[pure-silver] Re: Red Filter
- From: Laurence Cuffe <cuffe@xxxxxxx>
- To: pure-silver@xxxxxxxxxxxxx
- Date: Fri, 01 May 2009 09:55:54 +0100
I'm cheap.
My last red led safe light was a set of red Christmas tree lights
stuffed in pairs into empty white film canisters to act as diffusers,
and the while arrangement stuffed into a zip-lock bag stuck over a
nail on the wall.
Worked fine.
All the best
Larry Cuffe
On 1 May 2009, at 00:11, john stockdale wrote:
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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