Print
constrast
In
order to determine the normal ink film thickness
it is necessary to have a criterion, which
permits an unbiased evaluation. This criterion is known as print
contrast. It
is computed using the a formula. Normal
ink film thickness is obtained, where the print
contrast reaches its maximum.
Characteristic
curves of printing
The
characteristic curve of printing shows, to what
extent the printed dot differs in size from that
on the film respectively press and depends on
many factors such as the D0 the type of press,
(single-color or multi-color, paper, blanket,
impression pressure and others).
It serves as means of communication
between repro house and production printing.
The characteristic
curve of printing is established with the aid of
a screen grey-scale wedge. The screen density of the individual step
is known. The
Murray-Davies formula permits the computation of
the screen density in print. The values are entered in coordinate
system and the curve is plotted. This plot is a
convex curve, since a 100%
congruence, corresponding to the film, is not
possible; it is prevented by light trapping.
The characteristic curves of printing are also
important for matching proof print and
production print.
Ink
receptivity
Poor
ink receptivity when printing wet on wet results
in severe ink variations, when colors in the
area of 70-95% dot percentages are overprinted.
In such cases the results achieved on a proofing
press, printed dry, cannot be achieved in the
production run.
Dot gain
The
difference between dot percentages on film and
those on the production print is called dot
gain.
Two
methods are commonly used to determine it
1.
The computation, using the
Murray-Davies-formula
2.
Coarse screen – fine screen comparison,
by System Brunner
The dot gain for the individual tonal values,
established according to Murray-Davies, is
entered in the coordinate system and displayed
as characteristic printing curve.
The
System Brunner uses isometric diagrams to
represent the dot gain.
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