The Encyclopedia of Physical Science and Technology

ColorScience Robert M. Boynton University of California, San Diego I. Physical Basis of Perceived Color II. CIE System of Color Specification III. Color Rendering IV. Global Surface Properties V. Physical Basis of Surface Color VI. Color Difference and Color Order VII. Physiological Basis of Color Vision GLOSSARY Chromaticity Ratios x, y, z of each of the tristimulus values of a light to the sum of the three tristimulus val-ues X,Y, Z,thesebeingtheamountsofthreeprimaries required to match the color of the light. Chromaticity diagram Plane diagram formed by plot-ting one of the three chromaticity coordinates against another (usually y versus x). Color Characteristics of sensations elicited by light by which a human observer can distinguish between two structure-free patches of light of the same size and shape. Colorant A substance, such as a dye or pigment, that modifies the color of objects or imparts color to other-wise achromatic objects. Colorimetry Measurement and specification of color. Colormatching Actionofmakingatestcolorappearthe same as a reference color. Color order System of reference whereby the relation of onecolortoanothercanbeperceivedandthepositionof thatcolorcanbeestablishedwithrespecttotheuniverse of all colors. Color rendering General expression for the effect of a light source on the color appearance of objects in com-parison with their color appearance under a reference light source. Colortemperature Absolutetemperatureofablackbody radiator having a chromaticity closest to that of a light source being specified. Metamerism (1) Phenomenon whereby lights of differ-entspectralpowerdistributionsappeartohavethesame color.(2)Degreetowhichamaterialappearstochange color when viewed under different illuminants. Optimalcolors Stimulithatforagivenchromaticityhave the greatest luminous reflectance. Primaries (1) Additive: Any one of three lights in terms of which a color is specified by giving the amount of each required to match it by combining the lights. (2) Subtractive: Set of dyes or pigments that, when mixed in various proportions, provides a gamut of colors. Radiance Radiantfluxperunitsolidangle(intensity)per unitareaofanelementofanextendedsourceorreflect-ing surface in a specified direction. Reflectance Ratio of reflected to incident light. 289 290 Reflection Processbywhichincidentfluxleavesasurface or medium from the incident side, without change in wavelength. COLOR SCIENCE examines a fundamental aspect of human perception. It is based on experimental study un-dercontrolledconditionssusceptibletophysicalmeasure-ment. For a difference in color to be perceived between two surfaces, three conditions must be satisfied: (1) There must be an appropriate source of illumination, (2) the two surfaces must not have identical spectral reflectances, and (3) an observer must be present to view them. This arti-cle is concerned with the relevant characteristics of lights, surfaces, and human vision that conjoin to allow the per-ception of object color. I. PHYSICAL BASIS OF PERCEIVED COLOR Thephysicalbasisofcolorexistsintheinteractionoflight withmatter,bothoutsideandinsidetheeye.Thesensation ofcolordependsonphysiologicalactivityinthevisualsys-temthatbeginswiththeabsorptionoflightinphotorecep-tors located in the retina of the eye and ends with patterns ofbiochemicalactivityinthebrain.Perceivedcolorcanbe described by the color names white, gray, black, yellow, orange, brown, red, green, blue, purple, and pink. These 11 basic color terms have unambiguous referents in all fully developed languages. All of these names (as well as combinations of these and many other less precisely used nonbasiccolorterms)describecolors,butwhite,gray,and black are excluded from the list of those called hues. Col-orswithhuearecalledchromaticcolors;thosewithoutare called achromatic colors. Although color terms are frequently used in reference to all three aspects of color (e.g., one may speak of a sen-sation of red, a red surface, or a red light), such usage is scientifically appropriate only when applied to the sensa-tion;descriptionsoflightsandsurfacesshouldbeprovided in physical and geometrical language. II. CIE SYSTEM OF COLOR SPECIFICATION A. Basic Color-Matching Experiment The most fundamental experiment in color science entails the determination of whether two fields of light such as those that might be produced on a screen with two slide projectors, appear the same or different. If such fields are abutted and the division between them disappears to form asingle,homogeneousfield,thefieldsaresaidtomatch.A ColorScience match will, of course, occur if there is no physical differ-encebetweenthefields,andinspecialcasescolormatches arealsopossiblewhensubstantialphysicaldifferencesex-ist between the fields. An understanding of how this can happen provides an opening to a scientific understanding of this subject. Given an initial physical match, a difference in color can be introduced by either of two procedures, which are often carried out in combination. In the first instance, the radiance of one part of a homogeneous field is altered without any change in its relative spectral distribution. This produces an achromatic color difference. In the sec-ond case, the relative spectral distribution of one field is changedsuchthat,forallpossiblerelativeradiancesofthe twofields,nomatchispossible.Thisiscalledachromatic color difference. Whenfieldsofdifferentspectraldistributionscanbead-justedinrelativeradiancetoeliminateallcolordifference, the result is termed a metameric color match. In a color-matching experiment, a test field is presented next to a comparison field and the observer causes the two fields to match exactly by manipulating the radiances of so-called primariesprovidedtothecomparisonfield.Suchprimaries aresaidtobeadded;thiscanbeaccomplishedbysuperpo-sitionwithahalf-silveredmirror,bysuperimposedimages projectedontoascreen,byveryrapidtemporalalternation of fields at a rate above the fusion frequency for vision, or by the use of pixels too small and closely packed to be discriminated (as in color television). If the primaries are suitablychosen(nooneofthemshouldbematchedbyany possible mixture of the other two), a human observer with normal color vision can uniquely match any test color by adjustingtheradiancesofthreemonochromaticprimaries. Toaccomplishthis,it sometimesproves necessaryto shift one of the primaries so that it is added to the color being matched; it is useful to treat this as a negative radiance of that primary in the test field. The choice of exactly three primaries is by no means arbitrary: If only one or two primaries are used, matches are generally impossible, whereas if four or more primaries are allowed, matches are not uniquely determined. Theresultofthecolor-matchingexperimentcanberep-resented mathematically as t(T)=r(R)+g(G)+b(B), meaning that t units of test field T produce a color that is matched by an additive combination of r units of pri-mary R,g units of primary G, and b units of primary B, where one or two of the quantities r,g, or b may be negative. Thus any color can be represented as a vector in R, G, B space. For small, centrally fixated fields, ex-periment shows that the transitive, reflexive, linear, and associativepropertiesofalgebraapplyalsototheirempir-ical counterparts, so that color-matching equations can be manipulatedtopredictmatchesthatwouldbemadewitha ColorScience change in the choice of primaries. These simple relations break down for very low levels of illumination and also with higher levels if the fields are large enough to permit significant contributions by rod photoreceptors or if the fields are so bright as to bleach a significant fraction of cone photopigments, thus altering their action spectra. Matchesareusuallymadebyamethodofadjustment,an iterative, trial-and-error procedure whereby the observer manipulates three controls, each of which monotonically varies the radiance of one primary. Although such set-tings at the match point may be somewhat more variable than most purely physical measurements, reliable data re-sult from the means of several settings for each condition tested. A more serious problem, which will not be treated in this article, results from differences among observers. Although not great among those with normal color vi-sion, such differences are by no means negligible. (For those with abnormal color vision, they can be very large.) To achieve a useful standardization—one that is unlikely to apply exactly to any particular individual—averages of normal observers are used, leading to the concept of a standard observer. 291 In the color-matching experiment, an observer is in ef-fect acting as an analog computer, solving three simulta-neousequationsbyiteration,usinghisorhersensationsas a guide. Although activity in the brain underlies the expe-rience of color, the initial encoding of information related to wavelength is in terms of the ratios of excitations of three different classes of cone photoreceptors in the retina of the eye, whose spectral sensitivities overlap. Any two physical fields, whether of the same or different spectral composition, whose images on the retina excite each of the three classes of cones in the same way will be indis-criminable.Theactionspectraofthethreeclassesofcones in the normal eye are such that no two wavelengths in the spectrum produce exactly the same ratios of excitations among them. B. Imaginary Primaries Dependingonthechoiceofprimaries,manydifferentsets of color-matching functions are possible, all of which de-scribe the same color-matching behavior. Figure 1 shows experimental data for the primaries 435.8, 546.1, and FIGURE 1 Experimental color-matching data for primaries at 435.8, 546.1, and 700.0 nm. [From Billmeyer, F. W., Jr., and Saltzmann, M. (1981). “Principles of Color Technology,” 2nd ed. Copyright ©1981 John Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.] 292 ColorScience By simulating any of these sets of sensitivity func-tions in three optically filtered photocells, it is possible to remove the human observer from the system of color measurement (colorimetry) and develop a purely physical (thoughnecessarilyverylimited)descriptionofcolor,one that can be implemented in automated colorimeters. FIGURE 2 Estimates of human cone action spectra (Ko¨nig fun-damentals) derived by V. Smith and J. Pokorny. [From Wyszecki, G., and Stiles, W. S. (1982). “Color Science: Concepts and Meth-ods, Quantitative Data and Formulate,” 2nd ed. Copyright ©1982 John Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.] C. Chromaticity Diagram Ausefulseparationbetweentheachromaticandchromatic aspects of color was achieved in a system of colorimetry adopted by the CIE in 1931. This was the first specifica-tionofcolortoachieveinternationalagreement;itremains today the principal system used internationally for spec-ifying colors quantitatively, without reference to a set of actual samples. The color-matching functions x(λ), y(λ), and z(λ) are based on primaries selected and smoothed to force the y(λ) function to be proportional to the spectral luminous efficiency function V(λ), which had been standardized a decade earlier to define the quantity of “luminous flux” in lumens per watt of radiant power. The x(λ), y(λ), and z(λ) functions were then scaled to equate the areas under the curves, an operation that does not alter the predictions they make about color matches. To specify the color of a patch of light, one begins by integrating its spectral radiance distribution S(λ) in turn with the three color-matching functions: Z X = k S(λ)x(λ)dλ, 700.0 nm. Depicted in Fig. 2 are current estimates of the spectral sensitivities of the three types of cone pho-toreceptors. These functions, which have been inferred from the data of psychophysical experiments of various kinds,agreereasonablywellwithdirectmicrospectropho-tometric measurements of the absorption spectra of outer segments of human cone photoreceptors containing the photopigments that are the principal determinants of the spectral sensitivity of the cones. Theconespectralsensitivitiesmayberegardedascolor-matching functions based on primaries that are said to be imaginary in the sense that, although calculations of color matches based on them are possible, they are not phys-ically realizable. To exist physically, each such primary would uniquely excite only one type of cone, whereas real primaries always excite at least two types. Another set of all-positive color-matching functions, based on a different set of imaginary primaries, is given in Fig. 3. This set, which makes very similar predictions about color matches as the cone sensitivity curves, was adoptedasastandardbytheInternationalCommissionon Illumination (CIE) in 1931. Z Y = k S(λ)y(λ)dλ, Z Z = k S(λ)z(λ)dλ. The values X, Y, and Z are called relative tristimulus values; these are equal for any light having an equal-radiance spectrum. Tristimulus values permit the spec-ification of color in terms of three variables that are relatedtoconesensitivitiesratherthanbycontinuousspec-tralradiancedistributions,whichdonot.Like R,G,and B, the tristimulus values represent the coordinates of a three-dimensional vector whose angle specifies chromatic color and whose length characterizes the amount of that color. Chromaticity coordinates, which do not depend on the amount of a color, specify each of the tristimulus values relative to their sum: x = X/(X + Y + Z); y = Y(X + Y + Z); z = Z/(X + Y + Z) ColorScience 293 FIGURE 3 Tristimulus values of the equal-energy spectrum of the 1931 CIE system of colorimetry. [From Billmeyer, F. W., Jr., and Saltzmann, M. (1981). “Principles of Color Technology,” 2nd ed. Copyright ©1981 John Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.] Given any two of these, the third is determined (e.g., z = 1−x − y).Therefore,fullinformationaboutchromaticity can be conveniently represented in a two-dimensional di-agram, with y versus x having been chosen by the CIE for thispurpose.Theresultingchromaticitydiagramisshown in Fig. 4. If one wishes to specify the quantity of light as well,theY tristimulusvaluecanbegiven,allowingacolor tobefullyspecifiedas x, y,andY,insteadof X,Y,and Z. The manner in which the quantity of light Y is specified is determined by the normalization constant k. Depending on the choice of primaries for determining color-matching functions, many other chromaticity dia-gramsarepossible.Forexample,thesetofcolor-matching functions of Fig. 1 leads to the chromaticity diagram of Fig. 5. This so-called RGB system is seldom used. The affine geometry of chromaticity diagrams endows allofthemwithanumberofusefulproperties.Mostfunda-mentalisthatanadditivemixtureofanytwolightswillfall along a straight line connecting the chromaticities of the mixture components. Another is that straight lines on one such diagram translate into straight lines on any other re-latedtoitbyachangeofassumedprimaries.Thelocations oftheimaginaryprimaries X,Y,and Z areshowninFig.5, where one sees that the triangle formed by them com-pletely encloses the domain of realizable colors. The lines X–YandX–ZofFig.5formthecoordinateaxesoftheCIE chromaticity diagram of Fig. 4. Conversely, the lines B–G and B–R in Fig. 4 form the coordinate axes of the chro-maticitydiagramofFig.5.Theunevengridofnonoorthog-onal lines in Fig. 4, forming various angles at their inter-sections, translates into the regular grid of evenly spaced, orthogonal lines in Fig. 5. This illustrates that angles and areashavenoinstrinsicmeaninginchromaticitydiagrams. The CIE in 1964 adopted an alternative set of color-matchingfunctionsbasedonexperimentswithlarge(10◦) fields. Their use is recommended for making predictions about color matches for fields subtending more than 4◦ at the eye. ... - tailieumienphi.vn