Patent GB-624694-A (GB)
Title: Improvements relating to colour filters for use in colour television
IPC: H04N9/22
Filed: 1946-04-01
Abstract: 624,694. Colour filters. MARCONI'S WIRE LESS TELEGRAPH CO., Ltd. April 1, 1946, No. 9965. Convention date, March 31, 1945. [Class 97 (i)] [Also in Group XL (b)] A colour filter for use in a colour television system comprises one or more filter elements having spatially variable colour transmitting properties, and a further element, having alternate clear and opaque strips, which is relatively movable with respect to the colour filter element or elements. In one embodiment, Fig. 1, the filter comprises a member 26 bearing alternate opaque and light transmitting strips and a set of members 21, 22, 23 bearing alternate coloured and transparent strips, means being provided to bring the coloured strips of the different members sequentially into registry with the light transmitting strips of the member 26. The members 21, 22, 23 bear blue, green and red strips respectively so that the picture on the screen of the cathode-ray tube 16, seen through the filter, appears successively green, red and blue. Electric conductors 73, 74, 36, lying between the polepieces of an electromagnet 76 are attached to the lower ends of the members 21, 22, 23 while their upper ends are attached by springs to fixed supports. Synchronizing pulses from the receiver are applied to an electronic circuit which supplies current to the conductors in such a manner that the desired operation of the filter is obtained. Initially, no current is supplied so that the filter is in the position shown and appears green. Then current is applied to the conductor 74 to cause the member 22 to move downwards until it is impeded by the stop 68, and to the conductor 36 so that the member 23 moves downwards an equal distance. The colour then showing is red. The third position of the filter which then shows blue, is obtained by applying current to all three conductors so that member 22 remains as before and members 21 and 23 move downwards until impeded by their respective stops 69 and 61; alternatively, the current to conductor 36 may be cut off so that the member 23 reverts to its original position. The current to all the conductors is then cut off so that the filter reverts to its original green condition. In modifications of the system, a commutator driven by a synchronous motor may be used to supply current to the conductors or the members may be driven mechanically by a system of cams. Alternative embodiment, Fig. 7. The filter comprises members 128 and 134 of which fragments are shown. The member 128 comprises vertically arranged alternate opaque and transparent strips and the member 134 comprises cyclically arranged red, green and blue strips which are slanted with respect to those of the member 128. By a suitable arrangement of the members, horizontal red, green and blue bands which move vertically, upon relative horizontal motion between the members, may be produced. The relative horizontal motion may be produced by electromagnetic means as in the previous embodiment and is so arranged that during each frame a band of one colour, e.g. red, follows the scanning shot and then during the next frame a band of a different colour, e.g. green, follows the shot. Thus, the picture on the screen of the cathode-ray tube is presented successively in red, green and blue colouration. Specification 589,345, [Group XL], is referred to.