Color Management and Why We Need It - Color Management for Photographers - page 20

a particular color temperature would produce the same color of light. Because natural materi-
als are not theoretical blackbodies, heating them to a specific temperature creates deviates
from the theoretical color, from magenta to green. This is why we use the term correlated color
temperature (CCT), because many colors of white may correlate to the same blackbody color
temperature. This is illustrated in Fig. 1-6A. If you have a color meter that reports color tem-
perature of a light source, many light sources that appear different could read the same.
Different illuminants can have the same correlated color temperature. You may recall that
the definition of illuminants is that of a light source that is defined spectrally. This is one reason
why the CIE defined the standard illuminants , which have fixed chromaticities and spectra (see
Fig. 1-6B). These illuminants are defined spectrally, meaning a certain amount of energy at each
wavelength across the spectrum. It became clear that that in order to control the conditions
under which the colors were viewed, it was necessary to precisely specify the light (illuminants).
An example of standard illuminants are the D illuminants (D for daylight) such as D50 or D65.
Many people, and even software products, use D65 and 6500 K interchangeably as if they were
identical. This is not the case. D65 is an exact color, it is not a range of colors. There are differ-
ences in each definition! 2 If you examine both Figs. 1-6A and then 1-6B you will see the dif-
ferences. Some software products specify both D65 and 6500 K as choices for white point. When
you see this, the 6500 K option refers to the exact color of a theoretical blackbody at 6500
Kelvin. It's important to understand the principles of white point and color temperature when
we undertake the task of calibrating our displays or decide under what viewing conditions we
will view our prints.
2 I defer to an excellent post by Bruce Lindbloom on the ColorSync list that I think clarifies the difference
for those that need to know. Caution: geek alert!
a) D65 is a spectral power distribution (a certain amount of energy at each wavelength across the visible
spectrum).
b) D65 is a tristimulus value; the D65 spectrum, when viewed by the CIE standard observer, produces an
XYZ triplet (or xyY if you prefer).
c) 6500 K blackbody radiator is a spectral power distribution.
d) 6500 K is blackbody tristimulus value; the 6500 K blackbody spectrum, when viewed by the CIE stan-
dard observer, produces an XYZ triplet—similar to, but slightly different from, the one found in (b).
e) Correlated color temperature takes a color's chromaticity coordinate (x,y) and finds the particular black-
body temperature whose chromaticity coordinate (d) is closest to it. Note that there are many different
colors that have the same correlated color temperature. So a spectrum is very precise and unique. Its
xyY is less precise and unique. Its CCT is even less precise and unique.
D65 is a unique SPD (there exists only one). A color whose CCT is 6500 K is not unique (there are infinitely
many different xyY and SPDs that share it).