Spectral Color
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A spectral color is a
color Color (American English) or colour (British English) is the visual perceptual property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physical specifications of color are associ ...
that is evoked by ''
monochromatic light {{Short description, Electromagnetic radiation with a single constant frequency In physics, monochromatic radiation is electromagnetic radiation with a single constant frequency. When that frequency is part of the visible spectrum (or near it) the ...
'', i.e. either a single
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
of
light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...
in the
visible spectrum The visible spectrum is the portion of the electromagnetic spectrum that is visual perception, visible to the human eye. Electromagnetic radiation in this range of wavelengths is called ''visible light'' or simply light. A typical human eye wil ...
, or by a relatively narrow band of wavelengths (e.g.
laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fir ...
s). Every wavelength of visible light is perceived as a spectral color; when viewed as a continuous spectrum, these colors are seen as the familiar
rainbow A rainbow is a meteorological phenomenon that is caused by reflection, refraction and dispersion of light in water droplets resulting in a spectrum of light appearing in the sky. It takes the form of a multicoloured circular arc. Rainbows c ...
. All colors that do not qualify as a spectral color are called ''non-spectral colors'' or ''extra-spectral colors''. Extra-spectral colors cannot be evoked with a single wavelength of light, but rather by a combination of wavelengths. Likewise, light comprising several wavelengths cannot evoke a spectral color.


In color spaces

In
color space A color space is a specific organization of colors. In combination with color profiling supported by various physical devices, it supports reproducible representations of colorwhether such representation entails an analog or a digital represent ...
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-
dimensional In physics and mathematics, the dimension of a mathematical space (or object) is informally defined as the minimum number of coordinates needed to specify any point within it. Thus, a line has a dimension of one (1D) because only one coordi ...
color space (which including
luminance Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through, is emitted from, or is reflected from a particular area, and falls withi ...
), the spectral colors form a
surface A surface, as the term is most generally used, is the outermost or uppermost layer of a physical object or space. It is the portion or region of the object that can first be perceived by an observer using the senses of sight and touch, and is ...
. When excluding luminance and considering a two-dimensional color space (
chromaticity diagram Chromaticity is an objective specification of the quality of a color regardless of its luminance. Chromaticity consists of two independent parameters, often specified as hue (h) and colorfulness (s), where the latter is alternatively called ...
), the spectral colors form a
curve In mathematics, a curve (also called a curved line in older texts) is an object similar to a line (geometry), line, but that does not have to be Linearity, straight. Intuitively, a curve may be thought of as the trace left by a moving point (ge ...
known as the spectral locus. For example, the spectral locus of the
CIEXYZ The CIE 1931 color spaces are the first defined quantitative links between distributions of wavelengths in the electromagnetic visible spectrum, and physiologically perceived colors in human color vision. The mathematical relationships that defin ...
chromaticity diagram contains all the spectral colors (to the eye of the standard observer). A trichromatic color space is defined by three
primary color A set (mathematics), set of primary colors or primary colours (see American and British English spelling differences#-our, -or, spelling differences) consists of colorants or colored lights that can be mixed in varying amounts to produce a gamu ...
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
distance Distance is a numerical or occasionally qualitative measurement of how far apart objects or points are. In physics or everyday usage, distance may refer to a physical length or an estimation based on other criteria (e.g. "two counties over"). ...
between RGB pure
color wheel A color wheel or color circle is an abstract illustrative organization of color hues around a circle, which shows the relationships between primary colors, secondary colors, tertiary colors etc. Some sources use the terms ''color wheel'' & ' ...
colors and spectral colors shows a complicated dependence on the
hue In color theory, hue is one of the main properties (called color appearance parameters) of a color, defined technically in the CIECAM02 model as "the degree to which a stimulus can be described as similar to or different from stimuli that ...
. Due to the location of R and G primaries near the ‘almost flat’ spectral segment,
RGB color space An RGB color space is any additive color space based on the RGB color model. An RGB color space is defined by chromaticity coordinates of the red, green, and blue additive primaries, the white point which is usually a standard illuminant, and t ...
is reasonably good with approximating spectral orange, yellow, and bright (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 approaching IR or UV. Note that spectral color is universally included in
scientific Science is a systematic endeavor that builds and organizes knowledge in the form of testable explanations and predictions about the universe. Science may be as old as the human species, and some of the earliest archeological evidence for ...
color models such as CIE 1931, but industrial and consumer color spaces such as sRGB, CMYK, and
Pantone Pantone LLC (stylized as PANTONE) is a limited liability company headquartered in Carlstadt, New Jersey. The company is best known for its Pantone Matching System (PMS), a proprietary color space used in a variety of industries, notably graphic ...
, 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 The ProPhoto RGB color space, also known as ROMM RGB (Reference Output Medium Metric), is an output referred RGB color space developed by Kodak. It offers an especially large gamut designed for use with photographic output in mind. The ProPhoto ...
which use imaginary colors as primaries. In
color model A color model is an abstract mathematical model describing the way colors can be represented as tuples of numbers, typically as three or four values or color components. When this model is associated with a precise description of how the compon ...
s capable of representing spectral colors, such as CIELUV, a spectral color has the maximal saturation. In Helmholtz coordinates, this is described as 100%
purity Purity may refer to: Books * ''Pureza'' (novel), a 1937 Brazilian novel by José Lins do Rego * ''Purity'' (novel), a 2015 novel by Jonathan Franzen ** ''Purity'' (TV series), a TV series based on the novel *''Purity'', a 2012 novel by Jackson P ...
.


In dichromatic color spaces

In dichromatic
color vision Color vision, a feature of visual perception, is an ability to perceive differences between light composed of different wavelengths (i.e., different spectral power distributions) independently of light intensity. Color perception is a part of ...
there is no distinction between spectral and non-spectral colors. Their entire gamut can be represented by spectral colors.


Spectral color terms

The spectrum is often divided into ''
color term A color term (or color name) is a word or phrase that refers to a specific color. The color term may refer to human perception of that color (which is affected by visual context) which is usually defined according to the Munsell color system, or t ...
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 Sir Isaac Newton (25 December 1642 – 20 March 1726/27) was an English mathematician, physicist, astronomer, alchemist, theologian, and author (described in his time as a "natural philosopher"), widely recognised as one of the grea ...
. 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. 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 A mnemonic ( ) device, or memory device, is any learning technique that aids information retention or retrieval (remembering) in the human memory for better understanding. Mnemonics make use of elaborative encoding, retrieval cues, and imag ...
for it is commonly known as "
Roy G. Biv ROYGBIV is an acronym for the sequence of hues commonly described as making up a rainbow: red, orange, yellow, green, blue, indigo, and violet. There are several mnemonics that can be used for remembering this color sequence, such as the ...
". In modern divisions of the spectrum, indigo is often omitted and a
blue-green Blue-green is the color that is between green and blue. It belongs to the cyan family of colors. Variations Cyan (aqua) Cyan, also called aqua, is the blue-green color that is between blue and green on a modern RGB color wheel. The ...
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 In color theory, hue is one of the main properties (called color appearance parameters) of a color, defined technically in the CIECAM02 model as "the degree to which a stimulus can be described as similar to or different from stimuli that ...
degree range. Both instances deviate from the
basic color terms ''Basic Color Terms: Their Universality and Evolution'' (1969; ) is a book by Brent Berlin and Paul Kay. Berlin and Kay's work proposed that the basic color terms in a culture, such as black, brown, or red, are predictable by the number of color ...
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 A color term (or color name) is a word or phrase that refers to a specific color. The color term may refer to human perception of that color (which is affected by visual context) which is usually defined according to the Munsell color system, or t ...
s. The
hue In color theory, hue is one of the main properties (called color appearance parameters) of a color, defined technically in the CIECAM02 model as "the degree to which a stimulus can be described as similar to or different from stimuli that ...
that a given monochromatic light evokes is approximated at the right side of the table.


Extra-spectral colors

Among some of the colors that are spectral colors are: *
Grayscale In digital photography, computer-generated imagery, and colorimetry, a grayscale image is one in which the value of each pixel is a single sample representing only an ''amount'' of light; that is, it carries only intensity information. Graysca ...
(achromatic) colors, such as
white White is the lightest color and is achromatic (having no hue). It is the color of objects such as snow, chalk, and milk, and is the opposite of black. White objects fully reflect and scatter all the visible wavelengths of light. White on ...
,
gray Grey (more common in British English) or gray (more common in American English) is an intermediate color between black and white. It is a neutral or achromatic color, meaning literally that it is "without color", because it can be composed o ...
, and
black Black is a color which results from the absence or complete absorption of visible light. It is an achromatic color, without hue, like white and grey. It is often used symbolically or figuratively to represent darkness. Black and white have o ...
. * Any color obtained by mixing a gray-scale color and another color (either spectral or not), such as
pink Pink is the color of a namesake flower that is a pale tint of red. It was first used as a color name in the late 17th century. According to surveys in Europe and the United States, pink is the color most often associated with charm, politeness, ...
(a mixture of a
red Red is the color at the long wavelength end of the visible spectrum of light, next to orange and opposite violet. It has a dominant wavelength of approximately 625–740 nanometres. It is a primary color in the RGB color model and a secondar ...
dish color and white), or
brown Brown is a color. It can be considered a composite color, but it is mainly a darker shade of orange. In the CMYK color model used in printing or painting, brown is usually made by combining the colors orange and black. In the RGB color model used ...
(a mixture of
orange Orange most often refers to: *Orange (fruit), the fruit of the tree species '' Citrus'' × ''sinensis'' ** Orange blossom, its fragrant flower *Orange (colour), from the color of an orange, occurs between red and yellow in the visible spectrum * ...
and black or gray). *
Violet Violet may refer to: Common meanings * Violet (color), a spectral color with wavelengths shorter than blue * One of a list of plants known as violet, particularly: ** ''Viola'' (plant), a genus of flowering plants Places United States * Viol ...
-
red Red is the color at the long wavelength end of the visible spectrum of light, next to orange and opposite violet. It has a dominant wavelength of approximately 625–740 nanometres. It is a primary color in the RGB color model and a secondar ...
colors, which in
color theory In the visual arts, color theory is the body of practical guidance for color mixing and the visual effects of a specific color combination. Color terminology based on the color wheel and its geometry separates colors into primary color, seconda ...
include line of purples colors (such as, approximately,
magenta Magenta () is a color that is variously defined as pinkish- purplish-red, reddish-purplish-pink or mauvish-crimson. On color wheels of the RGB (additive) and CMY (subtractive) color models, it is located exactly midway between red and blue. I ...
and
rose A rose is either a woody perennial flowering plant of the genus ''Rosa'' (), in the family Rosaceae (), or the flower it bears. There are over three hundred species and tens of thousands of cultivars. They form a group of plants that can be ...
), and other variations of purple and red. *
Impossible color Impossible colors are colors that do not appear in ordinary visual functioning. Different color theories suggest different hypothetical colors that humans are incapable of perceiving for one reason or another, and fictional colors are rou ...
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 A metallic color is a color that appears to be that of a polished metal. The visual sensation usually associated with metals is its metallic shine. This cannot be reproduced by a simple solid color, because the shiny effect is due to the mater ...
s which reflect light by effect.


Notes


References

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