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

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Subject: [Leica] OT: Yellow gone gone gone
From: fredhess at (Fred Hess)
Date: Tue, 19 Apr 2011 19:42:58 +0200
References: <p06240815c9d370bc11cf@[]>

Hello Herb

I was the 'correspondent' who's lens was re-cemented. It took several
weeks before we got the lens back....b.t.w. bad balsam was not the problem,
it was
the kit around the lensparts, which cracked for some reason...

Nice to read you solved your problem with an IKEA led lamp...

Kind regards

Fred Hess

----- Original Message ----- 
From: "Herbert Kanner" <kanner op>
To: "Leica Users Group" <lug op>
Sent: dinsdag 19 april 2011 19:23
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.

Herbert Kanner
kanner op

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

Leica Users Group.
See for more information

Replies: Reply from jshulman at (Jim Shulman) ([Leica] OT: Yellow gone gone gone)
In reply to: Message from kanner at (Herbert Kanner) ([Leica] OT: Yellow gone gone gone)