Page:Journal of the Optical Society of America, volume 33, number 7.pdf/6

7500°K. The color of Illuminant D, found to be the optimum color for cotton grading, is also being widely used for agricultural grading and textile color matching. Its color closely matches that of the lightly overcast north sky most desired for such work. Illuminant S was designed as the blue end point for a series of illuminants representing the range from fully overcast to maximally clear sky. It was devised by weighting Abbot’s “‘sun-outside-atmosphere’” energy data by the inverse λ<r4> scattering relation. Illuminant S has been designated as “limit blue sky.” The colorimetric data on the Munsell samples for ICI Illuminant C, representative of average daylight, are of primary interest and the computations were carried out both at the National Bureau of Standards and in the U. S. Depart- ment of Agriculture. Those for the other three illuminants were made in the Department of Agriculture. All of the computations in the Department of Agriculture were done by using Hollerith cards and automatically punching sums obtained by the method of progressive digiting. The authors are indebted to Lila F. Knudsen, mathematical statistician of the Food and Drug Administration, for suggesting this rapid method of computation (20). All of the computations were made by the weighted ordinate method. The spectral energy distributions of the four illuminants are shown in Fig. 1, and in Table I are given the tristimulus data for the spectrum of each of the four illuminants used in the computations of X, Y, Z and x, y, z.

TABLE I. ICI tristimulus data for the four illuminants, A, C, D, and S, used in deriving the colorimetric data on the Munsell standards.
Wavelength (mμ)	For Illuminant A			For Illuminant C			For Illuminant D			For Illuminant S
	xE	yE	zE	xE	yE	zE	xE	yE	zE	xE	yE	zE
380	1		6	4		20	6		30	36		165
390	5		23	19		89	27	1	128	99	3	473
400	19	1	93	85	2	404	119	3	567	349	10	1658
410	71	2	340	329	9	1570	446	12	2125	1199	33	5719
420	262	8	1256	1238	37	5949	1504	45	7223	3567	107	17137
430	649	27	3167	2997	122	14628	3373	138	16461	6852	280	33442
440	926	61	4647	3975	262	19938	4202	277	21077	8143	538	40845
450	1031	117	5435	3915	443	20638	4100	463	21613	7652	865	40332
460	1019	210	5851	3362	694	19299	3476	717	19952	6194	1278	35554
470	776	362	5116	2272	1058	14792	2274	1059	14982	3870	1803	25503
480	428	622	3636	1112	1618	9461	1070	1556	9099	1742	2533	14815
490	160	1039	2324	363	2358	5274	347	2258	5049	530	3444	7703
500	27	1792	1509	52	3401	2864	52	3451	2906	74	4871	4102
510	57	3080	969	89	4833	1520	96	5214	1640	127	6870	2160
520	425	4771	525	576	6462	712	626	7023	774	781	8757	965
530	1214	6322	309	1523	7934	388	1533	7986	391	1847	9618	471
540	2313	7600	162	2785	9149	195	2610	8574	182	2958	9717	207
550	3732	8568	75	4282	9832	86	4062	9324	82	4070	9343	81
560	5510	9222	36	5880	9841	39	6072	10162	40	5148	8615	34
570	7571	9457	21	7322	9147	20	8160	10194	22	6092	7610	16
580	9719	9228	18	8417	7992	16	9310	8840	17	6798	6454	13
590	11579	8540	12	8984	6627	10	8946	6599	10	7090	5229	7
600	12704	74547	10	8949	5316	7	8343	4956	6	6798	4038	5
610	12669	6356	4	8325	4176	2	7800	3913	2	5871	2945	2
620	11373	5071	3	7070	3153	2	6372	2841	1	4585	2044	1
Σ =	109828	100000	35547	98041	10000	118103	96124	100000	124379	100078	100000	231410	x,y, z=	.44759	.40754	.14487	.31012	.31631	.37357	.29992	.31202	.38807	.23194	.23176	.53630