Archived posting to the Leica Users Group, 2007/11/07

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Subject: [Leica] OT: Fixing radioactive lenses
From: marcsmall at comcast.net (Marc James Small)
Date: Wed Nov 7 20:28:55 2007
References: <27347376-1AD0-40B7-B526-32A6572F1A6D@comcast.net> <200711072256.lA7MuoDn068946@server1.waverley.reid.org> <200711072003.48804.photo.forrest@earthlink.net>

At 08:03 PM 11/7/2007, Philip Forrest wrote:
 >Thorium oxidizes to a brown/black color and this could account for the
 >coloration of the lenses.  I'd say this would 
be a more logical reason behind
 >the yellowing of the glass with the radioactivity being coincidental.
 >I'd like to hear about the yellowing effect in these rare-earth lenses from
 >someone who worked with their manufacture as well.  My SMC Takumar 50mm 1.4
 >is one of my favorite lenses and probably better than any of the Leica 
 >glass
 >I own or dare say, better than any of the Leica glass produced up to that
 >point in time.
 >
 >The half-life of Thorium-232 is several billion 
years however there are trace
 >isotopes that have been used industrially (Th-234 and Th-231) with 
 >half-life
 >durations measuring about one day to one month.
 >

Thanks, Phil. But, again, the mythic statement 
always runs along the lines of "lenses made from 
radioactive glass always turn yellow as the 
radioactive elements decay", and I believe this 
to be false.  It is quite possible that these 
glasses have front surfaces which chemically turn 
yellow, but I do not believe that the 
radioactivity of the glasses has a bit to do with the yellowing, if any.

I suspect that it is a bit of urban lore, along 
with the tale of those WWII aerial recon 
photographers who all died from eye cancer from 
the use of Kodak lenses made from Thorium 
glasses, although they generally got no closer 
than five or six feet from their cameras and 
never once looked through the lenses, as these 
were fixed-focus suckers and the photographers 
only loaded and removed the film plates.

Urban myths abound.

Marc


msmall@aya.yale.edu
Cha robh b?s fir gun ghr?s fir!



In reply to: Message from len-1 at comcast.net (Leonard Taupier) ([Leica] OT: Fixing radioactive lenses)
Message from marcsmall at comcast.net (Marc James Small) ([Leica] OT: Fixing radioactive lenses)
Message from photo.forrest at earthlink.net (Philip Forrest) ([Leica] OT: Fixing radioactive lenses)