Archived posting to the Leica Users Group, 2007/11/07
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Marc, firstly obligatory disclaimer. I am no nuclear chemist. A quick Wiki shows half-lives for various isotopes from 25.5 hours to REALLY REALLY long (3 times the age of the earth)! I have seen it used in large optics in an aviation application with no functional issues despite many years of operation in very hostile environments (kinetic heating, vibration and more). I recall that the same equipment also contained a large mass of depleted uranium (ballast). Maybe there was some synergistic radioactive voodoo going on there. Back on Len's lens, I wonder if this is a coating, adhesive or lubricant affected rather than the element used within the particular glass compound? Weren't there some Leica lenses also using this glass type? Cheers H0 P2y -----Original Message----- From: lug-bounces+hoppyman=bigpond.net.au@leica-users.org [mailto:lug-bounces+hoppyman=bigpond.net.au@leica-users.org] On Behalf Of Marc James Small Sent: Thursday, 8 November 2007 08:54 To: Leica Users Group Subject: Re: [Leica] OT: Fixing radioactive lenses At 05:10 PM 11/7/2007, Leonard Taupier wrote: >I have an old Pentax 50/1.4 Super Takumar that has, over time, >yellowed due to one of the elements containing Thorium, a mildly >radioactive element. The addition of thorium makes a very high >refractive index glass, and a very good lens. This lens is probably >one of the most famous lenses with this element but I'm sure there >were more. Len I love you like a brother, but be conscious here. The yellow cast may well result from decaying coatings. Disassemble the lens, remove the lens coatings, and then contemplate the result. If an element is yellow, then, yes, your original thesis may be correct. It takes thousands of years for radioactive elements to decay. I am baffled at the thought that the passage of only a few decades can lead to some change in the color of glass elements. Maybe I am a babe in the woods -- you guys have locked my heels over the years on things ranging from the utility of Tilley hats to the vibrant necessity of US television -- but I just do not see this as change as a reality. Do we have any nuclear chemists on the List who can add anything to the discussion? Jerry Lehrer, I realize, will tell us of his experience at the Brooklyn Home For Broken Boys or whatever it is called, back in 1921 <he grins>, but I really would like to know the straight skinny from guys who regularly have worked with thorium or with thorium glasses. And, Jerry, you STILL owe me those promised serial numbers from your studio Rollei. <he grins, yet again> In short: can Thorium glass really change color in a few decades? We hear this all the time. But is it real? Marc msmall@aya.yale.edu Cha robh b?s fir gun ghr?s fir! _______________________________________________ Leica Users Group. See http://leica-users.org/mailman/listinfo/lug for more information