I have bought quite a few lights over the last year,and it seems like most of them suffer from the same issue from hard to clean glass.I got new reading glasses and I can’t believe how these lights come with dust on the reflector inside and on the glass inside.Even my high dollar ones have dust,so,one by one I have been taking them apart and cleaning them.A few lights that have never been taken apart seem like they have a ring that you can see on mid to high mode.Does the high output do something to the coating,because it seems like its in the glass.I used good mild cleaner ,cannon,and a soft scope glass cloth,and then blow it with compressed air,and it just don’t seem to go away.Any thoughts or suggestions would be appreciated?
Electronics and cables often gives off miscellaneous gases when heated. These gases may condense on the reflector or on the lens if they are cold.
I would suggest boiling in water with a little detergent. Or try different strong solvents like benzine or acetone.
You can often smell these gases, especially when the electronics is new and used for the first time. It is stuff like fire retardants, plastic softeners for PVC insulation in cables or other additives to plastics and electronic components. Typically this is very dangerous stuff, but I doubt that there is enough in a flashlight to be dangerous for a user. It may be different for the workers that build these flashlights…
For instance stuff like polychlorinated biphenyls, tetrabromo-bisphenol-A, polybrominated biphenyls, polybrominated diphenyl ethers, diisononyl phthalate, diisodecyl phthalate, and di-2-ethylhexyl phthalate. (PCB, TBBA, PBB, PBDE, DINP, DEHP, DIDP.)
Some people seem to be allergic to these compounds and gases. The allergic reaction may be confused with a reaction to high frequency electromagnetic radiation. Some people actually seems to have severe allergic reactions to fluorescent lighting, cellphones and computers, but it may actually be to the gases that these devices gives off when used, rather than to the high frequency electromagnetic radiation. And perhaps some people are allergic to high powered flashlights as well? :~
One way to minimize the problem could perhaps be to “burn in” a flashlight by removing the lens and the reflector and turn the flashlight on at max setting for a few hours or so, while ventilating. This is just speculation on my part. I have never tried it. But if someone buys two identical flashlights it could be interesting to test it. Do a burn-in on one, with the lens and reflector removed. And use the other as normal. And see if there is some difference on how the lens and the reflector looks after a while. I am not at all sure that a difference can be detected, but if it could, it would be nice to know.
That’s interesting,I was wondering if the heat produced by the led was causing the donut effect.I did not think about gases trapped in the light by manufacturing.
As I said, it is speculation on my part. I don’t know if the gases actually condense on the lens and reflector. But it is perhaps plausible? And perhaps even testable…
I have had spotlight reflectors get fog using HID the solution was to burn in the bulb for about 2 hours before installation, so it is plausible.
I may test it some day, if I ever buy two identical flashlights. Until then I think I might do a burn-in on new flashlights, without lens and reflector, it shouldn’t harm and it might be beneficial…
A new Uniquefire UF-T20 is famous for stuff evaporating in the head and condensing on the lens, I left mine on high for half an hour without the lens and it has been fine since.
I googled and I did indeed find mention of this on a well known forum for flashlight experts. 
It seems that grease and lubration also can evaporate and acerbate the effect.
Yes I think you are right,just for the heck of it I let my big m3x run and get hot without taking it apart and can see a difference in the glass.Next step will be to take it completely apart and clean the lens with a lens pen while letting it run turbo for quite a while.My m22 has the same fog ring in it,the only light that I own without dust on the reflector and sparkling clean glass is my nitecore srt7,go figure.I have the new thrunite tn32 and tn30 and both have dust down in the reflector,the glass is clean but they have only been run for short bursts.Its just a shame with the technology we have that these conditions exist,especially for what these things cost.Yes I know you can’t be so anal,but I am. LoL