COLOR AND ITS MEASUREMENT 651 1.6 • 1.2 0 400 500 600 700 • m•u Figure 1. Response of the "standard CIE observer" to an equal energy light source SPECTRUM COLORS • GREEN• 'x 540 søø • • RED I BLUE • [ 4•o •.'c' •• ,oo 400 Figure 2. Chromaticity diagram illustrating the visible spectrum locus measured with 10 spectrophotometric readings. The tristimu- lus chromaticity coordinates (x,y) for illumi- nant C are plotted near the center of the chart the three lights on a screen in the same location so that they may be mixed in proper proportions. A monochromatic light source is focused on a spot just adjacent to the mixture of the three colored lights. The observer views the screen through a cone angle of two degrees and is asked to adjust the three primary colors until the mixture matches the mono- chromatic light source, recording the relative amounts of the three lights necessary for the match. Figure 1 shows these relative amounts of red, green, and blue lights between 400 and 700 mu needed by the observer for the match. The monochromatic light is ideally a "white light," meaning that it is an equal energy source containing light from all visible wavelengths. Most standard light sources do contain light of every color, but not in equal amounts. Illuminant A, found in standard in- candescent or tungsten light sources, has light at all wavelengths, with much more energy at longer wavelengths. Illuminant B is a light source equivalent to the noon sun. Illuminant C, representing daylight on the north side of a building, contains light of all wavelengths with much more energy in the blue part of the spectrum. Figure 2 shows the three dimensions of the CIE color solid in two dimensions using x and y chro- marlcity coordinates. These chromaticity coordinates are calculated by expressing the tristimulus values X, Y, Z, as fractions of their total These tristimulus values of a sample are either calculated from diffuse reflectance measurements (9) to be discussed later or they may be mea-

652 JOURNAL OF THE SOCIETY OF COSMETI•2 CHEMISTS • • DOMINA ILL0 'C' ½• X F{gur½ 3. Chromaticity diagram (x,y) defining i]]um{nant C, dominant wavelength, and purity sured directly on tristimulus eolorimeters. Only two coordinates need to be specified since the total of the x, y, and z chromaticity coordinates equals unity. Here the color axis or chromaticity coordinates lie on the outside of the triangle going from blue to green to red, whereas the white to black axis or luminosity function is the center this brightness axis would be perpendicular to the screen with the white at the top and black at the bottom. Nonspectrum colors represent mixtures having the chromaticities represented at any point along the straight line joining the extremities of the spectrum locus and are produced by mixing suitable portions of radiant engery taken from the extreme short-wave (less than 440 m/•) and long-wave (greater than 680 m/•) regions of the spectrum. Colors represented by points on straight lines between the achromatic point C in Fig. 2 and the spectrum locus are called spectral colors. The CIE system divides the characteristics of light from which color is composed into several components including luminance, dominant wavdength, and purity. Luminance is the characteristic that differ- entiates the light reflected from a standard white sample, illuminated with a 100-W lamp, from that of light reflected from this same white sample when it is illuminated by a 200-W lamp with all other things being equal. The dominant wavelength may be defined as the wave- length that appears to be dominant in the light and is usually the most in- tense variety of radiant energy in the stimulus. Purity refers to the de- gree to which the dominant wavelength appears to predominate in the light. Figure 3 shows that the dominant wavelength is obtained by plotting the chromaticity coordinates of the illuminant and sample. A line is