FWIW,  here's a simple article on the topic - - it was easy to find via
Google search.

- - -  JimH.
   ************************

Print version of: http://www.schorsch.com/kbase/glossary/cct.html

Color Temperature
Correlated Color Temperature (CCT)
Kelvin
(Term of photometry)
Color temperature is a simplified way to characterize
the spectral properties of a light source. While in
reality the color of light is determined by how much
each point on the spectral curve contributes to its
output, the result can still be summarized on a
linear scale.

This value is useful eg. for determining the correct
film in photography depending on the lighting (resp.
for determining the white balance in digital photography),
and for specifying the right light source types in
architectural lighting design. Note, however, that
light sources of the same color (metamers) can vary
widely in the quality of light emitted. One may have
a continuous spectrum, while the other just emits light
in a few narrow bands of the spectrum. A useful way to
determine the quality of a light source is its color
rendering index.

Low color temperature implies warmer (more yellow/red)
light while high color temperature implies a colder
(more blue) light. Daylight has a rather low color
temperature near dawn, and a higher one during the
day. Therefore it can be useful to install an electrical
lighting system that can supply cooler light to supplement
daylight when needed, and fill in with warmer light at
night. This  also correlates with human feelings towards
the warm colors of light coming from candles or an open
fireplace at night.

Standard unit for color temperature is Kelvin (k).
(The kelvin unit is the basis of all temperature measurement,
starting with 0 k (= -273.16 deg C) at the absolute zero
temperature. The "size" of one kelvin is the same as that of
one degree Celsius, and is defined as the fraction 1/273.16 of
the thermodynamic temperature of the triple point of water,
which positions 0 deg Celsius at 273.16 k.)

Technically, color temperature refers to the temperature to
which one would have to heat a theoretical "black body" source
to produce light of the same visual color.

Some typical color temperatures are:
      1500 k Candlelight
      2680 k 40 W incandescent lamp
      3000 k 200 W incandescent lamp
      3200 k Sunrise/sunset
      3400 k Tungsten lamp
      3400 k 1 hour from dusk/dawn
      5000-4500 k Xenon lamp/light arc
      5500 k Sunny daylight around noon
      5500-5600 k Electronic photo flash
      6500-7500 k Overcast sky
      9000-12000 k Blue sky

References:
color rendering index
metamer
photometry
white balance

Print version of: http://www.schorsch.com/kbase/glossary/cct.html
Copyright © 1998-2002 Georg Mischler. All rights reserved.


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