h, time of day and concentration of atmospheric ice, dust or smoke. The distribution of chromaticities is again roughly parallel to the blackbody locus, but the average sky chromaticity is usually above a CCT of 10,000°K, corresponding to a dominant wavelength of about 470 to 475 nm (CIELUV hue angle of about 235° to 245°). The best paint matches to the typical blue sky color are a dulled cobalt blue (PB28) or iron blue (PB27), both visual complements of orange yellow.
The luminance of sky colors vary widely. As an upper bound, cumulus clouds that appear "pure white" have an albedo (reflectance) of around 70%-80%, equivalent to a light gray; it is the luminance contrast with the surrounding darker sky that makes them appear white. The sky typically appears relatively dark, with a luminance near the zenith that approximately matches a middle gray (reflectance 30%). Again, moderately diluted cobalt paints reproduce these values well.
Color Rendering Index. The orientation of the lines of constant CCT (above) shows that artificial lights with the same CCT can have a definite yellow, green, blue or purple tint, even though rated as "white" lights. This ambiguity is introduced in two ways: (1) by defining CCTs as metameric matches between a blackbody curve and a light, which can ignore radical irregularities in the spectral curve of the light, and (2) by defining CCTs as the nearest blackbody chromaticity to the chromaticity of the light, even when there is a visible color difference between them. Lights with the same CCT and even the same "white" chromaticity can have very different spectral emission curves, which will produce different appearances in surface colors.
These differences represent the color rendering quality of a light. The color rendering index (CRI) is a numerical rating of how closely the color appearance of surfaces viewed under the light matches the color appearance of the same surfaces viewed under a blackbody light of the same CCT. It is calculated by averaging the colorimetric differences (if any) between the reflectance of 8 to 14 standard colors as illuminated by the test light and a correlated light source of equal luminance and a flat, smooth emission profile, after chromatic adaptation to each light.
the warm/cool contrast and the opponent dimensions
relative visual response to illuminants A and D65
Fluorescent lights generally have reduced CRIs. They emit a broken spectrum that has an undulating shape across the spectrum and contains very high, sharp spikes at specific wavelengths (right). These hardly match the smoothly curving, domed curve of a blackbody spectrum, and the surface color reflectances at the "spike" wavelengths are grossly inflated in the total color appearance, producing a greenish or bluish cast that has an especially dulling effect on warm colors. "Cool white" fluorescent lights have a CRI from 65 to 85, "daylight" fluorescents around 80, and "warm white" fluorescents from 55 to 75. Metal halide bulbs range from the mid 60's to the mid 90's; and those beloved sodium vapor lamps, which transform all colors into a yellowish, cadaverous gray, have CRIs close to zero.
The best light sources are those that make all colors all hues at all levels of chroma and lightness appear "natural" or completely untinted. These are broadband, smooth spectral power distributions with chromaticities close to the daylight spectral path: incandescent lights (tungsten or halogen lights), propane lanterns, carbon or xenon arc lamps, burning magnesium. Incandescent or tungsten halogen bulbs all have a CRI of 100 and deliver a "true" color balance.
How can a light source that has an off white or "warm" color (such as a 120 watt incandescent bulb at 2860°K) still have a CRI of 100? As long as the light approximates a high temperature blackbody source, it generates light across the entire spectral range without spikes or gaps, and our eyes will be able to preserve the perception of "white" illumination by chromatic adaptation. That adaptation is the origin and principal basis of our warm/cool color sensitiv