Patent GB-658735-A (GB)
Title: Improvements in cathode-ray pick-up tubes
IPC: H01J31/36
Filed: 1947-05-21
Abstract: 658,735. Cathode-ray tubes. RADIO CORPORATION OF AMERICA. May 21, 1947, No. 13687. Convention date, June 28, 1946. [Class 39 (i)] In a pick-up cathoderay tube in which a potential image is formed by a photo-electric cathode 6, Fig. 1, on a target 5 which generates second - ary electrons under scanning by an electron beam from gun G, an accelerating electrode 14 accelerates the secondaries away from the target 5 and a grid electrode 11  is provided to repel the slower electrons into the electron multiplier 19, 20, 21, 22, 28, the faster electrons passing on and being collected by the plane end 12 of the gun anode 4. The composite stream from photo-cathode 6 impinges on target 5 which may be constructed as described in Specification 613,003 and generates secondaries which are collected by screen 10 so that varying positive charges are built up on target 5; when the latter is scanned by the beam from gun G, secondary electrons emitted from the " white " or most positive target areas will have a lower velocity than those emitted by the least positive or " black " areas since they are subjected to a smaller potential difference. The faster electrons are therefore removed and only the slower ones utilized. The target is discharged as described in Specification 636,250. Electrodes 7, 8 serve to accelerate the photoelectrons and screen 10 may be mounted on anode 8 or may be separate therefrom. The electron gun has control electrode 3, first anode 4, second anode 13, and accelerating electrode 14 which may be a metallic wall coating or a structurally separate metal cylinder. Screen 15 is mounted on cylinder 14 or is separate therefrom. Multiplier 19, 20, 21, 22, 28 is mounted on metal tube 30; each multiplying electrode comprises a disc of silver - magnesium alloy having a plurality of struck up radial vanes and associated with a fine mesh screen as described in Specification 634,639. The vanes are preferably stamped out over the whole of the disc but need only be stamped out over the operative sector of the surface thereof. Electrode 28 serves as the final collector and comprises a fine mesh screen only. Focusing coil 9 and deflecting coils 16 are provided; the scanning may be horizontal and vertical, or spiral. " Lifting plates " 17, 18, are provided to separate the primary and secondary beams and the reflected secondaries from the main secondary beam; they flare outwards at each end from the tube axis. If it is not desired to utilize the full potential swing of target 5, screen 15 may be operated at such a potential as to pass only a proportion of the emitted secondaries, the remainder being reflected back to target 5. In a modification, Fig. 5 (not shown), the grid 11 is replaced by an apertured disc which collects the faster secondary electrons and repels the slower electrons, and the first dynode is an apertured disc arranged between the beam lifting plates and the accelerating electrode 14 while the other dynodes surround the first anode of the electron gun. The Specification as open to inspection under Sect. 91 states that the higher velocity secondary electrons may be transmitted to the multiplier. This subject-matter does not appear in the Specification as accepted.