Patent GB-799425-A (GB)
Title: Improvements in or relating to arrangements embodying pick-up tubes
Inventor: PERKINS DENIS GORDON
IPC: H01J31/36
Filed: 1954-07-20
Abstract: 799,425. Cathode-ray storage tubes. ELECTRIC & MUSICAL INDUSTRIES, Ltd. July 6, 1955 [July 20, 1954], No. 21069/54. Class 39(1). In a cathode-ray tube arrangement of the orthicon type, the photoemissive storage target is scanned simultaneously by a light spot and an electron beam, the scanning electron beam being displaced by a number of lines from the light spot, so that the photoelectrons released in the light spot scanning are separated from the electrons returning from the target due to the longitudinal magnetic field and due to the relative displacement between the light spot scan and the electron beam scan, and a collecting electrode 18 for collecting only photoelectrons released by the light spot scanning. The scanning electron beam serves to restore the target elements to equilibrium (cathode) potential the return electrons being collected by gun anode 10. In one embodiment, Fig. 1, the charge storage target comprises transparent conducting layer 2a insulating sheet 2b and mosaic 2c with closely spaced mesh 2d and takes up a positive charge image when light image is focused thereon through lens 3 and halfsilvered mirror 4 the photoelectrons being collected by electrode 5. The scanning light spot is generated by cathode ray tube 6 and makes the mosaic areas still more positive and produces further photoelectrons which are modulated by the charge image and constitute the wanted signal output ; they return to and are collected by collector or multiplier 18 while the return electrons are collected by the gun anode 10. The scanning coils 12 and the scanning coils of cathode ray tube 6 are synchronized by feeding them from a common scan generator. The light spot scans about ten lines in advance of the scanning electron beam. In a modification, Fig. 2 (not shown), the light image impinges on a photocathode the photoelectrons from which form a positive charge image on a leaky dielectric sheet so that a mosaic on the far side thereof acquires this charge image, and the scanning light spot is directed on the target from the same side as the scanning electron beam. Further separation between the photoelectrons and returning electrons can be achieved by magnetic or electrostatic means in the vicinity of the scanning gun. In one such form, Fig. 3 (not shown) the focusing solenoid 11 terminates a short distance in front of the gun so as to set up a diverging magnetic field at the gun. This may be modified by having a solenoid 25, Fig. 4, surrounding the gun and giving a weaker magnetic field than solenoid 11. In another form, Fig. 6, the separation is achieved by magnets 29, 30 with internal polepieces 26, 27, 28 forming extensions of external polepieces 31, 32, 33, or two pairs of electrostatic lifting plates can be used, Fig. 7 (not shown). Electrode 18 is made of small surface area to minimise the number of photoelectrons intercepted which are due to the light image ; to assist in achieving this the electrode 18 may be put at a focal point in the photoelectron beam. The scanning electron beam is not finely focused on the target so that it is stated photoelectron collector 18 can be displaced along the tube relatively to return beam collector 10.