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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, illumination, colour, and colour spaces. It was established in 1913 a ...
(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. It is the fraction of incident electromagnetic power that is reflected at the boundary. Reflectance is a component of the response of the electronic ...
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. The perception is not linear to luminance, ...
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. The perception is not linear to luminance, ...
", 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 Munsell may refer to: * Albert Henry Munsell (1858–1918), American painter, teacher of art, and the inventor of the Munsell color system * Harvey M. Munsell, American soldier in the Civil War. *Munsell Color Company *Munsell color system In c ...
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 In color science, color difference or color distance is the separation between two colors. This metric allows quantified examination of a notion that formerly could only be described with adjectives. Quantification of these properties is of great ...
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 A color appearance model (CAM) is a mathematical model that seeks to describe the perceptual aspects of human color vision, i.e. viewing conditions under which the appearance of a color does not tally with the corresponding physical measurement of ...
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) or watercolour (British English; see spelling differences), also ''aquarelle'' (; from Italian diminutive of Latin ''aqua'' "water"), is a painting method”Watercolor may be as old as art itself, going back to t ...
), one can add white, black,
gray Grey (more common in British English) or gray (more common in American English) is an intermediate color between black and white. It is a neutral or achromatic color, meaning literally that it is "without color", because it can be composed o ...
, or the hue's
complement A complement is something that completes something else. Complement may refer specifically to: The arts * Complement (music), an interval that, when added to another, spans an octave ** Aggregate complementation, the separation of pitch-class ...
. 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 adopted by the International Commission on Illumination (CIE) in 1976, as a simple-to-compute transformation of the 1931 CI ...
, 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 defined by the International Commission on Illumination (abbreviated CIE) in 1976. (Referring to CIELAB as "Lab" without asterisks should be avoided to prevent confusion ...
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, CIECAM02 is the color appearance model published in 2002 by the International Commission on Illumination (CIE) Technical Committee 8-01 (''Color Appearance Modelling for Color Management Systems'') and the successor of CIECAM97s. ...
, 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 me ...
'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, RGB color model, designed in the 1970s by computer graphics res ...
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 of colorwhether such representation entails an analog or a digital represent ...
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 on ...
to the furthest point on the chromaticity diagram with the same
dominant wavelength In color science, the dominant wavelength is a method of characterizing a color's hue. Along with purity, it makes up one half of the Helmholtz coordinates. A color's dominant wavelength is the wavelength of monochromatic spectral light that evok ...
; 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, and physiologically perceived colors in human color vision. The mathematical relationships that defin ...
: 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

{{Color topics Color