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1950 EFFECTS OF COLOR TEMPERATURE 75
K becomes 154 Mireds, and in both cases the least perceptible difference would be approximately 5 Mireds. This least perceptible difference is not a constant throughout the whole of the Kelvin scale, but as Judd3 has pointed out, it is substantially constant from 1,800 to 11,000 K, which fully covers the range in which we are interested.
There would seem, therefore, to be every practical advantage in specifying light sources in terms of Mireds rather than degrees Kelvin, when we are discussing their visual appearance. However, the Mired, as a unit, still fails to convey directly the information in which we are interested for photographic purposes, and we shall propose a further standard later in this paper, though we shall relate it to the Mired.
One field which calls for definite investigation is the establishment of tolerances which will specify the permissible variation in color of the taking light. We know about what this should be when we are dealing with direct visual perception, but we do not know what difference in the color of the illuminant will produce a just perceptible difference in a color photograph taken by that illuminant.
Presumably, since the color photograph is of lower saturation, the tolerance is somewhat greater. Presumably, also, the better the subtractive primaries the more critical the balance, the poorer the primaries, the greater the tolerance. We know, for example, that when a letterpress printer has difficulty in controlling the balance of a threecolor job, he deliberately "grays" the process inks by contaminating them with each other, so that the balance will be less critical.
We may assume that in an ideal process of color photography, the photographic tolerances would be the same as the visual tolerances, or about 5 Mireds. As to actual processes, we have little data. Dr. Spencer, in an investigation made in England before the war, found that a density variation in a Carbro separation positive of about 5% represented the permissible limit. This is roughly equivalent to 10 Mireds illuminant color difference, or two visual steps. Eastman Kodak recommends a tolerance of about the same amount for Koda-. chrome.
So, while we should not infer too much from these unrelated observations, as good a guess as any at the present time would be that on current processes the photographic tolerance is about twice the visual tolerance.
However, it is not suggested that the use of this doubled tolerance in the exposure of color film would be good practice, for two reasons : first, because as processes improve, the photographic tolerance will approach the smaller visual tolerance, and second, because if we utilize