1 Physics of Color

Color This article is about the perceptual property. For a list of colors, see Lists of colors. For other uses, see Color (disambiguation). “Colorful” redirects here. For other uses, see Colorful (disambiguation). “Colour” redirects here. For other uses, see Colour (disambiguation). Colors can look differently depending on their surrounding colors and shapes. The two small squares have exactly the same color, but the right one looks slightly darker. numerically by their coordinates. Because perception of color stems from the varying spectral 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 stim- ulate these cells. These physical or physiological quan- tifications of color, however, do not fully explain the Colored pencils psychophysical perception of color appearance. The science of color is sometimes called chromatics, colorimetry, or simply color science. 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). 1 Physics of color Continuous optical spectrum rendered into the sRGB color space. Color effect - Sunlight shining through stained glass onto carpet Electromagnetic radiation is characterized by its wavelength (or frequency) and its intensity. When the Color (American English) or colour (Commonwealth wavelength is within the visible spectrum (the range of English) is the visual perceptual property corresponding wavelengths humans can perceive, approximately from in humans to the categories called red, blue, yellow, etc. 390 nm to 700 nm), it is known as “visible light”. Color derives from the spectrum of light (distribution of Most light sources emit light at many different wave- light power versus wavelength) interacting in the eye with lengths; a source’s spectrum is a distribution giving its the spectral sensitivities of the light receptors. Color cat- intensity at each wavelength. Although the spectrum of egories and physical specifications of color are also as- light arriving at the eye from a given direction determines sociated with objects or materials based on their physical the color sensation in that direction, there are many more properties such as light absorption, reflection, or emission possible spectral combinations than color sensations. In spectra. By defining a color space colors can be identified fact, one may formally define a color as a class of spectra 1 2 1 PHYSICS OF COLOR 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. 1.1 Spectral colors The familiar colors of the rainbow in the spectrum – named using the Latin word for appearance or apparition by Isaac Newton in 1671 – include all those colors 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 spec- The upper disk and the lower disk have exactly the same objective tral colors. The wavelengths listed are as measured in air color, and are in identical gray surroundings; based on context or vacuum (see refractive index). differences, humans perceive the squares as having different re- flectances, and may interpret the colors as different color cate- The color table should not be interpreted as a definitive gories; see checker shadow illusion. list – the pure spectral colors form a continuous spectrum, and how it is divided into distinct colors linguistically is a matter of culture and historical contingency (although scattered (that is, reflected with diffuse scattering), people everywhere have been shown to perceive colors or absorbed – or some combination of these. [2] in the same way ). A common list identifies six main • Opaque objects that do not reflect specularly (which bands: red, orange, yellow, green, blue, and violet. New- tend to have rough surfaces) have their color deter- ton’s conception included a seventh color, indigo, be- mined by which wavelengths of light they scatter tween blue and violet. It is possible that what Newton strongly (with the light that is not scattered being referred to as blue is nearer to what today we call cyan, absorbed). If objects scatter all wavelengths with and that indigo was simply the dark blue of the indigo dye [3] roughly equal strength, they appear white. If they that was being imported at the time. absorb all wavelengths, they appear black. The intensity of a spectral color, relative to the context in • which it is viewed, may alter its perception considerably; Opaque objects that specularly reflect light of dif- for example, a low-intensity orange-yellow is brown, and ferent wavelengths with different efficiencies look a low-intensity yellow-green is olive-green. 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 1.2 Color of objects but also be faintly reflective; examples are black objects coated with layers of enamel or lacquer. The color of an object depends on both the physics of the • Objects that transmit light are either translucent object in its environment and the characteristics of the (scattering the transmitted light) or transparent (not perceiving eye and brain. Physically, objects can be said scattering the transmitted light). If they also ab- to have the color of the light leaving their surfaces, which sorb (or reflect) light of various wavelengths differ- normally depends on the spectrum of the incident illu- entially, they appear tinted with a color determined mination and the reflectance properties of the surface, as by the nature of that absorption (or that reflectance). well as potentially on the angles of illumination and viewing. Some objects not only reflect light, but also transmit • Objects may emit light that they generate from light or emit light themselves, which also contribute to the having excited electrons, rather than merely re- color. A viewer’s perception of the object’s color depends flecting or transmitting light. The electrons not only on the spectrum of the light leaving its surface, may be excited due to elevated temperature but also on a host of contextual cues, so that color differ- (incandescence), as a result of chemical reactions ences between objects can be discerned mostly indepen- (chemoluminescence), after absorbing light of other dent of the lighting spectrum, viewing angle, etc. This frequencies ("fluorescence" or "phosphorescence") effect is known as color constancy. or from electrical contacts as in light emitting Some generalizations of the physics can be drawn, ne- diodes, or other light sources. glecting perceptual effects for now: To summarize, the color of an object is a complex result • Light arriving at an opaque surface is either reflected of its surface properties, its transmission properties, and "specularly" (that is, in the manner of a mirror), its emission properties, all of which contribute to the mix 2.2 Color in the eye 3 of wavelengths in the light leaving the surface of the ob- blindness and afterimages typically come in opponent ject. The perceived color is then further conditioned by pairs (red-green, blue-orange, yellow-violet, and black- the nature of the ambient illumination, and by the color white). Ultimately these two theories were synthesized in properties of other objects nearby, and via other charac- 1957 by Hurvich and Jameson, who showed that retinal teristics of the perceiving eye and brain. processing corresponds to the trichromatic theory, while processing at the level of the lateral geniculate nucleus corresponds to the opponent theory.[6] 2 Perception In 1931, an international group of experts known as the Commission internationale de l'éclairage (CIE) developed a mathematical color model, which mapped out the space of observable colors and assigned a set of three numbers to each. 2.2 Color in the eye Main article: Color vision The ability of the human eye to distinguish colors is 1.0 S ML 0.8 0.6 0.4 This image (when viewed in full size, 1000 pixels wide) contains 0.2 1 million pixels, each of a different color. The human eye can distinguish about 10 million different colors.[4] 0 400 450 500 550 600 650 700 2.1 Development of theories of color vision Normalized typical human cone cell responses (S, M, and L types) to monochromatic spectral stimuli Main article: Color theory based upon the varying sensitivity of different cells in the retina to light of different wavelengths. Humans be- Although Aristotle and other ancient scientists had al- ing trichromatic, the retina contains three types of color ready written on the nature of light and color vision, it was receptor cells, or cones. One type, relatively distinct not until Newton that light was identified as the source of from the other two, is most responsive to light that we the color sensation. In 1810, Goethe published his com- perceive as blue or blue-violet, with wavelengths around prehensive Theory of Colors in which he ascribed physi- 450 nm; cones of this type are sometimes called short- ological effects to color that are now understood as psy- wavelength cones, S cones, or blue cones. The other chological. two types are closely related genetically and chemically: In 1801 Thomas Young proposed his trichromatic the- middle-wavelength cones, M cones, or green cones are ory, based on the observation that any color could be most sensitive to light perceived as green, with wave- matched with a combination of three lights.

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