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The yellowing can be reversed by exposure to UV. Leave it out in the sun for a few weeks and it may reverse.
The radiation emitted is light and only a problem if you were using it as an eyepiece although that video would suggest its pretty radioactive.. Having said that there is no definitive research on what a minimum amount of radiation is for no physical effects only what is considered safe. No guarantee. You can google this and come up with a list of lenses that used it. Typically they were fast lenses as the thorium is like fluorite and allows better correction.
They are usually Pentax lenses and a couple of Minolta Rokkor lenses - the 56 F1.2 for example.
I wonder how these Lanthanum eyepieces and lenses would go if tested.
Greg.
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