Spectral color

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Prism: decomposition of “white light” into spectral colors

A spectral color is the color impression produced by monochromatic light selected from the visible part of the light spectrum . In every shade it is the most intense and therefore pure color .

Although there are theoretically an infinite number of spectral colors, we often only talk about a certain, relatively small number of spectral colors in everyday use, for example the seven rainbow colors red , orange , yellow , green , blue , indigo and violet . They are not closely selected from the light spectrum, in the best case all slightly different colors are meant for each wavelength range, and the wavelength ranges collide with one another without gaps.


Six spectral color ranges of light
Color name Wavelength range Frequency range
violet ≈ 450-400 nm ≈ 670–750 THz
blue / indigo ≈ 490-450 nm ≈ 610–670 THz
green ≈ 560-490 nm ≈ 540–610 THz
yellow ≈ 590-560 nm ≈ 510-540 THz
orange ≈ 630-590 nm ≈ 480-510 THz
red ≈ 700–630 nm ≈ 430-480 THz

The spectrum (as a concept of color theory) is the totality of all lines and bands of a certain frequency in a radiation event. In the physical sense, white light is light that is mixed with equal energy from parts of all wavelengths of the visible spectral range and is practically hardly feasible. This is usually understood to mean daylight (sunlight) and the standard types of light with their different color temperatures. The color sensation produced by a light source of the same energy is called precisely “achromatic”.

By refraction on the optical prism , by diffraction on gratings or in interference , it is possible to break down white light into spectral colors. Spectral colors as monochromatic light can also arise directly with suitable emission processes (sodium vapor lamp, laser). Newton presented this fact in his Opticks . He named seven spectral colors, namely red, orange, yellow, green, blue, indigo, violet, without definitely assigning a wavelength or a wavelength range to them.

“White radiation”, ie radiation of the same energy in photometry and related fields , is of particular importance . Here it is important to achieve this property in the necessary (considered) wavelength intervals. For the measurement of the eye sensitivity curve , in addition to radiation of the same energy, radiation of the same quantum intensity is required. Such measurements are used to derive standard spectral color values.


2 ° and 10 ° spectral color range in comparison

In order to be able to represent a three-dimensional color space , the color stimulus of the wavelength spectrum has to be mapped onto the three types of cones (type of color-sensitive photoreceptors ). A suitable system of color valences, the basic colors for red, green and blue sensitivities, must be set up for this. The calculation rule for this has been standardized with the tristimulus algorithm since 1931. Necessary parameters were specified in later investigations and confirmed in principle. The result in the graphic representation is the “shoe sole curve” of the standard color table , the so-called chromaticity diagram .

The spectral colors lie on the perimeter of this area. The spectral colors are therefore the most saturated colors of the respective dominant wavelength.

A dependence of the color coordinates on perception results from the different position of the spectral curve, depending on whether the 2 ° field of view or a 10 ° field of view is used. Within the 2 ° field, the “best color vision” is mapped onto the retinal surface; the cones in the retina are here closest to one another. In the 10 ° field of view the density of the cones already decreases and the sensation of the rods is added. This field of view corresponds to an A4 area at normal viewing distance.

Color perception

The cone cells in the human retina (as well as other living beings) have different sensitivity spectra depending on their type, which cover certain areas of the received light spectrum. The processing of the signals coming from the cones converts the received light of the different spectral ranges and intensities into perceived colors. Since the weighting of the spectral components depends on the perception areas of the cone types, the color perception is also directly dependent on it. If, for example, an almost monochromatic light is presented by LEDs , this light color is perceived as very pure (saturated) and bright, i.e. as a spectral color. In the case of human vision, there is no spectrum of light that stimulates a single type of cone. Since the areas of the spigot types overlap, several types are always addressed at the same time.

exemplary rectangular spectrum

The spectral color range according to CIE is supplemented in the standard color table with the purple line , which, however, contains mixed colors. Physically, there are no associated spectral colors for purple tones. These are only seen with mixed perception of short and long wave light. In the CIE diagram, the purple tones with the highest saturation correspond to this purple line. In principle, all non-spectral colors are mixed colors.

The human color perception during daytime vision ( photopic vision ) is limited to three types of receptors for short, medium and long wavelengths. Some animals, especially birds like pigeons, have four color receptors. This enables them to differentiate between more color variants and types of color than a person. Other animal species, such as dogs, only have two types of color receptors. This leads to a smaller range of distinguishable color tones.

The range of invisible infrared light borders on the long-wave red end of the visible spectrum . Due to the constant transition in sensitivity to irritating wavelengths, this limit is fluid (between 720 nm and 830 nm) and is subject to individual fluctuations. This is mainly determined by the chemical structure of rhodopsin (visual purple). The perceived hue changes only slightly from 650 nm.

The infrared part of the spectrum is known as thermal radiation, although every absorbed electromagnetic radiation generates its energy equivalent as heat, regardless of whether it is long-wave radio radiation or hard (very short-wave) gamma radiation. The heating effect of infrared and radio radiation is perceived by humans directly through other receptors. The depth of penetration of the radiation depends on the wavelength. Near infrared radiation penetrates only a few millimeters into the body, radio radiation is implemented throughout the body. This effect is used in shortwave radiation to heat internal organs.

At the short-wave violet end of the visible spectrum, at wavelengths from about 380 nm and smaller, the range of ultraviolet light borders. These wavelengths do not cause irritation to human receptors, so ultraviolet light is invisible. For the same reason as at the infrared end of the light spectrum, the visibility of the radiation between 360 nm and 410 nm is subject to individual and age-related fluctuations, even more than in the near-infrared range. The pigmentation of the cornea plays a major role here. In addition, the lens turns yellow with age, which filters short-wave light more strongly.

In cataract operations, the filter effect is eliminated after removal of the clouded lens ( aphakic vision ), which leads to a marked improvement in the perception of short wavelengths. After inserting the plastic lens , this special effect is canceled again.

Web links

Wiktionary: Spectral color  - explanations of meanings, word origins, synonyms, translations

References and comments

  1. ↑ It should be noted, however, that individual perception varies genetically and is particularly dependent on the yellowing of the lens. As a result, there is no exact limit violet / ultraviolet, but a range. The replacing lens provides better visibility of yellow tones.