1474699 Colour separating prism assembly RANK ORGANISATION Ltd 11 Dec 1974 [11 Dec 1973] 57285/73 Heading G2J A three colour beam-splitting assembly, e.g. for colour television, comprises four prisms P 1 -P 4 , the arrangement being such that the air-gap G 1 between first and second prisms is bounded by spectrally non-selective surfaces, the second air-gap G 2 between second and third prisms is bounded by spectrally selective dichroic surface, and the third and fourth prisms are separated by a further spectrally selective dichroic coating. The angles and arrangement of the prisms and the selection of dichroic coatings is such that green light entering the assembly is selectively dichroically reflected at layer D 1 and is subsequently totally internally reflected at G 1 ; blue (or red) light is selectively dichroically reflected at D 2 and is subsequently totally internally reflected at G 2 , and red, or blue, light is transmitted straight through the whole assembly, the characterising feature being that the spectrally "central" colour of the three be the first to be separated. The above desiderata give rise to a series of mathematical inequalities with which the prism properties must conform. UK Specification 1401476 is mentioned.
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