Page:Journal of the Optical Society of America, volume 30, number 12.pdf/37

 color system based upon photometry and disk mixture which will be in satisfactory agreement with the requirements of an ideal psychological system. It would seem to be worth while to study the psychophysical color system resulting from five new principal colors chosen so that their equal-area disk mixture will have the same trichromatic coefficients (x,y) as the illuminant. When this is done, we should be able to decide with certainty not only whether the 1919, 1926, or 1929 series of Munsell colors more closely approach a psychological system or the psychophysical system dealt with here, but also the more important question whether or not a simply defined psychophysical color system similar to it can be made to meet the requirements of the ideal psychological system envisaged by A. H. Munsell.

An examination has been made of the Munsell color system as it existed in 1919 and 1926. This examination has been based on colorimetric data derived from published and unpublished spectrophotometric measurements made in those years on samples representative of the Munsell Atlas. The relation between Munsell value and luminous apparent reflectance, $$V^2=100k( Y-0.007)$$, closely similar to that found by others, has been confirmed in a general way. While there are large individual variations, it is found that the above relation, with k close to unity, adequately represents the data as a whole. The white point on this scale corresponds to magnesium oxide, the black point to a sample reflecting diffusely about 0.7 percent.

The relation between Munsell Atlas hue and dominant wave-length depends upon the neutral (reference) point adopted. At best there are significant deviations from one-to-one correspondence. Therefore the terms for dominant wave-length and for Munsell hue as illustrated by the original papers of the Munsell system cannot be used interchangeably.

A psychophysical system has been developed, based on the inverse $$CV$$ weighting indicated in the Atlas instructions for a disk mixture to yield neutral gray.

In this psychophysical system the trichromatic coefficients $$(x, y)$$ of a Munsell color of a given hue depend only upon the ratio of value to chroma $$(V/C)$$. The following relations between excitation purity and colorimetric purity on the one hand and Munsell chroma and value on the other are obtained from this derivation:

$$P_{e(V/C)}=\frac{P_{e(5/5)}y_{5/0}}{y_{5/5}(V/C-1)+y_{5/0}}$$

$$P_{c(V/C)}=(C/V)P_{c(5/5)}$$

The 1919 and 1926 data indicate rough agreement with these relations, particularly for middle Munsell values; but there are consistent deviations from them, greater at low Munsell values than at high. Comparison of the data of this paper with the psychological data obtained by the Newhall subcommittee indicates that the above departures from the psychophysical system are in the direction to give chroma scales which are perceptually more uniform, and therefore that there has been an intentional departure from the psychophysical system.

1. Dorothy Nickerson, “,” J. Opt. Soc. Am. 30, 575 (1940).

2. I. G. Priest, K. S. Gibson and H. J. McNicholas, “An examination of the Munsell color system. I. Spectral and total reflection and the Munsell scale of value,” Bur. Stand. Tech. Paper No. 167 (1920).

3. A. H. Munsell, Atlas of the Munsell Color System (Wadsworth-Howland & Co., Malden, Mass., 1915). (Preliminary charts A and B published in 1910.)

4. H. J. McNicholas, “Equipment for routine spectral transmission and reflection measurements,” J. Research Nat. Bur. Stand. 1, 793 (1928); RP30.

5. G. F. A. Stutz, “Observations of spectrophotometric measurement of paint vehicles and pigments in the ultraviolet,"” J. Frank. Inst. 200, 87 (1925).

6. K. S. Gibson, “Spectrophotometry at the Bureau of Standards,” J. Opt. Soc. Am. 21, 564 (1931).

7. A. C. Hardy, Handbook of Colorimetry (Technology Press, Cambridge, Mass., 1936).

8. A. H. Munsell, “On the relation of the intensity of chromatic stimulus (physical saturation) to chromatic sensation,” Psychol. Bull. 6, 238 (1909).

9. A. H. Munsell, “A pigment color system and notation,” Am. J. Psychol. 23, 236 (1912).

10. A. H. Munsell, Color Diary, Bibliofilm Document No. 1307 ($2.50) obtainable from the American Docu