Patent GB-541751-A (GB)
Title: Improvements in or relating to projection screens for the production of pictures with stereoscopic effect
IPC: G03B21/60
Filed: 1940-05-03
Abstract: 541,751. Projection, screens. GABOR, D May 3, 1940, No. 7981. [Class 97 (i)] A screen for front projection comprises a base plate having vertical, or substantially vertical, cylindrical lenticulations on its front surface, the foci being on the rear; and diffusely reflecting surface; such a screen produces a multiplicity of images of the light aperture of the projector in a substantially vertical plane, and one or more arrays of plane mirrors, each array arranged in the manner of a Venetian blind, may be located in front of the base plate to reflect this plane into coincidence with one or more spectator planes. The screen may be used in conjunction with ordinary films, with films bearing images having a real relief, or with films, such as lenticular panoramagrams, appearing in relief and prepared, for example, as described in Specifications 410,515 or 541,752. Fig. 1 shows the functions of a screen 1 according to the invention, bands of non-overlapping images a<SP>1</SP>, b<SP>1</SP>, a<SP>11</SP>, b<SP>11</SP> ... of a projector aperture in the form of a slit AB being formed in the plane 2 occupied by the eyes of the spectators m a theatre pit, and, if desired, in the corresponding gallery plane 3. The length of each image is equal to that of the slit AB, and is such that the bands formed by them are wide enough to permit a spectator to move his head while still retaining both eyes in the band; the spectators' seats are arranged in the bands and either one behind the other or relatively staggered in successive rows. Figs. 8 and 8A show a screen base 5 embossed with cylindrical lenticulations 6 which are too small to be individually visible to a spectator and have their foci in the rear surface 10. The surface 10 may be frosted and covered with a reflecting metallic layer; if the base is used alone, the spectators must all be in a plane parallel to the screen passing through the projector. If the spectators are in a plane AC containing the projector A, the rear surface 10 may be formed of prismastic ledges, Fig. 9, having silvered concave or convex surfaces 15, and flat, unsilvered, surfaces 14, and a sheet 11, having an array of strip mirrors 12 therein, may be in optical contact with the lenticulations. The mirrors 12 &c. produce ray paths as shown from the projector to the plane AC, fine, closely-spaced lines or grooves may be ruled vertically across the surfaces 15 to produce horizontal diffusion. The mirror arrays may be produced by sheets 17, 18, Fig. 11, of plastic material moulded so as to leave gaps 20 when assembled, the gaps causing internal reflection of rays, having a large angle of incidence, and permitting passage of other rays. Screens comprising mirror arrays for use when the projector is not in a spectator plane, and for use with two spectator planes, are described in detail. Fig. 24 shows such a screen for two spectator planes, with the projector in neither of them, having a base plate 5 formed with small toroidal surface 26, compound lenticules 27 between sheets 28, 29 preferably having the same refractive index as the base 5, three mirror arrays 31, 32, 33, formed by gaps between separately moulded sheets, and prismatic ledges 42 on a front sheet. The ledges 42 cause any mirror image of the projector produced by the front surface of the screen to be unseen from the spectator planes ; the screen may be curved about a horizontal axis for the same purpose. Reflections due to differences of refractive indices of contacting sheets may be prevented by a transition layer between them wherein the refractive index gradually changes from one value to the other, the layer being at least a few light wave lengths thick. The layer may be produced by assembling the sheets with a suitable solvent between them, or by controlling the spraying of solutions of the two materials of the sheets. Lenticles enclosed between two sheets, as shown in Fig. 24, may be simple or compounds and the sheets may have a refractive index higher than that of the mirror array sheets. The base plate may be of wedge-shaped section having its edge coincident with the line of intersection of the spectator planes. Consequent variation of the focal length of the lenticles need not be continuous, but may be attained by the use of different sheets in building up the screen. Specifications 390,508 and 410,518 are referred to.