Understanding Delta E CMC Color Tolerance TB 130C

CMC Color Tolerance

CMC is not a color space but a color tolerance represented by a single numerical value, E. CMC is based on CIELCH where L is lightness, C is chroma, and H is hue.

Figure 1 Coordinates of color space (Eye-Color, 2002)

The CMC calculation defines an ellipsoid around the standard color while the volume of the ellipsoid represents acceptable color variation. The size and shape of the ellipsoid automatically varies depending on the location in color space. Colors in the orange area of color space are more sensitive to shifts in hue, which result in narrower ellipsoids, while colors in the green area of color space have broader ellipsoids due to their low sensitivity to hue. Additionally, lightness will decrease hue and chroma tolerances as it increases or decreases from standard.

Tolerance ellipsoides are tightly packed in the orange region.

Green Red Tolerance ellipsoides are larger in the green region.

Figures 2 + 3 CMC tolerance of color space (X-Rite, 2000) Blue

CMC vs CIELAB Color Tolerance

CIELAB color tolerance is defined by the change in color space on the L, a, and b axes where L is lightness, a is red/ green, and b is yellow/blue. Limits placed on the L, a, and b create a rectangular tolerance box around the color standard. This evenly segments color space into grids and assumes change in appearance is uniform throughout color space.

CMC vs CIELAB Color Tolerance

The Human Observer and Visual Perception The human eye does not detect color differences in hue, chroma, or lightness equally. Various conditions including viewing conditions, eye fatigue, color blindness, and color memory can all affect the eye’s ability to distinguish color differences. On average, the observer will recognize differences in the hue first, chroma second, and lightness third. The resulting visual acceptability from the eye’s ability to distinguish color is best represented by an ellipsoid.

While no color tolerance system is perfect, the ellipsoid tolerance of CMC best represents the differences observed by our eyes. Research has shown that CMC tolerance has a 95% agreement with visual assessment while CIELAB has a 75% agreement. (X-Rite, 2002) Based on the higher degree of agreement, SEKISUI KYDEX utilizes CMC tolerance to inspect the color of its thermoplastic sheet products.

References

Eye-Color. (2002). Color Spaces, CIELab_65percent.jpg retrieved online February 24, 2003, at http://www.i1color.com/knowledge/measuring_color.asp

X-Rite. (2000). A Guide to Understanding Color Communication.