[Leica] Img: Radioactive lens fix works

[henningw at archiphoto.com (Henning Wulff)]

>For those of you interested in the Pentax 50mm yellowing problem, 
>Here are photos taken before and after 116 hours exposure to UV 
>light to reverse the yellow cast problem due to the radioactive 
>element Thorium in one of the lens elements.
>
>Before photo. Lens has brownish, yellow cast.
>
><http://gallery.leica-users.org/v/LeonardT/Pentax/Takumar_50_2.jpg.html>
><http://tinyurl.com/233emd>
>
>Here is a photo taken after only 116 hours of being exposed to UV 
>light. The exposure was to 16 hours of sunlight and 100 hours of UV 
>from a blacklight.
>
><http://gallery.leica-users.org/v/LeonardT/Pentax/Takumar_116hrs.jpg.html>
><http://tinyurl.com/2bx7do>
>
>I did not expect any improvement this quickly if at all. I'll 
>probably keep the lens in front of the blacklight for a few days 
>more to see if there is any further improvement.
>
>Thanks for your interest.
>Len


Very impressive. I'm still not clear on the actual process of 
reversing this colour change, though. As Chris Saganich pointed out, 
available UV radiation is too long wavelength to have enough energy 
to boot electrons around to reverse these colour changes, as far as I 
know. On the other hand, clearly there is enough energy in the UV 
radiation. Something awfully low energy is happening here.

In any case, it looks like the lens is well on its way being 
'filterless' (internally) again.


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