Patent Publication Number: US-10788677-B2

Title: Fresnel assembly for light redirection in eye tracking systems

Description:
BACKGROUND 
     The present disclosure relates generally to light redirection, and specifically relates to a Fresnel assembly for light redirection in eye tracking systems. 
     Eye tracking refers to the process of detecting the direction of a user&#39;s gaze, which may comprise detecting an orientation of an eye in 3-dimentional (3D) space. Eye tracking in the context of headsets used in, e.g., virtual reality and/or augmented reality applications can be an important feature. Conventional systems commonly use, e.g., a number of infrared light sources to illuminate the eye light, and a camera is used to image a reflection of the light sources from the eye. Traditionally, eye tracking systems use beam splitters to redirect infrared light reflected from the eye to the camera. However, beam splitters are often large, cumbersome, and unsuitable for HMDs used in augmented reality (AR), mixed reality (MR), and virtual reality (VR) systems. 
     SUMMARY 
     A Fresnel assembly transmits (e.g., partially or fully) light in a first band (e.g., visible light) and redirects some, or all of light in a second band (e.g., infrared light) to one or more locations. The Fresnel assembly includes a plurality of surfaces that act to redirect light in the second band. The plurality of surfaces may be coated with a dichroic material that is transmissive in the first band and reflective in the second band. In some embodiments, an immersion layer is overmolded onto the Fresnel assembly to form an immersed Fresnel assembly. The immersion layer may be index matched to the Fresnel assembly. Additionally, in some embodiments, one or more surfaces of the immersed Fresnel assembly may be shaped (e.g., concave, convex, asphere, freeform, etc.) to adjust optical power of the immersed Fresnel assembly. The Fresnel assembly may be integrated into a head-mounted display (HMD). 
     In some embodiments, a HMD includes a display element, an optics block, a Fresnel assembly, an illumination source, and a camera assembly. The display element outputs image light in a first band (e.g., visible light) of light through a display surface of the display element. The optics block directs light from the display element to an eyebox (a region in space occupied by an eye of the user). The Fresnel assembly transmits light in the first band and directs light in a second band (e.g., infrared light) different than the first band to a first position. The illumination source (e.g., part of a tracking system) illuminates a target region (eyes and/or portion of the face) with light in the second band. The camera (e.g., part of an eye and/or face tracking system) is located in the first position, and is configured to capture light in the second band corresponding to light reflected from the target region of the user and reflected by the Fresnel assembly. 
     In some embodiments, the Fresnel assembly may be non-wavelength sensitive (e.g. it could be a non-immersed, uncoated surface, or it could have a partially-reflective coating and is immersed). It transmits a portion of the light from the display and directs it to the eyebox. It also directs parts of the display light outside of the eyebox. The camera is positioned such that the light that comes from the eye is reflected off of the Fresnels and is captured by the camera. Additionally, in some embodiments, the HMD includes a controller (e.g., part of the tracking system) that generates tracking information (e.g., gaze location and/or facial expressions). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of a cross section of a portion of a display block of an HMD (not shown), in accordance with an embodiment. 
         FIG. 2  is a diagram of a cross section of a portion of a display block of an HMD (not shown) with a canted Fresnel assembly, in accordance with an embodiment. 
         FIG. 3  is a diagram of a cross section of a portion of a display block of an HMD (not shown) with an immersed Fresnel assembly, in accordance with an embodiment. 
         FIG. 4  is a diagram of a cross section of a portion of an immersed Fresnel assembly  410 , in accordance with an embodiment. 
         FIG. 5  is a diagram of a cross section of a portion of an immersed Fresnel assembly that captures multiple view angles, in accordance with an embodiment. 
         FIG. 6  is a diagram of a cross section of a portion of an immersed Fresnel assembly configured to interact with multiple positions, in accordance with an embodiment. 
         FIG. 7A  is a diagram of an HMD, in one embodiment. 
         FIG. 7B  is a diagram of a cross-section of the HMD, in one embodiment. 
         FIG. 8A  is an example array of sub-pixels on an electronic display element, in accordance with an embodiment 
         FIG. 8B  is an image of an example array of sub-pixels adjusted by an optical block, in accordance with an embodiment. 
     
    
    
     The figures depict embodiments of the present disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles, or benefits touted, of the disclosure described herein. 
     DETAILED DESCRIPTION 
       FIG. 1  is a diagram of a cross section  100  of a portion of a display block of an HMD (not shown), in accordance with an embodiment. As shown in  FIG. 1 , the display block includes a display element  110 , a Fresnel assembly  120 , and a tracking system  130 . In some embodiments, the display block  100  may also include an optics block  140 . 
     The display element  110  displays images to the user. In various embodiments, the display element  110  may comprise a single electronic display panel or multiple electronic display panels (e.g., a display for each eye of a user). Examples of the display element  110  include: a liquid crystal display (LCD), an organic light emitting diode (OLED) display, an active-matrix organic light-emitting diode display (AMOLED), a quantum organic light emitting diode (QOLED) display, a quantum light emitting diode (QLED) display, a transparent organic light emitting diode (TOLED) display, some other display, or some combination thereof. In some embodiments, the display element  110  is a waveguide display. The display element  110  emits content within a first band of light (e.g., in a visible band). 
     The Fresnel assembly  120  transmits light in the first band and reflects light within a second band. For example, the Fresnel assembly  120  may transmit light within a visible band (e.g., 400-700 nanometers (nm)), and may reflect light within an infrared (IR) band (e.g., above 780 nm). The Fresnel assembly  120  comprises a first surface that includes plurality of reflective surfaces that form a Fresnel lens, a portion  145  of the plurality of reflective surfaces are illustrated in  FIG. 1 . In some embodiments, the plurality of reflective surfaces are coated with a dichroic film that is transmissive in the first band and reflective in the second band. In other embodiments, the plurality of the reflective surfaces are coated with a partial reflective coating. As discussed in detail below with regard to, e.g.,  FIGS. 4-6  the plurality of reflective surfaces can be configured to reflect light in the second band to different positions, and the positions. The Fresnel assembly  120  can partially transmit light, and partially reflect light, for any light band of interest. For example, without coating the Fresnel assembly  120  can weakly reflect light by Fresnel reflection, and transmit the rest of the light. In contrast, with coatings, the transmission/reflection ratio can be adjusted. Alternatively, the Fresnel assembly  120  can transmit light with one polarization, and reflect light with an orthogonal polarization. This can be useful especially for polarization based viewing optics, where light is transmitted in one single polarization. Additionally, while not shown in  FIG. 1 , in some embodiments, the Fresnel assembly  120  is coupled directly to a display surface of to the display element. 
     The Fresnel assembly  120  is at least partially transmissive to light in the first band. In some embodiments, the Fresnel assembly  120  may increase or decrease the optical power of light in the first band from the display element  110 . Additionally, for light in the first band, the Fresnel assembly  120  may reduce fixed pattern noise (i.e., the screen door effect). As is described in the descriptions of  FIGS. 8A and 8B , the Fresnel assembly  120  blurs light emitted from individual pixels in the display element  110 . This blurring allows for dark spots between individual colors to be masked. 
     The optics block  140  directs light to a target region. The target region includes a portion of the user&#39;s face. In some embodiments, the target region includes one or both eyes (e.g., the eye  155 ) of the user. For ease of illustration, the target region in  FIG. 1  is represented as an eyebox  150 . The eyebox  150  is a region in space that is occupied by an eye  155  of a user of the HMD. In an embodiment, the optics block  140  includes one or more optical elements and/or combinations of different optical elements. For example, an optical element is an aperture, a Fresnel lens, a convex lens, a concave lens, a filter, or any other suitable optical element that affects the image light emitted from the display element  110 . In some embodiments, one or more of the optical elements in the optics block  140  may have one or more coatings, such as anti-reflective coatings. 
     Magnification of the image light by the optics block  140  allows the display element  110  to be physically smaller, weigh less, and consume less power than larger displays. Additionally, magnification may increase a field of view of the displayed content. For example, the field of view of the displayed content is such that the displayed content is presented using almost all (e.g., 110 degrees diagonal), and in some cases all, of the user&#39;s field of view. In some embodiments, the optics block  140  is designed so its effective focal length is larger than the spacing to the display element  110 , which magnifies the image light projected by the display element  110 . Additionally, in some embodiments, the amount of magnification is adjusted by adding or removing optical elements. 
     In some embodiments, the optics block  140  is designed to correct one or more types of optical errors. Examples of optical errors include: two-dimensional optical errors, three-dimensional optical errors, or some combination thereof. Two-dimensional errors are optical aberrations that occur in two dimensions. Example types of two-dimensional errors include: barrel distortion, pincushion distortion, longitudinal chromatic aberration, transverse chromatic aberration, or any other type of two-dimensional optical error. Three-dimensional errors are optical errors that occur in three dimensions. Example types of three-dimensional errors include spherical aberration, comatic aberration, field curvature, astigmatism, or any other type of three-dimensional optical error. In some embodiments, content provided to the display element  110  for display is pre-distorted, and the optics block  140  corrects the distortion when it receives image light from the electronic display element  110  generated based on the content. 
     The tracking system  130  tracks movement of the target region. Some or all of the tracking system  130  may or may not be in a line of sight of a user wearing the HMD. The tracking system  130  is typically located off-axis to avoid obstructing the user&#39;s view of the display element  110 , although the tracking system  130  may alternately be placed elsewhere. Also, in some embodiments, there is at least one tracking system  130  for different portions of the user&#39;s face (e.g., a tracking system for the user&#39;s left eye and a second tracking system for the user&#39;s right eye). In some embodiments, only one tracking system  130  may track multiple target regions. 
     The tracking system  130  may include one or more illumination sources  160 , a camera assembly  165 , and a controller  170 . The tracking system  130  determines tracking information using data (e.g., images) captured by the camera assembly  165  of the target region (e.g., the eye  155 ). Tracking information describes a position of a portion of the user&#39;s face. Tracking information may include, e.g., facial expressions, gaze angle, eye orientation, inter-pupillary distance, vergence depth, some other metric associated with tracking an eye, some other metric associated with tracking a portion of the user&#39;s face, or some combination thereof. Some embodiments of the tracking unit have different components than those described in  FIG. 1 . 
     The illumination source  160  illuminates the target region (e.g., the eyebox  150 ) with light in the second band of light (i.e., source light  175 ) that is different from the first band of light associated with content from the display element  110 . Examples of the illumination source  160  may include: a laser (e.g., a tunable laser, a continuous wave laser, a pulse laser, other suitable laser emitting infrared light), a light emitted diode (LED), a fiber light source, a light guide, another other suitable light source emitting light in the second band, or some combination thereof. In some embodiments, the illumination source can be inside the field of view of the display, and is transparent to the display light. For example, the illumination source can be a transparent light guide with small light extraction features on top. In various embodiments, the illumination source  160  may also be configured to emit light in the first band. In some embodiments, the tracking system  130  may include multiple illumination sources  160  for illuminating one or more portions of the target region. In some embodiments, the light emitted from the one or more illumination sources  160  is a structured light pattern. 
     Reflected light  180  (inclusive of scattered light) from the illuminated portion of the target region (e.g., the eye  155 ) is light in the second band that is reflected by the target region towards the Fresnel assembly  120 . The reflected light  162  is redirected by the Fresnel assembly  120  toward a first position, and this redirected light is referred to as redirected light  185 . Some or all of the redirected light  185  is captured by the camera assembly  165  that is located at the first position. 
     The camera assembly  165  captures images of the target region (e.g., the eye  155  in the eyebox  150 ). The camera assembly  165  includes one or more cameras that are sensitive to light in at least the second band (i.e., light emitted by the illumination source  160 ). In some embodiments, the one or more cameras may also be sensitive in other bands of light (e.g., the first band). In some embodiments, a camera may be based on single-point detection (e.g., photodiode, balanced/matched photodiodes, or avalanche photodiode), or based on one or two-dimensional detector arrays (e.g., linear photodiode array, CCD array, or CMOS array). In some embodiments, the sensor plane of the camera is tilted with regards to the camera&#39;s lens, following the Scheimpflug condition, such that the image can be in focus across the whole sensor plane. In some embodiments, the camera assembly  165  may include multiple cameras to capture light reflected from the target region. The camera assembly  165  includes at least one camera located at the first position. Accordingly, the camera assembly  165  captures images of the target region (e.g., the eye  155 ) using the redirected light  164 . 
     The controller  170  determines tracking information using images from the camera assembly  165 . For example, in some embodiments, the controller  170  identifies locations of reflections of light from the one or more illumination sources  160  in an image of the target region. The controller  170  determines a position and an orientation of the eye  155  and/or portions of the face in the target region based on the shape and/or locations of the identified reflections. In cases where the target region is illuminated with a structured light pattern, the controller  170  can detect distortions of the structured light pattern projected onto the target region, and can estimate a position and an orientation of the eye  155  and/or portions of the face based on the detected distortions. The controller  170  can also estimate a pupillary axis, a gaze angle (e.g., corresponds to a foveal axis), a translation of the eye, a torsion of the eye, and a current shape of the eye  155  based on the image of the illumination pattern captured by the camera  155 . 
     Note that in the illustrated embodiment, the reflected light  180  passes through the optics block  140  prior to incidence on the Fresnel assembly  120 . In alternate embodiments (not shown), the Fresnel assembly  120  is closer to the eyebox  150  than the optics block  140  (i.e., a distance between the optics block  140  and the display element  110  is less than a distance between the Fresnel assembly  120  and the display element  110 ) and light in the second band reflected from the eye  155  does not pass through an optics block  140  before it is directed by the Fresnel assembly  120  to a first position. 
       FIG. 2  is a diagram of a cross section  200  of a portion of a display block of an HMD (not shown) with a canted Fresnel assembly  210 , in accordance with an embodiment. The portion of the display block includes the display element  110 , the canted Fresnel assembly  210 , the optics block  140 , and the tracking system  130 . The canted Fresnel assembly  210  is substantially the same as the Fresnel assembly  120 , except that it is canted with respect to an optical axis  220  and is positioned at a first distance  230  from the display element  110 . 
     The optical axis  220  passes through the display element  110 , the canted Fresnel assembly  210 , and the optics block  140 . In some embodiments, the optical axis  220  bisects one or more of the display element  110 , the canted Fresnel assembly  210 , the optics block  140 , or some combination thereof. The canted Fresnel assembly  210  is positioned at a tilt angle, α, with respect to the optical axis  220 . The tilt angle is determined by system level trade-offs between stray light from the draft facets, how normal the camera&#39;s view of the eye is needed, how compact the Fresnel assembly needs to be, etc. Reasonable tilt angles would be smaller than 30 degrees. 
     In this embodiments, the canted Fresnel assembly  210  is substantially closer to the optics block  140  than the display element  110 . The canted Fresnel assembly  210  is located the first distance  230  from the display element  110 , and a second distance  240  from the optics block  140 . The first distance  230  is substantially larger than the second distance  240 . If the Fresnel assembly is canted, it&#39;s preferred to put it behind the optics block  140 , and with a large distance from the display. If it&#39;s in between the eye and the optics block  140 , it wants to be close to non-canted such that it would not increase the design eye-relief. When it&#39;s put behind the optics block  140 , it is usually better to put Fresnel surfaces far away from the display, such that the eye would not be able to focus on the Fresnel grooves or imperfections of immersion. If the Fresnels/prisms also function as screen-door-reduction films, then it can potentially be put very close to the display. 
       FIG. 3  is a diagram of a cross section  300  of a portion of a display block of an HMD (not shown) with an immersed Fresnel assembly  310 , in accordance with an embodiment. The portion of the display block includes the display element  110 , the immersed Fresnel assembly  310 , and the tracking system  130 . 
     The immersed Fresnel assembly  310  is the Fresnel assembly  120  overmolded with an immersion layer  320 . The immersed Fresnel assembly  310  is coupled to the display element  110 . In some embodiments, the immersed Fresnel assembly  310  is directly coupled to a display surface of the display element  110 . In alternate embodiments, the immersed Fresnel assembly  310  and the display element  110  are separated by one or more intermediate layers (e.g., optical films). The immersed Fresnel assembly  310  can be located in any location between the display  110  and the eye  155 . 
     The immersion layer  320  is index matched to a material that makes up the Fresnel assembly  120 . As the immersion layer  320  is index matched to the Fresnel assembly  120 , light in the first band from the display element  110  is not affected by the Fresnel assembly  120 . For example, due to the substantially matched index, light in the first band is not refracted by the plurality of surfaces (e.g., surface  330 ) at the interface between the immersion layer  320  and the Fresnel assembly  120 . Instead light in the first band interacts with the immersed Fresnel assembly  310  as a block of a material of a single index. The immersion layer can be a glue or UV curable material that is of similar refractive index to the Fresnels. Or, a different process can be, two matching Fresnels can be sandwiched together, with index-matching glue in between. In some embodiments, both match Fresnels are the same material (such as a UV curable material), and no index-match glue is needed in between. 
     The immersed Fresnel assembly  310  includes an outer surface  340  (e.g., substrate). In this embodiment, the outer surface  340  is flat. In alternate embodiments, the outer surface  340  is shaped to provide some adjustment to optical power of light being transmitted by the immersed Fresnel assembly  310 . For example, the output surface may be concave, convex, an asphere, spherical, a freeform surface, flat, or some other shape. In some embodiments, the Fresnel assembly  120  is coupled to on a substrate that is not flat (e.g., curved, or some other shape). 
     In this embodiment, the plurality of surfaces of the Fresnel assembly  120  have a dichroic coating that is transmissive in the first band of light (e.g., visible light), but is reflective in the second band of light (e.g., IR light). In some embodiments, the coatings may be sputtered onto one or more of the surfaces, laminated onto one or more of the surfaces, etc. Accordingly, as discussed below with regard to  FIGS. 4-6 , light in the second band incident on the surfaces are reflected (e.g., toward the camera assembly  155 ). In some embodiments, the coating maybe a partial reflective coating. In other embodiments, the coating may be a polarization beam splitter coating. Moreover, in some embodiments, one or more of junctions between surfaces of the Fresnel assembly  120  may be softened (i.e., smoothed to have a radius of curvature v. a sharp edge formed by two intersecting surfaces). Softening of a junction can facilitate lamination of a coating onto the plurality of surfaces of the Fresnel structure. 
       FIG. 4  is a diagram of a cross section  400  of a portion of an immersed Fresnel assembly  410 , in accordance with an embodiment. The portion of the immersed Fresnel assembly  410  includes a portion of the Fresnel assembly  120  and a portion of the immersion layer  320  that is coupled to the Fresnel assembly  120 . 
     Light  430  in the second band is incident on a surface  420  of the plurality of surfaces of the Fresnel assembly  120 . In this embodiment, the plurality of surfaces have a dichroic coating that is transmissive in the first band and reflective in the second band. Accordingly, the light  430  is reflected toward a first position (e.g., occupied a camera of the camera assembly  165 ). Some or all of the surfaces of the Fresnel assembly  120  may be optimized to reduce stray light. For example, a slope of a draft facet  440  may be shaped such that it aligns with a chief ray angle exitance ray going into the camera&#39;s entrance pupil. Note that light might hit the wrong Fresnel facet in two scenarios, on its way to the Fresnel, and after it gets reflected by the Fresnel. In some embodiments where the view of the eye  155  is close to normal, and rays hit the Fresnel assembly  120  at close to normal and rarely hit the wrong Fresnel facet on the way in, so it&#39;s better to align the chief ray such that it would avoid hitting the wrong Fresnel facet on the way out and into the camera assembly  165 . In other embodiments, some tradeoffs may be made such that stray light can be minimized considering both scenarios. An alternative way to remove stray light is to deactivate the “unused” Fresnel draft facet, such that it wouldn&#39;t be reflective and give stray light. In some embodiments, only the “useful” facet is coated to reflect light, and the draft facet is not coated (this can be done by directional coating techniques) or painted black. Note that prior to arriving at a camera of the camera assembly  165 , in the immersed Fresnel assembly  410 , the light  430  is refracted at the outer surface  340 . Accordingly, by using a high index material (e.g., 2.5) for the immersion layer  320  and the Fresnel assembly  120 , the light  430  can be bent at a greater angle relative to a low index material (e.g., 1.3). And a greater bending of the light  430  may be useful in reducing a form factor of the HMD. 
     Another artifact to avoid is Fresnel visibility to the view&#39;s eye. If the “unused” draft facet reflects a lot of visible light into the viewer&#39;s eye, the Fresnel facets may be visible. In some embodiments, optimum co-designs of the coating and the Fresnel draft angle mitigates this artifact. In alternative embodiments, the draft facet is uncoated and becomes invisible for the see-through transmission path. In other embodiments, the Fresnel pitch is small (smaller than 400 microns) such that it is difficult for the eye to resolve the individual facets. 
     Note that in alternate embodiments, the first position is also occupied by the illumination source  165 . In these embodiments, the illumination source  160  illuminates the Fresnel assembly  120  with light in the second band, and the Fresnel assembly  120  redirects the light toward an eyebox. For further suppression of stray light into the camera, in some embodiments, crossed polarizers or other polarization diversity methods are used to block unwanted illumination light hitting the camera. 
       FIG. 5  is a diagram of a cross section  500  of a portion of an immersed Fresnel assembly  505  that captures multiple view angles, in accordance with an embodiment. The portion of the immersed Fresnel assembly  505  includes a portion of a Fresnel assembly  510  and a portion of the immersion layer  320  that is coupled to the Fresnel assembly  510 . The Fresnel assembly  510  is substantially similar to the Fresnel assembly  120 , except that, e.g., the plurality of surfaces are configured to direct light in a second band from different angles to the camera assembly  165 . In this manner the camera assembly  165  is able to capture using a single camera in a single image frame multiple view angles of a target region (e.g., eye, portion of the face, or both). In some embodiments, the camera assembly  165  may capture three dimensional (3D) information that describes a portion of the eye  155 . 
     In this embodiment, the Fresnel assembly  510  includes a plurality of surfaces. The plurality of surfaces have a dichroic coating that is transmissive in the first band and reflective in the second band. Light  515  in the second band is incident on the outer surface  340  of the immersion layer  320  at a first angle. The light  515  is refracted at the outer surface  340  toward a surface  520  of the plurality of surfaces of the Fresnel assembly  510 . The surface  520  reflects the light toward a surface  525  of the plurality of surfaces, and the surface  525  reflects the light  515  toward the outer surface  340  of the immersion layer  320 . The light  515  refracts at a refraction point  530  and propagates toward a first position that is occupied by a camera (e.g., a camera of the camera assembly  165 ). Note that the light might not follow the same paths in alternative embodiments, but in those cases, light coming from different paths can also allow for multiple views of the eye  155 , or allow for paths where an illuminator can be inserted. 
     Concurrent with the light  515 , light  535  in the second band is incident on the outer surface  340  of the immersion layer  320  at a second angle that is different than the first angle. The light  535  is refracted at the outer surface  340  toward a surface  540  of the plurality of surfaces of the Fresnel assembly  510 . The surface  540  reflects the light  535  back toward a portion of the outer surface  340  at an angle such that total internal reflection occurs at a TIR point  545 . The light  535  is reflected toward a surface  550  of the plurality of surfaces, and the surface  550  reflects the light  535  toward the outer surface  340  of the immersion layer  320 . The light  535  refracts at a refraction point  555  and propagates toward a first position that is occupied by a camera (e.g., a camera of the camera assembly  165 ). 
     Note that the light  515  and the light  535  are representative of different view angles of a target area (e.g., eye and/or portion of the face) being imaged by the camera. And that the camera is able to capture in a single image frame both view angles of the eye. The tracking system  130  may use the captured images for generating tracking information. 
     In alternate embodiments, a first set of surfaces are used for one view, and another set of surface is used for a different view. For example, the surfaces of the first set may be shaped to capture light specifically for a particular view, and the surface of the second set may be shaped to capture light specifically for a different view. In this embodiments, the surfaces of the first set and the surfaces of the second set may be positioned in separate portions of the Fresnel assembly  510 , interlaced across the entire Fresnel assembly  510 , etc. Additionally, in some embodiments, the surfaces may be shaped and/or coated such that different polarizations of light and/or wavelengths of light correspond to different view angles that are captured by the camera. 
       FIG. 6  is a diagram of a cross section  600  of a portion of an immersed Fresnel assembly  605  configured to interact with multiple positions, in accordance with an embodiment. The portion of the immersed Fresnel assembly  605  includes a portion of a Fresnel assembly  610  and a portion of the immersion layer  320  that is coupled to the Fresnel assembly  610 . The Fresnel assembly  610  is substantially similar to the Fresnel assembly  120 , except that, e.g., the plurality of surfaces include a first set of surfaces associated with a first position  615  and a second set of surfaces associated with a second position  620  that is different from the first position  615 . The first position  615  is on a first side of the optical axis  220 , and the second position  615  is on a second side of the optical axis  220 . In alternate embodiments, the first position  615  and the second position  620  are symmetric with respect to the optical axis  220 . 
     The first position  615  is occupied by a first device  625  and the second position  620  is occupied by a second device  630 . The first device  625  and the second device  630  may be a camera (e.g., of the camera assembly  165 ), an illumination source  160 , a display panel, or some combination thereof. The illumination source or the display panel can be LEDs, microLEDs, MicroOLEDs, display panels that show an arbitrary pattern (e.g., such as dots or sinusoidal patterns), some other suitable illuminators, or some combination thereof. However, at least one of the first position  615  or the second position  620  includes a camera. As illustrated in  FIG. 6 , the first device  625  is a camera, and the second device  630  may be a camera or an illumination source  160 . In an alternate embodiment, both the first position  615  and the second position  620  include cameras of the camera assembly  165 . In an alternate embodiment, the first position  615  includes a camera of the camera assembly  165  and the second position  620  includes an illumination source  160 . 
       FIG. 7A  shows a diagram of the HMD, as a near eye display  700 , in one embodiment. In this embodiment, the HMD is a pair of augmented reality glasses. The HMD  700  presents computer-generated media to a user and augments views of a physical, real-world environment with the computer-generated media. Examples of computer-generated media presented by the HMD  700  include one or more images, video, audio, or some combination thereof. In some embodiments, audio is presented via an external device (e.g. speakers and headphones) that receives audio information from the HMD  700 , a console (not shown), or both, and presents audio data based on audio information. In some embodiments, the HMD  700  may be modified to also operate as a virtual reality (VR) HMD, a mixed reality (MR) HMD, or some combination thereof. The HMD  700  includes a frame  710  and a display assembly  720 . In this embodiment, the frame  710  mounts the near eye display  700  to the user&#39;s head. The display  720  provides image light to the user. 
       FIG. 7B  shows a cross-section view  730  of the near eye display  700 . This view includes the frame  710 , the display assembly  720 , a display block  740 , and the eye  155 . The display assembly  720  supplies image light to the eye  155 . The display assembly  720  houses the display block  740 . For purposes of illustration,  FIG. 7B  shows the cross section  730  associated with a single display block  740  and a single eye  155 , but in alternative embodiments not shown, another display block which is separate from the display block  730  shown in  FIG. 7B , provides image light to another eye of the user. 
     The display block  740 , as illustrated below in  FIG. 7B , is configured to combine light from a local area with light from computer generated image to form an augmented scene. The display block  740  is also configured to provide the augmented scene to the eyebox  150  corresponding to a location of a user&#39;s eye  155 . The eyebox  150  is a region of space that would contain a user&#39;s eye while the user is wearing the HMD  700 . The display block  740  may include, e.g., a waveguide display, a focusing assembly, a compensation assembly, or some combination thereof. The display block  740  is an embodiment of the display blocks discussed above with regard to  FIGS. 1-3 . Additionally, the display block  720  includes may include an immersed Fresnel assembly as discussed above with regard to  FIGS. 4-6 . 
     The HMD  700  may include one or more other optical elements between the display block  740  and the eye  155 . The optical elements may act to, e.g., correct aberrations in image light emitted from the display block  740 , magnify image light emitted from the display block  740 , some other optical adjustment of image light emitted from the display block  740 , or some combination thereof. The example for optical elements may include an aperture, a Fresnel lens, a convex lens, a concave lens, a filter, a grating, a waveguide element, or any other suitable optical element that affects image light. The display block  740  may be composed of one or more materials (e.g., plastic, glass, etc.) with one or more refractive indices that effectively minimize the weight and widen a field of view of the HMD  700 . 
       FIG. 8A  is an example array  800  of sub-pixels on the display element  110 . The example array  800  shown in  FIG. 8A  includes red sub-pixels  810 , blue sub-pixels  820 , and green sub-pixels  830 . For example, the array  800  is portion of a PENTILE® display. In other embodiments, the array  800  may be in some other configuration (e.g., RGB). 
     A dark space  840  separates each sub-pixel from one or more adjacent sub-pixels. The dark space  840  is a portion of the array  800  that does not emit light, and may become visible to a user under certain circumstances (e.g., magnification) causing the screen door effect that degrades image quality. As discussed above in conjunction with  FIG. 1 , the Fresnel assembly  120  can serve to reduce fixed pattern noise so the dark space  840  between the sub-pixels is not visible to the user (e.g., by blurring each sub-pixel, creating a blur spot associated with each sub-pixel in the image). The blur spots are large enough so adjacent blur spots mask the dark space  840  between adjacent full pixels. In other words, for any display panel, the largest pixel fill-ratio is 100%, if there is no gap at all between sub-pixels. However, to completely get rid of the screen door artifact on the panel side, the pixel fill-ratio may be much greater (e.g., 300%), such that the sub-pixels of different colors are overlapping. This way, when only green pixels are emitting light, for example, when viewed with perfect viewing optics, there would be no gap between the sub-pixels. This is difficult to do for OLED and/or LCD display panels, but it is doable with a diffractive element such as the prism array redirection structure  125 . 
       FIG. 8B  is an example illustrating adjustment of image data of the array  800  of  FIG. 8A  by the Fresnel assembly  120 . As shown in  FIG. 8B , each of the sub-pixels has an associated blur spot. Specifically, the red sub-pixels  810  have a corresponding red blur spot  860 , the blue sub-pixels  820  have a corresponding blue blur spot  870 , and the green sub-pixels  830  have a corresponding green blur spot  880 . A blur spot is an area filled with an image of a blurred sub-pixel. So long as a blur spot does not overlap with a point of maximum intensity of an adjacent blur spot that is created by a sub-pixel of the same color, the two blur spots are resolvable as two adjacent pixels. In some embodiments, the three sub-pixels all overlap and creates a white pixel. The shape of the blur spot is not necessarily a circle, but is rather an area including the blurred image of a sub-pixel. The redirection structure  125  of  FIG. 1A  can be configured to blur each sub-pixel so the blur spots mask the dark space  940  between adjacent pixels. 
     Additional Configuration Information 
     The foregoing description of the embodiments of the disclosure has been presented for the purpose of illustration; it is not intended to be exhaustive or to limit the disclosure to the precise forms disclosed. Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the above disclosure. 
     Some portions of this description describe the embodiments of the disclosure in terms of algorithms and symbolic representations of operations on information. These algorithmic descriptions and representations are commonly used by those skilled in the data processing arts to convey the substance of their work effectively to others skilled in the art. These operations, while described functionally, computationally, or logically, are understood to be implemented by computer programs or equivalent electrical circuits, microcode, or the like. Furthermore, it has also proven convenient at times, to refer to these arrangements of operations as modules, without loss of generality. The described operations and their associated modules may be embodied in software, firmware, hardware, or any combinations thereof. 
     Any of the steps, operations, or processes described herein may be performed or implemented with one or more hardware or software modules, alone or in combination with other devices. In one embodiment, a software module is implemented with a computer program product comprising a computer-readable medium containing computer program code, which can be executed by a computer processor for performing any or all of the steps, operations, or processes described. 
     Embodiments of the disclosure may also relate to an apparatus for performing the operations herein. This apparatus may be specially constructed for the required purposes, and/or it may comprise a general-purpose computing device selectively activated or reconfigured by a computer program stored in the computer. Such a computer program may be stored in a non-transitory, tangible computer readable storage medium, or any type of media suitable for storing electronic instructions, which may be coupled to a computer system bus. Furthermore, any computing systems referred to in the specification may include a single processor or may be architectures employing multiple processor designs for increased computing capability. 
     Embodiments of the disclosure may also relate to a product that is produced by a computing process described herein. Such a product may comprise information resulting from a computing process, where the information is stored on a non-transitory, tangible computer readable storage medium and may include any embodiment of a computer program product or other data combination described herein. 
     Finally, the language used in the specification has been principally selected for readability and instructional purposes, and it may not have been selected to delineate or circumscribe the inventive subject matter. It is therefore intended that the scope of the disclosure be limited not by this detailed description, but rather by any claims that issue on an application based hereon. Accordingly, the disclosure of the embodiments is intended to be illustrative, but not limiting, of the scope of the disclosure, which is set forth in the following claims.