I have observed the effect you refer to in my own work. I suspect that
it has more to do with the lack of perfect contact between negative and
paper than it does with the nature of the light source, but I think it
would be hard to prove one way or the other, as just about any textured
paper would prevent perfect contact, even with a vacuum frame. (Has
anyone ever run a test - pt source vs UV tubes - with a vacuum frame?)
Some other point source light that I have used:
Carbon Arc - a good point source, but I can't recommend using it
because it is finicky to keep adjusted, gives off toxic
fumes, and requires high voltage. (and probably some other
problems too)
Platemakers usually have a Xenon light that is a fairly focused. I've
had good luck with them as far as sharpness goes. I've held
a roughly 200 line halftone screen without losing information,
though I was exposing a smooth surface plate, not textured paper.
I would also guess that you would have better sharpness with a sun lamp
than UV tubes, but that is just an opinion, not something I have ever
run tests on.
Carson Graves
carson@ileaf.com