Clear circle in glass lens

I tried doing a search on this in the forum but I couldn’t find the right terms to find an answer. I ran two of my Solarforce L2 series flashlights for about 20 minutes each to see how hot they would get. The first was a L2M Mini with a L2T head running a XM-L off a 18350. The second one was an L2P with a XM-L2 off a 18650. They each got warm, the L2T head was warmer than the L2P but neither was too hot to handle. What was curious is each had the same issue. The glass lens on each had a clear spot in the center where the hotspot is but the lens around the clear spot looked cloudy. After they cooled down there was nothing on the glass to explain why the outer ring was cloudy. These are plain glass lenses, not fixed to create a clear spot in the middle. So what created the cloudy area? How do I stop this from happening?

I will say I am impressed that neither flashlight got too hot. I haven’t wrapped the drop in with aluminum foil yet, maybe after I do that the heads will get hotter. Could this explain the cloudy glass? I was also impressed with the 18350 battery. I ran it down to 3.6v and it still had lots of power.

Perhaps some moisture evaporating?

The lens is dirty or mediocre clarity, when the light is on you see it. There's not enough light coming out the center to make it visible. The central 'hotspot' hasn't formed yet at that close range.

I use these in my Solarforce L2 series heads they are much better than the ones supplied.

When I look at the lens when the light is off it doesn’t look dirty or unclear. When I shine a light into the lens from the outside it all looks clear.

I had been looking at those lenses. How do you like them? Their coating is claimed to increase the lumens by 7 or 8%.

What Gj said, the ligt intensity on the lens is rather high and shows every irregularity, in the middle it is less, every reflector flashlight wil have this effect more or less. Try cleaning the lens on the inside and outside, the effect could be less.

All my lenses have been cleaned with windex and a soft cotton cloth. Over the week end I’m going to wrap the drop ins with aluminum foil and run them for 20 minutes again and see if that changes the heat transfer to the head and the cloudy glass issue. I may also order a couple of those coated lenses to see if the cloudy ring disappears.

First off,

This should tell you either your LEDs are under-driven or your heat-sinking needs a lot of work. 20 minutes should be long enough to reach thermal equilibrium. Or maybe you need to see a neurologist about that lack of feeling in your hands…

Does this mean you could see the cloudiness just after you turned them off? Meaning it only went away after the heads cooled down? Also, how long did/does it take for the effect to appear?

Just out of curiosity, how can you “tell” they’re “cloudy” looking at the shiny reflectors? Not saying you can’t, just curious, remembering N-Rays & wanting to repeat your experiment myself.

Interesting post! Thanks!


I ran the experiment again. It didn’t happen the second time. I must have not dried the lenses enough the first time and there was moisture in the head.

As far as the heat I wrapped the drop ins in ordinary aluminum foil and ran one experiemnt. I didn’t let it run for 20 minutes. After 12 minutes it was hot, not burning hot but hotter than it was the first time. And there was no cloudiness to the lens.

I have a couple of AR coated lenses coming from KD and will see if I can perceive any difference in the amount of bright. Is there a recommended way to clean AR lenses to not hurt the coating?

I cover the lens with soot from a wax candle and wipe it off with a clean microfiber. Those coated lens are cheap enough that I wouldn’t worry too much about giving them special treatment though.

Why do you do it that way? TIA

It’s a trick I learned from a photographer friend. He carries around a carbon “lens pen” for his cameras, but he pointed out that the soot from an ordinary candle or butane lighter works just as well (though I suppose the heat can pose a risk if one’s not careful). For some reason the carbon layer “clings” onto dirt and grease, and once the residue is wiped off you’re left with an absolutely spotless lens. I’m sure there’s some scientific reasoning behind why this works that I don’t know of :weary:

Good stuff, thanks!