Colour
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Colour or color is the visual perceptual property corresponding in humans to the categories called red, yellow, blue, black, etc. Colour derives from the spectrum of light (distribution of light energy versus wavelength) interacting in the eye with the spectral sensitivities of the light receptors. Colour categories and physical specifications of colour are also associated with objects, materials, light sources, etc., based on their physical properties such as light absorption, reflection, or emission spectra.
The science of colour is sometimes called chromatics.
Contents |
Physics of colour
| color | wavelength interval | frequency interval |
|---|---|---|
| red | ~ 630–700 nm | ~ 480–430 THz |
| orange | ~ 590–630 nm | ~ 510–480 THz |
| yellow | ~ 560–590 nm | ~ 540–510 THz |
| green | ~ 490–560 nm | ~ 610–540 THz |
| blue | ~ 450–490 nm | ~ 670–610 THz |
| violet | ~ 400–450 nm | ~ 750–670 THz |
| Color | <math>\lambda \,\!</math>/nm | <math>\nu \,\!</math>/1014 Hz | <math>\nu_b \,\!</math>/104 cm−1 | <math>E \,\!</math>/eV | <math>E \,\!</math>/kJ mol−1 |
|---|---|---|---|---|---|
| Infrared | >1000 | <3.00 | <1.00 | <1.24 | <120 |
| Red | 700 | 4.28 | 1.43 | 1.77 | 171 |
| Orange | 620 | 4.84 | 1.61 | 2.00 | 193 |
| Yellow | 580 | 5.17 | 1.72 | 2.14 | 206 |
| Green | 530 | 5.66 | 1.89 | 2.34 | 226 |
| Blue | 470 | 6.38 | 2.13 | 2.64 | 254 |
| Violet | 420 | 7.14 | 2.38 | 2.95 | 285 |
| Near ultraviolet | 300 | 10.0 | 3.33 | 4.15 | 400 |
| Far ultraviolet | <200 | >15.0 | >5.00 | >6.20 | >598 |
Electromagnetic radiation is characterized by its wavelength (or frequency) and its intensity. When the wavelength is within the visible spectrum (the range of wavelengths humans can perceive, approximately from 380 nm to 740 nm), it is known as "visible light".
Spectral colors
The familiar colours of the rainbow in the spectrum include all those colours that can be produced by visible light of a single wavelength only, the pure spectral or monochromatic colors. The table at right shows approximate frequencies (in terahertz) and wavelengths (in nanometers) for various pure spectral colors. The wavelengths are measured in vacuum (see refraction).
The colour table should not be interpreted as a definitive list – the pure spectral colours form a continuous spectrum, and how it is divided into distinct colours is a matter of culture, taste, and language. A common list identifies six main bands: red, orange, yellow, green, blue, and violet. Newton's conception included a seventh color, indigo, between blue and violet – but most people do not distinguish it, and most colour scientists do not recognize it as a separate colour; it is sometimes designated as wavelengths of 420–440 nm.
The intensity of a spectral colour may alter its perception considerably; for example, a low-intensity orange-yellow is brown, and a low-intensity yellow-green is olive-green.
Colour of objects
The colour of an object depends on both the physics of the object in its environment and the characteristics of the perceiving eye and brain. Physically, objects can be said to have the colour of the light leaving their surfaces, which normally depends on the spectrum of that light and of the incident illumination, as well as potentially on the angles of illumination and viewing. Some objects not only reflect light, but also transmit light or emit light themselves (see below), which contribute to the colour also. And a viewer's perception of the object's colour depends not only on the spectrum of the light leaving its surface, but also on a host of contextual cues, so that the colour tends to be perceived as relatively constant: that is, relatively independent of the lighting spectrum, viewing angle, etc. This effect is known as colour constancy.
Some generalizations of the physics can be drawn, neglecting perceptual effects for now:
- Light arriving at an opaque surface is either reflected "specularly" (that is, in the manner of a mirror), scattered (that is, reflected with diffuse scattering), or absorbed – or some combination of these.
- Opaque objects that do not reflect specularly (which tend to have rough surfaces) have their colour determined by which wavelengths of light they scatter more and which they scatter less (with the light that is not scattered being absorbed). If objects scatter all wavelengths, they appear white. If they absorb all wavelengths, they appear black.
- Opaque objects that specularly reflect light of different wavelengths with different efficiencies look like mirrors tinted with colors determined by those differences. An object that reflects some fraction of impinging light and absorbs the rest may look black but also be faintly reflective; examples are black objects coated with layers of enamel or lacquer.
- Objects that transmit light are either translucent (scattering the transmitted light) or transparent (not scattering the transmitted light). If they also absorb (or reflect) light of varying wavelengths differentially, they appear tinted with a colour determined by the nature of that absorption (or that reflectance).
- Objects may emit light that they generate themselves, rather than merely reflecting or transmitting light. They may do so because of their elevated temperature (they are then said to be incandescent), as a result of certain chemical reactions (a phenomenon called chemoluminescence), or for other reasons (see the articles Phosphorescence and List of light sources).
- Objects may absorb light and then as a consequence emit light that has different properties. They are then called fluorescent (if light is emitted only while light is absorbed) or phosphorescent (if light is emitted even after light ceases to be absorbed; this term is also sometimes loosely applied to light emitted due to chemical reactions).
To summarize, the color of an object is a complex result of its surface properties, its transmission properties, and its emission properties, all of which factors contribute to the mix of wavelengths in the light leaving the surface of the object. The perceived color is then further conditioned by the nature of the ambient illumination, and by the color properties of other objects nearby, via the effect known as color constancy and via other characteristics of the perceiving eye and brain.
Colour in the eye
Light, no matter how complex its composition of wavelengths, is reduced to three color components by the eye. For each location in the visual field, the three types of cones yield three signals based on the extent to which each is stimulated. These values are sometimes called tristimulus values.
The response curve as a function of wavelength for each type of cone is illustrated above. Because the curves overlap, some tristimulus values do not occur for any incoming light combination. For example, it is not possible to stimulate only the mid-wavelength/"green" cones; the other cones will inevitably be stimulated to some degree at the same time. The set of all possible tristimulus values determines the human color space. It has been estimated that humans can distinguish roughly 10 million different colors.[2]
The other type of light-sensitive cell in the eye, the rod, has a different response curve. In normal situations, when light is bright enough to strongly stimulate the cones, rods play virtually no role in vision at all.[3] On the other hand, in dim light, the cones are understimulated leaving only the signal from the rods, resulting in a colorless response. (Furthermore, the rods are barely sensitive to light in the "red" range.) In certain conditions of intermediate illumination, the rod response and a weak cone response can together result in color discriminations not accounted for by cone responses alone.
Colour in the brain
While the mechanisms of color vision at the level of the retina are well-described in terms of tristimulus values (see above), color processing after that point is organized differently. A dominant theory of color vision proposes that color information is transmitted out of the eye by three opponent processes, or opponent channels, each constructed from the raw output of the cones: a red-green channel, a blue-yellow channel and a black-white "luminance" channel. This theory has been supported by neurobiology, and accounts for the structure of our subjective color experience. Specifically, it explains why we cannot perceive a "reddish green" or "yellowish blue," and it predicts the color wheel: it is the collection of colors for which at least one of the two color channels measures a value at one of its extremes.
Nonstandard colour perception
Colour deficiency
If one or more types of a person's color-sensing cones are missing or less responsive than normal to incoming light, that person can distinguish fewer colors and is said to be color deficient or color blind (though this latter term can be misleading; almost all color deficient individuals can distinguish at least some colors).
Synesthesia
In certain forms of synesthesia, perceiving letters and numbers (grapheme–color synesthesia) or hearing musical sounds (music–color synesthesia) will lead to the unusual additional experiences of seeing colors. Behavioral and functional neuroimaging experiments have demonstrated that these color experiences lead to changes in behavioral tasks and lead to increased activation of brain regions involved in color perception, thus demonstrating their reality, and similarity to real color percepts, albeit evoked through a non-standard route.
Afterimages
After exposure to strong light in their sensitivity range, photoreceptors of a given type become desensitized. For a few seconds after the light ceases, they will continue to signal less strongly than they otherwise would. Colors observed during that period will appear to lack the color component detected by the desensitized photoreceptors. This effect is responsible for the phenomenon of afterimages, in which the eye may continue to see a bright figure after looking away from it, but in a complementary color.
Afterimage effects have also been utilized by artists, including Vincent van Gogh.
Colour constancy
There is an interesting phenomenon which occurs when an artist uses a limited colour palette: the eye tends to compensate by seeing any grey or neutral colour as the colour which is missing from the colour wheel. E.g., in a limited palette consisting of red, yellow, black and white, a mixture of yellow and black will appear as a variety of green, a mixture of red and black will appear as a variety of purple, and pure grey will appear bluish.
Colour naming
All languages that have two "basic" color names distinguish dark/cool colors from bright/warm colors. The next colors to be distinguished are usually red and then blue or green. All languages with six "basic" colors include black, white, red, green, blue and yellow. The pattern holds up to a set of twelve: black, grey, white, pink, red, orange, yellow, green, blue, purple, brown, and azure (distinct from blue in Russian and Italian but not English).
Associations
Individual colours have a variety of cultural associations such as national colors (in general described in individual color articles and color symbolism). The field of color psychology attempts to identify the effects of colour on human emotion and activity.
Health effects
When the colour spectrum of artificial lighting is mismatched to that of sunlight, material health effects may arise including increased incidence of headache. This phenomenon is often coupled with adverse effects of over-illumination, since many of the same interior spaces that have color mismatch also have higher light intensity than desirable for the task being conducted in that space.
Measurement and reproduction of colour
Relation to spectral colours
Most light sources are mixtures of various wavelengths of light. However, many such sources can still have a spectral colour insofar as the eye cannot distinguish them from monochromatic sources. For example, most computer displays reproduce the spectral colour orange as a combination of red and green light; it appears orange because the red and green are mixed in the right proportions to allow the eye's red and green cones to respond the way they do to orange.
Two different light spectra which have the same effect on the three colour receptors in the human eye will be perceived as the same colour. This is exemplified by the white light that is emitted by fluorescent lamps, which typically has a spectrum consisting of a few narrow bands, while daylight has a continuous spectrum. The human eye cannot tell the difference between such light spectra just by looking into the light source, although reflected colors from objects can look different. (This is often exploited e.g. to make fruit or tomatoes look more brightly red in shops.)
Most human colour perceptions can be generated by a mixture of three colours called primaries. This is used to reproduce colour scenes in photography, printing, television and other media. There are a number of methods or color spaces for specifying a color in terms of three particular primary colours. Each method has its advantages and disadvantages depending on the particular application.
No mixture of colours, though, can produce a fully pure colour perceived as completely identical to a spectral colour, although one can get very close for the longer wavelengths, where the chromaticity diagram above has a nearly straight edge. For example, mixing green light (530 nm) and blue light (460 nm) produces cyan light that is slightly desaturated, because response of the red colour receptor would be greater to the green and blue light in the mixture than it would be to a pure cyan light at 485 nm that has the same intensity as the mixture of blue and green.
Because of this, and because the primaries in colour printing systems generally are not pure themselves, the colours reproduced are never perfectly saturated colours, and so spectral colors cannot be matched exactly. However, natural scenes rarely contain fully saturated colors, thus such scenes can usually be approximated well by these systems. The range of colors that can be reproduced with a given color reproduction system is called the gamut. The CIE chromaticity diagram can be used to describe the gamut.
Another problem with colour reproduction systems is connected with the acquisition devices, like cameras or scanners. The characteristics of the colour sensors in the devices are often very far from the characteristics of the receptors in the human eye. In effect, acquisition of colours that have some special, often very "jagged," spectra caused for example by unusual lighting of the photographed scene can be relatively poor.
Pigments and reflective media
Pigments are chemicals that selectively absorb and reflect different spectra of light. When a surface is painted with a pigment, light hitting the surface is reflected, minus some wavelengths. This subtraction of wavelengths produces the appearance of different colours. Most paints are a blend of several chemical pigments, intended to produce a reflection of a given colour.
Pigment manufacturers assume the source light will be white, or of roughly equal intensity across the spectrum. If the light is not a pure white source (as in the case of nearly all forms of artificial lighting), the resulting spectrum will appear a slightly different color. Red paint, viewed under blue light, may appear black. Red paint is red because it reflects only the red components of the spectrum. Blue light, containing none of these, will create no reflection from red paint, creating the appearance of black.
Structural colour
Structural colours are colours caused by interference effects rather than by pigments. Colour effects are produced when a material is scored with fine parallel lines, formed of a thin layer or of two or more parallel thin layers, or otherwise composed of microstructures on the scale of the colour's wavelength. If the microstructures are spaced randomly, light of shorter wavelengths will be scattered preferentially to produce Tyndall effect colours: the blue of the sky, the luster of opals, and the blue of human irises. If the microstructures are aligned in arrays, for example the array of pits in a CD, they behave as a diffraction grating: the grating reflects different wavelengths in different directions due to interference phenomena, separating mixed "white" light into light of different wavelengths. If the structure is one or more thin layers then it will reflect some wavelengths and transmit others, depending on the layers' thickness.
Structural colour is responsible for the blues and greens of the feathers of many birds (the blue jay, for example), as well as certain butterfly wings and beetle shells. Variations in the pattern's spacing often give rise to an iridescent effect, as seen in peacock feathers, soap bubbles, films of oil, and mother of pearl, because the reflected color depends upon the viewing angle. Peter Vukusic has carried out research in butterfly wings and beetle shells using electron micrography, and has since helped develop a range of "photonic" cosmetics using structural color.[4]
Structural colour is studied in the field of thin-film optics. A layman's term that describes particularly the most ordered or the most changeable structural colors is iridescence.
Additional terms
- Hue: the color's direction from white, for example in a color wheel or chromaticity diagram.
- Colorfulness, chroma, or saturation: how "intense" or "concentrated" a color is; also known as chroma or purity.
- Value, brightness, or lightness: how light or dark a color is.
- Tint: a color made lighter by adding white.
- Shade: a color made darker by adding black.
External links and sources
- Why Should Engineers and Scientists Be Worried About Color?
- Color, Contrast & Dimension in News Design
References
- ↑ Craig F. Bohren (2006) Fundamentals of Atmospheric Radiation: An Introduction with 400 Problems, Wiley-VCH (ISBN: 3527405038)
- ↑ Judd, Deane B. & Wyszecki, Günter (1975) Color in Business, Science and Industry (third edition), Wiley-Interscience, New York (ISBN: 0471452122)
- ↑ Hirakawa, K. & Parks, T.W. (2005) Chromatic Adaptation and White-Balance Problem, IEEE ICIP
- ↑ ESRC Society Today - Science in the Dock, Art in the Stocks: http://www.esrc.ac.uk/ESRCInfoCentre/about/CI/events/FSS/2006/science.aspx?ComponentId=14867&SourcePageId=14865, Accessed: 2007-10-07
