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Color

Other meanings: Color (disambiguation).

Color (American English) or colour (British English) is the sensation caused by light as it interacts with the eye. The term color is also used for the property of objects that gives rise to these sensations.

Table of contents

1 The Physics of Color

2 Color Vision

3 Reproduction of color

3.1 Transmissive media
3.2 Reflective media

The Physics of Color

Electromagnetic radiation is a mixture of radiation of different wavelengths and intensities. When this radiation has a wavelength inside the human visibility range (approximately from 380 nm to 740 nm), that radiation is called light. The light's spectrum records each wavelength's intensity. The full spectrum of the incoming radiation from an object determines the visual appearance of that object, including its perceived color. As we will see, there are many more spectra than color sensations; in fact one may formally define a color to be the class of all those spectra which give rise to the same color sensation.

A surface that diffusely reflects all wavelengths equally is perceived as white, while a dull black surface absorbs all wavelengths and does not reflect (for mirror reflection this is different: a proper mirror also reflects all wavelengths equally, but is not perceived as white, while shiny black objects do reflect),

The familiar rainbow spectrum--named from the Latin word for image by Isaac Newton in 1666--contains all those colors that consist of visible light of a single wavelength only, the pure spectral or monochromatic colors:

colour	wavelength interval	frequency interval
red	~ 625-740 nm	~ 480-405 THz
orange	~ 590-625 nm	~ 510-480 THz
yellow	~ 565-590 nm	~ 530-510 THz
green	~ 520-565 nm	~ 580-530 THz
cyan	~ 500-520 nm	~ 600-580 THz
blue	~ 450-500 nm	~ 670-600 THz
indigo	~ 430-450 nm	~ 700-670 THz
violet	~ 380-430 nm	~ 790-700 THz

(The frequencies are approximations and given in tera hertz (THz). The wavelengths, valid in vacuum, are given in nanometers (nm). A list of other objects of similar size is available. )

The table above should not be interpreted as a definite list--the pure spectral colors form a continuous spectrum, and how it is divided into distinct colors is a matter of taste and culture. Similarly, the intensity of a spectral color may alter its perception considerably; for example, a low-intensity orange-yellow is brown, and a low-intensity yellow-green is olive-green.

Most colors are not pure spectral; these are created from mixtures of various wavelengths and intensities of light. Examples of non-spectral colors are the achromatic colors (black, gray and white), pastel colors[?], pink and magenta.

Color Vision

Although Aristotle and other ancient scientists speculated on the nature of light and color vision, it was not until Newton that light was correctly identified as the source of the color sensation. Goethe studied the theory of colors, and in 1801 Thomas Young proposed his trichromatic theory which was later refined by Hermann von Helmholtz. That theory was confirmed in the 1960s and will be described below.

The human eye contains three different types of color receptor cells, or cones. The first ("red") are most responsive to wavelengths around 565 nm, the second ("green") to those around 535 nm, and the third ("blue") to those around 445 nm. The sensitivity curves of the cones are roughly bell-shaped and overlap considerably. The incoming signal spectrum is thus reduced by the eye to three values, representing the intensity of the response of each of these types of color receptors.

Because of the overlap between the sensitivity ranges, not all combinations of stimuli are actually possible. For instance, it is not possible to only stimulate the "green" cone: at least one of the other cones will always be stimulated to some degree at the same time. The set of all combinations of stimuli that are possible make up the human color space.

One can picture this space as a region in three-dimensional Euclidean space if one identifies the X variable with the "red" stimulus, Y with "green" and Z with "blue". The origin (X,Y,Z) = (0,0,0) corresponds to black, and the point (X,Y,Z) = (1,1,1), i.e. full response of all three receptors, corresponds to white. The human color space is a region with these two points as corners, somewhat shaped like a pointy ellipsoid. The greys are located along a straight line connecting the two corners. The most saturated colors are located at the outer rim of the region, with brighter colors farther removed from the origin.

It has been estimated that humans can distinguish roughly 10 million different colors, although the identification of a specific color is highly subjective, since even the eyes of a single individual perceive colors slightly differently. If one or more types of a person's color-sensing cones isn't responding correctly to incoming light, that person has a smaller color space and is said to be color blind. Other animals may have more than three different color receptors (some birds and reptiles) or fewer (most mammals).

Reproduction of color

Two different light spectra which have the same effect on the three color receptors will be perceived as the same color. This is exemplified by the color cyan: cyan is a pure spectral color whose wavelength is located just between the responsitivity peaks of the "green" and "blue" cones. A cyan color experience can thus also be generated by an equal mixture of those two peak wavelengths, as long as these don't stimulate the red receptor. The human eye (as opposed to the bird's eye or the spectroscopist) then won't be able to tell the difference between pure spectral cyan and green-blue mixed cyan.

In the same way, most human color perceptions can be generated by a mixture of three colors called primaries. This is used to reproduce color scenes in photogaphy, printing, television, and other media.

Transmissive media

Media that transmit light (such as television) use additive color mixing with primary colors of red, green, and blue which are close to the wavelengths that generate peak responses of the eye's color receptors. This is called "RGB" color space. Mixtures of light of these primary colors cover a large part of the human color space and thus produce a large part of human color experiences. This is why color television sets or color computer monitors need only produce mixtures of red, green and blue light.

Other primary colors could in principle be used, but with red, green and blue the largest portion of the human color space can be captured. Unfortunately there is no exact consensus as to what frequency the red, green, and blue lights should be, so the same RGB values can give rise to slightly different colors on different screens.

Note that the color experience of a given light mixture may vary with absolute luminosity, due to the fact that both rods and cones are active at once in the eye, with each having different color curves, and rods taking over gradually from cones as the brightness of the scene is reduced. This effect leads to a change in color rendition with absolute illumination levels that can be summarised in the "Kruithof curve".

Reflective media

When producing a color print or painting a surface, the applied paint changes the surface; if the surface is then illuminated with white light (which consists of equal intensities of all visible wavelengths), the reflected light will have a spectrum corresponding to the desired color.

It is possible to achieve a large range of colors seen by humans by combining cyan, magenta, and yellow transparent dyes/inks on a white substrate. These are the subtractive primary colors. Often a fourth black is added to improve reproduction of some dark colors. This is called "CMY" or "CMYK" color space.

The cyan ink will reflect all but the red light, the yellow ink will reflect all but the blue light and the magenta ink will reflect all but the green light. This is because cyan light is an equal mixture of green and blue, yellow is an equal mixture of red and green, and magenta light is an equal mixture of red and blue.

HSV color space

The RGB and CMYK color spaces are most useful for technical reproduction of color scenes. A color space that more closely models the human experience is the HSV color space which arranges colors in a three-dimensional cone, somewhat similar to the human color space discussed above. The tip of the cone corresponds to black. If the pure spectral colors are extended by mixtures of red and blue, they can be arranged in a circle or "color wheel" (which was already known to Newton), the mouth of the cone. The position of a color on this circle is its hue. In the HSV space, every color is specified by its hue, saturation (distance from the circle's center) and value (luminosity).

The HSV color space was already used by 19th century physiologist Ewald Hering.

Color constancy

The trichromatric theory discussed above is strictly true only if the whole scene seen by the eye is of one and the same color, which of course is unrealistic. In reality, the brain compares the various colors in a scene, in order to eliminate the effects of the illumination. If a scene is illuminated with one light, and then with another, its colors will nevertheless appear constant to us. This was discovered by Edwin Land in the 1970s and lead to his retinex theory of color constancy.

Structural color

Structural colour is a property of some surfaces that are scored with fine parallel lines or formed of many thin parallel layers to make a diffraction grating. The grating absorbs some wavelengths more than others, causing white light to be reflected as colored light. Variations in the pattern's spacing often give rise to an iridescent effect, as seen in peacock feathers, films of oil, and mother of pearl.

Associations

Different colors are often associated with different emotional states, values or groups. These associations can vary among cultures and will be explained on the pages describing the individual colors.