Fixing Radioactive Pentax Lenses

It got a bit dusty in the two days it was under the light

Back in the day, if you wanted to add a bit of zing to the rear element of your camera lenses, dropping some Thorium into the glass mix was the thing to do. Never mind that Thorium is radioactive, it changes the refractive index of the glass and improve sharpness. However, after a few years the Thorium in the glass undergoes radioactive decay, causing it to change colour and give pictures a distinct yellow tinge. I noticed this in some pictures I took a while back. And for the last few days I’ve been doing something about it.

For the last three days or so I’ve been shining an Ikea led lamp onto the lens. This lamp apparently gives out a fairly bit of ultraviolet light which triggers another stage in the radioactive decay process and causes the Thorium to turn transparent again. I think it has made a difference, but I’ll need to take some more colour pictures to find out properly.