AATCC 173-2015 CMC: Calculation of Small Color Differences for Acceptability

AATCC 173-2015 CMC: Calculation of Small Color Differences for Acceptability. 1.1 The CMC (C:c) formula is a modification of the corresponding CIELAB color-difference formula. It has colordifference symbol AE me 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 (AE*) correlate poorly with visual assessments. The lack of correlation is caused by the nonuniformity of CIELAB color space (see Fig. 1). Significantly improved cor- relation between visual assessments and instrumentally measured color differences is given by the CMC (l:c) colordifference formula (see 7.1 to 73). The improvement in correlation between vi- sual and instrumental color-difference re- ports yielded by the CMC (0: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)。 1.2 The CMC (C: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 semiaxes (eSL, cSc, and Sm) makes the CMC (e:c) equation suitable for a wide range of uses. 2. Principle 2.1 The CMC (Ec) 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 semiaxes are lS, cSC, and Su in the directions of lightness, chroma, and hue differences, respectively, in CIELAB color space. The CMC (E:c) formula varies the ratio of the lengths of these three semiaxes systematically throughout CIELAB color space according equality of lengths with visually equal color differences, regardless of both the color of the standard and direction of difference of any trial from it. Around any given standard color, the ratio S:ScSH is fixed regardless of the end use of the trials. 3. Terminology 3.1 CIE 1976 L*a*b* equation, n.a commonly used equation which transforms CIE tristimulus values into a threedimensional opposed color space. Gener- ally abbreviated as CIELAB (see 8.1). 3.2 CMC unit, n.- in color difference evaluation, a measure of acceptability...