Bleaching a Yellow Takumar

Super Takumar lenses are known to get yellow with age. Actually, the issue is common to many radioactive lenses.
The Super-Tak 50.1/4 in my possession was very yellow, as you can tell from this picture. Note especially that the center is darker: this is because the lens is thicker in the middle. The glass itself yellows, not the coating. A bit of yellow gives warmer images, however such strong yellowing absorbs light and reduces the speed of the lens.

From what I gathered,  the radioactivity is caused by Thorium, used to improve the Abbe Index: Rare Earth elements allow excellent glass (LaK, for example or other Lanthanum based glass). Emitting  alpha particles tint the yellow.
Some people left their lenses in the sun for weeks and they cleared out! Fine, but I didn't want to leave my fine lens exposed to weather events.
I sought another source of UV radiation, so I bought a black light lamp. These lamps emit mostly UV-A and are safe. UV-B would probably be faster in action since higher in energy.

This is how a photograph can look.

I left the lamp close to the lens putting it in a desk lamp with a shade to direct most light down, the swing arm down closest to the lens.
The lens to be cleared is on the back, so I left the lens mostly upside down, but sometimes I turned it for a day upside.

This is how the lens looks after bleaching. Quite effective! Only a slight yellowing remains. I could have prolonged radiation, but am satisfied enough.