Narrow-beam LED lights and the Sun

I apologise if this has been covered already (I searched, but didn't find anything), but I was wondering what might happen if an LED light with a narrow beam is [accidentally or otherwise] pointed at the Sun on a clear day?

With optics being reversible things, presumably the LED at the optic's focus could end up being heated up, possibly to terminal temperatures?
I guess that reflector/optic frontal area is one factor that would dertermine how hot things might get, but I'm more uncertain what the effect of varying beamwidths might be.
Are there any rules of thumb that might indicate sunsafe/unsafe reflector/optic diameters, etc

If/when we next get some sun where I am, I may bodge together some test pieces from obsolete reflectors and LEDs and see what happens, but I'd be interested in any other opinions or information.

I'm fairly sure that even if you set the emmiter under a magnyfying glass for a split second it will not have any adverse effect... let alone optics.
I will have to say that your concern is more or less invalid.

Oh and i think this belongs more to the General Flashlight Discussion part of the forum...

Hi,

good question.
I would guess that it wouldn't be a problem, you would need quite a bit of luck for a significant amount of the light to hit the LED itself.
I doubt the warming would be anything the heatsinking could not dispose of.

Any particular reason you ask?

Any particular reason you ask?

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Just thinking about lights mounted on helmets lying around on the surface on a sunny day.

Actually, this is something to be careful of.

Considering that a 2" diameter lens can focus sunlight to the point of burning paper or melting solder, it can have an adverse affect on the LED.

Many LED domes are made of acrylic or epoxy - that temperature could start to melt or deform them. However, I'd say the real vulnerability would be the phosphor on white LEDs. You could end up "burning" the phosphor on a white LED quite easily.

Tell you what, today is going to be a pretty sunny day (kind of overcast). I'll take a crappy, disposable Luxeon high dome, and use a 2" mag reflector to focus sunlight onto it, then post the results here.

Playing with a 52mm diameter reflector and the [UK] sun today, it seemed easy to burn through black paper, somewhat harder with white paper.
I'd guess using half such a reflector with an LED (as I do), there could be enough heat to cause some damage, given the appropriate unlucky alignment, though possibly the thick glass front on the lamp might block some of the invisible radiation from the Sun. I'll have to make up a test piece and give it a try in an actual headset.

In reality, I doubt it would be a problem except in rare circumstances, but I'm wondering if I should put a small warning in my documentation just to cover myself.

I always make sure my spectacles (when not being worn) are not in direct sunlight, you never know.

I have two types, normal, and VDU, and swap depending on circumstance.

Well, some high clouds rolled in, so it's rather hazy here.
Even with that, I did get some results:

This is what happens with the 2.5" lens. This is 11AM sun with partially overcast skies (maybe 50% sun strength)






This is a high dome Luxeon emitter. The melted spots on the black ring happened within a second of the beam hitting them - they started melting and smoking immediately. The side of the acrylic dome has also melted as a part of the ring melting.

Direct exposure to the die doesn't seem to have burned the phosphor, but the dome appears to have formed small bubbles in it. With stronger sunlight I wouldn't doubt that the phosphor/die could experience some damage.

The mag reflector was much harder to focus, so I wasn't able to do much with a Luxeon emitter, but I was able to focus it on some wood, and it immediately started burning the wood at its focal point. So even though I couldn't get results today, a mag reflector still poses a risk to the device sitting in its focal point.

Given that your LEDs will be located exactly at the focal point of the optical system, then sunlight hitting the light's optical system on-axis will focus all of that optical energy to a small point. Even light not quite hitting on-axis will still be significantly focused, and could hit other parts, like the plastic outer ring, causing similar damage to that shown above.


So I would definitely protect LED lights from direct on-axis exposure to the sun - I think there is significant risk of some kind, given that parts of the body of the LED experienced damage immediately. It only takes a second or two of exposure to start melting plastic - if a light is left out and by chance receives on-axis light from the sun, exposure time could be several minutes of perfectly focused sunlight - that has the potential of causing significant damage.

evan9162 said:

but I was able to focus it on some wood, and it immediately started burning the wood.

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Wish we had weather like that here.

Using a lens I can see that happening.
Optics are designed to bounce the light out of the front pretty irelevant of what direction it comes in at. There is also quite a spread out section when the dome fits. Still dont think it would cause damage in a real life situation.

I would stick a sentence in to cover yourself anyway.
As no-one reads those things anyway throw in a few more lines about igniting sheep at 30 paces and making limestone smoke.

If an optic gave a fairly well-focussed image of an LED die at a distance, I'd take that to suggest that a light coming from a distant source could end up focussed onto some small area of the LED die?

The tests from evan9612 are interesting, I'll try and do some of my own when weather permits, both with my regular reflector setup, and maybe an optic, and I probably will put some kind of disclaimer/warning in my manuals.

With the sun past zenith, but minimal clouds between the reflector and the sun, I was able to destroy a seoul P4 in my Aspherical throw monster light, just by aiming it at the sun for less than a minute.

Its a 52mm aspherical lens, so if my super rough calculations are correct, useing only 400watts/M^2 (appearently 1000W/M^2 is common for noon on a cloudless day), my lens is possibily putting up to 8W onto my LED. Now, I know with the 5W laser I used to play with back in college, I could destroy just about anything made of non-reflective metals in short time.

I think this would explain why my P4 is now a dark bubbled puddle that smells terrible.

ok ok maybe I was totally wrong