If I had to bet, I would bet that if a manufacturer like Ilford made LED safelights, they would source LEDs that were manufactured to the required tolerances. That is what one pays for if one buys good quality darkroom specific equipment.
Originally Posted by Hexavalent
“Photography is a complex and fluid medium, and its many factors are not applied in simple sequence. Rather, the process may be likened to the art of the juggler in keeping many balls in the air at one time!”
Ansel Adams, from the introduction to The Negative - The New Ansel Adams Photography Series / Book 2
I saw ( and participated in ) the same original thread and ordered two of these. They arrived a few days ago. Less than $4 each and the shipping was very reasonable. I have one installed now and used it yesterday. It is mounted about 5 feet above my chemistry and enlarger baseboard in a fixture that has a reflective hood, aimed up at the ceiling. It provides enough light so that I can see what I'm doing. It passed the "coin test" ( yes I know, but if it fails this then there's no point at all, right? ) 10 minutes for: Ilford MGIV, WT MGIV, Adorama VC and Arista.edu VC.
I made two prints using Adorama VC. The prints have bright white highlights, just off paper white. The first print I made with the light on the entire time. The second print was made with the paper covered with black card while the safelight was on, and I kept it shielded during processing. So this is not quite as rigorous as what Bill suggested, but I can see no difference whatsoever.
( However, in the past, it's been the Arista.edu paper that I had problems with... and I haven't done anything except coin test with that so far... I'm hoping it will work. )
Anyhow, that's my 2 cents after using it for only 1 DR session. The real test is going to be the Arista.edu 11x14 because the way I process it requires quite a bit of time and I need to be able to see what I'm doing. In the past, I made a safe safelight for it by having the only light in the room go through R25 filter. The color of the light from this new led bulb seems very similar to that color to my eyes.
When the bulb comes, don't be surprised that the bulb part is made from white frosted glass. The red LEDs are inside and you can't really see them.
edit: I should have mentioned that I have calculated pre-flash times for all my papers using a standard height and bellows extension and filter for my enlarger. I will do the Kodak safelight test eventually for all of them.
The thing about LEDs is that they are bandgap devices, they (usually) emit light at exactly one wavelength only. No other wavelengths are possible, because the wavelength is defined by the photon energy, which is defined by the difference in chemical energy levels in the semiconductor.
Originally Posted by David Brown
Some manufacturers will deliberately mix multiple dopants to broaden the spectrum so this is clearly not true of all LEDs, but the basic simplest cheapest red LED you can make is a single-bandgap device. Obviously one must still test to make sure they don't have a deliberately spread-spectrum device or that the paper they're using doesn't somehow have a tiny bit of sensitivity out to the longer wavelengths.
Incandescents are fundamentally different in that they're a chunk of hot metal, i.e. a blackbody radiator which produces continuous spectrum all the way up to UV. One applies filtering to make it safe(r) but there is no such thing as perfect filtering cutoffs, etc, i.e. we can achieve only a certain dynamic range between the blocked and passed wavelengths. Attenuating the shorter wavelengths enough for paper to be safe to half an hour means attenuating the longer wavelengths down to annoyingly-dim levels.
LEDs though, if monochromatic, you can make them as blindingly bright as you want and the paper cannot pick it up. The photon energy is too small to chemically activate the paper, regardless of how many of the photons are present.
Or to re-state it: there is a bandgap in the LED that produces a photon, and a bandgap in the paper that can absorb the photon. If the energy available to create the photon is less than the energy required to activate an electron in the paper, the electron can never be activated. The number of photons (brightness) is completely irrelevant.
Last edited by polyglot; 06-14-2013 at 02:37 AM. Click to view previous post history.
FOR USE WITH STANDARD ON/OFF SWITCHES ONLY. Do not use with any type of dimmer, relay or other control circuit.
The item advertises this caveat. Does one not set up the safelight with the enlarger timer then?
"There are a great many things I am in doubt about at the moment, and I should consider myself favoured if you would kindly enlighten me. Signed, Doubtful, off to Canada." (BJP 1914).
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Originally Posted by cowanw
To get to the point, go to post #22.
If it helps at all, my DIY safelight (with LEDs and parts from superbright plus Kodak safelight filters) uses what they call a "Pulse Width Modulation" dimmer. No change in voltage or emission spectrum.
A relay is a electro-mechanical switch. So, no Problem with that. I guess a thyristor in on-off mode would work as well.
Originally Posted by cowanw
The LEDs usually having switching regulators in them because the LED itself needs about 1.5V at about 1A, not 115V. If you connect them to a triac dimmer, the regulator switching will turn off the triac and you get no light.
So no using them with electronic relays (which are also triacs) either. Mechanical relays with metal contacts are fine.
OK, I appear to be wrong! I received the red LED bulbs earlier this week. I re-tested my current setup with multiple 0C filters that had tested "safe" before, and it tested safe again. I then used the same procedure using the pair of new LED bulbs, and they, too tested safe.
Originally Posted by David Brown
(I probably should have tested just one LED bulb first, but I reasoned that if the pair passed, I wouldn't need to test just one by itself. Turned out to be true.)
I then combined the new LEDs with my current safelights and tested that, and also got a "safe" test. I was both surprised and pleased. I finally extended the test from 7 minutes (using the Kodak procedure) to 10 minutes and still got a clean, safe test!
I use the Kodak method http://www.kodak.com/global/en/consu...Safelite.shtml. This is very similar to the Ilford procedure http://www.ilfordphoto.com/aboutus/page.asp?n=148.
The Kodak test goes to 7 minutes. It was a simple step to add another 3 minute exposure to get a cumulative 10 minutes of safelight exposure. All of this was done with Ilford Multigrade paper.
Disclaimers: Different papers must be tested for, and each darkroom setup, or change thereto, must be tested.
Therefore, all this proves is that the specific LEDs mentioned by the OP work in my darkroom with Multigrade. It does not speak to all red LEDs, other types of red bulbs, strings of Christmas tree lights, etc.
My task in the near future is to re-arrange my safelight set-up for efficiency, incorporating the new LEDs (and probably eliminating some of the older units). This new set-up, of course, will require re-testing. I may write all of this up in detail and put it on my old darkroom blog, or maybe here on APUG as an article, or both. The procedure for testing is tedious, but thorough. The most difficult part (aside from distilling the slightly verbose instructions down to the actual steps) is determining what "enlarger exposure is required to produce a light gray tone on the photographic paper with standard processing". In other words, you have to determine the minimum threshold exposure of the paper and make a white-light exposure to push the paper to this level. Then there are two sets of safelight exposures made, one with the threshold exposure made before and one after the safelight exposure. I am guessing this is the most misunderstood (and probably neglected) part of testing safelights.
Anyway, bottom line: my darkroom is now about 3 times brighter when printing! And safe ...