Color Temperature & White Point

CCT ↔ CIE xy · Duv · Planckian & daylight loci · standard illuminants

Temperature CCT (K)

Duv (offset from locus)

Locus

CCT

–

Duv

–

x, y

–, –

u′, v′ (1976)

–, –

Nearest standard illuminant

–

CIE 1931 diagram · loci · illuminants

Planckian (blackbody) Daylight (D series)

Standard illuminants

	x	y	CCT

Method · formulas

Temperature → coordinates: the Planckian locus uses the piecewise cubic of Kim et al. (2002) — x is a polynomial in 1/T (break at 4000K), y a cubic in x (breaks at 2222K, 4000K). The daylight (D) locus uses CIE 15's x_D (break at 7000K) and y_D = −3x² + 2.87x − 0.275. Duv is applied by stepping along the locus normal in the CIE 1960 uv plane (+ toward green, − toward magenta).

Coordinates → temperature: CCT is McCamy's (1992) approximation CCT = 437n³ + 3601n² + 6831n + 5517, n = (x−0.3320)/(0.1858−y). Duv is the shortest (signed) distance from the input point, mapped to CIE 1960 uv, to a densely sampled Planckian locus.

※ The CCT approximations are valid roughly 1667–25000K and lose accuracy at the extremes. When |Duv| exceeds ~0.05 the very notion of a 'CCT' weakens (too far from the locus). Standard illuminant coordinates are CIE 15:2004, 2° observer.