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1950 EFFECTS OF COLOR TEMPERATURE 81
it was evident that the sensitivity in the red was one-tenth of the total sensitivity. Similar filter factor determinations in the green and blue would assign values for all three zones. However, since the notorious inefficiency of green and blue filters would make special correction factors necessary, Dr. White adopted the artifice used years before by Dr. Eder and used a red, a yellow and a colorless filter, of substantially equal efficiency. The unfiltered value minus yellow filtered value then gave a total blue reading, and yellow minus red gave the full green sensitivity.
The resulting percentages were expressed in a system of indices which it would be pointless to reproduce here, since the steps were too great for the differences which are significant in color photography. Let us see what happens, however, when we extend the system to light sources, color film and corrective filters.
Dr. White's proposal was to rate the sensitivity of black-and-white materials in relation to Illuminant C, which was a logical simplification. Color processes, however, introduce a completely new relationship between illuminant and sensitized material — a reciprocal relationship between the spectral distribution of energy in the light source and the spectral distribution of sensitivity in the film, so that an illuminant of the correct color will record balanced densities in the film.
Since the color of the light source becomes the most important variable, in this case, and since it is to the illuminant that corrective measures will be applied, it has seemed to us logical to take the illuminant as the point of departure for the entire system.
What we have done is, first, to evolve a numerical index which accurately describes the spectral distribution of energy in the light source, in terms of those attributes which have a bearing on color photography. The color film or process is then rated in terms of the index of the illuminant which will produce the most nearly neutral image of a neutral object. Corrective filters are rated in terms of the change which they introduce into the illuminant before it reaches the film. Thus, illuminant, filters and film are rated in identical units, all derived objectively from easily obtained data.
The first problem, then, is to find an index which will express the relative amounts of energy in three bands of the spectrum, suitably measured. For the isolation of these bands, the filters proposed by Dr. White seemed eminently suitable, with one modification. He proposed to use the Wratten Filter No. 25 for the red, the Wratton No. 12 for the yellow, and no filter for the white light exposure, a suitable correction factor being applied to the white light data to simulate the filter losses by surface reflection in the yellow and red light expo