Patent GB-675364-A (GB)
Title: Optical projection system, particularly for stereoscopic pictures, and screen therefor
IPC: G03B35/20
Filed: 1948-11-26
Abstract: 675,364. Projection screens. BEARD, E. G. Nov. 26, 1948 [Dec. 15, 1947; Dec. 31, 1947; Feb. 2, 1948; March 23, 1948], Nos. 30747/48, 30748/48, 30749/48 and 30750/48. Class 97 (i). A system for optical projection of images, for instance stereoscopic images, comprises a projector, for instance a stereoscopic projector, and a screen having, in substantially even distribution thereon, a set of concentric light ray deflecting elements, each of which is curved, at least in part, across its width, i.e. in transverse crosssection, said elements being so located in relation to the projector on the one hand and to a file of observers on the other hand that an observer looking towards the centre of the concentric elements sees the projector apparently in coincidence with the said centre. In the form shown in Fig. 5, the screen is formed with circular grooves m<SP>1</SP>, n<SP>1</SP>, o<SP>1</SP>, p<SP>1</SP> centred at C, the point of intersection with the line B on which a file of observers E, Em, En is situated. A similar set of grooves is provided for every such file of observers. The grooves are formed by moulding, embossing or engraving with ballpointed tools. The areas between the grooves may be polished or light absorbing. The grooves may be replaced by ribs of similar crosssection. The screen may be of aluminium, " Monel " metal (Registered Trade Mark), stainless steel or zinc or of a transparent material such as gelatine or glass having a mirror surface. To avoid reflection of light in unwanted directions the curved cross-section of the grooves may be incomplete (Fig. 12). For stereoscopic projection Pr, Pl project left and right eye views to the appropriate eyes, l, l, r, r, of viewers who must be situated beyond a distance from the screen A, such as d, so that the aggregate cross-section of the reflected light beams is sufficient to embrace both eyes of each viewer. There is no upper limit to the viewing distance provided that the projection beams are juxtaposed so that the reflected beams meet on the axis B. Mirrors as shown in Fig. 10 may be used to reflect the beams from the projectors Pr, Pl to change the effective separation which may be varied by movement of the mirrors Ir, Il in the direction of the arrow. A larger number of side-by-side projectors may be used presenting a succession of different views from different viewpoints. To increase the number of viewers who can be accommodated with a particular screen a rotating mirror J (Fig. 15) may reflect the beam or beams, from a projector or projectors Pl, Pr to mirrors U<SP>1</SP>l, U<SP>2</SP>l, &c. in turn and thence to the screen. These latter mirrors may themselves rotate to produce a stationary image on the screen, or intermittent projector illumination may be used. A transparent projection screen may be produced by forming concentric series of refracting elements on a transparent plate. The different series corresponding to different series of viewers may be formed on separate superposed plates. Fig. 20 illustrates a modification in which, instead of concentric circular elements the screen is formed with confocal elliptical elements. The separate series have a common focus Fl through which passes the projector axis P. The other foci F2, F<SP>3</SP>, F<SP>4</SP> of the separate series of elements lie on the lines B<SP>1</SP>, B<SP>2</SP>, B<SP>3</SP> defining files of viewers.