Are Red LED'S ok for red safe light ?

Discussion in 'B&W: Film, Paper, Chemistry' started by brucej, Nov 5, 2007.

  1. brucej

    brucej Member

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    There are a number of red LED's on the market now I have been wondering if anybody has tried them as a general purpose safe light ?
     
  2. Erik Hartmann

    Erik Hartmann Member

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    They use red LED's in the darkroom at Vraa-hoejskole (Denmark), and it is working OK....
    BUT I do not know the type.....
     
  3. GeorgesGiralt

    GeorgesGiralt Member

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    Hi !
    Yes, or no ;-)
    It depends on the LED. Some are emitting at the correct wavelength some don't.
    So if you want to try them, buy all from the same maker and test them for safety.
    Often, the salesclerk can't tell you either the maker nor the exact reference. They're just "red leds" for him....
     
  4. Lee L

    Lee L Member

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    Many film and paper manufacturers post sensitivity curves. Most LED manufacturers also post emission curves. You could match that way. The problem is finding out which particular LEDs are in the product you buy. You could also just purchase your own LEDs of known make and emission curve and make your own safelight, very easy to do with a simple resistor and power source. I've used both red and amber LEDs in the darkroom for years without problems, and really like them. LEDs typically don't have a very wide spectral output.

    http://superbrightleds.com/images/spectral_graph.gif shows the output of the LEDs I use. Half-power spectral bandwidth appears to be about 25 nm with both the red and amber at recommended voltage/current.

    Lee
     
  5. Bob F.

    Bob F. Member

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    By "LED" do you mean the commercially available LED based lamps? They do vary as suggested already, but they will be fine - it is not easy for an LED to emit very much light too far off its intended colour. Put it too close however and it will fog.

    If you make your own from quality LEDs and drive them at the correct current, out of band emission can be reduced to minimal levels allowing brighter light in theory than using bought lamps - but in practice I doubt it is worth the effort. Just do the usual safelight tests and you can certainly expect to end up with brighter light than the usual low-cost safelight with filters.

    I dislike red light (too gloomy) and use orange or amber LEDs in a home-made safelight. Unfortunately the only orange/amber ready-made LED lamp I could find locally emitted too much out of band to be useful (which just goes show how far you can take theory sometimes! :wink: )...

    Bob.
     
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  6. Erik Hartmann

    Erik Hartmann Member

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    Lee..... what number do your LEDs have....?????
     
  7. Pinholemaster

    Pinholemaster Member

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    The easiest test is to expose your print paper to a set of LEDs, red or amber, to see if they expose the paper. When you find a light that doesn't, you've found the correct light for your situation.

    Granted that can be expensive if you have to buy all the lights, so as mentioned above, matching spectral sensitivity to spectral emission is the first step.
     
  8. Lee L

    Lee L Member

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    Do you mean the product number from the website I mentioned in my post?

    If so, see this post: http://www.apug.org/forums/forum43/43983-darkroom-lighting.html#post541132

    Much of hat thread is closely related to this one. Search APUG for "led safelight" and you'll come up with other useful ones.

    Lee
     
  9. Tom Kershaw

    Tom Kershaw Subscriber

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    Bob,

    Would it be possible for you to put up on-line some kind of plan / design for your safelight?

    Tom.

    Edit) - Post 500; not exactly a major text in the field of photographic art and practice...
     
  10. Lee L

    Lee L Member

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    This site: http://www.hebeiltd.com.cn/?p=zz.led.resistor.calculator has a resistor calculator for LEDs in series or parallel. You need a DC power source (batteries or power supply), a resistor of the correct value, your LEDs, and a switch if you want to turn the LED off. As to the specific form the light takes, that's entirely up to the builder.

    Finding, buying, and shipping parts can get expensive if you source all the parts to build yourself. There are LED flashlight bulbs available in red (and perhaps yellow) or little portable battery powered arrays as seen here: http://superbrightleds.com/flashlights.htm that might be cheaper and just as useful as home-built. You could just slip a red LED into a flashlight and bounce it off the ceiling, or grab it an point it where you need it at the moment.

    Lee
     
  11. Sal Santamaura

    Sal Santamaura Member

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    I'm using the red (627nm) version of these:

    http://www.lightworld.com/optiled/index.asp?id=4

    Narrow spectrum, very bright and no fogging even with VC papers. They screw into a standard socket, consume only 2.5W and are rated to last 35,000 hours.
     
  12. gainer

    gainer Subscriber

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    I doubt that you could use amber LED's with VC paper, as it has yellow sensitivity, but I have been using red ones for years. You should test, of course, but I doubt that you will find a visibly red LED that fogs VC paper. If you want to do development by inspection of panchromatic film, use green LED's attenuated so you can just barely see by their light after you are dark adapted. BTW, red light least interferes with dark adaptation. If you want to do other things while film is in a closed tank and then inspect progress of development, use red light while working then switch to green for inspection.
     
  13. dpurdy

    dpurdy Member

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    this is probably a stupid question but just how bright are those led lights? Do they compare to a thomas safelight type brightness?

    and Sal how would you describe the brightness of that 35,000 hour bulb you recommend?

    I have for years used red christmas lights screwed into little night light units that plug into a regular wall plug. According to my testing they don't fog any kind of paper at all even at a distance of 3 feet. Very cheap and easy. A little dim though. I am very used to working in dim light so it is fine.
     
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  15. walter23

    walter23 Member

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    It depends on the exact spectrum emitted, which I don't know. My first darkroom experience was with a fairly bright red bulb (in a rental place) and I hwas having a horrible time getting my test strips consistent with my real exposures until I noticed that my safelight was actually not very safe at all, and that it would have an effect on the paper after maybe a minute of exposure. Obviously this was unacceptable so I switched safelights and suddenly darkroom work became a lot easier and my exposures weren't all over the map :wink:

    The simple solution is to buy one and test it. Put a quarter on a piece of paper (to make a circular shadow) and shine the red LED on it for a few minutes. If you see the outline your LED is no good, though you might be able to buy the right kind of red filter to put over the LEDs and restrict the spectrum.
     
  16. dpurdy

    dpurdy Member

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    I jsut called the place that Sal recommended and asked about that led and the salesman told me it would be about the same brightness as a christmas tree bulb. so if that is the case I will stick with the xmas tree bulbs as they are very cheap and easy and don't fog the papers I use.
     
  17. walter23

    walter23 Member

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    Better dim than too strong.

    I work in a tiny space (one rental darkroom closet unit just off of a big common wet sink area) and I moved my safelight to the very "back" of the room behind me. I shadow the paper while I'm working (though tests show my safelight is perfectly safe), and as a bonus this lets me read the dichroic filter setting dials on the colour head to dial in grade filtration under safe light, which was impossible when I had the safelight in front of me mounted above the wall behind the enlarger.
     
  18. buze

    buze Member

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    You might need a filter and sand paper it too, just because most leds beam is so narrow that it's more useful in a flashlight than in a general light device.

    I recently made a 60 LED 940nm 'safelight' for working in the darkroom with the infrared gogles, and it works best with a piece of ground glass in front to diffuse the light to a wider angle.
     
  19. Jon King

    Jon King Member

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    I made a RED LED safelight several years ago using a single 1 Watt Red Lumiled (www.luxeon.com), a resistor and a surplus wall wart. Bounced off the ceiling, it easily makes a 11x13 foot room bright enough to easily read in, while not fogging Ilford MG paper for at least 10 minutes (the limit of my testing - the paper did not fog).

    With this light, my darkroom is considerably brighter than the one at a local school that uses the orange mercury vapor lights. I do prefer the orange light to the red, but this quick hack has worked so well that I've never gotten around to testing Amber LED's.

    The LED resistor calculator website listed above would work well for anyone doing the same. High power LEDs will get warm, so I used a version of the LED that came mounted on a small star shaped circuit board. That board and the resistor got fastened down to a larger piece of a copper PCB, and it's been running well, warm but not uncomfortable to touch. The LED is rated for a current of 350mA, but I run it at about 300mA.
     
  20. Lee L

    Lee L Member

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    This is an overlay of the sensitivity curve of Ilford MG-IV with the amber LED emission spectrum (the blue curve) I've been using with that paper. The horizontal wavelength scale is matched. No attempt was made to correlate the vertical intensity/sensitivity scales. You can see that the paper sensitivity is rapidly heading toward zero as it approaches the output of the LED. You should, of course, test. But I find no fogging at very comfortable lighting levels and times for me with this combination. Note that there are different peak outputs for amber LEDs, mostly between 580 and 595 nm. Staying nearer the high end of that range is probably better.

    I did find the Thomas safelight to fog relatively quickly when I was using it in a lab with Kodak Poly RC-III. I had to close the vanes completely in a very large darkroom with high, dark ceilings. The person working that darkroom before me must never have produced a really clean white. Of course no one is using that combo much these days. Many of my musings are solely of historical interest. :smile:

    Lee
     
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  21. srs5694

    srs5694 Member

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    Just to add my experience, I'm using the red E27-R24 bulb from http://www.superbrightleds.com as one of my safelights and I have no problems. The bulb is mounted in a much older safelight housing that I bought used, but I've removed the filter from the safelight housing. The result is much brighter than the light the safelight used to produce with a tungsten bulb, but it doesn't fog my paper. (I did a safelight test out to 5 minutes, IIRC.)
     
  22. PeterB

    PeterB Subscriber

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    That graph has no value

    Hi Lee,
    unfortunately we can deduce nothing at all from that graph. You state the reason why yourself "No attempt was made to correlate the vertical intensity/sensitivity scales" in fact to make matters worse, one scale is linear the other log and they are different units!

    The best thing to do as you say is to test !!

    regards
    Peter
     
  23. chrisf

    chrisf Member

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  24. Sal Santamaura

    Sal Santamaura Member

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    I can't argue with your results but would advise that you not count on the christmas tree bulb's red paint maintaining its transmissive characteristics long term; do retest for fogging from time to time.

    As far as brightness, the red OptiLed puts out 45 lumens over a 130-degree angle. I don't know how many lumens a red 7 watt christmas tree bulb provides, but a similar white 7 watt night light version

    http://www.lightbulbemporium.com/proddetail.asp?prod=709007

    claims 35. The red paint must attenuate that quite a bit. I bounce one red OptiLed off the white ceiling of my roughly 6' x 8' bathroom when it's configured for printing. Things are comfortably bright, even for these aging eyes.
     
  25. Mike-D

    Mike-D Member

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    Going low tech, I used a short string of red LED Christmas lights to illuminate my cutting area which was a little dark. I removed a flasher circuit and added a resistor. Tested fine and works great.

    Mike D
     
  26. MurrayMinchin

    MurrayMinchin Membership Council Council

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    The superbrightleds.com E27-A24 (595nm) mentioned earlier is supposed to be equivilant to a 15 to 20 watt incandescent bulb. That would be far dimmer than the Thomas unit with the vanes open.

    Murray