Color plays a vitally important role in the world in which we live. Color can sway thinking, change actions, and cause reactions. It can irritate or soothe your eyes, raise your blood pressure or suppress your appetite.

When used in the right ways, color can save on energy consumption. When used in the wrong ways, color can contribute to global pollution.

As a powerful form of communication, color is irreplaceable. Red means "stop" and green means "go." Traffic lights send this universal message. Likewise, the colors used for a product, web site, logo, or most importantly, artwork, cause powerful reactions.

Color is the visual perceptual property corresponding in humans to the categories called red, yellow, white, etc. Color derives from the spectrum of light (distribution of light energy versus wavelength) interacting in the eye with the spectral sensitivities of the light receptors. Color categories and physical specifications of color are also associated with objects, materials, light sources, etc., based on their physical properties such as light absorption, reflection, or emission spectra.

Typically, only features of the composition of light that are detectable by humans (wavelength spectrum from 400 nm to 700 nm, roughly) are included, thereby objectively relating the psychological phenomenon of color to its physical specification.

Since perception of color stems from the varying sensitivity of different types of cone cells in the retina to different parts of the spectrum, colors may be defined and quantified by the degree to which they stimulate these cells. These physical or physiological quantifications of color, however, do not fully explain the psychophysical perception of color appearance.

The science of color is sometimes called chromatics. It includes the perception of color by the human eye and brain, the origin of color in materials, color theory in art, and the physics of electromagnetic radiation in the visible range (that is, what we commonly refer to simply as light).

Physics of Color

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."

A given light source may emit light at many different wavelengths (and most do); its spectrum is then a distribution giving its intensity at each wavelength. Although the spectrum of light arriving at the eye from a given direction determines the color perceived in that direction, there are many more possible spectral combinations than color sensations. In fact, one may formally define a color as a class of spectra that give rise to the same color 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 color in question.

Spectral colors

The familiar colors of the rainbow in the spectrum – named from the Latin word for appearance or apparition by Isaac Newton in 1671 – contains all those colors that can be produced by visible light of a single wavelength only, the pure spectral or monochromatic colors. The color table at right shows approximate frequencies (in terahertz) and wavelengths (in nanometers) for various pure spectral colors. The wavelengths are measured in vacuum.

The color table should not be interpreted as a definitive list – the pure spectral colors form a continuous spectrum, and how it is divided into distinct colors is a matter of culture, taste, and language. Furthermore, 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.

As discussed in the section on color vision, a light source need not actually be of one single wavelength to be perceived as a pure spectral color.

Color of objects
Surfaces appear to have the color of the light leaving them in the direction of the eye. Since the composition of this light may depend on the orientation of the surface and lighting conditions, the perceived color of an object also depends on these factors. However, some generalizations can be drawn.

Light arriving at an opaque surface is either reflected "specularly" (that is, in the manner of a mirror), or 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 color 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 color 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 chemo luminescence), or for other reasons.

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; and finally, by the permanent and transient characteristics of the perceiving eye and brain.

Copyright (c) Carla Dawson.
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