PATENT DOCUMENT

Publication Number: US-11740446-B2
Application Number: US-202117150919-A
Country: US
Kind Code: B2

Title: Optical system for head-mounted display

Abstract:
A head-mounted display may include a display system and an optical system in a housing. The display system may have a pixel array that produces light associated with images. The display system may also have a linear polarizer through which light from the pixel array passes and a quarter wave plate through which the light passes after passing through the quarter wave plate. The optical system may be a catadioptric optical system having one or more lens elements. The lens elements may include a plano-convex lens and a plano-concave lens. A partially reflective mirror may be formed on a convex surface of the plano-convex lens. A reflective polarizer may be formed on the planar surface of the plano-convex lens or the concave surface of the plano-concave lens. An additional quarter wave plate may be located between the reflective polarizer and the partially reflective mirror.

Claims:
What is Claimed is: 
     
       1. A head-mounted display, comprising:
 an array of pixels configured to produce light; 
 a first linear polarizer through which the light passes; 
 a first quarter wave plate that receives the light from the first linear polarizer; 
 a first lens element having a first surface that is convex and having an opposing second surface; 
 a partially reflecting mirror on the first surface; 
 a second quarter wave plate at the second surface; 
 a second lens element having a third surface at the second quarter wave plate and an opposing fourth surface that is concave; 
 a reflective polarizer at the fourth surface; 
 a third lens element having a fifth surface and opposing sixth surface, wherein the second lens element is interposed between the first and third lens elements; and 
 a second linear polarizer that is interposed between the second and third lens elements. 
 
     
     
       2. The head-mounted display defined in  claim 1 , wherein the third lens element is a dome lens. 
     
     
       3. The head-mounted display defined in  claim 1 , wherein the second linear polarizer and the reflective polarizer have parallel pass axes. 
     
     
       4. The head-mounted display defined in  claim 1 , wherein the reflective polarizer has a reflection axis and a pass axis that is orthogonal to the reflection axis. 
     
     
       5. The head-mounted display defined in  claim 1 , wherein the first linear polarizer has a pass axis and wherein the first quarter wave plate has a fast axis that is at a 45 degree angle relative to the pass axis. 
     
     
       6. The head-mounted display defined in  claim 1 , wherein the reflective polarizer has a pass axis and wherein the second quarter wave plate has a fast axis that is at a 45 degree angle relative to the pass axis. 
     
     
       7. The head-mounted display defined in  claim 1 , wherein the second surface is cylindrically concave and wherein the third surface is cylindrically convex. 
     
     
       8. The head-mounted display defined in  claim 1 , wherein the second and third surfaces are mating cylindrical surfaces. 
     
     
       9. The head-mounted display defined in  claim 1 , wherein at least one of the fifth surface and the sixth surface is an aspheric surface. 
     
     
       10. A head-mounted display, comprising:
 an array of pixels configured to produce light; 
 a first linear polarizer through which the light passes; 
 a first quarter wave plate that receives the light from the first linear polarizer; 
 a first lens element having a first surface that is convex and having an opposing second surface that is concave; 
 a partially reflecting mirror on the first surface; 
 a second quarter wave plate at the second surface; 
 a second lens element having a third surface that is convex and having an opposing fourth surface that is concave; 
 a reflective polarizer that is interposed between the first lens element and the second lens element; and 
 a second linear polarizer that is interposed between the first lens element and the second lens element. 
 
     
     
       11. The head-mounted display defined in  claim 10 , wherein the second linear polarizer and the reflective polarizer have parallel pass axes. 
     
     
       12. The head-mounted display defined in  claim 10 , wherein the reflective polarizer has a reflection axis and a pass axis that is orthogonal to the reflection axis. 
     
     
       13. The head-mounted display defined in  claim 10 , wherein the first linear polarizer has a pass axis and wherein the first quarter wave plate has a fast axis that is at a 45 degree angle relative to the pass axis. 
     
     
       14. The head-mounted display defined in  claim 10 , wherein the reflective polarizer has a pass axis and wherein the second quarter wave plate has a fast axis that is at a 45 degree angle relative to the pass axis. 
     
     
       15. The head-mounted display defined in  claim 10 , wherein the second lens element is a dome lens. 
     
     
       16. A head-mounted display, comprising:
 an array of pixels configured to produce light; 
 a first linear polarizer through which the light passes; 
 a first quarter wave plate that receives the light from the first linear polarizer; 
 a lens element having a first surface that is convex and having an opposing second surface that is concave; 
 a partially reflecting mirror on the first surface; 
 a second quarter wave plate at the second surface; 
 a reflective polarizer, wherein the second quarter wave plate is interposed between the reflective polarizer and the lens element; and 
 a second linear polarizer, wherein the reflective polarizer is interposed between the second linear polarizer and the second quarter wave plate. 
 
     
     
       17. The head-mounted display defined in  claim 16 , wherein the second linear polarizer and the reflective polarizer have parallel pass axes. 
     
     
       18. The head-mounted display defined in  claim 16 , wherein the reflective polarizer has a reflection axis and a pass axis that is orthogonal to the reflection axis. 
     
     
       19. The head-mounted display defined in  claim 16 , wherein the first linear polarizer has a pass axis and wherein the first quarter wave plate has a fast axis that is at a 45 degree angle relative to the pass axis. 
     
     
       20. The head-mounted display defined in  claim 16 , wherein the reflective polarizer has a pass axis and wherein the second quarter wave plate has a fast axis that is at a 45 degree angle relative to the pass axis. 
     
     
       21. An electronic device, comprising:
 an array of pixels configured to produce light; 
 a first linear polarizer through which the light passes; 
 a first quarter wave plate that receives the light from the first linear polarizer; 
 a lens element having a first surface that is convex and having an opposing second surface that is concave; 
 a partially reflecting mirror on the first surface; 
 a second quarter wave plate at the second surface; 
 a reflective polarizer, wherein the second quarter wave plate is interposed between the reflective polarizer and the lens element; and 
 a second linear polarizer, wherein the reflective polarizer is interposed between the second linear polarizer and the second quarter wave plate.

Description:
This application is a continuation of U.S. patent application Ser. No. 16/776,750, filed Jan. 30, 2020, which is a continuation of U.S. patent application Ser. No. 16/224,561, filed Dec. 18, 2018, now U.S. Pat. No. 10,591,707, which is a continuation of U.S. patent application Ser. No. 15/434,623, filed Feb. 16, 2017, now U.S. Pat. No. 10,203,489, which claims the benefit of provisional patent application No. 62/383,911, filed on Sep. 6, 2016 and provisional patent application No. 62/370,170, filed Aug. 2, 2016, all of which are hereby incorporated by reference herein in their entireties. 
    
    
     BACKGROUND 
     This relates generally to optical systems and, more particularly, to optical systems for head-mounted displays. 
     Head-mounted displays such as virtual reality glasses use lenses to display images for a user. A microdisplay may create images for each of a user&#39;s eyes. A lens may be placed between each of the user&#39;s eyes and a portion of the microdisplay so that the user may view virtual reality content. 
     If care is not taken, a head-mounted display may be cumbersome and tiring to wear. Optical systems for head-mounted displays may use arrangements of lenses that are bulky and heavy. Extended use of a head-mounted display with this type of optical system may be uncomfortable. 
     It would therefore be desirable to be able to provide improved head-mounted. 
     SUMMARY 
     A head-mounted display may include a display system and an optical system. The display system and optical system may be supported by a housing that is worn on a user&#39;s head. The head-mounted display may use the display system and optical system to present images to the user while the housing is being worn on the user&#39;s head. 
     The display system may have a pixel array that produces image light associated with the images. The display system may also have a linear polarizer through which image light from the pixel array passes and a quarter wave plate through which the light passes after passing through the linear polarizer. 
     The optical system may be a catadioptric optical system having one or more lens elements formed from clear materials such as glass or plastic and having reflective structures. The lens elements may include a plano-convex lens element and a plano-concave lens element. The plano-convex lens element may have a convex surface and an opposing planar surface. The plano-concave lens element may have a concave surface and an opposing planar surface that faces the planar surface of the convex lens element. 
     A partially reflective mirror may be formed on a convex surface of the plano-convex lens element. A reflective polarizer may be formed on the planar surface of the plano-convex lens or the concave surface of the plano-concave lens. An additional quarter wave plate may be located between the reflective polarizer and the partially reflective mirror. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram of an illustrative head-mounted display in accordance with an embodiment. 
         FIG.  2    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.  3    is a diagram of a head-mounted display with another illustrative optical system in accordance with an embodiment. 
         FIGS.  4  and  5    are cross-sectional side views of illustrative lens elements of the type that may be incorporated into a head-mounted display optical system in accordance with an embodiment. 
         FIGS.  6  and  7    are diagrams of additional illustrative head-mounted displays in accordance with embodiments. 
         FIGS.  8  and  9    are respectively top and side views of lens elements with cylindrical surfaces in accordance with an embodiment. 
         FIG.  10    is a diagram of an illustrative lens element and reflective polarizer during molding operations in a mold in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Head-mounted displays may be used for virtual reality and augmented 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 in which a head-mounted display such as a pair of virtual reality glasses is used in providing a user with virtual reality content is shown in  FIG.  1   . As shown in  FIG.  1   , virtual reality glasses (head-mounted display)  10  may include a display system such as display system  40  that creates images and may have an optical system such as optical system  20  through which a user (see, e.g., user&#39;s eyes  46 ) may view the images produced by display system  40  by looking in direction  48 . 
     Display system  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. Separate left and right displays may be included in system  40  for the user&#39;s left and right eyes or a single display may span both eyes. 
     Visual content (e.g., image data for still and/or moving images) may be provided to display system (display)  40  using control circuitry  42  that is mounted in glasses (head-mounted display)  10  and/or control circuitry that is mounted outside of glasses  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 system  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 display system  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 system  40  and optical system  20  of glasses  10 . 
     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 glasses  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 sensor, accelerometers, gyroscopes, and/or other sensors for measuring orientation, position, and/or movement of glasses  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). 
       FIG.  2    is a cross-sectional side view of glasses  10  showing how optical system  20  and display system  40  may be supported by head-mounted support structures such as housing  12  for glasses  10 . Housing  12  may have the shape of a frame for a pair of glasses (e.g., glasses  10  may resemble eyeglasses), may have the shape of a helmet (e.g., glasses  10  may form a helmet-mounted display), 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  40  in front of a user&#39;s eyes (e.g., eyes  46 ) as the user is viewing system  20  and display system  40  in direction  48  may sometimes be described herein as an example. If desired, housing  12  may have other suitable configuration. 
     Housing  12  may be formed from plastic, metal, fiber-composite materials such as carbon-fiber materials, wood and other natural materials, glass, other materials, and/or combinations of two or more of these materials. 
     Input-output devices  44  and control circuitry  42  may be mounted in housing  12  with optical system  20  and display system  40  and/or portions of input-output devices  44  and control circuitry  42  may be coupled to glasses  10  using a cable, wireless connection, or other signal paths. 
     Display system  40  and the optical components of glasses  10  may be configured to display images for user  46  using a lightweight and compact arrangement. Optical system  10  may, for example, be based on catadioptric lenses. 
     Display system  40  may include a source of images such as pixel array  14 . Pixel array  14  may include a two-dimensional array of pixels P that emits image light (e.g., organic light-emitting diode pixels, light-emitting diode pixels formed from semiconductor dies, liquid crystal display pixels with a backlight, liquid-crystal-on-silicon pixels with a frontlight, etc.), A polarizer such as linear polarizer  16  may be placed in front of pixel array  14  and/or may be laminated to pixel array  14  to provide polarized image light. Linear polarizer  16  may have a pass axis aligned with the X-axis of  FIG.  2    (as an example). Display system  40  may also include a wave plate such as quarter wave plate  18  to provide circularly polarized image light. The fast axis of quarter wave plate  18  may be aligned at 45 degrees to the pass axis of linear polarizer  16 . Quarter wave plate  18  may be mounted in front of polarizer  16  (between polarizer  16  and optical system  20 ). If desired, quarter wave plate  18  may be attached to polarizer  16  (and display  14 ). 
     Optical system  20  may include lens elements such as lens elements  26  and  32 . Lens element  26  may be a plano-convex lens (lens element) with a convex surface facing display system  40 . Optional lens element  32  may be a plano-concave lens (lens element) with a concave surface S 3  facing the user (eyes  46 ). 
     Optical structures such as partially reflective coatings, wave plates, reflective polarizers, linear polarizers, antireflection coatings, and/or other optical components may be incorporated into glasses  10  (e.g., system  20 , etc.). These optical structures may allow light rays from display system  40  to pass through and/or reflect from surfaces in optical system  20  such as surfaces S 1 , S 2 , and S 3 , thereby providing optical system  20  with a desired lens power. 
     Consider, as an example, image light ray R 1 . As image light ray R 1  exits display  14  and passes through linear polarizer  16 , ray R 1  becomes linearly polarized in alignment with the pass axis of linear polarizer  16 . The pass axis of linear polarizer  16  may be, for example, aligned with the X-axis of  FIG.  2   . After passing through polarizer  16 , ray R 1  passes through wave plate  18 , which may be a quarter wave plate. As ray R 1  passes through quarter wave plate  18 , ray R 1  becomes circularly polarized. 
     A partially reflective mirror (e.g., a metal mirror coating or other mirror coating such as a dielectric multilayer coating with a 50% transmission and a 50% reflection) such as partially reflective mirror  22  may be formed on the convex surface of lens element  26 . When circularly polarized ray R 1  strikes partially reflective mirror  22 , a portion of ray R 1  will pass through partially reflective mirror  22  to become reduced-intensity ray R 2 . Ray R 2  will be refracted (partially focused) by the shape of convex surface S 1  of lens element  26 . 
     Ray R 2  is circularly polarized. A second quarter wave plate such as quarter wave plate  28  may be included in optical system  20  to convert the circular polarization of ray R 2  into linear polarization. Quarter wave plate  28  may, for example, convert circularly polarized ray R 2  into a ray R 3  with a linear polarization aligned with the Y-axis of  FIG.  2   . 
     Reflective polarizer  30  may be formed adjacent to quarter wave plate  28 . With one illustrative configuration, reflective polarizer  30  and quarter wave plate  28  are planar layers and may be formed on the planer surface of lens element  26 . Reflective polarizer  30  may have orthogonal reflection and pass axes. Light that is polarized parallel to the reflection axis of reflective polarizer  30  will be reflected by reflective polarizer  30 . Light that is polarized perpendicular to the reflection axis and therefore parallel to the pass axis of reflective polarizer  30  will pass through reflective polarizer  30 . In the illustrative arrangement of  FIG.  2   , reflective polarizer  30  has a reflection axis that is aligned with the Y-axis, so ray R 3  will reflect from reflective polarizer  30  at surface S 2  as reflected ray R 4 . 
     Reflected ray R 4  has a linear polarization aligned with the Y-axis. After passing through quarter wave plate  28 , the linear polarization of ray R 4  will be converted into circular polarization (i.e., ray R 4  will become circularly polarized ray R 5 ). 
     Circularly polarized ray R 5  will travel through lens element  26  and a portion of ray R 5  will be reflected in the Z direction by the partially reflective mirror  22  on the convex surface S 1  of lens element  26  as reflected ray R 6 . The reflection from the curved shape of surface S 1  provides optical system  20  with additional optical power. At the same time, the portion of ray R 5  that is transmitted by partially reflective mirror  22  is converted from circularly polarized light to linearly polarized light by quarter wave plate  18 . This linearly polarized light has a polarization aligned with the Y axis so that it is absorbed by linear polarizer  16 . As a result, contrast degradation and stray light artifacts from the transmitted portion of ray R 5  are prevented in the image viewed by the user. 
     Ray R 6  is circularly polarized. After passing back through lens element  26  and quarter wave plate  28 , ray R 6  will become linearly polarized (ray R 7 ), where the linear polarization of ray R 7  is aligned with the X-axis of  FIG.  2   , which is parallel to the pass axis of reflective polarizer  30 . Accordingly, ray R 7  will pass through reflective polarizer  30  to provide a viewable image to the user. 
     If desired, glasses  10  may include an additional linear polarizer such as clean-up linear polarizer  34 . Clean-up linear polarizer  34  has a pass axis aligned with the pass axis of reflective polarizer  30  (i.e., parallel to the X-axis in this example) and will therefore remove any residual non-X-axis polarization from ray R 7  before ray R 7  reaches viewers eye  46 . 
     If desired, an additional lens element such as element  32  with an additional lens element surface (surface S 3 ) may be incorporated into optical system  20 . Surface S 3  may be concave and/or convex and may be used for additional focusing, distortion correction, etc. Element  32  may have a planar surface facing lens element  26  and a curved surface (S 3 ) facing viewer  46 . Surface S 3  may be concave, convex, aspherical, freeform, concave in parts and convex in parts, or may have other suitable shapes. Curved surfaces in system  20  such as surfaces S 1  and/or S 3  may be aspherical to improve sharpness or reduce distortion in the image presented to the user. Lens element  32  may, for example, be placed with its planar surface adjacent to reflective polarizer  30 , quarter wave plate  28 , and the planar surface of element  26  (i.e., reflective polarizer  30  and quarter wave plate  28  may be sandwiched between the planar surfaces of lens elements  32  and  26  without an air gap). 
     Although element  32  provides additional focusing power, optical system complexity and weight may, if desired, be reduced by omitting element  32 . Moreover, quarter wave plate  28  need not be located on the planar surface of element  26 , but rather may be located at any position between partially reflective mirror  22  and reflective polarizer  30 . For example, quarter wave plate  28  may be moved to position  24  between curved partially reflective mirror  22  and the convex surface of element  26 . 
       FIG.  3    is a cross-sectional side view of glasses  10  in an illustrative configuration in which optical system includes plano-convex lens element  26  and plano-curved lens element  32  (e.g., plano-concave, plano-aspherical, etc.) and in which reflective polarizer  30  is formed on curved surface S 3  of lens element  32 . Because surface S 3  is curved, additional optical power and/or distortion correction capabilities or a larger display field of view may be provided by allowing image light to reflect from reflective polarizer  30  when reflective polarizer  30  has been curved to the shape of surface S 3 . If desired, quarter wave plate  28  may be moved from the position shown in  FIG.  3    to a position adjacent to reflective polarizer  30  of  FIG.  3    (e.g., in location  50 ) or may be moved to position  24  of  FIG.  2   . The configuration of  FIG.  3    is merely illustrative. 
       FIG.  4    shows how lens elements  32  and  26  may, if desired, be separated by an air gap in system  20 . Antireflection coatings may be provided on the planar surfaces of element  32  and/or  26  to reduce reflections, if desired. 
     In the illustrative configuration of  FIG.  5   , system  20  is formed from lens elements with additional curved surfaces. In this arrangement, elements  32  and  26  are meniscus lenses and meet at curved mating surface  52 . The optical systems of  FIGS.  4  and  5    may include quarter wave plates, partially reflecting mirrors, and reflective polarizers to form catadioptric lenses as described in connection with catadioptric lens systems (lenses)  20  of  FIGS.  2  and  3   . 
     In the illustrative configuration of  FIG.  6   , reflective polarizer  30  has been formed on the surface of additional lens element  54 . Reflective polarizer  30  and lens element  54  may be attached to the adjacent curved surface of lens element  32  using optically clear adhesive (as an example). If desired, the surface of lens element  54  facing user  46  may have a curved surface. The thickness of lens element  54  may, if desired, be constant (e.g., the thickness of element  54  may vary by less than 10% or less than 5% or other suitable amount across its diameter). In addition, linear polarizer  34  may be formed on the curved surface of lens element  54  that faces user  46  to help suppress reflections of stray ambient light. Linear polarizer  34  may be oriented so that the pass axis of linear polarizer  34  is aligned with the X axis so that rays of image light such as light ray R 7  of  FIG.  2    will pass to user  46  for viewing while ambient light rays passing through polarizer  34  (in the −Z direction) will become X-polarized due to the X-axis pass axis orientation of linear polarizer  34  and will therefore not be reflected by reflective polarizer  30  (which has a reflective axis oriented with the Y axis), Obliquely oriented ambient light rays will also tend to be reflected away from user  46  due to the curved surfaces of the lens elements in system  20 . The presence of linear polarizer  34  will therefore help to reduce stray light reflections toward user  46  from the inwardly facing side of system  20 . 
     Outwardly facing surface S 4  of lens element  54  may be curved (e.g., convex) and opposing mating inwardly facing surface S 5  of lens element  32  may be correspondingly curved (e.g., concave). With one illustrative configuration, surfaces S 4  and S 5  may be rotationally symmetric about the Z axis of  FIG.  6    (e.g., lens elements  54  and  32  may be dome lenses and surfaces S 4  and S 5  may be dome lens surfaces). This allows lens element  54  to be rotated relative to lens element  32  (e.g., to align reflective polarizer  30  to quarter wave plate  28 , etc.). 
     In the example of  FIG.  6   , surfaces S 6  and S 7  are planar. This helps avoid imposing undesired stresses on quarter wave plate  28  (which may, as an example, be formed from a birefringent stretched film). Another illustrative arrangement for minimizing quarter wave plate stress is shown in  FIG.  7   . In the example of  FIG.  7   , surfaces S 6  and S 7  have a cylindrical curved shape (S 6  is convex and S 7  is concave so that these cylindrical shapes mate). Although quarter wave plate  28  of  FIG.  7    is curved, quarter wave plate  28  is only bent (curved) about a single axis (the Y axis) and is not bent about the X axis. As a result, quarter wave plate  28  does not have compound curvature that might impose undesired stresses on quarter wave plate  28 . For comparison,  FIGS.  8  and  9    show cross-sectional side views of lens elements  32  and  26  of  FIG.  7   .  FIG.  8    is a cross-sectional side view viewed along the Y axis. Quarter wave plate  28  is interposed between cylindrical surface S 6  of lens element  32  and cylindrical surface S 7  of lens element  26  and is bent about an axis parallel to the Y axis as shown in  FIG.  8   .  FIG.  9   , which is a cross-sectional side view of lens elements  32  and  26  of  FIG.  7    viewed along the X axis, shows how surfaces S 6  and S 7  do not bend about the X axis. Because surfaces S 6  and S 7  have this cylindrical shape, quarter wave plate  28  does not exhibit compound curvature and is not exposed to undesired amounts of stress so that a relatively uniform retardance is provided by the quarter wave plate  28  across the lens assembly. 
       FIG.  10    shows how lens elements such as lens element  54  may be formed by injection molding of plastic (polymer) or other material into mold  56 . Reflective polarizer  30  may be placed in mold  56  so that the reflective surface of reflective polarizer  30  bears against mold surface S 5 ′ as plastic is injected into the interior cavity of mold  56  to form lens element  54 . Mold surface S 5 ′ can be machined with high accuracy, so pressing reflective polarizer  30  against surface S 5 ′ during molding operations will help enhance the smoothness and accuracy of the reflective surface of reflective polarizer  30 . Similarly, if desired, reflective polarizer  30  can be formed by molding reflective polarizer  30  against opposing surface  58  during injection molding operations. 
     The lens elements used in optical system  20  may be relatively thin and formed of light-weight materials (e.g., plastic) and/or may be formed from materials such as glass. Reductions in weight may help provide user  46  with a comfortable viewing experience. It may be easier to mold the lens element(s) with uniform optical properties including low birefringence when lens elements such as element  54  have a uniform thickness. 
     As described in connection with  FIGS.  8  and  9   , quarter wave plate  28  may be interposed between lens elements  32  and  26  when elements  32  and  26  are bonded together (e.g., using adhesive layers on opposing sides of quarter wave plate  28 ). Surfaces S 6  and S 7  may be planar (e.g., element  32  may be a plano-concave element and element  26  may be a plano-convex element), as described in connection with  FIG.  6   , or surfaces S 6  and S 7  may be curved (see, e.g.,  FIG.  7   ). As described in connection with  FIGS.  8  and  9   , surfaces S 6  and S 7  can be cylindrical surfaces (surfaces bent around one axis). In this type of configuration, quarter wave plate  28  may bend along only one axis (e.g., quarter wave plate  28  may not have any compound curves), thereby reducing distortion in quarter wave plate  28  and helping to ensure that the retardation provided by quarter wave plate  28  is uniform. 
     During assembly of optical system  20 , a planar piece of quarter wave film may be placed between elements  32  and  26  with optical adhesive on either side of the quarter wave film. Elements  32  and  26  may then be forced together to distribute the adhesive and bend the quarter wave film about axis Y (an axis parallel to axis Y). Providing a cylindrically curved shape for surfaces S 6  and S 7  can enable the thickness of lens elements  32  and  26  to be reduced. The use of cylindrically curved shapes for surfaces S 6  and S 7  can help make for a more uniform thickness across the lens elements and thereby improve lens element moldability. When forming injection molded lens elements, uniformity of thickness in the mold cavity can help improve uniformity of flow of the molten plastic as it is being injected into the mold and the melt front flows across the mold cavity. The presence of a uniform flow during molding can be important for preventing flow lines in the molded lens, particularly when the lens element is thicker at the edge than the center. More uniform flow can also result in a lower birefringence in the molded lens elements. For catadioptric optical systems such as system  20 , low birefringence in the lens elements helps to maintain control of the polarization state of the image light, so that stray light and ghosts are reduced and so user  46  is thereby provided with a high contrast image without stray light artifacts. Moreover, the cylindrically curved shape of wave plate  28  in configurations of the type shown in  FIGS.  8  and  9    may help ensure that light rays in system  20  pass through quarter wave plate  28  with an angle that is closer to normal incidence than with planar wave plate configurations. As a result, the retardation provided by quarter wave plate  28  may be more uniform across the lens element and the image provided to user  46  may as a result be more uniform in contrast with fewer ghost artifacts. 
     In device  10 , image light is converted from unpolarized light to linearly polarized light, to circularly polarized light, then back to linearly polarized light, back to circularly polarized light and finally back to linearly polarized light. For the conversion from linearly polarized light to circularly polarized light to occur fully so that polarization ellipticity is reduced, it may be desirable for quarter wave plates  28  and  18  to be accurately oriented relative to the polarization axis of the polarizers. For example, it may be desirable to accurately orient the fast axis of the quarter wave plate  18  at 45 degrees to the polarization axis (pass axis) of linear polarizer  16  and the fast axis of quarter wave plate  28  at 45 degrees to the polarization axis (pass axis) of reflective polarizer  30 . The fast axes of the quarter wave plates may, for example, be oriented at 45 degrees to the polarization axes of the respective polarizers within +/−1.5 degrees or other suitable alignment tolerance. Accurate alignment of the quarter wave plates to the polarization axes of the polarizers helps ensure that light does not have a mixed polarization state (is not elliptically polarized). Accurate alignment therefore prevents portions of the image light from following unintended paths that form ghost images that degrade contrast and present stray light artifacts. 
     Linear polarizer  16  and quarter wave plate  18  may be aligned during lamination. For example, rolls of polarizer film and quarter wave film can be accurately aligned to one another in a rewinding process and laminated together with optically clear adhesive so that the alignment is maintained. The laminated polarizer/quarter wave film can then be attached to a substrate for mounting into the optical system or attached directly to a cover glass or other structure associated with pixel array  14 . Emissive displays such as organic light-emitting diode displays and light-emitting diode displays formed from arrays of crystalline semiconductor light-emitting diode dies may provide unpolarized image light so that attaching a laminated polarizer/quarter wave film to the pixel array allows the display system to emit circularly polarized light. 
     Quarter wave plate  28  can also be accurately aligned with reflective polarizer  30 . Reflective polarizer  30  can be formed in a curved shape (e.g., by thermoforming with heat and differential pressure or pressure forming), either directly to the concave surface of the lens element  32  or to a mold that matches the concave surface of the lens element  32  (e.g., mold  56  of  FIG.  10   ). Either the quarter wave plate  28  or reflective polarizer  30  can then be aligned during an assembly process in which two lens elements  32  and  54  are bonded with reflective polarizer  30  and curved quarter wave plate  28 . Either reflective polarizer  30  or the curved quarter wave plate  28  may, for example, be bonded to one of lens elements  32  or  54  and the remaining elements of system  20  may be oriented with a desired alignment accuracy. A polarimeter can be used to measure through optical system  20  during alignment to determine how much ellipticity is present and to use this information in guiding the alignment during assembly. In configurations of the type shown in  FIG.  6   , the interface between lens elements  32  and  26  is planar. In this type of configuration, quarter wave plate  28  can be bonded to the planar side of plano-convex lens element  26  and reflective polarizer  30  can be bonded to the concave side of plano-concave lens element  32 . Lens elements  32  and  26  can then be rotated relative to one another while polarization ellipticity is measured. Once it is determined that the quarter wave plate  28  is aligned satisfactorily to reflective polarizer  30 , the components of optical system  20  can be bonded together to preserve alignment. 
     If desired, dome optics (lens elements with dome-shaped surfaces) may be used to facilitate alignment of polarizer  30  and quarter wave plate  28 . For example, convex surface S 4  of element  54  and concave surface S 5  of element  32  may be dome shaped, allowing these dome lens elements to be rotated relative to each other during alignment operations. Quarter wave plate  28  may be bonded between lens elements  32  and  26 . Polarizer  30  may be formed on the surface of lens element  54 . Dome lens element  54  may then be bonded to surface S 5  of lens element  32  while aligning polarizer  30  and quarter wave plate  28 . Dome lens element  54  can be rotated as needed before bonding to lens element  32  while polarimetric measurements are made to assess alignment accuracy. If desired, reflective polarizer  30  can be molded to surface S 4  of lens element  54 , as described in connection with  FIG.  10    (e.g., using reflective polarizer  30  as an insert into mold  56 ). Applying pressure to the optical plastic for element  54  during molding forces reflective polarizer  30  against the wall of mold  56  during molding, so that the accuracy and smoothness of the reflective surface of reflective polarizer  30  (e.g., the outwardly facing surface of reflective polarizer  30 ) is determined by the accuracy and smoothness of the wall of mold  56 , which can be formed to optical specifications. After molding, the thickness of molded dome lens  54  (e.g. 1 to 3 mm) maintains the surface accuracy of the reflective surface of reflective polarizer  30  for ease of handling during assembly. The process of bonding dome lens  54  to the mating dome-shaped surface S 5  of lens element  32  (e.g. using liquid optically clear adhesive) can then be of sufficiently low force that the as-molded surface accuracy of the reflective surface of reflective polarizer  30  on surface S 4  of element  54  is not degraded. 
     As shown in  FIG.  6   , linear polarizer  34  may be formed on the eye side (concave surface S 8 ) of element  54  to help prevent spurious reflections of light from the environment. Linear polarizer  34  can be a separate layer, either flat or curved, positioned between optical system  20  and user&#39;s eye  46 . Linear polarizer  34  can also be attached to surface S 8  of lens element  54  (e.g., an inner dome lens surface) or can be laminated to reflective polarizer  30  before polarizer  30  is formed on surface  84  of lens element  54 . Linear polarizer  34  may be aligned relative to reflective polarizer  30  so the pass axes of the two polarizers are aligned. In this way, linear polarizer  34  absorbs light from the environment that has the polarization state that would be reflected by reflective polarizer  30 . Light from the environment that has the polarization state that is transmitted by both linear polarizer  34  and reflective polarizer  30  passes through quarter wave plate  28  and quarter wave plate  18  ending with a linear polarization state that is absorbed by linear polarizer  16 . This helps reduce stray light reflections, because reflective polarizer  30  reflects light of this polarization state with a high reflectivity, which has the potential to create distracting reflections of light entering device  10  from behind or to the side of user  46 . At the same time, linear polarizer  34  is aligned so the transmission axis of the linear polarizer  34  is parallel to the transmission axis of reflective polarizer  30  and thereby helps enable linear polarizer  34  to serve as a cleanup polarizer to improve the quality of images from pixel array  14  while reducing the brightness of the image light presented to the eye of the user by a relatively small amount (e.g., &lt;20% if linear polarizer  34  has a transmission of 40% and &lt;10% if linear polarizer  34  has a transmission of 45%). Linear polarizer  34  may be a high transmission polarizer with a transmission of at least 40%, at least 43%, or at least 45% compared to unpolarized light. 
     In an embodiment, quarter wave plates  28  in system  20  may be formed from multiple layers of retarder films laminated together. The layers of retarder films may be oriented at angles to one another so that together they act as a quarter wave plate with reduced variation in retardation as measured in waves over a broader spectral bandwidth, also known as an achromatic quarter wave. For example, the retardation of quarter wave plates  18  and/or  28  may be within +/−1.5° over a wavelength range of 450-650 nm). 
     A primer (e.g., an adhesion promoting polymer) may be applied to one or more surfaces of reflective polarizer  30  prior to insert molding of dome lens element  54 . This may help increase the bond strength between reflective polarizer  30  and dome lens element  54  after molding. 
     Reflective polarizer  30  may, if desired, have a substrate formed from a material such as polycarbonate or cyclic olefin that matches the thermal expansion of the lens elements in system  20  (e.g. acrylate or cyclic olefin lens elements), thereby reducing interfacial stress when optical system  20  is exposed to heat either from display system  40  or the environment. 
     If desired, lens element  26 , which is interposed between the other lens elements of system  20  and display system  40  may be made from glass (which may have lower thermal expansion and higher heat resistance capabilities than plastic) to help resist heat affects from display system  20 . In addition, a soft adhesive or an optical grease may be used to cement quarter wave plate  28  between lens elements  54  and  32  to enable some differential thermal expansion with reduced interfacial stress between the two lens elements and quarter wave plate  28 . 
     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: 20210115
Publication Date: 20230829
Grant Date: 20230829
Priority Date: 20160802
Inventors: KHAN, SAJJAD A.
ZHU, Nan
MYHRE, GRAHAM B.
BOLLMAN, BRENT J.
Anderson, Tyler
CHENG, Weibo
BORDER, JOHN N.
Assignee: APPLE INC
CPC Classifications: [{"code": "G02B17/0856", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B27/0018", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/0172", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/283", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/286", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G3/003", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B2027/0138", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B27/283", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B27/0172", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B17/0856", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B2027/0138", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B27/0172", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B17/0856", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/0018", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/283", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/286", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G3/003", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B2027/0138", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B27/283", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B2027/0138", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B27/0172", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/0018", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/286", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G3/003", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 61071438