Patent Publication Number: US-2021173204-A1

Title: Increased depth of field for mixed-reality display

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the benefit of priority to U.S. Provisional Patent Application No. 62/946,291, filed Dec. 10, 2019, entitled “INCREASED DEPTH OF FIELD FOR MIXED-REALITY DISPLAY,” the entire content of which is incorporated herein by reference for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     Modern computing and display technologies have facilitated the development of systems for so called “virtual reality” or “augmented reality” experiences, wherein digitally reproduced images or portions thereof are presented to a user in a manner wherein they seem to be, or may be perceived as, real. A virtual reality, or “VR,” scenario typically involves presentation of digital or virtual image information without transparency to other actual real-world visual input; an augmented reality, or “AR,” scenario typically involves presentation of digital or virtual image information as an augmentation to visualization of the actual world around the user. 
     Despite the progress made in these display technologies, there is a need in the art for improved methods, systems, and devices related to augmented reality systems, particularly, display systems. 
     SUMMARY OF THE INVENTION 
     The present disclosure relates generally to techniques for improving the performance and user experience of optical systems. More particularly, embodiments of the present disclosure provide systems and methods for operating a fixed focal plane optical system comprising a microdisplay and a leaky-grating light guide pupil-expanding eyepiece element with a scheme to disrupt human visual system accommodation cues by dynamically extending the depth of field of that system in a compact form factor. Although the present invention is described in reference to an optical system such as an augmented reality (AR) device, the disclosure is applicable to a variety of applications in computer vision and image display systems. 
     A summary of the invention is provided below in reference to a series of examples. As used below, any reference to a series of examples is to be understood as a reference to each of those examples disjunctively (e.g., “Examples 1-4” is to be understood as “Examples 1, 2, 3, or 4”). 
     Example 1 is a method of operating an optical system, the method comprising: defining, based on a vergence-accommodation conflict (VAC) limit, a delimited zone as a function of distance from the optical system, the delimited zone having at least one distance threshold; determining a virtual distance of a virtual depth plane from the optical system at which a virtual object is to be displayed; determining whether the virtual distance is outside the delimited zone by comparing the virtual distance to the at least one distance threshold; generating, by a projector of the optical system, a collimated pixel beam associated with the virtual object; based on determining that the virtual distance is outside the delimited zone, modifying the collimated pixel beam to generate a modified pixel beam, wherein modifying the collimated pixel beam includes at least one of: converging the collimated pixel beam; or reducing a diameter of the collimated pixel beam; injecting the modified pixel beam into an eyepiece of the optical system; and outputting the modified pixel beam from the eyepiece toward an eye of a user. 
     Example 2 is an optical system comprising: a projector configured to generate a collimated pixel beam associated with a virtual object; a light modifying device configured to modify the collimated pixel beam to generate a modified pixel beam; an eyepiece configured to output the modified pixel beam; and a processing module configured to perform operations comprising: determining a virtual distance of a virtual depth plane from the optical system at which the virtual object is to be displayed; comparing the virtual distance to at least one distance threshold; and based on comparing the virtual distance to the at least one distance threshold, causing the light modifying device to modify the collimated pixel beam to generate the modified pixel beam. 
     Example 3 is the optical system of example(s) 2, wherein modifying the collimated pixel beam includes: converging the collimated pixel beam. 
     Example 4 is the optical system of example(s) 2-3, wherein modifying the collimated pixel beam includes: reducing a diameter of the collimated pixel beam. 
     Example 5 is the optical system of example(s) 2-4, wherein the operations further comprise: defining a delimited zone as a function of distance from the optical system, the delimited zone including the at least one distance threshold. 
     Example 6 is the optical system of example(s) 5, wherein comparing the virtual distance to the at least one distance threshold includes: determining whether the virtual distance is outside the delimited zone. 
     Example 7 is the optical system of example(s) 5-6, wherein the delimited zone is defined based on a VAC limit. 
     Example 8 is the optical system of example(s) 7, wherein the VAC limit is defined by a user of the optical system. 
     Example 9 is the optical system of example(s) 2-8, wherein the at least one distance threshold includes an upper distance threshold. 
     Example 10 is the optical system of example(s) 9, wherein comparing the virtual distance to the at least one distance threshold includes: determining whether the virtual distance is greater than the upper distance threshold. 
     Example 11 is the optical system of example(s) 10, wherein modifying the collimated pixel beam based on comparing the virtual distance to the at least one distance threshold includes: in response to determining that the virtual distance is greater than the upper distance threshold, causing the light modifying device to modify the collimated pixel beam. 
     Example 12 is the optical system of example(s) 2-11, wherein the at least one distance threshold includes a lower distance threshold. 
     Example 13 is the optical system of example(s) 12, wherein comparing the virtual distance to the at least one distance threshold includes: determining whether the virtual distance is less than the lower distance threshold. 
     Example 14 is the optical system of example(s) 13, wherein modifying the collimated pixel beam based on comparing the virtual distance to the at least one distance threshold includes: in response to determining that the virtual distance is less than the lower distance threshold, causing the light modifying device to modify the collimated pixel beam. 
     Example 15 is the optical system of example(s) 2-14, wherein the eyepiece is configured to receive the modified pixel beam from the light modifying device. 
     Example 16 is the optical system of example(s) 2-15, wherein the light modifying device is positioned in an optical path between the projector and the eyepiece. 
     Example 17 is a method of operating an optical system, the method comprising: determining a virtual distance of a virtual depth plane from the optical system at which a virtual object is to be displayed; comparing the virtual distance to at least one distance threshold; generating, by a projector of the optical system, a collimated pixel beam associated with the virtual object; and based on comparing the virtual distance to the at least one distance threshold, modifying the collimated pixel beam to generate a modified pixel beam. 
     Example 18 is the method of example(s) 17, wherein modifying the collimated pixel beam includes: converging the collimated pixel beam. 
     Example 19 is the method of example(s) 17-18, wherein modifying the collimated pixel beam includes: reducing a diameter of the collimated pixel beam. 
     Example 20 is the method of example(s) 17-19, further comprising: defining a delimited zone as a function of distance from the optical system, the delimited zone including the at least one distance threshold. 
     Example 21 is the method of example(s) 20, wherein comparing the virtual distance to the at least one distance threshold includes: determining whether the virtual distance is outside the delimited zone. 
     Example 22 is the method of example(s) 20-21, wherein the delimited zone is defined based on a VAC limit. 
     Example 23 is the method of example(s) 22, wherein the VAC limit is defined by a user of the optical system. 
     Example 24 is the method of example(s) 17-23, wherein the at least one distance threshold includes an upper distance threshold. 
     Example 25 is the method of example(s) 24, wherein comparing the virtual distance to the at least one distance threshold includes: determining whether the virtual distance is greater than the upper distance threshold. 
     Example 26 is the method of example(s) 25, wherein modifying the collimated pixel beam based on comparing the virtual distance to the at least one distance threshold includes: in response to determining that the virtual distance is greater than the upper distance threshold, modifying the collimated pixel beam. 
     Example 27 is the method of example(s) 17-26, wherein the at least one distance threshold includes a lower distance threshold. 
     Example 28 is the method of example(s) 27, wherein comparing the virtual distance to the at least one distance threshold includes: determining whether the virtual distance is less than the lower distance threshold. 
     Example 29 is the method of example(s) 28, wherein modifying the collimated pixel beam based on comparing the virtual distance to the at least one distance threshold includes: in response to determining that the virtual distance is less than the lower distance threshold, modifying the collimated pixel beam. 
     Example 30 is the method of example(s) 17-29, further comprising: injecting the modified pixel beam into an eyepiece of the optical system. 
     Example 31 is the method of example(s) 17-30, further comprising: outputting the modified pixel beam from an eyepiece of the optical system toward an eye of a user. 
     Example 32 is the method of example(s) 17-31, wherein the collimated pixel beam is modified by a light modifying device positioned in an optical path between the projector and an eyepiece of the optical system. 
     Numerous benefits are achieved by way of the present disclosure over conventional techniques. For example, embodiments enable a single focal plane system to have several of the same benefits as a two-focal plane system, such as reduced VAC in both the near-field and far-field virtual depth planes. Additionally, since the pixel beam can be modified prior to injection into the eyepiece, embodiments are compatible with existing eyepieces that employ pupil-expansion combiner eyepiece technology. Embodiments also eliminate the need for clipping planes that are often employed for near field depth planes, thereby reducing the inconvenience to users due to virtual content disappearing. Other benefits of the present disclosure will be readily apparent to those skilled in the art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the disclosure, are incorporated in and constitute a part of this specification, illustrate embodiments of the disclosure and together with the detailed description serve to explain the principles of the disclosure. No attempt is made to show structural details of the disclosure in more detail than may be necessary for a fundamental understanding of the disclosure and various ways in which it may be practiced. 
         FIG. 1  illustrates an augmented reality (AR) scene as viewed through a wearable AR device. 
         FIG. 2A  illustrates an AR device having a single fixed focal plane. 
         FIG. 2B  illustrates an AR device having two fixed focal planes. 
         FIG. 3  illustrates the relationship between vergence-accommodation conflict (VAC) and the distance of the virtual depth plane. 
         FIG. 4  illustrates a schematic view of an example wearable AR device. 
         FIG. 5  illustrates an example function of a viewing optics assembly of an AR device and the resulting user visual percept of the system&#39;s output. 
         FIG. 6  illustrates an example function of a viewing optics assembly of an AR device and the resulting user visual percept of the system&#39;s output. 
         FIG. 7  illustrates an example function of a viewing optics assembly of an AR device and the resulting user visual percept of the system&#39;s output. 
         FIG. 8  illustrates an example function of a viewing optics assembly of an AR device and the resulting user visual percept of the system&#39;s output. 
         FIG. 9  illustrates an example function of a viewing optics assembly of an AR device and the resulting user visual percept of the system&#39;s output. 
         FIGS. 10A-10C  illustrate an example light modifying device for reducing the diameter of the collimated pixel beam. 
         FIG. 11  illustrates an example control scheme for a light modifying device and the corresponding user visual percept of the system&#39;s output. 
         FIG. 12  illustrates an example method for defining a VAC delimited zone. 
         FIG. 13  illustrates various examples of VAC delimited zones. 
         FIG. 14  illustrates an example method of operating an optical system. 
         FIG. 15  illustrates a simplified computer system. 
     
    
    
     In the appended figures, similar components and/or features may have the same numerical reference label. Further, various components of the same type may be distinguished by following the reference label with a letter or by following the reference label with a dash followed by a second numerical reference label that distinguishes among the similar components and/or features. If only the first numerical reference label is used in the specification, the description is applicable to any one of the similar components and/or features having the same first numerical reference label, irrespective of the suffix. 
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
     Mixed-reality (MR) and augmented reality (AR) wearable displays are capable of presenting virtual content to a user over a wide depth range. For many displays, a user may experience varying levels of accommodation-vergence conflict (VAC) at different depths, which occurs when the user&#39;s brain receives mismatching cues between the distance of a virtual object from the user&#39;s eyes and the focusing distance required for the eyes to focus on that virtual object. VAC leads to visual fatigue, headache, nausea, and eyestrain, and remains a significant source of discomfort for users. Accordingly, to maintain user comfort, modern MR and AR wearable displays may consider a VAC budget allowance when delivering virtual content over a depth range, which may result in a depth range that is significantly reduced. 
     Various approaches to mitigate VAC have been implemented. One approach includes adding a second depth plane and a vari-focal switch based on eye-tracking to the optical system. Another approach is to add a vari-focal element with the ability to sweep eyepiece focal planes across a broad range. These approaches come with increased volume in the form of additional eyepiece layers and/or through integration of liquid-Tillable tunable lens pairs straddling the eyepiece, as well as increased complexity due to complex illumination schemes. 
     Some embodiments of the present invention provide an optical system with a delimited zone, within which a limited amount of VAC is tolerated by a user, and outside of which an expanded depth of field can be switched on to disrupt human visual system accommodation cues. In some embodiments, the delimited zone can be defined based on a single or multiple fixed focal plane(s) or a single or multiple variable focus plane(s). Virtual content having an associated virtual depth plane that lies within the delimited zone may be projected to the user in a normal manner, whereas virtual content outside the delimited zone is modified by a light modifying device so as to reduce the reliability of the accommodation cues. 
     In some instances, the light modifying device may cause the collimated light generated by a projector to become converging when entering the eyepiece. This causes the virtual image light (i.e., light associated with a virtual image) that is outcoupled from the leaky-grating of the eyepiece to also be converging. However, the chief ray of each beamlet does not change direction, resulting in a virtual image with vergence cues but very weak accommodation cues. Such a virtual image can disrupt the vergence-accommodation response in areas of the depth of field where VAC would exceed the threshold tolerance. Thus, embodiments disclosed herein can extend the depth of field of the optical system, since the user&#39;s eye may not be able to focus on pixels at the virtual depth plane. Additionally or alternatively, the light modifying device may reduce the diameter of each collimated pixel beam generated by the projector. This can cause the light that is outcoupled from the leaky-grating of the eyepiece to likewise have pixel beams with reduced diameters, thereby disrupting the accommodation cues associated with the outcoupled light. 
     In some instances, optical see-through (OST) AR devices can improve virtual content being presented to a user by applying optical power to the virtual image light using one or more lens assemblies arranged within an optical stack. Embodiments of the present invention are compatible with existing systems that utilize lens assemblies to vary the virtual depth plane of the virtual object. 
       FIG. 1  illustrates an AR scene  100  as viewed through a wearable AR device, according to some embodiments. AR scene  100  is depicted wherein a user of an AR technology sees a real-world park-like setting  106  featuring various real-world objects  130  such as people, trees, buildings in the background, and a real-world concrete platform  120 . In addition to these items, the user of the AR technology also perceives that they “see” various virtual objects  102  such as a robot statue  102 - 2  standing upon the real-world concrete platform  120 , and a cartoon-like avatar character  102 - 1  flying by, which seems to be a personification of a bumble bee, even though these elements (character  102 - 1  and statue  102 - 2 ) do not exist in the real world. Due to the extreme complexity of the human visual perception and nervous system, it is challenging to produce a virtual reality (VR) or AR technology that facilitates a comfortable, natural-feeling, rich presentation of virtual image elements amongst other virtual or real-world imagery elements. 
       FIG. 2A  illustrates an AR device  200 A having a single fixed focal plane, according to some embodiments. During operation, a projector  214  of AR device  200 A may project virtual image light  223  (i.e., light associated with virtual content) onto an eyepiece  202 - 1 , which may cause a light field (i.e., an angular representation of virtual content) to be projected onto a retina of a user in a manner such that the user perceives the corresponding virtual content as being positioned at some location within the user&#39;s environment. For example, virtual image light  223  outcoupled by eyepiece  202 - 1  may cause the user to perceive character  102 - 1  as being positioned at a first virtual depth plane  210 - 1  and statue  102 - 2  as being positioned at a second virtual depth plane  210 - 2 . The user perceives the virtual content along with world light  232  corresponding to one or more world objects  230 , such as platform  120 . 
     In some embodiments, AR device  200 A includes a first lens assembly  205 - 1  positioned on the user side of eyepiece  202 - 1  (the side of eyepiece  202 - 1  closest to the eye of the user) and a second lens assembly  205 - 2  positioned on the world side of eyepiece  202 - 1 . Each of lens assemblies  205 - 1 ,  205 - 2  may be configured to apply optical power to the light passing therethrough. 
       FIG. 2B  illustrates an AR device  200 B having two fixed focal planes, according to some embodiments. During operation, projector  214  may project virtual image light  223  onto first eyepiece  202 - 1  and a second eyepiece  202 - 2 , which may cause a light field to be projected onto a retina of a user in a manner such that the user perceives the corresponding virtual content as being positioned at some location within an environment of the user. For example, virtual image light  223  outcoupled by first eyepiece  202 - 1  may cause the user to perceive character  102 - 1  as being positioned at a first virtual depth plane  210 - 1  and virtual image light  223  outcoupled by second eyepiece  202 - 2  may cause the user to perceive statue  102 - 2  as being positioned at a second virtual depth plane  210 - 2 . 
       FIG. 3  illustrates the relationship between VAC and the distance of the virtual depth plane for each of AR devices  200 A,  200 B described in reference to  FIGS. 2A and 2B , respectively. For AR device  200 B, the two-focal plane system provides switchable focal planes at 1.95 diopters (0.51 meters) and 0.65 diopters (1.54 meters), with a switch point at 1.3 diopters (0.77 meters), a near content limit (clipping plane) at 2.7 diopters (0.37 meters), and an ability to provide imagery never exceeding 1.0 diopter VAC between that plane and infinity. For AR device  200 A, the single fixed focal plane system has a focal plane location at 1.5 diopters (0.6 meters) and a near content limit of 2.5 diopters (0.4 meters) and a far content limit of 0.31 diopters (3.2 meters), assuming a maximum allowable VAC of 1.0 diopter. Such a configuration would have a usable range of 0.4-3.2 meters with content falling outside of that range requiring some solution to mitigate exceeding the VAC limit. 
       FIG. 4  illustrates a schematic view of an example wearable AR device  400 , according to some embodiments of the present invention. AR device  400  may include a left eyepiece  402 A and a left lens assembly  405 A arranged in a side-by-side configuration and a right eyepiece  402 B and a right lens assembly  405 B also arranged in a side-by-side configuration. In some embodiments, AR device  400  includes one or more sensors including, but not limited to: a left front-facing world camera  406 A attached directly to or near left eyepiece  402 A, a right front-facing world camera  406 B attached directly to or near right eyepiece  402 B, a left side-facing world camera  406 C attached directly to or near left eyepiece  402 A, and a right side-facing world camera  406 D attached directly to or near right eyepiece  402 B. In some embodiments, AR device  400  includes one or more image projection devices such as a left projector  414 A optically linked to left eyepiece  402 A and a right projector  414 B optically linked to right eyepiece  402 B. 
     Some or all of the components of AR device  400  may be head mounted such that projected images may be viewed by a user. In one particular implementation, all of the components of AR device  400  shown in  FIG. 4  are mounted onto a single device (e.g., a single headset) wearable by a user. In another implementation, one or more components of a processing module  450  are physically separate from and communicatively coupled to the other components of AR device  400  by one or more wired and/or wireless connections. For example, processing module  450  may include a local module  452  on the head mounted portion of AR device  400  and a remote module  456  physically separate from and communicatively linked to local module  452 . Remote module  456  may be mounted in a variety of configurations, such as fixedly attached to a frame, fixedly attached to a helmet or hat worn by a user, embedded in headphones, or otherwise removably attached to a user (e.g., in a backpack-style configuration, in a belt-coupling style configuration, etc.). 
     Processing module  450  may include a processor and an associated digital memory, such as non-volatile memory (e.g., flash memory), both of which may be utilized to assist in the processing, caching, and storage of data. The data may include data captured from sensors (which may be, e.g., operatively coupled to AR device  400 ) or otherwise attached to a user, such as cameras  406 , an ambient light sensor, eye trackers, microphones, inertial measurement units, accelerometers, compasses, GPS units, radio devices, and/or gyros. For example, processing module  450  may receive image(s)  420  from cameras  406 . Specifically, processing module  450  may receive left front image(s)  420 A from left front-facing world camera  406 A, right front image(s)  420 B from right front-facing world camera  406 B, left side image(s)  420 C from left side-facing world camera  406 C, and right side image(s)  420 D from right side-facing world camera  406 D. In some embodiments, image(s)  420  may include a single image, a pair of images, a video comprising a stream of images, a video comprising a stream of paired images, and the like. Image(s)  420  may be periodically generated and sent to processing module  450  while AR device  400  is powered on, or may be generated in response to an instruction sent by processing module  450  to one or more of the cameras. As another example, processing module  450  may receive ambient light information from an ambient light sensor. As another example, processing module  450  may receive gaze information from one or more eye trackers. As another example, processing module  450  may receive image information (e.g., image brightness values) from one or both of projectors  414 . 
     Cameras  406 A,  406 B may be positioned to capture images that substantially overlap within the field of view of a user&#39;s left and right eyes, respectively. Accordingly, placement of cameras  406  may be near a user&#39;s eyes but not so near as to obscure the user&#39;s field of view. Alternatively or additionally, cameras  406 A,  406 B may be positioned so as to align with the incoupling locations of virtual image light  422 A,  422 B, respectively. Cameras  406 C,  406 D may be positioned to capture images to the side of a user, e.g., in a user&#39;s peripheral vision or outside the user&#39;s peripheral vision. Image(s)  420 C,  420 D captured using cameras  406 C,  406 D need not necessarily overlap with image(s)  420 A,  420 B captured using cameras  406 A,  406 B. 
     Eyepieces  402 A,  402 B may comprise transparent or semi-transparent waveguides configured to direct and outcouple light generated by projectors  414 A,  414 B, respectively. Specifically, processing module  450  may cause left projector  414 A to output left virtual image light  422 A onto left eyepiece  402 A, and may cause right projector  414 B to output right virtual image light  422 B onto right eyepiece  402 B. In some embodiments, each of eyepieces  402 A,  402 B may comprise a plurality of waveguides corresponding to different colors. In some embodiments, lens assemblies  405 A,  405 B may be coupled to and/or integrated with eyepieces  402 A,  402 B. For example, lens assemblies  405 A,  405 B may be incorporated into a multi-layer eyepiece and may form one or more layers that make up one of eyepieces  402 A,  402 B. 
     In some embodiments, AR device  400  includes one or more light modifying devices  404 A,  404 B for modifying virtual image light  422 A,  422 B. Specifically, a left light modifying device  404 A may be positioned in an optical path between left projector  414 A and left eyepiece  402 A so as to modify left virtual image light  422 A prior to being outputted onto left eyepiece  402 A, and a right light modifying device  404 B may be positioned in an optical path between right projector  414 B and right eyepiece  402 B so as to modify right virtual image light  422 B prior to being outputted onto right eyepiece  402 B. In some embodiments, light modifying devices  404 A,  404 B may be integrated with projectors  414 A,  414 B. In some embodiments, light modifying devices  404 A,  404 B may be integrated with eyepieces  402 A,  402 B. 
     In some embodiments, projectors  414 A,  414 B may include a micro-electromechical system (MEMS) spatial light modulator (SLM) scanning device. In such embodiments, light modifying devices  404 A,  404 B may employ a varifocal mirror or lens that can be used in the laser beams prior to the scanning mirrors. If a relay optical system is used, one of the optical elements within the relay optics could be vari-focal and be switched to provide converging pixel rays to the ICG formed on the eyepieces. If a standard projection system is used with a pixel-based SLM (such as a liquid crystal on silicon (LCOS)), the SLM itself could be translated in the z-axis (perpendicular to the array), such that the projection lens produces a finite external focal plane (and thus convergent pixel rays). In some embodiments, a vari-focal lens could be incorporated between the projection/relay lens of the microdisplay and the ICG of the eyepiece itself, converting the output collimated pixel rays into convergent states. 
       FIG. 5  illustrates an example function of a viewing optics assembly  500  of an AR device and the resulting user visual percept of the system&#39;s output. Viewing optics assembly  500  includes a projector  514  and an eyepiece  502 . Projector  514  generates a collimated pixel beam  516  that is carried onto an eyepiece  502  at an input coupling grating (ICG)  503  formed on eyepiece  502 . After being diffracted by ICG  503 , collimated pixel beam  516  propagates in eyepiece  502  until an output grating formed on eyepiece  502  diffracts the light toward the user. 
     A leaky-grating light-guide, pupil-expanding eyepiece with no programmed optical power produces a virtual image at infinity. The percept is produced by multiple output “beamlets” (emitted replicants of the input pixel wavefronts) collected through the pupil and imaged onto the retina of the user&#39;s eye. In this case, when the user&#39;s eye is focused at infinity, a sharp image of the pixel is formed on the retina. When the eye is focused at another plane (for example at 1.33 meters from the user) a blurry image of the pixel is formed on the retina. 
       FIG. 6  illustrates an example function of a viewing optics assembly  600  of an AR device and the resulting user visual percept of the system&#39;s output. Viewing optics assembly  600  includes a projector  614  that generates a collimated pixel beam  616  that is carried onto an eyepiece  602  at an ICG  603  formed on eyepiece  602 . After being diffracted by ICG  603 , collimated pixel beam  616  propagates in eyepiece  602  until an output grating formed on eyepiece  602  diffracts the light toward the user. 
     Viewing optics assembly  600  includes a −0.75 diopters lens assembly  605  that modulates the wavefronts of the emitted beamlets, diverging them with respect to each other and diverging each ray independently, so as to both focus pixel light and converge beamlets at 1.33 meters from the user&#39;s eye. Lens assembly  605  shifts the chief rays of the emerging beamlets and diverges the collimated output to a single pixel focus position at the focal length of the lens. In this case, when the user&#39;s eye is focused at 1.33 meters, a sharp image of the pixel is formed on the retina. When the eye focuses at infinity, that image is blurred. 
     In the example illustrated in  FIG. 6 , the depth of focus of the image is determined by several factors, including the beamlet packing density (determined by the beam diameter, the eyepiece substrate thickness, along with several other factors), the size of the user&#39;s pupil, the optical quality of the lens assembly  605 , and the inherent depth of field of the user&#39;s eye. Each of these factors may be considered to determine an acceptable VAC budget figure for the system. In some embodiments, 1.0 diopters can be used as the VAC budget figure, although this value can be higher or lower in practice. 
       FIG. 7  illustrates an example function of a viewing optics assembly  700  of an AR device and the resulting user visual percept of the system&#39;s output. Viewing optics assembly  700  includes a projector  714  that generates a collimated pixel beam  716  that is modified by a light modifying device  704  to produce a modified pixel beam  752  having a converging wavefront. Modified pixel beam  752  is carried onto an eyepiece  702  at an ICG  703  formed on eyepiece  702 . After being diffracted by ICG  703 , modified pixel beam  752  propagates in eyepiece  702  until an output grating formed on eyepiece  702  diffracts the light toward the user. 
     In the example illustrated in  FIG. 7 , modifying the wavefronts of the imaged pixels introduces optical power to the projection system, transforming an infinity-focused system into a system that produces a finite image position in front of the projector. In such a configuration, a single pixel produces a converging (curved) wavefront at the pupil plane of the projector. When a converging pixel ray enters the eyepiece, the exiting beamlets maintain this convergence, however, the chief ray of each beamlet does not change direction. In this case, when the user&#39;s eye is focused either at 1.33 meters or at infinity, a blurred image of the pixel is formed on the retina. Additionally, the perceived pixel when the user&#39;s eye is focused at 1.33 meters may be different from the perceived pixel when the user&#39;s eye is focused at infinity, as depicted by different types of blur in  FIG. 7 . 
       FIG. 8  illustrates an example function of a viewing optics assembly  800  of an AR device and the resulting user visual percept of the system&#39;s output. Viewing optics assembly  800  includes a projector  814  that generates a collimated pixel beam  816  that is modified by a light modifying device  804  to produce a modified pixel beam  852  having a converging wavefront. Modified pixel beam  852  is carried onto an eyepiece  802  at an ICG  803  formed on eyepiece  802 . After being diffracted by ICG  803 , modified pixel beam  852  propagates in eyepiece  802  until an output grating formed on eyepiece  802  diffracts the light toward the user. Viewing optics assembly  800  further includes a −0.75 diopters lens assembly  805  positioned between eyepiece  802  and the user&#39;s eye that modulates the wavefronts of the emitted beamlets. 
     In the example illustrated in  FIG. 8 , lens assembly  805  collimates each beamlet output while simultaneously re-directing the chief ray of each beamlet to pivot around a point at the focal plane of the lens. As a result, when the user&#39;s eye is focused at 1.33 meters, a blurred image of the pixel is formed on the retina. When the user&#39;s eye is focused at infinity, a percept comprising a repeated structure of blurred images is produced. The user&#39;s eye is unable to bring the blurred image into focus, thereby disrupting the user&#39;s physiological vergence-accommodation cues and reducing the uncomfortable effects of vergence-accommodation conflict. This percept having a repeated structure allows virtual content to exist on planes outside of the VAC threshold. As a result, the depth of field of the optical system can extend beyond the VAC threshold without discomfort, since the user&#39;s eye will not be able to focus on pixels at the virtual depth plane. 
       FIG. 9  illustrates an example function of a viewing optics assembly  900  of an AR device and the resulting user visual percept of the system&#39;s output. Viewing optics assembly  900  includes a projector  914  that generates a collimated pixel beam  916  that is modified by a light modifying device  904  such as a spatial light modulator (SLM), relay optics, polarizers, beam splitters, lenses or a combination thereof, to produce a modified pixel beam  952  having a reduced diameter. Modified pixel beam  952  is carried onto an eyepiece  902  at an ICG  903  formed on eyepiece  902 . After being diffracted by ICG  903 , modified pixel beam  952  propagates in eyepiece  902  until an output grating formed on eyepiece  902  diffracts the light toward the user. Viewing optics assembly  900  further includes lens assemblies  905  including a −1 diopter component positioned between eyepiece  902  and the user&#39;s eye and a +1 diopter component positioned on the world side of eyepiece  902 . 
     In the example illustrated in  FIG. 9 , vergence-accommodation cues are disrupted and the depth of field of the system is extended by modulating the diameter of the laser beam, rather than through divergence/convergence of the image light. This may be performed by light modifying device  904  prior to injecting the light into ICG  903 . In this case, the percept is driven by the inability of the lens assembly between eyepiece  902  and the user&#39;s eye to provide a small focal spot due to the reduced size of the pixel beam. 
       FIGS. 10A-10C  illustrate an example light modifying device for reducing the diameter of the collimated pixel beam, according to some embodiments of the present invention. By varying the position of a second lens  1004  relative to a first lens  1002  and a third lens  1006 , the diameter of the input collimated pixel beam can be expanded, reduced, or left unmodified. In reference to  FIG. 10A , second lens  1004  is adjusted to be positioned closer to first lens  1002  than to third lens  1006  (e.g., adjacent to first lens  1002 ), causing the diameter of the collimated pixel beam to become expanded upon exiting the light modifying device. In reference to  FIG. 10B , second lens  1004  is adjusted to be positioned at a midpoint between first lens  1002  and third lens  1006 , causing the diameter of the collimated pixel beam to be left unmodified upon exiting the light modifying device. In reference to  FIG. 10C , second lens  1004  is adjusted to be positioned closer to third lens  1006  than to first lens  1002  (e.g., adjacent to third lens  1006 ), causing the diameter of the collimated pixel beam to become reduced upon exiting the light modifying device. 
     In some embodiments, the light modifying device illustrated in  FIGS. 10A-10C  is used to dynamically change the diameter of a MEMS laser beam. In some instances, the light modifying device may be positioned prior to the MEMS mirror(s) so as to modify the laser beam prior to entering the MEMS mirror(s). 
       FIG. 11  illustrates an example control scheme for a light modifying device and the corresponding user visual percept of the system&#39;s output, according to some embodiments of the present invention. In some embodiments, a VAC delimited zone  1102  is defined based on a desired VAC limit, such as 1 diopter. VAC delimited zone  1102  may include a lower distance threshold  1104 , below which the VAC experienced by a user exceeds the VAC limit, and an upper distance threshold  1106 , above which the VAC experienced by a user exceeds the VAC limit. 
     Under the control scheme, when it is determined that the distance of the virtual depth plane (from the AR device or user) is less than lower distance threshold  1104 , the light modifying device is caused to modify the wavefront of the collimated pixel beam. When it is determined that the distance of the virtual depth plane is greater than lower distance threshold  1104  and less than upper distance threshold  1106  (i.e., is within VAC delimited zone  1102 ), the light modifying device is caused to not modify the collimated pixel beam and to output the collimated pixel beam without modification. When it is determined that the distance of the virtual depth plane is greater than upper distance threshold  1106 , the light modifying device is caused to modify the wavefront of the collimated pixel beam. 
     The control scheme may optionally implement gradual modifications to the collimated pixel beam at or near the distance thresholds. For example, the light modifying device may impart partial modifications to the collimated pixel beam for virtual distances just before a distance threshold, greater modifications at the distance threshold, and full modifications well past the distance threshold. As one example, for an upper distance threshold of 3.2 meters, a control scheme may be implemented in which the collimated pixel beam is converged at 0% for a virtual distance of 2.8 meters, 25% for a virtual distance of 3.0 meters, 50% for a virtual distance of 3.2 meters, 75% for a virtual distance of 3.4 meters, and 100% for a virtual distance of 3.6 meters. In the same or a different example, for a lower distance threshold of 0.4 meters, a control scheme may be implemented in which the collimated pixel beam is converged at 0% for a virtual distance of 0.6 meters, 25% for a virtual distance of 0.5 meters, 50% for a virtual distance of 0.4 meters, 75% for a virtual distance of 0.3 meters, and 100% for a virtual distance of 0.2 meters. Control schemes with longer or shorter transition bands than the above examples may be implemented. One of ordinary skill in the art will see various variations, alternatives, and modifications. 
       FIG. 12  illustrates an example method for defining a VAC delimited zone  1202 , according to some embodiments of the present invention. First, the VAC experienced by a user is plotted as a function of the distance of the virtual depth plane from the AR device (alternatively referred to as the “VAC plot”). In some embodiments, the VAC plot is determined based on the focal plane design of the AR device. For the VAC plot illustrated in  FIG. 12 , a 0.75 meters focal plane is utilized. Next, the VAC limit is plotted alongside the VAC experienced by the user. Next, intersection points  1204 ,  1206  between the two plots are identified and the corresponding distances are used as lower and upper distance thresholds of VAC zone  1202 , respectively. 
       FIG. 13  illustrates various examples of VAC delimited zones that may be defined based on VAC plots for various single focal plane systems. As the focal plane of the AR device increases, both the lower distance threshold and the upper distance threshold of the VAC delimited zone increase, presenting a trade-off between near-field versus far-field performance. Additional depth planes can be added to the system to increase the VAC delimited zone. 
       FIG. 14  illustrates an example method  1400  of operating an optical system (e.g., AR device  400 ), according to some embodiments of the present invention. One or more steps of method  1400  may be performed in a different order than the illustrated embodiment, and one or more steps of method  1400  may be omitted during performance of method  1400 . Furthermore, two or more steps of method  1400  may be performed simultaneously or concurrently with each other. 
     At step  1402 , a VAC delimited zone (e.g., VAC delimited zones  1102 ,  1202 ) is defined. In some embodiments, the VAC delimited zone is defined based on the number of focal planes of the optical device and/or their corresponding focal plane locations. For example, the VAC associated with a single focal plane system with a focal plane location at 1.5 diopters can be estimated and used to determine the VAC delimited zone, which may be significantly smaller than the VAC delimited zone determined using the VAC associated with a multiple focal plane system, such as, for example, a two-focal plane system with focal plane locations at 1.95 diopters and 0.65 diopters. In some embodiments, the VAC delimited zone is additionally (or alternatively) defined based on a VAC limit, which may be specified by a user or may be predetermined for the system. In some embodiments, the VAC delimited zone is defined by finding the intersection point(s) (e.g., intersection points  1204 ,  1206 ) between the VAC associated with the optical system and the VAC limit, as described at least in reference to  FIGS. 3, 12, and 13 . 
     In some embodiments, the VAC delimited zone is defined as a function of distance from the optical system, where distances inside the VAC delimited zone correspond to virtual depth planes at which virtual content causes a user to experience VAC less than the VAC limit, and distances outside the VAC delimited zone correspond to virtual depth planes at which virtual content causes a user to experience VAC greater than the VAC limit. In some embodiments, the VAC delimited zone includes at least one distance threshold. For example, the VAC delimited zone may include a lower distance threshold (e.g., lower distance threshold  1104 ) and/or an upper distance threshold (e.g., upper distance threshold  1106 ), the lower distance threshold being less than the upper distance threshold. 
     At step  1404 , a virtual distance of a virtual depth plane (e.g., virtual depth planes  210 ) from the optical system at which a virtual object (e.g., virtual objects  102 ) is to be displayed is determined. The virtual distance may be expressed in meters, diopters, or some other unit that indicates physical displacement. In some embodiments, the virtual distance is determined by a processing module (e.g., processing module  450 ). In some embodiments, the virtual distance is determined prior to, during, or after the collimated pixel beam associated with the virtual object is generated by the optical system. 
     At step  1406 , the virtual distance is compared to the lower distance threshold and/or the upper distance threshold. In some embodiments, it is determined whether the virtual distance is less than the lower distance threshold, greater than the lower distance threshold and less than the upper distance threshold, or greater than the upper distance threshold. For example, in some embodiments, step  1406  may include determining whether the virtual distance is less than the lower distance threshold. As another example, in some embodiments, step  1406  may include determining whether the virtual distance is greater than the upper distance threshold. As another example, in some embodiments, step  1406  may include determining whether the virtual distance is less than the lower distance threshold or greater than the upper distance threshold. In some embodiments, step  1406  is equivalent to determining whether the virtual distance is outside the VAC delimited zone. 
     At step  1408 , a collimated pixel beam (e.g., collimated pixel beams  516 ,  616 ,  716 ,  816 ,  916 ) associated with the virtual object is generated by the optical system. In some embodiments, the collimated pixel beam is generated by a projector (e.g., projectors  214 ,  414 ,  514 ,  614 ,  714 ,  814 ,  914 ) of the optical system. The collimated pixel beam may contain color, brightness, and size information for displaying the virtual object. For example, the collimated pixel beam may include light from a single LED color source (e.g., red) or from multiple LED color sources (e.g., red, green, and blue). 
     At step  1410 , the collimated pixel beam is modified to generate a modified pixel beam (e.g., modified pixel beams  752 ,  852 ,  952 ). In some embodiments, the collimated pixel beam is modified by a light modifying device (e.g., light modifying devices  404 ,  704 ,  804 ,  904 ) of the optical system. In some embodiments, whether or not step  1410  is performed may depend on the comparison performed in step  1406 . For example, in some embodiments, step  1410  is performed only when it is determined that the virtual distance is outside the VAC delimited zone. For example, step  1410  may only be performed in response to determining that the virtual distance is less than the lower distance threshold or in response to determining that the virtual distance is greater than the upper distance threshold. In some embodiments, the light modifying device is integrated with the projector. In some embodiments, the light modifying device is separate from the projector. 
     In some embodiments, step  1410  includes step  1412  and/or step  1414 . At step  1412 , the collimated pixel beam is converged. In some embodiments, the collimated pixel beam is converged by the light modifying device. At step  1414 , a diameter of the collimated pixel beam is reduced. In some embodiments, the diameter of the collimated pixel beam is reduced by the light modifying device. 
     At step  1416 , the modified pixel beam is injected into an eyepiece (e.g., eyepieces  202 ,  402 ,  502 ,  602 ,  702 ,  802 ,  902 ) of the optical system. In some embodiments, the modified pixel beam is injected into an ICG (e.g., ICGs  503 ,  603 ,  703 ,  803 ,  903 ) formed on the eyepiece. 
     At step  1418 , the modified pixel beam is outputted from the eyepiece of the optical system. In some embodiments, the modified pixel beam is outputted from a leaky-grating formed on the eyepiece. In some embodiments, the modified pixel beam is outputted from the eyepiece toward a user&#39;s eye. 
       FIG. 15  illustrates a simplified computer system  1500  according to an embodiment described herein. Computer system  1500  as illustrated in  FIG. 15  may be incorporated into devices described herein.  FIG. 15  provides a schematic illustration of one embodiment of computer system  1500  that can perform some or all of the steps of the methods provided by various embodiments. It should be noted that  FIG. 15  is meant only to provide a generalized illustration of various components, any or all of which may be utilized as appropriate.  FIG. 15 , therefore, broadly illustrates how individual system elements may be implemented in a relatively separated or relatively more integrated manner. 
     Computer system  1500  is shown comprising hardware elements that can be electrically coupled via a bus  1505 , or may otherwise be in communication, as appropriate. The hardware elements may include one or more processors  1510 , including without limitation one or more general-purpose processors and/or one or more special-purpose processors such as digital signal processing chips, graphics acceleration processors, and/or the like; one or more input devices  1515 , which can include without limitation a mouse, a keyboard, a camera, and/or the like; and one or more output devices  1520 , which can include without limitation a display device, a printer, and/or the like. 
     Computer system  1500  may further include and/or be in communication with one or more non-transitory storage devices  1525 , which can comprise, without limitation, local and/or network accessible storage, and/or can include, without limitation, a disk drive, a drive array, an optical storage device, a solid-state storage device, such as a random access memory (“RAM”), and/or a read-only memory (“ROM”), which can be programmable, flash-updateable, and/or the like. Such storage devices may be configured to implement any appropriate data stores, including without limitation, various file systems, database structures, and/or the like. 
     Computer system  1500  might also include a communications subsystem  1519 , which can include without limitation a modem, a network card (wireless or wired), an infrared communication device, a wireless communication device, and/or a chipset such as a Bluetooth™ device, an 802.11 device, a WiFi device, a WiMax device, cellular communication facilities, etc., and/or the like. The communications subsystem  1519  may include one or more input and/or output communication interfaces to permit data to be exchanged with a network such as the network described below to name one example, other computer systems, television, and/or any other devices described herein. Depending on the desired functionality and/or other implementation concerns, a portable electronic device or similar device may communicate image and/or other information via the communications subsystem  1519 . In other embodiments, a portable electronic device, e.g. the first electronic device, may be incorporated into computer system  1500 , e.g., an electronic device as an input device  1515 . In some embodiments, computer system  1500  will further comprise a working memory  1535 , which can include a RAM or ROM device, as described above. 
     Computer system  1500  also can include software elements, shown as being currently located within the working memory  1535 , including an operating system  1540 , device drivers, executable libraries, and/or other code, such as one or more application programs  1545 , which may comprise computer programs provided by various embodiments, and/or may be designed to implement methods, and/or configure systems, provided by other embodiments, as described herein. Merely by way of example, one or more procedures described with respect to the methods discussed above, might be implemented as code and/or instructions executable by a computer and/or a processor within a computer; in an aspect, then, such code and/or instructions can be used to configure and/or adapt a general purpose computer or other device to perform one or more operations in accordance with the described methods. 
     A set of these instructions and/or code may be stored on a non-transitory computer-readable storage medium, such as the storage device(s)  1525  described above. In some cases, the storage medium might be incorporated within a computer system, such as computer system  1500 . In other embodiments, the storage medium might be separate from a computer system e.g., a removable medium, such as a compact disc, and/or provided in an installation package, such that the storage medium can be used to program, configure, and/or adapt a general purpose computer with the instructions/code stored thereon. These instructions might take the form of executable code, which is executable by computer system  1500  and/or might take the form of source and/or installable code, which, upon compilation and/or installation on computer system  1500  e.g., using any of a variety of generally available compilers, installation programs, compression/decompression utilities, etc., then takes the form of executable code. 
     It will be apparent to those skilled in the art that substantial variations may be made in accordance with specific requirements. For example, customized hardware might also be used, and/or particular elements might be implemented in hardware, software including portable software, such as applets, etc., or both. Further, connection to other computing devices such as network input/output devices may be employed. 
     As mentioned above, in one aspect, some embodiments may employ a computer system such as computer system  1500  to perform methods in accordance with various embodiments of the technology. According to a set of embodiments, some or all of the procedures of such methods are performed by computer system  1500  in response to processor  1510  executing one or more sequences of one or more instructions, which might be incorporated into the operating system  1540  and/or other code, such as an application program  1545 , contained in the working memory  1535 . Such instructions may be read into the working memory  1535  from another computer-readable medium, such as one or more of the storage device(s)  1525 . Merely by way of example, execution of the sequences of instructions contained in the working memory  1535  might cause the processor(s)  1510  to perform one or more procedures of the methods described herein. Additionally or alternatively, portions of the methods described herein may be executed through specialized hardware. 
     The terms “machine-readable medium” and “computer-readable medium,” as used herein, refer to any medium that participates in providing data that causes a machine to operate in a specific fashion. In an embodiment implemented using computer system  1500 , various computer-readable media might be involved in providing instructions/code to processor(s)  1510  for execution and/or might be used to store and/or carry such instructions/code. In many implementations, a computer-readable medium is a physical and/or tangible storage medium. Such a medium may take the form of a non-volatile media or volatile media. Non-volatile media include, for example, optical and/or magnetic disks, such as the storage device(s)  1525 . Volatile media include, without limitation, dynamic memory, such as the working memory  1535 . 
     Common forms of physical and/or tangible computer-readable media include, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, or any other magnetic medium, a CD-ROM, any other optical medium, punchcards, papertape, any other physical medium with patterns of holes, a RAM, a PROM, EPROM, a FLASH-EPROM, any other memory chip or cartridge, or any other medium from which a computer can read instructions and/or code. 
     Various forms of computer-readable media may be involved in carrying one or more sequences of one or more instructions to the processor(s)  1510  for execution. Merely by way of example, the instructions may initially be carried on a magnetic disk and/or optical disc of a remote computer. A remote computer might load the instructions into its dynamic memory and send the instructions as signals over a transmission medium to be received and/or executed by computer system  1500 . 
     The communications subsystem  1519  and/or components thereof generally will receive signals, and the bus  1505  then might carry the signals and/or the data, instructions, etc. carried by the signals to the working memory  1535 , from which the processor(s)  1510  retrieves and executes the instructions. The instructions received by the working memory  1535  may optionally be stored on a non-transitory storage device  1525  either before or after execution by the processor(s)  1510 . 
     The methods, systems, and devices discussed above are examples. Various configurations may omit, substitute, or add various procedures or components as appropriate. For instance, in alternative configurations, the methods may be performed in an order different from that described, and/or various stages may be added, omitted, and/or combined. Also, features described with respect to certain configurations may be combined in various other configurations. Different aspects and elements of the configurations may be combined in a similar manner. Also, technology evolves and, thus, many of the elements are examples and do not limit the scope of the disclosure or claims. 
     Specific details are given in the description to provide a thorough understanding of exemplary configurations including implementations. However, configurations may be practiced without these specific details. For example, well-known circuits, processes, algorithms, structures, and techniques have been shown without unnecessary detail in order to avoid obscuring the configurations. This description provides example configurations only, and does not limit the scope, applicability, or configurations of the claims. Rather, the preceding description of the configurations will provide those skilled in the art with an enabling description for implementing described techniques. Various changes may be made in the function and arrangement of elements without departing from the spirit or scope of the disclosure. 
     Also, configurations may be described as a process which is depicted as a schematic flowchart or block diagram. Although each may describe the operations as a sequential process, many of the operations can be performed in parallel or concurrently. In addition, the order of the operations may be rearranged. A process may have additional steps not included in the figure. Furthermore, examples of the methods may be implemented by hardware, software, firmware, middleware, microcode, hardware description languages, or any combination thereof. When implemented in software, firmware, middleware, or microcode, the program code or code segments to perform the necessary tasks may be stored in a non-transitory computer-readable medium such as a storage medium. Processors may perform the described tasks. 
     Having described several example configurations, various modifications, alternative constructions, and equivalents may be used without departing from the spirit of the disclosure. For example, the above elements may be components of a larger system, wherein other rules may take precedence over or otherwise modify the application of the technology. Also, a number of steps may be undertaken before, during, or after the above elements are considered. Accordingly, the above description does not bind the scope of the claims. 
     As used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise. Thus, for example, reference to “a user” includes a plurality of such users, and reference to “the processor” includes reference to one or more processors and equivalents thereof known to those skilled in the art, and so forth. 
     Also, the words “comprise”, “comprising”, “contains”, “containing”, “include”, “including”, and “includes”, when used in this specification and in the following claims, are intended to specify the presence of stated features, integers, components, or steps, but they do not preclude the presence or addition of one or more other features, integers, components, steps, acts, or groups. 
     It is also understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims.