At 14:56 12-11-2007 +0100, I wrote:
You have to be aware that (sometimes) it's pretty
difficult to obtain a very
straight linear plate and a very precise curve for your tonal correction.
Sometimes the addressebility (lightsource spot and resolution ratio) and
peaks are difficult to control. In the end it's more important that you get a
correct end result. The Fogra tools to check your CTP are important.
Fogra's warnings on the subject linear/tonal are (sometimes) even more
important.
Henk
Yes, you deserve the details:
During imaging of printing plates there are two main causes of
gradient flaws. These are, on the one hand, loss of adjustment and
dirt in the imaging unit, and on the other, poorly calibrated
linearization and color tone correction curves.
This data, stored in the RIP of a CtP system, is needed to achieve
the tone values in the print sheet that correspond to the standard
specifications for the relevant printing condition.
The purpose of linearization curves is often to reproduce a desired
tone value on a CtP plate, for instance to generate a 40 percent
swatch of the data set with exactly 40 percent on the plate.
Depending on the system configuration, what is needed first is to
enter a correction of up to ±7 percent. As a result of this correction,
it is not just on the plate that there is a shift in tone value, but in
printing as well. Next, a color tone correction is stored in the
RIP to compensate for the incorrect increase in tone value.
In mathematical terms, the application of linearization and color tone
correction curves represents an addition or subtraction in relation
to the tone value (linearization) followed immediately by subtraction
or addition in the RIP (color tone correction). Since there could
easily be two different arithmetical operations performed in the RIP
for these two steps, in unfavorable but frequently occurring cases
this leads to rounding errors, resulting in gradient flaws. These
f laws are not detected during technical measurement evaluations
of the tone value swatches, as they rarely occur in step swatches
(for instance a progress wedge in 5 percent steps).
Investigations by Fogra indicate that linearization curves are
only useful in exceptional cases. This means that in an ideal case
the measurement results which reproduce the tone values on
the developed printing plate should be based on a mean value which
is derived from a statistically reliable number of measurements
(around ten measurement swatches on at least four printing
plates). In addition, the measurement results may only serve as a
basis for a linearization curve if a compensating function
smoothes the curve gradient of the measurement result as well.
Before applying linearization curves, check whether stable
production cannot be achieved by another route as well
possibly via an adjustment.
Carry out a test with the Fogra CtP test chart and the twodimensional
gradient swatches contained in that.
Always perform visual comparative checks of the two-dimensional
gradients on plates with and without linearization curve.
If the gradients in the two imaged plates are perfect, then there
should at least be a four-color check plot performed on initial
application, prior to the linearization curve being used for
production orders.
This is described in detail (with images of the tools)
in an old Heidelberg News issue.
I experienced these problems in 2 different plants.
I will send the PDF to you directly.
I am pretty sure that the Fogra Aktuell wil have the same
type of information. Even in German.
Regards, Henk
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