GB886557A - Improvements in or relating to the control of illuminants - Google Patents

Abstract

886,557. Colour filters for colour measurement or photography. KODAK Ltd. July 17, 1959 [July 25, 1958], No. 23969/58. Classes 97(1) and 98(2). A method of controlling illuminants for use in colour measurements or colour reproduction comprises the steps of introducing into the light beam to a varying extent a coloured filter having a greater transmission in one region of the spectrum and a lesser transmission in another region of the spectrum and keeping constant the total absorption of light in the region of said greater transmission by maintaining in the part of the light beam not intercepted by the coloured filter, a correcting light filter having substantially the same transmission in the region where the said coloured filter has a greater transmission and having a transmission greater than that of the coloured filter in the region where the said coloured filter has its lesser transmission. As shown a coloured filter 1 is cemented to a correcting filter 2 so as to be movable across a light beam from a source 3 before entry into an integrating box 4. For instance, the filter 1 may be a cyan filter and the filter 2 may be a reddish filter having a greater transmission in the red than the cyan filter, but transmission in the blue and green which is substantially the same as that of the cyan filter. In this way, movement of the two filters across the light path varies the red component without alteration of the green and blue components of the light from the source 3. In a modification the two filters are each in the form of superimposed coloured wedges.