Warning, radioactive lenses!

[Pete Andrews]

I was recently reading some previous postings and web articles about certain len s glasses containing radioactive elements, and wondered if any of my collection might be 'hot'.Initially, I was more curious than worried, but after doing s ome tests, I'm now definitely worried, and I think my findings deserve wider att ention.

I have a sample of a 7" Kodak Aero-Ektar, which is a lens that has a b ig reputation for being potentially radioactive.However, all the articles I' ve read state that the radioactivity is mainly in the form of alpha emmission, a nd shouldn't give much call for concern, since alpha particles are quite low ene rgy, and easily stopped by any solid object. I'm told that alpha particles can o nly penetrate about 40 microns into human tissue.Anyway, to cut to the chase : I just got our radiation protection officer to run a geiger counter and a dosi meter over the Aero-Ektar, and he found that the rear element was quite hot, giv ing about 200 counts/second. Worse yet, we discovered that it was mainly GAMMA e mmission, since even an inch of perspex and a steel plate hardly affected the co unts at all. The active element used in making the glass is Thorium, and a quick check of the table of its decay products confirmed that they're mostly gamma em mitters, after about 6 years of decay.Dosimeter readings showed that within 1" of the rear lens surface, the dose was above the limit allowed for monitored radiation workers, and only fell to the publicly allowable safe limit at more th an 6" from the lens.I don't think I'll be using that lens as a paperweight, or handling it too much from now on. By all modern standards, that lens would be classed as downright hazardous, and not to be used without protective clothing! I'll be testing the rest of my lenses as potential gamma sources as well in the near future.

[pat krentz]

Since infinity is 7" and it is safe at 6" and you are not going to carry it around in your pocket, I would not worry about it. Pat

[Dave Mueller]

So, if you put it on a camera, and lock the shutter open, will it fog the film?

[dave bulmer]

Pete, since the part of my brain which is nearest the gg doesn't work anyway, I suggest you don't use the lens anymore and send it to me. That'll be two pints I owe you. Dave.

[Ed Buffaloe]

There was an article about this 4 or 5 years ago in Shutterbug, I believe. It reported that Apo-Lanthars, certain early Takumars, and many other lenses are somewhat radioactive. A simple test is to leave the lens on a sheet of photographic paper in a darkroom for several hours, then develop the paper. Dark spots from radiation exposure will be readily apparent.

[Dave Anton]

Is this only present in older lenses or in new ones as well?

[Dave Anton]

Is this only present in older lenses or in new ones as well?

[Ed Buffaloe]

I don't know if manufacturers are trying not to use radioactive materials, but the rare earths like lanthanium, thorium, yttrium, etc. are quite useful in adjusting refractive indexes, limiting dispersion, and absorbing UV in glass--it may be difficult to find replacements for them. I did some searching and can add Konica, Zuiko, and Steinheil to the list of companies that used rare-earth lenses--as well as the f/2.8 lenses in the Stereo Realist. Probably all the major lens companies experimented with them at one time.

[Geoffrey_1456]

Pete,

200 counts/second seems a bit high. You're sure it is not 200 counts/minutes. For LF, the other two "hot" lenses I know are Voigtlander Apo-Lanthar and Rodenstock Weitwinkel Perigon (1958 vintage, 130 mm/F12). Both contain Lanthanum. From personal experience, Apo-Lanthar and Perigon are wonderful lenses and will not expose films under normal usage. No worry. They are not hot enough. On second thought, I'd better send my hot lenses to NYU for a safety check-up. Thanks for the note. Regards,

[Paul Schilliger]

Gosh, I wasn't aware of that! Is that the reason my older Takumars for Pentax 67 are becoming encreasingly warm in color?