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    • Wisteria #1. Nikkormat FT2, Nikkor50mm f1.4, Ilford FP4+. 
    • Here's an account of my personal experiences, in case you are in a similar situation: I put a few weekly hours in a local camera shop checking and cleaning up old film equipment the shop receives. A while back we got two very interesting lenses; a Canon 35mm f/2 S.S.C "Concave" and a Konica Hexanon AR 57mm f/1.2. Both lenses are quite valuable, but both were also severely yellowed/browned to the point where light transmission must have been reduced by around 2 stops. Color photography would definitely also be very problematic. Glass yellowing is a typical phenomenon in lenses made with glass containing Thorium and Lanthanum, as the case was for the lenses in question. It is common advice on the internet to expose an affected lens to hours or days of sunlight as UV exposure is said to help reversing the yellow tint process. As I am living in Scandinavia where prolonged sunshine is a rare event and hardly ever happens during the short winter days, I was thinking of using a dedicated UV light to achieve the same. I asked in an unrelated thread here on photo.net and it was suggested using a UV-C light. That UV-C light would be effective made perfect sense although I have no understanding of what is going on in the glass. UV-C effectively kills DNA in organic material, and I imagined it would provide more "punch" over the other more gentle UV light options. I did try to search the internet on previous experiences with UV-C to clarify it's effect on radioactive glass and type of suited lamp, but there seemed to be little if anything. I also consulted some AI chat-bots to see if they could provide some input, but all I got was warnings against using UV-C due to the inherent dangers involved in exposure to that light spectrum. I decided to make my own empirical experiment and got a UV-C light tube from Amazon.de. I created a silver foil lined cardboard box with ventilation holes to keep direct UV-C light exposure contained inside the box - I had no idea of how hot such a tube (25W) would run in a small box, so I installed a temperature probe close to the lens location to make sure I didn't melt the lubricants. I did place the box in a room with an open window to allow the ozone produced by the light to escape. Observations: The UV-C light did not produce much heat - the inside of the box barely rose 2-3 degrees Celsius above ambient temperature. It is very inconvenient to work with UV-C light due to the dangers to you, your pets or plants if exposed to direct light (indirect reflections are OK, but not mirrored reflections from silver foil).  Also it produces ozone which shouldn't be inhaled so you don't want to be in the same room during the process. Most importantly: after more than 48 hours of very close direct exposure to the UV-C light source there were absolutely NO signs of reduced yellowing! (I do believe that all organic material that may have lived inside the lenses was effectively dead).   I re-consulted the internet, where I noticed someone claiming good results using a simple white modern household LED light bulb, and someone else reported good results using a black light UV-A panel (commonly used at parties). Without wanting to throw more money at the project than necessary, I tried with a very bright Philips LED bulb I had at hand which claimid to provide the equivalent light of a conventional 100W bulb. Fast forward 2 days of very close exposure in the foil-lined box: No visible improvement - whatever rays emitted from this particular bulb did not assist the project - other white light lamps may be different. Without much hope, nor a wish to buy a large Black-Light UV-A LED panel, I cheapened out and ordered a simple 11W black-light UV-A LED lightbulb. Lo and behold, after less than half a day in the silver foil box, the yellowing was visibly reduced. After two days, it may not have disappeared completely, but enough that I wouldn't notice it if I didn't know what to look for, and certainly useable for color with a tiny bit of warming if any at all. The bulb claimed to emit UV light in the 385-400 nm spectrum, which is what I would recommend to look for based my experiences - apparently you do not need a very powerful light either - which is convenient as you would want to avoid exposing the lens to high heat.    Further readings on use of radioactive glass in lenses: https://lenslegend.com/radioactive-lenses/
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