Patent GB-773203-A (GB)
Title: Improvements in or relating to colour television
Inventor: PERKINS DENIS GORDON
IPC: H04N9/04
Filed: 1954-06-02
Abstract: 773,203. Colour television. ELECTRIC & MUSICAL INDUSTRIES, Ltd. June 2, 1955 [June 2, 1954], No. 16267/54. Class 40 (3). Relates to colour television apparatus employing a plurality of pick-up tubes which are scanned by means of light from a common flying-spot scanner. In a first arrangement, Fig. 1, three pick-up tubes 1, 2, 3 are employed, the light from an image being projected on to the tubes by way of crossed dichroic mirrors 7, 8, whereby only light corresponding to the red, green and blue components of the image reach the respective tubes. Each tube incorporates only a target electrode and an anode, the target being formed of semi-insulating material, such as conductive glass, bearing on one face a mosaic of photo-electric elements and on the other a transparent signal electrode. The image builds up a charge pattern by photoemission to the anode, which is held positive, and in order to develop image signals, a flying spot of light of sufficient intensity to restore the mosaic elements to anode potential is caused to scan over the target. To this end, a cathoderay tube 4 is caused to scan a raster on its fluorescent screen and the light from the raster is introduced into the optical path to the pick-up tubes by partially reflecting mirror 9. A rotating colour filter 5 is provided in front of the cathode-ray tube and is arranged to pass in sequence light of colours the same as that of the component images on the pick-up tubes. In accordance with the type of filter used and its speed of rotation the pick-up tubes may be scanned in a sequence at field or line frequency. The signals derived from the tubes are suitably switched to an output channel by circuit 11 so that the noise of the output channel is limited to that due only to the tube being scanned at any instant. Two-colour operation may be obtained by omitting one of the pick-up tubes and changing filter 5. In a second arrangement, Fig. 3, two pick-up tubes are used, one, 13, to produce an output corresponding to the Y signal in accordance with the American N.T.S.C. standards, and the other, 12, to produce red and blue component signals. This tube is provided with a filter in the form of a mosaic or grating of red and blue elements, and the light from flying- spot scanner 16 is changed in colour in a corresponding sequence by providing a filter in front of its screen formed of yellow and cyan elements. Alternatively, the fluorescent screen may be constructed to fluoresce in these colours. The colour filter or phosphor pattern is accurately aligned with the filter in front of the pickup tube. Pulses to switch the red and blue signals from pick-up tube 12 into separate channels are derived by a photo-cell 19 associated with red or blue filter 20 and arranged to receive part of the light from the flying-spot scanner by means of half-silvered mirror 18. A modification of this arrangement, Fig. 4 (not shown), avoids the need for accurate alignment between filters in front of the flying spot scanner and pick-up tube by utilizing only a single filter disposed in that part of the optical path common to both image and flying spot scanner. In another modification, Fig. 2 (not shown), switching of the red and blue signals into separate channels is effected by an independent pulse source and the cathode-ray tube of the flying-spot scanner is of a type having two sets of interleaved grids adjacent a fluorescent screen composed of strips of different colour phosphors, pulses from the source being applied to the grids to control colour change in synchronism with the signal switching. Reference is made without giving specific details to the use of arrangements according to the invention, when operating in a field sequential sequence, for recording colour television images in monochrome on a single film which is subsequently projected through a rotating colour filter. The images may also be recorded at 75 fields per second, corresponding to 25 fields per second for a single colour, and regenerated as television signals at a higher field repetition rate.