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1950 LARGE-SCREEN TV PROJECTION 513
most London cinemas have a circle, and the film pictures are projected down from the operating box at steep angles to the screen, having sufficient depth of focus to accommodate the variation in projection distance and sufficient illumination to allow the use of nondirectional screens. The Schmidt system for television, however, cannot be more than 10° off normal axis to the screen, so that in most cinemas the television projector must be located either on the front of the circle or on the floor of the auditorium. The front of the circle or balcony is, of course, the ideal position, but the throw distance is peculiar to each cinema and such an installation would in general call for individual "tailored" design of the optics. The cost and time occupied to complete and manufacture each optical system would, under these conditions, be prohibitive.
The suggestion has been made that to maintain projection normal to the screen the projector could be hung from the roof on a hydraulic cylinder, which would lower it into position when required and retract it when the film projector was operating; but there are very few cinemas in London where this device would not obscure the viewing of at least 20% of the audience.
Optical System
The decision was made to standardize on a 40-ft throw from projector to screen, and the system designed by Imperial Chemical Industries, Ltd., to our specification, consists of a mirror 27 in. (68.5 cm) in diameter, and a correcting plate which has an aperture of 18 in. (45.7 cm).4 The speed of the system is //1. 14, the magnification being 27.7, which, to cover a screen diagonal of 20 ft requires an image on the cathode-ray tube of 8.66 in. diagonal. The angular field of the optical system is 14 ° each side of the axis.
Cathode-Ray Tube Assembly
Figures 2 and 3 illustrate the general assembly of the cathode-ray tube and optical system.
1. The 27-in. diameter glass mirror with the front surface aluminized and weighing 85 Ib (38.5 kg) is supported in rubber-lined clamps. These clamps are arranged so that the lower two which bear the weight of the mirror are spaced at 45 ° each side of the vertical, as with this disposition there is the minimum distortion of the mirror due to its own weight.
2. Is a black shield of the same diameter as the face of the tube placed on the axis of the mirror to prevent reflection of light back from the tube onto its own face, thereby reducing contrast in the picture.