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An illuminant is characterized by its relative spectral power distribution (SPD). The white point of an illuminant is the chromaticity of a white object under the illuminant, and can be specified by chromaticity coordinates, such as the x, y coordinates on the CIE 1931 chromaticity diagram (hence the use of the relative SPD and not the absolute SPD, because the white point is only related to ...
A list of standardized illuminants, their CIE chromaticity coordinates (x,y) of a perfectly reflecting (or transmitting) diffuser, and their correlated color temperatures (CCTs) are given below.
The spectrum of a standard illuminant, like any other profile of light, can be converted into tristimulus values. The set of three tristimulus coordinates of an illuminant is called a white point. If the profile is normalized, then the white point can equivalently be expressed as a pair of chromaticity coordinates.
For example, the white point of an sRGB display is an x, y chromaticity of (0.3127, 0.3290), where x and y coordinates are used in the xyY space. (u′, v′), the chromaticity in CIELUV, is a fairly perceptually uniform presentation of the chromaticity as (another than in CIE 1931) planar Euclidean shape.
The chromaticity components U* and V* are defined such that the white point maps to the origin, as in Adams chromatic valence color spaces. This arrangement has the benefit of being able to express the loci of chromaticities with constant saturation simply as ( U *) 2 + ( V *) 2 = C for a constant C .
In contrast, a color appearance model takes the white point of the illuminant into account (which is why a color appearance model requires this value for its calculations); if the white point of the illuminant changes, the color of the surface as reported by the color appearance model remains the same.
In addition, the SMPTE C RGB primaries used in North American standard definition are different than those of BT.709 (SMPTE C is commonly referred to as NTSC, however it is a different set of primaries and a different white point than the 1953 NTSC. [22]). The red and blue primaries for PAL and SECAM are the same as BT.709, with a change in the ...
Mathematically, for the spectral power distribution of a radiant exitance or irradiance one may write: =where M(λ) is the spectral irradiance (or exitance) of the light (SI units: W/m 2 = kg·m −1 ·s −3); Φ is the radiant flux of the source (SI unit: watt, W); A is the area over which the radiant flux is integrated (SI unit: square meter, m 2); and λ is the wavelength (SI unit: meter, m).