{{short description|Color evoked by a single wavelength of light in the visible spectrum}} {{Use American English|date=March 2021}} {{Use mdy dates|date=March 2021}} {{more citations needed|date=December 2009}} [[File:WhereRainbowRises.jpg|thumb|A [[rainbow]] is a [[dispersion (optics)|decomposition]] of white light into ''all'' of the spectral colors.]] [[File: Laser Pointer.jpg|thumb|Laser beams are monochromatic light, thereby exhibiting spectral colors.]]
A '''spectral color''' is a [[color]] that is evoked by [[monochromatic radiation|monochromatic light]], i.e. either a [[spectral line]] with a single [[wavelength]] or [[frequency]] of [[light]] in the [[visible spectrum]], or a relatively narrow [[spectral band]] (e.g. [[laser]]s). Every wave of visible light is perceived as a spectral color; when viewed as a [[continuous spectrum]], these colors are seen as the familiar [[rainbow]]. '''Non-spectral colors''' (or '''extra-spectral colors''') are evoked by a combination of spectral colors.
== In color spaces== [[File:CIE1931xy blank.svg|right|thumb| {{center|'''[[CIE 1931 color space#The CIE xy chromaticity diagram|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 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.]]
In [[color space]]s which include all, or most spectral colors, they form a part of boundary of the set of all real colors. When considering a three-[[dimension]]al color space (which includes [[luminance]]), the spectral colors form a [[surface (mathematics)|surface]]. When excluding luminance and considering a two-dimensional color space ([[chromaticity diagram]]), the spectral colors form a [[curve]] known as the '''spectral locus'''. For example, the spectral locus of the [[CIE 1931 color space#CIE xyY|CIE xy]] chromaticity diagram contains all the spectral colors (to the eye of the standard observer).
A trichromatic color space is defined by three [[primary color]]s, which can theoretically be spectral colors. In this case, all other colors are inherently non-spectral. In reality, the spectral bandwidth of most primaries means that most color spaces are entirely non-spectral. Due to different chromaticity properties of different spectral segments, and also due to practical limitations of light sources, the actual [[color distance|distance]] between RGB pure [[color wheel]] colors and spectral colors shows a complicated dependence on the [[hue]]. Due to the 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 green|bright (yellowish) green]], {{anchor|×}}but is especially poor in reproducing the visual appearance of spectral colors in the vicinity of central green, and between green and blue, as well as extreme spectral colors approaching [[Infrared|IR]] or [[ultraviolet|UV]].
Spectral colors are universally included in [[color science|scientific]] color spaces such as CIE 1931, but industrial and consumer color spaces/models such as [[sRGB]], [[CMYK]], and [[Pantone Matching System|Pantone]], do not typically include any spectral colors. Exceptions include [[Rec. 2020]], which uses three spectral colors as primaries (and therefore only includes precisely those three spectral colors), and color spaces such as the [[ProPhoto RGB color space]] which use imaginary colors as primaries.
In color spaces such as [[CIELUV]], a spectral color has maximal [[Colorfulness#Saturation|saturation]]. In [[Helmholtz–Kohlrausch effect#Helmholtz color coordinates|Helmholtz coordinates]], this is described as 100% [[Colorfulness#Excitation_purity|purity]].
===In dichromatic color spaces=== In [[Dichromacy|dichromatic]] [[color vision]] there is no distinction between spectral and non-spectral colors. Their entire visible [[gamut]] can be represented by spectral colors.{{NoteTag|This is true for dichromats with [[photoreceptor cell]]s with overlapping [[spectral sensitivity]] curves. If the spectral sensitivity curves do not overlap, then all colors (except the ones that only excite one type of conecell) would be non-spectral. However, there are no known vision systems where the [[cone cell|cones']] spectral sensitivity curves do not overlap.}}
== Spectral color terms ==
The spectrum is often divided into ''[[color term]]s'' or names, but aligning boundaries between color terms to a specific wavelength is very subjective.
The first person to decompose white light and name the spectral colors was [[Isaac Newton]], in the 1660s.<ref>{{Cite web |title=Newton and the Science of Color |url=https://www.thecolumbiasciencereview.com/blog/newton-and-the-science-of-color |access-date=2025-05-02 |website=Columbia Science Review |language=en}}</ref> Early in the study of radiometry, Newton was not able to measure the wavelength of the light, but his experiments were repeated contemporarily to estimate wavelengths where his color term boundaries probably lay.<ref name=mclaren/> [[Isaac Newton|Newton's]] color terms included red, orange, yellow, green, blue, indigo, and violet; this color sequence is still used to describe spectral colors colloquially and a [[mnemonic]] for it is commonly known as "[[Roy G. Biv]]".
In modern divisions of the spectrum, [[Indigo#Isaac Newton's classification of indigo as a spectral color|indigo is often omitted]] and [[cyan]] is often included. Some have argued that Newton's indigo would be equivalent to modern blue, and his blue equivalent to cyan.<ref>{{Cite journal |last=MacAdam |first=David L. |date=1975-05-01 |title=Color essays* |url=https://opg.optica.org/abstract.cfm?URI=josa-65-5-483 |journal=Journal of the Optical Society of America |language=en |volume=65 |issue=5 |pages=483-493 |doi=10.1364/JOSA.65.000483 |issn=0030-3941|url-access=subscription }}</ref> However, a more recent explanation is that Newton was working by analogy with the musical scale.<ref name="mclaren" /> In the table below, note how wavelength is not proportional to a perceptually uniform hue scale. On the other hand, Newton's sections include five which are approximately uniform in size as they would have physically appeared in the diffracted spectrum, and two which were approximately half the size of the others, namely orange and indigo. This corresponds in size and location to the five whole tones and two semitones of the musical scale in [[Dorian mode]]. In contrast, the sections in the [[ISCC-NBS]] spectrum vary greatly in wavelength range, but are more consistent in the [[hue]] degree range. Both instances deviate from the [[color term#Basic color terms|basic color terms]] used in English, only some of which are spectral colors.
The table below includes several definitions where the spectral colors have been categorized in [[color term]]s. The "Approximate appearance" column displays the color with the same [[OKLab|OKLCh]] hue and [[lightness]] as the corresponding spectral color for that wavelength, with maximal [[sRGB]] saturation, given a spectrum of equal [[radiant power]] and with the highest brightness as to, when projected onto sRGB, still have maximal saturation within this [[Gamut|color gamut]]. [[Gamut mapping]] is necessary because displays cannot produce all, if any, spectral colors (a display with sRGB gamut and [[Standard illuminant#Illuminant series D|D65 whitepoint]] is assumed for this approximation). The colors at the ends of the spectrum are shown as dark since the [[cone cells]] in the eye are progressively less sensitive to light as the extremes of the visible spectrum are approached.
{| class="wikitable" style="text-align: center;" |+Spectral color classifications ! rowspan="2" scope="col" |[[Wavelength]] {{br}} (nm) ! rowspan="2" scope="col" |Approximate {{br}} appearance !scope="col"|[[Isaac Newton|Newton]][[#×|<span style="color:black" title="Quantified by McLaren"></span>]]<ref name=mclaren>{{cite journal |last1=McLaren |first1=K. |title=Newton's indigo |journal=Color Research & Application |date=1985 |volume=10 |issue=4 |pages=225–229 |doi=10.1002/col.5080100411}}</ref> !scope="col"|[[ISCC-NBS]][[#×|<span style="color:black" title="Quantified by Kelly"></span>]]<ref>{{cite journal |last1=Kelly |first1=Kenneth L. |title=Color Designations for Lights |journal=Journal of the Optical Society of America |date=1 November 1943 |volume=33 |issue=11 |pages=627 |doi=10.1364/JOSA.33.000627}}</ref> !scope="col"|Malacara<ref>{{cite book |last1=Malacara |first1=Daniel |title=Color vision and colorimetry : theory and applications |date=2011 |publisher=SPIE |location=Bellingham, Wash. |isbn=9780819483973 |edition=2nd}}</ref> !scope="col"|[[CRC Handbook of Chemistry and Physics|CRC Handbook]]<ref>{{cite book |last1=Bruno |first1=Thomas J. |title=CRC handbook of fundamental spectroscopic correlation charts |date=2006 |publisher=CRC Press |location=Boca Raton, FL |isbn=9780849332500}}</ref> |- !1700 !1943 !2011 !2006 |- |380 | bgcolor="#010003" | | rowspan="5" |Violet | rowspan="6" |Violet | rowspan="5" |Violet | rowspan="7" |Violet |- |390 | bgcolor="#020009" | |- |400 | bgcolor="#080018" | |- |410 | bgcolor="#14002E" | |- |420 | bgcolor="#280053" | |- |430 | bgcolor="#3B007B" | | rowspan="2" |Indigo | rowspan="7" |Blue |- |440 | bgcolor="#3E0092" | | rowspan="5" |Blue |- |450 | bgcolor="#3200A4" | | rowspan="4" |Blue | rowspan="5" |Blue |- |460 | bgcolor="#002B9B" | |- |470 | bgcolor="#004260" | |- |480 | bgcolor="#004A55" | |- |490 | bgcolor="#005856" | | rowspan="4" |Green | rowspan="1" |Blue-green |- |500 | bgcolor="#006E5D" | | rowspan="5" |Green | rowspan="2" |Cyan | rowspan="7" |Green |- |510 | bgcolor="#008A65" | |- |520 | bgcolor="#00A56A" | | rowspan="5" |Green |- |530 | bgcolor="#00B865" | | rowspan="5" |Yellow |- |540 | bgcolor="#00C550" | |- |550 | bgcolor="#34CC00" | | rowspan="3" |Yellow-green |- |560 | bgcolor="#82C400" | |- |570 | bgcolor="#B1B500" | | rowspan="1" |Yellow | rowspan="2" |Yellow |- |580 | bgcolor="#D5A000" | | rowspan="4" |Orange | rowspan="1" |Yellow | rowspan="5" |Orange |- |590 | bgcolor="#EF8200" | | rowspan="2" |Orange | rowspan="3" |Orange |- |600 | bgcolor="#FE5D00" | |- |610 | bgcolor="#FF2B00" | | rowspan="13" |Red |- |620 | bgcolor="#EB001B" | | rowspan="8" |Red | rowspan="13" |Red |- |630 | bgcolor="#C90024" | | rowspan="11" |Red |- |640 | bgcolor="#A80022" | |- |650 | bgcolor="#87001B" | |- |660 | bgcolor="#680014" | |- |670 | bgcolor="#4C000C" | |- |680 | bgcolor="#370007" | |- |690 | bgcolor="#250003" | |- |700 | bgcolor="#180002" | | rowspan="6" bgcolor="#AAAAAA" | |- |710 | bgcolor="#0F0001" | |- |720 | bgcolor="#080001" | |- |730 | bgcolor="#040000" | |- |740 | bgcolor="#020000" | | rowspan="2" bgcolor="#AAAAAA" | | rowspan="2" bgcolor="#AAAAAA" | |}
==Extra-spectral colors==
Among some of the colors that are {{em|not}} spectral colors are:
* [[Grayscale]] (achromatic) colors, such as [[white]], [[grey|gray]], and [[black]]. * Any color obtained by mixing a gray-scale color and another real color (either spectral or not), such as [[pink]] (a mixture of a [[red]]dish color and white), or [[brown]] (a mixture of [[Orange (color)|orange]] and black or gray). * [[Violet (color)|Violet]]-[[red]] colors, which include colors in the [[line of purples]] (such as [[magenta]] and [[Rose (color)|rose]]), and other variations of purple and red. * [[Impossible color]]s, which cannot be seen under normal viewing of light, such as over-saturated colors or colors that are seemingly brighter than white. * [[Metallic color]]s which reflect light by effect.
== Notes == {{NoteFoot}}
== References == {{Reflist}}
{{-}} {{Color topics}}
{{DEFAULTSORT:Spectral Color}} [[Category:Color]]