I was keen to try this out so I went and bought a cheap no-name brand LED desk lamp from Woolworths, similar in style to the Ikea lamp that people have been posting about above, and tried it out on a significantly yellowed Takumar S-M-C 35mm f2 lens. I put the lens front element down on to a compact mirror and shone the LED lamp through the rear element from above. The results? I checked it ten hours later after just leaving it overnight and the improvement was amazing. Right now it's at the two and a half day mark and I don't think there's been much improvement since my initital check, but it's still early days. Maybe that last bit of yellow is hard to kill. I'll continue to leave it under the lamp and see what happens over the coming days.