From: David J. Romano (romano@agfa.com)
Date: 01/04/01-11:22:12 AM Z
Rachelle,
Depending on how long ago or where you heard of it, it may have been
called driography. Driography is an offset lithographic process that
prints images using only the grain structure of the plate as the
halftone screen. To be clear about things, there is really no such thing
as continuous tone, even in silver based film. We define continuous tone
as not being able to discern any image structure. To the eye it appears
smooth. It is still halftoning though: a collection of black silver
grains of different sizes and spacing against clear film. The offset
lithographic printing process is no different, it is still binary in the
sense that there is either ink or no ink present and their relative
amounts determine the perceived gray value.
Normal halftoning i.e., 150 lines per inch as might be used in a
magazine creates different gray values by varying the size of these ink
blotches (halftone dots) while leaving their spacing constant. Thins is
sometimes called AM screening (amplitude modulation). FM screening,
sometimes called stochastic varies the spacing, but not the size of the
dots. (although at some point, they are so close together that they do
touch, creating a larger dot) The dots are usually very small in order
to minimize graininess in the printed image. Their size is typically
about 20 microns. The printing plates are exposed either directly with a
laser beam or receive a contact exposure from a film containing the
image which has already been halftoned (broken down into small dots).
The process of driography uses a film contact exposure, but the image on
the film has not been halftoned, it is a regular "continuous tone"
image. The contact exposure is very carefully controlled to produce the
same image on the printing plate. The image is present only on the tips
of the plate's graining, thereby creating a halftone image. The plate is
printed dry, hence the name, with a very light impression. This is as
close to continuos tone lithography that can be done, but is really not
commercially viable. The FM screening is much easier to use as it can be
done digitally. Driography is completely analog.
Hope this helped more than it confused
Dave
Andy Darlow wrote:
>
> Hi Rachelle:
>
> Yes, there is a process called stochastic printing that gives a random
> series of tiny dots, like an inkjet printer. However, it is difficult to
> control on press, so it is hardly used.
>
> Please feel free to ask any further questions and for more info you may
> want to look into Scitex's version of stochastic output, FM screening.
> Agfa's is Crystal Raster.
>
> All the best!
>
> -Andy Darlow
>
> At 09:48 AM 1/4/01 -0500, you wrote:
> >Does anyone know of a process in which one prints continuous tone using an
> >offset printing press? I watched someone do it once, but can't remember the
> >name of the process or any other details. HELP.
> >
> >Rachelle
> >--
> >Rachelle A. Dermer, Ph.D. Candidate Art/Technology Teacher
> >Art History Department CAS 303 Weston High School
> >Boston University 444 Wellesley Street
> >725 Commonwealth Avenue Weston, MA 02493
> >Boston, MA 02119
> >
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