ebook img

Encyclopedia of Physical Science and Technology - Measurements, Techniques, and Instrumentation PDF

367 Pages·2001·7.08 MB·English
Save to my drive
Quick download
Download
Most books are stored in the elastic cloud where traffic is expensive. For this reason, we have a limit on daily download.

Preview Encyclopedia of Physical Science and Technology - Measurements, Techniques, and Instrumentation

P1: LLL/MBR P2: FPP Final Qu: 00, 00, 00, 00 Encyclopedia of Physical Science and Technology EN003H-945 June 13, 2001 22:9 Color Science 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 Color rendering General expression for the effect of a light source on the color appearance of objects in com- Chromaticity Ratios x, y, z of each of the tristimulus parison with their color appearance under a reference values of a light to the sum of the three tristimulus val- light source. ues X, Y , Z, these being the amounts of three primaries Color temperature Absolute temperature of a blackbody required to match the color of the light. radiator having a chromaticity closest to that of a light Chromaticity diagram Plane diagram formed by plot- source being specified. ting one of the three chromaticity coordinates against Metamerism (1) Phenomenon whereby lights of differ- another (usually y versus x). ent spectral power distributions appear to have the same Color Characteristics of sensations elicited by light by color. (2) Degree to which a material appears to change which a human observer can distinguish between two color when viewed under different illuminants. structure-free patches of light of the same size and Optimal colors Stimuli that for a given chromaticity have shape. the greatest luminous reflectance. Colorant A substance, such as a dye or pigment, that Primaries (1) Additive: Any one of three lights in terms modifies the color of objects or imparts color to other- of which a color is specified by giving the amount wise achromatic objects. of each required to match it by combining the lights. Colorimetry Measurement and specification of color. (2) Subtractive: Set of dyes or pigments that, when Color matching Action of making a test color appear the mixed in various proportions, provides a gamut of same as a reference color. colors. Color order System of reference whereby the relation of Radiance Radiant flux per unit solid angle (intensity) per one color to another can be perceived and the position of unit area of an element of an extended source or reflect- that color can be established with respect to the universe ing surface in a specified direction. of all colors. Reflectance Ratio of reflected to incident light. 289 P1: LLL/MBR P2: FPP Final Encyclopedia of Physical Science and Technology EN003H-945 June 13, 2001 22:9 290 Color Science Reflection Process by which incident flux leaves a surface match will, of course, occur if there is no physical differ- or medium from the incident side, without change in ence between the fields, and in special cases color matches wavelength. are also possible when substantial physical differences ex- ist between the fields. An understanding of how this can happen provides an opening to a scientific understanding COLOR SCIENCE examines a fundamental aspect of of this subject. human perception. It is based on experimental study un- Given an initial physical match, a difference in color der controlled conditions susceptible to physical measure- can be introduced by either of two procedures, which are ment. For a difference in color to be perceived between often carried out in combination. In the first instance, the two surfaces, three conditions must be satisfied: (1) There radiance of one part of a homogeneous field is altered must be an appropriate source of illumination, (2) the two without any change in its relative spectral distribution. surfaces must not have identical spectral reflectances, and This produces an achromatic color difference. In the sec- (3) an observer must be present to view them. This arti- ond case, the relative spectral distribution of one field is cle is concerned with the relevant characteristics of lights, changed such that, for all possible relative radiances of the surfaces, and human vision that conjoin to allow the per- two fields, no match is possible. This is called a chromatic ception of object color. color difference. When fields of different spectral distributions can be ad- justed in relative radiance to eliminate all color difference, I. PHYSICAL BASIS OF the result is termed a metameric color match. In a color- PERCEIVED COLOR matching experiment, a test field is presented next to a comparison field and the observer causes the two fields to The physical basis of color exists in the interaction of light match exactly by manipulating the radiances of so-called with matter, both outside and inside the eye. The sensation primaries provided to the comparison field. Such primaries of color depends on physiological activity in the visual sys- are said to be added; this can be accomplished by superpo- tem that begins with the absorption of light in photorecep- sition with a half-silvered mirror, by superimposed images tors located in the retina of the eye and ends with patterns projected onto a screen, by very rapid temporal alternation of biochemical activity in the brain. Perceived color can be of fields at a rate above the fusion frequency for vision, described by the color names white, gray, black, yellow, or by the use of pixels too small and closely packed to be orange, brown, red, green, blue, purple, and pink. These discriminated (as in color television). If the primaries are 11 basic color terms have unambiguous referents in all suitably chosen (no one of them should be matched by any fully developed languages. All of these names (as well as possible mixture of the other two), a human observer with combinations of these and many other less precisely used normal color vision can uniquely match any test color by nonbasic color terms) describe colors, but white, gray, and adjusting the radiances of three monochromatic primaries. black are excluded from the list of those called hues. Col- To accomplish this, it sometimes proves necessary to shift ors with hue are called chromatic colors; those without are one of the primaries so that it is added to the color being called achromatic colors. matched; it is useful to treat this as a negative radiance Although color terms are frequently used in reference of that primary in the test field. The choice of exactly to all three aspects of color (e.g., one may speak of a sen- three primaries is by no means arbitrary: If only one or sation of red, a red surface, or a red light), such usage is two primaries are used, matches are generally impossible, scientifically appropriate only when applied to the sensa- whereas if four or more primaries are allowed, matches tion; descriptions of lights and surfaces should be provided are not uniquely determined. in physical and geometrical language. The result of the color-matching experiment can be rep- resented mathematically as t(T ) = r(R) + g(G) + b(B), meaning that t units of test field T produce a color that II. CIE SYSTEM OF COLOR SPECIFICATION is matched by an additive combination of r units of pri- mary R, g units of primary G, and b units of primary A. Basic Color-Matching Experiment B, where one or two of the quantities r, g, or b may be The most fundamental experiment in color science entails negative. Thus any color can be represented as a vector the determination of whether two fields of light such as in R, G, B space. For small, centrally fixated fields, ex- those that might be produced on a screen with two slide periment shows that the transitive, reflexive, linear, and projectors, appear the same or different. If such fields are associative properties of algebra apply also to their empir- abutted and the division between them disappears to form ical counterparts, so that color-matching equations can be a single, homogeneous field, the fields are said to match. A manipulated to predict matches that would be made with a P1: LLL/MBR P2: FPP Final Encyclopedia of Physical Science and Technology EN003H-945 June 13, 2001 22:9 Color Science 291 change in the choice of primaries. These simple relations In the color-matching experiment, an observer is in ef- break down for very low levels of illumination and also fect acting as an analog computer, solving three simulta- with higher levels if the fields are large enough to permit neous equations by iteration, using his or her sensations as significant contributions by rod photoreceptors or if the a guide. Although activity in the brain underlies the expe- fields are so bright as to bleach a significant fraction of rience of color, the initial encoding of information related cone photopigments, thus altering their action spectra. to wavelength is in terms of the ratios of excitations of Matches are usually made by a method of adjustment, an three different classes of cone photoreceptors in the retina iterative, trial-and-error procedure whereby the observer of the eye, whose spectral sensitivities overlap. Any two manipulates three controls, each of which monotonically physical fields, whether of the same or different spectral varies the radiance of one primary. Although such set- composition, whose images on the retina excite each of tings at the match point may be somewhat more variable the three classes of cones in the same way will be indis- than most purely physical measurements, reliable data re- criminable. The action spectra of the three classes of cones sult from the means of several settings for each condition in the normal eye are such that no two wavelengths in the tested. A more serious problem, which will not be treated spectrum produce exactly the same ratios of excitations in this article, results from differences among observers. among them. Although not great among those with normal color vi- sion, such differences are by no means negligible. (For B. Imaginary Primaries those with abnormal color vision, they can be very large.) To achieve a useful standardization—one that is unlikely Depending on the choice of primaries, many different sets to apply exactly to any particular individual—averages of of color-matching functions are possible, all of which de- normal observers are used, leading to the concept of a scribe the same color-matching behavior. Figure 1 shows standard observer. 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.] P1: LLL/MBR P2: FPP Final Encyclopedia of Physical Science and Technology EN003H-945 June 13, 2001 22:9 292 Color Science 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 (though necessarily very limited) description of color, one that can be implemented in automated colorimeters. C. Chromaticity Diagram A useful separation between the achromatic and chromatic aspects of color was achieved in a system of colorimetry adopted by the CIE in 1931. This was the first specifica- tion of color to achieve international agreement; it remains 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 FIGURE 2 Estimates of human cone action spectra (Ko¨nig fun- the curves, an operation that does not alter the predictions damentals) derived by V. Smith and J. Pokorny. [From Wyszecki, they make about color matches. G., and Stiles, W. S. (1982). “Color Science: Concepts and Meth- To specify the color of a patch of light, one begins by ods, Quantitative Data and Formulate,” 2nd ed. Copyright ©1982 integrating its spectral radiance distribution S(λ) in turn John Wiley & Sons, Inc. Reprinted by permission of John Wiley & with the three color-matching functions: Sons, Inc.] ∫ X = k S(λ)x¯ (λ) dλ, ∫ 700.0 nm. Depicted in Fig. 2 are current estimates of Y = k S(λ)y¯ (λ) dλ, 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 Z = k S(λ)z¯(λ) dλ. kinds, agree reasonably well with direct microspectropho- tometric measurements of the absorption spectra of outer The values X, Y , and Z are called relative tristimulus segments of human cone photoreceptors containing the values; these are equal for any light having an equal- photopigments that are the principal determinants of the radiance spectrum. Tristimulus values permit the spec- spectral sensitivity of the cones. ification of color in terms of three variables that are The cone spectral sensitivities may be regarded as color- related to cone sensitivities rather than by continuous spec- matching functions based on primaries that are said to be tral radiance distributions, which do not. Like R, G, and B, imaginary in the sense that, although calculations of color the tristimulus values represent the coordinates of a three- matches based on them are possible, they are not phys- dimensional vector whose angle specifies chromatic color ically realizable. To exist physically, each such primary and whose length characterizes the amount of that color. would uniquely excite only one type of cone, whereas Chromaticity coordinates, which do not depend on the real primaries always excite at least two types. amount of a color, specify each of the tristimulus values Another set of all-positive color-matching functions, relative to their sum: based on a different set of imaginary primaries, is given in Fig. 3. This set, which makes very similar predictions x = X/(X + Y + Z); about color matches as the cone sensitivity curves, was y = Y (X + Y + Z); adopted as a standard by the International Commission on Illumination (CIE) in 1931. z = Z/(X + Y + Z) P1: LLL/MBR P2: FPP Final Encyclopedia of Physical Science and Technology EN003H-945 June 13, 2001 22:9 Color Science 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 = mixture components. Another is that straight lines on one 1 − x − y). Therefore, full information about chromaticity such diagram translate into straight lines on any other re- can be conveniently represented in a two-dimensional di- lated to it by a change of assumed primaries. The locations agram, with y versus x having been chosen by the CIE for of the imaginary primaries X, Y , and Z are shown in Fig. 5, this purpose. The resulting chromaticity diagram is shown where one sees that the triangle formed by them com- in Fig. 4. If one wishes to specify the quantity of light as pletely encloses the domain of realizable colors. The lines well, the Y tristimulus value can be given, allowing a color X–Y and X–Z of Fig. 5 form the coordinate axes of the CIE to be fully specified as x, y, and Y , instead of X, Y , and Z. chromaticity diagram of Fig. 4. Conversely, the lines B–G The manner in which the quantity of light Y is specified and B–R in Fig. 4 form the coordinate axes of the chro- is determined by the normalization constant k. maticity diagram of Fig. 5. The uneven grid of nonoorthog- Depending on the choice of primaries for determining onal lines in Fig. 4, forming various angles at their inter- color-matching functions, many other chromaticity dia- sections, translates into the regular grid of evenly spaced, grams are possible. For example, the set of color-matching orthogonal lines in Fig. 5. This illustrates that angles and functions of Fig. 1 leads to the chromaticity diagram of areas have no instrinsic meaning in chromaticity diagrams. Fig. 5. This so-called RGB system is seldom used. The CIE in 1964 adopted an alternative set of color- ◦ The affine geometry of chromaticity diagrams endows matching functions based on experiments with large (10 ) all of them with a number of useful properties. Most funda- fields. Their use is recommended for making predictions ◦ mental is that an additive mixture of any two lights will fall about color matches for fields subtending more than 4 at along a straight line connecting the chromaticities of the the eye. P1: LLL/MBR P2: FPP Final Encyclopedia of Physical Science and Technology EN003H-945 June 13, 2001 22:9 294 Color Science FIGURE 4 The XYZ chromaticity diagram showing the locations of the RGB primaries of Fig. 5 and the projection of the rectilinear grid of that figure onto this one. [From LeGrand, Y. (1957). “Light, Colour, and Vision,” 2nd ed., Wiley Interscience, New York.] Table I lists values of the CIE color-matching func- very close to being one of the worst possible. This illu- ◦ ◦ tions for 2 and 10 fields at 10-nm wavelength values. minant consists mainly of the paired sodium lines that lie ◦ ◦ Tables for 1-nm wavelength values for 2 and 10 fields very close together (at 589.0 and 589.6 nm) in the “yel- are available in Color Measurement, the second volume low” region of the spectrum; although some other spectral in the series Optical Radiation Measurements, edited by lines are also represented, these are present at such low F. Grum and C. J. Bartleson. relative radiances that low-pressure sodium lighting is for practical purposes monochromatic. For a surface that does not fluoresce, its spectral re- III. COLOR RENDERING flectance characteristics can modify the quantity and geometry of incident monochromatic light, but not its From an evolutionary viewpoint, it is not surprising that wavelength. Viewed under separate monochromatic light sunlight is an excellent source for color rendering. Its sources of the same wavelength, any two surfaces with strong, gap-free spectral irradiance distribution (Fig. 6) al- arbitrarily chosen spectral distributions can be made to lows the discrimination of a very large number of surface- match, both physically and visually, by adjusting the rel- color differences. Color appearance in sunlight provides ative radiances of incident lights. Therefore, no chro- the standard against which the adequacy of other sources matic color differences can exist under monochromatic for color rendering is often judged. illumination. The best sources for color rendering emit continuous A. Best and Worst Artificial Sources spectra throughout the visible region. Blackbody radia- for Color Rendering tion, which meets this criterion, is shown for three tem- Of the light sources in common use today, low-pressure peratures in Fig. 7. These curves approximate those for sodium is one of the poorest for color rendering, coming tungsten sources at these temperatures. P1: LLL/MBR P2: FPP Final Encyclopedia of Physical Science and Technology EN003H-945 June 13, 2001 22:9 Color Science 295 FIGURE 5 The RGB chromaticity diagram showing the locations of the XYZ primaries of Fig. 4. [From LeGrand, Y. (1957). “Light, Colour, and Vision,” 2nd ed., Wiley Interscience, New York.] B. Intermediate Quality C. Efficacy of Fluorescent Lighting The amount of visible light emitted by a source is mea- Much of the radiant flux produced by incandescence sured in lumens, determined by integrating its radiant emerges as infrared radiation at wavelengths longer than power output S(λ) with the spectral luminous efficiency those visible; this is not true of fluorescent light, which is function V (λ). The latter, which is proportional to y¯ (λ), more efficiently produced, accounting for its widespread peaks at ∼555 nm. Therefore, the theoretically most effi- use. This light results from the electrical energizing of mer- cient light source would be monochromatic at this wave- cury vapor, which emits ultraviolet radiation. Although it- length, with the associated inability to render chromatic self invisible, this radiation elicits visible light by causing color differences. Efficacy does not include power lost the fluorescence of phosphors suspended in a layer coating in the conversion from electrical input to radiant output, the inside of a transparent tube. which may vary independently of the efficacy of the light Fluorescent lamps emit energy at all visible wave- finally produced. lengths, which is a good feature for color rendering, but their spectra are punctuated by regions of much higher ra- D. Correlated Color Temperature diance whose spectral locations depend on the phosphors chosen and the visible radiations of mercury vapor. Radi- A blackbody, or Planckian radiator, is a cavity within a ant power distributions of six types of fluorescent lamps heated material from which heat cannot escape. No mat- are shown in Fig. 8. ter what the material, the walls of the cavity exhibit a P1: LLL/MBR P2: FPP Final Encyclopedia of Physical Science and Technology EN003H-945 June 13, 2001 22:9 296 Color Science TABLE I Spectral Tristimulus Values for Equal Spectral Power Source a. CIE 1931 Standard Observer b. CIE 1964 Supplementary Observer Wavelength Wavelength (nanometer) x¯(λ) y¯ (λ) z¯(λ) (nanometer) x¯10(λ) y¯ 10(λ) z¯10(λ) 380 0.0014 0.0000 0.0065 380 0.0002 0.0000 0.0007 385 0.0022 0.0001 0.0105 385 0.0007 0.0001 0.0029 390 0.0042 0.0001 0.0201 390 0.0024 0.0003 0.0105 395 0.0076 0.0002 0.0362 395 0.0072 0.0008 0.0323 400 0.0143 0.0004 0.0679 400 0.0191 0.0020 0.0860 405 0.0232 0.0006 0.1102 405 0.0434 0.0045 0.1971 410 0.0435 0.0012 0.2074 410 0.0847 0.0088 0.3894 415 0.0776 0.0022 0.3713 415 0.1406 0.0145 0.6568 420 0.1344 0.0040 0.6456 420 0.2045 0.0214 0.9725 425 0.2148 0.0073 1.0391 425 0.2647 0.0295 1.2825 430 0.2839 0.0116 1.3856 430 0.3147 0.0387 1.5535 435 0.3285 0.0618 1.6230 435 0.3577 0.0496 1.7985 440 0.3483 0.0230 1.7471 440 0.3837 0.0621 1.9673 445 0.3481 0.0298 1.7826 445 0.3687 0.0747 2.0273 450 0.3362 0.0380 1.7721 450 0.3707 0.0895 1.9948 455 0.3187 0.0480 1.7441 455 0.3430 0.1063 1.9007 460 0.2908 0.0600 1.6692 460 0.3023 0.1282 1.7454 465 0.2511 0.0739 1.5281 465 0.2541 0.1528 1.5549 470 0.1954 0.0910 1.2876 470 0.1956 0.1852 1.3176 475 0.1421 0.1126 1.0419 475 0.1323 0.2199 1.0302 480 0.0956 0.1390 0.8130 480 0.0805 0.2536 0.7721 485 0.0580 0.1693 0.6162 485 0.0411 0.2977 0.5701 490 0.0320 0.2080 0.4652 490 0.0162 0.3391 0.4153 495 0.0147 0.2586 0.3533 495 0.0051 0.3954 0.3024 500 0.0049 0.3230 0.2720 500 0.0038 0.4608 0.2185 505 0.0024 0.4073 0.2123 505 0.0154 0.5314 0.1592 510 0.0093 0.5030 0.1582 510 0.0375 0.6067 0.1120 515 0.0291 0.6082 0.1117 515 0.0714 0.6857 0.0822 520 0.0633 0.7100 0.0782 520 0.1177 0.7618 0.0607 525 0.1096 0.7932 0.0573 525 0.1730 0.8233 0.0431 530 0.1655 0.8620 0.0422 530 0.2365 0.8752 0.0305 535 0.2257 0.9149 0.0298 535 0.3042 0.9238 0.0206 540 0.2904 0.9540 0.0203 540 0.3768 0.9620 0.0137 545 0.3597 0.9803 0.0134 545 0.4516 0.9822 0.0079 550 0.4334 0.9950 0.0087 550 0.5298 0.9918 0.0040 555 0.5121 1.0000 0.0057 555 0.6161 0.9991 0.0011 560 0.5945 0.9950 0.0039 560 0.7052 0.9973 0.0000 565 0.6784 0.9786 0.0027 565 0.7938 0.9824 0.0000 570 0.7621 0.9520 0.0021 570 0.8787 0.9556 0.0000 575 0.8425 0.9154 0.0018 575 0.9512 0.9152 0.0000 580 0.9163 0.8700 0.0017 580 1.0142 0.8689 0.0000 585 0.9786 0.8163 0.0014 585 1.0743 0.8526 0.0000 590 1.0263 0.7570 0.0011 590 1.1185 0.7774 0.0000 595 1.0567 0.6949 0.0010 595 1.1343 0.7204 0.0000 600 1.0622 0.6310 0.0008 600 1.1240 0.6583 0.0000 605 1.0456 0.5668 0.0006 605 1.0891 0.5939 0.0000 610 1.0026 0.5030 0.0003 610 1.0305 0.5280 0.0000 continues P1: LLL/MBR P2: FPP Final Encyclopedia of Physical Science and Technology EN003H-945 June 13, 2001 22:9 Color Science 297 TABLE I (continued ) a. CIE 1931 Standard Observer b. CIE 1964 Supplementary Observer Wavelength Wavelength (nanometer) x¯(λ) y¯ (λ) z¯(λ) (nanometer) x¯10(λ) y¯ 10(λ) z¯10(λ) 615 0.9384 0.4412 0.0002 615 0.9507 0.4618 0.0000 620 0.8544 0.3810 0.0002 620 0.8563 0.3981 0.0000 625 0.7514 0.3210 0.0001 625 0.7549 0.3396 0.0000 630 0.6424 0.2650 0.0000 630 0.6475 0.2835 0.0000 635 0.5419 0.2170 0.0000 635 0.5351 0.2283 0.0000 640 0.4479 0.1750 0.0000 640 0.4316 0.1798 0.0000 645 0.3608 0.1382 0.0000 645 0.3437 0.1402 0.0000 650 0.2835 0.1070 0.0000 650 0.2683 0.1076 0.0000 655 0.2187 0.0816 0.0000 655 0.2043 0.0812 0.0000 660 0.1649 0.0610 0.0000 660 0.1526 0.0603 0.0000 665 0.1212 0.0446 0.0000 665 0.1122 0.0441 0.0000 670 0.0874 0.0320 0.0000 670 0.0813 0.0318 0.0000 675 0.0636 0.0232 0.0000 675 0.0579 0.0226 0.0000 680 0.0468 0.0170 0.0000 680 0.0409 0.0159 0.0000 685 0.0329 0.0119 0.0000 685 0.0286 0.0111 0.0000 690 0.0227 0.0082 0.0000 690 0.0199 0.0077 0.0000 695 0.0158 0.0057 0.0000 695 0.0318 0.0054 0.0000 700 0.0114 0.0041 0.0000 700 0.0096 0.0037 0.0000 705 0.0081 0.0029 0.0000 705 0.0066 0.0026 0.0000 710 0.0058 0.0021 0.0000 710 0.0046 0.0018 0.0000 715 0.0041 0.0015 0.0000 715 0.0031 0.0012 0.0000 720 0.0029 0.0010 0.0000 720 0.0022 0.0008 0.0000 725 0.0020 0.0007 0.0000 725 0.0015 0.0006 0.0000 730 0.0014 0.0005 0.0000 730 0.0010 0.0004 0.0000 735 0.0010 0.0004 0.0000 735 0.0007 0.0003 0.0000 740 0.0007 0.0002 0.0000 740 0.0005 0.0002 0.0000 745 0.0005 0.0002 0.0000 745 0.0004 0.0001 0.0000 750 0.0003 0.0001 0.0000 750 0.0003 0.0001 0.0000 755 0.0002 0.0001 0.0000 755 0.0001 0.0001 0.0000 760 0.0002 0.0001 0.0000 760 0.0001 0.0000 0.0000 765 0.0002 0.0001 0.0000 765 0.0001 0.0000 0.0000 770 0.0001 0.0000 0.0000 770 0.0001 0.0000 0.0000 775 0.0001 0.0000 0.0000 775 0.0000 0.0000 0.0000 780 0.0000 0.0000 0.0000 780 0.0000 0.0000 0.0000 Totals 21.3714 21.3711 21.3715 Totals 23.3294 23.3324 23.3343 characteristic spectral emission, which is a function of its perature, determined by calculating the chromaticity co- temperature. The locus of the chromaticity coordinates ordinates of the source and then locating the point on the corresponding to blackbody radiation, as a function of blackbody locus perceptually closest to these coordinates. temperature, plots in the chromaticity diagram as a curved line known as the Planckian locus (see Fig. 4). The spec- E. Color-Rendering Index tral distribution of light from sources with complex spectra does not approximate that of a Planckian radiator. Never- The CIE has developed a system for attempting to specify theless, it is convenient to have a single index by which the quality of color rendering supplied by any light source. to characterize these other sources of artificial light. For The calculations are based on a set of reflecting sam- this purpose the CIE has defined a correlated color tem- ples specified in terms of their reflectance functions. The P1: LLL/MBR P2: FPP Final Encyclopedia of Physical Science and Technology EN003H-945 June 13, 2001 22:9 298 Color Science The intermediate color-rendering properties of most flu- orescent light sources are closer to the best than to the worst. Mercury vapor and high-pressure sodium sources, widely used for street lighting, have poor color-rendering properties that fall between those of fluorescent and low- pressure sodium illumination. IV. GLOBAL SURFACE PROPERTIES The term reflection characterizes any of a variety of phys- ical processes by which less than 100% of the radiant energy incident on a body at each wavelength is returned without change of wavelength. Reflection is too compli- cated for detailed specification at a molecular level for most surfaces and wavelengths of light. For this reason and because the molecular details are unimportant for many practical purposes, methods have been devised for mea- suring the spectral reflectance of a surface—the spectral FIGURE 6 Spectral power distribution of typical daylight. [From distribution of returned light relative to that which is in- Billmeyer, F. W., Jr., and Saltzmann, M. (1981). “Principles of Color Technology,” 2nd ed., Copyright ©1981 John Wiley & Sons, Inc. cident. Reflectance depends on the wavelength and angle Reprinted by permission of John Wiley & Sons, Inc.] of incidence of the light, as well as the angle(s) at which reflected light is measured. calculations begin with the choice of a reference illumi- A. Specular and Diffuse Reflectance nant specified as a blackbody (or daylight) radiator having a color temperature (or correlated color temperature) as A familiar example of specular reflectance is provided by close as possible to the correlated color temperature of the a plane mirror, in which the angles of light incidence and test illuminant; the choice of reference illuminant depends reflectance are equal. An ideal mirror reflects all incident on the correlated color temperature of the test illuminant light nonselectively with wavelength. If free of dust and (daylight is used as a reference above 5000 K). For each suitably framed, the surface of an even less than ideal real of the samples, defined by their spectral reflectance func- mirror is not perceived at all; instead, the virtual image of tions, the amount of color shift E introduced in going an object located physically in front of the mirror is seen from reference to test illuminant is determined using the as if positioned behind. CIELUV formula described in Section VIB. There are 14 Although specular reflectance seldom provides infor- reference samples in all. A special color-rendering index mation about the color of a surface, there are exceptions. Ri , peculiar to each sample, is calculated as 100–4.6 E. In particular, highly polished surfaces of metals such as Most commonly a single-number index is calculated from gold, steel, silver, and copper reflect specularly. They also the mean of a subset of eight special color-rendering in- reflect diffusely from within but do so selectively with dices to provide a final value known as the general color- wavelength so that the specular reflection is seen to be rendering index Ra. The factor 4.6 was chosen so that tinged with the color of the diffuse component. More of- a standard warm white fluorescent lamp would have an ten, because highlights from most surfaces do not alter the Ra of ∼50; tungsten-incadescent sources score very close spectral distribution of incident light, specular reflection to 100. Table II gives Ra values for several commonly provides information about the color of the source of light used artificial sources. Despite its official status, Ra is of rather than that of the surface. limited value because of its many arbitrary features, espe- Diffuse reflectance, on the other hand, is typically selec- cially its dependence on so limited a set of color samples. tive with wavelength, and for the normal observer under It is most useful for distinguishing large differences in typical conditions of illumination it is the principal de- color rendering, but not so useful for discriminating among terminant of the perceived color of a surface. A surface sources of very high color-rendering properties. Individ- exhibiting perfectly diffuse reflectance returns all of the ual values of Ri can be useful for determining the man- incident light with the distribution shown in Fig. 9, where ner in which light sources differ in their color-rendering the luminance (intensity per unit area) of the reflected properties. light decreases a cosine function of the angle of reflection

See more

The list of books you might like

Most books are stored in the elastic cloud where traffic is expensive. For this reason, we have a limit on daily download.