More on press linearity
More on press linearity
- Subject: More on press linearity
- From: Brian Lawler <email@hidden>
- Date: Sun, 9 Nov 2008 12:28:37 -0800
Hi all,
May I stick my neck out again here?
The object of any effort to linearize, relative to offset printing,
has more to do with platemaking than presswork.
When we used film, we used to linearize the imagesetter to generate
near-straight-line film, where the halftone steps were within one
percent of the value desired.
With platesetters, we try to make a plate that is similarly linear.
This negates any error in the linearity of the lasers/attenuators/
shutter mechanisms on the platesetter.
With G7 there is a secondary effort to make the grayscale linear on
paper, which is two steps away from the platesetter (plate-blanket-
paper).
A G7 sheet will show "neutral" grays for every tonal step in a 0-100
percent gradient. This requires slight adjustment of the individual
color plates to a (non-linear) response to remove the tonal shifts
that normally take place on press. These shifts are a result of the
interaction of halftone patterns at different values, the interaction
of halftone screen angles, and of some other mysterious effects of
toning that occur on press when you overprint mineral-pigmented inks
in a non-clear ink vehicle.
The net of this is that press make-ready is much easier to achieve,
and color corrections on-press much less likely to be an issue.
So, G7 is in fact a tool that makes presses run to make-ready more
quickly, and stay in-balance more easily for an entire press run.
When we shift the responsibility for neutral gray to the plates, and
make plates where that color-neutrality is "built-in," then presswork
becomes more a science and less an art.
Another factor of linearization, especially when there is film
involved (which is unlikely these days) is that on some presses
holding a very small highlight dot is sometimes difficult. This is due
to the mechanical strength of the microscopic highlight dots on the
plate, and to some friction that can exist between the ink rollers
against the plate, and the plate against the blanket. The smallest
dots can be worn off the plate in normal use.
On modern platesetters it is much easier to make the plate behave in
such a way as to assure the strength of the smallest dots you want to
hold on the plate.
That process, even without G7, is a function of linearization. And,
because the laser platesetters we use today are so incredibly
accurate, they can do the same thing hundreds of times in a row with
amazing precision.
Add to that the quality of plate emulsions, processing systems (if
any) and you have a more reliable process for printing today than at
any time in history.
Best wishes,
Brian P. Lawler
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