[Leica] OT: Fixing radioactive lenses

[henningw at archiphoto.com (Henning Wulff)]

>At 08:03 PM 11/7/2007, Philip Forrest wrote:
>>Thorium oxidizes to a brown/black color and this could account for the
>>coloration of the lenses.  I'd say this would be a more logical reason 
>>behind
>>the yellowing of the glass with the radioactivity being coincidental.
>>I'd like to hear about the yellowing effect in these rare-earth lenses from
>>someone who worked with their manufacture as well.  My SMC Takumar 50mm 1.4
>>is one of my favorite lenses and probably better than any of the Leica 
>>glass
>>I own or dare say, better than any of the Leica glass produced up to that
>>point in time.
>>
>>The half-life of Thorium-232 is several billion years however there are 
>>trace
>>isotopes that have been used industrially (Th-234 and Th-231) with 
>>half-life
>>durations measuring about one day to one month.
>>
>
>Thanks, Phil. But, again, the mythic statement 
>always runs along the lines of "lenses made from 
>radioactive glass always turn yellow as the 
>radioactive elements decay", and I believe this 
>to be false.  It is quite possible that these 
>glasses have front surfaces which chemically 
>turn yellow, but I do not believe that the 
>radioactivity of the glasses has a bit to do 
>with the yellowing, if any.
>
>I suspect that it is a bit of urban lore, along 
>with the tale of those WWII aerial recon 
>photographers who all died from eye cancer from 
>the use of Kodak lenses made from Thorium 
>glasses, although they generally got no closer 
>than five or six feet from their cameras and 
>never once looked through the lenses, as these 
>were fixed-focus suckers and the photographers 
>only loaded and removed the film plates.
>
>Urban myths abound.
>
>Marc
>
>
>msmall@aya.yale.edu
>Cha robh b?s fir gun ghr?s fir!

I'm not too sure about this (haven't googled it 
and my background is physics, not chemistry) but 
the yellowing might come from a chemical change 
induced by the decay of the thorium. There are a 
lot of Thorium isotopes, with half lifes of 
seconds to millenia, and it might not take a lot 
of radiation (probably alpha particles for the 
most part) to cause nearby compounds to transmute.

In any case, it's not a coatings issue, but a 
bulk glass issue and is directly tied to the use 
of thorium in the glass.

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    *            Henning J. Wulff
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