Vintage lenses are wonderful things. I have a collection of about 50 various lenses that are all at least 40 or 50 years old in various mounts including Nikon F, M42 and Canon FD. Some lenses, though, can develop a yellow tint over the course of many years due to the radioactive thorium used in some of the elements and their coatings.
It is possible to reverse this yellowing, though, as Mathieu Stern demonstrates in this video. People often recommend leaving old lenses out in the sun to help solve this issue, but in much of the world right now, there aren’t copious amounts of high UV sunlight. So, Mathieu shows us how we can do it indoors, at home, using a UV blacklight.
The process is quite simple. Place a mirror down in a box and pop your lens on top. Mount the UV LED floodlight above that, turn it on and then leave it alone for 12 hours or so. After that, flip the lens upside down and run the process again for another 10-12 hours.
UV Blacklights, like the one Mathieu uses in his video, typically hover in the 385-415 nanometre range, which is right on the very edge of the visible light spectrum as we head towards the dangerous ultraviolet light that we can’t see. And while they are also available in strips, I would imagine Mathieu’s floodlight is more effective on lenses as it directs and pushes more of its light through the optics.
The LED strip option might be a good idea if you have a LOT of lenses you need to do at once, though. That being said, I suspect you’d probably be leaving them in there for a couple of weeks for enough UV light to hit the elements and clean them up. You’d want to line the inside of whatever tub you’re using with tin foil or some other reflective material to maximise exposure.
Do note, however, that while these such LEDs do present on the edge of the visible light spectrum, they usually also edge just outside of it, too. And while I don’t know the potentially harmful effects of exposure of these wavelengths to human skin and eyes over the course of 24 hours or more, I’d probably do it in a room that won’t see continuous human traffic for the duration or I’d do it inside something that won’t allow any light to leak out.