Archived posting to the Leica Users Group, 2007/11/12
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Hi Chris, You're right. I've been doing a lot of reading on the subject as to why it happens and why the fix works. I don't know how right this is but as I understand it the gamma radiation given off by the Thorium produces ionizing radiation which damages the glass. The attached page describes the process and was written by a gamma ray astro- physicist who does work for NASA and has an interest in the Aero- Ektar lens used in WW II. This lens is very radioactive due to Thorium glass, which was invented by Kodak in the late 30's and used by many lens makers years later including Leica in the original collapsable LTM Summicron. I don't have a clue how right he is. You might. <http://home.earthlink.net/~michaelbriggs/aeroektar/aeroektar.html> Len On Nov 12, 2007, at 10:47 AM, Chris Saganich wrote: > Ah yes UV A, electron absorption. I suppose a replacement of the > electrons kicked out by the thorium or perhaps some atomic level > annealing. Either way its better then heating up the lens to 650 > degrees in the oven. > Chris > > > At 11:21 AM 11/10/2007, you wrote: >> For those of you interested in the Pentax 50mm yellowing problem, >> Here are photos taken before and after 116 hours exposure to UV light >> to reverse the yellow cast problem due to the radioactive element >> Thorium in one of the lens elements. >> >> Before photo. Lens has brownish, yellow cast. >> >> <http://gallery.leica-users.org/v/LeonardT/Pentax/ >> Takumar_50_2.jpg.html> >> <http://tinyurl.com/233emd> >> >> Here is a photo taken after only 116 hours of being exposed to UV >> light. The exposure was to 16 hours of sunlight and 100 hours of UV >> from a blacklight. >> >> <http://gallery.leica-users.org/v/LeonardT/Pentax/ >> Takumar_116hrs.jpg.html> >> <http://tinyurl.com/2bx7do> >> >> I did not expect any improvement this quickly if at all. I'll >> probably keep the lens in front of the blacklight for a few days more >> to see if there is any further improvement. >> >> Thanks for your interest. >> Len >> >> >> >> _______________________________________________ >> Leica Users Group. >> See http://leica-users.org/mailman/listinfo/lug for more information > > Chris Saganich, Sr. Physicist > Weill Medical College of Cornell University > New York Presbyterian Hospital > chs2018@med.cornell.edu > Ph. 212.746.6964 > Fax. 212.746.4800 > Office A-0049 > > > _______________________________________________ > Leica Users Group. > See http://leica-users.org/mailman/listinfo/lug for more information