Page:Colorimetry104nime.djvu/28

3.4. Photoelectric Devices

If three photocells could be adjusted, as by glass filters, so that their responses were proportional throughout the visible spectrum to some linear combination [as in eq. (3) of the standard CIE distribution curves (see fig. 2)], then they could be used to test whether any two light beams have the same color according to eq. (2) and could be made to yield direct measurements of tristimulus values, $X,Y,Z$ [34, 45]. The $X$-function filter is the most difficult to design because it has two lobes, one short-wave lobe and one long-wave lobe. Two separate filters to cover a portion of one photocell have been designed for this bilobal function by Barnes [10] and by Nimeroff and Wilson [121]. Figure 12 shows the $X$-function match achieved in the Nimeroff-Wilson colorimeter. This function has also been fitted by a filter-photocell combination to approximate the long-wave lobe to which a portion of the $Z$-function is added, either electrically, as in the Color Difference Meter of Hunter [56, 57] and arithmetically, as in the Multipurpose refiectometer of Hunter [55], the Colormaster of Glasser and Troy [38] and the Color Eye of Bentley [11]. In these instruments the transformation equations to obtain $X,Y,Z$ for source C may be represented as:

${\begin{aligned}X&=0.80R+0.18B\\Y&=1.006G\\Z&=1.18B\\\end{aligned}}$

(7)

Fig. 12.Spectral transmittance of the required (light line) and the approximate filter (heavy line) for CIE function $X$ in the Nimeroff-Wilson colorimeter.

where R,G, and B are settings with red, green, and blue filters, respectively. In some of these instruments the filter for the long-wave lobe of the $X$-function may be amber, hence settings with this filter may be designated A. Van den Akker [152] has discussed the incompleteness of success of these colorimeters to duplicate the CIE standard observer system. Recent versions of Color Difference meters and the Color Eye are shown in figure 13.

Figure 14 shows the degree of success achieved by the filters designed by Hunter [55] to duplicate the CIE standard observer and simultaneously to adjust the spectral distribution of a projection lamp to that for CIE source C. Figure 15 shows the discrepancies that the filters of his multipurpose reflectometer introduce. These discrepancies are roughly proportional to the distance from the point representing the magnesium-oxide standard, and are frequently larger than 0.02 in $x$ or $y$; that is, more than 10 times a reasonable chromaticity tolerance for most colorimetric work. However, for the comparison of near-white surfaecs this degree of duplication is sufficient. Figure 16 refers to the small rectangle near the center of figure 15 and indicates that the discrepancies are less than 0.001 in $x$ or $y$ for comparison of near-white surfaces with magnesium oxide. In general, a similar agreement can be expected in using this photoelectric tristimulus colorimeter for the determination of small chromaticity differences between nonmetameric pairs. And even for measurement of fairly sizable nonmetameric chromaticity differences, such as analyzed spectrophotometrically in the upper portion of figure 17 (BPB 8/2 versus MgO, BG 7 /4 versus BG 6/4) , and small chromaticity differences with a moderate metameric component, such as shown in the lower left portion of figure 17 ($Y_{1}$ versus $Y_{2}$), the discrepancy is in the neighborhood of 0.002 in $x$ or $y$, which is negligible for many purposes. However, for highly metameric pairs, such as shown in the lower right portion of figure 17, the discrepancy may be expected to be in the neighborhood of 0.02, just as it is for large chromaticity differences.

If the limitations of photoelectric tristimulus colorimetry are appreciated, the method is most useful in product-control colorimetry of non-fluorescent specimens by difference from a working standard. The precision of the method is comparable, though perhaps not quite equal, to the best that can be done by eye. No unusual qualifications or extended special training is required by the operator; and, compared to visual colorimetry or to indirect colorimetry by the spectrophotometer, the results are obtained very rapidly.

4. Color Specification by Material Standards

4.1. General Principle

Because of the convenience of material standards of color, they are often used in commerce in preference to specification according to the more fundamental CIE system. Material standards may be carried from place to place, and, if the colors are sufficiently closely spaced in the neighborhood of the unknown color, the nearest match may be found by visual comparison. The color specification con-