Page:Journal of the Optical Society of America, volume 33, number 7.pdf/3

 Journal of the

OPTICAL SOCIETY of AMERICA

Volume 33, Number 7  July, 1943

Tristimulus Specification of the Munsell Book of Color from Spectrophotometric Measurements

KENNETH L. Kelly, Kasson S. Gibson, National Bureau of Standards, Washington, D. C., AND Dorothy Nickerson, Food Distribution Administration, U. S. Department of Agriculture, Washington, D. C.

The development of the Inter-Society Color Council-National Bureau of Standards (ISCC-NBS) system of color names, based on the standards in the Munsell Book of Color, made it necessary to specify the master standards of this book in fundamental terms. Accordingly, spectral reflection curves were run for each of the 421 master standards on the General Electric recording spectrophotometer at the National Bureau of Standards, using slit widths of approximately 4 millimicrons. Various corrections were applied to these spectrophotometric data in accordance with methods regularly used for such work at the bureau. Colorimetric computations were then made with these data, resulting in tristimulus specifications according to the 1931 ICI standard observer and coordinate system. Four illuminants were used: ICI Illuminants A and C, representative of incandescent-lamp light and average daylight, respectively, Illuminant D (lightly overcast north sky), and Illuminant S (extremely blue sky). The colorimetric specifications of the Munsell standards for all four illuminants are thus given. The trilinear coordinates for the Munsell standards calculated for ICI Illuminant C have been plotted on large chromaticity (x, y) diagrams and constant Munsell chroma lines drawn in. (Similar values obtained by Glenn and Killian at the Massachusetts Institute of Technology in 1935 for Munsell color standards bearing the same hue-value-chroma designations have also been plotted on the diagram and differences between the two sets of data are discussed.) These diagrams serve as means for determining the Munsell notation and thereby the ISCC-NBS color name for any color whose trilinear coordinates and apparent reflectance are given.

I. INTRODUCTION

WO of the official compendia of drugs and medicines, the United States Pharmacopoeia and the National Formulary, specify the purity and quality of drugs by a number of tests for which tolerance limits are set; with a crude drug, for example, these tests refer to ash, acid insoluble ash, size, chemical identification tests, taste, color, and so forth, these being indications of purity or quality. All of the tests except color have been under continuous study by committees entrusted with their revision. Color, on the other hand, presented a different type of problem whose solution was not attempted until 1931. Previously the color terms used in the USP and NF had enjoyed no official definition but contained among others such confusing terms as brownish green or blackish white, with seldom any reference to a color chart or standard. In the monograph of a drug, the pharmacognocist describes the colors of the outside and the inside, the colors of the various microscopic elements, and