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  1. #21
    Ken Nadvornick's Avatar
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    Quote Originally Posted by Bill Burk View Post
    Thanks for taking the time to do a test...

    Well, there you have it. I thought the sodium vapor bulb was inherently safe...

    Makes me wonder... Is your problem faded filters? Is it the extended spectral sensitivity of your VC paper (where maybe the Thomas would work great with Graded papers)?
    That was obviously a older thread, but the observations are still valid.

    Low-pressure sodium vapor discharge tubes have more than just sodium in them. They also have small amounts of neon and argon intentionally introduced during manufacture. These help with the initial striking of the lamp. It's called a Penning mixture. This mixture introduces additional emission spectra from the blue and green portions of the visible spectrum. (See here for a real-world illustration, 4th entry down, note the blues and greens.) These additional spikes are what is actually fogging the paper.

    If you have access to a Thomas Duplex try starting it up and letting it stabilize. Then remove all of the filters so you are looking at the bare bulb. Find an old CD/DVD disc. Use that disc to reflect the light from the bulb and look closely. The disc will act as a poor man's prism. In addition to the overwhelming orange sodium light you will also see much fainter, but still significant, pure bands of both blue and green light. Once you know what you are looking for, repeat the observation with the originally supplied filters for b&w back in place.

    The solution is to add an additional (inexpensive) layer of Roscoe Roscolux #19 Fire filter. It's a theatrical lighting item. It will remove all of the extraneous blues and greens while passing about 35% of the correct sodium D-line emission oranges. (See the filter technical data sheet here.) You lose about two-thirds of the "good" light in order to remove all of the "bad" light. But the Thomas is so darned bright to begin with that the loss is usually a good thing in a smaller darkroom.

    The Thomas Duplex is no longer manufactured. However, if you still wish to buy a new one of this type of safelight you can (as of the time of this post) still do so from the Sebastian Darkroom Products division of California Stainless Manufacturing. Click the following link to see a description of their OC-1012 sodium vapor safelight. It looks almost identical to the Thomas. California Stainless is the OEM manufacturer of many of the items offered by Arkay.

    Ken
    Last edited by Ken Nadvornick; 12-15-2012 at 02:21 PM. Click to view previous post history. Reason: Added additional information links...
    "They are the proof that something was there and no longer is. Like a stain. And the stillness of them is boggling. You can turn away but when you come back they’ll still be there looking at you."

    — Diane Arbus, March 15, 1971, in response to a request for a brief statement about photographs

  2. #22
    Bill Burk's Avatar
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    Quote Originally Posted by Ken Nadvornick View Post
    That was obviously a older thread, but the observations are still valid.
    Thanks for the additional detail, Ken - I always wondered why they'd need filters at all.

  3. #23
    Rafal Lukawiecki's Avatar
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    Bill, I found almost an identical experience to that described by Ken with bright orange LEDs. I did the same test with a CD, got green and blue spikes. I tested both a Lee lighting gel 19, fire orange, and a Lee primary red. Orange was safe to just 4 min, while red gives me 15 min safety with MGIV WT. Michael 1974 has been very helpful by sharing his filter experiences when he was building his LED safelight. I am delighted with mine now.
    Rafal Lukawiecki
    See rafal.net | Read rafal.net/articles

  4. #24
    Ken Nadvornick's Avatar
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    And I had the same problem with 635nm red LEDs. The CD/DVD test showed small pure blue and pure green spikes that led to unexplained fogging. The unaided eye could not see these. I then tried a single thickness of standard Rubylith as the filter. That did the trick. My tested safe time under the red LEDs (for both Ilford MGIV FB & RC) went to over 60 minutes.

    Ken
    "They are the proof that something was there and no longer is. Like a stain. And the stillness of them is boggling. You can turn away but when you come back they’ll still be there looking at you."

    — Diane Arbus, March 15, 1971, in response to a request for a brief statement about photographs

  5. #25

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    i BOUGHT ONE TOO ON THE EBAY SITE BUT i CAN'T FIX THE Cable TO THE EUROPEAN ELEC. NETWORK . HELLLLLPPPPP ....

    etiennedezutter@yahoo.com

  6. #26

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    You'll need an transformer to reduce the voltage to 110V.

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