GB458744A - Improvements in and relating to cathode ray tubes - Google Patents

Abstract

458,744. Discharge apparatus. KEYSTON, J. E., Andora, Pield Heath Road, Hillingdon, and NICOLL, F. H., 17, The Drive, Ickenham, both in Middlesex, and KLEMPERER, O., 70, Syke Ings, Iver, Buckinghamshire. March 19, 1935, Nos. 28088/36 and 28089/36. Divided out of 458,746. [Class 39 (i)] In cathode-ray tubes with electrons focused into a narrow beam and deflected to scan an equipotential screen 1 which is not spherical with its centre in the region of deflection, for example, a fluorescent screen or an inclined mosaic screen for television transmission, an inhomogeneous field d is produced in the path of the beam between the region of deflection and the screen 1 to increase the effective focal length of the lens 22, 24 for rays b directed to more distant parts of the screen 1 and so keep the beam sharply focused throughout its deflection over the screen. As shown, the inhomogeneous field is produced between a symmetrical silvering 27 on the walls of the tube and an inclined ring coating 28 near the screen 1 and may be an accelerating or retarding field. The surface of the screen 1 takes up the potential of the ring 28 due to secondary emission. In an image dissector for television transmission in which electrons are accelerated from a photo-electric cathode to a positive apertured diaphragm and are focused to form an electron image at the diaphragm by an electromagnetic lens, a steady inhomogeneous magnetic field is produced between the cathode and diaphragm to reduce changes of focus of the electron image at the aperture as the image is deflected over the aperture to be scanned thereby and as the distance travelled by the electrons from different parts of the cathode to the aperture consequently varies. The inhomogeneous magnetic field is stronger near the axis of the tube and may be produced by superimposing on the uniform field of a coil surrounding the tube the field of a permanent magnet or auxiliary coil. Specifications 445,094 and 458,746 are referred to. One Provisional Specification describes also fluctuating the focal length of the electron lens of an image dissector as described in Specification 458,746.