In this article I would like to elaborate about radioactivity in vintage manual lenses. Also, I will dive into radioactivity in general and talk about the color cast it can cause in certain lenses. I updated this section to include the real cause for the color cast.
There’s a lot of conflicting information considering radioactivity in photography gear around the web and most of it will likely be crap anyways.
What I can say for sure is that there ARE lenses out there that are radioactive because I measured their activity myself.
And if that didn’t get your attention, maybe this will:
The lenses in my collection measured MORE radioactive than our uranium samples we use at school to teach about radioactivity. You can read about my own collection of radioactive lenses in a later article.
Why would they build radioactive lenses then?
Thanks to Anu for correcting me. I took my info from the wrong source and checked with Wikipedia to put my infos right.
If you are interested in modern SLR lenses you might have stumbled upon the Fluorite term from time to time. Fluorite is a crystal formed by calcium fluoride that shows
impressive refractive very low dispersion properties (meaning that a ray of white light will mainly stay one ray and not split up, which would cause chromatic aberrations!) but also a low refractive index, which is bad if you want to focus light rays for photographic purposes (and also the reason, why Fluorite glass is almost exclusively being used in super tele lenses).
You will most likely also have heard about the legends that formed around this material?
It’s said that Canons 1’200mm f/5.6L USM lens uses optics made of Fluorite and that the crystals required for these big glass elements take one year to grow. (source)
So there you have the reason for the horrendous prices the lens manufacturers charge for their new models. (They have some Ph.D. of crystallography sitting next to an aquarium containing a saturated solution of calcium fluoride and waiting for the crystals to grow.)
Why is that even relevant, you may ask?
Well, as it happens, before Fluorite there was Thorium Oxide. (thorium dioxide, to be precise. See the Wikipedia article for more info.)
It’s a crystal that has
a similarly high a very high refractive index as todays calcium fluoride, which allows the lenses to be designed in a way that minimizes spherical aberration (which would result in uneven bokeh). Like Fluorite, Thorium Oxide causes only little dispersion and therefore should not cause too much chromatic aberrations. Lens manufacturers started experimenting and using thorium oxide in their higher tier lenses after 1940 and through the 1970s.
Why did they stop using it?
While radioactivity a hundred years ago was nowhere close to being understood by the people working with it (e.g. the “glow-in-the-dark” watch faces with paint containing radium, or the Revigator that was used to infuse drinking water with radon gas to make it more natural – though the danger in that device comes from the toxic byproducts of the radioactive decay and not the radioactivity itself) this changed after WW II.
When the reputation of radioactivity turned towards the one of an invisible monster/bogeyman the public started rejecting those lenses.
The reason (for not using thoriated lenses anymore) is that the manufacturers used a lot of thorium oxide for these lenses and thorium is an unstable element. With the vast majority of thorium being Th-232 the half-life of this element measures over 14 billion years, resulting in a really low number of decays per time. (the shorter the half-life, the more radioactive a substance is. the longer the half-life, the longer a substance will last. Ask Greenpeace…)
Thorium almost exclusively emits alpha-particles. These particles can be imagined as the cannonballs of radiation whereas beta-particles are bullets and gamma-rays are, well, laser beams.
Because air itself is not made of nothing but of atoms and molecules, those heavy and large cannonballs have to get past all these obstacles. You can imagine that this gets more and more difficult with increasing size and therefore the alpha-particles have a very limited reach in air and even more so the denser the material gets they have to pass through.
(rule of thumb: you need several feet of lead to block gamma radiation but only a few cm of acrylic glass to block beta-particles and can block most alpha-particles with a sheet of aluminium foil.)
On the other hand, it should be clear that the cannonball has the largest potential for destruction IF it finds its target.
– Therefore, sources of alpha-particles are most dangerous when they enter the body (imagine the “bull in a china shop”).
– Because the eyes are very sensitive, it’s important to keep radioactive sources away from this organ! (NEVER ever get a thoriated viewfinder/magnifier for your film camera!!!)
– The skin is very resistant against alpha-particles. They cannot simply pass through and the repair mechanisms there are pretty good. (They have to be when you consider all the cosmic radiation (UV and gamma-rays) they are exposed to all the time!)
Therefore, there were some safety concerns that had to be addressed regarding these thoriated lenses:
– If you were so stupid as to break such a lens (beware, it’s painful to look at these images) the radioactive thorium dioxide would no longer be contained and in powder form could easily be ingested. And that’s where it gets really dangerous!
– the reach of the radiation is limited and cannot pass through dense material, therefore you don’t get any radiation when the lens is mounted on the camera.
If you were, however, to use it as a magnifying glass (who does that?!) the lens comes close to the eyes and they don’t do well with alpha particles.
I will publish my measurements later next week and explain my findings.
But I can say so much already:
Unless you do the things I mentioned directly above, you will be perfectly safe!
A few comparative calculations to underline this claim:
– You would have to directly look through the lens for 3 – 10 hours at a time to absorb a similar level of radiation than you would when you get your jaw x-rayed at the dentist.
– To get to a “dangerous level” (20 mSv/y is the legal limit in switzerland for people working in radiology or nuclear power plants) you would have to look through the lens for almost 800 hours!
– If you were to hold the lens only 10cm away from your face (like when it’s mounted to the camera) you would have to look through the camera for more than 5’000 hours to reach the dose you absorb every year through natural sources of radiation (that’s with the lens cap instead of the much denser camera body in between face and lens!)
The real problem with thorium dioxide – UPDATE: 23.09.2015
The internet bursts with conspiracies as to why the thoriated lenses get a yellow color cast. (this image shows a grey card, photographed through the Fujinon 50mm f/1.4 with the white balance set according to the light source!)
This image show my Pancolar 50mm f/1.8 in front of a white background. You can clearly see the yellow glass.
As I’ve shown in this article, the color cast is real and can be dealt with.
It’s clear and has been shown by dozens of patient people that the color cast can be removed when the lens is exposed to sunlight for several weeks.
I, too, did show that you can also remove the color cast within a few days of exposure to the IKEA LED lamp “Jansjö”.
But why is there even a color cast?
Update: My former explanation made complete sense but turned out to be too far-fetched, obviously. After a fellow photographer on fredmiranda.com pointed me to the solution, it all seems clear now.
The color cast is caused by radiation. More specifically, it is caused by so-called color centers in the crystal which were created due to exposure to radiation (read the article in the link if you would like to know more). These color centers are, simply put, holes in the crystal structure that “swallow” some of the light passing through, thus resulting in a colored crystal.
The following article also explains why the color centers can be cured by exposure to heat or light of the absorbed wavelength.
Here’s a list containing many of the lenses known to be radioactive: