Dominant wavelength
Encyclopedia
In color science
, the dominant wavelength and complementary wavelength are ways of describing non-spectral (polychromatic) light mixtures in terms of the spectral (monochromatic) light
that evokes an identical perception of hue
.
On the CIE
color coordinate space
, a straight line drawn between the point for a given color and the point for the color of the illuminant can be extrapolated out so that it intersects the perimeter of the space in two points. The point of intersection nearer to the color in question reveals the dominant wavelength of the color as the wavelength of the pure spectral color at that intersection point. The point of intersection on the opposite side of the color space gives the complementary wavelength, which when added to the color in question in the right proportion will yield the color of the illuminant (since the illuminant point necessarily sits between these points on a straight line in CIE space, according to the definition just given).
In situations where no particular illuminant is specified, it is common to discuss dominant wavelength in terms of some standard (usually white
) illuminant, such as flat spectrum white light. For the purposes of this geometrical discussion, an analogy may be observed between the horseshoe shaped CIE 1931 color space and a circular slice of HSV color space, where the CIE flat spectrum white point at (1/3,1/3) is analogous to the HSV white point at (0,0). This comparison clarifies the derivation of the ideas of hue and complementary color common in uses of the HSV space.
is commonly thought of as a function of the power spectrum of light frequencies impinging on the photoreceptors of the retina
. In the simplest case of pure spectral light (also known as monochromatic), the spectrum of the light has power only in one narrow frequency band peak. For these simple stimuli, there exists a continuum of perceived colors which changes as the frequency of the narrow band peak is changed. This is the well known rainbow
spectrum, which ranges from red at one end to blue and violet at the other (corresponding respectively to the long-wavelength and short-wavelength extremes of the visible range of electromagnetic radiation
).
However, light in the natural world is almost never purely monochromatic; most natural light sources and reflected light from natural objects comprise spectra that have complex profiles, with varying power over many different frequencies. A naive perspective might be that therefore all these different complex spectra would generate color perceptions completely different from those evoked in the rainbow of pure spectral light. One can perhaps see intuitively that this is not correct: almost all hues in the natural world (purples being the exception, see below) are represented in the pure rainbow spectrum, although they may be darker or less saturated than they appear in the rainbow. How is it that all the complex spectra in the natural world can be condensed to hues in the rainbow, which only represents simple monochromatic band peak spectra? This is the result of the design of the eye
: the three color photoreceptors in the retina (the cone
s) reduce the information in the light spectrum down to three activity coordinates. Thus, many different physical light spectra converge psychologically to the same perceived color. In effect, for any single color perception, there is a whole parametric space in the power/frequency domain that maps to that one color.
For many power distributions of natural light, the set of spectra mapping to the same color perception also includes a stimulus that is a narrow band at a single frequency; i.e. a pure spectral light (usually with some flat spectrum white light added to desaturate). The wavelength of this pure spectral light that will evoke the same color perception as the given complicated light mixture is the dominant wavelength of that mixture.
Note that since purple
s (mixtures of red and blue/violet) cannot be pure spectral colors, no color mixture perceived as purple in hue can be assigned a proper dominant wavelength. However, purple mixtures can be assigned a dominant hue as coordinates along the line of purples
. See CIE
for the standard representation of color space, where the border is composed of a horseshoe curve representing the pure spectral colors, with a straight line completing the perimeter along the bottom and representing the mixtures of extreme red and blue/violet that give the pure purples. The same argument applies to complementary colors; for many coordinates in the green area of CIE color space, no complementary wavelength exists, but there is a complementary pure purple.
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...
, the dominant wavelength and complementary wavelength are ways of describing non-spectral (polychromatic) light mixtures in terms of the spectral (monochromatic) light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
that evokes an identical perception of 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,"...
.
On the CIE
International Commission on Illumination
The International Commission on Illumination is the international authority on light, illumination, color, and color spaces...
color coordinate 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....
, a straight line drawn between the point for a given color and the point for the color of the illuminant can be extrapolated out so that it intersects the perimeter of the space in two points. The point of intersection nearer to the color in question reveals the dominant wavelength of the color as the wavelength of the pure spectral color at that intersection point. The point of intersection on the opposite side of the color space gives the complementary wavelength, which when added to the color in question in the right proportion will yield the color of the illuminant (since the illuminant point necessarily sits between these points on a straight line in CIE space, according to the definition just given).
In situations where no particular illuminant is specified, it is common to discuss dominant wavelength in terms of some standard (usually 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...
) illuminant, such as flat spectrum white light. For the purposes of this geometrical discussion, an analogy may be observed between the horseshoe shaped CIE 1931 color space and a circular slice of HSV color space, where the CIE flat spectrum white point at (1/3,1/3) is analogous to the HSV white point at (0,0). This comparison clarifies the derivation of the ideas of hue and complementary color common in uses of the HSV space.
Explanation
The psychological perception of colorColor
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...
is commonly thought of as a function of the power spectrum of light frequencies impinging on the photoreceptors of the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
. In the simplest case of pure spectral light (also known as monochromatic), the spectrum of the light has power only in one narrow frequency band peak. For these simple stimuli, there exists a continuum of perceived colors which changes as the frequency of the narrow band peak is changed. This is the well known rainbow
Rainbow
A rainbow is an optical and meteorological phenomenon that causes a spectrum of light to appear in the sky when the Sun shines on to droplets of moisture in the Earth's atmosphere. It takes the form of a multicoloured arc...
spectrum, which ranges from red at one end to blue and violet at the other (corresponding respectively to the long-wavelength and short-wavelength extremes of the visible range of electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
).
However, light in the natural world is almost never purely monochromatic; most natural light sources and reflected light from natural objects comprise spectra that have complex profiles, with varying power over many different frequencies. A naive perspective might be that therefore all these different complex spectra would generate color perceptions completely different from those evoked in the rainbow of pure spectral light. One can perhaps see intuitively that this is not correct: almost all hues in the natural world (purples being the exception, see below) are represented in the pure rainbow spectrum, although they may be darker or less saturated than they appear in the rainbow. How is it that all the complex spectra in the natural world can be condensed to hues in the rainbow, which only represents simple monochromatic band peak spectra? This is the result of the design of the eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
: the three color photoreceptors in the retina (the cone
Cone cell
Cone cells, or cones, are photoreceptor cells in the retina of the eye that are responsible for color vision; they function best in relatively bright light, as opposed to rod cells that work better in dim light. If the retina is exposed to an intense visual stimulus, a negative afterimage will be...
s) reduce the information in the light spectrum down to three activity coordinates. Thus, many different physical light spectra converge psychologically to the same perceived color. In effect, for any single color perception, there is a whole parametric space in the power/frequency domain that maps to that one color.
For many power distributions of natural light, the set of spectra mapping to the same color perception also includes a stimulus that is a narrow band at a single frequency; i.e. a pure spectral light (usually with some flat spectrum white light added to desaturate). The wavelength of this pure spectral light that will evoke the same color perception as the given complicated light mixture is the dominant wavelength of that mixture.
Note that since purple
Purple
Purple is a range of hues of color occurring between red and blue, and is classified as a secondary color as the colors are required to create the shade....
s (mixtures of red and blue/violet) cannot be pure spectral colors, no color mixture perceived as purple in hue can be assigned a proper dominant wavelength. However, purple mixtures can be assigned a dominant hue as coordinates along the line of purples
Line of purples
In color theory, the line of purples or the purple boundary is the locus of non-spectral colors on the edge of the chromaticity diagram between red and violet. Like spectral colors, purples can be considered fully saturated in the sense that for any given point on the line of purples there exists...
. See CIE
International Commission on Illumination
The International Commission on Illumination is the international authority on light, illumination, color, and color spaces...
for the standard representation of color space, where the border is composed of a horseshoe curve representing the pure spectral colors, with a straight line completing the perimeter along the bottom and representing the mixtures of extreme red and blue/violet that give the pure purples. The same argument applies to complementary colors; for many coordinates in the green area of CIE color space, no complementary wavelength exists, but there is a complementary pure purple.