Home' FLEXO Magazine : January 2016 Contents 3-color sampling decreases—thereby providing less sample points at
high black levels, where Delta E differences are small, for the specific
purpose of enabling more sample points at low black levels, where
Delta E differences are large. Digital values are equally spaced for all
four colors, hence the targets are made for curves to be applied upon
output (the same curves that would be used for the production job).
The result is fairly uniform spacing of printed L*a*b* values when
aiming at a curve spec, such as G7 or FOGRA-39.
The distinction between a “stage” and a “step” is helpful in under-
standing EDK sampling logic. Referring to the eight stage descend-
ing patch set on the facing page, note the first stage of has eight
steps. From the first stage to the mid tone (where black is ≥ 50 per-
cent), each stage has one less step. Hence, the second stage has seven
steps, the third stage has six steps and the fourth stage has five steps.
However, at the fifth stage, black is 57 percent (which is ≥ 50 per-
cent). Above this point the stages “double.” Hence, the fifth and sixth
stages have five steps, and the seventh and eighth stages have four
steps. The increments of Black are equal to the 3-color increments in
the first stage (e.g . 14.3 percent for eight stage sampling).
TEST CHARTS, PATCH SETS, SAMPLING LOGIC
Users of color management have learned to distinguish between test
charts and patch sets:
• A test chart is a patch set in a very specific order of columns
and rows at a very specific size
• A patch set is a more general listing of tint build combinations
in no specific order with no specific size
The IT8.7-4 is actually a patch set containing definitions of 1,617
points. The IT8.7-4 patch set can be arranged in an infinite num-
ber of ways (rows and columns, random or ordered, large patches
or small patches) to make a specific test chart. There are many
IT8.7 -4 test charts being used in the flexo industry today and they
all look a little different. They contain the same color patches, but
they’re arranged in different ways.
Until now, the patch set has been the most general form of color pro-
file reference data. With the introduction of descending sampling log-
ic, we have an even more general form than the patch set. It is called
the “sampling logic.” Two examples of sampling logics are uniform
black sampling logic and descending black sampling logic. For a given
type of printing, one would logically first decide which sampling logic
is most appropriate, then decide which patch set of that sampling logic
is ideal, then make a specific test target for a specific press.
Uniform black sampling logic has the same number of 3-color
combination (CMY, OMY, CGY or CMV) points at every level of
black. Hence, a uniform black sampling logic patch set with nine steps
of four colors has 9 x 9 x 9 x 9 (94) = 6,661 patches (which is 26,244
for the four relevant combinations of CMYKOGV). A nine stage
descending black sampling logic is 2,205 points (which is 8,820 for
relevant combinations of CMYKOGV).
Uniform black sampling wastes a lot of sample patches at high levels
of black where the printed L*a*b* values are so close together that
nine steps are far more data than needed. A user willing to print and
measure more than 6,000 patches of CMYK would get far better data
using 12 stage descending logic (6,452 patches for four colors) than
nine stage uniform logic (6,661 patches for four colors). The descend-
ing logic has far more sample patches at low levels of black where
color differences are large.
WHICH STAGE TARGET IS BEST?
One advantage of the IT8.7-4 was simplicity in decision making. All
converters ran the same target regardless of press size. A disadvantage
was accuracy—particularly with colors containing black (see sidebar
Sequential press characterizations—
running each relevant combination of
four colors—is valuable for large
presses as well as small presses.
Esko Descending Logic Patch Set & Test Chart Sizes
* This is the number of patches and pages
for CMYK only. Multiply by four
(Omyk, cGyk, cmVk) for a 7-color profile.
JANUARY 2016 | FLEXO 47
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