Check: <http://medfmt.8k.com/mf/radioactive.html> which points out that
rare earths were used in the glass mixture to achieve high indices of
refraction. Some of these were mildly radioactive themselves or may
have contained contaminants that were radioactive. The contaminants can
vary considerably by glass batch so otherwise identical lenses can vary
considerably in their degree of radioactivity and amount of yellowing
over the years. In later years manufacturers learned to control the
contaminants.
But there are indeed other forms of yellowing caused by aging coatings
or aging optical cement.
Chuck Norcutt
Woburn, Massachusetts, USA
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