lOVIBOND red, yellow, and blue glasses' are widely used in color grading the various materials andarticles of commerce (vegetable oils, petroleum products, Naval stores, paint vehicles, and so forth). Thebasic plan of the system is that each glass is markedwith the number of unit glasses to which it is equivalentcalorimetrically; that is, the number of unit glassesthrough which light from the source must be passed toproduce the same color. Furthermore, the unit glassesof the red, yellow, and blue scales are related so thatcombinations of all three kinds of glasses of equal numbers on the Lovibond scales give a nearly neutral filter;that is, produce little or no change in chromaticity fromthat of the source. Because of the relation between thethree scales, the chromaticity of a color matched byLovibond glasses of all three sorts, say by glasses ofnumerals, R, Y, B, where R and Y both are greater thanor equal to B, corresponds closely to that of the twopart combination (Re, Ye), where RC= R-B, Y= Y-B.Thus all Lovibond chromaticities may be closely identified with those of two-part combinations, and no separate consideration of three-part combinations is requiredfor chromaticity.The numbers engraved on the glasses by the makerthroughout the years have followed the basic planclosely, but some of the uses to which the glasses havebeen put require a more precise grading than has alwaysbeen maintained by the maker. The present paper givesthe derivation of an ideal Lovibond color system consisting of colorimetric definitions in fundamental termsagainst which any actual glass can be compared and anew numeral in the ideal system assigned. This systemis based upon spectrophotometric determinations oftwenty glasses of each series (1,2,3, . . ., 20). Some of* A more complete report of this study appears in J. ResearchNatl. Bur. Standards 66C, 121 (1962).t Managing Director, The Tintometer Limited, The ColourLaboratory, Salisbury, England.I J. W. Lovibond, Measurement of Light and Colour Sensations(George Gill & Sons, London, 1893).these data are those' obtained at the National Bureau ofStandards for the glasses of the set (BS9940) purchasedTABLE I. Spectral internal transmittances of the Lovibondunit red, yellow, and blue glasses.
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