TRISTIMULUS COLOR MEASUREMENTS IN FADING STUDIES 869 velopment can be said to have started with Newton, and refinements are still being made in the accepted standards today. Without going into any great detail the following is a description of what is known as the CIE (Commission Internationale de l'Eclairage) color space. In the late 1920's and early 30's Wright and Guild, in England, in- dependently of each other had a number of observers match the color of the spectrum at each wavelength with three primary lights, red, green, and blue. The values obtained by these two experimenters were re- 2.0- 0.•3 0.4 0.0 .... 700 Wovelength Figure 2. CIE tristimulus values, for the spectrum colors markably close to each other even though they used different observers, primary lights, and instruments. Because of this close agreement on the values obtained the CIE adopted their average values as a standard. A number of mathematical transformations were performed on the values of the real primaries used in the studies of Wright and Guild in order to make subsequent calculations relatively simple and rapid. The results are the unreal or imaginary primaries shown in Fig. 2 as the tri- stimulus values g, 3, and •. At any wavelength the values X, Y, and Z correspond to the magni- tude of the CIE primaries g, 3, and • needed by the standard observer to match the color of the particular wavelength. Chromaticity coordinates

370 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS o.8 t 52O 54O 0,6 5OO 560 y 580 590 62O 650 0,2 O.C 012 I 1. I O0 O4 06 Figure $. CIE chromaticity diagram Wavelength TABLE I Sample Calculation (Weighted Ordinate Method) 400 6.3..3 O. 0144 O. 91 O. 09 O. 082 700 76..3 O. Ot 14 O. 87 O. 45 O. 391 • RE•oe = 420 = X' 758 = Y X 420 1603 Y 758 1603 Y actual 758 420.0 - 0.2620 = x 0.4729 = y 425 = Z Y = Yhypothetical - 1079- 0.7025 = 70.25%