Page:Colorimetry104nime.djvu/30

scale defined by each set is arbitrary, but the three units are related by being adjusted so that, for observation by daylight, subtractive combination of one unit of each of the red, the yellow, and the blue scales results in a filter perceived as neutral or achromatic. Each scale is exemplified by many glasses, each glass being marked with the number of unit glasses to which it is equivalent. Although the original purpose of the Lovibond color system was to aid in the color control of beer, these glasses are widely used today for other products such as vegetable oils, lubricating oils, and paint vehicles.

Fig. 14.Curves showing the approach of the Hunter-tristimulus filters [55] combined with incandescent lamp and barrier-layer cell to the CIE spectral tristimulus values for source C.

Fig. 15.Chromaticity discrepancies expected from the use of the muiltipurpose reflectometer with Hunter tristimulus filters [55].

Measurements are all referred to magnesium oxide as the standard white. O, spectrophotoinetric colorimetry, o, multipurpose reflectometer.

Fig. 16.Agreement between multipurpose reflectometer [55] and the chromaticity-difference (subtractive) colorimeter for near-white porcelain-enamel specimens measured relative to magnesium oxide.

A spectrophotometric analysis of the Lovibond color system was made by Gibson and Harris [32], and a scale of the red glasses used in combination with the 35-yellow glass has been constructed by Priest and Gibson [33, 50, 155], having the same unit as the original Lovibond red scale but embodying a closer approach to the principle that the