Radioactive glass.

Also colloquially “hot glass.” Lens glass which contains radioactive rare earth elements such as thorium (Th).

From the 1940s through to the early 60s lens makers (including Kodak/Aero Ektar, Leitz/Leica, Canon and Pentax/Takumar) sometimes included traces of thorium in certain lens elements because it lowers the refractive index of the glass. And some radioactive contaminants sometimes found their way into (usually non-radioactive) lanthanum (La) compounds used in other types of glass.

This is unfortunate for three reasons. First, thorium glass lenses tend to discolour with time, turning a light yellowish colour. Second, radioactive lenses can actually fog film in certain rare cases. And third and most importantly, the radioactive material in the lens actually increases for a period of time following manufacture as a buildup of thorium decay byproducts accumulates in the glass. And long-term exposure to radioactive materials is obviously not a very good thing from a health point of view. In fact there are definite health risks associated with using radioactive glass for telescope and microscope eyepieces, since your eye is extremely close to the glass.

If you’re concerned about your old lens or viewfinder (though nearly all commercial lenses built since the 1960s should be free of radioactive materials) you can try putting it on a piece of high-speed film or paper in a pitch-dark place for a week. If the developed emulsion shows fogging you should have the lens tested by a professional with a Geiger counter to determine how radioactive it is.

Note also that contaminated radioactive lenses also exist. Such lenses probably did not contain any radioactive materials at the time of manufacture but were subsequently contaminated by being exposed to radioactive materials. Severely contaminated equipment should have been disposed of as radioactive waste but lightly contaminated gear is sometimes sold to the public as surplus.

cf. “ecological” glass, rare earth glass, refractive index.

Entry last updated 2002-05-02. Term 988 of 1487.

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