Archived posting to the Leica Users Group, 2007/11/07
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]At 08:03 PM 11/7/2007, Philip Forrest wrote: >Thorium oxidizes to a brown/black color and this could account for the >coloration of the lenses. I'd say this would be a more logical reason behind >the yellowing of the glass with the radioactivity being coincidental. >I'd like to hear about the yellowing effect in these rare-earth lenses from >someone who worked with their manufacture as well. My SMC Takumar 50mm 1.4 >is one of my favorite lenses and probably better than any of the Leica >glass >I own or dare say, better than any of the Leica glass produced up to that >point in time. > >The half-life of Thorium-232 is several billion years however there are trace >isotopes that have been used industrially (Th-234 and Th-231) with >half-life >durations measuring about one day to one month. > Thanks, Phil. But, again, the mythic statement always runs along the lines of "lenses made from radioactive glass always turn yellow as the radioactive elements decay", and I believe this to be false. It is quite possible that these glasses have front surfaces which chemically turn yellow, but I do not believe that the radioactivity of the glasses has a bit to do with the yellowing, if any. I suspect that it is a bit of urban lore, along with the tale of those WWII aerial recon photographers who all died from eye cancer from the use of Kodak lenses made from Thorium glasses, although they generally got no closer than five or six feet from their cameras and never once looked through the lenses, as these were fixed-focus suckers and the photographers only loaded and removed the film plates. Urban myths abound. Marc msmall@aya.yale.edu Cha robh b?s fir gun ghr?s fir!