Archived posting to the Leica Users Group, 2007/11/12
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Yea, the radiation does cause stress in the glass but that isn't what causes the color. The color is caused by electrons re-bonding to different sights after the ionizing radiation kicks them out of their shells. The electrons need a new home in the structure and that changes molecular bonding. If we were talking about a stable crystal structure then the UV trick wouldn't work, but because glass is amorphous and liquid it is easy to restructure the molecular bonding...just add heat at the correct temperature...the annealing temperature. The UV can only do two things to glass either photoionize or be absorb as heat. Photoionization is kicking an electron out of its shell, but that's how the glass became colored in the first place. UV B and C have enough energy to photoionize, which is 4eV for glass. UV A or black light does not have enough energy to photoionize so all the energy is absorbed as heat, thus photo-annealing. So any radiation with energy less then 4eV would work, the time and intensity would have to be longer. I suppose UV A would be the most efficient since it has the highest energy, just under 4 eV. Chris At 12:34 PM 11/12/2007, you wrote: >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 > > >_______________________________________________ >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