COLOUR

also spelled color defined generally, the aspect of any object that may be described in terms of hue, lightness, and saturation. In physics, colour is associated specifically with electromagnetic radiation of a certain range of wavelengths visible to the human eye. Radiation of such wavelengths comprises that portion of the electromagnetic spectrum known as the visible spectrumi.e., light. Major regions of the visible spectrum are seen as different colours by the eye, so that the spectrum appears as a band of colours ranging from red at the long wave end, through orange, yellow, green, blue, and indigo, to violet at the short wave end. Sir Isaac Newton investigated this sequence of colours in a series of experiments on white light in the mid-1660s and made a classification of seven elemental colours in a prototype of the modern colour wheel. Three characteristics that are commonly used to distinguish one colour from another are hue, saturation, and brightness. Hue is an attribute associated with each of the dominant wavelengths of the spectrum. Saturation pertains to relative purity, or the amount of white light mixed with a hue. High-saturation colours contain little or no white light. Brightness refers to intensity, distinguished by the degree of shading. Hue and saturation, taken together, are called chromaticity. Accordingly, a colour may be characterized by its chromaticity and brightness. Many ways of specifying and classifying colour have been devised since Newton's invention of the colour wheel. Among the most significant is the Munsell colour system (q.v.) developed in 1913 by Albert H. Munsell of the United States. This system defines and arranges colours on the basis of hue, value, and chroma, characteristics that correspond respectively to dominant wavelength, brightness, and purity. Another widely consulted colour system was devised a few years later by Wilhelm Ostwald, a German chemist. Ostwald's method designates colours in terms of purity, whiteness, and blackness. The most widely used system of colorimetry is that adopted in 1931 by the Commission Internationale de l'clairage (CIE), commonly called the International Commission on Illumination. The CIE system, revised in 1964, employs three values that correspond to red, green, and blue as basic reference points, designating them as the three primary colours from which all other colours are derived. Any desired hue can be produced when various amounts of two of the primaries are mechanically combined either by addition or by subtraction. Addition involves mixing parts of the spectrum, whereas subtraction entails the removal or absorption of spectral components. (Left) Additive and (right) subtractive methods of colour combination Colours are said to be complementary if they form white when they are combined additively or black when mixed subtractively. The figure shows red, green, and blue-violet primaries mixed additively, and cyan (minus red), magenta (minus green), and yellow (minus blue) primaries mixed subtractively. It is apparent that the complement of any colour is the mixture of the other two in the triad. also spelled color, defined generally, the aspect of any object that may be described in terms of hue, lightness, and saturation. In physics, colour is associated specifically with electromagnetic radiation of a certain range of wavelengths visible to the human eye. Radiation of such wavelengths comprises that portion of the electromagnetic spectrum known as the visible spectrumi.e., light. Vision is obviously involved in the perception of colour. A person can see in dim light, however, without being able to distinguish colours. Only when more light is present do colours appear. Light of some critical intensity, therefore, is also necessary for colour perception. Finally, the manner in which the brain responds to visual stimuli must also be considered. The colour green has a quite different meaning for a resident of a tropical rain forest than it has for a desert dweller. Even under identical conditions, the same object may appear red to one observer and orange to another. Clearly, the perception of colour depends on vision, light, and individual interpretation, and an understanding of colour involves physiology, physics, and psychology. An object appears coloured because of the way it interacts with light. The analysis of this interaction and the factors that determine it are the concerns of the physics of colour. The physiology of colour involves the eye's and the brain's responses to light and the sensory data they produce. The psychology of colour is invoked when the mind processes the visual data, compares them to information stored in memory, and interprets them as colour. This article concentrates on the physics of colour. For a discussion of colour as a quality of light, see the articles light and electromagnetic radiation. For the physiological aspects of colour vision, see the article eye. See also the article painting for a discussion of the psychological and aesthetic uses of colour. Additional reading General works Isaac Newton, Optics; or, A Treatise of the Reflexions, Refractions, Inflexions, and Colours of Light, 4th ed. (1730, reissued 1979), the beginnings of the scientific study of colour; Johann Wolfgang von Goethe, Goethe's Theory of Colours (1840, reissued 1975; originally published in German, 1810), with excellent observations explained by an untenable theory; David L. MacAdam (ed.), Sources of Color Science (1970), covering theories developed in all periods but omitting Goethe and including only a little Newton; Ralph Merrill Evans, An Introduction to Color (1948, reissued 1965), an authoritative, highly readable introduction, with emphasis on technical applications; and Enid Verity, Color Observed (1980), a readable general introduction. A bibliography of colour studies is given in Mary Buckley, Color Theory: A Guide to Information Sources (1975). Current research on the subject, together with discussions of applications, is found in the magazines Color Research and Application (quarterly), Inter-Society Color Council News (bimonthly), and Journal of the Optical Society of America; Part A, Optics and Image Science (monthly). Colorimetry W.D. Wright, The Measurement of Colour, 4th ed. (1969), an authoritative outline of the trichromatic system; Deane B. Judd and Gnter Wyszecki, Color in Business, Science, and Industry, 3rd ed. (1975), an authoritative work for the specialist; and Faber Birren, A Grammar of Color: A Basic Treatise on the Color System of Albert H. Munsell (1969), a revision of the 1921 work. Sets of colour chips used as identifiers are collected in Munsell Book of Color (1929 ), a loose-leaf publication. See also Kenneth L. Kelly and Deane B. Judd, Color: Universal Language and Dictionary of Names (1976), a system of simple colour names with cross-references to thousands of commonly used names; Fred W. Billmeyer, Jr., and Max Saltzman, Principles of Color Technology, 2nd ed. (1981), a highly technical but readable text with an annotated bibliography; Gnter Wyszecki and W.S. Stiles, Color Science: Concepts and Methods, Quantitative Data and Formulae, 2nd ed. (1982), an advanced text; Deane B. Judd, Contributions to Color Science (1979); and David L. MacAdam, Color Measurement: Theme and Variations, 2nd rev. ed. (1985). Physics and chemistry of colour R. Daniel Overheim and David L. Wagner, Light and Color (1982), a brief survey; Francis A. Jenkins and Harvey E. White, Fundamentals of Optics, 4th ed. (1976); Leslie E. Orgel, An Introduction to Transition-Metal Chemistry: Ligand-Field Theory, 2nd ed. (1966); and Keith McLaren, The Colour Science of Dyes and Pigments (1983), authoritative intermediate to advanced texts. Kurt Nassau, The Physics and Chemistry of Color: The Fifteen Causes of Color (1983), is a comprehensive up-to-date treatment. Perception of colour G. Hugh Begbie, Seeing and the Eye: An Introduction to Vision (1969, reprinted 1973), a survey for the general reader; Gerald S. Wasserman, Color Vision: An Historical Introduction (1978); and Tom N. Cornsweet, Visual Perception (1970), more detailed treatments; Ralph Merrill Evans, The Perception of Color (1974); and Robert M. Boynton, Human Color Vision (1979), comprehensive advanced texts. Edward C. Carterette and Morton P. Friedman (eds.), Handbook of Perception, vol. 5 (1975), contains 12 chapters, written at the advanced level, on all aspects of colour perception. Colour in art Samuel J. Williamson and Herman Z. Cummins, Light and Color in Nature and Art (1983), a readable, wide-ranging intermediate-level textbook; Johannes Itten, The Art of Color: The Subjective Experience and Objective Rationale of Color (1961, reprinted 1973; originally published in German, 1961), an exposition of an influential aesthetic theory; M.E. Chevreul, The Principles of Harmony and Contrast of Colors and Their Applications to the Arts (1854, reissued 1981; originally published in French, 1839), with notes by Faber Birren; Faber Birren, Principles of Color: A Review of Past Traditions and Modern Theories of Color Harmony (1969), an introduction, and his History of Color in Painting: With New Principles of Color Expression (1965), an authoritative treatment; and George A. Agoston, Color Theory and Its Application in Art and Design (1979), a broad review valuable as a reference work. Kurt Nassau