Radioactive lenses update

Radioactive lenses update

You may remeber that recently I go ahold of a couple of lenses with Thorium in the glass, and successfully un-yellowed them. Well, the Thorium in the glass is supposedly radioactive, so I though it would be fun to get a hold of a Geiger Counter and see for myself- I was partly inspired by this amusing video…

So I got ahold of a Geiger Counter for myself and tried it on all my lenses, new and old, Russian, American and Japanese.

First I did a control check to see what the general background radiation in the apartment is, about 21 counts per minute or 0.14 micro-Sieverts/hour (uSv/h)-

Only 4 of my lenses turned out to be at all radioactive- all Takumars of some sort. My 8-element Super-Takumar 50mm f/1.4, which is not supposed to be radioactive at all, surprised me by reading at about 0.61 uSv/h-

Then two which I knew had Thorium glass, the Super-Multi-Coated Takumar 55mm f/1.8 and the 7-element Super-Takumar 50mm f/1.4 which read 3.89 uSv/h and 5.21 uSv/h respectively-

But the one that really set the Geiger Counter crackling (ignore the clothes dryer in the background) …

… was the 7-element Super-Multi-Coated Takumar 50mm f/1.4 which topped out at a whopping 7.87 uSv/h!

But to put this into perspective, you get 0.1 uSv from eating a banana, a chest CT scan is about 7 milli-Sieverts of exposure (that’s 7,000 uSv) and a radiation worker is permitted to get 50 milli-Sieverts per year of exposure (50,000 uSv), so even this lens is really not at all dangerous considering if you move the Geiger Counter a couple of feet or so away the reading drops to almost nothing. I would not recommend making a pillow out of 50 if them to sleep on every night, but casual use with a camera body between it and me is not anything to worry about. Even without the camera body to block it, considering my camera is probably held up to my eye for 10-20 seconds for each shot I take, if I take 100 pictures that’s less than half an hour, a maximum of 4 uSv in a session, less than a tenth of what you would receive flying from New York to LA! (if you’re interested in finding out more about radiation doses, this excellent XKCD Chart is very informative!)

But just to be on the safe side I have bought a nice metal box to store these four lenses in-

With the lid closed the maximum reading I could get with the Geiger Counter right against the box was 1.29 uSv/h, and move a foot away and it drops completely to 0.15 uSv/h, almost exactly the same as the background radiation I measured at the beginning 🙂


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