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Aspectral color is acolor that is evoked bymonochromatic light, i.e. either aspectral line with a singlewavelength orfrequency oflight in thevisible spectrum, or a relatively narrowspectral band (e.g.lasers). Every wave of visible light is perceived as a spectral color; when viewed as acontinuous spectrum, these colors are seen as the familiarrainbow.Non-spectral colors (orextra-spectral colors) are evoked by a combination of spectral colors.
Incolor spaces which include all, or most spectral colors, they form a part of boundary of the set of all real colors. When considering a three-dimensional color space (which includesluminance), the spectral colors form asurface. When excluding luminance and considering a two-dimensional color space (chromaticity diagram), the spectral colors form acurve known as thespectral locus. For example, the spectral locus of theCIE xy chromaticity diagram contains all the spectral colors (to the eye of the standard observer).
A trichromatic color space is defined by threeprimary colors, 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 actualdistance between RGB purecolor wheel colors and spectral colors shows a complicated dependence on thehue. 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, andbright (yellowish) green,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 approachingIR orUV.
Spectral colors are universally included inscientific color spaces such as CIE 1931, but industrial and consumer color spaces/models such assRGB,CMYK, andPantone, do not typically include any spectral colors. Exceptions includeRec. 2020, which uses three spectral colors as primaries (and therefore only includes precisely those three spectral colors), and color spaces such as theProPhoto RGB color space which use imaginary colors as primaries.
In color spaces such asCIELUV, a spectral color has maximalsaturation. InHelmholtz coordinates, this is described as 100%purity.
Indichromaticcolor vision there is no distinction between spectral and non-spectral colors. Their entire visiblegamut can be represented by spectral colors.[note 1]
The spectrum is often divided intocolor terms 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 wasIsaac Newton, in the 1660s.[1] 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.[2]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 amnemonic for it is commonly known as "Roy G. Biv".
In modern divisions of the spectrum,indigo is often omitted andcyan is often included. Some have argued that Newton's indigo would be equivalent to modern blue, and his blue equivalent to cyan.[3] However, a more recent explanation is that Newton was working by analogy with the musical scale.[2] 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 inDorian mode. In contrast, the sections in theISCC-NBS spectrum vary greatly in wavelength range, but are more consistent in thehue degree range. Both instances deviate from thebasic 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 incolor terms. The "Approximate appearance" column displays the color with the sameOKLCh hue andlightness as the corresponding spectral color for that wavelength, with maximalsRGB saturation, given a spectrum of equalradiant power and with the highest brightness as to, when projected onto sRGB, still have maximal saturation within thiscolor gamut.Gamut mapping is necessary because displays cannot produce all, if any, spectral colors (a display with sRGB gamut andD65 whitepoint is assumed for this approximation). The colors at the ends of the spectrum are shown as dark since thecone cells in the eye are progressively less sensitive to light as the extremes of the visible spectrum are approached.
| Wavelength (nm) | Approximate appearance | Newton[2] | ISCC-NBS[4] | Malacara[5] | CRC Handbook[6] |
|---|---|---|---|---|---|
| 1700 | 1943 | 2011 | 2006 | ||
| 380 | Violet | Violet | Violet | Violet | |
| 390 | |||||
| 400 | |||||
| 410 | |||||
| 420 | |||||
| 430 | Indigo | Blue | |||
| 440 | Blue | ||||
| 450 | Blue | Blue | |||
| 460 | |||||
| 470 | |||||
| 480 | |||||
| 490 | Green | Blue-green | |||
| 500 | Green | Cyan | Green | ||
| 510 | |||||
| 520 | Green | ||||
| 530 | Yellow | ||||
| 540 | |||||
| 550 | Yellow-green | ||||
| 560 | |||||
| 570 | Yellow | Yellow | |||
| 580 | Orange | Yellow | Orange | ||
| 590 | Orange | Orange | |||
| 600 | |||||
| 610 | Red | ||||
| 620 | Red | Red | |||
| 630 | Red | ||||
| 640 | |||||
| 650 | |||||
| 660 | |||||
| 670 | |||||
| 680 | |||||
| 690 | |||||
| 700 | |||||
| 710 | |||||
| 720 | |||||
| 730 | |||||
| 740 |
Among some of the colors that arenot spectral colors are:
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