View source for Spectral color

{{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, since [[Additive colors|when all colors of light are mixed together, they produce white]].]]

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 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 a [[blue-green]] color is sometimes included. Some have argued that Newton's indigo would be equivalent to our modern blue, and his blue equivalent to our blue-green. However, his nonintuitive choices can be better explained. In the table below, note how wavelength is not proportional to hue (which is approximately perceptually uniform). Color systems such as [[ISCC-NBS]] attempt to divide the spectrum into sections that appear perceptually uniform. On the other hand, Newton's sections are approximately uniform in size as they would have physically appeared in the diffracted spectrum, i.e. each about 40nm "wide".  In this theory, the sections were divided without influence of his own perception, and each section was then given a name that best suited its ''average'' color. 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 [[hue]] that a given monochromatic light evokes is approximated at the right side of the table.

{| class="wikitable" style="text-align: center;"
|+Spectral color classifications
!scope="col"|[[wavelength|nm]]
!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>
!scope="col"|[[Hue]][[#×|<span style="color:black" title="approximated hue degree of HSL space">*</span>]]
|-
|380
| rowspan="5" bgcolor="#7F00FF" |'''Violet'''
| rowspan="6" bgcolor="#7F00FF" |'''Violet'''
| rowspan="5" bgcolor="#7F00FF" |'''Violet'''
| rowspan="7" bgcolor="#7F00FF" |'''Violet'''
|250°
|-
|390
|250°
|-
|400
|250°
|-
|410
|249°
|-
|420
|249°
|-
|430
| rowspan="2" bgcolor="#3F00FF |'''Indigo'''
| rowspan="7" bgcolor="#0000FF" |'''Blue'''
|249°
|-
|440
| rowspan="5" bgcolor="#0000FF" |'''Blue'''
|247°
|-
|450
| rowspan="4" bgcolor="#1DA2DF" |'''Blue'''
| rowspan="5" bgcolor="#0000FF" |'''Blue'''
|245°
|-
|460
|242°
|-
|470
|238°
|-
|480
|226°
|-
|490
| rowspan="4" bgcolor="#00FF00" |'''Green'''
| rowspan="1" bgcolor="#00FFFF" |'''Blue-Green'''
|190°
|-
|500
| rowspan="5" bgcolor="#00FF00" |'''Green'''
| rowspan="2" bgcolor="#00FFFF" |'''Cyan'''
| rowspan="7" bgcolor="#00FF00" |'''Green'''
|143°
|-
|510
|126°
|-
|520
| rowspan="5" bgcolor="#00FF00" |'''Green'''
|122°
|-
|530
| rowspan="5" bgcolor="#FFFF00"|'''Yellow'''
|117°
|-
|540
|113°
|-
|550
| rowspan="3" bgcolor="#88FF00" |'''Yellow-Green'''
|104°
|-
|560
|93°
|-
|570
| rowspan="1" bgcolor="#FFFF00" |'''Yellow'''
| rowspan="2" bgcolor="#FFFF00" |'''Yellow'''
|62°
|-
|580
| rowspan="4" bgcolor="#FF8800" |'''Orange'''
| rowspan="1" bgcolor="#FFFF00" |'''Yellow'''
| rowspan="5" bgcolor="#FF8800" |'''Orange'''
|28°
|-
|590
| rowspan="2" bgcolor="#FF8800" |'''Orange'''
| rowspan="3" bgcolor="#FF8800" |'''Orange'''
|14°
|-
|600
|7°
|-
|610
| rowspan="13" bgcolor="#FF0000" |'''Red'''
|5°
|-
|620
| rowspan="8" bgcolor="#FF0000" |'''Red'''
| rowspan="13" bgcolor="#FF0000" |'''Red'''
|3°
|-
|630
| rowspan="11" bgcolor="#FF0000" |'''Red'''
|2°
|-
|640
|1°
|-
|650
|1°
|-
|660
|1°
|-
|670
|0°
|-
|680
|0°
|-
|690
|0°
|-
|700
| rowspan="6" bgcolor="#000000" |
|0°
|-
|710
|0°
|-
|720
|0°
|-
|730
|0°
|-
|740
| rowspan="2" bgcolor="#000000" |
| rowspan="2" bgcolor="#000000" |
|0°
|-
|750
| rowspan="1" bgcolor="#000000" |
|
|}

==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]]