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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 8.6₀/10.₆ for the greenish yellow printing-ink specimen indicates from the letter Y that the specimen is a yellow, from the value 8.6₀ 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 (5.4⁶ compared to 5.5⁰) 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.

d. ISCC-NBS System

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,