D65 is not 6500K
D65 is not 6500K
- Subject: D65 is not 6500K
- From: Parker Plaisted <email@hidden>
- Date: Sat, 30 Mar 2002 01:06:39 -0800
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From: "Shiloh Burton" <email@hidden>
To: email@hidden
Subject: Optical
Date: Thu, 28 Mar 2002 15:16:37 -0800
Just a quick question.
I thought D65 and 6500K were interchangeable. Yet, when I toggle back and
forth between the two when in optical there is a significant difference.
6500 K is much warmer and/or red than D65. What is the reason for this? Can
someone please explain to me the difference between the two?
Thank you in advance to the list!
Shiloh Burton
Digital Imaging Specialist
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Dear Shiloh,
Although I am not sure why you see a significant difference from OptiCal
when switching from D65 to 6500K, I can explain why D65 is not 6500K.
The following story is a brief history of a little detail that is often
overlooked in the relationship between CIE daylight standards and their
correlated color temperatures.
When the CIE defined D65, the xy chromaticity coordinates had a correlated
color temperature of 6500K. At that time, the CIE Daylight Locus in xy
space was very close to the Planckian Locus for blackbody radiation. In
1968, the scientific community changed Planck's constant from 0.0014350 to
0.0014388. The CIE decided not to change the standards for the relative
spectral power distribution of the daylight illuminants, therefore their
correlated color temperatures changed (because Planck's constant changed).
When Planck's constant changed in 1968, the correlated color temperature
for D65 became 6504K. To learn more about this, read selected pages in
Color Science: Concepts and Methods, Quantitative Data and Formulae by
Wyszecki and Stiles.
I know that OptiCal had the right correlated color temperatures for the CIE
daylight standards because I worked with them in 1998 to get it right
before they released the 2.5 version. I hope they have not undone the work
we did back then.
Sincerely,
Parker Plaisted
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