Color vision
Encyclopedia
Color vision is the capacity of an organism or machine to distinguish objects based on the wavelength
s (or frequencies
) of the light
they reflect, emit, or transmit. Color
s can be measured and quantified in various ways; indeed, a human's perception of colors is a subjective process whereby the brain
responds to the stimuli that are produced when incoming light reacts with the several types of cone photoreceptors
in the eye
.
discovered that white light splits into its component colors when passed through a prism, but that if those bands of colored light pass through another and rejoin, they make a white beam. The characteristic colors are, from low to high frequency: red, orange, yellow, green, cyan, blue, violet. Sufficient differences in frequency give rise to a difference in perceived hue
; the just noticeable difference
in wavelength varies from about 1 nm in the blue-green
and yellow
wavelengths, to 10 nm and more in the red and blue. Though the eye can distinguish up to a few hundred hues, when those pure spectral colors are mixed together or diluted with white light, the number of distinguishable chromaticities
can be quite high.
In very low light levels, vision is scotopic: light is detected by rod cell
s of the retina. Rods are maximally sensitive to wavelengths near 500 nm, and play little, if any, role in color vision. In brighter light, such as daylight, vision is photopic: light is detected by cone cell
s which are responsible for color vision. Cones are sensitive to a range of wavelengths, but are most sensitive to wavelengths near 555 nm. Between these regions, mesopic vision
comes into play and both rods and cones provide signals to the retinal ganglion cells. The shift in color perception from dim light to daylight gives rise to differences known as the Purkinje effect
.
The perception of "white" is formed by the entire spectrum of visible light, or by mixing colors of just a few wavelengths, such as red, green, and blue, or by mixing just a pair of complementary color
s such as blue and yellow.
, known as cone cell
s. In humans, there are three types of cones sensitive to three difference spectra, resulting in trichromatic color vision.
The cones are conventionally labeled according to the ordering of the wavelengths of the peaks of their spectral sensitivities
: short (S), medium (M), and long (L) cone types. These three types do not correspond well to particular colors as we know them. Rather, the perception of color is achieved by a complex process that starts with the differential output of these cells in the retina and it will be finalized in the visual cortex and associative areas of the brain.
For example, while the L cones have been referred to simply as red
receptors, microspectrophotometry has shown that their peak sensitivity is in the greenish-yellow region of the spectrum. Similarly, the S- and M-cones do not directly correspond to blue
and green
, although they are often depicted as such. It is important to note that the RGB color model
is merely a convenient means for representing color, and is not directly based on the types of cones in the human eye.
The peak response of human cone cells varies, even among individuals with 'normal' color vision;
in non-human species this polymorphic variation is even greater, and it may well be adaptive.
theory. The trichromatic theory, or Young–Helmholtz theory, proposed in the 19th century by Thomas Young
and Hermann von Helmholtz
, as mentioned above, states that the retina's three types of cones are preferentially sensitive to blue, green, and red. Ewald Hering
proposed the opponent process theory in 1872. It states that the visual system interprets color in an antagonistic way: red vs. green, blue vs. yellow, black vs. white. We now know both theories to be correct, describing different stages in visual physiology.
A range of wavelengths of light stimulates each of these receptor types to varying degrees. Yellowish-green light, for example, stimulates both L and M cones equally strongly, but only stimulates S-cones weakly. Red light, on the other hand, stimulates L cones much more than M cones, and S cones hardly at all; blue-green light stimulates M cones more than L cones, and S cones a bit more strongly, and is also the peak stimulant for rod cells; and blue
light stimulates S cones more strongly than red or green light, but L and M cones more weakly. The brain combines the information from each type of receptor to give rise to different perceptions of different wavelengths of light.
The opsins (photopigments) present in the L and M cones are encoded on the X chromosome
; defective encoding of these leads to the two most common forms of color blindness
. The OPN1LW
gene, which codes for the opsin present in the L cones, is highly polymorphic
(a recent study by Verrelli and Tishkoff found 85 variants in a sample of 236 men). A very small percentage of women may have an extra type of color receptor because they have different alleles for the gene for the L opsin on each X chromosome. X chromosome inactivation means that only one opsin is expressed in each cone cell, and some women may therefore show a degree of tetrachromat
ic color vision. Variations in OPN1MW
, which codes the opsin expressed in M cones, appear to be rare, and the observed variants have no effect on spectral sensitivity
.
Color processing begins at a very early level in the visual system (even within the retina) through initial color opponent mechanisms. Both Helmholtz's trichromatic theory, and Hering's opponent process theory are therefore correct, but trichromacy arises at the level of the receptors, and opponent processes arise at the level of retinal ganglion cells and beyond. In Hering's theory opponent mechanisms refer to the opposing color effect of red–green, blue–yellow, and light–dark. However, in the visual system, it is the activity of the different receptor types that are opposed. Some midget retinal ganglion cells oppose L and M cone activity, which corresponds loosely to red–green opponency, but actually runs along an axis from blue-green to magenta. Small bistratified retinal ganglion cells oppose input from the S cones to input from the L and M cones. This is often thought to correspond to blue–yellow opponency, but actually runs along a color axis from lime green to violet.
Visual information is then sent to the brain from retinal ganglion cells via the optic nerve
to the optic chiasma: a point where the two optic nerves meet and information from the temporal (contralateral) visual field crosses to the other side of the brain. After the optic chiasma the visual tracts are referred to as the optic tract
s, which enter the thalamus
to synapse at the lateral geniculate nucleus
(LGN).
The LGN
is divided into laminae (zones), of which there three types: the M-laminae, consisting primarily of M-cells, the P-laminae, consisting primarily of P-cells, and the koniocellular laminae. M- and P-cells received relatively balanced input from both L- and M-cones throughout most of the retina, although this seems to not be the case at the fovea, with midget cells synapsing in the P-laminae. The koniocellular laminae receive axons from the small bistratified ganglion cells.
After synapsing
at the LGN, the visual tract continues on back to the primary visual cortex
(V1) located at the back of the brain within the occipital lobe
. Within V1 there is a distinct band (striation). This is also referred to as "striate cortex", with other cortical visual regions referred to collectively as "extrastriate cortex". It is at this stage that color processing becomes much more complicated.
In V1 the simple three-color segregation begins to break down. Many cells in V1 respond to some parts of the spectrum better than others, but this "color tuning" is often different depending on the adaptation state of the visual system. A given cell that might respond best to long wavelength light if the light is relatively bright might then become responsive to all wavelengths if the stimulus is relatively dim. Because the color tuning of these cells is not stable, some believe that a different, relatively small, population of neurons in V1 is responsible for color vision. These specialized "color cells" often have receptive fields that can compute local cone ratios. Such "double-opponent" cells were initially described in the goldfish retina by Nigel Daw; their existence in primates was suggested by David H. Hubel
and Torsten Wiesel
and subsequently proven by Bevil Conway
. As Margaret Livingstone and David Hubel showed, double opponent cells are clustered within localized regions of V1 called blobs
, and are thought to come in two flavors, red–green and blue–yellow. Red–green cells compare the relative amounts of red–green in one part of a scene with the amount of red–green in an adjacent part of the scene, responding best to local color contrast (red next to green). Modeling studies have shown that double-opponent cells are ideal candidates for the neural machinery of color constancy
explained by Edwin H. Land
in his retinex theory.
From the V1 blobs, color information is sent to cells in the second visual area, V2. The cells in V2 that are most strongly color tuned are clustered in the "thin stripes" that, like the blobs in V1, stain for the enzyme cytochrome oxidase (separating the thin stripes are interstripes and thick stripes, which seem to be concerned with other visual information like motion and high-resolution form). Neurons in V2 then synapse onto cells in the extended V4. This area includes not only V4, but two other areas in the posterior inferior temporal cortex, anterior to area V3, the dorsal posterior inferior temporal cortex, and posterior TEO. (Area V4 was identified by Semir Zeki
to be exclusively dedicated to color, but this has since been shown not to be the case. Color processing in the extended V4 occurs in millimeter-sized color modules called glob
s. This is the first part of the brain in which color is processed in terms of the full range of hue
s found in color space
.
Anatomical studies have shown that neurons in extended V4 provide input to the inferior temporal lobe
. "IT" cortex is thought to integrate color information with shape and form, although it has been difficult to define the appropriate criteria for this claim. Despite this murkiness, it has been useful to characterize this pathway (V1 > V2 > V4 > IT) as the ventral stream or the "what pathway", distinguished from the dorsal stream ("where pathway") that is thought to analyze motion, among many other features.
An example of the subjectivity of color occurs in a rainbow. In a rainbow (or a spectrum of light projected from a prism), the changes between wavelengths of light are smooth and continuous; there are no breaks or boundaries corresponding to the "bands of color" which are seen subjectively by the eye. A black-and-white photograph of a rainbow shows no band stucture at all, demonstrating that the number of bands, and the bands themselves, are phenomena added to nature by the eye and the brain. They are not objectively real any more than "hot" or "cold."
The cone photoreceptors, which begin the process that results in the ultimate sensation of color in the brain, are sensitive to different portions of the visible spectrum
. For humans, the visible spectrum ranges approximately from 380 to 740 nm, and there are normally three types of cones. The number of colors that can be distinguished by these cones is in principle unlimited and in practice extremely large.
The sensation of color is also heavily dependent on contrast with surroundings. For example, a 'red' piece of construction paper or red filing folder does not reflect pure monochromatic "red" light. Rather, it absorbs a larger fraction of other frequencies of visible light shining upon it than those in a group of frequencies that are perceived as red when they are viewed alone. However, although the red paper reflects more of light near the red end of the spectrum, it reflects some of all frequencies (not only red), and a room lit with only the reflection from a red piece of paper would soon appear to be normally lit to an adapted eye, and would allow visualization of all of the other colors present in the room. A red-colored paper is therefore perceived to be red mainly by contrast with its surroundings, because the human eye
can distinguish between different wavelengths, and the brain creates the separate sensation of "red" as a helper in discriminating one object from another, or from its background. Color, which in nature is never constructed from pure frequencies of light, is a quality constructed by the visual brain from spectral reflectance, the objective property of objects. The advantage of color perception is the better discrimination of surfaces allowed by this aspect of visual processing, which makes quite similar surfaces into very different perceptions.
In most Catarrhini
(Old World monkeys and apes — primates closely related to humans) there are three types of color receptors (known as cone cell
s), resulting in trichromatic color vision. These primates, like humans, are known as trichromats. Many other primates and other mammals are dichromat
s, which is the general color vision state for mammals that are active during the day (i.e., felines, canines, ungulates). Nocturnal mammals may have little or no color vision. Trichromat non-primate mammals are rare.
Many invertebrates
have color vision. Honey- and bumblebees have trichromatic color vision, which is insensitive to red but sensitive in ultraviolet. In view of the importance of colour vision to bees one might expect these receptor sensitivities to reflect their specific visual ecology; for example the types of flowers that they visit. However, the main groups of hymenopteran insects excluding ants (i.e. bees, wasps and sawflies) mostly have three types of photoreceptor, with spectral sensitivities similar the honeybee’s.Papilio
butterflies possess six types of photoreceptors and may have pentachromatic vision. The most complex color vision system in animal kingdom has been found in stomatopods (such as the mantis shrimp
) with up to 12 different spectral receptor types thought to work as multiple dichromatic units.
Vertebrate animals such as tropical fish
and birds sometimes have more complex color vision systems than humans; thus the many subtle colors they exhibit generally serve as direct signals between fish or between birds, and are not intended to signal mammals. In the birds, tetrachromacy is achieved through up to four cone
types, depending on species.Each single cone contains one of the four main
types of vertebrate cone photopigment (LWS/ MWS, RH2, SWS2 and SWS1and has a coloured oil droplet in its inner segment. Brightly colored oil droplets inside the cones shift or narrow the spectral sensitivity of the cell. It has been suggested that it is likely that pigeons are pentachromat
s.
Reptiles and amphibians also have four cone types (occasionally five), and probably see at least the same number of colors that humans do, or perhaps more. In addition, some nocturnal gecko
s have the capability of seeing color in dim light.
In the evolution of mammals, segments of color vision were lost, then for a few species of primates, regained by gene duplication
. Eutherian mammals other than primates (for example, dogs, cats, mammalian farm animals) generally have less-effective two-receptor (dichromat
ic) color perception systems, which distinguish blue, green, and yellow—but cannot distinguish oranges and reds. The adaptation to see reds is particularly important for primate mammals, since it leads to identification of fruits, and also newly sprouting reddish leaves, which are particularly nutritious.
However, even among primates, full color vision differs between New World and Old World monkeys. Old World primates, including monkeys and all apes, have vision similar to humans. New World monkeys may or may not have color sensitivity at this level: in most species, males are dichromats, and about 60% of females are trichromats, but the owl monkeys are cone monochromats, and both sexes of howler monkey
s are trichromats. Visual sensitivity differences between males and females in a single species is due to the gene for yellow-green sensitive opsin
protein (which confers ability to differentiate red from green) residing on the X sex chromosome.
Several marsupial
s such as the fat-tailed dunnart
(Sminthopsis crassicaudata) have been shown to have trichromatic color vision.
Marine mammal
s, adapted for low-light vision, have only a single cone type and are thus monochromat
s.
s, particular flower types are often recognized by color as well. On the other hand, nocturnal mammals have less-developed color vision, since adequate light is needed for cones to function properly. There is evidence that ultraviolet
light plays a part in color perception in many branches of the animal kingdom
, especially insect
s. In general, the optical spectrum encompasses the most common electronic transitions
in matter and is therefore the most useful for collecting information about the environment.
The evolution of trichromatic color vision in primates
occurred as the ancestors of modern monkeys, apes, and humans switched to diurnal
(daytime) activity and began consuming fruits and leaves from flowering plants. Color vision, with UV discrimination, is also present in a number of arthropods – the only terrestrial animals besides the vertebrates to possess this trait.
Some animals can distinguish colors in the ultraviolet spectrum. The UV spectrum falls outside the human visible range, except for some cataract surgery
patients. Birds, turtles, lizards, many fish and some rodents have UV receptors in their retinas. These animals can see the UV patterns found on flowers and other wildlife that are otherwise invisible to the human eye.
UV and multi-dimensional vision is an especially important adaptation in birds. It allows birds to spot small prey from a distance, navigate, avoid predators, and forage while flying at high speeds. Birds also utilize their broad spectrum vision to recognize other birds, and in sexual selection.
s (in the visible range). Since there are, in principle, infinitely many distinct spectral colors, the set of all physical colors may be thought of as an infinite-dimensional vector space
, in fact a Hilbert space
. We call this space Hcolor. More technically, the space of physical colors may be considered to be the (mathematical) cone
over the simplex whose vertices are the spectral colors, with white at the centroid
of the simplex, black at the apex of the cone, and the monochromatic color associated with any given vertex somewhere along the line from that vertex to the apex depending on its brightness.
An element C of Hcolor is a function from the range of visible wavelengths—considered as an interval of real numbers [Wmin,Wmax]—to the real numbers, assigning to each wavelength w in [Wmin,Wmax] its intensity C(w).
A humanly perceived color may be modeled as three numbers: the extents to which each of the 3 types of cones is stimulated. Thus a humanly perceived color may be thought of as a point in 3-dimensional Euclidean space
. We call this space R3color.
Since each wavelength w stimulates each of the 3 types of cone cells to a known extent, these extents may be represented by 3 functions s(w), m(w), l(w) corresponding to the response of the S, M, and L cone cells, respectively.
Finally, since a beam of light can be composed of many different wavelengths, to determine the extent to which a physical color C in Hcolor stimulates each cone cell, we must calculate the integral (with respect to w), over the interval [Wmin,Wmax], of C(w)·s(w), of C(w)·m(w), and of C(w)·l(w). The triple of resulting numbers associates to each physical color C (which is an element in Hcolor) to a particular perceived color (which is a single point in R3color). This association is easily seen to be linear. It may also easily be seen that many different elements in the "physical" space Hcolor can all result in the same single perceived color in R3color, so a perceived color is not unique to one physical color.
Thus human color perception is determined by a specific, non-unique linear mapping from the infinite-dimensional Hilbert space Hcolor to the 3-dimensional Euclidean space R3color.
Technically, the image of the (mathematical) cone over the simplex whose vertices are the spectral colors, by this linear mapping, is also a (mathematical) cone in R3color. Moving directly away from the vertex of this cone represents maintaining the same chromaticity
while increasing its intensity. Taking a cross-section of this cone yields a 2D chromaticity space. Both the 3D cone and its projection or cross-section are convex sets; that is, any mixture of spectral colors is also a color.
In practice, it would be quite difficult to physiologically measure an individual's three cone responses to various physical color stimuli. Instead, a psychophysical approach is taken. Three specific benchmark test lights are typically used; let us call them S, M, and L. To calibrate human perceptual space, scientists allowed human subjects to try to match any physical color by turning dials to create specific combinations of intensities (IS, IM, IL) for the S, M, and L lights, resp., until a match was found. This needed only to be done for physical colors that are spectral (since a linear combination of spectral colors will be matched by the same linear combination of their (IS, IM, IL) matches. Note that in practice, often at least one of S, M, L would have to be added with some intensity to the physical test color, and that combination matched by a linear combination of the remaining 2 lights. Across different individuals (without color blindness), the matchings turned out to be nearly identical.
By considering all the resulting combinations of intensities (IS, IM, IL) as a subset of 3-space, a model for human perceptual color space is formed. (Note that when one of S, M, L had to be added to the test color, its intensity was counted as negative.) Again, this turns out to be a (mathematical) cone, not a quadric, but rather all rays through the origin in 3-space passing through a certain convex set. Again, this cone has the property that moving directly away from the origin corresponds to increasing the intensity of the S, M, L lights proportionately. Again, a cross-section of this cone is a planar shape that is (by definition) the space of "chromaticities" (informally: distinct colors); one particular such cross section, corresponding to constant X+Y+Z of the CIE 1931 color space
, gives the CIE chromaticity diagram.
It should be noted that this system implies that for any hue or non-spectral color not on the boundary of the chromaticity diagram, there are infinitely many distinct physical spectra that are all perceived as that hue or color. So, in general there is no such thing as the combination of spectral colors that we perceive as (say) a specific version of tan; instead there are infinitely many possibilities that produce that exact color. The boundary colors that are pure spectral colors can be perceived only in response to light that is purely at the associated wavelength, while the boundary colors on the "line of purples" can each only be generated by a specific ratio of the pure violet and the pure red at the ends of the visible spectral colors.
The CIE chromaticity diagram is horseshoe-shaped, with its curved edge corresponding to all spectral colors (the spectral locus
), and the remaining straight edge corresponding to the most saturated purple
s, mixtures of red
and violet
.
), while keeping other colors also looking realistic. For example, chromatic adaptation transforms are used when converting images between ICC profile
s with different white point
s. Adobe Photoshop
, for example, uses the Bradford CAT.
In color vision, chromatic adaptation refers to color constancy
; the ability of the visual system to preserve the appearance of an object under a wide range of light sources.
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s (or frequencies
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
) of the light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
they reflect, emit, or transmit. Color
Color
Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...
s can be measured and quantified in various ways; indeed, a human's perception of colors is a subjective process whereby the brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
responds to the stimuli that are produced when incoming light reacts with the several types of cone photoreceptors
Cone cell
Cone cells, or cones, are photoreceptor cells in the retina of the eye that are responsible for color vision; they function best in relatively bright light, as opposed to rod cells that work better in dim light. If the retina is exposed to an intense visual stimulus, a negative afterimage will be...
in the eye
Eye
Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement...
.
Wavelength and hue detection
Isaac NewtonIsaac Newton
Sir Isaac Newton PRS was an English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian, who has been "considered by many to be the greatest and most influential scientist who ever lived."...
discovered that white light splits into its component colors when passed through a prism, but that if those bands of colored light pass through another and rejoin, they make a white beam. The characteristic colors are, from low to high frequency: red, orange, yellow, green, cyan, blue, violet. Sufficient differences in frequency give rise to a difference in perceived hue
Hue
Hue is one of the main properties of a color, defined technically , as "the degree to which a stimulus can be describedas similar to or different from stimuli that are described as red, green, blue, and yellow,"...
; the just noticeable difference
Just noticeable difference
In psychophysics, a just noticeable difference, customarily abbreviated with lowercase letters as jnd, is the smallest detectable difference between a starting and secondary level of a particular sensory stimulus...
in wavelength varies from about 1 nm in the blue-green
Blue-green
Blue-green is a color that is a representation of the color that is between blue and green on a typical traditional old-fashioned RYB color wheel.Blue-green is belongs to the cyan family of colors....
and yellow
Yellow
Yellow is the color evoked by light that stimulates both the L and M cone cells of the retina about equally, with no significant stimulation of the S cone cells. Light with a wavelength of 570–590 nm is yellow, as is light with a suitable mixture of red and green...
wavelengths, to 10 nm and more in the red and blue. Though the eye can distinguish up to a few hundred hues, when those pure spectral colors are mixed together or diluted with white light, the number of distinguishable chromaticities
Chromaticity
Chromaticity is an objective specification of the quality of a color regardless of its luminance, that is, as determined by its hue and colorfulness ....
can be quite high.
In very low light levels, vision is scotopic: light is detected by rod cell
Rod cell
Rod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of visual photoreceptor, cone cells. Named for their cylindrical shape, rods are concentrated at the outer edges of the retina and are used in peripheral vision. On...
s of the retina. Rods are maximally sensitive to wavelengths near 500 nm, and play little, if any, role in color vision. In brighter light, such as daylight, vision is photopic: light is detected by cone cell
Cone cell
Cone cells, or cones, are photoreceptor cells in the retina of the eye that are responsible for color vision; they function best in relatively bright light, as opposed to rod cells that work better in dim light. If the retina is exposed to an intense visual stimulus, a negative afterimage will be...
s which are responsible for color vision. Cones are sensitive to a range of wavelengths, but are most sensitive to wavelengths near 555 nm. Between these regions, mesopic vision
Mesopic vision
Mesopic vision is a combination of photopic vision and scotopic vision in low but not quite dark lighting situations. Mesopic light levels range from luminances of approximately 0.001 to 3 cd m-2. Most night-time outdoor and traffic lighting scenarios are in the mesopic range.Humans see...
comes into play and both rods and cones provide signals to the retinal ganglion cells. The shift in color perception from dim light to daylight gives rise to differences known as the Purkinje effect
Purkinje effect
The Purkinje effect is the tendency for the peak luminance sensitivity of the human eye to shift toward the blue end of the color spectrum at low illumination levels.This effect introduces a difference in color contrast under different levels of...
.
The perception of "white" is formed by the entire spectrum of visible light, or by mixing colors of just a few wavelengths, such as red, green, and blue, or by mixing just a pair of complementary color
Complementary color
Complementary colors are pairs of colors that are of “opposite” hue in some color model. The exact hue “complementary” to a given hue depends on the model in question, and perceptually uniform, additive, and subtractive color models, for example, have differing complements for any given color.-...
s such as blue and yellow.
Physiology of color perception
Perception of color begins with specialized retinal cells containing pigments with different spectral sensitivitiesSpectral sensitivity
Spectral sensitivity is the relative efficiency of detection, of light or other signal, as a function of the frequency or wavelength of the signal....
, known as cone cell
Cone cell
Cone cells, or cones, are photoreceptor cells in the retina of the eye that are responsible for color vision; they function best in relatively bright light, as opposed to rod cells that work better in dim light. If the retina is exposed to an intense visual stimulus, a negative afterimage will be...
s. In humans, there are three types of cones sensitive to three difference spectra, resulting in trichromatic color vision.
The cones are conventionally labeled according to the ordering of the wavelengths of the peaks of their spectral sensitivities
Spectral sensitivity
Spectral sensitivity is the relative efficiency of detection, of light or other signal, as a function of the frequency or wavelength of the signal....
: short (S), medium (M), and long (L) cone types. These three types do not correspond well to particular colors as we know them. Rather, the perception of color is achieved by a complex process that starts with the differential output of these cells in the retina and it will be finalized in the visual cortex and associative areas of the brain.
For example, while the L cones have been referred to simply as red
Red
Red is any of a number of similar colors evoked by light consisting predominantly of the longest wavelengths of light discernible by the human eye, in the wavelength range of roughly 630–740 nm. Longer wavelengths than this are called infrared , and cannot be seen by the naked eye...
receptors, microspectrophotometry has shown that their peak sensitivity is in the greenish-yellow region of the spectrum. Similarly, the S- and M-cones do not directly correspond to blue
Blue
Blue is a colour, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 440–490 nm. It is considered one of the additive primary colours. On the HSV Colour Wheel, the complement of blue is yellow; that is, a colour corresponding to an equal...
and green
Green
Green is a color, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 520–570 nanometres. In the subtractive color system, it is not a primary color, but is created out of a mixture of yellow and blue, or yellow and cyan; it is considered...
, although they are often depicted as such. It is important to note that the RGB color model
RGB color model
The RGB color model is an additive color model in which red, green, and blue light is added together in various ways to reproduce a broad array of colors...
is merely a convenient means for representing color, and is not directly based on the types of cones in the human eye.
The peak response of human cone cells varies, even among individuals with 'normal' color vision;
in non-human species this polymorphic variation is even greater, and it may well be adaptive.
Theories of color vision
Two complementary theories of color vision are the trichromatic theory and the opponent processOpponent process
The color opponent process is a color theory that states that the human visual system interprets information about color by processing signals from cones and rods in an antagonistic manner...
theory. The trichromatic theory, or Young–Helmholtz theory, proposed in the 19th century by Thomas Young
Thomas Young (scientist)
Thomas Young was an English polymath. He is famous for having partly deciphered Egyptian hieroglyphics before Jean-François Champollion eventually expanded on his work...
and Hermann von Helmholtz
Hermann von Helmholtz
Hermann Ludwig Ferdinand von Helmholtz was a German physician and physicist who made significant contributions to several widely varied areas of modern science...
, as mentioned above, states that the retina's three types of cones are preferentially sensitive to blue, green, and red. Ewald Hering
Ewald Hering
Karl Ewald Konstantin Hering was a German physiologist who did much research into color vision and spatial perception...
proposed the opponent process theory in 1872. It states that the visual system interprets color in an antagonistic way: red vs. green, blue vs. yellow, black vs. white. We now know both theories to be correct, describing different stages in visual physiology.
Cone cells in the human eye
| Cone type | Name | Range | Peak wavelength |
|---|---|---|---|
| S | β | 400–500 nm Nanometre A nanometre is a unit of length in the metric system, equal to one billionth of a metre. The name combines the SI prefix nano- with the parent unit name metre .The nanometre is often used to express dimensions on the atomic scale: the diameter... |
420–440 nm |
| M | γ | 450–630 nm | 534–555 nm |
| L | ρ | 500–700 nm | 564–580 nm |
A range of wavelengths of light stimulates each of these receptor types to varying degrees. Yellowish-green light, for example, stimulates both L and M cones equally strongly, but only stimulates S-cones weakly. Red light, on the other hand, stimulates L cones much more than M cones, and S cones hardly at all; blue-green light stimulates M cones more than L cones, and S cones a bit more strongly, and is also the peak stimulant for rod cells; and blue
Blue
Blue is a colour, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 440–490 nm. It is considered one of the additive primary colours. On the HSV Colour Wheel, the complement of blue is yellow; that is, a colour corresponding to an equal...
light stimulates S cones more strongly than red or green light, but L and M cones more weakly. The brain combines the information from each type of receptor to give rise to different perceptions of different wavelengths of light.
The opsins (photopigments) present in the L and M cones are encoded on the X chromosome
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
; defective encoding of these leads to the two most common forms of color blindness
Color blindness
Color blindness or color vision deficiency is the inability or decreased ability to see color, or perceive color differences, under lighting conditions when color vision is not normally impaired...
. The OPN1LW
OPN1LW
Red-sensitive opsin is a protein that in humans is encoded by the OPN1LW gene.-External links:* -Further reading:...
gene, which codes for the opsin present in the L cones, is highly polymorphic
Polymorphism (biology)
Polymorphism in biology occurs when two or more clearly different phenotypes exist in the same population of a species — in other words, the occurrence of more than one form or morph...
(a recent study by Verrelli and Tishkoff found 85 variants in a sample of 236 men). A very small percentage of women may have an extra type of color receptor because they have different alleles for the gene for the L opsin on each X chromosome. X chromosome inactivation means that only one opsin is expressed in each cone cell, and some women may therefore show a degree of tetrachromat
Tetrachromat
Tetrachromacy is the condition of possessing four independent channels for conveying color information, or possessing four different types of cone cells in the eye...
ic color vision. Variations in OPN1MW
OPN1MW
Green-sensitive opsin is a protein that in humans is encoded by the OPN1MW gene.-External links:* -Further reading:...
, which codes the opsin expressed in M cones, appear to be rare, and the observed variants have no effect on spectral sensitivity
Spectral sensitivity
Spectral sensitivity is the relative efficiency of detection, of light or other signal, as a function of the frequency or wavelength of the signal....
.
Color in the human brain
Visual information is then sent to the brain from retinal ganglion cells via the optic nerve
Optic nerve
The optic nerve, also called cranial nerve 2, transmits visual information from the retina to the brain. Derived from the embryonic retinal ganglion cell, a diverticulum located in the diencephalon, the optic nerve doesn't regenerate after transection.-Anatomy:The optic nerve is the second of...
to the optic chiasma: a point where the two optic nerves meet and information from the temporal (contralateral) visual field crosses to the other side of the brain. After the optic chiasma the visual tracts are referred to as the optic tract
Optic tract
The optic tract is a part of the visual system in the brain.It is a continuation of the optic nerve and runs from the optic chiasm to the lateral geniculate nucleus....
s, which enter the thalamus
Thalamus
The thalamus is a midline paired symmetrical structure within the brains of vertebrates, including humans. It is situated between the cerebral cortex and midbrain, both in terms of location and neurological connections...
to synapse at the lateral geniculate nucleus
Lateral geniculate nucleus
The lateral geniculate nucleus is the primary relay center for visual information received from the retina of the eye. The LGN is found inside the thalamus of the brain....
(LGN).
The LGN
Lateral geniculate nucleus
The lateral geniculate nucleus is the primary relay center for visual information received from the retina of the eye. The LGN is found inside the thalamus of the brain....
is divided into laminae (zones), of which there three types: the M-laminae, consisting primarily of M-cells, the P-laminae, consisting primarily of P-cells, and the koniocellular laminae. M- and P-cells received relatively balanced input from both L- and M-cones throughout most of the retina, although this seems to not be the case at the fovea, with midget cells synapsing in the P-laminae. The koniocellular laminae receive axons from the small bistratified ganglion cells.
After synapsing
Synapse
In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another cell...
at the LGN, the visual tract continues on back to the primary visual cortex
Visual cortex
The visual cortex of the brain is the part of the cerebral cortex responsible for processing visual information. It is located in the occipital lobe, in the back of the brain....
(V1) located at the back of the brain within the occipital lobe
Occipital lobe
The occipital lobe is the visual processing center of the mammalian brain containing most of the anatomical region of the visual cortex. The primary visual cortex is Brodmann area 17, commonly called V1...
. Within V1 there is a distinct band (striation). This is also referred to as "striate cortex", with other cortical visual regions referred to collectively as "extrastriate cortex". It is at this stage that color processing becomes much more complicated.
In V1 the simple three-color segregation begins to break down. Many cells in V1 respond to some parts of the spectrum better than others, but this "color tuning" is often different depending on the adaptation state of the visual system. A given cell that might respond best to long wavelength light if the light is relatively bright might then become responsive to all wavelengths if the stimulus is relatively dim. Because the color tuning of these cells is not stable, some believe that a different, relatively small, population of neurons in V1 is responsible for color vision. These specialized "color cells" often have receptive fields that can compute local cone ratios. Such "double-opponent" cells were initially described in the goldfish retina by Nigel Daw; their existence in primates was suggested by David H. Hubel
David H. Hubel
David Hunter Hubel is the John Franklin Enders Professor of Neurobiology, Emeritus, at Harvard Medical School. He was co-recipient with Torsten Wiesel of the 1981 Nobel Prize in Physiology or Medicine, for their discoveries concerning information processing in the visual system; the prize was...
and Torsten Wiesel
Torsten Wiesel
Torsten Nils Wiesel was a Swedish co-recipient with David H. Hubel of the 1981 Nobel Prize in Physiology or Medicine, for their discoveries concerning information processing in the visual system; the prize was shared with Roger W...
and subsequently proven by Bevil Conway
Bevil Conway
Bevil Conway neuroscientist and artist. Conway specializes in visual perception in his scientific work, and he often explores the limitations of the visual system in his artwork. He is currently Knafel Assistant Professor at Wellesley College.Conway was educated at McGill University and Harvard...
. As Margaret Livingstone and David Hubel showed, double opponent cells are clustered within localized regions of V1 called blobs
Blob (visual system)
Blobs are sections of the visual cortex where groups of neurons that are sensitive to color assemble in cylindrical shapes. They were first identified in 1979 by Margaret Wong-Riley when she used a cytochrome oxidase stain, from which they get their name. These areas receive input from...
, and are thought to come in two flavors, red–green and blue–yellow. Red–green cells compare the relative amounts of red–green in one part of a scene with the amount of red–green in an adjacent part of the scene, responding best to local color contrast (red next to green). Modeling studies have shown that double-opponent cells are ideal candidates for the neural machinery of color constancy
Color constancy
Color constancy is an example of subjective constancy and a feature of the human color perception system which ensures that the perceived color of objects remains relatively constant under varying illumination conditions. A green apple for instance looks green to us at midday, when the main...
explained by Edwin H. Land
Edwin H. Land
Edwin Herbert Land was an American scientist and inventor, best known as the co-founder of the Polaroid Corporation. Among other things, he invented inexpensive filters for polarizing light, a practical system of in-camera instant photography, and his retinex theory of color vision...
in his retinex theory.
Semir Zeki
Semir Zeki is a professor of neuroesthetics at University College London. His main interest is the organization of the primate visual brain. He published his first scientific paper in 1967...
to be exclusively dedicated to color, but this has since been shown not to be the case. Color processing in the extended V4 occurs in millimeter-sized color modules called glob
Glob (visual system)
Globs are millimeter-sized color modules found beyond the visual area V2 in the brain's color processing ventral pathway. They are scattered throughout the posterior inferior temporal cortex in an area called the V4 complex. They are clustered by color preference, and organized as color columns...
s. This is the first part of the brain in which color is processed in terms of the full range of hue
Hue
Hue is one of the main properties of a color, defined technically , as "the degree to which a stimulus can be describedas similar to or different from stimuli that are described as red, green, blue, and yellow,"...
s found in color space
Color space
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...
.
Anatomical studies have shown that neurons in extended V4 provide input to the inferior temporal lobe
Temporal lobe
The temporal lobe is a region of the cerebral cortex that is located beneath the Sylvian fissure on both cerebral hemispheres of the mammalian brain....
. "IT" cortex is thought to integrate color information with shape and form, although it has been difficult to define the appropriate criteria for this claim. Despite this murkiness, it has been useful to characterize this pathway (V1 > V2 > V4 > IT) as the ventral stream or the "what pathway", distinguished from the dorsal stream ("where pathway") that is thought to analyze motion, among many other features.
Subjectivity of color perception
It has been established that the Himba people perceive colors differently from most Euro-Americans - they easily distinguish close shades of green, barely discernable for most people. The leading explanation is that the Himba created a very different color scheme which divides the spectrum to dark shades (Zuzu in Himba), very light (Vapa), Vivid blue and green (Buru) and dry colors - probably due to their specific way of life. However other explanations exist that have not been ruled out yet.An example of the subjectivity of color occurs in a rainbow. In a rainbow (or a spectrum of light projected from a prism), the changes between wavelengths of light are smooth and continuous; there are no breaks or boundaries corresponding to the "bands of color" which are seen subjectively by the eye. A black-and-white photograph of a rainbow shows no band stucture at all, demonstrating that the number of bands, and the bands themselves, are phenomena added to nature by the eye and the brain. They are not objectively real any more than "hot" or "cold."
The cone photoreceptors, which begin the process that results in the ultimate sensation of color in the brain, are sensitive to different portions of the visible spectrum
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...
. For humans, the visible spectrum ranges approximately from 380 to 740 nm, and there are normally three types of cones. The number of colors that can be distinguished by these cones is in principle unlimited and in practice extremely large.
The sensation of color is also heavily dependent on contrast with surroundings. For example, a 'red' piece of construction paper or red filing folder does not reflect pure monochromatic "red" light. Rather, it absorbs a larger fraction of other frequencies of visible light shining upon it than those in a group of frequencies that are perceived as red when they are viewed alone. However, although the red paper reflects more of light near the red end of the spectrum, it reflects some of all frequencies (not only red), and a room lit with only the reflection from a red piece of paper would soon appear to be normally lit to an adapted eye, and would allow visualization of all of the other colors present in the room. A red-colored paper is therefore perceived to be red mainly by contrast with its surroundings, because the human eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
can distinguish between different wavelengths, and the brain creates the separate sensation of "red" as a helper in discriminating one object from another, or from its background. Color, which in nature is never constructed from pure frequencies of light, is a quality constructed by the visual brain from spectral reflectance, the objective property of objects. The advantage of color perception is the better discrimination of surfaces allowed by this aspect of visual processing, which makes quite similar surfaces into very different perceptions.
In animals
The visible range and number of cone types differ between species. Mammals in general have color vision of a limited type, and are usually red-green color-blind, with only two types of cones. Humans, some primates, and some marsupials see an extended range of colors, but only by comparison with other mammals. Most non-mammalian vertebrate species distinguish different colors at least as well as humans, and many species of birds, fish, reptiles and amphibians, as well as some invertebrates, have more than three cone types and probably superior color vision to humans.In most Catarrhini
Catarrhini
Catarrhini is one of the two subdivisions of the higher primates . It contains the Old World monkeys and the apes, which in turn are further divided into the lesser apes or gibbons and the great apes, consisting of the orangutans, gorillas, chimpanzees, bonobos, and humans...
(Old World monkeys and apes — primates closely related to humans) there are three types of color receptors (known as cone cell
Cone cell
Cone cells, or cones, are photoreceptor cells in the retina of the eye that are responsible for color vision; they function best in relatively bright light, as opposed to rod cells that work better in dim light. If the retina is exposed to an intense visual stimulus, a negative afterimage will be...
s), resulting in trichromatic color vision. These primates, like humans, are known as trichromats. Many other primates and other mammals are dichromat
Dichromat
Dichromacy is the state of having two types of functioning color receptors, called cone cells, in the eyes. Organisms with dichromacy are called dichromats. Dichromats can match any color they see with a mixture of no more than two pure spectral lights...
s, which is the general color vision state for mammals that are active during the day (i.e., felines, canines, ungulates). Nocturnal mammals may have little or no color vision. Trichromat non-primate mammals are rare.
Many invertebrates
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
have color vision. Honey- and bumblebees have trichromatic color vision, which is insensitive to red but sensitive in ultraviolet. In view of the importance of colour vision to bees one might expect these receptor sensitivities to reflect their specific visual ecology; for example the types of flowers that they visit. However, the main groups of hymenopteran insects excluding ants (i.e. bees, wasps and sawflies) mostly have three types of photoreceptor, with spectral sensitivities similar the honeybee’s.Papilio
Papilio
Papilio is a genus in the swallowtail butterfly family, Papilionidae. The word papilio is Latin for butterfly.The genus includes a number of well-known North American species such as the Western Tiger Swallowtail...
butterflies possess six types of photoreceptors and may have pentachromatic vision. The most complex color vision system in animal kingdom has been found in stomatopods (such as the mantis shrimp
Mantis shrimp
Mantis shrimp or stomatopods are marine crustaceans, the members of the order Stomatopoda. They are neither shrimp nor mantids, but receive their name purely from the physical resemblance to both the terrestrial praying mantis and the shrimp. They may reach in length, although exceptional cases of...
) with up to 12 different spectral receptor types thought to work as multiple dichromatic units.
Vertebrate animals such as tropical fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
and birds sometimes have more complex color vision systems than humans; thus the many subtle colors they exhibit generally serve as direct signals between fish or between birds, and are not intended to signal mammals. In the birds, tetrachromacy is achieved through up to four cone
Cone cell
Cone cells, or cones, are photoreceptor cells in the retina of the eye that are responsible for color vision; they function best in relatively bright light, as opposed to rod cells that work better in dim light. If the retina is exposed to an intense visual stimulus, a negative afterimage will be...
types, depending on species.Each single cone contains one of the four main
types of vertebrate cone photopigment (LWS/ MWS, RH2, SWS2 and SWS1and has a coloured oil droplet in its inner segment. Brightly colored oil droplets inside the cones shift or narrow the spectral sensitivity of the cell. It has been suggested that it is likely that pigeons are pentachromat
Pentachromat
Pentachromacy describes the capability and capacity for capturing, transmitting, processing, and perceiving five independent channels of color information through the primary visual system. Organisms with pentachromacy are termed pentachromats...
s.
Reptiles and amphibians also have four cone types (occasionally five), and probably see at least the same number of colors that humans do, or perhaps more. In addition, some nocturnal gecko
Gecko
Geckos are lizards belonging to the infraorder Gekkota, found in warm climates throughout the world. They range from 1.6 cm to 60 cm....
s have the capability of seeing color in dim light.
In the evolution of mammals, segments of color vision were lost, then for a few species of primates, regained by gene duplication
Gene duplication
Gene duplication is any duplication of a region of DNA that contains a gene; it may occur as an error in homologous recombination, a retrotransposition event, or duplication of an entire chromosome.The second copy of the gene is often free from selective pressure — that is, mutations of it have no...
. Eutherian mammals other than primates (for example, dogs, cats, mammalian farm animals) generally have less-effective two-receptor (dichromat
Dichromat
Dichromacy is the state of having two types of functioning color receptors, called cone cells, in the eyes. Organisms with dichromacy are called dichromats. Dichromats can match any color they see with a mixture of no more than two pure spectral lights...
ic) color perception systems, which distinguish blue, green, and yellow—but cannot distinguish oranges and reds. The adaptation to see reds is particularly important for primate mammals, since it leads to identification of fruits, and also newly sprouting reddish leaves, which are particularly nutritious.
However, even among primates, full color vision differs between New World and Old World monkeys. Old World primates, including monkeys and all apes, have vision similar to humans. New World monkeys may or may not have color sensitivity at this level: in most species, males are dichromats, and about 60% of females are trichromats, but the owl monkeys are cone monochromats, and both sexes of howler monkey
Howler monkey
Howler monkeys are among the largest of the New World monkeys. Fifteen species are currently recognised. Previously classified in the family Cebidae, they are now placed in the family Atelidae. These monkeys are native to South and Central American forests...
s are trichromats. Visual sensitivity differences between males and females in a single species is due to the gene for yellow-green sensitive opsin
Opsin
Opsins are a group of light-sensitive 35–55 kDa membrane-bound G protein-coupled receptors of the retinylidene protein family found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical...
protein (which confers ability to differentiate red from green) residing on the X sex chromosome.
Several marsupial
Marsupial
Marsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...
s such as the fat-tailed dunnart
Fat-tailed Dunnart
The Fat Tailed Dunnart is a species of mouse-like marsupial of the Dasyuridae family, the family includes the Little Red Kaluta, quolls, and the Tasmanian Devil. It has an average body length of 60–90 mm with a tail of 45–70 mm. Ear length is 14–16 mm...
(Sminthopsis crassicaudata) have been shown to have trichromatic color vision.
Marine mammal
Marine mammal
Marine mammals, which include seals, whales, dolphins, and walruses, form a diverse group of 128 species that rely on the ocean for their existence. They do not represent a distinct biological grouping, but rather are unified by their reliance on the marine environment for feeding. The level of...
s, adapted for low-light vision, have only a single cone type and are thus monochromat
Monochromat
Monochromacy, also known as "total color blindness", is a complete inability to distinguish colors. This is distinguished from more common forms of color blindness, in which the affected individual can perceive color differences, but cannot make the same distinctions between colors as can an...
s.
Evolution
Color perception mechanisms are highly dependent on evolutionary factors, of which the most prominent is thought to be satisfactory recognition of food sources. In herbivorous primates, color perception is essential for finding proper (immature) leaves. In hummingbirdHummingbird
Hummingbirds are birds that comprise the family Trochilidae. They are among the smallest of birds, most species measuring in the 7.5–13 cm range. Indeed, the smallest extant bird species is a hummingbird, the 5-cm Bee Hummingbird. They can hover in mid-air by rapidly flapping their wings...
s, particular flower types are often recognized by color as well. On the other hand, nocturnal mammals have less-developed color vision, since adequate light is needed for cones to function properly. There is evidence that ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
light plays a part in color perception in many branches of the animal kingdom
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
, especially insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
s. In general, the optical spectrum encompasses the most common electronic transitions
Molecular electronic transition
Molecular electronic transitions take place when electrons in a molecule are excited from one energy level to a higher energy level. The energy change associated with this transition provides information on the structure of a molecule and determines many molecular properties such as color...
in matter and is therefore the most useful for collecting information about the environment.
The evolution of trichromatic color vision in primates
Evolution of color vision in primates
The evolution of color vision in primates is unique compared to most eutherian mammals. A remote vertebrate ancestor of primates possessed tetrachromacy, but nocturnal, warm-blooded, mammalian ancestors lost two of four cones in the retina at the time of dinosaurs...
occurred as the ancestors of modern monkeys, apes, and humans switched to diurnal
Diurnal animal
Diurnality is a plant or animal behavior characterized by activity during the day and sleeping at night.-In animals:Animals that are not diurnal might be nocturnal or crepuscular . Many animal species are diurnal, including many mammals, insects, reptiles and birds...
(daytime) activity and began consuming fruits and leaves from flowering plants. Color vision, with UV discrimination, is also present in a number of arthropods – the only terrestrial animals besides the vertebrates to possess this trait.
Some animals can distinguish colors in the ultraviolet spectrum. The UV spectrum falls outside the human visible range, except for some cataract surgery
Cataract surgery
Cataract surgery is the removal of the natural lens of the eye that has developed an opacification, which is referred to as a cataract. Metabolic changes of the crystalline lens fibers over time lead to the development of the cataract and loss of transparency, causing impairment or loss of vision...
patients. Birds, turtles, lizards, many fish and some rodents have UV receptors in their retinas. These animals can see the UV patterns found on flowers and other wildlife that are otherwise invisible to the human eye.
UV and multi-dimensional vision is an especially important adaptation in birds. It allows birds to spot small prey from a distance, navigate, avoid predators, and forage while flying at high speeds. Birds also utilize their broad spectrum vision to recognize other birds, and in sexual selection.
Mathematics of color perception
A "physical color" is a combination of pure spectral colorSpectral color
A spectral color is a color that is evoked by a single wavelength of light in the visible spectrum, or by a relatively narrow band of wavelengths...
s (in the visible range). Since there are, in principle, infinitely many distinct spectral colors, the set of all physical colors may be thought of as an infinite-dimensional vector space
Vector space
A vector space is a mathematical structure formed by a collection of vectors: objects that may be added together and multiplied by numbers, called scalars in this context. Scalars are often taken to be real numbers, but one may also consider vector spaces with scalar multiplication by complex...
, in fact a Hilbert space
Hilbert space
The mathematical concept of a Hilbert space, named after David Hilbert, generalizes the notion of Euclidean space. It extends the methods of vector algebra and calculus from the two-dimensional Euclidean plane and three-dimensional space to spaces with any finite or infinite number of dimensions...
. We call this space Hcolor. More technically, the space of physical colors may be considered to be the (mathematical) cone
Cone (topology)
In topology, especially algebraic topology, the cone CX of a topological space X is the quotient space:CX = /\,of the product of X with the unit interval I = [0, 1]....
over the simplex whose vertices are the spectral colors, with white at the centroid
Centroid
In geometry, the centroid, geometric center, or barycenter of a plane figure or two-dimensional shape X is the intersection of all straight lines that divide X into two parts of equal moment about the line. Informally, it is the "average" of all points of X...
of the simplex, black at the apex of the cone, and the monochromatic color associated with any given vertex somewhere along the line from that vertex to the apex depending on its brightness.
An element C of Hcolor is a function from the range of visible wavelengths—considered as an interval of real numbers [Wmin,Wmax]—to the real numbers, assigning to each wavelength w in [Wmin,Wmax] its intensity C(w).
A humanly perceived color may be modeled as three numbers: the extents to which each of the 3 types of cones is stimulated. Thus a humanly perceived color may be thought of as a point in 3-dimensional Euclidean space
Euclidean space
In mathematics, Euclidean space is the Euclidean plane and three-dimensional space of Euclidean geometry, as well as the generalizations of these notions to higher dimensions...
. We call this space R3color.
Since each wavelength w stimulates each of the 3 types of cone cells to a known extent, these extents may be represented by 3 functions s(w), m(w), l(w) corresponding to the response of the S, M, and L cone cells, respectively.
Finally, since a beam of light can be composed of many different wavelengths, to determine the extent to which a physical color C in Hcolor stimulates each cone cell, we must calculate the integral (with respect to w), over the interval [Wmin,Wmax], of C(w)·s(w), of C(w)·m(w), and of C(w)·l(w). The triple of resulting numbers associates to each physical color C (which is an element in Hcolor) to a particular perceived color (which is a single point in R3color). This association is easily seen to be linear. It may also easily be seen that many different elements in the "physical" space Hcolor can all result in the same single perceived color in R3color, so a perceived color is not unique to one physical color.
Thus human color perception is determined by a specific, non-unique linear mapping from the infinite-dimensional Hilbert space Hcolor to the 3-dimensional Euclidean space R3color.
Technically, the image of the (mathematical) cone over the simplex whose vertices are the spectral colors, by this linear mapping, is also a (mathematical) cone in R3color. Moving directly away from the vertex of this cone represents maintaining the same chromaticity
Chromaticity
Chromaticity is an objective specification of the quality of a color regardless of its luminance, that is, as determined by its hue and colorfulness ....
while increasing its intensity. Taking a cross-section of this cone yields a 2D chromaticity space. Both the 3D cone and its projection or cross-section are convex sets; that is, any mixture of spectral colors is also a color.
In practice, it would be quite difficult to physiologically measure an individual's three cone responses to various physical color stimuli. Instead, a psychophysical approach is taken. Three specific benchmark test lights are typically used; let us call them S, M, and L. To calibrate human perceptual space, scientists allowed human subjects to try to match any physical color by turning dials to create specific combinations of intensities (IS, IM, IL) for the S, M, and L lights, resp., until a match was found. This needed only to be done for physical colors that are spectral (since a linear combination of spectral colors will be matched by the same linear combination of their (IS, IM, IL) matches. Note that in practice, often at least one of S, M, L would have to be added with some intensity to the physical test color, and that combination matched by a linear combination of the remaining 2 lights. Across different individuals (without color blindness), the matchings turned out to be nearly identical.
By considering all the resulting combinations of intensities (IS, IM, IL) as a subset of 3-space, a model for human perceptual color space is formed. (Note that when one of S, M, L had to be added to the test color, its intensity was counted as negative.) Again, this turns out to be a (mathematical) cone, not a quadric, but rather all rays through the origin in 3-space passing through a certain convex set. Again, this cone has the property that moving directly away from the origin corresponds to increasing the intensity of the S, M, L lights proportionately. Again, a cross-section of this cone is a planar shape that is (by definition) the space of "chromaticities" (informally: distinct colors); one particular such cross section, corresponding to constant X+Y+Z of the CIE 1931 color space
CIE 1931 color space
In the study of color perception, one of the first mathematically defined color spaces is the CIE 1931 XYZ color space, created by the International Commission on Illumination in 1931....
, gives the CIE chromaticity diagram.
It should be noted that this system implies that for any hue or non-spectral color not on the boundary of the chromaticity diagram, there are infinitely many distinct physical spectra that are all perceived as that hue or color. So, in general there is no such thing as the combination of spectral colors that we perceive as (say) a specific version of tan; instead there are infinitely many possibilities that produce that exact color. The boundary colors that are pure spectral colors can be perceived only in response to light that is purely at the associated wavelength, while the boundary colors on the "line of purples" can each only be generated by a specific ratio of the pure violet and the pure red at the ends of the visible spectral colors.
The CIE chromaticity diagram is horseshoe-shaped, with its curved edge corresponding to all spectral colors (the spectral locus
Locus (mathematics)
In geometry, a locus is a collection of points which share a property. For example a circle may be defined as the locus of points in a plane at a fixed distance from a given point....
), and the remaining straight edge corresponding to the most saturated purple
Purple
Purple is a range of hues of color occurring between red and blue, and is classified as a secondary color as the colors are required to create the shade....
s, mixtures of red
Red
Red is any of a number of similar colors evoked by light consisting predominantly of the longest wavelengths of light discernible by the human eye, in the wavelength range of roughly 630–740 nm. Longer wavelengths than this are called infrared , and cannot be seen by the naked eye...
and violet
Violet (color)
As the name of a color, violet is synonymous with a bluish purple, when the word "purple" is used in the common English language sense of any color between blue and red, not including either blue or red...
.
Chromatic adaptation
In color science, chromatic adaptation is the estimation of the representation of an object under a different light source than the one in which it was recorded. A common application is to find a chromatic adaptation transform (CAT) that will make the recording of a neutral object appear neutral (color balanceColor balance
In photography and image processing, color balance is the global adjustment of the intensities of the colors . An important goal of this adjustment is to render specific colors – particularly neutral colors – correctly; hence, the general method is sometimes called gray balance, neutral balance,...
), while keeping other colors also looking realistic. For example, chromatic adaptation transforms are used when converting images between ICC profile
ICC profile
In color management, an ICC profile is a set of data that characterizes a color input or output device, or a color space, according to standards promulgated by the International Color Consortium...
s with different white point
White point
A white point is a set of tristimulus values or chromaticity coordinates that serve to define the color "white" in image capture, encoding, or reproduction. Depending on the application, different definitions of white are needed to give acceptable results...
s. Adobe Photoshop
Adobe Photoshop
Adobe Photoshop is a graphics editing program developed and published by Adobe Systems Incorporated.Adobe's 2003 "Creative Suite" rebranding led to Adobe Photoshop 8's renaming to Adobe Photoshop CS. Thus, Adobe Photoshop CS5 is the 12th major release of Adobe Photoshop...
, for example, uses the Bradford CAT.
In color vision, chromatic adaptation refers to color constancy
Color constancy
Color constancy is an example of subjective constancy and a feature of the human color perception system which ensures that the perceived color of objects remains relatively constant under varying illumination conditions. A green apple for instance looks green to us at midday, when the main...
; the ability of the visual system to preserve the appearance of an object under a wide range of light sources.
See also
- Color blindnessColor blindnessColor blindness or color vision deficiency is the inability or decreased ability to see color, or perceive color differences, under lighting conditions when color vision is not normally impaired...
- Color theoryColor theoryIn the visual arts, color theory is a body of practical guidance to color mixing and the visual impacts of specific color combinations. Although color theory principles first appeared in the writings of Leone Battista Alberti and the notebooks of Leonardo da Vinci , a tradition of "colory theory"...
- Inverted spectrumInverted spectrumInverted spectrum is the apparent possibility of two people sharing their color vocabulary and discriminations, although the colours one sees — their qualia — are systematically different from the colours the other person sees....
- Primary colorPrimary colorPrimary colors are sets of colors that can be combined to make a useful range of colors. For human applications, three primary colors are usually used, since human color vision is trichromatic....
- Visual perceptionVisual perceptionVisual perception is the ability to interpret information and surroundings from the effects of visible light reaching the eye. The resulting perception is also known as eyesight, sight, or vision...
External links
- Peter Gouras, "Color Vision", Webvision, University of Utah School of Medicine, May 2009.
- Kenneth R. Koehler, "Spectral Sensitivity of the Eye", College Physics for Students of Biology and Chemistry, University of Cincinnati Raymond Walters College, 1996.
- James T. Fulton, "The Human is a Blocked Tetrachromat", Neural Concepts, July 2009.
- Vurdlak, "Mega Color Blindness Test", Mighty Optical Illusions, March 2009.
- Clive Maxfield and Alvin Brown, "Color Vision: One of Nature's Wonders", DIYCalculator.com, 2006.
- Chenguang Lu, "The decoding model: a symmetrical model of color vision".
- Egopont, "Color Vision Test".