Patent Publication Number: US-2021191142-A1

Title: Electronic Devices With Optical Markers

Description:
This application is a continuation of U.S. patent application Ser. No. 16/143,812, filed Sep. 27, 2018, which claims the benefit of provisional patent application No. 62/640,495, filed Mar. 8, 2018, which are hereby incorporated by reference herein in their entireties. 
    
    
     BACKGROUND 
     This relates generally to electronic devices, and, more particularly, to the optical characteristics of electronic devices. 
     Mixed reality systems have head-wearable devices that display computer-generated content overlaid on real-world content. A user may have a cellular telephone or other electronic devices in addition to a head-wearable device. It can be difficult or impossible for mixed reality headsets to recognize the presence of cellular telephones and other such devices in the user&#39;s field of view. This may make it difficult or impossible to provide a user with mixed-reality features that take advantage of the presence of such electronic devices. In some situations, the visual appearance of cellular telephones and other electronic devices may not be satisfactory. 
     SUMMARY 
     An electronic device may be provided with optically distinguishable markers. A marker may be formed from a patterned coating or other structure with predefined optical characteristics. The marker may be visible in visible light illumination and/or may be detectable using infrared and/or ultraviolet light sensors. By processing sensor readings such as captured images that contain the markers, information on the electronic device can be obtained. For example, analysis of images containing the markers in a system may reveal information on device type, device location, device size, device orientation, and other information on a marked device. 
     Markers can be formed from coating layers such as thin-film interference coating layers, photoluminescent coating layers, and/or retroreflective coating layers. A marker may be patterned to form a two-dimensional bar code, may be patterned to form an outline or other recognizable marker structure, and/or may be used in forming other recognizable marker structures to help provide information about an electronic device. 
     In some configurations such as in mixed reality systems, a device with a sensor such as a depth sensor or other sensor may gather information on the markers of an electronic device. The device with the sensor can use this information in performing functions such as overlaying computer-generated content on real-world images that include marked devices. The information that is gathered may include images captured with an image sensor in a depth sensor while the electronic device is illuminated by one or more light beams from the depth sensor or other light sources. In this type of arrangement, markers may be configured to serve as mixed reality markers. 
     If desired, markers may be configured to highlight portions of an electronic device such as electronic device buttons (e.g., by placing the markers on movable button members or touch sensor button locations), may be configured to provide moving animated graphics (e.g., in lighting environments with pulsed light at different wavelengths), and/or may otherwise be used in providing electronic devices with desired optical properties. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an illustrative system with electronic devices in accordance with an embodiment. 
         FIG. 2  is a schematic diagram of an illustrative electronic device in accordance with an embodiment. 
         FIG. 3  is a diagram of an illustrative sensor in an electronic device in accordance with an embodiment. 
         FIG. 4  is a cross-sectional side view of an electronic device in accordance with an embodiment. 
         FIG. 5  is a cross-sectional side view of a layer of material made of one or more sublayers that may be formed on a structure in an electronic device in accordance with an embodiment. 
         FIG. 6  is a cross-sectional side view of an illustrative layer of material with particles in accordance with an embodiment. 
         FIGS. 7 and 8  are cross-sectional side views of illustrative layers of material with retroreflective surface properties in accordance with embodiments. 
         FIG. 9  is a graph showing illustrative light intensity profiles associated with light in an electronic device system of the type shown in  FIG. 1 . 
         FIG. 10  is a graph showing how light intensity may vary as a function of angle in a system with a retroreflective surface in accordance with an embodiment. 
         FIG. 11  is a perspective view of an illustrative electronic device with markers in accordance with an embodiment. 
         FIG. 12  is a top view of a corner portion of an electronic device with peripheral markers in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     A user may use one or more electronic devices. These electronic devices may include cellular telephones, accessories, and other devices. The user may also have a mixed reality head-mounted device. The head-mounted device may have cameras and other sensors that gather information on the location of real-world objects. The head-mounted device may use this information to overlay computer-generated content on top of real-world content. 
     To ensure accurate recognition of the presence of cellular telephones and other electronic devices in the field-of-view of a head-mounted device in a mixed reality system, cellular telephones and other electronic devices may be provided with optically distinguishable markers (sometimes referred to as optical markers or visual markers). The markers can be formed from patterned material that facilitates detection of the position of marked devices within the user&#39;s environment and/or that helps provide other information (e.g., device identifier information). 
     If desired, markers may be formed from material that exhibits recognizable spectral responses (sometimes referred to as spectral reflectance codes), retroreflective materials, photoluminescent material, thin-film interference filter layers, and other materials that help a head-mounted device or other device with sensors to gather information on electronic devices in the user&#39;s environment. Spectral reflectance codes formed in markers and other marker attributes may be used as device type identifiers, may convey information on the location, shape, orientation, and size of a device, or may be used to convey other information. 
     Markers may be formed from coatings or other structures. Markers may form coded patterns, patterns that serve to outline a device, and/or other patterns. Thin-film interference filter layers, layers of retroreflective material, thin-film interference filters, colored materials, and/or other types of structures may be used to highlight buttons and other device structures. Information on markers that is detected by an electronic device such as a head-mounted device may be used in registering computer-generated content to real-world content (e.g., presenting overlays of computer-generated content that are aligned with all or part of a cellular telephone or other device), may be used in identifying which accessories and/or other devices a user has available for use in a system, may be used in coordinating the operation of multiple electronic devices (e.g., to display content across multiple devices, to use one device as a master and one as a slave, to support system functions that depend on the relative orientation of multiple devices, etc.), and/or may be used to facilitate the implementation of other enhanced functionality in a system with multiple electronic devices. 
     An illustrative system that includes multiple electronic devices is shown in  FIG. 1 . As shown in  FIG. 1 , system  8  includes electronic devices such as electronic devices  10 . System  8  may, for example, include two or more devices  10 , three or more devices  10 , five or more devices  10 , fewer than 100 devices  10 , fewer than 25 devices  10 , fewer than 10 devices  10 , and/or other suitable number of devices  10 . Devices  10  may include one or more devices  10 A that acquire information on surrounding devices and one or more devices  10 B that have markers to facilitate the acquisition of such information by devices  10 A. As one example, device  10 A may be a head-mounted device that has a camera, structured light depth sensor, and other sensors for acquiring data in system  8  and device  10 B may be a cellular telephone, or other device in system  8  that has optical markers (e.g., optical mixed reality system markers) that help device  10 A identify and locate device  10 B relative to device  10 A. In this type of environment, device  10 A may provide a user with mixed reality content and system  8  may sometimes be referred to as a mixed reality system. 
     In general, devices  10 A and  10 B may be any suitable devices such as cellular telephones, electronic wrist watch devices, tablet computers, desktop computers, laptop computers, televisions, electronic devices with displays and other component that are part of furniture, vehicles or other embedded systems, wearable devices such as wrist bands, head bands, clothing, hats, head-mounted devices (e.g., glasses, googles, helmets, etc.), wireless pencils for providing a tablet computer with input, computer mice, keyboards, ear buds, and/or other accessories, and/or other electronic equipment. If desired, a device  10  may include both optical makers for enhancing recognition by other devices and sensors for recognizing such devices (e.g., the features of devices  10 A and  10 B need not be mutually exclusive). For example, a cellular telephone, head-mounted display, or other electronic device may include both markers for facilitating detection by sensors and may include sensors for performing detection operations. 
     As shown in  FIG. 1 , device  10 B may be illuminated by light  40 . Light  40  may be produced by devices such as device  10 A (e.g., light emitted from a sensor such as a depth sensor) and/or may be produced by one or more other light sources  11 . Light sources  11  may include the sun for producing natural light and/or may include artificial light sources (e.g., fluorescent lights, incandescent lights, visible light sources such as visible-light light-emitting diodes, halogen lights, and/or other indoor and/or outdoor artificial visible light sources). In some configurations, light sources  11  may include light sources (e.g., light-emitting diodes, lasers, lamps, etc.) that produce non-visible light (e.g., infrared light at one or more wavelengths and/or ultraviolet light at one or more wavelengths). In some configurations, light sources  11  may produce pulsed light at one or more desired wavelengths in one or more desired pulse patterns. In general, light sources  11  and/or light sources in devices  10  may include any one or more sources of artificial and/or natural light at one or more wavelengths (infrared, visible, and/or ultraviolet). 
     Light  40  that is emitted by external light sources  11  may be emitted in direction  42  to illuminate device  10 B. Light  40  that is emitted by light sources in device  10 A may be emitted in direction  44  to illuminate device  10 B. Device  10 B may have optical markers (markers that are recognizable optically under infrared light, visible light, and/or ultraviolet light illumination) that reflect light  40  (e.g., as specular reflections, retro-reflected light, and/or diffusely scattered light), Reflected light  40 ′ may be detected by sensors in device  10 A. If desired, device  10 B may also include light sources (light-emitting diodes, lasers, lamps, etc.) that produce light for detection by device  10 A (e.g., visible, infrared, and/or ultraviolet light). 
     To enhance the appearance of devices  10  (e.g., to enhance the appearance of a device such as device  10 B to the naked eye of a user), it may be desirable to provide devices  10  with patterned structures that are visible to the user and that provide devices  10  with a desired appearance. As an example, trim structures in devices  10 , buttons (e.g., movable button members or touch sensitive button locations), housing structures, and/or other structures may be provided with patterned structures. These patterned structures, which may sometimes be referred to as visible-light markers, may help distinguish particular buttons from each other, may help highlight the outline of a device or component, and/or may provide other information about devices  10  and/or the use of devices  10 . Markers may also be used as bar codes or other optical codes (e.g., two-dimensional matrix bar codes such as Quick Response Codes or other codes), alignment features (e.g., to inform device  10 A of the shape and position of device  10 B in system  8 ), etc. Markers such as these may operate at visible wavelengths and/or other wavelengths. For example, markers may operate at infrared wavelengths (while being visible or invisible to a user&#39;s naked eye at visible wavelengths), may operate at ultraviolet wavelengths, may operate at multiple wavelengths, etc. 
       FIG. 2  is a schematic diagram of an illustrative electronic device. Devices such as illustrative device  10  of  FIG. 2  may be used as device  10 A and/or device  10 B of  FIG. 1 . As shown in  FIG. 2 , electronic device  10  may have control circuitry  30 . Control circuitry  30  may include storage and processing circuitry such as processing circuitry associated with microprocessors, power management units, baseband processors, digital signal processors, microcontrollers, and/or application-specific integrated circuits with processing circuits. Control circuitry  30  may implement desired control and communications features in device  10 . For example, control circuitry  30  may be used in gathering data, processing gathered data, and taking suitable actions based on the gathered data. Control circuitry  30  may be configured to perform these operations using hardware (e.g., dedicated hardware or circuitry), firmware, and/or software. Software code for performing these activities may be stored on non-transitory computer readable storage media (e.g., tangible computer readable storage media). The software code may sometimes be referred to as software, data, program instructions, instructions, or code. The non-transitory computer readable storage media may include non-volatile memory such as non-volatile random-access memory (NVRAM), one or more hard drives (e.g., magnetic drives or solid state drives), one or more removable flash drives or other removable media, etc. Software stored on non-transitory computer readable storage media may be executed on the processing circuitry of control circuitry  30 . The processing circuitry may include application-specific integrated circuits with processing circuitry, one or more microprocessors, or other processing circuitry. 
     Device  10  may have communications circuitry  32 . Communications circuitry  32  may include wired communications circuitry (e.g., circuitry for transmitting and/or receiving digital and/or analog signals via a port associated with a connector) and may include wireless communications circuitry (e.g., radio-frequency transceivers and antennas) for supporting communications with wireless equipment. Wireless communications circuitry in circuitry  32  may include wireless local area network circuitry (e.g., WiFi® circuitry), cellular telephone transceiver circuitry, satellite positioning system receiver circuitry (e.g., a Global Positioning System receiver for determining location, velocity, etc.), near-field communications circuitry and/or other wireless communications circuitry. Using communications circuitry  32 , devices  10  in system  8  may communicate with each other (e.g., so that one device can gather input that is transmitted to another device to control that device). 
     Device  10  may use input-output devices  34  to receive input from a user and the operating environment of device  10  and to provide output. Input-output devices  34  may include one or more visual output devices such as display  14  (e.g., a liquid crystal display, an organic light-emitting diode display, or other display). Input-output devices  34  may also include buttons, joysticks, scrolling wheels, touch pads, key pads, keyboards, microphones, speakers, displays (e.g., touch screen displays), tone generators, vibrators (e.g., piezoelectric vibrating components, etc.), sensors, light-emitting diodes and other status indicators, data ports, etc. Sensors  36  in input-output devices  34  may include force sensors, touch sensors, gesture sensors, capacitive proximity sensors, optical proximity sensors, ambient light sensors, temperature sensors, air pressure sensors, gas sensors, particulate sensors, magnetic sensors, motion and orientation sensors (e.g., inertial measurement units based on one or more sensors such as accelerometer, gyroscopes, and magnetometers), strain gauges, sensors that include image sensors (cameras) operating at infrared, visible, and/or ultraviolet wavelengths, depth sensors (e.g., structured light depth sensors that emit beams of light in a grid or other pattern and that have image sensors to measure resulting spots of light produced on target objects), sensors that gather three-dimensional depth information using a pair of stereoscopic sensors, lidar (light detection and ranging) sensors, radar sensors, radio-frequency sensors (e.g., millimeter wave systems) for gathering two-dimensional or three-dimensional millimeter wave images of a user&#39;s surroundings, etc. 
       FIG. 3  is a diagram of an illustrative light-based sensor such as a depth sensor. As shown in  FIG. 3 , sensor  36  may include an image sensor or other light detector (see, e.g., two-dimensional digital image sensor  36 B). Image sensor  36 B may operate at infrared, visible, and/or ultraviolet wavelengths. Sensors such as image sensor  36 B can capture images of electronic devices, including any markers present on such electronic devices and/or can capture images of other items in a user&#39;s surroundings. In some situations, image sensor(s)  36 B can capture images of objects (e.g., device  10 B) that have been illuminated using blanket natural or artificial illumination from light sources  11 . If desired, image sensors  36 B can capture images of objects (e.g., device  10 B) that are illuminated by output from a camera flash or other light source in device  10 A. 
     In some illustrative configurations, sensor  36  (e.g., a sensor in device  10 A) may contain a light source that emits beams of collimated light (sometimes referred to as structured light) from light source  36 A. Light source  36 A may, as an example, contain a two-dimensional array of infrared lasers (or lasers operating at visible and/or ultraviolet light wavelengths). This two-dimensional array of lasers or other structured light source may emit an array of light beams (e.g., beams of light  40 ). Sensor  36 B can capture and process images that contain the spots formed when these light beams (structured light) illuminate markers on device  10 B and/or other portions of device  10 B and the objects surrounding device  10 B. By incorporating markers into device  10 B, the process of determining the three-dimensional shape of device  10 B and the position of device  10 B relative to device  10 A can be facilitated. In general, any suitable sensors  36  may be provided in devices  10 . The use of a structured light depth sensor is illustrative. 
       FIG. 4  is a cross-sectional side view of a portion of an illustrative device such as device  10  of  FIG. 2 . As shown in  FIG. 4 , device  10  may have a display such as display  14 . Display  14  may, if desired, be formed on one face of device  10  (e.g., a front side) and a housing structure such as layer  24  may be formed on an opposing face (e.g., a rear side). Other configurations for device  10  may be used, if desired. Device  10  may have sidewall portions such as portions  12 . These sidewall portions, the rear housing, and, if desired, other housing portions of the electronic device housing for device  10  may be formed from a material such metal, integral portions of front and/or rear glass layers, polymer, ceramic, other materials and/or combinations of these materials. 
     Display  14  may include display cover layer  16  (e.g., a layer of glass or clear polymer) and display module  18  (e.g., display layers that form an array of pixels that present images for a user of device  10 ). Display module  18  may be a liquid crystal display structure, an organic light-emitting diode display structure, an array of micro-light-emitting diodes, or other suitable display. Display  14  (e.g., cover layer  16 ) and the housing structures of device  10  such as housing  12  and rear layer  24  form an exterior surface for device  10  on which makers can be formed). During operation, module  18 , which may sometimes be referred to as a pixel array, may present images that are viewable through display cover layer  16 . Internal components in device  10  such as components  22  (e.g., electrical components such as integrated circuits, sensors, etc.) may be mounted on one or more substrates such as printed circuit  20  in the interior of device  10 . Device  10  may have a housing shaped to allow device  10  to be worn by a user (e.g., housing structures that are configured to be worn on a user&#39;s head or other body part), may have a housing with a rectangular outline (e.g., in configurations in which device  10  is a cellular telephone), and/or may have other suitable housings. 
     Markers may be formed from patterned coating layers or may be more deeply embedded into portions of devices  10 . If desired, markers may be formed from portions of housing structures by incorporating marker materials into an injection molded polymer housing structure, by embedding polymer filler material into holes, grooves, or other openings formed in a metal housing structure, by adding particles or other materials to glass or ceramic housing structures, or by otherwise patterning materials to form optical marker structures as part of an electronic device housing. In some configurations (e.g., when forming markers from coating layers), markers may be formed on the inner and/or outer surface of a substrate layer. As an example, transparent housing layers, transparent display layers, transparent window layers, and/or other transparent members in device  10  may have inner surfaces that are covered with patterned layers that form markers that are visible at an external surface of device  10  through the layers. External coatings can also be formed. Markers may be formed on functional members (e.g., button members that form part of buttons) and/or may be formed on other portions of device  10  (e.g., external surfaces such as those associated with device housing). Illustrative configurations in which markers for devices  10  are formed from coating layer(s) may sometimes be described herein as an example. 
     Coatings may be formed from metals, semiconductors, and/or dielectrics. Dielectric materials for the coatings may include organic materials such as polymer layers and/or inorganic materials such as oxide layers (e.g., silicon oxide, metal oxides such as aluminum oxide, etc.), nitride layers, and/or other inorganic dielectric materials. If desired, a coating may be formed from a thin-film interference filter layer (sometimes referred to as a dichroic layer). Photoluminescent structures, retroreflective structures, and/or other structures that impart markers with desired optical properties may also be used. In arrangements in which a shiny appearance is desired, a metal coating with a high reflectivity or a thin-film interference filter with dielectric layers (e.g., a stack of dielectric layers of alternating higher and lower refractive index values) may be configured to serve as a mirror coating (reflective coating). 
     An illustrative coating layer of the type that may be used in forming markers in devices  10  is shown in  FIG. 5 . As shown in  FIG. 5 , coating layer  62  may be supported by a structure such as structure  60 . Structure  60  may be a housing structure (e.g., a housing wall with an externally viewable surface), a transparent member (e.g., a transparent housing structure or a transparent display cover layer that has externally viewable and/or internally viewable surfaces), and/or other structures in device  10  (sometimes referred to as support layers, support structures, substrates, etc.). Layer  62  may be formed on the interior and/or exterior surfaces of structure  60 . Layer  62  may have one or more sublayers  62 ′. 
     In some configurations, the bulk properties of the one or more sublayers  62 ′ in layer  62  influence the optical properties of layer  62 . For example, layer  62  may include colored material (e.g., polymer containing dye and/or pigment) or material that exhibits photoluminescence (photoluminescent dye, phosphors, etc.). This material may be deposited in thin (near a wavelength of light) or thick layers and may be used to adjust light transmission, reflection, and absorption for layer  62 . 
     In other configurations, the thin-film interference properties of layers  62 ′ are used to provide coating layer  62  with desired optical properties. Layer  62  may, as an example, include N sublayers  62 ′ (where N is at least 3, at least 5, at least 7, at least 10, at least 20, at least 40, less than 200, less than 100, or other suitable number) that are each relatively thin (e.g., less than 10 microns, less than 3 microns, at least 0.05 microns, etc.) and that collectively form a thin-film interference filter with desired optical properties such as a desired reflection, transmission, and absorption across appropriate portions of the infrared, visible, and ultraviolet spectrums. 
     Coating layers  62 ′ may include dielectric layers (e.g., inorganic dielectric layers such as silicon oxide, silicon nitride, titanium oxide, tantalum oxide, zirconium oxide, aluminum oxide, etc. and/or polymer layers). In thin-film interference filter configurations for layer  62 , layers  62 ′ may have desired refractive index values such as alternating high and low indices of refraction (e.g., to form a thin-film interference mirror, to form a thin-film interference filter with higher reflectivity at infrared wavelengths than at visible light wavelengths, etc.). If desired, metal layers, semiconductor layers, and/or other layers of material may be incorporated into layer  62 . 
     As shown in  FIG. 6 , one or more of layers  62 ′ may be formed from material  64  with embedded particles  66 . Particles  66  may include light-scattering particles (e.g., materials with a refractive index of at least 1.8, at least 1.9, at least 2.0, less than 2.1, or other suitable materials that exhibit high scattering when embedded in materials of lower refractive index). Material  64  may be a dielectric such as a polymer or an inorganic material with a refractive index different than the refractive index of particles  66 . In some configurations, particles  66  may be formed from hollow microspheres, particles of low refractive index material, and/or other low-index structures. 
     In some configurations, particles  66  may be photoluminescent and may emit light at one or more selected wavelengths in response to application of pump light (e.g., light  40  at one or more wavelengths that are shorter than the emitted wavelengths). Particles  66  may also form reflective surfaces to help reflect light  40 . If desired, particles  66  and/or material  64  may have bulk absorption properties with desired optical characteristics. For example, particles  66  and/or material  64  may reflect more infrared light than visible light, so that infrared beams of light  40  from sensor  36  ( FIG. 3 ) will reflect strongly (see, e.g., reflected light  40 ′) and therefore have enhanced visibility to sensor  36 B without making the marker formed from layer  62 ′ unnecessarily visible to the user. 
       FIG. 7  is a cross-sectional side view of layer  62 ′ in an illustrative configuration in which layer  62 ′ has a retroreflective surface. Layer  62 ′ of  FIG. 7  may, as an example, be the uppermost or a nearly uppermost one of layers  62 ′ in layer  62 . Particles  66 ′ may be spheres (e.g., glass spheres) or other retroreflective particles. Using these particles, incoming rays of light  40  will be reflected backwards towards the source of light  40  (e.g., reflected rays  40 ′ will trace the paths of incoming rays  40 ). This type of arrangement may be used to help make markers on device  10 B strongly visible to sensor(s) in device  10 A (e.g., by enhancing the reflection of light  40  that is emitted by light-emitting component  36 A of  FIG. 3  towards image sensor  36 B of  FIG. 3 ). A retroreflective layer formed from layer  62 ′ of  FIG. 7  may be formed as the uppermost layer  62 ′ (or sole layer  62 ′) in coating layer  62  (as an example). 
     As shown in  FIG. 8 , other types of retroreflective surfaces may be formed for makers in devices  10 . In the example of  FIG. 8 , a textured surface has been formed on layer  62 ′. The textured surface has protrusions  68  and forms reflective angled surfaces  70  for retroreflectors. This causes incoming rays of light  40  to be reflected backwards (retroreflected) along their incoming paths as light  40 ′. If desired, retroreflective surfaces may be formed from combinations of the retroreflective surface structures of  FIGS. 7 and 8  and/or from other retroreflective coatings. The examples of  FIGS. 7 and 8  are illustrative. 
       FIG. 9  shows how photoluminescent material in layer  62  can be used to create emitted light at one or more wavelengths of interest. During operation of devices  10  in system  8 , light sources  11  and/or a light source(s) in device  10 A may emit pump light at one or more pump light wavelengths (see, e.g., pump light at wavelength WL 1 ). The pump light may be ultraviolet light, visible light, and/or infrared light. Due to the presence of photoluminescent material in layer  62 , the pump light causes the photoluminescent material to emit light at one or more longer wavelengths (e.g., wavelengths WL 2  . . . WLN) in accordance with the principals of photoluminescence. A marker that emits light at these particular known wavelengths can be detected using an image sensor (see, e.g., sensor  36 B of  FIG. 3 ) or other light-based detector(s). By analyzing the spectral composition of the reflected light (light  40 ′) from the marker and other attributes of the marker (e.g., the shape of the marker, etc.), device  10 A can obtain information on device  10 B (e.g., device type, device position relative to device  10 A, device shape, device location, etc.). As an example, a first coating layer that emits light at wavelengths w 1  and w 2  may be used in forming a marker for a cellular telephone and a second coating layer that emits light at different wavelengths w 3  and w 4  may be used in forming a marker for a wrist watch device. By analyzing the spectrum of the reflected light  40 ′ from a marker on a target device (e.g., using a color digital image sensor or other sensor that analyzes the spectrum of reflected light  40 ′ from the marker), device  10 A can determine whether a given device that is in the field of view of device  10 A is a cellular telephone or a watch or can extract other information about device  10 B. 
     In configurations for system  8  in which device  10 B includes retroreflective structures (e.g., markers formed from a retroreflective coating), incident light will be reflected back towards its source along the same path as original incident light. Consider, as an example, the scenario illustrated in  FIG. 10  in which light  40  is incident on device  10 B at an angle of AI (e.g., an angle AI relative to the surface normal of a retroreflective marker surface on device  10 B). This light  40  may have an intensity-versus-angle profile such as intensity profile  72 . In the absence of retroreflective marker structures, light  40  will be weakly reflected back along angle AI (see, e.g., light intensity profile  76 , corresponding to evenly scattered light from a non-retroreflective surface such as light that is diffusely reflected). In the presence of retroreflective structures (e.g., markers formed from a retroreflective coating layer  62  that includes retroreflective layer  62 ′ of  FIG. 7  and/or  FIG. 8 ), reflected light  40 ′ will have an intensity profile of the type shown by profile  74  (i.e., reflected light  40 ′ will have a strong intensity along a path with the same angular orientation relative to the surface normal of the marker  62  as incoming light  40 ). 
       FIG. 11  is a perspective view of an illustrative electronic device  10 B with illustrative optical marker structures (optical markers)  80 . Markers  80  may be formed from patterned coatings such as coating  62  of  FIG. 5  and/or other marker structures (e.g., markers formed in openings in housing layers, markers formed from portions of housing structures or other device structures containing marker material, etc.). Coatings such as coating  62  may be patterned using lift-off techniques, etching, shadow mask deposition techniques, machining, laser patterning, and/or other suitable patterning techniques. Coating  62  may be patterned to form crosses, dots, squares, asymmetrical shapes (e.g., asymmetrical crosses, asymmetrical spots, etc.), grids (see, e.g., the grid-shaped marker  80  in the center of device  10 B of  FIG. 11 ), rings (e.g., peripheral rings that run along some or all of the periphery of device  10 B), etc. Buttons with switches and movable button members and/or touch sensor buttons may be labeled with markers  80  (see, e.g., buttons  80 M). 
     In some configurations, markers  80  may be distributed evenly (e.g., there may be at least one marker  80  at each of the four corners of device  10  to help demarcate the outline of device  10 ). In other configurations, markers  80  may be distributed in a pattern that allows device  10 A to determine whether device  10 B has been rotated, flipped, etc. For example, different corners of the housing of device  10 B may be provide with different numbers of markers  80 , so that device  10 A can determine whether device  10 B has been rotated (e.g., in the plane of a surface on which device  10 B is resting). Markers  80  may be formed on a rear face of device  10 B, on a front face of device  10 B, on portions of a housing sidewall and/or sidewall-mounted buttons on device  10 B, and/or on other suitable portions of device  10 B. 
       FIG. 12  is a top view of a corner portion of device  10 B showing how markers  80  may be formed along a peripheral edge portion PP of device  10 B (e.g., a peripheral housing structure, such as a metal peripheral housing band, a peripheral housing structure such as a sidewall that runs around the periphery of device  10 B and that forms an integral portion of a planar rear wall, other structures that form a bezel or trim for display  14  or other components in device  10 B, etc.). In the example of  FIG. 12 , there are a series of markers  80  along the peripheral edge of device  10 B. If desired, a single strip-shaped marker or other marker pattern may be used to indicate where the periphery of device  10 B is located. 
     During operation, device  10 A can use sensors  36  to form digital images of device  10 B at one or more wavelengths or can otherwise use sensors  36  to make spectral measurements and/or other light-based measurements on device  10 B. Markers on device  10 B may be coded using predetermined patterns of color, reflectivity, light absorption, light transmission, marker shape, marker orientation, etc. In some configurations, markers  10 B may form two-dimensional bar codes or other encoded optical patterns that contain information about device  10 B (e.g., device type, device serial number or other identifying information, etc.). Such codes may, as an example, be device-specific or device-model-specific (device-type-specific) codes. When one or more devices  10 B are in the presence of device  10 A, device  10 A can sense each device  10 B (e.g., using imaging and other light-based detection techniques and, if appropriate, bar code decoding operations) using sensors  36 . This sensing operation may involve emitting structured light, scanning beam(s), blanket illumination, or other device-emitted light  40  and/or may involve the use of one or more natural and/or artificial external light sources  11  to emit light  40 . 
     Emitted light  40  may have predetermined spatial characteristics (e.g., light  40  may be structured light emitted in an array of parallel beams), may have predetermined spectral characteristics (e.g., light  40  may contain one or more predetermined wavelengths of light), may have predetermined temporal characteristics (e.g., light  40  at one or more different wavelengths may be emitted in a predetermined pattern of pulses), and/or may have other known characteristics. Markers  80  may contain spectral reflectance codes (known reflectance versus wavelength characteristics) and other predefined properties that help markers  80  convey desired information about devices  10 B to device  10 A. As a result of the interplay between emitted light  40  and the markers  80  on each device  10 B that is illuminated by emitted light  40 , reflected light  40 ′ will be produced with corresponding characteristics. For example, if light  40  contains light at wavelengths wa, wb, and wc and if a given marker  80  in device  10  is configured to reflect 80% of the light at wavelength wa, 30% of the light at wavelength wb, and 50% of the light at wavelength wc, device  10 A can determine whether the given marker (and device  10 B) are present by analyzing the amount of light  40  that is received as reflected light  40 ′ at each of these three wavelengths. 
     If it is desired to provide buttons or other device structures on device  10 B with different visible-light appearances during exposure to different lighting conditions, the buttons and other portions of device  10 B (e.g., surrounding housing structures) can be provide with appropriate marker coatings. If, as an example, it is desirable to highlight a button that can be pressed to help a user cause device  10 A to output warm (low color temperature) flash illumination during fluorescent ambient lighting conditions, that button and surrounding housing structures can be provided with respective coatings having tuned light reflection spectra so that the button appears brighter than the surrounding structures in device  10 B in fluorescent lighting, but that appears to have the same brightness than the surrounding structures in sunlight. 
     In some arrangements, coatings  62  can be patterned to reflect and/or luminesce in response to a time-varying pattern of light at one or more wavelengths from light sources  11 . Animated effects may be produced when light  40  from light sources  11  has a particular pulse sequence. For example, a first layer  62  may form a first pattern that reflects light in a narrow wavelength range around wavelength wla (e.g., by forming a thin-film interference filter that reflects light only in this range) and second and third layers  62  may form second and third patterns that respectively reflect light in narrow wavelength ranges around wavelengths wlb and wlc. Light sources  11  may cycle in sequence outputting light pulses of respective wavelengths wla, wlb, and wlc to produce an animated pattern formed by alternating the reflected light repeatedly between the first, second, and third patterns. This technique may be used to form chasing light patterns, moving animated characters, etc. 
     In mixed reality environments, device  10 A can use markers (e.g., retroreflective markers and/or markers tagged using photoluminescent material, thin-film interference filters with known spectral properties, markers with known asymmetrical patterns such as asymmetric shapes and/or layouts, etc.) to help identify each device  10 B in the field-of-view of the user (e.g., the field-of-view of sensor  36 B and/or other image sensor devices) and to help determine the locations of displays  14  and other components within these devices  10 B. Device  10 A can use image processing techniques (e.g., pattern recognition) to determine when markers  80  are present and to extract desired information from the detected markers. For example, device  10 A can process sensor data from sensors  36  to identify devices  10 B, to identify the location of devices  10 B (e.g., the direction and distance from device  10 B), to identify the rotational orientation of each device  10 B (e.g., its rotational orientation on a table or other support surface), to identify the outline of each device  10 B, to determine the location of housing edges, display edges, and/or other device features, and/or to extract other information about each of devices  10 B. Device  10 A can then use its display to take appropriate action (e.g., by overlaying computer-generated content that is precisely aligned with the detected edges of displays  14  on devices  10 B, to overlay content that completely or partly obscures each device  10 B, to outline or otherwise visually highlight the locations of detected devices  10 B (e.g., to allow a user to readily locate and use devices  10 B), etc. 
     Devices  10 B can be used as input devices (e.g., game controllers, navigation devices, microphones, etc.). The buttons, touch screens, gesture recognition devices and other input-output devices in each device  10 B can be used as input and/or devices  10 B can be used to gather input in other ways. As an example, a user wave a device  10 B (e.g., a keyboard) in the air and device  10 A may use the detected motion of device  10 B as input to move a virtual object being displayed by device  10 A to the user. As another example, device  10 A may overlay images over displays  14  in devices  10 B (e.g., to create an alternative reality in which all or part of the images on devices  10 B are altered by device  10 A). Yet another example involves highlighting a mouse or other input devices by creating a flashing icon or other visually prominent computer-generated object that is placed on or adjacent to a detected device  10 B in the user&#39;s field of view. Device  10 A can use this technique to help a user locate devices  10 B while operating in a mixed reality environment. Devices  10 B can also be provided with virtual buttons and other augmented features by overlapping virtual button regions with particular portions of devices  10 B (as identified using pattern recognition of markers  80  and/or other detectable features of devices  10 B that are sensed using sensors  36 ). Because markers  80  can be used as optical markers in mixed reality systems (e.g., markers that device  10 A can optically detect to determine the location, orientation, and other attributes of devices  10 B), markers  80  of this type may sometimes be referred to as mixed reality optical markers, optical mixed reality system markers, visual mixed reality markers, mixed reality markers, or mixed reality alignment features. To hide markers  80  from view in visible light conditions while allowing alignment markers  80  to be viewed by device  10 A at infrared wavelengths, markers  80  can be buried under a coating layer that is transparent at infrared wavelengths and opaque at visible wavelengths (e.g., an upper one or more of layers  62 ′ may be an infrared-light-transmitting-and-visible-light-blocking layer formed from a thin-film interference filter, a polymer layer with visible-light-absorbing material that is transparent to infrared light, etc.). 
     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.