Patent GB-1453134-A (GB)
Title: Liquid crystal phase transformation system
IPC: C09K19/50
Filed: 1974-01-23
Abstract: 1453134 Modulating light XEROX CORP 23 Jan 1974 [24 Jan 1973] 03142/74 Heading G2F [Also in Division H1] A layer of an optically negative liquid crystal composition which would exhibit dynamic scattering if placed directly between two conductive electrodes across which a potential difference is applied, and which comprises an optically active material and a nematic material, is transformed into an optically positive (uniaxial) state by being disposed between a pair of electrodes at least one of which has insulating properties sufficient to reduce the injection of charge carriers into the layer to a level at which dynamic scattering is substantially absent, and applying a field via the electrodes of a magnitude (generally 10<SP>3</SP>-10<SP>5</SP> volts/cm) and at a temperature such that the change occurs. The optically active material may or may not have a cholesteric mesophase. One electrode is transparent, the other may be transparent or opaque, for viewing via transmitted or reflected light respectively. The optically negative state may have a Grandjean texture, but preferably has the focal conic (light scattering and depolarizing state), whereas the positive state is clear and uniaxial, appearing isotropic to light at normal incidence. The change may be viewed directly or via crossed polarisers. In the first embodiment a layer of the composition up to 10 mils thick but preferably “-5 mils is disposed between one or two blocking electrodes each formed of a conductive layer on a substrate and covered by an insulating or semiconducting layer typically 20 A to 1Á thick. One or both conductive layers may be shaped or segmented (e.g. X-Y addressing strips) and/or the layer of composition may be shaped using a suitable spacer between the electrodes. A photoconductive insulating layer may be provided, for example between the conductive and insulating layers of a blocking electrode, whereby an optical image produces a corresponding image in the layer. In a further embodiment, a film of the liquid crystal composition is disposed on a single blocking electrode having its conductive layer earthed, the assembly forming the faceplate of a cathode ray tube with the liquid crystal layer innermost. Disposed over the liquid crystal layer may be a transparent protective insulating layer, or a secondary emission layer for providing the imaging field or an erasing field. Electron beam addressing may also be effected by disposing the liquid crystal layer on the exterior surface of a faceplate, for example of a pin tube which is only conductive through its thickness. The required electric field may also be generated by a combination of electron beam and electrode addressing. In a final embodiment, the applied electric field is constant and the temperature of the layer is altered, e.g. by projection of a thermal image, to bring it into the optically negative-positive transition region. In each case the electric field may be A.C. or D.C. On relaxation of the condition necessary to effect the transition, for example cessation of the electric field, the reverse change occurs over a period which may vary from less than a second to several hours depending the details of the system. As it does so, the birefringence of the liquid crystal layer alters and may be observed as changing colour patterns between polarisers.