- White point
A white point (often referred to as "reference white" or "target white" in technical documents) is a set of tristimulus values or
chromaticity coordinates that serve to define the color "white" in image capture, encoding, or reproduction. [cite book | title = Color and Mastering for Digital Cinema | author = Glenn Kennel | publisher = Focal Press | isbn = 0240808746 | year = 2006 | url = http://books.google.com/books?id=w__iiSih-zUC&pg=PA61&dq=white-point+chromaticity&lr=&as_brr=3&ei=s4qJR6iuFJq6tgPYmYnQBQ&sig=HeAw467xtg4Lrb24h4d9f1qKtJU ] Depending on the application, different definitions of white are needed to give acceptable results. For example, photographs taken indoors may be lit byincandescent light s, which are relatively orange compared todaylight . Defining "white" as daylight will give unacceptable results when attempting to color correct a photograph taken with incandescent lighting.__TOC__
Illuminants
An illuminant is characterized by its relative spectral power distribution (as opposed to the absolute SPD, because the brightness of the illuminant is allowed to vary). The white point of an illuminant is the
chromaticity of awhite object under the illuminant, and can be specified by chromaticity coordinates, such as the "x", "y" coordinates on theCIE 1931 chromaticity diagram . [cite book | title = The Manual of Photography: Photographic and Digital Imaging | author = R. E. Jacobson | publisher = Focal Press | year = 2000 | isbn = 0240515749 | url = http://books.google.com/books?id=HHX4xB94vcMC&pg=PA388&dq=white-point+chromaticity+cie+object&lr=&as_brr=3&ei=K46JR8XYJoeWtgOw-MTQBQ&sig=NbZh2Zi339EJCZ4oW9MdQgUEMY4#PPA388,M1 ]Illuminant and white point are separate concepts. For a given illuminant, its white point is uniquely defined. A given white point, on the other hand, generally does not uniquely correspond to only one illuminant. From the commonly used
CIE 1931 chromaticity diagram , it can be seen that almost all non-spectral colors, including colors described as white, can be produced by "infinitely many" combinations of spectral colors, and therefore by infinitely many different illuminant spectra.Although there is generally no one-to-one correspondence between illuminants and white points, in the case of the CIE D-series standard illuminants, the spectral power distributions are mathematically derivable from the chromaticity coordinates of the corresponding white points. [cite web | title = Spectral Power Distribution of a CIE D-Illuminant | url = http://www.brucelindbloom.com/index.html?Eqn_DIlluminant.html | author = Bruce Justin Lindbloom]
Knowing the illuminant's spectral power distribution, the
reflectance 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 anycolor 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 1964CIE XYZ color space s, its color coordinates are [K,K,K] where K is a constant, and its chromaticity coordinates are [x,y] = [1/3,1/3] .White point conversion
If the color of an object is recorded under one illuminant, then it is possible to estimate the color of that object under another illuminant, given only the white points of the two illuminants. If the image is "uncalibrated" (the illuminant's white point unknown), the white point has to be estimated. However, if one merely wants to
white balance (make neutral objects appear neutral in the recording), this may not be necessary.Expressing color as tristimulus coordinates in the
LMS color space , one can "translate" the object's color according to theVon Kries transform simply by scaling the LMS coordinates by the ratio of the maximum of the tristimulus values at both white points. This provides a simple, but rough estimate. Another method that is sometimes preferred uses aBradford transform or anotherchromatic adaptation transform ; in general, these work by transforming into an intermediate space, scaling the amounts of the primaries in that space, and converting back by the inverse transform.To truly calculate the color of an object under another illuminant, not merely how it will be perceived, it is necessary to record multi-spectral or hyper-spectral color information.
References
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