White point

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A white point is one of a number of reference illuminants used in colorimetry which serve to define the color "white". 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.

Each white point illuminant is ideally described as a spectral power distribution, that is, by giving the amount of power per unit wavelength at each wavelength of the visible spectrum. This will allow the coordinates of the white point in any color space to be defined. For example, one of the simplest white points to understand is the "E" or "Equal Energy" white point. Its spectral power distribution is flat, giving the same power per unit wavelength at any wavelength. In terms of the CIE XYZ color space its color coordinates are [K,K,K] where K is a constant, and its chromaticity coordinates are [x,y]=[1/3,1/3].

A list of common white points, their CIE chromaticity coordinates (x,y) and their correlated color temperature (CCT) 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 brightness Y=0.54, assuming correct sRGB display calibration.

White points
Name CIE 1931 CIE 1964 CCT
°K		 Hue Note
x y x y
E 1/3 1/3 1/3 1/3 5400 Equal energy
D50 0.34567 0.35850 0.34773 0.35952 5000
D55 0.33242 0.34743 0.33411 0.34877 5500
D65 0.31271 0.32902 0.31382 0.33100 6500 Television, sRGB color space
D75 0.29902 0.31485 0.29968 0.31740 7500
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

[edit] External links

[edit] 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. 
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