Dot gain and tonal control for printing
One of the most misunderstood experiences in the printing process
is dot gain, the effect of the printed halftone changing tone
on press relative to the original image and its electronic intermediates.
Dot gain is a part of every printing process - whether printing
on a laser printer or a multi-million-dollar printing press.
Dot gains occur at all tonal values, but are most visible in
the midtones and in the deep shadows; they are sometimes responsible
for making a job unacceptable. Dot gains are both real (gains
from printing processes) and perceived ("optical" dot
gain is the result of an optical illusion; though it is not physically
present, its illusion is, and its gain must be considered).
Controlling dot gain is a multi-step process, one which involves
the measurement and control of the tonality of each image destined
for print. Going to press without controlling dot gain is irresponsible
and will cause some jobs to be ruined.
The responsibility for controlling dot gain falls into the hands
of the person managing Photo CD and other images. By communicating
with the printer to determine the amount of gain experienced
on their presses and on common papers, a set of adjustment values
can be obtained, and then entered into the process to control
the effects of gain in printing.
The dot gain test
The simplest route to controlling dot gain is to measure the
performance of a printing press with a l00-step gray scale created
on an imagesetter. The imagesetter must be linear, and the halftone
screen frequency of the test must be the same as the frequency
used in production for the test to be valid. With this test halftone
film, a printing plate is made and the image printed on both
glossy and uncoated papers to test press performance.
With the assistance of the printer, a magnifier and a reflection
densitometer, the values on the l00-step test are viewed, and
measured for dot area (percentage). We are interested in gathering
information on highlight losses first; with the magnifier we
look at the highlight steps to determine the step on the scale
where the first highlight dot pattern reveals itself. Next, we
want to make note of the darkest shadow dot pattern. Make note
of both values.
Then, using the densitometer (set to dot are(l) we seek three
additional values on the scale. We want to find the step on the
scale which actually printed 25 percent, 50 percent, and 75 percent
reflective dot area values. These steps usually fall in the mid-teens,
the mid-30, and mid-60 ranges on the l00-step scale. A tolerance
of l-2 percent is acceptable.
With these values noted, a corrective curve can be made in the
software controlling the process to make the tonality of subsequent
images correct.
Where this corrective action is taken is a matter of preference.
It is possible to put curve controls into Photo.shop~~ software,
or to build corrective controls in QuarkXPress. In production,
the best location for tonal adjustment is probably the page make-up
arena, since the adjustment will apply itself to all images -
whether they are photographic or computer-generated.
The importance of measurement and control of press gains cannot
be overemphasized. The responsibility for controlling these effects
lies with the person making the conversion of images from Photo
CD disc to electronic documents. Designers and production people
cannot expect the imagesetter to fix the problems, or the printer
to fix the problems, because it is impossible for them to do
so at those late stages in production.
Other considerations when preparing files for printing
Preparing film for printing requires not only control over dot
gain, but correct settings for the printing process and press
chosen for the particular job. Among the considerations that
must be addressed are halftone screen frequencies, whether the
film should be made as positives or negatives, and whether the
emulsion should be up or down (RREU or RRED in printer's parlance,
for right-reading emulsion-up, and right-reading emulsion-down).
An industry trade book, Standard Rates and Data, found in the
public library can answer some of these questions; the printer
or a publication's Rate Sheet and Technical Specifications can
also provide specifications. Be certain to get a recent edition,
as publications change size occasionally.
