Substratum for liquid crystal displays
FIELD OF THE INVENTION 
     The invention relates to improvements in liquid crystal displays. There are provided integrated structures for use in large-scale liquid crystal displays which considerably improve the performance of same. The large-scale liquid crystal displays made possible by the present invention are of importance for use in matrix screens in computer industry and can be used in measuring instruments, electronic games, advertising displays and the like. 
     BACKGROUND OF THE INVENTION 
     The size of conventional liquid crystal displays is rather restricted. It has hitherto not been possible to produce large liquid crystal displays consisting of large integral glass plates as these are not of an adequate quality as regards the required flat surface which is imperative in order to ensure an even distance between the plates between which the liquid crystal is arranged at this must be of a certain uniform thickness, the absence of which affects adversely the quality of the display. The structures of the invention (termed &#34;substratum&#34;), make possible the mass production of large size liquid crystal displays of high quality. 
     SUMMARY OF THE INVENTION 
     According to the invention there is provided an integrated sandwich construction, termed &#34;substratum&#34; for use in large scale liquid displays, which makes possible the construction of large scale liquid crystal displays of high quality. 
     The term &#34;large scale&#34; is intended to define structures which are larger than hitherto possible employing flat integral glass plates. Generally this term defines displays of the order of about 10 cm by 10 cm, but much larger displays can be produced without difficulty. 
     The integrated sandwich constructions of the present invention consist essentially of three bonded layers of desired dimension namely a flat transparent sheet of adequate thickness to provide for rigidity and mechanical strength, a layer of an elastic transparent material and a further very thin glass plate bonded to said elastic layer. In a liquid crystal display such integrated structure is applied on top of the liquid crystal layer which is supported by another thick transparent sheet, preferably a glass sheet, there being provided spacing means and electrodes on said thin glass plate and on the support plate which are in contact with the liquid crystal. The combination of the resilient, elastic layer with the thin glass plate provides for a close and even contact of the said &#34;sandwich structure&#34; with said liquid crystal supported by another flat plate, resulting in said liquid crystal being in touch with both adjacent plates all over the surface thereof, there being provided an even distance between opposing electrodes and even thickness of the liquid crystal, substantially improving the performance of the display. 
     The three layers of the said substratum are hermetically cemented together by suitable means such as conventional adhesive. When this is used to form with a liquid crystal layer supported by a flat glass plate a liquid crystal display, providing for the required electrodes and spacers, the substratum and the said other components are secured with each other by suitable means, such as a suitable frame in the dead spaces of the pattern. 
     In order to indicate suitable sizes of the components the following guidelines are given: 
     a. The support plate is a large transparent plate, of glass or plastic, of adequate thickness to provide for mechanical strength and rigidity, for sheets of say 10×10 cm, a thickness of the order of 3 mm to 5 mm may be used. For a size of say 20×30 cm, a thickness of about 5 mm by 7 mm may be used, 
     b. The elastic transparent layer is made of a suitable rubber or similar compound having the required mechanical properties. Generally a thickness of from about 0.4 mm to about 0.7 mm will give satisfactory results; 
     c. The thin glass sheet is to be self-supporting, it ought to be as thin as feasible. Generally for displays of about 10×10 cm a sheet of about 0.1 to 0.6 mm, preferably about 0.2 mm to about 0.5 mm thickness will give satisfactory results. For larger displays a thickness of about 0.3 to about 0.6 mm will be satisfactory. The thin glass plate is sandwiched in the final structure between the elastic layer to which it is bonded over its entire area and the liquid display (with the electrodes and spacers) said spacers being the actual support and they also define the distance. 
     The three sheets are preferably hermetically bonded by a transparent elastic glue; resulting in a mechanically strong structure. Towards the outside the thick sheet provides firmness and rigidity. Towards the liquid crystal the structure is adequately resilient so as to provide for a close contact with the spacers, resulting in a uniform spacing of the electrodes of high accuracy. The liquid crystal is generally about 7μ to 10μ thick, preferably about 8μ. 
     Thus, spacers of suitable intervals with such height will provide for the required uniform spacing. 
     The invention is intended to extend to all arrangements based on transparent structures comprising a rigid support, a resilient layer and a thin transparent sheet adapted to adjust itself to the contours of a liquid crystal supported by a support sheet which is not entirely flat. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The invention is illustrated with reference to the enclosed schematical drawings, which are not according to scale as regards the various components and thickness and extent of these, wherein: 
     FIG. 1 is a sectional view of part of a conventional liquid crystal display, illustrating the problems of conventional liquid displays (not part of the invention); 
     FIG. 2 is a partial sectional view through a liquid crystal display of the invention; 
     FIG. 3 is a partial sectional view through a twin-layer liquid crystal display of the invention. 
    
    
     The conventional liquid crystal display of FIG. 1 consists of a glass plate 1, serving as cover plate and a support glass plate 2, between which there is arranged the liquid crystal layer 3, there being provided a plurality of spacers 4 to 9, which define the desired thickness of the liquid crystal layer 3, there being provided electrodes 10a to 10b respectively on glass plates 1 and 2 respectively. The uneven nature of the support plate 1 is shown in an exaggerated manner, and when the components are sealed by means of the frame 11 (only the left hand end being shown), due to the uneven nature of the support sheet 2, at some places like around spacers 4 and 5, 6 and 7, 7 and 8, and 8 to 9, the desired distance defined by the height of the spacers is not maintained, and the liquid crystal layer will be of uneven thickness resulting in a different distance between the electrodes at various locations. Without spacers the too small distance may manifest itself as rainbow colors, or even as shorts between opposite electrodes. It has been proposed in literature to use as cover plate a thin glass plate with spacers as shown in FIG. 1, but this does not result in an even spacing due to the uneven nature of large glass plates. This results also in the slowdown of switching time where the liquid crystal layer is too thick, and by rapid multiplexing it is possible that no switching will take place at all. Large thin glass plates in such structures are liable to be broken due to the vibrations or other stresses to which such arrangements are exposed. 
     The novel substratum, i.e. the sandwich structure of the present invention eliminates to a large extent the drawbacks of conventional structures and ensures an even uniform liquid crystal layer. 
     The embodiment illustrated in FIG. 2 comprises a triple-layer substrate sandwich construction comprising the base plate 12 which is a thick firm glass or plastic plate to which there is attached the transparent resilient elastic layer 13 which is covered with the thin glass plate 14 attached over its entire area to said layer 13. This structure is firm, and towards the outside of the liquid crystal display, the upper surface of the plate 12 presents a rigid structure. The lowermost layer of thin glass plate 14 is provided with a plurality of electrodes 23. 
     The triple-layer structure is attached to the lower part of the display structure which comprises a transparent glass plate 25 provided with a plurality of electrodes 22, and which supports the liquid crystal layer 15, there being provided a plurality of spacers 16 to 21 of the same height, which defines the spacing between the plates 14 and 25. The two assemblies are attached with each other by means of sealing frame 24, only the left-hand side of which is shown in the Figure. Due to the resilient nature of the layer 13 and the fact that glass plate 14 is a very thin one (of the order of 0.2 to 0.5 mm, for sizes of up to about 20×30 mm), the glass plate 14 is able to adjust itself to the contours of the lower plate 25, expressed by the spacers 16 to 21, so that said plate 14 is in firm contact with each of the spacers, resulting in a uniform spacing all over the surface of the liquid crystal 15, resulting in an identical distance between the opposite electrodes 22 and 23 all over the panel. The liquid crystal is generally about a thickness of 8μ, and this is maintained all over the surface of the liquid crystal layer. The distance is maintained during various conditions of operation, and due to the provision of the resilient layer 14 the thin glass plate is protected and will withstand mechanical stresses without deformation or breakage. Such liquid crystal display panels can be produced in this manner up to sizes which were hitherto infeasible with integral glass plates. 
     The arrangement according to the invention is applicable to the production of transmissive, reflective and transflective liquid crystal displays, of large size. 
     FIG. 3 illustrates another embodiment of the invention, consisting of a combination of two liquid crystal displays in an integral unit. This twin-display comprises in combination outer thick glass plates 36 and 37, on which there are provided respectively transparent electrodes 35 and 26, the inner surfaces of the two plates being in contact with liquid crystals 34 and 27, respectively, which are in touch with thin glass plates 32 and 29, respectively, to which there are attached transparent electrodes 33 and 28, respectively. A resilient elastic transparent layer 31 is provided between the two glass plates, while 30 designates a polarizer. In constructions wherein the liquid crystal substance is of the twisted nematic type, there are advantageously provided external polarizers 40 and 41, respectively, on the outer surfaces of the sheets 36 and 37. For large displays there are provided spacers between the plates 36 and 32, and plates 37 and 29, which are of the type shown in FIG. 2, but which are not shown in this Figure, and which define the spacing and thus the thickness of the liquid crystal layer. Due to the provision of the elastic layer 31, the thin glass plates adapt themselves to the said spacers and result in a uniform spacing between the electrodes facing each other. 
     With such twin-displays positioned back-to-back, interesting phenomena have been discovered: 
     a. When the liquid crystal layers are of the twisted nematic type, and when a third polarizer (such as 30) is provided between them, an extremely high contrast is obtained which approaches the product of multiplication of the contrast values of each of the displays. If one has a value of say 10:1, and the other 15:1, the resulting contrast value is of the order of 150:1. 
     b. This makes possible the production of a twin liquid crystal display with good contrast and excellent light output if three slightly polarizing sheets of high transmittance are used. 
     Reference is made to our U.S. Pat. Nos. 4,294,524 and 4,410,887. A color or multicolor liquid crystal display of very high contrast can be obtained when a color or multicolor layer as defined in the above patents, is provided for in the space between the two thin glass sheets 29 and 32 of FIG. 3. 
     EXAMPLE 1 
     A device was constructed corresponding essentially to the construction illustrated in FIG. 2, with a surface area of the structure of 178×254 mm. The transparent plate 25 was a glass plate of 4.8 mm thickness, and this was covered with electrode strips 22, of 5 mm width. There was provided a plurality of printed spacers 16 to 21 of an area each of 0.5 mm×0.5 mm and height 8μ, the step between these being 6 mm. The twisted nematic liquid crystal layer 15 was 8μ thick, and the width of the sealing frame 24 was 6 mm. The transparent electrode stripes 23 had a width of 5 mm and the transparent glass sheet 14 was 0.5 mm thick, while the transparent elastic rubber layer 13 was 0.6 mm thick and the transparent glass plate 12 was 4.8 mm thick. There was attained a close fit between the surfaces forming the boundary of the liquid crystal layer and these were maintained at a constant distance all over the surface of 8μ, giving a highly reliable performance. 
     EXAMPLE 2 
     A device was constructed according to FIG. 3, with a surface area of 52×56 mm. In this, 40 and 41, were conventional polarizers, 36 and 37 transparent glass plates of 1.3 mm thickness 26, 28, 33 and 35 transparent electrodes 0.8 mm wide; 27 and 34 twisted nematic liquid crystal layer of 10μ thickness, 30 a conventional transmissive polarizer, and 31 a transparent elastic rubber layer of 0.3 mm thickness. The integrated structure shows an extremely high contrast of about 115:1 and a clear image without parallax when viewed from a wide range of viewing angles.