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CRANDELL, FREUND AND MOEN
July
These are extreme cases, but it would be a mistake to assume that they are irrelevant. The rising popularity of lamps with strong monochromatic lines, even though these be superimposed on a continuous spectrum, makes it necessary to stress as strongly as may be that their use as illuminants for color photography leads to many unpleasant surprises, and may be misleading in the selection of fabrics, makeup, pigments and the like.
The need for a direct adequate method of specifying the red, green and blue energy content of an illuminant makes it worth while to review a proposal put forward a few years ago in connection with the spectral sensitivity of photographic materials. Dr. D. R. White,8 as a member of the Subcommittee on Sensitivity to Radiant Energy of the American Standards Assn., put forward a proposal which is extremely pertinent in this connection. Although his proposal was
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Fig. 4. Four monochromatic lines, at 400, 485, 585 and 700, visually match Illuminant C but would scarcely record at all on color film at ordinary exposure levels.
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Fig. 5. Transmission curves of standard Wratten filters proposed for division of spectrum into appropriate zones: dotted line, No. 0 filter; center No. 12; right, No. 25.
limited to monochrome reproduction and to a single illuminant, we should like to point out a way in which it could be extended to cover color film and color processes, the color balance of light sources, and the calibration of correction and compensation filters.
Since Dr. White's proposal is available in the literature, there is no need here for more than a brief summary. Basically, what he proposed was a simple means of measuring the relative percentages of the total sensitivity of an emulsion to Illuminant C, in the red, in the green and in the blue, and a simple index number in which three values would completely characterize that sensitivity.
This simple and direct approach involved only the determination of filter factors. Thus, if the material required ten times the intensity of Illuminant C through a No. 25 red filter to produce a suitable standard density that was required with no filter (a filter factor of 10)