Page:Colorimetry104nime.djvu/29

sists of identifying the particular member of the system yielding a match for the color to be specified. "We deal here both with systems of material standards of such scope that a considerable fraction of the colors possible in nonself-luminous objects are represented and with a few special small groups of material standards for particular purposes.

Fig. 13a.Tristimulus colorimeters by Gardner Laboratory.

Fig. 13b.Tristimulus colorimeters by Hunter Associates Laboratory that give results in nearly uniform color spacing.

Fig. 13c.Tristimulus colorimeters by Instrument Development Laboratories that provide narrow-band interference filters to aid in evaluating the spectral character of a color.

4.2. Transparent Media

Color systems based upon transparent media take advantage of the fact that it is possible with a fixed source to control the color of the transmitted light over a wide range by introducing varying amounts of three absorbing materials. This is done by permitting the light to pass through two or more elements of the absorbing medium instead of through a single element, and is called subtractive combination or mixture because the action of each element is to subtract a certain fraction of each part of the spectrum of the incident light. The color specification consists of the number of unit elements of each of the three absorbing components required to produce the color match.

a. Lovibond Glasses

The Lovibond color system consists of three sets of colored glasses, red, yellow, and blue [86, 87, 88], the principal coloring materials being gold, silver, and cobalt, respectively. The unit of the