First off I should have made my reply a little more clear. I made the
assumption that we were talking about building light banks and it would
be contained in a box with white sides. In such a case most of the light
would be reflected back on to the print and the only real loss would be
that which would be absorbed by the reflective surfaces of the unit
itself. My guess is that you would have only lost about half a stop.
If my memory serves me we used to use Glidden 5400 base white paint when
I worked at Spectrolab as a technician building solar simulators in the
mid 60's and I think we got better than 90% reflectivity from it. I
think it may have exceeded a front surface aluminum mirror as aluminum
is quite absorbtive, but then you can make mirrors out of aluminum and
you can't out of white paint. I do remember that silver was quite dark
and had a poor reflectivity. The tip here for anyone building a light
bank is to forget aluminum paint or foil as a reflective source. Foil is
particularly dangerous around electrical circuits
Without the sidewalls on the unit, and I am assuming you did not have
walls on it as I know your printing unit can move up and down and is
normally open, you will get some fall-off due to the illumination area
becoming larger. John Rudiak calculated that in your case a point source
would have dropped 6 stops where your figures show @ 1.5 stops.
I agree with your comments on theory meets reality. When I was in the
U.S. Marine Corps in the early 60's we were taught that when the terrain
disagrees with the map, go with the terrain. A simple maxim, but there
are plenty of cases where battles were lost because commanders believed
the map.
Dick Sullivan
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