Archived posting to the Leica Users Group, 2007/11/12
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Chris, That's the first explanation that makes sense to me. I've read much more on this subject then I ever wanted to. Nobody could exlain why the UV A fix works. By the way I have been leaving the lens 2" from the BLB tube for 135 hours now and I don't see any trace of the brown/ yellow coloration. I'm going to leave it there for another day and then start taking pictures with it. Thanks. Len On Nov 12, 2007, at 3:04 PM, Chris Saganich wrote: > 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 > > > _______________________________________________ > Leica Users Group. > See http://leica-users.org/mailman/listinfo/lug for more information