I don’t think it’s quite this cut and dried. By the time the OM 55/1.2 and
others of that vintage came out, they had switched to lanthanum glass, rather
than Thorium, I believe. Most of the thoriated lenses would have been
late 1940s vintage (Aero Ektars, etc.).
And, the yellowing also can be (or is caused by) a result of interaction of the
glass and the cement used to hold it together, rather than the glass itself
yellowing.
-Ed
On 11/20/16, 6:16 AM, "olympus on behalf of olympus-request@xxxxxxxxxxxxxxxxx"
<olympus-bounces+ed.sawyer=unh.edu@xxxxxxxxxxxxxxxxx on behalf of
olympus-request@xxxxxxxxxxxxxxxxx> wrote:
Yes. The yellowing radioactive lenses all used thorium dioxide in them.
This was replaced by lanthanum oxide, since it's much less radioactive
which is better for the people who make the lenses as well as not yellowing
over time. Natural lanthanum is slightly radioactive because it has a small
amount of a naturally occurring isotope, but is much less so than thorium
which has no stable isotopes.
Mark
On Sat, Nov 19, 2016 at 5:34 PM, Wayne Harridge <
wayne.harridge@xxxxxxxxxxxxxxxxxxx> wrote:
>Yes, but is it Thorium in the 55/1.2 ?
--
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