I think I shall do as you made: a very red light (Blood Red Filter) with
dimmable powerfull leds. So I shall get the brighter safe level which I shall
determine by testing.
Yes, I would be very interested if you could send an image of your dimmer made
with two leads of a potentiometer!
Thanks a lot in advance.
Best,
Thierry ARNAUD
Stark Alternative Photography
De : pure-silver-bounce@xxxxxxxxxxxxx [mailto:pure-silver-bounce@xxxxxxxxxxxxx] ;
De la part de John Stockdale
Envoyé : vendredi 8 octobre 2021 23:27
À : pure-silver@xxxxxxxxxxxxx
Objet : [pure-silver] Re: [pure-silver] Re : [pure-silver] Re: Light_in_the_Lab
LEDs produce a range of wavelengths. Some people seem to think that's it's a
narrow band but it's not. One problem I had when trying to do this was that the
manufacturer's graph of intensity is in arithmetic units and for the paper
sensitivity it's in logarithmic units. Unless you make a conversion it's hard
to do this. And it makes it worse than it appears. Another variable is angle
of incidence in different parts of the room. Also lots of reflected light on
the ceiling and around the room makes the impression of light different, and
easier on the eyes.
My approach is to make the very red light very bright and to use a dimmer to
bring it to a safe level determined by testing.
For me, the dimmer is simply a variable resistor (actually just two leads of a
potentiometer) which works but which electronically enlightened people will say
is primitive and inelegant.
I could send an image if that would be useful. I don't know if that's ok on
this list.
cheers, John
=================================================
On 9/10/2021 7:38 am, thierry.arnaud@xxxxxxxxxxxx wrote:
Thanks for the answer.
I plan to use leds. That is why I would like to know how many lumens can
support a paper when the light has a certain wavelenght (here with the Blood
Red Filter, about 670nm). Because there are now leds very powerfull and it is
possible to set many of them behind a large sheet of filter.
Apparently nobody knows this quantity of light and if there is a limit.
My idea is the following : if the paper is not sensitive to a certain
wavelenght of the light it is possible to push the intensity of the light (more
lumens) of this particular wavelenght without fogging the paper and BTW to work
in a pleasant lab.
I would prefer this setting because the sodium yellow light makes too noise but
the leds work silently.
Best,
Thierry ARNAUD
-------- Message original --------
De : John Stockdale <mailto:jstoc@xxxxxxxxxxxx> <jstoc@xxxxxxxxxxxx>
Date : jeu. 7 oct. 2021 à 23:20
À : pure-silver@xxxxxxxxxxxxx
Objet : [pure-silver] Re: Light_in_the_Lab
I built a number of safelights using discreet LEDs. Starting with graphs of
luminous outputs at the various wavelengths and the graphs of sensitivity of
the paper to various wavelengths I chose some orange LEDs, but they did not
pass a reasonable test. They caused fogging. I read that one could inspect the
colours of the light by reflecting off the back of a CD (the "rainbow effect")
and these orange LEDs showed yellow through to a hint of aqua.
Then I went to red and then to very red LEDs, and these are safe but of course
it's a more difficult light to work under. So I made up a hand held light with
the orange LEDs that I use for brief inspections. The difference is
astronomical.
If you use incandescent globes a dimmer can be useful. Dimming not only dims
but also alters the spectrum usefully.
I envy people who have the sodium vapour safelights because yellow is a lot
easier on the eyes.
Let us know how it goes,
John
==================================
On 8/10/2021 5:15 am, Tim Daneliuk (tundra) wrote:
On 10/7/21 11:01 AM, thierry.arnaud@xxxxxxxxxxxx wrote:
Interesting!
Thank you Dana!
But it gives no idea of the maximum quantity of lumens of a light which is
behind my red filter.
It's not that simple. The acceptable amount of light depends upon multiple
factors:
- The intensity of the light source in the relevant parts of the spectrum
- The spectral response of the filter
- The attenuation of the filter
- The distance of the filtered light source to the paper, worst case (shortest
distance)
- The highest sensitivity of paper you plan to use
In theory, you could figure this all out analytically, but why bother?
Build/buy a light source with the ability to replace bulbs and filter.
Take a shot at it and do a coin test.
As an aside, it is almost always preferable to have multiple, lower output
safelights in various parts of the darkroom, than one hanging from the ceiling.
In my case, I use one smaller Kodak "Bullet" light over the sink, and two 8x10
wall mounted ones at the far end of the darkroom. This allows brighter
illumination
far away from the most light sensitive end of things (unexposed paper by the
enlarger
and paper in development) while still illuminating the room nicely. I think I
ended up
using deep red X-Ray filters with a 25W bulb in the bullet and 40W in the 8x10s.
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