Patent GB-773217-A (GB)
Title: Optical projection or beam controlled object fields
IPC: G02B27/28
Filed: 1954-08-17
Abstract: 773,217. Optical systems. TECHNICOLOR MOTION PICTURE CORPORATION. Aug. 17, 1954 [Sept. 21, 1953], No. 23901/54. Class 97(1) [Also in Group XL(b)] In apparatus for modulating light from a separate source L, Fig. 1, employing a modulating means O such as a dark trace or variable opacity screen, or an obstructor screen of the type described in U.S.A. Specification 2,128,631, the light B from the source is directed towards the modulating means by a polarization selective device P which reflects a component beam of light Bs polarized at one orientation and transmits a component beam Bt polarized at the complementary orientation. A mirror R is then arranged behind the modulating means to return the modulated light in its own path, and an optical retardation device Q is provided in front of the modulating means for rotating the plane of polarization of the reflected beam Bp into the complementary orientation so that it may be directed towards a viewing screen S by the polarization selective device. The transmitted beam Bt is absorbed at A. As an alternative to modulating the reflected beam, this may be absorbed and the transmitted beam modulated, Fig. 2 (not shown). Such arrangements achieve a theoretical light efficiency of 50 per cent. By utilizing both the reflected and transmitted beams for modulation, an efficiency of 100 per cent may be obtained. The polarization selective device comprises a multiple-layer coating as described in Specification 605,489 provided at the interface of two prisms. The retardation device Q comprises a quarter-wave plate cut and arranged with its crystal axis at 45 degrees to the plane of polarization so that the initial plane polarized beam Bs is converted to circularly polarized light and the reflected beam Bp converted to plane polarized light but at the complementary orientation. Fig. 7 illustrates an application of the invention to colour television, the arrangement utilizing both the transmitted and reflected components of the original beam and there being six light modulating means in the form of variable opacity screens O controlled by scanning electron beams in cathode-ray tubes M1, C1, Y1, M2, C2 and Y2. The screen are backed by thin aluminium light reflecting layers R. Tubes M1, C1 and Y1 modulate the component Bt of the original beam from source L which is transmitted by the polarization selective device Pcm, whilst tubes M2, C2 and Y2 modulate the reflected component Bs. Additional selective devices Pcy and Pmy are necessary for directing the beams between the three tubes in each set. Modulation is effected by selective absorption, the screens reflecting light of all colours except where struck by the scanning electron beams when they absorb approximately one third of the spectrum, tubes Ml and M2 absorbing green light, tubes C1 and C2 red light, and tubes Y1 and Y2 blue light. The arrangement may be utilized for stereoscopic colour television tubes M1, C1, Y1 being modulated for one eye, and tubes M2, C2, Y2 for the other eye. The resulting images on the screen are distinguishable by their complementary polarizations. Fig. 8 (not shown) illustrates an alternative assembly of the parts shown in Fig. 7. Fig. 6 (not shown) illustrates another arrangement employing six cathode-ray tubes and employing both the transmitted and reflected components of the original beam. In this arrangement, the polarization selective devices comprise crossed selective coating formed at the interfaces of four prisms as described in U.S.A. Specification 2,449,287. Figs. 3 and 4 (not shown) illustrate two further arrangements for colour television utilizing three cathode-ray tubes to modulate only the reflected component of the original beam. The transmitted component is absorbed. Finally, Fig. 5 (not shown) illustrates an arrangement employing two cathode-ray tubes to modulate the two components of the original beam, the tubes being controlled in accordance with stereoscopic, colour, or background and foreground components of a scene.