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.
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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.
| Name | CIE 1931 | CIE 1964 | CCT (K) | Hue | Note | ||
|---|---|---|---|---|---|---|---|
| x | y | x10 | y10 | ||||
| A | 0.44757 | 0.40745 | 0.45117 | 0.40594 | 2856 | Incandescent tungsten | |
| F11 | 0.38054 | 0.37691 | 0.38543 | 0.37110 | 4000 | Narrow Band White Fluorescent | |
| F2 | 0.37207 | 0.37512 | 0.37928 | 0.36723 | 4200 | Cool White Fluorescent (CWF) | |
| B | 0.34842 | 0.35161 | 0.3498 | 0.3527 | 4874 | Obsolete, direct sunlight at noon | |
| D50 | 0.34567 | 0.35850 | 0.34773 | 0.35952 | 5003 | ||
| E | 1/3 | 1/3 | 1/3 | 1/3 | 5400 | Equal energy | |
| D55 | 0.33242 | 0.34743 | 0.33411 | 0.34877 | 5503 | ||
| F7 | 0.31285 | 0.32918 | 0.31565 | 0.32951 | 6500 | Broad-Band Daylight Fluorescent | |
| D65 | 0.31271 | 0.32902 | 0.31382 | 0.33100 | 6504 | Television, sRGB color space | |
| C | 0.31006 | 0.31616 | 0.31039 | 0.31905 | 6774 | Obsolete, north sky daylight | |
| D75 | 0.29902 | 0.31485 | 0.29968 | 0.31740 | 7504 | ||
| 9300 | 0.28480 | 0.29320 | 9300 | High-efficiency blue phosphor monitors | |||
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
- Agoston, George A. (1987). Color Theory and Its Application in Art and Design (Revised Edition). Springer-Verlag, New York. ISBN 0-387-17095-2.
- Wyszecki, Günter and Stiles, W.S. (2000). Color Science - Concepts and Methods, Quantitative Data and Formulae, 2nd edition, New York: Wiley-Interscience. ISBN 0-471-39918-3.
