Page:Journal of the Optical Society of America, volume 30, number 12.pdf/17

DECEMBER, 1940

Massachusetts Institute of Technology, Cambridge, Massachusetts

N 1919, the Munsell Color System was subjected to a spectrophotometric examination by Priest, Gibson, and McNicholas, who determined the spectral reflection characteristics of fifteen chromatic standards and nine neutral standards of the system. These spectrophotometric data have now been integrated in accordance with the 1931 I.C.I. standard observer and Illuminant C. The resulting tristimulus values and trichromatic coefficients are listed in Table I.

I. Tristimulus specifications (X, Y, Z) and trichromatic coefficients (x, y) of 15 Munsell Atlas colors computed from spectrophotometric data (1919) on the basis of the I.C.I. standard observer and coordinate system and for I.C.I. Illuminant C.

The instructions given by Professor Munsell for making a pigment color-solid are so complete that the entire color system can be constructed, given any five chromatic colors of the system which are unrelated in the sense that no two have either the same hue or complementary hues. The definitions of hue, value, and chroma are unambiguously established by the operations involved in constructing the system by means of additive mixtures on a Maxwell disk. The hue, value, and chroma of such a mixture are related to those of the components of the mixture as follows:

Hue.—When a chromatic color is mixed additively with a neutral (white, gray, or black), the hue of the mixture is the same as that of the chromatic color.

Value.—When two colors whose values are $$V_a$$ and $$V_b$$; occupy relative areas a and b on a Maxwell disk, the value of the mixture is given by the equation $$V^2=aV_a^2+bV_b^2$$.

Chroma.—When two complementary colors occupy areas on a Maxwell disk which are inversely proportional to the product of value by chroma, a neutral gray results.

By comparison of these quantities with the corresponding quantities in psychophysical systems, it is evident that hue is synonymous with dominant wave-length, that value is the square root of luminous apparent reflectance, $$Y$$, expressed in percent, and that, for a given hue and value, chroma is proportional to colorimetric purity in psychophysical systems.

Tristimulus specifications (X,Y,Z) and trichromatic coefficients (x,y) of the five principal Munsell colors taken from Table I, each set of tristimulus specifications being multiplied by a factor to make Y=0.2500.

When tristimulus specifications have been assigned to five unrelated colors in the Munsell system, the calculation of the tristimulus specifications for all other colors in the system involves no more than a hypothetical repetition of the procedure employed originally by Professor Munsell. In other words, the disk mixtures of Professor Munsell can be made hypothetically by the I.C.I. observer under carefully standard