Color

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Color (British spelling: colour) is the sensation caused by electromagnetic radiation as it interacts with the eye. The term color is also used for the property of objects that gives rise to these sensations.

Physics

Electromagnetic radiation (more commonly called "light") is a mixture of radiation of different wavelengths and intensities. The light's spectrum records each wavelength's intensity. The spectrum of the incoming radiation is the physical variable underlying the color experience. 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 in humans.

A white surface reflects all wavelengths equally, while a black surface absorbs all wavelengths and does not 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 chromatic colors:

red~ 625-740 nm
orange~ 590-625 nm
yellow~ 565-590 nm
green~ 520-565 nm
cyan~ 500-520 nm
blue~ 450-500 nm
indigo~ 430-450 nm
violet~ 380-430 nm

(The wavelengths are representative and are given in nanometers (nm). A list of other objects of similar size is available.)

There are many colors which are not pure spectral colors, for instance brown or pink, or are iridescent or fluorescent.

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.

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 somewhat. The incoming signal spectrum is thus reduced by the eye to three values: the human color space is three-dimensional. If one or more types of a person's color-sensing cones isn't responding correctly to incoming light, that person is said to be color blind, and has a smaller color space. Other animals may have more than three different color receptors (some birds and reptiles) or fewer (most mammals).

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 stimulatedto 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 resulting region has roughly a cone-like shape, and each point in the region corresponds to a color. Black sits at the tip of the color cone, located at the origin. As one moves up the cone, the colors get brighter, with white being located near the middle of the upper face of the color cone. The purest, most intense colors can be found along the rim of the upper face.

Most human color perceptions can be generated by a mixture of red, green and blue light alone; these are called the primary colors of additive light mixing. The red light is chosen because it will only stimulate "red" cones, the blue light will only stimulate "blue" cones, and the green light will stimulate all three, but mostly the "green" cones. In the color space mentioned above, red colors of increasing brightness form a ray, starting at the origin and ending at the upper face of the color cone. The same is true for green and blue colors. These three rays define a pyramid whose interior consists of all those colors which can be generated by a suitable mixture of the three primaries. The pyramid exhausts almost the entire color space, which 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 color space can be captured. The description of a color in terms of these primaries defines what is called the RGB color space; unfortunately there is no exact concensus 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.

When producing a color print or painting a surface, the applied paint changes the surface in such a way that, when 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, yellow and magenta paints on a white surface (primary colors of subtractive light mixture). The cyan paint will reflect all but the red light, the yellow paint will reflect all but the blue light and the magenta paint 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. To describe colors by subtractive mixture, the CMYK (Cyan, Magenta, Yellow, blacK) color space is used.

The RGB and CMYK color spaces are most useful for technical reproduction of colors. A color space that more closely models the human experience is the HSV color space which arranges colors in a three dimensional cone. If the pure spectral colors are extended by mixtures of red and blue, they can be arranged in a circle (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 luminosity (height inside the cone).

The color cyan exemplifies what was said earlier: the same color experience can be generated by different light spectra. Cyan is a pure spectral color, located just between the responsitivity peaks of the "green" and "blue" cones, and can thus also be generated by an equal mixture of the spectral colors green and blue. The human eye (as opposed to the bird's eye or the spectroscopist) can't tell the difference.

Note that the colour 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".

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.

References:


Color or color charge, in quantum chromodynamics, refers figuratively to a certain property of quarks. It can attain the three values "red", "green" and "blue". Quarks of different colors are attracted and quarks of like color are repelled by the strong nuclear force. Color charge is not related to electromagnetic radiation in any way.

Color, in journalism, refers to vivid, but peripheral commentary on an event, especially in broadcast sports.

Color, in law, refers to certain prima facie rights.

Color, in a deck of playing cards, refers to the four suits hearts, spades, diamonds and clubs.

Color, in politics, is associated with different parties or ideological factions. The classifications vary in different parts of the world, but red is generally associated with socialism or communism, blue with conservative parties and green with environmentalists.


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