Patent Publication Number: US-11029521-B2

Title: Head-mounted device with an adjustable opacity system

Description:
This application claims the benefit of provisional patent application No. 62/662,099, filed Apr. 24, 2018, which is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     This relates generally to devices with displays, and, more particularly, to head-mounted displays. 
     Head-mounted displays may be used to display virtual reality and augmented reality content. A head-mounted display that is displaying augmented reality content may overlay computer-generated images on real-world objects. If care is not taken, the computer-generated images may be difficult to see against the real-world objects, real-world objects may distract a viewer, or other issues may arise with displayed content. 
     SUMMARY 
     An electronic device such as a head-mounted device may have a transparent display. The transparent display may be formed from a display panel that provides images to a user through an optical coupler. A user may view real-world objects through the optical coupler while control circuitry directs the transparent display to display computer-generated content over selected portions of the real-world objects. 
     The head-mounted display may include an adjustable opacity system. The adjustable opacity system may include an adjustable opacity layer such as a photochromic layer that overlaps the optical coupler and a light source that selectively exposes the adjustable opacity layer to light to control the opacity of the adjustable opacity layer. The light source may emit ultraviolet light to control the adjustable opacity layer. The adjustable opacity layer may block or dim light from the real-world objects to allow improved contrast when displaying computer-generated content over the real-world objects. 
     The light source for the photochromic layer may share an optical coupler with a display unit that generates images for viewing by the user. Alternatively, the light source may emit light into a first optical coupler that redirects the light towards selected portions of the photochromic layer, whereas the display unit may emit display light into a second optical coupler that redirects the display light towards the viewer. A heating element may be positioned adjacent the adjustable opacity layer to heat the adjustable opacity layer. The optical coupler and adjustable opacity layer may be interposed between first and second filter layers that block light from the light source for the adjustable opacity system. Ultraviolet light absorbing material may also be included in the head-mounted device to prevent stray ultraviolet light from reaching the user&#39;s eyes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an illustrative head-mounted device in accordance with an embodiment. 
         FIG. 2  is a diagram of an illustrative transparent display with a tunable lens and a partially reflective element that serves as an optical coupler to direct images from one or more non-transparent display panels to a user in accordance with an embodiment. 
         FIGS. 3 and 4  are diagrams showing how a portion of a user&#39;s field of view may be modified by increasing opacity and/or overlaying computer-generated content in different regions of the field of view in accordance with an embodiment. 
         FIG. 5  is a diagram of an illustrative adjustable opacity system where a light source selectively exposes each light modulating pixel of a photochromic layer to light to control the opacity of each light modulating pixel in accordance with an embodiment. 
         FIG. 6  is a diagram of an illustrative head-mounted device showing how an optical coupler may be used to both direct display light to a user and direct light to a photochromic layer of an adjustable opacity system in accordance with an embodiment. 
         FIG. 7  is a top view of an illustrative head-mounted device with support structures that support an optical coupler and a photochromic layer for an adjustable opacity system in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Head-mounted devices and other devices may be used for virtual reality and augmented reality systems. These devices may include portable consumer electronics (e.g., portable electronic devices such as cellular telephones, tablet computers, glasses, other wearable equipment), head-up displays in cockpits, vehicles, etc., display-based equipment (projectors, televisions, etc.). Devices such as these may include transparent displays and other optical components. Device configurations in which virtual reality and/or augmented reality content is provided to a user with a head-mounted display are described herein as an example. This is, however, merely illustrative. Any suitable equipment may be used in providing a user with virtual reality and/or augmented reality content. 
     A head-mounted device such as a pair of augmented reality glasses that is worn on the head of a user may be used to provide a user with computer-generated content that is overlaid on top of real-world content. The real-world content may be viewed directly by a user (e.g., by observing real-world objects through a transparent display panel or through an optical coupler in a transparent display system that merges light from real-world objects with light from a display panel). Configurations in which images or real-world objects are captured by a forward-facing camera and displayed for a user on a display may also be used. 
     A schematic diagram of an illustrative head-mounted device is shown in  FIG. 1 . As shown in  FIG. 1 , head-mounted device  10  (sometimes referred to as head-mounted display  10 ) may have control circuitry  50 . Control circuitry  50  may include storage and processing circuitry for controlling the operation of head-mounted device  10 . Circuitry  50  may include storage such as hard disk drive storage, nonvolatile memory (e.g., electrically-programmable-read-only memory configured to form a solid state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Processing circuitry in control circuitry  50  may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors, power management units, audio chips, graphics processing units, application specific integrated circuits, and other integrated circuits. Software code may be stored on storage in circuitry  50  and run on processing circuitry in circuitry  50  to implement control operations for head-mounted device  10  (e.g., data gathering operations, operations involving the adjustment of components using control signals, etc.). 
     Head-mounted device  10  may include input-output circuitry  52 . Input-output circuitry  52  may be used to allow data to be received by head-mounted device  10  from external equipment (e.g., a tethered computer, a portable device such as a handheld device or laptop computer, or other electrical equipment) and to allow a user to provide head-mounted device  10  with user input. Input-output circuitry  52  may also be used to gather information on the environment in which head-mounted device  10  is operating. Output components in circuitry  52  may allow head-mounted device  10  to provide a user with output and may be used to communicate with external electrical equipment. 
     As shown in  FIG. 1 , input-output circuitry  52  may include a display such as display  26 . Display  26  may be used to display images for a user of head-mounted device  10 . Display  26  may be a transparent display so that a user may observe real-world objects through the display while computer-generated content is overlaid on top of the real-world objects by presenting computer-generated images on the display. A transparent display may be formed from a transparent pixel array (e.g., a transparent organic light-emitting diode display panel) or may be formed by a display device that provides images to a user through a beam splitter, holographic coupler, or other optical coupler (e.g., a display device such as a liquid crystal on silicon display). 
     The head-mounted device may include adjustable components stacked in series with display  26 . For example, the head-mounted device may include an adjustable polarizer (e.g., a polarizer with switches that allow selected regions of the adjustable polarizer to be configured to serve as vertical-pass linear polarizers, horizontal-pass linear polarizers, or non-polarizing regions), tunable lenses (e.g., liquid crystal tunable lenses, tunable lenses based on electrooptic materials, tunable liquid lenses, microelectromechanical systems tunable lenses, or other tunable lenses), and/or an adjustable color filter (e.g., an adjustable-color-cast light filter that can be adjusted to exhibit different color casts and/or a monochromatic adjustable-intensity light filter that has a single color cast). 
     Adjustable opacity system  20  may also be incorporated into head-mounted device  10 . The adjustable opacity system may include an adjustable opacity layer that is stacked in series with display  26  so that the adjustable opacity layer overlaps display  26  and so that the user may view real-world objects through the adjustable opacity layer and display  26 . The adjustable opacity system may be adjusted in real time using control signals from control circuitry  50 . 
     Adjustable opacity system  20  may be a photochromic light modulating device. 
     Adjustable opacity system  20  may include a light source that selectively exposes a photochromic layer to light. The photochromic layer (sometimes referred to as an adjustable opacity layer) may be controlled globally or may have an array of individually adjustable light modulator regions (sometimes referred to as light modulator pixels) that are adjusted between a transparent state and an opaque state. In the transparent state, transmission may be 100% or nearly 100% (e.g., greater than 99%, greater than 95%, etc.). In the opaque state, transmission is 0% or nearly 0% (e.g., less than 1%, less than 5%, etc.). Intermediate levels of light transmission (e.g., transmission values between 0% and 100%) may also be selectively produced by each of the pixels of the adjustable opacity layer. 
     There may be any suitable number of display pixels in display  26  and adjustable light modulator pixels in adjustable opacity system  20  (e.g., 0-1000, 10-10,000, 1000-1,000,000, 1,000,000 to 10,000,000, more than 1,000,000, fewer than 1,000,000, fewer than 10,000, fewer than 100, etc.). 
     Input-output circuitry  52  may include components such as input-output devices  60  for gathering data and user input and for supplying a user with output. Devices  60  may include a gaze-tracker such as gaze-tracker  62  (sometimes referred to as a gaze-tracking system or a gaze-tracking camera) and a camera such as camera  64 . 
     Gaze-tracker  62  may include a camera and/or other gaze-tracking system components (e.g., light sources that emit beams of light so that reflections of the beams from a user&#39;s eyes may be detected) to monitor the user&#39;s eyes. Gaze-tracker(s)  62  may face a user&#39;s eyes and may track a user&#39;s gaze. A camera in the gaze-tracking system may determine the location of a user&#39;s eyes (e.g., the centers of the user&#39;s pupils), may determine the direction in which the user&#39;s eyes are oriented (the direction of the user&#39;s gaze), may determine the user&#39;s pupil size (e.g., so that light modulation and/or other optical parameters and/or the amount of gradualness with which one or more of these parameters is spatially adjusted and/or the area in which one or more of these optical parameters is adjusted based on the pupil size), may be used in monitoring the current focus of the lenses in the user&#39;s eyes (e.g., whether the user is focusing in the near field or far field, which may be used to assess whether a user is day dreaming or is thinking strategically or tactically), and/or other gaze information. Cameras in the gaze-tracking system may sometimes be referred to as inward-facing cameras, gaze detection cameras, eye tracking cameras, gaze-tracking cameras, or eye monitoring cameras. If desired, other types of image sensors (e.g., infrared and/or visible light-emitting diodes and light detectors, etc.) may also be used in monitoring a user&#39;s gaze. The use of a gaze detection camera in gaze-tracker  62  is merely illustrative. 
     Cameras such as front-facing camera(s)  64  may be used to capture images of the real-world environment surrounding the user. For example, one or more front-facing cameras  64  may be used to capture images of real-world objects in front of a user and on the left and right sides of a user&#39;s field of view. The images of real-world objects that are gathered in this way may be presented for the user on display  26  and/or may be processed by control circuitry  50  to determine the locations of electronic devices (e.g., displays, etc.), people, buildings, and other real-world objects relative to the user. The real-world environment may also be analyzed using image processing algorithms. Information from camera  64  may be used in adjusting optical components such as adjustable opacity system  20  and controlling display  26 . 
     As an example, control circuitry  50  can identify the location of a real-world object such as a door to a building and can automatically overlay computer-generated content (e.g., a text label) on the door. As another example, control circuitry  50  may identify regions of the user&#39;s field of view that contain sources of glare. Control circuitry  50  may then adjust appropriate light modulator pixels in adjustable opacity system  20  to prevent the glare from reaching the eyes of the user. 
     In addition to adjusting adjustable components such as display  26  and adjustable opacity system  20  based on information from gaze-tracker  62  and/or front-facing cameras  64 , control circuitry  50  may gather sensor data and user input from other input-output circuitry  52  to use in controlling head-mounted device  10 . As shown in  FIG. 1 , input-output devices  60  may include position and motion sensors  66  (e.g., compasses, gyroscopes, accelerometers, and/or other devices for monitoring the location, orientation, and movement of head-mounted device  10 , satellite navigation system circuitry such as Global Positioning System circuitry for monitoring user location, etc.). Using sensors  66 , for example, control circuitry  50  can monitor the current direction in which a user&#39;s head is oriented relative to the surrounding environment. Movements of the user&#39;s head (e.g., motion to the left and/or right to track on-screen objects and/or to view additional real-world objects) may also be monitored using sensors  66 . 
     Light detectors  68  may include ambient light sensors that measure ambient light intensity and/or ambient light color. Input-output devices  60  may also include other sensors and input-output components  70  (e.g., force sensors, temperature sensors, touch sensors, buttons, capacitive proximity sensors, light-based proximity sensors, other proximity sensors, strain gauges, gas sensors, pressure sensors, moisture sensors, magnetic sensors, microphones, speakers, audio components, haptic output devices, light-emitting diodes, other light sources, etc.). Circuitry  52  may include wired and wireless communications circuitry  74  that allows head-mounted device  10  (e.g., control circuitry  50 ) to communicate with external equipment (e.g., remote controls, joysticks and other input controllers, portable electronic devices, computers, displays, etc.) and that allows signals to be conveyed between components (circuitry) at different locations in head-mounted device  10 . 
       FIG. 2  is a diagram showing how display  26  may be a transparent display that has a display unit  26 U that creates images that are reflected toward the eyes  12  of the user by optical coupler  26 C. Display unit  26 U may include, for example, a liquid crystal on silicon display and/or any other type of display with a pixel array that displays images. Display unit  26 U may include light-emitting diodes or other light sources for illuminating the images produced by a liquid crystal on silicon display (as an example). In some configurations, images may be produced by multiple display devices in display unit  26 U (e.g., multiple images that are combined using an optical combiner in display unit  26 U). 
     Optical coupler  26 C may be a beam splitter, a holographic coupler, a partially reflective element such as a partially reflective mirror, or other optical coupler. Optical coupler  26 C may be placed in front of the user&#39;s eyes  12  and may be partially transparent, so that the user can view external objects such as real-world object  30  through optical coupler  26 C. During operation, light from an array of display pixels in display unit  26 U such as light  82  may be directed to optical coupler  26 C. A waveguide, holographic coupling element, and/or other structures in coupler  26 C may direct light  82  towards user eyes  12 . Light  80  from real-world object  30  may also pass through the beam splitter or other coupling structures in optical coupler  26 C to the user&#39;s eyes  12 . In this way, the user may view both real-world content and overlaid images (e.g., computer-generated images) from display unit  26 U, creating an augmented reality environment. 
     Display  26  may include fixed and/or tunable lenses, as illustrated by lens  26 L. These lenses, which may include reflective elements, transparent lens elements, and/or other lens structures, may be dynamically adjusted during operation of head-mounted device  10  to place computer-generated images from display unit  26 U at multiple different focal planes using time-division multiplexing, thereby enhancing the realism of the user&#39;s augmented reality environment. Images may also be placed at multiple different focal planes by combining images from multiple different display devices in unit  26 U using a beam splitter or other optical combiner. 
     If desired, the functions of lens  26 L and/or display unit  26 U may be combined with optical coupler  26 C. As an example, optical coupler  26 C may have built-in lenses (e.g., embedded lens power) that work in combination with or replace lens  26 L in  FIG. 2 . In another example, optical coupler  26 C may include transparent display pixels (e.g., an array of transparent organic light-emitting diode display pixels) that generate the display light  82 . 
     In some situations (e.g., direct or indirect sunlight), real-world light  80  may have high brightness levels. In these situations, it may be difficult to generate display light  82  that has sufficient contrast with real-world light  80  (e.g., so that the computer-generated images on the display do not appear dim) without risking viewer discomfort or eye damage from high light intensity levels. Therefore, to reduce the brightness of the light from real-world objects the head-mounted device may include adjustable opacity system  20  (shown in  FIGS. 5-7 ). The adjustable opacity system may enable local opacity control in the head-mounted device (e.g., the adjustable opacity system has a plurality of individually controllable adjustable opacity pixels). 
       FIGS. 3 and 4  represent a user&#39;s field of view and show how the adjustable opacity system may be used to form different transparent and opaque regions in the display. As shown in  FIG. 3 , rectangle  90  represents a user&#39;s field of view. The entire field of view may be provided with light  80  (e.g., corresponding to real-world object  30 ). The field of view may have a first region  92  and second and third regions  94 - 1  and  94 - 2 , as shown in  FIG. 3 . These regions may be overlapped by display  26  (e.g., the display of  FIG. 2 ) so that computer-generated content can be displayed for the user. The regions are also overlapped by adjustable opacity system  20  so that different regions may be provided with different opacities. 
     As an example, an adjustable opacity layer of adjustable opacity system  20  may be configured to be transparent in region  92  (sometimes referred to as transparent region  92 ) of  FIG. 3  and to be opaque in regions  94 - 1  and  94 - 2  (sometimes collectively referred to as opaque region  94 ) of  FIG. 3 . As shown in  FIG. 3 , area  94 - 1  is located in the corner of field of view  90  whereas area  94 - 2  is located in a more central portion of field of view  90  (where area  94 - 2  is surrounded by area  92 ). When region  92  is transparent and regions  94 - 1  and  94 - 2  are opaque, the real world will be visible in region  92  and regions  94 - 1  and  94 - 2  will be black in the absence of displayed content from display  26 . This type of arrangement may be used, for example, to block objectionable or distracting content in the user&#39;s field of view from the user. Full or partial reductions in light transmission through selected areas of field of view  90  such as areas  94 - 1  and  94 - 2  may also be used to block glare from lights, glare from reflections, glare from the sun, or other sources of distracting and unwanted light. In  FIG. 3 , for example, area  94 - 1  may block glare from the sun whereas area  94 - 2  may block glare from reflections. If desired, display  26  may be used to display notifications, computer-generated content such as still and/or moving images corresponding to a game or other application, and/or other visual content in region  94 - 1  and/or  94 - 2 . For example, region  94 - 1  may block glare from the sun and computer-generated content may be displayed in region  94 - 2 . The dark background created by making region  94 - 2  opaque may help the user view display output in region  94 - 2  because the light associated with images in region  94 - 2  will not need to compete with light from underlying real-world objects. Display content may also be presented in region  92  or display  26  may not display any content in region  92 . Opaque region  94  may also be used to provide computer-generated content with black coloring. If the entire field-of-view is transparent, the real-world light provided to the field-of-view may prevent content with black coloring from being displayed. By making portions of the field-of-view opaque with the adjustable opacity system, displaying computer-generated content with black coloring becomes possible. 
     The size, shapes, and locations of the boundaries of regions  92 ,  94 - 1 , and  94 - 2  may be updated in real time by control circuitry  50  using information from input-output circuitry  52  (e.g., using information on the position of object  30  from front-facing camera  64  and other sensors in devices  60 , based on information from gaze detection system  62 , based on the orientation of the head-mounted device  10 , etc.). 
     In the example of  FIG. 4 , region  94  surrounds region  92 . Region  94  may be darkened (e.g., rendered partly or fully opaque using the adjustable opacity system) while region  92  is made completely transparent (or at least more transparent than region  94 ). This type of arrangement may be used, for example, when using head-mounted device  10  to highlight an object in the user&#39;s field of view. Real-world objects in region  94  may be blocked from the user&#39;s view. Transparent region  92  of  FIG. 4  may be dynamically aligned with object  30  ( FIG. 2 ). For example, control circuitry  50  may use image data gathered with front-facing camera  64  to detect the location of object  30  and, based on this information, may darken appropriate pixels of the adjustable opacity layer to visually highlight object  30  and to block out distracting peripheral objects. This may be done automatically or may be done only in the presence of bright light detected by ambient light sensor  68 . Computer-generated content (e.g., pre-recorded video, a still image, graphics, solid colors or patterns, computer-generated images for a game or other application, etc.) may be overlaid over darkened region  94  or region  94  may be left black (or at least partly darkened). 
     The size, shapes, and locations of the boundaries of regions  92  and  94  may be updated in real time by control circuitry  50  using information from input-output circuitry  52  (e.g., using information on the position of object  30  from front-facing camera  64  and other sensors in devices  60 , based on information from gaze detection system  62 , based on the orientation of the head-mounted device  10 , etc.). 
     As the foregoing examples demonstrate, region  90  (the user&#39;s field of view or part of the user&#39;s field of view) may be subdivided into multiple subregions such as regions  92  and  94 . There are two and three subregions in the examples of  FIGS. 3 and 4  respectively, but more subregions may be created if desired (e.g.,  2 - 10 , more than 2, more than 5, fewer than 20, etc.). In each subregion, control circuitry  50  can create abrupt and/or smoothly graded changes in light transmission. For example, in each of multiple portions of the user&#39;s field of view, control circuitry  50  can create a different light transmission (e.g., a visible light transmission T that is set to an adjustable value between 0% and 100%) using individually adjustable pixels of adjustable opacity system  20 . In each of these regions, display pixels of display  26  may be inactive and may display no content or may be active and used in displaying images. Adjustments may be made based on orientation, eye behavior, detected attributes of real-world objects, sensor input, user commands, or other parameters. 
     Consider, as another example, a scenario in which control circuitry  50  uses gaze-tracking system  62  to monitor the user&#39;s eyes. An eye tracking system may, as an example, monitor the location (e.g., the plane) at which the user&#39;s eyes  12  are focused in real time. In response to detection that eyes  12  are focused on display  26 , control circuitry  50  can enhance the opacity of adjustable opacity system  20  (e.g., the adjustable opacity layer can be made opaque), thereby enhancing the visibility of content on display  26  and blocking out real-world objects behind display  26 . In response to detection that eyes  12  are focused at a distance (e.g., at infinity or at another distance that is farther away from the user&#39;s eyes  12  than display  26  or the apparent position of display  26 ), control circuitry  50  can be configured to enhance the transparency of adjustable opacity system  20  (e.g., the adjustable opacity layer can be made transparent), thereby enhancing the visibility of real-world objects through display  26  and allowing pixels in display  26  to optionally be used to display computer-generated content over real-world objects that are visible through display  26 . 
     If desired, control circuitry  50  can be configured to adjust adjustable opacity system  20  to be transparent during a normal operating mode (so that object  30  can be viewed through display  26 ) and to be opaque in all but a subset of region  90  (e.g., to be transparent in region  92  of  FIG. 4  while being opaque in region  94  of  FIG. 4 ) during a highlighting mode, thereby allowing real-world objects that are aligned with region  92  to be visible and blocking real-world objects that are not overlapped by region  92 . This type of highlighting mode may be invoked in response to user input, in response to detection of an object of interest for highlighting, in response to orientation information, in response to detection of sources of glare outside of region  92  (e.g., using front-facing camera  64 ), in response to detection of a particular operating mode for display  10  (e.g., in response to running of a particular application or other software code on control circuitry  50 ), or in response to satisfaction of other criteria. 
     In some situations, it may be desirable to exclude background objects (e.g., by making one or more subregions such as region  94  opaque while making region  92  transparent). This type of arrangement may be use to remove glare, to block undesired distractions (text, moving objects, and/or other visual clutter) from view, etc. Background object exclusion operations can be performed automatically by control circuitry  50  based on information gathered by front-facing camera  64  and based on other information gathered by input-output devices  60 . 
     There are many possible arrangements for adjustable opacity system  20 . For example, adjustable opacity system  20  may be a cholesteric liquid crystal layer, may be a light modulator based on a switchable metal hydride film (e.g., an adjustable magnesium hydride mirror structure), may be a suspended particle device, may be an electrochromic light modulating device, may be a guest-host liquid crystal light modulator, or may be any other suitable light modulator layer for adjusting light transmission. 
       FIG. 5  is a diagram of one possible arrangement for adjustable opacity system  20 . In the embodiment of  FIG. 5 , adjustable opacity system  20  includes a photochromic layer such as photochromic layer  100  (sometimes referred to as adjustable opacity layer  100 ). Photochromic layer  100  is formed from a photochromic material that changes from transparent to opaque when exposed to light of a particular wavelength (e.g., ultraviolet light). Light source  102  emits light  110  that is selectively directed towards photochromic layer  100 . The photochromic layer  100  has a plurality of adjustable opacity pixels  20 P that each can be individually controlled to be either transparent or opaque (by exposing or not exposing each pixel to light from the light source. 
     In some cases, light source  102  may emit light that is projected directly on photochromic layer  100 . In other embodiments, the light source may emit light  110  that is redirected by a microelectromechanical (MEMS) mirror array such as MEMS mirror array  104 . MEMS mirror array  104  may be a digital micromirror device (DMD) with an array of mirrors that can be individually rotated to direct light from light source  102  to a desired location. In one illustrative example, each mirror of the mirror array may correspond to a respective adjustable opacity pixel  20 P in the photochromic layer  100 . An adjustable opacity pixel  20 P may be placed in a transparent state by having the corresponding mirror in MEMS mirror array  104  direct light from the light source towards absorber  106  (where the light is absorbed). Because the adjustable opacity pixel is not exposed to the light from the light source, the adjustable opacity pixel remains transparent (e.g., the adjustable opacity pixels are transparent in the absence of light from the light source). In contrast, an adjustable opacity pixel  20 P may be placed in an opaque state by having the corresponding mirror in MEMS mirror array  104  direct light from the light source towards the adjustable opacity pixel. When the adjustable opacity pixel is exposed to the light from the light source, the adjustable opacity pixel becomes opaque (e.g., at least more opaque than when the adjustable opacity pixel is not exposed to the light from the light source). The length of time the adjustable opacity pixel is exposed to the light from light source  102 , the wavelength of the light from the light source, and/or the intensity of the light from the light source may be adjusted to control the opacity of each adjustable opacity pixel. 
     The adjustable opacity system may also include an absorber  108  that absorbs light  110  that reflects off of the photochromic layer (or other stray light from light source  102 ). In this way, absorber  108  may prevent user eyes  12  from being exposed to the light. Absorbers  106  and  108  may be formed from any desired material that absorbs light from light source  102  (e.g., absorbs more than 70% of the light, more than 80% of the light, more than 90% of the light, more than 95% of the light, more than 99% of the light, etc.). For example, the absorbers may be formed from black ink or any other desired material. The absorbers may be formed as a coating on other device components if desired. 
     Light source  102  may emit any desired type of light. Light source  102  may emit ultraviolet light (e.g., light with a wavelength less than 380 nanometers), visible light (e.g., light with a wavelength between 380 and 750 nanometers), infrared light (e.g., light with a wavelength greater than 750 nanometers), or light of any other desired type that can control the opacity of photochromic layer  100 . Light source  102  may have built-in lenses and/or collimating optics. Additionally, optical components may be incorporated at other desired locations in the optical path between light source  102  and photochromic layer  100  (e.g., between light source  102  and MEMS mirror array  104 ). Other components and functionalities (e.g., the MEMS mirror array  104  and absorber  106 ) may be incorporated into the light source if desired. 
     Photochromic layer  100  may be formed from any desired photochromic material. In one example, the photochromic layer may include silver chloride (AgCl). When not exposed to ultraviolet light, the silver chloride may be transparent. When exposed to ultraviolet light of sufficient intensity, the silver chloride changes to a shape that absorbs visible light and is therefore opaque. This transition is reversible. Therefore, each pixel in the photochromic layer may be repeatedly switched between the transparent state and opaque state as desired. 
     As previously mentioned in connection with  FIG. 2 , an optical coupler ( 26 C) may be used to provide display light from a display unit to the user. If desired, the light (from light source  102 ) for controlling photochromic layer  100  may also be directed using optical coupler  26 C.  FIG. 6  is a diagram showing how an optical coupler may be used by both the display unit and the light source for the adjustable opacity system. The diagram of  FIG. 6  includes components for one of the user&#39;s eyes. Device  10  may use these components to provide images to both of a user&#39;s eyes or may contain two sets of such components to present images to both of a user&#39;s eyes. 
     As shown in  FIG. 6 , device  10  may include a display unit such as display  26 U for producing image light  82 . Image light  82  may be generated by illuminating a reflective display containing an array of pixels, as an example. Display  26 U may be a liquid-crystal-on-silicon display, a microelectromechanical systems (MEMs) display, or other type of display. The images presented on the array of pixels may be conveyed to input coupler region  112  which couples image light  82  into optical coupler  26 C (e.g., a waveguide). The image light coupled into waveguide  26 C is confined within waveguide  26 C in accordance with the principal of total internal reflection and travels towards output coupler region  114  as indicated by arrow  116 . Output coupler  114  couples light  82  (image light) out of waveguide  26 C and towards viewer  12  (an eye of a user) as output light. Additionally, light  80  from real-world object  30  may pass through waveguide  26 C to be viewed by viewer  12 . In this way, display unit  26 U may generate images that are viewed by the viewer and that overlap the real-world scene. 
     Light source  102  of adjustable opacity system  20  may also emit light ( 110 ) that is conveyed to input coupler  112 . Input coupler  112  couples the light from light source  102  into waveguide  26 C. The light from light source  102  may be confined within waveguide  26 C in accordance with the principal of total internal reflection and travel towards output coupler  114  as indicated by arrow  116 . Output coupler  114  may then couple light  110  out of waveguide  26 C and towards photochromic layer  100 . The output coupler  114  may selectively output the light from light source  102  to a desired adjustable opacity pixel  20 P (to selectively render each adjustable opacity pixel opaque or transparent). 
     Input coupler  112  and output coupler  114  may include, for example, structures such as Bragg gratings that couple light into waveguides and that couple light out of the waveguides. Couplers  112  and  114  may be formed from volume holograms or other holographic coupling elements (e.g., thin layers of polymers and/or other optical coupler structures in which holographic patterns are recorded using lasers). Each of the Bragg gratings or holographic coupling elements may be selective to a particular wavelength (e.g., will only effect light of the particular wavelength). For example, a first set of holographic coupling elements may effect ultraviolet light from light source  102  and a second set of holographic coupling elements may effect visible light from display unit  26 U. Couplers  112  and  114  may have infinite focal lengths (e.g., couplers  112  and  114  may be plane-to-plane couplers) or may have associated finite focal lengths. For example, optical coupler  112  and/or  114  can be powered (e.g., coupler  112  and/or  114  can be configured to form a lens of a desired finite focal length). 
     Light source  102  and display unit  26 U may share optical coupler  26 C (e.g., light source  102  and display unit  26 U may both emit light into optical coupler  26 C). In one embodiment where display unit  26 U and the ultraviolet light source  102  share the optical coupler, the display unit  26 U and light source  102  are formed separately. In this embodiment, the display unit  26 U may alternately emit red light, green light, and blue light (as an example). In another embodiment, however, the display unit  26 U and light source  102  may be combined into a single light source. For example display unit  26 U may alternately emit red light, green light, blue light, and ultraviolet light (or another wavelength of interest for the photochromic layer). In yet another embodiment, light source  102  and display unit  26 U may each have respective optical couplers (e.g., optical coupler  26 C in  FIG. 6  may represent one waveguide for directing light from light source  102  to photochromic layer  100  and another waveguide for directing light from display unit  26 U to the viewer). 
     To optimize the performance of head-mounted device  10 , it may be desirable for photochromic layer  100  to transition between opaque and transparent states as quickly as possible. In other words, it is desirable to reduce latency associated with the control of adjustable opacity pixels  20 P. Therefore, head-mounted device  10  may include a heating element  120  that is used to heat photochromic layer  100 . Heating element  120  may be formed from an ohmic heater (sometimes referred to as a resistive heater) that heats upon application of current, may be formed from a Peltier effect heating element, and/or may be formed from other heating structures. The heating element may be formed from a layer of indium tin oxide (ITO) that is sufficiently thin to be transparent to incoming light  80  from real-world objects. 
     During operation, control circuitry  50  may use heating element  120  to heat photochromic layer  100 . The photochromic layer may be more responsive when heated (e.g., the photochromic layer will switch between transparent and opaque states in a shorter length of time than if the photochromic layer was not heated). Heating element  120  may heat photochromic layer  100  globally (e.g., so that the photochromic layer has a uniform temperature or approximately uniform temperature across the photochromic layer). Alternatively, the heating element may have a number of separate and individually controllable portions that are each heated to desired temperatures (such that different portions of the photochromic layer can be heated to different temperatures). A temperature sensor (that measures the temperature of one or more portions of heating element  120  and/or photochromic layer  100 ) may be used in controlling heating element  120  if desired. 
     The components of head-mounted device  10  may be supported by a head-mountable support structure such as illustrative support structure  16  of  FIG. 7 . Support structure  16  may have the shape of the frame of a pair of glasses (e.g., left and right temples and other frame members), may have a helmet shape, or may have another head-mountable configuration. When worn on the head of a user, the user may view real-world objects such as object  30  through components such as filter layer  132 , substrate  134 , optical coupler  26 C, adjustable opacity layer  100 , heating element  120 , substrate  138 , and filter layer  140 . Filter layer  132 , substrate  134 , optical coupler  26 C, adjustable opacity layer  100 , substrate  136 , substrate  138 , and filter layer  140  may all be supported by support structure  16  and may be placed in front of user eyes  12  when worn on the head of the user. 
     Support structure  16  may support additional components at additional locations such as locations  38 ,  40 , and  42 . For example, components may be mounted on the front of support structure  16  in location  38 . Front-facing cameras  64  and/or sensors and other components in input-output circuitry may be mounted in location  38 . The components in location  38  may be used to detect the positions of real-world objects (e.g., object  30 ) and/or for capturing images of the real-world. Object  30  may include natural and manmade objects, people, buildings, sources of glare such as reflective objects, the sun, lights, etc. 
     Input-output devices  60  such as position and motion sensors  66 , light detectors  68 , or other desired input-output devices may be mounted in location  40 . Components in location  40  may face the environment of the user (e.g., outward facing components facing away from the user) whereas components in location  42  may face the user (e.g., inward facing components facing the user). Input-output devices  60  such as gaze-tracker  62  (image sensors), speakers (e.g., ear speakers) or other audio components that play audio (e.g., audio associated with computer-generated images and/or other content that is being displayed using display  26 , etc.) or other desired input-output devices may be mounted in location  42 . 
     Optical coupler  26 C may be interposed between the user&#39;s eyes  12  and adjustable opacity layer  100 . With this arrangement, the adjustable opacity layer is able to provide a dark background for display light provided to the user from optical coupler  26 C. Adjustable opacity layer  100  may be interposed between optical coupler  26 C and heating element  120 . 
     If desired, substrates such as substrates  134  and  138  may be provided in head-mounted device  10 . As shown in  FIG. 7 , optical coupler  26 C, photochromic layer  100 , and heating element  120  may be interposed between substrates  134  and  138 . Substrates  134  and  138  may be formed from glass or any other desired transparent material. Head-mounted device  10  may also include filter layers  132  and  140 . Filter layers  132  and  140  may serve to filter (e.g., block) light of a desired wavelength. In particular, filter layers  132  and  140  may be used to filter light of the wavelength that is used to control photochromic layer  100  (e.g., ultraviolet light  110  in  FIG. 6 ). To prevent the ultraviolet light from reaching the user&#39;s eyes  12  or others around the user, the photochromic layer  100  and optical coupler  26 C are interposed between ultraviolet filter layers  132  and  140  (e.g., filter layers  132  and  140  may transmit less than 5% of ultraviolet light, less than 1% of ultraviolet light, etc.). If a different type of light than ultraviolet light is used for the light source for the adjustable opacity system (e.g., infrared light), the filter layers  132  and  140  may filter that type of light. If desired, the filtering properties of filter layers  132  and  140  may be combined with another layer (e.g., filter layers  132  and  140  may be omitted and substrates  134  and  138  may have filtering properties). 
     Additional components of adjustable opacity system  20  and display  26  may be mounted on (supported by) support structure  16 . For example, light source  102  and display unit  26 U may be mounted on support structure  16 . Light source  102  and display unit  26 U may emit light into an input coupler region of optical coupler  26 C, for example. MEMS mirror array  104  and absorber  106  may also be supported by support structure  16 . As shown in  FIG. 7 , absorbing material for absorbing excess ultraviolet light from light source  102  may be coated on additional components in the head-mounted device. Coating  108  in  FIG. 7  coats the edge of optical coupler  26 C. This coating may absorb ultraviolet light (or other light form light source  102 ) and may prevent ultraviolet light from exiting the optical coupler and reaching the user&#39;s eyes. 
     If desired, additional lenses may be incorporated at any desired location within head-mounted device  10 . For example, additional lenses may be incorporated in the optical path of display light from display unit  26 U. Additional lenses may also be incorporated in the optical path of ultraviolet light from light source  102 . 
     The example of  FIG. 7 , where waveguide  26 C is used to both provide display light to the user and ultraviolet light to the photochromic layer, is merely illustrative. As previously mentioned, support structure  16  may instead support one waveguide for providing display light to the user and a second, separate waveguide for providing ultraviolet light to the photochromic layer. 
     Additionally, the examples of  FIGS. 5-7  where photochromic layer  100  is planar are merely illustrative. In general, the photochromic layer (as well as the optical coupler and any other layers in the head-mounted device) may have any desired shape. For example, the photochromic layer may be curved along one or more axes 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.