Patent GB-494333-A (GB)
Title: Improvements in and relating to optical systems for photography or cinematography
IPC: G02B27/10
Filed: 1937-01-22
Abstract: 494,333. Three-colour systems. HILLMAN, A. G., and JOHNSON, G. H. Jan. 22, 1937, No. 1987. [Class 97 (i)] A light-dividing system, for use with three lenses producing images in one plane, comprises two transmitting mirrors 4, 5 in front of the middle lens 2, a reflector 10 directing the beam reflected by the rear mirror 5 through one of the other lenses 1, a reflector 12 displaced outwardly with respect to the axis of the third lens 3, and means for directing the beam reflected by the reflector 12 through the lens 3. As shown, .the reflector 10 is a surface of a triangular prism 9, and the reflector 12 is a mirror, the beam from it being reflected by a mirror 13 and triangular prism 14. The mirrors 12, 13 may be replaced by a prism having corresponding reflecting surfaces, and this prism may be integral with or cemented to the prism 14. Fig. 7 shows a modification in which the prism 9 has an extension 9a to increase the optical-shortening effect of the glass in the paths of the reflected light beams, and the mirror 12 is replaced by the surface 20 of a prism 21 directing the beam through the lens 3 without further reflection. The prism 21 may be replaced by a mirror and a separate prism. The reflectors may be carried by side plates 47, Figs. 2 and 5, within a box 24 having a glass window 8, and the positions of the mirrors 4, 5 and prism 9 may be adjustable to centre the images. The adjustments for the mirrors 4, 5 may, as shown in Fig. 2, comprise screws 30, 37, each adjustable in a clamp 32 clamped to a cross rod 35, or to a bearing pivot 26, to turn the mirror about the pivots against the action of a spring 31 or 38. The position of one bearing pivot for each mirror may also be adjustable, and the pivot may be screwed into a disc 44 having a projection 45 slidable in a slot in one side plate 47 for this purpose. Opposing screws 48, 49, Fig. 2, may be provided to slide the disc 44, or screws 50, 51, Fig. 5, respectively engaging threaded holes in a block 52, and passing freely through a hole in the block to engage a threaded hole in the disc, may be provided. Alternative means for turning the mirror 5 about its pivot may comprise a screw 54, Fig. 5, opposite a spring 55, and held by a set screw 56. The prism 9 is fixed to a carrier 57 on a cross rod rotatably adjustable in the side plates. The mirrors may be of glass having aluminized front reflecting surfaces formed, for example, by the cathode sputtering method, or may be formed of perforated metal. The reflecting system may divide the beam into beams of unequal intensity, may be used with a green filter for the lens 1, a red filter for the lens 2,. and a blue filter for the lens 3, and may be rotatable through 180‹, in accordance with the nature of the illumination, to interchange the beams through the green and blue filters. If the reflecting system does not achieve complete correction of the optical lengths of the light paths, glass elements may be provided behind or in front of the lenses to achieve it. Specifications 478,500, 483,079, 483,817, and 494,334 are referred to.