Page:Colorimetry104nime.djvu/31

Fig. 17.Spectral reflectances of pairs of samples exhibiting various degrees of metamerism.Upper two pairs differ considerably In color, but show little or no metamerism. Lower two pairs are near matches; left, moderately metameric; right, strongly. Lovibond numeral should indicate the number of unit glasses to which the single glass bearing the numeral is equivalent. It is possible to adjust the glasses to slightly lower numerals, if desired, by reducing their thickness slightly [29]. Tintometer, Ltd., makers of the Lovibond glasses, have computed from the published spectrophotometric measurements [32] the chromaticity coordinates, $x,y$, of all the colors of the ideal Lovibond system [140] produced by illuminating the glasses with CIE standard sources B and C. Computation of these colors for source A has been done at the National Bureau of Standards [51]. Figure 18 shows the Lovibond network for source A. In addition to calibration of red glasses on the Lovibond scale, Tintometer Ltd. will also calibrate them on a scale, not precisely the same, set up in 1961 by agreement with the Color Committee of the American Oil Chemists' Society, based on the Priest-Gibson scale and known as the AOCS scale. Lovibond red glasses defining the AOCS scale are on deposit at Tintometer Ltd and at the National Bureau of Standards. The grading of vegetable oils in this country is carried out by means of this scale.

b. Arny Solutions

The Arny solutions consist of groups of solutions whose concentrations are adjusted to produce the color match. The required concentrations are the specifications of the color. The most used group is a triad consisting of half-normal aqueous solutions of cobalt chloride (red), ferric chloride (yellow), and copper sulfate (blue) in 1 percent Fig. 18.The chromaticities of various two-part combinations of Lovibond red, yellow, and blue glasses for source A as computed by Haupt and Douglas [51].Two-part combinations of Lovibond glasses will produce many commercially-important colors. hydrochloric acid. This group produces all colors except deep blue and deep red; it is supplemented by a triad of ammoniated aqueous solutions of potassium permanganate and potassium dichromate [8, 9]. Mellon and Martin [101] have reported the spectral transmittances for a number of solutions for colorimetric standards, including the Arny solutions at three or four concentrations, each for the spectral range 440 to 700 nm. By extrapolation of these data it is possible to find approximately the tristimulus values and chromaticity coordinates, $x$,$y$, on the standard coordinate system adopted in 1931 for these solutions, just as was done by Mellon [102] for the coordinate system used in America before the international agreement. In this way it is possible to transform color specifications from the standard system into the required concentrations of the Arny solutions, and the reverse. The Arny solutions are used in the 11th edition of the United States Pharmacopoeia as standards for the color of cod-liver oil and in carbonization tests with sulfuric acid for 28 organic compounds.

4.3. Pigmented or Dyed Surfaces

If the material standards are pigmented or dyed surfaces, no automatically convenient notation, such as suggested by additive combination of lights or subtractive combination of absorbing elements, is available. Any systematic aspect of the color specification must be derived from the method of identifying the various members of the set of colored surfaces serving as standards.

a. Color Dictionaries

From a color dictionary are obtained definitions of color names in terms of material standards. The primary aim is therefore to provide an array of