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ing Fig. 22.Chromaticities of ideal Munsell colors, value 5/, shown on the (x,y)-diagram.This chart serves to define Munsell renotation hue and chroma for colors having $Y/Y_{0}=0.198$. (Prepared by Color Measurement Laboratory, War Food Administration, U.S.D.A.) 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 based on the complete set of standards. Two methods by which to make sure transformations have been described. One, an ASTM method [7a], makes use 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