Patent GB-942306-A (GB)
Title: Improvements in electron beam system
IPC: H04N5/74
Filed: 1959-12-22
Abstract: 942,306. Colour television; light modulating. GENERAL ELECTRIC CO. Dec. 22, 1959 [Dec. 24, 1958], No. 43477/59. Heading H4F. [Also in Division H1] A cathode-ray tube for forming diffraction gratings in a light-modulating medium particularly for producing coloured television images, comprises an electron gun 3, Fig. 1, producing upper and lower electron beams, an arrangement 4 for splitting each beam into a plurality of electron beams having spaced virtual sources extending along a horizontal line, an arrangement 5 for controlling the widths of the beams and hence the densities of the lines of charge laid down on the modulating medium, an arrangement 6 for focusing the beams in a vertical direction and for combining the plurality of beams from both the upper and lower beams, a vertical deflection system 7, and an arrangement 8 for focusing the beams in the horizontal direction. The light-modulating medium 2 is moved in the horizontal direction and preferably comprises a material coated with a thermoplastic, e.g. polystyrene, which is heated to a plastic condition either before or after the formation of the lines of charge 9, but the medium may comprise a material which changes in physical characteristics, such as transparency or surface irregularity. The spacing between the split beams equals the grating spacing of the desired diffraction grating; the diffraction grating for the upper beam corresponds to a fixed primary colour, e.g. red, and the diffraction grating for the lower beam has a varying grating spacing corresponding to a colour that varies between two primary colours, e.g. blue and green. Each beam is split into at least three beams but preferably not more than seven or eight; in the example the upper beam is split into four beams and the lower into six. The electron gun comprises two hairpin filaments 10 and a beam current control electrode 12 provided with two apertures 13 and 14. Blanking signals are applied to the electrode 12 to cut-off the beams during the retracing periods. The beam splitting arrangement 4 comprises two acts 17, 18 of small diameter wires extending across apertures 19, 20 in anode 16. There may be 75 wires in set 17 and 110 wires in set 18 but each beam intercepts only, for example, four wires. The beam splitting action is produced by differing potential gradients between the wires intercepting the beam and the anode producing differing converging forces across the beam cross-section. The electric fields between the wire sets 17, 18 and the anode 16 are extended along the beam paths by electrodes 23, 32 connected to the anode. The spacing between the virtual sources formed by the splitting of the upper beam is determined by the voltage difference between anode 16 and wire set 17 and is set to equal the grating spacing of the red diffraction grating. For the lower beam the voltage difference between the anode 16 and the wire set 18 is equal to the sum of a constant and the logarithm of the ratio of the blue to green television signals. The beam width control arrangement 5 includes three deflection plates 33, 34 and 35. Plate 33 is energized by a Y-R signal, where the Y signal is the sum of the intensities of the red, green and blue video signals and the R signal is the intensity of the red video signal. Plate 34 is grounded and plate 35 is energized by a Y signal. The deflecting voltage for the upper beam, between the plates 33 and 35 corresponds to the intensity of the red video signal, and the deflecting voltage for the lower beam between the plates 33 and 34 corresponds to the sum of the blue and green video signal intensities. The vertical focusing and beam combining arrangement 6 comprises an einzel lens composed of two sets 38, 39 of three horizontal rod electrodes and a central positive combining rod electrode 41. The horizontal focusing arrangement 8 comprises a similar einzel lens with two sets 46, 47 of vertical rod electrodes. In another embodiment three diffraction gratings are produced corresponding to three fixed primary colours, the tube of Fig. 1 being adapted to a three beam system. In a further embodiment, Fig. 3, diffraction gratings are produced in a stationary light-modulating medium such as an oil film 60 on a transparent support 61 with a conducting surface. The arrangement includes a focusing coil 62 and a deflection coil system 64 for producing deflection in two directions. The filament 10 comprises line, instead of point, sources, the line sources being parallel with the diffraction lines formed on medium 60. The beam splitting and beam width controlling systems 4, 5 may be the same as shown in Fig. 1. The embodiment of Fig. 1 may be adapted for use with a fixed lightmodulating medium by the addition of a horizontal deflection system.