Patent GB-1201230-A (GB)
Title: Chromatic display or storage device
IPC: H01J29/14
Filed: 1969-01-31
Abstract: 1,201,230. Matrix displays. NATIONAL CASH REGISTER CO. 31 Jan., 1969 [23 Feb., 1968 (2)], No. 5382/69. Heading G5C. [Also in Divisions G1, H1 and H4] A chromatic display or storage device comprises disposed between a pair of spaced electrodes a cholesteric liquid crystalline member responsive to an electric filed and comprised of cholesteric liquid crystalline material enclosed in polymeric material. The member may comprise a plurality of individual minute droplets of cholesteric liquid crystalline material contained in a continuous polymeric film, or a plurality of minute polymeric capsules containing cholesteric liquid crystalline material, which capsules may be incorporated in and/or carried on a film of polymeric material. When an electric field is imposed on the cholesteric liquid crystalline member it instantaneously changes colour and/or reflective intensity (as viewed from incident white light) to shift from a first chromatic state to a second chromatic state. Upon removal of the electric field the colour changes from the second chromatic state to a third chromatic state which exhibits a colour (or reflective intensity) different from the colour given off in the second and first chromatic states. This third state does not fade readily and thus the retention ability of this third state by the device may be employed for storing information. By heating the member to the isotropic transition temperature of the cholesteric liquid crystals the third chromatic state is erasing and upon cooling the member returns to its original first state. A polychromatic display may be achieved by having a plurality of different cholesteric liquid crystalline materials in different layers, or in one layer, the droplets (or capsules) being formed of different material. The cholesteric liquid crystalline materials may comprise mixtures of different liquid crystalline substances. Polychormatic displays may also be achieved by using a plurality of switched electrodes as one of the electrodes, and by using a plurality of mixtures each responsive to a different field potential to exhibit different colours. The polymer matrix-retained or encapsulated cholesteric liquid crystal droplets may be prepared to be either flat or curved, and either rigid or flexible. The capsular wall material of the dropletretaining polymeric matrix may be tinted and thusa ct as a colour filter. Both electrodes may comprise non-transparent material although it is preferable that the first electrode (the electrode closer to the viewer) is transparent. When the first electrode is non-transparent e.g. in the form of thin electron-emitting resistive wire(s) it is preferably positioned so as not to obstruct the observed image. The second electrode need not be transparent nor coextensive with the first electrode, and it may for example comprise the pattern of a printed circuit. An opaque e.g. black, constrasting background is preferably provided behind the cholesteric liquid crystalline member, either in front or behind the second electrode. Specific embodiments described comprise, Figs. 1 and 2, a transparent insulating protective layer 1, a transparent first electrode 2, the cholesteric liquid crystalline member 3, a black lacquer insulating film 4, conducting element(s) 5 and a lower protective layer 6. In Fig. 2 to erase the third chromatic state a current is passed through leads 7 and 7<SP>1</SP>. The device may be disposed within a vacuum tube 9, Fig. 3, the conductive elements 5 being held at positive potentials and a current being passed through thin, non-transparent resistive wire 12 (which acts as the second electrode) so that it is heated and emits electrons which are drawn to the surface of the liquid crystal film 3. The third chromatic state may be erased by using a heat lamp or heat wires or by passing a current through elements 5. The device may comprise, coated on the inner surface of the faceplate of a CRT, a transparent electrode which is held at a positive potential, a layer of liquid crystal and a black insulator film, the black insulator film being scanned by the electron beam, Fig. 4 (not shown). Alternatively the latter device may be coated on the outside surface of the faceplate of a CRT, the faceplate having a plurality of conductive wires extending therethrough, Fig. 5 (not shown). The device of Fig. 6 comprises a transparent conductive coating 2 on a substrate 1, bus-bars 7, 7<SP>1</SP> attached to coating 2, a black insulator film 4, a cholesteric liquid crystal layer 3, and conductor elements 5 each having a pair of leads 8, 8<SP>1</SP>. In a first mode of operation a potential is applied between coating 2 and any one or more of resistive elements 5. The areas in the third chromatic state (when the field is removed) may be selectively erased by passing current through the elements 5. To erase the entire stored image the remaining elements may be heated or a current may be passed through coating 2. In a second mode of operation a current is passed through elements 5 to raise the temperature of the liquid crystal above the isotropic melting point. The heated portion becomes non-reflective in contrast to the coloured appearance of the unheated areas. When the current is interrupted the elements cools to ambient temperature and the liquid crystal returns to its initial coloured (first chromatic) state. Thus different characters corresponding to the shape of the elements 5 may be displaced in transient manner. To store an image a voltage source is connected between one of leads 8, 8<SP>1</SP> and electrode 2. Methods of preparation of the cholesteric liquid crystalline member and construction of the devices together with examples of materials used therein are given in the Specification.