Saturation (color theory)
Encyclopedia
In colorimetry
and color theory
, colorfulness, chroma, and saturation are related but distinct concepts referring to the perceived intensity of a specific color
. Colorfulness is the degree of difference between a color and gray
. Chroma is the colorfulness relative to the brightness
of another color that appears white under similar viewing conditions. Saturation is the colorfulness of a color relative to its own brightness. Though this general concept is intuitive, terms such as chroma, saturation, purity, and intensity are often used without great precision, and even when well-defined depend greatly on the specific color model in use.
A highly colorful stimulus is vivid and intense, while a less colorful stimulus appears more muted, closer to gray. With no colorfulness at all, a color is a “neutral” gray (an image with no colorfulness in any of its colors is called grayscale
). With three attributes—colorfulness (or chroma or saturation), lightness
(or brightness), and hue
—any color can be described.
Saturation is one of three coordinates in the HSL and HSV color space
s. Note that virtually all computer software implementing these spaces use a very rough approximation to calculate the value they call "saturation", such as the formula described for HSV and this value has little, if anything, to do with the description shown here.
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), one can add white
, black
, gray, or the hue's complement
.
Various correlates of saturation follow.
CIELUV : The chroma normalized by the lightness:
where is the chromaticity of the white point, and chroma is defined below.
By analogy, in CIELAB this would yield:
The CIE has not formally recommended this equation since CIELAB has no chromaticity diagram, and this definition therefore lacks direct correlation 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 formula is in agreement with the human perception of saturation:
The formula proposed by Eva Lübbe is in agreement with the verbal definition of Manfred Richter: Saturation is the proportion of pure chromatic color in the total color sensation.
where Sab is the saturation, L* the lightness and C*ab is the chroma of the color.
CIECAM02
: The square root of the colorfulness divided by the brightness:
This definition is inspired by experimental work done with the intention of remedying CIECAM97s's poor performance. M is proportional to the chroma C , thus the CIECAM02 definition bears some similarity to the CIELUV definition. An important difference is that the CIECAM02 model accounts for the viewing conditions through the parameter FL.
The excitation purity (purity for short) of a stimulus is its difference from the illuminant's white point
relative to the furthest point on the chromaticity diagram with the same hue (dominant wavelength
for monochromatic sources); using the CIE 1931 color space
:
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.
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 L*a*b* and L*u*v* color spaces
, the unnormalized chroma is the radial component of the cylindrical coordinate CIE L*C*h (lightness, chroma, hue) representation of the L*a*b* and L*u*v* color spaces, also denoted as CIE L*C*h(a*b*) or CIE L*C*h for short, and CIE L*C*h(u*v*). The transformation of to is given by:
and analogously for CIE L*C*h(u*v*).
The chroma in the CIE L*C*h(a*b*) and CIE L*C*h(u*v*) 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 L*a*b* and L*u*v* color spaces is very much different from the traditional sense of "saturation".
Colorimetry
Colorimetry is "the science and technology used to quantify and describe physically the human color perception."It is similar to spectrophotometry, but is distinguished by its interest in reducing spectra to the physical correlates of color perception, most often the CIE 1931 XYZ color space...
and color theory
Color theory
In the visual arts, color theory is a body of practical guidance to color mixing and the visual impacts of specific color combinations. Although color theory principles first appeared in the writings of Leone Battista Alberti and the notebooks of Leonardo da Vinci , a tradition of "colory theory"...
, colorfulness, chroma, and saturation are related but distinct concepts referring to the perceived intensity of a specific color
Color
Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...
. Colorfulness is the degree of difference between a color and gray
Grey
Grey or gray is an achromatic or neutral color.Complementary colors are defined to mix to grey, either additively or subtractively, and many color models place complements opposite each other in a color wheel. To produce grey in RGB displays, the R, G, and B primary light sources are combined in...
. Chroma is the colorfulness relative to the brightness
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...
of another color that appears white under similar viewing conditions. Saturation is the colorfulness of a color relative to its own brightness. Though this general concept is intuitive, terms such as chroma, saturation, purity, and intensity are often used without great precision, and even when well-defined depend greatly on the specific color model in use.
A highly colorful stimulus is vivid and intense, while a less colorful stimulus appears more muted, closer to gray. With no colorfulness at all, a color is a “neutral” gray (an image with no colorfulness in any of its colors is called grayscale
Grayscale
In photography and computing, a grayscale or greyscale digital image is an image in which the value of each pixel is a single sample, that is, it carries only intensity information...
). With three attributes—colorfulness (or chroma or saturation), lightness
Lightness (color)
Lightness is a property of a color, or a dimension of a color space, that is defined in a way to reflect the subjective brightness perception of a color for humans along a lightness–darkness axis. A color's lightness also corresponds to its amplitude.Various color models have an explicit term for...
(or brightness), and hue
Hue
Hue is one of the main properties of a color, defined technically , as "the degree to which a stimulus can be describedas similar to or different from stimuli that are described as red, green, blue, and yellow,"...
—any color can be described.
Saturation
Color space
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...
s. Note that virtually all computer software implementing these spaces use a very rough approximation to calculate the value they call "saturation", such as the formula described for HSV and this value has little, if anything, to do with the description shown here.
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
Subtractive color
A subtractive color model explains the mixing of paints, dyes, inks, and natural colorants to create a full range of colors, each caused by subtracting some wavelengths of light and reflecting the others...
system (such as watercolor), one can add white
White
White is a color, the perception of which is evoked by light that stimulates all three types of color sensitive cone cells in the human eye in nearly equal amounts and with high brightness compared to the surroundings. A white visual stimulation will be void of hue and grayness.White light can be...
, black
Black
Black is the color of objects that do not emit or reflect light in any part of the visible spectrum; they absorb all such frequencies of light...
, gray, or the hue's complement
Complementary color
Complementary colors are pairs of colors that are of “opposite” hue in some color model. The exact hue “complementary” to a given hue depends on the model in question, and perceptually uniform, additive, and subtractive color models, for example, have differing complements for any given color.-...
.
Various correlates of saturation follow.
CIELUV : The chroma normalized by the lightness:
where is the chromaticity of the white point, and chroma is defined below.
By analogy, in CIELAB this would yield:
The CIE has not formally recommended this equation since CIELAB has no chromaticity diagram, and this definition therefore lacks direct correlation 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 formula is in agreement with the human perception of saturation:
The formula proposed by Eva Lübbe is in agreement with the verbal definition of Manfred Richter: Saturation is the proportion of pure chromatic color in the total color sensation.
where Sab is the saturation, L* the lightness and C*ab is the chroma of the color.
CIECAM02
CIECAM02
Published in 2002 by the CIE Technical Committee 8-01 , as of 2008 CIECAM02 is the most recent color appearance model ratified by the CIE, and the successor of CIECAM97s.| quote=The CIECAM97s model was adopted by the CIE in 1997 for color imaging applications. It includes forward and reverse modes...
: The square root of the colorfulness divided by the brightness:
This definition is inspired by experimental work done with the intention of remedying CIECAM97s's poor performance. M is proportional to the chroma C , thus the CIECAM02 definition bears some similarity to the CIELUV definition. An important difference is that the CIECAM02 model accounts for the viewing conditions through the parameter FL.
Excitation purity
White point
A white point is a set of tristimulus values or chromaticity coordinates that serve to define the color "white" in image capture, encoding, or reproduction. Depending on the application, different definitions of white are needed to give acceptable results...
relative to the furthest point on the chromaticity diagram with the same hue (dominant wavelength
Dominant wavelength
In color science, the dominant wavelength and complementary wavelength are ways of describing non-spectral light mixtures in terms of the spectral light that evokes an identical perception of hue....
for monochromatic sources); using the CIE 1931 color space
CIE 1931 color space
In the study of color perception, one of the first mathematically defined color spaces is the CIE 1931 XYZ color space, created by the International Commission on Illumination in 1931....
:
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.
Chroma in CIE 1976 L*a*b* and L*u*v* color spaces
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 nonlinear in terms of psychovisually perceived color differenceColor difference
The difference or distance between two colors is a metric of interest in color science. It allows people to quantify a notion that would otherwise be described with adjectives, to the detriment of anyone whose work is color critical...
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 L*a*b* and L*u*v* color spaces
Lab color space
A Lab color space is a color-opponent space with dimension L for lightness and a and b for the color-opponent dimensions, based on nonlinearly compressed CIE XYZ color space coordinates....
, the unnormalized chroma is the radial component of the cylindrical coordinate CIE L*C*h (lightness, chroma, hue) representation of the L*a*b* and L*u*v* color spaces, also denoted as CIE L*C*h(a*b*) or CIE L*C*h for short, and CIE L*C*h(u*v*). The transformation of to is given by:
and analogously for CIE L*C*h(u*v*).
The chroma in the CIE L*C*h(a*b*) and CIE L*C*h(u*v*) 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 L*a*b* and L*u*v* color spaces is very much different from the traditional sense of "saturation".