Patent GB-696615-A (GB)
Title: Apparatus for reproduction of television pictures
IPC: H04N5/74
Filed: 1951-09-04
Abstract: 696,615. Colour television; light modulating. GRETENER, E. Sept. 4, 1951 [Sept. 4, 1950], No. 20867/51. Class 40 (iii). The invention relates to an optical projection system for television pictures of the type in which light from a separate source 1 is modulated by means of a Schlieren optical system comprising a mirror bar 3 and a concave mirror 8 bearing a control layer capable of being modified in response to television signals by means of a beam of scanning radiation. In accordance with the invention, a colour splitter 6 formed of dichroic filters is inserted between the mirror bar and concave mirror so as to produce red, blue and green component colour light beams 24, 25, 26 which illuminate the control layer over separate areas 29, 10, 30 which are then scanned to produce point-bypoint variations in accordance with corresponding component colour picture signals. In this manner a single mirror bar system is utilized for all the component pictures. As illustrated, the colour splitter comprises two crossed dichroic filters 21, 22, filter 22 passing red and green and reflecting blue light whilst filter 22 reflects red but passes green and blue light. The component beams 24, 25, 26 impinge on three reflection mirrors 27, 7, 28 and are reflected towards the control layer. The reflection mirrors and filters are disposed and inclined so that the virtual image of the centre 13 of the mirror bar as seen from each of the areas 29, 10, 30 coincides approximately with the focus 15 of the concave mirror. The line of intersection 102 of the filters 21, 22 passes through the centre of curvature 15 of the concave mirror and intersects the optical axis 49 of the projection lens 18 at right angles. The planes of mirrors 27, 7, 28 intersect in a common point on line 102. This positioning of the optical parts produces the necessary imaging of the mirror bar on itself to effect modulation of the light flux in accordance with the method of the Schlieren optical system, and furthermore, ensures that the three projected images of the areas appear in register on screen 19. Since light beam 25 is reflected once whereas beams 24 and 26 are reflected twice it is necessary to reverse the direction of line scan over area 10. The intersection of colour filters 21, 22 introduces a blank zone in filter 21 and disturbs the operation of the splitter. This blank zone may be avoided by replacing the assembly by four rectangular prisms mounted in an adjustable frame and having filter coatings applied to the prism surfaces. Alternatively the zone of intersection may be rendered opaque or masked and the disturbing effect of resulting sharp boundary lines eliminated by increasing the filter transparency gradually away from the zone. In another embodiment, Fig. 3 (not shown), the colour splitter is formed of parallel filters. This arrangement requires an assembly of five reflection mirrors to deflect the component light beams on to the concave mirror. In a modification of this arrangement, Fig. 5 (not shown), the filters are not parallel and only three reflection mirrors are necessary. In yet another embodiment, Fig. 7 (not shown), three separate concave mirrors are employed for the component pictures. The control layer on the concave mirror may be a medium deformable by an electrostatic force with the scanning effected by a cathode-ray beam. The areas may be scanned simultaneously by separate cathode-ray sources as shown at 31, 11 and 32 in Fig. 1. Alternatively, a single source may scan the areas sequentially. Alternative control layers may comprise a medium having an index of refraction variable in response to the intensity of a scanning cathode-ray beam or a medium capable of control by infra-red or ultra-violet light. Specifications 543,485 and 678,307 are referred to. The Journal of the Society of Motion Picture and Television Engineers, Vol. 54, April 1950, pages 393-406 also is referred to.