GB586957A - Improvements in or relating to three colour cameras - Google Patents

Abstract

586,957. Optical systems; colour photography. COOTE, J. H. Dec. 14, 1943, No. 20937. [Classes 97 (i) and 98 (ii)] In an optical dividing system for tricolour separation, of the kind comprising two inclined metallised transmitting-reflectors, the metals deposited on the reflectors are different, and at least one exhibits spectral selection in the transmitted and reflected beams. As shown, the beam from the lens 1 of a tricolour camera falls upon a metallised reflector 2 which reflects a portion of the beam containing at least 50 per cent blue light to a blue-sensitive layer 3, and transmits a high proportion of red and green light to a second metallised reflector 4 which reflects a portion containing at least 50 per cent red light to a red-sensitive layer 5, the remaining mainly green light passing through the reflector 4 to a green-sensitive layer 6. Colour filters 13, 15, 16 are associated with the various sensitive layers. The reflectors may be enclosed in an all-glass or transparent plastic material prism block system extending from lens 1 to the planes of the filters. The metal of reflector 2 may be Al, Be or Ag, and that of reflector 4 Au, Cu or Zn, and may be deposited, by thermal, evaporation or cathodic sputtering in a high vacuum, upon the optically-flat surface of a transparent pellicle as of gelatin or cellulose nitrate, or upon optically - flat glass or transparent plastic material such as polymethyl methacrylate in plate or prism form. When only one of the reflectors exhibits spectral selection, the other, exhibiting high and uniform spectral reflection, may be a metal of Group VIII of the Periodic System; thus, the metal of 2 may be rhodium, and that of 4 gold. Interchange of the positions of the reflectors is possible if the composition of the incident light renders this desirable. The reflectors are preferably inclined at angles greater than 45 degrees to the lens axis, e.g. at 50 and 60 degrees respectively. For daylight work with fast panchromatic films and filters all having factors of from 5 to 6, surface 2 should reflect about 30 per cent of the light incident upon it, and surface 4 about 48 per cent. Further forms of the system are described in which (a) the reflectors consist of opticallyflat glasses meeting along a line intersecting the optical axis of the lens 1, with appropriate compensation (e.g. varying thickness of colour filters) for the differences in the lengths of the paths of the divided beams, and (b) the reflectors are formed on the internal surfaces of a compound prism block made up, e.g. of four similar glass or plastic material prisms of right-angled isosceles form cemented together with their refracting edges in contact, and in this form the colour filters may be in the prism block behind the metallised surfaces. Specifications 440,006 and 475,415 are referred to.