Thomas Duplex Issues

Discussion in 'Darkroom Equipment' started by Sundowner, Sep 3, 2013.

  1. Sundowner

    Sundowner Subscriber

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    Someone help me troubleshoot what's going on here, because I'm stumped; I inherited a literally-brand-new-except-for-the-box Thomas Duplex, and after going over the thing with a fine-toothed comb I'm still having issues with it burning the paper during a very basic eight-minute coin test.

    I'm not kidding when I say that the safelight is new; it doesn't have a single scratch on it. In fact, the only thing that needed replacement/rebuilding were the old gelatin filters that were crinkled and thoroughly useless, which is expected with these lights. I rebuilt the inner filters by using the factory glass on the outside and a piece of 50% transmission diffusion plastic on the inside, with the suggested #19 Fire gel sandwiched between the two. For the outer filters, I used a #27 Medium Red between two pieces of factory glass. I used a black vinyl tape to secure the glass and plastic plates together; it works pretty well.

    After rebuilding the filters I tested the safelight and got a light burning of the paper in less than a minute's time with the outer vanes slightly open; I used a CD as a poor man's prism and found that there was a faint band of green light making its way either through or around the #19 inner filter. This green band was also present without the filter in place, although it was much stronger and joined by some blue/purple stuff; with the outer vanes closed there was nothing except the yellow/orange/red spectrum to be seen. So, I tried a second test with the vanes fully closed and got exactly the same results.

    At this point I was confused because there should be no way that any paper-exposing light could be making it's way through the filters, so I chalked it up to light leakage in he housing and went over all of the safelight's seams with black tape. After that, I taped the inner filters in place - I found a couple of spots where light could easily leak around the filter - and tried a third test with the outer vanes slightly open again. Same results; the one-minute coin silhouette was easily visible.

    In a slight bit of desperation I taped the outer vanes down to prevent any and all chances of light leakage, and I ran a fourth test...and got the same results again. I've got no light leaks, no light skipping around the filters, so it seems that the only thing that could be happening is that the filters themselves are...well, not filtering. I find this hard to believe because people have a lot of success with rebuilding Thomas filters with the #19 and #27 gels, but I don't know what else it could be.

    Anyone got any thoughts that might help me out, here? Am I doing something wrong that I don't know about? :confused:
     
  2. Ken Nadvornick

    Ken Nadvornick Subscriber

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    Which paper are you using for testing? I know some of them (Slavich, for example) specifically state that only red light is safe. For those I use a DIY 635nm red LED safelight.

    Your description sounds thorough and correct to me. Is your tape lifting up from the heat perhaps? Is it truly opaque?

    I suppose you could try temporarily covering the outside of the outer filters with some taped down red Rubylith. What you need to find is some level of baseline configuration that works so you can then work backwards from that to isolate the problem.

    Ken
     
  3. zsas

    zsas Member

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  4. Neal

    Neal Subscriber

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    Dear Sundowner,

    Probably a stupid suggestion on my part but are you sure the fogging is from the safelight rather than another source?

    Neal Wydra
     
  5. Greg Davis

    Greg Davis Subscriber

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    The filters are wrong. The pair on the moveable vanes should be two sheets of the Rosco #19 with diffusion, but the bottom pair should be a sheet of Rosco #3406 and #3407 with diffusion. This matches the filters that are sold commercially for this safelight.
     
  6. Ken Nadvornick

    Ken Nadvornick Subscriber

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    I dunno' Greg. For b&w all I use is a single layer of Roscoe #19 Fire between glass sheets without any diffusion. The Roscoe spectral transmission chart says this should be sufficient. And for both my CD and pre-flashed fogging tests of Ilford MGIV, it is.

    This filter is located in the lower filter position. The vanes contain only sheets of black mount board. That allows me to adjust down to total darkness for easel framing and focusing.

    Ken
     
    Last edited by a moderator: Sep 4, 2013
  7. Mark_S

    Mark_S Subscriber

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    Some thoughts -

    1) Test Neal's thought that the fogging may be coming from something other than the Thomas Duplex - try a fog test with the light off, to make sure that your darkroom is dark when the lights are off.

    2) I use an Agfa Sodium Vapour light - which is similar, but smaller than the Thomas Duplex. My lights take a long time to warm up, and until they are warmed up, will fog paper - I turn the safelight on 20 minutes before I start printing to allow it to stabilize - are you doing your test immediately after turning the light on?

    3) Are there flourescent lights in the room (even if they are turned off). It may be that the light from the safelight is exciting the phosphors in the flourescent lights and they are emitting light which is fogging your paper.
     
  8. Greg Davis

    Greg Davis Subscriber

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    I tried that after your posts in the other thread. I had fog after very short times. I test using Kodak's method of using safelight exposure before a medium gray exposure, and again after a medium gray exposure on separate pieces of paper. Using the filters I mentioned gave me safe times after exposure of up to three minutes with the vanes fully open, and well past seven minutes with them fully closed. With just the #19 filter in place I had safe times of less than one minute fully closed after the paper received printing exposure. Just my experience.

    I should note that I was using Ilford MGWT paper. Regular MG gave slightly longer safelight times, though I don't know why since printing times are shorter with MG than MGWT.

    And, yes, I was meticulous about other sources of light being an influence. Even the glow from my watch face was eliminated from the darkroom first.
     
  9. Sundowner

    Sundowner Subscriber

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    Wow! All kinds of responses! Thank you, everyone!

    I'm currently printing on Oriental, and it's OC-safe for over an hour with my Kodak D safelights.

    The tape isn't lifting and it's opaque; I just checked to see whether or not either could be a factor. I ordered some rubylith to see if that helps, but I don't have it on-hand yet.

    I have seen those filters; I already had the gels on-hand and the diffusion plastic, so I was hoping to use them. Also, the rubylith I ordered only cost $10 for a rather large sheet.

    I'm pretty sure, Neal; I've had no issues with this paper in the exact same conditions, with the exception of the safelight change. That tells me that something, as we've discovered, is indeed in need of re-wacking.

    That's different than what I've seen so far. Hmm.

    This is more in-line with what I'd been reading; I was hoping to use the outer vanes to control the overall light level but I haven't made it that far, yet. :D

    1) Just now I tried one strip that was left out in the dark for eight minutes, and another directly from the paper safe; both printed perfect, unexposed white.
    2) I let the light warm up for about ten minutes or so; I can try it after a longer time, though. That won't be a problem.
    3) No fluorescent stuff anywhere near the darkroom, for exactly the reason you mentioned.
     
  10. Greg Davis

    Greg Davis Subscriber

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    I don't know what you have been reading, but I purchased a set of filters from Thomas Duplex and cut them open to find the Rosco gel numbers still printed on them.
     
  11. Sundowner

    Sundowner Subscriber

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    Wow...that's odd. I just looked up those filters and the wavelengths they allow to pass are not what I would have expected at all. The factory filters that came with mine were horrendously cracked/crinkled gelatin with no printed numbers to be found, and I looked pretty hard to find one just in case I had to cross-reference something. That's very interesting info; thank you!
     
  12. Ken Nadvornick

    Ken Nadvornick Subscriber

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    Hi Greg,

    This is the first chance I've had to look into this further. I went and checked my set of original FBD filters for my Duplex. I purchased my unit new before they were discontinued so I know they were OEM pieces. They showed no indication of Roscoe product or serial numbers. Could it be that the construction of the filters changed over the product lifetime of these safelights? Mine was purchased quite a few years ago.

    I then had a look at the spectral transmission graphs made available online by Roscoe. There are two very slightly different versions of the #19 Fire filter listed. I've referenced the version I purchased. Here are the three in question for others who may not have seen them:

    Rosco E-Colour+ #019 Fire

    Rosco Roscolux #3406 Sun85

    Roscoe Roscolux #3407 Sun CTO

    And here again is that LPS line spectrum from Brian Niece, Associate Professor of Chemistry at Assumption College in Massachusetts (fourth item down the page):

    Low Pressure Sodium Line Spectrum

    From the generated line spectrum of an LPS lamp the extra blues and greens caused by the inclusion of the argon/neon Penning mixture are readily apparent. And a look at the #19 Fire filter chart shows that it should be almost perfectly opaque to those wavelengths all by itself. For standard OC-rated b&w materials this filter should work fine. And in practice, at least for me, it does.

    On the other hand, I'm not sure what additional safeness the other two filters would add, given their respective charts. Especially in the green regions for VC papers. Filter #3407 in particular allows a substantial percentage of blue-greens and greens to pass. This should be an obvious risk factor for VC papers if the #19 were not present.

    Might these second two filters be intended for different materials? I'm really curious regarding your observed results.

    Ken
     
  13. frobozz

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    Well, we need a cross-reference between Thomas' "edge tape color" based ID scheme, and the various gel colors, but as a clue, here are the official instructions from Thomas (I scanned the sheets that came with mine):

    http://backglass.org/duncan/darkroom/thomas_duplex_super_safelight_instr.pdf

    Duncan
     
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  15. Greg Davis

    Greg Davis Subscriber

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    I purchased a set from Freestyle while Thomas was still in business. I bought a yellow tape, a red tape, and a black tape. I took them apart a few years later to see if they were filters I could replace myself for less money. In the yellow tape filter I found two sheets of gel filter material. One had the number 3407, the other 3406. I recognized them as Rosco filters from my sample book. The red tape had "ROSCO 19" printed on the edge of the filter. The black tape did not have any markings, but were the same colors as the yellow tape, but used more layers of filter material inside. All of them used the tissue paper that comes with Rosco filters as the diffusion material.

    The numbers were not present in both filters of a pair. I don't think they were intended to be left on the filter material when cut and put in place. I had a very old set of Thomas Duplex filters, and they were different material. It was far too old and faded to tell what they once were. It is possible that the filters sold for these safelights now and just before Thomas Duplex stopped selling their lights are made by someone else and they just use what they think may work, but if you buy a set of filters from Freestyle or KHB Photographix, these are what you get.

    You could always buy a set, open them, and tell us what is in them. If they are different than what I have, then I was sold the wrong filters. But I doubt it.
     
  16. Sundowner

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    This is the part that confuses me. The 19 filter is only supposed to allow the passage of a slight percentage of green-ish wavelengths, and this was very evident in my not-at-all scientific test; there was a definite green band right where one would expect it, were they looking at the chart for the 19 filter. I was under the impression that this wouldn't be an issue when I ordered the 19 and 27 filters, because I'd read so many reports of them being great cutoff filters for the sodium lamp...but somehow I still got a definite graying of the paper after less than a minute's time. With that said, I can't figure out how the other two filter numbers would help, because they seem to be letting in a lot of blue/green wavelengths.

    I just found that online not too long ago; thanks for posting it!

    I think the filters I had in mine when I got it were the older style, as well...and they were desiccated beyond recognition. I'm almost tempted to buy the inner set and see what comes in them...but that may have to wait until I have some extra cash. For now, I'm hoping that the rubylith I ordered will come in soon and hopefully allow me to actually use the safelight, rather than just look at it. :D
     
  17. Ken Nadvornick

    Ken Nadvornick Subscriber

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    And it's confusing me as well. The #19 filter should cut off virtually ALL of the blues and greens. In other words, when in place you should see no blue and/or green bands at all.

    Before my successful (pre-fogged) safelight tests I fired up the Duplex, let it warm for ~30 minutes, then looked at the CD reflections unfiltered. In addition to the massive yellow sodium doublet, I saw the much, much fainter blue and green spikes. I also saw the extra deep red spike.

    Then while looking directly at those spikes I interposed a single layer of #19 filter. The blue and green spikes disappeared. When I removed the #19, they returned. Back and forth.

    Later, when I performed the safelight test I was able to leave a sheet of Kentmere Bromide #3 graded paper out for ~30 minutes with no hint of fogging.* As you may know, bromide papers are very fast. It should have responded to any blue light present. But it didn't. It remained pure white. (Even checked with a reflection densitometer to factor out my subjective eyes.)

    So as I said, I too am confused.

    Also keep in mind that if you use a cover sheet of Rubylith you will likely be filtering out all of the yellow sodium light as well. At least in a perfect world. I've never tried it myself, so I don't know how much will pass. Maybe only that secondary red spike I mentioned above?

    Ken

    * Apologies, as I earlier identified the test paper as Ilford MGIV. It was actually the Kentmere Bromide. The Ilford was used for my red LED safelight testing.
     
  18. Greg Davis

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    In regards to the paper that you left out for 30+ minutes, did you flash it at all to see if there was any effect on top of exposure? My tests didn't show much, if anything, in blank white areas. The degradation was in areas that were already exposed by the enlarger.
     
  19. Ken Nadvornick

    Ken Nadvornick Subscriber

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    Yes, I did. That's what I meant when I said "pre-fogged." Perhaps not the best choice of words by me. Probably should have said "threshold fogged."

    A test was first made to determine threshold exposure for the Bromide #3 paper. Then the test sheet was pre-exposed to just below that threshold level. Any additional safelight fogging exposure should have therefore been immediately visible after development. Either to my eye, or to the far more sensitive reflection densitometer. Comparisons were made against a similarly pre-exposed control sheet that was not subjected to the safelight.

    It should also be noted that 30 minutes was an arbitrary cutoff by me. Because there was no detectable fogging, the actual safe time limit was therefore an unknown time beyond 30 minutes.

    When I later performed similar testing for a DIY red LED safelight under Rubylith, I extended the test out to 60 minutes without any apparent fogging on MGIV. That's where my earlier incorrect reference to MGIV came from.

    Ken
     
  20. Sundowner

    Sundowner Subscriber

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    Well, allow me to make it even more confusing: I did the exact same thing that you did with filtered and unfiltered reflections on the CD. Unfiltered, I saw the blue and green bands, and also the massive yellow sodium band and the deep red...exactly as expected. With the #19 filter in place, the yellow and red were unaffected and the blue was entirely cut out...but the green band was still dimly present. I actually managed to do a side-by-side comparison with the #19 filter halfway covering the sodium lamp, and although the green band was attenuated it was, as aforementioned, present. So, I don't know what to make of that, because there should be very, very little green coming through the #19 filter...but it was most assuredly there, and likely - in my opinion - burning the paper.
     
  21. Jim Noel

    Jim Noel Member

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    I simplified the solution in my 10x10' darkroom. I use the supplied yellow/tan filter over the tube. I inserted black foam core in the vanes to attenuate the light. I have never had a problem with fogging in the 20+ years I have used this light.
     
  22. Sundowner

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    That's basically the point that I wanted to get to, down the road...but I haven't gotten to "never had a problem with fogging" yet. :laugh:
     
  23. Ronald Moravec

    Ronald Moravec Member

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    Go with the filter sold by manufacturer. Rosco gels come in large sheets and the code number is printed only in one corner. Depending on where the cuts are made, it may or may not show up.

    I use the gels in green and blue as VC filters for Aristo cold light head using all the grades of each, 1/8 to full. The man who made Aristo heads put me on to it.
     
  24. Greg Davis

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    I did another safelight test this morning. First, I used a CD as Ken spells out to check for other colors of light. With the vanes wide open, and the Yellow tape filter in place in the bottom, the Red tape filter in the vanes, I saw no color except yellow. Still with the vanes wide open, I exposed two pieces of paper to the safelight, one was flashed before the safelight exposure, the other afterwards. Both showed signs of fog after 3 minutes in the flashed area, none in the borders. Both showed fogging at 7 minutes in the flashed area, none in the borders. My previous test was done in a different darkroom and I cannot guarantee there wasn't a light leak.

    I expect better performance with the vanes fully or partially closed.
     
  25. Ken Nadvornick

    Ken Nadvornick Subscriber

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    Hmm...

    If the set of filters you are using, regardless of where they came from or what material they are made of, are successfully removing all of the extraneous higher wavelengths above the sodium D-lines doublet at ~589nm, then the only other immediate explanation is that the paper emulsion sensitivity is bleeding over to some degree into that wavelength. Either because it was designed to do that, or it has aged to do that.

    Or something else silly is going on. Like, say, a leaky paper safe. When I did my testing I did take new sheets of fresh paper directly from the box it came in while in total darkness, then held them in a clamshell safe that I also occasionally use to store loaded film hangers, while the Duplex warmed up. I don't really think this is it, but it is a very slight possibility.

    I am now looking for some time here to go down to my darkroom and also repeat the CD wavelength check. Maybe tonight.

    Perhaps my color vision is slightly different than his and I missed the very faint residual green line that 'Sundowner' observed. Since I tested on Kentmere Bromide, which should be blue-sensitive only, I would still have seen no fogging if, as he observed, all of the blue was removed.

    Tests on VC paper (which I haven't done under the Duplex) would be a different story however, if there was some minor residual green remaining. And I do note that if one looks at the Rosco transmission chart for the #19 filter, there IS a very slight transmission of greens. It looks to maybe only be 2-3%. But it's definitely there, if the chart is accurate.

    For the record, of late I've been using only the red LEDs because I got into working with some Slavich graded bromide, and that paper is marked as red-only safe. Then when using MGIV I've just continued with those LEDs.

    Ken
     
  26. Greg Davis

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    My paper is a brand new box of MGWT, the paper is taken from the box, not a safe. PM me your address and I will send you some of the filter material I am using, which is the same stuff in the filters sold by Freestyle and others for these safelights. You can try it out yourself.

    And as I said, I get some fogging when the vanes are wide open after three minutes, but closed down, I get much longer times while still bright enough to work easily.

    I should note that I am using these Kodak instructions to perform the test.