Colorfulness,chroma andsaturation are attributes of perceived color relating to chromatic intensity. As defined formally by theInternational Commission on Illumination (CIE) they respectively describe three different aspects of chromatic intensity, but the terms are often used loosely and interchangeably in contexts where these aspects are not clearly distinguished. The precise meanings of the terms vary by what other functions they are dependent on.
As colorfulness, chroma, and saturation are defined as attributes of perception, they can not be physically measured as such, but they can be quantified in relation to psychometric scales intended to be perceptually even—for example, the chroma scales of theMunsell system. While the chroma and lightness of an object are its colorfulness and brightness judged in proportion to the same thing ("the brightness of a similarly illuminated area that appears white or highly transmitting"), the saturation of the light coming from that object is in effect the chroma of the object judged in proportion to its lightness. On aMunsell hue page, lines of uniform saturation thus tend to radiate from near the black point, while lines of uniform chroma are vertical.[7]
The naïve definition of saturation does not specify its response function. In the CIE XYZ and RGB color spaces, the saturation is defined in terms of additive color mixing, and has the property of being proportional to any scaling centered at white or the white point illuminant. However, both color spaces are non-linear in terms of psychovisually perceivedcolor differences. It is also possible — and sometimes desirable — to define a saturation-like quantity that is linearized in term of the psychovisual perception.
In theCIE 1976 LAB and LUV color spaces, the unnormalizedchroma is the radial component of the cylindrical coordinateCIE LCh (lightness, chroma, hue) representation of the LAB and LUV color spaces, also denoted as CIE LCh(ab) or CIE LCh for short, and CIE LCh(uv). The transformation of to is given by:and analogously for CIE LCh(uv).
The chroma in the CIE LCh(ab) and CIE LCh(uv) coordinates has the advantage of being more psychovisually linear, yet they are non-linear in terms of linear component color mixing. And therefore, chroma in CIE 1976 Lab and LUV color spaces is very much different from the traditional sense of "saturation".
Another, psychovisually even more accurate, but also more complex method to obtain or specify the saturation is to use acolor appearance model like CIECAM02. Here, thechroma color appearance parameter might (depending on the color appearance model) be intertwined with e.g. the physical brightness of the illumination or the characteristics of the emitting/reflecting surface, which is more sensible psychovisually.
The CIECAM02 chroma for example, is computed from a lightness in addition to a naively evaluated color magnitude In addition, a colorfulness parameter exists alongside the chroma It is defined as where is dependent on the viewing condition.[8]
The saturation of a color is determined by a combination of light intensity and how much it is distributed across the spectrum of different wavelengths. The purest (most saturated) color is achieved by using just one wavelength at a high intensity, such as in laser light. If the intensity drops, then as a result the saturation drops. To desaturate a color of given intensity in asubtractive system (such aswatercolor), one can add white, black,gray, or the hue'scomplement.
Various correlates of saturation follow.
InCIELUV, saturation is equal to thechroma normalized by thelightness:where is the chromaticity of the white point, and chroma is defined below.[9]
By analogy, inCIELAB this would yield:
The CIE has not formally recommended this equation since CIELAB has no chromaticity diagram, and this definition therefore lacks direct connection with older concepts of saturation.[10] Nevertheless, this equation provides a reasonable predictor of saturation, and demonstrates that adjusting the lightness in CIELAB while holding(a*,b*) fixed does affect the saturation.
But the following verbal definition of Manfred Richter and the corresponding formula proposed by Eva Lübbe are in agreement with the human perception of saturation:Saturation is the proportion of pure chromatic color in the total color sensation.[11]where is the saturation, the lightness and is the chroma of the color.
InCIECAM02, saturation equals the square root of thecolorfulness divided by thebrightness:
This definition is inspired by experimental work done with the intention of remedyingCIECAM97s's poor performance.[8][12] is proportional to the chroma thus the CIECAM02 definition bears some similarity to the CIELUV definition.[8]
Saturation is also one of three coordinates in theHSL and HSVcolor spaces. However, in the HSL color space saturation exists independently of lightness. That is, both a very light colorand a very dark color can be heavily saturated in HSL; whereas in the previous definitions—as well as in the HSV color space—colors approaching white all feature low saturation.
Theexcitation purity (purity for short) of a stimulus is the difference from the illuminant'swhite point to the furthest point on the chromaticity diagram with the samedominant wavelength; using theCIE 1931 color space:[13]where is the chromaticity of the white point and is the point on the perimeter whose line segment to the white point contains the chromaticity of the stimulus. Different color spaces, such as CIELAB or CIELUV may be used, and will yield different results.
excitation purity.
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