[Leica] OT: Fixing radioactive lenses

[henningw at archiphoto.com (Henning Wulff)]

>I have an old Pentax 50/1.4 Super Takumar that has, over time, 
>yellowed due to one of the elements containing Thorium, a mildly 
>radioactive element. The addition of thorium makes a very high 
>refractive index glass, and a very good lens. This lens is probably 
>one of the most famous lenses with this element but I'm sure there 
>were more.
>
>The fix for removing the yellow cast to the lens is to expose it to 
>a UV source like sunlight or a UV light for a week or more. I have 
>been putting the lens in a window during the day and exposed to a 
>black light at night.
>
>My question is concerning the glass used in my windows. They are 
>made of high E glass and have an air space between the double panes. 
>I know high E glass is supposed to suppress  Infra Red but I see no 
>info on if it passes Ultra Violet which is at the opposite end of 
>the light spectrum. Does anybody know?
>
>Thanks,
>Len
>
>

Most glass is quite poor at UV transmission. Just try getting a tan 
through a window!

A black light (flourescent tube type; the incandescents put out very 
little UV) is ususally better than putting the lens in the sun behind 
glass. If you can put it out in the sun without glass in front of it, 
that would be best.

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    *            Henning J. Wulff
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