Patent GB-446661-A (GB)
Title: Improvements in or relating to television transmitting systems
IPC: H01J31/34
Filed: 1934-08-03
Abstract: 446,661. Television transmitters; solidelectrode apparatus. BLUMLEIN, A. D., 7, Courtfield Gardens, and McGEE, J. D., 9, King's Avenue, both in Ealing, London. Aug. 3, 1934, No. 22724. [Classes 39 (i) and 40 (iii)] In a television transmitting system of the kind in which an optical image of an object is projected upon a photo-electric mosaic screen adapted to be scanned by a cathode ray, the latter serves periodically to bring the potential of the individual elements of the screen to approximately the potential of the cathode, with the result that, as each element is discharged in the scanning operation, impingement of the electrons upon the element ceases. In addition, means are provided to prevent impulses due to the variations in the general brightness of the image from being transmitted with the picture signals. In the arrangement shown in Fig. 1, a doublesided mosaic screen 7 is employed, the side receiving the image being associated with an annular anode 8 in a battery circuit 21 with bridging condenser 24, while the scanning ray traversing the opposite side of the screen is derived from an earthed cathode 3 associated with the usual anode 5 and deflecting coils 9, 10. The mosaic screen comprises a perforated metal signal plate 11 fitted with conductive inserts 12, which are insulated at 13 and faced with photo-electric material 14, such as caesium. Near the screen on the opposite side is disposed a grid 6 composed, e.g. of wire gauze and maintained at a moderate positive potential. The signal plate 11 is earthed through lead 26 and an impedance 19 connected to the negative pole of a weak, e.g. 4-volt, battery 20. A second grid 27 maintained at cathode potential may be arranged between the grid 6 and screen 7 to suppress secondary emission from the elements 12. In operation, the separate elements 12, which with the signal plate 11 form a series of condensers of very small capacity, receive charges proportional to the brightness of the incident light. During the scanning, the cathode ray effects an almost instantaneous discharge of the condensers in succession, and corresponding voltage variations are thereby input by leads 26, 25 to an amplifier. As soon as each element is discharged, no further electrons impinge upon it, all being collected by the adjacent portions of the grid 6 ; while, when the ray has passed, the element again acquires its appropriate charge due to the emission of photo-electrons to the anode 8. The condenser 24 functions to prevent impulses due to the changing brightness of the image from being transmitted with the picture signals. If desired, the negative pole of battery 21 may be earthed instead of being connected to lead 26. In a modification, Fig. 4, the condenser 24 is omitted and, instead, an impedance 23 corresponding to the impedance 19 is arranged in a second circuit, the voltages arising across the same being amplified to counterbalance the impulse voltages across the impedance 19, leaving the picture signals undisturbed. In another modification, the image is projected and the scanning ray operates on one side of a mosaic screen, and the grid 6 performs the functions of the omitted anode 8. The screen in this case comprises a mica sheet 13a, Fig. 6, coated on one side with silver to form a signal plate 11a, and having mounted on the other side a mosaic of photo sensitized elements 12a. The method of equalizing the potentials of the cathode and mosaic elements may be applied to transmitters of the kind described in Specification 442,666. Specification 446,664 also is referred to.