Many so-called Lanthanum glasses are actually Lanthanum/Thorium glasses. Many of the manufacturers preferred to emphasize the Lanthanum content and downplay the Thorium content. Period literature makes it clear that Thorium was intentionally included for the desirable optical properties of this glass: a high index of refraction with a low dispersion. This helps reduce chromatic and spherical abberations.

The radioactive lenses that I have measured are much to radioactive to be explained by the extremely slight radioactivity of Lanthanum, or by contamination with Thorium. The amount of Thorium is too large to be contamination.

Most major lens manufacturers used Thorium glass: Kodak, Voightlander, Schneider, Pentax, Nikon, etc. The Apo-Lanthars definitely contain Thorium. I don't know whether Russian lens manufacturers used thorium glass. One possible sign is that the glass ages to a tea color. This can be cleared via exposure to UV light.

Thorium glass is no longer used -- optically similar glasses without thorium are available.

For more on thorium glass, see my Aero-Ektar webpage: http://home.earthlink.net/~michaelbr...aeroektar.html