PATENT DOCUMENT

Publication Number: US-11054646-B1
Application Number: US-201815886623-A
Country: US
Kind Code: B1

Title: Head-mounted display device with Fresnel lenses

Abstract:
A head-mounted device may include a display system and an optical system in a housing. The display system may have displays that produce images. The optical system may have Fresnel lenses through which a user of the head-mounted device may view the images. The Fresnel lenses may have concentric rings with slope facets and draft facets angled parallel to the chief rays. Light scattering in the Fresnel lenses may be reduced by coating the draft facets with opaque masking material and/or by aligning concentric rings of the opaque masking material that are supported on a transparent substrate with the draft facets. A central portion of the Fresnel lens that is free of facets may be enlarged to reduce scattering. The Fresnel lenses may have wedge-shaped cross-sectional profiles and may have outer portions that are thicker than inner portions. Gradient-index material may be used in forming the Fresnel lenses.

Claims:
What is claimed is: 
     
       1. A lens having a maximum lateral dimension with opposing first and second ends, comprising:
 a center dome portion; and 
 an outer Fresnel lens portion that radially surrounds the center dome portion and that has slope facets and draft facets, wherein the draft facets are coated with absorptive material, wherein the slope facets are free of the absorptive material, and wherein the center dome portion and the outer Fresnel lens portion have a wedge shaped cross-sectional profile in which the outer Fresnel lens portion has a first thickness at the first end of the maximum lateral dimension of the lens and a second thickness at the second end of the maximum lateral dimension of the lens, the first thickness being greater than the second thickness. 
 
     
     
       2. The lens defined in  claim 1  wherein the center dome portion and the Fresnel lens portion are formed from a gradient-index-of-refraction material. 
     
     
       3. The lens defined in  claim 2  wherein the draft facets have an angle that varies radially across the Fresnel lens. 
     
     
       4. The lens defined in  claim 1 , wherein the outer Fresnel lens portion comprises a first outer Fresnel lens portion that forms the first end of the maximum lateral dimension and a second outer Fresnel lens that forms the second end of the maximum lateral dimension, wherein the center dome portion is laterally interposed between the first and second outer Fresnel lens portions. 
     
     
       5. A device, comprising:
 a display including pixels configured to display images; 
 a Fresnel lens through which the images are viewable; and 
 a housing configured to support the display and the Fresnel lens, wherein the Fresnel lens has an inner Fresnel lens element and an outer Fresnel lens element, wherein the inner Fresnel lens element has first Fresnel lens rings, wherein the outer Fresnel lens element has second Fresnel lens rings, wherein the first Fresnel lens rings of the inner Fresnel lens element face the second Fresnel lens rings of the outer Fresnel lens element, wherein the inner Fresnel lens element has a concave surface opposite the first Fresnel lens rings, and wherein the outer Fresnel lens element has a convex surface opposite the second Fresnel lens rings. 
 
     
     
       6. The device defined in  claim 5  wherein the inner Fresnel lens element and the outer Fresnel lens element have an equal number of Fresnel lens rings. 
     
     
       7. The device defined in  claim 5  wherein the first Fresnel lens rings of the inner Fresnel lens element have slope facets that are free of absorptive masking material and have draft facets covered with absorptive masking material. 
     
     
       8. The device defined in  claim 5  wherein the first Fresnel lens rings of the inner Fresnel lens element have first draft facets with surfaces parallel to chief rays, and wherein the second Fresnel lens rings of the outer Fresnel lens element have second draft facets, each of the second draft facets being parallel to each other and non-parallel with respect to the first draft facets. 
     
     
       9. The device defined in  claim 5  wherein the Fresnel lens has a diameter, wherein the inner and outer Fresnel lens elements each have a central dome portion without Fresnel lens rings and an outer portion with Fresnel lens rings, and wherein the central dome portion of the inner and outer Fresnel lens elements has a diameter of at least 20% of the diameter of the lens. 
     
     
       10. The device defined in  claim 5  further comprising a field lens interposed between the display and the Fresnel lens.

Description:
This patent application claims the benefit of provisional patent application No. 62/504,729, filed on May 11, 2017, which is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     This relates generally to optical systems and, more particularly, to optical systems for head-mounted devices. 
     Head-mounted devices such as virtual reality glasses and augmented reality glasses use displays to generate images. The displays may be positioned close to a user&#39;s eyes, so lenses are typically placed between the displays and the user&#39;s eyes to bring the displays into focus. 
     If care is not taken, lenses for head-mounted devices may be bulkier and heavier than desired and may not exhibit satisfactory optical performance. Use of a head-mounted device with such lenses could be uncomfortable and tiring. 
     SUMMARY 
     A head-mounted device may include a display system and an optical system in a housing. The head-mounted device may form a virtual reality or augmented reality device. 
     The display system may have displays that produce images. The displays may be either planar or curved. The optical system may have Fresnel lenses through which a user of the head-mounted device may view the images. The Fresnel lenses may have curved convex surfaces that face the displays or may be planar. 
     The Fresnel lenses may have concentric rings with slope facets and draft facets. Light scattering in the Fresnel lenses may be reduced to improve image contrast by coating the draft facets with opaque masking material and/or by aligning concentric rings of the opaque masking material that are supported on a transparent substrate with the draft facets. 
     A central dome portion of the Fresnel lens that is free of facets may be enlarged to reduce scattering. The Fresnel lenses may have wedge-shaped cross-sectional profiles with outer portions that are thicker than inner portions. Gradient-index material may be used in forming the Fresnel lenses. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of an illustrative head-mounted device in accordance with an embodiment. 
         FIG. 2  is a cross-sectional side view of an illustrative Fresnel lens with draft facets that have been coated with opaque masking material in accordance with an embodiment. 
         FIG. 3  is a cross-sectional side view of an illustrative Fresnel lens and associated mask formed from concentric rings of opaque masking material aligned with draft facets in the Fresnel lens to prevent the draft facets from scattering light in accordance with an embodiment. 
         FIG. 4  is a cross-sectional side view of an illustrative Fresnel lens having draft facet surfaces that are aligned with and parallel to chief rays in a head-mounted device in accordance with an embodiment. 
         FIG. 5  is a cross-sectional side view of an illustrative Fresnel lens with a dome-shaped central region in accordance with an embodiment. 
         FIG. 6  is a cross-sectional side view of an illustrative gradient-index material of the type that may be used in forming lenses in accordance with an embodiment. 
         FIG. 7  is a cross-sectional side view of an illustrative Fresnel lens of the type that may be formed using the gradient-index material of  FIG. 6  in accordance with an embodiment. 
         FIG. 8  is a diagram of an illustrative Fresnel lens with two elements in accordance with an embodiment. 
         FIGS. 9 and 10  are diagrams showing illustrative field lenses used in conjunction with Fresnel lenses in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Head-mounted devices such as head-mounted displays may be used for virtual reality and augmented reality systems. For example, a pair of virtual reality glasses or other head-mounted device 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 device such as a pair of virtual reality glasses or other device is used in providing a user with virtual reality content or other content is shown in  FIG. 1 . As shown in  FIG. 1 , head-mounted display  10  may include a display system such as display system  40  that provides images comprised of image light and may have an optical system such as lens system  20  through which a user (see, e.g., user&#39;s eyes  46 ) may view the images produced by display system  40  in direction  48 . Lens system  20  may include fixed and/or tunable lenses for focusing the images on display  40  for viewing by the user. With one illustrative configuration, which is described herein as an example, lens system  20  includes Fresnel lenses. The Fresnel lenses may have low weight, small thicknesses, and other desirable properties relative to smooth surfaced lenses so that the bulk and weight of device  10  are reduced and viewer comfort is enhanced. 
     Display system  40  may be based one or more displays. Each display may be a liquid crystal display, an organic light-emitting diode display (e.g., a flexible organic light-emitting display that can be curved), a display having an array of crystalline semiconductor light-emitting diode dies on a curved or plane substrate, a liquid-crystal-on-silicon display, a microelectromechanical systems (MEMs) display, and/or displays based on other display technologies. Separate left and right displays may be included in system  40  in front of a user&#39;s left and right eyes, respectively, 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  40  using control circuitry  42  that is mounted in head-mounted device  10  and/or control circuitry that is mounted outside of head-mounted 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 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. 
     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, and sensors (e.g., 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). Devices  44  can include cameras (digital image sensors) for capturing images of the user&#39;s surroundings, cameras for performing gaze detection operations by viewing eyes  46 , and/or other cameras. 
     Optical system components such as left lens  40 L and right lens  40 R and display system components such as left display  40 L and right display  40 R for device  10  may be mounted in a housing such as housing  12 . Housing  12  may have the shape of a frame for a pair of glasses (e.g., head-mounted device  10  may resemble eyeglasses), may have the shape of a helmet (e.g., head-mounted device  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 optical 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 configurations. 
     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. 
     Displays  40 L and  40 R each include an array of pixels P for generating images. As shown in  FIG. 1 , Fresnel lenses  20 L and  20 R may have wedge shapes (wedge-shaped cross-sectional profiles in the horizontal X-Z plane of  FIG. 1 ) in which the outer portions of the lenses such as outer ends  21  near the opposing left and right sides of device  10  are thicker than inner portions of the lenses such as inner ends  22 . The wedge shape of lenses  20 L and  20 R may be produced by adding a prism to the rear of planar Fresnel lenses that do not have a wedge shape or may be provided by forming lenses  20 L and  20 R from unitary wedge-shaped structures. As a result of the wedge-shaped cross-sectional profile of lenses  20 L and  20 R, respective rays of light from displays  40 L and  40 R such as rays  49 L and  49 R bend towards the users&#39; eyes (e.g., into alignment with the Z axis) when propagating from pixels P to eyes  46 . During manual and/or automatic set-up operations, the lateral location (e.g., the locations along the X-axis of  FIG. 1 ) of lens  20 L and display  40 L relative to the lateral location of lens  20 R and display  40 R may be adjusted to accommodate a user&#39;s interpupillary distance. Because the rays of light from the pixel arrays of displays  40 L and  40 R are bent and directed towards the user&#39;s eyes by wedge-shaped Fresnel lenses  20 L and  20 R, respectively, ends  20  of lenses  20 L and  20 R can be placed close to each other (e.g., when needed to accommodate a small interpupillary distance for a user) and larger displays  40 L and  40 R can be used without undesired placement constraints that might be present in lenses that are located directly in front of a user&#39;s eyes that don&#39;t bend light rays from displays  40 L and  40 R. Fresnel lenses  20 L and  20 R with wedge-shaped cross-sectional profiles and/or planar Fresnel lenses for device  10  may have curved outer surfaces (e.g., convex surfaces facing displays  40 L and  40 R) that match corresponding curved shapes for displays  40 L and  40 R. Displays  40 L and  40 R may, for example, be formed from one or more flexible display panels that are bent to wrap around the front portion of the head of the user. 
     A cross-sectional side view of an illustrative Fresnel lens  20  (e.g., lens  20 L or lens  20 R) is shown in  FIG. 2 . As shown in  FIG. 2 , lens  20  may be characterized by slope facets  50  and draft facets  52 . Facets  50  and  52  are formed from the surfaces of ring-shaped Fresnel lens portions such as concentric Fresnel lens rings  24 . There may be any suitable number of rings  24  in lens  20  (e.g., 50, 10-100, at least 5, at least 10, at least 15, at least 20, at least 30, at least 45, at least 75, fewer than 200, fewer than 150, or other suitable number). Slope facets  50  have a curved lens-shaped profile and exhibit relatively small angles (e.g., less than 45°, less than 30°, less than 20°, 0-20°, 0-30°, 0-40°, at least 2°, etc.) with respect to lateral dimensions such as dimension  51  (e.g., a lateral dimension in a plane aligned with the surface of lens  20  that is facing display  40 ). Draft facets  52 , which may have straight cross-sectional profiles, typically have relatively small angles (e.g., 2°) with respect to vertical dimension  53  (e.g., a vertical dimension that is normal to the plane of the surface of lens  20  and that is therefore orthogonal to dimension  51  can be referred to as the optical axis of the lens). Because of these orientations, draft facets  52  may sometimes be referred to as vertically extending facets and slope facets  50  may sometimes be referred to as horizontally extending or laterally extending facets wherein the angle is typically &lt;45 degrees from the plane of the lens. 
     During operation, light propagates from display  40  through lens  20 . The rays of light from display  40  may make glancing contact with draft facets  52 , which can lead to undesirable light scattering, which decreases image contrast. To reduce light scattering and thereby enhance lens performance, an opaque masking material or other absorptive masking material  54  may be formed as a coating on some or all of draft facets  52  while leaving slope facets  50  uncovered by opaque masking material  54 . In this way, light passing through slope facets  50  may be focused by the shape of facets  50  and light incident on draft facets  52  may be absorbed by opaque material  54  without scattering from facets  52 . 
     Any suitable technique may be used for forming opaque masking material  54  on facets  52 . Material  54  may be, for example, a polymer with dark particles such as a black ink formed from a polymer binder and carbon black particles, other black pigment, and/or black dye. Other types of dyes and pigments and/or other binders may be used in forming opaque masking material  54 , if desired. Material  54  may also be formed from metal layers and/or other opaque masking structures. If desired, polymer binder for material  54  may be photosensitive to facilitate patterning by light exposure (photo-patterning). With photo-patterning, a blanket film of material  54  (e.g., positive photoresist) may be deposited over the surface of lens  20  that contains facets  52  and  50  followed by exposure to ultraviolet light (or other suitable light) and development. The light that is applied to the photosensitive material will be absorbed by the photosensitive material on slope facets  50  but will be self-shadowed by the very high incident angle of draft facets  52  relative to the light. As a result, the more exposed material on slope facets  50  will be removed following development and only the shadowed and therefore less exposed material (draft facet material  54  of  FIG. 2 ) will remain following development. 
     If desired, other fabrication techniques may be used to selectively apply opaque masking material  54  to draft facets  52 . For example, pad printing, needle dispensing, ink-jet printing, or other material application techniques may be used to selectively apply material  54  to facets  52 . Facets  52  and facets  50  may also be coated with a blanket layer of material  54  that is subsequently selectively removed from facets  50  (e.g., using laser removal techniques, machining techniques, chemical treatment, etc.). With another illustrative approach, electrostatic charge may be selectively written onto facets  52  using a charged electrode (e.g., a probe with a narrow tip or other electrode structure) or by selectively rubbing facets  52  to generate charge so that charged material  54  will selectively be attracted to facets  52  during electrostatic printing operations. Material can also be selectively removed from facets  50  by spraying or otherwise selectively applying a release coating onto facets  50  without coating facets  52 . With this type of configuration, a blanket layer of material  54  that is applied over the face of lens  20  can be selectively released from facets  50  to leave facets  52  covered with material  54 . 
     As shown in  FIG. 3 , light scattering from draft facets  52  may be reduced by blocking facets  52  from exposure to image light using patterned masking material  54  on a separate substrate such as substrate  58 . Patterned masking material  54  may, for example, form a series of concentric rings that are each aligned with a respective concentric ring-shaped draft facet  52  in lens  20 . Lens  20  and/or substrate  58  may be formed from transparent glass, clear plastic (e.g., transparent molded or machined plastic), and/or other transparent materials. Substrate  58  (and lens  20 ) may have planar shapes and/or substrate  58  and/or lens  20  may have curved surfaces (e.g., curved cross-sectional profile shapes that match a curved shape for a wrap-around display such as display  40  of  FIG. 1 ). Material  54  may be patterned by photolithography, shadow mask deposition, machining, laser patterning, selective printing, and/or other suitable mask formation techniques. When assembled into housing  12 , the mask structure formed from substrate  58  and patterned opaque masking material  54  may block light from display  40  that would otherwise strike draft facets  52 , thereby reducing light scattering in lens  20 . By positioning the mask structure relatively close to the user&#39;s eye  46  the patterned opaque masking material  54  will not degrade the image quality perceived by the user other than to slightly reduce the brightness of the image. 
       FIG. 4  shows an example of how draft facets  52  may be designed to reduce scattering and thereby increase image contrast. Image light provided by pixels P in display  40  (e.g., a left of right display) pass through lens  20  (e.g., a left or right Fresnel lens) and is viewed as an image by user&#39;s eye  46 , which is located in eyebox  47  wherein the image is viewable. Chief rays  62  comprise central rays within the cone of image light provided by each pixel and as such, the chief rays pass from the pixels P to the center of eyebox  47 . As the image light passes through a Fresnel lens, the slope facets provide the intended function of the lens by focusing the image light at the eyebox  47 . At the same time, image light can be scattered by the draft facets because the image light is defocused and redirected away from the eyebox  47 . In an embodiment, scattering of the image light by the draft facets  52  can be reduced by aligning at least some of the surfaces of draft facets  52  so that they are parallel to chief rays  62 . Because the angle of the chief rays varies radially from the optical axis of the lens, in a preferred embodiment the angle of the draft facets in lens  20  is varied radially in correspondence to the local angle of the chief ray, and as a result draft facets near the optical axis of the lens can have a small angle (e.g. 2 degree) while draft facets near the edge of the lens have a larger angle (e.g. at least 10 degrees). However, in some lens designs such as a freeform lens, the angle associated with the chief rays of the image light may vary nonlinearly with position across the lens and the draft facet angles will need to be varied in correspondence. 
     As shown in  FIG. 5 , lens  20  can be a hybrid lens, comprised of an outer Fresnel portion that includes concentric rings  24  with slope facets  50  and draft facets  52  that surround a smooth dome-shaped central portion that is free of rings and facets such as lens portion  28  where dome portion  28  is designed to provide a focal plane that is coplanar to the focal plane associated with the Fresnel portion that has concentric rings and facets. Typically this requires that the dome portion  28  have a different base curvature than the outer Fresnel portion as shown in  FIG. 5  where the dome portion  28  is more curved than the outer Fresnel portion where the concentric rings  24  are shown with a planar base curve. Because dome portion  28  is free of facets, light scattering may be reduced in dome portion  28  relative to the portions of lens  20  that contain rings  24 . To help reduce overall light scattering in lens  20 , the fraction of the diameter of lens  20  that is occupied by dome portion  28  can be enlarged (e.g., the ratio of dome portion diameter DC to lens diameter D) may be at least 20%, at least 30%, at least 40%, at least 50%, at least 60%, less than 80% or other suitable fraction. In a preferred embodiment, the dome portion of the lens and the Fresnel portion of the lens are designed to provide a smooth optical transition to insure that the viewing experience is uninterrupted as the user&#39;s eye moves to view different portions of the displayed image, where the smooth optical transition can be provided by matching field curvatures between the dome portion  28  and the Fresnel portion to within 0.5 diopters and matching distortion within 1% of each other. For configurations of Fresnel lens  20  that do not have circular outlines, diameter D may be taken as the largest lateral dimension of lens  20  and diameter DC may be taken as the largest lateral dimension of central dome portion  28 . Characterized in another way, the diameter of portion  28  may be at least 10 times, at least 20 times, fewer than 100 times, 5-100 times, or other suitable number of times greater than the width of the innermost of rings  24  and/or diameter DC may be configured to fall outside of an angular range of +/−300 with respect to a ray passing through the center of eyebox  47 . 
     The rings of lens  20  may have a constant height arrangement in which each of rings  24  has a common height H, may have a constant width arrangement in which each of rings  24  has a common width PT, and/or may have a configuration in which some or all of ring heights H and/or some or all of ring widths W differ from each other. Dome portion  28  may have a height that matches height H of rings  24  or that differs from that of rings  24  (e.g., the height of dome portion  28  may be greater than H). 
     If desired, a gradient-index material, comprised of material that has a laterally or radially varying refractive index, may be used in forming lens  20 . Consider, as an example, the arrangement of  FIG. 6 . As shown in  FIG. 6 , gradient-index-of-refraction material  70  may be formed from multiple bent layers of glass, plastic, and/or other transparent material such as layers  30 ,  32 ,  34 , and  36 . There may be any suitable number of layers (e.g., 2-100, at least 5, at least 25, at least 100, fewer than 500, fewer than 100, fewer than 30, etc.) in structure  70 . The example of  FIG. 6  is illustrative 
     The index of refraction of the layers in the structure of  FIG. 6  may decrease progressively as a function of distance outward in directions  93  from inner layer  30 . For example, layer  30  may have a refractive index of 1.5, layer  32  may have a refractive index of 1.45, layer  34  may have a refractive index of 1.40, and layer  36  may have a refractive index of 1.35 (as an example). Material  70  of  FIG. 6  may be formed by laminating multiple layers of material of different refractive indices together and bending these layers under heat and/or pressure (as an example). Lens  20  may then be formed from lower portion  38  of material  70  (e.g., by machining). A Fresnel lens  20  formed from gradient-index material  70  in this way is shown in  FIG. 7 . 
     As shown in  FIG. 7 , lens  20  may have central portions such as portions  72  and peripheral (edge) portions such as portions  74 . When a gradient-index material is used in forming lens  20 , central portion  72  will have a higher index of refraction (e.g., n=1.5) than peripheral portions  74  (e.g., n=1.35). The gradient-index material may also be comprised of alternate layers of at least two materials with different refractive index, where the at least two materials can be birefringent so that the refractive index through the thickness of the layer is different from the refractive index within the layer, and the layers are oriented at different angles across a lateral dimension of the lens  20 . Between portions  72  and  74 , the index of refraction of lens  20  may vary progressively as described in connection with portion  38  of gradient-index material  70  of  FIG. 6 . Index discontinuities can be minimized by using numerous thin layers in forming gradient-index material  70 . The use of gradient-index material  70  in forming Fresnel lens  20  of  FIG. 7  allows the strength of the Fresnel structures in lens  20  (e.g., the slope of the slope facets and therefore the heights H of rings  24  of a given width) to be reduced relative to a Fresnel lens formed from a material with a uniform index of refraction. This helps allow lens thickness to be reduced, allows draft facet scattering to be reduced by widening rings  24  for a given height H, etc. Gradient-index configurations for lens  20  may be used for planar Fresnel lens shapes, curved Fresnel lens shapes, wedge-shaped Fresnel lenses (with curved and/or planar outer surfaces facing display  40 ), and/or other suitable Fresnel lenses for device  10 . Non-Fresnel lenses  20  that are formed from gradient-index material  70  may also be used in device  10 , if desired. 
       FIG. 8  is a diagram of an illustrative configuration for lens  20  in which lens  20  has two elements: inner lens  20 - 1  and outer lens  20 - 2 . Lens  20 - 1  and/or lens  20 - 2  may be Fresnel lenses and/or other suitable lenses. In the example of  FIG. 8 , lens  20 - 1  has Fresnel lens rings (facets)  24  on outer (outwardly facing) surface SF 2 , but not on inner (inwardly facing) surface SFL. If desired, both surfaces SF 1  and SF 2  or only surface SF 1  may have Fresnel lens facets. In the example of  FIG. 8 , lens  20 - 2  has Fresnel lens rings (facets)  24  on inner (inwardly facing) surface SF 3 , but not on outward (outwardly facing) surface SF 4 . If desired, both surfaces SF 3  and SF 4  or only surface SF 4  may have Fresnel lens facets. Lens  20  may also have one or more additional lens elements (e.g., one-sided and/or two-sided Fresnel lenses, lenses with smooth surfaces, etc.), as indicated by dots  80 . 
     To help minimize light scattering, lens  20  may be configured so that the light that illuminates a given Fresnel lens ring  24  on lens  24 - 2  evenly illuminates a corresponding Fresnel lens ring  24  on lens  24 - 1 , as illustrated by rays  82 . With this type of arrangement, there are an equal number of Fresnel lens rings (facets) on outer lens  20 - 2  and inner lens  20 - 1 . This one-to-one correspondence between the Fresnel lens rings on lens  20 - 1  and the Fresnel lens rings on lens  20 - 2  (the equal number of Fresnel rings on these two lens elements) helps reduce scattering and enhance lens performance. 
     Draft facets  52  in lens  20 - 1  may have surfaces that are aligned with and parallel to chief rays as described in connection with  FIG. 4 . In an illustrative embodiment, draft facets  52  in lens  20 - 2  are horizontal (parallel to the Z axis) to facilitate release of lens  20 - 2  (e.g., a polymer lens) from a mold. Other configurations for the facets of lenses  20 - 1  and  20 - 2  may be used, if desired. 
     Lens  20  may be a single element lens or a multielement lens. Multielement lenses may, for example, have inner and outer lens elements as shown in  FIG. 8 . Two-element lenses such as these may sometimes be referred to as two-element objectives and may be formed using any suitable objective configuration (e.g., a Petzval objective configuration with two positive lenses, a telephoto objective configuration with a positive inner lens and negative outer lens, or a reverse telephoto objective configuration with a negative inner lens and a positive outer lens). The center smooth portion of the Fresnel lens surface used in lens  20  may be relatively large as described in connection with  FIG. 5 . Draft facet masking techniques may also be used in lens  20  to help reduce scattering. As the example of  FIG. 8  demonstrates, lens  20  may have a meniscus Fresnel lens design (e.g., with a hybrid Fresnel configuration having an enlarged smooth center portion) that has an inner lens with Fresnel lens structures on its convex surface and an outer lens with Fresnel lens structures on an opposing concave surface. Other arrangements may be used for lens  20 , if desired. 
     As shown in  FIGS. 9 and 10 , device  10  may have lenses such as lens  20 ′ of  FIG. 9  that each include a lens  20  (e.g., a single-element objective or a multi-element objective) and that each include a field lens (e.g., field lens  20 F) interposed between display  40  and lens  20 . Display  40  may be flat or curved. In the  FIG. 9  example, field lens  20 F is a biconvex lens. In the  FIG. 10  example, field lens  20 F of lens  20 ′ is a plano-convex lens that is attached to the planar front surface of display  20 . Other field lens configurations may be used, if desired. 
     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: 20180201
Publication Date: 20210706
Grant Date: 20210706
Priority Date: 20170511
Inventors: CHAN, VICTORIA C.
BORDER, JOHN N.
OLSON, JEFFREY C.
PETROV, YURY A.
HUO, EDWARD S.
CLARKE, BRANDON
Assignee: APPLE INC
CPC Classifications: [{"code": "G02B2027/0123", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B3/08", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/0172", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B2027/0178", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06T19/006", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B27/0172", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B27/0955", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06T19/006", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B27/0955", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/0172", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B2027/0178", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 76658182