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convert from luminance purity to excitation purity, and the reverse.

Table 7. Spectral Reflectances of Greenish Yellow Printing-ink Specimen (see fig. 4b) Read for the Selected Ordinates for Source G (see table 6)

Number	X		Y		Z
1	0 076		0 102		0.078
2*	.076	0.076	.240	0.240	.076	0.076
3	.079		.460		.076
4	.088		.615		.075
5*	.096	.096	.683	.683	.075	.075
6	.117		.711		.076
7	.727		.720		.076
8*	.739	.739	.725	.725	.077	.077
9	.745		.728		.077
10	.750		.731		.078
11*	.758	.758	.735	.735	.080	.080
12	.765		.738		.082
13	.772		.742		.084
14*	.779	.779	.745	.745	.086	.086
15	.783		.747		.088
16	.785		.750		.089
17*	.787	.787	.755	.755	.091	.091
18	.788		.760		.093
19	.789		.765		.095
20*	.790	.790	.772	.772	.097	.097
21	.791		.778		.100
22	.792		.782		.103
23*	.793	.793	.785	.785	.106	.106
24	.795		.787		.110
25	.797		.788		.119
26*	.800	.800	.790	.790	.133	.133
27	.803		.792		.155
28	.807		.796		.210
29*	.812	.812	.802	.802	.342	.342
30	.819		.812		.650
Totals	19.298	6.430	21.136	7.032	3.677	1.163
Times factor	0.630	0.630	0.704	0.703	0.145	0.137

*r1Values for calculation with 10 selected ordinates.

Fig. 6.Dominant wavelenth and purity evaluated from the (x,y)-chromaticity diagram.

The four points indicated by circles represent the colors of the four printing-ink specimens shown in figures 9.4 and 9.5.

Table 8. Dominant wavelengths and excitation purities of four printing-ink specimens (see fig. 4)

Hue designation of specimen	Chromaticity coordinates (from table 5c)		Dominant wave­length, nm	Excita­tion purity, percent
	x	y
Red purple	0.430	0.239	498. 9C[1]	53.7
Greenish yellow	.426	.476	573.2	74.0
Greenish blue	.194	.248	483.6	49.6
Blue	.190	.213	479.7	55.1

Figure 6 indicates how dominant wavelength and excitation purity of the four printing-ink specimens would be found from their chromaticity coordinates $x,y$, relative to source C taken as the fixed light. Table 8 gives the dominant and complementary (C) wavelengths found as in figure 10 by the intersections of the straight lines with the spectrum locus. Table 8 also gives the excitation purities found by dividing the distance from the fixed point (source C) to the specimen point by the total distance from the fixed point to the boundary (spectrum locus plus straight line connecting its extremes). Large-scale charts for reading dominant wavelength and purity relative to source C are provided in the Hardy Handbook of Colorimetry [49].

Apparatus for the direct measurement of dominant wavelength and luminance purity has been designated by Nutting [125] and by Priest [132]. The degree of metamerism ordinarily obtained with such apparatus leaves it open to the same objections as have prevented tristimulus colorimeters with single sets of primaries from being useful for production control. There is a further disadvantage in the direct measurement of luminance purity in that the luminance of the spectrum component has to be determined relative to the luminance of the mixture by separate photometry. Since there is usually a large chromatic difference between these two fields, simple equality-of-brightness settings are not reliable, and an auxiliary flicker photometers, as in Priest's apparatus [132], must be used. This method has been found to exaggerate individual-observer differences; often-times observers will differ only slightly in the mixtures of spectrum light and fixed light that they find to be equivalent to an unknown color, but they will disagree importantly in their photomerty of the components.

1. C denotes complementary wavelength.