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Colorfulness, chroma and saturation are attributes of perceived color relating to chromatic intensity. As defined formally by the
International Commission on Illumination The International Commission on Illumination (usually abbreviated CIE for its French name, Commission internationale de l'éclairage) is the international authority on light Light or visible light is electromagnetic radiation within the port ...
(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. * Colorfulness is the "attribute of a visual perception according to which the perceived color of an area appears to be more or less chromatic"., page 87. The colorfulness evoked by an object depends not only on its spectral
reflectance The reflectance of the surface of a material is its effectiveness in reflecting radiant energy In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion ...

reflectance
but also on the strength of the illumination, and increases with the latter unless the
brightness Brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light. In other words, brightness is the perception elicited by the luminance of a visual target. It is not necessarily proportional to lumina ...

brightness
is very high ( Hunt effect). * Chroma is the "colorfulness of an area judged as a proportion of the brightness of a similarly illuminated area that appears white or highly transmitting". As a result, chroma is mostly only dependent on the spectral properties, and as such is seen to describe the object color. It is how different from a grey ''of the same lightness'' such an object color appears to be. * Saturation is the "colorfulness of an area judged in proportion to its
brightness Brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light. In other words, brightness is the perception elicited by the luminance of a visual target. It is not necessarily proportional to lumina ...

brightness
", which in effect is the perceived freedom from whitishness of the light coming from the area. An object with a given spectral reflectance exhibits approximately constant saturation for all levels of illumination, unless the brightness is very high. 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 the Munsell 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 a Munsell hue page, lines of uniform saturation thus tend to radiate from near the black point, while lines of uniform chroma are vertical.


Chroma

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 perceived
color difference The difference or distance between two color Color (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native t ...
s. It is also possible — and sometimes desirable — to define a saturation-like quantity that is linearized in term of the psychovisual perception. In the CIE 1976 LAB and LUV color spaces, the unnormalized chroma is the radial component of the cylindrical coordinate CIE 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 (a, b) to \left(C_, h_\right) is given by: C_^* = \sqrt h_ = \operatorname\left(,\right) 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".


In color appearance models

Another, psychovisually even more accurate, but also more complex method to obtain or specify the saturation is to use a
color appearance model #REDIRECT Color appearance model A color appearance model (CAM) is a mathematical model that seeks to describe the perception, perceptual aspects of human color vision, i.e. viewing conditions under which the appearance of a color does not tally wi ...
like CIECAM02. Here, the chroma 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 C, for example, is computed from a lightness J in addition to a naively evaluated color magnitude t. In addition, a colorfulness M parameter exists alongside the chroma C. It is defined as M = CF_B^, where F_L is dependent on the viewing condition.


Saturation

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 a subtractive system (such as
watercolor Watercolor (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of variety (linguistics), varieties of the English language native to the United States. ...

watercolor
), one can add white, black,
gray Grey or gray (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United States. Currently, American En ...

gray
, or the hue's
complement A complement is often something that completes something else, or at least adds to it in some useful way. Thus it may be: * Complement (linguistics), a word or phrase having a particular syntactic role ** Subject complement, a word or phrase addi ...
. Various correlates of saturation follow.


CIELUV and CIELAB

In
CIELUV In colorimetry, the CIE 1976 ''L''*, ''u''*, ''v''* color space, commonly known by its abbreviation CIELUV, is a color space A color space is a specific organization of colors. In combination with color profiling supported by various physical de ...
, saturation is equal to the ''chroma'' normalized by the ''lightness'': s_ = \frac = 13 \sqrt where \left(u_n, v_n\right) is the chromaticity of the white point, and chroma is defined below. By analogy, in
CIELAB The CIELAB color space also referred to as ''L*a*b*'' is a color space A color space is a specific organization of colors. In combination with color profiling supported by various physical devices, it supports reproducible representations ...
this would yield: s_ = \frac = \frac 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. Nevertheless, this equation provides a reasonable predictor of saturation, and demonstrates that adjusting the lightness in CIELAB while holding 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. S_ = \frac 100\% where S_ is the saturation, L^* the lightness and C^*_ is the chroma of the color.


CIECAM02

In
CIECAM02 In colorimetry Colorimetry is "the science and technology used to quantify and describe physically the human color perception". It is similar to spectrophotometry Spectrophotometry is a branch of electromagnetic spectroscopy concerned w ...
, saturation equals the square root of the ''colorfulness'' divided by the ''brightness'': s = \sqrt\frac This definition is inspired by experimental work done with the intention of remedying
CIECAM97s A color appearance model (CAM) is a mathematical model that seeks to describe the perception, perceptual aspects of human color vision, i.e. viewing conditions under which the appearance of a color does not tally with the corresponding physical mea ...
's poor performance. M is proportional to the chroma C, thus the CIECAM02 definition bears some similarity to the CIELUV definition.


HSL and HSV

Saturation is also one of three coordinates in the
HSL and HSV HSL (for hue, saturation, lightness) and HSV (for hue, saturation, value; also known as HSB, for hue, saturation, brightness) are alternative representations of the RGB color model s The RGB color model is an additive color, additive color ...
color space A color space is a specific organization of color Color (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language ...
s. However, in the HSL color space saturation exists independently of lightness. That is, both a very light color 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.


Excitation purity

The excitation purity (purity for short) of a stimulus is the difference from the illuminant's
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. Depending ...
to the furthest point on the chromaticity diagram with the same hue (
dominant wavelength Image:dominant wavelength.png, frame, Dominant/complementary wavelength example on the CIE color spaceThe "x" marks the color in question. For the white point indicated, the dominant wavelength for "x" is on the nearer perimeter, around 600 nm, whi ...

dominant wavelength
for
monochromatic during the 1889 Exposition Universelle File:Parrot EGA monochrome palette.png, A photograph of a μονόχρωμος.html" ;"title="lightness">values of one color). The term ''monochrome'' comes from the grc">μονόχρωμος">monochrom ...

monochromatic
sources); using the
CIE 1931 color space The CIE 1931 color spaces are the first defined quantitative links between distributions of wavelengths in the electromagnetic visible spectrum The visible spectrum is the portion of the that is to the . in this range of s is called ' or ...
: p_e = \sqrt where \left(x_n, y_n\right) is the chromaticity of the white point and \left(x_I, y_I\right) 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.


References

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