[Leica] OT: Yellow gone gone gone

[jhnichols at lighttube.net (Jim Nichols)]

Hi Herb,

I'm glad that I was mistaken about the balsam, and that the Ikea light 
worked.  Sounds like a pretty cheap fix.  My college courses of the late 
40s-early 50s didn't offer any explanations that would "shed any light" on 
how it works.  I have tried sunlight on my Pentax 50/1.4 Super Multi Coated 
Takumar, but couldn't tell much difference.  For lenses as valuable as those 
that you have, the light seems a great investment.

Jim Nichols
Tullahoma, TN USA
----- Original Message ----- 
From: "Herbert Kanner" <kanner at acm.org>
To: "Leica Users Group" <lug at leica-users.org>
Sent: Tuesday, April 19, 2011 12:23 PM
Subject: [Leica] OT: Yellow gone gone gone


> So I belatedly discovered that my 90mm chron was yellow compared to my 
> 35mm chron, which was clear of any color cast. Then I read about the 
> yellowing from thorium glass. I put a query up on the web site that 
> recommended that Ikea LED lamp and after a few days got a response saying 
> that the treatment probably did not apply to my lens, that the problem was 
> aging balsam, and showed a photograph of an affected lens. Well, the 
> defect was close to the rim of the glass and looked like oil on water. I 
> don't believe that could be aging changing the balsam's color. That is 
> interference fringes showing the beginning of separation of the lens 
> elements that had been cemented.
>
> 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.
>
> The physics of all this partially baffles me. Thorium is an alpha emitter. 
> Alpha particles are helium nuclei. In glass, the helium would be trapped. 
> Alpha particles have a very short range and only the ones coming from 
> thorium atoms at the surface could leave. So now, I have no idea whether, 
> in glass, these helium nuclei would eventually pick up in time the normal 
> complement of orbital electrons, despite glass being an insulator,. Nor do 
> I know whether the absorption spectrum of helium would result in yellowish 
> transmission. Finally, UV being of relatively low energy compared to the 
> binding energy of all but the outermost electrons, how does UV disrupt the 
> electron structure sufficiently to kill the yellow, and how long will this 
> disruption last, the helium being present forever?
>
> One thing is clear. The correspondent who said that Leica Wetzlar fixed 
> his yellow lens for free must have had this phenomenon. If bad balsam were 
> the problem, it seems to me that the lens would have had to be completely 
> disassembled and the cemented element pair immersed in a solvent until the 
> balsam softened to where the elements could be safely separated and 
> recemented.
>
> Herb
> -- 
> 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!
>
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