Info 08: Yellow cast on older lenses

From Ray Allen:

Speaking of the Super-Takumar 50mm f/1.4, I am unsure whether you are aware that the model 2 (seven element) and subsequent models contain a rare earth glass element which over time has made these lenses turn yellow. The model 1 (eight element) does not and will not turn yellow.
I run the Spotmatic group on Yahoo! and we call this condition the Golden Glow or Yellow Curse. However we were recently told that prolonged exposure to bright sunlight would cure it. Naturally, we were a little sceptical but we feel differently now. After about two weeks either on a window sill or out in direct sunlight a number of us have reported complete removal of the yellow cast and a return to "water clear" glass. The lenses that were definitely unusable for slide photography can now be used again.
Of course, we are not sure at this stage how long the cure will last. The original onset of the yellowing was slow but it may return more quickly. This is not a great problem though as another week or so in the sun should cure it again. The offending element is towards the rear of the lens so it is best to expose the rear element to the sun.

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From Gene Poon:

I've been following the discussion, and today a Web page was presented with an explanation of how the yellowing, here called "radiation browning," happens.  It also says that the condition is reversed by exposure to heat and light, which agrees with our observations that the cure of putting the lens into the sun really does work.

http://www.birns.com/gis/gisbrowning.htm

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The yellowing of lenses using rare earth glass elements is well known to many users and collectors of old cameras, while I believe the "cure" was unknown to most of them, including myself. Many thanks, Gene and Ray, for such interesting information on "how it happens" and "how to cure it".

Dario