Color Temperature
The color temperature of an illuminant or color is the temperature
of a black-body radiator that radiates light of an equivalent hue to
that color. Color temerature is a one dimensional scale, and throws
away information about the luminance and greenness or purpleness
hue, relaying information only about the redness or blueness of a
color or illuminant.
Traditionally Color Temperature is measured as the temperature in
degrees Kelvin of the black body radiator that has the closest color
in the (obsolete) perceptual CIE 1960 UCS. If the color being
measured is close to the black body color temperature locus, then
the use of an obsolete measure is of no consequence, but if color
far from the locus are being measured, this can introduced a
noticeable disparity between the CCT and the visual closest
temperature. The Kelvin scale of color temperature is very visually
non- linear, becoming highly compressed at high values. For these
reasons, ColorMeter offers a number of alternative ways of
evaluating Color Temperature:
An alternative to using the obsolete Correlated (CIE 1960 UCS)
metric for determining the closest color is to choose Visual, which
uses the modern CIE DE2000 color difference metric. This will give a
more accurate visual correspondence for light sources that are not
close to the spectrum locus.
The number shown in bracket after the Correlated Color Temperature
is the distance in CIE 1960 UCS uv value ("Duv") of the
point from the black body locus, while for Visual Color Temperature
it is the distance in CIEDE2000 delta E. A tolerance of 0.0054 Duv
is regarded as satisfactory in many lighting standards,
corresponding to roughly 7-8 CIEDE2000 (Note that the MacAdam
Ellipse "step" number correlates with the Delta E 2000. This is not
a coincidence.) A positive value indicates that the measurement is
above the black body locus (green direction), while a negative value
is below it (purple direction).
In many situations (particularly if the illuminant is at or above
5000ºK) it is of interest to know what the equivalent Daylight color
temperature to the measurement is, since daylight white points are
the ones most often used as standards. While the black body locus is
quite close to the daylight locus, they are slightly different. The
Daylight locus is specified over a smaller range of temperatures,
since this reflects the realistic range of color temperatures that
natural daylight can range over.
While degrees Kelvin is highly visually non-linear, it turns out
that inverse temperature is much more linear, and is therefore
better in comparing degrees of color temperature error or
difference. The conventional unit used for this is the micro
reciprocal degree or Mired, which is 1000000 divided
by the temperature.