Defining QC Setups . . . continued
It is this automatic adjustment to the "volume of acceptability",
compensating throughout the color space making CMC tolerancing so
desirable, for matching human sensitivity to color difference. The CMC
equation is summarized as follows:
The volume of acceptance is defined by DE
CMC
≤ cf where:
Where S
L
, S
c
, S
H
are the semi-axes lengths for a 1.0 DE
CMC
ellipsoid. (If
complete equations for generating DE
CMC
are desired, see AATCC TEST
Method 173-1989.)
The CMC adjustment menu consists of the three parameters that can be
set: Commercial factor (cf), Lightness factor (ι), and Chromaticity factor
(c).
* The commercial factor is the tolerance limit that each sample is
not to exceed. (e.g., if cf = 1.00 then any sample which has a ∆E
value greater than 1.00 would be commercially unacceptable.) The
"cf " will have to be adjusted to meet the requirements of each
measuring situation. For example, the color match of formal
clothing may require a "cf " of .50 or less. However, a "cf " of
2.00 or more may be an adequate color match when measuring
blue denim. A starting point value of .90 for "cf " is
recommended.
* The lightness factor is normally set to 2.00 (default) but other
values may be required when surface characteristics differ
dramatically. This value of " ι " should be assumed best for textile
measurement until actual results indicate a need for adjustment.
Increasing " ι " or "c" will increase the tolerance to variations of the
lightness or chroma components of the color error respectively.
For the textile industry it is unlikely that the chromaticity factor normally
will require any adjustment from the default setting of 1.00.
3-4
DE
CMC
=
∆L
S
L
2
+
∆C
cS
c
2
+
∆H
S
H
2
.5
ι
