Patent GB-1167486-A (GB)
Title: Electro-Optical Device
Inventor: HEILMEIER GEORGE HARRY
IPC: G02F1/1335
Filed: 1968-03-27
Abstract: 1,167,486. Monogrammic signs. RADIO CORPORATION OF AMERICA. 27 March, 1968 [31 March, 1967], No. 14650/68. Heading G5C. [Also in Divisions H1 and H4] Relates to optical devices including a nematic liquid crystal layer which exhibits non-destructive turbulent motion when an electric current is caused to flow therethrough upon the application of a voltage across the layer. This turbulence which is restricted to the region of the applied voltage causes scattering of light incident on that region (the dynamic scattering effect), the degree of scattering being varied by varying the magnitude of the applied voltage between a threshold value necessary to initiate turbulence and a saturation value above which there is little or no change in the degree of light scattering. The preferred nematic compositions are high purity materials having resistivities in the range of about 10<SP>8</SP> to 10<SP>11</SP> ohm-centimetres in their nematic state; examples of the compositions are given in the Specification. The nematic liquid crystal compositions contain an excess of mobile ions to provide the current flow between electrodes in the devices, these ions being permanently present in the composition or being induced by injection of electrons from the cathode upon the application of a sufficiently high voltage. In the first embodiment, Fig. 1, the device comprises transparent plates 11 and 12 supporting orthogonally arranged transparent electrodes 15 and 16, the nematic liquid crystal layer being disposed therebetween. By means of commutator switches 31 and 35 the electric voltage is applied to selected regions of the liquid crystal layer. In a transmissive mode, Fig. 2 (not shown), a light source is positioned on one side of the device and the device is observed from the other side. At less than a threshold field the observer sees the entire plate area as uniformly bright, but when a voltage of sufficient magnitude is applied between a pair of electrodes, current flows in the liquid crystal layer giving rise to scattering of the incident light which results in this region of the observers field of view becoming darker. By sequentially or simultaneously energizing a plurality of selected electrode strips a plurality of areas are darkened. The degree of darkening for each element may be modulated by controlling the magnitude of the applied voltage above the threshold thereby affording a grey scale. In an absorptive mode, Fig. 3 (not shown), one support plate, e.g. the back plate 11 is coated with a layer of dark lightabsorbing material or is itself of dark coloured composition so that the device appears dark to the observer except in the regions where a voltage is applied causing the light to be reflected due to back scattering. In a reflective mode, Fig. 4 (not shown), the back support plate is made reflective and the light source preferably is arranged to produce a collimated beam of light which is at such an angle that the reflected light does not strike the viewer unless a voltage is applied causing the light to be scattered. The conductive strips may be replaced by wires embedded in the support plates or conductive plugs forming an integral part of the support plate. In operation the device is maintained at a temperature at which the nematic-liquid crystal composition is in its mesomorphic state. To improve the contrast ratio of the device operating in the reflective mode it is arranged that most of the light striking the device which is unscattered is reflected into a light absorbtive background, Fig. 5 (not shown). The devices are operable with A.C., D.C. or pulsed D.C. voltages. In a further embodiment, Fig. 6, the voltage across the nematic liquid crystal layer 17 is generated by the charge deposited by an electron gun 63, the device comprising a front support plate 12 having an earthed transparent conductive layer 61, and a back support plate 11c having a plurality of holes therein containing conducting epoxy resin plugs 62 which are coated with evaporated aluminium. Instead of epoxy plugs, metal wires may be embedded in the back support plate. The electron beamscanning system may be made into a colour system by arranging the plugs in a triad configuration, each plug in a triad being coated with an aluminium layer which reflects a different colour. Alternatively the front support plate may selectively transmit a particular primary colour. A three-colour gun system may be employed or instead of electron beamscanning, matrix scanning may be employed.