PATENT DOCUMENT

Publication Number: US-11885959-B1
Application Number: US-201916550108-A
Country: US
Kind Code: B1

Title: Optical system with ghost image mitigation

Abstract:
An electronic device may include a display system with a pixel array and a catadioptric lens. The display system may include a linear polarizer through which image light from the pixel array passes and a first quarter wave plate through which the light passes after passing through the polarizer. The lens may include a partial mirror, a second quarter wave plate, and a reflective polarizer. A third quarter wave plate may be formed between the linear polarizer and the pixel array to mitigate ghost images. Control circuitry may predict potential ghost images based on the geometry of the lens and data from an image frame. Tone mapping circuitry may adjust contrast of the image frame within a region overlapping the predicted ghost image. The control circuitry may adjust luminance of the image frame outside of the region overlapping the predicted ghost image.

Claims:
What is claimed is: 
     
       1. An electronic device comprising:
 an array of pixels configured to produce light; 
 a linear polarizer configured to receive the light; 
 a first quarter wave plate configured to receive the light from the linear polarizer; 
 a first lens configured to transmit the light and having a first surface and an opposing second surface; 
 a second lens configured to transmit the light and having a third surface and an opposing fourth surface; 
 a partially reflective mirror between the first surface and the second lens; 
 a reflective polarizer, the second lens being disposed between the reflective polarizer and the first lens; and 
 a second quarter wave plate between the fourth surface of the second lens and the first lens. 
 
     
     
       2. The electronic device defined in  claim 1 , further comprising:
 a third quarter wave plate located between the array of pixels and the linear polarizer. 
 
     
     
       3. The electronic device defined in  claim 2 , wherein the linear polarizer has a pass axis and the third quarter wave plate has a fast axis that is aligned at 45 degrees to the pass axis of the linear polarizer, and wherein the first quarter wave plate has a fast axis that is aligned at 45 degrees to the pass axis of the linear polarizer. 
     
     
       4. The electronic device defined in  claim 3 , wherein the second quarter wave plate has a fast axis that is aligned at 90 degrees to the fast axis of the first quarter wave plate. 
     
     
       5. The electronic device defined in  claim 4 , wherein the reflective polarizer has a reflection axis aligned with the pass axis of the linear polarizer. 
     
     
       6. The electronic device defined in  claim 5 , wherein the reflective polarizer has a pass axis orthogonal to the reflection axis. 
     
     
       7. The electronic device defined in  claim 1 , further comprising a third lens between the partially reflective mirror and the second quarter wave plate. 
     
     
       8. The electronic device defined in  claim 1 , further comprising:
 a layer of optically transparent adhesive that attaches a third quarter wave plate to the pixel array. 
 
     
     
       9. The electronic device defined in  claim 1 , further comprising:
 control circuitry coupled to the display, wherein the control circuitry is configured to:
 receive an image frame; 
 identify a ghost image location based on a predetermined geometry of the first and second lenses and content of the image frame; 
 generate an adjusted image frame by adjusting contrast of a region of the image frame that overlaps the identified ghost image location; and 
 provide the adjusted image frame to the display. 
 
 
     
     
       10. The electronic device defined in  claim 9 , wherein the control circuitry comprises:
 a ghost prediction engine configured to generate information identifying the ghost image location; 
 a local tone mapping grid generation engine configured to identify a region of the image frame overlapping the identified ghost image location; 
 a look-up table configured to select a tone mapping curve from a plurality of tone mapping curves based on the image frame and the information generated by the ghost prediction engine; and 
 a tone mapping engine configured to generate the adjusted image frame by applying the selected tone mapping curve to the identified region of the image frame. 
 
     
     
       11. The electronic device defined in  claim 1 , further comprising:
 control circuitry coupled to the display, wherein the control circuitry is configured to:
 receive an image frame; 
 identify a ghost image location based on a predetermined geometry of the first and second lenses and content of the image frame; 
 generate an adjusted image frame by adjusting luminance of the image frame except within a region that overlaps the identified ghost image location; and 
 provide the adjusted image frame to the display. 
 
 
     
     
       12. The electronic device defined in  claim 1  wherein the second quarter wave plate is formed as a coating on the fourth surface of the second lens and wherein the fourth surface of the second lens is a convex surface. 
     
     
       13. The electronic device defined in  claim 12  wherein the reflective polarizer is formed as a coating on the third surface of the second lens and wherein the third surface of the second lens is a concave surface. 
     
     
       14. The electronic device defined in  claim 1 , wherein the array of pixels has a curved surface. 
     
     
       15. An electronic device comprising:
 an array of pixels configured to produce light; 
 a linear polarizer; 
 a first quarter wave plate, the linear polarizer being optically coupled between the first quarter wave plate and the array of pixels; 
 a second quarter waveplate coupled to the array of pixels; 
 a lens having a first surface and an opposing second surface, the first quarter wave plate being optically coupled between the lens and the linear polarizer; and 
 a partially reflective mirror on the lens. 
 
     
     
       16. The electronic device of  claim 15 , further comprising:
 a reflective polarizer; and 
 a third quarter wave plate, wherein the reflective polarizer is laminated directly onto the third quarter wave plate and wherein the third quarter wave plate is located between the second surface of the lens and the reflective polarizer. 
 
     
     
       17. The electronic device of  claim 15 , wherein the second quarter waveplate is adhered to the array of pixels. 
     
     
       18. The electronic device of  claim 15 , wherein the array of pixels comprises light-emitting diodes. 
     
     
       19. The electronic device of  claim 15 , wherein the light comprises red, green, and blue wavelengths. 
     
     
       20. The electronic device of  claim 15 , further comprising:
 control circuitry coupled to the display, wherein the control circuitry is configured to:
 receive an image frame; 
 identify a ghost image location based on a predetermined geometry of the lens elements and content of the image frame; 
 generate an adjusted image frame by adjusting contrast of a region of the image frame that overlaps the identified ghost image location; and 
 provide the adjusted image frame to the display. 
 
 
     
     
       21. An electronic device comprising:
 an array of pixels configured to produce light; 
 a linear polarizer; 
 a first quarter wave plate that receives the light from the linear polarizer, the linear polarizer being optically coupled between the array of pixels and the first quarter wave plate; 
 a first lens having a first surface and an opposing second surface, the first quarter waveplate being optically coupled between the first lens and the linear polarizer; 
 a second lens having a third surface and an opposing fourth surface, the first lens being optically coupled between the second lens and the first quarter wave plate; 
 a partially reflective mirror optically coupled between the first lens and the second lens; 
 a reflective polarizer, the second lens being optically coupled between the reflective polarizer and the first lens; and 
 a second quarter wave plate optically coupled between the array of pixels and the linear polarizer, wherein the linear polarizer has a pass axis, the second quarter wave plate has a first fast axis that is oriented at first non-zero angle with respect to the pass axis, and the first quarter wave plate has a second fast axis that is oriented at a second non-zero angle with respect to the pass axis.

Description:
This application claims the benefit of provisional patent application No. 62/726,035, filed Aug. 31, 2018, which is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     This relates generally to optical systems and, more particularly, to optical systems for devices with displays. 
     Lenses may sometimes be used to allow a viewer to view a nearby display. For example, electronic devices such as virtual reality glasses use lenses to display images for a user. 
     If care is not taken, lenses and other optical components in these electronic devices may be bulky and heavy and may not exhibit satisfactory optical performance. 
     SUMMARY 
     An electronic device such as a head-mounted device or other electronic device may include a display system and an optical system. The display system and optical system may be supported by support structures that are configured to be worn on the head of a user. The electronic device may use the display system and optical system to present images to the user while the device is being worn by the user. 
     The display system may have a pixel array that produces image light associated with the images. The pixel array may have a curved or planar surface. The display system may also have a linear polarizer through which image light from the pixel array passes and a first quarter wave plate through which the light passes after passing through the linear polarizer. The light may then pass through a catadioptric lens having a partial mirror, a second quarter wave plate, and a reflective polarizer. A third quarter wave plate may be formed between the linear polarizer and the pixel array to mitigate ghost images associated with the partial mirror in the catadioptric lens. 
     The optical system may be coupled to control circuitry that receives an image frame. The control circuitry may identify (predict) potential ghost images associated with the catadioptric lens based on the geometry of the lens and image data in the image frame. The control circuitry may generate a grid having a region overlapping the predicted ghost image. A look-up table may select a tone mapping curve from a set of tone mapping curves based on the grid and the image frame. Tone mapping circuitry may adjust contrast of the image frame using the selected tone mapping curve to produce an adjusted image frame that is displayed by the display. If desired, luminance adjustment circuitry may adjust luminance of the image frame outside of the region overlapping the predicted ghost image. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram of an illustrative head-mounted display showing components of an illustrative optical system in the head-mounted display in accordance with an embodiment. 
         FIG.  2    is a diagram of an illustrative head-mounted device in one example where a curved pixel array is incorporated into the head-mounted device in accordance with an embodiment. 
         FIG.  3    is a diagram of processing circuitry for mitigating ghost images in a head-mounted device in accordance with an embodiment. 
         FIGS.  4 A- 4 D  are diagrams showing how luminance adjustments may be performed on an image frame to mitigate ghost images in accordance with an embodiment. 
         FIG.  5    is a flow chart of illustrative steps involved in mitigating ghost images in accordance an embodiment. 
         FIG.  6    is a graph showing different tone mapping curves that may be applied to an image frame to mitigate ghost images in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Electronic devices such as head-mounted display devices may be used for virtual reality and augmented reality systems (sometimes referred to as mixed reality systems). For example, a pair of virtual reality glasses that is worn on the head of a user may be used to provide a user with virtual reality content. 
     An illustrative system that includes an electronic device such as a head-mounted device is shown in  FIG.  1   . As shown in  FIG.  1   , electronic device  10  (e.g., a head-mounted device with support structures configured to be worn on the head of a user such as glasses, goggles, a helmet, hat, etc.) may include a display system with one or more displays  40  (e.g., a display for each of a user&#39;s eyes such as eye  46 ). A single display  40  is shown in  FIG.  1   . Systems with a pair of displays  40  may present images to a user&#39;s left and right eyes simultaneously. 
     Device  10  may include an optical system  20  and a display system  11  supported by head-mounted support structures such as housing  12 . Housing  12  may have the shape of a frame for a pair of glasses, may have the shape of a helmet, may have the shape of a pair of goggles, or may have any other suitable housing shape that allows housing  12  to be worn on the head of a user. Configurations in which housing  12  supports optical system  20  and display system  11  in front of a user&#39;s eyes (e.g., eye  46 ) as the user is viewing system  20  and display system  11  in direction  48  may sometimes be described herein as an example. If desired, housing  12  may have other suitable configurations. 
     Display system  11  may include a display  40 . Display  40  has an array of pixels P (pixel array  39 ) that present images to a user (see, e.g., user eye  46 , which is viewing display  40  in direction  48  through optical system  20  which may be formed using a catadioptric lens). Pixel array  39  of display  40  may be based on a liquid crystal display, an organic light-emitting diode display, an emissive display having an array of crystalline semiconductor light-emitting diode dies, and/or displays based on other display technologies. In a preferred embodiment, the display is a self-emitting display, which can be more compact since illumination optics are not required. Separate left and right displays may be included in device  10  for the user&#39;s left and right eyes. Each display such as display  40  of  FIG.  1    may be planar or may have a curved shape. 
     Visual content (e.g., image data for still and/or moving images) may be provided to display  40  using control circuitry  42  that is mounted in device  10  and/or control circuitry that is mounted outside of device  10  (e.g., in an associated portable electronic device, laptop computer, or other computing equipment). Control circuitry  42  may include storage such as hard-disk storage, volatile and non-volatile memory, electrically programmable storage for forming a solid-state drive, and other memory. Control circuitry  42  may also include one or more microprocessors, microcontrollers, digital signal processors, graphics processors, baseband processors, application-specific integrated circuits, and other processing circuitry. Communications circuits in circuitry  42  may be used to transmit and receive data (e.g., wirelessly and/or over wired paths). Control circuitry  42  may use display  40  to display visual content such as virtual reality content (e.g., computer-generated content associated with a virtual world), pre-recorded video for a movie or other media, or other images. Illustrative configurations in which control circuitry  42  provides a user with virtual reality content using displays such as display  40  may sometimes be described herein as an example. In general, however, any suitable content may be presented to a user by control circuitry  42  using display  40 . 
     Input-output devices  44  may be coupled to control circuitry  42 . Input-output devices  44  may be used to gather user input from a user, may be used to make measurements on the environment surrounding device  10 , may be used to provide output to a user, and/or may be used to supply output to external electronic equipment. Input-output devices  44  may include buttons, joysticks, keypads, keyboard keys, touch sensors, track pads, displays, touch screen displays, microphones, speakers, light-emitting diodes for providing a user with visual output, sensors (e.g., a force sensors, temperature sensors, magnetic sensors, accelerometers, gyroscopes, and/or other sensors for measuring orientation, position, and/or movement of device  10 , proximity sensors, capacitive touch sensors, strain gauges, gas sensors, pressure sensors, ambient light sensors, and/or other sensors). If desired, input-output devices  44  may include one or more cameras (e.g., cameras for capturing images of the user&#39;s surroundings, cameras for performing gaze detection operations by viewing eyes  46 , and/or other cameras). 
     A polarizer such as linear polarizer  56  may be placed in front of pixel array  39  and/or may be laminated to pixel array  39  to provide polarized image light. Linear polarizer  56  may have a pass axis aligned with the X-axis of  FIG.  1    (as an example). Display system  11  may also include a wave plate such as quarter wave plate  59  to provide circularly polarized image light. The fast axis of quarter wave plate  59  may be aligned at 45 degrees to the pass axis of linear polarizer  56 . Quarter wave plate  59  may be mounted in front of polarizer  56  (between polarizer  56  and optical system  20 ). If desired, quarter wave plate  59  may be attached to polarizer  56  (and display  40 ). Display  40  may emit image light  26  that is circularly polarized after passing through quarter wave plate  59 . 
     Optical system  20  may include lens elements such as lens elements  28  and  22 . Lens element  28  may be a plano-convex lens (lens element) with a convex surface V 2  facing display system  11 . Optional lens element  22  may be a plano-concave lens (lens element) with a concave surface V 1  facing the user (eye  46 ). This example is merely illustrative. Surfaces V 2  and V 1  may be convex, concave, planar, spherical, aspherical, freeform, or have other curved shapes. 
     Optical structures such as partially reflective coatings, wave plates, reflective polarizers, linear polarizers, antireflection coatings, and/or other optical components may be incorporated into device  10  (e.g., system  20 , etc.). These optical structures may allow light rays from display system  11  to pass through and/or reflect from surfaces in optical system  20  thereby providing optical system  20  with a desired lens power (e.g., for image light  24  that passes to eye box  43  and eye  46 ). For example, optical system  20  may include a reflective polarizer  70 , a quarter wave plate  66 , and a partially reflective mirror  62 . Optical system  20  may form a catadioptric lens for display system  11  and may sometimes be referred to herein as catadioptric lens  20  or lens  20 . 
     If care is not taken, reflections within lens  20  may produce ghost images G that are visible to eye  46 . Display system  11  may include ghost image mitigation structures that serve to eliminate or minimize ghost images such as ghost image G. For example, display system  11  may include quarter wave plate  52  between polarizer  56  and display  40  that serves to mitigate or eliminate ghost image G. The operation of quarter wave plate  52  in mitigating ghost image G is described in greater detail below in connection with  FIG.  2   . The example of  FIG.  1    in which lens  20  includes lens elements  22  and  28  is merely illustrative. If desired, one or more additional lens elements may be incorporated into lens  20 . For example, an additional lens element may be interposed between reflective polarizer  70  and quarter wave plate  66 . If desired, an additional lens element may be formed over partially reflective mirror  62 . Lens  20  may include any desired number of one or more lens elements. Each lens element may include concave, convex, planar, spherical, aspherical, freeform, and/or other curved surfaces that may be used for additional focusing, distortion correction, etc. Each lens element may be attached together, or one or more of the lens elements may be separated from the other lens elements in lens  20  by an air gap or other structures. 
     The example of  FIG.  1    in which display  40  has a planar shape is merely illustrative. If desired, display  40  may have other shapes such as a spherical shape, aspherical shape, freeform shape, curved shape, etc.  FIG.  2    is a diagram illustrating how quarter wave plate  52  may mitigate ghost images such as ghost image G in an example where display  40  has a curved shape. This is merely illustrative and, in general, similar ghost mitigation operations may be performed for displays  40  having any desired shape and for lenses  20  having any number of lens elements with any desired shapes. 
     As shown in  FIG.  2   , display  40  and pixel array  39  may have a surface S 7  facing eye  46 . Surface S 7  may be spherically, aspherically, or freeform curved, e.g., surface S 7  of display  40  may be concave. With one illustrative configuration, concave surface S 7  of pixel array  39  and display  40  may be a spherical surface and may be radially symmetric about axis  41 . Aspherical or freeform surface shapes may also be used for surface S 7 . 
     Catadioptric lens  20  may be configured to focus image light from pixel array  39  into eye box  43  (e.g., a circle of about 10-20 mm in diameter). Eye  46  may be located about 10-30 mm from the innermost surface of lens  20 . The field of view of lens  20  may be characterized by angles A 1  and A 2  with respect to axis  41 . Angle A 1  may be at least 70° or at least 80° and angle A 2  may be at least 30° or at least 40° (e.g., when eye  46  is a right eye and when lens  20  is being viewed from above). Nasal (nose-facing) angle A 2  is preferably less than about 50°, because the user&#39;s nose prevents a wider nasal field of view. The temporal (temple-facing) angle of view A 1  may be larger (e.g., at least 80°) to expand a user&#39;s peripheral vision. Overall, the field of view of each lens  20  (e.g., the field of view per eye) may be at least 120°, at least 125°, less than 160°, or other suitable value and the resulting binocular field of view (the field of view for both of a user&#39;s eyes taken together) may be at least 150°, at least 160° or other suitable value. 
     Catadioptric lens  20  may include lens elements such as lens elements  60 ,  64 , and  68 . 
     Lens elements  60 ,  64 , and  68  may be formed from glass, polymer, or other materials. One or more of lens elements  60 ,  64 , and  68  may be omitted if desired. Additional lens elements may be mounted to lens elements  60 ,  64 , and/or  68  if desired. Lens elements  60 ,  64 , and  68  may be characterized by curved surfaces S 1 , S 2 , S 3 , S 4 , S 6 , and S 6 . Curved surfaces S 1  and S 2  of lens element  68 , curved surfaces S 3  and S 4  of lens element  64 , curved surfaces S 5  and S 6  of lens element  60 , and curved surface S 7  of display  40  may be spherical. If desired, one or more of these surfaces may be aspherical, planar, or freeform. 
     In the example of  FIG.  2   , surfaces S 6 , S 4 , and S 2  are convex surfaces and face concave surface S 7  of pixel array  39 . Lens element  68  may have opposing convex and concave surfaces. Surface S 2  may form the convex surface of lens element  68  and surface S 1  may form the opposing concave surface of lens element  68 . Lens element  64  may also have opposing convex and concave surfaces. Surface S 3  may form the concave surface of lens element  64  and surface S 4  may form the convex surface of lens element  64 . Lens element  60  may also have opposing convex and concave surfaces. Surface S 5  may form the concave surface of lens element  60  and surface S 6  may form the convex surface of lens element  60 . Concave surface S 5  may have a curvature that matches that of convex surface S 4  and concave surface S 3  may have a curvature that matches that of convex surface S 2 , so concave surface S 5  may sometimes be referred to as surface S 4  or surfaces S 5  and S 4  may collectively be referred to as the curved surface between elements  64  and  60 . Similarly, concave surface S 3  may sometimes be referred to as surface S 2  or surfaces S 3  and S 2  may collectively be referred to as the curved surface between elements  64  and  68 . 
     Optical structures such as partially reflective coatings, wave plates, reflective polarizers, linear polarizers, antireflection coatings, and/or other optical components may be incorporated into device  10 . These optical structures may allow light rays from display  40  to be emitted from surface S 7  of display  40  and to pass through and/or reflect from surfaces in lens  20  such as surfaces S 1 -S 6 . The radius of curvature of surfaces S 1 -S 6  may be about 10-70 mm, at least 20 mm, less than 60 mm, 15-35 mm, 20-30 mm, 20-40 mm, or other suitable size. As shown in  FIG.  2   , lens elements  60 ,  64 , and  68  may be concentric dome lenses that together form a cemented triplet with films and coatings on the various surfaces to control the path of the image light as it passes through the catadioptric lens  20 . 
     Lens elements  60 ,  64 , and  68  may have respective thicknesses TH 3 , TH 2 , and TH 1 . Thickness TH 1  may be uniform throughout element  68  (e.g., TH 1  may vary by less than 5%, less than 10%, or less than another suitable amount throughout element  26 ). Thickness TH 2  may be uniform throughout element  64  (e.g., TH 2  may vary by less than 5%, less than 10%, or less than another suitable amount throughout element  26 ). Thickness TH 3  may be uniform throughout element  60  (e.g., TH 3  may vary by less than 5%, less than 10%, or less than another suitable amount throughout element  26 ). Additional lens elements may be mounted to surfaces S 1  and/or S 6  if desired. Lens element  60  may be omitted in one suitable arrangement. 
     Chromatic aberrations may be minimized by forming most of the lens power of lens  20  from the reflective structures of lens  20  and by forming only a small amount (e.g., negligible amount) of the lens power of lens  20  through refraction by lens elements  60 ,  64 , and  68 . As an example, lens  20  may be characterized by a refractive effective focal length of −170 mm and a total effective focal length of +35 mm. With this type of configuration the overall focal length of lens  20  has a positive sign rather than a negative sign when the reflective contribution and the refractive contribution are combined because the reflective structures of lens  20  dominate the overall performance of the lens. This helps reduce chromatic aberrations which are associated with refractive lens power. In general, lens  20  may have any suitable focal length (e.g., 30-40 mm, at least 15 mm, at least 25 mm, less than 45 mm, less than 55 mm, etc.). The reflective contribution to the lens power of lens  20  may be greater than the refractive contribution to the lens power (e.g., the reflective contribution may be at least three times, at least five times, at least ten times, or more than the refractive contribution). 
     Linear polarizer  56 , a retarder such as a quarter wave plate  59 , and a retarder such as quarter wave plate  52  may be located between pixel array  39  and lens  20  (e.g., within display system  11 ). Linear polarizer  56  may be interposed between quarter wave plates  59  and  52 . Linear polarizer  56  and quarter wave plate  59  may be used to circularly polarize light emitted by display  40 . Linear polarizer  56  may have a pass axis aligned with the X-axis of  FIG.  2    (as an example) and the fast axis of quarter wave plate  59  is aligned at 45 degrees to the pass axis of the linear polarizer. The fast axis of quarter wave plate  52  is also aligned at 45 degrees to the pass axis of linear polarizer  56  (e.g., parallel or orthogonal to the fast axis of quarter wave plate  59 ). Quarter wave plate  52  may serve to mitigate ghost images in device  10 . 
     With the illustrative arrangement of  FIG.  2   , quarter wave plate  52  is formed on surface S 7  of pixel array  39 . Quarter wave plate  52  may be a film or coating that is attached to surface S 7  with a layer of adhesive such as optically clear adhesive  50 . Linear polarizer  56  may be formed from a polarizer film that is thermoformed into a shape to match concave surface S 7  and attached to quarter wave plate  52  using a layer of adhesive such as optically clear adhesive  54 . Quarter wave plate  59  may be a film or coating that is attached to linear polarizer  56  with a layer of adhesive such as optically clear adhesive  58 . 
     Adhesive layers  58 ,  54 , and/or  50  may be replaced using any desired substrates or may be omitted if desired. Layers  58 ,  54 , and  50  may have any desired thicknesses and are illustrated in  FIG.  2    with a relatively large thickness for the sake of clarity. If desired, quarter wave plate  59 , linear polarizer  56 , and/or quarter wave plate  52  may be formed on convex surface S 6  of lens element  60 , on layer  62  of lens  20 , or may be located at other suitable locations between surfaces S 4  and S 7 . Optional antireflection coating may be formed on any surfaces that are exposed to air (e.g., the surface of quarter wave plate  59  and/or lens element  60 ) to enhance light transmission. 
     Surface S 7  may have significant curvature, so the use of a coating process may help ensure satisfactory formation of quarter wave plates  59  and  52 . With one illustrative configuration, quarter wave plates  59  and  52  may be liquid-crystal-based retarder layers (e.g., birefringent coatings formed from liquid crystals in a liquid polymer binder that is applied to surface S 7  by spin coating or other suitable deposition techniques followed by ultraviolet light curing and/or thermal curing). In either embodiment, associating quarter wave plate  59  with a linear polarizer such as linear polarizer  56  will cause the image light entering lens  20  to be circularly polarized, provided that the fast axis of the quarter wave plate is oriented at 45 degrees to the pass axis of the linear polarizer  56 . 
     For example, as shown in  FIG.  2   , pixels P in display  40  may emit light (e.g., image light), as shown by ray R 1 . Light R 1  may be un-polarized and may pass through quarter wave plate  52 . Linear polarizer  56  linearly polarizes the emitted light (e.g., based on the pass axis of linear polarizer  56 ), as shown by linearly polarized ray R 2 . Quarter wave plate  59  may circularly polarize the linearly polarized light R 2 , as shown by circularly polarized ray R 3 . In the example of  FIG.  2   , light R 3  is circular-polarized in a first direction (e.g., light R 3  may be right-hand circular polarized). 
     A partially reflective mirror coating may form partially reflective mirror  62  of lens  20 . As shown in  FIG.  2   , partially reflective mirror  62  may be formed on convex surface S 4  of lens element  64 . The coating for mirror  62  is a metal mirror coating or other mirror coating layer such as a dielectric multilayer coating with a 50% transmission coefficient and a 50% reflection coefficient or other suitable light transmission and reflection values. When circularly polarized image light (e.g., ray R 3 ) strikes partially reflective mirror  62 , a portion of ray R 3  will pass through partially reflective mirror  62  to become reduced-intensity ray R 6 . Simultaneously, a portion of ray R 3  will be reflected by the partially reflective mirror  62 , as shown by ray R 4 . Reflected portion R 4  of ray R 3  will be circularly polarized in a second direction (e.g., left-hand circular polarized). Circularly polarized light R 4  may pass back through the quarter wave plate  59  such that the circularly polarized light is converted to linearly polarized light R 5  with the opposite polarization state so that it will be absorbed by the linear polarizer  62 , thereby trapping the reflected light and reducing stray light in the optics of the electronic device  10 . 
     Ray R 6  is circularly polarized (e.g., right-hand circular polarized). A third quarter wave plate such as quarter wave plate  66  may be included in optical system  20  between the partially reflective mirror  26  and a reflective polarizer  70 . Quarter wave plate  66  may convert the circular polarization state of ray R 6  into a linear polarization state, as shown by linearly polarized ray R 7  (e.g., the fast axis of quarter wave plate  66  may be aligned at 90 degrees with respect to the fast axis of quarter wave plate  59 ). Quarter wave plate  66  may be formed under the partially reflective mirror  62  on surface S 4  (not shown), on convex surface S 2  of lens element  68  (as shown in  FIG.  2   ), on concave surface S 3  of lens element  64 , and/or may be formed on the concave surface S 1  of lens element  68  (not shown) with reflective polarizer  70  under the quarter wave plate. 
     Reflective polarizer  70  may be formed on concave surface S 1  of lens element  68 . Alternatively, a thin (about 1 mm) curved spherical dome lens (not shown) may be provided with an optically clear adhesive that adhesively bonds the reflective polarizer to it. The dome lens with reflective polarizer  70  can then be adhesively bonded to surface S 1 . In the illustrative configuration of  FIG.  2   , quarter wave plate  66  has been formed from a coating layer (e.g., a birefringent liquid-crystal-based polymer layer) on surface S 2 . Optically clear adhesive layers (not shown) may be used to attach lens elements  68 ,  64 , and  60  together. 
     Quarter wave plate  66  may convert circularly polarized ray R 6  into a linearly polarized ray R 7  having a polarization aligned with the X-axis of  FIG.  2   . Reflective polarizer  70  may be a polymer film (e.g., a multilayer reflective polarizer film or a wire-grid polarizer film) that is thermoformed onto concave surface S 1  of lens element  68 . However, surface S 1  may have significant curvature making thermoforming undesirable due to the large distortion imparted to the reflective polarizer film, as a result, it may be desirable to form reflective polarizer  70  from a coating layer. With one illustrative configuration, reflective polarizer  70  may be a wire-grid polarizer formed using a sol-gel process. During formation of reflective polarizer  70 , a glass-based sol-gel liquid is applied to surface S 1  and is patterned using a stamper with a nanoscale polarizer pattern, where the solgel can included electrically conductive components or electrically conductive materials can be preferentially applied to the solgel pattern to form an array of nanoscale wire conductors that together form the wire-grid polarizer. Other reflective polarizer coating techniques may be used, if desired. 
     Reflective polarizer  70  may have orthogonal reflection and pass axes. Light that is polarized parallel to the reflection axis of reflective polarizer  70  will be reflected by reflective polarizer  70 . Light that is polarized perpendicular to the reflection axis and therefore parallel to the pass axis of reflective polarizer  70  will pass through reflective polarizer  70 . In the illustrative arrangement of  FIG.  2   , reflective polarizer  70  has a reflection axis that is aligned with the X-axis, so ray R 7  will reflect from reflective polarizer  70  at surface S 1  as reflected ray R 8 . 
     Reflected ray R 8  has a linear polarization aligned with the X-axis. After passing through quarter wave plate  66 , the linear polarization of ray R 8  will be converted into circular polarization (i.e., ray R 8  will become circularly polarized ray R 9 ). Circularly polarized light R 9  may be circularly polarized in the first direction (e.g., light R 9  may be right-hand circular polarized). 
     Circularly polarized ray R 9  will travel through lens element  64  and a portion of ray R 9  will be reflected in the Z direction by the partially reflective mirror  62  on the convex surface S 4  of lens element  64  (as reflected ray R 24 ). The reflection from the curved shape of surface S 4  provides optical system  20  with additional optical power. Ray R 24  is circularly polarized in a second direction (e.g., ray R 24  is left-hand circular polarized). After passing back through lens element  64  and quarter wave plate  66 , ray R 24  will become linearly polarized, as shown by ray R 25 . The linear polarization of ray R 25  is aligned with the Y-axis of  FIG.  2   , which is parallel to the pass axis of reflective polarizer  70 . Accordingly, ray R 25  will pass through reflective polarizer  70  to provide a viewable image to the user. 
     If desired, device  10  may include an additional linear polarizer such as a clean-up linear polarizer (not shown) positioned between the reflective polarizer  70  and the user&#39;s eye  46 , where the clean-up linear polarizer has a pass axis aligned with the pass axis of reflective polarizer  70  (i.e., parallel to the Y-axis in this example) and will therefore remove any residual non-Y-axis polarization from ray R 25  before ray R 25  reaches viewers eye  46 . The clean-up polarizer will also absorb any light from the environment that would otherwise be reflected by the reflective polarizer  70 . The clean-up linear polarizer may be a polarizer film that is thermoformed onto reflective polarizer  70  and attached using adhesive or may be located elsewhere between the reflective polarizer  70  and eye  46 . 
     The portion of ray R 9  that is transmitted by partially reflective mirror  62  is shown by ray R 10 . Ray R 10  is converted from circularly polarized light to linearly polarized light R 11  by quarter wave plate  59 . Linearly polarized light R 11  has a polarization aligned with the X-axis. Linear polarizer  56  (which has a pass axis aligned with the X-axis) may pass linearly polarized light R 11  as ray R 12 . Quarter wave plate  52  may circularly polarize light R 12  to produce circularly polarized light R 13 . Light R 13  may be circularly polarized in the first direction (e.g., light R 13  may be right-hand circular polarized). Light R 13  may reflect off of surface S 7  of display  14 , as shown by ray R 14 . Light R 14  may be circularly polarized in the second direction due to the reflection at surface S 7  (e.g., reflected light R 14  may be left-hand circular polarized). When circularly polarized reflected light R 14  passes through quarter wave plate  52 , quarter wave plate may convert the circularly polarized light into linearly polarized light R 15 . Because reflected light R 14  has an opposite circular polarization to light R 13 , quarter wave plate  52  produces linearly polarized light R 15  having a polarization aligned with the Y-axis of  FIG.  2   . Linear polarizer  56 , which has a pass axis orthogonal to the linear polarization of light R 15 , may thereby absorb light R 15 . This may serve to mitigate the generation of ghost images associated with light reflected off of display  40 . 
     Consider, for example, a scenario where quarter wave plate  52  is omitted. In this scenario, linear polarized light R 12  reflects off of display  40  as linear polarized light R 16  (e.g., reflected light having a polarization aligned with the polarization of light R 12 ). Linear polarized light R 16  passes through linear polarizer  56  as linear polarized light R 17  and is converted into right hand circular polarized light R 18  by quarter wave plate  59 . A portion of light R 17  is transmitted through partial mirror  62 , as shown by ray R 19 . Quarter wave plate  66  converts circularly polarized light R 19  into linear polarized light R 20 . The linear polarization of light R 20  is aligned with the X-axis of  FIG.  2    and is thereby reflected off of reflective polarizer  70 , as shown by reflected light R 21 . 
     Reflected light R 21  is linearly polarized and is converted into right hand circular polarized light R 22  by quarter wave plate  66 . A portion of right hand circular polarized light R 22  is reflected off of partial mirror  62 , as shown by reflected light R 23 . Reflected light R 23  is left-hand circular polarized, due to the reflection off of partial mirror  62 . Linear polarizer  66  thereby converts left-hand circular polarized light R 23  into linear polarized light R 24  having a polarization aligned with the pass axis of reflective polarizer  70  (i.e., aligned with the Y-axis of  FIG.  2   ). Reflective polarizer  70  thereby passes light R 24  to the user&#39;s eye  46 . This light may be off-axis with respect to image light R 25  and may form an undesirable ghost image G 2  that is visible to the user. By forming quarter wave plate  52  between linear polarizer  56  and display  40 , ghost images such as ghost image G 2  may be eliminated from system  10 . 
     Deposition techniques that may be used in forming coatings in lens  20  and on display  40  include liquid coating techniques (ink-jet printing, screen printing, pad printing, spinning, dipping, dripping, painting, and spraying), atomic layer deposition, physical vapor deposition techniques such as sputtering and evaporation, chemical vapor deposition, plasma-enhanced chemical vapor deposition, and/or other thin-film deposition techniques. The configuration of  FIG.  2    (e.g., the curved concave emitting surface S 7  of display  40 ) may help improve optical performance for device  10 . As an example, curved surface S 7  may help reduce field curvature across the displayed field of view so that the user is presented an image with more uniform sharpness. 
     The example of  FIG.  2    is merely illustrative. Quarter wave plate  52 , linear polarizer  56 , and quarter wave plate  59  may have any other desired shapes such as curved shapes, planar shares, or other shapes (e.g., shapes matching the shape of display  40 ). Display  40  may have other shapes. Quarter wave plate  52  may serve to mitigate ghost images such as ghost image G 2  regardless of the shape of elements  59 ,  56 ,  52 , and  40  and regardless of the number and shape of lens elements in lens  20 . 
     In practice, some of the relatively-high intensity light R 7  may leak through reflective polarizer  70 , forming an off-axis ghost image G 1  that may be visible to the user. If desired, control circuitry  42  may adjust the images that are displayed using display  40  to compensate for potential ghost images G 1 , ghost images G 2 , and/or other ghost images associated with reflections in lens  20 . 
       FIG.  3    is a diagram of ghost mitigation circuitry  80  in device  10  that may be used to adjust images that are displayed using display  40  for mitigating potential ghost images. Ghost mitigation circuitry  80  may, for example, be implemented using control circuitry  42  and/or other circuitry in device  10 . 
     As shown in  FIG.  3   , ghost mitigation circuitry  80  may include ghost prediction circuitry such as ghost prediction engine  90 , local tone mapping grid circuitry such as local tone mapping grid generation engine  94 , a tone mapping look-up-table (LUT) such as tone mapping LUT  98 , tone mapping circuitry such as tone mapping engine  102 , and image adjustment circuitry such as luminance adjustment engine  120 . 
     Ghost prediction engine  90  may receive an image frame such as image frame  82  (e.g., an image frame from a stream of video data to be displayed using display  40 ). Image frame  82  may include one or more objects such as object  84 . Ghost prediction engine  90  may predict whether a given image frame  82  is likely to produce a ghost image (e.g., ghost images such as ghost images G 1  and G 2  of  FIG.  2   ) based on the content of image frame  82  and the predetermined (known) geometry of catadioptric lens  20 . 
     As an example, ghost prediction engine  90  may store calibration data (e.g., predetermined data generated during manufacture and/or testing of device  10 ). The calibration data may identify how ghost images are likely to be generated for certain pixel values at different locations across a given input image frame (e.g., the calibration data may be generated by measuring ghost images that appear at eye box  43  in response to different calibration image frames for the particular geometry of catadioptric lens  20 ). Ghost prediction engine  90  may compare input image  82  to this predetermined calibration data to predict the presence of ghost images when image frame  82  is eventually displayed using display  40 . Ghost prediction engine  90  may, for example, predict the presence, location, shape, and/or intensity (strength) of ghost images in image frame  82  when displayed using display  40  based on the content of image frame  82  and the predetermined geometry of lens  20 . 
     Ghost prediction engine  90  may convey ghost prediction information  88  to local tone mapping grid generation engine  94  over path  92 . Information  88  may identify the presence, location, shape, and/or intensity of one or more ghost images such as ghost image  86  that are expected to be visible in image frame  82  when displayed using display  40 . Local tone mapping grid generation engine  94  may generate a tone mapping grid  106  based on information  88 . For example, engine  94  may generate a grid  106  that divides input image  82  into two or more cells  108 . One or more cells  108  may overlap with the expected ghost image  86  (see, e.g., cell  110  of  FIG.  3   ). By dividing input image frame  82  into a grid in this way, image adjustments may be performed to portions of the image frame having expected ghost image  86  without altering other portions of the image. Engine  94  may provide grid  106  to tone mapping LUT  98  over path  96 . Ghost prediction engine  90  may provide information  88  to tone mapping LUT  98  over path  92 . 
     Tone mapping LUT  98  may store a set of tone mapping curves that can be applied to input image  82  to adjust the contrast of portions (regions) of input image  82 . Tone mapping LUT  98  may select a given tone mapping curve from the set of tone mapping curves based on grid  106  received from engine  98  and information  88  received from engine  90 . For example, tone mapping LUT  98  may select a tone mapping curve to apply to the cell  110  of grid  106  that includes expected ghost image  86  based on the strength, size, shape, and/or position of expected ghost  86  and/or based on the content of image frame  82 . The tone mapping curve may be a tone mapping curve that adjusts the contrast of cell  110  in the image frame to minimize the visibility of expected ghost  86  when image frame  82  is eventually displayed using display  40 . Tone mapping LUT  98  may provide information identifying the selected tone mapping curve to tone mapping engine  102 , as shown by paths  100 . 
     Tone mapping engine  102  may apply the identified tone mapping curve to cell  110  of input image frame  82 . This may serve to adjust the contrast of cell  110  in input image frame  82  (e.g., in a way such that the presence of the ghost image is minimized when image frame  82  is displayed by display  40 ). Tone mapping engine  102  may output adjusted image frame  112  having adjusted contrast within cell  110  over path  104 . Adjusted image frame  112  may include object  84  and any other image data from image frame  82  (e.g., without contrast adjustments provided to regions other than region  110  of the image frame). Adjusted image frame  112  may be provided to additional processing circuitry to perform other image processing operations prior to being displayed or may be provided to display  40 . Display  40  may display adjusted image frame  112 . The adjusted contrast within region  110  may serve to minimize visibility of predicted ghost  86  within region  110  of the image frame. 
     If desired, tone mapping engine  102  may adjust the contrast of pixels adjacent to cell (region)  110  in image frame  112 . For example, tone mapping engine  102  may mix (weight) the tone mapping curve used for region  110  with a tone mapping curve applied to pixels adjacent to region  110  (e.g., a linear tone mapping curve). In other words, engine  102  may interpolate contrast adjustments for pixels adjacent to region  110 . If desired, the tone mapping curve for region  110  may be weighted more heavily for pixels closer to region  110  than for pixels farther from region  110 . This may serve to blur the adjusted-contrast of region  110  with surrounding regions in the image frame (e.g., for aesthetic purposes so that the entire image frame appears as a smooth, unaltered image despite being adjusted to mitigate potential ghost images). 
     In practice, ghost images may be particularly visible when the background of image frame  82  is dark whereas foreground objects are bright. Such high contrast image data is relatively common in video data that is to be displayed over display  40  (e.g., during end credits for a film, during dark scenes, video game menu interfaces, etc.). If care is not taken, ghost images (e.g., faint halos or other ghost images) for this type of image data may be difficult to mitigate by adjusting local contrast (e.g., using engine  102 ), because bright ghost images superimposed on a black background by lens  20  cannot be removed in an additive fashion. In these scenarios, the brightness (luminance) of the entire image frame may be increased except at the location of the predicted ghost image. This may increase the luminance of the black background to match the expected luminance of the ghost image so that the ghost image blends in with the background and is no longer visible to a user. 
     As shown in  FIG.  3   , ghost mitigation circuitry  80  may include optional luminance adjustment engine  120 . Luminance adjustment engine  120  may receive ghost information  88  from ghost prediction engine  90  over path  116 . Luminance adjustment engine  120  may adjust the luminance (brightness) of input image frame  82  at all locations in image frame  82  except at the predicted location for the ghost image (e.g., the location of expected ghost image  86  identified by information  88 ). Engine  120  may output the adjusted image frame to display  40  over path  118 . When display  40  displays the adjusted image, the ghost image (e.g., ghost image G 1  of  FIG.  2   ) may blend in with the increased luminance surrounding the ghost image such that it is no longer visible to the user at eye box  43 . If desired, ghost prediction engine  90  may process the content of image frame  82  to determine whether to use tone mapping engine  102  or luminance adjustment engine  120  to adjust image frame  82 . Both engines  102  and  120  may be used to adjust the same image frame if desired (e.g., to output an image frame having an adjusted luminance outside of the predicted ghost location and locally-adjusted contrast within and around the predicted ghost location). Circuitry  102 ,  94 , and  98  may be omitted in another suitable arrangement if desired. 
       FIGS.  4 A- 4 D  are diagrams showing how luminance adjustment engine  120  may mitigate ghost images in an image frame having a bright object over a dark (e.g., black) background.  FIG.  4 A  shows an example of an input image frame  130  (e.g., input image frame  82  of  FIG.  3   ) having a bright object  132  over a dark background  134 . The presence of bright object  132  may generate a ghost image over dark background  134  when received at eye box  43  (e.g., a faint glow or halo associated with bright object  132 ). 
       FIG.  4 B  shows an example of how image frame  134  may be received at eye box  43  after passing through lens  20  of  FIGS.  1  and  2   . As shown in  FIG.  4 B , object  132  produces a ghost image  138  over dark background  134 . Ghost image  138  may still be present even when quarter wave plate  52  of  FIGS.  1  and  2    is formed over display  40  (e.g., due to leakage of light R 7  through reflective polarizer  70  as ghost image G 1 ). Adjusting the contrast within region  138  (e.g., using local tone mapping engine  102  of  FIG.  3   ) may be not be able to remove ghost image  138  from the frame. 
       FIG.  4 C  shows an example of an adjusted image frame  140  that may be output by luminance adjustment engine  120  to mitigate ghost image  138 . As shown in  FIG.  4 C , the luminance (brightness) of image frame  136  may be adjusted (increased) across the entire image frame except within region  142  overlapping expected ghost image  138 . In this way, background  134  of the frame may have an increased luminance, as shown by background region  134 ′ of  FIG.  4 C . 
       FIG.  4 D  shows an example of how image frame  130  of  FIG.  4 A  would appear to a user (e.g., at eye box  43  of  FIGS.  1  and  2   ) after being adjusted by luminance adjustment engine  120  (e.g., by displaying adjusted image frame  140  of  FIG.  4 C  using display  40 ). As shown in  FIG.  4 D , the brightness of ghost image  138  ( FIG.  4 B ) matches the increased luminance of background region  134 ′ so that ghost image  138  is no longer visible in the image frame  142  when viewed at eye box  43 . The example of  FIGS.  4 A- 4 D  are merely illustrative. In general, any desired number of expected ghost images having any desired shapes and sizes may be mitigated in this way using luminance adjustment engine  120 . 
       FIG.  5    is a flow chart of illustrative steps that may be performed by ghost mitigation circuitry  80  of  FIG.  3    to mitigate potential ghost images that are visible at eye box  43 . At step  140 , ghost prediction engine  90  in ghost mitigation circuitry  80  may receive input image frame  82 . 
     At step  142 , ghost prediction engine  90  may predict the presence, strength, shape, size, and/or location of one or more ghost images that will be visible at eye box  43  when image frame  82  is displayed by display  40  (e.g., ghost images such as ghost image G 1  of  FIG.  2   ). Prediction engine  90  may predict this ghost image information based on the content of image frame  140  and the predetermined geometry of lens  20  (e.g., based on calibration data stored at circuitry  80 ). If no ghost images are predicted to be present for image frame  82 , processing may loop back to step  140  as shown by path  144  to receive another image frame  82  (e.g., the next image frame in a stream of video data). 
     If one or more ghost images are predicted to be present for image frame  82 , processing may proceed to step  148  as shown by path  146 . At step  148 , prediction engine  90  may determine whether the luminance of the entire image frame needs to be adjusted using engine  120  of  FIG.  3   . For example, prediction engine  90  may determine whether image frame  82  includes a dark background and one or more light objects over the dark background. If the luminance of the image frame needs to be adjusted (e.g., the image frame includes a relatively dark background and a relatively light foreground), processing may proceed to step  160  as shown by path  158 . 
     At step  160 , luminance adjustment engine  120  may generate an adjusted image frame (e.g., adjusted image frame  140  of  FIG.  4 C ) by increasing the luminance of image frame  82  except at locations overlapping the predicted ghost image (e.g., except within region  142  of  FIG.  4 C ). 
     At step  162 , luminance adjustment engine  120  may output the adjusted image frame to display  40 . Display  40  may subsequently display the adjusted image frame. By the time the adjusted image frame is received at eye box  43 , the ghost image generated by lens  20  may no longer be visible to the user (e.g., as shown in  FIG.  4 D ). If the luminance of the image frame does not need to be adjusted (e.g., the image frame does not include a relatively dark background and a relatively light foreground), processing may proceed to step  152  as shown by path  150 . 
     At step  152 , local tone mapping grid generation engine  94  may generate tone mapping grid  106  of  FIG.  3   . Grid  106  may include a number of cells  108 . One or more cells  110  may overlap the predicted ghost location. 
     At step  154 , tone mapping LUT  98  may identify a tone mapping curve for the cell(s)  110  in the grid overlapping the predicted ghost location. The tone mapping curve may be selected based on ghost information  88  and/or the content of image frame  82  to adjust the contrast of the image frame within cell  110  such that the ghost image will be minimized when received at eye box  43 . 
     At step  156 , tone mapping engine  102  may output adjusted image frame  112  of  FIG.  3   . Engine  102  may generate adjusted image frame  112  by adjusting the contrast within cell(s)  110  of the image frame using the selected tone mapping curve. If desired, pixels adjacent to cell  110  may be adjusted by weighting the selected tone mapping curve with another tone mapping curve (e.g., a linear tone mapping curve) to blur the adjusted contrast region within the image frame. Processing may subsequently proceed to step  162 , at which engine  102  may output the adjusted image frame to display  40 . Display  40  may display the adjusted image frame. By the time the adjusted image frame is received at eye box  43 , the presence of the ghost image generated by lens  20  may be minimized (e.g., may be invisible or unperceivable the user due to the contrast adjustments performed by the tone mapping engine). 
     The steps of  FIG.  5    are merely illustrative. Two or more of the steps of  FIG.  5    may be performed concurrently. The steps of  FIG.  5    may be performed in other orders if desired. Steps  148  and  160  may be omitted or steps  148 ,  152 ,  154 , and  156  may be omitted. Step  160  may be performed on an image frame adjusted at steps  152 - 156  or steps  152 - 156  may be performed on an image frame adjusted at step  160  if desired. 
       FIG.  6    is a graph of illustrative tone mapping curves that may be used in mitigating ghost images (e.g., while processing steps  152 - 156  of  FIG.  5   ). As shown in  FIG.  6   , pixel input values are plotted on the horizontal axis and pixel output values are plotted on the vertical axis. Tone mapping LUT  98  of  FIG.  3    may select a desired tone mapping curve such as curves  174 ,  172 ,  170 , or other tone mapping curves to adjust the localized contrast within cell  110  of image frame  82 . If no contrast adjustment is to be performed, linear tone mapping curve  170  may be selected. When curve  170  is applied to a given pixel value in an image frame, the output of the tone mapping operation will be the same given pixel value. When non-linear curves such as curves  172 ,  174 , or other curves are applied to a given pixel value in an image frame, the output of the tone mapping operation (e.g., the adjusted pixel value in the adjusted image frame) will be given based on the vertical coordinate corresponding to that given pixel value on the horizontal axis. In this way, the contrast of the pixel values within cell  110  may be adjusted by engine  102  of  FIG.  3    using a corresponding tone mapping curve. 
     Different tone mapping curves may adjust contrast in different ways and some tone mapping curves may mitigate different types of ghost images from different input pixel values differently. Tone mapping LUT  98  of  FIG.  3    may select an optimal tone mapping curve that best mitigates (minimizes) the predicted ghost image. As different input frames are received, different tone mapping curves may be used as necessary to mitigate ghost images even as the ghost images change over time (e.g., based on the different input images that are received over time for the given lens geometry). In this way, ghost images that may not be mitigated by quarter wave plate  52  of  FIGS.  1  and  2    such as ghost image G 1  may be actively mitigated in device  10  regardless of the image data to be displayed. 
     The foregoing is merely illustrative and various modifications can be made to the described embodiments. The foregoing embodiments may be implemented individually or in any combination.

Metadata:
Filing Date: 20190823
Publication Date: 20240130
Grant Date: 20240130
Priority Date: 20180831
Inventors: HSU, WEI-LIANG
ZHANG, SHENG
FLYNN, MARK F.
PETROV, YURY A.
WANG, CHAOHAO
Assignee: APPLE INC
CPC Classifications: [{"code": "G02B27/0018", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B1/115", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B5/003", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B13/002", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/0018", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B5/003", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B13/002", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B1/115", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B1/115", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B13/002", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B5/003", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/0018", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/0172", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/286", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 89666198