I am using a bank of six *PHILLIPS TL 20W 05* uv-tubes (24") approx. 6 inches
from the glass to expose my HANFSTAENGL carbon tissues, which are sensitized in
1-2.5 percent potassium dichromate or, more often, 1-3% ammonium dichromate in
water and alcohol 1:1, 3 min, 15 degrees C.. Density range of my negs is around
1.5-1.7, exposure times around 10 minutes. I never had problems with low
contrast but I'll give those daylight tubes a try...
BTW, I just looked into my old Eder IV/2 from 1926 (german version...): He
mentiones a maximum sensitivity of dichromate sensitized colloids at 450 nm
(with daylight exposure) and 420 nm (with carbon arc) (p. 508). Eder and others
found some sensitivity even until to the green part of the spectrum. If the UV
tubes waste a lot of energy at 350 nm, where the glass justs begins to let the
light go through and the daylight tubes are strong at 450, that could be an
explanation for their similar efficiency, especially if thick glass is used.
But the interdependance of contrast and wavelength could only be shown (but
probably never explained, as the tanning mechanism of dichchromated colloids is
not yet fully understood), if somebody would do some tests with narrow band
filters between 350 and 500 nm... But I 'll not be disapointed, if nobody feels
touched by this proposal - printing good negatives is more fun ;-) And maybe
these tests have allready been done years ago and the results are sleeping in
some scientific literature? Maybe someone in Britain could ask somebody at the
Autotype research dept.?
Klaus Pollmeier