Spectral color

The CIE xy chromaticity diagram. The spectrum colors are the colors on the horseshoe-shaped curve on the outside of the diagram. All other colors are not spectral: the bottom straight line is the line of purples, whilst within the interior of the diagram are unsaturated colors that are various mixtures of a spectral color or a purple color with white, a grayscale color. White is in the central part of the interior of the diagram, since when all colors of light are mixed together, they produce white.

A spectral color is a color that is evoked by a single wavelength of light in the visible spectrum, or by a relatively narrow band of wavelengths, also known as monochromatic light. Every wavelength of visible light is perceived as a spectral color, in a continuous spectrum; the colors of sufficiently close wavelengths are indistinguishable.

The spectrum is often divided into named colors, though any division is somewhat arbitrary: the spectrum is continuous. Traditional colors include: red, orange, yellow, green, blue, and violet.

The division used by Isaac Newton, in his color wheel, was: red, orange, yellow, green, blue, indigo and violet; a mnemonic for this order is "Roy G. Biv". In modern divisions of the spectrum, indigo is often omitted.

One needs at least trichromatic color vision for there to be a distinction between spectral and non-spectral colours : trichromacy gives a possibility to perceive both hue and saturation in the chroma. In color models capable to represent spectral colors,^[1] such as CIELUV, a spectral color has the maximal saturation

In color spaces

This metrically accurate diagram shows that the spectral locus is almost flat on the red – bright green segment, is strongly curved around green, and becomes less curved between green/cyan and blue

In color spaces which include all, or most spectral colors, they form a part of boundary of the set of all real colors. If luminance is counted, then spectral colors form a surface, otherwise their locus is a curve in a two-dimensional chromaticity space.

Theoretically, only RGB-implemented colors which might be really spectral are its primaries: red, green, and blue, whereas any other (mixed) color is inherently non-spectral. But due to different chromaticity properties of different spectral segments, and also due to practical limitations of light sources, the actual distance between RGB pure color wheel colors and spectral colors shows a complicated dependence on the hue. Due to location of R and G primaries near the almost "flat" spectral segment, RGB color space is reasonably good with approximating spectral orange, yellow, and bright (yellowish) green, but is especially poor in reaching a visual appearance of spectral colors between green and blue, as well as extreme spectral colors. The sRGB standard has an additional problem with its "red" primary which is shifted to orange due to a trade-off between purity of red and its reasonable luminance, so that the red spectral became unreachable. Some samples in the table below provide only rough approximations of spectral and near-spectral colors.

CMYK is usually even poorer than RGB in its reach of spectral colors, with notable exception of process yellow, which is rather close to spectral colors due to aforementioned flatness of the spectral locus in the red–green segment.

Note that spectral color are universally included to scientific color models such as CIE 1931, but industrial and consumer color spaces such as sRGB, CMYK, and Pantone, do not include any of spectral colors.

Table of spectral or near-spectral colors

Most of the colors listed do not reach the maximal (spectral) colorfulness, or are not usually seen with it, but they can be saturated enough to be perceived closely to their dominant wavelength spectral colors. Ranges of wavelengths and frequencies are only approximate.

Wavelengths and frequencies in gray indicate dominant wavelengths and frequencies, not actual range of spectrum composing a specified color, which extends farther to both sides and is averaged by receptors to give a near-spectral appearance.


	Color term, light source, or dye	Sample ^[2]	Wavelength, nm	Frequency, THz	Hue ^[3]	Comments
	Red		740–625^[4]	405–479		A traditional, broad color term, which includes some nearby non-spectral hues. The short-wave boundary can extend to 620 or even about 610 nanometres
	• Extreme spectral red = red (CIE RGB)	×	740	405	?	Exact spectrum has more influence on luminance than on chromaticity in this band; chromaticities are almost the same for these two variants
	• red (Wide-gamut RGB primary)^[5]	×	≈ 700	≈ 428	?
	• Helium–neon laser	×	633	473	?
	• Some carmine dyes	×	NIR–602^[6]	NIR–497	?	Near-spectral, but other parts of carmine (color) are purple
	• red (sRGB primary)		614–609	488–492	0°	Noticeably non-spectral
	Orange		620–585 625–590^[4]	483–512 479–508	0°–30°	The short-wave (yellowish) part corresponds to amber, the long-wave (reddish) side nears (or includes) RGB red above.
	Yellow		585–560 590–565^[4]	512–540 508–530		A traditional color term
	• Sodium-vapor lamp		≈ 589	≈ 508	?
	• yellow (NCS)		?	?	50°	Gold has almost identical chromaticity at h = 51°
	• Munsell 5Y for V = 10, C = 22^[7]		≈ 577	≈ 519	?
	• process (canary) yellow		?	?	56°
	• yellow (sRGB secondary)		≈ 570	?	60°
	• Chartreuse yellow		?	?	68°
	Lime		≈ 564	?	≈ 75°	May be classified as either green or yellow
	Green		565–???	530–???		A traditional, broad color term
	• Chartreuse green		?	?	90°
	• Bright green		≈ 556	?	96°
	• Harlequin		≈ 552	?	105°
	• green (sRGB primary)		≈ 549	≈ 547	120°	Noticeably non-spectral
	• green (Wide-gamut RGB primary)^[5]	×	≈ 525	≈ 571	?	Almost spectral
	• Spring green (sRGB definition)	×	?	?	150°	May lie rather far from the spectrum
	• green (NCS)	×	?	?	160°	May lie rather far from the spectrum
• Munsell 5G for V = 4, C = 29^[7]^[8]	×	≈ 503	≈ 597	(?)≈ 163° (extrap.)
	Cyan		500+–480^[9] 520–500^[4]	593–624 576–600		Sometimes included (or overlaps) with blue, terminological distinction between the two is inconsistent
	• Turquoise	×	?	?	≈ 175°	Most of "turquoise" lies far away of the spectrum
	• cyan (sRGB secondary)	×	488	?	180°	Lie rather far from the spectrum
	• process cyan	×	?	?	193°	Lie rather far from the spectrum
	Blue		490–450 500–435^[4]	610–666 600–689		A traditional, broad color term, which used to include cyan
	• blue (NCS)	×	?	?	197°	Lies rather far from the spectrum
	• Azure (sRGB definition)	×	≈ 488	≈ 614	≈ 210°	May lie rather far from the spectrum
	• Munsell 5B for V = 5, C = 20^[7]	×	≈ 482	≈ 622	(?)≈ 225° (extrap.)
	• blue (RGB primary)		466–436^[10]	?	240° (of sRGB)	May be classified as indigo or (if indigo is omitted) as violet
	Indigo		≈ 446	≈ 672	(?)≈ 243° (extrap.)	Definition is controversial, this wavelength least disputably belongs to "indigo"
	Violet	×	450–400 435–380^[4]	666–750 689–788	up to 277° (extrap.)	Far spectral violet is very dim and rarely seen. The term also extends to purples

Non-spectral colors

Among some of the colors that are not spectral colors are:

Grayscale (achromatic) colors, such as white, gray, and black.
Any color obtained by mixing a gray-scale color and another color (either spectral or not), such as pink (a mixture of a reddish color and white) or brown (a mixture of orange and black or gray).
Violet-red colors, which in color theory include line of purples colors (such as, approximately, magenta and rose), and other variations of purple and red.
Impossible colors, which can not be seen under normal viewing of light, such as over-saturated colors or colors that are seemingly brighter than white.

References

↑ HSL and HSV do not qualify because many spectral colors lie rather far from its gamut.
↑ Samples (allegedly in sRGB) currently rely on Wikipedia data which sometimes use poor, unprofessional sources, misinterpretation of sources, or occasionally contain original researches.
↑ Values for the hue (HSL and HSV or an extrapolation, where necessary) currently rely on Wikipedia data which are prone to miscalculation and other irregularities. Also note that RGB is not an absolute color space, and certain specific standard (such as sRGB) is necessary to map RGB hues to near-spectral colors.
1 2 3 4 5 6 Thomas J. Bruno, Paris D. N. Svoronos. CRC Handbook of Fundamental Spectroscopic Correlation Charts. CRC Press, 2005. http://hyperphysics.phy-astr.gsu.edu/hbase/vision/specol.html#c1
1 2 A proprietary color space
↑ Bisulca, Christina (2008). "UV-Vis-NIR reflectance spectroscopy of red lakes in paintings" (PDF). 9th International Conference on NDT of Art. ndt.net. Retrieved 2013-06-19.
1 2 3 http://www.cis.rit.edu/research/mcsl2/online/munsell_data/all.dat and commons:File:CIE1931xy blank.svg
↑ Linearly interpolated between two tabulated values.
↑
↑ Different definitions of RGB give significantly different wavelengths for blue primary, but this does not change the chromaticity greatly.

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