On Mar 10, 2014, at 8:43 AM, Andrew Rodney <andrew@digitaldog.net> wrote:
I've got an email from this list a few years back specifying the various locations and I'll see if I can dig it up. But the idea it's one measurement taken in one location at one time doesn't seem correct.
Check the archives for a topic called: The history of Standard Illuminants, June 2011. Posted by John (No last name), it was quite interesting. A portion of what John wrote is below:
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.).
Andrew Rodney http://www.digitaldog.net/