Colour

This page is about the concept of colour. For the pharmaceutical company, see Colour (pharmaceutical company). For other pages titled "colour", see Colour (disambiguation).
Colour is the visual perceptual property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Colour categories and physical specifications of colour are associated with objects or materials based on their physical properties such as light absorption, reflection, or emission spectra. By defining a colour space colours can be identified numerically by their coordinates.
Because perception of colour stems from the varying spectral sensitivity of different types of cone cells in the retina to different parts of the spectrum, colours may be defined and quantified by the degree to which they stimulate these cells. These physical or physiological quantifications of colour, however, do not fully explain the psychophysical perception of colour appearance.
The science of colour is sometimes called chromatics, colourimetry, or simply colour science. It includes the perception of colour by the human eye and brain, the origin of colour in materials, colour theory in art, and the physics of electromagnetic radiation in the visible range (that is, what is commonly referred to simply as light).
== Physics of colour ==
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 390 nm to 700 nm), it is known as "visible light".
Most light sources emit light at many different wavelengths; a source's spectrum is a distribution giving its intensity at each wavelength. Although the spectrum of light arriving at the eye from a given direction determines the colour sensation in that direction, there are many more possible spectral combinations than colour sensations. In fact, one may formally define a colour as a class of spectra that give rise to the same colour sensation, although such classes would vary widely among different species, and to a lesser extent among individuals within the same species. In each such class the members are called metamers of the colour in question. This effect can be visualized by comparing the light sources' spectral power distributions and the resulting colours.
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