WHITE POINT

A 'white point' is a set of tristimulus values or chromaticity coordinates that serve to define the color "white" in image capture or reproduction. Depending on the application, different definitions of white are needed to give acceptable results. For example, photographs taken indoors may be lit by incandescent lights, which are relatively orange compared to daylight. Defining "white" as daylight will give unacceptable results when attempting to color-correct a photograph taken with incandescent lighting.

Contents

White points and illuminants

White points for some illuminants

External links

References

White points and illuminants

Each illuminant is ideally described as a spectral power distribution, that is, by giving the amount of power per unit area per unit wavelength at each wavelength of the visible spectrum, often normalized to unity at 560 nanometers. A white point is a set of coordinates, usually CIE tristimulus values or CIE chromaticity coordinates, of a white object, often a perfect reflecting (or transmitting) diffuser, under that illuminant, viewed by a specified observer (often the CIE 1931 Standard Observer or the CIE 1964 Supplementary Standard Observer). Thus, white points are not illuminants and illuminants are not white points, though they are closely related.
Knowing the illuminant's spectral power distribution, the radiance ratio spectrum of the specified white object (often taken as unity), and the numerical definition of the observer allows the coordinates of the white point in any color space to be defined. For example, one of the simplest illuminants is the "E" or "Equal Energy" spectrum. Its spectral power distribution is flat, giving the same power per unit wavelength at any wavelength. In terms of both the 1931 and 1964 CIE XYZ color spaces, its color coordinates are [K,K,K] where K is a constant, and its chromaticity coordinates are [x,y]=[1/3,1/3].

White points for some illuminants

A list of standardized illuminants, their CIE chromaticity coordinates (x,y) of a perfect reflecting (or transmitting) diffuser, and their correlated color temperatures (CCTs) are given below. The CIE chromaticity coordinates are given for both the 2 degree field of view (1931) and the 10 degree field of view (1964). The color swatches represent the hue of each white point, calculated with luminance Y=0.54, assuming correct sRGB display calibration.

'White points'

Name	CIE 1931		CIE 1964		CCT (K)	Hue	Note
	x	y	x10	y10
E	1/3	1/3	1/3	1/3	5400		Equal energy
D50	0.34567	0.35850	0.34773	0.35952	5003
D55	0.33242	0.34743	0.33411	0.34877	5503
D65	0.31271	0.32902	0.31382	0.33100	6504		Television, sRGB color space
D75	0.29902	0.31485	0.29968	0.31740	7504
A	0.44757	0.40745	0.45117	0.40594	2856		Incandescent tungsten
B	0.34842	0.35161	0.3498	0.3527	4874		Obsolete, direct sunlight at noon
C	0.31006	0.31616	0.31039	0.31905	6774		Obsolete, north sky daylight
9300	0.28480	0.29320			9300		High-efficiency blue phosphor monitors
F2	0.37207	0.37512	0.37928	0.36723	4200		Cool White Fluorescent (CWF)
F7	0.31285	0.32918	0.31565	0.32951	6500		Broad-Band Daylight Fluorescent
F11	0.38054	0.37691	0.38543	0.37110	4000		Narrow Band White Fluorescent

External links

★ CVRL Tabulated data- for illuminants A and D65 (click on "CIE Functions" menu choice)

★ A Review of RGB color spaces (BabelColor).

★ CIE color calculator

References

★ Color Theory and Its Application in Art and Design (Revised Edition), Agoston, George A., , , Springer-Verlag, New York, 1987, ISBN 0-387-17095-2

★ Color Science - Concepts and Methods, Quantitative Data and Formulae, Wyszecki, Günter and Stiles, W.S., , , Wiley-Interscience, 2000, ISBN 0-471-39918-3