<p>I have a theory that goes like this:<br /> A number of lens manufacturers, Asahi for example, produced SLR lens designs for a time that included rare earth compounds in the glass. Certain examples of certain lens designs give off enough radiation to cause yellowing or read high on a radiation meter, while other examples of the same designs do not. Why?...<br /> ...If the glass contained some thorium oxide by design, in a fixed proportion to the other ingredients, all lenses of a given design made with that glass should show the same telltale signs of radioactivity.<br /> But what if the thorium oxide in it was only an incidental impurity more present in some batches of a rare earth compound than in others, because of lot-to-lot purity variations in a raw material? Or possibly the raw material sources changed over time, or there was some alternation among different sources depending on cost and availability. You could then have lookalike lenses varying significantly in radioactivity.<br /> Someone familiar with Japanese lens-making in that era might be able to support or refute this.<br /> I worked for companies that made partially stabilized zirconium oxide powders that were plasma sprayed to create thermal barrier coatings on jet engine turbine blades. It was known that some zirconium oxide raw materials contained more residual radioactive oxide impurities than others, depending on the original sources of the ore, subsequent purification steps, etc.</p>