Patent GB-845288-A (GB)
Title: Improvements relating to cathode ray tubes
IPC: H01J31/16
Filed: 1956-10-29
Abstract: 845,288. Cathode ray tubes. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Oct. 29, 1956 [Oct. 27, 1955], No. 32999/56. Class 39(1). In a cathode ray tube wherein symbols or characters are to be projected on to a fluorescent display screen the sheet like electron beam is passed through an electrode 18, Fig. 4, having a plurality of perforations shaped in the form of symbols and/or characters (letters, numerals or punctuation marks) and is then subjected to selective deflection by an equal plurality of deflecting electrodes 30 so that only the part of the beam passing through a perforation shaped as the desired symbol or character is deflected through a right angle by the associated deflecting electrode 30 and projects the desired symbol or character on to a fluorescent display screen, The device thus acts as an electronic type setting machine e.g. an electronic typewriter in which symbols appear one by one in a line or other desired pattern on the screen the selective deflection being accomplished by depressing the potential of selected deflecting electrodes 30 in turn. The remaining deflecting electrodes 30 and accelerating electrode are maintained at the same potential, i.e. +500 volts, so that the selected portion of the beam cross-section is not spuriously deflected in its passage towards the target. The deflecting electrodes 30 may be U-shaped, Fig. 4, planar, Fig. 6 (not shown), planar but inclined Fig. 7 (not shown), or in three portions 30, 38, 39, Fig. 8, and the symbols or characters are stamped or etched in the accelerating electrode. The input to the selected deflecting electrodes is a coded signal such as data to be printed or recorded photographically. The input may in one case be a sequential pulse code and may be fed to the tube by a perforated paper tape perforated cards or, magnetic tape. In another case it may represent a radar signal. The individual leads to the deflecting electrodes may be cabled and are connected to a suitable selector circuit ; there may be 64 deflecting electrodes each slightly staggered with respect to 64 corresponding characters in the accelerating electrode i.e. the deflecting electrodes are slightly further away from the fluorescent screen than their associated perforations to improve the deflecting action. If in a modification the deflecting electrodes are directly above their associated perforations then the potentials of both the deflecting electrode above a perforation and the next adjacent deflecting electrode are lowered to zero to effect the selection. The deflectionselection system is followed by an accelerating and focusing electrode at 50 volts connected to a regulated power supply, horizontal and vertical electrostatic scanning plates and a fluorescent screen carrying a suitable applied potential. In modifications, the cathode, beam forming and accelerating electrodes carrying the symbols or characters are inclined and a further electrode having a slot running along its entire length is disposed between the accelerating electrode and deflecting electrodes, is negative to the former electrode and parallel to the latter electrodes and assists in the selective deflection action, Fig. 6 (not shown). In the modification of Fig. 8, each deflecting electrode has two parts 38, 39 connected together and insulated from the third part 30 and from the accelerating electrode 18 all these parts being at +500 V, it is stated that this improves the focusing action on the selected electrons passing to the screen. The focussing action is produced by lowering the potential of the second or third deflecting electrode to the left of the selected deflecting electrode from +500 to 350 volts so that with the adjacent deflecting electrodes it functions as a three electrode lens. There may be more than or less than 64 symbols or characters provided each has an associated deflecting electrode. To display a group of symbols or characters simultaneously, the apertures in the accelerating electrode 18 are so clustered that a single deflecting electrode deflects the required part of the beam simultaneously. In a complete system, Fig. 5, the information to be displaced is represented by a perforated tape 44, the perforations being arranged in transverse rows across the tape in alignment with photomultiplier tubes 48, the amount of light from source 46 reaching tubes 48 varying with each transverse row of perforations which represents a particular symbol. The output from the tubes is fed to a code converter 50 which energises symbol selector 56 to selectively lower the potential of each of the corresponding deflecting electrodes 30 in turn. The code converter 50 also separates out the horizontal and vertical magnetic scan waveforms and applies them in proper phase relation so that a single dot, single line or complete raster is traced out.