Patent GB-829255-A (GB)
Title: Improvements in or relating to electroluminescent display screens
IPC: H01L31/14
Filed: 1957-12-06
Abstract: 829,255. Electroluminescence. WESTING- HOUSE ELECTRIC CORPORATION. Dec. 6, 1957 [Dec. 14, 1956 (2)], No. 38004/57. Class 39(1). [Also in Group XXXVI] A device for producing light images in response to electrical video signals comprises a plurality of independently controllable light producing elements each comprising a first member of electroluminescent material and a second member of a material the capacitance of which is a function of applied voltage or charge. The first member is adapted to receive time varying electric fields thereacross and the second member is adapted to be controlled in its effective capacitance by video signals, the two members being operatively associated with each other so that the magnitude of the time varying electric field across the first member is controlled in accordance with the varying capacitance of the second member. The electroluminescent material which may be zinc sulphide activated by copper with or without manganese may be dispersed in a suitable plastic dielectric matrix or an inorganic material such as glass and may be in the form of strips 12, Fig. 2. A light transmitting conducting material 14, such as stannic oxide, is deposited on the front surface of each phosphor strip and elemental areas 16 of conducting material, such as silver or aluminium, are deposited on the rear surface. On each electrode 16 is a layer 18 of suitable dielectric material, preferably a non-linear substantially reactive ferroelectric material such as barium titanate. A light power supply source is connected between conducting bars 20 which are earthed and conducting strips 14 by means of main supply bus bars. A video signal may be supplied and distributed to each of the elements of the display screen in a sequential manner by means of a switching member connected to leads 33 provided from each electrode 16. The control voltage or charge may contain an additional bias provided by a battery in series with the video signal to assure that the control element operates over the optimum portion of its characteristic. The entire display device may be embedded in a suitable transparent resin. The device may be prepared by soldering a brass sheet to each side of a sheet of the ferroelectric material; spraying a solution of the phosphor, e.g. zinc sulphide activated by copper admixed with butyl acetate and a polyvinyl chloride lacquer, on to one surface of this sandwich and evaporating electrode 14 on to the phosphor layer. The structure shown in Fig. 2 may be obtained by property cutting and milling this sandwich. Alternatively a sintered, enamelled or evaporated phosphor layer may be used of 1 to 5 mm. in thickness. A sintered layer may be prepared by mixing appropriate quantities of zinc sulphide, zinc oxide, potassium chloride and copper chloride, firing this mixture in SO 2  and after washing in dilute hydrochloric acid, pressing and sintering in nitrogen. In a modified screen, which eliminates the need for a light transmitting electrode, Fig. 3 (not shown), the layers of each light producing element are arranged perpendicular to the viewing surface so that the viewer observes the laminated edge of the display areas. If the magnitude of the light power supply voltage is not small in comparison with the control potential it is desirable to decouple the light power supply from an individual element when the control signal is applied to this element. Circuit means for achieving this may be incorporated into the screen structure and may comprise an apertured dielectric sheet 61, Fig. 8, having fillings of resistive material 65, such as carbon, in alignment with each light producing element, which is connected to the screen by means of conductive elements 73. Contact elements 67 and an apertured conductive coating 69 form the plates of the decoupling capacitor which is connected across the video input terminals and in conjunction with the isolating resistor formed by resistance material 65 effectively decouples the light power supply during application of the control potential. The screen comprises a transparent electrode 53, a phosphor layer 51, conducting elements 54, 56, 60 and ferroelectric layer 58, 62. The light power supply is connected between conducting strips 64 and electrode 53 and the control signal is applied to electrodes 60. In a modified screen, Fig. 12 (not shown), the light transmitting electrode and phosphor layer are in the form of elemental areas and sandwiched between these layers and the control electrodes are two leg portions, one comprising an electrode and a ferroelectric layer and the other comprising an insulating layer sandwiched between two electrodes across which the light power supply is connected so that the transparent electrode may be extremely thin. In a further embodiment of the invention the device shown in Fig. 8 may constitute the screen of a cathode-ray tube 111, Fig. 14, the video signals being applied to the electron gun or to the grid 150 as shown. The surfaces of the electrodes 60 or electrodes 67, if the decoupling structure is incorporated in the screen, are arranged to have a secondary emission ratio greater than one. Alternatively the decoupling structure alone may constitute the screen of the cathode-ray tube, the remainder of the display device being disassociated from the tube and operated by remote control. A modified structure for use as the screen of a cathode-ray tube comprises L-shaped control electrodes 186, Fig. 16, which support elements 194 of a material having the property of electron bombardment induced conductivity. The video signals are applied to bus bars 196 and a constant current scanning beam is used to open the individual circuits. A device for use as the screen of a cathode-ray storage tube comprises atransparent electrode 224, Fig. 18,an insulating grid structure 226, the even and odd horizontal rows of apertures of which are filled with ferroelectric material 232 and ultra-violet light emitting phosphor 234, such as rhombohedral aluminium oxide activated with aluminium, respectively. Horizontal strips 236 of thin conductive material such as gold having a high secondary emission ratio are applied to grid 226 and support electroluminescent strips 238 ; conductive strips 240, an insulating grid 242 similar to and in alignment with grid 226, phosphor 244 similar to phosphor 234 being deposited through the even rows of apertures thereof, and gold foil strips 246. A deflection grid is arranged between the screen and the electrongun and is operated to deflect the modulated scanning beam on to the even or odd rows of apertures so that it acts to charge or discharge the dielectric capacitors 232 according as it is desired to write or erase information. The ultra-violet light emitting phosphor layers when bombarded by electrons irradiate the ultra-violet sensitive gold strips causing multiplication of the beam current, thus decreasing the time required to write or erase information. An alternative storage tube screen comprises control electrodes 254, Fig. 19, each having an electron sensitive portion of which the upper half 260 has a secondary emission ratio greater than one and the lower half less than one. A deflection grid is arranged to deflect the modulated beam on to the lower portions 262 for writing and the full electron beam on to the upper portions 260 for erasing.