Information about White Point
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.
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].
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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 | ||||
| E | 1/3 | 1/3 | 1/3 | 1/3 | 5400 | Equal energy | |
| D50 | 0.34567 | 0.35850 | 0.34773 | 0.35952 | 5003 | ||
| D55 | 0.33242 | 0.34743 | 0.33411 | 0.34877 | 5503 | ||
| D65 | 0.31271 | 0.32902 | 0.31382 | 0.33100 | 6504 | Television, sRGB color space | |
| D75 | 0.29902 | 0.31485 | 0.29968 | 0.31740 | 7504 | ||
| 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 | |
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.
The tristimulus values of a test color are the amounts of the three primary colors in a three-component color model needed to match that test color. The tristimulus values are most often given in the CIE 1931 color space, in which they are denoted , , and .
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Chromaticity is the quality of a color as determined by its purity and hue.[1]
"Purity" in this context, called "colorfulness" in CIECAM02, and "chroma" in Munsell, can be measured as the distance of a color from the neutral axis in a color space such as CIE
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"Purity" in this context, called "colorfulness" in CIECAM02, and "chroma" in Munsell, can be measured as the distance of a color from the neutral axis in a color space such as CIE
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In color science, the power per unit area per unit wavelength of a radiant object. Mathematically, one may write:
where is the spectral exitance (or emittance) of the source (SI units: watt meter–3
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where is the spectral exitance (or emittance) of the source (SI units: watt meter–3
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1 metre =
SI units
1000 mm 0 cm
US customary / Imperial units
0 ft 0 in
The metre or meter[1](symbol: m) is the fundamental unit of length in the International System of Units (SI).SI units
1000 mm 0 cm
US customary / Imperial units
0 ft 0 in
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CIE is an acronym which can stand for:
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- Council on Islamic Education
- Córas Iompair Éireann, the Irish Transport System
- the International Commission on Illumination (Commission Internationale de l'Eclairage)
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color space. For example, Adobe RGB and sRGB are two different absolute color spaces, both based on the RGB model.
In the most generic sense of the definition above, color spaces can be defined without the use of a color model.
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In the most generic sense of the definition above, color spaces can be defined without the use of a color model.
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CIE 1931 XYZ color space (also known as CIE 1931 color space), created by the International Commission on Illumination (CIE) in 1931.
The human eye has receptors (called cone cells) for short (S), middle (M), and long (L) wavelengths.
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The human eye has receptors (called cone cells) for short (S), middle (M), and long (L) wavelengths.
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Hue is one of the three main attributes of perceived color, in addition to lightness and chroma (or colorfulness). Hue is also one of the three dimensions in some colorspaces along with saturation, and brightness (also known as lightness or value).
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Luminance is a photometric measure of the density of luminous intensity in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle.
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sRGB is a standard RGB (Red Green Blue) color space created cooperatively by HP and Microsoft for use on monitors, printers, and the Internet. It was originally proposed in 1995 by Ralf Kuron of FOGRA as a pragmatic approach in connection to ICC.
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The kelvin (symbol: K) is a unit increment of temperature and is one of the seven SI base units. The Kelvin scale is a thermodynamic (absolute) temperature scale where absolute zero — the coldest possible temperature — is zero kelvins
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Hue is one of the three main attributes of perceived color, in addition to lightness and chroma (or colorfulness). Hue is also one of the three dimensions in some colorspaces along with saturation, and brightness (also known as lightness or value).
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Television (often abbreviated to TV, T.V., or more recently, tv; sometimes called telly, the tube, boob tube, or idiot box in British English) is a widely used telecommunication system for broadcasting and receiving moving pictures
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sRGB is a standard RGB (Red Green Blue) color space created cooperatively by HP and Microsoft for use on monitors, printers, and the Internet. It was originally proposed in 1995 by Ralf Kuron of FOGRA as a pragmatic approach in connection to ICC.
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