Patent GB-1390925-A (GB)
Title: Optical display device
IPC: G02F1/13363
Filed: 1971-09-01
Abstract: 1390925 Matrix displays RANK ORGANISATION Ltd 21 June 1972 [1 Sept 1971] 40757/71 Heading G5C [Also in Division H4] A layer 4 of nematic liquid crystal material having a positive dielectric anisotropy is disposed between two surfaces 5, 6, prepared, e.g. by rubbing, to have linear surface textures in different respective directions, whereby the polarization plane of incident light is rotated through the angle # between the two directions on traversing the layer, and means are provided for applying an electric field to selected regions of the layer and of above a threshold level whereat the nematic molecules tend to align along the field direction, destroying the optical activity of the layer. As particularly described, surfaces 5, 6 are of transparent electrically conductive material coated on glass walls 2, 3 and connectible to a voltage source 7. The coatings may be shaped. Layer 4 may be of 4<SP>1</SP>-ethoxybenzylidene 4-amino benzonitrile, typically 10Á thick. The incident light preferably has a plane of polarization parallel or normal to the direction of the texture of the surface it first encounters. In a first arrangement, Fig. 2 (not shown), #=90 degrees and the cell 1 is disposed between crossed or parallel polarizers. The light source may include a ground glass screen. Alternatively, Fig. 3, a polarizer 12, a #/4 plate 13 and a reflector 14 are provided, and #=45 degrees. One of the preferred axes of plate 13 may be parallel or at 45 degrees to the direction of polarizer 12 depending on whether light is to be transmitted when the cell 1 is unenergized or energized In Fig 4 (not shown), #=90 degrees, and reflector 14 is replaced by a cell (15) containing a cholesteric material for reflecting circularly polarized light of a particular hand, and within a narrow spectral range. In practice the cholesteric material is an approximately equal cholesteric-nematic mixture for broadening the spectral range and for thermal stabilization, the actual spectral range depending on the ratio of the components. Plate 13 is disposed so that when cell 1 is not energized, it produces circularly polarized light of the hand which is unaffected by cell (15) i.e. all the incident light is transmitted On energizing cell 1, light within the narrow spectral range is reflected at cell (15), i.e. the complementary colour is transmitted. Several such arrangements for passing different complementary colours may be arranged along an optical path, for selective operation, with an intensity modulator of the Fig. 2 type and a further passive filter optionally in series therewith, Figs. 5 and 6 (not shown). In a further arrangement, Fig. 7, cell 1 (#=90 degrees), and cholesteric cell 15 are disposed between crossed or parallel polarizers 9, 12. Regardless of the state of cell 1, plane polarized light therefrom or within the narrow spectral range of cell 15 will be equally reflected and transmitted. Light outside this range is absorbed or transmitted by analyser 9 depending on the state of cell 1, so that either saturated narrow spectral range light or an approximately 75% saturated complementary secondary thereof is transmitted. The colour switching cells may be used in a frame sequential television system. A display screen, useful for monochrome or colour television may comprise a matrix of cells 1, formed by first and second electrodes on opposite sides of a nematic liquid crystal layer. The first electrode comprises an array of discrete electrode elements each defining a cell 1, each column of elements being connected via respective diodes to a respective column conductor strip. The second electrode comprises parallel strip electrode elements corresponding to respective rows of cells 1. The screen is scanned one cell or one line at a time by reverse biasing of all cells except those addressed, to which a forward bias is applied. The charge applied to a selected cell 1 is maintained for a frame period by the capacitance of the diode and cell 1. Alternatively, the diode may be replaced by more complex thin film circuitry, the supply return therefor being made by connection to further strips on the first electrode, or to the second electrode. Self scanning may be achieved if the memory circuits include shift registers, whereupon the second electrode need not be divided into row strips. If the incoming signal is digital in form, said element of the first electrode may itself be divided into a plurality of portions with areas increasing exponentially, addressing of individual portions being by a further thin film circuit for each element. The addressing strips of the first electrode are masked, or insulated, or corresponding portions of the second electrode are rendered non-conductive, to avoid unwanted liquid crystal conductivity. A plurality of screens may be stacked for colour reproduction.