roducing familiar changes in the appearance of our environment. Hues appear to shift warm or cool, colors become more or less saturated, lighter or darker, and complementary shadow colors change from violet to blue green.
Blackbody Color. The obvious next step is to find a method that can describe or define the relative amounts of yellow or blue bias in a "white" light. A simple way to do this, for natural light and most "white" artificial lights, is by the light's blackbody temperature.
In 1900 the Austrian physicist Max Planck mathematically described the spectral power distribution that would be produced at different temperatures by a perfectly radiating object, called a "black body" because no light would reflect from it. (This work led to the development of quantum mechanics, which won Planck the Nobel Prize in 1918.)
These blackbody curves approximately match the spectra radiated by many natural light sources, including heated metals, electrical discharges and stars. In all these cases, an entire spectral emission curve can be specified by its blackbody temperature alone.
blackbody spectral emittance curves
for temperatures of 2860°K, 5000°K and 6500°K, corresponding to CIE illuminants A, D50 and D65; as the blackbody temperature increases, the peak emittance increases and shifts from infrared to ultraviolet wavelengths
Shown above are the blackbody curves corresponding to three standard "white" illuminants published by the CIE. They illustrate the three characteristics of all blackbody radiation: (1) a continuous, smooth curve with a single peak emittance; (2) a shift of the peak emittance as temperature increases, from a peak in the far infrared (7200 nm) for a blackbody at room temperature to a peak in ultraviolet (30 nm) for a blackbody at 100,000°K (degrees Kelvin); and (3) an enormous increase in total radiant flux as temperature increases.
color changes in daylight
(top) noon; (bottom) late afternoon; note change in the sky color
color changes in daylight
(top) morning; (bottom) midday diurnal color changes are best observed through the window of a darkened room
If we convert the blackbody emittance to lumens, then standardize the vast energy differences to get light profiles of equal luminance, these relative spectral emittance profiles (or illuminants) can be assigned a hue and chroma location in a CIE chromaticity diagram, just like any other colored light. The changes in the shape and peak energy of the blackbody curves then produce a characteristic color sequence as temperature increases a curved line called the blackbody locus (right).
The essential features of the blackbody locus are: (1) the curve is closest to the equal energy white point at a temperature of about 5800°K; (2) above 5000°K the curve is nearly straight; (3) this straight portion is aligned from blue to yellow (approximately from 470 nm to 575 nm); (4) below 4000°K the curve arcs sharply into orange and red, and becomes much more saturated, as temperature decreases; (5) an equal temperature difference defines a smaller color difference as the blackbody temperature increases; and therefore (6) blackbody radiation never reaches a violet or purple hue: at an infinitely high temperature, the blackbody chromaticity has a dominant wavelength of about 470 nm (CIELUV hue angle of 249).
Correlated Color Temperature. The blackbody locus provides the method necessary to specify the color of almost any naturally occurring light source. The temperature (curve shape) of the blackbody is adjusted until its standardized spectral emittance curve produces a visual or metameric match between the blackbody and light source their chromaticity points are the same. Note that the match is not between the shape of the two spectral emittance curves, but between the apparent color of the two curves as they appear, at equal luminance, to a normal observer. Then the temperature of the blackbody curve, expressed in degrees Kelvin (K), is the correlated color temperature (abbreviated rK or CCT) of the matching light.
The blackbody temperature has proven very useful 