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:

ColorTempurature Settings

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 being on 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.

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