Patent GB-750911-A (GB)
Title: Optical projection system and apparatus
Inventor: GABOR DENNIS
IPC: G03B21/60
Filed: 1953-05-29
Abstract: 750,911. Projection screens. NATIONAL RESEARCH DEVELOPMENT CORPORATION. May 25, 1954 [May 29, 1953; Oct. 14, 1953; Oct. 14, 1953], Nos. 15053/53, 28367/53 and 28368/53. Class 97 (1). An optical projection screen comprises at least one surface which diffuses each incident ray, by reflection, into a fan of rays in a vertical plane, such fans then being rotated about vertical axes by rotating the diffusing surface, or being effectively rotated by subdivision into similar fans in vertical planes relatively inclined. In the form shown in Fig. 3 a screen 10 is covered with horizontal corrugations to diffuse incident rays vertically. The screen is rotated by a motor 20 about an axis 14, being enclosed in a transparent cylinder 11 to reduce wind resistance. It is shown that if an image is projected by a projector with a limited aperture an observer in any position will see an image, slightly curved about a vertical axis symmetrically. about the line of sight. Instead of a rotating reflector a stack of transparent partially reflecting foils 1, Fig. 12, may be used to produce from any vertical fan of rays 90 emergent fans in different horizontal directions as a result of repeated reflections. In the transmission screen shown the " diffuser " 30a producing the vertical fans of rays consists of a sheet covered with horizontal cylindrical lenticulations 26 having their foci at the opposite face of the sheet which is silvered except in the vicinity of the focal lines, the silvering increasing the reflections between the foils 1. The reflections are also increased by a " reverser " 33a which consists of a transparent sheet covered with right-angled prismatic ridges 92, which reverse light rays incident on them from the foils 1, separated by plane regions 93 through which rays emerge to the audience. The reflecting screen, Fig. 28, comprises a diffuser 30, the foils 1 and a reverser 33, which in this case is covered with corner prisms so as to be wholly reflecting, the emergent light rays reaching the audience through regions of the diffuser which may simply be apertures, as shown, or may be regions of the sheet 30 which are plane or spherically curved with or without the addition of prismatic shape towards the screen edges. Both Figs. 12 and 28 show only half of the screen which is completed by a second half symmetrically disposed by its side so that the foils 1 converge to the centre of the screen. The diffuser and foils may be concave. The diffuser may have a continuous curvature, the two halves of each foil may be separately curved with a central cusp. Thus in the construction shown in Fig. 41 the diffuser 47 is secured to rods 45 and tensioned by screws 55 against curved girder assemblies 50, 51,50a, 51a, being laced to the members 51, 51a and held in shape by the members 50, 50a. Other assemblies 58, 59, 58a, 59a and 60, 61, 60a, 61a, are provided in symmetrical pairs carried on pivots 63 secured in positions of angular adjustment by screws 64, 65 engaging slotted members 66, 67. The transparent foils 48, 49 are similarly supported by those assemblies, being tensioned round a central post 69. The reverser 33 is supported at the rear of the screen on straight members 68, 68a. The small number of foils used in such a screen necessitates a projection objective assembly having a large effective aperture in the horizontal plane in order that the viewing zone may be filled with viewing positions in which the whole image is visible. Moreover, such a system may be used for steroscopic projection to provide alternate left- and right-eye viewing zones corresponding to pictures projected from separate films or from lenticular film. It is shown that a suitable large aperture system consists of telescopic pairs of positive lenses 1 and negative lenses 2, Fig. 44, arranged with their axes converging to O at a distance from the object AB, of f = L/M where L is the projection distance and M the required magnification. The lenses 1 are in a ring with its centre at O, the lenses 2 in a ring with its centre at C in the object. In a practical embodiment as shown in Fig. 47c for projecting a lenticular film 100, an extended light source 101 is combined with a condenser 102 and a preliminary magnification is produced by the lens 3. The main vertical magnification is produced by a toroidal system 4, 5 and the horizontal magnification by cylindrical telescopic systems 1, 2. The latter may be combined with a Fresnel lens which permits their axes to be parallel instead of converging. Specifications 541,751, 541,752 and 541,753 are referred to.