GB552389A - Improvements in or relating to cathode ray tubes and circuit arrangements therefor - Google Patents

Abstract

552,389. Cathode-ray tubes. FREEMAN, G. S. P., and HENSON, A. F. Aug. 2, 1941, Nos. 9871/41 and 2263/42. [Classes 39 (i) and 40 (iii)] In a television transmission system a cathode ray tube has (i) a photo cathode 5, Fig. 1, on to which an optical image is projected and (ii) an electron gun 2 for forming a fasciculated beam ; a composite electron stream corresponding to the optical image impacts a screen 7-which may be plain mica or a mosaic-and the fasciculated beams scans the electron image on the screen, the stream and beam both impacting the screen at the same angle and on the same side. The stream and beam move between a pair .of deflecting plates 9, the whole tube being surrounded by a solenoid 10, which gives a strong magnetic field at right angles to the electrostatic field between the plates 9. The cathode 5 is at a much lower potential relative to the screen than the gun 2 so that the deflection of the stream between the plates is small compared with the " lift " of the beam, the movements of the electrons with respect to the solenoid 10 being as described in Specification 530,409. The scanning movement of the beam is effected by plates 3 for the line scan and electromagnet coils 4 for the frame, but it may be obtained wholly electrostatically or electromagnetically; the plates 3 may be plane or curved as described in Specification 542,219. In a modification the fasciculated beam is reflected by an electrode 8a, Fig. 3, the photo cathode 5 being on the opposite side of the deflector plates 9 to the gun 2 ; more than one stage of reflection and/or electron multiplication may be used in an arrangement similar to that described in Specification 536,417. Secondaries from the screen are collected by a wall coating. The deflection plates 9, Fig. 1, are wide enough apart to accommodate the beam in its full displacements and the stream ; the electrostatic field between these plates is varying and would give rise to distortion of the electron image and of the area scanned by the beam. To avoid this distortion means such as described in Specification 543,890 are used ; the plates 9 receive their potential from a source 17 through a resistance 18 whose mid point is connected to a potentiometer 16 receiving current from a shaping network 15 supplied from the scanning current generator 14 which feeds the frame deflection coils 4. The photo cathode 5 is also connected to the potentiometer 16 through a condenser 21. The network 15 may be replaced by a source of a.c. of the desired form and independent of the generator 14. According to the second Provisional Specification if electrostatic scanning only is used the compensating device described in Specification 542,497 may be used. According to the first Provisional Specification scanning may be effected by high velocity electrons and secondaries emitted are less in number than the impinging primaries from the composite stream.