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.