2)
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 landscape surface colors.
blackbody and daylight color series
chromaticities of blackbody spectra under D65 adaptation; adapted from Charity (1997)
The major difference between the blackbody and daylight color series (when defined against a D65 "white" standard) is the visibly increased "green" component in the daylight colors from deep yellow to white, and from white through middle blue, which produces the noticeable yellow or cerulean tints around daylight colors close to white. These colors appear because "green" light shifts all "red" wavelengths toward yellow and all "blue" wavelengths toward blue green (cerulean).
chromaticities of CIE illuminants when eye is adapted to equal energy "white" (EE)
The yellow is visible in afternoon sunlight, especially in shafts of light into a darkened room, and in this range does not match the orange color of the blackbody spectrum. Ceruleans and very subtle greens are often visible in the sky close to the horizon, especially around sunset when the "violet" component of daylight is heavily filtered. At these times the low sky can appear to shade from a grayish deep yellow into scarlet (photo, right).
It is intriguing that green and rose hues (shown in the diagram immediately around the white point) appear in the afterimage colors of specular reflections, and commonly in the glint of iridescence or refraction colors in water and high altitude clouds near the sun, and the rose colors of dawn. These are subtle shifts in "white" color that are not within the daylight series of colors but are perpendicular to it (along a magenta/green dimension).
The artist can only marvel that the solar light and atmosphere can weave such a narrow chromaticity path and yet create so many landscape color changes and light contrasts. The color variations between dawn and sunset, or the sky seen from a seagoing ship and a mountain peak, or a desert sky before and after a cleansing cloudburst, or the light of spring and fall, or noon and twilight, or the colors of rainbow and iridescent sundog, are