Abstract:
An electroluminescent lamp includes two electrodes and at least three connectors, a first connector coupled to a first electrode and a second connector and a third connector coupled to the second electrode. A first electric current flows through the lamp between the first connector and the second connector, causing the lamp to emit light. A second electric current flows through the second electrode between the second connector and the third connector, whereby the second electrode increases in temperature in response to the second electric current. When backlighting a liquid crystal display, the second electrode provides heat to warm the display for proper operation. Either electrode can be used as a heater.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to electroluminescent (EL) lamps and, in particular, to an EL lamp having a heater element as one electrode As used herein, an EL “panel” is a single substrate including one or more luminous areas, wherein each luminous area is an EL “lamp.” One or more lamps in a panel can include a heater as an electrode. 
     An EL lamp is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is transparent. The dielectric layer includes a phosphor powder or there is a separate layer of phosphor powder adjacent the dielectric layer. The phosphor powder emits light in the presence of a strong electric field, using very little current. 
     A modern EL lamp is a thick film device, typically including a transparent substrate of polyester or polycarbonate material having a thickness of about 7.0 mils (0.178 mm.). A transparent, front electrode of indium tin oxide (ITO) or indium oxide is vacuum deposited onto the substrate to a thickness of 1000 Å or so. A phosphor layer is screen printed over the front electrode and a dielectric layer is screen printed over phosphor layer. A rear electrode is screen printed over the dielectric layer. Other methods for making an EL lamp can be used instead, e.g. roll coating. 
     The inks used for making an EL lamp include a binder, a solvent, and a filler, wherein the filler determines the nature of the printed layer. A typical solvent is dimethylacetimide (DMAC) or ethylbutylacetate (EB acetate). The binder is typically a fluoropolymer such as polyvinylidene fluoride/hexafluoropropylene (PVDF/HFP), polyester, vinyl, or epoxy. A phosphor layer is typically deposited from a slurry containing a solvent, a binder, and zinc sulphide particles. A dielectric layer is typically deposited from a slurry containing a solvent, a binder, and barium titanate (BaTiO 3 ) particles. A rear (opaque) electrode is typically deposited from a slurry containing a solvent, a binder, and conductive particles such as silver or carbon. Because the solvent and binder for each layer are chemically the same or similar, there is chemical compatibility and good adhesion between adjoining layers. 
     In many applications, e.g. automotive displays and portable devices such as wrist watches, radios, global positioning systems, and compact disk players, an EL lamp is used for backlighting a liquid crystal display (LCD); e.g., as disclosed in U.S. Pat. No. 4,580,877 (Washo) and U.S. Pat. No. 5,121,234 (Kucera). The combination is popular because of the uniformity of the back lighting and the resolution and contrast of the LCD. There is a problem in that the liquid crystal material becomes too viscous for proper operation at low temperatures, e.g., below 0° C. It is known to provide a heaters for an LCD but a heater often increases the bulk of the display, which is undesirable. Also, heaters typically apply heat nonuniformly to the display. 
     It is known in the art to vary the resistivity of an ITO layer by changing thickness As disclosed in U.S. Pat. No. 4,880,475 (Lindmayer), relating to a thin film (glass substrate) electroluminescent device; an ultra-thin, high resistivity ITO layer is protected by another metal oxide layer, such as nickel oxide. 
     It is also known in the art to use an ITO layer as a heater in an LCD. As disclosed in U.S. Pat. No. 5,559,614 (Urbish et al.), the heater electrode is coplanar with but separate from the rear electrode in the LCD. The heater electrode is disclosed as a combination of ITO and a copper oxide layer. Connections to the heater are only described generically but at least two additional connectors are required for the heater. More than two additional connections are likely required for a patterned heater electrode. 
     It is also known in the art for an LCD and an EL lamp to share a common electrode, e.g. as disclosed in U.S. Pat. No. 4,500,173 (Leibowitz et al.), but it is not known that the common electrode be a heater. 
     In view of the foregoing, it is therefore an object of the invention to provide an EL lamp with a heater electrode. 
     Another object of the invention is to provide an EL lamp wherein the front electrode and the heater electrode are the same electrode. 
     A further object of the invention is to provide an EL lamp wherein the rear electrode and the heater electrode are the same electrode. 
     Another object of the invention is to provide an EL lamp with an integral heater requiring a single, additional connector. 
     A further object of the invention is to provide a liquid crystal display with an EL lamp for backlighting and for heating the liquid crystal display. 
     Another object of the invention is to provide a liquid crystal display backlit by an EL lamp having a transparent, heating, front electrode. 
     A further object of the invention is to provide a liquid crystal display with a heater covering the entire area of the display. 
     SUMMARY OF THE INVENTION 
     The foregoing objects are achieved in this invention wherein an electroluminescent lamp includes two electrodes and at least three connectors, a first connector coupled to a first electrode and a second connector and a third connector coupled to the second electrode. A first electric current flows through the lamp between the first connector and the second connector, causing the lamp to emit light. A second electric current flows through the second electrode between the second connector and the third connector, whereby the second electrode increases in temperature in response to the second electric current. When backlighting a liquid crystal display, the second electrode provides heat to warm the display for proper operation. Either electrode can be used as a heater. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a cross-section of an electroluminescent lamp; 
     FIG. 2 is a cross-section of an electroluminescent lamp and a liquid crystal display constructed in accordance with a preferred embodiment of the invention; 
     FIG. 3 is a plan view of an electroluminescent lamp constructed in accordance with the invention; 
     FIG. 4 is a cross-section of an electroluminescent lamp and a liquid crystal display constructed in accordance with an alternative embodiment of the invention; and 
     FIG. 5 is a cross-section of an electroluminescent lamp and a liquid crystal display constructed in accordance with an alternative embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a cross-section of an EL lamp. In none of the cross-sections are the several layers shown in proportion or to scale. Lamp  10  includes transparent substrate  11  of polyester or polycarbonate material. Transparent electrode  12  overlies substrate  11  and includes indium tin oxide or indium oxide. Phosphor layer  16  overlies electrode  12  and dielectric layer  15  overlies the phosphor layer. The phosphor layer and the dielectric layer can be combined into a single layer, as indicated by reference number  13 . Overlying dielectric layer  15  is rear electrode  18  containing conductive particles such as silver or carbon in a resin binder. 
     During operation, an alternating current is applied to electrodes  12  and  18 , causing a minute current to flow between the electrodes, through the lamp, causing the phosphor in layer  16  to emit light. 
     In accordance with the invention, a small current is applied to opposite edges of transparent electrode  12 , causing current to flow across the electrode, orthogonal to the current through the lamp. The amount of current is determined by the resistivity of electrode  12  and the area of the lamp. A resistivity of approximately 50-500Ω per square is preferred. The applied voltage can be alternating or direct. In most applications, it is assumed that the applied heater voltage is 3-12 volts DC. 
     FIG. 2 illustrates an EL lamp attached to a liquid crystal display for back-lighting the display. Lamp  10  is constructed as described above. Liquid crystal display  20  includes transparent electrode  21 , liquid crystal material  22 , and transparent electrode  23 . Other layers, e.g. polarizing layers, are not shown but are well known in the art. Liquid crystal material is typically somewhat viscous and, at temperatures below 0° C., tends to gel. Liquid crystal displays operate by changing the alignment of the molecules of the material when an electric field is applied. At low temperatures, the viscosity of the material increases greatly and the material may not respond to an applied field. By providing a small amount of heat, the temperature of the liquid crystal material can be raised sufficiently for proper operation in cold temperatures. Displays, such as instrument panels in vehicles, tend to trap heat. Thus, little heat is necessary to raise the temperature sufficiently for the display to operate normally. 
     FIG. 3 is a plan view of an EL lamp constructed in accordance with the invention. EL lamp  30  includes connectors  31  and  32  for the front electrode and connectors  33  and  34  for the rear electrode. EL lamps typically have bus bars surrounding an electrode to provide a low resistance connection to the electrode. With a heater electrode, a bus bar completely around the perimeter of an electrode is not possible, unless a second connection is made at a part of the lamp hidden from a viewer. Bus bar  36  is located along one edge of EL lamp  30  and is electrically coupled to connector  31 . Similarly, bus bar  37  is located along the opposite edge of the lamp and is electrically coupled to connector  32 . Thus, a voltage coupled across connectors  31  and  32  will cause a current to flow across the front electrode, warming the electrode. The heating current can be alternating or direct. 
     Depending upon the power supply used, only a single, additional connector is needed. One of connectors  33  and  34  can be eliminated unless kept for reasons unrelated to the invention. A lamp constructed in accordance with the invention is compatible with a driver having either a single-ended output; i.e. a driver producing alternating current between an output terminal and ground, a half-bridge output, or a transformer output. If a driver has a full bridge output, (four switching elements), then a floating bias is necessary for the heater, e.g. the voltage is coupled across the front electrode by a transformer. 
     In the prior art, great efforts have been made to obtain a uniform output from an EL lamp. The voltage used to provide heat does not introduce a significant non-uniformity across the face of an EL lamp because the heater voltage is small relative to the peak-to-peak voltage used to drive the EL lamp; e.g. 3 volts vs. 120 volts. 
     FIG. 4 illustrates an alternative embodiment of the invention in which a transparent rear electrode is used for the EL lamp and the lamp is attached to a liquid crystal display by the rear electrode. This moves the phosphor layer closer to the liquid crystal display and provides a slightly more diffuse light source because the light is passing through the dielectric layer, as illustrated in FIG.  4 . The dielectric layer and the phosphor layer can be combined. 
     In particular, lamp  41  includes transparent electrode  42  in contact with rear electrode  43  of liquid crystal display  44 . Phosphor/dielectric layer  46  overlies electrode  42  and rear electrode  47  overlies layer  46 . Substrate  48  overlies rear electrode  47 . A reflective layer, such as layer  49 , is optionally provided to redirect light toward a viewer. Electrode  42  is preferably the heater electrode for liquid crystal display  44 . 
     Instead of being constructed on substrate  47 , the lamp materials, i.e. layers  42 ,  46 , and  47 , can be deposited on a temporary substrate, e.g. by roll coating, and laminated to the rear electrode of liquid crystal display  44 , thereby eliminating substrate  48 . Electrodes  42  and  43  can be separated by a thin insulating layer, not shown, if it is desired to pattern either electrode. An adhesive layer can serve as both an adhesive and as an insulator. 
     FIG. 5 illustrates an alternative embodiment of the invention in which an electrode is shared with a liquid crystal display. Liquid crystal display  51  includes transparent conductive layer  52  that is both the rear electrode of the liquid crystal display and the front electrode of EL lamp  53 . Phosphor layer  54  overlies layer  52  and dielectric layer  55  overlies the phosphor layer. Rear electrode  56  can be deposited from a conductive ink or can be a layer of metal foil, such as aluminum, which would also provide a reflective function. Layer  52  is preferably the heater layer. 
     EL lamp  53  is preferably made by depositing the lamp materials, in reverse order, either on a metal foil, which then becomes the rear electrode, or on a temporary substrate, starting with layer  56 . The materials are then laminated to the rear of liquid crystal display  51  and the temporary substrate, if present, is removed. 
     The invention thus provides an EL lamp having a heater electrode as either the front electrode, the rear electrode, or both electrodes. The lamp requires a minimum of a single, additional connector for the heater. Combined with a liquid crystal display, the invention provides an EL lamp for backlighting and for heating the liquid crystal display. 
     Having thus described the invention, it will be apparent to those of skill in the art that many modifications can be made with the scope of the invention. For example, the rear electrode of lamp  30  can be a heater electrode, if desired, as an alternative to the front electrode or in addition to the front electrode. The heater electrode can be patterned and still serve as both a front, or rear, electrode and a heater electrode. A bimetallic switch, or other apparatus, can be added near the display for sensing when the display has warmed sufficiently and opening the heater circuit when warm.