Archived posting to the Leica Users Group, 2011/04/19

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Subject: [Leica] OT: Yellow gone gone gone
From: kanner at (Herbert Kanner)
Date: Tue, 19 Apr 2011 10:23:16 -0700

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.

Herbert Kanner
kanner at

Do not meddle in the affairs of cats,
for they are subtle and will pee
on your computer!

Replies: Reply from fredhess at (Fred Hess) ([Leica] OT: Yellow gone gone gone)
Reply from imagist3 at (George Lottermoser) ([Leica] OT: Yellow gone gone gone)
Reply from jhnichols at (Jim Nichols) ([Leica] OT: Yellow gone gone gone)