In the 1940s, scientists and lens designers at Kodak, a world-famous camera and filmmaker, started experimenting with mixing rare earth elements into their lenses. After a series of tests, they found out that by adding Thorium 232 to lenses they can improve the quality of images produced. Not long after, other companies followed and as a result, many vintage lenses produced between the 1940s-1970s contain Thorium which makes them radioactive.
Kodak M18 Instamatic
Very popular, hand-held camera which was manufactured between 1967 and 1969 and it has a small Thorium lens inside.
Thorium oxide (ThO2) was added to lenses due to its optical properties such as high refractivity and low dispersion which allowed to minimize chromatic aberration. Some companies experimented also with Lanthanum, however, these lenses weren’t so widely manufactured as Throium lenses and they don’t have significant activity since only a very small percentage of natural Lanthanum is radioactive.
These lenses often are pretty spicy in terms of radioactivity which can sound scary first but in reality, they are 100% safe to use unless you grind the thoriated glass into a powder and inhale it or eat it. Thorium and its decay products emit alpha beta and gamma radiation. Alpha and beta particles are easily stopped by the glass itself as well as the metal body however there is a small amount of gamma leaking through. So should we be worried? Not at all! The amount of gamma radiation leaking from the lens is actually smaller than standing on granite bricks. This being said, I wouldn’t leave the lens mounted on a camera for long periods of time simply to reduce radiation exposure to the sensor.
Takumar SMC f/1.8 55mm
A fantastic lens which can be found for not a lot of money. It is worth mentioning that not all of these lenses contain Thorium so finding one that does is a lottery.
Yellowing of radioactive lenses
One of the most characteristic features of radioactive lenses besides their radioactivity is their yellowed glass. The reason why the glass in Thoriated lenses has a slightly yellowish/brownish tint is simply because of radiation coming from the Th232 alternating atomic structure of glass. This yellowing can be easily reversed by exposing the lens for few hours to a UV light but I personally do enjoy the tint so I will leave my lens as it is.
While yellowed glass is a good indicator for a lens being radioactive, it is not always true. As mentioned earlier, some people restore their lenses to their original form, also during my research I also have found some yellowed lenses that did not contain Thorium. If you want to buy a radioactive lens I would suggest visiting your local flea market and bringing a small Geiger counter with you or doing a lot of research on different lens models and comparing the serial numbers with lenses known to be hot.
Gamma spectroscopy of the Takumar SMC f/1.8 55mm radioactive lens
As expected, gamma spectroscopy of the Takumar SMC lens clearly shows the presence of thorium and its decay products.
Extra links that might be useful
A list of radioactive lenses [Click Here]
A list of radioactive Takumar & Pentax lenses [Click Here]
Great YouTube video on radioactive lenses by “Simon’s utak” [Click Here]
A few photos shot on a Canon M50 with a K&F adapter and a radioactive Takumar SMC f/1.8 55mm lens.
All these photos were shot with a Canons standard picture style and no post processing except balancing the exposure in some cases