HunterLab EasyMatch QC, User Manual

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EasyMatch QC User’s Manual Version 2.1
A-2 Measurement Values
Opponent-Color Scales (Hunter Lab, CIE 1976 L*a*b*, and CIELCh)
The opponent-color scales give measurements of color in units of approximate visual uniformity
throughout the color solid. Thus, in the
Hunter scale , L measures lightness and varies from 100 for
perfect white to zero for black, approximately as the eye would evaluate it. The chromaticity
dimensions (a and b) give understandable designations of color as follows:
a measures redness when positive, gray when zero, and greenness when negative.
b measures yellowness when positive, gray when zero, and blueness when negative.
The relationship between the Hunter Lab Scale and the CIE XYZ Scale for the CIE 1931 2° Standard
Observer and the CIE 1964 10° Standard Observer is as follows:
n
Y
Y
100 L
=
n
n n
a
Y
Y
Y
Y
-
X
X
K a
=
n
n n
b
Y
Y
Z
Z
-
Y
Y
K b
=
where:
X, Y, and Z are the CIE tristimulus values obtained for the sample.
X
n
, Y
n
, and Z
n
are tristimulus values of the standard illuminant as listed in ASTM E308 with Y
n
always equal to 100.00 (normalized).
K
a
and K
b
are the ASTM E308 chromaticity coefficients for the illuminant used.
Illuminant A represents incandescent (tungsten) lamplight with an approximate color
temperature of 2854 K. Illuminant C represents average, or north sky, daylight with a correlated
color temperature of approximately 6770 K. Illuminants D65 , D50 , D55 , and D75 represent
daylight with correlated color temperatures of approximately 6500 K, 5000 K, 5500 K, and 7500
K, respectively. Illuminants
F02 (cool white fluorescent), F07 , and F11 are fluorescent
illuminants. TL84 and Ultra3000 are custom illuminants.
The Hunter Lab total color difference (dE) and chromaticity difference (dC) for any illuminant and
observer are calculated as follows:
2 2 2
db da dL = dE
+ +
2 2
db + da = dC