White point
Encyclopedia
A white point is a set of tristimulus values or chromaticity
coordinates that serve to define the color "white" in image capture, encoding, 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.
. The white point of an illuminant is the chromaticity
of a white
object under the illuminant, and can be specified by chromaticity coordinates, such as the x, y coordinates on the CIE 1931 chromaticity diagram (hence the use of the relative SPD and not the absolute SPD, because the white point is only related to color and unaffected by intensity).
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.
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 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].
Expressing color as tristimulus coordinates in the LMS color space
, one can "translate" the object's color according to the Von 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 a Bradford transform or another chromatic 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.
Chromaticity
Chromaticity is an objective specification of the quality of a color regardless of its luminance, that is, as determined by its hue and colorfulness ....
coordinates that serve to define the color "white" in image capture, encoding, 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
Daylight
Daylight or the light of day is the combination of all direct and indirect sunlight outdoors during the daytime. This includes direct sunlight, diffuse sky radiation, and both of these reflected from the Earth and terrestrial objects. Sunlight scattered or reflected from objects in outer space is...
. Defining "white" as daylight will give unacceptable results when attempting to color correct a photograph taken with incandescent lighting.
Illuminants
An illuminant is characterized by its relative spectral power distributionSpectral power distribution
In color science and radiometry, a spectral power distribution describes the power per unit area per unit wavelength of an illumination , or more generally, the per-wavelength contribution to any radiometric quantity .Mathematically, for the spectral...
. The white point of an illuminant is the chromaticity
Chromaticity
Chromaticity is an objective specification of the quality of a color regardless of its luminance, that is, as determined by its hue and colorfulness ....
of a white
White
White is a color, the perception of which is evoked by light that stimulates all three types of color sensitive cone cells in the human eye in nearly equal amounts and with high brightness compared to the surroundings. A white visual stimulation will be void of hue and grayness.White light can be...
object under the illuminant, and can be specified by chromaticity coordinates, such as the x, y coordinates on the CIE 1931 chromaticity diagram (hence the use of the relative SPD and not the absolute SPD, because the white point is only related to color and unaffected by intensity).
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
Spectral color
A spectral color is a color that is evoked by a single wavelength of light in the visible spectrum, or by a relatively narrow band of wavelengths...
, including colors described as white, can be produced by infinitely many combinations
Convex combination
In convex geometry, a convex combination is a linear combination of points where all coefficients are non-negative and sum up to 1....
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
Standard illuminant
A standard illuminant is a theoretical source of visible light with a profile which is published. Standard illuminants provide a basis for comparing images or colors recorded under different lighting.-CIE illuminants:...
, the spectral power distributions are mathematically derivable from the chromaticity coordinates of the corresponding white points.
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 any color space
Color space
A color model is an abstract mathematical model describing the way colors can be represented as tuples of numbers, typically as three or four values or color components...
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 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
LMS Color Space
LMS is a color space represented by the response of the three types of cones of the human eye, named after their responsivity at long, medium and short wavelengths....
, one can "translate" the object's color according to the Von 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 a Bradford transform or another chromatic 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
Multi-spectral image
A multispectral image is one that captures image data at specific frequencies across the electromagnetic spectrum. The wavelengths may be separated by filters or by the use of instruments that are sensitive to particular wavelengths, including light from frequencies beyond the visible light range,...
or hyper-spectral
Hyperspectral imaging
Hyperspectral imaging collects and processes information from across the electromagnetic spectrum. Much as the human eye sees visible light in three bands , spectral imaging divides the spectrum into many more bands...
color information.