GB651728A - Improvements in or relating to cathode-ray tubes - Google Patents

Abstract

651,728. Cathode-ray tubes. QUERVAIN, A. DE. Dec. 24, 1947, No. 34184. Convention date, Feb. 14, 1939. [Class 39 (i)] In a hard cathode-ray tube having a fluorescent screen which is liable to deterioration under bombardment by ions generated during operation, the screen is provided on its surface facing the beam with a metallic film, preferably of aluminium, which is of such thickness as to block the ions but allows the electrons to pass through to excite the screen. The fluorescent screen may be deposited on the end wall of the envelope in which case the aluminium can be vaporized on to it; alternatively the thin film can serve as a carrier for the fluorescent material which is deposited thereon. In a specific example of the former arrangement an aluminium layer of 0.056 microns in thickness is produced by evaporation from a filament arranged centrally in the tube at a pressure of 10<SP>-4</SP> mms., and is effective at a beam voltage of 5.5 kilovolts and a maximum beam current of 300 microamperes. The tube is finally evacuated to 10<SP>-6</SP> or 10<SP>-7</SP> mms. and will then operate 4-5 times longer without appreciable ion burn with magnetic focusing and deflection, or still longer than this with electrostatic focusing and magnetic deflection, as compared with a similar tube having no ion trap. The Specification as open to inspection under Sect. 91 refers in addition to cellulose layers and suggests that the layer need not be in contact with the screen. The screen may be a photographic film. This subject-matter does not appear in the Specification as accepted.