Archived posting to the Leica Users Group, 1999/08/02
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Use of rare earth compounds, including radioactive elements such as thorium, was quite common in glass manufacturing prior to the development of more modern APO glass formulas. At least one poster cites a Leica 35mm f2.0 summicron as triggering radiation alarms at their nuclear plant site. Other radioactive glass lenses include the popular kodak aero-ektar series, and even some mid-1960s pentax 50mm lenses and the Canon Pellix lenses are radioactive. Some reported beta activity levels are quite high by modern safety standards (up to one+ millirem per hour reported, which would exceed the recommended lifetime dosage rate for industrial radiation workers in under a year). You would need a special license to own/buy/store such radioactive sources (hopefully EPA/OSHA isn't reading this list - otherwise, will they impound/ bury all of our older radioactive Leica lenses or forbid selling them?) When used in telescope/microscope eyepieces, these same glasses have been held responsible for eye cancers, due to the close proximity, long duration of contact in professional use, and relatively high beta activity levels. I wouldn't leave one sitting around next to fast film in my camera bags, either. Distance or moderate thicknesses of shielding (e.g., 1 inch of paper) will block most or all of the beta radiation, but the lower levels of gamma radiation will penetrate SIMA lead shield bags and most other shielding, up to 2-3 feet of earth, and so on. Fortunately, these gamma ray levels are much less, but any exposure would carry an increased risk of cancers and other damage and be difficult to shield against. The coloration of the glass is related to the tendency of thorium and certain rare earth glass compounds to be unstable and break down over time, rather than any direct effect of the radiation itself. The radiation would most likely only create microscopic particle tracks in the glass, but no macroscopic color shifts or other effects visible to the eye. see my Lens Faults pages at http://www.smu.edu/~rmonagha/bronfaults.html and glass mfg pages at http://www.smu.edu/~rmonagha/mf/glassmfg.html * Robert Monaghan POB752182 Dallas Tx 75275-2182 rmonagha@post.cis.smu.edu * * Medium Format Cameras: http://www.smu.edu/~rmonagha/mf/index.html megasite*