Digital configuration: was Blurry Plates
- Dear Dan
Couple of considerations for this. I've not specifically encountered
this before but based on technical information mixed with a bit of
theory I'll take a stab at it, though I'd be quite interested in
comments from others. Let me know if this seems to fit what you are
doing or if I am completely off base here.
First, in terms of the printing surface. What you see (on the
negative) is what you get (or, more to the point, what you need to
strive for). Halation (cross-filtering as the molecular structure
reforms and cross-links) is a filling-in process that increases with
exposure. This primarily effects the shoulder. Thus the relief depth
actually decreases with more exposure. Halation provides stability to
the shoulder during washout which will then need post-exposing to
completely strengthen it for printing. It does not affect the printing
surface per se.
You would normally want less exposure on a broad surface because you
would have more of an ink drain. The UV does not get bounced around as
you suspect. The effect is on the exposed image. The "new" technology
"sheet polymer" is already back-exposed so there is no reflection back
up from the metal-backing. "Older" technology required back-exposing.
The reason you can't actually wash away that thin layer of polymer at
the plate's "floor" is because it was pre-photopolymerized at the
manufacturer. This is technically known as the anti-halation layer. So
I don't think its the plates here, though exposure times do have a
significant effect on image. A thinning effect with increased exposure
and vice versa.
I believe this to be a pre-processing problem/solution. Essentially it
has to do with ink gain. If you are working with linocuts I assume the
image is coming from a printed reproduction. Already you have a
certain amount of ink gain. If you are furnishing this proof to be
then made into a negative you are also then dealing with camera
distortion and an added set of exposure considerations.
The solution here, I think, is to digitally scan the proofs and rework
them in an image-editing program like Photoshop. We had talked here
earlier about the letterpress configuration of digital type. I believe
this to be crucial for best results. The same applies to images. A
good percentage of the commission work that comes in to my shop is
just that. Digitally reconstructing images for photopolymer printing.
Here, you need to open up the white areas a bit. You do not have to
alter them just increase their relative size in proportion to the
black. Not a lot of increase is required. You could, for example,
click on an internal white area of your image, select all similar
areas, and then increase the relative size of the white (apply pixel
growth). Note that you need to do this from a grayscale scan (at
600dpi, or, if you can, at 1200dpi) not a black/white scan (as you
cannot change the black/white w/o actually altering the image). When
you are finished with your alterations, increase the dpi to 1200 then
convert the image to black/white. All you are really trying to do is
visually compensate for gain, not alter the image.
Maybe you know all this, but a lot of folks think that images don't
need to be adjusted. The reason we now use photopolymer for
letterpress is because it has been tied into digital imaging. The
process has been around for some forty odd years but never caught on
for fine printing previously because it was dependent on printed
reproduction with no way to independently control output. Now we have
that. Aren't we lucky :-).