GB661162A - Improvements in or relating to electric discharge devices sensitive to infra-red rays - Google Patents

Abstract

661,162. Cathode-ray tubes. BRITISH THOMSON-HOUSTON CO., Ltd. Aug. 6, 1948 [Feb. 7, 1947], No. 3722/47. Class 39 (i). An electron discharge device sensitive to infra-red rays comprises an electrode provided with a dielectric containing a titanate of the alkaline earth metals in co-operation with a signal plate and a mosaic, means being provided for detecting the capacity changes produced by the action of the infra-red rays on the various parts of the dielectric. The dielectric may comprise a composition of two molecules of BaTiO 3 with one molecule of SrTiO 3 , which has a high dielectric constant and a high temperature coefficient of dielectric constant. Fig. 2 shows a mosaic screen in an electron image tube according to the invention in which a metallic gauze 1 serves as the support for a very thin film 2 of insulating material, e.g. a varnish or bentonite. The film 2 is coated on its side in contact with the gauze with a thin layer 3 of metal, e.g. aluminium or silver, of such thickness that it absorbs at least an appreciable part of the infra-red radiation which falls on it, but does not greatly increase the heat conductivity of the metal layer. The metal layer serves as the signal electrode and is coated with a fine powder 4 comprising BaTiO 3 and SrTiO 3 , which is in turn coated with patches 5 of a photoelectric material, e.g. a caesium-silveroxygen composition. In a modification the dielectric comprises an insulating film containing a titanate powder as a filler, a thin metal layer and a mosaic of insulated metal globules being arranged at one and the other side respectively of the film. A layer of photoelectric material can be provided on the side of the mosaic which is opposite to the dielectric layer. In the construction shown in Fig. 3 the grid 1 with the mosaic is mounted in an evacuated envelope 6, sealed at one end with a window 7 of infra-red transmitting material such as potassium bromide or iodide. The flange 8 of the envelope and the lead 9 to the signal electrode may be sealed to the window by wax. The inside of the envelope is coated with a conductive coating 10. Photo-electrons are released by visible or ultra-violet radiation emitted by a light source 11 so as to be approximately uniform over the surface of the mosaic which acquires a positive charge and is brought back periodically to a lower datum level by a scanning electron beam 12 emitted by the gun 13 and deflected by the plates 14. Infra-red radiation falls through the window 7 on to the back of the mosaic, causing an increase in the temperature and capacity of the titanate powder particles. In a modification an auxiliary electron source is used instead of the light source 11, which sprays the mosaic with an approximately uniform stream of slow electrons, whereby the potential of the mosaic is reduced to a value only slightly higher than the potential of the auxiliary electron source. The scanning beam emitted by the gun 13 raises the potential to the level of the anode coating 10. The potential of any mosaic element thus alternates between two fixed potentials and the currents in the signal plate associated with the fixed potential increase are proportional to the capacity and thus to the dielectric constant of the mosaic element which varies approximately in proportion to the intensity of the infra-red radiation. In this form no photoemissive coating of the mosaic is required. The uniform stream of light or electrons can be dispensed with if the film 2 has sufficient conductivity to restore the potential of the mosaic between two scans approximately to the potential of the signal plate, which is preferably somewhat lower than the potential of the anode 10.