Patent GB-504268-A (GB)
Title: Improvements in or relating to television reception
IPC: H01J31/12
Filed: 1937-07-15
Abstract: 504,268. Television ; cathode - ray tubes. RUDKIN, E. P., and HELLINGS, G. M. July 15, 1937, Nos. 19671 and 21242. [Classes 39 (i), 40 (iii), and 40 (v)] To increase the brightness and freedom from flicker of television pictures, the modulated cathode-ray at the receiver sets up at each point of the scanned screen a self-sustaining exchange of secondary electrons between associated surfaces so as to retain the picture element after the ray has passed to other picture points, and this electron exchange is positively interrupted to extinguish the picture element before the ray returns to the point during the next scan. Two surfaces 1, 2, Fig. 1, supplied with oscillations from a valve 3 form a to-and-fro electron multiplier at the scanned screen of the receiving tube. The surface 1 is of wire mesh made highly emissive with caesium and the surface 2 is fluorescent and poorly emissive the arrangement is such that the number of electrons in the to-and-fro exchange increases slightly or is maintained constant until the exchange is stopped by application of the frame synchronizing impulse to the grid of the oscillator valve 3. As the first line of the picture is retained longer than the last, the resulting unevenness of the brightness requires compensation by grading the sensitivity of the fluorescent surface or of associated optical filters or by reversals of the direction of scanning. Alternatively or addi. tionally, the electron exchange is interrupted over sections of the picture in succession. The surface 1 is then divided into sections each supplied by a separate oscillating valve or by a separate part of a divided anode in a pentode oscillator ; the surface 2 may be similarly divided and operated in phase opposition to the sections of the surface 1. The electron exchange in each successive section may be interrupted by reducing the oscillating voltages supplied to it. To effect this, a second cathoderay in the receiver tube is deflected to fall on the sections successively and is quenched during each of its fly-backs, Fig. 2 (not shown), or a cathode-ray in a separate tube is deflected over segments to provide negative, bias interrupting the successive oscillator circuits, Fig. 3 (not shown). In another modification, Fig. 4 (not shown), the surfaces 1, 2 are separated at each section by a grid which may be periodically provided with negative bias, to interrupt the electron exchange, by a. separate cathode-ray tube with small output anodes; and the surfaces 1, 2 are subjected to the oscillating magnetic field of a surrounding coil supplied at the same frequency as that of the oscillators. In a further modification, Fig. 5, four secondary emissive surfaces 56 ... 59 are provided, the first 56 being scanned by the modulated cathode-ray and any of the others forming the fluorescent screen. The picture elements are retained by closed cycles of secondary electrons proceeding round rectangles as shown and are extinguished sectionally by grids 29 ... 32. The secondary emission is focused by coils 60 and electrodes 61, 62. According to the first Provisional Specification, retention of a picture element until it is positively extinguished may also be attained by using as the luminescent screen of known receiver tubes, a material which phosphoresces on and after excitation by the cathode-ray until it is subjected to surge potential or ultra high-frequency radiation or X-rays to render the material momentarily fluorescent and so extinguish its luminescence ; the fluorescent screen may then be formed by gas in a separate spherical envelope. According to the second Provisional Specification, sectional extinction of picture elements is produced by a commutator connected to strip sections of the scanned screen or controlling oscillators connected to screen sections ; or the picture is extinguished point by point shortly before the return of the scanning ray. The scanned surface may then be, a mosaic of emissive globules in a lattice to which oscillations are applied from a magnetron, an auxiliary cathoderay producing the extinction.