Description

AATCC 173 – CMC: Calculation of Small Color Differences for Acceptability

The CMC (l:c) formula is a modification of the corresponding CIELAB color-difference formula. It has color-difference symbol DEcmc. Even though use of CIELAB 1976 L*a*b* (CIELAB) equation provides industry with a “standard” means of calculating and communicating color differences of trials from their standards (see 8.1), it is recognized that CIELAB color difference values (DE*) correlate poorly with visual assessments. The lack of correlation is caused by the non-uniformity of CIELAB colorspace (see Fig. 1). Significantly improved correlation between visual assessments and instrumentally measured color differences is given by the CMS (l:c) color difference formula. The improvement in correlation between visual and instrumental color-difference reports yielded by the CMC (l:c) equation will usually permit the adoption of a single-number tolerance for judging the acceptability of a color match in most situations, regardless of both the color of the standard and the direction of the color difference of any trial from it. Another equation, CIE94 has been considered, but it has not shown any significant improvement over CMC (see 8.2).

The CMC (l:c) formula retains, in modified form, the partitioning of overall color difference into differences in lightness, chroma, and hue components (see Fig. 2) which is present in the CIELAB color-difference formula. Using the ellipsoidal semi-axes (lSL, cSc, and SH) makes the CMC (l:c) equation suitable for a wide range of uses.