Thomas Duplex Safelight Questions

[Kevin Crisp]

As I mentioned, I got a Craigslist pair of these for $12.50 each. I am a big spender. They both needs bulbs and filters and some cleaning up. Or maybe new cords and not bulbs, I haven't started on them yet.

My question is, if I only use them for b&w printing, with the vanes always open some to let the orange-ish light out, does it really matter at all what is in the upper/outer slots of the moving vanes? I didn't realize (I have had one of these for years) that the vanes apparently do have some deep, dark nearly opaque filters in them. Why couldn't I just put together new orange filters for the fixed in-body slots, and put black cardboard in the vanes so I can close it up and make it dark when I want to? Why wouldn't that work just fine?

Thanks.

[Luis-F-S]

Try it and see if it fogs the paper. There have been posts in the past on making the filters with Roscoe gels. L

http://www.largeformatphotography.info/forum/showthread.php?114156-thomas-duplex-safelight-Top-Filter&highlight=duplex

Bulbs:

http://www.topbulb.com/35w-low-pressure-sodium

[lenser]

The bulb man, Reno Nv. I bought a new bulb from them a couple of years ago for only $29.99. No idea if the price is still the same.

[StoneNYC]

Luis those are sodium bulbs not met curry vapor bulbs. I know people use the sodium as a substitute but isn't there a difference?

[Old_Dick]

Just took a look. Mercury vapour lamp (380-780 nm) , sodium vapour lamp is concentrated in the yellow part of the visible spectrum (around 580-590 nm)

[Jim Noel]

Unless your darkroom is very large you probably don't need all teh light the lamp can produce. In my 10x10' room I replaced the filters with pieces of mat board painted flat black in order to attenuate the light to a reasonable level.

[Old_Dick]

I used Rubylith as a filter, works great. I got the Thomas almost for free.

[EdSawyer]

These do take Sodium bulbs by design, and the price on them from good vendors is still about $30-35.

I think the upper vanes did have a different color filter than the lower filter windows, not sure if they were just for attenuation or further filtering or ?

in big darkrooms I have been in, they had one of these running wide open (vanes all the way up, just normal orange filters in place) with no fogging issues.

[HMG]

If you haven't used one, you're in for a treat. I was going to sell mine, thinking way overkill for my 8x8 darkroom. But I wanted to test it first. Hung it up, plugged it in, and it's been there every since. I made new filters with hardware store glass and rubylith. No issues with fogging.

[StoneNYC]

Just took a look. Mercury vapour lamp (380-780 nm) , sodium vapour lamp is concentrated in the yellow part of the visible spectrum (around 580-590 nm)

Thanks, can you help me understand, what spectrum of light does to print paper which is NOT sensitive to... Amber/red light but IS sensitive to other light colors, so yellow is (I thought) in the spectrum of "fogging" (let's use Ilford VC/MG type modern papers which prefer amber) papers.

And also let's deal with the light and how it "feels" in the darkroom. One of the nice things about the Thomas' over a red safelight to me is that my eyes handle it better, but amber is not yellow, so can you educate me on both the fogging and light output color in these terms? If you don't mind, thanks! :)

[Michael R]

Stone -

Typical graded papers are predominantly (ie not "perfectly") blue-sensitive, while typical VC papers are predominantly (ie not "perfectly") sensitive to blue and green.

Here's a very rough summary of the wavelength ranges of visible light (it can't be precise because it is a continous spectrum) in nm:

Violet ~400-450
Blue ~450-500
Green ~500-570
Yellow ~570-590
Orange ~590-620
Red ~620-700

(Below violet you get to UV, then x-rays, then gamma rays. Above red you get to IR, then microwaves, then radio waves)

Based on the above approximate ranges, VC paper can generally be considered sensitive roughly to a range of say <400-570nm. These are not hard cutoffs of course, and safelight filters, most LEDs etc. aren't "sharp cutting" either (and they can emit wavelenghts outside their primary spectrum too), which is why a proper test of safe time for any paper/lighting scenario should always be conducted, both initially, and at periodic intervals thereafter.

The Thomas is based on a sodium vapour lamp. Mercury vapour would be one of the worst possible safelights.

[Chauncey Walden]

They will fog at full output. After a John Sexton class where he called them a Thomas paper burner I did more careful checking of mine. Bring the paper up to threshold and then check it for how long you can expose it to the safelight without fogging. I had to leave the vanes closed (full internal and vane filters) and cover the vane on the enlarger side about 75% to be safe. The darkroom is still plenty light.

[StoneNYC]

Stone -

Typical graded papers are predominantly (ie not "perfectly") blue-sensitive, while typical VC papers are predominantly (ie not "perfectly") sensitive to blue and green.

Here's a very rough summary of the wavelength ranges of visible light (it can't be precise because it is a continous spectrum) in nm:

Violet ~400-450
Blue ~450-500
Green ~500-570
Yellow ~570-590
Orange ~590-620
Red ~620-700

(Below violet you get to UV, then x-rays, then gamma rays. Above red you get to IR, then microwaves, then radio waves)

Based on the above approximate ranges, VC paper can generally be considered sensitive roughly to a range of say <400-570nm. These are not hard cutoffs of course, and safelight filters, most LEDs etc. aren't "sharp cutting" either (and they can emit wavelenghts outside their primary spectrum too), which is why a proper test of safe time for any paper/lighting scenario should always be conducted, both initially, and at periodic intervals thereafter.

The Thomas is based on a sodium vapour lamp. Mercury vapour would be one of the worst possible safelights.

Hmm, interesting, I was under the impression the Mercury Vapor lamps were how they came and Sodium Vapor was the "cheaper and less good" replacement.

These are all of course filtered, so does it matter since they are filtered? Isn't it actually "better" with the mercury vapor because you are getting more light waves outside the papers range and only filtering the waves that would affect the paper? And so the Sodium vapor (after filtering) would appear less-bright to the human eye than the Mercury vapor would?

I'm asking because that's how I interpret that.

[Michael R]

Stone: The spectral output of these types of bulbs can be complicated because it depends on pressure and how the bulb is constructed etc. For example, a "naked" mercury vapour lamp produces a distinctly bluish-green white light (along with UV). It would typically be wrapped in a second layer of glass which filters out the UV. This outer layer can also be phosphorescent to correct for the bluish-greenish cast.

Think about what the filtering does. It removes all the light you don't want. Suppose you start with a bulb which naturally emits most of its light (ie peak emission) in the yellow or orange or red part of the visible spectrum, and only a small amount in the blue or green. That's good because you don't need to filter as much of the light (which can also extend filter life). On the other hand, suppose the bulb emits most of its light in the blue and green part of the spectrum. That means you need to filter most of the light the bulb is emitting.

All that said, I'm no expert specifically on the Thomas units, so it is possible they came with mercury vapour lamps. I'd be really surprised though. It doesn't make sense to me. I always thought they came with sodium vapour.

+1 to Greg's reference to Kodak's safelight test. That is the proper way to test for safe time. And it should be repeated periodically because any filter which works by absorption will fade with use.

Don't overlook proper safelight testing, no matter how safe you assume a safelight to be. For example people sometimes use red LEDs and don't realize they can emit blue even though you don't see it. Etc. And of course different materials will have different sensitivites.

[StoneNYC]

Stone: The spectral output of these types of bulbs can be complicated because it depends on pressure and how the bulb is constructed etc. For example, a "naked" mercury vapour lamp produces a distinctly bluish-green white light (along with UV). It would typically be wrapped in a second layer of glass which filters out the UV. This outer layer can also be phosphorescent to correct for the bluish-greenish cast.

Think about what the filtering does. It removes all the light you don't want. Suppose you start with a bulb which naturally emits most of its light (ie peak emission) in the yellow or orange or red part of the visible spectrum, and only a small amount in the blue or green. That's good because you don't need to filter as much of the light (which can also extend filter life). On the other hand, suppose the bulb emits most of its light in the blue and green part of the spectrum. That means you need to filter most of the light the bulb is emitting.

+1 to Greg's reference to Kodak's safelight test. That is the proper way to test for safe time. And it should be repeated periodically because any filter which works by absorption will fade with use.

Got it, thanks!

[Luis-F-S]

The Thomas is based on a sodium vapour lamp. Mercury vapour would be one of the worst possible safelights.

Hmm, guess I've been fogging my prints for the last 30 or so years. Wonder how come I, (or the galleries that have carried my work) haven't noticed....L

[Michael R]

Try reading first. It helps. On the other hand if you're actually using mercury vapour lamps instead of sodium, can't help you.

As for your remark about fogging, you'd be surprised how many people don't realize they've got safelight issues. You are assuming it would always show up as outright fog. Not necessarily. It can also be more subtle, acting like a pre/post flash and therefore reducing paper contrast. You would compensate without necessarily realizing it, and neither would your gallery.

[Rolfe Tessem]

A couple of notes:

B&H lists the replacement bulb at $99.00, although there are undoubtedly less expensive sources.

Second, if you use an analyzer, normally the safelights have to be off for the analyzer reading. You can't switch a Thomas light on and off through a timer or analyzer like a normal safelight because sodium lights take time to strike. You would need to have other safelights that can be switched by the analyzer and use the Thomas strictly for the print developing part of the session.

[EdSawyer]

The Thomas lights have always been sodium vapor, not mercury vapor. (save the Merc. vapor for nice tube rectifiers like 866s and the like. ;-)

[bob carnie]

thanks Greg

For anyone interested, here is the manual for the Thomas safelight.

[Michael R]

For anyone interested, here is the manual for the Thomas safelight.

Thanks for posting this. It is strange that they differentiate VC papers from orthochromatic papers, since VC papers are orthochromatic. Regardless, it is always a good idea to do a proper safe time test with any paper, whether the safelight is red, amber, sodium vapour yellow.

[Luis-F-S]

The Thomas lights have always been sodium vapor, not mercury vapor. (save the Merc. vapor for nice tube rectifiers like 866s and the like. ;-)

+1 Yup, that's correct!

[EdSawyer]

Glad that somone got the rectifier reference! ;-)

[Kevin Crisp]

These certainly are durable. I spent about 90 minutes on one of my $12.50 ones. Mice had been living in it and it was obviously exposed to the elements. The MASSIVE French transformer inside is why these weigh so much. I tested it and still good at 400V on the secondary. For all the on line comments about this or that unit having a bad ballast it turns out there isn't one. It is just a transformer and a socket. Unsoldered and took the transformer out, then pressure washed it out and let it dry. Transformer back in and a new bulb and it works fine. I am making the replacement filters in a couple days and I'm in business. I'm going to try the Roscoe "fire" filters many recommend.

If you're in the LA area and have any interest in the second one (similar condition but I don't know about mice, I haven't looked under the reflector...) let me know. It needs a bulb, but the transformer on that one is working too.

[stradibarrius]

Thanks Greg for that!

For anyone interested, here is the manual for the Thomas safelight.