Patent GB-698107-A (GB)
Title: Apparatus for projecting television pictures
IPC: H04N9/16
Filed: 1951-09-04
Abstract: 698,107. Television ; light-valves. GRETENER, E. Sept. 4, 1951 [Sept. 18, 1950], No. 20868/51. Class 40 (iii). In an optical projection system for television images of the type in which light from a separate source is modulated by a Schlieren optical system comprising a mirror bar and one or more concave mirrors each bearing a control layer, the mirror bar is formed with reflecting surfaces on both sides of the bars and additional reflecting surfaces are located in the light beam between the bar system and concave mirror system whereby both the light beam reflected from the bars and that passing between them is utilized. The system is applied to the projection of components of stereoscopic, colour and stereoscopic colour television pictures. The control layer may comprise a medium deformable by a modulated cathode-ray beam as described in Specification 543,485, a medium having a variable index of refraction, or a medium capable of control by electromagnetic radiation such as ultra-violet or infra-red radiation. Fig. 3 shows a system arranged for the projection of three colour components together with a fourth black-and-white component. Light from lamp 1 illuminates mirror bar system 3 through condenser 2 and is split into two beams 27, 28. Beam 27 which is reflected from the surfaces of the bars illuminates the three-colour component images, while beam 28 which passes between the bars illuminates the black-and-white component. Beam 27 is split into blue, green and red beams 32, 33 and 34 by a system of dichroic filters 30, 31, the three partial beams being deflected towards the concave mirror 9 by mirrors 35, 36 and 37 so as to illuminate the areas 38, 39 and 40 with the respective colours. The areas are scanned by cathode-ray guns 41, 42, 43, the modulation of the beams producing deformations in the control layer whereby the reflected light which returns to the mirror bar is caused to pass in corresponding amounts between the bars to produce images of the areas on a screen 26. For correct operation of the Schlieren optical system the mirror bar must be located at the focus of the concave mirror, and mirrors 35-37 and filters 30, 31 are so positioned and inclined that the virtual image of the centre of the upper surface of the bar system as seen from each of the areas 38-40 coincides with the focus 21. Light, beam 28 which illuminates the black-and-white component is reflected towards the concave mirror by mirrors 44, 45 and illuminates an area 46 which is scanned by cathoderay gun 47. Mirrors 44 and 45 are arranged so that the virtual image of the centre of the lower surface of the bar system coincides with the focus 22. To enable the component pictures to be projected on the screen in registration the optically effective distance between the areas on the concave mirror and the projection lens 25 must be equal for components due to the upper and lower surfaces of the bar system. To allow for the thickness of the bar system, the concave mirror is made in two parts 106, 107 having different radii, the difference 23 between the two radii being equal to the optically effective distance between upper and lower surfaces. Owing to the repeated reflection of the light beam it is necessary for the direction of scanning to be different for the individual components. This is illustrated in Fig. 4 (not shown). Filter system 30, 31 may be replaced by a system of four prisms, or a system of parallel dichroic filters. The filters and deflection mirrors may be combined as described in Specification 696,615. In another embodiment, Figs. 1 and 2 (not shown), the system is arranged for the projection of stereoscopic television components. The two beams produced by the mirror bar system illuminate the left- and right-eye component images respectively, filters being included in the light beams to polarize the light in planes at right angles. The areas on the concave mirror may be scanned simultaneously by two cathode-ray guns or sequentially by a single gun. In yet another embodiment, Figs. 5 and 6 (not shown), the system is arranged for the projection of colour stereoscopic images in which the left- and right-eye pictures are polarized in planes at right angles and each comprise three colour components. A separate concave mirror may be employed for each of the component pictures as described in Fig. 7 of Specification 696,615. Specification 678,307 also is referred to. The journal of the Society of Motion Picture and Television Engineers, Volume 54, April 1950, pp. 393-406 also is referred to.