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The possibility of using photocells to measure color has attracted widespread attention ever since the photoelectric cell became available in practical form. Photoelectric cells have now been used for a number of years in spectrophotometry to measure the spectral reflective and transmissive properties of materials. From the spectrophotometric curves of samples, one may obtain the tristimulus values of color by multiplying the separate spectral values of any sample by the three distribution functions of the standard observer and totaling the three sets of products throughout the visible spectrum (see Eq. 2, below, and (11, p. 365). Although the use of photocells has considerably diminished the time needed to obtain spectrophotometric curves, and although devices such as the one developed by Sears (19) are available to accelerate the integration, a considerable expenditure of time and effort is still required (13, p. 259) to obtain the accurate colorimetric specification of a sample by the method of spectrophotometry and integration.
Hunter, Richard S.
Assistant Physicist, National Bureau of Standards, Washington, D. C.