Archived posting to the Leica Users Group, 2001/07/27
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Hugh Thompson writes: > Talking to a dealer this evening and he told me > that glass in some 1950's M lenses is radioactive, > and the lenses have added value because of their > rarity. Leica and most other manufacturers of good lenses use special glasses to obtain the optical characteristics they require. At one time, Leica had its own glass lab, and at least one of the glasses it produced contained thorium oxide. Thorium is a mildly radioactive metal similar to uranium in its radioactivity. The use of thorium and lanthanum oxides made it possible to obtain very high indices of refraction with low dispersion, which is useful for correction of many aberrations. Unfortunately I can't be more specific on which lenses used thorium-containing glasses; I only know (thanks again to Erwin Puts) that a few lenses were designed with such glasses. These may only have been the few that incorporated glass from Leica's own small glass lab. The early Noctilux incorporated special glass from this lab, but I don't know if it used a thorium-containing glass. In any case, that Noctilux is extraordinarily rare today (and should not be confused with the current Noctilux, which uses off-the-shelf optical glasses without thorium). Thorium is not a health hazard unless you eat it or inhale it, and even then, it presents only a tiny risk, compared to most radioactive isotopes. It is still used for fabrication of specialty optical glass (but I don't know if Leica ever uses it). It is also widely used for the mantles of gas lamps, as an alloying ingredient with magnesium, in refractories for crucibles and furnaces, and the like.