Re: Sunlight -- footnote
Re: Sunlight -- footnote
- Subject: Re: Sunlight -- footnote
- From: John Ward <email@hidden>
- Date: Sat, 29 Dec 2007 16:10:28 -0700
Dear List (history buffs only),
Earlier this year (Nov 18-20) Roger Breton compared his spectral
measurements of noon indoor sunlight at Montreal, Canada with D50
standard illuminant, noting a lack of UV below about 500 nm in his
measurements. In the ensuing discussion he raised the question of the
time of year "when D50 was measured."
Since none of the knowledgeable color scientists on this list answered,
I will step in where experts fear (or refuse) to tread.
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.).
As to Roger's question, nowhere in the original paper do the authors
make note of the observation times. They do remark that the Rochester
measurements "refer to sky conditions not yielding any visual evidence
of industrial contamination" -- not sure we could do that today. The
Kodak data by Condit and Grum was published in the same volume of JOSA
(p. 937) and was taken between early June and July 3, 1962. The Enfield
data was published but I don't have that paper and I do not know if the
Ottawa data was separately published. However, between late February and
early April 1962, Nayatani and Wyszecki (also published in JOSA) used a
colorimeter to measure the chromaticities of near north sky at Ottawa,
Canada with results similar to the spectral measurements used by Judd
et. al. In the early 1960s it would seem that everyone was out measuring
daylight but no one seemed to be concerned about season or UV absorption
with solar azimuth.
Judd et. al did note that, "It is our view that ultraviolet content is
poorly correlated with correlated color temperature, and that the amount
of ultraviolet indicated by the composite data should be taken as
typical." They go on to explain that "typical" means the best that can
be derived from exising experimental data -- and that is our standard to
this day.
Sharp eyes will note that the measurements were analyzed over the range
from 330 to 700 nm whereas the current CIE standard extends from 300 to
830 nm. The extrapolations were done by Judd et. al. using existing data
on atmospheric absorption due to water vapor and ozone.
Finally, it might be worth noting that even though Roger didn't observe
much difference in the UV content between indoor (filtered by window
glass) and outdoor illumination, the paper industry apparently still
prefers the reduced UV content of Illuminant C to represent indoor
illumination for daylight filtered through window glass (ISO TC6/WG3,
2003).
John
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