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 (1982)
In general there is an extremely close fit between the daylight and blackbody curves. This is not surprising, because the solar spectrum is one of many natural light sources that resemble a blackbody radiator. But the most essential point is that the blackbody locus describes the entire sequence of landscape illumination across diurnal and seasonal cycles.
blackbody (rK), mired (M) and
mired deviations specify wratten filters used to neutralize light temperature for color film balanced at 5500°K; these filters are visual complements of the blackbody color
However, there is one problem. Compared to an "unbiased" or equal energy illuminant, the blackbody locus and the path of all daylight spectra are shifted toward green by a small, consistent amount. This occurs because the equal energy illuminant is perfectly flat, while the daylight spectra are typically peaked near the center or short wavelength end of the spectrum. When these chromaticities are reproduced at luminances matching your computer screen, they produce a distinct yellowish green or bluish green tint (right); in fact, the dominant wavelength for sunlight is about 530 nm. Sunlight is not yellow, it is a very pale green!
Can the correlated color temperature describe lights that do not exactly match a blackbody chromaticity? The answer is yes, provided the color difference is not large. We simply determine the closest matching CCT to the light we want to describe. In a uniform color space, such as the CIE 1976 UCS, a line through the light's chromaticity that is perpendicular to the blackbody locus passes through the closest matching CCT value. In a nonuniform color space, such as the CIE 1931 Yxy chromaticity diagram used above, these lines (shown in green) are not perpendicular to the blackbody locus but slanting to it.
The subtle chromaticity differences between the blackbody curve and the aggregate swath of daylight spectra can be compared in the daylight color series shown below. This represents the actual colors of light that mix with and alter 