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72 CRANDELL, FREUND AND MOEN July
ing illumination is not all of one color, and it is desired to obtain a practical average value.
When measurement of the green is desired as well, for illuminants which are not a good approximation of a black-body radiator, it is a simple matter to add a green filter and a second trigger position. This is now being done on a new professional model which will be ready shortly. The significance of this green measurement, and the relation of readings to the use of corrective filters, will become clearer in a later section of this paper, in the discussion of a proposal for an improved method of specifying the color balance of an illuminant. We will first consider some of the possible applications of color temperature measurement to actual studio procedure.
When we speak of color temperature today in the motion picture industry, as already mentioned, we may mean any one of three quite different things: (1) We may mean true black-body radiation, or a close approximation thereof. (2) We may mean a light source which is a visual match for such black-body radiation, even though its spectral distribution is vastly different (and such a light source corresponds to the meaning of the adopted American nomenclature) . (3) In purely photographic and cinematographic terminology, by color temperature we may mean merely the proportions of red, green and blue radiation in a light source, which balance the sensitivities of a particular type of color film — a light which may be neither blackbody radiation nor a visual match for such radiation.
This third usage of the term is not met with in the scientific literature, properly speaking, but is the common usage in motion picture practice, and as such is worthy of a little more consideration than has as yet been given to this subject. In motion pictures, and in color photography generally, we are not concerned with the conformity of a light source to a particular black-body distribution of radiation. What we are interested in is a continuous spectrum in the three broad bands or zones of the spectrum which will be recorded on the film, and it is desirable that the general curve of spectral energy distribution be reasonably smooth, so that it will not show any unpleasant surprises in connection with colors having narrow absorption or reflectance bands. With this qualification it is only necessary that the energy which falls in the band of the blue filter or sensitization, the green filter or sensitization, and the red filter or sensitization have a proper ratio to the total of the three or to each other, since this will ensure color balance. Actually, it would be more accurate and correct, and more easily intelligible, to the average nonspecialist in colorimetry if we were to use the term "color balance" of the illumi