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These slightly radioactive elements such as thorium or rare earth elements such as lanthanum were used to produce desired highly refractive glasses.Other uses for rare earth elements in photography include color enhancing filters. Ross' Romancing the Rare Earths for a history of enhancing filters and the use of various rare earth elements therein.
The resulting "daughter" products of that radioactive decay series also produce both alpha and beta particles.
Related rare earth such as lanthanum are often produced from the same sources, with monazite being up to 25% lanthanum.
This decay process means these thoriated glass lenses can gradually become more radioactive over time, as the more highly radioactive decay products build up in the glass. You would expect the radioactivity to decrease over time.
Over the course of the development of this site, I have come upon and linked to many other sites containing interesting and related information.
Sometimes in checking links, I find that those sites have disappeared.
Sometimes I have kept copies in different formats of information on those sites.
Rather than let such information disappear into the ether, occassionally I have reproduced some or all of it here. If I have reproduced this inappropriately and you are its original author, please contact me and I will remove it.
Notes: The site still exists, but many of the pages seem to be no longer maintained and available and I have not been able to reach the original site developer, Robert Monaghan, who collected and edited the following information from use net postings.
This was restored from a copy I made in November 2003; Bob may have added to or edited this page after that. Industar 61 lens (lanthanum glass) Lens Faults Pages Photo. Maybe some of those bargains you got on EBAY are "hot" in a different sense than you might have thought!
Net on Radioactive Lenses (gamma levels..) Radioactive Fed-5s (slight..) Radioactive Optics Posting (WJM) Radioactive Pentax Lens (GM tube sound file..) Romancing the Rare Earths On Enhancing Filters by Howard Ross U. Govt Surplus "Hot" Cameras and Lenses You will find two kinds of radioactive lenses - those that were contaminated and sold as government surplus, and those that are naturally "hot" or radioactive due to the radioactive elements or radioactive impurities used in their manufacture. I am assuming that heavily contaminated and radioactive government surplus items end up in government low level radioactive waste land-fills or other burial sites.
If you want to buy some slightly radioactive lenses and cameras, the U. So if your government surplus bargain camera or lens is contaminated, it is probably only "lightly" contaminated. "Hot" Lenses made from Radioactive Materials Many specialty optical glasses have radioactive elements added to them, either as desired components or as impurities in the mixtures being used.