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This article is about the perceptual property. For a list of , 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 of color stems from the varying 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, , or simply color science. It includes the per- ception of color by the and brain, the origin of color in materials, in art, and the physics of electromagnetic radiation in the visible range (that is, what we commonly refer to simply as ).

1 Physics of color

Continuous optical spectrum rendered into the sRGB .

Color effect - shining through stained glass onto carpet Electromagnetic radiation is characterized by its (or frequency) and its intensity. When the Color (American English) or colour (Commonwealth wavelength is within the (the range of English) is the visual perceptual property corresponding humans can perceive, approximately from in humans to the categories called , , , 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 in the spectrum – named using the Latin 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 ta- ble 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, , yellow, , blue, and . New- tend to have rough surfaces) have their color deter- ton’s conception included a seventh color, , 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 , absorbed). If objects scatter all wavelengths with and that indigo was simply the dark blue of the indigo [3] roughly equal strength, they appear . If they that was being imported at the time. absorb all wavelengths, they appear . 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 , and ferent wavelengths with different efficiencies look a low-intensity yellow-green is -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 ob- jects 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 view- ing. 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 spectrum, viewing angle, etc. This frequencies ("fluorescence" or "phosphorescence") effect is known as . 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 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 , 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: The ability of the human eye to distinguish colors is

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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 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 . This theory lengths around 540 nm, while the long-wavelength cones, was later refined by and Hermann L cones, or red cones, are most sensitive to light we per- von Helmholtz. As Helmholtz puts it, “the principles of ceive as greenish yellow, with wavelengths around 570 Newton’s law of mixture were experimentally confirmed nm. by Maxwell in 1856. Young’s theory of color sensa- Light, no matter how complex its composition of wave- tions, like so much else that this marvelous investigator lengths, is reduced to three color components by the eye. achieved in advance of his time, remained unnoticed un- [5] For each location in the visual field, the three types of til Maxwell directed attention to it.” cones yield three signals based on the extent to which each At the same time as Helmholtz, Ewald Hering developed is stimulated. These amounts of stimulation are some- the opponent process theory of color, noting that color times called tristimulus values. 4 2 PERCEPTION

The response curve as a function of wavelength varies for : it is the collection of colors for which at least each type of cone. Because the curves overlap, some tris- one of the two color channels measures a value at one of timulus values do not occur for any incoming light com- its extremes. bination. For example, it is not possible to stimulate only The exact nature of color perception beyond the process- the mid-wavelength (so-called “green”) cones; the other ing already described, and indeed the status of color as cones will inevitably be stimulated to some degree at the a feature of the perceived world or rather as a feature of same time. The set of all possible tristimulus values de- our perception of the world – a type of qualia – is a matter termines the human color space. It has been estimated of complex and continuing philosophical dispute. that humans can distinguish roughly 10 million different colors.[4] The other type of light-sensitive cell in the eye, the rod, has a different response curve. In normal situations, when light is bright enough to strongly stimulate the cones, rods 2.4 Nonstandard color perception play virtually no role in vision at all.[7] On the other hand, in dim light, the cones are understimulated leaving only 2.4.1 Color deficiency the signal from the rods, resulting in a colorless response. (Furthermore, the rods are barely sensitive to light in the Main article: “red” range.) In certain conditions of intermediate illu- mination, the rod response and a weak cone response can together result in color discriminations not accounted for If one or more types of a person’s color-sensing cones are by cone responses alone. These effects, combined, are missing or less responsive than normal to incoming light, summarized also in the , that describes the that person can distinguish fewer colors and is said to be change of color perception and pleasingness of light as color deficient or color blind (though this latter term can function of temperature and intensity. be misleading; almost all color deficient individuals can distinguish at least some colors). Some kinds of color deficiency are caused by anomalies in the number or na- 2.3 Color in the brain ture of cones in the retina. Others (like central or cortical ) are caused by neural anomalies in those Main article: Color vision parts of the brain where visual processing takes place. While the mechanisms of color vision at the level of the

2.4.2 Tetrachromacy

Main article: Tetrachromacy

While most humans are trichromatic (having three types of color receptors), many animals, known as tetrachromats, have four types. These include some species of spiders, most marsupials, birds, reptiles, and many species of fish. Other species are sensitive to only two axes of color or do not perceive color at all; these are called dichromats and monochromats respectively. A The visual dorsal stream (green) and ventral stream () are distinction is made between retinal tetrachromacy (hav- shown. The ventral stream is responsible for color perception. ing four in cone cells in the retina, compared to three in trichromats) and functional tetrachromacy retina are well-described in terms of tristimulus values, (having the ability to make enhanced color discrimina- color processing after that point is organized differently. tions based on that retinal difference). As many as half A dominant theory of color vision proposes that color in- of all women are retinal tetrachromats.[8]:p.256 The phe- formation is transmitted out of the eye by three opponent nomenon arises when an individual receives two slightly processes, or opponent channels, each constructed from different copies of the gene for either the medium- the raw output of the cones: a red–green channel, a blue– or long-wavelength cones, which are carried on the x- yellow channel, and a black–white “” channel. chromosome. To have two different genes, a person must This theory has been supported by neurobiology, and ac- have two x-chromosomes, which is why the phenomenon counts for the structure of our subjective color experi- only occurs in women.[8] For some of these retinal tetra- ence. Specifically, it explains why we cannot perceive chromats, color discriminations are enhanced, making a “reddish green” or “yellowish blue”, and it predicts the them functional tetrachromats.[8] 2.7 Color naming 5

2.4.3 Synesthesia adapts to changes in the viewing environment.

In certain forms of synesthesia/ideasthesia, perceiving let- ters and numbers (grapheme–color synesthesia) or hear- 2.7 Color naming ing musical sounds (music–color synesthesia) will lead to the unusual additional experiences of seeing colors. Be- Main article: havioral and functional neuroimaging experiments have demonstrated that these color experiences lead to changes Colors vary in several different ways, including in behavioral tasks and lead to increased activation of (, orange, yellow, green, blue, and violet), brain regions involved in color perception, thus demon- saturation, , and gloss. Some color are strating their reality, and similarity to real color percepts, derived from the name of an object of that color, such albeit evoked through a non-standard route. as "orange" or "", while others are abstract, like “red”. 2.5 Afterimages In the 1969 study : Their Universal- ity and Evolution, and Paul Kay describe a After exposure to strong light in their sensitivity range, pattern in naming “basic” colors (like “red” but not “red- photoreceptors of a given type become desensitized. For orange” or “dark red” or “blood red”, which are “shades” a few seconds after the light ceases, they will continue of red). All languages that have two “basic” color names to signal less strongly than they otherwise would. Col- distinguish dark/cool colors from bright/warm colors. ors observed during that period will appear to lack the The next colors to be distinguished are usually red and color component detected by the desensitized photore- then yellow or green. All languages with six “basic” col- ceptors. This effect is responsible for the phenomenon of ors include black, white, red, green, blue, and yellow. The afterimages, in which the eye may continue to see a bright pattern holds up to a set of twelve: black, gray, white, figure after looking away from it, but in a complementary , red, orange, yellow, green, blue, purple, brown, and color. (distinct from blue in Russian and Italian, but not English). effects have also been utilized by artists, in- cluding Vincent van Gogh. 3 Associations 2.6 Color constancy Individual colors have a variety of cultural associations Main article: Color constancy such as national colors (in general described in individual color articles and ). The field of attempts to identify the effects of color on When an artist uses a limited color palette, the eye tends human emotion and activity. is a form to compensate by seeing any gray or neutral color as the of alternative medicine attributed to various Eastern tra- color which is missing from the color wheel. For exam- ditions. Colors have different associations in different ple, in a limited palette consisting of red, yellow, black, countries and cultures.[11] and white, a mixture of yellow and black will appear as a variety of green, a mixture of red and black will appear Different colors have been demonstrated to have effects as a variety of purple, and pure gray will appear bluish.[9] on cognition. For example, researchers at the University of Linz in Austria demonstrated that the color red signif- The trichromatic theory is strictly true when the visual icantly decreases cognitive functioning in men.[12] system is in a fixed state of . In reality, the is constantly adapting to changes in the en- vironment and compares the various colors in a scene to reduce the effects of the illumination. If a scene is il- 4 Spectral colors and color repro- luminated with one light, and then with another, as long duction as the difference between the light sources stays within a reasonable range, the colors in the scene appear relatively Most light sources are mixtures of various wavelengths constant to us. This was studied by Edwin Land in the of light. Many such sources can still effectively produce 1970s and led to his retinex theory of color constancy. a spectral color, as the eye cannot distinguish them from It should be noted, that both phenomena are readily ex- single-wavelength sources. For example, most computer plained and mathematically modeled with modern theo- displays reproduce the spectral color orange as a combi- ries of and color appearance (e.g. nation of red and green light; it appears orange because CIECAM02, iCAM).[10] There is no need to dismiss the the red and green are mixed in the right proportions to trichromatic theory of vision, but rather it can be en- allow the eye’s cones to respond the way they do to the hanced with an understanding of how the visual system spectral color orange. 6 6 SUBTRACTIVE COLORING

three particular primary colors. Each method has its ad- 0.9 520 vantages and disadvantages depending on the particular application. 0.8 540 No mixture of colors, however, can produce a response 0.7 truly identical to that of a spectral color, although one can 560 get close, especially for the longer wavelengths, where the 0.6 CIE 1931 color space diagram has a nearly 500 straight edge. For example, mixing green light (530 nm) 580 0.5 and blue light (460 nm) produces cyan light that is slightly y desaturated, because response of the red color receptor 0.4 600 would be greater to the green and blue light in the mixture 620 than it would be to a pure cyan light at 485 nm that has 0.3 490 700 the same intensity as the mixture of blue and green.

0.2 Because of this, and because the primaries in color print- ing systems generally are not pure themselves, the colors 480 0.1 reproduced are never perfectly saturated spectral colors, 470 and so spectral colors cannot be matched exactly. How- 460 0.0 380 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 ever, natural scenes rarely contain fully saturated colors, x thus such scenes can usually be approximated well by these systems. The range of colors that can be repro- The CIE 1931 color space chromaticity diagram. The outer duced with a given color reproduction system is called curved boundary is the spectral (or monochromatic) locus, with the . The CIE chromaticity diagram can be used to wavelengths shown in nanometers. The colors depicted depend describe the gamut. on the color space of the device on which you are viewing the image, and therefore may not be a strictly accurate representa- Another problem with color reproduction systems is con- tion of the color at a particular position, and especially not for nected with the acquisition devices, like cameras or scan- monochromatic colors. ners. The characteristics of the color sensors in the de- vices are often very far from the characteristics of the receptors in the human eye. In effect, acquisition of col- A useful concept in understanding the perceived color of ors can be relatively poor if they have special, often very a non-monochromatic light source is the dominant wave- “jagged”, spectra caused for example by unusual lighting length, which identifies the single wavelength of light that of the photographed scene. A color reproduction system produces a sensation most similar to the light source. “tuned” to a human with normal color vision may give is roughly akin to hue. very inaccurate results for other observers. There are many color that by definition can- The different color response of different devices can be not be pure spectral colors due to desaturation or be- problematic if not properly managed. For color informa- cause they are (mixtures of red and violet light, tion stored and transferred in digital form, color manage- from opposite ends of the spectrum). Some examples of ment techniques, such as those based on ICC profiles, can necessarily non-spectral colors are the achromatic colors help to avoid distortions of the reproduced colors. Color (black, gray, and white) and colors such as pink, tan, and management does not circumvent the gamut limitations . of particular output devices, but can assist in finding good mapping of input colors into the gamut that can be repro- Two different light spectra that have the same effect on duced. the three color receptors in the human eye will be per- ceived as the same color. They are metamers of that color. This is exemplified by the white light emitted by fluorescent lamps, which typically has a spectrum of a 5 Additive coloring few narrow bands, while daylight has a continuous spec- trum. The human eye cannot tell the difference between is light created by mixing together light of such light spectra just by looking into the light source, al- two or more different colors. Red, green, and blue are the though reflected colors from objects can look different. additive primary colors normally used in additive color (This is often exploited; for example, to make fruit or systems such as projectors and computer terminals. tomatoes look more intensely red.) Similarly, most human color perceptions can be gener- ated by a mixture of three colors called primaries. This 6 Subtractive coloring is used to reproduce color scenes in , print- ing, television, and other media. There are a number of Subtractive coloring uses , , pigments, or filters methods or color spaces for specifying a color in terms of to absorb some wavelengths of light and not others. The 7

the red paint, creating the appearance of a black object.

7 Structural color

Further information: and

Structural colors are colors caused by interference effects rather than by pigments. Color effects are produced when a material is scored with fine parallel lines, formed of one or more parallel thin layers, or otherwise composed of microstructures on the scale of the color’s wavelength. If the microstructures are spaced randomly, light of shorter wavelengths will be scattered preferentially to produce Tyndall effect colors: the blue of the sky (Rayleigh scat- Additive : adding red to green yields yellow; adding tering, caused by structures much smaller than the wave- all three primary colors together yields white. length of light, in this case air molecules), the luster of opals, and the blue of human irises. If the microstruc- tures are aligned in arrays, for example the array of pits in a CD, they behave as a diffraction grating: the grating re- flects different wavelengths in different directions due to interference phenomena, separating mixed “white” light into light of different wavelengths. If the structure is one or more thin layers then it will reflect some wavelengths and transmit others, depending on the layers’ thickness. Structural color is studied in the field of thin-film op- tics. A layman’s term that describes particularly the most ordered or the most changeable structural colors is . Structural color is responsible for the and of the feathers of many birds (the blue jay, for example), as well as certain butterfly wings and beetle shells. Variations in the pattern’s spacing often give rise to an iridescent effect, as seen in peacock feathers, soap bubbles, films of oil, and mother of pearl, because the re- flected color depends upon the viewing angle. Numerous scientists have carried out research in butterfly wings and beetle shells, including Isaac Newton and Robert Hooke. mixing: adding magenta to yellow yields red; Since 1942, electron micrography has been used, advanc- adding all three primary colors together yields black ing the development of products that exploit structural color, such as "photonic" cosmetics.[13] color that a surface displays comes from the parts of the visible spectrum that are not absorbed and therefore re- main visible. Without pigments or dye, fabric fibers, paint base and paper are usually made of particles that 8 Mentions of color in social media scatter white light (all colors) well in all directions. When a or is added, wavelengths are absorbed or According to , the top three colors in social media “subtracted” from white light, so light of another color for 2012 were red (186 million mentions; accredited to reaches the eye. Taylor Swift's Red album, NASA's landing on Mars, and If the light is not a pure white source (the case of nearly red carpet coverage), blue (125 million mentions; accred- all forms of artificial lighting), the resulting spectrum will ited to the United States presidential election, 2012, Mars appear a slightly different color. Red paint, viewed under rover Curiosity finding blue rocks, and blue sports teams), blue light, may appear black. Red paint is red because it and Green (102 million mentions; accredited to “environ- scatters only the red components of the spectrum. If red mental friendliness”, Green Bay Packers, and green eyed paint is illuminated by blue light, it will be absorbed by girls).[14] 8 12 EXTERNAL LINKS AND SOURCES

9 Additional terms [2] Berlin, B. and Kay, P., Basic Color Terms: Their Univer- sality and Evolution, Berkeley: University of California • Color wheel: an illustrative organization of color Press, 1969. in a circle that shows relationships. [3] Waldman, Gary (2002). Introduction to light : the physics of light, vision, and color (Dover ed.). Mineola: Dover • , chroma, purity, or saturation: how Publications. p. 193. ISBN 978-0-486-42118-6. “intense” or “concentrated” a color is. Tech- nical definitions distinguish between colorfulness, [4] Judd, Deane B.; Wyszecki, Günter (1975). Color in Busi- chroma, and saturation as distinct perceptual at- ness, Science and Industry. Wiley Series in Pure and Ap- tributes and include purity as a physical quantity. plied Optics (third ed.). New York: Wiley-Interscience. These terms, and others related to light and color p. 388. ISBN 0-471-45212-2. are internationally agreed upon and published in the [5] Hermann von Helmholtz, Physiological Optics – The Sen- [15] CIE Lighting Vocabulary. More readily available sations of Vision, 1866, as translated in Sources of Color texts on colorimetry also define and explain these Science, David L. MacAdam, ed., Cambridge: MIT Press, terms.[10][16] 1970.

• : a phenomenon where the hue is de- [6] Palmer, S.E. (1999). Vision Science: Photons to Phe- pendent on concentration and/or thickness of the ab- nomenology, Cambridge, MA: MIT Press. ISBN 0-262- sorbing substance. 16183-4. [7] “Under well-lit viewing conditions (), • Hue: the color’s direction from white, for example cones ...are highly active and rods are inactive.” Hi- in a color wheel or chromaticity diagram. rakawa, K.; Parks, T.W. (2005). Chromatic Adap- tation and White-Balance Problem (PDF). IEEE ICIP. • Shade: a color made darker by adding black. doi:10.1109/ICIP.2005.1530559. Archived from the original (PDF) on November 28, 2006. • Tint: a color made lighter by adding white. • [8] Jameson, K. A.; Highnote, S. M.,; Wasserman, L. Value, brightness, , or luminosity: how light M. (2001). “Richer color experience in observers or dark a color is. with multiple photopigment opsin genes.” (PDF). Psy- chonomic Bulletin and Review. 8 (2): 244–261. doi:10.3758/BF03196159. PMID 11495112.

10 See also [9] Depauw, Robert C. “United States Patent”. Retrieved 20 March 2011. • [10] M.D. Fairchild, Color Appearance Models, 2nd Ed., Wi- • (art) ley, Chichester (2005).

• [11] “Chart: Color Meanings by Culture”. Retrieved 2010-06- 29. • Complementary color [12] Gnambs, Timo; Appel, Markus; Batinic, Bernad. (2010). Color red in web-based knowledge testing. Computers in • Human Behavior, 26, p1625-1631. • International Color Consortium [13] “Economic and Social Research Council - Science in the • Dock, Art in the Stocks”. Archived from the original on International Commission on Illumination November 2, 2007. Retrieved 2007-10-07.

• Lists of colors (compact version) [14] “Celebrate Color”. pantone.com. Pantone. Retrieved 7 December 2014. • Neutral color [15] CIE Pub. 17-4, International Lighting Vocabulary, 1987. • including Metal effect pigments http://www.cie.co.at/publ/abst/17-4-89.html

• Primary, secondary and tertiary colors [16] R.S. Berns, Principles of Color Technology, 3rd Ed., Wi- ley, New York (2001).

11 References 12 External links and sources [1] Craig F. Bohren (2006). Fundamentals of Atmospheric Radiation: An Introduction with 400 Problems. Wiley- • Bibliography Database on Color Theory, Buenos VCH. ISBN 3-527-40503-8. Aires University 9

• Maund, Barry. “Color”. Stanford Encyclopedia of Philosophy. • “Color”. Internet Encyclopedia of Philosophy.

• Why Should Engineers and Scientists Be Worried About Color?

• Robert Ridgway's A Nomenclature of Colors (1886) and Color Standards and Color Nomenclature (1912) - text-searchable digital facsimiles at Linda Hall Li- brary

• Albert Henry Munsell's A Color Notation, (1907) at Project Gutenberg

• AIC, International Colour Association

• The Effect of Color | OFF BOOK Documentary produced by Off Book (web series)

• Study of the history of colors 10 13 TEXT AND IMAGE SOURCES, CONTRIBUTORS, AND LICENSES

13 Text and image sources, contributors, and licenses

13.1 Text

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