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74 CRANDELL, FREUND AND MOEN July
negative image and a certain ratio of dye densities in the finished positive.
Nothing will emphasize the absurdity of the present color temperature nomenclature in relation to photography and cinematography more than the necessity of explaining the matter to someone previously quite ignorant of it. At the present time, one is first obliged to explain the concept of a black-body radiator, an abstraction not too easily grasped by the nontechnical mind, then the complex mathematics of determining the distribution of energy in the radiation from a black body at different temperatures, the character of light emitted at the different temperatures, and so on. After this somewhat lengthy beginning, it is then necessary to confuse completely the person to whom the explanation is being given by explaining that what we have just told him is color temperature, but that what we are talking about is not that but something quite different. Then, it is necessary to go into the explanation of equivalent color temperature and light sources which are a visual match for a particular color temperature, after which we must again explain that that is not what we mean either, but that color temperature in relation to photography actually means the balance of red, green and blue in certain important sections of the spectrum, such balance to be similar to the balance of those same zones in a black-body radiator.
"Color balance of the illuminant" is probably too clear and simple a term to find favor as a new nomenclature, but there assuredly is a drastic need for a simple term which will make it clear that we are concerned with the balance of three specific zones and not with blackbody radiation or a visual match for it. If, as we believe to be wise, a new term is sought, it would seem desirable at the same time to adopt a more rational system of numerical evaluation than that employed in the color temperature system. Judd3 pointed out 14 years ago that the use of the reciprocals of Kelvin temperatures would give rise to a scale in which the least perceptible difference in color temperature to the observer would remain more or less constant, whereas on the Kelvin scale it differs sharply from zone to zone. At 3200 K (degrees Kelvin) for example a difference of slightly less than 50 K is a perceptible difference, whereas at 6500 K, the least perceptible difference is in excess of 200 K. On the other hand, if these are expressed in terms of Micro-Reciprocal Degrees or Mireds (obtained by dividing the Kelvin temperatures into 1,000,000) we find that the least perceptible difference represents about the same number of Mireds over the entire portion of the scale in which we are interested. A temperature of 3200 K becomes 312 Mireds, and 6500