Tonight I just learned about the use of thorium in lenses up until 1980's. This prompted me to go dig out my 60's vintage civilian defense geiger counter, which I bought years ago after some goof ball medical technician made a wise crack about the half life of the liter of barium I had just quaffed for a cat scan. (fwiw, the drinkable stuff is from a non-radioactive isotope). Anway, I put my old lenses to the test. I can say after scanning for alpha and beta particle that I believe the following lenses are Thorium-free: Nikkor-N 24/2.8 pre-AI Nikor-S.C 50/1.4 pre-AI Nikkor-P 105/2.5 pre-AI Nikkor-Q C 200/4 pre-AI FWIW, I checked a few other lenses: Zuiko OM 50/1.4 Minolta Rokkor 200/3.5 Minolta Rokkor 50/1.7 After reading that Thorium in lenses goes back to 1937, I checked my Exakta lenses: Steinheil 135/3.5, Westenar 50/2.5..... nope. In hindsite, after reading wikipedia, it doesn't seem like much of an issue anyway. It says alpha particles from thorium can't even penetrate human skin, so even if I did have a thorium lens, it likely wouldn't be a concern unless I breathed the pulverized dust of a broken lens. Given that the use of it in commercial lenses stopped around 1980 makes me wonder if three-mile island incident in 1979, might have made marketing thorium lenses impossible, and lead to the end of it's use. Interesting diversion for the evening though.