For context I have a film camera that is pretty old and has a “yellow” build up in the lens. The glass of the lens is made of thorium and from what I understand is that lenses that are constructed with Thorium have a high refractive index based on what google has shared with me.
How in the world does the lens become yellow? How is UV light a means to deyellow it?
There are videos out on youtube that provide solutions to use UV light as a means to “Deyellow” a lens but not too many of these camera guys explain too much about it.
In: Physics
This one is pretty interesting.
The lens is not “made of thorium” but instead is mostly glass with some thorium added to it which increases the refractive index. Thorium is radioactive however and is constantly throwing out alpha and beta particles, as well as gamma radiation.
That radiation is capable of knocking electrons out of place in the glass’s lattice, and glass is an insulator to the point that electrons within that lattice are not mobile enough to fill in those gaps. As a result it creates “color centers” where those electron holes impact how light passes through the glass, creating the yellowing effect in old thorium lenses. Each decay event is capable of causing tens or even hundreds of thousands of color centers. The lenses can also be pretty significantly radioactive so you probably want to take some care where you store them to reduce your own dose.
Exposing the lens to UV light adds enough energy to the glass’s lattice that electrons are able to move around to fill those color center electron holes, reversing the yellowing effect by essentially repairing the damage caused by the decaying thorium.
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