COLOR AND ITS MEASUREMENT 657 represent the amounts of primary red, green, and blue color that the standard observer would need to get a match. Ii each of these tri- stimulus values is divided by the sum of three, the resulting values x, y, and z, called chromaticity coordinates, give the proportion of the total stimulus attributed to each primary color. Furthermore, since the sum of the three chromaticity coordinates is unity, the values of x and 3' plotted on the chromaticity diagram may be used alone to specify the color. The third achromatic dimension of lightness or darkness is specified by the ¾ tristimulus value. The manipulations included in tristimulus colorimetry are mathematically presented below: Weighted Ordinate Calculation of Method Chromaticity Coordinates 700 X X = • ExC'Rx .•X x = •oo X + Y + Z 700 y Y = • 'Ex c'RX':?x y = •oo X + Y + Z 700 a Z = • Ex c'RX.zX z = •oo X + Y + Z z = I-- (x-t-y) The Hardy General Electric Spectrophotometer, previously dis- cussed, provides spectral reflectance curves in addition to tristimulus values. This combination results in nonmetameric matches in color formulations which are colors similar in appearance under all light sources. Tristimulus instruments alone often produce metameric matches which look the same only if similar viewing conditions are used. Instrument Development Laboratories (IDL) market the Color-Eye which is an abridged spectrophotometer giving 10 or 16 visible wave- length points as well as tristimulus values. The instrument is also de- signed to measure fluorescence of a sample since the tristimulus filters are placed between the sample and detector as illustrated in Fig. 8. A com- puter may also be purchased with the instrument to directly measure color difference in MacAdam units. A competitive tristimulus colorimeter called the Hunterlab D-25 Color Difference Meter (19),* schematically presented in Fig. 9, in- cludes vacuum phototubes, calibrated tristimulus filters, and a transis- * Hunter Associates Laboratory, Inc., 9529 Lee Highway, Fairfax, Va. 22080.

658 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS SPECULAR INSERT '•'-•----.._••GHT SOURCE REFLECTANCE -----,,,•f --lt SAMPLE /• .... T•NS•SSION ZEROING SLIT SPLIT LENS •'• ROTATING "FLICKER" MmmO• , •, ,,,,• • , STOP • t PHOTOMULTIPLIER I I REFLECTANCE STANDARD DIRECT LICHT STOPS STRAY LIGHT STOPS TP•NSMISSION STANDARD - MICROMETRIC SLIT (VERTICAL ApERATURE) . TRISTIMULUS and ABRIDGED SPE CTROPHOTOME TER FILTERS Figure 8. Block diagram of the I.D.L. Color-Eye torized color measurement circuit. The readout is presented in X, ¾, Z tristimulus values as well as L, a, b scales (20) which closely correlate with the Munsell system. The L measures lightness or value, whereas the a and b readings represent chromaticity coordinates. A computer may be purchased with the instrument to give direct color difference measurements in NBS units. The Hunterlab Color Difference Meter does not offer the 10 or 16 wavelength spectrophotometric points offered in the Color-Eye, which are useful where nonmetameric matches are re- quired. The Hunter colorimeter may, however, be used in combina- tion with a Beckman DU or other diffuse reflectance spectrophotometer if nonmetameric matches are required and may be applied to routine control of pharmaceutical color formulations. Another commercial instrument, called the Davidson and Hemmend- inger Colorant Mixture Computer (COMIC), enables the colorist to measure reflected light from a sample of various wavelengths, using K/S