Page:Colorimetry104nime.djvu/21

angular size of field severely limits the precision of setting compared to what is possible by direct comparison of large specimens in daylight. Furthermore, the metamerism also prevents one normal observer from getting the same reading as another except by accident. If a reasonable approximation to the standard values of $X,Y,$ and $Z$ is to be assured, either the readings of a group of five or ten observers must be averaged, or a color standard yielding a spectral composition similar to that of the unknown specimen must be used. Because of industrial interest in large-field color matching Stiles and Burch [146] and Speranskaya [144] determined the color-matching functions for 10°-field viewing. In this determination either the observers were instructed to ignore the Maxwell spot [73, 98, 103, 104, 156] or it was masked. The color-matching functions thus found are significantly different from the 2°-field functions of the 1931 CIE Standard Observer. The difference is chiefly that expected from the removal of an intervening yellow filter, the macular pigment, from the field of view.

Table 5a. Spectral reflectances of four printing-ink specimens

Wave­length, nm	Spectral reflectance relative to magnesium oxide
	Red purple	Greenish yellow	Greenish blue	Blue
380	0.375[1]	0.091	0.150	0.230[1]
390	.375[1]	.089[1]	.187	.293[1]
400	.376	.085	.228	.354
410	.379	.079	.269	.415
420	.381	.077	.306	.458
430	.373	.076	.353	.505
440	.345	.077	.407	.563
450	.295	.086	.467	.616
460	.235	.095	.520	.639
470	.174	.108	.552	.645
480	.120	.145	.560	.635
490	.083	.250	.548	.608
500	.066	.445	.523	.568
510	.061	.635	.483	.508
520	.057	.708	.432	.438
530	.054	.725	.363	.353
540	.055	.733	.292	.272
550	.062	.743	.220	.198
560	.071	.752	.162	.145
570	.095	.768	.128	.117
580	.220	.782	.113	.106
590	.440	.787	.102	.102
600	.597	.790	.093	.098
610	.676	.793	.088	.097
620	.715	.798	.088	.103
630	.739	.803	.098	.122
640	.756	.809	.110	.147
650	.768	.814	.124	.172
660	.776	.818	.136	.187
670	.780	.822	.145	.186
680	.782	.824	.147	.172
690	.783	.827	.149	.162
700	.788	.829	.160	.169
710	.794	.832	.177	.192
720	.799	.883	.196	.221
730	.805	.885	.218	.256
740	.809	.836	.258	.304
750	.812	.837	.298	.362
760	.815[1]	.838[1]	.338[1]	.422[1]
770	.817[1]	.839[1]	.375[1]	.484[1]

Table 5b. Computation of tristimulus values, X, Y, Z, and chroniaticity coordinates, x, y, for the greenish-yellow printing-ink specimen under source C

(The computation form given as table 4b has been used.)

Wave­length, nm	Reflect­ance ($\rho$)	$\rho E_{c}{\overline {x}}$	$\rho E_{c}{\overline {y}}$	$\rho E_{c}{\overline {z}}$
380	0.091	0		2
390	.089	2		8
400	.085	7	0	34
410	.079	26	1	124
420	.077	95	3	458
430	.076	228	9	1112
440	.077	306	20	1535
450	.086	337	38	1775
460	.095	319	66	1833
470	.108	245	114	1617
480	.145	161	235	1372
490	.250	91	590	1319
500	.445	23	1513	1274
510	.635	57	3069	965
520	.708	408	4575	504
530	.725	1104	5752	281
540	.733	2041	6706	143
550	.743	3182	7305	64
560	.752	4422	7400	29
570	.768	5623	7025	15
580	.782	6582	6250	13
590	.787	7070	5215	8
600	.790	7070	4200	6
610	.793	6602	3312	2
620	.798	5642	2516	2
630	.803	4263	1759	0
640	.809	2988	1167
650	.814	1912	721
660	.818	1113	412
670	.822	582	213
680	.824	304	110
690	.827	141	51
700	.829	639
710	.832	68
720	.833	32
730	.835	16
740	.836
750	.837
760	.838
770	.839
Tristimulus values	$X,Y,Z$	63,076	70,395	14,495
Chromaticity coordinates	$x,y,z$	0.4263	0.4758	0.0980

Table 5c. Tristimulus values, $X$, $Y$, $Z$, under source C, luminous reflectance relative to magnesium oxide, $Y/Y_{0}$, and chromaticity coordinates, x, y, computed from the spectral reflectance of four printing-ink specimens as in table 5b

Hue designa­tion of specimen	Tristimulus values, arbitrary units			Lumi­nous reflect­ance, $Y/Y_{0}$	Chromaticity coordinates
	$X$	$Y$	$Z$		$x$	$y$
Red purple	39788	22124	30570	0.221	0.430	0.239
Greenish yellow	63076	70395	14495	.704	.426	.476
Greenish blue	19003	24245	54529	.242	.194	.248
Blue	21948	24633	69010	.246	.190	.213

It will be seen that tristimulus colorimeters give only poor information regarding the unknown specimen. Their application to product-control problems is negligible, but because of the ease of calibration and simplicity of the theory they are very useful research tools. Tristimulus colorime-

1. Extrapolated.