Radioactive Lens Treatment

[Kevin Crisp]

As most of you know, some lenses used radioactive glass. In time, the radioactive glass gets damaged, turning brownish yellow. Kind of like iced tea color. This is common in R Claron lenses, which are among my favorites.

I thought I'd quantify how long it takes to get rid of this. I happened onto a SMC Takumar 50mm Pentax lens, the rear element of which was really brown. It put the lens, rear element up, on tin foil and arranged a UV light in an aluminum reflector about 5" above the lens. The bulb was a compact florescent, Feit brand, BPESL15T/BLB. After one week, the glass is clear and the brown is gone. I don't know that the Thorium glass is on the outside on an R Claron like it is on this Pentax lenses, so it might take longer on one of those.

I have tried doing this on other lenses with one of the bright LED party lights, and it seemed to barely make a difference. But this Feit CF bulb does the trick.

And, learned the hard way, if you have any sort of dimmer on your reflector socket, even if you turn it up all the way, you'll kill the bulb in about two days. Don't try it that way.

The brown will slowly come back, but it takes years to get bad again.

[Joseph Kashi]

VERY helpful - thank you. I have one of those 300mm R Clarons as well as a suspect Ektar or two from that time frame.

[Dan Fromm]

Interesting. My 20W BLB unbranded (not marked on the base, box long gone) fluorescent took much, much longer to clear three TTH Apo-Tessars (1 Apotal, 1 Cooke Copying Lens, 1 Copying Lens) and one 55/8 Repro-Claron. Did the job but the lenses basked under it for two months. The TTH lenses are still clear, the R-Claron was sold long ago.

[Kevin Crisp]

If the UV has to pass through some other glass to get to the thorium one, I'd expect it to take a lot longer. On the 50mm Takumar, is is obvious the rear element is the issue. So this one week is probably a best case scenario.

[Mark Sawyer]

I leave my Aero-Ektars on the windowsill of my studio, with the back elements facing out and the fronts covered with aluminum foil so they don't focus the sun and start a fire.

[reddesert]

Interesting information. Have any of you tried a similar test in the sun, without window glass intervening?

There is an Ikea desk lamp that some people have recommended for this purpose. I don't have any of these lenses and haven't tried it.

[Kevin Crisp]

I did a little more research -- the takumar had several thorium lens elements, clustered toward the back. So the one week treatment did manage to clear up all of them.

[Havoc]

Might give my 50 takumar some light then, it begins to get a slight tea color.

[PRJ]

I have a 50 Takumar that was yellowed. I used a few small UV leds that were left over from a UV exposure unit I had built. They cleared the lens in a couple days. I just put them on some tin foil and put that on the front of the lens. If'n I'd have known that the back of the lens was causing the offending issue, it probably would have gone even faster.

[Kevin Crisp]

A friend of mine just sent me back a camera I sold him in 1974. It was my first nice camera, a Spotmatic. Its non-MC 50mm takumar is REALLY brown, even though many lists of thoriated (is that a word?) lenses don't have the regular takumars on there. I'm going to give it a go with the LED 60w blacklight I got and see what happens after a week. If it is still bad I'll use that Feit-brand bulb, which clearly works very well. I can tell they have UV at different frequencies, with the Feit bulb the back elements glow greenish and this is much less pronounced with the leds.

[Kevin Crisp]

After two days under a 60W LED UV light I can see that the brown is much improved. Maybe 70% better, so that seems to be an option. But the CF bulb was cheaper. Ebay has a ton of LED ones sold as "party lights."