Patent GB-1475735-A (GB)
Title: Matrix-address liquid crystal display device
IPC: G02F1/1337
Filed: 1974-07-31
Abstract: 1475735 Controlling light NIPPON TELEGRAPH & TELEPHONE PUBLIC CORP 31 July 1974 [6 Aug 1973 7 Sept 1973 8 March 1974 11 June 1974] 33767/74 Heading G2F [Also in Division G5] In a matrix addressed display device comprising an optically scattering cholesteric liquid crystal phase material which is converted to a clear homotropically aligned nematic phase by electric fields above a threshold level, and for which there exists a stabilizing electric field at or close to the threshold level at which an existing phase, whether nematic or cholesteric, is metastable, the liquid crystal material is initially rendered nematic by a field above threshold and thereafter subjected to a field controlled so as effectively to maintain the nematic phase, and addressed points are rendered scattering by reducing the applied field to below the threshold and stabilizing levels for a time # and subsequently raising it to the stabilizing level, # being greater than a critical value determined by the reduced field amplitude, such that the nematic to cholesteric phase transition is initiated, but less than the time taken for completion of the transition, which latter proceeds in the presence of the stabilizing field. Since # is significantly less than the transistion time, scanning of the matrix may be relatively rapid compared with related phase change display devices. If the electrode plates of the device are given homotropic orienting properties # is further reduced, whereas it is increased for homogeneously orienting surfaces. The homotropic orientation, to which the drains are restricted, may be induced by rendering the plates hydrophobic e.g. by treatment with various specified silanes, directly or after vacuum deposition, or with lecithin, silicone resin or sodium oleate. The sodium oleate may be added to the liquid crystal material. Alternatively the plates may be rendered hydrophilic by treatment with chromic acid or sodium hydroxide. It is believed that the nematic to cholesteric transition is facililated by allowing the cholesteric helix axis or axes to be formed parallel to the electrode plates. The cholesteric material may include a cholesteric component and a nematic component with positive di-electric arisotropy. If additionally a nematic component with negative di-electric arisotropy is present, the transmission intensityvoltage curve of Fig. 1C is given, where VH is the threshold voltage. Depending on whether the applied voltage is reduced rapidly or slowly, the nematic phase H will return along arrows 8 and 9 respectively, to assume a clear (I) or scattering cholesteric texture, while the scattering cholesteric phase between VL and VH always returns along arrow 6 and remains scattering. Thus by scanning a display matrix and rapidly removing the voltage applied at all points, the image may be retained for some time at least. Two basic matrix addressing schemes are described. In Fig. 10 (not shown) column electrodes of an X-Y matrix are addressed in parallel with (+V) or (-V), according to image information, where V is the stabilizing voltage amplitude. The row electrodes are scanned with a voltage selected from (+V) and (-V), and are otherwise kept at zero volts. At selected points in the addressed row, the reduced field amplitude is therefore zero, and the applied voltage at unselected points Œ 2V, i.e. well above V H . Other points in the array are maintained at Œ V, the stabilizing voltage. The voltages may be D.C., or A.C. in which case (+V) and (-V) are replaced by equal amplitude A.C. voltage 180 degrees out of phase. Fig. 13B shows the second scheme in which the Y3 row electrode is being addressed and at least one following row electrode Y 4  is preaddressed, each element thereof being subjected to a voltage + V which in this case is less than V H , increasing the tendency for the nematic to cholesteric transition to occur and decreasing #. An A.C. voltage V of amplitude V is applied to unaddressed row electrodes, the effective voltage #V<SP>2</SP> + v<SP>2</SP> being equal to the stabilizing voltage, whereas 2V # V H . By reversing the polarity of the voltage applied to one set of electrodes the negative image is formed. Reference has been directed by the Comptroller to Specification 1387928.