Radioactive lenses
There are a significant number of lenses produced from the 1940s through the 1970s that are measurably radioactive. Main source of radioactivity is the use of thorium oxide as a component of the glass used in the lens elements. Thoriuum oxide has crystalline structural similar to calcium fluoride (fluorite). Like fluorite, its optical properties of high refractivity and low dispersion allows lens designers to minimize chromatic aberration and utilize lenses of lower curvature, which are less expensive to produce. Contrary to often seen statements to the otherwise, lenses containing lanthanum are not appreciably radioactive - lanthanum is only 1/10,000th as radioactive as thorium. Radioactivty in lanthanum containing lenses is due to the intentional inclusion of thorium in the optical glass mix. The presence of thorium can sometimes, depending on the mixture of other elements in the lens, cause moderate to severe browning of the lens element(s).
Typical radiation levels can approach 1 mR/hr as measured at the lens element's surface, decreasing substantially with distance. For reference, a typical chest x-ray consists of about about 10 mR.
By far the most prolific producer of radioactive lenses was Eastman Kodak. Substantial numbers of amateur cameras were produced and sold with thoriated (containing thorium oxide) lenses, including some of the Kodak Pony, Signet, and high end Instamatic cameras. In addition, many professional level Ektar lenses from this era contain thorium. Perhaps the
