Archived posting to the Leica Users Group, 2011/04/19[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]
So I belatedly discovered that my 90mm chron was yellow compared to my 35mm chron, which was clear of any color cast. Then I read about the yellowing from thorium glass. I put a query up on the web site that recommended that Ikea LED lamp and after a few days got a response saying that the treatment probably did not apply to my lens, that the problem was aging balsam, and showed a photograph of an affected lens. Well, the defect was close to the rim of the glass and looked like oil on water. I don't believe that could be aging changing the balsam's color. That is interference fringes showing the beginning of separation of the lens elements that had been cemented. So now I concluded that the balsam was fine, and that this lens had at least one thorium glass element. I headed to Ikea, where I had never been before. I felt like a lab rat in a maze; everyone should have that experience ONCE. By some miracle I found a desk version of that lamp $9.89, assembly required! I applied the recommended twelve hours of exposure, and the yellow is gone gone gone. The physics of all this partially baffles me. Thorium is an alpha emitter. Alpha particles are helium nuclei. In glass, the helium would be trapped. Alpha particles have a very short range and only the ones coming from thorium atoms at the surface could leave. So now, I have no idea whether, in glass, these helium nuclei would eventually pick up in time the normal complement of orbital electrons, despite glass being an insulator,. Nor do I know whether the absorption spectrum of helium would result in yellowish transmission. Finally, UV being of relatively low energy compared to the binding energy of all but the outermost electrons, how does UV disrupt the electron structure sufficiently to kill the yellow, and how long will this disruption last, the helium being present forever? One thing is clear. The correspondent who said that Leica Wetzlar fixed his yellow lens for free must have had this phenomenon. If bad balsam were the problem, it seems to me that the lens would have had to be completely disassembled and the cemented element pair immersed in a solvent until the balsam softened to where the elements could be safely separated and recemented. Herb -- Herbert Kanner kanner at acm.org 650-326-8204 Do not meddle in the affairs of cats, for they are subtle and will pee on your computer!