Patent GB-1502783-A (GB)
Title: Liquid crystal display screens
IPC: G02F1/13357
Filed: 1975-01-23
Abstract: 1502783 Alpha-numeric displays; liquid crystal displays SIEMENS AG 23 Jan 1975 [29 Jan 1974 16 Dec 1974] 2898/75 Heading G5C [Also in Division G2] In an electro-optic device comprising two or more tandem liquid crystal layers 9, 10 between a pair of end-plates 1, 2, pairs of adjacent layers are separated by intermediate plates 5 having electrodes 7, 8 on each surface, the or each plate 5 being thin relative to the plates 1, 2, to reduce parallax when the incident light is not collimated. In Fig. 1 a number of spaced U-shaped metal strips 6 (of an ironnickel soft glass sealing alloy) are placed over each of two opposed edges of each of glass plates 1, 2 to determine the thickness of layers 9, 10. With channel section strips of equal thickness at the other edges of the plates, the assembly is then sealed with glass solder 12. Some strips 6 may contact conductive paths leading to separate segments of the electrodes 7, 8 and externally may act as plug-in contacts. Contact surfaces are preferably plated with a ductile metal such as gold or tin. The conductive paths to electrodes 7 and 8 may extend to the same or opposite edges of the plate 5. As shown, electrodes 3, 4 are provided on the end-plate. In one form of construction, the layers may have identical electro-optic characteristics. With identical types of display greater contrast would be obtained for the same switching time. In Fig. 1, electrodes 7, 8 are of the seven segment type, and are in register, electrodes 3, 4 being continuous. Alternatively electrodes 3, 4, 7, 8 may each comprise a conductive layer divided into two sectors by a continuous line so that each electrode represents a different character, i.e. by selective addressing of a particular sector a corresponding region of the associated cell becomes optically scattering. In a further alternative electrode pairs 3 and 7, and 4 and 8, are each in the form of registered sets of parallel strip electrodes, the strips of one pair being transverse to those of the other pair. Each layer 9, 10 is a 45 degrees twisted nematic layer with positive dielectric anisotropy (DA), and the device is between crossed polarizers. The device is operated by coincidence addressing of a row and column, half-addressed points differing little in brightness from the background. In a second form of construction, the layers differ in electro-optic response. One layer may be a 90 degrees twisted nematic layer with positive DA. The second layer may then be a 45 degrees twist layer with positive DA, with a birefringent intermediate plate 5 of mica, so that between polarizers a variable colour filter is obtained, e.g. for disposition before a monochrome cathode-ray tube. Alternatively the second layer may be an homotropically aligned nematic layer with negative DA exhibiting variable birefringence, also for colour modulation between polarizers. The second layer may also be a dye-containing layer of nematic material with positive DA, normally homogeneously aligned along a predetermined direction. Each of the adjacent electrodes is in the form of an interdigitated comb pair for applying a field transversely to the predetermined direction so that the layer acts as a (coloured) polarizer with either of two preferred polarizing directions. In use a polarizer is disposed on the side of the first layer remote from the second layer (contiguous to the exterior of the end plate). For a reflective display the rearmost electrode may have a mirror finish, or a further cell containing a reflective layer is provided. The reflective layer may be deposited by heating a solution of a silverammonium coupler, or a (shaped) printed layer of an organic silver compound which decomposes when the solder glass seal of the device is melted. Alternatively a diffuse reflector can be provided in the dye containing (polarizing) liquid crystal cell. Particular display applications include (a) the reproduction of moving images, by arranging for cells located one after the other on the intended optical path to be successively switched, particularly if the successive image parts partially overlap; (b) the display of patterns whose electrode systems would otherwise intersect; (c) the provision of redundant displays, e.g. by arranging for each segment of the display to be formed of two or more parts corresponding to the number of cells in the stack.