The series of D-illuminants was adopted by the CIE in 1971 based on 622

measurements from the early 1960s: 249 at Rochester, NY (Kodak); 274 at

Enfield, England (Thorn Electrical Industries); and 99 at Ottawa, Canada

(National Research Council). Each of these labs contributed spectral

measurements taken with different kinds of instruments measuring at

different spectral intervals over slightly different ranges. The data

were combined into a master set consisting of averages over 10 nm

intervals from 330 to 700 nm from which the average and four

characteristic vectors were calculated. The average and first two of

these vectors account for most of the variance in the observed data and

live on as the S0, S1, and S2 vectors used to calculate the

D-illuminants in the CIE standard (see Wyszecki & Stiles, 2nd. Ed., page

146). S0 is the mean, S1 provides a yellow-blue variation relating to

cloud cover and inclusion/exclusion of direct sunlight, and S2 provides

a pink-green variation which was thought at the time to derive from

variations in atmospheric water vapor and haze.

All of this was reported by Judd, MacAdam and Wyszecki, J. Opt. Soc.

Am., Vol. 54, p. 1031 (1964) and was incorporated without change into

the 1971 CIE standard except for the addition of the formula for

illuminant chromaticities in terms of correlated color temperature due

to Kelly at NBS (now NIST, Washington, D.C.).