[pure-silver] Re: Red Filter
- From: john stockdale <j.sto@xxxxxxxxxxx>
- To: pure-silver@xxxxxxxxxxxxx
- Date: Fri, 01 May 2009 09:11:29 +1000
RobC wrote:
....................
I would add that these days LED safelights which have light output completely
outside the range of B+W materials are the preferred choice and they can be
bright enough to read by in the darkroom. They are also pretty cheap to make up
yourself. Check the datasheets for your paper wavlength range and look out some
LED's which output above that. Google is your friend.
The spectrum of LEDs is wider than most people suppose. The graphs of
spectrum spread usually published have an arithmetic vertical axis,
whereas graphs of photographic sensitivity are in logarithmic units
(e.g. "density"). The result of this is that if you're trying to match
the LED spectrum to a published paper sensitivity graph, what looks like
a good match is often far from it. I have made several LED safelights,
and have gone from orange to red to "very red". All but the last have a
sheet of Rubylith over them to make them safe with mainstream VC papers.
Try this with a sample of your proposed LED: using the underside of a
CD, view its light at the angle that produces the "rainbow" effect. You
will probably be surprised to see how far the spread goes into orange,
yellow and aqua.
I have read, but can't really test, that LEDs operating at less than
their rated maximum current have a narrower spectrum spread (and are
less bright, of course). So my last safelight had about 80 LEDs and
operate at about half rated current.
John Stockdale
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