This works, submerge the lamps in hot water and strong bleach solution, half a day should do it, this will remove yellowness from the glass, inside and out, and also return the adhesive sealer back to white again. When done, rinse with very hot tap water, and allow to drain, final drying can be done with a hair dryer/hot air gun... your lamps will be sparkly like new again 
Remember to use gloves and goggles if using bleach, and out of anyones way. Safety first 
Thom
Thanks Thom but does this not damage the silver reflective backing?
No, doesn't touch it, done it many times over the years, and never once had a problem with any headlamps, obviously if a reflector is already pitting, flaking, or corroding, it wont make it any better!
Below are my lights, that had they'd been available some years ago, I would not have hesitated to replace them, they were that bad! the results are Crystal clear (pun intended) outer lamps were brand new items when pic was taken, about 4 or 5 years ago...
Those lamps were around 20 years old at the time, and still look as fresh today
Thom
