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 equation (see ) for the radiation emitted by a “black body” provide a system of stimuli, easily specifiable, and evoking a characteristic series of colors (including gray). However, since the system is virtually unidimensional it can only be applied to a very limited range of colors, practically only to those due to stimuli whose relative spectral distributions fit the Planckian equation. The value of &Tau; required for the black body to produce the color match is used as an index of the color, being called the “color temperature.”

F. .—Methods of color specification based on color matching with arbitrary standards are at present of great technical importance. Such standards include selectively transmissive solutions of definite composition, as well as colored glasses—as in the Lovibond Tintometer—and variegated pigments—as in the Munsell, Ridgway, and Ostwald systems. The final results of measurement by means of one of these methods are expressed in terms of a number or numerical symbol, standing for the particular standard which most nearly approximates the sample in color. These devices are simple in their practical applications, but tend to be unreliable and inaccurate, while the results obtained by different systems are difficult of intercomparison.

One of the main interests of the present Committee is to provide means by which color specifications in terms of different systems can be reduced to a common denominator and, so far as possible, be interconverted (). Spectrophotometric data are potentially convertible into the data of any other system whatsoever, but no specifications which are based upon simple color-matching can be reduced to spectrophotometric terms, without additional information. However, a satisfactory common denominator for all systems is apparently provided by the elementary color excitations. Values of these excitations can be found which will specify completely the color characteristics of any stimulus, and each