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 to specify the chromaticity of a wide range of artificial "white" lights and natural daylight phases, as summarized below. Note that an equal temperature change produces a smaller color change at higher temperatures.
blackbody locus in the CIE UCS
adapted from Hunt (2004)
The CIE standard illuminants are based on averaged photometric measurements that characterize common light distributions. The 2860°K (A) illuminant represents light from a standard domestic incandescent light bulb; the 5000°K (D50) illuminant represents a warm daylight distribution and is preferred in graphic arts applications, and the 6500°K (D65) illuminant represents a cool daylight distribution that is preferred for industrial colorimetric applications (for example, matching automotive colors or architectural paint colors).
Notice the fact inconvenient for color theory and sometimes confusing when using CCT values in lighting and photography: as the temperature becomes warmer, the color becomes cooler! To avoid confusion, photographers instead use the mired (M), defined as:
M = 1,000,000/rK
The mired has two desirable properties: values increase as the color becomes warmer (diagram, right), and an equal numerical difference on the mired scale represents an approximately equal visual change in the color temperature across the practical range of CCT values.
Solar & Daylight Color. How well do correlated color temperatures describe the chromaticities of actual landscape illumination? The diagram below shows that the blackbody locus closely parallels the aggregate chromaticity variations across a large sample of daylight spectra the colors of natural light, measured in different sky directions across different seasons and geographic regions at different times of day.
loci of daylight and blackbody spectra
chromaticities of daylight spectra measured by Budde (1963), Condit & Grum (1964) and Henderson & Hodgkiss (1963), CIE illuminants D50 and D65, and solar CCT (5780°K) in the CIE 1931 Yxy chromaticity diagram; adapted from Wyszecki & Stiles (198