ADDENDUM
i do not quite trust memory ad numbers, so I looked up Th 232 and it has a 1,41x10E16 years Zerfallsperiode; quite a long time until its physical and chemical properties are significantly changed hence losing its refractive and dispersion influence on the relevant lens elements. By that time it will not matter; more than Kodak will be out of business. As to measuring, even a sheetof paper migt be enough screening, so my ancient geiger counter-probe certainly has no chance to detect alpha radiation. p.
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