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. In the study of the perception of Color, one of the first mathematically defined Color spaces was the CIE 1931 XYZ color space (also known as CIE 1931 color space Chromaticity is an objective specification of the Quality of a Color regardless of its luminance that is as determined by its Colorfulness (or saturation [1] 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. The incandescent light bulb, incandescent lamp or incandescent light globe is a source of electric Light that works by Incandescence, (a general Daylight or the light of day is the combination of all direct and indirect Sunlight outdoors during the daytime (and perhaps Twilight) Defining "white" as daylight will give unacceptable results when attempting to color correct a photograph taken with incandescent lighting.
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Illuminants
- See also: Standard illuminant
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). A standard illuminant is a profile or Spectrum of Visible light which is published in order to allow images or colors recorded under different lighting to be compared In Color science, a spectral power distribution describes the power per unit area per unit wavelength of an illumination (radiant exitance or more generally the per-wavelength 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. Chromaticity is an objective specification of the Quality of a Color regardless of its luminance that is as determined by its Colorfulness (or saturation White is a Color, the perception which is evoked by Light that stimulates all three types of color sensitive Cone cells in the Human eye In the study of the perception of Color, one of the first mathematically defined Color spaces was the CIE 1931 XYZ color space (also known as CIE 1931 color space [2]
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. In the study of the perception of Color, one of the first mathematically defined Color spaces was the CIE 1931 XYZ color space (also known as CIE 1931 color space 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 A convex combination is a Linear combination of points (which can be vectors scalars, or more generally points in an Affine space)
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. A standard illuminant is a profile or Spectrum of Visible light which is published in order to allow images or colors recorded under different lighting to be compared [3]
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. In photometry and Heat transfer, reflectivity is the fraction of incident radiation reflected by a surface 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 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]. In the study of the perception of Color, one of the first mathematically defined Color spaces was the CIE 1931 XYZ color space (also known as CIE 1931 color space
White point conversion
- Further information: Chromatic adaptation
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. In color science chromatic adaptation is the estimation of the representation of an object under a different light source than the one in which it was recorded 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. In Photography and Image processing, color balance is the global adjustment of the intensities of the colors (typically red green and blue Primary colors
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. LMS is a Color space represented by the response of the three types of cones of the human Eye, named after their Responsivity (sensitivity Color vision is the capacity of an organism or machine to distinguish objects based on the Wavelengths (or frequencies) of the Light they reflect or emit 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. In color science chromatic adaptation is the estimation of the representation of an object under a different light source than the one in which it was recorded
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. Multi-spectral imaging is a technology originally developed for space-based imaging Hyperspectral imaging collects and processes information from across the Electromagnetic spectrum.
References
- ^ Glenn Kennel (2006). Color and Mastering for Digital Cinema. Focal Press. ISBN 0240808746.
- ^ R. E. Jacobson (2000). The Manual of Photography: Photographic and Digital Imaging. Focal Press. ISBN 0240515749.
- ^ Bruce Justin Lindbloom. Spectral Power Distribution of a CIE D-Illuminant.
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