Clewer,Brian wrote:

> My question is 'how long would be a reasonable time for exposure'

Of course, it depends on your particular situation.  Faced with enlarger
lamps of unknown power and photographic paper of unknown sensitivity in
the darkroom, photographers use an experimental techhnique.  (I think
it's called a "test strip" but I don't remember for sure).  Set
everything up as usual, except use an opaque cover to cover all of the
print except one thin strip.  Make a short exposure.  Pull the cover away
a little and expose again for the same short time.  The first section has
been now been exposed twice, and the newly uncovered section once.
Repeat this until the entire piece of paper has been exposed, and then
develop it.  The area that has been exposed every time will be
overexposed, and the last one, that was only done once, will be
underexposed.  One of the areas in between should be close to properly
exposed.  Since photo paper (and probably PCB resist) tends to just
integrate the amount of light it receives, a single exposure of the same
total duration will have the same effect.

Unless the budget is unlimited, the test is done with a thin strip of
photo paper so each test area is maybe 1-2 cm square.  If the usual time
is 1-2 minutes, test exposures of 10-15 seconds each should give enough
resolution.  If you are wondering whether the glass plate has any effect,
put half of the test strip under glass and the other half not so each
test exposure area is half covered.  Science rules!


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