screening, if present
, Prinergy respects the PostScript spot function
included with the screen specification.
About dot shapes
You can specify the dot shape in the
Calibration & Screening
section of
output process templates.
Note: This topic only applies to regular (rational tangent) screening. For IS
screening, see
on page
.
With CTP there is little difference between dot shapes in the plating
process, but there are some subtle differences on press.
In film workflow, mechanical gain in the imaging and plating process
exacerbated mechanical gain on the plates and caused nonlinearities
where neighboring dot structures touched, causing a tonal jump right
on the plate. This problem was further compounded by mechanical
gain on press, leading to even larger visual tone jumps where the dots
met.
Thermal CTP eliminates all mechanical gain in the plating process.
Even where neighboring dot structures touch, there is no bleeding of
the pixels or dot shape into one another, like there is in an analog and/
or Gaussian-based exposure system. The result is accurate tonal
reproduction onto the plate, so that Round, Euclidean, and Elliptical all
produce the same physical dot area on the plate. However, there are
subtle mechanical differences between the dot shapes, based on how
they respond on press, because the compounding effect of plating gain
has been eliminated and the tone jumps are not nearly as obvious.
The accuracy of pixel-for-pixel reproduction, and therefore the edges
of each halftone dot, is wholly dependant on the optical resolution of
the device and media. For CTP devices that do not deliver accurate
reproduction of each pixel, differences will be less subtle.
The choice of dot shape is more critical when the plating system
produces mechanical gain or loss on plate greater than 4%. Most
thermal CTP systems produce linear output and do not affect your
choice of dot shapes. Photopolymer plates such as those found on
violet CTP and modern high speed negative thermal plates produce
measurable levels of gain on plate and users should take note of tone
jumps in the tints where neigbouring dot structures touch. Choosing a
dot shape that avoids tone jumps in critical areas may be important in
selecting the best dot shape for your print application or typical subject
matter.
Generally, Round, Euclidean, and Elliptical dots produce similar
physical dot area on the plate, but they may respond differently on
press. Also, subtle mechanical differences may be seen where the dots
touch.
638
Chapter 12—Outputs
