GB977428A - Image tubes - Google Patents

Abstract

977,428. Image converter tubes; cathode materials and processing. WESTINGHOUSE ELECTRIC CORPORATION. April 2, 1962 [April 28, 1961], No. 12587/62. Heading H1D. In an electron discharge tube utilizing a charge image formed on a target, e.g. an image converter tube with at least one secondary electron emissive member between the input photocathode and the output phosphor screen the secondary electron emissive member comprises a thin continuous conductive (e.g. aluminium) layer 32 transmissive to the primary electrons and a secondary electron emissive insulating layer 36 (e.g. barium fluoride, potassium chloride or magnesium oxide) which has the property of providing a greater contrast over its surface between light and dark areas than existing in corresponding parts of the bombarding electron image and, further, the property that such contrast increases with time. An intervening aluminium oxide layer 34 may or may not be provided. A spongy or porous barium fluoride layer 36 is formed by evaporating the material from a tungsten boat at a distance of 3 inches in an argon atmosphere under 1 mm. Hg. In operation a positive grid opposite the layer 36 limits the potential to which the surface can rise. Storage properties of the layer 36 may be utilized, the radiation image source being removed after formation of the change pattern and the change pattern being utilized to modulate the flood beam produced by a subsequently applied uniform radiation image. The stored pattern may be erased (1) by bombardment with primary electrons which penetrate right through the layer 36 and are directed back into the surface by a negative electrode; (2) by heating; or (3) by reducing the velocity of the primary electrons to provide a secondary electron yield less than unity. A focusing magnet around the tube is provided by a tubular permanent magnet. Other types of discharge tube in which a flying spot scanner modulated with image information is utilized to excite the cathode or in which a scanning electron beam modulated with the image information is utilized are referred to. The tube may be of the direct viewing type. A flying spot scanner may be used in the reading operation and the information may be read out as electrical signals instead of as a visible output. Specifications 792,507 and 862,211 are referred to. Reference has been directed by the Comptroller to Specification 911,560.