CIE 1931 white points

"White" is not one color. A sheet of paper looks white under a tungsten bulb and in daylight, although it reflects very different light. The eye adapts to the illumination, and colorimetry has to account for that. That is why every color space has a defined white point.

Illuminant versus light source

A light source is a physical lamp or the sun. An illuminant is a table: a spectral distribution defined numerically by the CIE, which may or may not correspond to a real lamp. The white point is the color of a perfectly white surface lit by that illuminant, expressed in CIE XYZ or xy coordinates.

The main illuminants

Illuminant	Description	Temperature	x	y	X, Y, Z
A	tungsten incandescent	2856 K	0.44757	0.40745	109.850, 100, 35.585
C	old "daylight", now obsolete	6774 K	0.31006	0.31616	98.074, 100, 118.232
D50	morning daylight, printing	5003 K	0.34567	0.35850	96.422, 100, 82.521
D55	daylight, photography	5503 K	0.33242	0.34743	95.682, 100, 92.149
D65	average daylight, displays, sRGB	6504 K	0.31271	0.32902	95.047, 100, 108.883
E	equal energy at every wavelength	about 5455 K	0.33333	0.33333	100, 100, 100
F2	cool white fluorescent	4230 K	0.37208	0.37529

Values for the 2° observer. D65 is 6504 K rather than a round 6500 K because in 1968 the value of one of the constants in Planck's law was revised while the spectrum definition stayed the same.

Color temperature

A heated black body glows with a color that depends only on its temperature: orange at 2000 K, nearly white at 5000 K, bluish at 10,000 K. On the chromaticity diagram these colors form the Planckian locus. The nearest temperature on that curve is the correlated color temperature (CCT) of a given light. That is why "warm" light has a lower kelvin number than "cool" light.

Why it matters

Displays and sRGB use D65. A monitor white set to 6500 K is this point.
Printing and ICC profiles use D50. Prints are judged in D50 viewing booths.
CIELAB is always computed relative to some white. The same color has different Lab values under D50 and D65.

Chromatic adaptation

To move a color from one white to another the way the eye would, chromatic adaptation is used. Most often the Bradford method: XYZ is converted to approximate cone responses, each is scaled by the ratio of destination to source white, and the result goes back to XYZ:

| 0.8951 0.2664 -0.1614 | M_B = | -0.7502 1.7135 0.0367 | | 0.0389 -0.0685 1.0296 | (ρ, γ, β) = M_B · (X, Y, Z) (ρ', γ', β') = (ρ · ρ_d/ρ_s, γ · γ_d/γ_s, β · β_d/β_s) (X', Y', Z') = M_B⁻¹ · (ρ', γ', β') ρ_s, γ_s, β_s: source white after M_B ρ_d, γ_d, β_d: destination white after M_B

Example: RGB (200, 80, 40), which in sRGB (D65) has XYZ = (27.07, 18.17, 4.09). After Bradford adaptation to D50:

D65 white in M_B: (0.9414, 1.0404, 1.0895) D50 white in M_B: (0.9963, 1.0204, 0.8186) XYZ (D50) = (28.58, 18.73, 3.10)

The color now has more X and less Z, because D50 white is warmer than D65. This is the XYZ that goes into an ICC profile.

The calculator above computes everything relative to D65, like sRGB. Graphics software working with ICC profiles will show slightly different Lab values for the same color, because it computes relative to D50.