Archived posting to the Leica Users Group, 2011/04/24
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Wow! Time flies. I did a lot of research on Thorium yellowing in 2007 and took these two photos of my Pentax 50mm f1.4 Super Takumar lens. It took me a total of 116 hours to remove the yellow cast, exposing the lens to UV at night and sunlight during the day. Here are the before and after photos. http://gallery.leica-users.org/v/LeonardT/Pentax/ It was during this timeframe that Jim Nichols investigated his Pentax lens for yellowing. I believe I have a technical paper explaining the phenomena somewhere. If I find it I'll post it. I was not aware that the 90mm Summicron used Thorium. It must be an early model as leica stopped using Thorium in the crossover period between the 50mm Summitar and Summicron around 1954. The 90mm Summicron was introduced in 1957. Len On Apr 20, 2011, at 4:30 PM, Herbert Kanner wrote: > Thanks for the kind word, George. Byron Rakitzis suggested that my > speculation as to the cause of the yellow was probably incorrect, > and suggested a Google search. I found a lot of information > indicating that the yellow was due to radiation damage from gamma > rays emitted by some of the daughter products of the decayed > thorium. No information as to why a few hours application of UV > cures this. > > Herb > > >> On Apr 19, 2011, at 12:23 PM, Herbert Kanner wrote: >> >> > 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. >> >> >> Kudos! Herbert. >> and thanks for the detailed and useful information. >> >> Regards, >> George Lottermoser >> george at imagist.com >> http://www.imagist.com >> http://www.imagist.com/blog >> http://www.linkedin.com/in/imagist >> >> >> >> >> >> >> _______________________________________________ >> Leica Users Group. >> See http://leica-users.org/mailman/listinfo/lug for more information > > -- > 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! > > _______________________________________________ > Leica Users Group. > See http://leica-users.org/mailman/listinfo/lug for more information