Page:Tristimulus specification of the Munsell book of color from spectrophotometric measurements (IA jresv31n1p55).pdf/4



Spectral reflection curves of all of the samples noted and listed in table 2 were run on the General Electric recording spectrophotometer at the National Bureau of Standards. The samples were run relative to magnesium oxide [16], with approximately 4 mJ.l slits and over a wavelength range from 400 to 750 m J.l. The samples were backed with black paper for these measurements. Calibration curves were run on each sheet, enabling corrections to be applied to the data for wavelength errors, for 100-percent and zero curve deviations, and for aging of the magnesium oxide comparison surface, in accordance with methods regularly used at the National Bureau of Standards [17, 18].

As already noted, the colorimetric computations were made for four different illuminants. ICI illuminants A and 0 have become well established in colorimetric work. llluminant A is the Plankian radiator or blackbody at 2,842 0 K (02 = 14,320 micron-degrees, or 2,848 0 K with O2 = 14,350); the color temperatures of common incandescent illuminants vary from about 2,600 0 to about 3,100 0 K. llluminant 0 is that produced by a source at 2,842° K combined with a certain Davis-Gibson daylight liquid filter [19]. On the "OSA excitations" basis (used in the design of the Davis-Gibson filters) the resulting color matched that of a Plankian radiator at 6,500 0 K. On the basis of the ICI data the approximate color t emperature of this lamp-and-filter combination is 6,800 0 K. The color and spectmlenergy distribution of I CI illuminant 0 satisfactorily match those of overcast sky or average daylight for colorimetric use.

Iliuminant "D" is that produced by an illuminant at 3,000° K combined with a Macbeth (Corning) daylight glass filter giving a color temperature of approximately 7,500 0 K. The color of illuminant "D", found to be the optimum color for cotton grading, is also being widely used for agricultural grading and textile color matching. Its color closely matches that of the lightly overcast north sky most desired for such work. Illuminant "S" was designed as the blue end point for a series of illuminants representing the range from fully overcast to maximally clear sky. It was devised by weighting Abbot's "sun-outsideatmosphere" energy data by the inverse A4 scattering relation. llluminant "S" has been designated as "limit blue sky."

The colorimetric data on the Munsell samples for ICI illuminant C, representative of average daylight, are of primary interest and the computations were carried out both at the National Bureau of Standards and in the U. S. Department of Agriculture. Those for the other three illuminants were made in the Department of Agriculture. All of the computations in the Department of Agriculture were done by using Hollerith cards and automatically punching sums obtained by the method of progressive digiting. The authors are indebted to Lila F. Knudsen, mathematical statistician of the Food and Drng Administration, for snggesting this rapid method of computation [20]. All of the computations were made by the weighted ordinate method.

The spectral-energy distributions of the four illuminants are shown in figure 1, and in table 1 are given the tristimulus data for the spectrum of each of the four illuminants used in the computations of X, Y, Z and x, y, z.