# Colorimetry/Chapter 4

Because of the convenience of material standards of color, they are often used in commerce in preference to specification according to the more fundamental CIE system. Material standards may be carried from place to place, and, if the colors are sufficiently closely spaced in the neighborhood of the unknown color, the nearest match may be found by visual comparison. The color specification con- ​sists of identifying the particular member of the system yielding a match for the color to be specified. "We deal here both with systems of material standards of such scope that a considerable fraction of the colors possible in nonself-luminous objects are represented and with a few special small groups of material standards for particular purposes.
Color systems based upon transparent media take advantage of the fact that it is possible with a fixed source to control the color of the transmitted light over a wide range by introducing varying amounts of three absorbing materials. This is done by permitting the light to pass through two or more elements of the absorbing medium instead of through a single element, and is called subtractive combination or mixture because the action of each element is to subtract a certain fraction of each part of the spectrum of the incident light. The color specification consists of the number of unit elements of each of the three absorbing components required to produce the color match.
The Lovibond color system consists of three sets of colored glasses, red, yellow, and blue [86, 87, 88], the principal coloring materials being gold, silver, and cobalt, respectively. The unit of the ​scale defined by each set is arbitrary, but the three units are related by being adjusted so that, for observation by daylight, subtractive combination of one unit of each of the red, the yellow, and the blue scales results in a filter perceived as neutral or achromatic. Each scale is exemplified by many glasses, each glass being marked with the number of unit glasses to which it is equivalent. Although the original purpose of the Lovibond color system was to aid in the color control of beer, these glasses are widely used today for other products such as vegetable oils, lubricating oils, and paint vehicles.
A spectrophotometric analysis of the Lovibond color system was made by Gibson and Harris [32], and a scale of the red glasses used in combination with the 35-yellow glass has been constructed by Priest and Gibson [33, 60, 155], having the same unit as the original Lovibond red scale but embodying a closer approach to the principle that the ​Fig. 17.Spectral reflectances of pairs of samples exhibiting various degrees of metamerism.Upper two pairs differ considerably In color, but show little or no metamerism. Lower two pairs are near matches; left, moderately metameric ; right, strongly. Lovibond numeral should indicate the number of unit glasses to which the single glass bearing the numeral is equivalent. It is possible to adjust the glasses to slightly lower numerals, if desired, by reducing their thickness slightly [29]. Tintometer, Ltd., makers of the Lovibond glasses, have computed from the published spectrophotometric measurements [32] the chromaticity coordinates, x,y, of all the colors of the ideal Lovibond system [140] produced by illuminating the glasses with CIE standard sources B and C. Computation of these colors for source A has been done at the National Bureau of Standards [51]. Figure 18 shows the Lovibond network for source A. In addition to calibration of red glasses on the Lovibond scale, Tintometer Ltd. will also calibrate them on a scale, not precisely the same, set up in 1961 by agreement with the Color Committee of the American Oil Chemists' Society, based on the Priest-Gibson scale and known as the AOCS scale. Lovibond red glasses defining the AOCS scale are on deposit at Tintometer Ltd and at the National Bureau of Standards. The grading of vegetable oils in this country is carried out by means of this scale.
The Arny solutions consist of groups of solutions whose concentrations are adjusted to produce the color match. The required concentrations are the specifications of the color. The most used group is a triad consisting of half-normal aqueous solutions of cobalt chloride (red), ferric chloride (yellow), and copper sulfate (blue) in 1 percent Fig. 18.The chromaticities of various two-part combinations of Lovibond red, yellow, and blue glasses for source A as computed by Haupt and Douglas [51].Two-part combinations of Lovibond glasses will produce many commercially-important colors. hydrochloric acid. This group produces all colors except deep blue and deep red; it is supplemented by a triad of ammoniated aqueous solutions of potassium permanganate and potassium dichromate [8, 9]. Mellon and Martin [101] have reported the spectral transmittances for a number of solutions for colorimetric standards, including the Arny solutions at three or four concentrations, each for the spectral range 440 to 700 nm. By extrapolation of these data it is possible to find approximately the tristimulus values and chromaticity coordinates, $x$,$y$, on the standard coordinate system adopted in 1931 for these solutions, just as was done by Mellon [102] for the coordinate system used in America before the international agreement. In this way it is possible to transform color specifications from the standard system into the required concentrations of the Arny solutions, and the reverse. The Arny solutions are used in the 11th edition of the United States Pharmacopoeia as standards for the color of cod-liver oil and in carbonization tests with sulfuric acid for 28 organic compounds.
If the material standards are pigmented or dyed surfaces, no automatically convenient notation, such as suggested by additive combination of lights or subtractive combination of absorbing elements, is available. Any systematic aspect of the color specification must be derived from the method of identifying the various members of the set of colored surfaces serving as standards.
From a color dictionary are obtained definitions of color names in terms of material standards. The primary aim is therefore to provide an array of ​named surface colors adequate for the purpose, and any arrangement or organization of the colors serves only the secondary purpose of assisting the user to find the one which most nearly matches.
The Maerz and Paul Dictionary of Color [97] is the foremost authority on color names. It contains about 7000 different samples of color printed on semiglossy paper, and there are listed about 4000 color names which are keyed to one or another of the color samples. These names are drawn from usage in many fields: paint, textile, ceramic, scientific, technical, and artistic. The samples are also identified by plate, column, and row, and because of their large number and fairly uniform color distribution it is usually possible to find among them a sample approaching what is called a "commercial color match" for any given uniform opaque surface. On this account the Maerz and Paul Dictionary finds a considerable application as a collection of color standards quite separate from its primary function of defining color names. There are noticeable color differences between corresponding samples in different copies of the Dictionary, but the differences have been held to a reasonably small amount by discarding the less satisfactory printed sheets.
The accepted authority for color names in the textile and allied industries is the Color Association of the United States. This association has issued nine editions of a standard color card since 1915, the current edition [149] containing 216 color samples of pure dye silk. Furthermore, the association issues to its members several seasonal cards each year. All colors of these standard and seasonal cards are identified by name and cable number. The standard colors have been measured by spectrophotometric and colorimetric procedures, and luminous reflectance, $Y/Y_{o}$, relative to magnesium oxide, and chromaticity coordinates, $x,y$, for source C have been published [137].
A color dictionary much used for the specification of the colors of flowers, insects, and birds was prepared in 1921 by Eidgway [139]. This outstanding pioneer work contains about 1000 named color samples of paper painted by hand. Each chart shows columns of colors of the same dominant wavelength progressing from each chromatic color at the middle of the column toward white and the top, and toward black at the bottom; and there are five series of such columns, each one encompassing the entire hue circuit, but at different purities. Many of the names were coined at the time of publication to fill in gaps in popular color nomenclature and so have not much descriptive value. Each sample is arbitrarily identified by column, row, and series, however. In addition, there is an alphabetical list of the color names giving this identification.
The notation of the Ostwald system is based on the properties of idealized pigment surfaces having spectral reflectance constant at a certain value between two complementary wavelengths and reflectance constant at a certain other value at other parts of the spectrum [30, 128]. The full colors are those that have the low values of spectral reflectance equal to zero and the high ones equal to 100 percent. The difference between these two reflectances for other idealized pigment surfaces is the full color content, the value of the low reflectance is the white content, and the difference between the high reflectance and 100 percent is the black content. The complete Ostwald notation consists of a number and two letters. The number indicates dominant (or complementary) wavelength on an arbitrary but approximately uniform perceptual scale, and is called Ostwald hue number. The first letter indicates white content, a being a white content of 89.13 percent, which is as near to 100 percent as is practicable for usual pigment-vehicle combinations, and other letters in alphabetical sequence indicating decreasing white content on a logarithmic scale. The second letter indicates black content, a being a black content of 10.87 percent, which is as near to zero as is practicable, and other letters in alphabetical sequence indicating increasing black content on a logarithmic scale. The logarithmic scales were thought by Ostwald to insure uniform color scales, but this is true only to a rough approximation. Since the percent white content, black content, and full-color content must necessarily add up to 100, no explicit indication of the latter is required.
The Ostwald ideas have been a considerable aid in thinking about color relationships on the part of those who duplicate colors by mixtures of chromatic pigments with white and black pigments, and they have served as a guide in the selection of combinations of such colors to produce pleasing effects. However, the use of these idealized pigment surfaces as a basis for a system of colorimetry has been hampered by the fact that actual pigment surfaces approximate them rather poorly, and by the fact that not all actual pigment surfaces can be color matched by one of these ideal surfaces. Still, color charts made up more or less in accord with the Ostwald principles have been widely used for color standards and for the selection of harmonizing colors [63, 127, 129, 148]. Of these, the Jacobson Color Harmony Manual [63] is pre-eminent not only because of its technical excellence, but also because Foss [30] has given a clear statement of which of the somewhat contradictory Ostwald principles were followed in its construction, and Granville and Jacobson [47] have made a spectrophotometric study of the color chips and have published luminous reflectance, $Y/Y_{0}$, relative to magnesium oxide, and chromaticity coordinates, $x,y$ for every chip. These chips are therefore valuable for use in colorimetry by difference from a working standard (see sec. 3), and the fact that the chip is a member of an orderly arrangement of; colors facilitates the selection of a working standard for any particular purpose.
​Fig. 19.Dimensions of the surface-color solid: (a) opaque surf aces (b) transparent volumes.
The basis of the Munsell system is description of colors perceived to belong to surfaces in terms of hue, lightness, and saturation. Each such tridimensional description can be represented by a point plotted in a space diagram known as the surface-color solid, as shown in figure 19. In the surface-color solid the central axis represents the grays extending from black at the bottom to white at the top. Lightness of a chromatic (nongray) color determines the gray to which it is equivalent on this scale. Lightness is represented in the color solid by distance above the base plane. Hue determines whether a color is perceived as red, yellow, green, blue, purple, or some intermediate; it is represented in the color solid by angle about the central axis. Saturation indicates the degree of departure of a surface-color perception from the gray of the same lightness; it is represented by distance from the central gray axis.
The Munsell color system specifies a surface color by giving for usual viewing conditions its position on more or less arbitrary hue, lightness, and saturation scales having perceptually nearly uniform steps [107]. The Munsell term corresponding to lightness is Munsell value, that for saturation is Munsell chroma, and that for hue is Munsell hue. Munsell value is zero for the ideal black surface I having luminous reflectance equal to zero, and it is for the ideal white diffusing surface having luminous reflectance equal to 1. Munsell chroma is expressed in arbitrary units intended to be perceptually of the same size regardless of value and hue. The strongest known pigment colors have chromas of about 16; neutral grays have zero chroma as do black and white. Munsell hue is expressed on a scale intended to divide the hue circuit (red, yellow, green, blue, purple, back to red) into 100 perceptually equal steps. According to one convention the 100 Munsell hues are identified simply by a number from 1 to 100, and on this scale hues that differ by 50 are nearly complementary. Fig. 20.Diagram of hue circle with Munsell hue notation. Fig. 21.Alternate ways of expressing Munsell hue notation. The most common convention, however, is to divide these 100 hues into 10 groups of 10 hues each, and identify each group by initials indicating the central member of the group, thus: red R, yellow red YR, yellow Y, green yellow GY, green G, blue green BG, blue B, purple blue PB, purple P, and red purple RP. The hues in each group are identified by the numbers 1 to 10. Thus, the most purplish of the red hues (1 on the scale of 100) is designated as IR, the most yellowish as 10R, and the central hue as 5R, or often simply as R; see figures 20 and 21. The transition points (10R, 10 YR, 10Y, and so on) between the groups of hues are also sometimes designated by means of the initials of the two adjacent hue groups, thus: R-YR = 10R, YR-Y = 10YR, Y-GY = 10Y, and so on; but this convention is little used. The Munsell notation is commonly written: Hue value/chroma, this is, the hue notation, such as 6R, then the value, such as 7, and finally the chroma, such as 4, the latter two being separated by a solidus: 6R 7/4. More precise designations are given in tenths of the arbitrary steps of the scales, thus: 6.2R 7.3/4.4. The grays are indicated by the symbol N for neutral followed by the value notation, thus: N 7/ or N 7.3/; the chroma being zero for neutrals is not specifically noted.
Three representations of the Munsell system ​have been published, the original Atlas in 1915 [105], now chiefly a collector's item, the Munsell Book of Color in 1929 and 1942 [106], and the Munsell Color Standards in High-Gloss Surface in 1957. The book consists of rectangles of mat-finish handpainted paper permanently mounted on charts in a loose-leaf binding. The high-gloss color standards are detachably mounted. The neutrals form a one-dimensional color scale extending from N 1/ to N 9/. Each chromatic sample, of which there are about 1000, takes its place on three color scales: a hue scale, a value scale, and a chroma scale; and the spacing of these scales is intended to be perceptually uniform. The pocket edition, adapted for determining Munsell notation of unknown colors by visual comparison, consists of forty constant-hue charts, so called because all the samples on each chart have the same Munsell hue. These samples are arranged in rows and columns, the rows being chroma scales at constant Munsell value, the coiumns bemg value scales at constant Munsell chroma. Comparison of an unknown color with these two families of scales gives by interpolation the Munsell value and Munsell chroma of the unknown. Interpolation between adjacent constant-hue charts gives the Munsell hue. Unknowns not too far outside the range of the Munsell charts may be evaluated with some reliability by extrapolation along the value and chroma scales. Table 9 gives Munsell Book notations of the four printing;-ink specimens reproduced on figure 4. The greenish yellow specimen was evaluated by interpolation, the other three by extrapolation of varying degrees of uncertainty, the red-purple specimen being furthest outside the range of the Munsell charts and hence the least certainly evaluated by visual estimate.
To facilitate the comparison of the unknown color with those of the permanently mounted paper rectangles on the Munsell charts, either the unknown color must be brought into juxtaposition with the rectangle and held in nearly the same plane, or, if the form of the unknown prevents such juxtaposition, two masks of thin cardboard having a rectangular opening to fit the paper rectangles should be used. One mask should be placed over the unknown color; the other over one or another of the Munsell colors in succession to obtain the interpolated Munsell Book notation.
Kelly [69, Y6] made effective use of a form of mask with three rectangular openings particularly adapted to comparisons involving powdered chemicals and drugs viewed through a cover glass. It is advantageous to have the color of the mask fairly close to that of the unknown, particularly in Munsell value; that is, if the unknown color is dark, the mask should be of a dark color also. Use of a light mask for a dark color prevents the observer from making as precise a visual estimate as he can make with a mask more nearly a color match for the unknown.
Because of the visual uniformity of the scales, the estimates of Munsell notation for unknown colors within the color range of the charts have a reliability corresponding to the use of a much larger collection of unequally spaced color standards. On this account the pocket edition of the Munsell Book of Color is widely used as a practical color standard for general purposes.
The standard or library edition shows the same colors as the pocket edition, but it shows them on constant-value charts in a polar-coordinate system, and on constant-chroma charts in a rectangular coordinate system, as well as on contant-hue charts. This edition has full explanatory matter in the text and is adapted particularly for teaching color. It is too bulky for convenient practical use in determining the Munsell notation of an unknown color.
The samples of the 1929 Munsell Book of Color have been measured by means of the spectrophotometer twice independently with generally concordant results [39, 78]. In both these studies luminous reflectance, $Y/Y_{0}$, relative to magnesium oxide and chromaticity coordinates, $x$,$y$, of the roughly 400 samples were computed for source C. Glenn and Killian [39] have also published the dominant wavelength and purity for each of the colors; Kelly, Gibson, and Nickerson [78] have published specifications ($Y$,$x$,$y$) for three additional sources (source A, Macbeth daylight, and limit blue sky. Nickerson and Wilson [118] have extended them to nine sources. Furthermore, they have published [78] a series of ($x$,$y$) chromaticity diagrams showing the position of the Munsell colors for each of the Munsell values from 2/ through 8/. From these diagrams it is possible to find the chromaticity coordinates, $x$,$y$, correspond-
$Y/Y_{0}$
ing to any Munsell Book notation; and the reverse transformation is also possible. The samples of the 1942 supplement to the Munsell Book of Color together with many special Munsell standards have been measured spectrophotometrically by Granville, Nickerson, and Foss [46] and by Nickerson, Tomaszewski, and Boyd [119]. These Munsell standards, together with those of the 1929 Munsell Book of Color, number nearly 1500 and comprise the largest systematic set of color standards of known luminous reflectance and chromaticity coordinates ever made. These standards are commercially available separately in disk form and on large sheets, and they make practical the general colorimetry of opaque specimens not only by disk mixture, but also by difference from a standard.
From the charts themselves, luminous reflectance and chromaticity coordinates of an unknown color may also be found quickly and with an accuracy sufficient for many purposes by obtaining first the Munsell Book notation of the unknown and then transforming it by reference to ($x$,$y$) interpolation charts ba.sed on the complete set of standards. Two methods by which to make sure transformations have been described. One, an ASTM method [7a], makes u.se of tables and charts of corresponding CIE chromaticity coordinates and Munsell notations. The other is an automatic-computer program [75] by which CIE notations are transformed to Munsell notations.
The Munsell color standards may also be used, though less conveniently, in the colorimetry of light-transmitting [69, 76] elements (gelatin films, crystal and glass plates, solutions, and so on); and, conversely, such elements may be given Munsell Book notations from their CIE specifications by means of interpolation charts. Table 12, to be discussed presently in another connection, shows Munsell Book notations so derived from the luminous transmittances, T, and chromaticity coordinates, of the glass standards of the ASTM Union colorimeter. The last four book notations given are relatively uncertain because the colors to be specified are far outside the range of the Munsell standards.
The spacing of the Munsell colors has been examined in detail by a subcommittee of the Colorimetry Committee of the Optical Society of America [111] . This committee work confirmed the many local irregularities in spacing revealed by the spectrophotometric studies and established the need for some more general but minor adjustments to make the colors of the Munsell charts correlate more nearly perfectly under ordinary conditions (adaption to daylight, gray surrounding field, and so on) with the surface-color solid. The subcommittee found it possible from this study to recommend specifications ($Y$,$x$,$y$) on the standard coordinate system defining an ideal Munsell system [112]. It has also given to every Munsell standard a revised notation, called the Munsell renotation, indicating exactly in what way and how much each color chip deviates from the ideal. Furthermore, the recommended definition of the ideal Munsell system has been extended beyond the color ranges covered by the present Munsell charts so as to include all colors theoretically producible from nonfluorescent materials [83] under source C. The connection between luminous reflectance, $Y$ , and Munsell renotation value is given in Table 10. Note that N 0/ corresponds to $Y$ = 0, N 9.91/ corresponds to magnesium oxide (Y = 1.00), and N 5.0/ corresponds to $Y$ = 0.198 (quite different from 0.50, the halfway point). Figure 22 shows one of the ($x$,$y$)-chromaticity charts (that for Munsell renotation value equal to 5.0) defining the ideal system. From these charts combined with the data given in table 10 it is possible to find the ideal Munsell notation for any color specified in terms of luminous reflectance, $Y$, and chromaticity coordinates, $x$,$y$. Table 9 shows these Munsell renotations for the four printing-ink specimens of figure 4. Note that they agree in a general way, though not perfectly, with the notations found by direct visual comparison with the Munsell Book of Color. Some of the discrepancies are ascribable to local irregularities of the color spacing of the Munsell charts, but most of them may be laid to the uncertainty of the visual estimates, all but that for the greenish yellow specimen requiring extrapolation over a considerable color range.
Munsell renotations, such as these, have a unique usefulness as color specifications. Because of their definition in terms of the standard CIE coordinate system they are capable of nearly the precision of the $Y$,$x$,$y$ form of specification, and like that form they may be extended to apply to all object colors, both opaque and transparent objects. For opaque surfaces $Y/Y_{0}$ is luminous reflectance relative to magnesium oxide; for transparent objects $Y/Y_{0}$ is luminous transmittance relative to an equivalent thickness of air; for solutions $Y/Y_{0}$ is luminous ​transmittance relative to the same thickness of distilled water or solvent. For any of these objects Munsell renotation value, $V$, may be found from $Y/Y_{0}$ in accord with table 10. Because of their close correlation with the color solid, Munsell renotations are capable of being quickly understood. Thus the renotation 8.3Y 8.6$_0$/10.$_6$ for the greenish yellow printing-ink specimen indicates from the letter Y that the specimen is a yellow, from the value 8.6$_0$ that it is a relatively light color, being close to the top of the value scale, 0 to 10, and from the chroma 10.$_6$, that it is a strong or nongrayish color, being more than 10 Munsell chroma steps away from the gray of the same Munsell value. Munsell renotation hue and chroma serve more adequately for object colors the purposes formerly served by dominant wavelength and purity. Munsell renotation hue correlates significantly better under ordinary conditions of daylight observation with the hue of the perceived color than does dominant wavelength, and Munsell renotation chroma is by far superior to purity in its correlation to saturation. This correlation with the color-perception solid does not, however, necessarily hold under all observing conditions. Ordinarily this printing-ink specimen will be perceived to have a light, strong greenish yellow color, but it is not so perceived under all conditions. If this specimen be viewed next to a brilliant yellow-green area such as is provided by a fluorescent fabric, it will be perceived to take on a darker color of yellowish orange hue and moderate saturation. Thus the lightness, hue, and saturation of the color perception depend upon the surroundings and upon the adaptive state of the eye; and lightness, hue, and saturation are taken correctly to be psychological terms. But the Munsell renotation refers only to the light that is reflected from the specimen and stays constant regardless of these changes in observing conditions. It is therefore a psychophysical characterization of the specimen according to the light reflected from it, just as are luminous reflectance,$Y$, and chromaticity coordinates, $x$,$y$ from which it can be derived.
​Another advantage of expressing spectrophotometric results in the form of the Munsell renotation is that the amount and kind of the color difference between two specimens can be found immediately from the two renotations in an easily understandable form. Thus, from the hue difference between the two blue printing-ink specimens (5.6B compared to 0.8PB)), the former is seen to be more greenish (less purplish) by about five Munsell hue steps. From the value difference (5.4$^6$ compared to 5.5$^0$) the two are seen to be of the same value to the nearest one-tenth Munsell value step; and from the chroma difference (8.,5 compared to 9.6) the greenish blue is seen to be more grayish by one Munsell chroma step. Such differences as these in terms of Munsell hue, value, and chroma may be combined info a single index, I, of color difference [115,117]:
$I={\frac {C}{5}}(2\Delta H)+6\Delta V+3\Delta C$
(8)
where $C$ is Munsell chroma, and $\Delta H$, $\Delta V$, $\Delta C$, are the differences between the two colors in Munsell hue, value, and chroma, respectively. The difference between the colors of the two blue printing-ink specimens would be found by this formula as:
${\begin{aligned}I&={\frac {9.0}{5}}(2\times 5.2)+6\times 0.04+3\times 1.1\\&=18.7+0.2+3.3=22.2\ {\text{units.}}\\\end{aligned}}$
These units are of such size that color differences of less than one unit would ordinarily not be of commercial importance; that is, pairs of colors exhibiting such differences would be considered to be commercial matches. Note that these two blue printing-ink specimens are far from being a commercial match; also note that the hue difference is far more important than the chroma difference, and that the value equivalence is well within commercial toleration.
A method devised at the request of the American Pharmaceutical Association and the United States Pharmacopoeial Convention for designating the colors of drugs and chemicals is coming into use for general purposes. The general plan of the method was worked out by the Inter-Society Color Council, and the details were developed at the National Bureau of Standards; the method is therefore referred to as the ISCC-NBS method of designating colors [69, 79]. This method provides a designation for every color perceived as belonging to an object (either an opaque surface, or a light-transmitting layer), and it has been extended to the colors of self-luminous areas by Kelly [77]; see figure 3. The number of color designations was purposely made small, 267, for the sake of simplicity. Since about fen million surface colors can be distinguished by the normal human observer with optimum observing conditions, the ISCC-NBS method falls far short of supplying a different designation for each distinguishable color, or even for all colors (numbering perhaps half a million) considered to be commercially different.
The plan of the method is to divide the surface-color solid (see fig. 19) arbitrarily into 267 compartments, and assign a designation to each in as good conformity as possible to color nomenclature currently used in art, science, and industry. The compartments embracing the black-white axis are given the following designations: black, dark gray, medium gray, light gray, and white. The compartments adjacent to these are given similar designations formed by adding an adjective indicating the hue, such as yellowish white, dark purplish gray, or greenish black. All other compartments take designations consisting of a hue name (red, orange, yellow, green, blue, purple, pink, ​Fig. 23.The modifiers used in the color designations of the ISCC-NBS system [75]. brown, olive) preceded by modifiers (light, dark, grayish, strong) indicating the lightness and saturation of the perceived color. Figure 23 shows the complete list of modifiers used in the ISCC-NBS system; note that vivid is a substitute for very strong; pale, a substitute for light grayish; deep, for dark and strong, and so on.
The boundaries between the groups of colors known by these designations have been adjusted to accord as closely as possible with common usage and have been expressed in terms of Munsell Book notation. Figure 23 also shows these boundaries for colors of purple hue.
The ISCC-NBS designations are not to be considered a substitute for numerical designation of color resulting from application of a suitable colorimetric method, but they do supply a certain precision to verbal color designations that has previously been lacking. Table 11 gives the ISCC-NBS designations of the four printing-ink specimens shown on figure 4 and the ISCC-NBS hue designations have been used throughout the text of this chapter. These color designations are used in the current edition (N.F. VII) of the National Formulary, and they are being inserted into the 'United States Pharmacopoeia and into a textbook of qualitative analysis [130]. They have been used for describing the colors of building stone [83] and soils [138], and for a considerable variety of research purposes such as the description of mica colors after heat treatment [53]. Kelly [80] has found the Munsell notation for the centroid color of each compartment assigned an ISCC-NBS color designation and has recommended a system of abbreviations; see table 11.
The ISCC-NBS designations are generally unsuited for use in sales promotion. The method has been approved for color description of drugs and chemicals by the delegates of nine national societies, and it has been recommended for general use by the United States of America Standards Institute [161a].
In 1958 the method was recommended by the Subcommittee on the Expression of Historical Color Usage [60], Inter-Society Color Council, for the statistical expression of color trends. The adaptation of the method for this purpose involved the tabulation of six distinct, but correlated, degrees or levels of accuracy in color description [82]. In the first level the color solid is divided into just 13 parts, ten described by a generic hue name and three neutrals, white, gray and black. In the second level, the color solid is divided into 29 parts, ten of the original 13 parts being further divided and assigned intermediate hue names. In the third level each part of the color solid described by a generic or intermediate hue name and the appropriate modifier descriptive of its lightness and saturation; that is, in the third level of accuracy, the 267 color designations of the ISCC-NBS method are used without abridgement. Level four is illustrated by the Munsell Book of Color [100] with its more than 1,000 uniformly spaced color samples. Level five is illustrated by the interpolated Munsell book notation by which about 100,000 different colors can be reliably specified. Level six, the level in which the greatest accuracy of color identification is possible, is illustrated by two basic methods: the CIE method, and the interpolated Munsell renotations. This degree of accuracy is realizable only through the measurement of the color with a spectrophotometer, and is equivalent to the division of the color solid into about 5,000,000 parts. Figure 24 gives further information about the six levels: (A) level of fineness of color identification, (B) number of divisions of color solid, (C) type of color designation, (D) example of color designation, and (E) alternate color-order systems usable in that level.
Through the cooperation of the Inter-Society ​Color Council and the Tobey Color Card Co., St. Louis, Mo., the National Bureau of Standards commenced issuing the ISCC-NBS Centroid Color Charts [109] in February 1965. Each set consists of 18 charts displaying samples closely approximating 215 of the 267 centroid colors, and fair approximations of 36 others, making 251 color samples in all. The samples are in the form of one-inch squares of paper coated with glossy-finish paint affixed to a variable gray background so that each color is on a neutral background of approximately its own lightness. The Munsell renotation of each color sample is supplied in the cover pages to the charts. Duplicates of each of the 251 ISCC-NBS centroid colors can be obtained in 9 by 12 in sheets from the Munsell Color Company, Inc. of Baltimore, Md. These centroid colors have been used as standards for a wide variety of purposes. They facilitate color description at level three, and have found their most important application so far in the recording of data for establishing trends of public acceptance of color in various lines of merchandise.