Patent GB-636250-A (GB)
Title: Improvements in or relating to television and like pick-up tubes
IPC: H01J31/36
Filed: 1946-08-06
Abstract: 636,250. Cathode-ray tubes. RADIO CORPORATION OF AMERICA. Aug. 6, 1946, No. 23359. Convention date, Aug. 3, 1945. [Class 39 (i)] An electric discharge pick-up tube of the orthicon type in which the beam velocity is reduced to substantially zero in the vicinity of the target has means on the gun-side of the target producing a potential pattern corresponding to the charge image formed on the remote side of the target which pattern controls the returning beam, and grid means preventing collection of electrons, by the actual target. The tube comprises an electron gun 2, a target 3 and a photo cathode 4. The end 8 of an anode 7 forms the first dynode of the electron multiplier, and there are the electrode 10, wall coating 11, deflecting unit 12 adjustable by a rod 13, compensating coil 14 and a focusing coil 15. The multiplier consists of additional fan-like dynodes 16, 17, 18, 19 and a collecting screen electrode 20 connected to the grid of an amplifier tube 20<1>. The photocathode 4 is a semi-transparent coating of lightsensitive emissive material. A frame 23 supports the thin glass target 3 and a collecting screen 24. Between a decelerating ring 25 and the target 3 is the discharge screen 27 which is at zero or gun potential. The electrons in the beam 9 do not therefore land on the target 3 in substantial quantities but on the screen 27 which is under the potential pattern of the charge image on the target 3. The scene focused on the photo-cathode 4 produces electrons which are projected at high velocities through screen 24 on to the unscanned side of target 3 liberating secondary electrons and leaving a charge image on the target. The secondary electrons proceed to collecting- screen 24 but enough rain down on the elemental areas of the charge pattern between frames of moving pictures to discharge these areas. With stationary pictures, the charges build up to saturation. The electrons of the modulated beam, which do not fall on the targets 27 or 3 return towards the gun and fall on the first dynode of the multiplier and are finally utilized in the circuit exemplified by the amplifier 20<1>. The multiplier unit may be omitted. By controlling the potential pattern, the charge image for the light regions of the picture can be made to attract the electrons towards the mesh openings of the screen 27, giving maximum multiplier current for the white regions, stated to improve the signal/ noise relationship. If the screen 27 is biassed 2-5 volts positive relative to the gun cathode, secondary electrons are emitted under impact of the beam electrons and accelerate to the multipliers in greater numbers for the bright areas than for the dark areas and substantially no beam electrons land on the relatively negative glass target. Image amplification may be obtained by the arrangement shown in Fig. 4, in which the focussing coil 15 is subdivided to allow light to fall on the sensitized screens 27, 27<1>, 27<11>. Photo-electrons liberated from the screens are proportional to the charge image pattern induced on the targets 3, 3<1>, 3<11> by liberation of secondary electrons arising from the impacting photo-electrons. The amplified photo-electrons from screen 27<1> fall on target 3<1> to produce further amplification of photo-electrons from screen 27<1> and so on. The screens 27<1> and 27<11> may be made thermally emissive, in the form of stranded screens and so avoid further flooding by light and the consequential division of the focusing coil. The .Specification as open to inspection under Sect. 91 also describes the omission of the screen 27 (Fig. 4) and the coating of target 3<11> with a fluorescent material. This subjectmatter does not appear in the Specification as accepted.