Page:Journal of the Optical Society of America, volume 33, number 7.pdf/6

 7500°K. The color of Illuminant D, found to be the optimum color for cotton grading, is also being widely used for agricultural grading and textile color matching. Its color closely matches that of the lightly overcast north sky most desired for such work. Illuminant S was designed as the blue end point for a series of illuminants representing the range from fully overcast to maximally clear sky. It was devised by weighting Abbot’s “‘sun-outside-atmosphere’” energy data by the inverse λ scattering relation. Illuminant S has been designated as “limit blue sky.” The colorimetric data on the Munsell samples for ICI Illuminant C, representative of average daylight, are of primary interest and the computations were carried out both at the National Bureau of Standards and in the U. S. Depart- ment of Agriculture. Those for the other three illuminants were made in the Department of Agriculture. All of the computations in the Department of Agriculture were done by using Hollerith cards and automatically punching sums obtained by the method of progressive digiting. The authors are indebted to Lila F. Knudsen, mathematical statistician of the Food and Drug Administration, for suggesting this rapid method of computation (20). All of the computations were made by the weighted ordinate method. The spectral energy distributions of the four illuminants are shown in Fig. 1, and in Table I are given the tristimulus data for the spectrum of each of the four illuminants used in the computations of X, Y, Z and x, y, z.