Patent GB-589345-A (GB)
Title: Improvements in optical filters and masks
IPC: H01J29/89
Filed: 1944-06-12
Abstract: 589,345. Colour and stereoscopic television. GENERAL ELECTRIC CO., Ltd., and JESTY, L. C. June 12, 1944, Nos. 11201/44 and 9756/45. [Classes, 39 (i) and 40 (iii)] In a colour or stereoscopic television system, the receiver or transmitter screen is viewed or illuminated through a mask comprising two or more superposed screens having different distributions of their radiation transmitting properperties, at least one of the screens being movable to cause a cyclic variation in the appearance of or radiation received by the area. The mask may comprise a fixed apertured screen 10, Fig. 1, with a cooperating movable screen 12, Fig. 2, the latter having three sets of apertures coloured blue, green and red respectively and arranged in groups 13, one aperture of each set being included in each group. Each aperture in each group has its centre on a common circle and the screen 12 is given a circular movement, e.g. by cranks 14, 15 to present successive apertures of screen 12 in the apertures 11 of screen 10. The arrangement of'the filters in each group is such that as screen 12 moves coloured bands appear to sweep over the mask in a predetermined sequence. The overall size of each group of apertures should correspond to the width of a few scanning lines. The arrangement may comprise apertures in two positions in each group, the third being omitted so that the mask operates to produce the effect of sequences of two sweeping bands of colour followed by a dark band. The arrangements of the apertures in the groups may be identical so that the colour change is uniform over the whole area of the mask. The arrangement represented in Fig. 3 may be used for flicker reduction. It comprises two screens 17, 18 each having opaque bands 19 alternating with transparent bands 20, the pitches of the bands being such that when two opaque bands in two screens register, some transparent band in screen 17, say the fifth, will be obturated by an opaque band in screen 18. Thus if screen 18 is moved downwardly over screen 17 a band of light will appear to pass down the area followed by an opaque band. The screen 18 may be oscillated with slow forward and quick return strokes or it may be formed as an endless band. If the bands on screens 17, 18 have the same pitch, the opacity change will be uniform over' the mask. A mask similar to that shown in Fig. 3 but in which the screens have coloured transparent elements is represented in Fig. 4 (not shown). In the form shown in Fig. 5, one screen is formed with colour bands 26, 27, 28 and the co-operating screen has inclined slots of the same pitch as the groups of colour bands and upon relative movement of the screens colour bands appear to sweep across the mask transversely with respect to the direction of movement. In Fig. 6 the mask comprises screens 30, 31, 32 each having alternate coloured and transparent arcuate bands, each screen having bands of one colour. The screens are mounted for circular movement with phases displaced by 120 degrees, the radius of movement, r, being w cot 60 degrees, where w is the width of the bands, and each subtends an angle of 60 degrees at its centre. One screen may be fixed and two rotated if desired. Fig. 7 shows diagrammatically the end of a cathode-ray tube including a mask according to the invention comprising a fixed screen 18 and movable screen 17, the latter being supported on ligaments 43 and driven from solenoids 44 outside the tube operating on armatures 45 on the screen. The transparent elements of the screens may be fitted with lenses to improve the optical efficiency. In the arrangement of Fig. 6 arcuate cylindrical lenses of width equal to twice the width of one colour band may be arranged contiguously and supported in front of the filter in such position that the upper and lower ends of the lenses are in register with the pairs of colour bands 35, 33 when the screens are in the relative positions shown in the drawing. If one of the colour screens is fixed, the lens screen must move. Screens for use in masks according to the invention may be produced by moulding or photographically from suitable masters. Specification 571,061 is referred to.