AATCC 173:2005 pdf download.CMC: Calculation of Small Color Differences for Acceptability
1.1 The CMC formula is a modi- fication of the corresponding CIELAB color-difference formula. It has color- difference symbol ∆E cmc . Even though use of the CIE 1976 L*a*b* (CIELAB) equa- tion provides industry with a ‘standard’ means of calculating and communicating color differences of trials from their stan- dards (see 8.1), it is recognized that CIELAB color-difference values (∆E*) correlate poorly with visual assessments. The lack of correlation is caused by the non-uniformity of CIELAB color space (see Fig. 1). Significantly improved cor- relation between visual assessments and instrumentally measured color differ- ences is given by the CMC color- difference formula (see 7.1 to 7.3). The improvement in correlation between vi- sual and instrumental color-difference re- ports yielded by the CMC equation will usually permit the adoption of a sin- gle-number tolerance for judging the ac-ceptability of a color match in most situa- tions, 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 improve- ment over CMC (see 8.2).
1.2 The CMC formula retains, in modified form, the partitioning of overall color difference into differences in light- ness, chroma, and hue components (see Fig.2) which is present in the CIELAB color-difference formula. Using the ellip- soidal semi-axes (eSL, cSc. and Sμ) makes the CMC equation suitable for a wide range of uses. 2. Principle 2.1 The CMC modification to CIELAB provides a unit of measurement for the acceptance volume about a stan- dard color. This volume takes the shape of an ellipsoid whose semi-axes are lSL, cSc, and SH in the directions of lightness, chroma, and hue differences, respec- tively, in CIELAB color space. The CMC formula varies the ratio of the lengths of these three semi- axes system- atically throughout CIEI AB color space according equality of lengths with visu- ally equal color differences,AATCC 173 pdf download.