[...]
> Nevertheless,
> I think that if one could find a transparent substance one could put in
> a print frame that whould cut the exposure time in half, a lot of people
> on this list would be very interested. Pure quartz would work, but I
> don't know if you can buy a sheet of it. There might be a variety of
> glass or plastic that does a better job of transmitting UV. Maybe someone
> on the list knows of something like this.
Here is something I've done that might fit the bill, though I did for
different reasons than minimizing UV absorption.
Years ago, I was teaching a photogravure course and needed very precise
contact between the sensitized resist and the continuous tone positive
and especially the screen during the secondary exposure. The "screen"
was a piece of lith film with a very fine random dot pattern, and if
there were any gaps between the film and the resist, the dot
information would simply be lost. Standard contact frames didn't
work, no matter what I tried (extra backing and padding, different
types of glass - the thicker the glass, generally the flatter it is,
etc.)
By chance, I happened to have access to a vacuum easel that was left
over from someone's attempt to do dye transfer. This was a professional
dye transfer easel (made by Condit, I guess - lots of $), but in
principle it shouldn't be hard to fabricate on your own. Anyway, this
was the type of easel that had concentric grooves cut into it for the
vacuum that corresponded to the standard paper sizes. This is designed
to hold the paper flat at the edges rather than lots of little holes
all over that would end up showing in the image. The vacuum comes from
a single point in the groove (you could apply vacuum to any single
groove by moving the tube from the pump, but the point was to only use
the size groove that fit the exterior dimensions of your paper. I
realize that this is a sketchy description, it is really simple. If
there is interest, I'll try to do an ascii drawing and post it, but it
will take time.
What I ended up doing was using a groove that was larger than my sensitized
resist, placing the resist down first, then the film (which was smaller
than the resist) and over that placing a very thin sheet of mylar that was
larger than the vacuum groove. The mylar was not only smooth, but, coming
fresh off the roll, was free of surface defects (at least that I could see).
Once the vacuum pump was turned on, the mylar was sucked down tight on
the film and resist, essentially shrink wrapping it against the flat
baseboard of the easel. It worked great. The dot information in the resist
was very crisp once it was developed.
Now, on the assumption that very thin mylar transmits much more UV light
than glass, I think this might also serve as an answer to the question of
UV absorption.
Carson Graves
carson@ileaf.com