In color science, the dominant wavelength is a method of characterizing a color's hue. Along with

purity Purity may refer to: Books * ''Pureza'' (novel), a 1937 Brazilian novel by José Lins do Rego * ''Purity'' (novel), a 2015 novel by Jonathan Franzen ** ''Purity'' (TV series), a TV series based on the novel *''Purity'', a 2012 novel by Jackson P ...

, it makes up one half of the Helmholtz coordinates. A color's dominant wavelength is the

wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, t ...

of monochromatic spectral light that evokes an identical perception of

hue In color theory, hue is one of the main properties (called color appearance parameters) of a color, defined technically in the CIECAM02 model as "the degree to which a stimulus can be described as similar to or different from stimuli that ...

Determination

Helmholtz coordinates

The Helmholtz Coordinates are a

Polar coordinate system In mathematics, the polar coordinate system is a two-dimensional coordinate system in which each point on a plane is determined by a distance from a reference point and an angle from a reference direction. The reference point (analogous to th ...

for defining a 2D

chromaticity Chromaticity is an objective specification of the quality of a color regardless of its luminance. Chromaticity consists of two independent parameters, often specified as hue (h) and colorfulness (s), where the latter is alternatively called ...

plane. The circumferential coordinate is the dominant wavelength, which is analogous to

of 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, designed in the 1970s by computer graphics researchers to mor ...

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 represen ...

s. The radial coordinate is the

, which is analogous to

saturation Saturation, saturated, unsaturation or unsaturated may refer to: Chemistry * Saturation, a property of organic compounds referring to carbon-carbon bonds **Saturated and unsaturated compounds ** Degree of unsaturation **Saturated fat or fatty aci ...

of the HSL and HSV color spaces.

Color space

Any chromaticity diagram from any color space can be used for calculating the dominant wavelength as long as it is bound by a spectral locus defined by

. However, the values will change depending on which color space is used. Unless otherwise stated, 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 ...

(

CIEXYZ 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 ...

) is used., but the CIELUV color space is sometimes used.

Calculation

To calculate the dominant wavelength of a chromaticity (or color), a straight line is drawn on the chromaticity diagram between the chromaticity's coordinates and the

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 ...

. The line is then extrapolated so it intersects the perimeter of the diagram at two points, where the perimeter comprises the spectral locus and/or line of purples. The point of intersection that is nearer to the chromaticity in question defines its ''dominant wavelength''. The purity can then be calculated as defined

here Here is an adverb that means "in, on, or at this place". It may also refer to: Software * Here Technologies, a mapping company * Here WeGo (formerly Here Maps), a mobile app and map website by Here Technologies, Here Television * Here TV (form ...

White point

The white point is generally defined as—or assumed to be— equal energy white. This is defined as ,y(0.33,0.33) in

color space and is analogous to the HSV white point at (0,0). However, other white points may be used, generally defined by "white"

standard illuminant A standard illuminant is a theoretical source of visible light with a spectral power distribution that is published. Standard illuminants provide a basis for comparing images or colors recorded under different lighting. CIE illuminants The Inter ...

s or a

color temperature Color temperature is the color of light emitted by an idealized opaque, non-reflective body at a particular temperature measured in kelvins. The color temperature scale is used to categorize the color of light emitted by other light sources ...

such as 6500 K.

Complementary wavelength

When the chromaticity lies within the triangle with vertices at the white point, spectral blue (~400 nm) and pure spectral red (~700 nm), the dominant wavelength is indeterminate because the point of intersection that is nearer to the chromaticity lies on the line of purples instead of the spectral locus. The colors on the line of purples cannot be defined by wavelength because they do not represent monochromatic light. Instead, the dominant wavelength is replaced with the ''complementary wavelength'', which will represent the

complementary color Complementary colors are pairs of colors which, when combined or mixed, cancel each other out (lose hue) by producing a grayscale color like white or black. When placed next to each other, they create the strongest contrast for those two co ...

. To calculate it, the point of intersection that is ''further'' from the chromaticity in question is used. If the nearer intersection is on the line of purples, the further intersection must be on the spectral locus.

Application

Dominant wavelength is used to define the color of light sources, such as the LEDs, that do not lie along the Planckian locus (which would otherwise be defined by

Color temperature Color temperature is the color of light emitted by an idealized opaque, non-reflective body at a particular temperature measured in kelvins. The color temperature scale is used to categorize the color of light emitted by other light sources ...

). These light sources are also often described by their ''peak wavelength''—the wavelength of highest

radiometric Radiometry is a set of techniques for measuring electromagnetic radiation, including visible light. Radiometric techniques in optics characterize the distribution of the radiation's power in space, as opposed to photometric techniques, which ch ...

Spectral flux (highest peak in the power spectrum)—but the dominant wavelength is a photometric quantity, and therefore intuitively conveys what color the light will appear without relying on inexact color naming.

References

{{Color topics Color