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 technical applications is confined almost exclusively to the first two sets of factors, although the ultimate goal is always to be found in the consciousness of some observer. Practical colorimetry 1s therefore concerned with means for the unambiguous designation of those properties of objects and radiation which determine color perception. Most of the means actually employed, however, utilize the visual apparatus of an observer as an essential element—in determining an equation of colors—and hence the results are frequently not independent of the nature and special condition of this apparatus. For this reason it is necessary as in photometry, that the observers should be tested as average and normal.

A. .—A method of specifying the physical characteristics of objects and samples of radiation, for the purposes of colorimetrics, which is actually independent of the observer is found in various applications of spectrophotometry or spectroradiometry. These devices enable us to establish the spectral distributions of reflection or transmission for objects, or of energy for radiation, and thus to specify perfectly the essential factors in their values as color stimuli. On the side of the stimulus, pure and simple, spectrophotometry is the fundamental method of colorimetric specification. All other methods (except spectroradiometry) fail to give an equally complete account of the stimulus characteristics. The excuse for their use, however, lies in the fact that the detail of spectral distributions is not actually required if our concern is solely with final color result produced in the observer's consciousness, and methods which dispense with this detail have the advantage of increased simplicity, both in practical application and in expression of results. (Since there is a special Committee of the Optical Society on the subject of Spectrophotometry, it can not be a function of the present Committee to consider this topic in detail.)

Complete data based upon spectrophotometry specify the colorimetric value of a stimulus in terms of the identical radiant power actually evoking the color, both in regard to total amount and