Patent Publication Number: US-2022221747-A1

Title: Display system with variable power reflector

Description:
INCORPORATION BY REFERENCE 
     This application is a continuation of U.S. application Ser. No. 17/175,566, filed on Feb. 12, 2021 entitled “DISPLAY SYSTEM WITH VARIABLE POWER REFLECTOR,” which is a divisional of U.S. application Ser. No. 15/902,814 filed on Feb. 22, 2018 entitled “DISPLAY SYSTEM WITH VARIABLE POWER REFLECTOR,” which claims the priority benefit of U.S. Provisional Patent Application No. 62/462,850 filed on Feb. 23, 2017 entitled “VARIABLE-FOCUS VIRTUAL IMAGE DEVICES,” which is incorporated by reference herein in its entirety. This provisional patent application includes the following sections both of which are incorporated by reference and form a part of this patent application: 
     1. SECTION I: Specification and Drawings for the portion of the application entitled “DISPLAY SYSTEM WITH VARIABLE POWER REFLECTOR.”
 
2. SECTION II: Specification and Drawings for the portion of the application entitled “VARIABLE-FOCUS VIRTUAL IMAGE DEVICES BASED ON POLARIZATION CONVERSION.”
 
     Sections I and II both discuss variable focus or variable power devices and features associated with the components of these devices and both Sections equally form part of the disclosure of this application. Accordingly, various features, elements, structures, methods, etc. described in Section I can be used with, combined with, incorporated into, or are otherwise compatible with features, elements, structures, methods, etc. described in Section II in any combination. Likewise, various features, elements, structures, methods, etc. described in Section II can be used with, combined with, incorporated into, or are otherwise compatible with features, elements, structures, methods, etc. described in Section I in any combination. 
     This application also 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; and U.S. application Ser. No. 14/331,218 filed on Jul. 14, 2014. 
    
    
     BACKGROUND 
     Field 
     The present disclosure relates to optical devices, including 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, in which digitally reproduced images or portions thereof are presented to a user in a manner wherein they seem to be, or may be perceived as, real. A virtual reality, or “VR”, scenario typically involves the presentation of digital or virtual image information without transparency to other actual real-world visual input; an augmented reality, or “AR”, scenario typically involves presentation of digital or virtual image information as an augmentation to visualization of the actual world around the user. A mixed reality, or “MR”, scenario is a type of AR scenario and typically involves virtual objects that are integrated into, and responsive to, the natural world. For example, an MR scenario may include AR image content that appears to be blocked by or is otherwise perceived to interact with objects in the real world. 
     Referring to  FIG. 1 , an augmented reality scene  1  is depicted. The user of an AR technology sees a real-world park-like setting  20  featuring people, trees, buildings in the background, and a concrete platform  30 . The user also perceives that he “sees” “virtual content” such as a robot statue  40  standing upon the real-world platform  1120 , and a flying cartoon-like avatar character  50  which seems to be a personification of a bumble bee. These elements  50 ,  40  are “virtual” in that they do not exist in the real world. Because the human visual perception system is complex, it is challenging to produce AR technology that facilitates a comfortable, natural-feeling, rich presentation of virtual image elements amongst other virtual or real-world imagery elements. 
     Systems and methods disclosed herein address various challenges related to AR and VR technology. 
     SUMMARY 
     This application include discussions of systems and methods that can be employed to provide variable optical power. Variable focus or variable power devices may find application in certain head mounted display devices that project images as if the images originated from different depths. By changing the optical power of an optical element in the head mounted display device, images presented to the wearer of the head mounted display device appear as if located at different distances from the wearer. The variable focus or variable power optical device can thus be modulated to cause different image content to be displayed as if the image content is situated at different locations with respect to the user. Some variable power elements comprise reflectors comprising movable membranes. Other variable power elements comprise liquid crystal switchable devices that can switch between optical power levels using switchable liquid crystal elements. Some variable focus devices described herein utilize the polarization properties of light to facilitate switching from one focus to another. 
     The systems, methods and devices of this disclosure each have several innovative aspects, no single one of which is solely responsible for the desirable attributes disclosed herein. A variety of example systems and methods are provided below. 
     Embodiment 1: A wearable augmented reality head-mounted display system configured to pass light from the world forward a wearer wearing the head-mounted system into an eye of the wearer, said wearable augmented reality head mounted display system comprising: 
     an optical display configured to output light to form an image; 
     one or more waveguides disposed to receiving said light from said display; 
     a frame configured to dispose the waveguides forward of said eye such that said one or more waveguides have a forward side and a rearward side, said rearward side closer to said eye than said forward side; 
     a variable power reflector disposed on said forward side of said one or more waveguides, said variable optical power reflector configured to have an optical power that is adjustable upon application of an electrical signal; and 
     one or more out-coupling elements disposed with respect to said one or more waveguides to extract light from the one or more waveguides and direct at least a portion of said light propagating within said waveguide to the variable power reflector, said light being directed from said variable power reflector back through said waveguide and into said eye to present an image from the display into the eye of the wearer. 
     Embodiment 2: The system of Embodiment 1, wherein said optical display comprise a scanning fiber display or a liquid crystal display. 
     Embodiment 3: The system of any of the Embodiments 1-2, wherein said variable power reflector comprises a deformable reflective surface having an adjustable shape. 
     Embodiment 4: The system of Embodiment 3, wherein said variable power reflector includes at least one electrode to apply an electric field to deform said reflective surface. 
     Embodiment 5: The system of any of the Embodiments 1-4, wherein said variable power reflector comprises silicon nitride. 
     Embodiment 6: The system of any of the Embodiments 1-5, wherein said variable power reflector comprises one or more switchable optical elements configured to switch between two states. 
     Embodiment 7: The system of Embodiment 6, wherein said two states comprise two levels of reflection. 
     Embodiment 8: The system of Embodiments 6 or 7, wherein said two states comprise two levels of optical power. 
     Embodiment 9: The system of any of Embodiments 6, 7 or 8, wherein said two states comprise two levels of transmission. 
     Embodiment 10: The system of any of Embodiments 6-9, wherein said one or more switchable optical elements comprises a diffractive optical element. 
     Embodiment 11: The system of any of Embodiments 6-10, wherein said one or more switchable optical elements comprises a reflective element. 
     Embodiment 12: The system of any of Embodiments 6-11, wherein said one or more switchable optical elements comprises liquid crystal element comprising liquid crystal. 
     Embodiment 13: The system of any of Embodiments 6-12, wherein said one or more switchable optical elements comprises a switchable polarization grating or a switchable polymer dispersed liquid crystal element. 
     Embodiment 14: The system of any of the Embodiments 1-13, wherein said variable power reflector comprises a stack of switchable optical elements, different of said switchable optical elements associated with different optical powers such that said stack of switchable optical elements can switch between different optical powers. 
     Embodiment 15: The system of any of the Embodiments 1-14, wherein said variable power reflector is configured to reflect light from said display while transmitting light from the world forward the wearer. 
     Embodiment 16: The system of any of the Embodiments 1-15, wherein said variable power reflector includes wavelength selective filters to reflect light from said display while transmitting light from the world forward the wearer. 
     Embodiment 17: The system of any of the Embodiments 1-16, wherein said variable power reflector has a transmission spectrum that includes a notch, light having a wavelength corresponding to said notch being reflected from said variable power reflector more than wavelengths adjacent to said notch. 
     Embodiment 18: The system of Embodiment 17, wherein the wavelength corresponding to said notch is reflected at least two times as much as light having wavelengths adjacent to said notch. 
     Embodiment 19: The system of Embodiment 17, wherein the wavelength corresponding to said notch is reflected at least five times as much as light having wavelengths adjacent to said notch. 
     Embodiment 20: The system of any of Embodiments 17-19, wherein said display comprises a color light source having a color corresponding with said notch wavelength. 
     Embodiment 21: The system of Embodiment 20, wherein the color of said color light source corresponds to red, green, or blue light. 
     Embodiment 22: The system of any of the Embodiments 1-21, wherein said variable power reflector includes one or more polarization selective filters to reflect light from said display while transmitting light from the world forward the wearer. 
     Embodiment 23: The system of any of the Embodiments 1-22, wherein said one or more out-coupling elements are configured to direct more light extracted from the one or more waveguides to the forward side of the one or more waveguides in comparison to the reverse side. 
     Embodiment 24: The system of any of the Embodiments 1-23, wherein said one or more out-coupling elements comprise one or more metasurfaces, one or more liquid crystal polarization gratings, one or more volume phase holograms or any combination thereof. 
     Embodiment 25: The system of any of the Embodiments 1-24, further comprising electronics in electrical communication with said variable power reflector to drive said variable power reflector, said electronics configured to change said optical power so as to present different image content as if originating from different depth planes. 
     Embodiment 26: The system of Embodiment 25, wherein said electronics is configured to change said optical power on a frame by frame basis. 
     Embodiment 27: The system of Embodiment 25, wherein said electronics is configured to change said optical power on a pixel by pixel basis. 
     Embodiment 28: The system of any of the Embodiments 1-27, further comprising a light recycling system configured to re-direct light extracted from the one or more waveguides toward the rearward side of the one or more waveguides toward the forward side such that said light can be incident on said variable power reflector and directed to said eye of the wearer. 
     Embodiment 29: The system of Embodiment 28, wherein said light recycling system comprises one or more polarization control elements or polarization selective elements. 
     Embodiment 30: The system of Embodiment 28, wherein said light recycling system comprises a polarization selective reflector disposed between said one or more waveguides and said eye. 
     Embodiment 31: The system of Embodiment 30, wherein said light recycling system comprises one or more retarders disposed between said one or more light guides and said polarization selective reflector. 
     Embodiment 32: The system of Embodiment 31, wherein said one or more retarders is disposed between said one or more light guides and said polarization reflector introduces a net retardance of about a half wave. 
     Embodiment 33: The system of any of Embodiments 29-32, wherein said recycling system comprises one or more retarders disposed between said one or more light guides and variable power reflector. 
     Embodiment 34: The system of Embodiment 33, wherein said one or more retarders disposed between said one or more light guides and variable power reflector introduces a net retardance of about a quarter wave. 
     Embodiment 35: A display device, comprising: 
     a waveguide configured to propagate visible light under total internal reflection in a direction parallel to a major surface of the waveguide and to outcouple the visible light in a direction normal to the major surface; 
     a deformable mirror configured to reflect visible light having a first polarization, wherein the deformable mirror comprises at least one of a notch reflector or one or more cholesteric liquid crystal (CLC) layers, wherein each of the CLC layers comprises a plurality of chiral structures, wherein each of the chiral structures comprises a plurality of liquid crystal molecules that extend in a layer depth direction and are successively rotated in a first rotation direction, wherein arrangements of the liquid crystal molecules of the chiral structures vary periodically in a lateral direction perpendicular to the layer depth direction such that the one or more CLC layers are configured to Bragg-reflect incident light. 
     Embodiment 36: The display device of Embodiment 35, wherein the waveguide is configured to outcouple the visible light selectively towards the deformable mirror. 
     Embodiment 37: The display device of any of Embodiments 35 or 36, wherein different ones of the one or more CLC layers are configured to reflect visible light having a wavelength in a wavelength range corresponding to different ones of red, green or blue light, while being configured to transmit light having a wavelength outside the wavelength range. 
     Embodiment 38: The display device of any of Embodiments 35-37, wherein each of the chiral structures of the CLC layers comprises a plurality of liquid crystal molecules that extend in a layer depth direction by at least a helical pitch, wherein different ones of the one or more CLC layers have different helical pitches. 
     Embodiment 39: The display device of any of Embodiments 35-38, wherein different ones of the one or more CLC layers have substantially the same optical power. 
     Embodiment 40: The display device of any of Embodiments 35-39, wherein the display device comprises a plurality of notch reflectors, wherein each of the notch reflectors is configured to reflect visible light having a first polarization, wherein each of the notch reflector comprises one or more cholesteric liquid crystal (CLC) layers, wherein each of the CLC layers comprises a plurality of chiral structures, wherein each of the chiral structures comprises a plurality of liquid crystal molecules that extend in a layer depth direction and are successively rotated in a first rotation direction, wherein arrangements of the liquid crystal molecules of the chiral structures vary periodically in a lateral direction perpendicular to the layer depth direction such that the one or more CLC layers are configured to Bragg-reflect incident light. 
     Embodiment 41: The display device of Embodiment 40, wherein different ones of the plurality of notch reflectors have different optical powers. 
     Embodiment 42: The display device of Embodiment 40, further comprising a half-wave plate corresponding to each of the plurality of notch reflectors. 
     Embodiment 43: A display device, comprising: 
     a light projector configured to output light to form an image; 
     a waveguide assembly configured to receive light from the light projector, the waveguide assembly configured to propagate the received light via total internal reflection in a direction parallel to a major surface of the waveguide assembly and to outcouple the received light in a direction normal to the major surface, the waveguide assembly having a rearward side disposed closer to an eye of an user and a forward side disposed opposite the rearward side; and 
     a deformable mirror configured to reflect a portion of the outcoupled light having a characteristic, the deformable mirror disposed on the forward side of the waveguide assembly. 
     Embodiment 44: The display device of Embodiment 43, wherein the characteristic of portion of the outcoupled light comprises a polarization characteristic. 
     Embodiment 45: The display device of Embodiments 43 or 44, wherein the characteristic of portion of the outcoupled light comprises a wavelength characteristic. 
     Embodiment 46: The display device of any of Embodiments 43-45, wherein the deformable mirror comprises a polarizing reflector configured to reflect a portion of the outcoupled light having a polarization characteristic. 
     Embodiment 47: The display device of any of Embodiments 43-46, wherein the deformable mirror comprises a notch reflector configured to reflect light within a wavelength range. 
     Embodiment 48: The display device of any of Embodiments 43-47, further comprising a polarizer disposed on the rearward side of the waveguide assembly. 
     Embodiment 49: The display device of any of Embodiments 43-48, wherein the deformable mirror comprises one or more cholesteric liquid crystal (CLC) layers, wherein each of the CLC layers comprises a plurality of chiral structures, wherein each of the chiral structures comprises a plurality of liquid crystal molecules that extend in a layer depth direction and are successively rotated in a first rotation direction, wherein arrangements of the liquid crystal molecules of the chiral structures vary periodically in a lateral direction perpendicular to the layer depth direction such that the one or more CLC layers are configured to Bragg-reflect incident light. 
     Embodiment 50: The display device of any of Embodiments 43-49, wherein the waveguide assembly comprises one or more liquid crystal layers. 
     Embodiment 51: The system of any of Embodiments 6-11, wherein said one or more switchable optical elements comprise a cholesteric liquid crystal (CLC). 
     Embodiment 52: The system of any of the Embodiments 1-23, wherein said one or more out-coupling elements comprise a cholesteric liquid crystal (CLC). 
     Embodiment 53: A display device, comprising: 
     a waveguide configured to propagate visible light under total internal reflection in a direction parallel to a major surface of the waveguide and to outcouple the visible light in a direction normal to the major surface; 
     a notch reflector configured to reflect visible light having a first polarization, wherein the notch reflector comprises one or more cholesteric liquid crystal (CLC) layers, wherein each of the CLC layers comprises a plurality of chiral structures, wherein each of the chiral structures comprises a plurality of liquid crystal molecules that extend in a layer depth direction and are successively rotated in a first rotation direction, wherein arrangements of the liquid crystal molecules of the chiral structures vary periodically in a lateral direction perpendicular to the layer depth direction such that the one or more CLC layers are configured to Bragg-reflect incident light. 
     Embodiment 54: The display device of Embodiment 53, wherein the waveguide is configured to outcouple the visible light selectively towards the notch reflector. 
     Embodiment 55: The display device of any of Embodiments 53 or 54, wherein the notch reflector comprises a deformable mirror having the one or more CLC layers formed thereon. 
     Embodiment 56: The display device of any of Embodiments 53-55, wherein different ones of the one or more CLC layers are configured to reflect visible light having a wavelength in a wavelength range corresponding to different ones of red, green or blue light, while being configured to transmit light having a wavelength outside the wavelength range. 
     Embodiment 57: The display device of any of Embodiments 53-56, wherein each of the chiral structures of the CLC layers comprises a plurality of liquid crystal molecules that extend in a layer depth direction by at least a helical pitch, wherein different ones of the one or more CLC layers have different helical pitches. 
     Embodiment 58: The display device of any of Embodiments 53-58, wherein different ones of the one or more CLC layers have substantially the same optical power. 
     Embodiment 59: The display device of Embodiment 53, wherein the display device comprises a plurality of notch reflectors, wherein each of the notch reflectors is configured to reflect visible light having a first polarization, wherein each of the notch reflector comprises one or more cholesteric liquid crystal (CLC) layers, wherein each of the CLC layers comprises a plurality of chiral structures, wherein each of the chiral structures comprises a plurality of liquid crystal molecules that extend in a layer depth direction and are successively rotated in a first rotation direction, wherein arrangements of the liquid crystal molecules of the chiral structures vary periodically in a lateral direction perpendicular to the layer depth direction such that the one or more CLC layers are configured to Bragg-reflect incident light. 
     Embodiment 60: The display device of Embodiment 59, wherein different ones of the plurality of notch reflectors have different optical powers. 
     Embodiment 61: The display device of Embodiments 59 or 60, further comprising a half-wave plate corresponding to each of the plurality of notch reflectors. 
    
    
     
       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 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  illustrates a wearable augmented reality head-mounted display system that includes a variable power reflector disposed to receive light from the light projector system. 
         FIG. 10B  illustrates the light projection system of  FIG. 10A  where the variable power reflector creates virtual images as if emanating from an object at infinity. 
         FIG. 10C  illustrates a variable power reflector that is polarization selective and that uses the polarization of light to determine what light is reflected and what light is transmitted. 
         FIG. 10D  illustrates an example display device comprising a deformable mirror and a waveguide assembly with a liquid crystal grating. 
         FIG. 11  illustrates a display device including light recycling system comprising an arrangement of retarders that are configured to redirect injected light from the waveguide that is outcoupled towards the user&#39;s eye towards the reflector. 
         FIG. 12  illustrates a variable power reflector that can comprise one or more switchable optical elements such as a stack or a plurality of switchable reflective elements. 
         FIGS. 13A-13C  illustrate example reflective diffraction lenses that can be implemented as part of a display device, where the reflective diffraction lenses are formed of patterned CLC materials serving as a reflective polarizing mirror. 
         FIG. 14  illustrates an embodiment of a stack or plurality of switchable reflective elements having a plurality of switchable reflective elements. 
     
    
    
     DETAILED DESCRIPTION 
     A wearable augmented reality head-mounted display system can be configured to pass light from the world forward a wearer wearing the head-mounted system into an eye of the wearer. The head-mounted display system can include an optical display that is configured to output light to form an image. The display system may include a frame that includes waveguides that are forward of the eye such that said one or more waveguides have a forward side and a rearward side and such that the rearward side is closer to the eye than the forward side. The system may include one or more waveguides that are disposed to receiving the light from the display. A variable power reflector can be disposed on the forward side of the one or more waveguides. The reflector can be configured to have an optical power that is adjustable upon application of an electrical signal. The system may further include one or more out-coupling elements that are disposed with respect to the one or more waveguides to extract light from the one or more waveguides. The elements may direct at least a portion of the light propagating within the waveguide to the variable power reflector. The light being directed from the variable power reflector may be reflected back through the waveguide and into the eye to present an image from the display into the eye of the wearer. 
     The optical display may include a scanning fiber display or a liquid crystal display. In some variants, the variable power reflector includes a deformable reflective surface that has an adjustable shape. The variable power reflector may include at least one electrode to apply an electric field to deform the reflective surface. The variable power reflector may include silicon nitride. The variable power reflector may include one or more switchable optical elements that are configured to switch between two states. The two states may include two levels of reflection, two levels of optical power, and/or two levels of transmission. 
     The one or more switchable optical elements may include a diffractive optical element, a reflective element, and/or a liquid crystal element that includes liquid crystal. 
     The head-mounted display system may include one or more switchable polarization gratings or switchable polymer dispersed liquid crystal elements. The variable power reflector may include a stack of switchable optical elements associated with different optical powers such that the switchable optical element can switch between different optical powers. 
     In some cases, the variable power reflector is configured to reflect light from the display while transmitting light from the world forward the wearer. The variable power reflector may include wavelength selective filters to reflect light from the display while transmitting light from the world forward the wearer. The variable power reflector may have a transmission spectrum that includes a notch in such a way that light that has a wavelength corresponding to the notch is reflected from the variable power reflector more than wavelengths that are adjacent to the notch. The wavelength corresponding to the notch may be reflected at least five times as much as light having wavelengths adjacent to the notch. 
     The head-mounted display may include a color light source that produces a color corresponding with the notch wavelength. For example, the color of the color light source may correspond to red, green, or blue light. 
     The variable power reflector may include polarization selective filters to reflect light from the display while transmitting light from the world forward the wearer. 
     In some designs, the one or more out-coupling elements are configured to direct more light that is extracted from the one or more waveguides to the forward side of the one or more waveguides in comparison to the reverse side. The one or more out-coupling elements may include one or more metasurfaces, one or more liquid crystal polarization gratings, one or more volume phase holograms or any combination thereof. 
     The display system may include electronics that are in electrical communication with the variable power reflector to drive the variable power reflector. The electronics can be configured to change the optical power so as to present different image content as if originating from different depth planes. The electronics may be configured to change the optical power on a frame by frame basis or a pixel by pixel basis. 
     The system may include a recycling system. The recycling system may be configured to re-direct light that is extracted from the one or more waveguides toward the rearward side of the one or more waveguides toward the forward side such that the light can be incident on the variable power reflector and directed to the eye of the wearer. The recycling system may include one or more polarization control elements or polarization selective elements. The recycling system may include a polarization selective reflector that is disposed between the one or more waveguides and the eye. 
     The recycling system can include one or more retarders that are disposed between the one or more light guides and the polarization reflector. The one or more retarders that are disposed between the one or more light guides and the polarization reflector can be configured to introduce a net retardance of about a half wave. 
     In some designs, the recycling system may include one or more retarders that are disposed between the one or more light guides and the variable power reflector. The one or more retarders that are disposed between the one or more light guides and the variable power reflector can be configured to introduce a net retardance of about a quarter wave. 
     Reference will now be made to the figures, in which like reference numerals refer to like parts throughout. 
       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, may optionally be positioned adjacent the other ear canal of the user to provide stereo/shapeable sound control). 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 a physiological state of the user  90  in some embodiments. For example, the sensor  120   a  may be an electrode. 
     With continued reference to  FIG. 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. 
     With reference now to  FIG. 3 , 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 different presentations 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. 
       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. In addition, the waveguide assembly  260  may also be referred to as an eyepiece. 
     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. It will be appreciated that the image injection devices  360 ,  370 ,  380 ,  390 ,  400  are illustrated schematically and, in some embodiments, these image injection devices may represent different light paths and locations in a common projection system configured to output light into associated ones of the waveguides  270 ,  280 ,  290 ,  300 ,  310 . 
     In some embodiments, the 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 . 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 with out-coupling optical elements that out-couple light to form images that appear to be set on a depth plane at a large distance (e.g., optical infinity) from the eye  210 . Other waveguides or other sets of out-coupling optical elements may output an exit beam pattern that is more divergent, which would require the eye  210  to accommodate to a closer distance to bring it into focus on the retina and would be interpreted by the brain as light from a distance closer to the eye  210  than optical infinity. 
     In some embodiments, a full color image may be formed at each depth plane by overlaying images in each of the component colors, e.g., three or more component colors.  FIG. 8  illustrates an example of a stacked waveguide assembly in which each depth plane includes images formed using multiple different component colors. The illustrated embodiment shows depth planes  240   a - 240   f , although more or fewer depths are also contemplated. Each depth plane may have three or more component color images associated with it, including: a first image of a first color, G; a second image of a second color, R; and a third image of a third color, B. Different depth planes are indicated in the figure by different numbers for diopters (dpt) following the letters G, R, and B. Just as examples, the numbers following each of these letters indicate diopters (1/m), or inverse distance of the depth plane from a viewer, and each box in the figures represents an individual component color image. In some embodiments, to account for differences in the eye&#39;s focusing of light of different wavelengths, the exact placement of the depth planes for different component colors may vary. For example, different component color images for a given depth plane may be placed on depth planes corresponding to different distances from the user. Such an arrangement may increase visual acuity and user comfort and/or may decrease chromatic aberrations. 
     In some embodiments, light of each component color may be outputted by a single dedicated waveguide and, consequently, each depth plane may have multiple waveguides associated with it. In such embodiments, each box in the figures including the letters G, R, or B may be understood to represent an individual waveguide, and three waveguides may be provided per depth plane where three component color images are provided per depth plane. While the waveguides associated with each depth plane are shown adjacent to one another in this drawing for ease of description, it will be appreciated that, in a physical device, the waveguides may all be arranged in a stack with one waveguide per level. In some other embodiments, multiple component colors may be outputted by the same waveguide, such that, e.g., only a single waveguide may be provided per depth plane. 
     With continued reference to  FIG. 8 , in some embodiments, G is the color green, R is the color red, and B is the color blue. In some other embodiments, other colors associated with other wavelengths of light, including magenta and cyan, may be used in addition to or may replace one or more of red, green, or blue. 
     It will be appreciated that references to a given color of light throughout this disclosure will be understood to encompass light of one or more wavelengths within a range of wavelengths of light that are perceived by a viewer as being of that given color. For example, red light may include light of one or more wavelengths in the range of about 620-780 nm, green light may include light of one or more wavelengths in the range of about 492-577 nm, and blue light may include light of one or more wavelengths in the range of about 435-493 nm. 
     In some embodiments, the light source  530  ( FIG. 6 ) may be configured to emit light of one or more wavelengths outside the visual perception range of the viewer, for example, infrared and/or ultraviolet wavelengths. In addition, the in-coupling, out-coupling, and other light redirecting structures of the waveguides of the display  250  may be configured to direct and emit this light out of the display towards the user&#39;s eye  210 , e.g., for imaging and/or user stimulation applications. 
     With reference now to  FIG. 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 total internal reflection (TIR) of light through the waveguides  670 ,  680 ,  690  (e.g., TIR between the top and bottom major surfaces of each waveguide). In some embodiments, the layers  760   a ,  760   b  are formed of air. While not illustrated, it will be appreciated that the top and bottom of the illustrated set  660  of waveguides may include immediately neighboring cladding layers. 
     Preferably, for ease of manufacturing and other considerations, the material forming the waveguides  670 ,  680 ,  690  are similar or the same, and the material forming the layers  760   a ,  760   b  are similar or the same. In some embodiments, the material forming the waveguides  670 ,  680 ,  690  may be different between one or more waveguides, and/or the material forming the layers  760   a ,  760   b  may be different, while still holding to the various refractive index relationships noted above. 
     With continued reference to  FIG. 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. The in-coupling optical elements  700 ,  710 ,  720  each deflect the incident light such that the light propagates through a respective one of the waveguides  670 ,  680 ,  690  by TIR. In some embodiments, the incoupling optical elements  700 ,  710 ,  720  each selectively deflect one or more particular wavelengths of light, while transmitting other wavelengths to an underlying waveguide and associated incoupling optical element. 
     For example, in-coupling optical element  700  may be configured to deflect ray  770 , which has a first wavelength or range of wavelengths, while transmitting rays  1242  and  1244 , which have different second and third wavelengths or ranges of wavelengths, respectively. The transmitted ray  780  impinges on and is deflected by the in-coupling optical element  710 , which is configured to deflect light of a second wavelength or range of wavelengths. The ray  790  is deflected by the in-coupling optical element  720 , which is configured to selectively deflect light of third wavelength or range of wavelengths. 
     With continued reference to  FIG. 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. The light rays  770 ,  780 ,  790  propagate through the respective waveguide  670 ,  680 ,  690  by TIR until impinging on the waveguide&#39;s corresponding light distributing elements  730 ,  740 ,  750 . 
     With reference now to  FIG. 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 within the waveguides  670 ,  680 ,  690 , respectively. The light rays  770 ,  780 ,  790  then impinge on the light distributing elements  730 ,  740 ,  750 , respectively. The light distributing elements  730 ,  740 ,  750  deflect the light rays  770 ,  780 ,  790  so that they propagate towards the out-coupling optical elements  800 ,  810 ,  820 , respectively. 
     In some embodiments, the light distributing elements  730 ,  740 ,  750  are orthogonal pupil expanders (OPE&#39;s). In some embodiments, the OPE&#39;s deflect or distribute light to the out-coupling optical elements  800 ,  810 ,  820  and, in some embodiments, may also increase the beam or spot size of this light as it propagates to the out-coupling optical elements. In some embodiments, the light distributing elements  730 ,  740 ,  750  may be omitted and the in-coupling optical elements  700 ,  710 ,  720  may be configured to deflect light directly to the out-coupling optical elements  800 ,  810 ,  820 . For example, with reference to  FIG. 9A , the light distributing elements  730 ,  740 ,  750  may be replaced with out-coupling optical elements  800 ,  810 ,  820 , respectively. In some embodiments, the out-coupling optical elements  800 ,  810 ,  820  are exit pupils (EP&#39;s) or exit pupil expanders (EPE&#39;s) that direct light in a viewer&#39;s eye  210  ( FIG. 7 ). It will be appreciated that the OPE&#39;s may be configured to increase the dimensions of the eye box in at least one axis and the EPE&#39;s may be to increase the eye box in an axis crossing, e.g., orthogonal to, the axis of the OPEs. For example, each OPE may be configured to redirect a portion of the light striking the OPE to an EPE of the same waveguide, while allowing the remaining portion of the light to continue to propagate down the waveguide. Upon impinging on the OPE again, another portion of the remaining light is redirected to the EPE, and the remaining portion of that portion continues to propagate further down the waveguide, and so on. Similarly, upon striking the EPE, a portion of the impinging light is directed out of the waveguide towards the user, and a remaining portion of that light continues to propagate through the waveguide until it strikes the EP again, at which time another portion of the impinging light is directed out of the waveguide, and so on. Consequently, a single beam of incoupled light may be “replicated” each time a portion of that light is redirected by an OPE or EPE, thereby forming a field of cloned beams of light, as shown in  FIG. 6 . In some embodiments, the OPE and/or EPE may be configured to modify a size of the beams of light. 
     Accordingly, with reference to  FIGS. 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., EP&#39;s)  800 ,  810 ,  820  for each component color. The waveguides  670 ,  680 ,  690  may be stacked with an air gap/cladding layer between each one. The in-coupling optical elements  700 ,  710 ,  720  redirect or deflect incident light (with different in-coupling optical elements receiving light of different wavelengths) into its waveguide. The light then propagates at an angle which will result in TIR within the respective waveguide  670 ,  680 ,  690 . In the example shown, light ray  770  (e.g., blue light) is deflected by the first in-coupling optical element  700 , and then continues to bounce down the waveguide, interacting with the light distributing element (e.g., OPE&#39;s)  730  and then the out-coupling optical element (e.g., EPs)  800 , in a manner described earlier. The light rays  780  and  790  (e.g., green and red light, respectively) will pass through the waveguide  670 , with light ray  780  impinging on and being deflected by in-coupling optical element  710 . The light ray  780  then bounces down the waveguide  680  via TIR, proceeding on to its light distributing element (e.g., OPEs)  740  and then the out-coupling optical element (e.g., EP&#39;s)  810 . Finally, light ray  790  (e.g., red light) passes through the waveguide  690  to impinge on the light in-coupling optical elements  720  of the waveguide  690 . The light in-coupling optical elements  720  deflect the light ray  790  such that the light ray propagates to light distributing element (e.g., OPEs)  750  by TIR, and then to the out-coupling optical element (e.g., EPs)  820  by TIR. The out-coupling optical element  820  then finally out-couples the light ray  790  to the viewer, who also receives the out-coupled light from the other waveguides  670 ,  680 . 
       FIG. 9C  illustrates a top-down plan 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. However, as discussed herein, the in-coupling optical elements  700 ,  710 ,  720  are not vertically aligned; 
     rather, the in-coupling optical elements are preferably non-overlapping (e.g., laterally spaced apart as seen in the top-down view). As discussed further herein, this nonoverlapping spatial arrangement facilitates the injection of light from different resources into different waveguides on a one-to-one basis, thereby allowing a specific light source to be uniquely coupled to a specific waveguide. In some embodiments, arrangements including nonoverlapping spatially-separated in-coupling optical elements may be referred to as a shifted pupil system, and the in-coupling optical elements within these arrangements may correspond to sub pupils. 
     Variable Power Reflector 
     As illustrated in  FIG. 10A , the wearable augmented reality head-mounted display system  60  may include a variable power reflector (or variable focus reflector)  1032  disposed to receive light from the light projector system  2000 , e.g., via the waveguide  2700 . The variable power reflector  1032  may be configured to have an optical power that is adjustable upon application of an electrical signal. For example, the variable power reflector  1032  shown in  FIG. 10A  comprises a deformable reflector having a deformable reflective surface (e.g., surface closest to waveguide  2700 ) having a shape that is adjustable. For example, the shape of the deformable reflector  1032  may be configured to bend to produce different convex shapes corresponding to different optical power. The variable power reflector  1032 , may for example, comprise a deformable membrane or other thin member that is flexible and can vary its shape. In some designs, the deformable reflector  1032  is electrically actuated. The deformable reflector may, for example, include at least one electrode  1036  to apply an electric field to deform the reflective surface possibly via electro-static forces or by some other. As illustrated in  FIG. 10A , another electrode  1042  is included on the surface of the waveguide  2700 . Application of a voltage across the two electrodes  1036  and  1042  may provide such an electric field and may thus cause the deformable reflector to deform. With deformation of the deformable membrane or member, the reflective surface (e.g., surface closest to the waveguide  2700 ) may be deformed. As illustrated, the shape can be convex and thus provide a reflective surface having negative optical power. Although in some examples the surface closest to the waveguide  2700  may comprise a reflective surface for reflecting light from the light projection system (display)  2000 , other surfaces associated with the variable power reflector may reflect light from the light project system. The variable power reflector  1032  may, for example, include multiple layers that include one or more reflective surface that need not be the surface closest to the light guide  2700 . The variable power reflector  1032  may be wavelength selective and/or polarization selective such that light having certain wavelengths and/or polarization states are reflected by the variable power reflector  1032  while light having other wavelengths and/or polarization states are transmitted through the variable power reflector  1032 . 
     The waveguide  2700  may have a forward side and a rearward side with the rearward side being closest (more proximal) to the eye of the wearer and the forward side being farther (or more distal) to the eye of the wearer and closer to the world in front of the wearer and the head mounted display system. As illustrated the variable power reflector  1032  may be disposed on a forward side of the waveguide. The waveguide may include outcoupling elements configured to extract light from the waveguide and direct this light toward the forward side of the waveguide so as to be incident on the variable power reflector. Since the variable power reflector is disposed on the forward side of the waveguide, the variable power reflector is configured to transmit light from the world forward the wearer such that the head-mounted display system can pass light from the world forward a wearer wearing the head-mounted system into an eye of the wearer. Despite being transmissive to light from the world forward the wearer, the variable power reflector  1032  is configured to reflect light from said display. As discussed below, in some examples, the variable power reflector may include one or more wavelength selective filters to reflect light from the light projection system (display)  2000  while transmitting light from the world forward the wearer. Additionally, in some examples, the variable power reflector may include one or more polarization selective filters to reflect light from the light projection system (display)  2000  while transmitting light from the world forward the wearer. 
     As illustrated, the variable power reflector may be caused to deform its shape to cause light from the display  2000  that is reflected from the variable power reflector to diverge. In particular, light from the light source  530  is being directed by the beam splitter  550  to the spatial light modulator  540  and to an image injection device  4000  disposed with respect to the waveguide  2700  to couple spatially modulated light containing an image into the waveguide. This light  1022  may propagate through the waveguide  2700  by total internal reflection (TIR) and be extracted therefrom via one or more outcoupling optical elements (e.g., an exit pupil expander comprising, for example, diffractive features). In some configurations, the light from the display  2000  that is injected into the waveguide  2700  may be collimated and thus collimated light  1022  may exit the waveguide and propagate toward the variable power reflector  1032  as illustrated in  FIG. 10A . In the example shown in  FIG. 10A , the variable power reflector (or variable focus reflector) is deformed so as to create a convex reflective surface that produces a negative optical power (e.g., produces a virtual image at a distance from the eye  210 ), thus causing reflected light rays  1024  to diverge. The reflected light  1024  may then enter the eye  210  of an individual (e.g., a wearer) and appear to be formed by an object a short distance from the eye on the other side of the waveguide and deformable mirror. As a result, the image formed by the display may appear to originate from an object relatively close to the wearer. 
     In the example shown in  FIG. 10A , light exits the image injection device  4000  and is injected into an input surface  4600  of the waveguide  2700  that is on the edge of waveguide  2700 . In other examples, however, the light may be injected into a major surface of the corresponding waveguide (e.g., one of the waveguide surfaces directly facing the world  144  or the viewer&#39;s eye  210 ). See, for example,  FIGS. 9A-9C . In various embodiments, light from the image injection device  4000  can be polarized prior to being injected into the waveguide  2700 . For example, the light injected into the waveguide  2700  can be configured to be linearly polarized along the vertical (or horizontal) direction. 
     In certain configurations, the one or more outcoupling optical elements  5700  may be configured to direct substantially all outcoupled light in a distal direction from the eye of the viewer, e.g. towards the world. The one or more outcoupling optical elements  5700  may include one or more asymmetrical diffraction gratings. 
     Although the variable power reflector  1032  may be disposed so as to reflect light outcoupled in a distal direction from the eye  210  of the user back towards the eye  210  of the user potentially altering the wavefront of the light reflected therefrom, the variable power reflector transmits light from the world in front of the wearer to the user&#39;s eye without introducing similar changes to the wavefront. For example, although the variable power reflector may be configured have optical power that acts on light from the display  2000  that is reflected from the variable power reflector, the variable power reflector may not similarly act on the light from the world in front of the wearer that is transmitted therethrough to the eye. In some cases, for example, the thickness of the variable power reflector  1032  may be chosen sufficiently low so as not to significantly alter the optical properties of light passing through the variable power reflector  1032  (e.g., from the world). Advantageously, this may allow for light passing through the variable power reflector  1032  from the world to be observed at the proper depths while controlling the depths at which images from the display  2000  appear to be originating. This configuration may therefore exclude any additional lenses to compensate for a change in the radius of curvature or optical power of the variable power reflector for light from the world in front of the wearer, and accordingly make the system lighter, cheaper and less complex. 
     The reflector  1036  may be comprised of one or more transparent materials, such as, for example, silicon mononitride (SiN). The material at the appropriate thickness may also be sufficiently flexible to be deformed, for example, by electrostatic forces with application of an electric field. Other methods of deforming the variable power reflector may be used. Similarly, as discussed below, other ways of providing for variable optical power different from deforming the shape of the reflector may also be employed. 
     In some embodiments, the resiliency of the variable power reflector  1032  allows for rapid temporal transitions among one or more optical powers. For example, rapid transitions from one focal length (e.g., curvature) to another may allow for a time-multiplexing of different focal planes as seen by a viewer (e.g., the eye  210 ). In some embodiments, transitions may toggle between two or more focal lengths many times a minute. 
     As discussed above, in some cases, the curvature of the variable power reflector  1032  is controlled by two or more electrodes. Although a distal electrode  1036  may be disposed on a distal side of the variable power reflector  1032 , as illustrated by  FIG. 10A , and a proximal electrode  1042  may be disposed proximal to the reflector  1032  (e.g., disposed on the waveguide  2700 , as shown In  FIG. 10A ), other electrode configuration are possible. The distal electrode  1036  may be disposed on a proximal side of the variable power reflector  1032 . The electrodes  1036 ,  1042  may be comprised of a material that can conduct electricity and that is transparent to visible light. One or more materials may be used for the electrodes  1036 ,  1042 , such as, for example, indium tin oxide (ITO). 
     The electrodes may be in electrical communication with electronics configured to drive the variable power reflector. These electronics may be configured to change the optical power of the variable optical power reflector so as to present different image content as if originating from different depth planes. In certain configurations, for example, the controller  560  or other electronics controls the operation the variable power reflector and may control, in conjunction, one or more of the waveguide  2700  (e.g., stacked waveguide assembly of  FIG. 6 ) and/or other parts of the light projector system  2000  (e.g., the image injection device  4000 , the light source  530 , the light modulator  540 , the camera assembly  630 ). In some cases, the controller  560  regulates the timing and provision of image information to the waveguide  2700  and thus to the variable power reflector and also controls the timing of the actuation or variation in optical power of the variable power reflector and synchronization therebetween. 
     Accordingly, the variable power reflector (or the variable focus reflector)  1032  may have optical power that is adjustable and that can be controlled electronically. This capability allows the depth plane corresponding to image outcoupled from the waveguide  2700  to be dynamically adjusted. For example, the light outcoupled from the waveguide  2700  may be substantially collimated, as if the object generating the light is located at an infinite distance (or at least far) from the viewer. If the desired focal plane corresponding to the variable power reflector (or the variable focus reflector) in at an infinite distance, the reflector  1032  can be set in a planar state so as, in this case where collimated light is output from the waveguide, to leave the wavefront curvature of the reflected light substantially unchanged. This situation is shown in  FIG. 10B . Alternatively, if the desired focal plane is closer to the viewer than infinity, the variable power reflector  1032  may be adjusted (e.g., flexed, electrically influenced) to assume an appropriate amount of convex curvature. This mode is shown in  FIG. 10A . 
     Accordingly,  FIG. 10B  illustrates the light projection system  2000  of  FIG. 10A  where the variable power reflector  1032  creates virtual images as if at infinity. The light rays  1022  exit the waveguide  2700  parallel or substantially parallel to each other and propagate toward the variable power reflector  1032 . Because the electrodes  1036 ,  1042  have caused the reflector  1032  to have a flat reflective surface, the reflected light rays  1024  are also parallel to each other as they propagate toward the eye  210 . In some embodiments, like in  FIG. 10A , the variable power reflector  1032 , the waveguide  2700 , and the outcoupling optical element  5700  are configured not to substantially alter the degree of divergence of the light  1024 . 
     As discussed above, the variable power reflector may be configured to reflect light from the display  2000  while transmitting light from the world forward the wearer. For example, the variable power reflector may include wavelength selective filters to reflect light from the display while transmitting light from the world forward the wearer. In some designs, for example, the variable power reflector  1032  comprises one or more wavelength selective filters (e.g., notch filters, band-pass filters) corresponding to one or more colors or combinations of colors. For example, one or more notch filters may be tuned to reflect light at a range of wavelengths corresponding to the color green (e.g., a narrow band within the range between 520-560 nm), the color red (e.g., a narrow band within the range between 635 nm-700 nm), and/or the color blue (e.g., a narrow band within the range between 450 nm-490 nm), while allowing light to be transmitted through the reflector  1032  at other visible wavelengths. Because it may be advantageous to allow most visible wavelengths of light passed through the reflector (e.g., from the “world”), the one or more wavelength selective filters can be tuned to reflect only a narrow range of visible wavelengths. For example, the reflector may comprise one or more notch filters each configured to reflect a band of wavelengths encompassing a range of between about 0.1 nm and 2 nm, between about 1 and 5 nm, between about 2 nm and 8 nm, between about 5 nm and 10 nm, and/or between about 10 and 25 nm. Any range formed by any of these values is possible. Other size bands are also possible. 
     The narrow bands in the visible spectrum where the variable power reflector is reflective may coincide with the color of light emitted by the display  2000 . The display, for example, may comprise one or more light sources (e.g., LEDs or lasers) that emit light of a particular color. The narrow band reflected by the variable power reflector may overlap and/or coincide with the color and wavelengths emitted by these light sources in the display. 
     Although the spectral transmission of the variable power reflector  1032  reflects light having a wavelength within these narrow bands, light outside these bands may be transmitted. Accordingly, much of the ambient visible light from the world forward the wearer and the head-mounted display is transmitted through the variable power reflector. To provide such reflectivity characteristics, the variable power reflector may include one or more reflective layers. For example the variable power reflector may include a plurality of layers having different spectral reflectivity. 
     Similarly, the variable power reflector may include one or more polarization selective filters to reflect light from the display while transmitting light from the world forward the wearer.  FIG. 10C , for example, illustrates a variable power reflector  1032  that is polarization selective and that uses the polarization of light to determine what light is reflected and what light is transmitted. In this example, the variable power reflector is configured to reflect one polarization state and transmits another polarization state. In particular, the variable power reflector is configured to reflect one linear polarization state (vertical) and transmits another linear polarization state (horizontal). As shown in  FIG. 10C , the light source  530  in the display emits light  1170   a  with both horizontal and vertical polarization. The light  1170   b  output from the waveguide by the output coupling elements, however, is shown as vertically polarized. The variable power reflector  1032 , in this example, is be configured to reflect vertically polarized light. As shown, the vertically polarized light  1170   b  is reflected by the reflector  1032 . The light  1170   c  from the display  2000  entering the eye  210  may thus be vertically polarized. As discussed above, the variable power reflector may be wavelength selective in addition to being polarization selective. 
     In contrast, in the example shown in  FIG. 10C , light from the world  510  may comprise both vertically and horizontally polarized light. If the variable power reflector  1032  is configured to transmit horizontally polarized light and/or reflect (and/or absorb) vertically polarized light, horizontally polarized light  1170   d  may successfully pass (e.g., transmit) through the reflector  1032  without corresponding vertically polarized light. As such, the light  1170   e  from the world in front of the wearer entering the eye  210  may be horizontally polarized. 
     In this manner, the variable power reflector may both reflect light from the display  2000  as well as pass visible light from the world in front of the wearer and the head-mounted display. Other techniques, however, may be employed. 
     For example, in some embodiments, the waveguide  2700  can comprise a liquid crystal grating. The liquid crystal grating can comprise a cholesteric liquid crystal (CLC) layer having a plurality of liquid crystal molecules that are arranged to have successive azimuthal twists of the director as a function of position in the film in a layer normal direction, or a layer depth direction, of the liquid crystal layer. As described herein, the liquid crystal molecules that are arranged to have the successive azimuthal twists are collectively referred to as a chiral structure. As described herein, an angle (ϕ) of azimuthal twist or rotation is described as the angle between the directors the liquid crystal molecules, relative to a direction parallel to the layer normal. The spatially varying director of the liquid crystal molecules of a chiral structure can be described as forming a helical pattern in which the helical pitch (p) is defined as the distance (e.g., in the layer normal direction of the liquid crystal layer) over which the director has rotated by 360°, as described above. The liquid crystal grating can be configured as a diffraction grating as discussed below. 
     Without being bound to any theory, under a Bragg-reflection condition, the wavelength of the incident light (λ) may be proportional to the mean or average refractive index (n) of a CLC layer and to the helical pitch (p), and can be expressed as satisfying the following condition under some circumstances: 
       λ≈ np   [1]
 
     Thus, by controlling the pitch p of the liquid crystal molecules, different ones of the switchable reflective elements can be tuned to reflect different wavelengths. In addition, the bandwidth (Δλ) of Bragg-reflecting wavelengths may be proportional to the birefringence Δn (e.g., the difference in refractive index between different polarizations of light) of CLC layer and to the helical pitch (p), and can be expressed as satisfying the following condition under some circumstances: 
       Δλ=Δ n·p   [2]
 
       FIG. 10D  illustrates an implementation of a display device  3000  configured to output image information to a user using a waveguide assembly  2904  and a deformable mirror  3004 . The display device  3000  comprises a waveguide assembly  2904 , where the waveguide assembly  2904  is interposed between a curved or deformable mirror  3004  (so that it has optical power) and an optional clean-polarizer  3008 . The waveguide assembly  2904  comprises a cholesteric liquid crystal diffraction grating (CLCG)  2905 . The CLCG  2905  can be disposed between the waveguide assembly  2904  and the deformable mirror  3004  in some implementations. The waveguide assembly can comprise one or more waveguides configured to propagate light via total internal reflection. The eyepiece  2904  may be configured to asymmetrically project light, in this example, toward the left (world-ward) rather than to the right (eye-ward). The eyepiece  2904  may comprise DOEs, metamaterials, holograms, etc. that can preferentially project light in the desired, asymmetric direction and/or desired polarization state (e.g., linear or circular). Although, not illustrated, the display device  3000  can comprise an image injection device similar to the image injection device  4000  discussed above with reference to  FIGS. 10A-10C . 
     In operation, some of the light propagating in the x-direction within one or more waveguides within the waveguide assembly  2904  may be redirected, or out-coupled, in the z-direction as a light beam  3012  having a uniform circular polarization (e.g., RHCP). The waveguide assembly  2904  projects the light beam  3012  of a virtual image toward the curved or deformable mirror  3004  (in the opposite to side of the user&#39;s eye  4 ). In some embodiments, the deformable mirror  3004  is coated with a polarizing reflection layer (e.g., multi-layer linear polarization reflectors or broadband cholesteric liquid crystal circular polarization reflectors) to reflect light having a designated polarization, e.g., light having the same polarization as the out-coupling polarization of the CLCGs, and to allow light from the real world  1114  transmitted toward the eye  4 . In some other embodiments, instead of a polarizing reflection layer, the deformable mirror  3004  is coated with a notch reflection layer or CLC reflection layer, which is designed to reflect light within a narrow bandwidth Δλ that matches the virtual image bandwidth of the out-coupled light from the waveguide assembly  2904 . In some embodiments, a clean-up polarizer  3008  can be placed as shown in  FIG. 10D  to eliminate any ghost images without going through the deformable mirror. 
     Although in the various examples shown in  FIGS. 10A-10D , a single waveguide  2700  or a waveguide assembly  2904  is shown, a stack of waveguides may be used for outputting image information to user (e.g., as shown in  FIG. 6 ). In certain designs, different waveguide in the stack may correspond to a particular color (e.g., range of wavelengths) and/or depth plane of the display. Similarly, the image injection device  4000 , the waveguide  2700 , the waveguide assembly  2904  and other elements of the assembly may represent multiple image injection devices, waveguides, etc., respectively. For example, the image injection device  4000  can be a discrete display that produces image information for injection into a corresponding waveguide  2700 . In some cases, the image injection device  4000  may comprise an output end of a single multiplexed display which may, e.g., pipe image information via one or more optical conduits (e.g., fiber optic cable) to the image injection device  4000 . The image information provided by the image injection device  4000  may include light of different wavelengths, or colors (e.g., different component colors and/or subcolors). Additional information on a stacked waveguide assembly such as one used for waveguide  2700  or waveguide assembly  2904  may be found above in connection with  FIG. 6 . 
     As discussed above, the outcoupling elements  5700  discussed above with reference, e.g., to  FIGS. 10A and 10B , can be asymmetrical such that the amount of light injected in to the waveguide  2700  from the image injection device  4000  that is directed by the outcoupling elements  5700  towards the forward side of the waveguides and toward the reflector  1032  is greater than the amount of light injected in to the waveguide  2700  from the image injection device  4000  that is directed by the outcoupling elements  5700  towards the rearward side of the waveguide  2700  and the eye  210  of the user. For example, greater than 50% of the light injected into the waveguide  2700  can be directed by the outcoupling elements  5700  towards the reflector  1032 . In various embodiments, the amount of injected light that is directed towards the reflector  1032  by the outcoupling elements can be greater than 60%, greater than 70%, greater than 80%, greater than 90%, greater than 95% and/or any other percentage between about 50% and about 99.9%. Embodiments of the outcoupling elements  5700  that can be configured to asymmetrically diffract light such that more light is diffracted in one direction than in another direction can include liquid crystal polarization gratings, volume phase holograms and/or metasurfaces. As discussed above, light injected into the waveguide  2700  that is directed towards the reflector  1032  and to the user&#39;s eye  210  can have the benefit of being focus modulated by the reflector  1032  (the wavefront of light can be changed for example such that the light appears to come from an object at another depth). Thus, it can beneficial in some cases if most, e.g., all or substantially all, light injected into the waveguide  2700  is directed towards the reflector  1032 . 
     However, in some embodiments of the display device, most of the light injected into the waveguide  2700  from the image injection device  4000  may not be directed towards the reflector  1032  by the outcoupling elements  5700 . In some such embodiments, some of the injected light may be directed towards the user&#39;s eye  210 . In some such embodiments, a light recycling system such as a polarization based light recycling system comprising an arrangement of retarders (e.g., retarder layers) can be used to recycle the light that is directed towards the user&#39;s eye  210  by the outcoupling elements  5700  such that it is directed towards the reflector  1032  to be focus modulated before being incident on the user&#39;s eye  210 . 
       FIG. 11  illustrates an embodiment of a display device including a light recycling system, namely a polarization based light recycling system, comprising an arrangement of retarders that are configured to redirect injected light from the waveguide that is outcoupled towards the user&#39;s eye  210  towards the reflector  1032 . In various embodiments, the arrangement of retarders includes a quarter-wave retarder or quarter-wave plate  1104 , which converts linearly polarized light into circular or elliptically polarized light and vice-versa, disposed between the waveguide  2700  and the reflector  1032  and a half-wave retarder or half-wave plate  1108 , which is configured to shift (e.g., rotate) the direction of polarization of linearly polarized light, disposed between the waveguide  2700  and the user&#39;s eye  210 . In various embodiments, the quarter-wave plate  1104  and/or the half-wave plate  1108  can be rigid structures. In some embodiments, the quarter-wave plate  1104  and/or the half-wave plate  1108  can be flexible structures (e.g., films). In some cases, one or more elements or layers having a desired net retardance, such as a net retardance of a quarter of a wavelength (λ/4) or a net retardance of a half of a wavelength (λ/2), can be used. The display device illustrated in  FIG. 11  further includes a polarization selective reflector  1112  disposed between the half-wave plate  1108  and the user&#39;s eye  210 . In various embodiments, the polarization selective reflector  1112  can comprise a wavelength selective component (e.g., a notch filter) or reflective liquid crystal device. In various embodiments, the polarization selective reflector  1112  can comprise a metasurface. Embodiments of the polarization selective reflector  1112  comprising a wavelength selective component, a reflective liquid crystal component and/or a metasurface can be configured to reflect light having certain characteristics (e.g., a narrow band of wavelengths and/or specific polarization states) and transmit light not having those certain characteristics. 
     Consider a portion of light injected into the waveguide  2700  that is outcoupled by the outcoupling optical elements  5700  towards the reflector  1032  as beam  1172   a . The beam  1172   a  is incident on the quarter-wave (λ/4) plate that converts the polarization state of the beam  1172   a  from linear to circular/elliptical polarization or vice-versa. Thus, the beam  1174   a  that is output from the quarter-wave plate  1104  has circular/elliptical polarization if the beam  1172   a  is linearly polarized (or vice-versa). The beam  1174   a  is reflected by the reflector  1032  and undergoes focus modulation as discussed above with reference to  FIGS. 10A and 10B . The focus modulated reflected beam  1174   a  passes through the quarter-wave plate  1104  and undergoes a polarization shift from circular/elliptical to linear (or vice-versa). The polarization state of the beam  1176   a  that is output from the quarter-wave plate  1104  is orthogonal to the polarization state of the beam  1172   a . As the beam  1176   a  traverses the half-wave (λ/2) plate  1108 , the direction of polarization is shifted (or rotated). Thus, the beam  1178   a  that exits the half-wave plate  1108  has a polarization state that is orthogonal to the polarization state of the beam  1176   a . The polarization state of the beam  1178   a  is identical to the polarization state of beam  1172   a . The polarization selective reflector  1112  can be configured to transmit light having the polarization state of beam  1178   a  (or the polarization state of the beam  1172   a ) and can reflect light having a polarization state that is orthogonal to the polarization state of beam  1178   a  (or  1172   a ). 
     Consider a portion of light injected into the waveguide  2700  that is outcoupled by the outcoupling optical elements  5700  towards the user&#39;s eye  210  as beam  1172   b . The beam  1172   b  is incident on the half-wave plate  1108  that shifts (or rotates) the direction of polarization of the beam  1172   b . For example, if beam  1172   b  is linearly polarized along the vertical direction, then beam  1176   b  that exits from the half-wave plate is linearly polarized along the horizontal direction. As discussed above, the polarization selective reflector  1112  is configured to transmit light having the polarization state of beam  1172   b  and reflect light that is orthogonal to the polarization state of beam  1172   b . Accordingly, beam  1176   b  having a polarization state that is orthogonal to the polarization state of  1172   b  is reflected by the polarization selective reflector  1112  towards the reflector  1032 . The direction of the polarization state of the beam  1176   b  is shifted (e.g., rotated) as it traverses the half-wave plate  1108 . Thus, the beam  1178   b  has the same polarization state as the beam  1172   b . The polarization state of the beam  1178   b  is converted from linear polarization to circular/elliptical polarization as it traverses through the quarter-wave plate  1104 . Thus, beam  1174   b  that exits from the quarter-wave plate  1104  is circularly/elliptically polarized and is reflected by the reflector  1032 . The beam reflected from the reflector  1032  is focus modulated and undergoes a shift in polarization from circularly/elliptically polarized to linear polarization as it traverses through the quarter-wave plate  1104 . The beam  1180   b  that exits the quarter-wave plate  1104  has a polarization that is orthogonal to the beam  1178   b . For example, if the beam  1178   b  (or the beam  1172   b ) is linearly polarized along the vertical direction, then the beam  1180   b  is linearly polarized along the horizontal direction. The direction of the polarization state of the beam  1180   b  is shifted (or rotated) as it traverses through the half-wave plate  1108 . For example, if the beam  1180   b  is linearly polarized along the horizontal direction, then the beam  1182   b  is linearly polarized along the vertical direction. The beam  1182   b  has the same polarization state as the polarization state of the beam  1172   b . The polarization selective reflector  1112  is configured to transmit the focus modulated beam  1182   b  towards the user&#39;s eye  210 . 
     As discussed above, the reflector  1032  can comprise coatings that are configured to reflect light in a narrow band of (visible) wavelengths while transmitting light outside the narrow band of (visible) wavelengths. For example, the reflector  1032  can be configured to reflect light in a narrow range of wavelengths of red, green and/or blue. As another example, the reflector  1032  can be configured to reflect light in a wavelength range of about 5-10 nm around a red center wavelength, for example, 630 nm. As yet another example, the reflector  1032  can be configured to reflect light in a wavelength range of about 5-10 nm around a green center wavelength, such as for example, 550 nm. As another example, the reflector  1032  can be configured to reflect light in a wavelength range of about 5-10 nm around a blue center wavelength such as for example 470 nm. In various embodiments, the reflector  1032  can be configured to reflect light in a wavelength range of about 5-20 nm around a central wavelength in the visible spectral range. In various designs, these narrow wavelength bands may overlap and/or coincide with the wavelengths emitted by the light sources of the display  2000 . The display  2000 , for example, may include a plurality of color light sources and the narrow band regions of reflectivity of the variable power reflector may coincide or overlap with the color of light emitted by the light sources. For instance, for the example cited above, the light sources in the display  2000  may emit red light that is centered about 630 nm or within 5-10 nm thereof, green light that is centered about 550 nm or within 5-10 nm thereof, and blue light that is centered about 470 nm or within 5-10 nm thereof. Different designs may have light sources and reflector filters having different spectral characteristics, however, in various cases, the wavelength emitted by the light source and the wavelengths reflected by the variable power reflector overlap or coincide while many other visible wavelengths are not reflected and transmit through the variable power reflector. If many other visible wavelengths are transmitted, much of the light from objects forward the wearer and the head mounted display will be visible to the viewer through the head mounted display. 
     Also as discussed above, the variable power reflector can comprise a polarization selective reflector to selectively reflect light from the display  2000  while transmitting light from objects forward the wearer and the head mounted display. In some designs, for example, the reflector  1032  can be configured to comprise a polarization selective reflector that is configured to reflect light of a specific polarization. For example, the reflector  1032  can be configured to reflect light having particular polarization state while being transmissive to other polarization states, in some cases, many, most, all, or substantially all other polarization states. 
     In certain designs, the variable power reflector can comprise one or more switchable optical elements such as a stack or a plurality of switchable reflective elements  1232  as depicted in  FIG. 12 . Such switchable elements may be configured to switch between two states such as two levels of reflection, for example, a low level of reflection (0% or more) or a higher level of reflection. Similarly, the switchable elements may be configured to switch between two levels of transmission, for example, a low level of transmission (0% or more) or a higher level of transmission. Such switchable optical elements may comprise for example switchable reflectors, switchable diffractive optical elements such as switchable diffractive reflectors or switchable reflective gratings or holographic optical elements. The switchable optical elements may be configured to switch between two power levels such as a low power lever (0 Diopters or higher magnitude) and a higher power level. These power levels may be positive or negative and may switch between two positive powers, two negative power or a positive power and a negative power or a zero power and a non-zero power. Such switchable optical elements may comprise, for example, switchable diffractive optical elements such as switchable diffractive optical lenses. 
     In various cases, one or more switchable optical elements comprise one or more liquid crystal elements comprising liquid crystal such as a switchable polymer dispersed liquid crystal element. The switchable optical elements may comprise a switchable polarization grating. In certain configurations, the variable power reflector comprises a stack of switchable optical elements wherein different of said switchable optical elements are associated with different optical powers such that said switchable optical element can switch between different optical powers. These optical powers may be provided, for example, by lenses such as diffractive lenses. 
     In the example illustrated in  FIG. 12 , different of the plurality of switchable reflective elements have a negative optical power (although one or more the optical powers could be positive or zero in different designs). For example, the plurality or stack of switchable reflective elements can have a first switchable reflective element having a first negative optical power, a second switchable reflective element having a second negative optical power, a third switchable reflective element having a third negative optical power, . . . , a n th  switchable reflective element having a n th  negative optical power. In certain configurations, one of the first, second, third, . . . , or the n th  switchable reflective element can be configured to reflect light outcoupled from the waveguide  2700  while the remaining switchable reflective elements can be configured to be transmissive. In this manner, light outcoupled from the waveguide  2700  that is directed away from the user&#39;s eye  210  is reflected from the selected one of the first, second, third, . . . , or the n th  switchable reflective element such that it appears to originate from a certain depth plane to the user. In various embodiments, each of the plurality of switchable reflective elements can be configured to reflect specific wavelengths of light such that the specific wavelengths of light appear to originate from a particular depth. 
     As described above with respect to  FIG. 12 , some display devices comprise a plurality or a stack of switchable reflective elements. Advantageously, each of the switchable reflective elements can comprise liquid crystals, as described elsewhere in the specification and in the following with respect to, e.g.  FIGS. 13A-13C . For example, the switchable reflective elements may be formed of unpolymerized or  tsp}q ivmefpi$ y$  gv}wxe sv$viegx i$qiwskirw$,VQ - {lswi$gsrj y rw$ger$fi$epxivih$f}$er$  i|xivre y g $ 
     In some embodiments, switchable reflective elements can comprise a liquid crystal, such as, for example, a cholesteric liquid crystal (CLC) layer as discussed above.  FIGS. 13A and 13B  illustrate an example switchable reflective element  3100 A that can be implemented as part of a display device, where switchable reflective element  3100 A is formed of a patterned CLC layer serving as a reflective polarizing mirror.  FIG. 13A  illustrates local orientations of liquid crystal directors (arrows) on top of a binary Fresnel lens pattern. Accordingly, the switchable reflective element  3100 A can be configured to have optical power that is adjustable using an applied electric field. Embodiments of the switchable reflective elements  3100 A can be used as an alternative to a deformable mirror illustrated with respect to, e.g.,  FIGS. 10A-10C and 11 .$ 
     Referring to  FIG. 13B , when the switchable reflective element  3100 A is illuminated with circularly polarized incident light  3012  having a circular polarization that corresponds to (e.g., having the same handedness as) the handedness of the CLC chirality (e.g., RHCP), the reflected light  3016  exhibits a lens effect similar to curved mirror reflectors described above. On the other hand, light with the orthogonal polarization (e.g., LHCP) is transmitted without interference. The switchable reflective element  3100 A can be configured to have a bandwidth in a range of less than about 10 nm, less than about 25 nm, less than about 50 nm, less than about 100 nm, or some other range. 
       FIG. 13C  illustrates a switchable reflective element  3100 C comprising a plurality of switchable reflective elements  3100 -R,  3100 -G and  3100 -B. In the illustrated embodiment, the switchable reflective elements  3100 -R,  3100 -G and  3100 -B are in a stacked configuration and are configured to reflect light within a range of wavelengths Δλ within the red, green and blue spectra, respectively. When the switchable reflective element  3100 C is illuminated with circularly polarized incident light  3012  having a circular polarization that corresponds to the handedness of the CLC chirality (e.g., RHCP) and a wavelength within a range of wavelengths Δλ within the red, green and blue spectra, the reflected light  3016  exhibits a lens effects similar to curved mirror lenses. On the other hand, light with the orthogonal polarization (e.g., LHCP) is transmitted without interference. 
       FIG. 14  illustrates an embodiment of a stack or plurality of switchable reflective elements  1232  having a plurality of switchable reflective elements  1404   a ,  1404   b ,  1404   c ,  1408   a ,  1408   b  and  1408   c . The reflective elements  1404   a - 1404   c  are configured to have a first negative optical power and the reflective elements  1408   a - 1408   c  are configured to have a second negative optical power. The reflective elements  1404   a  and  1408   a  can be configured to reflect a first range of wavelengths (e.g., about 5-10 nm around red). The reflective elements  1404   b  and  1408   b  can be configured to reflect a second range of wavelengths (e.g., about 5-10 nm around green). The reflective elements  1404   c  and  1408   c  can be configured to reflect a third range of wavelengths (e.g., about 5-10 nm around blue). Each of the reflective elements  1404   a ,  1404   b ,  1404   c ,  1408   a ,  1408   b  and  1408   c  is sandwiched between a pair of electrodes  1173 . A selected one of the reflective elements  1404   a ,  1404   b ,  1404   c ,  1408   a ,  1408   b  and  1408   c  can be switched on or off by applying an electrical voltage across the pair of electrodes  1173  bounding the selected one of the reflective elements  1404   a ,  1404   b ,  1404   c ,  1408   a ,  1408   b  and  1408   c.    
     In various embodiments, each of the plurality of switchable reflective elements in the stack  1232  can comprise, for example, liquid crystal diffraction gratings, polymer dispersed liquid crystal grating structures and/or polarization grating structures. In various embodiments, each of the plurality of switchable reflective elements can be switched on or off in a frame sequential manner of on a pixel-by-pixel basis. The switchable reflective elements may comprise diffractive optical elements such as diffractive lenses. 
     Additionally, in various embodiments, the stack of reflective elements  1232  can be integrated with a deformable membrane (e.g., the variable power reflector  1032 ) to provide additional control to adjust the divergent optical power of the reflective stack. 
     Referring to  FIG. 12 , image content from an image projection device  1282  (e.g., a spatial light modulator) is injected into the waveguide  2700  via an optical coupling element  2050 . In various embodiments, the optical coupling element  2050  can comprise a polarization beam splitter. Light from the waveguide  2700  is directed towards the reflector  1232  by outcoupling elements associated with the waveguide  2700  (e.g., outcoupling elements  5700 ). In various embodiments, an light recycling system such as a polarization based light recycling system comprising arrangement of retarders as discussed above with  FIG. 11  can be used in the example of the display device illustrated in  FIG. 12  to redirect any light from the waveguide  2700  that is directed towards the user&#39;s eye  210  towards the reflector  1232 . Light outcoupled from the waveguide  2700  and directed to the variable power reflector can be reflected from the variable power reflector by switching one of the reflective elements so that the user&#39;s eye  210  receives light from the display at a particular divergence (possibly including no divergence but rather collimation) and thus perceives the light as originating from a object at a particular depth in front of the wearer such as near the wearer or far away. It is noted that light from the ambient environment (also referred to herein as the world)  510  surrounding the display device is transmitted through the reflector  1232  towards the user&#39;s eye  210 . 
     Although not illustrated, the waveguide  2700  in  FIGS. 11 and 12  can include a stack of waveguides as described above with reference to  FIG. 6 . The combination of the stack of waveguides and the variable reflector  1032  or the stack of switchable reflective elements  1232  can possibly be useful to provide increased ability to provide the desired depth. 
     It will be appreciated that each of the processes, methods, and algorithms described herein and/or depicted in the figures may be embodied in, and fully or partially automated by, code modules executed by one or more physical computing systems, hardware computer processors, application-specific circuitry, and/or electronic hardware configured to execute specific and particular computer instructions. For example, computing systems may include general purpose computers (e.g., servers) programmed with specific computer instructions or special purpose computers, special purpose circuitry, and so forth. A code module may be compiled and linked into an executable program, installed in a dynamic link library, or may be written in an interpreted programming language. In some embodiments, particular operations and methods may be performed by circuitry that is specific to a given function. 
     Further, certain embodiments of the functionality of the present disclosure are sufficiently mathematically, computationally, or technically complex that application-specific hardware or one or more physical computing devices (utilizing appropriate specialized executable instructions) may be necessary to perform the functionality, for example, due to the volume or complexity of the calculations involved or to provide results substantially in real-time. For example, a video may include many frames, with each frame having millions of pixels, and specifically programmed computer hardware is necessary to process the video data to provide a desired image processing task or application in a commercially reasonable amount of time. 
     Code modules or any type of data may be stored on any type of non-transitory computer-readable medium, such as physical computer storage including hard drives, solid state memory, random access memory (RAM), read only memory (ROM), optical disc, volatile or non-volatile storage, combinations of the same and/or the like. In some embodiments, the non-transitory computer-readable medium may be part of one or more of the local processing and data module ( 140 ), the remote processing module ( 150 ), and remote data repository ( 160 ). The methods and modules (or data) may also be transmitted as generated data signals (e.g., as part of a carrier wave or other analog or digital propagated signal) on a variety of computer-readable transmission mediums, including wireless-based and wired/cable-based mediums, and may take a variety of forms (e.g., as part of a single or multiplexed analog signal, or as multiple discrete digital packets or frames). The results of the disclosed processes or process steps may be stored, persistently or otherwise, in any type of non-transitory, tangible computer storage or may be communicated via a computer-readable transmission medium. 
     Any processes, blocks, states, steps, or functionalities in flow diagrams described herein and/or depicted in the attached figures should be understood as potentially representing code modules, segments, or portions of code which include one or more executable instructions for implementing specific functions (e.g., logical or arithmetical) or steps in the process. The various processes, blocks, states, steps, or functionalities may be combined, rearranged, added to, deleted from, modified, or otherwise changed from the illustrative examples provided herein. In some embodiments, additional or different computing systems or code modules may perform some or all of the functionalities described herein. The methods and processes described herein are also not limited to any particular sequence, and the blocks, steps, or states relating thereto may be performed in other sequences that are appropriate, for example, in serial, in parallel, or in some other manner. Tasks or events may be added to or removed from the disclosed example embodiments. Moreover, the separation of various system components in the embodiments described herein is for illustrative purposes and should not be understood as requiring such separation in all embodiments. It should be understood that the described program components, methods, and systems may generally be integrated together in a single computer product or packaged into multiple computer products. 
     In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than restrictive sense. 
     Indeed, it will be appreciated that the systems and methods of the disclosure each have several innovative aspects, no single one of which is solely responsible or required for the desirable attributes disclosed herein. The various features and processes described above may be used independently of one another, or may be combined in various ways. All possible combinations and subcombinations are intended to fall within the scope of this disclosure. 
     Certain features that are described in this specification in the context of separate embodiments also may be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment also may be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination may in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. No single feature or group of features is necessary or indispensable to each and every embodiment. 
     It will be appreciated that conditional language used herein, such as, among others, “can,” “could,” “might,” “may,” “e.g.,” and the like, unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements and/or steps. Thus, such conditional language is not generally intended to imply that features, elements and/or steps are in any way required for one or more embodiments or that one or more embodiments necessarily include logic for deciding, with or without author input or prompting, whether these features, elements and/or steps are included or are to be performed in any particular embodiment. The terms “comprising,” “including,” “having,” and the like are synonymous and are used inclusively, in an open-ended fashion, and do not exclude additional elements, features, acts, operations, and so forth. Also, the term “or” is used in its inclusive sense (and not in its exclusive sense) so that when used, for example, to connect a list of elements, the term “or” means one, some, or all of the elements in the list. In addition, the articles “a,” “an,” and “the” as used in this application and the appended claims are to be construed to mean “one or more” or “at least one” unless specified otherwise. Similarly, while operations may be depicted in the drawings in a particular order, it is to be recognized that such operations need not be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. Further, the drawings may schematically depict one more example processes in the form of a flowchart. However, other operations that are not depicted may be incorporated in the example methods and processes that are schematically illustrated. For example, one or more additional operations may be performed before, after, simultaneously, or between any of the illustrated operations. Additionally, the operations may be rearranged or reordered in other embodiments. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments, and it should be understood that the described program components and systems may generally be integrated together in a single software product or packaged into multiple software products. Additionally, other embodiments are within the scope of the following claims. In some cases, the actions recited in the claims may be performed in a different order and still achieve desirable results. 
     Accordingly, the claims are not intended to be limited to the embodiments shown herein, but are to be accorded the widest scope consistent with this disclosure, the principles and the novel features disclosed herein.