Abstract:
A light shutter includes a first electrode, a second electrode, and a third electrode, wherein the first and second electrodes are capacitively coupled to each other by the third electrode. The first electrode and the second electrode can be segmented and the segments separately addressed. In a display having a light shutter and an EL panel stacked on the same side of a substrate, there is a common front electrode and separate rear electrodes for operating the light shutter and the EL panel. The common electrode is electrically floating and the rear electrodes are capacitively coupled to each other by the common front electrode. The rear electrodes can be segmented and the segments separately addressed.

Description:
BACKGROUND TO THE INVENTION 
       [0001]    This invention relates to liquid crystal displays and, in particular, to a display having an electrode that is split and capacitively coupled to a common electrode. 
       GLOSSARY 
       [0002]    As used herein, a “display” is a device that provides information in visual form to a viewer. 
         [0003]    A “graphic” can be text, a symbol, an arbitrary shape, or some combination thereof. A graphic can be translucent, diffuse, shaded, colored, a silhouette or outline, or some combination thereof. Graphics can be positive (black on white) or negative (white on black), where white is diffuse, shaded, etc. 
         [0004]    A “phosphor” is not restricted to a single type of phosphor or dopant and does not exclude cascading phosphors or dyes for color enhancement. 
         [0005]    An “EL layer” is a layer containing phosphor and dielectric or separate layers of phosphor and dielectric. 
         [0006]    An “EL lamp” is a thick film, capacitive device including an EL layer between two electrodes, at least one of which is transparent. The phosphor is luminous when a voltage is applied to the electrodes. A “panel” is a plurality of EL lamps on a commons substrate. The lamps may be operated independently of one another or in unison. Alternating current must be used to drive an EL lamp. 
         [0007]    A “thick film” EL lamp refers to one type of EL lamp and “thin film” EL lamp refers to a different type of EL lamp. The terms only broadly relate to actual thickness and actually identify distinct disciplines. A thin, thick film EL lamp is not a contradiction in terms and such a lamp is much thicker than a thin film EL lamp. 
         [0008]    “Opaque” does not mean that no light is transmitted, only that the amount of light transmitted is substantially reduced, e.g. to fifteen percent of incident light. 
         [0009]    “Overlying” or “underlying” do not imply orientation and merely describe a situation wherein layers of materials have major surfaces facing each other, with or without contact. The surfaces are not necessarily planar. 
         [0010]    A liquid crystal display is a capacitive structure, having a dielectric (liquid crystal) between two electrodes, at least one of which is transparent. Often both electrodes are transparent and typically are made from indium tin oxide (ITO) sputtered on a transparent substrate, such as a dimensionally stable, transparent sheet of plastic. In order to provide graphic or alpha-numeric information, at least one of the electrodes is patterned. Typically, this includes screen printing a mask and etching the ITO layer. Etching is a chemical process with attendant problems, and cost, of handling and waste treatment. 
         [0011]    In the last twenty years, a particular class of materials, known as polymer dispersed liquid crystals (PDLC), has been developed for displays; e.g., see U.S. Pat. No. 4,992,201 (Pearlman). Devices using these materials operate on alternating current at 60-120 volts peak-to-peak, unlike earlier liquid crystal materials that operated at much lower voltages, and provide contrast without the need for polarizers. Sometimes referred to as “optical shutters,” polymer dispersed liquid crystals have applications outside the realm of displays. 
         [0012]    U.S. Pat. No. 6,842,170 (Akins et al.) discloses a liquid crystal display combined with an electroluminescent (EL) back light and a touch screen. The liquid crystal display is part of a keypad, containing a mask layer with images of the buttons on a telephone (0-9, * and #) and other control buttons. It is also disclosed that the liquid crystal display and the EL back light can share a common substrate. 
         [0013]    International Publication WO 2005/121878 discloses a liquid crystal display and an EL back light on the same side of a substrate, as does U.S. Application Publication 2002/0163606 (Kitai et al.). Other permutations are known in the art, with devices on opposite sides of a substrate; e.g., see U.S. Pat. No. 5,121,234 (Kucera) and U.S. Pat. No. 6,441,551 (Abe et al.). Various interlayers or outer layers for affecting optical performance, e.g. color, reflectance, and dispersion, are also known in the art. These devices are typically built front to rear, which means that the first layer on the substrate is a transparent front electrode. Subsequent layers cover the front electrode, making electrical contact with the front electrode difficult. Also, bus bars must be used to reduce resistance from the electrode to a point of contact, which can introduce additional problems with location and uneven thickness, particularly for multi-segment displays. 
         [0014]    An EL device emits light at almost any applied voltage. Useful amounts of light generally require more than sixty volts rms. Liquid crystal devices are more like switches: they do not change state until a threshold voltage is reached, e.g. eighty volts rms. For PDLC, the voltage is 10-40 volts rms. The response curve is not perfectly square—that would be impossible because of the voltage gradient across the thickness of the material—but the change is relatively abrupt, certainly compared with the roughly linear response of an EL lamp to voltage; e.g. see U.S. Application Publication 2006/0250534 (Kutscher et al.). 
         [0015]    Driving EL and PDLC between a single pair of electrodes, as disclosed in the &#39;234 patent and the &#39;551 patent, is not entirely straightforward. For some applications, the answer is simple. Apply a large enough voltage to open shutters and cause the EL lamps to emit light. For multi-segment displays, there was no simple answer to driving EL and PDLC simultaneously, e.g. because of problems routing conductive traces. 
         [0016]    The choice of a technology for a particular display is a balance of competing interests, not the least of which is cost. In the case of cellular telephones, the choice is often based on the presumption that the user will be indoors or at least not in direct sunlight when the telephone is used. In other words, the content of the display all but vanishes in bright light because the display relies on luminous back lighting for visibility. Many liquid crystal displays rely on reflective back lighting. Thus, the back lighting increases or decreases with ambient light and the content of the display remains visible. Some displays try for the best of both worlds with a “transflective” layer between a back light and a liquid crystal module. 
         [0017]    Screen printing and roll coating are both well known processes for manufacturing EL lamps and other devices. A problem with screen printing is the cost and handling of product in a piecemeal fashion. A problem with roll coating liquid crystal material is the need to provide access to the first deposited electrode for electrical contact. Etching is costly, both in the number of steps and in the possibly environmentally unfriendly materials used for etching. Other solutions, e.g. such as disclosed in U.S. Pat. No. 5,821,691 (Richie et al.) are useful but restrict the location of the first electrode. It is known in the art of electroluminescent lamps to capacitively couple to the first deposited electrode by using a split electrode on the opposite side of a panel; e.g. see U.S. Pat. No. 2,928,974 (Mash). It is also known that there is a problem with this approach because brightness is proportional to area; e.g. see U.S. Pat. No. 5,508,585 (Butt). This means that two or more electrodes must be equal in area in order for the lamps to be equal in luminance, all other factors being the same. 
         [0018]    U.S. Pat. No. 6,934,313 (Deacon) discloses an optical waveguide of PDLC material having two electrodes on one side of the PDLC material and a third electrode on the other side of the PDLC material. The two electrodes are not capacitively coupled to each other by the third electrode because the third electrode does not float electrically. Transparent electrodes are also disclosed. 
         [0019]    In view of the foregoing, it is therefore an object of the invention to provide a PDLC light shutter in which one electrode is split. 
         [0020]    Another object of the invention is to provide a split electrode capacitively coupled to a floating electrode for controlling a PDLC layer. 
         [0021]    A further object of the invention is to enable roll coating of the front electrode of a light shutter. 
         [0022]    Another object of the invention is to provide a common, floating, front electrode for stacked PDLC and EL layers. 
         [0023]    A further object of the invention is to provide capacitively coupled electrodes for PDLC and EL layers. 
         [0024]    Another object of the invention is to provide optimum drive voltages for each layer in a stack including a PDLC layer and an EL layer. 
         [0025]    A further object of the invention is to simplify the construction of displays having light shutters as part of the display. 
         [0026]    Another object of the invention is to provide a light shutter and EL panel on the same side of a substrate with a common front electrode and separate rear electrodes. 
       SUMMARY OF THE INVENTION 
       [0027]    The foregoing objects are achieved by this invention in which a light shutter includes a first electrode, a second electrode, and a third electrode, wherein the first and second electrodes are capacitively coupled to each other by the third electrode. The first electrode and the second electrode can be segmented and segments separately addressed. In a display having a light shutter and an EL panel stacked on the same side of a substrate, there is a common front electrode and separate rear electrodes for operating the light shutter and the EL panel. The common electrode is electrically floating and the rear electrodes are capacitively coupled to each other by the common front electrode. The rear electrodes can be segmented and the segments separately addressed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    A more complete understanding of the invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings, in which: 
           [0029]      FIG. 1  is a cross-section of a light shutter constructed in accordance with a preferred embodiment of the invention; 
           [0030]      FIG. 2  is a schematic of the embodiment of  FIG. 1 ; 
           [0031]      FIG. 3 ,  FIG. 4 ,  FIG. 5 ,  FIG. 6 , and  FIG. 7  illustrate the operation of a device constructed in accordance with the invention; 
           [0032]      FIG. 8  is a chart illustrating the operation of a layer of PDLC material; 
           [0033]      FIG. 9  is a cross-section of a back lit shutter constructed in accordance with a preferred embodiment of the invention; and 
           [0034]      FIG. 10  is a cross-section of a back lit shutter constructed in accordance with an alternative embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0035]      FIG. 1  illustrates a light shutter constructed in accordance with a preferred embodiment of the invention. Specifically, light shutter  10  includes transparent substrate  11  and transparent electrode  12  overlying one major surface of the substrate. Liquid crystal layer  13  overlies electrode  12 . Overlying liquid crystal layer  13  are electrodes  15 ,  16 , and  17 . Electrodes  15  and  16  are coupled to source  18  of alternating current and are driven to opposite polarities. Electrode  17  is coupled by switch  19  to the same terminal on source  18  as electrode  16 . 
         [0036]    Substrate  11  can be glass or any transparent polymeric material, such as polycarbonate or polyethylene terephthalate (PET). It is only required that the substrate be transparent and can be coated with an adherent layer of transparent, conductive material. The thickness of substrate  11  is not critical and depends upon application. Transparent electrode  12  is preferably indium tin oxide. Liquid crystal layer  13  is preferably a polymer dispersed liquid crystal. 
         [0037]    In accordance with one aspect of the invention, layer  13  is roll coated and is not patterned to provide clearance for contact to electrode  12 . In accordance with another aspect of the invention, electrode  12  is floating; i.e. there is no ohmic connection to layer  12 . In accordance with a third aspect of the invention, the electrodes on the upper surface of layer  13  are capacitively coupled to each other by means of electrode  12 . 
         [0038]      FIG. 2  is a schematic of the circuit illustrated in  FIG. 1 , with corresponding elements bearing the same reference number as in  FIG. 1 . Capacitor  21 , which includes electrode  15 , is coupled in series with capacitor  22 , which includes electrode  16 . Capacitor  23 , which includes electrode  17 , is coupled in parallel with capacitor  22 . 
         [0039]    With switch  19  open, capacitors  21  and  22  form a voltage divider that produces a voltage at their junction, i.e. on electrode  12 , in inverse proportion to the ratio of their capacitances. Only when the capacitances are substantially equal is the voltage from source  18  halved by the voltage divider. 
         [0040]    As known in the art, capacitance is proportional to area and inversely proportional to the distance between the plates or electrodes. As a first approximation, it is presumed that the areas of the plates in a given capacitor are equal even though it is obvious from  FIG. 1  that electrode  12  is vastly bigger than the other electrodes. 
         [0041]    It has been discovered that the coupling effect can not only be exploited for selectively switching liquid crystal material, it can also be used to control both liquid crystal and electroluminescent layers from a common electrode, thereby eliminating the need to make ohmic contact with the front, transparent, electrode in a display. This greatly reduces costs, reduces thickness, and simplifies the construction of a shuttered light source for a display. The layers can be on the same side of a substrate, whereby the substrate can be the exposed, outer surface of a display. The need for protective layers, special handling, and other problems is thus avoided. 
         [0042]      FIG. 3  is essentially the same, electrically, as  FIG. 1  except that the common electrode has been divided into several smaller, electrically connected electrodes and switch  19  has been eliminated. The common electrode is floating. For the sake of discussion, all electrodes are presumed to have the same area. 
         [0043]    In  FIG. 4 , the structure has been modified slightly. Electrodes  51  and  53  are positioned lower than electrodes  41  and  43  ( FIG. 3 ). This decreases the distance to electrodes  46  and  48  and increases the capacitance of these capacitors. 
         [0044]    In  FIG. 5 , electrode  51  is displaced from electrode  32 . Although it may appear that the electric field should extend from electrode  51  to electrode  32 , this is not the case, at least not in any meaningful way. Capacitance is determined by area of overlap. The capacitance associated with electrodes  51  and  55  is far greater than the capacitance associated with electrodes  32  and  55 . Thus, the alternating current flows from electrode  51  to electrode  55  and from electrode  56  to electrode  32  (ignoring the other electrodes for the moment). 
         [0045]    The displacement of the electrodes is such that two chambers are created, chambers  58  and  59 . If these contain air, for example, the dielectric constants of the chambers are equal and the capacitance of electrodes  51  and  55  is much greater than the capacitance of electrodes  32  and  56 . 
         [0046]    In  FIG. 6 , the segmented lower electrode has been replaced with electrically identical, continuous electrode  61 . This is to simplify the drawing. Layer  63  is interposed between electrodes  32  and  34  and electrodes  51  and  53 . Similarly, layer  64  is interposed between electrodes  51  and  53  and common electrode  61 . 
         [0047]    The capacitor formed by electrodes  32  and  61  and the capacitor formed by electrodes  34  and  61  have two dielectrics, whereas the capacitor formed by electrodes  51  and  61  and the capacitor formed by electrodes  53  and  61  have only one dielectrics. If the materials of layers  63  and  64  are the same, this is of no consequence. If, in accordance with another aspect of the invention, the materials are not the same, a whole new world of possibilities opens up. 
         [0048]    In particular, layer  63  is preferably an EL layer and layer  64  is preferably PDLC. The dielectric constant of an EL layer is generally lower than the dielectric constant of PDLC. The path from electrode  51  to electrode  32  includes two capacitors and three dielectrics. Voltage drop can be calculated as though there were three capacitors in series. 
         [0049]    As known in the art, the capacitance of capacitors connected in series is equal to the reciprocal of the sum of the reciprocals of the capacitances, as in the following equation. 
         [0000]    
       
         
           
             C 
             = 
             
               1 
               
                 
                   1 
                   
                     C 
                     1 
                   
                 
                 + 
                 
                   1 
                   
                     C 
                     2 
                   
                 
                 + 
                 
                   1 
                   
                     C 
                     3 
                   
                 
               
             
           
         
       
     
         [0000]    The voltage divides as the inverse of the capacitances. Thus, for equal areas, equal dielectric constants, and equal thicknesses, the voltage drop across each dielectric is the same. Thus, the voltage across electrode  51  and  61  is one third the applied voltage and the voltage across electrodes  61  and  32  is two thirds the applied voltage. As the dielectric constants, distances, and areas change, the situation becomes more complicated but the same rules apply. In accordance with the invention, these rules are exploited to provide novel displays. 
         [0050]    For example, in  FIG. 7 , electrode  71  is substantially larger than electrodes  72 ,  73 , or  74 . This means that the capacitance between electrode  71  and  75  is larger than the capacitance between electrode  73  and electrode  75 . With switches  76  and  78  closed, the voltage across electrodes  71  and  75  is therefore lower than the voltage across electrodes  73  and  75 . As noted above, liquid crystal material behaves like a switch. Thus, the volume of PDLC between electrodes  71  and  75  remains opaque, indicated by the darker stippling, while the volume of PDLC between electrodes  73  and  75  is transparent. This selectivity, which applies to the electroluminescent layer also, enables one to design displays of considerable sophistication. 
         [0051]      FIG. 8  illustrates the operation of a layer of representative PDLC material. As indicated by the curved line, the material remains opaque until the voltage across the layer exceeds some threshold, e.g., twenty volts rms. Then, the layer becomes substantially transparent. Increasing the voltage above the threshold does not change transparency significantly, as indicated by comparison with the dashed line. Normally transparent material works oppositely. 
         [0052]    In  FIG. 7 , some electrodes are coupled to one side of AC source  70  and some to the other. Any number of electrodes can be coupled to either side of the AC source. The electrodes are coupled to source  70  by switches. A switch is indicated as being in series with each electrode. This may not be the case in practice, depending upon the nature of the display. The electrodes are selected in combinations that need not include all electrodes. Stated another way, the entire set of electrodes can be selected in any number of sub-sets to produce a desired display. 
         [0053]    Although a given design may seemed fixed by the physical construction of a display, this is not the case. Once constructed, one cannot change spacing or dielectric constant but one can, in effect, change area by selectively combining areas, e.g. by closing or opening switches  76 ,  77 ,  78 , and  79 , thereby increasing capacitance and reducing voltage or decreasing capacitance and increasing voltage, affecting the EL layer, the PDLC layer, or both layers. 
         [0054]    For example, assume that layer  63  is an EL layer and layer  64  is a layer of PDLC material. Closing switches  76  and  78  can render the layer  64  transparent in the area of electrode  73  (ignoring the relative areas of the electrodes for the moment), allowing incident light to be reflected from the EL layer. Thus, a device constructed as illustrated in  FIG. 7  can operate in a “daytime” mode, with no backlighting, and in a “night time” mode, with backlighting. 
         [0055]    In the following examples, change is effected by changing areas to adjust voltage. These and many other combinations are possible in a multi-segment display without access to the front electrode. Construction is simplified yet highly variable displays can be produced in accordance with the invention. 
         [0056]    Assume that switches  77  and  79  are closed. EL layer  63  emits light in the regions beneath electrodes  72  and  74  and PDLC layer  64  is transparent in the regions beneath electrodes  72  and  74 . Closing switch  76  adds greatly to the capacitance on the left side of supply  70  (assuming it has a relatively large area as illustrated), reducing the voltage. The regions under electrodes  71  and  72  are opaque. The region in EL layer  63  under electrode  72  dims and the region in EL layer  63  under electrode  74  brightens. 
         [0057]    Assume that switches  77  and  79  are closed. EL layer  63  emits light in the regions beneath electrodes  72  and  74  and PDLC layer  64  is transparent in the region below of electrode  72 . Closing switch  78  adds capacitance to the right hand side of supply  70 , thereby reducing the voltage across electrode  74  and electrode  75 , dimming EL layer  63  in the area of electrode  74  and brightening EL layer in the area of electrode  72 . 
         [0058]    Depending upon the relative areas, PDLC layer  64  can be made opaque because of the reduced voltage, even when the region above in EL layer  63  is luminous. A region of EL layer  63  can remain luminous when the underlying shurtter layer is opaque because of the relatively linear response of the EL layer to voltage and the very non-linear response of the PDLC layer to voltage. Thus, one can use electrode  74 , or any electrode, to vary the net capacitances of other lamps and shutters in a set or sub-set of electrodes. 
         [0059]    Without trying to exhaust all possible combinations, operation can be likened to the common electrode acting somewhat like the fulcrum of a see-saw. Decreasing brightness or opacity on one side increases brightness or opacity on the other side. This feature allows one to affect a display remotely from the affected area and greatly increases the flexibility of the design of a display. 
         [0060]      FIG. 9  is a cross-section of a preferred embodiment of the invention. In this embodiment, the basic structure is as shown in  FIG. 7 , with the addition of isolation layer  81  and bus bars  83 ,  84 , and  85 . Isolation layer  81  modifies the dielectric constant between the electrodes for the EL lamps and can include a graphic, such as graphic  87 . Isolation layer  81  is optional and provides a convenient way to modify dielectric constant without reformulating inks for the phosphor layer or the dielectric layer. Layer  81  also provides a convenient way to add a graphic. 
         [0061]      FIG. 10  is a cross-section of an alternative embodiment of the invention.  FIG. 9  differs from  FIG. 8  in the location of isolation layer  91 . In  FIG. 8 , isolation layer  81  is between the phosphor layer and the front electrode. In  FIG. 9 , isolation layer  91  is between the dielectric layer and the rear electrode. 
         [0062]    The invention thus provides a PDLC light shutter in which one electrode is split and the split electrode is capacitively coupled to a floating electrode. Because an electrode is floating, a PDLC layer can be roll coated over the floating electrode, reducing costs, providing greater uniformity, and simplifying construction. Two layers of split electrodes can separately control a layer of PDLC and a layer of EL material. By controlling capacitance, one can optimize drive voltages for the PDLC layer and the EL layers. 
         [0063]    Having thus described the invention, it will be apparent to those of skill in the art that various modifications can be made within the scope of the invention. For example, the several illustrations are not exhaustive of the series-parallel combinations of capacitors that can be made in an actual embodiment of the invention. Although described in terms of the electrodes defining the pattern, the phosphor layer can be patterned (e.g. screen printed) instead of or in addition to having patterned electrodes. Gaps in the pattern can be used as “dummy” loads for adjusting capacitance and voltage.