Defining QC Setups . . . continued
L*C*h° - color space is calculated from L*a*b*. The "L*" defines
lightness, "C*" specifies chroma, and "h°" denotes hue angle. L*C*h°
sometimes offers an advantage over L*a*b* in that it’s easy to relate to
the earlier system that is based on physical samples, like the Munsell
color space.
XYZ - X, Y and Z are the CIE tristimulus values calculated per ASTM
E308-90. X,Y and Z are used in calculating the other color spaces and
indices. If the tristimulus values are equal for a given pair of samples,
under a specified observer, illuminant, and geometry of illumination and
view, the samples will visually match under these conditions.
Lab (Hunter) - is an opponent-colors based system similar to L*a*b*,
developed in 1942 by the late Richard Hunter.
Wht - whiteness index is used as a preference rating for how white a
material should appear. Whiteness is calculated per ASTM E313 (We)
and per CIE 15.2 (W).
Yel - the ASTM’s E313 yellowness index (Ye) is used to determine the
amount to which a sample color can shift away from an ideal white. The
D1925 yellowness index (Yd) is used for measuring plastics.
CMC - is an acronym for the Colour Measurement Committee of the
Society of Dyers and Colourists. CMC equations are mathematical
modifications to the formula used to calculate small color differences or
color error (DE) in CIE L*C*h° color space. The purpose of the CMC
equations is to appropriately distort the scaling of the L*C*H*
(Lightness, Chroma, & Hue) components for the DE (color error)
calculation. This provides a much better agreement between human visual
assessment and instrumental measurement of small color differences.
A single CMC DE tolerance value describes, in L*C*h° color space, an
ellipsoid around the standard color where the semi-axes correspond to
the Lightness, Chroma, and Hue of the standard. This ellipsoid represents
the "volume of acceptability" for that given color standard with a DE
CMC
less than or equal to the given tolerance. For any given tolerance, the
ellipsoid will vary in both size and shape depending on the position of
the standard in L*C*h° color space.
3-3
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