Patent Publication Number: US-2021181395-A1

Title: Coatings for Transparent Substrates in Electronic Devices

Description:
This application is a continuation of U.S. patent application Ser. No. 15/900,575, filed Feb. 20, 2018, which claims the benefit of provisional patent application No. 62/556,243, filed Sep. 8, 2017, which are hereby incorporated by reference herein in their entireties. 
    
    
     FIELD 
     This relates generally to electronic devices and, more particularly, to coatings for transparent substrates in electronic devices. 
     BACKGROUND 
     Electronic devices often contain displays. A display may have an active area with pixels that display images for a user and an inactive area alongside the active area. A layer of glass may serve as a protective display cover layer. The layer of glass may overlap the active area and the inactive area. A layer of glass may also form part of a housing for an electronic device. To hide internal components from view, surfaces in an electronic device such as the inner surface of a layer of glass forming a housing for an electronic device and the inner surface of the protective display cover layer in the inactive area of a display may be covered with a layer of ink. 
     It may be desirable to improve the outward appearance of the display cover layer in the inactive area or the output appearance of a glass housing layer. This can be challenging, because glass is sensitive to stress. If care is not taken, a coating on a glass layer in an electronic device may make the glass layer susceptible to cracking. It can also be difficult to control the appearance of coating layers, which can make it difficult to manufacture electronic devices of uniform appearance. 
     SUMMARY 
     An electronic device may have a housing in which a display is mounted. The housing may be formed from housing structures that surround an interior region in the electronic device. Electrical components may be mounted in the electronic device interior. 
     The display may be coupled to the housing structures on a front face of the electronic device. The housing structures may include a rear wall on an opposing rear face of the electronic device. 
     A display cover layer for the display may have a surface that faces the interior of the housing. The rear wall may also have a surface that faces the interior of the housing. Structures in the electronic device such as the display cover layer and rear housing wall may be formed from transparent glass layers. Coatings may be formed on the inwardly facing surfaces of the transparent glass layers. 
     A coating on a transparent glass layer may be formed from a thin-film interference filter having a stack of dielectric layers. The coating may also include an ink layer on the thin-film interference filter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an illustrative electronic device in accordance with an embodiment. 
         FIG. 2  is a cross-sectional side view of a portion of an illustrative electronic device having transparent substrates with coatings in accordance with an embodiment. 
         FIG. 3  is a cross-sectional side view of a coating layer formed from a multilayer dielectric stack in accordance with an embodiment. 
         FIG. 4  is a graph of light transmission spectrums for illustrative coating layers in accordance with an embodiment. 
         FIG. 5  is a cross-sectional side view of an illustrative transparent substrate having an interior surface coated with an illustrative coating in accordance with an embodiment. 
         FIG. 6  is a cross-sectional side view of an illustrative transparent substrate having an interior surface coated with another illustrative coating in accordance with an embodiment. 
         FIG. 7  is a perspective view of an illustrative electronic device having housing walls surrounding a coated transparent glass layer in accordance with an embodiment. 
         FIG. 8  is a cross-sectional side view of a portion of the electronic device of  FIG. 7  in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Electronic devices such as cellular telephones often include glass members such as display cover glass layers and glass housing members. These layers are traditionally coated with materials such as ink. The ink may be opaque to hide internal device components from view, but may not always have a desired appearance. The appearance of glass layers in an electronic device can be altered by depositing inorganic layers such as physical vapor deposition (PVD) inorganic layers onto the glass layers. Challenges arise, however, in ensuring that the deposited layers produce desired optical effects (e.g., desired transmission, opacity, and reflection values at various viewing angles) while minimizing undesired manufacturing variations. 
     To address these challenges, a device such as electronic device  10  of  FIG. 1  may have transparent glass layers or other substrates coated with coatings that include thin-film interference filters and ink layers. In these coatings, thin-film interference filter layers may be arranged to produce non-neutral colors or to produce neutral colors. The thin-film interference filter layers may be coated with ink such as neutrally colored ink or ink with a non-neutral color. Optional buffer layer material may be included in the coatings. In some configurations, thin-film interference layers may be supported by a polymer film and attached to a transparent glass layer using a layer of adhesive. 
     An illustrative electronic device of the type that may have one or more coated structures is shown in  FIG. 1 . The coated structures in device  10  of  FIG. 1  may include transparent structures such as transparent glass structures (e.g., transparent glass substrates or other transparent substrates that form display cover layers, rear housing walls, other housing structures, camera windows, lenses with curved surfaces and/or other curved members, and/or other structures). If desired, other substrates may be coated (e.g., opaque structures, structures formed from materials other than glass, etc.). Illustrative configurations in which transparent glass substrates in device  10  are coated are described herein as an example. 
     Coated substrates such as transparent glass substrates may be oriented in device  10  so that the coatings face outwardly or inwardly. For example, coatings may be located on the inner (interior) surfaces of the substrates (the sides of the substrates facing inwardly into the interior of device  10 ) so that these coatings may be viewed through the substrates from outside the device. 
     Electronic device  10  may be a computing device such as a laptop computer, a computer monitor containing an embedded computer, a tablet computer, a cellular telephone, a media player, or other handheld or portable electronic device, a smaller device such as a wristwatch device, a pendant device, a headphone or earpiece device, a device embedded in eyeglasses or other equipment worn on a user&#39;s head, or other wearable or miniature device, a television, a computer display that does not contain an embedded computer, a gaming device, a navigation device, an embedded system such as a system in which electronic equipment with a display is mounted in a kiosk or automobile, equipment that implements the functionality of two or more of these devices, an accessory (e.g., earbuds, a remote control, a wireless trackpad, etc.), or other electronic equipment. In the illustrative configuration of  FIG. 1 , device  10  is a portable device such as a cellular telephone, media player, tablet computer, or other portable computing device. Other configurations may be used for device  10  if desired. The example of  FIG. 1  is merely illustrative. 
     In the example of  FIG. 1 , device  10  includes display  14 . Display  14  has been mounted in housing  12 . Housing  12 , which may sometimes be referred to as an enclosure or case, may be formed of plastic, glass, ceramics, fiber composites, metal (e.g., stainless steel, aluminum, etc.), other suitable materials, or a combination of any two or more of these materials. Housing  12  may be formed using a unibody configuration in which some or all of housing  12  is machined or molded as a single structure or may be formed using multiple structures (e.g., an internal frame structure, one or more structures that form exterior housing surfaces, bezel structures, housing sidewall structures, rear housing walls formed from glass plates or other planar transparent members, metal, plastic, and/or other materials, and/or other housing members). Openings may be formed in housing  12  to form communications ports, holes for buttons, and other structures. 
     Display  14  may be a touch screen display that incorporates a layer of conductive capacitive touch sensor electrodes or other touch sensor components (e.g., resistive touch sensor components, acoustic touch sensor components, force-based touch sensor components, light-based touch sensor components, etc.) or may be a display that is not touch-sensitive. Capacitive touch sensor electrodes may be formed from an array of indium tin oxide pads or other transparent conductive structures. 
     Display  14  may have a central active area that includes an array of pixels. The array of pixels may be formed from liquid crystal display (LCD) components, an array of electrophoretic pixels, an array of plasma display pixels, an array of organic light-emitting diode pixels or other light-emitting diode pixels, an array of electrowetting pixels, or pixels based on other display technologies. In some configurations, an inactive border area that is free of pixels may run along one or more edges of display  14 . 
     Display  14  may be protected using a display cover layer such as a layer of transparent glass, clear plastic, transparent ceramic, sapphire or other transparent crystalline material, or other transparent layer(s). The display cover layer may have a planar shape, a convex curved profile, a concave curved profile, a shape with planar and curved portions, a layout that includes a planar main area surrounded on one or more edges with a portion that is bent out of the plane of the planar main area, or other suitable shape. If desired, one or more openings may be formed in the display cover layer to accommodate optional components such as button  16 , ports such as speaker port  18 , and other structures. In some configurations, display  14  may have an outer layer such as a color filter layer or a thin-film transistor layer in a liquid crystal display that is sufficiently thick and strong to serve as a display cover layer. In other configurations, the outermost layer of display  14  may be a separate cover layer that does not have any color filter elements or thin-film transistor circuitry. 
     Illustrative device  10  of  FIG. 1  has a rectangular footprint (outline when viewed from above) with four peripheral edges. Housing  12  may have sidewalls  12 SW that run along the four peripheral edges of device  10 . Sidewalls SW may be vertical sidewalls, curved sidewalls, integral portions of a rear housing wall that extend fully or partly up the sides of housing  12 , and/or other suitable sidewall structures. In some configurations, display  14  has peripheral portions that extend down some or all of the side edges of device  10 . Housing  12  may have a rear wall such as rear wall  12 R. Rear wall  12 R may be formed from integral portions of sidewalls  12 SW and/or from separate structures. Rear wall  12 R may have a substantially planar surface on a rear face of device  10 . Display  14  may include a parallel planar surface on the opposing front face of device  10 . 
     A cross-sectional side view of device  10  taken along line  20  and viewed in direction  22  of  FIG. 1  is shown in  FIG. 2 . As shown in  FIG. 2 , device  10  may have an interior in which electrical components  50  are housed. Electrical components  50  may include integrated circuits, sensors, and other circuitry. Components  50  may be mounted on one or more printed circuits such as printed circuit  48 . 
     Display  14  of  FIG. 2  may have a transparent layer such as display cover layer  30  (i.e., the outermost layer of display  14 ). Display cover layer  30  may be formed from a transparent material such as glass, plastic, sapphire or other crystalline material, transparent ceramic, etc. In the active area of display  14 , display  14  may contain pixel array structures  32  (e.g., an organic light-emitting diode display layer, a liquid crystal display module, etc.) with an array of pixels  31  for displaying images. 
     Rear housing wall  12 R may be formed from a planar member such as a transparent glass substrate (transparent glass member  34 ). Transparent glass substrates such as display cover layer  30  and/or member  34  may be provided with coatings. In the example of  FIG. 2 , the underside of display cover layer  30  in the inactive area of display  14  has been coated with coating  28 . The inner surface of member  34  (e.g., a glass plate) has been provided with coating layer  36  and coating  52 . Coating  52  may be formed from the same coating materials as coating layer  36  and/or may be formed differently so that coating  52  has a different visual appearance than coating  36 . Coating  52  may, as an example, be patterned to form text, a logo, or other suitable visual element on the rear of housing  12 . A user such as viewer  42  who is viewing the front face of device  10  in direction  40  may view coating  28  through display cover layer  30 . A user such as viewer  46  who is viewing the rear face of device  10  in direction  44  may view coatings such as coating  36  and coating  52  through member  34 . If desired, sidewalls  12 SW may be formed from transparent glass structures (e.g., sidewall members or portions of layer  30 ) and coatings such a coatings  28 ,  36 , and/or  52  may be formed on the inner surfaces of these members (as an example). 
     Coatings such as coatings  28 ,  26 , and  52  may be formed from dielectric layers, metal layers, and/or other layers of material. These layers may be deposited by spraying, printing (e.g., screen printing, inkjet printing, pad printing, etc.), dripping, painting, chemical vapor deposition (e.g., plasma enhanced chemical vapor deposition,), physical vapor deposition (e.g., evaporation and/or sputtering), atomic layer deposition, electroplating, lamination, and other deposition techniques. Coatings may be patterned using shadow mask deposition, printing patterning techniques, photolithography (lift-off, etching, etc.), laser patterning (e.g., ablation), mechanical patterning (e.g., drilling, grinding, milling, etc.) and/or other patterning techniques. 
     In some arrangements, multiple thin-film layers for a coating may be formed in a stack. Thin-film stacks such as these may form thin-film interference filters (sometimes referred to as dichroic filters or dichroic layers). The optical properties of each of the layers in a thin-film stack (e.g., the index of refraction of each layer) and the thickness of each layer may be selected to provide the thin-film interference filter with desired characteristics (e.g., a desired light transmission spectrum, a desired light reflection spectrum, a desired light absorption spectrum). These characteristics may provide a coating with a desired appearance when present on the inner surface of a transparent substrate (e.g., a desired color, etc.). A thin-film stack may, as an example, be configured to reflect light of a particular color or to exhibit a color-neutral behavior (e.g., to serve as a neutral-color partially reflective mirror). 
       FIG. 3  is a cross-sectional side view of an illustrative thin-film stack configured to form a thin-film interference filter. The thin-film stack of  FIG. 3  has multiple layers  56 . Layers  56  may have thicknesses of 0.01-1 micron, at least 0.05 microns, at least 0.1 microns, at least 0.15 microns, less than 1.5 microns, less than 1 micron, etc. Layers  56  may be inorganic dielectric layers (e.g. oxides such as silicon oxide, niobium oxide, titanium oxide, tantalum oxide, zirconium oxide, magnesium oxide, etc., nitrides such as silicon nitride, oxynitrides, and/or other inorganic dielectric materials). Organic dielectric layers (e.g., clear polymer layers) and/or other materials (thin metal films, semiconductor layers, etc.) may also be included in the thin-film stack, if desired. 
     In the example of  FIG. 3 , the thin-film stack formed from layers  56  forms thin-film interference filter  54 . Filter  54  may be formed form dielectric materials such as inorganic dielectric layers deposited with physical vapor deposition techniques and may therefore sometimes be referred to as a physical vapor deposition layer, physical vapor deposition coating, or physical vapor deposition stack. Other techniques for forming filter  54  may be used, if desired. 
     Filter  54  may be configured to exhibit high reflectivity (e.g., filter  54  may be configured to form a dielectric mirror that reflects a relatively large amount of light (see, e.g., reflective light  12 ) relative to incident light Il, may be configured to exhibit low reflectivity (e.g., filter  54  may be configured to form an antireflection coating so that a relatively large amount of light  13  passes through filter  54  relative to incident light I 1 ), may be configured to form a colored (tinted) layer (e.g., by reflecting one or more selected colors of light such as when configuring filter  54  to serve as a bandpass filter, band-stop filter, low pass filter, or high pass filter), and/or may be configured to from a light-blocking layer (e.g., by exhibiting a high opacity). Layers  56  may also be configured to adjust the optical properties (transmission, reflection, absorption) of filter  54  at multiple different values of angle A (e.g., an angle A with respect to surface normal n for filter  54  that is associated with an incident angle of incoming light such as ray R 1  and that is also associated with corresponding angle of view for a viewer viewing reflected light such as ray R 2 ). 
       FIG. 4  is a graph containing curves  60  and  58  for two respective illustrative light transmission spectrums for filter  54  (e.g., visible light transmission spectrums at an illustrative angle A of) 0°. As shown by illustrative curve  58 , there may be complex features at multiple different wavelengths (e.g., peaks, valleys, etc.) in the light transmission spectrum for filter  54  (e.g., over visible light wavelengths λ.). In other configurations (e.g., curve  60 ), filter  54  is configured to exhibit a neutral color spectrum. 
     The optical characteristics of filter  54  can be tuned (at one or more values of angle A) by adjusting the attributes of layers  56  (e.g., index of refraction, thickness, etc.). The optical properties of filter  54  may also be adjusted by adjusting the number of layers  56  in filter  54 . With one illustrative configuration, the overall thickness of filter  54  is maintained at a relatively low value (e.g., 80-300 nm, less than 3 microns, less than 2 microns, less than 1 micron, at least 0.1 micron) by limiting the thicknesses of each of layers  56  (e.g., to less than 1.5 microns, less than 1 micron, less than 0.5 microns, less than 0.4 microns, etc.) and by limiting the number of layers  56  in filter  54  (e.g., to 2-6, at least 2, at least 3, at least 4, at least 5, fewer than 20, fewer than 14, fewer than 10, fewer than 7, etc.). In general, filter  54  need not be restricted to these configurations and may contain any suitable types of layers  56  and/or may include layers  56  of any suitable thickness, index of refraction, etc. 
     In some arrangements, it may be desirable for filter  54  to be configured to exhibit a color tint (in reflection and/or transmission). For example, it may be desirable for filter  54  to reflect red light so that filter  54  has a pink color or to reflect light that provides filter  54  with a gold appearance in reflection. In other arrangements, it may be desirable for layer  54  to exhibit a neutral color (e.g., white, gray, black, etc.) and/or a color that is relatively constant in color cast over a wide range of angles A (e.g., a wide range of viewing angles). 
     The apparent color of filter  54  may be characterized by a color in Lab color space. With one illustrative configuration, filter  54  operates as a partially reflective mirror (e.g., a mirror of 10-20% reflectivity, or a reflectivity of at least 5%, at least 15%, at least 20%, less than 85%, less than 60%, less than 50%, less than 35%, or other suitable value) and exhibits a gray color in reflection (e.g., the reflectivity of filter  54  is neutral in color so that filter  54  forms a color-neutral partially reflective mirror). In this configuration, for example, the color of reflected light may be characterized by Lab color coordinates a and b that are less in magnitude than 5,less in magnitude than 3, or other suitable neutral values (e.g., the value of color coordinate “a” may be about −1 and the value of color coordinate “b” may be about −2). If desired, filter  54  may be configured to exhibit an angularly invariant color. For example, the changes in the magnitudes of color coordinates a and b (e.g., Δa and Δb, respectively) may be maintained at values less than 2, less than 3, less than 4, or other suitable values over a range of viewing angles (reflected light angle A) of 0-60°. 
     Layers  56  may include inorganic materials such as oxides. For example, layers  56  may include one or more silicon oxide layers and one or more niobium oxide layers. Niobium oxide can be deposited consistently using sputtering and may allow filter  54  to exhibit good color control. Other oxides may be used (e.g., one or more tantalum oxide layers  56  may be interspersed with one or more silicon oxide layers in filter  54 , one or more titanium oxide layers  56  may be interspersed with one or more silicon oxide layers, etc.). In some arrangements, higher and lower refractive index materials alternate in the stack of layers forming filter  54 . For example, filter  54  may include alternating niobium oxide layers and silicon oxide layers, may include alternating titanium oxide and silicon oxide layers, or may include alternating tantalum oxide layers and silicon oxide layers. 
     Filter  54  may form part of a coating on a transparent glass substrate in device  10 . In this type of configuration, the most inwardly facing layer  56  of filter  54  (e.g., the last layer  56  that is deposited on filter  54  in an illustrative configuration in which filter  54  is formed on a transparent glass substrate) may be formed from a layer of silicon oxide to enhance adhesion with subsequent layers such as a subsequent ink layer. The ink layer may be a polymer containing colorant such as dye and/or pigment. The colorant may have a neutral color such as white, gray, or black, may have a non-neutral color such as red, blue, green, yellow, gold, or may have another suitable color. 
     A cross-sectional side view of an illustrative coated substrate for device  10  is shown in  FIG. 5 . As shown in  FIG. 5 , substrate  68  (e.g., a transparent glass substrate, etc.) may have an outer surface such as outer surface (exterior surface)  90  that faces a user such as viewer  62  who is viewing substrate  68  in direction  64  and that therefore faces the exterior regions surrounding device  10 . Substrate  68  may also have an opposing inner surface  90  (interior surface) that faces the interior of device  10  and housing  12  away from viewer  62 . Coating  74  may be formed on inner surface  92  and may face the interior of housing  12  and device  10 . Substrate  68  may be, for example, substrate  30  of  FIG. 2 , substrate  34  of  FIG. 2 , and/or other substrate in device  10 . Coating  74  may be coating  28  of  FIG. 2 , coating  36  of  FIG. 2 , coating  52  of  FIG. 2 , and/or another coating. 
     In the illustrative configuration of  FIG. 5 , thin-film interference filter  54  has initially been deposited on a separate substrate (substrate  76 ). Substrate  76  may be, for example, a sheet of polymer. After forming reflective layer  72  by forming thin-film interference filter  54  on flexible substrate  76 , reflective layer  72  may be laminated to inner surface  92  of substrate  68  using a layer of adhesive such as adhesive layer  70  (e.g., a polymer layer). Optional ink layer  78  may be formed on the interior surface of layer  72  (e.g., on the inner surface of substrate  76 ). If desired substrate  76  may be omitted to help reduce the thickness of layer  74  (e.g., the layers of filter  54  may form layer  72  without using an additional polymer film substrate such as substrate  76 ). Filter  54  of  FIG. 5  may be formed using 2-6 layers  56  or any other suitable number of layers  56 . Filter layers in device  10  such as filter  54  may be patterned to form logos, text, icons, and/or other patterns. 
     Another illustrative arrangement for providing substrate  68  with a coating on interior surface  92  is shown in  FIG. 6 . In the example of  FIG. 6 , coating  80  has been formed on inner surface  92  of substrate  68 . Viewer  62  may view coating  80  in direction  64  through the transparent material of substrate  68 . 
     Substrate  68  may be coated with an optional buffer layer such as layer  70 . Layer  70  may be a clear polymer and may have a thickness of 50 nm -3 microns, at least 0.1 microns, at least 0.2 microns, at least 0.3 microns, at least 0.5 microns, at least 1 micron, at least 2 microns, 1-3 microns, less than 5 microns, or other suitable thickness. When filter  54  is relatively thick, the presence of optional buffer layer  70  may help reduce stress-induced cracks that might damage substrate  68 . If the amount of stress imparted by filter  54  is relatively low, buffer layer  70  may be omitted. 
     Ink layer  78  may be deposited on the inner surface of filter  54 . Filter  54  may include layers  56  such as inorganic dielectric layers of alternating refractive index values. The layer  56  in filter  54  that is immediately adjacent to ink layer  78  may be formed from silicon oxide to promote adhesion (e.g., to ensure that ink layer  78  securely adheres to layer  54 ). When ink layer  78  is formed from white material or other brightly colored material, ink layer  78  may help reflect light that has been transmitted through substrate  68  outwardly towards viewer  62 . When ink layer  78  is black, light transmitted through substrate  68  may be absorbed, so that the color of the light related from filter  54  towards viewer  62  dominates. Gray ink reflects some but not all of the light that has been transmitted through substrate  68 . In configurations in which ink  78  has a non-neutral color (e.g., red, green, blue, yellow, gold, etc.), the color of coating  80  will be tinted accordingly. By using a color-neutral and angularly insensitive design for filter  54  (e.g., a gray mirror with a reflectively of 10-30%), filter  54  and therefore coating  80  will be relatively insensitive to performance fluctuations due to manufacturing variations in layers  56 . This helps ensure consistency when manufacturing numerous devices  10 . 
     If desired, the overall thickness of layer  80  may be minimized by using a relatively small number of layers  56  in filter  54 . This approach may be facilitated by using ink  78  to provide coating  80  with a desired color rather than relying on filter  54  to impart the desired color. 
     Other configurations may be used for coating  80 , if desired. The configuration of coating  80  described in connection with  FIG. 6  is merely illustrative. Buffer layer  70  may be included in coating  80  or may be omitted. Filter  54  may form a neutral-color angularly insensitive thin-film interference filter or may have a color and/or exhibit angular changes in color. Filter  54  may be configured to serve as a highly reflective mirror, a partially reflective mirror, an antireflection coating, etc. Layers  56  of filter  54  may be deposited by physical vapor deposition and/or other techniques. 
     The properties of the substrate coatings in device  10  may allow the substrate to be visually matched to nearby structures such as portions of housing  12  at one or more viewing angles. Consider, as an example, the arrangement of device  10  of  FIG. 7 . As shown in  FIG. 7 , substrate  100  (e.g., substrate  30 , substrate  34 , or other suitable substrate in device  10 ) may be mounted adjacent to a band of housing structures or other exposed portions of housing  12 .  FIG. 8  is a cross-sectional side view of device  10  of  FIG. 7  showing how substrate  100  may have a coating such as coating  106  (e.g., coating  74  of  FIG. 5  or coating  80  of  FIG. 6 ). 
     Coating  106  may include thin-film interference filter  54  and optional ink  78  that are selected to provide substrate  100  with an appearance that matches that of housing  12  and/or that contrasts or otherwise complements that of housing  12 . This matching (or contrasting) may occur at one or more viewing angles A. For example, the appearance of housing  12  and coated substrate  100  may match (or contrast in a predetermined fashion) when a viewer such as viewer  102  is viewing substrate  100  at normal incidence and may contrast in a predetermined fashion (or match) when viewer  102  is viewing substrate  100  in direction  104  at an off-axis angle A (e.g., when A is not zero and has another value such as a value of at least 45°). 
     As an example, housing  12  may have a gold color and coating  106  of substrate  100  may have a configuration that provides substrate  100  with a gold appearance (e.g., at an on-axis viewing angle where A is zero or at an off-axis viewing angle such as when A is at least 45°). In this type of configuration, the appearances of substrate  100  and housing  12  are deliberately blended. 
     As another example, consider a scenario in which the appearance of coated substrate  100  is configured to be gold at an on-axis viewing angle and in which ink  78  has a contrasting color such as blue. When substrate  100  is viewed at normal incidence (on-axis), a relatively small amount of underlying blue color from ink  78  will be visible so substrate  100  will appear to be gold. When substrate  100  is viewed at an off-axis viewing angle (e.g., at least 45°), however, the blue color of the underlying ink  78  in coating  106  will be visible to the user through coating  106 . As a result, the appearance of substrate  100  (e.g., rear housing wall  12 R, etc.) will change from gold to blue, depending on the angle at which substrate  100  is viewed. 
     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.