Thorium glass?

[Terrence Brennan]

I was in a Habitat for Humanity shop in Stittsville, Ontario yesterday and spotted a Asahi Pentax for sale.

It was in its ever ready case, and had a 50mm f/1.4 Takumar lens. The camera itself was a bit rough, and the shutter lagged a bit opening on the 1 second exposure speed.

When I looked through the viewfinder, it was yellow! It looked as if a Wratten number 8 filter was over the lens, but there was no filter on it! I removed the lens, and sure enough, it was yellow. A peek through the viewfinder, without the lens, looked normal.

What could cause a lens to become discoloured as that one was? I have read here on Phototrio about some of the Kodak 7-inch Aero Ektars that have become discoloured with age, due to thorium in some of the lens elements.

I am just curious; I own Nikkor lenses, some of them at least 60 years old, and have never seen this before.

 

[RLangham]

Yes that is a famously (and harmlessly) radioactive lens with thoriated glass. I’ve had at least two of them, they’re very nice but not necessarily any nicer than a comparable Nikkor, Canon and Minolta.

Thoriated glass was a fad; the old Soviet thoriated Industar-61 was no different in performance to my eyes than the older non-thoriated Industar-26m that it evolved out of.

 

[Dan Fromm]

Relatively few lenses were made with thorium glass lenses. ReproClarons, TTH ApoTessar clones, and a few others come to mind.

Michael Briggs in Huntsville, Alabama

aeroektar/aeroektar.html Aero-Ektars and radiation hazards therefrom.

If you buy the lens, don't carry it in a pants pocket.

 

[Dan Fromm]

Thoriated glass was a fad;

Not a fad, a solution to a problem.

 

[Paul Howell]

I have the 50 1.4 and found to be very sharp with excellent contrast, really nice with Kodachrome. Mine turned yellow, living in the desert Southwest I left mine in the sun for week and it cleared.

 

[DREW WILEY]

Thoriated glass from its era was more common than you might think. I even have one old glass skylight filter which has it, which has appreciably ambered. Thoriated glass wasn't a gimmick, but a relevant design option.

 

[Andreas Thaler]

I even have one old glass skylight filter which has it, which has appreciably ambered.

This ties in with the adventure I described on Reddit:

Destructive removal of a stuck lens filter; testing for radioactivity

 

[Paul Howell]

Swiss Alpa used the Pentax thorium 50 1.4 for the Alpa all models. Along with a Schneider 50, 1.8 or 2.0 and the famous Kern Swiss 50 1.7 Macro. These lens were tested then torn down and rebuilt and adjusted for best performance then test and glass plate negative was taken and kept on file in case was ever returned for repair.

 

[DREW WILEY]

Aliens arriving from other galaxies were helping with lens design at that time. They glowed in the dark; but the radioactivity gradually faded away - that's why we no longer see aliens hanging around unless drinking or smoking something radioactive ourselves.

I have a little bottle of uranyl nitrate powder on a darkroom shelf. I was curious just how radioactive it was, so left a high speed sheet of film directly beneath the bottle for a couple of months and then developed it. No effect at all. Apparently, just the thickness of the glass of the bottle blocked all the radioactivity. You probably get more rad exposure from the luminous hands of a Mickey Mouse watch.

 

[tcolgate]

Some of the Zuiko Pen F lenses are thoriated (the 100 I think?). I've certainly not found and problems with it.

 

[brbo]

Some of the Zuiko Pen F lenses are thoriated (the 100 I think?). I've certainly not found and problems with it.

I don't think 100/3.5 has it, but the 40/1.4 definitely has thoriated glass.

 

[Paul Howell]

Kodak made lens with thorium, not just the areo lens but a few of the consumer grade cameras as well.

 

[isaac7]

I have the 50 1.4 and found to be very sharp with excellent contrast, really nice with Kodachrome. Mine turned yellow, living in the desert Southwest I left mine in the sun for week and it cleared.

You can use regular light instead of sunlight. I'm not sure where the idea that it had to be UV light came from. I put my 50mm f1.4 SMC Takumar under an LED lamp for about a week and it is completely clear now.

 

[DREW WILEY]

UV is what counts. Many LED's output quite a bit of UV too, and so do CFL's and halogens. Ordinary old fashioned tungsten light bulbs don't.

 

[Dan Fromm]

UV is what counts. Many LED's output quite a bit of UV too, and so do CFL's and halogens. Ordinary old fashioned tungsten light bulbs don't.

Second the motion. I've used a UV-B compact fluorescent, also a cheapie Ikea LED desk lamp, to clear yellowed lenses.

 

[AZD]

For the technical answer to your question, look up “f-center”, which explains the mechanism.

Hardware stores and similar now sell LED bulbs in colors and UV (black light). The UV one works exceptionally well at removing the yellow tint from thoriated lenses. It works much faster than sunlight.

I have fixed three: Super Multi Coated Takumar 50/1.4 and 105/2.4, and an early 35/1.4 Nikkor. All excellent lenses, BTW. Gimme that gamma radiation!

 

[isaac7]

UV is what counts. Many LED's output quite a bit of UV too, and so do CFL's and halogens. Ordinary old fashioned tungsten light bulbs don't.

Not according to Lawrence Livermore. Visible light is all that is needed. Check out table 2: Bleaching parameters and you'll see that the wavelengths needed are in visible light spectrum for all glass types. https://www.osti.gov/servlets/purl/10178461

I don't believe the Ikea LED lamp I used emits any UV light to speak of. Doesn't surprise me that incandescent lamps don't work, they are too warm. Any sufficient amount of mostly full spectrum light should work fine.

 

[blee1996]

Let the yellow stay on as a filter for black and white film.

 

[koraks]

Many LED's output quite a bit of UV too

No they don't. Typical white LEDs have a hard fairly cutoff at around 400nm since the emitter is a blue one and this doesn't have side-bad emissions below that wavelength. This may still be sufficient to 'erase' the thorium 'stain'; I don't know what wavelengths are effective. UV LEDs are plentiful and cheap, however.

The 'magic trick' about thorium glass was of course its refractive index, which is relatively high.

 

[dynachrome]

I must have a number of radioactive lenses but my favorite is a concave front Canon 35/2 FD SSC . I put a piece of aluminum foil, shiny side up, under the lens and kept it under a daylight balanced CFL bulb for months. It worked pretty well. I also have a later 35/2 convex front FD SSC which is not radioactive and three 35/2 New FD examples. All five are very good. Of the newer designs, I prefer the FD SSC because it has better build quality than the New FD design. The most severely discolored of my radioactive lenses must be the 28/2.5 MC Rokkor and MC Rokkor-X. They are sharp lenses but are beyond yellow or dark yellow and into the dark green area. I have not been as successful in clearing these. The explanation I have read is that the radioactive element or elements are in the middle. With this much darkening, an f/2.5 lens might be transmitting only somewhere between f/4 and f/5.6. If I want to use an f/2.5 28 which doesn't have this problem. I have many examples of the Vivitar Fixed Mount 28/2.5 (62 and 67). These are also very good. The later 28/2.5 Vivitar TX is not bad either but it is not built as well. I also have some Panagor 28/2.5s which are the same as the Fixed Mount Vivitars.

 

[brbo]

When I got my Zuiko 40/1.4 (for Pen F) it was visibly yellowed and quite obviously so when compared to my 38/1.8, though nowhere near to what you can see in some other lenses.

I never got around to getting an UV light to clear the yellowing, I just use it regularly and store it without caps.

It's now only a bit more yellow than my other lenses.

 

[Nitroplait]

I have cleared several yellowed thorium lenses from Konica, Canon and Pentax with a cheap 11W Led UV-A (Black party light) purchased from Amazon.
24 hours or less is all it takes - even for lenses with the color of a dehydrated drunk persons piss.

At the moment I am actually treating a Canon 35mm f/1.5 LTM lens which is the first one I have seen with fluorescent green edges around the glass when exposed to UV-A. Spicy!

 

[titrisol]

When I looked through the viewfinder, it was yellow! It looked as if a Wratten number 8 filter was over the lens, but there was no filter on it! I removed the lens, and sure enough, it was yellow. A peek through the viewfinder, without the lens, looked normal.

What could cause a lens to become discoloured as that one was? I have read here on Phototrio about some of the Kodak 7-inch Aero Ektars that have become discoloured with age, due to thorium in some of the lens elements.

Thorium glass was used in some of the elements of this lens, it looks like tobacco stain
But it can be "cleaned"by exposing it to UV light or a very intense LED (Ikea Jansjo)

Ikea Lamp and Takumar Lens - PentaxForums.com

After reading of experiences of people who have had success removing the Yellowish Colour cast from various vintage lenses , decided to give it a go.

www.pentaxforums.com

The theory on clearing glass was developed in the 40s, https://www.osti.gov/servlets/purl/10178461/Intense light works fine, UV is faster and sunlight was the old-timers cure
All work.

 

[pentaxuser]

From what I read, probably Google AI, the warning it gives is that the yellowing does come back after UV clearing but I could see no mention of how long this re-yellowing takes. As no-one has mentioned this, it may be a wrong statement so can I ask, has anyone who has successfully cleared a thorium noticed this effect?

Thanks

pentaxuser

 

[Paul Howell]

I just checked my Pentax M42 50mm 1.4, I treated under daylight well over 10 years ago and it is still clear.