asured by H, the angle around the vertical axis (I), television system is based on the 1931 specifications. with red at 0° (for consistency with the HSV model, we Color perception was measured by viewing combina- have changed from the Tektronix convention of blue at 0°). tions of the three standard CIE (International Commis- The value of S ranges from 0 on the vertical axis (I) to 1 on sion on Illumination or Commission Internationale de the surfaces of the hexcone. The grays all have S = 0, but l'Eclairage) primary colors: red with a 700-nm maximum saturation of hues is at S = 1, I = 0.5. Table 7.2-5 wavelength, green at 546.1 nm, and blue at 435.8 nm. lists the 75% amplitude, 100% saturated HSI color bars. These primary colors, and the other spectrally pure colors resulting from mixing of the primary colors, are Chromaticity diagram located along the curved outer boundary line (called the spectrum locus), shown in Figure 7.2-14. The ends of the spectrum locus (at red and blue) The color gamut perceived by a person with normal are connected by a straight line that represents the vision (the 1931 CIE Standard Observer) is shown in purples, which are combinations of red and blue. The Figure 7.2-14. The diagram and underlying mathematics area within this closed boundary contains all the colors that can be generated by mixing light of different colors. The closer a color is to the boundary, the more saturated WHITE it is. Colors within the boundary are perceived as be- coming more pastel as the center of the diagram (white) is approached. Each point on the diagram, represent- ing a unique color, may be identified by its X and y coordinates. In the CIE system, the intensities of red, green, and blue are transformed into what are called the tristimulus GREEN YELLOW values, which are represented by the capital letters X, Y, and Z. These values represent the relative quantities of the primary colors. The coordinate axes of Figure 7.2-14 are derived from the tristimulus values: = X/(X+Y+Z) MAGENTA red/(red + green + blue