Patent Publication Number: US-2022225006-A1

Title: Electronic Devices With Skin Sensors

Description:
FIELD 
     This relates generally to electronic devices, and, more particularly, to electronic devices with sensors. 
     BACKGROUND 
     Electronic devices often have sensors. For example, sensors may be used in earbuds to help detect when earbuds are being worn in a user&#39;s ears. It can be challenging for such sensors to distinguish between scenarios in which earbuds are located in a user&#39;s ears and scenarios in which earbuds are located in another confined space such as a user&#39;s pocket. 
     SUMMARY 
     Electronic devices may be provided with skin sensors. The electronic devices may include ear buds, wristwatches, and other electronic devices. 
     A skin sensor may use optical measurements to detect the presence of skin adjacent to an electronic device. Actions may be taken by the device in response to detection of the presence of skin. For example, in a pair of earbuds, the initiation of audio playback and the pausing of audio by the earbuds may be controlled based on whether the skin sensor detects skin, indicating that the earbuds are being worn in a user&#39;s ears. 
     A skin sensor may have first and second light-emitting devices such as infrared devices that emit light at respective first and second infrared light wavelengths. Reflected light is monitored by a photodetector. The sensor may have a thin-film interference filter or other optical structures overlapping the first and second light-emitting devices to narrow the angular spread of light emitted from the skin sensor. This reduces tilt sensitivity and helps enhance skin sensor accuracy. 
     In an illustrative configuration, a thin-film interference filter overlapping the first and second light-emitting devices has a first bandpass filter with a first pass band overlapping the first light-emitting device to pass light from the first light-emitting device and has a second bandpass filter with a second pass band overlapping the second light-emitting device to pass light from the second light-emitting device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an illustrative electronic device in accordance with an embodiment. 
         FIG. 2  is a front view of an illustrative earbud in accordance with an embodiment. 
         FIG. 3  is a side view of an illustrative view of an illustrative wristwatch in accordance with an embodiment. 
         FIG. 4  is a cross-sectional side view of an illustrative electronic device with an optical skin sensor in accordance with an embodiment. 
         FIG. 5  is a perspective view of an illustrative printed circuit on which a pair of light sources of first and second respective wavelengths and a corresponding photodetector have been mounted in accordance with an embodiment. 
         FIG. 6  is a cross-sectional side view of an illustrative skin sensor that includes the printed circuit and components of  FIG. 5  in accordance with an embodiment. 
         FIG. 7  is a graph showing how skin sensor measurements may be more or less susceptible to tilt-induced variations in accordance with embodiments. 
         FIG. 8  is a cross-sectional side view of an illustrative thin-film interference filter that may be used in a skin sensor in accordance with an embodiment. 
         FIG. 9  is a graph in which light transmission has been plotted as a function of wavelength for an illustrative bandpass filter in accordance with an embodiment. 
         FIG. 10  is a graph of output intensity versus output angle for illustrative skin sensors with and without bandpass filters overlapping their light-emitting devices in accordance with an embodiment. 
         FIGS. 11, 12, 13, 14, and 15  show illustrative operations involved in forming bandpass filters in accordance with embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Electronic devices may be provided with skin sensors. For example, skin sensors may be included in wearable devices such as earbuds. The skin sensors may be used to detect when the earbuds are located in a user&#39;s ears. When it is determined that the earbuds are located in a user&#39;s ears, audio may be played for a user. If, on the other hand, skin sensors do not detect the presence of skin, it may be concluded that the earbuds are not in a user&#39;s ears so that audio playback can be halted. To help avoid false positives, the skin sensors may use a multi-wavelength design that helps to distinguish between scenarios in which the sensors are located adjacent to skin and scenarios in which the sensors are located next to other materials (e.g., fabric in a user&#39;s pocket). 
     The spectral response of human skin is characterized by peaks and valleys. for example, the reflectivity of human skin is relatively high (e.g., about 50-60%) at a wavelength of 1065 nm and is relatively low (e.g., about 5-10%) at a wavelength of 1465 nm. As a result, the presence of skin can be monitored by a sensor that emits light at 1065 nm and 1465 and that measures the amount of light reflected from a target object at these wavelengths. With an illustrative arrangement, the ratio R of reflected light at 1065 nm to reflected light at 1465 nm can be monitored and compared to a threshold TH (e.g., 2.0 or other suitable value). When the ratio R is less than TH, it can be concluded that the target object is not skin. When the ratio R is greater than TH, it can be concluded that skin is present. To help avoid false positives in the presence of non-skin objects, it may be desirable to control the light output from the skin sensors. In particular, false positives may be suppressed by narrowing the angular spread of emitted light. This may help avoid tilt dependencies in the skin sensor readings. 
       FIG. 1  is a schematic diagram of an illustrative electronic device of the type that may use a skin sensor. The skin sensor may be used to detect the presence of a human body part that has skin such as an ear, arm, head, or other skin-covered body part. The detected presence of skin may be used to trigger an action such as the presentation of audio through a speaker and/or may be used to activate or inactivate one or more other functions in a device (e.g., turning on or off input-output devices, initiating or halting the presentation of media, adjusting on-screen options on a touch sensitive display, adjusting other user selectable options, etc.). 
       FIG. 1  is a schematic diagram of an illustrative electronic device. Electronic device  10  may be operated in a system with one or more electronic devices. For example, device  10  may receive media content (e.g., audio and/or video) from a companion device (e.g., a cellular telephone, tablet computer, laptop computer, desktop computer, etc.) or device  10  may be a stand-alone device. Configurations in which device  10  is a stand-alone device may sometimes be described herein as example. Device  10  may be an earbud, a wristwatch, a head-mounted device, or other wearable electronic device. In some arrangements, device  10  may be a portable device such as a cellular telephone, tablet computer, or laptop computer, may be a desktop computer, may be a television, may be an accessory such as a trackpad, computer mouse, computer stylus, or other accessory, or may be any other suitable electronic equipment. Illustrative configurations in which device  10  is an earbud or a wristwatch may sometimes be descried herein as an example. 
     Device  10  may include control circuitry  20 . Control circuitry  20  may include storage and processing circuitry for supporting the operation of device  10 . The storage and processing circuitry may include storage such as nonvolatile memory (e.g., flash memory or other 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  20  may be used to gather input from sensors and other input devices and may be used to control output devices. The processing circuitry may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors and other wireless communications circuits, power management units, audio chips, application specific integrated circuits, display control circuits, etc. During operation, control circuitry  20  may use a display and/or other output devices in providing a user with visual output and/or other output. 
     To support communications between device  10  and external equipment, control circuitry  20  may communicate using communications circuitry  22 . Circuitry  22  may include antennas, radio-frequency transceiver circuitry (wireless transceiver circuitry), and other wireless communications circuitry and/or wired communications circuitry. Circuitry  22 , which may sometimes be referred to as control circuitry and/or control and communications circuitry, may support bidirectional wireless communications between device  10  and external equipment over a wireless link (e.g., circuitry  22  may include radio-frequency transceiver circuitry such as wireless local area network transceiver circuitry configured to support communications over a wireless local area network link, near-field communications transceiver circuitry configured to support communications over a near-field communications link, cellular telephone transceiver circuitry configured to support communications over a cellular telephone link, or transceiver circuitry configured to support communications over any other suitable wired or wireless communications link). Wireless communications may, for example, be supported over a Bluetooth® link, a WiFi® link, a wireless link operating at a frequency between 6 GHz and 300 GHz, a 60 GHz link, or other millimeter wave link, cellular telephone link, wireless local area network link, personal area network communications link, or other wireless communications link. Device  10  may, if desired, include power circuits for transmitting and/or receiving wired and/or wireless power and may include batteries or other energy storage devices. For example, device  10  may include a coil and rectifier to receive wireless power that is provided to circuitry in device  10 . 
     Device  10  may include input-output devices such as devices  24 . Input-output devices  24  may be used in gathering user input, in gathering information on the environment surrounding the user, and/or in providing a user with output. Devices  24  may include media playback devices such as speakers  14  and/or displays. Sensors  16  in input-output devices  24  may include one or more skin sensors  26  that detect the presence of human skin. Skin sensors  26  may use optical measurements involving two or more probe wavelengths. Because skin has an identifiable reflection spectrum, optical measurements with a skin sensor can differentiate between the presence of skin and other (non-skin) target objects. 
     If desired, sensors  16  may include force sensors (e.g., strain gauges, capacitive force sensors, resistive force sensors, etc.), audio sensors such as microphones, touch and/or proximity sensors such as capacitive sensors (e.g., a two-dimensional capacitive touch sensor integrated into a display, and/or a touch sensor that forms a button, trackpad, or other input device not associated with a display), and other sensors. If desired, sensors  16  may include optical sensors such as optical sensors that emit and detect light, ultrasonic sensors, optical touch sensors, optical proximity sensors, and/or other touch sensors and/or proximity sensors, monochromatic and color ambient light sensors, image sensors, fingerprint sensors, temperature sensors, sensors for measuring three-dimensional non-contact gestures (“air gestures”), pressure sensors, sensors for detecting position, orientation, and/or motion (e.g., accelerometers, magnetic sensors such as compass sensors, gyroscopes, and/or inertial measurement units that contain some or all of these sensors), health sensors, radio-frequency sensors, depth sensors (e.g., structured light sensors and/or depth sensors based on stereo imaging devices that capture three-dimensional images), optical sensors such as self-mixing sensors and light detection and ranging (lidar) sensors that gather time-of-flight measurements, humidity sensors, moisture sensors, gaze tracking sensors, and/or other sensors. In some arrangements, device  10  may use sensors  16  and/or other input-output devices to gather user input. For example, buttons may be used to gather button press input, touch sensors overlapping displays can be used for gathering user touch screen input, touch pads may be used in gathering touch input, microphones may be used for gathering audio input, accelerometers may be used in monitoring when a finger contacts an input surface and may therefore be used to gather finger press input, etc. 
     If desired, electronic device  10  may include additional components (see, e.g., other devices  18  in input-output devices  24 ). The additional components may include haptic output devices, light-emitting diodes for status indicators, light sources such as light-emitting diodes that illuminate portions of a housing and/or display structure, other optical output devices, and/or other circuitry for gathering input and/or providing output. Device  10  may also include a battery or other energy storage device, connector ports for supporting wired communication with ancillary equipment and for receiving wired power, and other circuitry. 
       FIG. 2  is a front view of device  10  in an illustrative configuration in which device  10  is an earbud configured to be worn in a user&#39;s ear. Earbud  10  of  FIG. 2  has a housing such as housing  12  formed from polymer, metal, glass, ceramic, fabric, and/or other materials. Housing  12  may include main body portion  12 M and an optional protruding stalk portion  12 T. Portion  12 M is configured to be received in a user&#39;s ear so that a user may listen to audio (sound) presented by speaker  14 . Control circuitry  20  can control audio playback (e.g., by initiating and halting sound output) based on the detected location of device  10  (e.g., in a user&#39;s ear, in a pocket, resting on a table, etc.). For example, audio can be turned off when device  10  is on a table or in a pocket or other enclosure and can be turned on only when device  10  is being worn in a user&#39;s ear adjacent to the user&#39;s skin. 
     To differentiate between scenarios in which device  10  is resting adjacent to fabric in a pocket or other inanimate object from scenarios in which device  10  is being worn in a user&#39;s ear, device  10  may have one or more skin sensors  26 . Sensors  26  may be optical sensors that operate through transparent housing walls in portion  12 M and/or that operate through openings in housing walls or localized transparent window structures. 
     Skin sensors  26  may be formed on one or more sides of device  10  to detect when device  10  is in a user&#39;s ear. In an illustrative configuration, skin sensors  26  include a first skin sensor on one side of portion  12 M that faces a user&#39;s concha when device  10  is in a user&#39;s ear and include a second skin sensor on another side of portion  12 M that faces a user&#39;s tragus when device  10  is in a user&#39;s ear. Arrangements in which multiple sensors  26  are used may help device  10  distinguish between scenarios in which device  10  is in a user&#39;s ear and in which device  10  is out of a user&#39;s ear. If desired, other configurations may be used. For example, device  10  may have only a single skin sensor that measures only the concha, only the tragus, or only another portion of the user&#39;s ear. 
       FIG. 3  is a side view of device  10  in an illustrative configuration in which device  10  is a wristwatch device configured to be worn on a wrist of a user. In the example of  FIG. 3 , device  10  has housing  12 . Housing  12  includes main unit  12 H and strap (band)  12 B. A display or other input-output device  24  may be mounted on front face F of housing portion  12 H. Skin sensor may be mounted on opposing rear face R of housing portion  12 M, on a portion of strap  12 B, or elsewhere in housing  12 . When device  10  is worn on a user&#39;s wrist, skin sensor  26  of  FIG. 3  may be used to detect the presence of the user&#39;s wrist. 
       FIG. 4  is a cross-sectional side view of an illustrative electronic device with a skin sensor (sometimes referred to as a skin sensor module). In the example of  FIG. 4 , device  10  has housing  12 . Housing  12  has structures such as walls that separate exterior region  46  from interior region  40 . Electrical components  42  (e.g., control circuitry  20 , communications circuitry  22 , and input-output devices  24 ) may be mounted in interior region  40  (e.g., using one or more printed circuits such as printed circuit  44 ). 
     The electrical components mounted in interior region  40  may include one or more skin sensors such as skin sensor  26 . Skin sensor  26  may have a package such as package  52 . Two or more light-emitting devices  54  may be mounted in package  52  and one or more photodetectors such as photodetector  56  may be mounted in package  52  (e.g., an opaque polymer package or a package formed from other materials). Openings in the portion of the package housing wall on top of package  52  allow light emitted by devices  54  to exit package  52 . 
     As shown in  FIG. 4 , package  52  may be aligned with a transparent portion of housing  12  such as portion W (e.g., a through hole, an inserted clear window, a transparent housing material that forms part of a housing wall, or other area through which light may pass). Portion W is transparent to light at the wavelengths emitted by devices  54 . In an illustrative arrangement, this light is infrared light (e.g., near infrared light at wavelengths between 900 nm and 2000 nm, as an example). Infrared light is invisible to users and is therefore not distracting. If desired, visible wavelengths and/or other infrared wavelengths may be used. There may be two devices  54  in sensor  26  each of which emits light at a different respective wavelength or, if desired, there may be three or more devices  54  emitting light at different respective wavelengths (e.g., for more spectral measurement accuracy). Illustrative arrangements in which sensor  26  has a pair of devices  54  emitting light at respective first and second wavelengths of, respectively 1065 nm (corresponding to a high skin reflectance) and 1465 nm (corresponding to a lower skin reflectance) are sometimes described herein as an example. 
     After passing through transparent portion W (sometimes referred to as a window or transparent region), the light emitted by sensor  26  may reflect off of nearby objects such as illustrative target object  50 . Reflected light from object  50  will again pass through window W and will be detected by photodetector  56  in skin sensor  26 . 
     The emitted light from sensor  26  is emitted in direction nm (which may be, for example, the surface normal of the upper planar surface of sensor  26 ) while spreading over a cone characterized by angular size A (e.g., an angular spread of +/−A/2). Light sensor  56  may likewise be pointed in direction nm. 
     When direction nm is parallel to surface normal ns of the surface of object  50 , geometric effects from tilting will not tend to impact the amount of emitted light that is reflected back towards sensor  26 . Sensor  26  can therefore make accurate measurements of the relative intensity of the reflected light at each wavelength of interest (e.g., at 1065 nm and at 1465 nm). If, however, direction nm and direction ns are not parallel (e.g., when device  12  and sensor  26  are tilted with respect to the surface of the skin or other object being measured), there is a potential that geometrical effects will unevenly impact the amount of reflected light from one of light-emitting devices  54  versus the amount of reflected light from the other of light-emitting devices  54 . This effect is exacerbated when the size of angle A is large, giving rise to a risk that geometrical light collection efficiency effects will obscure underlying spectral reflectivity effects. 
     To ensure that accurate spectral reflectance measurements can be made over a wide range of tilt angles, sensor  26  may be provided with an optical structure that helps to narrow the angular spread of emitted light such as a bandpass filter. Consider, as an example, the arrangement of  FIGS. 5 and 6 .  FIG. 5  is a perspective view of an illustrative printed circuit on which components for sensor  26  have been mounted. In the example of  FIG. 5 , first and second light-emitting devices  54  have been mounted at one side of printed circuit  60  and photodetector  56  has been mounted on another side of printed circuit  60 . Devices  54  and  60  may be semiconductor devices. As an example, devices  54  may be light-emitting diodes and photodetector  56  may be a photodiode. Other arrangements may be used, if desired. For example, devices  54  may be lasers (e.g., vertical cavity surface emitting lasers or other laser diodes). 
       FIG. 6  is a cross-sectional side view of printed circuit  60  of  FIG. 5  mounted in sensor package  52 . As shown in  FIG. 6 , package  52  may have transparent package portions PW (e.g., through-hole openings, clear portions of the walls of package  52 , inserted transparent window members, etc. Devices  54  may be configured to emit light through a first of portions PW and photodetector  56  may be configured to receive reflected light through a second of portions PW. 
     Bandpass filter  62  may have first and second areas with first and second bandpass filters each with a respective passband for a corresponding emitted wavelength from one of devices  54 . For example, in a configuration in which there are two devices  54  that emit light at two respective wavelengths (e.g., when sensor  26  is a dual-wavelength skin sensor), bandpass filter  62  may have a first region with a first bandpass filter having a first pass band that is aligned with the first wavelength of light emitted by a first of devices  54  and may also have a second region with a second bandpass filter having a second pass band that is aligned with the second wavelength of the light emitted by a second of devices  54 . In another illustrative configuration, filter  62  may be formed from a single coating (e.g., a stack of thin-film layers forming a thin-film interference filter) that exhibits pass bands at both the first and second wavelengths. Configurations in which filter  62  has separate first and second areas with respective first and second bandpass filters are sometimes described herein as an example. 
     Filter  63  may overlap photodetector  56  and may be formed from a stack of thin-film layers that form a dual-band bandpass filter (e.g., a thin-film interference filter formed from a stack of thin-film dielectric layers of with refractive index values and thicknesses selected to form first and second passbands to pass reflected light at the first and second wavelengths) or may have another configuration that allows reflected light from the target object at the first and second wavelengths to pass to photodetector  56  (e.g., two side-by-side bandpass filters with respective first and second passbands, etc.). Filters  62  and  63  may be formed from separate substrates or may be formed from coatings deposited and patterned onto a single substrate. 
     By covering photodetector  56  with filter  63 , extraneous ambient light will be blocked (e.g., light at wavelengths other than 1065 nm and 1465 nm will be rejected, allowing photodetector  56  to measure only light from the first of devices  54  or the second of devices  54  and not stray ambient light at other wavelengths). To distinguish between measurements associated with the first of devices  54  and measurements associated with the second of devices  54 , the first and second devices can emit light at different times (e.g., using time-division multiplexing). As an example, the first and second devices can emit light in alternation. The measurements of photodetector  56  can then be synchronized to the emitted light pattern so that separate measurements for the first and second wavelengths can be made. 
     The presence of bandpass filter  62  over devices  54  helps to narrow the light emission angles (cone spread angles) of the light beams emitted from devices  54 . The narrowing of the angular spread of the emitted light reduces tilt dependencies in the reflected light measurements and thereby helps ensure that skin sensor  26  can make accurate measurements.  FIG. 7  is a graph which illustrates the impact of narrowing the light emission cone from skin sensor  26 . During operation of skin sensor  26 , the ratio R of the amount of light reflected at 1065 nm to the amount of light reflected at 1465 nm is computed and compared to a threshold TH (e.g., a threshold value of 2 or other suitable value). When R is less than TH, control circuitry  20  can conclude that target object  50  is not skin, whereas when R is greater than TH, control circuitry  20  can conclude that target object  50  is skin. Dashed line  70  shows how the value of R for an illustrative target  50  that has an on-axis R value of about 1 is relatively insensitive to the amount of tilt between skin sensor  26  and the target (e.g., R varies by less than 10% or less than 5% while the angle between skin sensor light emission direction nm and surface normal ns of the illuminated surface of target  50  changes from +45° to −45°). This is due to the relatively narrow value of cone emission angle A that arises from use of bandpass filter  62 . As an example, when bandpass filter  62  is present, the value of A may be less than 60° (e.g., the emitted light may be characterized by an angular spread of less than +/−30°), less than 50°, or less than 40° (e.g., light may be emitted over a range of less than −20° to +20°). In the absence of filter  62  and a resulting larger emitted light spread (e.g., 120°), geometric effects could cause the value of R to vary by a relatively large amount when tilted as shown by line  72 , which could cause skin measurements to be less accurate than desired. 
       FIG. 8  is a cross-sectional side view of an illustrative bandpass filter for skin sensor  26 . In the example of  FIG. 8 , bandpass filter  62  is configured to pass light at first and second passbands. Arrangements in which bandpass filter  62  has three or more pass bands may be used, if desired. 
     As shown in  FIG. 8 , filter  62  has a stack of layers  62 - 5  on substrate  62 - 4 . Substrate  62 - 4  may be formed from glass, ceramic, polymer, semiconductor, crystalline material such as a sapphire, other material, and/or combinations of these materials. Layers  62 - 5  may be stacked on substrate  62 - 4  to form thin-film coating layer  62 - 3 . Layers  62 - 5  may be formed from dielectric, semiconductor, and/or metal that is transparent at the operating wavelengths of skin sensor  26  (e.g., 1065 nm and 1465 nm in this example). In an illustrative configuration, layers  62 - 5  may be dielectric layers such as layers of oxide or nitride (e.g., silicon oxide, aluminum oxide, zirconium oxide, titanium oxide, silicon nitride, etc.). In general, any suitable organic or inorganic dielectric materials and/or other materials may be used in forming stacks of layers  62 - 5  on substrate  62 - 4 . 
     Layers  62 - 5  may be thin-film layers formed by physical vapor deposition and other thin-film material deposition techniques. Layers  62 - 5  may have subwavelength thicknesses and may be configured (e.g., by selection of thickness values and/or refractive index values) to form the pass bands of filter  62  in accordance with thin-film interference filter principals. 
     In the example of  FIG. 8 , coating  62 - 3  has two parts, which correspond to the two separate areas overlapping respective first and second devices  54 . The two separate areas form first bandpass filter  62 - 1 , which overlaps a first of devices  54 , and adjacent second bandpass filter  62 - 2 , which overlaps a second of devices  54 . 
     In this type of thin-film interference filter, the values of refractive index for layers  62 - 5  in each filter may alternate (e.g., between higher and lower values). There may be any suitable numbers of layers  62 - 5  in each portion of layer  62 - 3  (e.g., at least 30, at least 40, at least 100, 20-300, fewer than 500, fewer than 250, fewer than 100, etc.). The layers of filter  62 - 1  form a first bandpass filter (e.g., a filter with a 1065 nm pass band suitable for overlapping the 1065 nm light-emitting device) and the layers of filter  62 - 2  form a second bandpass filter (e.g., a filter with a 1465 nm pass band suitable for overlapping the 1465 nm light-emitting device). 
       FIG. 9  is a graph of the transmission spectrum of an illustrative bandpass filter. Curve  74  represents the transmission T of filter  62  as a function of wavelength. As shown in  FIG. 9 , curve  74  has a first pass band  76  centered at 1065 nm (formed by the first bandpass filter  62 - 1 , which is overlapping the 1065 nm light-emitting device) and a second passband  78  centered at 1465 nm (formed by the second bandpass filter  62 - 2 , which is overlapping the 1465 nm light-emitting device). The full width half maximum (FWHM) of the light emitted by devices  54  may be 30-150 nm (as an example). In this configuration, the FWHM of each pass band may have a value in the range of 30-150 nm. The position of curve  74  in  FIG. 9  corresponds to the filter characteristics of filter  62  at normal incidence (light passing through filter  62  parallel to direction nm, which is perpendicular to the planar surface of filter  62 ). The passbands will shift to lower wavelengths (see, e.g., shift direction  80  of  FIG. 9 ) for light at different off-axis orientations. In general, emitted light rays that have greater angles with respect to direction nm will exhibit greater shifts, resulting in significant attenuation for those light rays as the passbands moves away from its nominal position. On-axis light rays (e.g., light rays in a narrow cone surrounding direction nm) will not be affected and will continue to be passed through pass bands  76  and  78 . As a result of these properties, bandpass filter  62  will pass on-axis light rays and will block off-axis light rays, resulting in a narrowing of the angular spread of the light rays in the emitted cone of light from devices  54  as this light passes through filter  62 . 
       FIG. 10  is a graph showing how the presence of filter  62  helps narrow the range of emitted light angles for the light emitted by devices  54 . Dashed curve  84  corresponds to the nominal output of light-emitting devices  54  in the absence of filter  62 . In this example, there is a 120° angular spread (e.g., a wide Lambertian distribution extending +/−60° with respect to direction nm) of emitted light intensity. This wide range of emitted light angles can lead to undesired tilt angle dependencies when measuring target objects as described in connection with curve  72  of  FIG. 7 . When filter  62  is present in skin sensor  26  (e.g., when filter  62  overlaps devices  54  and filters the light emitted from devices  54 ), the range of angles covered by each beam of emitted light is reduced (see, e.g., narrowed curve  86 , which, in this example is characterized by a 40° angular spread, which is significantly narrower than the uncorrected 120° output from devices  54  when not filtering is present). This narrowing of the emitted light from skin sensor  26  may effectively remove tilt angle dependencies and thereby help to enhance the accuracy of skin reflectivity measurements. 
     If desired, other narrowing arrangements may be used in addition to (or instead of) using bandpass filter  64 . For example, devices  54  may be provided with narrow angular emission characteristics (e.g., when devices  54  are lasers or resonant cavity light-emitting diodes), apertures PW in the top of package  52  may be narrowed to restrict the beamwidth of emitted light, a lens, light pipe, and/or other optical elements may be used to restrict emitted beam width, and/or other structures may be added to sensor  26  to help restrict the angular spread of emitted light and/or otherwise reduce tilt dependence in the measured values of R for sensor  26 . If desired, a diffuser may be formed on filter  62  or may be incorporated into sensor  26  in place of filter  62  to help reduce tilt dependence in the measured values of R. 
     Thin-film interference filter coatings such the coating for filter  62 - 3  of  FIG. 8  may be formed by depositing layers  62 - 5  by physical vapor deposition or other thin-film-layer deposition techniques and patterning using photolithography (lift-off, etching, etc.), shadow masking, and/or other suitable patterning techniques. Illustrative fabrication operations for filter  62  are illustrated in  FIGS. 11, 12, 13, 14, and 15 . 
     As shown in  FIG. 11 , with one illustrative arrangement, filter  62 - 2  may be deposited by depositing and patterning a photoresist layer on the left side of substrate  62 - 4  followed by deposition of the layers of filter  62 - 2  and lift-off to remove the left-hand portion of the deposited layers. This leaves filter  62 - 2  of  FIG. 11  on substrate  62 - 4 . 
     As shown in  FIG. 12 , this lift-off patterning technique can then be repeated to form filter  62 - 1  adjacent to filter  62 - 2 . First, photoresist is deposited and patterned to cover filter  62 - 2  while leaving the left half of substrate  62 - 4  uncovered. Second, the layers of filter  62 - 1  are deposited. Third, using lift-off, the portion of the layers of filter  62 - 1  that overlap filter  62 - 2  are removed, leaving filters  62 - 1  and  62 - 2  on adjacent portions of substrate  62 - 4 , as shown in  FIG. 12 . 
     It may be desirable to form an opaque masking layer on the upper and/or lower surfaces of filter  62 . This opaque masking layer may run along the seam between filters  62 - 1  and  62 - 1 , where filter structures may overlap (and may therefore not be exhibiting their desired optical characteristics). In the example of  FIG. 13 , a strip of black ink or other opaque material (opaque masking strip  90 ) has been formed on the surface of filter  62  covering the seam between filters  62 - 1  and  62 - 2 . To form strip  90 , photoresist  92  was deposited and patterned to leave a gap for strip  90 . An opaque masking layer was then deposited as a blanket film (see, e.g., portions  90 ′ of the film, which cover photoresist  92 , and strip  90 , which is deposited in the gap). Following lift-off operations, photoresist  92  and portions  90 ′ are removed, leaving only strip  90 . 
     In the example of  FIGS. 14 and 15 , ink-jet printing, pad printing, or other printing techniques have been used to deposit opaque masking strip  94  (e.g., a layer of black ink). In the example of  FIG. 14 , strip  94  has been formed by printing a strip of black ink on the surface of filter  62  running along the seam between filters  62 - 1  and  62 - 2 . In the example of  FIG. 15 , strip  94  has been printed on the exposed lower surface of substrate  62 - 4  so as to overlap and run along the seam between filters  62 - 1  and  62 - 2  formed on the opposing front side of filter  62 . 
     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.