ′, as well as to cause the light to be diverging or collimated as if the light emanated from a depth different from the depth of the remainder of the EPE  800 ′,  810 ′,  820 ′. As shown in the top-down plan views of  FIGS. 10B-10E , the sub-regions  805 ,  815 ,  825  may be vertically aligned such that they occupy the same lateral region when viewed from the top down, e.g., the same region as projected on the x-y coordinate plane as depicted in  FIG. 10A . 
     Out-coupling of light from the waveguides as if emanating from different depths may be accomplished by providing sub-regions  805 ,  815 ,  825  with an optical power different from the optical power (if any) of the remainder of the EPE  800 ′,  810 ′,  820 ′. For example, the sub-regions or zones  805 ,  815 ,  825  may have a higher or lower optical power so as to focus the virtual image at a different position within the eyebox, for example, as if the virtual images in the two regions are projected from different depths. For instance, the sub-regions  805 ,  815 ,  825  may include negative optical power that causes the light to diverge as if the light is emanating from a location a couple of feet in front of the viewer, while the remainder of the EPE  800 ′,  810 ′,  820 ′ has no optical power or less negative optical power that causes the light to be collimated as if the light originated from a very distant object. The optical power of the sub-region  805 ,  815 ,  825  may be selected based on the desired depth of the sub-region  805 ,  815 ,  825  relative to the remainder of the EPE, e.g., region or zone  800 ′,  810 ′,  820 ′. In some embodiments, more than two regions of different optical power may be provided for out-coupling of light  770 ,  780 ,  790 . As described above, the optical power of each region may be determined by the structure of the out-coupling elements or other elements of the system. 
     Regions may have various shapes or locations within an EPE. For example, sub-region  805 ,  815 ,  825  may be located along a top, side (e.g. left or right), or bottom edge of the EPE, and/or may be surrounded within the region  800 ′,  810 ′,  820 ′. Region  800 ′,  810 ′,  820 ′ and sub-regions  805 ,  815 , 825  may be square, rectangular, trapezoidal, elliptical, etc., as viewed in an x-y plane or other shapes. For example, the implementation depicted in  FIG. 10C  includes a sub-region similar to the sub-region of  FIG. 10B , but configured to include the extent of the left side of the EPE disposed to the left of the partition line. The implementation depicted in  FIG. 10D  includes a sub-region located near the top of the EPE, and surrounded within the region  800 ′,  810 ′,  820 ′. The implementation depicted in  FIG. 10E  includes an elliptical sub-region located near the right side of the EPE and similarly surrounded within the region  800 ′,  810 ′,  820 ′. Any of the shapes, locations, or other qualities of the sub-regions described and depicted with reference to  FIGS. 10B-10E  may be combined in various implementations. For example, sub-regions  805 ,  815 ,  825  may be included on both the left and right of the EPE, or left and bottom, right and bottom, left and top, or right and top of the EPE. The different sub-regions  805 ,  815 ,  825  may have different sizes and/or shapes when multiple sub-regions  805 ,  815 ,  825  are included. It may be preferable in some embodiments to locate the sub-region  805 ,  815 ,  825  near the periphery of the EPE, e.g., along an edge or nearer to the periphery than to the center of the EPE, so as to avoid interfering with primary content displayed in region  800 ′,  810 ′,  820 ′. The size of the sub-region  805 ,  815 ,  825  may be any suitable size for displaying content, such as between 5%-10%, 10%-15%, 15%-20%, or 20%-25%, of the entire lateral extent of the EPE (e.g., the horizontal and vertical or x-y lateral extent of the EPE area) or more or less, or any combination of these ranges. Moreover, the transition between the optical power of region  800 ′,  810 ′,  820 ′ and the optical power of region  805 ,  815 ,  825  may be abrupt or discrete at a border between the optical power regions, or may be a smooth or gradual transition. 
     As described above, the out-coupling optical elements  800 ,  810 ,  820  may comprise one or more reflective, diffractive, and/or holographic optical elements, such as a holographic, diffractive, and/or reflective turning feature, reflector, or mirror configured to turn or redirect light, for example, so as to be guided with the light guide. Deflection may in some cases be caused by microstructure such as diffractive features in one or more gratings, and/or holographic and/or diffractive optical elements. The optical elements  800 ,  810 ,  820  may be configured to reflect, deflect, and/or diffract the light rays  770 ,  780 ,  790  so that they propagate out of the waveguide toward the users eye. In addition, the out-coupling optical elements  800 ,  810 ,  820 , can include optical power to cause the light to be collimated or to diverge as if the light emanated from a far or close distance, depth, or depth plane. For example, the out-coupling optical elements  800 ,  810 ,  820 , may include holographic, diffractive, and/or reflective optical elements that in addition to deflecting or re-directing the light out of the waveguides, these holographic, diffractive, and/or reflective optical elements may further include optical power to cause the light to be collimated or diverging. The out-coupling optical elements  800 ,  810 ,  820  may also comprises a refractive surface that refracts or bends the light to provide a lensing effect. As described above, different optical power can be provided to different sections of the out-coupling optical elements  800 ,  810 ,  820 . Also as described above the transition between the optical power of region  800 ′,  810 ′,  820 ′ and the optical power of region  805 ,  815 ,  825  may be abrupt or discrete at a border between the optical power regions, or may be a smooth or gradual transition. In examples where a refractive surface provided optical power, the two regions or zones of different optical powers may be formed from and comprise a freeform surface. 
     Also as described above, in certain embodiments, optical elements, for example, diffractive optical element, holographic optical elements, refractive lens surfaces, or other structures may be disposed with respect to the out-coupling optical elements  800 ,  810 ,  820  to provide the optical power cause the collimation or divergence of the light. A layer with optical power such as a layer with a refractive surface or a layer with diffractive and/or holographic features may for example be disposed with respect to (e.g., on top of) the out-coupling optical elements  800 ,  810 ,  820  to additionally provide optical power. A combination of contributions from both the out-coupling optical elements  800 ,  810 ,  820  having optical power and an additional layer with optical power such as a layer with a refractive surface or a layer with diffractive and/or holographic features is also possible. As described above, different optical power can be provided to different sections of the out-coupling optical elements  800 ,  810 ,  820 . Also as described above the transition between the optical power of region  800 ′,  810 ′,  820 ′ and the optical power of region  805 ,  815 ,  825  may be abrupt or discrete at a border between the optical power regions, or may be a smooth or gradual transition. Accordingly, in examples where a refractive surface provides optical power, the two regions or zones of different optical powers may be formed from and comprise a freeform surface. 
     A multiple-depth EPE including regions  805 ,  815 ,  825  configured to project light as if emanating from a depth or depth plane other than the depth or depth plane associated with of another portion of out-coupling elements  800 ′,  810 ′,  820 ′ may provide several advantages. In some embodiments, the multiple-depth EPE may be provided within a head mounted display to facilitate the simultaneous viewing of a plurality of different content types (e.g., distant objects versus text) by a wearer. The multiple-depth EPE may provide a more realistic presentation of content at multiple depth planes, and may avoid increasing computer hardware and/or software processing and/or processing requirements, including additional electrical power usage, to generate content at different depths or depth planes. More specifically, projecting images appearing at different depths or depth planes in different regions of the display using multiple stacked sets of waveguides, one set for each depth or depth plane, where each waveguide provides a full frame, may potentially involve significant additional computation and/or processing, which may be a burden on processing resources and frame rate of the display. Thus, the multiple-depth EPE wherein multiple depths or depth planes are provided for a single waveguide and a single full frame, can provide content appearing at a plurality of depth planes with reduced impact on processing resources or frame rate in comparison to a system that provides multiple depths or depth planes using different waveguides presenting full frames. 
     In one example, sub-regions  805 ,  815 ,  825  of the EPE&#39;s  800 ′,  810 ′,  820 ′ can have a lens function that projects light that appears to be coming from a focal plane closer inward to the eye than the light projected from the remainder of the EPE&#39;s  800 ′,  810 ′,  820 ′. The nearer focal plane of sub-regions  805 ,  815 ,  825  can be used to project detailed and/or readable content. For example, a user may watch virtual content, such as a movie or other visual content, while simultaneously checking e-mail or viewing other detailed content in sub-region  805 ,  815 ,  825 . Further implementations can include presenting text, captions, a “ticker” display or icons in one sub-region at the bottom or side of the eyepiece containing readable content such as news, sports information, financial information, icons for providing user selection or the like. In one example, an augmented reality system may use various recognition software functions to identify people and/or things within a wearer&#39;s field of view such that these items may be tagged, flagged, or otherwise identified. Augmented content, such as an arrow, highlighted region, or other visual content displayed through the EPE&#39;s  800 ′,  810 ′,  820 ′ may direct the user&#39;s attention to a person or item of interest, while detailed content, such as words, images, symbols, or other information associated with the person or item may be displayed at a closer depth plane in a sub-region  805 ,  815 ,  825 . 
     For various types of near and distant content as described above, it may be desirable to selectively project only the near content through the sub-region  805 ,  815 ,  825 , while projecting only the distant content through the remainder of the EPE&#39;s  800 ′,  810 ′,  820 ′. For example, in some situations the sub-region  805 ,  815 ,  825  may be on the lower portion of the EPE&#39;s  800 ′,  810 ′,  820 ′ in a location where a viewer would gaze downward to view. This downward gaze is also commonly used to read or to view close-up objects. Accordingly, the sub-region  805 ,  815 ,  825 , may be configured to present near image content while the remainder of the EPE&#39;s  800 ′,  810 ′,  820 ′ may be configured to present far image content. Various approaches may be utilized to separate far and near image content into the two regions of each EPE  800 ′,  810 ′,  820 ′, the upper and lower sub-regions. 
     In one example, as depicted in  FIG. 11A , the head mounted display further comprises a multiplexing system  901  configured to distribute a first portion of image content such as a first portion of a frame (e.g., video frame) through a first region of the exit pupil expander  800 ′,  810 ′,  820 ′ and a second portion of image content such as a second portion of a frame through a second region of the exit pupil expander  800 ′,  810 ′,  820 ′. The first portion of the image content may include image content to be presented in the portion of the field of view of the wearer coinciding with the first region of the exit pupil expander  800 ′,  810 ′,  820 ′ while the second portion of the image content may include image content to be presented in the portion of the field of view of the wearer coinciding with the second region  805 ,  815 ,  825  of the exit pupil expander  800 ′,  810 ′,  820 ′. Additionally, the image content presented through the first portion may be projected as if located at a first depth, and the image content presented through the second portion may be projected as if located at a second depth that is different from the first. Accordingly, the second region may have different optical power than the first region. This different optical power may cause the light from the second region to diverge in a manner that is different from the propagation of the light from the first region. In one example, the first region is forward and central while the second region  805 ,  815 ,  825  is below the first region and the image content presented through the first region corresponds to far image content while the image content presented through the second region corresponds to near image content. 
     In the example shown in  FIG. 11A , the multiplexing system  901  comprises a polarization modulator (as well as control electronics configured to drive/control the polarization modulation and establish the appropriate timing of the modulation) and respective first and second polarization filters associated with respective first and second regions of the exit pupil expander  800 ′,  810 ′,  820 ′ that are configured to present a first portion of the image content corresponding to first (e.g., far) image content through the first region while blocking the first portion of the image content from exiting the second region  805 ,  815 ,  825 . Similarly, the multiplexing system  901  is configured to present a second portion of the image content corresponding to second (e.g., near) image content through the second region  805 ,  815 ,  825  while blocking the second portion of the image content from exiting the first region. In this example, this multiplexing is accomplished by modulating the polarization using, for example, the control electronics depending on whether the first portion of the image content/first image content (e.g., the far image content) or second portion of the image content/second image content (e.g., the near image content) is to be presented. In particular, sub-regions  805 ,  815 ,  825  include a polarizing filter  920  (often referred to herein as the second polarizer) configured to selectively pass light having a second polarization, while the remainder of the EPE&#39;s  800 ′,  810 ′,  820 ′ may include a polarizing filter  915  (often referred to herein as the first polarizer) configured to selectively pass light having a first polarization orthogonal or perpendicular to the second polarization passed by the second polarizing filter  920  of the sub-regions  805 ,  815 ,  825 . Polarizing filters  915 ,  920  may be included in or integrated with the out-coupling element of each waveguide in the waveguide stack, or may be a single filter located at the exterior eye-facing surface of the EPE&#39;s  800 ′,  810 ′,  820 ′. The polarization filters  915 ,  920 , for example, may comprise polarization gratings that couple light of a particular polarization out of the waveguide. Image content for both the sub-regions  805 ,  815 ,  825  and the remainder of the EPE&#39;s  800 ′,  810 ′,  820 ′ may be injected into the waveguide stack  660  in a polarization-multiplexed beam. A near component of the polarization-multiplexed beam can include the image content to be displayed in the sub-region  805 ,  815 ,  825 , and is polarized in the orientation that is selectively passed by the polarizing filter of the sub-region  805 ,  815 ,  825 . Similarly, a distant component of the polarization-multiplexed beam can include the image content to be displayed in the remainder of the EPE  800 ′,  810 ′,  820 ′, and is polarized in the orientation that is selectively passed by the polarizing filter of the remainder of the EPE  800 ′,  810 ′,  820 ′. A polarization-multiplexed beam as described herein can be generated, for example, by sending light from a display  900 , such as a fiber scanning display described herein, through a polarizer  905  and a polarization modulator  910  to create polarization modulated beams  770 ,  780 ,  790 . The polarization modulator  910  may comprise, for example, a liquid crystal modulator comprising a liquid crystal layer disposed between electrodes configured to apply a voltage (e.g., from the control electronics) across the liquid crystal layer so as to cause linearly polarized light to rotate. The polarizer  905  can polarize the light providing as an output, for example, linearly polarized light. The polarization modulator can then rotate the polarization (e.g., rotate the polarization angle of linearly polarized light) and thereby modulate the amount of rotation of the polarization. The polarization modulator  910  may thus modulate or change the polarization from a first polarization state or the first polarization corresponding to the polarization that is passed by the first polarization filter  915  and blocked by the second polarization filter  920  to a second polarization state or the second polarization that is passed by the second polarization filter  920  and blocked by the first polarization filter  915 . Beams  770 ,  780 ,  790  can then be sent into the waveguide stack  660  at in-coupling elements  700 ,  710 ,  720  as described herein. When the light is out-coupled at EPEs  800 ′,  810 ′,  820 ′ including sub-regions  805 ,  815 ,  825  or the other portions of the EPE&#39;s, the light passes through either of the first polarizing filter  915  or the second polarizing filter  920  to selectively project one of the components of the polarization-multiplexed beam (e.g., the first or second portions of the image content corresponding to the far or near image content) to the eye of a viewer. 
     In another example, as depicted in  FIG. 11B , a time division multiplexing system comprising first and second shutters associated with first and second regions of the exit pupil expander that are configured to present far image content through the first region while blocking the second region with the second shutter and present near image content through the second region at a different time while blocking the first region with the first shutter. The time division multiplexing system may include control electronics configured to drive/control the shutters and establish the appropriate timing of the presentation of the different image content from the different regions. Separation of image content between the near and distant regions may be achieved by providing liquid crystal shutters  930 ,  925  on the sub-region  805 ,  815 ,  825  and the remainder of the EPE  800 ′,  810 ′,  820 ′. Similar to the polarizing filters described above, separately controllable liquid crystal shutters  930 ,  925  may be provided for each of the regions of the EPE  800 ′,  810 ′,  820 ′. In the example configuration of  FIG. 11B , liquid crystal shutter  930  is coextensive with the sub-region  805 ,  815 ,  825 , while liquid crystal shutter  925  is coextensive with the remainder of the EPE  800 ′,  810 ′,  820 ′. The liquid crystal shutters  925 ,  930  may each comprise liquid crystal layers disposed between electrodes arranged to apply a voltage (e.g., from the control electronics) across the liquid crystal layers. The liquid crystal shutters  925 ,  930  further may comprise a polarizer and an analyzer (another polarizer). Each liquid crystal film or layer may be controllable by applying a voltage (e.g., from the control electronics) across the film to change the polarization of the film, for example, between two alternate perpendicular orientations. For example, light initially propagates through the polarizer to provide polarized light such as linearly polarized light. The liquid crystal film receives this linearly polarized light. The liquid crystal film may rotate the polarization (e.g., the polarization angle of linearly polarized light) depending on the voltage applied to the liquid crystal film via the electrodes. Accordingly, the linear polarization of the light may be made to be parallel or perpendicular to the linear polarization state that is transmitted by the analyzer. Thus, when distant image content is being displayed, the voltages across the two liquid crystal shutters may be selected such that the out-coupled light in the sub-region  805 ,  815 ,  825  is blocked while the out-coupled light in the remainder of the EPE  800 ′,  810 ′,  820 ′ is passed. Similarly, when near image content is being displayed, the voltages across the two liquid crystal shutters may be selected such that the out-coupled light in the sub-region  805 ,  815 ,  825  is passed while the out-coupled light in the remainder of the EPE  800 ′,  810 ′,  820 ′ is blocked. 
     The shutters need not completely block ambient light from the world forward of the wearer and the head mounted display. For example, instead of comprising two polarizers (a polarizer and an analyzer), the shutters may comprise an analyzer and no polarizer preceding the liquid crystal film. Accordingly, un-polarized ambient light from the objects in the environment or world forward of the wearer and the head mounted display may pass through the analyzer regardless of whether the liquid crystal film rotates linearly polarized light or not. In such a design, by contrast, the light from the display may comprise linearly polarized light. For example, the head mounted display device may include a polarizer disposed in the optical path between the display and the in-coupling optical element of the waveguide, or the waveguide itself may include a polarizing element. The in-coupling element may, for example, be polarization selective and thus comprise a polarizer. Such a polarizer or polarizing element may yield linearly polarized light that may be rotated by the liquid crystal film depending on the voltage applied (e.g., from the control electronics) to the liquid crystal film. Accordingly, the liquid crystal film and the analyzer can operate as a shutter for the light from the display that may be selectively open and closed to allow light from the display to pass or be blocked; however, ambient light from objects forward the wearer and the head mounted display may pass through regardless of the state of the shutter. In this manner, when the shutter is closed to light from the display, the user can still see through the eyepiece to the world forward the viewer and the head mounted display. 
     In a third example, separation of image content between the near and distant regions of the EPE  800 ′,  810 ′,  820 ′ may be achieved by selectively projecting image content to the sub-regions  805 ,  815 ,  825  and remainder of the EPE  800 ′,  810 ′,  820 ′ based on a wearer&#39;s gaze direction and/or head pose. As illustrated in  FIG. 6 , the head mounted display device may include an eye tracking camera  630  and/or head pose orientation sensor. Such a head pose orientation sensor may comprise, for example, one or more accelerometers, gyros, motion sensors, GPS components, etc., configured to assist in determining head movement, orientation, and/or position. The head mounted display may also comprise one or more eye tracking cameras  630  and processing electronics configured to determine the wearer&#39;s gaze direction. Thus the wearer&#39;s gaze direction may be detected by an eye tracking camera  630  configured to determine a gaze direction of the eye  210  and the gaze direction may be used to determine what image content to present. (Alternatively and/or additionally, head pose sensors may provide information regarding the wearer&#39;s gaze.) For example, when the wearer&#39;s gaze is directed to the sub-region  805 ,  815 ,  825 , the electronics may cause the display to in-couple light including only the near image content and not the distant image content. If the wearer is looking at the sub-region  805 ,  815 ,  825 , the wearer will see the near image content at the proper near depth as provided by the sub-region  805 ,  815 ,  825 . This near image content may be at the lower portion of the frame or field or field of view. The upper portion of the frame or field or field of view may not include image content. Similarly, when the wearer&#39;s gaze is directed toward the remainder of the EPE  800 ′,  810 ′,  820 ′, the electronics may cause the display to in-couple light including only the distant image content and not the near image content. If the wearer is looking at the remainder of the EPE  800 ′,  810 ′,  820 ′, the wearer will see the far image content presented at the proper far depth as provided by the remainder of the EPE  800 ′,  810 ′,  820 ′. The far image content may be in the upper portion of the frame or field or field of view. The lower portion of the frame or field or field of view may not include image content. 
     In some embodiments, the system may be configured to project a combination of both near and distant image content. For example, a wearer may be looking at distant image content, which is in the upper portion of the frame or field or field of view (or EPE  800 ′,  810 ′,  820 ′), when the wearer receives image content more suitable for viewing at a near depth such as an email, text, etc. If the email is generally displayed as near image content, the display may notify the wearer by simultaneously displaying both the existing distant image content and the email content. The wearer may then see the email content, which will appear distant, for example, due to the optical power of the remainder of the EPE  800 ′,  810 ′,  820 ′, and may thereby be prompted to look down to the sub-region  805 ,  815 ,  825 . The eye tracking may detect this movement of the eye downward and may cause the display to present the email content alone without the distant image content. The wearer gazing, downward, will see the e-mail through the sub-region  805 ,  815 ,  825  and thus the e-mail will appear to be nearer because of the optical power associated with the sub-region  805 ,  815 ,  825 . The e-mail (or other image content more suitable for viewing at a near depth plane) can thus be viewed in detail. 
     It is contemplated that the innovative aspects may be implemented in or associated with a variety of applications and thus includes a wide range of variation. Variations, for example, in the shape, number, and/or optical power of the EPE&#39;s are contemplated. The structures, devices and methods described herein may particularly find use in displays such as wearable displays (e.g., head mounted displays) that can be used for augmented and/or virtually reality. More generally, the described embodiments may be implemented in any device, apparatus, or system that can be configured to display an image, whether in motion (such as video) or stationary (such as still images), and whether textual, graphical or pictorial. It is contemplated, however, that the described embodiments may be included in or associated with a variety of electronic devices such as, but not limited to: mobile telephones, multimedia Internet enabled cellular telephones, mobile television receivers, wireless devices, smartphones, Bluetooth® devices, personal data assistants (PDAs), wireless electronic mail receivers, hand-held or portable computers, netbooks, notebooks, smartbooks, tablets, printers, copiers, scanners, facsimile devices, global positioning system (GPS) receivers/navigators, cameras, digital media players (such as MP3 players), camcorders, game consoles, wrist watches, clocks, calculators, television monitors, flat panel displays, electronic reading devices (e.g., e-readers), computer monitors, auto displays (including odometer and speedometer displays, etc.), cockpit controls and/or displays, camera view displays (such as the display of a rear view camera in a vehicle), electronic photographs, electronic billboards or signs, projectors, architectural structures, microwaves, refrigerators, stereo systems, cassette recorders or players, DVD players, CD players, VCRs, radios, portable memory chips, washers, dryers, washer/dryers, parking meters, head mounted displays and a variety of imaging systems. Thus, the teachings are not intended to be limited to the embodiments depicted solely in the Figures, but instead have wide applicability as will be readily apparent to one having ordinary skill in the art. 
     Various modifications to the embodiments described in this disclosure may be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of this disclosure. Thus, 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. The word “exemplary” is used exclusively herein to mean “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. Additionally, a person having ordinary skill in the art will readily appreciate, the terms “upper” and “lower”, “above” and “below”, etc., are sometimes used for ease of describing the figures, and indicate relative positions corresponding to the orientation of the figure on a properly oriented page, and may not reflect the proper orientation of the structures described herein, as those structures are implemented. 
     Certain features that are described in this specification in the context of separate embodiments also can be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment also can be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
     Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. Further, the drawings may schematically depict one more example processes in the form of a flow diagram. However, other operations that are not depicted can be incorporated in the example processes that are schematically illustrated. For example, one or more additional operations can be performed before, after, simultaneously, or between any of the illustrated operations. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products. Additionally, other embodiments are within the scope of the following claims. In some cases, the actions recited in the claims can be performed in a different order and still achieve desirable results. 
     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 true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation, material, composition of matter, process, process act(s) 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 end user. In other words, the “providing” act merely requires the end 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 which is logically possible, as well as in the recited order of events. 
     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 true 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. Except as specifically defined herein, all technical and scientific terms used herein are to be given as broad a commonly understood meaning as possible while maintaining claim validity. 
     The breadth of the present invention is not to be limited to the examples provided and/or the subject specification, but rather only by the scope of claim language associated with this disclosure.