Regarding Keith Schreiber's message of a few days back about using
ordinary cool white fluorescent tubes for nonsilver processes, I think
Phil Davis was probably pulling your leg, as Judy has clearly
demonstrated. However, to give him the benefit of the doubt it's possible
that he's got a defective tube or two, or perhaps an ancient variety
without the standard modern coating. I knew somebody who got very badly
sunburned under supposedly ordinary office fluorescents, which must have
had something pretty wrong with them in terms of their uv output.
I don't have any suggestions regarding specific point light sources and
can't offer any comparisions between the sun and BL fluoros re sharpness,
but I wonder if Mike Ware's Euclidean geometry isn't a bit too simple. It
assumes that a single point in the negative will have a very large circle
of confusion caused by the dimensions of the area covered by his bank of
tubes. But since the inverse square law kicks in here - light from the end
of tubes T1 and T4 has to travel a much longer distance than light from T2
or T3 directly above point X - then perhaps the effective circle of
confusion (as against the potential) must be much smaller.
(t1) (t2) (t3) (t4) Bank of four tubes 320 mm long
80 mm between tubes and paper
_____________________ glass
----------x---------- negative
~~~~~~~~~~^~~~~~~~~~~ paper
Is it possible there's some difusion going on in the paper itself? Perhaps
the light is even bouncing back up into the sensitized portion of the
substrate after being reflected from the back of the printing frame. Does
the thickness of the paper or a dark backing make a difference? Or is it
possible the lack of sharpness is caused by the paper fibres swelling on
wet processing and not necessarily drying down with a 0.15 mm point
perfectly resolved in exactly the same position? It might be worth
undertaking a test or two contact printing a negative of a lens test chart
on conventional photographic paper or printing out paper. It shouldn't be
too hard to see if there's that much difference between contacting
printing normal photographic paper under a fluorescent fixture or a single
incandescent bulb (the theory should still hold), even in those bleak
sunless northern lattitudes you have to endure this time of year. Down
here the sun will soon be hot enough to fry a negative, so we'll be
interested to hear what you find.
Yours from sunny down under,
Philip Jackson
pjackson@nla.gov.au